Advanced Ceramic Coatings: Fundamentals, Manufacturing, and Classification [1 ed.] 9780323996594, 0323996594

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Elsevier Series on Advanced Ceramic Materials

Advanced Ceramic Coatings Fundamentals, Manufacturing, and Classification Edited by

Ram K. Gupta Associate Professor, Department of Chemistry, Pittsburg State University, Pittsburg, KS, USA

Amir Motallebzadeh Surface Science and Technology Center (KUYTAM), Koç University, Turkey

Saeid Kakooei Head of the Centre for Corrosion Research (CCR) and Senior Lecturer in the Mechanical Engineering Department at the University Technology PETRONAS

Tuan Anh Nguyen Principal Research Scientist, Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam

Ajit Behera Assistant Professor, Metallurgical and Materials Engineering Department, National Institute of Technology, Rourkela, Odisha, India

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2023 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-323-99659-4 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals Publisher: Matthew Deans Acquisitions Editor: Gwen Jones Editorial Project Manager: Emily Thomson Production Project Manager: Anitha Sivaraj Cover Designer: Christian J. Bilbow Typeset by TNQ Technologies

List of contributors

Vinit Kumar Agarwalla Darmstadt, Germany

Institute of Materials Science, Technische Universit€at,

Mohd Faizal Ali Akhbar Department of Naval Architecture and Maritime Technology, Faculty of Ocean Engineering Technology and Informatics (FTKKI), Universiti Malaysia Terengganu, Kuala Nerus, Terengganu, Malaysia Rodianah Alias Faculty of Innovative Design and Technology, Universiti Sultan Zainal Abidin, Gong Badak Campus, Kuala Terengganu, Terengganu, Malaysia Yousef Alshammari School of Engineering, Faculty of Science & Engineering, University of Waikato, Hamilton, New Zealand Mohammad Azadi Semnan, Iran T. Avanish Babu

Faculty of Mechanical Engineering, Semnan University,

SAS, Vellore Institute of Technology, Vellore, Tamil Nadu, India

Ajit Behera Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela, Odisha, India Vijaykumar S. Bhamare Centre for Nanoscience and Nanotechnology, Department of Chemistry, KLS Gogte Institute of Technology, Belagavi, Karnataka, India Aarti S. Bhatt Department of Chemistry, NMAM Institute of Technology, Affiliated to NITTE (Deemed to be University), Nitte, Karnataka, India Mansoor Bozorg Faculty of Chemical and Materials Engineering, Shahrood University of Technology, Shahrood, Iran Pasquale Cavaliere Lecce, Italy

Department of Innovation Engineering, University of Salento,

Arunkumar Chandrasekhar Department of Sensors and Biomedical Technology, Vellore Institute of Technology, Vellore, Tamil Nadu, India Maria Covei

Transilvania University of Brasov, Bras¸ov, Romania

xii

List of contributors

R.C. Cozza S~ao Paulo State Technological College, Faculdade de Tecnologia de Maua, Centro Estadual de Educaç~ao Tecnol ogica “Paula Souza”, S~ao Paulo, Brazil; FEI University CentereEducational Foundation of Ignatius Priest Saboia de Medeiros Department of Mechanical Engineering, S~ao Bernardo Do Campo, S~ao Paulo, Brazil Bogatu Cristina Transilvania University of Brasov, Bras¸ov, Romania Soumen Das Materials Chemistry and Metal Fuel Cycle Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, India T. Dharini Centre for Nanoscience and Nanotechnology, Centre of Excellence for Energy Research, Sathyabama Institute of Science and Technology, Chennai, Tamil Nadu, India Anca Duta

Transilvania University of Brasov, Bras¸ov, Romania

Kaniz Fatma Department of Chemistry, School of Applied Sciences, KIIT University, Bhubaneswar, Odisha, India L.C. Fontana Universidade do Estado de Santa Catarina, Centro de Ciências Tecnologicas, Departamento de Física, Laborat orio de Plasmas, Filmes e Superfícies, Joinville, Santa Catarina, Brazil Santhosh G Nano-Materials and Energy Devices Lab (NMEDL), Department of Mechanical Engineering, NMAM Institute of Technology, Nitte (Deemed to be University), Nitte, Karnataka, India Mehran Ghasempour-Mouziraji TEMA - Centre for Mechanical Technology and Automation, University of Aveiro, Portugal Mohd Hamdi Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia Manjunath S. Hanagadakar Department of Chemistry, S.J.P.N. Trust’s Hirasugar Institute of Technology, Nidasoshi, Karnataka, India Morteza Hosseinzadeh Department of Engineering, Ayatollah Amoli Branch, Islamic Azad University, Amol, Iran Tismanar Ioana

Transilvania University of Brasov, Bras¸ov, Romania

Ashish Jain Materials Chemistry and Metal Fuel Cycle Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, India; Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai, India Vaishnavi Khade

SAS, Vellore Institute of Technology, Vellore, Tamil Nadu, India

A. M. Kamalan Kirubaharan Centre for Nanoscience and Nanotechnology, Centre of Excellence for Energy Research, Sathyabama Institute of Science and Technology, Chennai, Tamil Nadu, India; Coating Department, Fun Glass-Centre for Functional and Surface Functionalised Glass, Alexander Dubcek University of Trencin, Trencin, Slovakia

List of contributors

xiii

Raviraj M. Kulkarni Centre for Nanoscience and Nanotechnology, Department of Chemistry, KLS Gogte Institute of Technology, Belagavi, Karnataka, India P. Kuppusami Centre for Nanoscience and Nanotechnology, Centre of Excellence for Energy Research, Sathyabama Institute of Science and Technology, Chennai, Tamil Nadu, India Pay Jun Liew Faculty of Manufacturing Engineering, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, Durian Tunggal, Melaka, Malaysia P. Mallick Department of Physics, Maharaja Sriram Chandra Bhanja Deo University, Baripada, Odisha, India Amit Mallik Department of Chemistry, Acharya Jagadish Chandra Bose College, Kolkata, West Bengal, India Ehsan Marzban Shirkharkolaei Department of Mechanical Engineering, Isfahan University of Technology, Isfahan, Iran J.M.C. Miscione Universidade de S~ao Paulo Escola Politécnica, Departamento de Engenharia Metal urgica e de Materiais, S~ao Paulo, Brazil Supratim Mukherjee Materials Chemistry and Metal Fuel Cycle Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, India Mojtaba Najafizadeh The State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, China; Department of Innovation Engineering, University of Salento, Lecce, Italy Tadachika Nakayama Extreme Energy-Density Research Institute, Nagaoka University of Technology, Nagaoka, Japan Son Thanh Nguyen Department of Creative Engineering, National Institute of Technology-Kushiro College, Kushiro, Japan Ayahisa Okawa Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Japan Rupayana Panda Department of Chemistry, School of Applied Sciences, KIIT University, Bhubaneswar, Odisha, India Subhasis Pati

Regional Institute of Education, Bhubaneswar, Odisha, India

Muhammad Rizwan Faculty of Chemical and Process Engineering, NED University of Engineering and Technology, Karachi, Pakistan J.C. Sag as Universidade do Estado de Santa Catarina, Centro de Ciências Tecnologicas, Departamento de Física, Laborat orio de Plasmas, Filmes e Superfícies, Joinville, Santa Catarina, Brazil Priyatosh Sahoo Germany

Institute of Materials Science, Technische Universit€at, Darmstadt,

xiv

List of contributors

C.G. Sch€ on Universidade de S~ao Paulo Escola Politécnica, Departamento de Engenharia Metal urgica e de Materiais, S~ao Paulo, Brazil Humair Ahmed Siddiqui School of Engineering, Faculty of Science & Engineering, University of Waikato, Hamilton, New Zealand; Faculty of Chemical and Process Engineering, NED University of Engineering and Technology, Karachi, Pakistan F.C. Silva Universidade de S~ao Paulo Escola Politécnica, Departamento de Engenharia Metal urgica e de Materiais, S~ao Paulo, Brazil; S~ao Paulo State Technological College, Faculdade de Tecnologia de Cotia, Centro Estadual de Educaç~ao Tecnologica “Paula Souza”, S~ao Paulo, Brazil; S~ao Paulo State Technological College, Faculdade de Tecnologia de S~ao Paulo, Centro Estadual de Educaç~ao Tecnologica “Paula Souza”, S~ao Paulo, Brazil Gheorghita Silvioara

Transilvania University of Brasov, Bras¸ov, Romania

Hisayuki Suematsu Extreme Energy-Density Research Institute, Nagaoka University of Technology, Nagaoka, Japan Masahiro Todoh

Faculty of Engineering, Hokkaido University, Sapporo, Japan

Jasaswini Tripathy Department of Chemistry, School of Applied Sciences, KIIT University, Bhubaneswar, Odisha, India Ion Visa

Transilvania University of Brasov, Bras¸ov, Romania

Madhuri Wuppulluri India

CFM, Vellore Institute of Technology, Vellore, Tamil Nadu,

Jiwang Yan Department of Mechanical Engineering, Faculty of Science and Technology, Keio University, Hiyoshi, Kohoku-ku, Yokohama, Japan Ching Yee Yap Faculty of Manufacturing Engineering, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, Durian Tunggal, Melaka, Malaysia

Characterization methods and characterization of the coatings

1

Rodianah Alias 1 , Mohd Faizal Ali Akhbar 2 , Yousef Alshammari 3 , Humair Ahmed Siddiqui 3,4 , Muhammad Rizwan 4 , Mohd Hamdi 5 and Masahiro Todoh 6 1 Faculty of Innovative Design and Technology, Universiti Sultan Zainal Abidin, Gong Badak Campus, Kuala Terengganu, Terengganu, Malaysia; 2Department of Naval Architecture and Maritime Technology, Faculty of Ocean Engineering Technology and Informatics (FTKKI), Universiti Malaysia Terengganu, Kuala Nerus, Terengganu, Malaysia; 3School of Engineering, Faculty of Science & Engineering, University of Waikato, Hamilton, New Zealand; 4Faculty of Chemical and Process Engineering, NED University of Engineering and Technology, Karachi, Pakistan; 5Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia; 6Faculty of Engineering, Hokkaido University, Sapporo, Japan

1. Introduction In materials science, characterization refers to the process of probing and measuring a material’s structure and properties. It is a fundamental process in the field of materials science, without which no scientific understanding of engineering materials could be gained. A wide range of techniques can be used to characterize and evaluate surfaces. Different properties are given relative importance depending on a variety of factors, such as a coating’s development stage, the need for further optimization, and the intended application [1]. A coating that has been in full production for several years is likely to undergo more rigorous testing than one that is new. In that case, the emphasis shifts to nondestructive testing. In general, the importance of each surface coating property to be characterized and evaluated varies depending on whether the coating is intended for production or research and development use [2]. While the former is concerned with quality control and repeatability, the latter aims to evaluate specific properties and how process variables affect them. A coating that has been optimized and put into routine production requires effective process control, following the adage that good quality should be built in rather than tested out [3]. To ensure consistency, it is important, however, that producers use standardized and repeatable tests. In the future, property validation will become increasingly important as the need for it increases.

Advanced Ceramic Coatings. https://doi.org/10.1016/B978-0-323-99659-4.00017-6 Copyright © 2023 Elsevier Ltd. All rights reserved.

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2.

Advanced Ceramic Coatings

Ceramic coatings

Ceramic materials are the emerging ideal materials for a wide range of engineering applications such as cutting tools, engines, turbines, space vehicles, and biomedical applications due to their superior properties when compared to traditional ceramics [4,5]. The properties of advanced ceramics mainly differ from those of traditional ceramic materials in their processing, composition, and microstructure [6]. Due to their excellent properties, ceramic coatings have been used extensively in surface modification over the last decades [7,8]. Coating metal surfaces with a thin ceramic coating has always proven useful for improving the mechanical properties of metallic substrates. Ultrasonic cleaning, acid dipping, mechanical and electrochemical polishing, thermal treatment, laser surface melting, and plasma exposure are the most used surface treatments [9e11]. Ceramic coatings are primarily used to prevent corrosion and oxidation of base alloys as well as minimize wear damage. In the case of thermal barrier coatings (TBCs), ceramic coatings can also reduce the base metal temperature. Ceramic materials are known for their heat resistance, corrosion resistance, wear resistance, and electrical insulation [12]. At present, there are a variety of ceramic coating characterization methods for protective coatings. Hence, to gain a better understanding of advanced ceramic materials and further develop them for a specific engineering application, extensive research is needed to evaluate their microstructural, mechanical, chemical, optical, and biomedical properties. In this chapter, the main properties of characterization and evaluation will be reviewed.

3.

Characterization method of ceramic coatings

Ceramic coatings are characterized mainly by surface roughness, thickness, and mechanical properties [13]. The stylus profilometer is typically used to measure surface roughness. Despite its limitations, this method is widely used. A stylus used in a profilometer may scratch the surface of very soft films [14]. A noncontacting optical method of profile assessment has been developed in order to avoid the problems caused by stylus contact. Based on two-beam optical interference, the surface profile is measured by a reference surface that is mounted on a piezo-electric transducer, enabling the phase of the interference pattern to be measured, which is proportional to the height at that location [15]. Profiling can also be done using optical principles using confocal scanning microscopy, which uses a tandem scanning microscope in conjunction with a confocal imaging method to provide a 3D topographic image [16]. In this way, both of these optical techniques provide a highly sensitive method of characterizing surface profiles. The scanning tunneling microscope (STM) is even more sensitive. This technique detects atom-scale topography by monitoring electrons flowing between a probe and the surface. The piezoelectric ceramics can be used to move the probe in three dimensions, thereby enabling an atom-by-atom analysis of the surface [17]. Unlike the STM, the atomic force microscope (AFM) can probe insulating surfaces. In this method, too, a probe is used to scan a surface,

Characterization methods and characterization of the coatings

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but instead of a repulsive force, a force that varies with relief is used to deflect the tip, whose movement is monitored by a laser beam, which, in turn, is monitored by a feedback control to produce near-atomic resolution images [18]. The development of various scanning probe microscopes will substantially improve our understanding of surfaces and their behavior. Surface thickness can be measured in many ways, such as optical, removal, electromagnetic, scattering, and excitation [19]. Even though some of the thickness measurement methods achieve great accuracy, they all have strengths and weaknesses. As an example, X-ray fluorescence requires careful calibration but is particularly helpful in coating facilities that produce nominally identical coatings [20]. A multi-layered coating can present problems due to the differences in the definition of the substrate surface. If sections are observed directly by optical or scanning electron microscopy, this can be avoided, but the component must be cut to produce the section [21]. One of the most accurate methods involves using a stylus profilometer with a step-height measurement feature, which of course requires an uncoated region over which to measure the step height [22]. Also available are optical techniques, but they are usually for transparent films. Interference techniques are sometimes used to measure the thickness of opaque coatings. A masking step must also be left during the coating process. Commercially, there are various ways to assess the thickness, generally appropriate for thicker films, as are techniques based on gravimetric measurements [23]. The mechanical properties of coatings are determined by hardness, adhesion, pulloff, and scratch indentation tests [24,25]. The most common hardness measure for coatings is the Vickers or Knoop microhardness using GPa or kg/mm-2 units [26]. Some think that adhesion is the most important coating property, since if it is inadequate, all functionality of the surface may be lost. Various methods have been proposed to measure adhesion. Many people believe that adhesion is the most important coating property since, if it is inadequate, all functionality of the surface may be lost. Many methods have been proposed to measure adhesion. These technologies have their origins in technologies where bonding forces are relatively low, such as painting, and are therefore limited in their application. An adhesion pull-off test is an industrial standard that is used to determine the durability of protective coatings and if they are suitable for service. The scratch test, sometimes called the stylus method, has been widely accepted by scientists and the industry for adhesion assessment [27]. Generally, an indenter is pulled across the coating surface under increasing normal loads until it detaches. This load gives an indication of adhesion strength and is often referred to as the critical load [28]. As coating processes have improved in recent years, the range and capabilities of surface analysis techniques have improved as well. Tribologists use these measurements not only to identify and optimize new coatings, but also to better understand friction, wear, and corrosion. Physicochemical analysis techniques in conjunction with mechanical characterization are crucial to the development of future models that correlate production parameters with structural and material properties for optimized performance.

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Advanced Ceramic Coatings

3.1

Physicochemical characterization

The physicochemical properties for ceramic coatings comprise microstructure, elemental composition, hydrophobicity, adhesion, topography, phase, crystal structure analysis, and surface chemistry, as tabulated in Table 1.1.

3.1.1

Microstructure

The microstructural characterization of the developed ceramic coatings was carried out using field emission scanning electron microscopy (FESEM). A surface microstructure can reveal material structures on a microor nanoscale from either a surface morphology view or cross-sectional images. To view the cross-sectional images, particularly for flat specimens, the specimens were ground on edge when holding the sample vertically. As a standard case, cross-sectional images of ceramic coating were obtained by scratching the specimen’s surface [29]. The chemical element concentration was determined using FESEM equipped with energy dispersive X-ray spectrometry (EDX). EDX analysis is useful in failure analysis cases where spot analysis is crucial to arriving at a valid conclusion. With a FESEM/EDX system, secondary electrons and backscattered electrons can be used for morphological analysis, as well as X-rays for identifying and quantifying chemical concentrations present at detectable levels. The detection limit in EDX is determined by the surface characteristics of the sample. The smoother the surface, the higher the detection limit [30].

3.1.2

Phase and crystal

The phases were identified by XRD (Panalytical Empyrean, Netherlands) with Cu-Ka radiation. The recommended setting to be used for ceramic coating is 40 kV and 40 mA over a 2q range of 10e90 degrees [31]. Crystal structure analysis may comprise lamellar preparation and selected area electron diffraction (SAED). By milling with a focus ion beam (FIB), very thin lamellae were prepared for transmission electron microscopy (TEM). The platinum (Pt) layer was deposited at 30 kV with an ion beam. Pt was deposited at dimensions of 10
1.5
1 m with a current control between 1.6 and 3.2 nA. The material was Table 1.1 List of physicochemical properties with the characterization equipment. Properties

Characterization equipment

Microstructure Elemental composition Hydrophobicity Adhesion Topography Phase Crystal structure Surface chemistry

Field emission scanning electron microscopy (FESEM) Energy dispersive X-ray spectrometry (EDX) Video-based optical contact angle measuring system Microscratch tester Atomic force microscopic (AFM) X-ray diffraction (XRD) Transmission electron microscopic (TEM) X-ray photoelectron spectrometry (XPS)

Characterization methods and characterization of the coatings

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then milled away from the front and back of the region of interest using high-current beams of 6.5e32 nA. On the left side, only a small tab of material retained the lamellae, leaving the bottom and right edges free. The sample was lifted from the bulk material by using the easy lift needle, which had been deposited with Pt. Copper grids were used to attach the lamellae. The probe was milled free of the lamellae once it was secured to the grid. Afterward, the lamellae were thinned to 1500 C); surpassing this temperature barrier would be a substantial development for engineering applications. Moreover, it is necessary to have a universal method for measuring the ceramic coating toughness. To date, the universal method is not yet established for thin films or coatings [123], and it remains a challenging task for researchers to fulfill these global demands.

Acknowledgment The authors gratefully acknowledge the Universiti Sultan Zainal Abidin and University of Malaya for providing the equipment and necessary services for this research. This study is funded by the Fundamental Research Grant Scheme (FRGS) Grant No: Ref: FRGS/1/2020/ STG05/UNISZA/02/1.

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Fracture mechanics of ceramic coating

2

Mohammad Azadi Faculty of Mechanical Engineering, Semnan University, Semnan, Iran

Abbreviations APS BC EB-PVD HVOF PEO TBC TC TGO YSZ

air plasma spraying bond coat electron beam physical vapor deposition high-velocity hydrogen fuel plasma electrolytic oxidation thermal barrier coating top coat thermally grown oxide yttria-stabilized zirconia

1. Introduction Ceramic coating systems could be commonly applicable for two objectives: the first one is for thermal barriers, and the other one is for wear or corrosion resistance in combustion engines and gas turbines [1]. Notably, the coating thickness should be higher in the first-mentioned application to fabricate a barrier to heat transfer. Thermal barrier coating (TBC) could be applied on the surface of the combustion chamber in diesel engines or gasoline engines, besides on the surface of the blades in a gas turbine, in order to allow higher temperatures through combustion, enhance the thermal efficiency, or obtain lower base metal temperatures. Then, a coating layer leads to an enhancement in the fatigue lifetime of high-temperature parts and also reducing in emissions and fuel consumption [2,3]. The application history of TCB on superalloys in the aero industry could be seen in Fig. 2.1. A TBC system generally includes two layers. One layer is a ceramic top coat (TC) for heat-resisting the base material. The TC layer is mostly yttria-stabilized zirconia, or ZrO2-8%Y2O3, with 350 mm thickness. The second layer is a metallic bond coat (BC), gradually from Ni-Cr-Al-Y with a thickness of 150 mm [3]. The coating thickness could be wider from 0.1 to 2.0 mm [1]. The process for fabricating the coating layers on the surface could be air plasma spraying (APS), high-velocity hydrogen fuel, electron beam physical vapor deposition (EB-PVD), or plasma electrolytic oxidation (PEO) techniques [1,2,5]. Other similar types of the TC layer could be considered as Al2O340%TiO2 or Al2O3-40%ZrO2 with the BC layer of Ni-Cr-Al [6] or Ni-Co-Cr-Al-Y [1]. Advanced Ceramic Coatings. https://doi.org/10.1016/B978-0-323-99659-4.00013-9 Copyright © 2023 Elsevier Ltd. All rights reserved.

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Figure 2.1 The application history of TCB on superalloys in the aero industry [1,4].

For the protection of the components from wear and corrosion phenomena, Attarzadeh et al. [5] presented a review on ceramic coatings. In this article, a list of investigations that studied the wear and corrosion performances of PEO coatings was also reported on steels in various mediums.

2.

Failures in coatings

2.1

General root causes

There are several root causes for failures in coating systems, including no proper preparation of the base surface (or no appropriate bonding between the coating layers and the substrate, besides the roughness); environmental effects (maybe using coatings in a false application) such as medium composition, cleanliness, humidity, and temperature; no proper process parameters (or using a wrong technique or due to the residual stress or according to the coating porosity); no proper formulation or material type for the coating layers; and finally, the coating thickness, as the most important variable in the coating performance. One example of the wrong technique is to utilize the EB-PVD approach. Since the coating layers are fabricated as a needle shape, the heat could be transferred and no barrier would be made. However, by APS, the coating layers are fabricated layer by layer with a vacuum confined inside the coating porosities, and consequently, a lower amount of heat could be transferred. These coating processes could be comparably seen in Fig. 2.2. Due to the mentioned aspects, various issues for the exposure conditions and also for damage mechanisms in TBC systems should be investigated to predict or model the fatigue lifetime. As previously mentioned as a major issue, the weakness of TBC

Fracture mechanics of ceramic coating

29

Figure 2.2 The difference between (A) EB-PVD and (B) APS technique [7].

systems is the interfaces between BC and TC layers as well as between the BC layer and the substrate. In these areas, high stresses will be occurred according to the mismatch of the thermal expansion between different materials. One other reason for high stresses could be the roughness through interface regions. As another notable issue, a damage mechanism could be developing in the thermally grown oxide (TGO) layer. This TGO layer will be fabricated at the interface of TC and BC layers, which is formed due to the BC oxidation at 900
C [3]. This application is for coating the blades of gas turbines. Lower than this temperature, no TGO would be fabricated, especially for combustion engines.

2.2

Roughness and thickness

Sfar et al. [8], Bialas [9], and Ranjbar-Far et al. [10] simulated the roughness at the TCBC interface, the oxide layer and also its volume growth, the cyclic material behavior, and the creep relaxation phenomenon to estimate their influences on the distribution of the stress. Moridi et al. [3] investigated the thickness influence of coatings on the stress distribution applied on A356 aluminum alloy under both thermal and thermomechanical loads, without considering TGO, in the cylinder head application. They have demonstrated the surface roughness influence of the interfaces between BC and TC layers and also between the substrate and BC. Their results illustrated the following issues: • • •

In the ceramic TC layer, the stress was roughly higher at the peaks of the BC profile than ones which are existed in the smooth interface. The rough interface in TBC systems increased the initial adherence between the coating layers and therefore, caused a decrease in the fatigue lifetime. Increasing the TC thickness resulted in enhancing the stress. The thickness of coatings had a better fatigue lifetime both in finite element simulations and thermal shock tests.

30

• • • • • •

•

Advanced Ceramic Coatings

Waves positioning (in-phase or out-of-phase with various penetrations of interfacial undulations) improved comprehension of the distribution of the stress and also the roughness influence. Spalling and cracking were damage modes in the vital TBC systems, which were affected by the magnitude and sign of the stress, compared to yield strength. Quantitative values of the stress increased when the roughness amplitude was enhanced and wavelength shortened due to the enhancement of the stress, which led to cracking in the tensile area. Besides, high compressive stress may not allow crack growth in the valley. Through in-phase positioning, when the BC thickness increased according to the differentiation in the penetration of interfacial asperities, an enhancement in developing stress occurred to a maximum value and then reduced afterward. In out-of-phase positioning, the developing stress firstly decreased to the minimum value and then increased subsequently. In-phase positioning of mutual waves for the roughness produced more severe stress than that of out-of-phase positioning. However, the contour pattern was less likely to cause crack growth since tensile regimes were surrounded by compressive stresses, which led to stopping the cracks. On the contrary, out-of-phase positioning prepared a more proper path for the cracks to grow. The BC-substrate interface was more prone to the nucleation of the microcracks and also the subsequent delamination than the interface at the BC and TC layers [11].

2.3

Porosity and crack

Rezvani Rad et al. [12] investigated the effects of the real roughness and the porosity in TBC. They depicted the following issues by finite element modeling of the coating process: • •

•

Considering the pore in the TC layer caused a remarkable reduction in the distribution of the axial stress in the substrate. However, it leads to local stress concentration in the TC layer with respect to the BC layer according to irregular porosities. Considering the porosity in the TC layer showed an influence on the stress in TBC systems. Three aspects were investigated including the stress concentration in the TC layer around the irregular pores; less value for the axial stress inside the TC layer (according to the less elastic module); less compressive axial stress inside the BC layer; and finally less tensile stress inside the substrate. These observations were due to less thermal conductivity for the porous model of the TC layer than one without pores. This issue causes less heat transfer from the TC layer to the BC layer and the substrate. The influence of large and irregular porosities in the TC layer on the stress concentration was much more than the small pores in the BC layer.

In another similar work, Rezvani Rad et al. [11] performed a stress analysis of TBC systems, under out-of-phase thermo-mechanical loading, with the consideration of the effects of the porosity and the roughness after the coating process. They showed that •

When the TC temperature was at its peak (250
C), utilizing TBC systems could reduce the temperature in the substrate by more than 50
C. This could be due to the porosity, which led to no heat transfer through the coating layer.

Fracture mechanics of ceramic coating

•

31

The stress distribution of the TC layer was mostly affected by the shape and also the percentage of the porosity. The reason is that the porous model possessed a significantly less elastic module compared to the models without porosity.

It was proven that, with the objective of maximizing the power and efficiency besides minimizing the heat loss in a coated Stirling engine, the coating with 50% of the porosity was better than the dense coating layer, with the optimal value of 1500 m for the thickness. Under this working condition in the Stirling engine, the efficiency is enhanced by 140%, with respect to the base engine [13]. Wei et al. [14] checked the effect of the near-spherical 3D-porosity on the damage mechanism of atmospheric or air plasma-sprayed TBCs by micro- and macro- integrated modeling. The parameters of the porosity characteristics were considered as the orientation angle and the aspect ratio, which affected the crack propagation. Their results showed the following notes: • • • • • •

Near spherical porosities, the stress concentration could induce crack initiation in the early stage. Regular spherical porosities of 10% were more conducive for improving the coating fatigue lifetime. The total crack length exhibited firstly an enhancing trend and then a reducing trend versus the orientation angle. For the orientation angle of 30 degrees, the total crack length had the largest value in the TC layer. The total crack length exhibited firstly a reducing trend and then an enhancing trend versus the porosity content. For the pore content of 10%, the total crack length has the smallest value in the TC layer. Increasing the aspect ratio in the TC layer, the total crack length demonstrated an enhancing trend. The total crack length showed a continuous enhancement with TGO thickening. This growth of the TGO layer is due to premature spalling through coating layers in the porous model.

2.4

Process parameters

Azadi et al. [15] optimized air plasma sprayed TBC parameters in diesel engine applications, including the nozzle distance to specimen surfaces, the feed rate of coating powders, and the coating thickness through two types of experiments (thermal shock fatigue testing and bending testing). Their investigation showed the following issues: • • • • •

The nozzle distance and the feed rate were optimized at 30 g/min and 80 mm, respectively when the objective was the maximizing process of the bending strength. Thermal shock experiments demonstrated that in coating layers, lower thicknesses had better fatigue lifetimes. Enhancing the coating thickness led to a decrease in the thermal shock fatigue lifetime due to higher stress around the interface between the BC layer and the substrate. The stress was at its maximum and in the tensile mode at the interface between the BC layer and the substrate, where cracks initiated. According to the failure analysis, the separation of coating layers from the substrate occurred during thermal shock fatigue cycles due to the mismatch of material properties between the coating layers and the substrate.

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Advanced Ceramic Coatings

As mentioned, Rezvani Rad et al. [12] examined the effects of the cooling rate and the preheating temperature on the distribution of the residual stress during the fabrication of TBC. Notably, two steps, including BC and TC deposition processes, were considered in the following results: • • • • •

In the substrate, the distribution of the residual stress was different based on two steps of the coating process. No significant effect could be found by the convective heat transfer through the deposition of the BC layer. However, for the TC layer fabrication, it was an effective parameter. The preheating temperature before the deposition of BC and TC layers had a pivotal role in the distribution of stresses in all coating layers. To have no critical points with significant tensile stress in the coating layers, preheating of the substrate should be done before the fabrication of the BC layer. Preheating of both BC and substrate before the TC deposition was not proper. Since this process will lead to critical tensile stress in the substrate and also the bulking phenomenon of the BC layer based on the compressive axial stress.

Mehboob et al. [1] also reported residual stresses as one of the major parameters that lead to failures and damages like delamination, spalling, and surface cracks in TBCs.

3.

Fracture observations

Fracture surfaces of coated materials could be investigated after fatigue tests by scanning electron microscopy (SEM). A sample for microscopic views of coated specimens under cyclic mechanical (isothermal) and thermo-mechanical (nonisothermal) loadings is demonstrated in Figs. 2.3 and 2.4, respectively. In Figs. 2.3 and 2.4, the cracks are demonstrated by white arrows in the coating layers. Moreover, the separation of coating layers is illustrated by black arrows at the interface of the substrate and the BC layer. In room temperature low-cycle fatigue testing, no separation at interfaces was seen due to isothermal experiments (a constant temperature during testing). For high-temperature, low-cycle fatigue and thermo-mechanical fatigue testing, the separation at the interface of the BC layer and the substrate (depicted in Fig. 2.3 with white arrows) was seen due to the mismatch of material properties. Finally, the separation was not found at the interface of the TC and BC layers through high-temperature fatigue testing. In addition, the mismatch of material properties between the substrate and the BC layer led to hoop stress in the BC layer [16]. The investigation of the fracture surface consequently illustrates that the dominant damage mechanism in the samples was based on the crack initiation at the interfaces of the substrate and the BC layer. Moreover, some cracks also propagated inside the substrate [16]. This mismatch could be found in the elastic module, the thermal conductivity, and the hardness of materials [1]. In Fig. 2.5A, the circumferential cracks are observed on low-cycle fatigue testing samples due to high mechanical strains during cyclic loadings. Fig. 2.5B demonstrates also the longitudinal cracks on the surface of coated specimens under thermo-

Fracture mechanics of ceramic coating

33

Figure 2.3 The fractography of coated specimens after (A) room temperature low-cycle fatigue testing, (B) high-temperature low-cycle fatigue testing, (C) out-of-phase thermo-mechanical fatigue testing at low magnification, and (D) high magnification [16].

mechanical fatigue loading based on the fact that the substrate was expanded by transient thermal (nonisothermal) loads. This substrate expansion led to radial stresses in the substrate. In this case, the coating layers had a constraint role for the substrate, which could be compressed. Another fact for crack types was the loading speed, which was about 102 1/s for low-cycle fatigue experiments, compared to 104 1/s for thermo-mechanical fatigue testing [16]. Notably, the sensitivity of the TC layer, as a brittle ceramic material, was significantly high in relation to the loading speed [17]. Experimental fatigue data demonstrated the following issue presented by Azadi et al. [16] for TBC systems, which consisted of the BC layer (Ni-Mo-Al) and the TC layer (ZrO2-8%Y2O3) with 250 and 500 mm of the total coating thickness on A356 aluminum alloy. •

•

The coating system increased the low-cycle fatigue lifetime at room temperature. However, at high temperatures, two different behaviors were observed; under higher strain amplitudes, the coating layers had a beneficial influence, while under lower strain amplitudes, the coating layers had an opposite influence. In the low-cycle fatigue situation, a thicker coating system had a detrimental influence.

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Advanced Ceramic Coatings

Figure 2.4 SEM micrographs of coated specimens after (A) high-temperature low-cycle fatigue testing at low magnification and (B) high magnification, (C) out-of-phase thermo-mechanical fatigue testing at low magnification, and (D) high magnification [16]. • • • •

•

The coating layers increased effectively the out-of-phase thermo-mechanical fatigue lifetime. Nevertheless, 500 mm thick coats gave superior performance and lifetime to the material, while 250 mm thick coats caused a decrease in the fatigue lifetime. In thermo-mechanical fatigue testing, the dominant issue was debonding of the coating system based on transient thermal strains, and then, thicker coats had a superior bond to the substrate and, therefore, a better thermo-mechanical fatigue lifetime. 250 mm thick coats gave a better low-cycle fatigue lifetime, whereas 500 mm thick coats offered a better thermo-mechanical fatigue lifetime, which could be explained by the behavior of the TBC system under the variation of the temperature. The fractography of fracture surfaces showed that the separation of coating layers at the interface of the BC layer and the substrate was introduced as the damage mechanism in the coated A356 aluminum alloys, under both out-of-phase thermo-mechanical fatigue and hightemperature low-cycle fatigue conditions. Considering the three common damage parameters of fatigue, creep, and oxidation, in hightemperature, low-cycle fatigue testing on coated samples, oxidation and creep failures had approximately a similar rule. In addition, they were less than the ones in the pure fatigue damage at higher mechanical strain amplitude. Finally, they were more at lower mechanical strain amplitude [18].

Fracture mechanics of ceramic coating

35

Figure 2.5 Cracks on coated samples under (A) high-temperature low-cycle and (B) out-ofphase thermo-mechanical fatigue testing [16].

As another crack root cause, the delamination was reported by Kramer et al. [19] in thick TBCs, applied on aero-engines, as shown in Fig. 2.6. In this application, cracks between the interface of the BC and TC layers were presented by Yu et al. [20], as another failure mechanism, which is depicted in Fig. 2.7.

Figure 2.6 The delamination through the TC layer [19].

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Advanced Ceramic Coatings

Figure 2.7 The cracks between BC and TC layers of coating [20].

Figure 2.8 SEM images of PEO coating under (A) corrosion [21] and (B) wear phenomena [22].

For failure mechanisms in PEO coatings with the application of corrosion and wear resistance, SEM images in Fig. 2.8 were reported by Yang et al. [21] and also Zhang and Kong [22], respectively. Yang et al. [21] examined the surface morphology of an aluminate PEO coating after salt spray experiments for 300 h. No obvious destruction appeared on the surface. However, the products of the corrosion phenomenon enhanced, and then several microcracks disappeared. During the process of salt spray testing, a PEO coating could isolate low-carbon steel from the corrosive environment. But it was not a full barrier since the defects (porosities or cracks) existed. Several corrosive mediums still could penetrate the PEO coating layers through cracks or either micro-porosity to the substrate and led to substrate corrosion. Afterward, the products of the corrosion phenomenon were followed through the cracks or microporosities. Thus, they were accumulated in these regimes. This behavior would make the corrosive environment more difficult to penetration into the coating layer. Consequently, the aluminate PEO coating layers could withstand the salt spray corrosion at least, for 300 h. Zhang and Kong [22] illustrated the wear phenomenon that the worn tracks were fine and smooth in the PEO coating. However, some alternative stresses occurred by rubbing between the coating layers and the tribo-pair. That was not severe enough

Fracture mechanics of ceramic coating

37

to induce plastic deformation due to the high hardness of the surface for PEO coating. As a conclusion to these observations, the coating layer had small damage on the contact zone. Then, the aggregate of this damage plus the fatigue cracks initiation, the total mechanism was introduced as fatigue wear. In addition, the fatigue wear phenomena were evident since the sheetlike debris was fully separated from the PEO coating, and then, the pits showed up on the worn track of the sample.

4. Conclusions In the presented chapter, the fracture mechanics behavior of ceramic coatings is reported under different fracture, creep, fatigue, corrosion, and wear phenomena. Various failure mechanisms were introduced by using scanning electron microscopy. Then, different parameters were investigated as important issues, as follows: • • • •

The surface preparation of the substrate is important to have a better bond strength. The roughness of the surface had a significant role in the stress distribution. A corrosive environment should be considered for designing TBC systems based on the medium composition, cleanliness, humidity, temperature, etc. Process parameters have the most important role in coating performance and lifetime. These variables could be the technique type, the residual stress during coating, the coating porosity, the feed rate, the nozzle distance, the cooling rate, etc. As another issue is the material type for coating layers. Besides, the coating thickness should be designed based on the application.

Based on these design variables, various failure mechanisms could be observed on coated samples. The reason could be due to cracks, delamination, and the pore. These failures were caused by the residual stress, the mismatch of material properties, the oxidation, the fabrication of TGO, etc. However, engineering designers have always tried to solve such a problem with different methods or an optimum process.

References [1] G. Mehboob, M.J. Liu, T. Xu, S. Hussain, G. Mehboob, A. Tahir, A review on failure mechanism of thermal barrier coatings and strategies to extend their lifetime, Ceramics International 46 (2020) 8497e8521. [2] M. Azadi, M. Baloo, G.H. Farrahi, S.M. Mirsalim, A review of thermal barrier coating effects on diesel engine performance and components lifetime, International Journal of Automotive Engineering 3 (1) (2013) 305e317. [3] A. Moridi, M. Azadi, G.H. Farrahi, Thermo-mechanical stress analysis of thermal barrier coating system considering thickness and roughness effects, Surface and Coatings Technology 243 (2014) 91e99. [4] H. Zhao, C.G. Levi, H.N.G. Wadley, Molten silicate interactions with thermal barrier coatings, Surface and Coatings Technology 251 (2014) 74e86. [5] N. Attarzadeh, M. Molaei, K. Babaei, A. Fattah-Alhosseini, New promising ceramic coatings for corrosion and wear protection of steels: a review, Surfaces and Interfaces 23 (2021) 100997.

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[6] T. Hejwowski, A. Weronski, The effect of thermal barrier coatings on diesel engine performance, Vacuum 65 (2002) 427e432. [7] L. Latka, Thermal barrier coatings manufactured by suspension plasma spraying - a review, Advances in Materials Sciences 18 (3) (2018) 95e117. [8] K. Sfar, J. Aktaa, D. Munz, Numerical investigation of residual stress fields and crack behavior in TBC systems, Materials Science and Engineering A 333 (1e2) (2002) 351e360. [9] M. Bialas, Finite element analysis of stress distribution in thermal barrier coatings, Surface and Coatings Technology 202 (24) (2008) 6002e6010. [10] M. Ranjbar-Far, J. Absi, G. Mariaux, F. Dubois, Simulation of the effect of material properties and interface roughness on the stress distribution in thermal barrier coatings using finite element method, Materials and Design 31 (2) (2010) 772e781. [11] M. Rezvani Rad, G.H. Farrahi, M. Azadi, M. Ghodrati, Stress analysis of thermal barrier coating system subjected to out-of-phase thermo-mechanical loadings considering roughness and porosity effect, Surface and Coatings Technology 262 (2015) 77e86. [12] M. Rezvani Rad, G.H. Farrahi, M. Azadi, M. Ghodrati, Effects of preheating temperature and cooling rate on two-step residual stress in thermal barrier coatings considering real roughness and porosity effect, Ceramics International 40 (10) (2014) 15925e15940. [13] F. Ahadi, M. Azadi, M. Biglari, S.N. Madani, Study of coating effects on the performance of Stirling engine by non-ideal adiabatic thermodynamics modeling, Energy Reports 7 (2021) 3688e3702. [14] Z.Y. Wei, X.X. Dong, H.N. Cai, S.D. Zhao, Influences of the near-spherical 3D pore on failure mechanism of atmospheric plasma spraying TBCs using a macro-micro integrated model, Surface and Coatings Technology 437 (2022) 128375. [15] M. Azadi, G.H. Farrahi, A. Moridi, Optimization of air plasma sprayed thermal barrier coating parameters in diesel engine applications, Journal of Materials Engineering and Performance 22 (2013) 3530e3538. [16] M. Azadi, G.H. Farrahi, G. Winter, W. Eichlseder, Experimental fatigue lifetime of coated and uncoated aluminum alloy under isothermal and thermo-mechanical loadings, Ceramics International 39 (8) (2013) 9099e9107. [17] ASM Handbook, Mechanical Testing and Evaluation, vol. 8, ASM International, USA, 2000. [18] M. Azadi, G.H. Farrahi, G. Winter, P. Huter, W. Eichlseder, Damage prediction for uncoated and coated aluminum alloys under thermal and mechanical fatigue loadings based on a modified plastic strain energy approach, Materials and Design 66 (B) (2015) 587e595. [19] S. Kramer, S. Faulhaber, M. Chambers, D.R. Clarke, C.G. Levi, J.W. Hutchinson, A.G. Evans, Mechanisms of cracking and delamination within thick thermal barrier systems in aero-engines subject to calcium-magnesium-alumino-silicate (CMAS) penetration, Materials Science and Engineering A 490 (1e2) (2008) 26e35. [20] C. Yu, H. Liu, C. Jiang, Z. Bao, S. Zhu, F. Wang, Modification of NiCoCrAlY with Pt: Part II. Application in TBC with pure metastable tetragonal (t0 ) phase YSZ and thermal cycling behavior, Journal of Materials Science and Technology 35 (3) (2019) 350e359. [21] W. Yang, Q. Li, W. Liu, Z. Peng, B. Liu, Characterization and properties of plasma electrolytic oxidation coating on low carbon steel fabricated from aluminate electrolyte, Vacuum 144 (2017) 207e216. [22] J. Zhang, D. Kong, Effect of micro-arc oxidation on friction-wear behavior of cold-sprayed Al coating in 3.5 wt.% NaCl solution, Journal of Materials Engineering and Performance 28 (2019) 2716e2725.

Band-gap engineering of ceramic coatings

3

P. Mallick Department of Physics, Maharaja Sriram Chandra Bhanja Deo University, Baripada, Odisha, India

1. Introduction Ceramics are generally crystalline and nonmetallic inorganic compounds. These materials possess excellent physicochemical properties, such as high strength and hardness, high thermal stability, low thermal expansion, good chemical stability, resistance to erosion, corrosion, and wear, etc. [1,2]. Low maintenance along with longer life is the major advantage associated with ceramics [2]. Ceramic coating is generally used as a protective layer which can either be made out of oxide or nonoxide ceramic compounds [3,4]. Even though the large number of nonoxides are used for surface functionalization, the majority of the ceramic coatings are employed by using oxide-based ceramic coating for the protection as well as surface functionalization of metallic materials, devices, etc. [3,4]. The development of nanostructured ceramic coatings has continued to attract the attention of the scientific community due to their diverse implication, starting from protective layers to devise functionalization [5]. For example, the nanostructured ceramic coating could be used as (i) repellent to dry particles, oil, water, etc., (ii) scratch resistance, (iii) bacteria resistance, (iv) corrosion resistance, etc. [4]. Titania (TiO2) coating has been shown to prevent from corrosion of stainless steel (316L) in chloride solution [6]. Nanostructured TiO2 coating has been reported to exhibit tremendous photoactive antibacterial properties [7,8]. Al2O3 ceramic-coated AZ31PH Mg alloy has been shown to exhibit wear and corrosion resistance along with high hardness properties [9]. Similarly, SiO2 ceramic-coated alloys are reported to exhibit corrosion resistance properties [10]. The surface of superhydrophobic MnO2-coated Mg alloy has been shown to exhibit excellent self-cleaning properties along with chemical stability and mechanical durability [11]. The ceramic coatings are not only used for surface protection purposes but also as a transparent conductive oxide layer for the development of display panels, solar cells, optoelectronic devices, etc. [12,13]. Transparent ceramic coating can also be used in the facades as a photocatalyst in order to keep the exterior of the building clean by using solar energy [12]. In addition, the ceramics and/or their coatings can also be used as biosensors and drug delivery systems [14]. Further, ceramics can also be used in high-technology systems operated under extreme conditions such as space, aeronautics, nuclear, missile, etc. [1].

Advanced Ceramic Coatings. https://doi.org/10.1016/B978-0-323-99659-4.00007-3 Copyright © 2023 Elsevier Ltd. All rights reserved.

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Advanced Ceramic Coatings

Among different ceramic materials, metal oxides such as zinc oxide (ZnO), nickel oxide (NiO), cadmium oxide (CdO), TiO2, alumina (Al2O3), zirconia (ZrO2), tin oxide (SnO2), gallium oxide (Ga2O3), ferric oxide (Fe2O3), etc. have been considered as invaluable materials due to their application potentials [14]. Engineering of properties of these materials is required to develop more suitable ceramic coatings for specific application points of view. Among the different properties, a band gap could be engineered in these oxide ceramics for advanced practical applications such as light emission and absorption [15], energy harvesting [16,17], electronics [18], photocatalysis [19], optoelectronics [20], CO oxidation [19,21], gas sensor [22], etc.

2.

Band gap engineering

A band gap is one of the important characteristic features associated with ceramic materials; it is basically an energy interval between the top of the valence band and the bottom of the conduction band [23]. Band-gap engineering has been considered one of the invaluable tools for obtaining tunable properties for several decades [24]. One can modify the band gap of ceramics/semiconductors by employing numerous important strategies [24] such as (i) changing composition by doping [25e27], (ii) through strain generation [28e30], and (iii) quantum confinement by size alteration [24,31e34]. As a result, one can achieve two consequences: (i) band gap widening and (ii) band gap narrowing. Both these features have been preferred owing to achieve specific applications prospective. Numerous studies have been undertaken to tune the band gap of ceramic materials to make use of them for different applications, as reflected later.

2.1

Importance of band gap engineering

Physical and chemical properties of materials are significantly related to their corresponding band gap, which substantially needs to be altered for achieving desired materials properties [24e27]. The band gap of a semiconductor associated with an optoelectronic device governs its ultimate efficiency to exploit the use of sunlight in different applications such as solar cells, photocatalysts, etc. [24]. Research on photocatalysts has been growing in recent years due to their tremendous impact on photocatalytic energy generation, water, and air purification, self-cleaning and antifogging products for the automobile industry, destroying bacteria and viruses, etc. [35]. Engineering of band gap has been considered a powerful key for the photocatalytic processes [36e40]. The band-gap engineering strategy has also been employed to improve the properties of optoelectronic devices like solid-state lasers, lightemitting diodes (LEDs), etc. [41,42]. Improvements in the efficiency of solar cells have been shown to be achieved by adopting the band-gap engineering strategy [43e48].

Band-gap engineering of ceramic coatings

2.2

41

Determination of band gap

The energy required to excite the electron from the valence band to the conduction band is termed band gap energy [49]. The correct determination of the band gap energy of the particular semiconductor material is essential to probing its photochemical and photophysical features. Generally, the band gap of a semiconductor can be determined using the Tauc method [50], and this method was further advanced by Mott and Davis [51]. In this method, the optical band gap energy (Eg ) of the semiconductor is related to its optical absorption coefficient (a) by the following relation [49]:
 ða:hvÞp ¼ B0 hn  Eg

(3.1)

where hn is the incident photon energy and B0 is a constant parameter that varies from material to material. The value of p is taken to be 2 for direct transition and 1/2 for indirect transition. However, it is proper to use the KubelkaMunk function (FðRN Þ) [49,52]: FðRN Þ ¼

ð1  RN Þ2 2RN

(3.2)

instead of a while calculating the Eg from diffuse reflectance spectra. Therefore, one can write Eq. (3.1) as
 ðFðRN Þ:hvÞp ¼ B0 hn  Eg

(3.3)

for such cases. In order to determine the Eg , one needs to extrapolate the linear portion of the curve (ðFðRN Þ:hvÞp w hn or ðahnÞp w hn) to zero photon energy. It has also been reported that the materials for which the above approach is less effective due to a lack of sufficient linear portion in the curve, one needs to follow alternative approach as discussed follows [53]. Unlike the former case, here, one needs to plot the first derivative of transmittance spectra of the sample with respect to the wavelength (dT dl ) versus l. The peak position in the plot would correspond to the band edge of the sample [53,54].

3. Band gap engineering in some ceramic oxides In recent years, investigations on band gap alternation in ceramic oxides have been actively pursued due to their diverse technological applications. The band-gap engineering on some important ceramic oxides due to various influences has been reflected in the followings:

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Advanced Ceramic Coatings

3.1

Band gap engineering in zinc oxide

ZnO featured with n-type conductivity, larger exciton binding energy (60 meV), and a wide direct band gap of 3.37 eV has been considered one of the most popular ceramic materials as far as application prospects are concerned [55e59]. The area of applications of ZnO spans various fields such as optoelectronics [60e64], solar cells [65], catalysts [66], sensors [62,67,68], pharmaceuticals [69], energy harvesting piezoelectric devices [70], etc. Further, the ZnO has also several other advantages like high electron mobility, good transparency [71], low cost, environment-friendly [72], etc. Various forms of nanostructures like wires, rods, tubes, and belts of ZnO can easily be prepared [59]. Most importantly, one can use most of the atoms present in the periodic table to tune the properties of ZnO for specific applications [73]. Furthermore, ZnO has been considered a promising alternative material to replace transparent conductive tindoped indium-oxide (ITO) [74]. However, one needs to address specific issues of ZnO through doping in order to realize ZnO-based devices replacing ITO [74]. The band gap of ZnO is one of the important features that can have an influence on its electrical, optical, and photocatalytic properties [55]. Herein, we discuss the evolution of the band gap of ZnO in the followings sections.

3.1.1

Band gap widening in zinc oxide

Several dopants as well as size reduction have a significant effect on the widening of the band gap of ZnO. Singh et al. [75] reported the enhancement of the band gap of ZnO from 3.35 to 3.65 eV with increasing Mg doping concentrations from 0% to 20%. Similarly, Gowrishankar et al. [76] reported that the increase of Mg doping concentration from 0% to 20% in ZnO led to the broadening of the band gap from 3.35 to 3.94 eV. Further, Rana et al. [77] reported the upsurge of the band gap from 3.14 to 3.44 eV up on increasing Mg doping concentration from 0% to 16% in ZnO. In addition, Wei et al. [78] studied the effect of Mg doping on the band gap of ZnO and reported that the doping concentration-dependent evolution of the band gap of ZnO follows the following trend: Eg ðxÞ ¼ 1:36x þ 3:28; 0  x  0:7

(3.4)

Eg ðxÞ ¼ 8:4x  2:064; 0:75  x  0:95

(3.5)

Seide et al. [79] studied the effect of Fe, Co, and Mn doping on the band gap of ZnO. Their study indicated that the Fe doping in ZnO also led to the enhancement of band gap from 3.24 to 3.34 eV with increasing doping concentrations from 0% to 8%. The authors also reported the increment of the band gap of ZnO from 3.24 to 3.3.0 eV with increasing Mg doping concentration from 0% to 8%. Though their study on Co-doping led to the widening of the band gap of ZnO from 3.24 to 3.32 eV with increasing doping concentrations from 0% to 4%, the same remains unaltered at higher Co-doping concentrations (i.e., at 8%). The enhancement of the band gap of ZnO from 3.27 to 3.40 eV upon increasing the Al doping concentration from

Band-gap engineering of ceramic coatings

43

0% to 1.5% is reported [80]. Similarly, the upsurge of the band gap from 3.26 to 3.76 eV has been reported while increasing Cu doping concentration from 0% to 7% in ZnO [28]. In addition, Ni and Cr doping-induced widening of the band gap in ZnO has also been reported. The band gap of ZnO is shown to increase from 3.29 to 3.35 eV with increasing Ni doping concentrations from 0% to 10% [81]. The enhancement of Ni doping concentration also led to the shape transformation of ZnO from nanorod to spherical morphology. Further, the enhancement of Cr doping concentration from 0% to 10% in ZnO led to the suppression of crystallite size from 37.95 to 16.46 nm, which eventually resulted in the increase of band gap of ZnO from 3.23 to 3.39 eV due to the quantum confinement effect [31]. It has also been reported that the Al-doped ZnO showed a higher band gap as compared to pure ZnO [82]. However, the authors showed that the band gap decreased with increasing particle size in both cases (undoped and doped), obeying the following relations [82]: Eg;undoped ðeVÞ ¼ 3:46 þ Eg;doped ðeVÞ ¼ 3:46 þ

3.1.2

3:23 D1:65 ðnmÞ

3:23 þ 0:06 D1:65 ðnmÞ

(3.6)

(3.7)

Band gap narrowing in zinc oxide

Like the band gap widening, certain dopant also causes band gap narrowing in ZnO. For example, Ji et al. [83] reported the reduction of the band gap of ZnO from 3.16 to 2.71 eV with increasing Co-doping concentration from 0% to 10%. Similarly, Riaz et al. [80] reported the suppression of the band gap from 3.27 to 3.21 eV up on increasing Ag doping concentration from 0% to 1.5% in ZnO. Unlike the result of Azfar et al. [81], the increase of Ni dopant from 0% to 6% has also caused band gap narrowing in ZnO from 3.2 to 3.13 eV, respectively [84]. On the other hand, Sm doping from 0% to 6% led to shape transformation from cubic to rod-like morphology in ZnO along with band gap narrowing from 3.19 to 2.74 eV [85]. Similarly, the shape of ZnO is reported to be transformed from hexagonal prism to spherical morphology along with band gap narrowing from 3.15 to 2.95 eV with increasing Al doping concentration from 0% to 6% [86]. On the other hand, the increase of Al doping concentration from 0% to 5% is reported to cause band gap narrowing from 3.26 to 3.23 eV along with the enhancement of porosity in ZnO [87]. Unlike the case of Cr dopant-induced size reduction and band gap widening as discussed above [31], Ga [88] and Cd [76] dopants caused size reduction along with band gap narrowing in ZnO. In general, the size reduction causes band gap widening due to quantum confinement [31], the band gap narrowing along with size reduction in a doped system could possibly be due to the effect of dopant [76,88].

44

3.2

Advanced Ceramic Coatings

Band gap engineering in nickel oxide

NiO has been considered one of the excellent materials due to its low toxicity, good thermal stability, wide span of optical density, low cost, durability, chemical stability, etc. [89,90]. NiO exhibits p-type semiconducting characteristics with a high exciton binding energy of w110 meV [91]. The direct band gap energy of NiO can be varied from 3 to 4.3 eV [92e94]. Wide band gap along with deep valance band edge (w5.4 eV) and work function (w5.0 eV) proved that NiO as an interfacial anode material in perovskite solar cells [95e97]. The research on NiO has actively been pursued by the scientific community due to diverse technological applications such as gas sensors [98,99], electrochromic display devices [100], p-type transparent conductors [101,102], transparent heat mirrors [103], organic photovoltaics [104,105], resistance switching [106], supercapacitors [107], dye-synthesized solar cells [102,108], thermoelectric devices [109], catalysis [110], etc. Band-gap engineering in NiO is essential in order to tune its properties for specific applications. For example, one needs to dope NiO with monovalent atoms such as Li in order to reduce its band gap and thereby increase its conductivity [96,111]. Further, Li doping in NiO would result in an increase in the open-circuit voltage in solar cells along with a shift of deep valance band edge (5.3 eVe5.4 eV) [96,112]. The reduction of the band gap in NiO could be useful for achieving the energy-harvesting optoelectronics devices such as photovoltaic and solar cells [113,114]. The electrochromic characteristics of NiO could be improved by increasing the band gap of NiO through doping [115]. The p-type semiconductivity of NiO could be retained by doping I to V group elements [116,117]. Further, the p-type to n-type transformation in NiO is reported to be achieved by Zn doping beyond a certain concentration [118]. Therefore, several important features of NiO will be emerge for different specific applications through band-gap engineering. Herein, we discuss the evolution of the band gap of NiO in the followings.

3.2.1

Band gap widening in nickel oxide

The band gap of NiO is shown to be enhanced from 3.75 to 3.95 eV with increasing Mg doping concentrations from 0% to 4.9% [119]. In another work, Panigrahi et al. also reported a similar type of information, i.e., the enhancement of the band gap of NiO from 3.64 to 3.90 eV with increasing Mg doping from 0% to 5% [120]. Further, the authors also reported the upsurge of the band gap of NiO from 3.64 to 4.03 eV uon increasing Zn doping concentrations from 0% to 5% [120]. In accordance with their study, Benhamida et al. [121] also showed the enhancement of the band gap from 3.76 to 3.93 eV with increasing Zn doping from 0% to 5% in NiO. Patel et al. [122] reported the enhancement of the band gap of NiO from 3.98 to 4.08 eV with increasing Fe doping concentrations from 0% to 5%. Ponnusamy et al. [123] also reported that Fe doping induced a similar variation of the band gap in NiO. In addition, Fe and Zn doping were also reported to cause band gap narrowing in NiO, which is discussed later.

Band-gap engineering of ceramic coatings

3.2.2

45

Band gap narrowing in nickel oxide

Like in the case of ZnO, size enhancement as well as doping would lead to the shrinking of the band gap of NiO. For example, Hashem et al. [124] reported thermal annealing-induced enhancement of size from w15 to 35 nm would cause the shrinking in the band gap of NiO from 3.60 to 3.51 eV. Mallick and Biswal reported the shrinking of the band gap in NiO from 3.65 to 3.43 eV with increasing Fe doping concentrations from 0% to 2% [125]. Gavale et al. [126] also reported the diminishing of the band gap of NiO from 3.48 to 2.86 eV with increasing the concentration of Fe doping from 0% to 10%. Like Fe, the increase of Zn doping from 0% to 10% causes a shrinking of the band gap of NiO from 4.04 to 3.56 eV [118]. The authors reported that the Zn doping-induced enhancement of oxygen vacancies in NiO could be the cause of the suppression of the band gap in NiO by introducing additional defect states [118]. The co-dopant has been shown to cause band gap narrowing in NiO. The band gap of NiO decreased from 3.70 to 3.31 eV up on increasing Co doping concentrations from 0% to 10.87% [127]. In another study, Bakr et al. [128] reported identical variation of NiO, i.e., decrease of band gap from 3.66 to 3.58 eV with increasing Co doping concentration from 0% to 8%. Further, the increase of Al dopant concentration from 0% to 2% would lead to a decrease in the band gap of NiO from 3.693 to 3.645 eV [129]. Similarly, the increase of Nd doping from 0% to 5% led to the suppression of the band gap of NiO from 3.67 to 3.48 eV [113]. Also, the band gap of NiO is shown to decrease from 3.94 to 3.85 with increasing Li doping concentrations from 0% to 5% [96]. Unlike the case of other dopants, the increase of Ga doping from 0% to 5% has been reported to cause band gap narrowing from 3.55 to 3.47 eV along with shape transformation from rod to spherical morphology in NiO [130].

3.3

Band gap engineering in copper oxide

Copper oxide (CuO) has attracted the attention of the scientific community as an industrial material due to its abundance, low cost, ease to produce, nontoxic, and wide range of technological applications [131e134]. CuO is one of the important few semiconductors showing a p-type character, which has been exploited for optoelectronic applications [135]. CuO has been shown to exhibit both direct and indirect band gap features. The direct band gap of CuO is reported to vary from 2 to 4 eV, whereas its indirect band gap is reported to vary from 1 to 1.4 eV [135,136]. CuO with a wide range of optical band gaps could be considered a promising material for different optoelectronic device applications [137]. CuO has also been shown its potential for its use as magnetic media [138], gas sensor [139e141], electronic material [142], catalyst [140], supercapacitor [143], battery [144,145], photoelectrochemical, photothermal, and photoconductive applications [132,146], p-type electrode [147], etc. Since the band gap plays a crucial role in detecting the performance of several devices like photodetectors, gas sensors, biosensors, LEDs, solar cells, antimicrobials, etc. [133,148e150], it is therefore desired to tune the band gap of CuO for specific applications. Evolutions of both direct and indirect band gaps of CuO are discussed in the followings.

46

3.3.1

Advanced Ceramic Coatings

Band gap widening in copper oxide

It has been reported that the occurrence of quantum confinement due to size reduction would lead to a widening of the band gap of CuO nanostructures, which in turn would provide an excellent opportunity for a wide range of application possibilities, including photovoltaic [151e158]. Velusamy et al. [159] synthesized CuO nanoparticles of size w1.9 nm employing the microplasma synthesis route and reported their direct band gap to be w2.9 eV. Mallick and Sahu [160] reported the effect of solvents (ethanol and propanol) on the band gap of CuO. The authors [160] reported the occurrence of a direct band of w3.57 eV, while the samples prepared showed an indirect band of 1.18 and 1.21 eV, respectively, prepared by using ethanol and propanol as solvent. The existence of a higher direct band gap in CuO could be due to the occurrence of intragap states as well as the quantum confinement effect [132]. Tripathi et al. [136] reported to show a transition from an indirect transition of w1 eV to a direct band gap of w4eV by tuning the morphology of nanostructure as well as midgap defects. In addition, the replacement of Cu2þ by Al3þ in the CuO matrix led to band gap (direct) widening due to the formation of oxygen vacancies and defects [161]. Rahaman et al. [134] reported the widening of direct band gap of CuO from 2.67 to 2.90 eV with increasing Mn doping from 0 to 4 at. % while the same slightly reduced upto 2.72 eV with further increasing Mn doping concentration to w8 at.%. The authors suggested the Mn-doping-induced band gap widening in CuO could be due to the emergence of the Cu2O phase with increasing Mn-doping concentration [134]. Further, the change of conductivity from P to n-type at and above 4 at.% Mn doping in CuO has also been evidenced [134]. Similarly, Babu et al. [131] also reported the conversion of P to n-type conductivity up on Cd-doping in CuO. The authors also reported the Cd-doping-induced elongation in both direct and indirect band gaps of CuO [131].

3.3.2

Band gap narrowing in copper oxide

The decrease of the band gap of CuO from w3.76 to 3.41 eV with increasing Zn doping concentration from 0 to 10M has been reported [133]. Ponnar et al. [162] reported that the increase in size from 18 to 24 nm and decrease in band gap from 3.63 to 3.13 eV with increasing Ce doping in CuO. The authors suggested the Ce doping-induced modulation in size and band gap well agreed with the quantum confinement effect [163,164]. The increase of La doping from 0% to 6% induced a decrease in the band gap of CuO from 1.48 to 1.46 eV, which has also been reported due to La3þ ion doping-induced impurity band broadening [165]. On the other hand, the authors [165] also reported the widening of the band gap in these samples upon annealing due to the partial elimination of Cu vacancies [166].

3.4

Band gap engineering in cadmium oxide

Among the different transparent conducting oxide (TCO) ceramics, CdO has been considered the most promising material for optoelectronics [167e170]. Further,

Band-gap engineering of ceramic coatings

47

CdO possesses several characteristic features like high chemical stability, nontoxic, cost-effective material, etc. CdO has also been considered one of the major components for paint pigments and electroplating baths [171,172]. CdO is shown to exhibit n-type conductivity with direct band energy varying from 2.2 to 2.79 eV [173e175] and indirect band gap energies varying from 0.84 eV [176] to 1.98 eV [177]. CdO, with high optical transparency in the visible region of the solar spectrum along with low electrical resistivity (w102 to 104 U cm) has been considered as one of the potential materials for the construction of electro-optical devices [178]. Based on the band gap energy and low resistivity, CdO offers widespread technological possibilities such as window materials in solar cells [176,179e182], gas sensors [183], photodiodes [184], optoelectronic devices, liquid crystal displays [185], smart windows, low-emissive windows, optical communications, IR heat mirror, thin-film resistors [169,186e188], photosensors and phototransistors [189,190], transparent electrodes [191,192], supercapacitors [193], etc. Since the TCO ceramics remain the center of attraction due to their band-gap engineering [194], therefore band engineering in CdO could improve its optical, electrical, magnetic, and photocatalytic properties from a specific application point of view [175,192,195e200].

3.4.1

Band gap widening in cadmium oxide

Alahmed et al. [167] reported an increase in the band gap of CdO from 2.23 to 2.45 eV with increasing Mn doping concentrations from 0% to 8%. Similarly, Aydm et al. [169] reported the enhancement of the band gap of CdO from 1.89 to 2.08 eV with increasing Fe doping concentration from 0% to 20%. The Sr dopant has a dramatic effect on the widening of the band gap in CdO except at 1% doping concentration (band gap of 2.35eV). The upsurge of band gap in CdO has been reported from 2.44 to 2.61 eV with increasing Sr dopant concentration from 0% to 3% [168]. The enhancement of Tb doping concentration from 0% to 5% has caused the upsurge of the band gap of CdO from 2.79 to 2.91 eV [175]. Likewise, the increment in Cu [190] and Ni [201] dopants from 0% to 2.56% and 0%e6% caused the enhancement of the band gap of CdO from 2.24 to 2.70 eV and 2.26e2.60 eV respectively. Similarly, the Sn doping-induced blue shift in the band gap of CdO has also been reported [202]. In a systematic study, Dhankhar et al. [203] reported the enhancement of the band gap of CdO from 2.42 to 3.28 eV with increasing Zn doping concentration from 0% to 100%.

3.4.2

Band gap narrowing in cadmium oxide

Unlike the band widening [190,201] discussed above, the Cu [204] and Ni [205] dopants have also been reported to suppress the band gap in CdO. However, the systematic narrowing has not been reported with increasing dopant concentration but the band gap is lower as compared to the pure CdO. Bader et al. [206] reported the suppression of the band gap of CdO from 2.63 to 2.46 eV with increasing In-doping concentrations from 0% to 3%. Similarly, the systematic narrowing of the band gap of CdO from 3.95 to 3.18 eV and 3.50 to 2.87 eV, with increasing Co [198] and Ga [207] doping

48

Advanced Ceramic Coatings

concentrations from 0% to 10% and 0%e9%, respectively. The decrease of the band gap of CdO from 2.76 to 2.36 eV due to an increase in the Al dopant concentration from 1.32% to 7.24% has also been reported [208]. The suppression of the band gap of CdO by w20% and 17% has also been reported, even with a small amount of Yb (w0.03 at.%) and Sm (0.4%) doping, respectively [209,210]. Jeejamol et al. [197] have undertaken the effects of Zr doping as well as thermal annealing on the optical band gap of CdO. The authors reported that the Zr doping caused the narrowing of the band gap of CdO, irrespective of annealing temperature.

3.5

Band gap engineering in tin oxide

The TCO ceramics possess conflicting properties comprising high conductivity and transparency [211e213]. As stated above, the TCO ceramics are the essential components in optoelectronics [212], and their band gap has to be more than 3.1 eV in order to achieve visible light transparency. SnO2 has been considered one of the good choices to replace ITO [212] due to its large abundance [214] and less expensive [215]. SnO2 possesses intrinsic n-type conductivity, a direct band of 3.6 eV [216], a carrier density of w1021 cm3, high visible light transparency (w97%) [217], good chemical stability, infrared spectrum reflector, a transparent thermal barrier [218], etc. The SnO2 has proved the suitable material for solar cells [218e220], flat panel displays [218,221], touch screen sensors [222], chemical sensors [223,224], photocatalysis [225], photoanodes for water oxidation [226], anodes for lithium-ion batteries [227], etc. However, most of the studies report the existence of dominant broad visible emission at w 540 nm in contrast to the near band edge UV emission for bulk SnO2 due to the dipole-forbidden nature associated with the band edge quantum states [218]. This limits its optoelectronic potentiality [218,228]. In order to overcome this, one needs SnO2 nanostructures [218]. The band structure engineering could provide the pathway for tunning optoelectronic properties [218] as well as other applicationoriented properties [218,223e226,229,230].

3.5.1

Band gap widening in tin oxide

The band gap of SnO2 increased from 3.91 to 4.22 eV with increasing Bi doping concentration from 0 to 3 at% [231]. Similarly, a gradual blue shift in the band gap of SnO2 with increasing Co doping concentration has been reported [232]. Santhakumari et al. [233] also reported the blue shift in the band gap of SnO2 from 3.79 to 3.98 eV with increasing Mn doping concentration from 0% to 10%. The authors attributed the same to Mn doping-induced decrease in particle size of SnO2. Sukriti et al. [85] studied the effect of pH during synthesis on the optical band gap of SnO2 nanostructures. The authors reported the progressive blue shift in the band gap of SnO2 from 3.76 to 3.98 eV with increasing pH from 3 to 11. Their study indicated the higher pH led to size reduction, which in turn enhances the band gap due to quantum confinement. The authors further suggested the utility of their material for lightemitting diodes, photo-detectors, etc. [85]. Zhou et al. [230] reported the gradual increase of the band gap of SnO2 with an increasing thickness, which is in contrast

Band-gap engineering of ceramic coatings

49

to the case of ref. [234]. The authors correlated the band gap with strain theoretically and suggested that the development of c-axis strain from w-4% to 14% led to the suppression of the band gap from 3.74 to 2.41 eV or vice-versa. Rus et al. [235] also reported the gradual blue shift of the band gap of SnO2 with increasing film thickness. The authors also correlated the strain with the band gap and suggested that the development of compressive strain led to band gap reduction, i.e., compressive strain of w1% would lead to a reduction of the band gap by w0.38 eV.

3.5.2

Band gap narrowing in tin oxide

In a theoretical study, Ganose and Scanlon [212] showed the narrowing of the band gap of SnO2 from w3.67 to 3.17 eV with increasing Pb doping concentrations from 0% to 12.5%. The authors suggested their prediction could be achieved experimentally due to the high enthalpy of mixing the systems. Further, band gap narrowing could be achieved due to the conduction band with reference to vacuum level, and such band gap narrowing could be useful for the application of the system in organic photovoltaics [212]. In agreement, Sarangi et al. [229] reported the suppression of the band gap of SnO2 from w3.64 to 2.87 eV with increasing Pb doping concentrations from 0% to 15%. Similarly, the band gap of Co-doped SnO2 decreased from w3.79 to 3.62 eV with increasing Co-doping concentration from 2% to 6% [236]. Also, the band gap of SnO2 decreased from w4.2 to 4.1 eV with increasing In doping concentrations from 0% to 16% [237]. Islam et al. [234] the effect of thickness on the optical band gap of 4% Ba doped SnO2 and reported that the band gap of the sample decreased from 3.94 to 3.64 eV with increasing thickness from 100 to 200 nm. The authors attributed the formation of oxygen vacancies to the band gap narrowing at higher thickness [238]. It has also been reported that electrons are less confined in thick films, which in turn favors band gap narrowing [218]. Further, Kamble et al. [239] studied the effect of precursor concentration on the optical band gap of SnO2. Their study indicated the suppression of the band gap of SnO2 from w4.06 to 3.40 eV by increasing the precursor concentration of stannic chloride in water from 0.1 to 0.5 M during the preparation. Similarly, Pacheco-Salazar et al. [240] reported a progressive band gap reduction which is w23% in 10 mol% Ce-doped SnO2. The authors attributed the same to the contribution of surface polarization as well as quantum confinement.

3.6

Band gap engineering in zirconia

ZrO2 is shown to exhibit five different polymorphs [241,242]. It exhibits a baddeleyite crystal structure with a monoclinic P21/c unit cell at ambient temperature. The firstorder structural phase transition from monoclinic to a tetragonal phase (P42/nmc) would occur beyond 1480K which is further converted into the cubic fluorite phase (Fm3m) at 2650K [243]. In addition, the orthorhombic Pbca and Pnma phases could be observed under the influence of pressure above 3 and 20 GPa, respectively [244,245]. The ZrO2 has shown diverse industrial application possibilities, which basically depend on its crystal structure [246]. Among the different crystal structures, the

50

Advanced Ceramic Coatings

ZrO2 with stable monoclinic structure has no practical applications as the decomposing of ceramic components is commonly detected during the cooling from the tetragonal phase [247]. However, the ZrO2 with a tetragonal and cubic crystal structures are shown to exhibit excellent mechanical (bulk modulus and high fracture toughness), chemical (chemically inert and corrosion resistant), thermal (low thermal conductivity and extremely refractory), and dielectric [248] properties which make the ZrO2 as an excellent candidate for technological applications [246]. ZrO2 has been considered a potential candidate for use as a solid oxide fuel cell [249], the solid electrolyte in an oxygen sensor [250], thermal barrier coating [251], confinement of nuclear waste [252], gate dielectric in combination with hafnium [253], catalyst [254,255], active photon absorber [256], etc. Further, ZrO2 has been considered a potential candidate for use in ceramic technology [257] as well as broadband filters and electro-optical devices due to its excellent optical properties [258,259]. Researchers are trying to achieve the application-oriented high symmetric ZrO2 by either doping it or reducing its dimension to the nanoscale [246]. However, no concrete general information on the mechanism of phase stabilization is available [260e262]. ZrO2 is shown to exhibit n-type semiconducting properties [263] with two direct band-to-band transitions at 5.2 and 5.79 eV [259]. Further, the band gap of ZrO2 is also reported to vary from 2.3 to 3.8 eV [264]. The band gap of ZrO2 is highly influenced by native defects, crystal phases, crystallite sizes [265], etc. The band-gap engineering in ZrO2 is of great interest to improve device-oriented properties, especially photocatalytic performance [266,267].

3.6.1

Band gap widening in zirconia

Kumar et al. [256] studied the effect of Ni doping on the band gap of ZrO2, and their result indicated that the band gap of ZrO2 increased from 5.69 to 5.91 eV with increasing Ni doping concentration from 0% to 1%. The authors attributed such a rise in the bad gap to quantum confinement [268,269]. On the other hand, the band gap of ZrO2 showed a decreasing trend beyond 1% Ni doping concentration, though the same is higher than that of pure ZrO2. The authors assigned such variation to the combined effect of quantum confinement as well as the formation of interstitial defects in ZrO2 with the rise in Ni doping concentration. Further, the authors [256] also suggested that the occurrence of a higher band gap of 5% Ni-doped ZrO2 as compared to undoped ZrO2 could be due to the Ni doping-induced enhancement of monoclinic phase fraction in ZrO2 which is in accordance with the literature [270]. In agreement with this result, Joy et al. [268] also reported the enhancement of the band gap in Agdoped ZrO2 samples synthesized with an Ag/Zr molar ratio of 0.16% widening of band gap from 5.7 to 5.8 eV. Ilanchezhiyan et al. [271] reported that the increase in Gd doping from 0% to 5% led to the enhancement of the band of ZrO2 from 4.64 to 4.80 eV. The authors attributed the upsurge in band gap to the Gd doping-induced invasion of degenerated electrons into the conduction band of ZrO2. As a result, the shift in the quasi-Fermi level takes place toward the conduction band of ZrO2 and the widening of the energy band caused due to the increase in carrier concentration [271].

Band-gap engineering of ceramic coatings

3.6.2

51

Band gap narrowing in zirconia

Xiao et al. [263] reported the band gap narrowing in ZrO2 from 5.0 to 2.56 eV upon increasing Fe doping concentration from 0% to 12.7%. The authors attributed that the positioning of Fe ions either in the lattice site or in the interstitial position causing new conduction band minimum or valence band maximum to cause band gap narrowing [263]. Similarly, Navio et al. [264] also reported the suppression of the band gap of ZrO2 from 3.65 to 2.39 eV upon increasing Fe doping concentration from 0% to 5%, and the authors suggested that the introduction of additional energy levels into the band gap due to the incorporation of impurities or defects would lead to band gap narrowing. In agreement with this observation [264], Hussain et al. [272] reported the band gap narrowing in ZrO2 from 4.31 to 3.05 eV upon increasing Co doping concentration from 0% to 12%. Also, Ag doping causes suppression of the band gap from 5.7 to 5.3 eV with an increasing Ag/Zr ratio of 0%e0.8% [268]. Further, 15% of Ti doping also caused the red shift of the band gap by 1.3eV [273]. In addition, Sudrajat et al. [254] reported that the N-doping-induced generation of midgap state would lead to the suppression of the band gap of ZrO2 from 4.8 to 3.8 eV. Further, Berlin et al. [274] studied the effect of Mn doping on the band gap of ZrO2 thin film. Their study indicated that the film thickness and band gap decreased from 270 to 177 nm and from 5.72 to 4.42 eV, respectively, with increasing Mn doping concentrations from 0% to 20%. The authors suggested the Mn-doping-induced band gap narrowing could be due to the combined effect of quantum confinement along with the presence of oxygen vacancies.

3.7

Band gap engineering in cerium oxide

Cerium oxide (CeO2) is an n-type wide bandgap semiconductor whose band gap varies from 2.6 to 3.4 eV [275,276]. It possesses several interesting characteristics like good visible light transparency, nontoxic, photocatalysis, a high dielectric constant (ε ¼ 26) [277], abundance in the Earth’s crust [278,279], etc. Further, it has the ability to release and uptake oxygen through the change of valance state from Ce4þ to Ce3þ, modulating the oxygen vacancies [280]. Such peculiarities make CeO2 the catalyst for UVblocking material in cosmetic/sunscreen products [281] and cotton fabrics [282], UV-aging resistance layer [283], anticorrosion layer [283], removing vehicle exhaust gases [279], oxygen sensors [284], oxygen ion conductors in conductors in solid oxide fuel cells (SOFCs) [285], functionalization of silk fiber for antibacterial activity [286], etc. A wide bandgap of CeO2 makes it an active material in the UV region [287]. In order to make the wide-spread use of CeO2 as an electronic material for logic operations as well as photocatalytic applications under visible light, one needs to tune its band gap [277,283,288e292]. In fact, band gap tuning has shown its utility in solar cell modules [293,294].

52

3.7.1

Advanced Ceramic Coatings

Band gap widening in cerium oxide

Kumar et al. [295] reported the Sn doping-induced size reduction in CeO2 nanostructures, which in turn affect the band gap, which could possibly be due to the quantum confinement effect. The authors [295] showed the enhancement of the band gap of CeO2 from 4.85 to 5.04 eV with increasing Sn doping from 0% to 6%. Soni et al. [277] studied the effect of Gd doping on the band gap of CeO2. The authors reported that the band gap of pure CeO2 (2.60 eV) is less than that of its bulk counterpart (3.15 [296]). The authors attributed this discrepancy to the increase in the Ce3þ state on the grain boundary. Further, the authors have reported the band gap widening when 2% and 4% of Gd ions are incorporated into the lattice of CeO2. Like Kumar et al. [295], these authors [277] also have seen Gd doping induced size reduction in CeO2 and thereby band gap widening due to confinement of charge carriers in the small size particles.

3.7.2

Band gap narrowing in cerium oxide

Tian et al. [283] reported the La doping-induced initial suppression of the band gap of CeO2 from 3.2 to 2.97 eV (up to 5% doping) beyond which the same moderately increased to 3.04 eV (up to 15% doping). However, the band gap of doped samples is lower than that of a pure sample. The authors suggested that the 5% La-doped CeO2 could be a better candidate for the UV-shielding purpose. Soni et al. [277] reported that Gd doping-induced size reduction, as well as band gap narrowing in CeO2 at higher doping concentrations, could be due to the migration of more Ce3þ ions at the grain boundary, which upsurges with decreasing particle size [297]. Ansari et al. [287] studied the band gap properties of pure and electrochemically active filmmodified CeO2. The authors also reported that the band gap of a pure sample is 3.19 eV, which is reduced to 2.87 eV upon modification, which could be due to the generation of a high amount of Ce 3þ ions at the grain boundary [297]. Kumari et al. [298] reported the red shift of the band gap of CeO2 from 3.94 to 2.64 eV with increasing Ni doping concentrations from 0% to 10%. Similarly, Fe doping also caused red shift of the band gap of CeO2 from 2.9 to 1.1 eV with increasing Fe doping concentrations from 0% to 20% [278]. Wu et al. [299] studied the effect of thermal annealing on the particle size and band gap of the CeO2 nanostructure. Their study indicated the size of the sample enhanced from w4 to 15.3 nm with increasing annealing temperature from 0 to 650 C. The size enhancement has influenced the optical band gap and the same decreased from 2.80 to 1.19 eV. Also, Huang et al. reported the red shift of the band gap of CeO2 octahedra from 3.42 to 2.94 eV with increasing size from 52 to 110 nm, respectively.

3.8

Band gap engineering in titania

TiO2 is a well-studied ceramic oxide owing to its nontoxicity, biocompatibility, stability, low cost, multifunctional applications, etc. [36,300e308]. TiO2 is shown to exhibit three major polymorphous, namely (i) rutile having a tetragonal structure, (ii) anatase

Band-gap engineering of ceramic coatings

53

having a tetragonal structure, and (iii) brookite having the orthorhombic structure [300,309,310]. Both anatase and rutile are the commonly observed phases of TiO2. However, the observation of brookite of TiO2 is rarely reported [305]. The crystal structure of TiO2 would detect its applications. For example, the anatase phase is shown to exhibit a higher photocatalytic activity as compared to other phases [300]. The rutile TiO2 is shown to exhibit a high refractive index along with thermal stability [311]. TiO2 with brookite phase has been considered a deserving candidate for photovoltaic applications [312,313]. It has been reported that the TiO2 showing the rutile and brookite phases exhibit a direct band gap, while the same with anatase phase exhibits an indirect band gap [300]. It is an n-type semiconductor having a wide band gap of 3.0 eV for rutile, 3.13 eV for brookite, and 3.2 eV for anatase phases [300,302,306]. TiO2 has shown its potentiality in various applications such as white pigment [308], self-cleaning material [314,315], solar energy conversion [314,316], waste-water purification [314,317], photocatalyst [318e321], lithium-ion battery [322], solar cell [323], photo anode [324], gas sensor [305], hydrogen production [325], spintronics, [301,326] etc. Though TiO2 has shown its potentiality as one of the future materials in terms of multifunctional properties, the major hindrance for its applications is its wide band gap, which makes it active in the UV region [307]. Since UV light represents only w4% to5% of solar spectral irradiance [300,308]. Further, TiO2 has been considered a versatile photosensitive material [302,308] for many solar light-driven clean energy industries and environmental technologies [325]; it is therefore highly desireable to reduce the band gap of TiO2 in order to make it efficient in the visible region comprising the 40% of the solar spectrum [300,302].

3.8.1

Band gap widening in titania

In this system, the occurrence of both direct and indirect band transitions has been reported. Gultekin et al. [327] reported that the direct band gap of TiO2 changed from 3.74 to 3.89 eV with increasing Au doping concentrations from 0% to 50%. The direct band gap of TiO2 is blue-shifted from 3.25 to 3.458 eV with increasing Co doping concentrations from 0% to 7% [328]. Also, the increase in Mg doping concentration from 0% to 10% led to the increment in the band gap of TiO2 from 3.33 to 3.44 eV [329]. In agreement with this observation, Zhang et al. [330] reported the increase of the direct band gap of TiO2 increased from 3.20 to 3.27 eV with increasing Mg doping concentration from 0 to 1 mol% [330]. The authors attributed the band gap widening to a doping-induced negative shift of the conduction band edge (EC ), which can be estimated as [331,332] EC ¼ 2:94  Eg

(3.8)

Further, Kaleji et al. [333] reported the occurrence of indirect transition in TiO2 that can upsurge from 3.21 to 3.39 eV with increasing Nb doping concentrations from 0% to 20%.

54

3.8.2

Advanced Ceramic Coatings

Band gap narrowing in titania

Kharoubi et al. [334] reported that the direct band gap of TiO2 redshifted from 3.33 to 3.44 eV with increasing Mn doping concentrations from 0% to 10%. Similarly, Kamble et al. [335] reported the narrowing of the direct band gap of TiO2 from 3.18 to 2.34 eV with increasing Cu doping concentration from 0 to 1.71 eV. Cu doping has also been shown to regulate the indirect transition in TiO2. For example, the Cu doping-induced narrowing of the indirect band gap of TiO2 has also been reported [336,337]. In contrast to the result obtained by Kaleji et al. [333] as discussed above, Su et al. [323] reported the narrowing of the indirect band gap of TiO2 with increasing Nb doping concentrations up to 2 mol%. However, the same slightly increased at 5 mol% Nb-doped TiO2 sample and the authors attributed the same to the Nb doping-induced structural defect at higher concentrations (5 mol%). Mo et al. [325] studied the effect of hydrogenation on the indirect band gap of TiO2 by annealing the sample at different hydrogen atmosphere pressures. Their study indicated that the band edge of the samples decreased from 2.7 to 1.5 eV by changing the annealing atmosphere from vacuum to a 0.95 bar hydrogen atmosphere. Similarly, Vallejo et al. [308] reported the suppression of the indirect band gap of TiO2 from 3.25 to 2.84 eV with increasing N doping concentrations from 0% to 5%. Though the 0.1 mol % of Er dopant causes the red shift in the indirect band gap in TiO2 from 3.25 to 2.85 eV, the same remains unaltered up to Er doping concentration of 0.5 mol% [338]. However, the band gap of TiO2 decreased to 2.81 eV when doped with 0.7 mol% of Er. On the other hand, the band gap of TiO2 increased to 2.89 eV at higher Er doping concentration (1 mol%) [338].

3.9

Band gap engineering in gallium oxide

Ga2O3 is shown to exhibit several polymorphisms such as a-Ga2O3, b-Ga2O3, g-Ga2O3, ε-Ga2O3 [339], and k-Ga2O3 [340]. Among these polymorphous, the monoclinic b-Ga2O3 phase is the most stable one [339,341,342]. It is also possible to synthesize a high-quality phase of a-Ga2O3 due to the advancement of synthesis procedures [343e349]. Being an ultrawide band gap semiconductor [350], a band gap of the a-Ga2O3 varies from 5.1 to 5.3 eV [345,347,348,351,352], while the b-Ga2O3 phase has the band gap of 4.8 eV [353]. Ga2O3 has shown research interest because of its utility in various possible applications such as solar-blind flame detection [354], deep ultraviolet (UV) photodetectors [355e358], deep-UV transparent electrodes [359,360], a transparent n-type layer for solar cells [361], a transparent top contact for GaN LEDs [362], field-effect transistors [363], power-electronic devices [364,365], solar water splitting films [366e368], etc. Further, the Ga2O3 could be useful for ultrahigh power device (>1 MW) applications in hybrid propulsion, electric power transmission, etc. [369,370] due to its high electric field breakdown strength [370e372]. The band- gap engineering in Ga2O3 is highly essential in order to design and optimize the devices [373,374]. Band- gap widening through alloying/doping in this system could able to push the photo-detection cut-off limit of wavelength further into the UV region [372,375]. Tunable conductivity with band gap widening via

Band-gap engineering of ceramic coatings

55

doping in Ga2O3 could be achieved for the improvement of high-power MESFETs [364,376] and MOSFETs [363,377].

3.9.1

Band gap widening in gallium oxide

Feng et al. [378] reported the blue shift of the band gap of b-Ga2O3 from 4.89 to 5.29 eV with increasing Al doping concentration from 0 to w35%. Similarly, Ito et al. [379] reported the band gap of a-Ga2O3 can be blue-shifted to 7.8eV with increasing Al doping concentration w81%. Krueger et al. [372] have undertaken a systematic study on the Al doping-induced evolution of structural and optical properties of b-Ga2O3. Their study indicated the b-Ga2O3 phase is retained for Al doping concentration of less than 80%. Beyond this doping concentration, the mixed phase is evidenced and the a-Al2O3 phase is obviously observed at 100% doping. Further, the authors [372] reported that the band gap of the sample in a wide range of Al doping concentration (x) evolves as per the following relation: Eg ðeVÞ ¼ ½4:75 þ 1:87x

(3.9)

The authors also claimed that they found out the experimental band gap of the qAl2O3 phase (6.62  0.05 eV) for the extrapolation of band gap variation data [372]. Further, Zhang et al. [373] reported that the band gap of b-(Ga1-xAlx)2O3 can be varied from w5 to 7 eV with increasing Al concentration. In a theoretical study, Wang et al. [380] reported that the band gaps of a-(Ga1-xAlx)2O3 and b-(Ga1-xAlx)2O3 can be increased from 5.26 to 8.56 eV and 4.69 to 7.03 eV, respectively. Their results showed good agreement with the reported literature (Fig. 3.1). Similar observations have also been reported by Quarto et al. [381]. In addition, Zhang et al. [370] reported that the band gap of (AlxGa1x)2O3 can be increased from 4.85 to 8.8 eV. The authors also reported that the b-Ga2O3 phase is persisted up to x ¼ 0.8 and the corundum phase is preferred x > 0.8.

3.9.2

Band gap narrowing in gallium oxide

Barthel et al. [350] reported the red shift of the direct band gap of a-Ga2O3 from 5.04 to 4.77 eV with increasing Ti doping concentration from 0 to w5%. The authors also reported a similar trend in band gap with further increasing Ti concentration, i.e., band gap of the sample decreased up to 3.91 eV for 50% Ti doping. However, their study indicated the crystallinity starts diminishing and samples show complete amorphization at and above 20% Ti doping. The band gap of b-Ga2O3 is shown to redshifted from 4.56 to 3.34 eV with increasing Fe doping concentration from 0 to 30 at.% [382]. Similar results were also reported by Zhang et al. [383]. The authors [382] attributed strong p-d and s-d hybridization in Fe-doped b-Ga2O3 to band gap narrowing. He et al. [384] presented a systematic study on band gap narrowing in In-doped bGa2O3 (Fig. 3.2). Further, Zhang et al. [370] reported that the In-doped Ga2O3 can vary as per the following relation: 
Eg ðeVÞ ¼ 4:87  ð2:16  xÞ þ 1  x2 ; 0 3 x31

(3.10)

56

Advanced Ceramic Coatings

Figure 3.1 Band gaps of (A) a and (B) b-(AlxGa1x)2O3 as a function of Al fraction x [380]. Reprinted with permission from [T. Wang, W. Li, C. Ni, A. Janotti, Phys. Rev. Appl. 10 (2018) 011003.] Copyright (2022) by the American Physical Society.

The authors also suggested that the In doping caused mixed phase in the range, 0:443x30:90 [370].

3.10

Band gap engineering in a-Fe2O3

Among the various crystallographic forms of iron oxide, the a-Fe2O3 is the most thermodynamically stable phase [385]. It is a well-studied ceramic material featured with low cost, nontoxicity, high chemical stability, high resistance, photo-corrosion stability, abundance, etc. [386e388]. The a-Fe2O3 exhibits n-type conductivity [389] whose direct band gap and indirect band gap energy vary from w2.0e2.7 eV [385,390] and 1.38e2.09 eV [391], respectively. a-Fe2O3 has proven its potentiality for diverse technological applications, such as anode material for lithium-ion batteries [392], gas sensors [393], photocatalysts [394], water splitting [395], supercapacitor [396], electron transport layer in a solar cell [397], drug delivery applications [391,398,399], field emission devices, magnetic resonance imaging, magnetic recording, tissue repair engineering, pigments, spin electronic devices [391], etc. Due to its low band gap, the a-Fe2O3 could be used as a potential candidate for solar energy water splitting than other materials [387]. However, the short charge diffusion

Band-gap engineering of ceramic coatings

57

Figure 3.2 Variation of band gap of in-doped b-Ga2O3. The theoretical and experimental results are represented by the open and solid symbols, respectively [384]. Reprinted with permission from W. He, Z. Wang, T. Zheng, L. Wang, S. Zheng, Origin of the band gap reduction of indoped b-Ga2O3, Journal of Electronic Materials 50 (2021) 3856e3861. https://doi.org/10. 1007/s11664-021-08899-4, Springer Nature.

distance [390,400,401], high electron-hole recombination [386], poor reaction kinetics [402], less conductivity, and higher bias potential required [403] associated with a-Fe2O3 reduced its efficiency. In order to overcome these properties, one needs to adopt the band-gap engineering strategy to improve the electronic, magnetic as well as optical properties of a-Fe2O3 [385,386,404e408].

3.10.1 Band gap widening in a-Fe2O3 Sharmin et al. [407] reported the blue shift in the direct band gap of a-Fe2O3 from 2.19 to 2.98 eV with increasing Al dopant concentration from 0 to 6 at. %. The authors attributed such an upsurge in band gap to Al substitution indued level degeneracy. Further, their study indicated the suppression of the same to 2.81 and 2.67 eV with further increasing Al doping concentrations to 8 and 10 at.%, respectively, could possibly be due to Al clustering or interstitial substitution at higher doping concentrations [407]. The band of Fe2O3 is shown to increase from 2.00 to 2.64 eV with increasing the activated carbon content from 0% to 25% [409]. It has also been reported the direct band gap of a-Fe2O3 nanostructure remains invariant (w2.67 eV) under thermal annealing. However, the indirect band gap of a-Fe2O3 increased from 1.67 to 1.94 eV with increasing annealing temperature from 500 to 600 C and the band gap remains the same (w1.94 eV) with further increasing the annealing temperature to 700 C [391].

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Advanced Ceramic Coatings

3.10.2

Band gap narrowing in a-Fe2O3

Reddy et al. [404] reported the decrease of the direct band gap of a-Fe2O3 nanostructure from 1.92 to 1.49 eV with increasing Y dopant concentration from 0 to 0.4 mol. In agreement, Reveendran et al. [405] also reported the decrease of the direct band gap of a-Fe2O3 nanostructure from 2.06 to 1.87 eV with increasing Cu dopant concentration from 0 to 20 at.%. Hsu et al. [387], reported the red shift of the indirect band gap of aFe2O3 nanostructure from 2.03 to 1.98 eV with increasing Pt dopant concentration from 0 to 1.5 at.5%. Kumar et al. [410] also reported the red shift of the indirect band gap of a-Fe2O3 from 2.1 to 1.6 eV in a wide range of Cr doping concentrations. Kocher et al. [385] reported the occurrence of both direct (O2 2PeFe3þ 3d) and indirect (Fe3þ 3-3d spin forbidden) transition in the Ni-doped a-Fe2O3. Their study indicated that the red shift of both the direct and indirect band gap from 2.14 to 2.06 eV and 1.94 to 1.75 eV, respectively, with increasing Ni dopant concentration from 0 to 4 wt.%. The authors stated that the Fe3þ and O2 ions are octahedrally coordinated in a-Fe2O3 and the energy levels of 3 d5 orbitals can be split into two eg and three t2g. The existence of a higher number of electrons (5 electrons) in t2g orbitals of Ni2þ ions than that of Fe3þ ions (3 electrons) would lead to the development of lowlying unoccupied molecular orbitals (LUMOs) in the energy gap [385,411,412]. This would lead to the band gap narrowing. A decrease in the direct band gap of 5% Ni-doped a-Fe2O3 from 2.86 to 2.70 eV with increasing annealing temperature from 0 to 500 C has been reported [386]. In a theoretical study, Xia et al. [413] predicted that sulfur doping in place of oxygen would also lead to a red shift in the band gap of a-Fe2O3-xSx from w2.05 to 1.28 eV with increasing x from 0 to 0.33.

4.

Calculation of band gap related parameters

Several optical parameters such as refractive index (n) [414], nonlinear optical refrac
 tive index ðn2 Þ [415], electron polarizability (a0 ) [56], first-order susceptibility c1 , third-order nonlinear optical susceptibility (c3 ) [414,416e419], static dielectric constant (ε0 ), high-frequency dielectric constant (εN ) [420,421], oscillator strength (E0 ) [422,423], etc., have relevance in different device applications. These parameters can be calculated from the value of Eg as per the following relations: n ¼ KEgC n2 ¼

B Eg4

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 12:41  3 Eg  0:365 M  0:395  1024 cm3 a ¼ r 12:41 0

(3.11) (3.12)



(3.13)

Band-gap engineering of ceramic coatings

59

n2  1 4p

(3.14)

 4 c3 ¼ A c1

(3.15)

ε0 ¼ 18:52  3:08Eg

(3.16)

εN ¼ 11:26  1:42Eg

(3.17)

E0 ¼ 2Eg

(3.18)

c1 ¼

5. Conclusion Band-gap engineering has been considered one of the invaluable tools for designing the material’s features for specific application purposes. Brief reviews of the literature on band-gap engineering on certain ceramic oxides have been presented with the application perspective. The influences of certain parameters, like size, stress, thermal annealing, doping, etc., on the band-gap of ceramic oxides have been discussed. In addition, the information for the determination of band-gap-related parameters has been presented in view of their relevance in different device applications.

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Anticorrosion and antiwear ceramic coatings

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Santhosh G Nano-Materials and Energy Devices Lab (NMEDL), Department of Mechanical Engineering, NMAM Institute of Technology, Nitte (Deemed to be University), Nitte, Karnataka, India

1. Introduction Steel, an alloy of iron and carbon with a carbon concentration of up to 2%, has several applications in the automobile, household appliances, industrial machine, and heavy construction sectors like the chemical and marine industries. It is the material that is used to construct most of the world’s infrastructure and industries, from oil tankers to sewing needles so that the globe produced 1877.5 million tonnes (Mt) of crude steel in 2020, which provides some perspective on the relative significance of this material. Mild steel (MS) is chosen for machinery, automobile manufacturing, pipelines, construction because of its affordable pricing, machinability, and mechanical capabilities, but they must resist corrosion phenomena [1]. Temperature difference, air, chemical vapor, fumes, and humidity all have an impact on how quickly steel corrodes. The nature and moisture of the soil have an impact on the rate and intensity of corrosion in submerged steel structures and pipes. Steel becomes brittle and flaky because of the corrosive reaction, which alters the steel’s microstructure on its surface. It gradually loses mechanical strength and flexibility. The outcome is a significantly shorter useable life for steel buildings and other applications. Therefore, depending on the application type and environment, it is necessary to apply a suitable protective coating [2e4]. Hence, increasing the life of metals and metal alloys used in the industrial applications is inevitable. These days, high-performance coating reinforced with secondary phase materials has made huge impact and created an interesting research field for the researchers. Nanofillers are promising materials that bring new dimensions and opportunities to the anticorrosion and antiwear coating applications. Nanofillers are multifunctional materials used in matrix that provides enhanced coating behaviors (Fig. 4.1). Corrosion and wear are two foremost areas, which are responsible for the decline in performance and life of the engineering materials and systems. Hence, finding a better solution for corrosion and wear problems is very much essential. The corrosion process involves oxidation of the surface of the metals and alloys, simultaneously the reduction of the protons, oxygen, and water on the same surface. The corrosion process is mainly divided into two categories: (i) chemical corrosion and (ii) electrochemical corrosion. Chemical corrosion happens because of the chemical reaction of atmospheric gases with the materials surface, while the electrochemical corrosion is due to the destruction

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Figure 4.1 Schematic diagram of anticorrosion and antiwear coating. Graphical representation of corrosion and wear phenomenon in metal parts.

of anodic parts due to oxidative dissolution. However, the electrochemical corrosion is much faster than the chemical corrosion. Wear is the successive loss of material from the surface of solids. The wear loss is classified into two categories: (i) loss of material reducing the dimension and (ii) detaching material or wear debris from the worn surface. The loss of material leads to excessive vibration, noise, and increases fatigue fracture in the moving parts. While the detaching wear or wear debris are very harmful for the system as they remain trapped in valves and pipes, these wear debris can accumulate as an electrical contacting point as well. From the mechanism, one can understand that the reason for the wear and corrosion is the bare surface of the materials which are in contact with the atmosphere or with another bare surface. Thus, protecting the surface of the engineering materials from wear and corrosion by anticorrosive and antiwear coating is extremely important. Fig. 4.1 shows the schematic representation of antiwear and anticorrosive coating used on metal surface. Normally, the coatings are provided to prevent the surface form oxidating and reducing agents present in the external environment. The antiwear and anticorrosion coatings used to protect the materials surface have mainly different components: a pigment, binder, and additives [5e16]. For various applications, the surface needs modifications to perform better in the harsh environment. Considering the wide area of applications, metal and their alloy coating can be prepared by various other materials such as polymers, ceramics, alloys, and metals. Polymers are one such material that have the general advantages of being cheap, easy to process, and easy to coat with simple techniques. Metallic coatings also have gained lot of interest due to their electrical and thermal conductivity, but the high processing cost with complexity associated with oxidation makes them infrequent in coating applications. In contrast, ceramics are highly adopted materials due to their good hardness with highly appreciated wear and temperature resistant with corrosion and oxidation stability, but their higher processing cost and brittleness limit them being used widely as protective coating. However, the choice of material depends on the desired properties, compatibility, environmental conditions, and material processing cost. Considering all these aspects, polymer-based coating materials have acquired

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their place in the research world. The polymer-based coating exhibits better thermal and chemical resistance [17].

2. Polymer-based coatings Polymers are the most popular options in anticorrosion and antiwear coating applications. Various polymers like acrylics, polyurethane, alkyds, and epoxy are widely studied polymers as coating to protect the surface from corrosion and wear. Few conducting polymers like polypyrene, polyaniline (PANI), and its derivatives have been studied because of their good electrical conductivity, reversible electrochemical behavior, electrical, and optical capabilities. The conducting polymer has received major industrial interest and has potential uses in a wide range of applications because of its versatility and beneficial attributes, such as its physical, chemical, and mechanical capabilities, safety, and low cost. Conducting polymers have been used in a variety of applications, including sensors, electronic devices, batteries, and anticorrosive additive inorganic coatings [18]. By using suitable fillers, the barrier properties of organic coatings can be enhanced. Polybenzoxazines are employed as corrosion-protective coatings due to their unique properties, which are superior to those of epoxy resins and conventional phenolics. These characteristics include almost minimal shrinkage, low surface free energy, low water absorption, and exceptional dielectric properties [19]. An ultrafine microstructure with all its components (area, crystals, phases, etc.) less than 100 nm is what is referred to as a nanocoating. It is possible to construct these coatings using layers that are even more thin than 100 nm. There are numerous grain boundaries, interphase boundaries, dislocations, and other structures in them where the separations between them are nearly interatomic [20]. Due to their distinct advantages over more traditional, larger-grained coatings, nanostructured coatings were able to outperform them in terms of mechanical and corrosion properties [21]. This nanocoating is more successful in filling voids and stopping corrosive elements from diffusing through the surface of the substrate due to the relatively small sizes of the particles used in it [22]. Additionally, the increased density of the nanocoating crystal structure offers greater adhesive qualities, extending the coating’s lifespan. Nanocoating gives products stronger, harder, and higher resistance to surroundings with corrosion and wear because it has superior mechanical and electrical qualities [23]. In some situations, nanocoatings might not be effective as protective surfaces. Due to high density of their grain boundaries, which allow for quick diffusion routes for passivated ions and improved adherence of the protecting oxide layer to the substrate’s surface, nanocoatings are an efficient physical barrier in high-temperature applications. However, there are more anodic sites to the greater grain boundary fraction, the surface is more vulnerable to corrosion attack. Additionally, by incorporating into the voids, dislocations, and grain/interphase barriers, nanocoatings create a protective structure. Due to the rapid diffusion of passivating ions, these characteristics have the advantage of generating a passivation layer that is more effective. On the other side, the agglomeration of these nanoscale materials may result from the rapid diffusion of hostile ions, which leads to

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uneven surfaces and raises the likelihood that active sites will form, reducing corrosion resistance. Such a disagreement highlights the need to investigate the corrosion rate of each nanocoating while considering all relevant environmental factors. The best resistance against abrasion, disturbance, corrosive chemicals, and severe temperatures is provided by epoxy coatings. Epoxy coating is not just long-lasting but also corrosive substance resistant [24]. They are among the most common materials used to prevent corrosion in a variety of environmental conditions. Due to increased research in this area, epoxy coating has grown in popularity in recent years. Epoxy coatings are significantly more effective than polymeric and cathodic corrosion prevention. Epoxy has the additional effect of reducing chemical erosion in the right circumstances; epoxy is a very economical corrosion protection option. Epoxy and different nanomaterials are successfully mixed to increase corrosion resistance.

3.

Nanomaterials in coating

Nanomaterials have been used as a successful candidates to halt corrosion due to their morphological properties. Those nanomaterials that at least one of their morphological properties, such as grain size, particle size, structure size, etc., have. They can be two dimensional (nanoparticles) or one dimensional (nanowires, nanorods, and nanotubes) (nanoplatelet, nanosheets, and nanofilms). Enhancements in nanomaterials’ thermal, mechanical, physical, chemical, magnetic, electronics, and optical characteristics have been noted. They are mostly to blame for this because of their tiny diameters, which enable bigger mass fraction at the surfaces and hence greater contact areas. It has been discovered that nanomaterials have the potential to slow down the corrosion of metal substrates by modification with coatings that have nanocrystalline structures.

3.1

Nanoparticle preparation methods

The methods used to create the nanoparticles, which are divided into three main categories: physical, chemical, and mechanical processes, have significantly improved over time. These technologies are used for both research and commercial purposes. Self-healing, self-cleaning, high scratch, and wear resistance are introduced to nanocoating technology [19]. They are designed to release controlled amounts of inhibitors in response to environmental stimuli including pH, moisture levels, heat, stress, coating deformation [25], etc., to heal and cure flaws and damages. The corrosion behavior of traditional coatings with microparticles or thicknesses would be different from that of nanocoating. As depicted in Fig. 4.2, there are three primary deposition processes that can be used to create nanocoating. The cheapest method of deposition is mechanical, which can be done by spraying, painting, spin-coating, or dip-coating. Bonding, condensation, and sputtering are all methods of physical deposition. While lubricants are added during brazing bonding at higher temperatures, moderate pressure and temperature are used during physical diffusion bonding [26].

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Figure 4.2 Thin film deposition methods [26]. The figure depicts the classification of deposition techniques. From D. Abdeen, M. Hachach, M. Koc, M. Atieh, A review on the corrosion behavior of nanocoatings on metallic substrates, Materials 12 (2) (2019).

For metal substrates, sol-gel protective coatings have demonstrated superior chemical stability, oxidation control, and increased corrosion resistance. Additionally, the sol-gel approach is a surface protection technology that is favorable to the environment and has historically been used to increase metals’ resistance to corrosion. The ideal method is sol-gel since it is a reasonably inexpensive coating and has moderate pressing temperatures, but it demands expensive raw materials and a regulated processing environment [22].

4. Anticorrosion and antiwear behavior of nanoparticles The anticorrosion and antiwear behavior of various nanoparticles are studied by combining them with epoxy resin. By lowering porosity and zigzagging the diffusion pathway for harmful compounds, the inclusion of a second phase that is soluble with epoxy polymer can enhance the barrier effectiveness of epoxy coatings. For instance, an epoxy nanocomposite can be made by spreading inorganic filler particles with a nanoscale size throughout the epoxy resin matrix. Epoxy resins impregnated with eco-friendly nanoparticles can be utilized to increase the durability and integrity of coatings (epoxy). Each test can provide some corrosion parameter measurements and aid in understanding the corrosion mechanism for the material being examined. ZnO is one of the promising metal oxides that forms a thin layer on the metal surface to increase corrosion resistance among the various metal oxides [21]. Zinc oxide is an extremely powerful noncorrosive substance [27]; it acts as a good corrosion barrier and

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is a well-known protective corrosion product. In comparison to bare steel, ZnOalginateecoated steel exhibits a larger polarization potential and a lower corrosion current, indicating that the coating offers better corrosion protection [21]. Zinc oxide has a very thin layer of extremely corrosion-resistant oxide on its surface, and the development of hydroxide creates a barrier protection that is stable across a broad pH range. This is the reason there is interest in employing zinc oxide in organic coatings to prevent steel from rusting [27] and the purpose of almost half of all produced zinc is used in corrosion prevention [28]. The uniform distribution of PANI (polyaniline) and potential for the creation of uniform passive layers on the surface of a metallic substrate give ZnO greater corrosion prevention capabilities. PANI-metal oxide nanocomposite compounds are therefore interesting candidates to incorporate into organic coatings as cutting-edge anticorrosion additives [18]. When long-term protection of the metal structure is necessary, zinc is frequently utilized in anticorrosion coating industries. Due to its special ability to protect the metal even after minor mechanical damage to the coating, zinc-rich primers (ZRPs) are frequently utilized in a range of harsh situations, including shipbuilding, offshore platforms, and industrial settings [29]. The ZRP offers good cathodic protection in the early phases of service life, and as service life progresses, the stable zinc corrosion products add additional barrier protection [29]. Like ZnO, the SiO2 is one of the most promising nanoparticles. They have many great qualities, including a high hardness (7 Mohs), a low refractive index (1.46), and a fair price compared to other nanoparticles, which make them perfect for producing scratch-resistant, transparent coatings that are also cost-effective, as well as a high absorption of ultraviolet radiation [19].

5.

Coating techniques

There are many general coating techniques used to coat thin layer on materials surface; methods like “electrochemical deposition,” “dip coating,” “spin coating,” film casting and layer-by-layer printing and spraying techniques, etc., are used by many researchers [30]. To understand the nature of anticorrosive and antiwear applications, various methods like arc discharge technique, gas fame spray technique, vacuum deposition, and vapor deposition techniques are used to create various coatings on materials [31e37].

5.1

For anticorrosion application

In most of the coating methods, the material surface is thoroughly cleaned; at first, the grounded surface is cleaned by ultrasonication in absolute alcohol and then dried in an oven in the presence of nitrogen [38]. The hydroxy acrylic melamine dissolved in water is coated onto the cold rolled MS to protect the MS surface from corrosion [38]. ZnO nanoparticle reinforced poly(dimethyl siloxane) nanocomposite of varying concentration of hexadecyltrimethoxysilane as curing agent was created by spin coating technique; the coating of the nanocomposites was found to protect the material surface from corrosion [39]. Further, various nanoparticles reinforced with epoxy resin were

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also used as coating on steel surface to reduce corrosion. Graphene oxidee functionalized Fe3O4 hybrid nanoparticles mixed with epoxy resin were uniformly coated using spray coating technique on steel surface found to be the most effective method to reduce corrosion [40]. Zinc polyphosphate nanopigmenteincorporated epoxy coatings prevented steel from corrosion [41].

5.2

For antiwear applications

The nanomaterials are widely appreciated materials for antiwear applications. The nanomaterials mixed with lubricating oils were also used to coat the metal surface for better protection. Hence, the coating techniques used for antiwear applications are same as nanoparticle preparation techniques and few techniques used to prepare antiwear coating are discussed here. The magnetron sputtering and cosputtering techniques are used to prepare antiwear nanocomposite coating of TiN and TiN/Cu on Si substrate [42]. The chemical precipitation technique is also considered one of the most efficient ways of creating antiear coating on materials surface. This method involves the reaction between material used in the solvent and suitable surfactants are used to keep the materials separate before precipitation. Further, several other methods like codeposition and electrodeposition methods are also used to create antiwear coatings. The codeposition method involves the deposition of inert particles into metallic matrix to create composite coating, but the coating created by this method will lead to the agglomeration and insolubility of the particles in the matrix. To overcome these issues, electrodeposition technique is used instead of codeposition technique. Several particles like Al2O3, TiO2, SiO2, diamond, and carbon nanotube, etc., were used and electrochemically deposited into NieW metal alloy matrix [43e46]. Nanomaterials prepared by electrodeposition methods like chemical vapor deposition (CVD) and physical vapor deposition (PVD) are also used in creating antiwear coatings. The PVD method was used to deposit CrN/nitriding coating on steel surface [47]. Many researchers have used PVD and CVD to create antiwear coatings on various metal surfaces by depositing thin layer of nanomaterials [48e51].

6. Surface morphology of coated surface The surface morphology of the steel surface before and after ZnO nanoparticles at pH 7.5 is presented in Fig. 4.3. Fig. 4.3B shows the image of electro-deposited ZnO coating at pH 4.5e5.0 (coating 1); Fig. 4.3D shows the ZnO-derived nanocontainers with corrosion inhibitor safranin on metal surfaces at pH 7.5 (coating 2). The crystalline structure of the coating is more apparent at increasing magnification. The SEM insets for the deposits of polymeric-modified ZnO nanoparticles and ZnO-based nanocontainers (Fig. 4.3A and C) show a comparatively homogenous population of the particles, with size close to that of the original nanoparticles and nanocontainers. ZnO cluster formation, which is more obvious for ZnO nanocontainers, is also seen. They most likely arise during the electrodeposition process on the steel surface

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Figure 4.3 SEM micrographs prepared by electrodeposition (A) ZnO NPs without PEI covering, (B) ZnO NPs with PEI covering and deposition of a standard zinc coating, (C) ZnO-based nanocontainers with safranin, (D) ZnO-based nanocontainers without safranin [21]. Surface morphology of coated and uncoated steel surface to understand the behavior of nanoparticles in improving the surface property. From K. Kamburova, N. Boshkova, N. Boshkov, T.S. Radeva, Composite coatings with polymeric modified ZnO nanoparticles and nanocontainers with inhibitor for corrosion protection of low carbon steel, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 609 (2021) 125741, https://doi.org/10.1016/j.colsurfa.2020.125741.

because these agglomerates (clusters) are not detected in the suspensions prior to the electrodeposition process on the cathode surface. The zinc-terminated coating morphology is identical in the SEM micrographs of both composite coatings (Fig. 4.3B and D). It should be highlighted that a limited number of dispersed microscopic pores also developed on the surface of the coating, most likely as a result of water electrolysis during the deposition of the ZnO nanoparticles and ZnO nanocontainers [52]. The results of this investigation on the surface morphology of the coating and their corrosive behavior demonstrate that potentiodynamic polarization and polarization resistance provide greater corrosion protection than bare zinc (Fig. 4.3).

7.

Electrochemical investigation of coating

7.1

Polyethylenimine-based coating

The potentiodynamic polarization curves of the newly created composite zinc coatings containing ZnO nanoparticles before (coating 1) and after (coating 2) deposition of standard zinc on steel surface are presented in Fig. 4.4. It can be seen that the corrosion

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potential of steel is higher than that of coatings. From the studies, it is revealed that the corrosion potential (Ecorr) values for coating 1 and coating 2 are 1.109 V and
1.096 V, respectively. Steel and regular zinc have corrosion potential values of
0.720 V and
1.065 V, respectively. In comparison to coating 1, which has a corrosion current density (Icorr) of 1.1  10
5 A/cm2, coating 2 has a lower Icorr value of 6.7  10
6 A/cm2, i.e., about 1.6 times. The existence and distribution of polymeric-altered ZnO particles as well as ZnO-based nanocontainers containing impregnated inhibitors in the metal matrix could be the reason for this discovery under the conditions of external anodic polarization. Both types of composite coating have very similar anodic curves and hardly any noticeable differences between them are seen. After extended corrosive treatment, it is possible that the variations between the composite samples will become more pronounced. To put things into perspective, the corrosion current densities (Icorr) of common zinc and steel substrate are 1.8  10
5 and 7.8  10
6 A/cm2, respectively (Fig. 4.4).

7.2

Chitosan-based coating

John et al. [22] have done potentiodynamic polarization studies to know and understand more about the kinetics of anodic and cathodic reactions. The polarization measurements were carried out in 0.1 N HCl solution. The potentiodynamic polarization curves for MS coated with a thin layer of chitosan (CS)/nano-ZnO hybrid and

Figure 4.4 Potentiodynamic polarization curves of coating 1 (C1), coating 2 (C2), Zn coating, and steel substrate [21]. The nanoparticle coating on steel surface and their analysis using potentiodynamic polarization curves. From K. Kamburova, N. Boshkova, T.S. Radeva, Composite coatings with polymeric modified ZnO nanoparticles and nanocontainers with inhibitor for corrosion protection of low carbon steel, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 609 (2021) 125741, https://doi.org/10.1016/j.colsurfa.2020.125741.

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untreated MS were measured [8]. The results were recorded by sweeping the potential from the equilibrium potential toward positive and negative potential against saturated calomel electrode in 0.1 N HCl electrolyte. The electrochemical corrosion parameters for MS for both uncoated and coated are shown in Fig. 4.4. These parameters include corrosion current densities (Icorr), corrosion potential (Ecorr), anodic Tafel slope (ba), and cathodic Tafel slope (bc). The corrosion current and potential values of uncoated and coated MS samples are shown in Table 4.1. It is evident that the CS/nano-ZnO hybrid coating reduces the corrosion current values of plain MS samples. However, the corrosion potential’s Tafel lines are used to extrapolate the uncoated and coated cathodic densities, respectively. In acidic solutions, the cathodic reaction is the discharge of hydrogen ions to form hydrogen gas or to reduce oxygen, and the anodic reaction of corrosion is the passage of metal ions from the metal surface into the solution. The inhibitor may have an impact on the cathodic as well as the anodic reaction. When CS/nano-ZnO is added, the polarization curves for both the anodic and cathodic sections are inhibited, and the anodic and cathodic curves are shifted to lower current densities, as can be seen in Fig. 4.5A and B. This could be explained by the inhibitor adhering to the metal surface. CS/nano-ZnO composite can therefore be regarded as a mixed type inhibitor. So the potentiodynamic polarization tests firmly show that CS/ ZnO nanoparticles have the ability to suppress corrosion (Fig. 4.5).

7.3

Epoxy-based coating

Hao et al. [24] studied the behavior of MS upon coating zinc phosphate (ZP) with epoxy to inhibit the corrosion on MS surface. The polarization curves of bare MS at various immersion times in artificial seawater with and without ZP are shown in Fig. 4.6. The corrosion potentials in the presence and absence of the pigment’s aqueous extract are E corr and E0 corr, respectively. The corrosion current densities in the presence and absence of the pigment’s aqueous extract are, respectively, i corr and i0 corr (Fig. 4.6) (Table 4.2). The outcomes of three parallel tests demonstrate the experimental data of low dispersion and high repeatability. With the passing of the immersion period in the pigment extract, the corrosion current density reduced and the corrosion potential Table 4.1 Polarization parameters in 0.1 N HCL for MS with or without CS/ZnO coating. Sl. no. 1 2 3

Sample Bare CS alone CS/ ZnO

LPR (W cm2)

Icorr (mA cm2)

Ecorr (mV)

ba (mVdecL1)

bc (mVdecL1)

22.69 35.99

0.1 0.068


484
507

73 73

164 120

100.38

0.0263


513

56

110

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Figure 4.5 (A) potentiodynamic and (B) linear polarization curves obtained in 0.1 N HCl for mild steel with and without CS/nano-ZnO films [22]. Potentiodynamic and linear polarization studies of Chitosan/ZnO coated steel surface. From Z. Sharifalhoseini, M.H. Entezari, M. Shahidi, Direct growth of ZnO nanostructures on the Zn electroplated mild steel to create the surface roughness and improve the corrosion protection of the electroless NieP coating, Materials Science and Engineering: B 231 (2018) 18e27, https://doi.org/10.1016/j.mseb.2018.07.001.

Figure 4.6 Polarization curves of the bare mild steel at different immersion times. In artificial sea water with and without ZP [24]. From Y. Hao, F. Liu, E.H. Han, S. Anjum, G. Xu, The mechanism of inhibition by zinc phosphate in an epoxy coating, Corrosion Science 69 (2013) 77e86, https://doi.org/10.1016/j. corsci.2012.11.025.

100

Table 4.2 Polarization values of MS with different immersion times in the extract solution [24]. Sl. no.

Time (h)

I8 corr (mA cm2)

E8 corr (mV)

I¢corr (mA cm2)

E¢corr (mV)

ba (mVdecL1)

bc (mVdecL1)

b0 a (mVdecL1)

b0 ¢c (mVdecL1)

1 2 3

0 18 36

15.4 4.1 13.0


729
747
732

14.5 2.3 0.7


710
674
734

52 85 57

189 303 117

63 87 88

323 147 164

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increased. Nevertheless, over the course of the immersion in artificial seawater, the current density and corrosion potential barely altered. Additionally, after 36 h of immersion in the pigment extract, P increased from the initial 6% to 94%. ZP clearly has a corrosion-inhibiting effect, as shown by the electrochemical measurement findings of exposed MS in simulated seawater with and without ZP.

8. Conclusion The nanomaterials are gaining lot of attention as protective coating materials like other conventional anticorrosive and antiwear materials. This is because of their size with unique chemical and physical properties compared to bulk materials. In this chapter, we have discussed about few nanoparticles used to protect the steel surface. The incorporation of nanoparticles in the coating enhances the protective features of the coating materials. The use of nanoparticles is also come with certain problems, like poor dispersion and homogeneity in size and shape along with the properties when used in polymer matrix. Researchers are still working to develop a complete protective coating material having repeatability and reproducibility. The future of special coating materials is still open for many researchers to explore and find new efficient materials with the broad perspective to use them in various fields like marine, building, defense, and automobiles.

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[24]

[25]

[26] [27]

[28]

[29]

[30]

[31]

[32]

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[36]

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Self-healing ceramic coatings 1

2

3

5

Son Thanh Nguyen , Ayahisa Okawa , Tadachika Nakayama and Hisayuki Suematsu 3 1 Department of Creative Engineering, National Institute of Technology-Kushiro College, Kushiro, Japan; 2Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Japan; 3Extreme Energy-Density Research Institute, Nagaoka University of Technology, Nagaoka, Japan

There is a crack, a crack in everything. That’s how the light gets in Anthem (song) dLeonard Cohen.

1. Background Self-healing is the ability that a material can repair the damages such as surface cracks, and recover its mechanical properties autonomously during service. Depending on the healing approaches whether require a preembedded healing agent, such as silicon carbide (SiC) nanoparticles, self-healing can be classified as extrinsic- or intrinsic-selfhealing [1]. The earliest works of self-healing ceramics were reported for UO2 [2], MgO [3], and Al2O3 [4], when monolithic ceramics showed crack disappearance and strength recovery after being thermally shocked at high temperatures for many hours. The self-healing mechanism was the mass diffusion during grain growth that was associated with the disappearance of voids or cracks. This intrinsic healing method requires heating at sintering temperature and for a long time, thus it was not very favored. Later, in 1992, Choi and Tikare Choi [5] developed a silicon nitride (Si3N4)-based ceramic composite that can perform self-healing ability. They dispersed 10% of the MgO healing agent into the Si3N4 matrix (extrinsic self-healing), then the oxide transformed into MgSiO3 at high temperature and filled in the cracks. After that, Chu et al. [6] reported the self-healing ability of SiC/mullite composites, in which the dispersed SiC particles were turned into SiO2 by oxidation: SiC þ 2O2 ¼ SiO2 þ CO2

(5.1)

This reaction is accompanied by the volume expansion of glassy phase SiO2, which leads to the closure of surface cracks and subsequently the strength recovery of the composite. Since then, much research on the self-healing function of ceramicmatrix-composites using SiC or Si3N4 as the healing agent has been conducted [7e12]. The self-healing function in these composites mainly comes from the volume Advanced Ceramic Coatings. https://doi.org/10.1016/B978-0-323-99659-4.00011-5 Copyright © 2023 Elsevier Ltd. All rights reserved.

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expansion generated by the oxidation of healing agents into SiO2. This mechanism enables the self-healing to occur at a temperature range of 1100e1400 C, easily attainable than the resintering method, therefore promoting the progress of research on selfhealing ceramic. Not only carbide or nitride but also metallic nanoparticles, such as Ni or Co, were also researched as healing agents to be dispersed into the ceramic matrix to generate the self-healing ability [13e15]. In these metal/ceramic composites, besides the volume expansion of oxide products, the outward diffusion of metallic ions to the surface also contributes to the self-healing behavior of the composite. The diffusion also contributes to the self-healing ability of MAX-based ceramics (Cr2AlC, Ti2AlC, etc.), when the weakly bonded atoms of A-site (e.g., Al3þ) can fastly diffuse to the crack surfaces and form oxidation products (e.g., Al2O3) [16e18]. Fig. 5.1 summarizes the underlying mechanisms for self-healing ceramic materials (Fig. 5.1). Research on self-healing ceramic coatings (SHCCs) is an emerging topic in the field of self-healing ceramics. While the healing mechanism of SHCCs is similar to that of ceramic bulks, i.e., the volume expansion of oxidation products, there are some works focusing on SHCC, but many investigations, including further life extension and fly ash reaction data are still required. SHCCs are considered promising materials that can revolutionize many industries such as aerospace, automotive, and construction. For example, self-healable thermal/environmental barrier coatings (T/EBCs) are desired for airplanes’ gas turbine engines. The fuel efficiency of the engines is directly related to the inlet temperature, where a higher temperature favors better performance [19].

Figure 5.1 Self-healing mechanism in ceramics. From bottom (clockwise): resintering (mass diffusion), volume expansion, outward diffusion.

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In order to further increase the inlet temperatures, to an extent that exceeds the capability of the nickel-based superalloys [20,21], ceramic matrix composites (CMCs) have been introduced [22,23]. Among the CMCs, silicon carbide fiber/silicon carbide matrix (SiCf/ SiCm), well-known for its high-temperature capability and low density, is considered the most promising material for aircraft gas turbine blades [24,25]. However, when SiC is exposed to oxidative environments, it reacts with oxygen to form SiO2, which in turn reacts with steam in the combustion environment to generate gaseous silicon hydroxides and leads to SiC recession [25,26]. Therefore, it is necessary to prevent the SiCf/SiCm substrate from receding during many cycles of operation by using T/EBCs. This is a structure composed of at least two layers, a bond coat layer to connect with the blade’s substrate and a top coat layer to protect the blade against heat and corrosion from the environment. During operation at high-temperatures, stresses induced by coefficient thermal expansion (CTE) mismatch between T/EBCs and the substrate are unavoidable. Consequently, cracks can occur, develop, and lead to the spallation of the T/EBCs. With self-healing ability, T/EBCs can seal these cracks, hinder their interconnection, and hence avoid failure and prolong the service life of the turbine blades. Generic principles governing the self-healing behaviors and strength recovery of engineering ceramics, such as thermodynamic, thermochemical, and thermokinetic aspects, have been reviewed by Greil [27,28]. In addition, Tanvagrian et al. [29] also summarized the past advancements in the field of crack-healing in ceramics. However, there is still a lack of comprehensive reviews on the latest research developments of the SHCCs, such as the healing behaviors of the coatings in a steam environment or the renewability of healing agents, etc. Those findings indeed can greatly expand the range of applications of SHCCs not only for T/EBCs but also for many other applications. The goal of this chapter is to provide an overview of cutting-edge research related to SHCCs. The chapter is structured to have four sections: the first one is the background section, briefly introducing the SHCCs and their typical applications. In the second section, the methodology that is used to design a self-healing ceramic system will be discussed. The third section features the major accomplishments that have been reported about SHCCs. This section is divided into several subsections concerning to T/EBCs, and other applications. The future perspectives of SHCCs will be also discussed in the last section of the chapter.

2. Methodology 2.1

Fabrication of bulk samples and self-healing tests

Because coating ceramic is made by an expensive and sophisticated process, the early stages of the research on SHCCs usually involve the fabrication of bulk samples from powders of the potential material to confirm their crack-healing ability. The bulk samples can be sintered by several methods like hot-pressing [30e32], pulsed-electriccurrent-sintering [33e35], or cold isostatic pressing (CIP)dpressureless sintering [36,37]. The crack-healing test usually involves introducing surface cracks

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intentionally using Vickers [7,9,10,14,38] or Knoop [16,39] indentations. Then the surface cracks were healed by annealing in air or ambient gas (H2O, O2, .) at a designated temperature. Before and after the annealing, the cracks were observed by SEM or optical microscope to evaluate the healing effect. Fig. 5.2 shows the indentation cracks

Figure 5.2 SEM micrographs showing the Vickers indentation cracks on the surface of 10% SiC/Yb2Si2O7 composites (left column) low magnification and (right column) high magnification; (top) as-indented (center) annealed in air, and (bottom) annealed in Ar. From S.T. Nguyen, T. Nakayama, H. Suematsu, T. Suzuki, L. He, H.B. Cho, K. Niihara, Strength improvement and purification of Yb2Si2O7-SiC nanocomposites by surface oxidation treatment, Journal of the American Ceramic Society 100(7) (2017) 3122e3131. https://doi.org/ 10.1111/jace.14831

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on the surface of a SiC-dispersed ceramic composite, before and after annealing in air or argon (Ar). The surface cracks almost disappeared after annealing in the air. However, the crack does not change when annealing the composite in Ar, indicating that in this ceramic composite, the annealing in an inert atmosphere had a very limited effect on crack healing. The bending strength test can give another clue of healed cracks therefore often used in the verification of crack-healing test [14,15,32]. In preparation for the strength measurement, the sintered composites were ground, polished, and cut into bar specimens, then their long edges were chamfered at 45 degrees. The bending strength of the composites was measured by three- or four-point bending methods. The bending strength of a specimen is given by: s

3b ¼ 3PL=2wt

2

(5.2)

for the three-point testing method, or by: s

4b ¼ 3PðL  lÞ=2wt

2

(5.3)

for the four-point testing method. In the above equations, P is the load at which the specimen is fractured, L and l are the outer and inner spans, and w and t are the width and thickness of the specimen, respectively. In a bending test, particularly in the case of a four-point test, the fracture may not develop through the center of the bar but cross the weakest point on the tensile surface. Fig. 5.3AeD shows the optical and SEM images after the bending test of as-sintered, as-indented, annealed-in-air, and annealedin-Ar specimens with their fractures. Because the indentation cracks are the most vulnerable flaws, they become the originated sites for the fracture path of the bar. Therefore, the cracks are considered the reason for the degradation of the bending strength in the as-indented bar. Also, if the fracture is initiated from the indentation, crack healing could be insufficient. On the other hand, the fact that the fracture did not develop from the indentation but from another site means the cracks were healed (Fig. 5.3D). In addition, a beneficial effect of the self-healing process is that it can enhance the mechanical properties of the materials. Many reports have pointed out that after the self-healing process, the strength of the ceramic composites was improved due to the residual compressive stress around the healed cracks [14,38,40]. In these cases, to break the specimen, it is required higher tensile stress to overcome that residual compressive stress. Consequently, the bending strength is much higher than that of the as-indented specimen, and in some cases even higher than the original strength (as-sintered specimen). Weibull distribution (Fig. 5.3E) is very useful in analyzing the strength and predicting the fracture probability of a material, thus can be employed to evaluate the self-healing and strength recovery behaviors [40e42].

Figure 5.3 Fracture images after bending test and fracture probability of specimens (A) optical image of specimens, two parts of each fractured specimen were assembly to illustrate the fracture paths (BeD) SEM micrographs showing the fracture path and indentation site on: (B) As-indented specimen; (C) specimen annealed in argon; (D) specimen annealed in air; (E) Weibull distribution for the strength of 10 vol% SiC nanocomposites regarding fracture site. From S.T. Nguyen, T. Nakayama, H. Suematsu, H. Iwasawa, T. Suzuki, Y. Otsuka, L. He, T. Takahashi, K. Niihara, Self-healing behavior and strength recovery of ytterbium disilicate ceramic reinforced with silicon carbide nanofillers, Journal of the European Ceramic Society 39(10) (2019) 3139e52.

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113

Methods for fabricating SHCCs

After confirming the excellent self-healing ability of the sintered bulk samples, fine powder of the materials can be used to fabricate SHCCs, usually by a coating method such as atmospheric plasma spraying (APS), suspension plasma spraying (SPS), highvelocity oxygen fuel sprayingapying (HVOF), or very low-pressure plasma spraying (VLPPS) [43]. Compared to the two formers, HVOF and VLPSS can produce coating layers without vertical cracks (Fig. 5.4). However, SPS and particularly APS are still

Figure 5.4 Backscattered SEM (BSE-SEM) micrographs of as-sprayed Yb2Si2O7 coatings by different methods (A) APS, (B) SPS, (C) HVOF, and (D) VLPPS. Black arrows indicate the vertical cracks, green arrows indicate the interpassage crack, white arrows indicate branch cracks, blue arrows indicate the partially or nonmolten particles, and gray arrows indicate the pores in the images. From E. Bakan, D. Marcano, D. Zhou, Y.J. Sohn, G. Mauer, R. Vaben, Yb2Si2O7 environmental barrier coatings deposited by various thermal spray techniques: a preliminary comparative study, Journal of Thermal Spray Technology, 26(6) (2017) 1011e24.

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the preferred coating technique to fabricate T/EBCs for their simple set-up and wide range of material choices [44,45]. Besides the above-mentioned coating methods, SHCCs can be developed on a substrate’s surface by modifying them with physical deposition or chemical deposition techniques. Depending on the geometry and properties of the components/applications, the most appropriate method can be selected. In the next section, the most popular applications of SHCCs and their fabrication methods will be introduced and discussed in detail.

3.

SHCCs for T/EBCs and other applications

3.1

Self-healing ceramic coatings for T/EBCs

3.1.1

Development of T/EBCs and material candidates

Gas turbine engines have significantly contributed to the development of various fields, such as power generation and aerospace industries. The core power and fuel efficiency of a gas-turbine engine used in aircraft are directly related to the turbine inlet gas temperature because a higher temperature provides better turbine performance [46]. Thermal barrier coatings (TBCs) were an important invention that significantly improved the inlet of gas turbine engines. However, the gas temperature of the turbine is expected to exceed 1500 C soon, and nickel-based superalloys cannot endure that high temperature even with the support of TBC and cooling technologies, as shown in Fig. 5.5. Compared to superalloys, CMCs have a higher temperature capability (can work at 1300e1500 C without cooling air) and much lower density for improving thrust-to-weight ratio; therefore they are materials of interest for gas turbine blades. Among the most promising CMCs for the turbine blade’s substrate is SiCf/ SiCm, because this material has high damage tolerance, good fatigue resistance, superior thermal conductivity, and is thermally stable at elevated temperatures [47,48]. However, a major drawback of SiC composites is their recession in the hightemperature steam environment of a gas turbine engines, as follows. Therefore, EBCs have been devised to protect SiCf/SiCm blades from hot water vapor. For T/EBCs, the top coat layer is very important and decisive to their performance. The following criteria are often considered when selecting top coat materials: (i) A promising material must possess high chemical stability at high-temperatures to avoid corrosion and the formation of unwanted reaction products that can deteriorate the structural integrity of the top coat and hence reduce its protective capability; (ii) the top coats must have low thermal conductivity to insulate the substrate from hot gas; (iii) the CTE value of the top coat must closely match that of the substrate (e.g., SiC). The third criterion is very important and must be considered carefully for the sake of minimizing the thermal stress between coating layers and the substrate, which is considered the main cause for the appearance of cracks and delamination. It is noteworthy that in the case of TBC, the role of SiC substrate is substituted by a superalloy.

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Figure 5.5 The evolution of turbine blade materials. Coatings, cooling concepts, and turbine inlet temperature versus year of entry. SiCðsÞ þ 3 = 2O2 ðgÞ / SiO2 ðsÞ þ COðgÞ[

(5.4)

SiCðsÞ þ 3H2 OðgÞ / SiO2 ðsÞ þ COðgÞ [ þ 3H2 ðgÞ[

(5.5)

SiO2 ðsÞ þ 2H2 OðgÞ / SiðOHÞ4 ðgÞ[

(5.6)

From B.T. Richards, H.N.G. Wadley, Plasma spray deposition of trilayer environmental barrier coatings, Journal of the European Ceramic Society, 34 (2014) 3069e3083.

Table 5.1 lists candidate materials for T/EBCs and substrates (SiC and IN718 superalloys), accompanied by their CTE, thermal conductivity, and important mechanical properties values. The first generation of EBC is a layer of mullite (3Al2O3$2SiO2) directly coated on a SiCf/SiCm substrate. But SiO2 is quickly vaporized at high temperatures leaving only the Al2O3 scale. Alumina is readily spalled, therefore this EBC only provides a very short protection for the substrate [75]. To avoid the volatilization of SiO2, two layers of EBC were employed, in which a layer of yttria-stabilized zirconia (YSZ) is added on the mullite layer [76]. However, the CTE mismatch between either YSZ/mullite and mullite/SiC leads to the appearance of cracks in their interfaces, and as a result, the delamination of the upper layers often occurs [77,78]. In the third generation, YSZ is replaced by barium-strontium aluminosilicate (BSAS), and a Si bond-coat layer is inserted between mullite and SiC substrate to mitigate the CTE mismatch between layers [24]. This multi-layered EBC is effective in protecting turbine blades at temperatures lower than 1000 C. However, the low melting eutectics occur near 1300 C, and many cracks and pores appear, thus limiting the usage of this EBC at high temperatures for a long time (Fig. 5.6). Because mullite and BSAS are not ideal candidates, it is necessary to find new materials for the top coat. Among the proposed materials, rare earth silicates have been identified as the most suitable family of materials for EBC top coat. The family has

Table 5.1 Thermal and mechanical properties (melting point Tm, thermal conductivity l, thermal expansion coefficient CTE, Vickers hardness HV, Young’s modulus E, bending strength s, fracture toughness KIc) of T/EBC’s candidate materials and substrate. K

Material

Tm[8C]

l[W/mK]

CTE[310L6/8C]

H v[GPa]

E[GPa]

s[MPa]

Ic[MPam

Yb2Si2O7

1850

2.1 (1000 C)

2.76  0.22 2.5  0.2 2.3  0.1

Y2Si2O7 Y2SiO5

1775 1980

2.12  0.05 1.85  0.17

[55,56] [49,52,55,57,58]

Lu2Si2O7 Lu2SiO5

1860 2100

168  10 218  7

2.6  0.2 2.2  0.2

[59,60] [52,54,58]

Er2SiO5 Gd2SiO5 Y2Ti2O7 La2Zr2O7 BaZrO3 Mullite SiC IN718(PECS) IN718(SLM) IN718(Cast)

1980 1950 2321 2300 2690 1830 2830 n/a n/a n/a

2.3e1.5 (RTe927 C) 1.71(RTe1300 C) 4.9e1.9 (200e1000 C) 1.34 (600e900 C) 1.65(RTe1300 C) n/a 5.92(RTe1200 C) 1.63(RTe1300 C) 1.42(0e1300 C) 1.45(RTe1300 C) 7.2(RTe700 C) 1.6 (1000 C) 3.4 (1000 C) 6 (RTe1400 C) 55.1e26.3(500e150 C) 10.1 9.1 10.3

159  5 152 215  12 207  20 135  4 116  3

[38,49e51]

1950

7.3  0.2 8.3 6.4  0.1

168

Yb2SiO5

3.7e4.5(527e1327 C) 3.6 6.3(200e1400 C) 7.2(200e1350 C) 3.9  0.4(200e1250 C) 8.36  0.5(RTe1300 C) 6.9  0.2(200e1350 C) 5.48(RTe1200 C) 6.7  0.6(200e1350 C)

156  4 n/a 206 n/a n/a 200 359  54 n/a n/a n/a

n/a n/a 1.35 1.1  0.2 n/a 2.5 3.1  0.3 n/a n/a n/a

[52,54,61] [52,62] [63e67] [68] [68] [69,70] [71e73] [74]

n/a: not applicable;

PECS: by pulsed electric current sintering;

5.9  0.5(200e1350 C) 10.3  0.4(200e1350 C) 10 (RTe1000 C) 9.1 (RTe1000 C) 7.9 (RTe1000 C) 4.5(RTe1400 C) 3.5 (RT) 13.9 (25e600 C) 11.9 (25e600 C) 11.8 (25e600 C)

SLM: by selective laser melting;

6.2  0.1 5.3  0.1 7  0.3 8.9  0.2 n/a n/a 10.6e12.2 9.9  0.4 11.1  1.9 n/a 32  4.8 290 349 250

158 155  3 124  2 155 178  2 176 172 157 n/a 253 175  11 175  11 30 415  12 n/a n/a n/a

Cast: by casting technique.

1/2

]

References

[49,52e54]

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Figure 5.6 Cross section of Si/mullite/BSAS on MI CFC after 1000 h in 90% H2Oebalance O2 at 1300 C with 1 h cycles. (A) and (B) are from two different batches of coatings. From K.N. Lee, D.S. Fox, J.I. Eldridge, D. Zhu, R.C. Robinson, N.P. Bansal, R.A. Miller, Upper temperature limit of environmental barrier coatings based on mullite and BSAS, Journal of the American Ceramic Society, 86(8) (2003) 1299e1306.

common chemical formulas such as RESiO5 (monosilicate) or RE2Si2O7 (disilicate), in which RE is a rare-earth element such as Y, Yb, Lu, Ho, etc. Fig. 5.7 presents the relation between the corrosion rate and CTE values of ceramic materials. Ceramics showing the best performance in corrosion tests are in the yellow region; however, all of them have CTE value higher than 6  106 K1, unsuitable with the low value of SiC (3.5  106 K1) [72] and hence not preferred for EBC top coat. Among other ceramics, Yb2Si2O7 and Y2Si2O7 have CTE closest to SiC, and the former presents a

Figure 5.7 Corrosion behavior of ceramic materials. Investigated at 1450 C, depicted by the corrosion rate ordered by the CTE of the materials. From H. Klemm, Silicon nitride for high-temperature applications, Journal of the American Ceramic Society, 93(6) (2010) 1501e22.

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Figure 5.8 Apparent bulk CTE of RE2Si2O7 compounds. Temperature ranges used for the ABCTE determination are 303e1873K, varying depend on each polymorph. From A.J. Fernandez-Carrion, M. Allix, A.I. Becerro, Thermal expansion of rare-earth pyrosilicates, Journal of the American Ceramic Society, 96(7) (2013) 2298e2305.

better corrosion behavior. The CTE values of RE2Si2O7 were also calculated, and the results also pointed out that b-Y2Si2O7 and b-Yb2Si2O7, beside b-Er2Si2O7, b-Lu2Si2O7, g-Y2Si2O7, and g-Ho2Si2O7 are disilicates, having CTE values approximate to 3.5  106 K1, appropriate for coating on SiC substrate (Fig. 5.8). In addition, as shown in Fig. 5.9, Yb2Si2O7 and Lu2Si2O7 are single-component disilicates showing only one polymorphic structure upon temperature variation, which means they could maintain their phases in a wide range of temperatures. Yb2Si2O7 is also more stable than other rare-earth silicate candidates for EBCs, such as Yb2SiO5 and Y2Si2O7, when it interacts with the molten calcium-magnesium-alumina-silicates (CMAS), the airborne materials ingested into turbine engines during their operation [79,80]. For these reasons, Yb2Si2O7 attracts much interest from scientists and is widely considered the most promising material for EBC. Recently, high-entropy ceramics (HECs) with multicomponent rare-earth silicates such as (Yb0.2Y0.2Lu0.2Sc0.2Gd0.2)Si2O7 [81] or (Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6)2Si2O7 [82] with high-temperature stability and excellent water-vapor corrosion resistance were synthesized. It is also found that these HECs have a lower thermal conductivity than Yb2Si2O7 [83], which means they can be even better candidates for EBC. However, the synthesis of HECs is a more laborious task, and presently Yb2Si2O7 is still the most favorite material in the field.

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Figure 5.9 Schematics of the polymorphic transformation temperatures and melting points of RE2Si2O7 disilicates. 6RE1/6 ¼ Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6 (the leftmost column). From L. Sun, Y. Luo, X. Ren, Z. Gao, T. Du., Z. Wu, J. Wang, A multicomponent g-type (Gd 1/6 Tb 1/6 Dy 1/6 Tm 1/6 Yb 1/6 Lu 1/6) 2 Si 2 O 7 disilicate with outstanding thermal stability, Materials Research Letters, 8(11) (2020) 424e30.

3.1.2

Self-healing ceramic coatings for TBCs

YSZ is the standard topcoat material for TBC applied on the superalloy blades of a gas turbine engine. YSZ ceramic composites dispersed with MoSi2 (doped with Al and B) were spark plasma sintered, and their self-healing ability was investigated [84]. The role of Al is to form an Al2O3 shell preventing MoSi2 healing particles from premature oxidation, while that of B is to increase the fluidity of MoSi2, which is beneficial for crackhealing. During operation at high temperatures, chemical reactions lead to the formation of SiO2, which can heal the crack. In addition, the formed SiO2 can interact with the ZrO2 matrix to form ZrSiO4, which helps to strengthen the healed crack. Later, very recently, the degradation and lifetime of a self-healing TBC system tested under realistic conditions were investigated [85]. The MoSi2 healing particles were introduced within the first 150 mm (in the total 500 mm) of the YSZ coating matrix directly on the top of the bond coat using a dual particle feed APS system (Fig. 5.10). The system allows powder injection at two different locations of the plasma jet [86]. Compared to standard YSZ coatings, the lifetime of the self-healing TBC was considerably increased for both the furnace cycling test and the realistic burner rig test. The results are very promising, and the intermediate self-healing layer can be adapted to other T/EBC systems.

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Figure 5.10 Schematic diagram of the additional injection holder to inject powder at a different location of the plasma jet. The distance between the two injection points (l) and the distance from the second injector to the plasma jet axis (d) can be adjusted. From D. Koch, G. Mauer, R. Vaben, Manufacturing of composite coatings by atmospheric plasma spraying using different feed-stock materials as YSZ and MoSi2, Journal of Thermal Spray Technology, 26(4) (2017) 708e16.

3.1.3

Self-healing ceramic coatings for EBCs

Notwithstanding the recent optimistic results that EBCs have shown on protecting the substrate against the environment and CMAS attacks, the quest for further innovation for EBCs is always required, and self-healing ability is one of the important targets. Because EBC was researched and developed later than TBC, so far the research on SHCCs for EBC is still in the early stages, mainly focusing on the investigation of self-healing ability in sintered bulks of candidate materials. One of the first studies on the self-healing performance of EBC composite materials was reported for SiC/ Yb-silicate nanocomposites [38]. The mechanism underlying the self-healing function of the composite is illustrated in Fig. 5.11 [87]. The healing mechanism is based on the addition of 10-vol% SiC to the Yb2Si2O7/ Yb2SiO5 matrix. Once a crack appears, as seen in Fig. 15.10A, it provides access for the atmospheric oxygen into the material. During annealing at 1250 C in air, the SiC nanofillers react with oxygen, creating viscous SiO2 glass (Fig. 15.10B). This viscous amorphous SiO2 is capable of filling the cracks, and then reacts with the minor phase Yb2SiO5 present within the matrix to form Yb2Si2O7, effectively sealing the crack due to the associated volume expansion (Fig. 15.10C). This is a specific case of selfhealing by multiple reactions, and the cracks were healed by the volume expansion of reaction products (SiO2 and Yb2Si2O7). In a similar research, scientists successfully performed self-healing and strength recovery in the composites of Yb2Si2O7/Yb2SiO5 and 1 vol% SiC [88]. Normally, it is required to disperse more than 15 vol% SiC to obtain a ceramic composite that can completely recover its strength by self-healing [10,89,90]. In the case of Yb2Si2O7/Yb2SiO5-SiC composites, full recovery of strength can be achieved with only 1e10 vol% SiC. This is because the synergic effect of the volume expansion and the residual compressive stress rapidly seals the cracks and

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Figure 5.11 Crack-healing mechanism in SiC/Yb-silicate nanocomposites. (A) Before healing, (B) during healing, and (C) after healing. Note the transformation of the Yb2SiO5 grain to Yb2Si2O7 at the surface. From S.T. Nguyen, T. Takahashi, A. Okawa, H. Suematsu, K. Niihara, T. Nakayama, Improving self-healing ability and flexural strength of ytterbium silicate-based nanocomposites with silicon carbide nanoparticulates and whiskers, Journal of the Ceramic Society of Japan, 129(4) (2021) 209e16.

increases the strength. Later, the self-healing ability of the Yb2Si2O7-Yb2SiO5-SiC composite has been also performed in a superheated water vapor environment [91]. A similar approach was reported [92], where crack self-healing was demonstrated on sintered bodies of Y2SiO5-Y2Si2O7 matrix with 5 vol% of SiC nanoparticles after exposure to air at temperatures ranging between 1000 C and 1300 C for 1e24 h. The authors also remarked that ion diffusion is also a crack-healing mechanism in the composite. Despite the above results being promising, there are some difficulties that must be overcome. First, the consumption of healing agents such as SiC or Yb2SiO5 limits the self-healing function to a single annealing cycle. The remaining SiC and Yb2SiO5 would not be insufficient for self-healing when a new crack appears. The decomposition of Yb2Si2O7 to Yb2SiO5 in a steam environment was investigated and confirmed that Yb2SiO5 can be regenerated after each annealing cycle [93]. Using this concept, the scientists successfully prolonged the self-healing function for many cycles. Because the environment for the recycling process is superheated water vapor, which is very similar to the environment inside the gas turbine engine, this finding brings an optimistic view about the EBC with permanent self-healing ability. Second, it is necessary to confirm that the self-healing function still works well when the sintered bodies are replaced by coating layers. Thermal spray often results in coating layers with higher porosity, which might change the rate of oxidation, and volume expansion, leading to the failure of the self-healing function. Very recently, scientists investigated the protective effect of a bilayer EBC, consisting of a Si bond coat and Yb2Si2O7/Yb2SiO5 top coat, fabricated by APS on a SiCf/SiCm substrate [94]. The EBC was exposed to water vapor at 1300 C for 40e200 h, and then

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the structural change was evaluated. The finding that the cracks at the topcoat, resulting from the APS, disappear after the water vapor exposure reveals the self-healing property of the EBC. Although many efforts are still required to obtain a realistic system of EBC with self-healing ability, these findings prove that SHCCs for the application of EBC are very possible.

3.2

Self-healing ceramic coatings for friction surfaces

The frictional sliding contact can wear out the top lubricous layer of conventional twolayered systems, leaving an underneath hard ceramic surface with a high coefficient of friction. Consequently, cracks occur and propagate, leading to the breakage of the system. Surface self-organization during sliding contact has recently received more attention from researchers as this novel method can maintain the thin lubricous layer, while self-healing the damages of underneath hard ceramic. The silver migration to the surface, triggered by temperature, was used to provide enhanced lubricity of carbide/ nitride-based ceramic composites like TiC-Ag [95], CrN-Ag [96], Ta2AlC-Ag, Cr2AlC-Ag [97], CrAlN-Ag [98], MoCN-Ag [99], etc. Later, it has been found that tantalum oxide/silver (Ta2O5/Ag) coatings, produced by unbalanced magnetron (UBM) sputtering Ta2O5 with 14 at.% Ag content on an Inconel 718 substrate, also exhibits outstanding tribological properties at elevated temperatures [100]. The results pointed out the migration of Ag to the surface and the form of a lubricious Ag-Ta-O ternary phase, which contributes to the low values of the wear rate and coefficient of friction (COF). The observed wear performance suggests that this surface reconstruction was accompanied by a self-healing process. Later, it was demonstrated that niobium oxide (Nb2O5) exhibits the ability of self-healing, or the so-called precipitation-induced stimulated-healing, activated by adding Ag2O in the crack site followed by a heat treatment at 950 C for 12 h [101]. The formation of a ternary oxide (AgNbO3) at an elevated temperature helps to heal the crack. Scientists then explored the mechanism of lowering the healing temperature by assisting the process of crackhealing with normal load and shear stresses [102]. During a pin-on-disk test of an Ag2O-Nb2O5 system (Fig. 5.12), when the temperature reached 600 C, the formation of a lubricious ternary AgNbO3 phase at the interface was observed and accompanied by a threefold decrease in COF. This temperature is about 350 C lower than the one required in purely thermodynamic processes. The formation of the lubricious Nb-AgO ternary phase at a much lower than thermodynamically required temperature suggests that the self-healing process can be triggered with mechanically induced stresses. The findings provided new insights into the self-healing processes and proposed a novel method for in situ healing ceramic coatings. Recently, hybrid dual-phase coatings composed of an A356 aluminum alloy modified by plasma electrolytic oxidation (PEO) and burnished with graphite-MoS2-Sb2O3 chameleon solid lubricant powders have been produced [103]. The PEO layer provides high hardness and load support, while the solid lubricant powders reduce friction. COF values for the coating sliding against a Si3N4 ball decreased from 0.2 at room temperature down to 0.02 at 300 C.

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Figure 5.12 Preparation of the bulk niobium oxide sample by (A) pressing and (B) sintering the pellet. The sample was further tested for the tribological performance (C). Tribology test of Nb2O5 with and without the presence of Ag at (D) 25 C, (E) 400 C, and F) 600 C. Results indicate a reduction in the coefficient of friction in case of silver presence at 600 C (F). From A. Shirani, J. Gu, B. Wei, J. Lee, S.M. Aouadi, D. Berman, Tribologically enhanced selfhealing of niobium oxide surfaces, Surface and Coatings Technology, 364 (2019) 273e78. https://doi.org/10.1016/j.surfcoat.2019.03.002

The increase in test temperature forced the material of the chameleon layer to fill the PEO layer’s pores and cracks to provide easier sliding of the films. This process helped to not only promote crack healing and improve the PEO-Chameleon coating’s resistance to fracture and fatigue in high-pressure contacts but also protect MoS2 and graphite lubricous phases from oxidation.

3.3

SHCC for oxidation and corrosion protection

SHCCs have been researched for the purpose of protecting the underlying materials from oxidation or corrosion. In 2014, a self-healing oxidation-resistant coating with a double layer SiC by the combination of chemical vapor reaction (CVR) and chemical vapor deposition (CVD) was reported [104]. There was no mass loss of the doublelayer SiC coating after 24 h static oxidation in air at 1500 C. In addition, it exhibits a better oxidation resistance compared to single-layer SiC coating. The reasons are the self-healing effect of the microcracks, the good adherence between the CVR and CVD SiC layers, and the presence of the C/SiC gradient layer. Later, ZrB2-SiO2 coatings on the SiC-coated C/C composites were prepared by infiltrating SiO2 sol into a porous ZrB2 layer [105]. The oxidation of the ZrB2 phase leads to the formation of ZrO2 and ZrSiO4, generating compressive stress and weakening the thermal stress upon cooling. The compressive stress helps to restrain the crack formation and heal

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Figure 5.13 Schematic of the oxidation resistance mechanism of the ZrB2-SiO2 coating. The compressive stress helps to restrain the crack formation and heal minicracks. From O. Haibo, L. Cuiyan, H. Jianfeng, C. Liyun, F. Jie, L. Jing, X. Zhanwei, Self-healing ZrB2-SiO2 oxidation resistance coating for SiC coated carbon/carbon composites, Corrosion Science, 110 (2016) 265e72. https://doi.org/10.1016/j.corsci.2016.04.040

minicracks, hence improving the oxidation resistance of the coating (Fig. 5.13). These coatings were proven to be effective in protecting the underlying composite from oxidation at 1500 C for 330 h. Glass usually has low viscosity at high temperatures, and thus can be used to heal the coated composites. For example, the thermally stable borosilicate glass with excellent wetting properties and viscosity was employed to protect SiCf/SiC composite from oxidation [106]. The glass-coated composites were prepared by a simple and low-cost slurry technique, then processed in air at 1200 C for 100 h. Three-point bending test pointed out that after the heat treatment, the mechanical behavior of the glass-coated composite remained almost virtually unaffected, unlike the uncoated composite, which showed a dramatic decrease in strength. In addition, the glass-coated SiCf/SiC composites still show their initial fiber/matrix interface. These results confirmed the selfhealing effect and oxidation protection of the glass coating. Recently, B4C was used to add in borosilicate glass matrix to improve the thermal protective effect for C/C composite [107]. It was revealed that the self-healing coating exhibited variable oxidation resistance at temperatures from 700 to 900 C, depending on the carbide content. The B2O3 formed from B4C can be dissolved into the borosilicate glass, which significantly changes the CTE value of the glass and softens its temperature. When B4C content was 25 wt%, the protective coating exhibited the lowest porosity and oxygen permeability. In a similar study [108], Si-C-N/borosilicate glass-B4C-Al2O3 coating was deposited on C/C aircraft brake materials (by polymeric-derived ceramic (PDC)

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Figure 5.14 Schematic illustration of self-healing process in a thin-film glassy coating. (A) Vanadium boride (VB) layers are embedded between glassy layers. Wherever any damage occurs, a crack is formed in the coating; oxidation of the healing layer occurs by reaction with oxygen. (B) V2O5 and B2O3 produced by the oxidation of VB at the operating temperature (typically 700e800 C). (C) The low viscosity of V2O5 and B2O3 enables them to spread and leads to the crack healing. From S. Castanié, T. Carlier, F.O. Méar, S. Saitzek, J.F. Blach, R. Podor, L. Montagne, Selfhealing glassy thin coating for high-temperature applications, ACS Applied Materials and Interfaces 8(6) (2016) 4208e15.

technique combined with slurry brushing) to protect them from oxidation and thermal shock. The carbide reacts with the oxygen diffused into the coating to form B2O3, which can heal the coating’s defects, hence improving its oxidation resistance. The role of Al2O3 is to inhibit the volatilization of B2O3, thereby maintaining the selfhealing ability at high temperatures (900 C). Glassy thin films with self-healing ability can be also developed by pulsed laser deposition (PLD). This method was used [109] to embed thin layers (50 nm) of vanadium boride (VB) healing agent between glassy layers (Fig. 5.14). Self-healing was obtained through the oxidation of VB at the operating temperature (700e800 C) to form V2O5 and B2O3, whose low viscosity enables them to spread to the crack sites easily. The crack of nanometric dimension was confirmed to be healed within a few minutes at 700 C.

4. Summary and perspectives For the past few decades, the research trend of self-healing materials has been a hot topic in the field of materials science with thousands of research articles published annually. However, the topic of SHCCs is still in its early stages and would attract

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more interest for the foreseeable future. This chapter has reviewed the healing mechanisms and methodology to develop SHCCs and summarized the cutting-edge research findings in this area, including applications like T/EBCs, friction surfaces, oxidation protection films, etc. These optimistic results (summarized in Table 5.2) not only prove the possibility but also suggest the bright future of SHCCs. Normally, the road from basic research to practical application is very long. This is also true for SHCCs; the materials still have many aspects that need to be improved before being applied in many industries such as aerospace, automobiles, electronic and mechanical devices manufacturing, etc. For example, to improve the performance of SHCCs, scientists are seeking methods to lower the self-healing temperature or to accelerate the healing process. Recently, scientists succeeded in making Al2O3/SiC bulks that can be fully healed after a heat treatment at 1000 C for only a few minutes [110]. By adding MnO, an oxide with a low melting point, they created a self-healing system in the ceramic material that works like the recovery of bone in the human body. In conventional SiC-based self-healing ceramics, the carbide will react with oxygen gas at high temperatures to form SiO2, which is liquid and movable. However, at 1000 C or lower temperatures, the estimated time is hundreds of hours. To increase the melting rate of SiC, small amounts of MnO are added to the material. As shown in Fig. 5.15AeC, this oxide reacts with SiO2 and the substrate (Al2O3), forming a SiO2-Al2O3-MnO complex with much better fluidity than SiO2, which can seal cracks for a very short time. Most self-healing methods for ceramic materials use high temperatures to stimulate oxidation, generating glassy phases to seal the cracks. However, scientists have succeeded in using electricity to trigger the healing process at room temperature [111]. They conducted the anodization of the Ti/Al2O3 composite in an H3PO4 electrolyte solution. As shown in Fig. 5.15DeF, the metallic particles were attracted toward the crack surface, where TiO2 was formed from the oxidation reaction that eventually helps to seal the crack, at room temperature. In addition, the group added carbon nanotubes (CNTs) as the carbon source into the matrix to form Al2O3/Ti/TiC multiphase composites with enhanced fracture toughness [112]. This method can be applied to heal cracks even at room temperature; therefore it is believed to be very promising and will pave the way for developing new applications of SHCCs in the future. The SHCCs research community will also have to deal with questions about the reliability of their materials. Ceramic materials are brittle; therefore, their mechanical behavior from sample to sample is very scattered depending on the distribution of the defects. The variation of the self-healing behaviors and the scatter of mechanical properties (strength, toughness, etc.) must be investigated intensively by experiments, which can be extremely high-cost and time-consuming. Finite element analysis [113], kinetic models [114], and machine learning approaches [115] can be useful tools to predict the healing behavior and mechanical properties, hence can improve the reliability of SHCCs.

Application

Materials (structure)

T/EBCs

MoSi2-YSZ (coat) SiC-Yb2Si2O7-Yb2SiO5 (bulk) SiC-Yb2Si2O7 (bulk) SiC-Y2Si2O7-Y2SiO5 (bulk) Yb2Si2O7-Yb2SiO5/Si (topcoat/bond coat) Ta2O5/Ag (coat) Ag2O-Nb2O5 (bulk)

Friction surface

Nb2O5-Ag2O-Nb2O5 (multilayer coat)

Oxidation protection

A356/Graphite-MoS2-Sb2O3 (hybrid dual-phase coat) SiC/SiC ZrB2-SiO2 (coat) Borosilicate (glass coat) B4C-borosilicate (glass coat) SiCN/borosilicate-B4C-Al2O3 (glass coat) VB (glassy thin films)

Method of fabrication

Self-healing/oxidation resisting temperatures

APS Hot-pressed Hot-pressed PECS APS

1100e1400 C 850e1400 C 1150 C 1000e1300 C 1300 C

UBM sputtering CIP-pressureless sintering Magnetron sputtering PEO and burnished CVR and CVD Infiltration method Slurry technique Slurry brushing PDC þ slurry brushing PLD

Ambient

References

Air Air Air/Steam

[85,86] [38,42,87,88] [50,91]

750 C 950 C

Air Steam Air Air

[92] [94] [100] [101]

600 C

Air

[102]

300 C

Air

[103]

1500 C (24h) 1500 C (330h)

Air Air

[104] [105]

1200 C (100h) 700e900 C 900 C

Air Air Air

[106] [107] [108]

700e800 C

Air

[109]

Self-healing ceramic coatings

Table 5.2 Summary of discussed SHCCs, their applications and working conditions.

n/a: not applicable

127

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Figure 5.15 Fast self-healing in Al2O3/SiC composites and effect of healing activator network (AeC) and schemes of the room temperature crack-healing mechanism induced by anodization (DeF). (A) Oxygen penetrates cracked surfaces, and oxidizes SiC to SiO2 (defined as the inflammation stage). (B) Al2O3 and MxOy dissolve into SiO2 to form a mechanically weak, low-viscosity supercooled melt, which completely fills irregularly shaped gaps (defined as the repair stage). (C) Mechanically strong crystals nucleate and grow in the supercooled melt (defined as the remodeling stage). (D) A dramatic increase of the current density triggers the oxidation, resulting in the healing of the crack tip. (E) Cracks are gradually healed from the tip; Ti particles distributed on the sides of the crack are oxidized and expanded to form a bridge. (F) Ti particles distributed in all the possible current pathways are oxidized. From T. Osada, K. Kamoda, M. Mitome, T. Hara, T. Abe, Y. Tamagawa, W. Nakao, T. Ohmura, A novel design approach for self-crack-healing structural ceramics with 3D networks of healing activator. Scientific Reports, 7(1) (2017). https://doi.org/10.1038/s41598-017-17942-6 (DeF) From S. Shi, T. Goto, S. Cho, T. Sekino, Electrochemically assisted room-temperature crack healing of ceramic-based composites, Journal of the American Ceramic Society, 102 (2019) 4236e46.

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[3] T.K. Gupta, Crack healing in thermally shocked MgO, Journal of the American Ceramic Society 58 (3e4) (1975), https://doi.org/10.1111/j.1151-2916.1975.tb19578.x, 143e143. [4] T.K. Gupta, Crack healing and strengthening of thermally shocked alumina, Journal of the American Ceramic Society 59 (5e6) (1976) 259e262, https://doi.org/10.1111/ j.1151-2916.1976.tb10949.x. [5] S.R. Choi, V. Tikare, Crack healing behavior of hot pressed silicon nitride due to oxidation, Scripta Metallurgica et Materialia 26 (8) (1992) 1263e1268, https://doi.org/ 10.1016/0956-716X(92)90574-X. [6] M.C. Chu, S. Sato, Y. Kobayashi, K. Ando, Damage healing and strengthening behaviour in intelligent mullite/SiC ceramics, Fatigue and Fracture of Engineering Materials and Structures 18 (9) (1995) 1019e1029, https://doi.org/10.1111/j.1460-2695.1995.tb00924.x. [7] K. Ando, M.C. Chu, K. Tsuji, T. Hirasawa, Y. Kobayashi, S. Sato, Crack healing behaviour and high-temperature strength of mullite/SiC composite ceramics, Journal of the European Ceramic Society 22 (8) (2002) 1313e1319, https://doi.org/10.1016/S09552219(01)00431-9. [8] K. Ando, K. Furusawa, K. Takahashi, S. Sato, Crack-healing ability of structural ceramics and a new methodology to guarantee the structural integrity using the ability and prooftest, Journal of the European Ceramic Society 25 (5) (2005) 549e558, https://doi.org/ 10.1016/j.jeurceramsoc.2004.01.027. [9] Z. Chlup, P. Flasar, A. Kotoji, I. Dlouhy, Fracture behaviour of Al2O3/SiC nanocomposite ceramics after crack healing treatment, Journal of the European Ceramic Society 28 (5) (2008) 1073e1077, https://doi.org/10.1016/j.jeurceramsoc.2007.09.007. [10] K. Houjou, K. Ando, S.P. Liu, S. Sato, Crack-healing and oxidation behavior of silicon nitride ceramics, Journal of the European Ceramic Society 24 (8) (2004) 2329e2338, https://doi.org/10.1016/S0955-2219(03)00645-9. [11] W. Nakao, K. Takahashi, K. Ando, Threshold stress during crack-healing treatment of structural ceramics having the crack-healing ability, Materials Letters 61 (13) (2007) 2711e2713, https://doi.org/10.1016/j.matlet.2006.04.122. [12] K. Takahashi, M. Yokouchi, S.-K. Lee, K. Ando, Crack-healing behavior of Al2O3 toughened by SiC whiskers, Journal of the American Ceramic Society 86 (12) (2003) 2143e2147, https://doi.org/10.1111/j.1151-2916.2003.tb03622.x. [13] D. Maruoka, T. Itaya, T. Misaki, M. Nanko, Recovery of mechanical property on nanoCo particles dispersed Al2O3 via high-Temperature oxidation, Materials Transactions 53 (10) (2012) 1816e1821, https://doi.org/10.2320/matertrans.MAW201202. [14] D. Maruoka, M. Nanko, Recovery of mechanical strength by surface crack disappearance via thermal oxidation for nano-Ni/Al2O3 hybrid materials, Ceramics International 39 (3) (2013) 3221e3229, https://doi.org/10.1016/j.ceramint.2012.10.008. [15] H.V. Pham, M. Nanko, W. Nakao, High-temperature bending strength of self-healing Ni/ Al2O3 nanocomposites, International Journal of Applied Ceramic Technology 13 (5) (2016) 973e983, https://doi.org/10.1111/ijac.12569. [16] S. Li, L. Xiao, G. Song, X. Wu, W.G. Sloof, S. van der Zwaag, Y. Zhou, Oxidation and crack healing behavior of a fine-grained Cr 2 AlC ceramic, Journal of the American Ceramic Society 96 (3) (2013) 892e899, https://doi.org/10.1111/jace.12170. [17] R. Pei, S.A. McDonald, L. Shen, S. van der Zwaag, W.G. Sloof, P.J. Withers, P.M. Mummery, Crack healing behaviour of Cr2AlC MAX phase studied by X-ray tomography, Journal of the European Ceramic Society 37 (2) (2017) 441e450, https:// doi.org/10.1016/j.jeurceramsoc.2016.07.018.

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[104] D. Huang, M. Zhang, Q. Huang, L. Xue, P. Zhong, L. Li, Corrosion Science 87 (2014) 134e140, https://doi.org/10.1016/j.corsci.2014.06.024. [105] O. Haibo, L. Cuiyan, H. Jianfeng, C. Liyun, F. Jie, L. Jing, X. Zhanwei, Self-healing ZrB2eSiO2 oxidation resistance coating for SiC coated carbon/carbon composites, Corrosion Science 110 (2016) 265e272, https://doi.org/10.1016/j.corsci.2016.04.040. [106] M. Ferraris, M. Montorsi, M. Salvo, Glass coating for SiCf/SiC composites for hightemperature application, Acta Materialia 48 (18e19) (2000) 4721e4724, https:// doi.org/10.1016/S1359-6454(00)00263-9. [107] J. Deng, K. Hu, B. Lu, X. Ma, H. Li, J. Wang, S. Fan, L. Zhang, L. Cheng, Effect of B4C addition on the oxidation behavior of borosilicate glass repairing coating for C/C brake materials, Ceramics International 46 (10) (2020) 14496e14504, https://doi.org/10.1016/ j.ceramint.2020.02.248. [108] S. Fan, X. Ma, B. Ji, Z. Li, Z. Xie, J. Deng, L. Zhang, L. Cheng, Oxidation resistance and thermal shock properties of self-healing SiCN/borosilicate glass-B4C-Al2O3 coatings for C/C aircraft brake materials, Ceramics International 45 (1) (2019) 550e557, https:// doi.org/10.1016/j.ceramint.2018.09.207. [109] S. Castanié, T. Carlier, F.O. Méar, S. Saitzek, J.F. Blach, R. Podor, L. Montagne, Selfhealing glassy thin coating for high-temperature applications, ACS Applied Materials and Interfaces 8 (6) (2016) 4208e4215, https://doi.org/10.1021/acsami.5b12049. [110] T. Osada, K. Kamoda, M. Mitome, T. Hara, T. Abe, Y. Tamagawa, W. Nakao, T. Ohmura, A novel design approach for self-crack-healing structural ceramics with 3D networks of healing activator, Scientific Reports 7 (1) (2017), https://doi.org/10.1038/ s41598-017-17942-6. [111] S. Shi, T. Goto, S. Cho, T. Sekino, Electrochemically assisted room-temperature crack healing of ceramic-based composites, Journal of the American Ceramic Society 102 (7) (2019) 4236e4246, https://doi.org/10.1111/jace.16264. [112] S. Shi, S. Cho, T. Goto, T. Sekino, CNT-induced TiC toughened Al 2 O 3/Ti composites: mechanical, electrical, and room-temperature crack-healing behaviors, Journal of the American Ceramic Society 103 (8) (2020) 4573e4585, https://doi.org/10.1111/ jace.17152. [113] S. Ozaki, K. Yamagata, C. Ito, T. Kohata, T. Osada, Finite element analysis of fracture behavior in ceramics: prediction of strength distribution using microstructural features, Journal of the American Ceramic Society 105 (3) (2022) 2182e2195, https://doi.org/ 10.1111/jace.18201. [114] T. Osada, T. Hara, M. Mitome, S. Ozaki, T. Abe, K. Kamoda, T. Ohmura, Self-healing by design: universal kinetic model of strength recovery in self-healing ceramics, Science and Technology of Advanced Materials 21 (1) (2020) 593e608, https://doi.org/10.1080/ 14686996.2020.1796468. [115] W. Wang, N.G. Moreau, Y. Yuan, P.R. Race, W. Pang, Towards machine learning approaches for predicting the self-healing efficiency of materials, Computational Materials Science 168 (2019) 180e187.

Self-cleaning photocatalytic ceramic coatings

6

Maria Covei, Ion Visa and Anca Duta Transilvania University of Brasov, Brașov, Romania

1. Photovoltaic modules’ soilingdan issue A report from the International Energy Agency states that, despite being the second year of the COVID-19 pandemic, the global photovoltaic (PV) market has grown in 2021, by at least 175 GW of newly installed or commissioned systems. This brings the total global power output to at least 942 GW, with PVs saving as much as 1100 million tons of CO2 [1]. The performance of these solar energy converters is directly affected by soiling produced by dirt, bird droppings, algae, moss, fungi, pollen, exhaust gases, and industrial and agricultural emissions. The accumulation of inorganic (e.g., mineral dust) and organic matter (e.g., volatile organic compounds) on the PV protective cover leads to the reflection/scattering/absorption of the incident solar radiation, which, in turn, leads to a decrease in the power output and in the energy yield, which results in an overall increase in the PV system cost. In the beginning, it was considered that the natural wash-off due to rain would be enough to maintain optimal cleaning of the solar energy convertors. However, recent studies have shown that mild rain showers or dew could actually contribute to the PV cover soiling [2e4]. Soiling mitigation is extremely important also from an economic point of view, as reports state that up to 7 billion EUR will be encountered in losses by 2023 in India and China alone, due to fouling, resulting in 4%e7% soiling losses in the annual power production [5]. Soiling mitigation can be reached through active or passive measures. Active cleaning of the installed PV modules can be done using manual, mechanic, or electrostatic methods. By far, the most commonly employed active cleaning method involves highpressure water jets, sometimes also using detergents, followed by brushing. Some of the disadvantages of using mechanical cleaning are related to the time and resources (water and cleaning solutions) consumed, thus being not particularly environmentalfriendly, the power required for the movement of the robotic arms and wipers, and, most importantly, the formation of microscratches [4,6e8]. Another active cleaning method is using a power source to generate a charge on the PV cover that repels the dust particles. The first use of the electrostatic method was in 1967 when NASA employed it for its Mars-stationed PV panels [4]. The most noticeable disadvantage of using this method is the cost associated with generating a strong enough electric field to overcome the dust particles’ cohesive and gravitational forces [4]. Due to these drawbacks, passive cleaning methods have gained more interest lately. These involve the deposition of thin films on the outer PV glass, which act as Advanced Ceramic Coatings. https://doi.org/10.1016/B978-0-323-99659-4.00012-7 Copyright © 2023 Elsevier Ltd. All rights reserved.

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self-cleaning (SC) and antisoiling coatings. These can be either superhydrophilic or superhydrophobic and must simultaneously meet the following criteria: (a) They must be highly transparent (with an optical transmittance equal to or larger than that of the solar glass); (b) They must be antireflective; (c) They must be highly durable, ideally up to 20e25 years; (d) They must be easily redeposited on the PV cover (either in-factory or in-situ, on the installed panels); (e) They must be cost-effective.

A discussion of what exactly these requirements entail is below provided, with examples of coatings reported in the literature, as well as a case study based on a TiO2 (titanium)-WO3-rGO multifunctional composite coating deposited on a commercial PV module.

2.

Self-cleaning coatings

There are two main categories of SC coatings: superhydrophobic and superhydrophilic thin films. These two types of surfaces have different water wettability, which is directly dependent on their chemical composition (polarity) and morphology. In order to understand the basics of SC, it is important to talk about the contact angle between a liquid (in the case of SC coatings for PV modules, the liquid is water, namely rain) and the solid (the SC coating). The contact angle (q) is the angle measured between the solid-liquid interface and the tangent to the liquid-vapor interface. According to Young’s equation (which applies in ideal cases), the contact angle can be expressed as follows [9]: cos q ¼ ðgSV  gSL Þ = gLV

(6.1)

where: g is the surface energy at the Sdsolid, Ldliquid or Vdvapor interphase. When the contact angle is lower than 90 , the surface is considered hydrophilic, while for values above 90 , the surface becomes hydrophobic. Moreover, at very low (150 ) contact angles, the surfaces have extreme wetting properties and are called superhydrophilic and superhydrophobic, respectively [10]. Since surfaces are rarely ideal, two other models were proposed considering the contact angle, namely: the Wenzel and the Cassie-Baxter models. The Wenzel model suggests that the surface roughness plays an important part in the wettability of a coating. Therefore, the contact angle is defined as [11]: cos qa ¼ r cos qs

(6.2)

where: qa is the apparent contact angle, qs is the true surface contact angle, and r is the roughness factor, defined as the ratio between the actual and geometrical surface areas. In the frame of the Wenzel model, the water droplets tend to infiltrate the surface

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cavities, resulting in a roughness factor higher than one (r > 1). This leads to an increase in the contact angle for hydrophobic surfaces and, conversely, a decrease for hydrophilic surfaces. While this model is useful for relatively homogeneous surfaces, it does not fully meet the requirements of heterogeneous ones. For these, the CassieBaxter model is employed, which states that the contact angle can be estimated as follows [11]: cos qa ¼ fs cos qs þ fv cos qv

(6.3)

where: qv is the water contact angle (WCA) in air and fS and fV are the friction coefficients of the water droplet in contact with the solid surface and the air (vapor) trapped at the surface, respectively. For highly porous surfaces, the water droplets can infiltrate the pores (capillary activity), forming a layer of liquid over the surface and thus enhancing hydrophilicity [12]. The SC mechanisms of the two types of coatings (hydrophilic and hydrophobic) are very different, as illustrated in Fig. 6.1. For the hydrophobic surfaces, the water droplets carry the dust and dirt particles away from the surface through a sliding motion. It is preferable to get a rolling motion, which is the case for spherical water droplets that are formed on superhydrophobic surfaces [13]. The hydrophilic coatings, on the other hand, clean the surface by creating a water sheet that washes away the dirt, with or without degrading it through photocatalysis. Extreme wetting (q < 10 ) is desired for the thin films as this leads to improved contact between the surface and the liquid, benefitting the sheeting process.

2.1

Superhydrophobic coatings

A surface can become superhydrophobic following two mechanisms: (a) lowering the surface energy or (b) increasing the surface roughness. The surface energy can be tailored by using (in)organic materials, such as alkyl ketenes, polycarbonates, polyamides, polydimethylsiloxanes, or fluorinated polymers (e.g., Teflon) [10,14e19], but also using metal oxides such as ZnO when the thin film comprises nanorods or nanowires [10,20]. Additionally, creating a rough surface for these or other materials can increase the WCA at the surface, resembling “the lotus flower effect” that can be met in nature on the eponymous plant, but also on the butterfly wings, shark skin, fish scales, etc. For coatings, very good control over the morphology has been reported

(a)

(b)

Figure 6.1 Self-cleaning mechanisms of (A) superhydrophobic and (B) superhydrophilic coatings.

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when using wet chemical deposition methods, such as hydrothermal, electrospinning, or chemical vapor deposition, etc. [21e23]. Since most superhydrophobic surfaces are polymeric, one of the main concerns is related to their durability under constant weathering in possibly corrosive/erosive environments. It was reported that the average lifetime of a superhydrophobic coating is about 3e4 years [24], which is not enough for solar energy converters that have a lifetime of 20e25 years. Therefore, recent studies focused on the development of self-healing polymeric coatings that can withstand wet conditions, high-temperatures, and high UV environments [25e27].

2.2

Superhydrophilic coatings

Some of the most well-known SC coatings are deposited on glass and are commercialized by Pilkington (Pilkington Activ™) or PPG Industries, Inc. (SunClean), which have been in the market for over 20 years now. The superhydrophilic SC mechanism considers the degradation of the organic pollutant molecules at the surface of the coating through photocatalysis and/or by sheeting the water that carries away the dirt molecules and/or the PC byproducts. One of the most intensively researched materials for this application is TiO2 due to its low-cost, high availability, nontoxicity, and chemical stability. Alongside TiO2, other metal oxides such as ZnO, WO3, SnO2, etc. have also been investigated as photocatalytic SC materials. Some of these materials may not be as efficient as TiO2 in the photodegradation of organic pollutants but may have better wetting properties that also recommend them for SC applications. It is the case with many metal oxides, for which the coating’s hydrophilicity can be turned into superhydrophilicity under UV irradiation. This process can be explained as follows: - When the photocatalyst is irradiated with an irradiation source of equal or higher energy than the band gap energy (Eg), electrons will jump from the valence band (VB) to the conduction band (CB), leaving behind holes in the VB; - The metal centers are reduced by the CB electrons, e.g., Ti þ IV turns into Ti þ III; - The VB holes oxidize O2 anions to O0 atoms, which are ejected, leaving behind oxygen vacancies; - Meanwhile, VB holes also react with the water molecules at the surface, leading to the formation of hydroxyl radicals (HO•); - The water molecules, along with the HO• migrate to the surface oxygen vacancies, where they adsorb, forming chemical bonds (e.g., TieOH bonds) that lead to the superhydrophilic behavior of the thin film.

As can be noticed from the above description of the mechanism, holes play a more significant role in the development of the superhydrophilic behavior of the SC layers compared to electrons. This is why the rate of the photo-induced conversion directly depends on the amount of hole scavengers in the film, which should be as low as possible. Furthermore, it was found that this type of mechanism is not permanent, with the coatings reverting to (or close to) their original wettability, in the absence of the UV radiation, as the hydroxyl radicals desorption takes place in the form of H2O2 molecules or H2O and O2 molecules [12,28]. Besides this process that takes place under irradiation, the metal oxide thin films will also act as photocatalysts,

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thereby degrading the organic pollutant molecules previously adsorbed at the film surface. The PC process takes place in much the same way as the previously photoinduced mechanism was described. However, the main difference is that the hydroxyl radicals resulting from the VB holes interacting with water react with the pollutant and break it down, ideally forming carbon dioxide and water (mineralization) or other oxidation by-products. These are removed from the solid adsorbent surface through desorption and/or wash-off. It should be noted that, along with the highly reactive hydroxyl radicals, to a lesser extent, the superoxide radical anions that are obtained through the reaction of the CB electrons with the adsorbed O2 molecules, also contribute to pollutant degradation. Some of the main issues faced by the mostly used photocatalysts are either their physical or chemical instability in water under certain conditions (e.g., ZnO is not stable over a large pH range), their fast electron-hole recombination (notoriously, TiO2 is such an example), or the required UV-activation due to being wide band gap semiconductors (most metal oxide photocatalysts can be included in this category). As UV radiation represents only 4%e10% of the solar spectrum, most photocatalysts are not highly efficient under natural sunlight. Therefore, there are multiple studies aimed at increasing the photocatalytic efficiency and lowering the photocatalyst band gap through doping with a different metal or nonmetal ions, creating hybrids or heterojunctions, dye sensitization, etc. [12,29e34]. As a preliminary conclusion on SC thin films, one may say that it is rather difficult to correctly evaluate their performance in a global framework, as there are no specific parameters or standardized methods for SC testing [6,34]. The SC performance of a coating subjected to real conditions will be affected by the type and size of the dust and dirt particles and by the weathering conditions such as UV radiation, rainfall, temperature, wind, etc. The long-term durability of such coatings becomes extremely important, and there are contradictory reports [4,24] stating that the superhydrophilic coatings have a longer lifetime (up to 25 years) than the superhydrophobic ones (up to 4 years), while other reports disagree with such a statement [6]. A consensus, in this case, is hard to reach given the different conditions considered in each case.

3. Antireflective coatings The solar panels have covers with a thickness of 2.0 mm, 3.2 mm, or 4.0 mm, with the light transmittance progressively decreasing as the glass thickness increases. Low-iron cover glass thickness tends to usually be 3.2 mm, which leads to 8%e10% of the incident radiation being reflected [6,35]. In order to improve the amount of radiation that reaches the solar cell(s) or the solar-thermal absorber, antireflective coatings (ARCs) are applied to the glass cover. This leads to a reduction in the reflection at the interface between the two (or multiple, if layered ARCs are used) media with different refractive indexes. According to Fresnel’s theory, when considering a single-layer coating and assuming negligible absorption and scattering of the (single-wavelength) radiation, the refractive index of the coating is calculated as follows [36]:

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nc ¼ ðna ns Þ1=2

(6.4)

where: nc, na and ns are the refractive indexes of the coating, the air (na ¼ 1) and the glass substrate (ns ¼ 1.5), respectively. This equation is valid when the optical thickness of the coating is tuned to be a quarter of the wavelength of the incident radiation (d nc ¼ l/4). The real thickness of the thin film is therefore: d ¼ l=4nc

(6.5)

For multilayered coatings, the reflection between the i and j layers is [37]: 
 Rij ¼ jRx j exp  2 di þ dj

(6.6)

where: |Rx| ¼ [(ni-nj)/(ni þ nj)] and di ¼ (2p ni di cos qi)/l. qi is the refraction angle and di is the thickness of the i layer.

Considering this, there are three major strategies to develop efficient ARC (Fig. 6.2): for single-layered films, tailoring the optical thickness to a quarter of the incident wavelength or including graded-index coatings; for multi-layered coatings, tailoring the thickness and the refractive index of the individual layers [38]. Using quarter-wave single-layered films means that the coating’s thickness will be very small and therefore durability becomes an issue. Moreover, the ideal refractive index, in this case, is nc ¼ 1.224, which falls below the refractive index values of most ARC materials currently in use. Two solutions are suggested in this case. (1) the incorporation of pores into a thicker film as this will decrease the refractive index or (2) the incorporation of stronger materials into the thin oxide film [6,38].

In the first case, one must consider that increasing porosity too much can result in a stability loss of the thin film as well as in multiple light scattering, thus defeating the initial purpose. For the second solution, it was found that using a graded index coating, with decreasing refractive index from the substrate to the air, may have a beneficial R1 R2 R3 R4 nair

R1 nair

n1 n2

R2

nc

n3

Glass substrate

(a)

Glass substrate

(b)

Figure 6.2 Schematic diagram of (A) single layer and (B) gradient layer antireflective coatings.

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effect on the overall performance of the ARC. This requires advanced deposition methods that allow very good control over the optical properties of the material. Finally, one of the mostly employed methods for developing efficient ARC is to use multi-layered stacks. The mostly used materials for this application are SiO2 (silica) and TiO2 [6,39]. Alongside these materials, the use of other oxides such as ZnO, WO3, Al2O3, and Cr2O3 has also been reported [38]. The SiO2/TiO2 thin film is a preferred combination of metal oxides for ARCs because it brings together two very useful individual properties of the materials that complement each other: SiO2 has a low refractive index (1.46 at 588 nm) and has great optical transmission, whereas TiO2 has a much higher refractive index (2.6) that is inappropriate for ARCs. Moreover, as already discussed, TiO2 is one of the best available photocatalysts, and therefore, is extremely attractive for PV cover coatings that merge SC and antireflective properties. Through optical simulations, multilayered (up to four or five) coatings based on TiO2 and SiO2 with different thicknesses can be conceived. Further tuning the nanoparticles’ size and the film porosity can lead to a further decrease in the refractive index. Atomic layer deposition and DC/RF magnetron sputtering were reported as efficient deposition techniques for the multilayered stacks [40,41], but they have a significant cost and cannot be applied on too large surface areas.

4. Multifunctional thin films Multifunctional coatings are single- or multi-layered thin films that simultaneously bring together a larger number of properties, such as SC, antifouling, antiicing, selfhealing, antireflective, scratch-resistant, antimicrobial, etc. In order to maximize the amount of radiation that reaches the solar cells in a PV module, any additional layer deposited on the glass cover (either for SC or AR purposes or both) needs to maintain a high transmittance level (over 80%) over a wide spectral range. Most of the materials that are used to deposit SC and/or ARC are metal oxides, and these can be deposited as thin, highly transparent films through a variety of methods. Spin-coated superhydrophobic layers with 88% transmittance in the visible spectral range have been reported to recover the solar cell efficiency by more than 90% after being washed with water [42]. Since some of these layers are slowly degraded over the course of a few years, it is often preferred to consider deposition methods that can be applied to in-field, already installed PV modules glazing. This means that reapplication, in ambient conditions becomes an option, moving away from the current classical deposition in-factory, at the beginning of the PV module lifetime. A report was issued on sprayed hydrophobic SiO2 layers that can be cured in ambient conditions and were tested in Egypt, proving that the SC, antireflective, and transparent coating led to an increased PV power output by 15% [43].

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5.

Case study

5.1

Materials and methods

An example of a multifunctional thin film based on TiO2, WO3, and reduced graphene oxide (rGO) deposited by spray coating on the solar glass will be further discussed. The choice of the coating material can be explained as follows: TiO2 is a very good PC material that has been used as (part of) a SC or antireflective coating. However, considering its UV activation and the fast charge carriers’ recombination, it still can be improved. When adding WO3 (a narrower bandgap n-type semiconductor, Eg ¼ 2.6e3.0 eV) the extent of the TiO2 activation range from the UV towards the VIS spectral range is expected to occur. Finally, adding rGO (a superconductor), will support avoiding the charges recombination and extend the lifetime of the holes so that not only will the PC performance of the TiO2-WO3 heterojunction be improved, but also the photo-induced superhydrophilicity. The composite thin film has SC and antireflective properties and is highly transparent and durable making it an ideal coating for PV glazing. The thin film was obtained using a powdersddispersiondthin film route. Titanium oxide and tungsten oxide sol-gel powders were obtained as previously described in Refs. [44,45] and were then mixed together with the rGO commercial dispersion (Sigma Aldrich, 10 mg/mL) to obtain a one-batch mixed powder with the gravimetric ratio TiO2: WO3: rGO ¼ 50:50:1. Magnetic stirring, followed by ultrasonication under heating at 100 C was employed until completely drying the powder that was then milled (MLW, KM1), for 24 h. The powder was then dispersed in water-ethanol mixture (3:1 v/v) to get a 5% concentration and was ultrasonicated for 3 h. This dispersion was then deposited on a demonstrator-size (20  30 cm2) solar glass substrate using 10 spraying sequences (P ¼ 1.2 bar) with a 60-s break between two consecutive sequences to allow the evaporation of the continuous medium. The spraying temperature was 80 C. The surface properties were investigated using scanning electron microscopy (SEM, Hitachi SEM S-3400 N type 121 II equipped with a Thermo Scientific UltraDry X-ray spectrometer for the EDX analysis) and atomic force microscopy (AFM, NT-MDT model NTGRA PRIMA EC); the optical properties were identified using UV-Vis spectroscopy (Perkin Elmer Lambda 950), and the wetting properties using contact angle measurements (OCA20 instrument) using water as a reference liquid. The photocatalytic experiments used simulated solar radiation at a laboratory (low) irradiance value (Gtotal ¼ GUV þ GVIS ¼ 3 þ 31 ¼ 34 W/m2). Before irradiating the samples for 8 h, the thin films were kept in dark for 1 h to reach the adsorption/desorption equilibrium. The initial pollutant (phenol, Ph) concentration was 10 ppm. The pollutant removal efficiency was calculated as: h ¼ ðA0  AÞ = A0  100 ½%

(6.7)

where: A0 is the initial absorbance of the solutions, and A is the absorbance after 1 h in dark followed by up to 8 h of irradiation measured each hour at l ¼ 270 nm, using a UV-VIS-NIR Perkin Elmer Lambda 950 spectrometer.

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The key parameters that were investigated are: (1) the PC efficiency of the film in phenol removal, (2) the WCA, (3) the reflectance (R%), and (4) the transmittance (T%) over a wide spectral range (250e2500 nm) aiming at R80%, values that can be accepted for glazing. The durability under variable relative humidity (RH ¼ 10e90%), irradiance (G ¼ 300e1000 W/m2), and temperature (T ¼ 10e40  C) was estimated by remeasuring these parameters after successive tests of up to 120 h. During these tests, a homemade climatic chamber with good control over RH and T was used, while the radiation source was a homemade vertical solar simulator, which allows control over the radiation intensity (irradiance, G) and composition (UV to VIS ratio).

5.2

Thin film characterization before durability testing

The homogeneity of the deposited thin film was evaluated by performing full characterization in two different areas: in the center of the spraying cone and at its edge. As the surface roughness and the elemental composition values (estimated based on the EDX spectra) in Table 6.1 show, there is not any significant difference between the two selected areas. This is the result of the previous optimization of the spraying parameters (number of spraying sequences, average distance between two successive spraying lines, time between two spraying sequences, etc.). The significant contribution of the elements in the glass substrate (e.g., Si, Na, Ca, Mg, etc.) demonstrates that the deposited films are consistently thin. Also, the very small contact angle (around 11 ) outlines that the film is highly hydrophilic, which is well required for PC and SC thin films. Moreover, the contact angle measurements were done without any prior UV-conditioning of the thin films. It is therefore expected that under solar radiation testing, UV-induced superhydrophilicity will most likely occur. Similarly, the surface topography, investigated for the two areas of the film in Fig. 6.3 shows many similarities. The film consists of a continuous, relatively homogeneous layer that contains both TiO2 and WO3, as well as rGO inserts (as supported by the C content confirmed by the EDX data). On top of this, multiple aggregates occur, differing in shape and size, as the coarse ones may be attributed to a higher TiO2 content, while the more spherical ones contain a higher amount of WO3. This Table 6.1 Mean square roughness (RMS), water contact angle (WCA) and elemental composition for the two selected areas.

Area position

RMS [nm]

WCA [o]

Ti [%at]

W [%at]

O [%at]

C [%at]

Substrate elements [%at]

1dat the cone center 2dat the cone edge

40.5

11.0

0.64

0.45

67.41

3.26

28.24

44.1

11.8

0.46

0.33

62.66

2.98

33.57

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(a)

(b)

Figure 6.3 SEM images of two areas on the thin film: (A) The cone center and (B) the cone edge.

attribution is consistent with SEM images of the individual powders before mixing and thin film deposition (now shown here). These aggregates increase the specific surface of the thin film thus offering a larger area for pollutant adsorption and this will further improve the photocatalytic efficiency. Moreover, the aggregates may be responsible for the roughness increase, and this may be advantageous to the antireflective properties, as previously discussed. However, it is important to consider the aggregates’ adhesion at the surface, as some of these aggregates may be washed off during weathering, as will be further discussed in relation to the durability tests. The results in Fig. 6.4 outline the optical properties of the thin film that is highly transparent (T > 80%) over the entire investigated spectral range (UV-A, Vis, and NIR). The reflectance values are mostly lower than 15% but exceed this value slightly in the VIS spectral range. This may be the consequence of the light scattering on the surface aggregates. Again, it can be noticed that almost no difference is observed in the values of the optical parameters at the cone center or edge. This suggests that any small difference in the film thickness in those areas does not significantly impact the optical properties of this coating. 100

30

80

25 20

R [%]

T [%]

60 40

15 10

20 0

centru con

Cone center margine con Cone edge

Cone center centru con Cone edge con margine 500

1000

1500

λ [nm]

(a)

2000

2500

5 500

1000

1500

2000

2500

λ [nm]

(b)

Figure 6.4 (A) Transmittance and (B) reflectance of the thin films, at the cone center and the cone edge.

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Table 6.2 Durability testing the multifunctional thin films in simulated weathering conditions.

5.3

Test no.

RH [%]

G [W/m2]

T [8C]

1 2 3 4

10 50 90 10

300 50 50 1000

28 24 24 50

Durability testing

The testing conditions are inserted in Table 6.2, outlining different relative humidity (RH), and irradiance (G) values, and the temperature (T) values employed to simulate the Brasov weather.

5.3.1

Test 1dsimulation of a cloudy but dry day

The results in Fig. 6.5A show that the PC efficiency has an opposite trend to the WCA values, and this well supports the idea that improved wetting (lower contact angle)

Figure 6.5 Variation of (A) the water contact angle (black) and photocatalytic efficiency (red), (B) transmittance, and (C) reflectance variation with the testing time under RH ¼ 10%, G ¼ 300 W/m2 at T ¼ 28 C.

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leads to better adsorption of the pollutant molecules on the coating surface, where photo-degradation takes place. The decrease in the contact angle value after a longer testing time (120 h) can be linked to the UV-induced superhydrophilicity of both TiO2 and WO3. All the key investigated parameters indicate high stability of the film under these mild conditions.

5.3.2

Test 2dsimulation of a relatively humid morning

Tests two and three were run in order to evaluate the impact that humidity has on the coating’s key properties while minimizing, as much as possible, the impact of irradiance and temperature. The results in Fig. 6.6 show that the humidity has a more significant impact on the stability of the films compared to irradiance (according to the data collected during Test 1). Although the film transmittance is relatively constant over 120 h of testing (Fig. 6.6B), the reflectance decreases, reaching values lower than 15% after 120 h of testing (Fig. 6.6C). This may be the follow-up of the removal of several aggregates from the coating surface, thus decreasing the multiple reflections; consequently, the antireflective properties improve after the coating is kept in a

Figure 6.6 Variation of (A) the water contact angle (black) and photocatalytic efficiency (red), (B) transmittance and (C) reflectance variation with the testing time under RH ¼ 50%, G ¼ 50 W/m2 at T ¼ 24 C.

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relatively humid environment for a certain period of time. However, given the decrease in the specific surface of the coating, the PC efficiency was found to decrease (because of fewer available adsorption centers for the phenol molecules), as it can be observed in Fig. 6.8A. The significant increase in contact angle from 11 to 43 degree also contributes to this effect.

5.3.3

Test 3dsimulation of a highly humid morning (e.g., after rainfall)

The third test completes the results recorded during Test 2 by increasing the relative humidity even more, to 90%, to observe if the optical, wetting, and photocatalytic properties are further affected at such extreme humidity values. As Fig. 6.7A shows, the very high relative humidity (90%) has a similar effect on the contact angle, as the average RH (50%), in the short term. After 48 h, however, the contact angle starts to decrease, leading to an improved PC behavior, compared to that recorded during Test 2. Although reflectance is also affected (Fig. 6.7C) in a similar

Figure 6.7 Variation of (A) the water contact angle (black) and photocatalytic efficiency (red), (B) transmittance and (C) reflectance variation with the testing time under RH ¼ 90%, G ¼ 50 W/m2 at T ¼ 24 C.

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manner as it was during Test 2, there is almost no discernible difference between the values obtained in these two tests. These results allow concluding that an increase in humidity does not attract a further decrease in the reflectance as only the small particles are removed from the surface in the humid environment. The larger aggregates, due to their significant mass or volume, remain fixed, even at higher RH.

5.3.4

Test 4dsimulation of a sunny summer day

The fourth test was designed to identify the impact of the maximum irradiance that can be encountered in Brasov, Romania, where the coatings will be tested in outdoor conditions at a later date. The global irradiance value of 1000 W/m2 was recorded as a combination of 15e20 W/m2 of UV and 880 W/m2 of VIS radiation using a solar simulator that mimics a clear, summer day. The values in Fig. 6.8A outline that, similarly with the first test, the UV radiation supports the superhydrophilicity at the coating surface. The WCA slowly decreases from 11 to 10 degree after 48 h and then remains relatively constant. The phenol

Figure 6.8 Variation of (A) the water contact angle (black) and photocatalytic efficiency (red), (B) transmittance and (C) reflectance variation with the testing time under RH ¼ 10%, G ¼ 1000 W/m2 and T ¼ 50 C.

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(PC) degradation efficiency increases up to about 25%, as possible changes may occur at the film surface, resulting in TieO and WeO bonds that charge the surface and attract the phenol molecules. The transmittance (Fig. 6.8B) slightly increases in the VIS spectral range, and this may be correlated to a thinning of the film (removal of the surface aggregates) or to the previously mentioned reorganization of the particles at the film surface. The reflectance (Fig. 6.8C) values also slightly decrease after the first 24 h but remain afterward constant. Following the experimental tests, it can be concluded that the relative humidity has a more significant effect on the key parameters of the SC, antireflective TiO2-WO3rGO thin film compared to irradiance or temperature. This is particularly important for locations such as Brasov, a mountain city with humid weather. It is worth mentioning that the high relative humidity (90%) did not seem to impact the optical or photocatalytic properties of the films any more significantly than the average humidity (50%).

6. Conclusions and future outlook The ever-increasing interest in developing efficient solar energy converters with longer lifetimes has outlined the need to investigate diverse antisoiling alternatives. This led to a slow but certain shift from the active methods, based on manual and mechanical labor, to the passive ones, based on thin film coatings. The SC coatings can be divided into two major categories: superhydrophobic and superhydrophilic. While the SC mechanism significantly differs between them, both can be successfully used on PV glazing to prevent soiling, freezing, fogging, etc., thus maintaining a high power output for the module. While superhydrophobic thin films are mostly obtained using polymeric materials, superhydrophilic ones are based on metal oxides, preferably the ones that also possess (PC) activity. Due to this added bonus of degrading the organic pollutants instead of just removing them from the PV cover surface, the (PC) superhydrophilic coatings are sometimes preferred to the superhydrophobic ones. Moreover, there are reports that state the formers have a much longer lifetime. The antireflective layers represent another alternative for increasing the power output of the PV modules, by increasing the amount of radiation that reaches the solar cell. It was found that multiple-layered coatings have an advantage in this area as the refractive index of the composite can be tailored by optimizing the layers’ thickness and porosity. The choice of individual materials is obviously very important. One of the most commonly used metal oxides is TiO2 (alongside SiO2). Titanium dioxide-based composites represent strong candidates in the area of SC coatings. By selecting the appropriate match for TiO2, the spectral activation range can be extended from the UV to the UV þ VIS spectral range, and the charge carrier’s recombination can be minimized. The durability of these coatings is often a more serious issue than upscaling from laboratory level to large-area substrates. One effective example of a multifunctional thin film that is simultaneously SC, antireflective, and highly stable in environmental conditions was discussed. The TiO2WO3-rGO thin film sprayed over the solar glass as substrate proved to be highly stable,

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even after up to 120 h, to the impact of irradiance (50e1000 W/m2) and temperature (24e50 C). The relative humidity was investigated as an environmental factor that affects the optical properties, possibly due to the reorganization of the aggregates at the surface of the films. However, overall, these coatings are recommended for application on PV solar glass and for outdoor testing, as they proved not only antireflective but also photoactive and superhydrophilic under UV radiation and this indicates good SC properties. In the area of multifunctional coatings for PV glazing, it is important to investigate various and different durability testing results in harsh conditions (chemical stability in corrosive/erosive environments, mechanical stability and antistatic properties in highdust containing environments, antiicing and antifreezing properties in low-temperature conditions, etc.). Moreover, flexible solar cells have also gained attention, so the adherence and stability of these multifunctional coatings to other substrates (e.g., polymer sheets) represent another point of interest. Further, the deposition of SC coatings on other materials, such as alumina, cement, wood, or different types of glazing (glass, mirrors, windshields, etc.), and other materials used in construction, could also open up new and exciting research domains.

Acknowledgment This work was supported by a grant of the Ministry of Research and Innovation, CNCSUEFISCDI, project number PN-III-P1-1.1.-PD-2016-0289, within PNCDI III, Contract no. 78/2018, which is gratefully acknowledged.

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[26] Q. Rao, J. Zhang, X. Zhan, F. Chen, Q. Zhang, UV-driven self-replenishing slippery surfaces with programmable droplet-guiding pathways, Journal of Materials Chemisrty 8 (2020), 2481e2419. [27] X. Tan, Y. Wang, Z. Huang, S. Sabin, T. Xiao, L. Jiang, X. Chen, Facile fabrication of a mechanical, chemical, thermal, and long-term outdoor durable fluorine-free superhydrophobic coating, Advanced Materials Interfaces 8 (2021) 2002209. [28] N. Sakai, A. Fujishima, T. Watanabe, K. Hashimoto, Quantitative evaluation of the photoinduced hydrophilic conversion properties of TiO2 thin film surfaces by the reciprocal of contact angle, Journal of Physical Chemistry B 107 (2003) 1028e1035. [29] S. Prabhu, L. Cindrella, O. Joong Kwon, K. Mohanraju, Superhydrophilic and selfcleaning rGO-TiO2 composite coatings for indoor and outdoor photovoltaic applications, Solar Energy Materials and Solar Cells 169 (2017) 304e312. [30] A.O.T. Patrocinio, L.F. Paula, R.M. Paniago, J. Freitag, D.W. Bahnemann, Layer-byLayer TiO2/WO3 Thin films as efficient photocatalytic self-cleaning surfaces, ACS Applied Materials & Interfaces 6 (2014) 16859e16866. [31] N. Serpone, Is the band gap of pristine TiO2 narrowed by anion- and cation-doping of Titanium Dioxide in second-generation photocatalysts? Journal of Physical Chemistry B 110 (2006) 24287e24293. [32] Q. Guo, Z. Zhang, X. Ma, K. Jing, M. Shen, N. Yu, J. Tang, D.D. Dionysiou, Preparation of N,F-codoped TiO2 nanoparticles by three different methods and comparison of visiblelight photocatalytic performances, Separation and Purification Technolgies 175 (2017) 305e313. [33] M. Gr€atzel, Dye-sensitized solar cells, Journal of Photochemistry 4 (2003) 145e153. [34] F. Tang, A review on the self-cleaning glass technology applied in automobile, in: IEEE Symposium on Product Compliance Engineering-Asia (ISPCE-CN), 2018, pp. 1e4. [35] P. Buskens, M. Burghoorn, M.C.D. Mourad, Z. Vroon, Antireflective coatings for glass and transparent polymers, Langmuir 32 (2016) 6781e6793. [36] L. Yao, J. He, Recent progress in antireflection and self-cleaning technology e from surface engineering to functional surfaces, Progress in Materials Science 61 (2014) 94e143. [37] H.A. Macleod, Thin-film Optical Filters, Institute of Physics Publishing, London, 2010, ISBN 0-7503-0688-2. [38] C. Garlisi, E. Trepci, X. Li, R. Al Sakkaf, K. Al-Ali, R.P. Nogueira, L. Zheng, E. Azar, G. Palmisano, Multilayer thin film structures for multifunctional glass: self-cleaning, antireflective and energy-saving properties, Applied Energy 264 (2020) 114697. [39] C. Tao, X. Zou, K. Du, G. Zhou, H. Yan, X. Yuan, L. Zhang, Fabrication of robust, selfcleaning, broadband TiO2-SiO2 double-layer antireflective coatings with closed pore structure through a surface sol-gel process, Journal of Alloys and Compounds 747 (2018) 43e49. [40] K. Pfeiffer, L. Ghazaryan, U. Schulz, A. Szeghalmi, Wide-angle broadband antireflection coatings prepared by atomic layer deposition, ACS Applied Materials & Interfaces 11 (2019) 21887e21894. [41] D. Zambrano, R. Villarroel, R. Espinoza-Gonzalez, N. Carvajal, A. Rosenkranz, A.G. Monta~ no-Figueroa, M.J. Arellano-Jiménez, M. Quevedo-Lopez, P. Valenzuela, GacituacMechanical and microstructural properties of broadband anti-reflective TiO2/ SiO2 coatings for photovoltaic applications fabricated by magnetron sputtering, Solar Energy Materials and Solar Cells 220 (2021) 110841. [42] e Q. Xu, Q. Zhao, X. Zhu, L. Cheng, S. Bai, Z. Wang, L. Meng, Y. Qin, A new kind of transparent and self-cleaning film for solar cells, Nanoscale 8 (2016) 17747e17751.

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[43] H.R. Alamri, H. Rezk, H. Abd-Elbary, H.A. Ziedan, A. Elnozahy, Experimental investigation to improve the energy efficiency of solar PV panels using hydrophobic SiO2 nanomaterial, Coatings 10 (2020) 503. [44] C. Bogatu, D. Perniu, C. Sau, O. Iorga, M. Cosnita, A. Duta, Ultrasound assisted sol-gel TiO2 powders and thin films for photocatalytic removal of toxic pollutants, Cerammics International 43 (2017) 7963e7969. [45] M. Covei, C. Bogatu, D. Perniu, A. Duta, I. Visa, Self-cleaning thin films with controlled optical properties based on WO3-rGO, Ceramics International 45 (2019) 9157e9163.

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Vijaykumar S. Bhamare and Raviraj M. Kulkarni Centre for Nanoscience and Nanotechnology, Department of Chemistry, KLS Gogte Institute of Technology, Belagavi, Karnataka, India

1. Introduction Ceramic coatings were studied first time in the midof the 20th century [1e4]. Thereafter, ceramic coatings were decorated on the metallic surface of X-15 rocket nozzles in the 1960s [5]. These coatings were also utilized to decorate different parts of gas turbine engines to withstand higher temperatures [6]. During the mid-1970s, these coatings were effectively utilized in research engines where extremely high temperatures were involved [7,8]. Subsequently, these coatings were utilized for commercial purposes in gas turbines during the 1980s [9]. During the 1990s, science and technology were used for the development of ceramic coatings with enhanced heat resistance and fire retardant features. These ceramic coatings are called heat-resistant fireretardant ceramic coatings (HRFRCCs). During this period, there was more focus on simple designing, life modeling, enhanced performance, and cost-effective ceramic coatings to accomplish the objective. This development of ceramic coatings was reported in earlier review papers [10,11]. There are many advantages to using ceramic coatings with an excellent heat resistant feature on a film-cooled turbine vane, as reported in the literature [12]. The demand for ceramic coatings has increased in the last few decades across the world due to their unique features, such as inexpensive, anticorrosive, wearresistant, heat resistant, fire retardant features, etc. These special properties of ceramic coatings make them promising materials for coating metallic surfaces in different fields. HRFRCCs are found to be very useful in gas turbine blades and vanes. These are also utilized in diesel engines where very high temperatures are involved due to the unique features of HRFRCCs [13]. The literature review revealed that there are several methods reported for the fabrication of HRFRCCs on metallic surfaces as per the requirements [14e16]. These days, ceramic coatings using nanotechnology are utilized to offer outstanding thermal features for coated metallic surfaces, which can withstand very high temperature and can be utilized in aircraft [17e20]. There are so many advantages or merits of HRFRCCs. It has attracted the attention of the research community across the world due to its better performance, effectiveness, higher efficiency, longer durability, etc. Ceramic coatings that offer protection to steel against oxidation at drastic conditions of temperatures are preferred. These coatings should provide excellent heat resistance to ceramic-decorated metallic substrates at very high temperatures. These ceramic Advanced Ceramic Coatings. https://doi.org/10.1016/B978-0-323-99659-4.00008-5 Copyright © 2023 Elsevier Ltd. All rights reserved.

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coatings should be thinner and antioxidative. It should strongly stick to metallic substrates. There should be no cracking and chipping at very drastic conditions of temperature. It should be light in weight. Ceramic coatings were decorated on the surface of mild steel and usedutilized during the war to protect the exhaust systems of aircraft. Ceramic coatings with higher heat resistance and fire retardant features can be used in burners, furnace parts, heat interchangers, etc. It was found that HRFRCCs did not increase the base metal temperature. This is due to the involvement of cooling systems, which are utilized in different hot parts of engines. As a result, it makes the engine efficient to work at drastic conditions of temperature [21]. Hence, there has been an increase in the demand for HRFRCCs with special features such as antisintering, thermal stability, phase stability, and poor thermal conductivity so that these coatings can be used efficiently at very high inlet gas temperature conditions in engines [22]. HRFRCCs are decorated on the surface of substrates in the form of multi-layers to offer heat resistant properties in gas turbines [23,24]. Normally, there are two layers involved in HRFRCCs. The first layer is a ceramic top coat and the second layer is a metallic bond coat. The bond coat is present in between the top coat and the metallic surface of engine components. The metallic bond coat plays a significant role in HRFRCCs and offers antioxidative, anticorrosive features at drastic conditions of temperature. Moreover, the bond coat helps to strengthen the coating of the ceramic top coat on the surface of the metal alloy [25]. The choice of material for HRFRCCs depends upon the thermal loading conditions and plays a very crucial role to enhance the efficiency of the engine and improve emission reduction [26]. Myoung et al. [27] reported that there is an enhancement in thermal stability and durability of ZrO2-8% Y2O3 HRFRCCs decorated on the surface of Ni-based superalloy. Yttriastabilized zirconia (YSZ) coatings have many failures at very high-temperature conditions [28,29]. Many oxide stabilizers [30] and innovative materials [31e33] are utilized for overcoming the drawbacks of YSZ coatings. The literature survey indicates that huge numbers of manuscripts, review papers, and chapters are published in esteemed journals on HRFRCCs as per the ScienceDirect database (Fig. 7.1). The previous investigations show that metals and their alloys have low thermal stability, poor durability, and less wear resistant at drastic conditions of higher temperatures, which limits their uses for various applications [34e39]. In view of the above, many metals and their alloys are coated with ceramic materials to improve physical and mechanical features [11,40e44]. The present chapter demonstrates the main requirement of HRFRCCs, applications of HRFRCCs in different fields, degradation mechanism, various factors affecting HRFRCCs, various deposition methods, and ceramic materials utilized for HRFRCCs on metallic substrates. Current progress in the field of HRFRCCs is also included in this chapter.

2.

Major necessities for HRFRCCs

There are particular requirements of HRFRCCs so that they can work effectively and efficiently in drastic conditions of temperature [45]. There is huge demand for

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Years Figure 7.1 Science direct database (date: April 26, 2022) indicates manuscripts published in various esteemed journals from 2012 to 2021 with the content “heat resistant ceramic coatings”.

HRFRCCs that are light in weight and exhibit lower thermal conductivity. These coatings should not be affected at drastic conditions of temperature and also due to thermal shock. These coatings should work properly and remain undisturbed in oxidizing atmosphere. It should offer insulation to the metallic substrate at very high temperatures. The expansion of coatings due to high temperatures should be at its lowest. These coatings should possess strain compliance at drastic conditions of temperature. Moreover, these coatings should not absorb more heat at hot conditions and avoid it to reach at metallic alloy’s surface. Furthermore, HRFRCCs should shield the surface of metallic alloys for longer durations and thermal cycles without any damage [46]. These coatings should form a thin layer on a metallic substrate. HRFRCCs should exhibit sufficient porosity. These coatings must exhibit phase stability to avoid change in a volume otherwise it may lead to cracking in coatings. These coatings should be antioxidative and anticorrosive. They must have better wear resistance. They must have very high melting points. These coatings must be chemically inert. These coatings must have strong adherence to the metallic surface. They should have a lower sintering rate. In order to meet these features, there is an involvement of proper selection of materials for HRFRCCs [47].

3. Structure of HRFRCCs Generally, HRFRCCs contain four layers. The fourth layer at the bottom is metallic alloy. The third layer is the metallic bond coat. Thermally grown oxide (TGO) is

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Figure 7.2 Design of HRFRCCs indicates different types of layers.

formed on the metallic bond coat. The topmost layer is a ceramic top coat as shown in Fig. 7.2. The cold air is passed through the metallic substrate to decrease the temperature. The topmost layer is usually made of YSZ due to its lower thermal conductivity. This layer remains undisturbed during the normal temperatures maintained for various applications. The topmost layer helps to maintain the temperature of second and third layers at lower level as compared to surface. Nevertheless, HRFRCCs get disturbed beyond the temperature of 1200
C due to transformation in phases. As a result, cracks are observed on the topmost layer. Therefore, there was a need to find out promising and innovative materials that can withstand very high temperatures beyond 1200
C. Nowadays, mullite or alumina materials are utilized to develop extra coatings on the topmost layer and offer a better antioxidative feature to HRFRCCs [47]. Normally, the thickness of the metallic bond coat is 75e150 mm thick. It is made up of nickelbased alloy. Nickel and platinum aluminides are also utilized for the formation of metallic bond coats. This coat makes the surface of metallic materials antioxidative and anticorrosive. The metallic bond coat could not withstand temperatures beyond 700
C. As a result, the TGO layer is formed due to the oxidative reaction of the metallic bond coat. HRFRCCs are fabricated in such a way that the oxidation of the bond coat should be lowest at higher operating temperatures. This TGO layer will not allow oxygen gas to pass through it and reaches to metallic bond coat [48]. HRFRCCs can be customized at the interface of TGO and metallic bond coat.

4.

Different processing techniques utilized for HRFRCCs

There are different processing techniques reported in the literature for the fabrication of HRFRCCs to improve their mechanical properties. These techniques are presented in Fig. 7.3. HRFRCCs can be performed normally using two methods. Atmospheric plasma spray (APS) is preferred due to its cost-effectiveness and low thermal conductivity as compared to the electron beam physical vapor deposition (EB-PVD) method. Thermal spraying methods are widely used for the fabrication of HRFRCCs. These methods are categorized on the basis of chemical, electrical, and kinetic energies

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Electrophore c deposi on

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Electron beam physical vapour deposi on (EBPVD)

Atmospheric plasma sprayed (APS) Plasma-enhanced chemical vapour deposi on (PECVD)

plasma laser hybrid spraying

Detona on gun spray

Electrosta c spray-assisted vapour deposi on (ESAVD)

Different processing techniques for HRFRCCs

Solu onprecursor plasma spray (SPPS)

Cathodic plasma electroly c deposi on

Sol-gel

Thermal plasma Low-pressure plasma spraying

Spark plasma sintering (SPS)

Composite sol-gel

Figure 7.3 Different deposition techniques utilized for HRFRCCs on metallic substrates.

supplied to particles. It was also reported that depositions of HRFRCCs on metallic substrates by using different thermal spray methods have unique features. Classification of thermal spray techniques on the basis of chemical, electrical, and kinetic energies supplied to particles is presented in Fig. 7.4.

5. Applications of HRFRCCs HRFRCCs are widely utilized in hot sections of engines as presented in Fig. 7.5 due to their mechanical properties such as sintering rate, thermal insulation, expansion coefficient, chemical robustness, thermal conductivity, elastic-plastic features, cohesive strength, fracture, resistant to corrosion, wear resistant, adhesive strength, erosion, light in weight, antioxidative, thin layer, sufficient porosity, microstructure, phase stability, etc. These mechanical properties offer stability and durability to HRFRCCs to be applied on metallic substrates.

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Flame spray technique

Chemical energy supplied to particles

Detonation Gun technique

Flame wire spray technique Flame powder spray technique

HIgh velocity oxy fuel technique HIgh velocity air fuel technique Electric arc wire spray technique

Thermal Spraying coating techniques

Electrical energy supplied to particles

Air plasma spraying technique Liquid generated plasma technique

Transferred plasma arc spraying technique Non-transferred plasma arc spraying technique

Controlled atmospheric plasma technique

Kinetic energy supplied to particles

Cold spray techniques

HIgh pressure cold spray technique Low pressure cold spray technique

Figure 7.4 Classification of thermal spray techniques on the basis of chemical, electrical and kinetic energies supplied to particles.

6.

Different factors responsible for the failure of HRFRCCs

HRFRCCs using various materials which are utilized for coating on metallic substrate undergo many changes such as change in chemical, phase transformation, and microstructural properties at higher temperatures. Aluminum is responsible for the formation of TGO due to its diffusion from the bond coat. The speed of different processes enhances exponentially with an increase in temperature. As a result, HRFRCCs undergo degradation, which is responsible for their failure and limits their applications in different fields where extreme temperatures are involved. Mainly, there are three factors responsible for the failure of HRFRCCs. The first factor is bondcoat degradation.

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Figure 7.5 Applications of HRFRCCs in hot sections of engines.

Combustor

Rotating blades

Burners

Applications of HRFRCCs in hot sections of engines

Stationary guide vanes

Shrouds

Blade outer air-seals

The life of any HRFRCCdepends on the condition of the bond coat. Bond coat undergoes oxidation, which affects its stability and efficiency. The bond coat made up of nickel-chromium-aluminum yttrium undergoes oxidation, which forms a layer of TGO on its surface. This oxidized TGO leads to the breakdown of HRFRCCs when it reaches a particular thickness. This is due to the cracking of the oxidized TGO layer. The second crucial factor responsible for the failure of HRFRCCs is residual stress. The function of residual stress is not easy and simple. It was observed that residual stress is produced among three layers due to a mismatch of thermal expansion coefficients. There is a lot of scope available in this field to investigate innovative materials with suitable chemical compositions to avoid failure of HRFRCCs due to higher residual stress [49]. K. Sfar et al. studied the influence of residual stress and crack behavior on the stability and efficiency of HRFRCCs [50]. The third factor responsible for the failure of HRFRCCs is top coat degradation. This layer undergoes crack because of residual stress created from a mismatch of thermal expansion coefficients among different layers of HRFRCCs. Due to the cracking of the top coat, oxygen reaches the bond coat which is responsible for the collapse of HRFRCCs. L. Yang et al. studied and analyzed the influence of higher residual stress on YSZ ceramic coatings at temperature of 1150
C [51]. G. Pujol et al. studied the enhancement of microstructure, thermal stability, and mechanical features of the top coat. Furthermore, another important and crucial factor studied was corrosion by melted CaeMgeAl silicates (CMASs), which is also responsible for the degradation of HRFRCCs [52].

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Materials utilized for HRFRCCs

The following materials have been investigated and examined for the fabrication of HRFRCCs to improve their performance in different fields at drastic conditions of temperature.

7.1

YSZ

The literature survey revealed that 7%e8% yttria stabilised zirconia (YSZ) material is investigated by many researchers and utilized for the fabrication of HRFRCCs. The experimental results of many investigations reported that 7%e8% YSZ coating withstands very high temperature conditions in diesel engines and also in gas turbines [49e55]. Kaspar and Ambroz [51] reported that YSZ coatings are very effective and anticorrosive for sodium sulfate and vanadium oxide as compared to zirconium dioxide coating, which is stabilized by calcium oxide or magnesium oxide. Voyer et al. [56] and Troczynski et al. [57] investigated and examined 18%e20% YSZ coatings. The main drawback of YSZ coatings is that they can withstand temperatures below 1473 K for a longer duration. YSZ coatings undergo phase transformations at drastic conditions of temperature, which are responsible for the cracks on the surfaces of coatings [58,59]. Miller [60] fabricated a zirconia dioxide coating that is stabilized by calcium oxide and magnesium oxide and utilized it at 1223 K in a gas turbine. In YSZ coatings, there is more concentration of O2 vacancies, which is responsible to form TGO. As a result, YSZ coatings cannot withstand very high temperatures in gas turbine applications. This is the main drawback of the YSZ coatings applied on a metallic surface. Therefore, there was a need of other coating materials with better durability at drastic conditions of temperature. Ramaswamy et al. [61] reported that antioxidation bond coats like Al2O3 and mullite can resolve this problem of TGO formation on the coated surface. Vassen et al. [62] fabricated 10%e15% YSZ coatings using the vacuum plasma spraying method. This study revealed that a layer of TGO is formed due to the easy transport of O2 and hence oxidation of bond coat takes place. In order to understand and determine the stress development of coatings, the finite element method was applied and results were discussed. It was also highlighted that a crack propagation model can be used to study and analyze crack development in YSZ coatings the applied on metallic surface at drastic conditions of temperature [62]. In another investigation performed by R. Vassen et al. the same year, the graph of the number of thermal cycling lives of YSZ coatings versus the temperature of substrate in Kelvin was plotted to understand the durability of YSZ coatings [63]. The adverse effect of SiO2 impurity present in YSZ coatings on the thermal cycling life was studied and examined by many researchers [64,65]. The experimental results revealed that there is an enhancement of the life cycle of YSZ coatings with a decrease in the SiO2 level. This study includes the influence of SiO2 impurities on the mechanical features of YSZ coatings. The increase in the amount of SiO2 affects the electrical conductivity and sintering rates. This investigation also reported that the SiO2 impurity affects creep rates. The experimental data indicates SiO2 impurity in YSZ coatings has

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affected the shape and size of YSZ grains and also dissolves yttrium oxide from the coating. This affects the performance of YSZ coating, which contains SiO2 impurities. He et al. [66] reported that rare earth silicates can be utilized to enhance the efficiency and effectiveness of HRFRCCs. Silicates reduce the transport of oxygen and hence prevent the oxidation of bond-coat. This investigation indicated that silicates are a promising material for oxygen barriers in HRFRCCs [66]. Chen et al. [67] synthesize YSZ coatings with the help of the APS technique? During this investigation, tensile testing was done to check the strength of the coating. The characterization of synthesized YSZ coatings indicates that the greater the proportion of columnar grains, the stronger the bond coat. This investigation concluded that the strength of the bond coat is higher when the porosity of the synthesized material is smaller. Varghese et al. [68] reported that laser-remelted YSZ HRFRCCs are found chemically nonreactive in melted Na. Xu et al. [69] reported YSZ as a promising material to be applied in the extremely hot components of gas turbines. A machine learning model was designed to determine the strength of bond coat in YSZ coatings.

7.2

Mullite

Mullite is a very significant and promising material that has been investigated, examined, and utilized for HRFRCCs due to its special features. It is lighter and thermally stable. It is chemically inert and exhibits poor thermal conductivity [70]. Mullite powders are prepared from alumina, silica, and chromium oxide powders. The chemical composition is 3 (Al2O3) 0.2 (SiO2). The powder mixtures are sintered at a higher temperature for a period of 72 h. It was reported that mullite is found to be superior material than YSZ because of its antioxidative feature. Mullite can withstand very high temperatures in diesel engines as compared to YSZ coatings. Many researchers reported that the mullite coatings are more durable than YSZ coatings based on diesel engine tests that were carried out using both materials [71,72]. Ramaswamy et al. [61] reported that the thermal cycling life of YSZ coating is longer as compared to mullite coating at drastic temperature conditions beyond 1273 K. It was highlighted that mullite coating underwent volume reduction due to crystallization at temperature of 1023e1273 K. Consequently, there was debonding and cracking found in mullite coating. Lee et al. [73] fabricated mullite coatings using the plasma spraying method and reported that mullite material is a potential candidate for silicon carbide substrates due to similarities in thermal expansion coefficients. Adenan et al. [74] fabricated 8% YSZ and mullite coatings on the surface of carbon steel. Both the coats are examined using different tests. Mullite coating gets damaged after 14 thermal cycles. On the other hand, 8% YSZ coating remains stable up to 64 thermal cycles. This indicated that 8% YSZ coatings are more stable and durable as compared to mullite coating [74]. Jech et al. [75] studied and analyzed microstructural modifications in traditional YSZ and YSZ-mullite HRFRCCs at different temperatures. During this investigation, YSZ and YSZ-mullite coatings were synthesized with the help of the APS method. The synthesized materials were characterized by different sophisticated techniques.

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Both the coatings withstand up to 1050
C for a period of 500 h. Thereafter, YSZ coating underwent oxidation at 1150
C after 100 h and could not retain its integrity. On the other hand, YSZ-mullite coating was found to be thermally stable even at 1150
C for the period of 200 h. The experimental data indicated that the TGO layer is found to be thicker in traditional YSZ coatings as compared to YSZ-mullite coatings [75]. Huanjie Fang et al. fabricated a double ceramic layer made of YSZ-mullite materials and examined the CMAS attack on the coating. The experimental results indicated that a double ceramic layer shows a bigger contact angle as compared to YSZ coating. This offers more resistance to YSZ-mullite coatings toward CMAS attack. CaAl2Si2O8 layer is formed in YSZ-mullite coating which is also responsible to improve resistance toward CMAS [76].

7.3

Alumina

Aluminum oxide is cheaper and easily available which can be utilized for coatings. This material exists in different phases. Alpha-aluminum oxide is found to be stable, chemically nonreactive, and hard in nature. Kingswell et al. [77] fabricated and studied the erosion behavior of aluminum oxide coatings prepared by using different thermal spraying methods. Many researchers investigated the influence of aluminum oxide on YSZ coatings and discussed it. The experimental results of these studies reported that the presence of a small quantity of aluminum oxide in YSZ coatings enhanced the bonding strength and hardness. It was also highlighted that there was no considerable change in Young’s modulus and stiffness [78e80]. A transmission electron microscope was applied to study and analyze the microstructure of alumina oxide coatings. These papers also reported investigations of the thermal and mechanical features of these coatings. During these investigations, other two phases like gamma- and deltaaluminum oxides are also observed. These two phases are found to be nonstable. Both phases change into stable alpha-aluminum oxide in the thermal cycling process. There was a considerable change in the volume of the bondcoat, which is responsible to form cracks on the surface of the coating [81,82]. The stability of alpha-phase aluminum oxide coating could be enhanced partly by using dopants like titania, ferric oxide, and chromium oxide. Vassen et al. [58] highlighted that aluminum oxide coatings exhibit higher thermal conductivity than YSZ coatings. In addition, aluminum oxide coatings show lower values of thermal expansion coefficient than YSZ coatings [58]. Steffens et al. found that the inclusion of SiC fibers into the microstructure of aluminum oxide coating enhances considerably the mechanical features of the coating [83]. Xu et al. [84] highlighted that aluminum oxide is not a very potential material for the coating. However, the inclusion of aluminum oxide in the YSZ coating enhances the hardness and oxidative resistance of the substrate. The experimental results indicated that the thermal cycling life of 8% YSZ-alumina coating is longer than that of 8% YSZ coating [84]. Ahmadi et al. examined the influence of laser surface treatment on the thermal shock performance of alumina/YSZ HRFRCCs. The synthesized multilayer HRFRCCs were characterized and analyzed using different sophisticated methods.

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The experimental results indicated enhancement in the thermal shock resistance of the alumina/YSZ HRFRCCs [85]. Zhang et al. [86] studied and utilized alumina-modified 7% YSZ HRFRCCs for hot parts of engines. This investigation revealed that alumina-modified 7% YSZ HRFRCCs are found to be superior as compared to conventional 7% YSZ coatings [86]. Ariharan and Balani [87] studied the fretting wear behavior of hard materials such as alumina and proposed the same for fabricating HRFRCCs. Alumina material is utilized to enhance the tribological features of the coated metallic surface [87]. In this research work, alumina-8% YSZ-CNT-based ceramic matrix composite coatings were fabricated, studied, and analyzed. The experimental results indicated that alumina-based coatings such as alumina-3% YSZ and alumina-8% YSZ-CNT exhibited good wear resistance. It may be because of superior mechanical features.

7.4

Ceric oxide and yttria stabilised zirconia

Plasma-sprayed HRFRCCs are utilized to coat hot parts of gas turbines. These coatings are formed by combining ceric oxide with YSZ material. This combination improves the performance of YSZ coating and enhances the efficiency of engines at drastic conditions of temperature. Ceric oxide shows better mechanical features than YSZ material. It was also found that ceric oxide stabilizes YSZ coating with higher fracture toughness. This type of coating is considered an advanced HRFRCC, which has a higher heat shielding ability. Many researchers investigated and compared the mechanical and thermal features of ceric oxide and YSZ coatings to find out suitable and promising materials for HRFRCCs to be used in extremely hot engine parts. Ceric oxide-YSZ coatings are found to be significant due to their outstanding thermal shockresistant property. It was also observed that the thermal barrier ability and hardness of ceric oxide-YSZ coating are enhanced [88e90]. Kim et al. [91] reported that ceric oxide-YSZ coating has excellent thermal shock resistance features as compared to YSZ coating. This is due to a higher value of the thermal expansion coefficient, little phase transformation, and stress produced. It was also found that there is a decline in hardness and stoichiometry change in the ceric oxide-YSZ coating. The reason behind it may be the vaporization of ceric oxide material [88]. It may be also due to the conversion of ceric oxide to dicerium trioxide and the increase in the rate of sintering of ceric oxide-YSZ coating [92,93]. Mukherjee et al. [94] studied and compared YSZ, ceria-YSZ, and alumina-YSZ coatings for their performances. During this investigation, hot corrosion tests were conducted for these coatings and analyzed. Ceria-YSZ coatings are more resistant tothe infiltration of the molten salt as compared to YSZ and alumina-YSZ coatings. This investigation also revealed that ceria-YSZ coatings have excellent thermal shock resistance as compared to YSZ and alumina-YSZ coatings [94]. Paik et al. [95] fabricated YSZ and ceria-YSZ coatings with the help of the plasma spraying method and applied them to parts of the combustion chamber of a diesel engine. It was highlighted that release of NOx is found to be more in ceria-YSZ coatings than in uncoated pistons of diesel engines [95].

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Islam et al. [96] reported that cerium oxide (ceria) is a promising material for HRFRCCs to be applied on hot sections of engines due to its significant features. It was highlighted that ceria-YSZ coatings are fabricated on the metallic substrate using the plasma spraying method. In this investigation, the efficiency of YSZ and ceria-YSZ coatings is compared and analyzed. The experimental results indicated that the thermal insulation and resistance to CMAS infiltration provided by ceria-YSZ coatings were enhanced by 45% as compared to traditional YSZ coatings due to the formation of CeAlSiO [96].

7.5

Lanthanum zirconate

Many research investigations examined lanthanum zirconate (LZC) as a potential candidate for the fabrication of HRFRCCs to be applied on hot sections of engines [58,63,97e101] LZC possesses a pyrochlore structure. It has the general formula A2B2O7 [102]. LZC is found to be better than 8% YSZ coating because of its salient features such as low thermal conductivity, low oxygen ion diffusivity [98], no phase transformation, and higher sintering resistance. This shows that LZC coatings are having good phase stability. They are thermally stable. LZC possesses low fracture toughness as compared to 8% YSZ coatings. As a result, cracks are formed in LZC coatings at drastic conditions. Hence, different techniques are utilized to avoid cracks and enhance the toughness of LZC coatings. Many researchers suggested composite coatings to resolve this issue. A literature survey also revealed that multi-layer gradient coating architectures can also enhance the toughness of LZC coatings. Cao [101] reported that YSZ coatings have higher thermal expansion coefficient values as compared to LZC coatings [101]. In order to enhance the thermal cycling life of LZC coatings, CeO2 is used to replace ZrO2. As a result, there is an improvement in the thermal expansion coefficients as well as the fracture toughness of CeO2-substituted LZC coatings. It was also suggested that doped LZC has better fracture toughness and long life. Many researchers are investigating the effect of different dopants and their doping ratios on the fracture toughness of LZC coatings. Doped LZC coatings possess a higher coefficient of thermal expansion, which decreases residual stress to enhance the stability of the coatings at drastic conditions [63]. Cihan et al. [103] utilized rare earth elements for doping into LZC to form a top coat using the APS technique, along with ceria-YSZ material as the second material for double coat. During this investigation, tests were conducted at different engine speeds. The parts of cylinder along with the piston are decorated. The experimental data showed that the efficiency of the engine was enhanced at higher pressure and temperature for doped LZC-ceria-YSZ decorated chamber. It was also reported that the performance of doped LZC-ceria-YSZ coatings is excellent due to the maximum torque of coated engines [103]. Cheng et al. [104] studied the sintering behavior of HRFRCCs using five different LZC-YSZ materials. The experimental data indicated the vertical cracks on 8% of YSZ HRFRCCs after 2200 cycles [104].

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7.6

169

Silicates

Chraska et al. [105] reported spraying of ZrSiO4 for HRFRCCs. Thereafter, Ramaswamy et al. [106] highlighted the application of zircon sand for HRFRCCs. Zircon is a natural sand that is found in Kerala state of India. It is a mixture that mainly contains ZrSiO4 as the major phase. There are many metal oxide impurities present in zircon sand. This sand is widely used for many applications [107]. A literature survey revealed that ZrSiO4 breaks down into ZrO2 and SiO2 during plasma spraying [108e111]. Ault [109] observed involvement of c-ZrO2 during flame spraying of zircon. Chraska et al. [111] found c-ZrO2, m-ZrO2, and t-ZrO2 on the plasmasprayed zircon. This investigation highlighted that the rate of cooling decides the formation of modifications such as c-ZrO2 or t-ZrO2 on the plasma-sprayed zircon [110]. Ramaswamy et al. [112,113] investigated zirconia coatings and checked their applications in hot sections of diesel engines. The experimental results indicated that zirconia coatings are found to be more stable at drastic conditions of temperature, stress, and chemical atmosphere in hot sections of diesel engines. Calosso and Nicoll reported that SEM characterization of zirconia shows narrow particle size and round shape. This played a vital role in plasma spraying zirconia coatings [114]. Subbarao, in his overview of zirconia, highlighted that an increase in volume is observed when the tetragonal phase transforms into the monoclinic phase in zirconia [115]. Heuer and Ruhle noticed that the dissolution of magnesium oxide and calcium oxide in ZrO2 helps to stabilize the tetragonal phase [116]. ZrO2eSiO2 recombination reaction is possible due to rapid heating and annealing. The values of rate of reaction are found to be higher because of the Hedvall effect [117]. P. Vincenzini reported that the tetragonal phase of ZrO2 plays a vital role in HRFRCCs for engine applications [118]. However, the tetragonal phase is not found stable at drastic conditions of temperature for a longer time. The addition of Y2O3 in t-ZrO2 was suggested to stabilize the zirconia coating. A suitable amount of natural zircon sand and Y2O3 were added to obtain 8 wt% Y2O3 in ZrO2. Ramaswamy et al. studied the mixture of natural zircon sand8 wt% Y2O3 (ZS8Y) and carried out characterization using different sophisticated techniques [106]. The XRD pattern of this sample ZS8Y shows peaks for tetragonal ZrO2, ZrSiO4, and Y2O3. The analysis of the XRD pattern indicates that zircon breaks down into tetragonal ZrO2. M. Matsui et al. reported that SEM images of ZS8Y show properly formed grains [119]. The characterization of ZS8Y using EDX shows 85% ZrO2, 8% Y2O3, and 5% silica. It was also reported that Y2O3 is found to be dissolved in ZrO2 at very high temperatures (1400
C). This stabilizes the tetragonal phase of ZS8Y. This ZS8Y is also found to be nonreactive toward silica. On the contrary, magnesium oxide-stabilized ZrO2 is found to be unstable at drastic conditions of temperature [120]. This unstable ZrO2 reacts with silica to produce zirconium silicate. Zircon is found to be chemically inert. It has higher thermal conductivity. Generally, it does not react with any acids except HF. It reacts with bases at higher conditions of temperature. There is no sharp melting point for zircon since it undergoes dissociation before melting. Zircon is found to be unstable at a higher temperature of 1949 K or a lower temperature of 1585 K, which depends on the purity of the sample [121]. I. Kvernes et al. reported that plasma-sprayed ZrSiO4 breaks down into ZrO2 and silica.

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The amorphous silica is problematic. It may be because of the evaporation of SiO and Si(OH)2 [122]. N. Mifune et al. utilized and examined composite oxides for coatings in hot sections of gas turbine vanes. This investigation indicated that composite oxides are promising materials for silicate coatings due to their outstanding thermal shock resistance and anticorrosion features [123]. Liu et al. [124] reviewed YSZ-ZrO2 coatings and different fabrication methods. In this review paper, the properties of YSZ-ZrO2 coatings are discussed. This review paper highlighted the different materials used for coatings and the challenges faced by research community to fabricate YSZ-ZrO2 coatings [124]. Boissonnet et al. [125] investigated YSZ-zirconia, which was fabricated using the plasma spray method. The experimental data indicated deep sintering with CMASs deposits. As a result, the thermal diffusivity of CMASss was enhanced [125]. Wang et al. [126] synthesized different mole percentage Y2SiO5eZrSiO4 coatings at a temperature of 1500
C. During this work, the effect of ZrSiO4 dopant concentration on the structure, sintering rate, and features of Y2SiO5eZrSiO4 was studied. The experimental data indicated that initially there was an increase in toughness and elastic modulus of coatings, and thereafter the values declined with an increase in ZrSiO4 dopant concentration [126].

7.7

Rare earth oxides

Rare earth oxides (REOs) are inexpensive and easily accessible. REOs such as lanthanum (III) oxide, ceric oxide, praseodymium (III) oxide, and niobium (V) oxide can be used for HRFRCCs to be applied in extremely hot sections of gas engines. REOs coating exhibits low thermal diffusivity as compared to zirconium dioxide. Ding et al. revealed that REOs have high thermal expansion coefficient as compared to zirconium dioxide [127]. I. Warshaw and Roy noticed that REOs are found to be polymorphic at drastic conditions of temperature [128]. It adversely influences the thermal shock resistance feature of REOs coatings. Zhou et al. [129] studied and analyzed high-entropy rare-earth zirconate coatings (HEREZCs) thoroughly. LZC was chosen as reference material for these HEREZCs. The experimental data revealed that there is an enhancement in the thermal stability and thermal shock resistance of HEREZCs. It was highlighted that HEREZCs exhibit fluorite structure [129]. Zhen et al. [130] studied the thermal cycling performance of rare earth oxides codoped zircon dioxide HRFRCCs using the EB-PVD method. During this study, different parameters of HRFRCCs were investigated thoroughly and analyzed. The experimental data indicated that rare earth oxides codoped zircon dioxide HRFRCCs can withstand very high temperatures without any change in phase. The synthesized HRFRCCS are having pyramidal shapes [130]. Ogawa et al. [131] studied the low thermal conductivity of double perovskite rareearth tantalates. These materials are having small thermal conductivity and are found to be stable at drastic conditions of temperature. The nanostructure size of spontaneously formed materials was noted by performing characterization of materials [131].

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7.8

171

Metal-glass composite

There are many demerits of ZS8Y coatings on hot sections of engines. The choice of HRFRCC materials is very significant [132]. ZS8Y is responsible for poor adherence [133], crack formation [134], low durability, less stability, phase transformations, volume change, TGO [135] formation, and spalling of the coatings. MGCs are studied, examined, and utilized for the HRFRCCs in different applications. MGC is utilized to enhance thermal expansion match with the metallic substrate [136]. This is done by changing the proportion of metal/glass materials. MGC is also used to enhance the adherence of coating on the metallic substrate. MGC is superior than ZS8Y materials for HRFRCCs due to a lack of porosity. It protects metallic substrate as well as bond-coat from the action of corrosive gases. So, MGC avoids the oxidation of bond-coat which prevents TGO formation. MGC contains normal flat glass and MCrAlY alloy with a higher Al percentage. MGC is a new approach for HRFRCCs [136,137] in which this complex mixture is coated on a metallic substrate by plasma spraying under vacuum. Cao et al. [47] reported that MGC coatings offer higher durability and better efficiency as compared to ZS8Y coatings. The experimental results were found superior for thermal cycling tests [47]. Li et al. [138] investigated the influence of crystallization on the mechanical features of HRFRCCs, which were synthesized by using ceramic-metal-glass coatings. The experimental results implied that the mechanical features of coatings at microlevel are enhanced as compared to the macro-level. The hardness was found to be more at the nano-particle level [138]. Qiaolei Li et al. [139] utilized alumina-titania ceramic-glass metal composite for the fabrication of HRFRCCs. In this work, characterization of synthesized HRFRCCs was carried out using sophisticated techniques for the elemental and microstructure analysis. In this study, the fracture toughness of HRFRCCs was studied and found to be excellent as compared to traditional gradient coatings. The thermal spraying method was used to fabricate HRFRCCs made up of alumina-titania-glass metal [139].

7.9

Garnet ceramics

Nitin et al. [140] suggested another material, Y3AlxFe5-xO12 known as garnet ceramics (GC), for HRFRCCs. This investigation studied and analyzed the influences of different parameters, such as composition and temperature on the values of thermal conductivity of coatings and found them to be low. GC are utilized in different fields due to its salient features [141,142]. Y3Al5O12 (YAO) is found to be a promising material for HRFRCCs due to its salient features such as outstanding phase and thermal stability. YAO withstands very high temperatures 2243 K [143e145]. It possesses lower thermal conductivity. It was also reported that YAO is found to be higher oxygen-resistant as compared to ZrO2 coatings. As a result, it avoids the oxidation of bondcoat and enhances the durability of the coating. The experimental data revealed that oxygen diffusivity is around 10 folds higher in ZrO2 than in YAO coatings. It was also highlighted that lower values of melting point and thermal expansion coefficient restrict the utilization of this material in many applications.

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Zhou et al. [146] synthesized and studied Y3þxAl5O12 (YAlO) ceramic materials. During this research work, the microwave dielectric features of YAlO were investigated and analyzed. In addition, microstructure and phase composition features of synthesized YAlO were studied by varying cation compositions. The experimental data revealed that a small quantity of Y2O3 helps to reduce the internal stresses of coatings [146].

7.10

Zirconates

Vassen et al. [98] suggested zirconates as novel materials for HRFRCCs due to salient features. Different types of zirconates, such as LZC, strontium zirconate (SZC), and barium zirconate (BZC), which are having higher melting points were prepared and sintered. These prepared zirconates exhibit low-sintering activity. In this investigation, thermal expansion coefficients for zirconates were calculated and found to be low as compared to YSZ coatings. The experimental data shows that the thermal conductivity values of BZC and SZC are found to be higher as compared to YSZ coatings. It was noticed that LZC shows low thermal conductivity. During this investigation, it was reported that SZC underwent phase transformation between 700
and 800
C. Therefore, SZC is not a very promising material for HRFRCCs. Indentation methods were applied to examine important mechanical features of BZC and LZC. The experimental data indicated that YSZ has higher hardness and Young’s modulus than BZC and LZC. BZC and LZC materials were utilized for the fabrication of HRFRCCs by plasmasprayed techniques. It was also reported that a huge attack was observed in the case of BZC coating at 1200
C temperature. Due to this, there was a loss of barium oxide. On the contrary, LZCs are found to be a potential candidate for HRFRCCs due to their exceptional thermal stability as well as thermal shock resistance. Liu et al. [147] studied and analyzed the thermal as well as mechanical features of perovskite barium zirconate (PBZ). The experimental data revealed that PBZ is very stable from 25
C to its melting point. PBZ is considered a potential material in different fields at drastic conditions of temperature. PBZ exhibits lower thermal conductivity as compared to other ceramic materials. This paper highlighted that thermal conductivity values for BZC are higher at a lower temperature (298 K) and lower at a higher temperature (1473 K) [147]. Liu et al. [148] synthesized SrZrO3eLa2Ce2O7 composites as HRFRCCs and characterized them using different sophisticated methods. In this research work, coprecipitation-calcination process was used to fabricate HRFRCCs. The experimental data revealed that SrZrO3eLa2Ce2O7 composite exhibit lower thermal conductivity than 8% YSZ. This investigation concluded that SrZrO3eLa2Ce2O7 composite is a promising candidate for the fabrication of HRFRCCs as compared to 8% YSZ material. The fracture toughness was found to be more in SrZrO3eLa2Ce2O7 composite as compared to SrZrO3 and La2Ce2O7 [148].

7.11

Lanthanum aluminates

Friedrich et al. [149] projected the new material lanthanum hexaaluminate for HRFRCCs. This type of new coating contains lanthanum (III) oxide, alumina, and

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magnesium oxide. Lanthanum aluminates (LA) coating is found to be thermochemically stable at temperature 1673 K [63,149,150]. The experimental results indicated that the rate of sintering is higher in YSZ coatings as compared to LA coatings. The microstructure of LA coatings shows lower thermal conductivity. Gadow and Lischka [151] proposed LA as a promising material for HRFRCCs in gas turbines. LA which has a magnetoplumbite structure, is found to be a potential candidate for the fabrication of HRFRCCs as compared to YSZ material. LA can be utilized beyond 1300 C due to its higher thermal stability as compared to YSZ. Vourdas et al. [152] studied Perovskite and tailored its mechanical features. In this research work, LA was synthesized to study its mechanical features for its application in HRFRCCs. The characterization of synthesized materials indicated the perovskite structure of LA. During this investigation, the mechanical and fracture features of LA were studied and analyzed thoroughly. Elastic modulus was recorded along with ultimate stress. Apparent porosity was also estimated in this study [152]. Sun et al. [153] studied the influences of laser-glazing to understand and analyze the microstructure, elemental composition, and surface properties based on experimental data obtained from characterization of samples. This investigation highlighted the fact that LaMgAl11O19 is found to be thermally more stable. It has superior interface compatibility. This investigation also revealed that lanthanum magnesium aluminate (LMA) shows enhanced CMAS resistance at a higher temperature. Lanthanum, magnesium, and aluminum from the LMA layer arrested the molten CMAS successfully [153]. Khan et al. [154] studied the mechanical features of LaMgAl11O19/YSZ. The experimental tests were conducted and analyzed for LaMgAl11O19 and dual-layered YSZ. It was observed that LaMgAl11O19 and YSZ coating materials exhibit higher thermal stability and excellent thermal shock resistance. It was concluded that laser glazing improves the thermal behavior of HRFRCCs. This work suggested that LaMgAl11O19/ YSZ material can be utilized for coating hot parts of gas engines [154]. Shishkin et al. [155] studied SrCe0.95M0.05O3 (SCMO), where M ¼ praseodymium, yttrium, lanthanum, and tin. SCMO was utilized for the fabrication of HRFRCCs to be operated at an extremely hig temperature. The experimental data implied that the smaller size of dopant affects the thermal expansion coefficient of synthesized materials. The small size of SCMO decreases the sintering rate and average grain size of particles. The bigger size of dopants affects the anticorrosion property of SCMO as silicates and aluminates are formed. It was observed that tin dopant SCMO is not found stable due to the leaching of the element tin. This research work highlighted a better thermal expansion coefficient, smaller average grain, porous nature, and higher hardness of SrCe0.95Y0.05O3 which makes it a potential candidate for the fabrication of HRFRCCs [155].

7.12

(Ca1-XMgX) Zr4 (PO4)6

Hirschfeld et al. investigated and tested CMZP for fabricating HRFRCCs [156]. The experimental data showed the value of the thermal expansion coefficient is around zero for CMZP. In addition, CMZP possesses low thermal conductivity as compared to zirconium dioxide. CMZP is found to be lighter as compared to zirconium dioxide.

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There was not any strength loss found after air quenching from higher temperatures. It indicates better a thermal shock-resistance property exhibited by CMZP coatings.

7.13

Lanthanum (III) phosphate

Monoclinic LaPO4 (monazite) has a higher melting point. It is not soluble in water. Morgan and Marshall [157] projected a monoclinic LaPO4 (monazite)-alumina (LPA) composite for HRFRCCs at drastic conditions of temperatures of 1750
C in air. The LPA composite exhibits lower fracture resistance. LPA coating is found to be stable at drastic conditions of temperature in oxidizing as well as reducing chemical surroundings [157]. Sudre et al. projected LP as a promising material for coating the surfaces of nickel based superalloys [158]. In this investigation, the mechanical features of LP were examined thoroughly. The experimental results of this investigation reported that LP is more thermally stable than YSZ. The thermal expansion coefficients of LP are found to be higher as compared to YSZ. LP is an anticorrosive material. This study concluded that LP is a suitable candidate to replace zirconium dioxide for thermal insulation coating. LP coating was decorated on a substrate using EB-PVD and laser ablation methods. The different conditions required for good adherence to LP coating on the surface of the substrate were analyzed and discussed. Foroushani et al. [159] studied YSZ and YSZ/LP coatings with different percentages, such as 10% and 20% LP. In this investigation, the atomic plasma spray process was utilized for the fabrication of coatings. The characterization of synthesized HRFRCCs indicated a higher porous structure. The experimental data implied that sintering resistance was found to be higher in YSZ-LP coating as compared to YSZ coating. This study also revealed that the TGO layer is thicker in YSZ-LP coating as compared to YSZ coating [159]. Guo et al. [160] fabricated LP/YSZ HRFRCCs using the air plasma spray technique. During the investigation, CMAS corrosion behavior of LP/YSZ HRFRCCs was analyzed at 1250
Ce1350
C. This research revealed that LP/YSZ HRFRCCs are more resistant toward CMAS infiltration at the temperature of 1250
C. It may be due to the sealing layer that is formed. The experimental data highlighted that CMAS is less viscous at higher temperatures, which helps to promote infiltration [160]. Sun et al. [161] studied and utilized LP for the improvement of features of ceramics. LP acts as a toughening agent. In this research work, the different features, such as compatibility and thermal conductivity, of LaMgAl11O19/LP are studied from 25
C to 1600
C. The characterizations of materials were carried out to study the structure and chemical composition and thermal conductivity. The experimental data implied that thermal conductivity enhances at the temperature of 1600
C. It may be due to the diffusion of lanthanum changing the composition of LP [161].

8.

Conclusions

There are many materials available as elaborated and discussed in this present chapter, for the fabrication of HRFRCCs to withstand extremely high temperatures. These

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materials provide better thermal insulation, higher durability, more efficiency, higher resistance to corrosion, and higher stability to HRFRCCs. Ceramic coatings with higher heat resistance and fire retardant features can be used in burners, furnace parts, heat interchangers, etc. Ceramic coatings can be decorated on the surface of mild steel to protect the exhaust systems of aircraft. There should be no cracking and chipping at very drastic conditions of temperature. These ceramic coatings should be thinner and antioxidative. It should strongly stick to metallic substrates. There are different processing techniques available as per the requirements of HRFRCCs. APS and EBPVD techniques are widely used. APS is preferred over EB-PVD due to its costeffectiveness and low thermal conductivity. The mechanical features of HRFRCCs can be tailored using suitable ceramic materials and appropriate deposition techniques. HRFRCCs using different materials and various deposition techniques for coating on metallic substrate undergo many changes, such as a change in chemical, phase transformation, and microstructure, at operating conditions. The speed of different processes enhances exponentially with an increase in temperature. As a result, HRFRCCs undergo degradation, which is responsible for their failure and limits their applications in different fields where extreme temperatures are involved. Mainly, there are three factors, such as bond-coat degradation, higher residual stress, and top coat degradation, are responsible for the failure of HRFRCCs at extremely high temperatures. However, there is an urgent need for research in this field to design and develop advanced ceramic materials and their composites for enhancing the mechanical features of HRFRCCs using appropriate deposition processes to withstand extremely hightemperatures without any failure in degradation, stability, efficiency, reliability, and durability of coatings. It is also highlighted that serious issues such as degradation mechanisms and factors responsible for the failure of HRFRCCs should be addressed while designing and developing future-generation advanced ceramic materials.

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[138] Q. Li, P. Song, Q. Dong, et al., Effect of partial crystallization of an amorphous layer on the mechanical properties of ceramic/metal-glass coating by thermal spraying, Ceramic International 45 (2019) 18803e18813. [139] Q. Li, P. Song, K. Lu, et al., Fracture behaviour of ceramicemetallic glass gradient transition coating, Ceramic International 45 (2019) 5566e5576. [140] P.P. Nitin, P.G. Klemens, Low thermal conductivity in garnets, Journal of American Ceramic Society 80 (1977) 1018e1020. [141] S. Geller, Crystal and static magnetic properties of garnets, in: A. Paoletti (Ed.), Physics of Magnetic Garnets, North-Holland Publishing Co., Amsterdam, Netherlands, 1978, pp. 1e55. [142] G. deWith, Translucent Y3Al5O12 ceramics: something old, something new, in: P. Vincenzini (Ed.), High Tech Ceramics, Elsevier, Amsterdam, Netherlands, 1987, pp. 2063e2075. [143] T.A. Parthasarathy, T.-I. Mah, K. Keller, Creep mechanism of poly crystalline yttrium aluminum garnet, Journal of American Ceramic Society 75 (1992) 1756e1759. [144] J.D. French, High Temperature Deformation and Fracture Toughness of Duplex Ceramic Microstructures, Ph.D. Thesis, Lehigh University, Bethlehem, PA, 1993. [145] J.S. Abell, I.R. Harris, B. Cockayne, et al., An investigation of phase stability in the Y2O3-Al2O3 system, Journal of Materials Science 9 (1974) 527e537. [146] M. Zhou, H. Chen, X. Zhang, et al., Phase composition, microstructure, and microwave dielectric properties of non-stoichiometric yttrium aluminium garnet ceramics, Journal of the European Ceramic Society 42 (2022) 472e477. [147] Y. Liu, W. Zhang, B. Wang, et al., Theoretical and experimental investigations on high temperature mechanical and thermal properties of BaZrO3, Ceramic International 44 (2018) 16475e16482. [148] Y. Liu, Y. Bai, E. Li, et al., Preparation and characterization of SrZrO3-La2Ce2O7 composite ceramics as a thermal barrier coating material, Materials Chemistry and Physics 247 (2020) 122904. [149] C.J. Friedrich, R. Gadow, M.H. Lischka, Lanthanum hexaaluminate thermal barrier coatings, in: M. Singh, T. Jessen (Eds.), The 25th Annual International Conference on Composites, Advanced Ceramics, Materials, and Structures: B (Cocoa Beach of Florida, Jan. 2001), American Ceramic Society, Westerville, OH, USA, 2001, pp. 372e375. [150] R. Gadow, G.W. Schafer, Thermal Insulating Materials and Method for Producing Same, German Patent No, 1999. WO 99/42630. [151] R. Gadow, M. Lischka, Lanthanum hexaaluminate-novel thermal barrier coatings for gas turbine applications-materials and process development, Surface and Coatings Technology 151-152 (2002) 392e399. [152] N. Vourdas, E. Marathoniti, P.K. Pandis, et al., Evaluation of LaAlO3 as top coat material for thermal barrier coatings, Transactions of Nonferrous Metals Society 28 (2018) 1582e1592. [153] Y. Sun, H. Wu, H. Zhao, et al., High-temperature degradation of the in-situ laser-glazed plasma sprayed LaMgAl11O19 thermal barrier coating exposed to Ca-Mg-Al-silicate deposits, Corrosion Science 176 (2020) 108934. [154] M.A. Khan, V.A. Annakodi, D. Muthukannan, et al., Thermal shock resistance and thermal insulation capability of laser-glazed functionally graded lanthanum magnesium hexaluminate/yttria-stabilised zirconia thermal barrier coating, Materials 14 (2021) 3865. [155] R.A. Shishkin, O.G. Reznitskikh, A. Yu Suntsov, et al., Properties of SrCe0.95M0.05O3 (M ¼ La, Pr, Y, Sn) thermal barrier materials, Ceramic International 48 (2022) 27003e27010.

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Photocatalytic ceramic coatings

8

Aarti S. Bhatt Department of Chemistry, NMAM Institute of Technology, Affiliated to NITTE (Deemed to be University), Nitte, Karnataka, India

1. Introduction Ceramics are an essential class of materials that find extensive applications in the fields of automotive, electronic, aerospace, and personal protection. Among the various ceramic products available, ceramic tiles are known for their strength and toughness, making them ideal for diverse usage in hospitals, restaurants, offices, and residential and commercial areas. However, the surface cleaning of the ceramic tiles demands time, labor, and the excessive use of chemical detergents. This can pose a major challenge to the economy as well as the environment. The past decade has seen an advent in modern technology with much of the research in the ceramic field directed toward developing self-cleaning ceramic tiles that are more durable and long-lasting. The most feasible approach to achieve self-cleaning properties is applying photocatalytic coating on the ceramic surface. With the help of natural sunlight and oxygen and moisture present in the atmosphere, the photocatalytic tiles possess the ability to keep the pollutants at bay and thereby create a cleaner environment at an affordable cost.

2. The science behind the photocatalytic technology Photocatalysis is basically the conversion of solar energy to chemical energy in presence of a catalyst. Semiconductors, such as metal oxides, are preferably employed as photocatalysts. The interaction of light energy with the photocatalysts, leads to the absorption of photons, resulting in the generation of electron-hole pair. The electron migrates to the conduction band to oxidize the atmospheric oxygen, while the hole stays in the valence band to reduce the atmospheric moisture. The end result of the simultaneous redox reaction is the production of highly reactive hydroxyl (OH•) and superoxide (O
• 2 ) radicals that are substantial in the decomposition of complex organic matters such as hydrocarbons, aromatics, phenolic compounds, dyes, etc. into simpler molecules such as CO2 and H2O (Fig. 8.1). The fate of the electron-pair duo depends on the separation of the valence and conduction bands of the photocatalyst; with a risk of recombination for shorter separation. Metal oxides, due to their semiconductor nature, possess electronic structure and charge transport characteristics desirable of a photocatalyst [1,2]. TiO2 is the most popular and widely used photocatalyst due to its low energy band gap and high stability [3]. Moreover, the superhydrophilicity and visible light transparency of TiO2 makes it Advanced Ceramic Coatings. https://doi.org/10.1016/B978-0-323-99659-4.00001-2 Copyright © 2023 Elsevier Ltd. All rights reserved.

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Figure 8.1 Depiction of photocatalytic pathway undertaken by TiO2 photocatalyst when subjected to sunlight. The electrons in the conduction band and the holes in the valence band undergo reduction and oxidation respectively to finally convert the pollutants into harmless compounds. Reproduced from Applied Catalysis B 176e177 (2015) 396e428 with permission from Elsevier.

all the more desirable for surface coating applications. Additionally, TiO2 is also wellknown for its antibacterial activity. Out of the three stable crystal structures of TiO2, viz., rutile, anatase, and brookite, the anatase phase is found to be the most active photocatalytic material [4]. A major hindrance in the performance of TiO2 is its wide band gap (3.2 eV), which curtails the instant separation of electrons and holes postgeneration, thereby dampening its quantum efficiency and also restricting the employment of solar light for photocatalysis. A likely solution is to introduce a dopant in the TiO2 structure, which may possibly introduce a new donor/acceptor level, thereby allowing the charge to transport under visible light. Dopants such as transition metal ions [5,6], rare earth metal ions [7e9], and even non-metals [10] have been attempted. In several cases, the inclusion of dopant has also reportedly shown stronger surface adhesion. Unfortunately, the photocatalytic results obtained for TiO2 studied in suspension and on the final coating do not match well. This is because, during the sintering of TiO2, much of the surface active sites are destroyed. The few literatures that report the successful photocatalytic coating of doped TiO2 on ceramic tiles have used Ag [11], Zn [6], W [12], and Nb2O5 [2,13] as dopants.

3.

Deposition techniques

The deposition technique is an important criterion for achieving a highly stable and efficient photocatalytic coating on the ceramic surface. The choice of the deposition

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technique decides the final thickness and surface roughness of the coating. It is also possible to control the concentration of TiO2 present in the coating layer during deposition. While these parameters are desirable for good photocatalytic performance, external features such as texture, brightness, transparency, and color improve the market value of the end-product. With the advent of modern technology, the tile industry is showing interest in developing ceramics possessing high durability, antifouling, and self-sterilizing properties. In the following section, some of the common deposition techniques employed for the fabrication of photocatalytic coatings on ceramic tiles have been discussed. These do not include liquid-phase deposition [11] and dip coating techniques [14], as they are not considered versatile enough to be adapted at the industrial level. Also, the working difficulties associated with these techniques viz. long-time immersion and unnecessary dual-side coatingdmake them less approachable. i) Spraying: Spraying is the most widely employed method for industrial deposition of coatings on the ceramic tile surface due to the simplicity and cost-effective operation of this method. Spraying of the metal oxide suspensions on the glazed ceramic surface generally results in a coating of 0.1e3.0 mm thickness, which is considered to be the thinnest coating possible when compared to other techniques [15e17]. Though this method gives the freedom to regulate the amount of metal oxide depositing on the ceramic, there lies a large scope for better optimization of the parameters so as to achieve a uniform and continuous film rather than the habitually obtained grid-fashioned and discontinuous pattern of deposit. At the same time, the photocatalytic activity of the coating will depend on the TiO2 used (commercially available or synthesized), annealing temperature, and also the agglomeration and diffusion processes that may occur during deposition. ii) Screen printing: Although the screen-printing technique was developed in the early 1970s, its employment for coating the ceramic surface has garnered interest in recent times. This technique produces a film with a thickness of 5.0e35.0 mm [16,17]. Though coatings with higher thickness are less favorable, this method enjoys the benefit of generating a rough coating surface with granular texture, which implies a higher active surface area for photocatalysis. It is not surprising that screen-printed photocatalytic coatings on ceramic tiles display better functional properties, but at the same time, it becomes necessary to keep a check on the surface morphology as excessive roughness can make cleaning process tedious. iii) Inkjet printing: Inkjet printing is a modern, fast process technique that is most suitable for generating ceramic tiles with attractive, colorful, and designer coatings. The noteworthy feature of this technique is being a no-contact coating process, it can be carried out on any kind of surfacedhard, rough, glazed, or unglazed. This technique gives a film with an approximate thickness of 70.0 mm with a nonuniform coating of titania on the ceramic surface. A high temperature annealing is shown to smoothen the surface and also improve the hydrophilicity [18]. iv) Roller printing: Unlike the inkjet printing technique, roller printing involves the transfer of the ink by direct contact to the ceramic surface. Though the hydrophilic nature of the final material is similar to that obtained through inkjet printing, the thickness of the coating layer generally exceeds 70.0 mm and may reach a value as high as 100 mm which is quite undesirable [18]. Further, the coating overall has an imprinted gridlike appearance with empty spaces at regular intervals. For efficient photocatalytic performance, it becomes essential to upgrade the current technique so as to obtain a thinner and uniform film covering the entire ceramic surface.

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Other than the above-mentioned common techniques, deposition of titania on the ceramic surface is also achieved by mixing fine powders of titania in glazing suspension. However, it becomes difficult to control the titania concentration as most of it gets dissolved in the molten glaze during firing. Thus, as can be observed, each method has its own merits and demerits. It is demanding to strike a balance between the various operating parameters so as to obtain a coating with thin, uniform, and high surface active site, leading to better photocatalytic activity. Besides, the thermal sintering carried out on the ceramic tiles in terms of temperature (ideally 650e800 C) and firing exposure time is also crucial in achieving the desirable TiO2 anatase phase with rich surface active sites and a highly crystalline structure [19e21]. Also, the upgradation of laboratory-scale synthesis to the industrial level without cutting down on the performance efficiency and final appearance (with respect to color, gloss, cleanability, roughness, and durability) is another major challenge. Though surface modification of ceramic tiles was initially limited to glazing and/or a decorative purpose, the demand for coatings with antimicrobial and antifouling properties is rising, especially after the post-pandemic time.

4.

Properties of the photocatalytic ceramic coatings

The main intention of imparting a photocatalytic coating on the ceramic surface is to achieve self-sanitizing properties. For this, other than a high photocatalytic action, superhydrophilicity also becomes a major contributing factor (Fig. 8.2). An increased wettability assures the formation of a thin water layer washing away any kind of dirt accumulating on the ceramic surface. It is always a mammoth job to create a fine balance between the superhydrophilicity and photocatalytic superiority of a given ceramic coating. There are recognized standard procedures available to evaluate the photocatalytic efficiency of the ceramic tiles, such as ISO 10678 (based on the degradation of methylene blue (MB) and UNI 11259 (based on the degradation of Rhodamine B). The degradation of MB dye is the most widely tested technique for assessing the photocatalytic activity on the ceramic tile surface. Basically, the surface is subjected to UV light, and the difference in the specific degradation rate of MB in the presence and absence of UV light is reported. On the other hand, photocatalytic studies with several deviations with respect to light source intensity, dye concentration, and exposure time have also been reported. As an example, it is shown that a sample irradiated with a 10 Wm
2 UV light source could degrade 70% of the total MB, while at a lower intensity of 3 Wm
2, the efficiency plummeted to a mere 30% [22]. Other than the experimental parameters, the nature of the titania deposited on the ceramic also plays a crucial role in governing the photocatalytic activity. Most of the literature claims the anatase phase of TiO2 to be the most suitable for high photocatalytic efficiency [4,23]. A major reason is the availability of highly dense localized states favoring a high-level surface adsorption of OH• radicals. A large band gap also restricts electron-hole recombination. A comparative study between the anatase and rutile phases of TiO2 has shown that

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Figure 8.2 Images depicting UV light induced (A) and (B) hydrophilicity and (C) and (D) antifogging effect of the TiO2 surface. Reproduced from Applied Catalysis B 176e177 (2015) 396e428 with permission from Elsevier.

the O
and Oe 2 species substantially undergo photosorption on the anatase TiO2 while the rutile TiO2 has a poor hold on the ionic species [23]. A standard procedure to assess the hydrophilicity of a surface is provided in ISO 27448. Herein, the basic concept is to measure the contact angle between the water molecule and the metal oxide film surface. An angle below 5 degrees is considered optimum for a superhydrophilic feature [18,24]. An enhancement in the wettability is possible by the presence of TiO2. On exposure to UV light, titanium ions can undergo reduction to generate holes. The so-generated holes can oxidize the Oe 2 ions thereby creating oxygen vacancy. The oxygen vacancies serve as probable sites for water occupancy, making the surface hydrophilic [19,20,25,26]. Alternatively, the generation of oxygen vacancies can be further intensified by doping TiO2 with metal ions such as Ni2þ and Fe3þ [27]. A high wettability confers self-cleaning and antifogging attributes to the coating surface [1,13]. However, the hydrophilicity is also dependent on the roughness of the coating surface. It has been observed that a rough surface exhibits better hydrophilic behavior than a smooth surface [18,28]. In many modern civil structures, technology based on photoinduced superhydrophilicity has been adapted. For instance, Hydrotech first breaks down the pollutants on the ceramic surface under the influence of UV light, which can be then easily removed by giving water wash (Fig. 8.3). Nowadays, TiO2 coating is also being analyzed for its good antibacterial response. ISO 27447 is a standard procedure that studies the elimination of bacteria when

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Figure 8.3 Illustrative depiction of the self-cleaning mechanism of photocatalytic active surface (from left to right) step 1: accumulation of dirt and pollutants on the surface; step 2: photoinduced degradation of the accumulated dirt particles; step 3: washing off of the by-products by rain water. Reproduced from Applied Catalysis B 176e177 (2015) 396e428 with permission from Elsevier.

subjected to UV radiation [29]. As discussed earlier, the interaction of UV light with TiO2 generates a large amount of radical species, which have the ability to rupture the cell wall and cell membrane of these microorganisms, thereby denying bacterial growth. Here too, the anatase phase of TiO2 has been reported to confer a higher antibacterial activity [30]. Furthermore, doping TiO2 with N, S [31,32], Ag [30], carbon [33], and even quaternary ammonium salts [34] have shown to enhance the antibacterial activity.

5.

Conclusion and future challenges

With an urge to lead a greener and healthier life, the tile industry is devoted to developing coated tiles with photocatalytic, self-cleaning, and antibacterial properties. This chapter gives a brief overview of the common deposition techniques of TiO2 on ceramic tile surfaces, which have an influential impact on the coating properties. Though spraying is the most widely accepted approach, there exists a possibility of fine-tuning the technique and finding a reasonable solution to achieve favorable thickness, roughness, and particle size distribution. These parameters are very crucial in determining the photocatalytic activity, hydrophilicity, and antibacterial properties

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of the coating surface. The literature gives ample evidence on tailoring and improvising these properties through doping or by bringing about phase transitions. However, all of the study is restricted to only one and/or two properties. making it difficult to arrive at a common conclusion. In addition, hardly few of the reported literature could be extended further toward industrial-scale manufacturing as the properties of TiO2 in suspension decline when converted to the layered form. It is also worth mentioning that, though ISO standards are available to assess the different properties of coating films, each of the methods suffers from ambiguity in terms of methodology, defining the variables/parameters, or result interpretation. Also, it is high time to switch over to sunlight-induced photoreactions rather than the traditionally employed UV light source. As a forward measure to improve the commerciality of the photocatalytic ceramic tiles, cost-effective coatings with properties suitable for the native environmental conditions must be developed. Nevertheless, the demand for an easy solution to retain cleaner and brighter infrastructure will never subside, and in this regard, photocatalytic ceramic coating appears as a promising strategy.

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[28] J. M€a€att€a, M. Piispanen, H.R. Kym€al€ainen, A. Uusi-Rauva, K.R. Hurme, S. Areva, A.M. Sjöberg, L. Hupa, Effects of UV-radiation on the cleanability of titanium dioxidecoated glazed ceramic tiles, Journal of the European Ceramic Society 27 (16) (2007) 4569e4574. [29] In: ISO 27447, Fine ceramics, advanced technical ceramicseTest method for antibacterial activity of semiconducting photocatalytic materials, 2009. [30] H. Li, Q. Cui, B. Feng, J. Wang, X. Lu, J. Weng, Antibacterial activity of TiO2 nanotubes: influence of crystal phase, morphology and Ag deposition, Applied Surface Science 284 (2013) 179e183. [31] J.A. Rengifo-Herrera, K. Pierzchała, A. Sienkiewicz, L. Forr o, J. Kiwi, C. Pulgarin, Abatement of organics and Escherichia coli by N, S co-doped TiO2 under UV and visible light: implications of the formation of singlet oxygen (1O2) under visible light, Applied Catalysis B 88 (3e4) (2009) 398e406. [32] J.A. Rengifo-Herrera, C. Pulgarin, Photocatalytic activity of N.S co-doped and N-doped commercial anatase TiO2 powders towards phenol oxidation and E. coli inactivation under simulated solar light irradiation, Solar Energy 84 (1) (2010) 37e43. [33] D. Mitoraj, A. Janczyk, M. Strus, H. Kisch, G. Stochel, P.B. Heczko, W. Macyk, Visible light inactivation of bacteria and fungi by modified titanium dioxide, Photochemical and Photobiological Sciences 6 (2007) 642e648. [34] X. Chen, K. Cai, J. Fang, M. Lai, J. Li, Y. Hou, Z. Luo, Y. Hu, L. Tang, Dual action antibacterial TiO2 nanotubes incorporated with silver nanoparticles and coated with quaternary ammonium salts (QAS), Surface and Coatings Technology 216 (2013) 158e165.

Anti-corrosion and anti-wear ceramic coatings

9

Rupayana Panda, Kaniz Fatma and Jasaswini Tripathy Department of Chemistry, School of Applied Sciences, KIIT University, Bhubaneswar, Odisha, India

1. Introduction Ceramic coatings are two-dimensional layered structures that are applied to the surface of a substrate to increase the durability and performance of engineering materials that are exposed to various corrosive environments. Ceramic coatings have gained a lot of economic and technical relevance recently because of their unique features, which enhance their mechanical properties like strength and hardness, making them more durable over course of time. Coatings on materials are widely accepted for sustainable manufacturing to lower production costs and increase productivity. Ceramics are rigid materials simply made up of nonmetallic inorganic components. A usual characteristic of all ceramic materials is that they are subjected to high temperatures throughout the manufacture or utility. Other important characteristics of ceramics include the insulation abilities of heat and electricity, corrosion resistance, fragility, rupture behavior without deformation, and high hardness [1]. Briefly, coating refers to a substance that is implemented to certain substances to enhance the surface characteristics such as pigment, shine, wear resistance, chemical resistance, or porosity without affecting the size and shape. They are generally categorized as decorative or preventive based on whether they are used to change (or retain) the surface’s appearance or defend it. In the fields of engineering, industrial, and energy production, prolonging the life of machinery composed of materials, particularly metals or metal alloys, is undoubtedly beneficial. To overcome the material quality degradation due to wear and corrosion, ceramic coatings on metals and alloys are introduced by creating high-performance oxide layers that alleviate corrosion, wear, temperature, insulator, and resistance issues. Ceramic coatings provide excellent mechanical protection against abrasion and wear. These coatings are also essential for applications in microelectronic circuits and heating elements for the insulation property, which is focused on the inertness of corrosionresistant ceramic coatings, which is a prerequisite property against harsh corrosive environments used in specific zones of gas turbine engines, nuclear reactors, and so forth. Ceramic materials have several benefits, including increasing the lifetime of components, preventing rust, minimizing heat on raising components, lowering friction, preventing thermal and acidic corrosion, and improving surface attractiveness [2]. The fundamental goal of coating development throughout the years has been to achieve better corrosion and wear resistance and minimize coating thickness.

Advanced Ceramic Coatings. https://doi.org/10.1016/B978-0-323-99659-4.00006-1 Copyright © 2023 Elsevier Ltd. All rights reserved.

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Corrosion is the word used to describe the surface deterioration of metals and their alloys brought on by chemical or electrochemical interactions. Several metals have higher corrosion resistance than others, which can be related to a variety of reasons, including chemical composition, the type of electrochemical processes, and other factors [3]. However, a basic coating’s performance against long-term corrosion is a bit challenging and requires additional work. In this process, the erosion medium seeps into the coating micropores and harms the substrate as the erosion period lengthens and gradually coating ruptures, allowing the eroding media to directly enter the substrate. Similarly, wear is the process in which progressive loss of materials occurs due to the interaction between the solid surfaces because of the counterbody’s mechanical motion. Diverse forms of wear, like severe wear, moderate wear, impact wear, etc., occur in the industry and daily life. To address these issues, increase the effectiveness of coatings, lengthen the life span of metals, preventive agents, and self-replicating abilities must be added to the coating to give its better anticorrosion and antiwear proficiency [4e8]. By preventing or delaying metal degradation by limiting the entry of corrosive elements to the metallic substrate, a variety of coating layers, along with biodynamics, transitions, and electroplating, have been produced [9]. This chapter discusses several nanocomposite-based ceramic coatings for anticorrosion and antiwear applications, along with their specifications, application techniques, operating principles, etc.

2.

Significance of anticorrosion and antiwear coatings

A system or component of engineering can be as large as a giant machine or as little as a microscopic electronic gadget, but wear and corrosion are the two main variables limiting its life and function. It is vital to comprehend the underlying mechanics of corrosion and wear to overcome these issues. Comparing the mechanisms of corrosion to those of wear, they are relatively straightforward. The strong attack of local corrosion can occur in a very small area of an uncoated surface [10,11]. If the damaged areas are not repaired quickly, they will spread throughout the metal surface and lead to the early failure of coatings [12]. Emerging research interests are provoked by the growing need for in-situ replaceable anticorrosion materials that can prevent expensive mechanical restoration and continual replacement in demanding applications [13]. Li et al. (2016) created several catechol-based acrylate polymers that effectively performed self-repair underground and could quickly couple with calcium ions or magnesium ions in water [14]. Du et al. (2016) created a liquid self-rehabilitation covering by combining polyethylene glycol (PEG) and tannic acid. When tannic acid and PEG come into touch with water, their hydrogen bonds may weaken, giving the coating a slight fluidity to fill in fractures [15]. The adaptive anticorrosive coating responds to environmental stimuli by changing the characteristics of the material, and it can recover or even improve the coating’s anticorrosion capacity with minimum operator intervention [16,17]. The corrosion antibiotic agent included in the matrix actively fills the deficiency because the adaptive

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anticorrosive coating is promptly activated when corrosion starts to occur in the system. They slow down the corrosion process by preventing the electrocatalytic processes of corrosion. As a result of inevitable wear and tear from use, the coating is broken, opening a new entry point for corrosive materials. Also contributing to its failure are the coating’s inherent porosity and various routes of penetration. Fillers with a resistance, sacrifice or corrosion inhibition to the coating are thus extremely desirable to increase the coating’s protective capacity [18,19]. Despite being injected directly into the coating medium, the corrosion inhibitor nevertheless has a better anticorrosion impact on the metallic substrate. Additionally, it will cause gaps in the coating, which significantly lowers its ability to act as a barrier [20]. Wear is the rapid decrease of composites as they come into contact with the hard surfaces caused by the mechanical action of an opposing body. The two primary types of wear damage are (1) content breakdown that reduces characteristics and (2) the detachment of materials from the wear rate or wear debris. The second one is as dangerous because contamination can result in food or beverage spoilage throughout the production, irritation from particles that are stuck within the conductive wall, obstruction of a nozzle, vital pipeline, or filter, accumulation on an electronics command tacking point, etc. Diverse types of wear, including adhesive wear and moderate wear, wobbling wear, and implications of wear, exist in the industry and daily life according to various wear-processing methods. Most of these friction coefficients are obtained by applying any one of the wear mechanisms, such as adhesive, corrosive, erosive, or fatigue, or a combination of them. Whereas a tougher material rubs as opposed to a smoother one, either two-body wear or three-body wear results in abrasion [21e23]. According to research done by Guan and Buchheit (2004), a vanadate transformation covering the coating’s creation increases corrosion resistance to pitting and inhibits oxygen reduction reactions [24]. According to Guosheng et al. (2013), ZnNi coating has a long lifespan, can be used as a relatively low cathodic coating for steel substrates, and satisfies the criteria for cathodic protection. A steel surface plated with pure aluminum, pure zinc, a Zn/silicon (Si) alloy, or a Zn/Al alloy is referred to as metallic coated steel [25]. A general schematic representation of the concept is shown in Fig. 9.1. The key characteristic of long-lasting abrasive-resistant ceramic coatings, for example, is their excessive hardness combined with their good toughening character of ceramic coating materials. These coatings are primarily used on tool materials in casting and machining to provide excellent mechanical protection against abrasion and erosion. In addition to the insulation property of electric insulation ceramic coatings, which is essential for applications in microelectronic circuits and heating elements, the superior chemical inertness of corrosion-resistant ceramic coatings is a prerequisite property against hostile corrosive environments, as well as hightemperature resistance properties, which are essential for thermal barrier ceramic coatings used in different zones of gas turbine engines, nuclear reactors, and so on. The impact of the coating’s hardness, adhesion, and cohesion as a whole on wear resistance is what gives a coating its ability to resist wear. A coating that exhibits strong wear resistance also exhibits good corrosion resistance [26,27]. The surface roughness of coatings that are attached firmly plays a key function in enhancing their antiwear

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Figure 9.1 Schematic representation of ceramic coating nanoparticles for anticorrosion and antiwear.

qualities and lowering friction when they are sliding. The creation of surface roughness in the form of nanoscale shapes or designs is therefore greatly suitable to enhance the material’s tribological capabilities. According to Kustandi et al. (2009), the roughness of surfaces can decrease the friction coefficient by up to 35% when compared to unstructured surfaces. Furthermore, reciprocating wear tests showed that decreased wear depth and width occurred when roughness was present on the polymer surface and there was very little substance passed on to the sliding surface [28].

3.

Evaluation of the performance of coatings

3.1

Methods for evaluation of anticorrosion coating

There are generally three characteristics that define the manner of corrosion such as the types of corrodent, the method of corrosion, and the way corroded material appears. The anticorrosion characteristics of coatings are investigated on the basis of those properties using the following approaches.

3.1.1

Weight-loss technique

The corrosion investigation is carried out using weight-loss (WL) techniques for coated substrates submerged in corrosive electrolytes for a specific amount of time. In general, due to the lack of substances from the surface of structural damage, the weight of the conceptual adsorbent must drop significantly. As a result, the coating’s performance is visually assessed by estimating the WL via weighing. The layer covering can be used to defend against corrosion. Before being immersed in a corrosive electrolyte, the weight of the uncoated and coated specimens is first recorded on

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an electronic balance. After spending 60 days submerged in the test solution, the airdried specimens are weighed once more. This is followed by thorough dilution with distilled water and acetone. The following formula can be used to get WL per unit area per unit time and the corrosion rate (CR) in millimeters per year: WL ¼

w0  w1 at

CR ðmm = yearÞ ¼

ðw0  w1 Þ  87:6 atd

Where w0 represents the sample’s initial weight before immersion, w1 represents the sample’s weight following immersion, a represents the specimen’s surface area, t represents the conclusion of each experiment, and d represents the metal sample’s density.

3.1.2

Surface study technique

With the help of several characterization techniques, the structure, chemical makeup, and surface morphology of the coated substrate’s surface are examined both before and after corrosion. As a whole, characterization methods such as electron scanning microscopy, field emission scanning electron microscope, the examination of electron probes, optical microscopy, ellipsometry, and atomic force microscopy (AFM) are employed [29]. To understand how the variances affect how well the coatings resist corrosion, the differences are observed and studied.

3.1.3

Electrochemical technique

In the electrochemical technique, Tafel extrapolation and chronoamperometry techniques are used to accomplish corrosion inhibition at ambient temperature in a typical three-electrode configuration. Julius Tafel demonstrated an experimental relationship between the current I, and the overpotential, ղ, during an electrocatalytic test of the reduction reaction of hydrogen [30]. h ¼ a þ b log I Where the overpotential ղ is defined as the difference between the potential of the working electrode, E, and the equilibrium potential.

3.1.4

Linear polarization resistance

Stern and Geary were the first authors to develop theoretically a linear correlation between the polarization resistance and the CR based on the kinetics of electrochemical reactions (i.e., corrosion current under open circuit conditions) and the idea of mixed potential theory, which was first introduced by Wagner and Traud in 1938 (i.e., parameters of the cathodic and anodic E/I relations). The benefits and drawbacks of their

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approach have been discussed in a number of articles, and the experimental evidence has confirmed the linearity of the slope of the current-potential plot around the corrosion potential, thereby avoiding the issue of high current densities. The polarization curve can be derived from the Butler-Volmer equation, which relates current density and overpotential [31].

3.1.5

Salt spray test

The anticorrosion functioning of the protecting layer can be assessed by implementing a salt spray testing machine by the ASTM B368 standard. According to a standard procedure, experimental chambers are continually sprayed with compressed air while being passed through a 5-wt% NaCl solution. In the test chamber, the pH was regulated between 6.5 and 7.0, and the airflow is held at 1.0e2.0 mL/h per 80 cm2.

3.1.6

Electrochemical noise analysis

The measurement of electrochemical noise resulting from the study of fluctuations in corrosion potential or current provides an exciting method for the investigation of corrosion processes in reactive media such as aqueous media or hot aggressive gases. This method makes it especially appropriate for localized corrosion monitoring, general corrosion detection, crevice study, stress corrosion cracking, fretting corrosion, or the evaluation of anticorrosive organic coatings, as well as other surface inhomogeneity case studies [32].

3.2

Methods for evaluation of antiwear coating

The antiwear testing procedures are carried out under practically exact replicas of the manufacturing or application environments, where several wear characteristics are taken into account when evaluating the antiwear efficiency of trimming manufacturing equipment such as factories, punches, saws, lathes, and drills [33,34]. These parameters include adhesion, scraping, warm rust, shock, and impact. As an illustration, the antiwear efficiency of TiO2 nanoparticles having water-based lubricant for austenitic stainless steel was examined at room temperature on a ball-on-disk tribometer [35]. Laboratory analysis can be done using tools like exertion testers, ultrasonic and wear test equipment, and nano- and microhardness testers to assess the antiwear effectiveness of sealants on substances that are not being used in machining operations [36]. The empirical equation shown below can be used to determine the wear rate: K¼

V FxL

Where K, V, F, and L stand for the corresponding wear rate (mm3/N/m), wear volume (mm3), applied load (N), and sliding distance (m). In addition to the tests used for quality control, the antiwear testing process may additionally include tests for permeability, width, adhesion, toughness, strength, ductility, chemical properties, pressure, and wear resistance.

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4. Methods for fabrication of anticorrosion and antiwear coatings A variety of general coating techniques are carried out, including electrochemical deposition, self-assembly techniques, spin coating, film casting, linear polarization resistance, printing, layer-by-layer deposition, anodization, Langmuir Blodgett film casting, electrochemical noise analysis, spraying, pellet preparation, and panting [37]. Several coating techniques, including physical vapor deposition, thermal spraying, electroplating, film of microarc oxidation (MAO), chemical conversion coating, and plating of the minimum electron, have been utilized to coat magnesium alloys for anticorrosion applications. Corrosion defense coatings are usually focused on boundary vigilance, restrictive lubricants, or electrochemically active metal composites. Oxygen deprivation or resistance inhibition are effects of barrier protective coatings. Electrochemically active metal composites are often made of Zn, and their high surface electrode activity allows them to stop the electrical flow from releasing from the material and producing galvanic corrosion [38]. Anticorrosion coatings must resist strong resistance to corrosive agents such as water, biochemical oxygen demand, and ions [39]. Zhang et al. (2020) have used cold spray and ultrasonic shot peening (USSP) for anticorrosive copper (Cu)-based coatings modified Mg alloys as shown in Fig. 9.2. The surface of the alloy was coated with a variety of Cu/Ni (nickel) composite coatings that varied in the amount of Ni. The outcome showed that USSP improved the structural homogeneity of pure Cu coating which is coated through the cold-spray method. The Ni concentration of the coated Cu/ Ni composite improved their microhardness values, and the coated pure Cu corrosion resistance was intensified by the 16-vol% of Ni particles. Therefore, a coated, coldsprayed Cu/Ni composite exhibited outstanding corrosion resistance which was applied to the surface of a Mg-alloy [40]. Arukalam et al. (2018) spin-coated steel substrates with the poly (dimethylsiloxane) ZnO nanoparticle nanocomposite using a hexadecyltrimethoxysilane healing agent. The aforementioned coating on the substrate was continually baked at 60 C for 4 days to provide adequate and thorough posthealing [41]. Chen et al. (2022) used a phosphate ceramic coating that was enhanced with chromate-passivated aluminum particles which was resulting in greater corrosion performance. Aluminumemodified phosphate ceramic coating withstands the salt-spray method for 14 days in 3.5 wt% solutions of NaCl. After 14 days submersion of the aluminum-modified phosphate ceramic coating maintained its initial low-frequency impedance. The produced coating’s initial low-frequency impedance shows great high-temperature resistance to corrosion [42]. Qian et al. (2019) have used mesoporous SiO2 particles loaded with corrosion inhibitor benzotriazole. The nanoparticles were further coated with polydopamine (PDA). The resulting inorganic nanocapsule was then mixed with the aqueous alkyd coating. PDA, which is pH-sensitive, managed the release of benzotriazole. To fix flaws and stop additional corrosion, the released benzotriazole and dissolved PDA were combined to get complexes with Fe2O3 [43].

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Figure 9.2 Diagrammatic illustration of the corrosion process for the Cu coating applied as sprayed and the Cu coating that was ultrasonically shot peened and submerged in solutions containing 3.5 mass percent NaCl. Reproduced from L. Zhang, Y. Zhang, H. Wu, S. Yang, X. Jie, Structure and corrosion behavior of cold-sprayed Cu/Ni composite coating posttreated by ultrasonic shot peening, SN Applied Sciences, 2 (2) (2020), 1e14.

The nanomaterial-based coating has been applied to the metal’s surface either by combining the nanomaterials with the lubricants or by bonding the thin-film layer to the metallic surface. As a result, the techniques used to prepare nanomaterials for coating and antiwear applications are very similar. A few of the more popular techniques are covered here. Typically, the approaches are followed by the plastic deformation of coarse-grained materials. A popular and simple method for creating the antiwear layer is cosputtering. For instance, coverings of Ti-N solidified Ti-Cu-N, and the Ti-N/Cu nanoparticles have been coated on Si substrates using cosputtering of Ti and Cu targets [44]. The liquid lubricant with 4.0 wt% TiO2 nanoadditions had the best antiwear characteristics, along with the least friction coefficient and the greatest antiwear capacity [35].

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5. Corrosion mechanism 5.1

Anticorrosion mechanism

To comprehend and manage corrosion, thermodynamics, and electrochemistry are crucial concepts. The degree to which something resists corrosion is frequently strongly influenced by metallurgical considerations. The study of matter and its different fields is particularly convenient for analyzing the mechanisms for corrosion-related reactions, the condition of metal surfaces, and other essential characteristics. As was already said, corrosion can be divided into three groups which are based on the way that its reactions happen: electrochemical, chemical, and physical. Chemical corrosion is the term for corrosion in situations when no electric current is generated, such as when metals corrode in nonelectrolytes or dry gases. Chemical corrosion is simply governed by the fundamental rules of chemical kinetics of heterogeneous processes. Another illustration of a chemical attack-induced corrosion process is the attack on metal surfaces during etching. Metallic corrosion, which occurs when solid metals come into contact with liquid metals, is a prime example of the physical mechanism underlying corrosion. The attack on solid metal is occasionally caused by liquid metal penetrating the solid metal’s grain boundaries. In most cases, molten metal and solid metal combine to produce an alloy. Anodic and cathodic electrochemical processes can be distinguished [45]. Thus, an efficient anticorrosion property could be achieved through a ceramic coating of metallic materials. According to Yuan et al. (2020), the friction coefficient dramatically dropped after the epoxy resin (ER) was mixed with graphene oxide (GO), nano titanium (Nano-Ti), and graphene oxideetitanium (GO-Ti) composites, as shown in Fig. 9.3. Two explanations can be given for this. Nanofillers use their hardness to sustain the structure

Figure 9.3 The anticorrosion mechanisms of GO-Ti/epoxy composite coatings. Reproduced from H. Yuan, F. Qi, N. Zhao, P. Wan, B. Zhang, H. Xiong, X. Ouyang, Graphene oxide decorated with titanium nanoparticles to reinforce the anticorrosion performance of epoxy coating, Coatings, 10 (2) (2020), 129.

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under load to control the mobility of the polymer chain and the adjacent EP polysaccharides. The friction between the coating and the friction pair, on the other hand, can instead be primarily caused by the solid lubricant film layers since the nanofillers can have a lubricating effect on the resin. When ER cures, it is simple to create micropores as a result of the diluted solvent’s volatilization, which makes it simpler for the corrosive medium to come into touch with the substrate surface. Even though GO and Nano-Ti by themselves can partially block micropores, they are unable to diffuse into resin or adequately screen the corrosive medium. The composite has exceptional dispersion performance due to the addition of nano-Ti. A “maze effect” can be created to stop corrosive media from penetrating, lengthen the path of caustic substances in the coating, and ultimately delay the occurrence due to the sheet structure, excellent dispersion, and interfacial compatibility of composites [46]. Although the hydrophilic groups were kept in the waterborne coating after the film formed, the shielding effects on water, oxygen, and chlorine ions were diminished, making the anticorrosion performance of a waterborne coating even worse than the solvent one. The study and the anticorrosion mechanism indicate that there are two causes for the results. The first explanation could be that the evenly scattered graphite in the epoxy matrix serves as a perfect shield and has hydrophobic properties. The Hþ cannot further approach the metal because, secondly, the Nþ in the lignin sulfonate (LAI) polymer got adsorbed on it by electrostatic contact. As a result of the nonshared electron pairs that the N, O, and other atoms in LAI possess, their ability to form a coordinate bond mostly with the d orbital of the metal, the abundance of N, O, and other atoms bonded to the metallic surface, and the spatial network structure of LAI macromolecular stretched into the interior of the solution, the LAI polymer played the role of a corrosion inhibitor [47].

2Fe3+ + O + 2H+

2Fe3+ + H2O

(Dissolved oxygen)

Fe3+ + LAI

Fe3+ [ LAI ]

Additionally, the phenolic hydroxyl groups, alcohol phenolic hydroxyl groups, ether linkages, and amine groups in the LAI molecule could specifically bind with Fe3þ on the metallic surface, which has an ion-shielding effect. So, compared to an epoxy coating made only of epoxy, the LAI-G/epoxy system exhibits greater corrosion resistance, as shown in Fig. 9.4.

5.2

Antiwear mechanism

Chang et al. (2015) incorporated Si into the CrAlN films, which had a great impact on the microstructure, leading to improvements in the mechanical and tribological characteristics as shown in Fig. 9.5 [48,49]. When (Cr0.5Al0.5)1xSixN covering is heated to a high temperature and then subsequently Si content is added to the coating, the friction coefficient increases, and the wear rate decreases. Although an increase in friction coefficient was seen, Cr25.8Al24.7N49.5 and Cr21.9Al21.7Si9.0N50.1 had friction

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Figure 9.4 Schematic illustration of the anticorrosion mechanism of graphene and LAI polymer. Reproduced from J. Ding, S. Shi, H. Yu, Study on modification of lignin as a dispersant of aqueous graphene suspension and corrosion performance in waterborne G/epoxy coating. International Journal of Advanced Engineering Research and Science, 3 (9) (2016), 236841.

Figure 9.5 The schematic representation of the wear mechanism in (Cr0.5Al0.5)1xSixN coating. Reproduced from C.C. Chang, H.W. Chen, J.W. Lee, J.G. Duh, Development of Si-modified CrAlSiN nanocomposite coating for antiwear application in extreme environments, Surface and Coatings Technology, 284 (2015), 273e280.

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coefficient differences that were just 0.1. The (Cr0.5Al0.5)1xSixN covering with Si is a shielded covering with the lowest wear rate and the best friction coefficient [50]. Nanomaterials reduce friction and wear and can help reduce direct interaction. Another key factor of the nanomaterials in the lubricant is that their viscosity can be increased under low applied pressure. As a result, the properties of the adhesive layer that lubricates the interaction on the ground of the shifting components are enhanced. By spinning in between the moving components, spherical nanomaterials lubricated with oil functionally reduce friction. The diameter, morphology, and nature of particles, along with their concentration in solvents, all have a strong influence on friction and wear reduction.

6.

Coatings for anticorrosion applications

To prevent the substances from rusting, coatings are frequently placed on the surface. Ceramic coatings can be added to the polymer or metal/alloy matrix, which frequently help to increase anticorrosion performances.

6.1

Polymer matrix coating

The development of polymer nanocomposite-based ceramic coatings finds extensive applications in anticorrosion applications. The examples are shown in Table 9.1.

6.1.1

Epoxy coating

ER was observed to be widely used as a corroded-preventive coating in industrial and raw material usage due to its resistance to rust, high bond strength, inertness, excellent thermal conductivity, exceptional mechanical strength, reduced healing strain rate, and suitability for waterborne techniques [58,59]. Moreover, accompanying limitations, including weak tensile properties, excessive abrasiveness, and minimal fatigue resistance have made ER usage for anticorrosive coatings more constrained [60e62]. Xia et al. (2020) suggested that agglomeration of the nanostructures occurred when pristine g-C3N4 nanostructures were added to waterborne epoxy because of the poor accessibility and intercellular interaction between the two materials [63]. This increased the number of microchannels the corrosive electrolyte could pass through and sped up the production of corrosion products [64]. The anticorrosion effectiveness of composite coatings was examined using a salt spray test. On the surface of each sample, a scratch was etched before the test.

6.1.2

Polyacrylic acid-based coating

The category of polymers, composites, or variants of acrylic and methacrylic acids, including esters, amides, and nitriles, is known as polyacrylic-based coatings. Suspension of polyacrylic is observed to get many applications in various fields of the coating due to its benefits, including outstanding ductility, development of the polymer film

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Table 9.1 Examples of nanocomposite ceramic coating for anticorrosion.

Polymer matrix Epoxy

Nanocomposites

Ceramic coating method

Additive for lubricating oil that has been carbonized

Electrostatic system spraycoating

Graphene or hybrids of graphene oxide and Fe3O4

Spray coated

Quantum dots made of silanefunctionalized graphene

Sprayed with air

Halloysite clay, SiO2, Zn, and Fe2O3

Dip coated

Significant results

References

Coatings were further enhanced in terms of wear resistance and anticorrosion. Significant improvements were made in microhardness and anticorrosive performance. The addition of highly adjustable silanefunctionalized graphene quantum dots significantly increased corrosion resistance. The best anticorrosive materials include halloysite clay and iron oxide, while epoxy/ silicon dioxide coatings have a higher Young’s modulus than other materials.

[51]

[52]

[53]

[54]

Continued

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Table 9.1 Examples of nanocomposite ceramic coating for anticorrosion.dcont’d

Polymer matrix Polyacrylic acid

Nanocomposites

Ceramic coating method

Ferrite (Fe2O3) with acrylatebutylated melamine formaldehyde

Simple coated

Silica nanostructures in a copolymer matrix containing 2-ethyl hexyl acrylate and glycidyl methacrylatecontaining epoxy moieties. Zinc particles and multi-walled carbon nanotubes are employed in the first layer, and hexagonal boron nitride is used in the second layer with a polystyreneacrylic matrix.

Sol-gel coated

Simple twolayer covering

Significant results

References

Enhancing the weight percentage of Fe2O3 improved the corrosionprotection effectiveness. The use of optimized blended films resulted in excellent anticorrosion protection.

[55]

The corroded region on the conductive disbandment was reduced by up to 90% using the nanocomposite composition.

[56]

[57]

after evaporation, connectivity between other components, superior adhesive properties, excellent antisolvent ability, water, oxygen, ultraviolet light, and ions. Generally speaking, such polymeric substances with outstanding abrasion resistance in both an acidic and an alkaline medium are very prominent as a crucial base coating in the automotive industry. The morphological properties of various coatings were displayed in Fig. 9.6. As could be seen from Fig. 9.6A and B, the pure hydroxyapatite (HAp) coating and the gentamicin sulfate (GS-HAp) coating both displayed a unique bifacial coating pattern, with the top layer made up of HAp crystals that resembled flowers and the bottom layer of HAp crystals. When polyacrylic acid (PAA) was introduced as part of the

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Figure 9.6 Scanning electron microscope images of (A) pure HAp, (B) GS-Hap, and (C) PAA/ GS-HAp coatings. (D) EDS spectra that correspond to each coating. Reproduced from X.J. Ji, Q. Cheng, J. Wang, Y.B. Zhao, Z.Z. Han, F. Zhang, Z.L. Wang, Corrosion resistance and antibacterial effects of hydroxyapatite coating induced by polyacrylic acid and gentamicin sulfate on magnesium alloy, Frontiers of Materials Science, 13 (1) (2019), 87e98.

hydrothermal process, however, a dense HAp layer was created, and circular HAp nanoparticles were generated on its surface (Fig. 9.6C). Furthermore, the PAA/GSHAp coating’s denser surface topography may result in greater interfacial bonding and improved anticorrosion efficiency [65]. For usage in anticorrosion applications, the coating of an improved nanocomposite of silica with a 2-ethylhexyl acrylate copolymer and glycidyl methacrylate demonstrated a high-quality barrier characteristic [56]. In terms of the environment, a nanocomposite covering made of polyacrylate latex (also known as vinyl acrylic terpolymers) and Naþ-montmorillonite (MMT) nano clay was found to be effective against corrosion [66].

6.2 6.2.1

Metal/alloy matrix coating Magnesium matrix coating

Magnesium alloys have been drawing particular attention due to their exceptional qualities, including their low weight, great specific strength, amazing electromagnetic shielding property, and stiffness, and are considered one of the suitable materials

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used in the areas of optical and aeronautical instruments. For better corrosion resistance and absorption properties of light, magnesium alloy coatings are used as protective media [67]. Zhao-hui et al. (2010) used an ultrasonic process to create a magnesium matrix composite enhanced with SiC nanoparticles. According to the microstructural analysis, ultrasonic vibration can effectively and uniformly disperse nanoparticles into magnesium alloys [68]. Bai et al. (2019) used MAO with asymmetric bipolar pulses to create black integrated ceramic coatings against corrosion and absorption on AZ31 magnesium alloy to increase the magnesium alloy’s corrosion resistance and broaden its applications in the field of light absorption [69].

6.2.2

Titanium metal matrix

Titanium matrix shows particular strength and rigidity as compared to steel and Ni-based matrix. Titanium and its alloys are used in various applications, such as aerospace industries and automotive industries which are gaining a lot of scientific attention. These materials are thin and frequently offer desirable qualities, including strong specificity, high resistance to chemicals, and highly effective biocompatibility. They are the perfect choice for structural, chemical, petrochemical, maritime, and biological applications because of the combination of these qualities [70]. Rahman et al. (2017) reinforced a Ti-5Al5Mo-5V-3Cr metal matrix composite with the SiC fiber. They asserted that the material’s anticorrosion performance outperformed MMCs created using Ti21S or Ti64 matrices. Therefore, zero fundamental research regarding this enhancement of anticorrosion performance is revealed [71].

7.

Coatings for antiwear application

It has been reported that nanocomposite coatings have a good chance of preserving metal in environments that are hostile to corrosion/wear [72]. The nanocrystalline Ni-W/SiC nanocomposite coating had improved tensile characteristics, anticorrosion behavior, and abrasion resistance. Tuten et al. (2019) [73] created a thick (HfNbTiVZr)Nx HEA coating with a total thickness of 800 nm Fig. 9.7. When using a bare Ti-6Al-4V substrate, the authors discovered that raising the load from 1 to 3 N increased the wear depth Fig. 9.7A. The profilometer was unable to pick up the extremely slight wear depth due to its size. The AFM images display the surface topography Fig. 9.7B. Comparing the TiTaHfNbZr HEA coatings to the Ti-6Al-4V substrate, they discovered that wear decrease volume was minimal for all loads.

8.

Future perspectives

The investigation of protective ceramic coating use has gained much attention because of its worldwide impact on production and manufacturing. This is because, in comparison to bulk-size materials, the incorporation of nanoscale materials with special

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Figure 9.7 (A) Wear track profiles of Ti-6Al-4V and (B) AFM images of 800 nm thick TiTaHfNbZr HEA coatings at different loads. Reproduced from N. T€uten, D. Canadinc, A. Motallebzadeh, B.U.R.A.K. Bal, Microstructure and tribological properties of TiTaHfNbZr high entropy alloy coatings deposited on Ti6Al4V substrates, Intermetallics, 105 (2019), 99e106.

chemical, physical, and physicochemical properties may increase anticorrosion or antiwear performance. The optimally dense ceramic coating has outstanding corrosion resistance. On the one hand, the prepared ceramic covering itself has pores and fissures, which allow corrosion ions to invade and cause damage. On the other hand, because of its low resilience, it is simple to develop flaws or even fall off in a hard environment over time, failing the coating. Therefore, it is challenging to produce dense, hard-yet-tough ceramic coatings suitable for industrial applications. The possibility is provided by the nano-multilayer composite ceramic coatings. Future research on energy savings will be particularly interested in the potential problems brought on by remarkable advancements in a variety of new wearresistant materials in fundamentals and applications. The characteristics of nanocomposite coatings are also linked to basic difficulties, like the efficient assembly of nanoparticles in their composite for specific applications. The key challenges to the

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effective implementation of those two factors into a single part to enhance the feature are equal distribution and regulation of the size, structure, and surface properties of tiny nanomaterials, as well as inadequate control over surface interactions. The creation of a highly effective, nontoxic, and economical corrosion prevention technology is essential for the automotive and transportation sectors. Nanotechnology and materials science applications have immensely aided in the development of upgrading highly effective corrosion prevention systems. As efficient, nontoxic low-cost coating systems, polymer and sol-gel coatings, along with metallic coatings, exhibit significant potential. Experts’ ability to address the challenges posed by the protective use in many industries, including construction, defense, marine, and residential, will determine the future of these specific coating markets.

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[56] F. Khelifa, et al., Solegel incorporation of silica nanofillers for tuning the anti-corrosion protection of acrylate-based coatings, Progress in Organic Coatings 76 (5) (2013) 900e911. [57] M.B. Kale, et al., Waterborne polyurethane/graphene oxide-silica nanocomposites with improved mechanical and thermal properties for leather coatings using screen printing, Polymer 170 (2019) 43e53. [58] S. Kang, et al., Preparation and characterization of epoxy composites filled with functionalized nanosilica particles obtained via solegel process, Polymer 42 (3) (2001) 879e887. [59] A. Allaoui, N.-E. El Bounia, How carbon nanotubes affect the cure kinetics and glass transition temperature of their epoxy composites?ea review, Express Polymer Letters 3 (9) (2009) 588e594. [60] A. Todoroki, Y. Tanaka, Delamination identification of cross-ply graphite/epoxy composite beams using electric resistance change method, Composites Science and Technology 62 (5) (2002) 629e639. [61] M.D. Maksimovic, V.B. Miskovic-Stankovic, The corrosion behaviour of epoxy-resin electrocoated steel, Corrosion Science 33 (2) (1992) 271e279. [62] A. Gergely, et al., Corrosion protection of cold-rolled steel by zinc-rich epoxy paint coatings loaded with nano-size alumina supported polypyrrole, Corrosion Science 53 (11) (2011) 3486e3499. [63] Y. Xia, et al., Co-modification of polydopamine and KH560 on g-C3N4 nanosheets for enhancing the corrosion protection property of waterborne epoxy coating, Reactive and Functional Polymers 146 (2020) 104405. [64] H. Zheng, et al., Reinforcing the corrosion protection property of epoxy coating by using graphene oxideepoly (ureaeformaldehyde) composites, Corrosion Science 123 (2017) 267e277. [65] X.-J. Ji, et al., Corrosion resistance and antibacterial effects of hydroxyapatite coating induced by polyacrylic acid and gentamicin sulfate on magnesium alloy, Frontiers of Materials Science 13 (1) (2019) 87e98. [66] M.-C. Lai, et al., Advanced environmentally friendly anticorrosive materials prepared from water-based polyacrylate/Naþ-MMT clay nanocomposite latexes, European Polymer Journal 43 (10) (2007) 4219e4228. [67] C.E. Castano, et al., Microstructural evolution of cerium-based coatings on AZ31 magnesium alloys 246 (2014) 77e84. [68] Z.-H. Wang, et al., SiC nanoparticles reinforced magnesium matrix composites fabricated by ultrasonic method, Transactions of Nonferrous Metals Society of China 20 (2010) s1029es1032. [69] L. Bai, et al., Effect of positive pulse voltage on color value and corrosion property of magnesium alloy black micro-arc oxidation ceramic coating, Surface and Coatings Technology 374 (2019) 402e408. [70] M. Peters, C. Leyens, Titanium and Titanium Alloys: Fundamentals and Applications, John Wiley & Sons, 2006. [71] K.M. Rahman, et al., A high strength TieSiC metal matrix composite 19 (7) (2017) 1700027. [72] B. Li, et al., Preparation of Ni-W/SiC nanocomposite coatings by electrochemical deposition, Journal of Alloys and Compounds 702 (2017) 38e50. [73] N. T€uten, et al., Microstructure and tribological properties of TiTaHfNbZr high entropy alloy coatings deposited on Ti6Al4V substrates, Intermetallics 105 (2019) 99e106.

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Priyatosh Sahoo 1 , Vinit Kumar Agarwalla 1 and Ajit Behera 2 1 Institute of Materials Science, Technische Universit€at, Darmstadt, Germany; 2Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela, Odisha, India

1. Introduction to surface coatings Surface coatings are the surface modification materials that are applied on the surface of the substrate materials. Depending on the application, these coatings can be either for decorative or functional purposes. In the automotive industry, coatings were primarily used to protect the materials against corrosion and wear phenomena, while at the same time they also secondarily serve decorative purposes. However, in recent times, smart functional coatings are being employed which can enhance the strength, electronic properties, and thermal shock resistance of the substrate materials. In a typical vehicle, different parts are coated with different materials depending on the work environment to which they are subjected. Now-a-days advanced vehicles are developed with the help of many smart coating materials, as shown in Fig. 10.1. Coatings are used in various parts of the automotive industry, like piston rings, shift forks, synchronizing rings, and cast engine cylinder bores [1]. Different types of coatings are used based on the types of protection that are required. Thermal spray coatings are used to reduce the wear in engines, reducing fuel consumption by

Figure 10.1 Applications of coating in different parts of a car. Advanced Ceramic Coatings. https://doi.org/10.1016/B978-0-323-99659-4.00014-0 Copyright © 2023 Elsevier Ltd. All rights reserved.

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reducing its coefficient of friction [2,3]. Even within the thermal spray coatings, different materials can be used depending on the requirements of the engine. To reduce the weight of the piston assembly, aluminum is used in place of iron, but it requires a certain specific wear resistance, which is again provided by thermal spray coating of chromium on the aluminum surface [4]. In valve trains, frictional losses account for a huge amount of the total loss for the system. The main of which arises in the cam and follower interface; although there is an oil film between them, it is very thin, hence an antiwear coating can help reduce the huge number of frictional losses [2,4]. Nickel coating is also used in reducing the wear resistance in brakes, gear systems, etc. [4,5]. Another type of coating is used for protection against corrosion. Zinc coatings are one of the most important types of corrosion coatings used in the automotive industry. They enable a smooth combination of the bulk and surface properties of the materials used in the industry [5]. Zinc coatings are further protected by a chromate coating to protect them from corrosion. Parts where corrosion resistance coatings are include brake calipers, fluid delivery tubes, and fasteners, preventing them from reducing the life of the parts to a greater extent. One more type of coating used in the automotive industry is the electronically functional coating, which is used to ensure greater safety of modern automobiles, making them smarter as per the requirements [5]. Different types of alloys are used in electronics coatings, like gold, silver, palladium, etc., depending on the type of use and the economics of the coatings. It may include applications like anti vibration sensors and fuel injectors. Another type of coating is conformal coating used for the protection of electronics exposed to an environment in automotive applications. Atmospheric moisture is one of the most important aspects to be considered for these applications, and for these, the use of hydrolytic stability in coatings is of prime importance [6]. Decorative coatings provide another important type of coating, used as the aesthetics of the vehicle are of much importance to the consumer. The decorative coating varies over a wide range, like the nickel or copper plating of wheels, using different types of paint on vehicles. But these decorative coatings can also be used to provide protection from a type of filiform corrosion [5]. Apart from these, there are many other types of coatings used in the automotive industry.

2.

Various smart ceramic coatings in the automotive industry

Smart Coatings or intelligent coatings are coatings which can intelligently sense the changes in their surroundings and react in a response to the change [7]. While conventional coatings serve as a passive stationary barrier between the substrate and the environment, these smart coatings dynamically adapt to any external stimuli. Smart coatings can be designed to respond to a multitude of stimuli ranging from changes in pH or temperature to changes in electrical or magnetic fields. They can respond through a change in their geometry, temperature, or mechanical properties. However, upon removal of the stimuli, ideally, the coatings should retain their original state [8].

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Figure 10.2 Forecast of the smart coatings industry [4].

Smart coatings are presently being used for advanced applications in many industries [9]. Their potential use and improvement are currently being explored and researched. The potential use of smart coatings in the automobile industry has been shown in Fig. 10.2.

2.1

Smart icephobic coatings

In harsh cold environments, the formation, adhesion, and accumulation of ice on surfaces can cause economic and safety issues. Ice on surfaces can lead to increased energy consumption or even mechanical and electrical failures [10]. This can be a major concern for electric vehicles operating in regions of cold climates. The presence of ice can also increase the wear of materials, which can increase their maintenance cost [11]. Ice formation on surfaces occurs through homogeneous and heterogeneous nucleation. Understanding these phenomena during ice formation can help in developing antiice coatings [12]. Numerous procedures have been adopted to limit the accumulation of ice and reduce adhesion between the ice and the surface. These procedures can cause icephobicity of the surfaces, i.e., lower the adhesion strength between surface and ice [13]. The icephobicity of a surface can be increased significantly by surface roughness. Introducing micro-nanoscale textures on surfaces may promote superhydrophobicity by trapping air packets under the water droplets. However, they also increase the risk of heterogeneous nucleation of ice [14]. Alternatively, passive methods such as surface-engineered antiicing coatings can be used to reduce the ice adhesion strength with minimum operational and maintenance costs [15]. Antiicing coatings must have the following properties: limit water accumulation on the surface, prevent heterogeneous nucleation, and lower ice adhesion strength [13].

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Figure 10.3 Strategies for icephobicity.

All the parameters majorly associated with the coating surface to enhance the icephobicity has been shown in Fig. 10.3. Smart Icephobic coatings can be categorized into numerous categories depending on the type of stimuli and response to them. Some of the different categories of Icephobic coatings are discussed below.

2.1.1

Thermoresponsive coatings

These coatings respond to stimuli (such as electric or magnetic fields) by generating the heat required to melt or restrict the accumulation of ice. These are the most common type of smart icephobic coatings. Some of the plants, such as eastern skunk cabbage have these thermoresponsive icephobic coatings. The coatings have thin aqueous layers, due to which ice slips off easily. These thermoresponsive coatings can be further subdivided depending on the stimuli that they act upon.

2.1.1.1 Electrosensitive thermoresponsive coatings These coatings use resistive heating to melt ice from the surface of electrically conductive materials. Due to heating at the surface, ice melts, which weakens the adhesion strength of ice and causes it to be removed by external forces such as gravity. A potential material that could be used for the automotive industry was fabricated by Ref. [16]. They produced a composite by mixing epoxy and graphene nanoribbon stacks. The composite was conductive due to the presence of graphene nanomaterials. Resistive heating can be generated in the composite by applying a constant voltage.

2.1.1.2 Magnetosensitive thermoresponsive coatings These coatings work under the principle of the magnetothermal effect. This effect is related to Néel or Brownian relaxation. Due to this effect, magnetic nanoparticles generate heat when an external magnetic field is applied [17]. Resistive heating is an effective method but also requires high costs and high energy consumption.

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Figure 10.4 Synthesis of multifunctional magnetic hybrid coatings.

In comparison to it, the heating capacity of magnetic nanoparticles is good and costefficient, and hence, can be considered to be used for icephobic applications. Such material was prepared by Ref. [18]. They fabricated magnetosensitive coatings by mixing fluorinated copolymer-tethered epoxy groups with amino-modified Fe3O4 nanoparticles, which were then cross-linked with diethylenetriamine (Fig. 10.4).

2.1.2

Electromechanical icephobic coatings

These types of coatings use piezoelectric materials to remove ice from the surface. When an electric field is applied to the coatings, the piezoelectric materials expand or contract. These deformations can produce very high stresses, which can remove the bond between the ice and the surface of the material [19]. These stresses can be either greater than the adhesion strength of the ice, which can cause ice delamination, or they can produce tensile strength greater than that of ice, which results in crack

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formation in ice. The generated stresses can be modified by controlling the frequency of the applied electric field. Currently, piezoelectric actuators are being used in the aerospace industry for antiicing applications [20]. Such piezoelectric materials after further research could also be used for automotive applications.

2.2

Smart thermal insulation coatings

Heat loss through the combustion chamber wall of diesel engines accounts for around 14%e19% of the total fuel energy [21]. Minimizing these losses can increase thermal efficiency and reduce the usage of fuel. Reducing heat loss can also increase the available exhaust energy, which can be used to recover waste heat energy [22]. Conventional methods to reduce heat losses are by applying a ceramic coating with low heat conductivity and high heat capacity on the combustion chamber wall. Due to this, the high temperature could be maintained throughout the combustion processes, which reduces the heat loss from the combustion chamber wall to the cooling system. But this also increases the combustion chamber wall temperature during all strokes and will lead to a reduction in volumetric efficiency. Also, the reduced volumetric efficiency can sometimes lower thermal efficiency. To counter the above issues related to the use of conventional thermal insulation coatings (TIC) with low heat conductivity and high heat capacity, a new Smart TIC with low heat conductivity and low heat capacity can be used to achieve the adiabatic effect without losing volumetric efficiency. Due to the above-mentioned properties of Smart TIC, the surface temperature of the coatings can change promptly with that of the gas temperature. Similar to conventional TIC, the Smart TIC also maintains a high temperature during the combustion processes. However, in comparison to conventional TIC, during the intake stroke as the gas temperature is also low, the temperature of the Smart TIC decreases, which maintains the volumetric efficiency of the engine. [23]. They investigated the effect of using Smart TIC on a two-stroke marine diesel engine (Fig. 10.5). They also compared the results with the effect of using Smart TIC on a high-speed four-stroke engine. Some of their results are discussed below. Figure 10.5 Two-stroke diesel engine.

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Cao compared the effect of the thickness of smart TICs of 0.1, 0.5, and 1 mm, conventional TICs of 0.1 mm, and base engine. For the base engine without any coatings, the combustion chamber wall temperature was almost constant. This is due to the high heat capacity and heat conductivity of the base metal. The wall temperature of the combustion chamber with conventional TIC was found to be higher than that of the base engine due to the low heat conductivity and high heat capacity of the coatings. When using smart TIC of different thicknesses, they found that during the scavenging and compression processes, the wall temperature was lower than conventional TIC, while during the combustion processes, the wall temperature was higher than conventional TIC. Also, they observed that as the thickness of the coating decreases the surface temperature of coatings reduces more rapidly. This occurs because as the thickness of smart TIC increases the heat capacity also increases. They also noticed that the thermal efficiency increases by 3.55% in comparison to the base engine when using the 0.1-mm Smart TIC. This is because with a 0.1-mm Smart TIC, the heat transfer losses of the engine are significantly lower without affecting the volumetric efficiency of the engine. The thermal efficiency of the engine also increases significantly when coated with Smart TIC. However, they also saw a small increase in NOx emissions. Cao also compared the results of using smart TIC in the two-stroke engine and highspeed four-stroke engine models. They observed an increase of 8.2% in thermal efficiency in the four-stroke engine when using smart TIC. The increase in thermal efficiency in the four-stroke engine is way higher than that of the two-stroke engine. They reasoned that this increase to be due to the high surface-to-volume ratio of the four-stroke engine [24].

2.3

Smart wear resistant coatings

Antiwear coatings are very important for increasing the lifeline of the machinery and reducing the load on the automobiles and other machinery used in most of the industry, thus saving huge amounts of money and resources. For protection from wearing, generally, a thin liquid film is used between moving particles, which can be called antiwear coatings [25]. Smart antiwear coatings are often referred to as self-healing coatings. Materials like MgeLi are emerging as a material of prime importance in the automotive industry, but they have very poor wear resistance and thus some surface treatment is needed to make the materials be used efficiently in the industry [26]. Recovery of the coatings is one of the most difficult processes, and this is ensured by the process of smart coatings. Currently, no single component material is known to serve the above process, and it can be ensured only by the use of composite materials [27]. There are three basic steps behind the formation of coatings first is creating a transition layer between the substrate and the coatings, followed by introducing solid crystalline grains into the matrix, and then creating a nanostructure with a large number of grain boundaries [27]. One such surface treatment method used in the industry is the plasma electrolytic oxidation technique to develop ceramic coatings with high corrosion and wear resistance properties [26]. This is also done by the process of anodizing, which provides the foundation for PEO techniques. It was mostly done using the DC

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electrolysis technique, but now many AC techniques have surpassed the DC technique in terms of coatings with improved properties. The most recent of which is the technique PEO, which can be considered as an intermediate between aqueous solution anodizing having low energy and high-energy plasma deposition in the dry state [28]. It occurs generally in areas with high local temperatures and local pressure. PEO coatings are very adhesive, hard, and wear-resistant [28]. As already stated, it occurs by AC electrolysis the reaction occurring at the anode is oxide formation, but in reality, they are not just oxides but rather hydrated oxides. Hydrogen evolution occurs at the cathode [28]. Fig. 10.6 shows the nanocomposite functional coating. The hard phase in the nanocomposite helps to prevent surface degradation from various types of loads. Some examples of ceramic coatings are tungsten carbides, Cr2O3, Al2O3, and ZrO2. And as per the studies by Ramzan Kose and his colleagues, it was found that Cr2O3 was best among them when the wearing rate was low [29]. But there are many more coatings that have not been tested to make a conclusive suggestion on which is the best coating. Also, epoxy-based coatings are explored as self-healing coatings for both antiwear and anticorrosion properties [30]. studied the effect of adding TiO2 to the epoxy and found that the addition of different percentages of nano-TiO2 has different effects on the properties. They found that initially, the addition of nano-TiO2 degraded the antiwear properties because it acted as debris. After the addition of more, it improved the antiwear properties by forming composite self-suspense [30]. Zr-xTi is also one of the premium coatings used for wear-resistant properties, and its wear resistance is improved significantly if the coatings are thermally oxidized. It is due to the fact that oxidation leads to the formation of ZrO2 and ZrTiO4, which enhance the adhesion of the coating to the surface [31].

Figure 10.6 Nanocomposite coating combining amorphous and functional gradient structures.

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2.4

227

Smart corrosion resistant coatings

Corrosion is one of the major sources of loss in the world; it costs around 5% of the world’s gross national product [32]. Various types of corrosion in the auto-vehicle parts are shown in Fig. 10.7. It is also rightly said that a coating without corrosion protection does not serve any other purpose [33]. Magnesium and alloys are one of the major materials to be used in the automotive industry, but they are highly prone to corrode [34]. To solve this issue, there are a number of coating techniques that can be used, like conversion coating and electrochemical coating [35,36]. Plasma electrolytic oxidation is one of the most prominent techniques used for ceramic coating, especially for magnesium alloys [37]. The advantage of these coatings is that they stick strongly and impart excellent properties to the parts. The addition of tungsten produces a thicker ceramic coating [37]. They also studied the effect of oxidation on the coatings and found that, after one and a half hours, the porous ceramic coating covered almost the entire surface of the part and removed all the cracks. Again, the effect of tungsten addition on corrosion resistance was studied by Zhijun Li using an SEM, and its observation is shown in Fig. 10.8. He observed that with the addition of tungsten, the corrosion potential increases, and the corrosion current density decreases, hence clearly the corrosion resistance of the coating increases, which he attributed to the formation of the WO3 phase, which is more stable thermodynamically [37]. Another coating technique used for the deposition of ceramic is cathode plasma electrolytic deposition (CPED). The main advantage of CPED is to produce a thicker coating in a short duration, also it’s a low-cost method, has excellent adhesive properties, and can also be applied to highly intricate shapes [38]. Al2O3eZrO2 coatings by this method show excellent anticorrosion properties on magnesium alloys [39]. They deposited Al2O3 using aluminum isopropoxide on magnesium alloy and found that the coated magnesium alloys had two parts inner Al2O3 and an outer organic layer. He also found that the corrosion resistance of the magnesium alloy improved significantly after the deposition of alumina coatings.

Figure 10.7 Corrosion of car at various part.

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Figure 10.8 SEM image of coating (A) without tungsten (B) with tungsten addition. Adopted from Z. Li, Y. Yuan, P. Sun, X. Jing, Ceramic coatings of LA141 alloy formed by plasma electrolytic oxidation. ACS Applied Materials and Interfaces, 3 (9) (2011), 3682e3690.

Hot pressing is another method of depositing corrosion-resistant coatings on materials [36]. When the effect of hot-pressed ZSM-5 zeolite coating on MgeLi alloy was studied, it was found to have an ellipsoid structure and it showed excellent corrosion resistance properties. ZSM-5 is a siliceous aluminum silicate zeolite having the chemical formula NanAlnSi96-nO192.16H2O with a very large range of Al and Si atoms as the value of n varies from 0 to 27. It is a microporous material with a linked tetrahedral structure (Fig. 10.9). The main advantage of this structure is that it shows directional dependence, and as a consequence, it shows distinctive hollow crystals. It shows corrosion resistance in the directions of {100} and {102} because of the aluminum enrichment during the synthesis using the hydrothermal process.

Figure 10.9 Twinned structure of ZSM-5.

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3. Summary Coatings are being widely used in the automotive industry. The conventional coatings being used today are only designed for specific environmental conditions and cannot function in changing environments. Smart coatings, on the other hand, can be used for a variety of applications and can be tailored to respond to different environments. The potential use of these smart coatings is currently being researched to make them economical and optimize for multiple functionalities. For regions with severe cold environments, smart icephobic coatings can be potentially used to reduce the formation and accumulation of ice on automotive materials. In the above sections, the use of thermoresponsive and electromechanical icephobic coatings was discussed, both of which act with different mechanisms. With further research, these materials could be integrated into the automotive industry. Around 14%e19% of the total fuel energy is lost as heat losses through the combustion wall of diesel engines. Conventional TIC can reduce these losses, but they also reduce the volumetric efficiency of the engine. When using Smart TIC, the heat losses are minimized without affecting the volumetric efficiency. However, it was also observed that using Smart TIC also slightly increased the exhaust NOx formation of the engine. Antiwear coatings increase the lifelines of all the machines. We discussed some materials, such as WC, Cr2O3, Al2O3, and ZrO2, that can be used as smart coatings and their deposition techniques. Among them, plasma electrolytic oxidation is the most useful technique owing to its improved adhesion and wear-resistant qualities. Nano TiO2 improves the self-healing of smart epoxy coatings by the formation of composite self-suspense and also due to the formation of some oxides in some cases. We also discussed some of the techniques for coatings and coating material that can be used for corrosion resistance. Currently, magnesium is one of the most important materials, and we discussed techniques and materials mostly for magnesium alloys. An important coating technique is CPED. It produces thick coatings, is economically efficient, and can be applied to highly intricate shapes. Tungsten shows an improvement in the performance of coatings due to the formation of tungsten oxide. One of the crucial coatings for magnesium alloys is Al2O3. ZSM-5 is another material that is used for corrosion resistance of MgeLi alloys.

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[22] S. Sittichompoo, H. Nozari, J.M. Herreros, N. Serhan, J.A.M. da Silva, A.P.E. York, P. Millington, A. Tsolakis, Exhaust energy recovery via catalytic ammonia decomposition to hydrogen, Fuel 285 (2021) 119111, https://doi.org/10.1016/j.fuel.2020.119111. [23] J. Cao, T. Li, X. Zhou, A study of smart thermal insulation coating on improving thermal, Fuel 304 (2021) 120760, https://doi.org/10.1016/j.fuel.2021.120760. [24] M. Moran, H. Shapiro, D. Boettner, M. Bailey, Fundamentals of Engineering Thermodynamics, ninth ed., Wiley, 2020. [25] P. Kar, Anticorrosion and Antiwear. Nanomaterials- Based Coatings, 2019, pp. 195e236, https://doi.org/10.1016/b978-0-12-815884-5.00008-9. [26] Z. Li, Q. Kuang, X. Dong, T. Yuan, Q. Ren, X. Wang, J. Wang, X. Jing, Characteristics of high- performance anti- corrosion/anti- wear ceramic, Surface and Coatings Technology 375 (2019) 600e607, https://doi.org/10.1016/j.surfcoat.2019.07.066. [27] I.A. Podchernyaeva, D.V. Yurechko, V.M. Panashenko, Some trends in the development of wear- resistant functional coatings, Powder Metallurgy and Metal Ceramics 52 (3e4) (2013) 176e188, https://doi.org/10.1007/s11106-013-9511-0. [28] F.C. Walsh, C.T.J. Low, R.J.K. Wood, K.T. Stevens, J. Archer, A.R. Poeton, A. Ryder, Plasma electrolytic oxidation (PEO) for production of anodised coatings on, Transactions of the IMF 87 (3) (2009) 122e135, https://doi.org/10.1179/174591908x372482. [29] R. Köse, L. Urtekin, A. Ceylan, S. Salman, F. Findik, Three types of ceramic coating applicability in automotive industry for, Industrial Lubrication & Tribology 57 (4) (2005) 140e144, https://doi.org/10.1108/00368790510601680. [30] M. Ghorbani, H. Ebrahimnezhad- Khaljiri, R. Eslami- Farsani, H. Vafaeenezhad, The synergic effect of microcapsules and titanium nanoparticles on the, Surfaces and Interfaces 23 (2021) 100998, https://doi.org/10.1016/j.surfin.2021.100998. [31] W.F. Cui, C.J. Shao, The improved corrosion resistance and anti- wear performance of Zre xTi, Surface and Coatings Technology 283 (2015) 101e107, https://doi.org/10.1016/ j.surfcoat.2015.10.051. [32] M. Abdolah Zadeh, S. van der Zwaag, S.J. Garcia, Adhesion and long- term barrier restoration of intrinsic self- healing, ACS Applied Materials and Interfaces 8 (6) (2016) 4126e4136, https://doi.org/10.1021/acsami.5b11867. [33] Interview: “Corrosion protection is and will remain the key” (2021). https://www. european-coatings.com/articles/archiv/interview-corrosion-protection-is-and-will-remainthe-key. [34] H. Guo, M. An, S. Xu, H. Huo, Microarc oxidation of corrosion resistant ceramic coating on a magnesium, Materials Letters 60 (12) (2006) 1538e1541, https://doi.org/10.1016/ j.matlet.2005.11.066. [35] A.L. Rudd, C.B. Breslin, F. Mansfeld, The corrosion protection afforded by rare earth conversion coatings, Corrosion Science 42 (2) (2000) 275e288, https://doi.org/10.1016/ s0010-938x(99)00076-1. [36] D. Song, X. Jing, J. Wang, Y. Wang, P. Yang, M. Zhao, M. Zhang, Corrosion-resistant ZSM- 5 zeolite coatings formed on Mge Li alloy by, Corrosion Science 53 (5) (2011) 1732e1737, https://doi.org/10.1016/j.corsci.2011.01.047. [37] Z. Li, Y. Yuan, P. Sun, X. Jing, Ceramic coatings of LA141 alloy formed by plasma electrolytic oxidation, ACS Applied Materials and Interfaces 3 (9) (2011) 3682e3690, https://doi.org/10.1021/am200863s. [38] R. Ji, M. Ma, Y. He, C. Liu, T. Fang, Z. Zhang, Y. Wang, Y. He, J. Wu, Improved corrosion resistance of Al2O3 ceramic coatings on AZ31 magnesium, Ceramics International 44 (13) (2018) 15192e15199, https://doi.org/10.1016/j.ceramint.2018.05.159. [39] P. Liu, X. Pan, W. Yang, K. Cai, Y. Chen, Al2O3e ZrO2 ceramic coatings fabricated on WE43 magnesium alloy by cathodic, Materials Letters 70 (2012) 16e18, https://doi.org/ 10.1016/j.matlet.2011.11.087.

Visible- and solar-active photocatalytic ceramic coatings

11

Tismanar Ioana, Bogatu Cristina, Gheorghita Silvioara and Anca Duta Transilvania University of Brasov, Brașov, Romania

1. State of the art on photocatalytic materials A broad variety of photocatalysts is mentioned in heterogeneous photocatalytic processes, and the most efficient materials are mentioned to be wide band gap semiconductors that can be activated only by UV radiation. However, this spectral range represents only 5%e8% of the available solar radiation, thus industrial-scale processes must use artificial radiation sources that significantly increase the costs of the photocatalytic processes, which is one of the reasons why they have not been widely implemented yet. Moreover, the production costs for efficient photocatalytic materials are relatively high, which is another limitation in scaling up these processes. Alternatives for producing photocatalytic materials using low-cost precursors and techniques are currently being investigated with the aim to activate the photocatalysts under visible (Vis) radiation or with solar radiation to limit the costs of the photocatalytic process. In addition to the cost prerequisites, the performance in the photocatalytic processes is important. The main reason for the low efficiency in the photocatalytic processes is the recombination of the charge carriers therefore the limitation of electron-hole recombination represents a must when designing a photocatalytic material. Moreover, even if this material is Vis- or solar-active and efficient because of the reduced electron-hole recombination, another important aspect is its stability in the working conditions, and this is why the research target was also set on the development of stable materials in the aqueous environment over a relatively wide pH range (pH ¼ 5 e9). Based on these requirements, nowadays research focuses on the development of efficient and stable photocatalytic materials in the aqueous environment, that can be activated by solar radiation (particularly its Vis component, along with the UV component) to be used in advanced wastewater treatment targeting reuse as an answer to the sustainable development conditions imposed to the industrial processes.

1.1

Photocatalysts

Photocatalytic materials are semiconductors that, when irradiated with Vis and/or UV radiation, generate electron-hole (eehþ) pairs that can independently participate in redox reactions generating reactive species as, e.g., the hydroxyl radical, responsible for the degradation of organic compounds [1,2]. The energy required to activate the semiconductors depends on the value of the band gap energy, and, depending on

Advanced Ceramic Coatings. https://doi.org/10.1016/B978-0-323-99659-4.00016-4 Copyright © 2023 Elsevier Ltd. All rights reserved.

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the semiconductor, the photocatalytic process is activated with the formation of eehþ pairs by radiation with specific energy, the activation condition being: Erad  Eg semiconductor For being efficiently used in viable photocatalytic processes, a set of minimum prerequisites was formulated for the photocatalyst. - to be stable in the working environment. The photocatalytic materials should not be involved as reactants in chemical reactions in the aqueous environment, including at extreme pH values (pH < 3 or pH > 11), therefore the range of the semiconductors used in photocatalytic processes is relatively limited. Depending on the targeted application, the stability domain of the photocatalytic materials has to be clearly and specifically defined; - to be activated by Vis- or solar radiation (w6% UV and w45% Vis), to limit the process costs; - to have a significant capacity to generate eehþ pairs, in order to be able to produce oxidizing species as hydroxyl radicals; - to have an adequate surface morphology, with a high specific surface area, and a suitable surface charge to well support the adsorption of the pollutant as a first step in the photocatalytic mechanism; - to have a highly crystalline structure that supports the charge carriers’ mobility, thus avoiding their recombination (in an amorphous structure, the lifetime of the charge carriers is shorter due to the presence of many recombination centers); - to be nontoxic for avoiding environmental problems and to avoid the use of rare elements; - to have relatively low production costs.

Starting from this set of minimum conditions, photocatalytic materials were developed so that they can be used for advanced wastewater treatment targeting reuse, further aiming at up-scaling. Many wide bandgap (Eg > 2eV) semiconductors, insoluble and stable in water (at least at pH ¼ 5e9) proved to be effective in photocatalytic processes under radiation with adequate wavelength. These include titanium dioxide (TiO2), zinc oxide (ZnO), tin dioxide (SnO2), or tungsten trioxide (WO3). As the data in Table 11.1 show, the activation range of most of the metal oxides semiconductors, such as TiO2 or ZnO corresponds to the UV spectral range. The use of ferric oxide (Fe2O3) that is visibly-active is limited by the rate of the eehþ recombination mainly due to the low diffusion rate of the holes [5]. It should be also noted that the semiconductors activated by radiation in the VIS spectral range, such as WO3, Fe2O3, or CdS in Table 11.1, are not highly stable in water (CdS) and/or in a more acidic (pH < 5) or more alkaline (pH > 9) aqueous environment. Among the semiconductors, the mostly used in photocatalytic processes is TiO2 both in laboratory investigations and in pilot installations [6]. It is the most investigated oxide semiconductor for its use in various fields, initially for hydrogen production [7], and further on in other applications using heterogeneous photocatalysis, in solar cells and in electronic devices, being also used to obtain white pigments and for anticorrosion protective layers, or as a gas sensor [8].

Semiconductors used in photocatalytic processes

Valence band, VB [eV vs. NHEa]

Conduction band, CB [eV vs. NHE]

Band gap energy, Eg [eV]

Wavelength l [nm]

Corresponding spectral range

TiO2 anatase (A) TiO2 rutile (R) ZnO SnO2 WO3 CdS Fe2O3

þ2, þ2, þ3, þ4, þ2, þ2, þ2,

0, 3 0, 1 0, 2 þ0, 3 þ0, 2 0, 4 þ0, 3

3, 2 3, 0 3, 2 3, 8 2, 6 2, 5 2, 2

388 413 388 326 476 486 563

UV UVeVIS UV UV VIS VIS VIS

9 9 0 1 8 1 5

Visible- and solar-active photocatalytic ceramic coatings

Table 11.1 Position of the energy bands, the band gap energy and the wavelength of the radiation required for photoexcitation for different semiconductors [3,4].

NHE ¼ normal hydrogen electrode.

a

235

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TiO2 is widely used in photocatalytic processes [9,10], because: - it is stable in the aqueous environment, including at extreme pH values, which makes it an optimal candidate for advanced wastewater treatment, including very alkaline or acidic ones; - it has a high capacity to produce hydroxyl radicals able to oxidize organic compounds or microorganisms, due to the high band gap energy value (Eg ¼ 3.0e3.2eV) which results in a relatively high lifetime of the electrons and holes, before recombination; - it can be relatively easily obtained in the laboratory and at an industrial scale as a powder or thin film with controlled properties and does not involve very high production costs; - it has good thermal stability, as its melting temperature is Tt ¼ 1843 C and it is nontoxic.

Because of the activation energy value, the photocatalytic processes that use efficient semiconductors such as TiO2 are not broadly considered for up-scaling because of the cost of the UV radiation produced using artificial sources. The need to replace conventional energy sources with renewable energy sources led to the formulation of alternative solutions for photocatalyst activation, focusing on the use of solar radiation for initiating photocatalytic reactions. Considering the information summarized in Fig. 11.1, it can be noticed that the process efficiency will be diminished by the recombination of the photo-generated electrons with the holes; however, the recombination rate is lower for a wide bandgap semiconductor as TiO2 is. As the use of stable (pH ¼ 5e9) and efficient semiconductors is limited due to the UV activation, different strategies were proposed for Vis-activation.

1.2

Vis-active photocatalytic structures

The activation strategies using Vis or solar radiation include the doping or coupling of UV-active semiconductors with other ones with narrowe(er) band gaps. The structures thus obtained have the important advantage represented by the limitation of the electron-hole recombination.

1.2.1

Doping the wide band gap semiconductor

Reducing the energy of the band gap targeting the Vis-activation of the TiO2 photocatalytic structures can be reached by doping with nonmetal spices or with metal cations. Figure 11.1 Development of the electron-hole pairs and the reactions of these species produced using the TiO2 photocatalyst, the anatase polymorph (A).

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Nitrogen doping is the most commonly reported nonmetal doping agent. Substitutional nitrogen generates a new energy band near the valence band of the TiO2 semiconductor, and thus the e can be initially promoted on this newly generated band and then make the transition to the conduction band using energy corresponding to the Vis spectral range [11]. Interstitial nitrogen reacts with the oxygen in the network and forms NO [11e13]. F doping of TiO2 generates oxygen vacancies and Ti3þ species with the energy levels near the conduction band leading to activation under Vis radiation by capturing the photo-generated electrons, [14]. Similarly, Cl doping introduces new energy levels into the band gap of TiO2, allowing the successive transition of the electrons into the CB [11,15]. When doping with carbon, the band gap of TiO2 is narrowed by generating in this band gap an energy level near the valence band. C doping can also lead to the formation of carbon-containing species (as, e.g., the black titanium carbide, TiC) at the TiO2 surface that facilitates the absorption of visible radiation [11]. Doping with metal cations has been extensively studied because it slows down the recombination of the electron-hole pair by replacing the semiconductor cations with other metal cations with different charges, thus generating new charges that contribute to a more efficient separation of the electron-hole pairs [16]. The process that takes place may involve the following reactions: 3þ Ti4þ þ e CB 4 Ti

(11.1)

ðn1Þþ Mnþ þ e CB 4 M

(11.2)

ðnþ1Þþ Mnþ þ hþ VB 4 M

(11.3)

where: Mnþ is the doping metal ion. To be effective, the energy level of the Mnþ/M(n1)þ should be below the conduction band of TiO2, and the energy level of Mnþ/M(nþ1)þ should be above the valence band. Thus, the energy level of the transition metal ions influences the efficiency of the electrons capture. The capture of the electron allows the holes to react with the hydroxide anions in water to form the hydroxyl radical (Eq. 11.4) used in organic compound oxidation reactions [17]. • HO þ hþ VB 4 HO

1.2.2

(11.4)

Coupling the semiconductors

Limited recombination, along with the Vis-activation of the photocatalytic structures, can also occur when coupling UV-active n-type semiconductors with other semiconductors, either n-type to form n-n tandem structures or p-type to form n-p diode heterostructures [18]. In the case of tandems, the Vis-activation occurs only at the level of one of the n-type semiconductors that have the band gap energy corresponding to the Vis spectral range; therefore, it is preferable to use n-p diode heterostructures.

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1.2.2.1 Diode type heterostructures (n-p heterojunction) Diode-type structures such as those used in photovoltaic cells are Vis-active. To be able to work, the energy bands of the semiconductors that form the heterojunction must be suitably aligned to facilitate the transition of the electrons and holes, thus avoiding their recombination. As the example in Fig. 11.2 shows, the band gap of the p-type semiconductor (narrower, corresponding to Vis-activation) is positioned so that the conduction band of this (p-type) semiconductor is at higher absolute values than the conduction band of the n-type semiconductor, and the valence bands respect a similar rule. Following the prerequisites formulated for PV structures, the Fermi levels (EF) of both semiconductors should be aligned. These conditions have to be met for a structure able to operate under Vis radiation. By transferring the electrons from the conduction band of one component of the composite structure to the conduction band of the other component, it is given time for the photo-generated holes to react with H2O or OH to form the hydroxyl radical. Thus, any PV cell heterostructure stable in the aqueous environment can act as a Vis-active photocatalyst.

2.

Vis-active photocatalytic materials used in advanced wastewater treatment targeting reuse

2.1

The photocatalytic process

Nowadays, wastewater is treated using traditional processes. The conditions that have to be met for the discharge of the treated wastewater into natural effluents are those imposed by the acceptable discharge concentrations as formulated in the legal frame(s). However, these very low concentrations accepted for discharge are not allowed if the water reuse is targeted. This is why a tertiary treatment step called

Figure 11.2 Transition of electrons to more stable energy levels and reactions where the charge carriers participate in photocatalytic composites with n-p heterojunction.

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advanced wastewater treatment has to be applied, and this step should be effective in removing pollutants at very low concentrations (ppm, ppb). Among the advanced wastewater treatment processes, mostly investigated are the advanced oxidation processes (AOPs) that are effective in removing organic pollutants at low concentrations so these processes can be applied to bring the treated wastewater to the reuse conditions. Heterogeneous photocatalysis, as an AOP, is a process where the catalyst is not in the same aggregation state as the reactants. This process experienced a remarkable development during the past 25e30 years, as it is an effective solution for the degradation/mineralization of organic pollutants at low concentrations in wastewater or in humid air with the formation of common, nontoxic gaseous compounds. Since 1972, when Fujishima and Honda have drawn attention on the possibility of obtaining oxidative species by irradiating a semiconductor [7], heterogeneous photocatalysis has been extensively studied as an oxidation process focusing on the removal of organic pollutants from water or air [19,20]. Besides its use in advanced wastewater treatment, heterogeneous photocatalysis can also be applied in civil engineering applications as self-cleaning surfaces, in the automotive industry as antifogging surfaces and in sanitary applications as antibacterial surfaces [21]. In the case of advanced wastewater treatment involving heterogeneous photocatalytic processes, the reuse of the water at the end of this oxidation step represents the targeted application. Being a surface process, heterogeneous photocatalysis takes place following several steps. 1) Adsorption of the organic pollutant molecules on the photocatalytic surface: this step may run in the dark or under irradiation, and it only depends on the main factors that influence the adsorption process. 2) Irradiation of the semiconductor with radiation of suitable energy (wavelength) to generate the electronehole (eehþ) pairs; 3) Redox reactions between the water molecules or the ions resulted from water dissociation and the photo-generated electrons and holes at the photocatalyst surface, resulting in active species in the degradation of the organic pollutant(s); 4) Oxidation reaction(s) of the adsorbed pollutant molecules on the catalyst surface with the active species, resulting oxidation intermediates and/or final mineralization products; 5) Desorption of these reaction products from the catalyst surface [22].

The detailed mechanism of the photocatalytic processes may include various steps, but the ones below highlighted are common to any heterogeneous photocatalysis mechanism. a) The activation step by irradiating the semiconductor with suitable radiation (Erad  Eg semi þ conductor) to form the electron (e ) and hole (h ) pairs (Eq. 11.5):  semiconductor þ hn4semiconductor e CB þ hVB




(11.5)

b) The possible recombination of the electrons with the photo-generated holes (Eq. 11.6):
þ semiconductor e CB þ hVB 4 semiconductor þ hn

(11.6)

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c) The oxidation-reduction reactions at the photocatalytic surface (Eqs. 11.7 and 11.8): - The photo-generated electrons may be involved in the following reactions:  e CB þ Ox2 4Red2 Reduction

(11.7)

• e CB þ O2 4O2 þ • O• 2 þ H 4HO2

2HO2 4 O2 þ H2 O2 H2 O2 þ hv42HO - The photo-generated holes may participate in the following reactions: þ hþ VB þ Red1 4Ox1 Oxidation

(11.8)

: þ hþ VB þ H2 O4HO þ H  . hþ VB þ OH 4HO

The radical species generated through the reactions described by Eq. (11.8) are strong oxidants and can decompose, up to mineralization, the organic pollutants (e.g., pesticides, herbicides, dyes, drugs, etc.) adsorbed at the semiconductor surface. The expected reaction products are water, carbon dioxide, and other nonmetal oxide species. Thus, the overall reaction mechanism in photocatalysis [10] is detailed in (Eq. 11.9).


þH2 =HO þ semiconductor þ h / semiconductor e ƒ! semiconductor þ HO CB þ hVB ƒƒƒƒ HO. þ organic pollutantðsÞ þ O2 /CO2 þ H2 O þ intermediate products (11.9) The efficiency of the photocatalytic processes is strongly influenced by: (a) The recombination of the electrons with the holes (Eq. 11.6), which is considered to be the main cause for the low efficiencies in the photocatalytic processes [23]. Any solution to limit or avoid this recombination has a significant and positive effect on increasing the process efficiency and considerably supports applications such as advanced wastewater treatment or humid air purification. (b) The radiation used in the photocatalytic process. The spectral range of the radiation (UV, UV þ VIS, or VIS) should be selected depending on the band gap value of the photocatalytic material and the intensity of this radiation (the irradiance value) has an influence on the process rate as the higher the irradiance value, the higher the amount of photoexcited charge carriers.

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Optimizing the response of the photocatalytic material under different irradiation conditions can lead to increased efficiency of the photocatalytic process, as the results in Fig. 11.3 comparatively show for the photodegradation efficiencies of phenol using different photocatalytic materials: TiO2, CuxS, fly ash (Fly1 and Fly2), as well as mixtures of these in Visactive composite materials. The type of radiation used was UV (28% for Simulator 1 and 9% for Simulator 2) þ VIS (72% for Simulator 1 and 91% for Simulator 2): the total irradiance value using Simulator 1 was 25 W/m2 (out of which 7 W/m2 was UV radiation), and the irradiance value using Simulator 2 was 443 W/m2 (with 40 W/m2 UV). The figure outlines an efficiency increase with increasing the irradiance value, especially on the TiO2 sample [24]. (c) The stability of the photocatalytic material in the aqueous environment under UV or Vis radiation is also important. (d) The adsorption of the pollutant on the catalyst surface, as the first step in the heterogeneous photocatalysis mechanism. Reduced adsorption of the pollutant at the photocatalytic surface will lead to limited overall efficiency of the entire process. Therefore, photocatalytic materials with a large specific surface area represent a must for efficient processes, along with ensuring favorable conditions for the pollutants’ adsorption by optimizing the surface load of the photocatalytic material [25]. Various research groups analyzed the influence of the photocatalyst surface charge on the process efficiency as Robert, Piscopo, and Weber, who discussed the influence of the pH on the adsorption of a pollutants mix consisting of 4-hydroxybenzoic acid and benzamide; they outlined that at different pH values the pollutants were completely differently degraded: at pH ¼ 4 and at pH ¼ 6, the 4-hydroxybenzoic acid had a removal rate significantly higher than benzamide, while at pH ¼ 8, the degradation rate of benzamide was two times higher than that of the acid. In conclusion, considering a mix of significantly different pollutants, the removal of a single component/pollutant is possible through adsorption [20], depending on the solution pH and the substrate’s point of zero charge that electrostatically favor the adsorption.

All these factors have to be considered in the process design not only in laboratory but also at industrial scale. Up-scaling the heterogeneous photocatalysis processes targeting applications in advanced wastewater treatment has not been rigorously investigated yet because there are many barriers to overcome, such as the low process

Figure 11.3 Photodegradation efficiency of phenol (Vfenol ¼ 500 mL, cfenol ¼ 20 mg/L, irradiation duration ¼ 2 h, dispersed photocatalytic material with c ¼ 0.2 g/L) [24].

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efficiency, the high material(s) and process costs, and the lack of a regulatory framework to impose such processes. However, when imposing advanced wastewater treatment targeting the reuse as mandatory, this stage becomes important. Initially, to match the real conditions, the implementation process is aimed at the demonstrator or pilot level, considering the results firstly optimized at laboratory level; moreover, the operating regime should be dynamic (continuous flow) to mimic the conditions that can be expected at the industrial level. Large-scale implementation should be monitored using photocatalytic testing standards. Currently, the standard pollutant used in the photocatalytic degradation processes involving wastewater is methylene blue (MB) (ISO Standard 10678: 2010 [23]). Only a few studies are reported for up-scaled process, and those published used suspended photocatalytic powders under (simulated) solar radiation [24] or UV radiation with irradiance value similar to the UV component in the solar radiation, investigating the influence of the pollutant flow rate [26]. Results recorded using photocatalytic thin films in continuous flow processes in conditions similar to the real ones are scarce, mainly because in this case the focus is on the photocatalyst durability in the operating conditions [27], besides the high process efficiency.

2.2

Multilayered composite thin films based on wide band gap n-type semiconductors (TiO2) and narrow band gap p-type semiconductors (CuxS, CuInS2, and Cu2ZnSnS4)

The most widely used sulfide in diode-type heterostructures is copper sulfide. As a cocatalyst, copper sulfide was considered in composites aiming at modifying the activation of TiO2 to obtain significant improvements in the photocatalytic process [28]. Copper sulfide is a complex composed of five stable compounds found in nature, namely: covelline (CuS), anilite (Cu1.75S), digenite (Cu1.8S), djurleite (Cu1.95S), and chalcocite (Cu2S), therefore copper sulfide is generically written as CuxS (2>x > 1). The CuxS powder can be obtained using various methods, ranging from solid-state reactions to processes at high temperatures or hydrothermal methods [29]. The use of copper sulfide(s) with TiO2 represents an alternative for shifting the TiO2 activation energy toward Vis [30]. The example in Fig. 11.4A shows the

Figure 11.4 Alignment of the energy bands in the TiO2 AeCuxS ¼ and the activation of the composite when irradiated with: (A) Vis radiation; (B) UV radiation [31].

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activation of TiO2 only by the migration of electrons from its valence band to the valence band of CuxS to fill the holes resulting from the Vis activation of the p-type semiconductor. However, the small energy difference between the semiconductor VBs leads to a fast recombination of the e with the hþ. Comparatively, Fig. 11.4B outlines the activation of the composite using UV radiation, when the activation of the n-type semiconductor (TiO2) with the formation of the ee hþ pair can be observed. However, the use of UV radiation raises the process cost, as these composites do not meet the condition of activation with radiation corresponding to the Vis spectral range. Moreover, the stability of the photocatalytic material in the working conditions is not fulfilled by this type of composite because sulfides are less stable in the aqueous environment, which limits the use of this type of composite photocatalytic (powder) structures. One potential alternative is the use of composite layered thin films with a suitable position of the semiconductor layers: the outer layer corresponds to a stable and Vistransparent semiconductor, without affecting the alignment of the energy bands. The CuInS2 (CIS)eTiO2 structures [32], met these requirements, as presented in Fig. 11.5 where the Vis-activation of these composites is presented for a structure with the outer layer of TiO2 (Vis-transparent) that allows the passage of this type of radiation to the Vis-active petype semiconductor where the electron-hole pairs are formed. The transition of the electrons to limit the recombination process will take place from the CIS conduction band to the TiO2 conduction band, while the holes will eventually occur at the TiO2 surface, as a result of the electrons’ migration from their valence band to the CuInS2 VB to neutralize the holes formed during the electron photoexcitation. The main issue related to the TiO2eCIS type structures is the use of a rare element (In) that is also used in the photovoltaic industry, and this competition can hardly be accepted for up-scalable products. Therefore, alternative solutions were proposed

Figure 11.5 Alignment of the energy bands in TiO2 AeCIS (CuInS2) composite structures [33].

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involving CuSbS2, but the composites showed limited efficiency mainly because of the poor interfaces [34]. The TiO2eCu2ZnSnS4 (CZTS) thin film structure [27,35,36] represents another viable alternative that replaces the toxic (e.g., Cd in CdS) or rare (In in CIS) elements in the narrow band gap semiconductor structure. The CZTS thin film deposited in composite layered structures with the TiO2 layer exposed to the polluting environment (Fig. 11.6), results in stable composites without affecting the proper alignment of the energy bands as the activation is similar to those of the TiO2eCIS type structures in Fig. 11.5. The main issue related to CZTS is that it requires a carefully selected set of deposition conditions to avoid the formation of oxides (e.g., CuO) as a major impurity in the CZTS structure. The development of unwanted by-products leads to an unsuitable alignment of the energy bands in the diode TiO2 (n)eCZTS (p) structure; moreover, the deposition conditions for CZTS with low impurity content significantly increase the production costs of the composite. In the case of Vis-active multilayered composite thin films with a stable semiconductor exposed to the polluting environment (e.g., TiO2), the structure and morphology of the outer layer significantly influence the efficiency of the photocatalytic processes. The challenge is to deposit materials that simultaneously meet the Vis-activation requirements along with increased stability and durability in the working environment, in a continuous flow operating regime.

2.2.1

3D composite thin films with wide band gap ceramic semiconductor (TiO2) matrix and graphene oxide, reduced graphene oxide, or graphitic carbon nitride filler

Current research on heterogeneous photocatalytic processes is aimed at the use of graphene/graphene derivatives, and this choice is well justified by the unique properties of these materials. By coupling with metal oxide semiconductors (e.g., TiO2), the composites are expected to show high photocatalytic efficiency due to the limitation of

Figure 11.6 Alignment of the energy bands in the TiO2 AeCu2ZnSnS4 (CZTS) structure [37].

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the eehþ recombination; moreover, these composites are Vis-active and have high stability in the aqueous environment even at extreme pH values being obtained using simple, up-scalable techniques, at an acceptable cost. Therefore, the composites with graphene/graphene derivatives may represent ideal candidates for meeting the set of requirements for Vis-active photocatalytic materials. The semiconductors (e.g., TiO2) coupling with graphene was proposed, in particular, to avoid the electron-hole recombination, the carbon polymorph playing the role of an electron scavenger to limit recombination and was less considered Visactivator. However, in any composite, one major prerequisite is related to the quality of the interface developed between the components, and the compatibility between a metal oxide matrix (ionic compound) and graphene (nonpolar structure) is very low. Therefore, in photocatalytic composites, the use of graphene oxide (GO) was considered, thus supporting the attraction between the matrix species and the filler with the development of stable chemical bonds. In order to simultaneously get the properties of graphene and those of GO, composites with a metal oxide matrix and reduced graphene oxide (rGO) filler were additionally investigated. Reduced graphene oxide has properties similar to those of graphene (good electrical conductivity, good stability), but, due to the residual O atoms in the structure, it also has properties specific to graphene oxide (semiconductor behavior, hydrophilicity). Composites with metal oxide matrix and graphene derivatives (GO, rGO) fillers were intensively investigated in recent years, in photocatalytic applications for the degradation of organic pollutants and for self-cleaning surfaces [38,39]. The studies outlined that the obtaining routes of these composites have to be selected considering the relatively low thermal stability of graphene/graphene derivatives that at temperatures higher than 160e 170 C react with the oxygen in the air and oxidize to CO2 [40]. By coupling the oxide photocatalyst with graphene derivatives, the recombination of electrons and holes can be significantly reduced, and the band gap is narrowed, the composite thus becoming Vis-active [41,42]. GO behaves as a p-type semiconductor and forms n-p (diode-type) Vis-active heterojunctions with TiO2 [42]. These structures limit the e  hþ recombination by transferring electrons from the GO conduction band in the composite structure to the TiO2 conduction band, as the energy bands are suitably aligned to support the e and hþ transition Fig. 11.7.

Figure 11.7 Alignment of the energy bands in TiO2 (anatase)eGO composite structures with (A) 50% functionalization, EgGO ¼ 1.2eV, [43]; (B) 50%, EgGO ¼ 1.8eV [44].

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Different band gap values were reported depending on the degree of functionalization, as the band gap value in graphene derivatives varies with the degree of oxidation; therefore, it is difficult to predict the behavior of these composites only based on the alignment of the energy bands. Various studies even outlined different values of the band gap energy at the same percentage of functionalization as presented in Fig. 11.7. However, experimental measurements using the composites demonstrate shifts to longer wavelengths in the absorbance spectra, indicating the Vis-activation of the composites [45e47]. Yeh et al. demonstrated that the position of the CB does not vary with the oxidation degree in GO and can be found at 0.5 eV versus NHE [48].

2.2.1.1 Photocatalytic activity of TiO2eGO composite thin films There are two routs of using photocatalytic materials in advanced wastewater treatment processes: as powders or as thin film(s) deposited on an inert substrate. When using the photocatalytic powders, the process requires in the end an additional step of separation (filtration) that involves additional costs and leads to the discontinuity of the advanced wastewater treatment flow, for recirculating the powders. Therefore, thin films represent a viable option to be used as photocatalytic materials. Photocatalytic thin films were deposited on glass substrates covered with fluorinedoped tin oxide (FTO). Before deposition, the substrate was cleaned using deionized water and detergent in an ultrasound bath; further on, it was rinsed in ethanol and dried in air. An overall thin film surface of 20  30 cm2 was placed in the demonstrator photoreactor, Fig. 11.8 [27]. The surface contains five 10  10 cm2 plates, one 8  10 cm2, and five 2  2 cm2 plates used for analyses before and after the process. Parallel experiments were run at laboratory scale, in a static (no flow) regime, using 1.5  1.5 cm2 glass/FTO substrates. The photocatalytic thin films were deposited following two steps.

Figure 11.8 Demonstrator reactor before (left) and during the photocatalytic process (central and right).

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(1) A first TiO2 layer was deposited by spray pyrolysis using a precursor system of titanium tetra-isopropoxide, acetylacetone, and ethanol mixed in a volume ratio of 1:1:15 [49]. This precursor system was deposited at 400  C using eight spraying sequences with a 60 s break between two consecutive deposition pulses. After deposition, high-temperature annealing was applied for 3 h at 450 C to obtain an acceptable crystallinity degree. (2) A second layer was deposited over the first TiO2 layer by spraying a diluted solution of the composite with a TiO2 matrix and GO filler (TiO2eGO composite). The solution was prepared using titanium tetra-isopropoxide, ethanol, acetylacetone, and acetic acid in a volume ratio of 1:0.8:0.04:0.009 [49]. An aqueous GO dispersion (30 mg/mL) was slowly added under continuous stirring in the TiO2 precursor system to get the composite sol with a GO weight ratio of 5%wt. After synthesis, the composite solution was sonicated for 1.5 h and was then diluted using ethanol in a volume ratio of sol:EtOH ¼ 1:5 [49]. The diluted sol was then sprayed over the first TiO2 layer using 12 spraying sequences with a 60 s break between two consecutive deposition pulses. The sol was sprayed at 100 C to support the ethanol vaporization, followed by 1 h of thermal treatment at 150 C to further remove the possible by-products in the thin film without oxidizing the GO filler [50].

The thin film characterization used X-ray diffraction (XRD, Bruker D8 Discover) and the DIFFRAC-EVA 5.0 software to evaluate the crystallinity degree. The surface morphology was investigated using scanning electron microscopy (SEM, Hitachi model S-3400 N type II), and the average roughness of the film (RMS) was measured using atomic force microscopy (AFM, NT-MDT model BL222RNTE). The surface elemental composition was evaluated using energy dispersive X-ray spectrometry (EDX, Thermo). The photo-degradation experiments were run in a static regime, at laboratory scale, and in continuous flow regime, using the demonstrator reactor and a 10 ppm MB aqueous solution, as recommended by the ISO 10678:2010 [23], standard. In the demonstrator reactor there were used 5 L of MB solution with a constant flow of 1 L/min, and the thickness of the aqueous layer over the photocatalyst was 20 mm [27]. In the beginning, both processes (in static and dynamic regimes) ran for 1 h in dark to reach the adsorption/desorption equilibrium [49]. Then, the photocatalytic reactors were continuously irradiated for 8 h using a solar simulator Fig. 11.8, and the pollutant residual concentration was evaluated each hour; based on the absorbance value recorded using a UVeVis-NIR spectrophotometer (Perkin Elmer Lambda 950), the efficiency in degrading the MB pollutant was calculated. The average irradiance value over the photocatalytic demonstrator reactor was G ¼ 810 W/m2, out of which 24 W/m2 corresponded to the UV radiation and the rest to VIS. The experiments in static regime used another homemade reactor (overall volume of 20 mL) irradiated with radiation with significantly lower irradiance value (G ¼ 55 W/m2, out of which 3 W/m2 corresponded to the UV radiation). The results of the XRD analyses are presented in Fig. 11.9. The thin film before photocatalysis in the (a) graph in Fig. 11.9 outlines the peaks corresponding to TiO2, the anatase polymorph, and the characteristic peaks of SnO2 from the FTO substrate, well confirming the deposition of the thin film matrix. The absence of the peak corresponding to GO (at 2q ¼ 12.1 [51]) may be the result of the chemical bonds (partially) developed between TiO2 and GO.

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1-TiO2 anatase 2-SnO2 cassiterite 3-C orthorhombic (GO)

2

3 Intensity [a. u.]

2

2

1

Thin film

(a)

Before photocatalysis

44.6

(b)

After photocatalysis

41.0

2 2

2

(b)

2

(a) 5

10

15

20

25

30

35

40

45

50

55

Crystallinity degree [%]

60

65

70

2Theta [degree]

Figure 11.9 XRD patterns and crystallinity degree of the thin film: (A) Before and (B) after photocatalysis.

Considering the rather high crystallinity degree of the material that wasn’t annealed at high-temperature after the overall deposition, it may be assumed this film to be potentially efficient in photocatalysis as an ordered structure well supports the reduced recombination of the photo-generated electrons and holes. The XRD results recorded after photocatalysis (Fig. 11.9, graph b) outline almost similar patterns as before the photocatalytic process and a slight decrease in the crystallinity degree confirming the good stability of the photocatalysts in the working condition. The morphology, the surface elemental composition, and the roughness of the thin film before and after photocatalysis are inserted in Table 11.2. The SEM images outline large TiO2 aggregates that well cover the GO platelets suggesting good compatibility between the filler and the matrix. Even with these large aggregates stacked on the photocatalyst surface, the EDX results outline the substrate elements (SEs), suggesting that the film is thin. After photocatalysis, the EDX results outline a small decrease in the titanium (Ti) content without significant modifications in the carbon (C) content. This may suggest that part of the titanium oxide aggregates leave the surface, possibly in the MB pollutant solution, as also confirmed by the decrease in the RMS value. The very low amount of sulfur (S) on the photocatalyst surface may result from the MB adsorption and outline that the pollutant was well decomposed during the process. The photocatalytic efficiencies in continuous flow and in static regimes are shown in Fig. 11.10. The results outline that reaching the adsorption-desorption equilibrium requires a longer time (over 1 h) in a dynamic regime, in continuous flow. A fast adsorption was observed in the beginning, and adsorption took place for about 1 h more, after irradiation was started. This may suggest that the irradiated substrate requires a certain duration for the development of the oxidation species and this period is significantly higher than in the static regime. As the results show, after 2 h, the process in continuous flow begins to run faster as compared to the process in a static

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Table 11.2 Morphology, elemental composition and roughness for the composite thin film glass/ FTO/TiO2/TiO2eGO before and after the photocatalytic process. Thin film

Surface elemental composition, EDX [at%]

SEM images

Before photocatalysis

C: 9.22 Ti: 31.02 O: 54.34 Substrate elements (F, Sn, Si) SE*: 5.41

After photocatalysis

C: 9.97 Ti: 29.18 O: 56.11 S: 0.20 SE*: 4.55

Efficiency in continous regime Efficiency in static regime

15

DARK

ηt [%]

20

284.2

239.9

Figure 11.10 Photocatalytic efficiencies in continuous flow (dynamic) regime and in static regime using photocatalytic this films of glass/FTO/TiO2/TiO2eGO.

30 25

Roughness, RMS [nm]

10 5 0 0

1

2

3

4

5

6

7

8

Time t [hours]

regime. However, there are only small differences between the results recorded in the static regime and the results in the dynamic regime, although the irradiance value was about 15 times higher during the dynamic regime as compared with the static regime (810 W/m2/55 W/m2). This may be the result of an increased recombination rate of the photo-generated holes and electrons, deactivated before being involved in chemical oxidation reactions, as a possible consequence of the crystallinity degree that reaches only 44.6%.

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To overcome this issue, a similar filler with much higher thermal stability to make possible the thermal treatment after the deposition should be investigated. An alternative for obtaining composite structures with high crystallinity degree is represented by the coupling of TiO2 (the mostly used photocatalyst) with graphitic carbon nitride (g-C3N4), a narrow bandgap (Eg w 2.7 eV) n-type semiconductor. The suitable energy band alignment allows the VIS-activation of the composite along with a limited electronehole recombination [52]. By coupling TiO2 with g-C3N4, Tie OeN, TieO ¼ C, or TieOeC bonds may be formed. These bonds support the development of continuous interfaces between the matrix and the filler [53]. The resulting tandem structure, through the suitably positioned VBs and CBs of the two semiconductors, allows separate circuits for the electrons (and for the holes) and gives time to the holes produced in the VB of the g-C3N4 to react with water and/or with the hydroxide anions (HO) to form the oxidizing species (hydroxyl radicals, HO•). For the TiO2eg-C3N4 composites, Yang et al. [54], investigated the stability in the aqueous environment by photodegrading rhodamine B (RhB) and concluded that after four successive photocatalytic cycles (each cycle lasting 4 h), the photo-degradation efficiency is only slightly lower than during the first cycle. Moreover, the TEM analyzes after the four photocatalytic cycles show insignificant morphology changes. These findings are further supported by Hao et al. [55], who also used TiO2eg-C3N4 composites for the RhB photo-degradation and observed that, after five photocatalytic cycles (with a total duration of 400 min), the photo-degradation efficiency remained relatively constant. These results confirm the high stability of the composite structure and support the reuse option in consecutive photocatalytic cycles, which is a major asset when considering up-scaled applications.

3.

Concluding remarks

The current context of excessive consumption of drinking water resources requires novel solutions to solve the problems raised by the water “stress.” The most obvious path to limit the fresh water consumption is related to the wastewater reuse. The reused water must meet certain requirements for being considered in usual applications, mainly related to the elimination of pollutants at very low but still unacceptable concentrations when an additional step of advanced wastewater treatment is required. However, this step is not available yet (for organic pollutants) or is unaffordable and makes the process unacceptable. A viable option for this treatment is represented by the AOPs, when oxidizing species (e.g., the hydroxyl radical) are obtained by reactions produced on an irradiated photocatalyst. Among the AOPs, the most investigated is heterogeneous photocatalysis, mainly because at the end of the process no by-products result in the form of sludge as in the case of homogeneous photocatalysis. The photocatalytic materials are semiconductors that produce electron-hole pairs involved in the generation of the oxidizing species. Photocatalytic materials must meet a set of minimum requirements for being viable in the heterogeneous

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photocatalysis process: to be stable in the working (aqueous) environment; to be Visactive, thus limiting the activation costs by using the solar radiation (UV þ Vis) for activation; to have a high capacity to produce electron-hole pairs and to support only a highly limited recombination of the charge carriers; to have a high specific surface area and a high crystallinity degree; to be nontoxic; and to have acceptable production costs. Most of these requirements are fulfilled by TiO2, which is the most commonly used photocatalyst. The main issue when selecting this material is related to its wide band gap (3.0e3.2 eV), which makes activation possible only using UV radiation. The UV percentage in the solar radiation spectrum is low thus UV activation occurs using artificial irradiation sources that increase the cost of the photocatalytic process. To overcome this issue, solutions are investigated to prepare Vis-active photocatalytic materials by doping or coupling two or more semiconductors. The most common Vis-activation strategies include the development of n-p diode heterostructures by coupling an n-type with a p-type semiconductor. TiO2 is a wide bandgap n-type semiconductor, highly stable in the aqueous environment, thus the focus is to find viable solutions for efficient composites in Vis-activated photocatalysis. Such composites can be obtained by coupling TiO2 with p-type semiconductors with a narrower band gap, such as CuInS2 (CIS), Cu2ZnSnS4 (CZTS), or graphene derivatives such as GO or rGO. These composites are VIS-active following the suitable alignment of the energy bands that allow the formation of diode-type heterostructures. The composites containing graphene derivative fillers exhibit promising photocatalytic results, but the thermal instability of these fillers prohibits the annealing at a suitable temperature, leading to low crystallinity degree values followed by average photocatalytic efficiencies in the advanced wastewater treatment targeting the reuse. To overcome this issue, the (r)GO filler can be replaced with g-C3N4 a thermally stable, narrow bandgap n-type semiconductor. By coupling TiO2 with g-C3N4, Vis-active composites result, with the advantage of a limited electron-hole recombination.

Acknowledgments This work was supported by two grants of the Romanian Ministry of Research and Innovation, CCCDIeUEFISCDI: contract no. 42 PCCDI/2018 and contract no. 124 PED/2017.

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Amit Mallik Department of Chemistry, Acharya Jagadish Chandra Bose College, Kolkata, West Bengal, India

1. Introduction The rapid progress of science and the technological impact of combining substrate tools with the properties of functional advanced ceramic materials are immense given their relevant role in the progress of the novel and more efficient electronic devices [1]. The semiconductor industry currently has a $584 billion (B) turnover approximately worldwide. At a much later stage of historical development, there was available a range of products that were similarly formed from nonmetallic, inorganic solid materials but which relied on very substantial raw materials for modification and refinement or even on the design of utterly new compositional systems in order to provide properties matched to more specific and exacting requirements [2]. The applications of developed advanced ceramic materials have many directions. Types of ceramic materials are shown in Fig. 12.1. The initial application of ceramic materials in the electrical perspective was as an insulator, and electrical porcelains and aluminas sustain a significant role in this connection. In the period since 1940 however, a great divergence in the job has occurred with innovations in materials improvement and in the associated solid-state theory taking place in recital.

Figure 12.1 Type of ceramic materials. Advanced Ceramic Coatings. https://doi.org/10.1016/B978-0-323-99659-4.00009-7 Copyright © 2023 Elsevier Ltd. All rights reserved.

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Recently, the countless advances being made in mechanical, coating related applications at higher temperatures, and the protection of devices are seen as perhaps the main target for the realization of the promise of advanced ceramics [3e6]. The properties that lie behind this promise include wear resistance, hydrophobicity, hardness, stiffness, compressive hardness, mechanical strength [7], corrosion resistance, environmental barrier [8,9], and density. The main attraction, however, is the refractoriness of ceramicsdthat is, the high melting point and the retention of mechanical strength to high temperatures and irradiations of waves. In Table 12.1. the melting temperatures of some ceramic materials are shown. A general flow chart of the fabrication of ceramics is shown in Fig. 12.2.

2.

Ceramic composites

Humans discovered ceramics material in 24,000 BC dated back. At those time advanced ceramics take account one of the strongest and toughest materials [1,6]. The use of advanced ceramic products in construction, electronics, scientific and industrial equipment, defense systems, cars, home appliances, and many other things [1]. Nowadays, ceramic has a broader meaning and takes into account various materials, such as glass, cement, and advanced ceramic composites. Here are some examples of ceramics: (i) Glass-ceramics, (ii) fired bricks, (iii) silicon, (iv) silicon carbide, (v) aluminum nitride, (vi) titanium carbide, (vii) tungsten carbide, and (vii) composite ceramics.

3.

Ceramic coatings

Various substrates, such as ceramic and nonceramic ceramic coatings, are used in the fabrication of wafer equipment [10]. The advantages of metallic substrates contribute to the performance of ceramics, whereas it uses the ease of manufacture of the metal component. It is generally useful for ceramic substrates where a high purity of ceramic is needed. The two beautiful methods, such as thermal spray, chemical vapor deposition (CVD), or physical vapor deposition (PVD), can be used for fabricating ceramic

Table 12.1 Melting temperature of some materials use in ceramic engineering applications. Sr. No.

Material

Composition

Melting point (8C)

1. 2. 3. 4. 5.

Silicon carbide Zirconia Alumina Silicon nitride Aluminum

SiC ZrO2 Al2O3 Si3N4 Al

2650 2770 2054 1900 660

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Figure 12.2 General flow chart of the fabrication of ceramics.

coatings on the substrates. The thickness and consistency of ceramic coatings depend on the method of fabrication [11].

3.1

Ceramic coating by thermal spray method

Thermal spray coatings are generally line-of-vision processes (Fig. 12.3). Thermal spray coatings are extensively used for different applications like wear, corrosion, thermal and electrical insulation, etc. The nature of ceramic coatings is typically different from their bulk materials or even from other types of coatings. The feature of thermal spray coatings is dependent on the microstructure, porosity, and rough as-sprayed surface [12]. Thermal spray coating rates are very fast, and manufacture costs may be

Figure 12.3 Schematic of thermal spray coating.

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quite lower than those of other coating methods. The additional treatment is necessary for prepared thermal spray coatings due to the features of the applications [13]. The three techniques such as flame, plasma, and detonation are involved in the thermal spray coatings. In these techniques, the plasma spray is the easiest and simplest technique. It is generally used for the fabrication of ceramic coatings on metal substrates. Plasma spray coatings are widely used in the application of wear resistance. The disadvantages of thermal spray coating are in porous structure and fewer uniform films than those produced by CVD and PVD.

3.2

Ceramic coating by chemical vapor deposition or physical vapor deposition

CVD and PVD are generally vaporization coating methods that transfer material on an atomic level [14]. These two methods processes are quite similar except for the starting raw materials/precursors in CVD (Fig. 12.4). The precursors are first introduced as the gaseous state in the reaction chamber. But in PVD, the coating material is vaporized by different mechanisms such as heating, the bombardment of an electron gun, or high energy ionized gas, and the vapor phase is shifted to the substrate, resulting formation of a coating (Fig. 12.5). The CVD method produces a compact and high-quality coating. But it is very expensive and the rates of formation of coating are very slow. PVD coating methods also produce a compact and high-quality coating by using thermal vaporization, electron beam vaporization, or sputtering. The disadvantages of the PVD method are the slow rate of deposition and trouble applying the oxide coating well.

3.3

Plasma spray ceramic coatings for semiconductors

The plasma spray technique (shown in Fig. 12.6) is generally used for the fabrication of micro- and nano substrates, particularly in dry etching, but a higher plasma power source is required for the fabrication of large-scale substrates, including high etching rates. The tendency of the large requirement of plasma erosion-resistant coatings on the inside wall of the chamber. There are two pure oxide ceramics, such as alumina (Al2O3) and yttria (Y2O3) that behave as the best coating materials against plasma erosion [15e19]. These highpurity oxide coatings are generally useful for the plasma spraying agglomerated and sintered powders. Figure 12.4 Schematic of chemical vapor deposition (CVD).

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Figure 12.5 Schematic of physical vapor deposition (PVD).

Figure 12.6 Schematic of plasma spray process.

4. Materials coatings for semiconductors 4.1

Oxide ceramics coatings

Oxide ceramics of quartz and alumina are generally used for ceramic coatings [20e23]. Quartz is an extensively used ceramic in the semiconductor industry. Generally, quartz is considered as a crystalline solid material, in the point of view it is widely considered as high-purity silica glass. It is used in the semiconductor industry due to its many properties, such as high purity, chemical resistance, thermal and electrical insulation, etc. The production of quartz parts can be divided into two sections: one is blank or ingot forming, and the second is a fabrication. High-quality silica powder is required for the formation of ingot, which is generally from a melt. The silica powder is kept in a zircon bowl and melted at approx. 2000 C by gas firing. The obtained molten mass is then drawn through a zircon die to produce a big, continuous ingot. To obtain a regular surface of ingots, cutting and machining are essential. Acid cleaning of as-prepared

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ingots is essential for the removal of impurities, which are generally introduced during the processes. The superb quality of alumina is widely used in the wafer-processing semiconductor industry. In wafer processing, the purity of alumina is used in the range between 99.5% and 99.9%. It has versatile properties such as high strength, hardness, stiffness, electrical and thermal insulation, and is corrosion resistance [22,23]. Above properties of alumina can help to make a large number of applications in the semiconductor industry. The parts of alumina are mostly prepared from blanks or adjacent net-shaped pieces by machining. The blanks may be prepared from high purity raw materials using cold isostatic pressing (CIP) or dry pressing, and after that constricted by pressureless sintering. Machining is the significant step for manufacturing high-quality alumina components than the sintering of blanks is comparatively straightforward.

4.2

Carbide ceramic coatings for semiconductors

Among other carbide materials, silicon carbide is vastly used in the production of semiconductor single wafers. Silicon carbide has been widely used in diffusion furnaces due to its high quality and longer temperature strength [24]. Silicon carbide is not only a lighter material but also superb hardest ceramic material than other carbide materials. It has extraordinary thermal conductivity, high elastic modulus, low thermal expansion, and is unaffected by acids and lyes. Silicon carbide offers a wide bandgap semiconductor material and numerous numbers of wonderful characteristics for high voltage power semiconductor [25]. It has also been used in large vacuum chucks due to itsfantastic properties, which are mentioned above. Though, it is a brilliant candidate for plasma etch chambers due to its plasma corrosion resistance and high quality. Manufacturers of large complex parts for silicon wafer process chambers may be able by using an amalgamation of slip casting and machining. Dry pressing, CIP, and extrusion methods are used for the manufacture of simple shapes of silicon wafers. Large parts of silicon wafers can be produced by CIP, which depends on the size of the equipment, and some positive features can be produced by the mold. The protective coating layer of CVD SiC on the surface is essential for plasma etch environments due to silicon carbide (SiC) etches more slowly than silicon. This coating layer can be kept on both large surface areas and on the inner walls of the tubes and partial enclosures. Boron carbide shows similar properties with silicon carbide, but boron carbide has a lower resistivity and lower thermal conductivity. Fabrication of boron carbide parts can be made by hot pressing blanks, with also dense machining used to get large, complicated shapes. Hot pressing provides the limitation of shapes available in boron carbide parts, relying mostly on condensed grinding to carry the desired shape [26]. Smaller shapes can be formed through injection molding and hot isostatic pressing. This method is very fruitful for high-volume runs where the shape is fixed. A plastic binder is mixed with boron carbide powder to allow better flow properties. The mixture easily distorts and flows into the cold mold under high pressure when heated.

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The resultant parts are identical in shape and size. Boron carbide is not widely used in the semiconductor industry, but it has probable applications in plasma technology due to its corrosion resistance and composition.

4.3

Nitride ceramic coatings for semiconductors

In the beginning, silicon nitride was used in the semiconductor industry instead of alumina due to its low thermal expansion and high corrosion resistance. Traditionally, it is used as a form of ceramic ball bearing in the semiconductor industry. Ceramic balls get a wide range of characteristics than standard ball bearings due to their fantastic properties, i.e., less lubrication, slower friction, electrical insulation, and high corrosion resistance. Silicon nitride and alumina both have similar properties like resistivity and dielectric constant, but it shows stronger due to it have exceptional microstructure [27,28]. Silicon nitride shows better resistance to etch environments than alumina. Fabrication of large, intricate parts from silicon nitride shows difficulty due to the high temperature and pressure required for densification. Silicon nitride may be formed as green by CIP and dry pressing, with the ultimate densification, gets under high temperatures and pressures for high purity, or using the liquid phase sintering with the help of MgO and Y2O3 as sintering aids. Aluminum nitride is used in electrostatic chucks (ESCs) due to its beautiful electrical properties. Aluminum and silicon both have similar properties, i.e., high thermal conductivity and better thermal expansion, which helps to make a desirable material for ESCs that can offer fixturing for the silicon wafer in vacuum environments in the absence of mechanical clamps [29]. Fabrication of fixed electrodes can be made by different processes, but cofiring method is the most common approach. In the semiconductor industry, hexagonal boron nitride is used in a diffusion furnace, where boron nitride is used to aggressively dope silicon wafers. Boron nitride is also used in ion implant equipment as an insulator. Boron nitride also shows the expectational properties, i.e., high dielectric strength, thermal conductivity, machinability, and low dielectric loss, which help to make a desirable part [30].

5. EMI shielding mechanism of ceramic coated materials The prominence of ceramic coatings is increasing due to their attractive physical and chemical properties, such as low density, outstanding hardness, good mechanical strength at high temperature, high thermal conductivity, good thermal shock resistance, EMI shielding, semiconductivity, and high-temperature oxidation resistance [1,31e33]. Different types of electromagnetic (EM) radiation are shown in Fig. 12.7. EMI shielding materials have shown a vital role in the present society for the extensive use of various electronic equipment and semiconductor devices in commercial and military fields [34e36]. Excessive EM radiation will not only have a negative effect on people’s nervous systems but also affect the endocrine system and eventually cause

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Figure 12.7 Various types of EM waves.

serious harm to our health [37e39]. An EMI shielding mechanism of ceramic-coated materials is shown in Fig. 12.8. EM waves are unwanted EM radiation triggered by extensive use of alternating current or voltage, which creates corresponding induced signals in the nearby electronic device and tries to spoil its performance [39,40]. EMI radiation is blocked, which means the generated noises are stopped by using shielding materials [41]. The conducted EMI uses filters to impede these noise formations in electronic circuits [42]. Propagation of EM waves and interaction shielding coatings are shown in Fig. 12.9. Various terminologies related to EMI shielding and EM waves are very important to discuss and elaborate on the EMI shielding of a material [43]. Wang et al. reported a 3-dimensional graphene/SiBCN/SiC ceramic composite for effective resistance at high temperatures [44]. Fig. 12.10 represents the EMI shielding mechanism of a ceramiccoated material layer on a semiconductor device.

6.

Future perspective

The coating technology was developed on a larger scale for electronics device coating applications to prevent the growth of fouling, shielding the outer EM radiation, or

Figure 12.8 EMI shielding mechanism of ceramic coating layers on semiconductor.

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Figure 12.9 Propagation of electromagnetic waves and interaction with shielding material.

Figure 12.10 Schematic representation of EMI shielding process of a ceramic coating material layers on semiconductor devices.

other damaging phenomena. The coating technologies for semiconductor coating applications are of large interest in preventing damage or disturbances of the devices. The modern studies on advanced coatings are motivated in the area of nontoxic, nonbiocide, environment-friendly-based coatings. Though the upcoming research is more prospective concerns on the time span of the recently established engineering technologies. Several innovative coating technologies are based on principles or optimization in the direction of the extreme-performance theme, valuing their performances

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utilizations in the electrical field (electronic devices), which is supposed to be the greatest promising entrant between the various ranges of possibilities for ceramic coatings as known. The ceramic coating technologies require more research in the following areas, such as: (a) Developments on photo-curing ceramic coatings by following stringent marine policies that reduce toxic compound emissions, environmental impact, production, and maintenance costs. (b) Development and advancement of self-healing ceramic coatings, which will bring superb performance along with a longer lifespan to the ceramic coatings. (c) Newly developed hybrid-like ceramic coatings to improve the efficiency of the coatings for semiconductor coating applications.

7.

Conclusion

Advanced ceramic materials will continue to play a crucial role in protecting semiconducting devices that are increasingly being used in hot sections of the electronic industry due to ever-increasing global demands. The major desires for ceramic coating systems are hot-corrosion and recession resistance for protection from outer harsh environmental conditions. The coating systems must have good shielding properties to reduce the effects of EM radiation, and to protect the devices from other environmental effects (e.g., temperature, moisture, etc.) on semiconducting devices at the interface stresses.

Acknowledgment A. Mallik thankfully acknowledges to Department of Chemistry, Acharya Jagadish Chandra Bose College for the facility of lab and computer.

Declaration The authors have no conflict of interest.

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[38] D. Lu, Z. Mo, B. Liang, L. Yang, Z. He, H. Zhu, et al., Flexible, lightweight carbon nanotube sponges and composites for high-performance electromagnetic interference shielding, Carbon 133 (2018) 457e463. [39] J.T. Orasugh, C. Pal, M.S. Ali, D. Chattopadhyay, Electromagnetic interference shielding property of polymer-graphene composites, in: Polymer Nanocomposites Containing Graphene, Woodhead Publishing, 2022, pp. 211e243. [40] P. Saini, M. Arora, Microwave Absorption and EMI Shielding Behaviour of Nanocomposites Based on Intrinsically Conducting Polymers, Graphene and CarbonNanotubes, New Polymers for Special Applications, 2012, pp. 71e112. [41] H. Mei, X. Zhao, X. Gui, D. Lu, D. Han, S. Xiao, L. Cheng, SiC encapsulated Fe@ CNT ultra-high absorptive shielding material for high temperature resistant EMI shielding, Ceramics International 45 (14) (2019) 17144e17151. [42] G. Jose, P.V. Padeep, Electromagnetic shielding effectiveness and MechanicalCharacteristics of polypropylene based CFRP, IRD India 3 (2014) 2319. [43] H.W. Ott, Electromagnetic Compatibility Engineering, John Wiley and Sons, New Jersey, 2009. [44] C. Wang, Y. Liu, M. Zhao, F. Ye, L. Cheng, Effects of upgrading temperature on electromagnetic shielding properties of three-dimensional graphene/SiBCN/SiC ceramic composites, Ceramics International 45 (17) (2019) 21278e21285.

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Soumen Das 1 , Supratim Mukherjee 1 and Ashish Jain 1,2 1 Materials Chemistry and Metal Fuel Cycle Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu, India; 2Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai, India

1. Corrosion and wear Modernization of technological development demands materials to perform satisfactorily under extreme operating conditions. Hostile environments cause gradual degradation of materials by corrosion in the form of wear and tear due to physical, chemical, and electrochemical interactions. Wear corrosion refers to surface degradation and loss of materials from the surface due to mechanical or chemical actions, resulting in a shorter lifetime and reduced performances of materials. Hence, from a technical and economical views, it has become an absolute necessity to ensure protection of structural materials against aggressive environments using corrosion-resistance coating layers. Though the development of anticorrosion coating is known since ancient times, there is relentless effort to meet the ever-growing demand for new coating materials and coating technology to cater to numerous industrial requirements [1,2]. Continuous development of modern infrastructure, technological advancement, an unregularized industrial boom, acidification of natural water and air have intensified the corrosion problems [3e6]. Moreover, major industries, such as automobiles, steel, nuclear power plants, aerospace, and offshore oil and gas extraction suffer continuous corrosion issues that lead to substantial economical and functional burdens [7e10]. Economic data of the US Government reveal that almost 4% of the gross national product of developed countries goes into prevention, maintenance and replacement of corrosion-affected components [11]. Corrosion is the gradual degradation of a material caused by chemical and electrochemical reactions with its environment. Since metals are electrically conductive, their corrosion is generally electrical in nature. On the other hand, for ceramics and other nonmetallic materials, the deterioration happens due to chemical causes. Corrosion is categorized according to [12,13] (a) (b) (c) (d)

The mechanism of the corrosion process, The nature of the environment, Type of corrosion deterioration and Type of corrosion reaction.

If one delves deep into these phenomena in detail, then it is clear that the mechanism of corrosion involves chemical corrosion, electrochemical corrosion, crevice Advanced Ceramic Coatings. https://doi.org/10.1016/B978-0-323-99659-4.00015-2 Copyright © 2023 Elsevier Ltd. All rights reserved.

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corrosion, deposit corrosion, waterline corrosion, intercrystalline corrosion, erosioncorrosion, etc. The electrochemical corrosion involves: (i) (ii) (iii) (iv) (v)

Galvanic corrosion or bimetallic corrosion, Structural selective corrosion, Selective dissolution Layer corrosion or exfoliation, Thermogalvanic corrosion.

On the basis of environment, the corrosion phenomena can be categorized as (a) atmospheric, (b) marine, (c) underground, (d) biological, (e) radiation, (f) liquid metal, (g) molten salt, (h) acid, alkali, and (i) electrolyte, etc. [13e15]. The chemical composition, dust in the atmosphere and relative humidity are the three complementary factors that affect atmospheric corrosion. Biological corrosion is initiated due to the activity of the living microorganism that produces metabolites, such as organic acids, sulfides, and sulfates. Banerjee et al. [12] have pointed out that unlike surface corrosion, which is mainly confined to the metal surface, local corrosion or pitting occurs on a very small area. The intercrystalline corrosion, on the other hand is centered on the grain boundaries [16,17]. On the basis of its heterogeneous nature, certain areas of the metal surface may corrode at a higher rate than others. Apart from this, one of the most important environmental factors that determine the corrosion behavior of metals is the pH value of the electrolyte. If a metal surface is in contact with a fluid that has a variation of pH due to many factors, either spontaneous or intentional, or due to the (i) nature of chemical reactions and (ii) variations of concentrations, etc., then areas with different potentials initiate electrochemical corrosion. Fig. 13.1 shows various types of corrosion on metal surfaces. Noble metals are immune to atmospheric corrosion. Materials such as aluminum, chromium, stainless steel, and titanium which are passivated by in situ formation of a thin oxide barrier layer are resistant to acidic pollutants in the air. They suffer corrosion when chlorides are present in the air or when conditions are conducive to galvanic corrosion [13]. Oxidation and tarnishing of metals, in a way, are unavoidable corrosion events due to the presence of oxidizing gas such as oxygen and halogens. When the metal surface is exposed to such gas at atmospheric pressure and room temperature, it reacts with gas and corrodes even under dry conditions. Thus, the surfaces of most metals when exposed to oxygen in the air are immediately covered with a thin film of oxide. Owing to the fact that the oxides in the grain boundaries dilate the boundary or lattice, thus it stresses the metals. Moreover, changes in the nature and composition of oxides (in the case of transition metals and elements with multivalence states) can generate stresses at various points facilitating interfacial voids and spalling of the scale, as observed in the case of some Fe-Cr alloys [13]. At high temperatures, the interactions of metal surfaces with oxygen or hot oxidizing gases render hightemperature oxidation rather uniformly leading to intergranular or localized corrosion in combustion equipment, heat exchangers, furnace, boilers, and turbines. Apart from the wet and dry corrosions, the relative movement between solid bodies or flow or sliding of hard particles against another surface, or hard bodies involved in impact invariably damages the surfaces in contact. In general, wear depends on (1) the

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Figure 13.1 Representative images of (A) surface corrosion, (B) corrosion cracking, (C) exfoliation corrosion (photograph supplied courtesy of L. Hahara, Hawaii corrosion laboratory), (D) intergranular corrosion below a metal surface, (E) air cooler tube with severe pitting, (F) irradiated (770 Gyh-1 60 h) copper surface. Images (A)e(D) were taken with permission from P. Adrian. Mouritz (Eds.), Corrosion of aerospace metals in Introduction to aerospace materials, Woodhead Publishing Limited, 2012 498e520, 2012 Woodhead Publishing Limited, Image (E), (F) was taken with permission from Å. Björkbacka, S. Hosseinpour, M. Johnson, C. Leygraf, M. Jonsson, Radiation induced corrosion of copper for spent nuclear fuel storage, Radiation Physics and Chemistry, 92 (2013) 80e86. M. Stewart, O.T. Lewis, Heat Exchanger Equipment Field Manual: Common Operating Problems and Practical Solutions, Elsevier Inc, NY, 2013, pp 253-419, respectively, 2013 Elsevier Ltd.

structure of the materials and the geometry of the interaction, (2) the forces, stresses, and duration of the interaction, and (3) the environment and surface conditions i.e., the ambient temperature, surface chemistry, topography, and environment [18]. These types of wear are categorized as adhesive, abrasive, erosive, corrosive, and impact wear. There are also other types of wear viz. fatigue, fretting, cavitation, diffusive, and melting wear. Almost any interaction involving contact between solid bodies will cause wear. In general, the terms scuffing, scoring, and galling are associated with severe sliding wear. In erosion, wear is caused by hard particles striking the surface, either carried by a gas stream or entrained in a flowing liquid. The oxidative wear is related to the oxygen as a corroding agent as discussed above [19]. If two parallel surfaces are brought in contact, the surfaces move closer together as the normal load is increased. The macroscopic elastic stress causes subsurface damage owing to plastic deformation, fracture, or fatigue. When the stresses on the surface vary owing to repeated movement of the contact, the radial tensile stress on the surface may result in fatigue or cracking of brittle material. On the other hand, shear stress on the axis of loading below the surface results in plastic deformation. For higher normal load, the plastic deformation zone extends up to the free surface. Crystal structure,

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dislocation vacancies, and stacking friction determine the plastic deformation, which in turn affects the adhesion, friction, and wear behavior of ceramics. The profile and rate of wear are influenced by the stresses and mechanical damage, thermal effects, chemical reactions and interactions of the surfaces. These factors, in turn, are governed by both the physical and chemical responses of the surfaces [20]. For example, partially stabilized zirconia affects wear in aqueous environments, with a change in the coefficient of friction [21]. The metal-ferrite adhesive bond at the interface is a chemical bond between the metal atoms and the large oxygen ions in the ferrite surface [22,23]. The strength of this bond is related to the oxygen-to-metal bond strength in the metal oxide. With transition metals, a higher d-bond character reduces metal activity, adhesion, and friction. Thus, finally, the primary objective boils down to protecting useful equipment and materials from corrosion, wear, and tear to minimize economic impact and to enhance their lifetime. In a nutshell, the primary requirement is to isolate the metal or nonmetal surfaces from the corrosive environment and in the process reduce the wear and tear. Researchers have identified five primary methods of corrosion control for specified desired objectives [12]. These are: (a) Material selection, i.e., selection of proper materials for a particular environment based on the characteristic features of materials and the medium (b) Coatings, i.e., applying suitable protective coatings on the metal surface to isolate it from the abrasive and surrounding corrosive medium to prevent immediate contact, thus resisting damaging the metal surface. (c) Inhibitors, i.e., adding certain chemicals in small quantity to reduce the corrosion rate of a metal (d) Cathodic protection, i.e., lowering the operating temperature, changing the concentration, addition of corrosion retarding substances, etc. (e) Design, i.e. design of equipment to take care of preventive action, plan for replacement before failure, avoid galvanic action and stray current by providing proper electrical insulation, efficient plan for joint and junctions to avoid crevice corrosion, etc.

The function of protective coatings is to keep out the air and moisture from the metal surface and also to isolate the underlying metal surface from the corrosive media. Based on the type of coating used to serve the purpose, the protective coating may be categorized into two different groups-metallic and nonmetallic. The metallic coatings can be divided into two parts: 1. Cathodic coating with electrochemically passive metals such as Cu, Ni, Cr, Pb, Sn, or Ag which keep out air, and moisture from the base surface. 2. Anodic or sacrificial coating with Zn, Cd, and Al which work as a barrier layer and only corrodes itself slowly over time.

The nonmetallic coatings include organic and inorganic materials. Coating of metal surface with organic materials particularly paints and lacquers is very popular and effective as well [24e29]. It is seen that the pigments and grease present in the paint often hinder the electrochemical and other corrosion processes. In the group of inorganic coatings, ceramic materials are widely used as shielding coatings for their

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high strength and hardness, and higher thermodynamic stability, which leads to the chemical inertness of coating, high-temperature withstand, less abrasion, and oxidation stability, among others [30e32]. The effects of surface films are very significant in ceramics. Diamond and TiN display high friction when sliding against surface films in a vacuum; however, in air, much lower friction is measured. Surface oxidation in TiN and changes in the nature of the surface are due to the adsorption of gaseous species in the case of diamonds, which are reported to be responsible for this low friction [20]. The wear rate (Q) is defined as the volume removed/sliding distance to the load W acting normally and is expressed as Q ¼ KW=H

(13.1)

Where H is the hardness. K is the dimensionless wear coefficient. K is high for metals sliding against metals and is low for nonmetals to metals or nonmetals contacts. Eq. (13.1) also reveals that the wear rate varies inversely with H. However, hardness alone does not always predict the relative wear and erosion resistance. Other factors, for example, microstructural features such as carbides in steels or graphite in cast irons also play important roles. Ceramics are identified as ionic solids, and they exhibit a low wear coefficient. Coupled with their greater hardness, ceramics can offer significantly lower sliding or abrasive wear rates than metals. While some ceramics, such as MgO, CaF2, or CsF, exhibit primary ionic bonding, others, such as diamond, SiC, SiN, and TiC have strong covalent bonding. Interestingly, Al2O3, SiO2, TiO2, and ZrO2 exhibit a combination of ionic and covalent bonding characteristics [13]. The corrosion resistance of ceramic coatings depends on the surface properties of the coating as well as those of the substrate, chemical composition, structure, and defects of the coating. The coating method also plays a major role. For example, PVD-coated TiN, CrN, and (TiAl)N films on metal are popular for their high corrosion resistance, superior hardness, chemical stability, and high adhesion [1]. Sol-gel-based nanoscale graphene nanocomposite and carbon nanotubes (CNTs) show promising corrosion protection of aluminum-based alloys in aerospace industries, whereas a reduced graphene oxide (rGO) layer is used between Al2O3 and AISI 304 stainless steel substrates for its high chemical corrosion resistivity. The disadvantages of ceramic coatings are their susceptibility to crack formation under mechanical and thermal shock. However, for elbows, manifolds, and laterals that might need extra protection against abrasion, the exterior ceramic coatings may extend the life of these parts. A component can be surface engineered using three basic process techniques [20] (a) Modifying the surface without altering the composition, such as surface melting and transformation hardening, (b) Modifying the existing surface by changing its composition or that directly results in the formation of new phases and (c) Applying a coating material to the surface to create a hard surface to resist wear or surface damage or to introduce a low-friction surface. This facilitates the use of the ceramic coating in tribological applications by lowering the wear resistance yet retaining the desired bulk properties.

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Metal-ceramic interface and ceramic coating

The wear and corrosion resistance of ceramics are affected due to, (i) the structure of the material itself and (ii) external factors such as load, temperature, and atmosphere. The hardness reflects the bond strength of grain boundaries, though the microstructures of materials often have a substantial impact on the macroscopic properties of materials. For example, the grain size, the porosity, the grain boundary phase, and intercrystalline impurities have a certain influence on the wear resistance of ceramic materials [33e35]. Similarly, ionic species present at the surface, electronic surface states, the chemistry of the material, cohesive bonding energy crystallography, and the defects influence the bond of adhesion between ceramic coating and metal surface. In addition, the coefficient of thermal expansion (CTE) of ceramics is significantly different than that of metals, causing a large thermal expansion mismatch. Interfacial stresses associated with this thermal expansion mismatch can induce interfacial cracks or weaken the joint [36]. One way to reduce this interfacial tension is to keep the reaction layer as thin as possible [37]. A great deal of work has been undertaken to understand the adhesion strength between oxides or nonoxide ceramics with metal substrates based on the interfacial activities and electronic structures [38e42]. The bonding mechanism can be categorized as (a) mechanical interlocking and anchoring, (b) metal-metal or ceramic-metal bonding, and (c) chemical bonding with the substrates. Researchers find that the mechanism of metal-carbide adhesion is likely to differ essentially from the case of oxides owing to the fact that, as mentioned before, the majority of the oxides are ionic crystals. The transition-metal carbides, on the other hand, are characterized by covalent bonding mixed with metallicity and an insignificant amount of iconicity [43]. Dudiy et al. have shown that in the case of a carbide-metal system, interface bond strength depends on the interfacial electronic structure [40]. Song et al. [39] have shown that the substantial adhesion between Nb and single crystalline Al2O3 is due to a strong ionic bond. First-principle calculations by Smith et al. [41] showed that in metalceramic interfaces, mixtures of metallic, covalent, and ionic bonds may be formed involving a number of different elements. Theoretical studies on MgO/Al and MgO/ Ag revealed that changes in equilibrium adhesive energies ranged from 9% to 61%. Although interstitial C in MgO/Al increases the adhesive energy, in most instances, C and S impurities decreased the adhesive energies of both MgO/Al and MgO/Ag [44e46]. The importance of metal-ceramic adhesion led to many pioneering computational and experimental works. This is important because physical and chemical interactions at the interface of a metal to a ceramic determine the integrity of the joints. Earlier, the study was hampered mostly owing to the lack of available experimental data. However, thanks to the untiring work of dedicated researchers, there is substantial qualitative information on the complex phenomena occurring at the interface [38,47,48]. So, basically, the quality of a metal-ceramic bond depends on the structure of the interface, which is influenced by the mutual crystallographic orientation of the metal and ceramic surfaces. For the Nb/Al2O3 system, Burger et al. [49] showed diffusion bonding at the interface at high temperatures. The authors identified Al2O3

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precipitating in the Nb at a distance of 15 mm from the interface. High-resolution transmission electron microscope (TEM) images revealed perfect lattice matching of Nb and Al2O3. Morozumi et al. [50] observed an interlayer of NbO, several pm in thickness [51]. The purity of the material determines the bonding conditions [52,53]. In a similar trend, the bonding of Cu to Al2O3 requires a thin layer of oxygen on the surface of Cu prior to bonding [54], while the bonding of Ti to Al2O3 results in the formation of the intermetallic phases TiAI or Ti3Al, which probably also include oxygen. The thickness of such a reactive layer depends on the bonding time and temperature [51]. The interface of Fe and Al2O3 surfaces was studied by Brydson et al. [55]. Authors pointed out that (i) maximum mechanical interlocking happens if metal wets the alumina, (ii) bonding between the metal and alumina surface takes place through metal-oxygen interactions, and (iii) large-scale spinel formation should be avoided as the spinel-metal interface is weak. The author found out that the prevention of large-scale spinel formation by preoxidation and partial reduction of metal cations at the surface. In this context, semiconductor TiO2 is supposed to be a very good item for Ti4þ is reduced by a large number of metals [54]. Fig. 13.2 shows the crosssectional TEM images of Nb-TiO2 and Cu-TiO2 interfacial regions. Economos et al. [56] reported that for metal-oxide systems, considerable interaction happens at the interface at high temperatures (w1600e1800 C), though the possibility is less at w1400 C. Authors pointed out that the order of reactivity of the oxides is ThO2< BeO < ZrO2< Al2O3< MgO < TiO2 and below 1400 C no reaction in neutral atmospheres occurs. Table 13.1 presents the results of the interfacial reactions of metals and oxides. There are reports on the reactive metal-ceramic matrix to learn about the interfacial interaction and work of adhesion (Wd) [57,58]. Wd is expressed as the sum of localized chemical bonds at the interface, van der Waals interaction, and the nonequilibrium term resulting from the formation of a new phase. Chen et al. [59] compiled

Figure 13.2 High resolution TEM images of (A) Nb-TiO2 interfacial zone showing clear reaction layer which is highly defective and consists of reduced TiO2 and oxidized Nb, (B) the Cu-TiO2 interface which is atomically flat and shows no strain field or misfit dislocations. Images taken with permission from M. Wagner, T. Wagner, D.L. Carroll, J. Marien, D.A. Bonnell, M. Ruhle, Model systems for metal-ceramic interface studies, MRS Bulletin, (August) (1997) 43.

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Table 13.1 Reactions of metal and oxides. Microscopic observations of interfaces

Reactants

Temp (8C)

Microscopic observations of interfaces

Ni-BeO, NiThO2 Ni-A12O3, NiTiO2, NiZrO2 Ni-MgO

1600,1800

No reaction

-

1400,1600, 1800

No reaction; adherence

-

1800

Sharp interface; black discoloration of MgO concentrated around grain boundaries; good adherence No reaction No reaction

-

Mo-BeO, MoAl2O3, MoTiO2, MoZrO2, Mo-MgO, MoThO2 Mo-Al2O3

1600,1400 1600,1400, 1800

2100

Be-BeO

1800,1600

Be-Al2O3

1800

Be-MgO

1600,1400 1600

Be-ThO2

1400 1600

Be-ZrO2

1600

Be-TiO2

1400 1600

Slight black discoloration of Al2O3 at interface Penetration and black discoloration of BeO; no interfacial product or corrosion Definite interfacial layer of new product, probably chrysoberyl; slight darkening of Al2O3 Surface discoloration of plaque; slight oxide on metal Penetration of metal around grains and black discoloration of oxide concentrated around grain boundaries; very slight new interfacial layer No interfacial reaction Penetration around grains of oxide and discoloration of oxide; no interfacial phase Slight discoloration of oxide; no corrosion or interfacial phase Very slight discoloration Reaction with decrease in porosity of oxide near interface and some corrosion of surface

-

BeO (?)

BeO.3Al2O3, Al2O3 BeO MgO

ThO2

ZrO2, BeO -

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Table 13.1 Reactions of metal and oxides.dcont’d

Reactants

Temp (8C)

Si-BeO

1600

Si-Al2O3

1400 1600

Si-MgO

1400 1600

Si-ThO2

1400 1600

Si-ZrO2 Si-TiO2

1600 1400 1600

Nb-BeO

1400 1800

Nb-Al2O3

1600 1800

Ti-BeO

1400,1600 1800

Ti-Al2O3

1600 1800

Ti-MgO

1600 1400 1800

Microscopic observations of interfaces Slight corrosion of oxide; no interfacial phase; slight penetration along grain boundaries No reaction Interfacial film on surface; slight penetration; no corrosion Slight surface alteration Definite interfacial layer of Mg2SiO4; corrosion of oxide Slight reaction and adherence Penetration of metallic phase between grain boundaries and black discoloration of grains; corrosion of surface Interfacial layer of new phase Slight surface discoloration Slight reaction with decrease in porosity of oxide near surface; no new phase Slight adherence Very slight interfacial layer and corrosion; penetration of new phase around grain boundaries; darkening of BeO grains Slight penetration Little reaction, if any; no alteration of oxide structure No reaction Some corrosion; penetration of opaque phase between grains; black discoloration of grains Slight reaction Some corrosion; penetration of opaque phase between grains; black discoloration of grains. Particularly around main boundaries Slight corrosion-and discoloration Very slight reaction Some corrosion; new phase formed at interface; black discoloration proceeding along the grain boundaries

Microscopic observations of interfaces -

SiO2, Al2O3, 3Al2O3.2SiO2 MgO Mg2SiO4

ThO2

ZrSiO4, ZrO2 -

BeO, Nb2O5

Nb2O5 Al2O3 -

Al2O3 TiO

MgO MgTiO3

Continued

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Table 13.1 Reactions of metal and oxides.dcont’d

Reactants

Temp (8C)

Ti-ThO2

1400,1600 1800

Ti-ZrO2

1400,1600 1800

Ti-TiO2

1600 1400 1600 1400 1800

Zr-BeO

Zr-Al2O3

1600 1800

Zr-ThO2

1600 1400 1800

Zr-MgO

1800

Zr-ZrO2

1600, 1400 1800

Zr-TiO2

1600, 1400 1800

Microscopic observations of interfaces Slight reaction No surface alteration; adherence; slight penetration along grain boundaries No reaction Corrosion; penetration of opaque phase along grain boundaries; considerable black discoloration of grains; no interfacial phase Black reaction area Slight surface discoloration Deep corrosion of oxide Slight reaction Penetration of metal phase along grain boundaries; some corrosion Slight penetration and corrosion Corrosion of surface; black discoloration proceeding mainly along grain boundaries and enveloping grains Slight corrosion Slight surface discoloration; no corrosion Very slight corrosion; slight discoloration along grain boundaries Considerable corrosion; no interfacial layer; no change in MgO structure No reaction Slight discoloration of oxide; no corrosion or new phase No reaction Deep corrosion of surface; no interfacial phase

Microscopic observations of interfaces

TiO " # Ti O2 Zr

BeO ZrO2

Al2O3 ZrO2

ThO2 ZrO2

MgO ZrO2 -

-

Reproduced with permission from G. Economos, W. D. Kingery, Metal-ceramic interactions: II, metal-oxide interfacial reactions at elevated temperatures, Journal of the American Ceramic Society, 36 (1953) 403e409.

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Figure 13.3 (A) Comparison of experimental and calculated values of work of adhesion. (B) Inverse correlation between thermodynamic work of adhesion and interfacial free energy between metal and nonmetal. (A) Taken with permission from J.M. Howe, Bonding, structure and properties of metal/ceramic interfaces, MRS Online Proceedings Library, 314 (1993) 27e37. (B) taken with permission from J. M. Howe, Bonding, structure and properties of metal/ceramic interfaces, MRS Online Proceedings Library, 314 (1993) 27e37.

experimental and calculated data for the work of adhesion that is shown in Fig. 13.3A as pointed out by the author. Depending on the magnitude of interfacial free energy, Wd for the Al/SiO2 system is the highest among Al/Al2O3, Al/CaO, and Al/SiO2 systems, and Wd for the Al/C system is the highest among Al/Al4C3, Al/SiC, and Al/C systems. Naidich et al. [61] proposed that the metallicity of the ceramic increases metal/ceramic adhesion. The interaction between iono-covalent compounds (such as Al2O3 and AlN) and the metal matrix is predominantly physical and is negligible compared to chemical interaction. In such a case, the interfacial free energy determines the work of adhesion (see Fig. 13.3B). Thus, the higher the reactivity, the higher the Wd. It is thus revealed that in the case of Al/CaO, Al/SiO2, and Al/BN, Wd is highest for the Al/BN as its interfacial free energy is calculated as most negative at 956 N/m and smallest for Al/CaO [59,60]. As discussed, by establishing the covalent bond across the interface and the ionicity of the metal, the interfacial bond strength between metal and ceramic can be enhanced. As such, Brydson et al. [55] suggested that an oxidizing atmosphere promotes M-Obonds, thus enhancing transition metal bonding to an alumina surface. Tight-binding band structure calculations by Alemany et al. [62] suggested that transition metal/ alumina interfaces formed under oxidizing conditions are stronger than those formed in a vacuum, a phenomenon also observed by Economos et al. [56]. It is concluded that partial oxidation of O2 surface anions to O reduces the O-M repulsive interaction and allows charge transfer from the metal to these anions, resulting in stronger bonding. Exclusive work by Jarvis et al. [63] showed that chemical bonding at the interface is enhanced for compounds exhibiting substantial covalent bonding character

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(such as SiO2) relative to those formed using more ionic oxides (such as Al2O3). The latter interactions are related to van der Waals, image charge contributions, and the modifications to the local electronic structure of the interface. These well-documented studies on the interaction between ceramic coatings and metal surfaces reveal that such bonding depends on the fabrication process. Generally, ceramic coatings are deposited by physical vapor deposition (PVD) [64e66], chemical vapor deposition (CVD) [67e69], thermal spray coating [70e73], plasma spray coating [74e77], sputter coating [78e80], wet chemical [81e83], electrochemical coatings [84e86], and pack cementation diffusion coating [87e89]. The basic requirement is to maintain the adherence, continuity, and general durability of the protective coatings. As the deposition process will be discussed comprehensively elsewhere in this handbook, in the present chapter we briefly touch upon various methods of coating techniques. PVD and CVD render high-quality coatings owing to interaction between the surface atoms and the reactants in the gas phase. The nature of deposition may be powder, film, polycrystal, or single crystal depending on deposition parameters such as the composition of the reaction mixtures, substrate temperature, gas surface composition, flow rate, chamber pressure, etc [90]. Xu et al. [91] discussed that for a given system, among several possible reactions, the one with the most negative Gibbs energy value leads to the most stable reaction products. For instance, the deposition of Si3N4 is, hence, favored by the utilization of SiH4 as a Si-containing precursor instead of SiCl4. The thin film formation and the microstructural growth of the deposit are governed by (i) VolmereWeber mode, (ii) FrankeVan Der Merwe mode, and (iii) StanskieKrastanov mode. An extensive discussion on nucleation and growth in films is taken up by Das and Bera et al. [92,93]. Spray processing is used to obtain a thick coating of a wide range of materials for a variety of applications ranging from gas turbine technology to the electronic industry. The principle behind the spray is to melt materials and propel them toward the surface, where they form a deposit and solidify. According to the choice of starting materials and heat source employed for melting the spray coating process can be of the following types [1]: (i) high-velocity oxy-fuel spraying, (ii) combustion flame spraying, (iii) vacuum plasma spray, (iv) plasma spraying, and (v) two wire electric arc spraying. Owing to the high rate of impact and rapid solidification of high flux, spray deposits are comprised of cohesively bonded splats. Houben et al. [94] identified various types of splat morphologies, such as “pancake type” and “flower type”, which basically depend on the velocity of the impinging droplet. The physical properties of such deposits depend on the cohesive strength, the size and morphology of the porosity, the occurrence of cracks and defects, the cooling rate, the nucleation and growth of crystals, phase formation, etc. [95,96] The cooling and solidification rates of the deposits depend on the splats and substrate conditions [97]. The high solidification rate renders heterogeneous nucleation of the solid phase followed by columnar growth. This leads to the formation of metastable phases along with nanometer to submicrometer grain size distributions. On the nucleation and grain growth Bera et al. [93] suggested that the anisotropic energy of the crystal faces, oriented attachment of the neighboring grains affect densification and nucleation. Adhesion of the films with the substrates too is determined by their chemical potential and the surface free energy. As the

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growth process progresses, there is the diffusion of oxidizing species into the bulk of the substrate through defects or grain boundaries at high temperatures. In both the primary and secondary grain growth, some grains grow larger, whereas smaller ones shrink and disappear. Those aggregated on the surface lead to the formation of coatings. Work of Krell et al. [35] revealed a substantial contribution of the grain boundaries to inelastic deformation. It is reported that thin coatings consisting of small particles formed by CVD and PVD processes provide useful abrasive wear resistance. Musil et al. [98] also described that depending on the size of the constituent grains of the coating, the hardness is enhanced. Fig. 13.4 shows that for a critical value of the grain size dc w10 nm, the maximum value of the hardness Hmax of coating is achieved [99]. The important mechanisms responsible for the hardness enhancement are: (1) grain growth-related evolution of compressive macro stress in the coating, (2) plastic deformation due to dislocation, (3) small grain sizes, (4) the solid solution hardening, and (5) cohesive forces among neighboring grains. Authors, [98,99] however noted that the brittle feature of hard coatings strongly limits their practical utilization. Thus, researchers look for developing ceramic coatings with enhanced toughness that exhibit superior resistance to cracking and appreciably high (w20 GPa) hardness, thermal stability, oxidation resistance, low friction, wear resistance, and corrosion resistance. As mentioned before, Ziegler et al. [33] studied the effects of grain-boundary chemistry on the mechanical properties of high-purity silicon nitride ceramics. The author investigated the role of oxygen, present along the grain boundaries, in influencing the fracture behavior. It is noted that grain boundaries in most silicon nitrides have a typical width of 1e5 nm and that toughness is significantly affected by grain boundary chemistry. Table 13.2 highlights the various oxide, nonoxide, or composite materials used as ceramic barriers to prevent corrosion and wear of metals and alloys.

3. Protective ceramic coating for various applications Ceramic coatings are used in a variety of applications with the sole purpose of protecting the base materials from corrosion. Fig. 13.5, shows a diagram of these basic roles.

Figure 13.4 Hardness H of the coating as a function of the size d of grains. Taken with permission from J. Musil, S. Miyake, Nanocomposite coatings with enhanced hardness, in Novel Materials Processing (MAPEES’04): S. Miyake (Ed.), Elsevier Ltd Amsterdam, 2005, 345e356.

282

Table 13.2 List of various materials used as coating for corrosion and wear resistance. Materials

Purpose

Substrate

Thickness

Process

References

1.

SiC

RF magnetron sputtering

[100]

Tungsten carbide (WC), WC-Co, WC-Ni and WC-Co-Cr

AISI 304 stainless steel (SS), carbon steel (CS) Metallic substrates

2.3e3.0 mm

2.

Micron level

Hexagonal-boron nitride (h-BN)

Thermal spray method, lowpower plasma torch CVD

[101]

3.

4.

Si3N4

5.

TiO2

6.

Al2O3

7.

Multilayer Al2O3/TiO2

Electrochemical corrosion protection against 0.8 M HCl aqueous solution Electrochemical corrosion protection and wear resistance against water/ vapor Electrochemical corrosion protection and wear resistance against water/ vapor Electrochemical corrosion protection and wear resistance against water/ vapor Excellent corrosion resistance in chloride containing solution at the room temperature. Resistance to corrosion in acidic, neutral and alkaline solutions Electrochemical corrosion protection against NaCl solution

20 nm.

[102]

AZ31 magnesium alloys

50e150 nm.

Plasma electrolytic oxidation (PEO)

[103]

316L stainless steel

375 nm

SoleGel and dipcoating technology

[104]

Mild and stainless steel

10e100 nm

Atomic layer deposition (ALD)

[105]

AISI 316 austenitic stainless steel

10e300nm

Atomic layer deposition (ALD)

[106]

Advanced Ceramic Coatings

Metallic substrates (steel, Ni)

TiO2/Al2O3 mono/ multilayers

9.

80% Al2O3-13TiO2 þ 20% ZrO2

10.

Yttria stabilized zirconia (YSZ)

11.

YSZ, YSZ þ Al2O3 and YSZ/Al2O3

12.

CeO2

13.

Al2O3eCr2O3

14.

Cr2O3-20YSZ (CZ) and Cr2O3-20YSZ-10SiC (CZS)

Electrochemical corrosion protection against 0.05-M NaCl aqueous medium Electrochemical corrosion protection of biomedical alloy Electrochemical corrosion resistance against 0.5 M H2SO4, 3.5 wt% NaCl, and 0.1 M KOH solutions Hot corrosion resistance against 55 wt.% V2O5 and 45 wt.% Na2SO4 at 1050 C for 40 h Scratch and abrasion resistance and electrochemical corrosion resistance Electrochemical corrosion resistance against 3.5 wt% NaCl solution (pH ¼ 7) after 48 h of immersion and wear resistance Ball-on-disk wear resistance up to a load of 10 N and electrochemical corrosion resistance against 3.5 wt% NaCl solution at room temperature

AZ-31 magnesium alloy

130 mm

Atomic layer depositions

[107]

Ti-13Nb-13Zr biomedical alloy

100 mm

Plasma spray coating technique

[108]

AA7075 aluminum alloy

w5 mm

Aerosol deposition (AD)

[109]

Nickel based superalloy (inconel 738)

w350 mm

Plasma spray coating technique

[110]

AA2024-T3 aluminum alloy

w10 mm

Solegel coating

[111]

Ti-6Al-4V

300 mm

Detonation spray coating (DSC) and air plasma spray techniques

[112]

304 L stainless steel

110e295 mm

Atmospheric plasma sprayed

[113]

283

Continued

Ceramic coated surface for corrosion and wear resistance

8.

284

Table 13.2 List of various materials used as coating for corrosion and wear resistance.dcont’d Materials

Purpose

Substrate

Thickness

Process

References

15.

SnO2eTiO2

304 stainless steel

15  5 mm.

Solegel method

[114]

16.

SnO2

304 stainless steels

60  5 mm.

Paint coating

[115]

17.

LaTi2Al9O19 (LTA)/ yttria stabilized zirconia (YSZ) Yttria stabilized zirconia (YSZ) as top coat and NiCrAlY as bond Cr2O3-20YSZ (CZ) and Cr2O3-20YSZ-10SiC (CZS) composite Sm2SrAl2O7 (SSA)

Photo electrochemical corrosion protection Electrochemical corrosion protection against 3.5 wt% NaCl aqueous solution Thermal barrier coatings molten salt of Na2SO4 and NaCl Thermal shock protection up to 400 C

Ni-based superalloy K3

w170 mm

[116]

AZ91D magnesium alloy

–

Atmospheric plasma spray coating technique Atmospheric plasma spraying (APS)

Electrochemical corrosion protection against 3.5 wt% NaCl solution The hot corrosion protection against in simulated gas molten mixtures (50 wt% Na2SO4 þ 50 wt% V2O5 and 90 wt% Na2SO4 þ 5 wt% V2O5 þ 5 wt% NaCl) up to 900 C in turbine environments

304 L stainless steel

w300 mm

Plasma spray coating technique

[113]

NiCrAlY bond coated inconel 718 superalloy

200  20 mm

Air plasma spray process

[118]

18.

19.

20.

[117]

Advanced Ceramic Coatings

TiCrN

22.

Al2O3e(Y2O3) ZrO2/ SiO2 nanocomposite

23.

NiP-ZrO2

24.

TiC

25.

TiB2eTiCeAl2O3/Al

26.

TiO2-Al2O3 nanocomposite

27.

Zn3(PO4)2/GO composite

Electrochemical corrosion resistance of the coatings in 3.5% aqueous NaCl solution High temperature corrosion resistance against corrosion molten salt (50 V2O5 þ 25 Na2SO4 þ 25 NaCl) wt.% at 1000 C for 100 h. Electrochemical corrosion resistance of the coatings in 3.5% aqueous NaCl solution Electrochemical corrosion resistance of the coatings in 1 M aqueous H2SO4 solution Electrochemical corrosion resistance Electrochemical corrosion resistance against 40 g/L H2SO4 (4.0  101 mol L 1) and 40 g/L NaCl (6.8  101 mol L 1) aqueous solutions Electrochemical corrosion resistance against 3.5 wt% of the aqueous solution of NaCl

Mild steel

4 mm

SS 316L

PVD

[119]

Hot-air spray pyrolysis technique

[120]

Low alloy steel (30CrMnSi)

w7ew15 mm

Pulse electrodeposition technique

[121]

304 stainless steel

Less than 1 mm

High-energy microarc alloying process

[122]

Magnesium alloy

w300e350 mm

[123]

Titanium sheets

w10ew30 mm

Plasma-sprayed coating Electrophoretic enhanced micro arc oxidation (EEMAO) technique

Steel plate

w50  5 mm

Wet chemical coating

Ceramic coated surface for corrosion and wear resistance

21.

[124]

[125]

Continued 285

Table 13.2 List of various materials used as coating for corrosion and wear resistance.dcont’d

29.

Purpose

Substrate

Thickness

Process

References

NiPZnX (ZnX ¼ ZnSnO3, Zn3(PO4)2, ZnSiO3) Titanium-nitride (TiN)

Corrosion and tarnish in 10% NaCl aqueous solution

Steel surface

w275 nm

Wet chemical coating

[126]

Corrosion protection against biochemical medium

Orthopedic implants and implant material (CoCrMo) 304L steel, high speed steels (HSSs) M2, silicon wafers, AISI D2, copper, and cemented carbides (WC/ Co) 316 L SS, magnesium (Mg) and titanium (Ti) AISI-430 ferritic stainless steel Ferrite-based alloy (AISI C1115 steel) DZ125 nickelbased superalloys AA6061 alloy

w8e10 mm

PVD

[127]

w1e4 mm

PVD, CVD

[128]

–

-Phosphate chemical conversion (PCC) method Pulse electrodeposition Flame-sprayed coatings

[129]

Ti(C,N), (Ti,Al)N), TiCN, TiAlN, AlTiN, TiSiN and TiCNO

High temperature resistance to wear, erosion, and corrosion

31.

Scholzite (SZ, CaZn2 (PO4)2$2H2O)

Resistance to wear, erosion, and corrosion dental and orthopedic field

32.

CuMn2O4

33.

La2O3

34.

Forsterite-type Mg2SiO4

Surface oxidation protection up to 750 C Electrochemical corrosion resistance 3% NaCl aqueous solution Thermal barrier coating up tp 1200 C

35.

Ni-La2O3

Electrochemical corrosion resistance against 3.5 wt% NaCl solution at room temperature

– 1e15 mm

[130] [131]

200 mm

Thermal spraying

[132]

–

Electrodeposition

[133]

Advanced Ceramic Coatings

30.

286

28.

Materials

LaMgAl11O19 (LaMA)

37.

YSZ/LaMgAl11O19 (LaMA)

38.

Fcc-AlCrTiSiN

39.

Ca2SiO4/Al2O3

40.

MgAl2O4/R-SiOx (MALSI) Multiphase PEO (Mg2SiO4, MgO, MgAl2O4) Y2O3-doped mulliteZrSiO4 (Y2Si2O7eZrSiO4)

41.

42.

43. 44.

45.

ZrSiO4 ZrTiO4 and ZrTiO4ePMMA coatings ZrTiO4/ZrO2 nanocomposite

Thermal barrier coating in molten V2O5-Na2SO4 mixture up to 1100 C Thermal barrier coatings (TBCs) in 50 wt.% Na2SO4 þ 50 wt.% V2O5 molten salt at 950 C for 60 h Electrochemical corrosion resistance against 10 vol% HCl aqueous solution Electrochemical corrosion resistance against sea water Electrochemical corrosion resistance Electrochemical corrosion resistance against 3.5 wt% NaCl solution Thermal barrier coatings (TBCs) in the presence of moisture (95% H2O-5% O2) at temperatures of 1500 C Water corrosion protection Hydrophilicity, corrosion resistance in biological environment Electrochemical corrosion

Steel

100 nm

Plasma-spray method

[134]

Ni-based superalloy substrate

200 mm

Plasma-spray method

[135]

316L austenitic stainless steel (ASS) Mild steel and SS

0.5 mm

PVD

[136]

–

[137]

Metallic substrate

2 mm

Mg alloy

20 mm

Air-plasma-spray method Plasma enhanced (CVD) (PECVD) Plasma electrolytic oxidation (PEO)

C/CeSiC composites

50 mm

Slurry painting

[140]

SiC/SiC Medical-grade stainless steel

120 mm 50 mm

Air plasma method Solegel coating

[141] [142]

Ti-6Al-4V

–

Plasma electrolytic oxidation (PEO)

[143]

[138] [139]

Ceramic coated surface for corrosion and wear resistance

36.

287

288

Advanced Ceramic Coatings

Figure 13.5 Ceramic coatings are used to meet the core purpose of protecting a material from corrosion.

3.1

Superhydrophobic coating

Ceramic superhydrophobic (SH) coating is frequently used to protect metal substrates against wet corrosions, such as severe marine corrosion and microbiologically influenced corrosion (MIC). Marine corrosion is a combination of physical corrosion and chemical/electrochemical corrosion. Physical corrosion such as erosion and cavitation corrosion take place mainly due to rapid liquid flow, the impact of sand on surfaces, and the high-speed rotation of the propeller. Apart from these, the presence of higher concentrations of different types of salt (e.g., NaCl, KCl, MgCl2, MgSO4, K2SO4), CO2, and O2, significantly increases the electrical conductivity and promotes chemical corrosion [144]. MIC or biocorrosion is a result of the presence or activity (or both) of microorganisms in biofilms on the surface of metals, causing substantial economic concern. Microbes basically enhance the Fe0 oxidation by indirect mechanism, and abiotically consumption of H2 generated from the oxidation makes it thermodynamically favorable [145]. SH coatings have a strong water-repellent property and can achieve remarkable anticorrosion performance by isolating the corrosion media and the substrate. Nanoparticles-based SH coatings have air pockets, which further enhance the anticorrosion properties. Trimethylethoxysilane (TMES)-modified SiO2 superhydrophobic coating on aluminum alloy shows remarkable water-repelling features and greater long-term corrosion resistance against saline water with more than

Ceramic coated surface for corrosion and wear resistance

289

99.5% efficacy even after immersion for 5 days [146]. The remarkable effect of SH coatings can be further understood from the effective increase in corrosion current density (icorr) values. SH coating with a static contact angle of 154.3 degrees on magnesium substrate shows 10 times higher icorr (icorr ¼ 4106 A$cm2) over that of the layered double hydroxide (LDH) film (icorr ¼ 4105 A$cm2) and the Mg substrate (icorr ¼ 9105 A$cm2). Magnesium and its alloys are used frequently in the medical industry owing to their biodegradability, biocompatibility, and mechanical properties [147].

3.2

Corrosion protection against corrosive nonaqueous medium

Though generally the concept of corrosion is associated more with aqua-based media (acidic, alkaline, saline water, water vapor), nonaqueous media also cause severe damage to the materials. For example, titanium and its alloys experience severe corrosion in nonaqueous media, namely acetic anhydride or methanol, while exhibiting superior corrosion resistance against salty water or marine atmosphere [148]. This is due to the formation of a protective oxide/hydroxide film that is unlikely to occur in nonaqueous electrolytes. The mechanism of corrosion in a nonaqueous electrolyte primarily follows three steps: (1) rupture, (2) reaction with organic molecules, and (3) removal of corrosion products. The corrosion resistance of metals against nonaqueous electrolytes is found to be immensely important due to the prevalent usage of nonaqueous electrolytes in battery applications, including Li-ion batteries. SS 304, Ti, Ni, and Al find extensive application in Li-ion batteries and redox flow batteries (RFB) as current collectors, casing materials, electrodes, and bipolar plates, but undergo severe corrosion in the presence of organic electrolytes such as propylene carbonate (PC, C4H6O3), acetonitrile (AN, C2H3N), and g-butyrolactone (GBL, C4H6O2) [149,150]. Since ceramic coatings are chemically inert and thermodynamically stable, they have better compatibility with organic solvents. A ceramic coating of graphene oxide is used to inhibit the corrosion of aluminum current collectors in lithium-ion batteries [151]. Oxide-based surface coatings such as (Al2O3, ZrO2, TiO2, B2O3, MoO3, and WO3) can prevent the corrosion of cathode materials from the electrolyte but drastically decrease battery capacity by preventing the migration of lithium-ions and electrons through the coating layer [152e154]. Advanced ceramic materials such as LiAlO2, Li3ZrO2, Li2O-2B2O3, Li3PO4, and Li2WO4 not only prevent corrosion but also provide Liþ at the interface, resulting in superior ionic conduction and increase in resistance during the charging/discharging process. Thus, Li-based oxide materials are used extensively for the surface coating of Ni/Ni-rich cathode materials in lithium-ion batteries [155].

3.3

Protection against liquid sodium

The sodium-cooled fast reactor uses liquid sodium metal as a coolant instead of water. Hence from the structural materials to clad of fuel pins such as Ni-based superalloys,

290

Advanced Ceramic Coatings

Ti-based alloys, stainless steels (D9, T91), etc., are directly in contact with the liquid sodium. These materials are intermittently or continuously exposed to liquid sodium at high temperature with differential thermal effects; the flow of liquid sodium and mechanically induced vibrations for several many thousands of hours and hence suffers severe wear and corrosion damage. Corrosion by liquid sodium follows a complex mechanism that includes (1) dissolution and leaching of some selective alloying element from the alloys, (2) penetration of the liquid sodium through the grain boundaries, and (3) corrosive action by the impurities present in the liquid sodium [156]. CrC diffusion coating with nichrome binder (Cr3C2-15 vol% nichrome) on nickel-based Inconel 718 alloy exhibits excellent compatibility with liquid sodium and reduces the average frictional corrosion rate and hence being used commercially [157]. P. Varghese et al. [158] have shown that a dense and smooth thermal barrier coating (TBC) of yttria stabilized zirconia (YSZ) on AISI type 316LN stainless steel can be used for corrosion protection from molten sodium due to its better chemical inertness and stability for fast breeder reactor applications. Recently, Y. Chen et al. [159] have reported high corrosion resistance of hard CrN coating on a 316L stainless steel surface in liquid sodium up to 550 C . Liquid metal batteries are a novel device for grid-scale energy storage. However, tremendous corrosion damage due to the usage of liquid sodium is one of the major concerns. CVD assisted SiC and Si3N4 coatings offer better protective barriers against dissolution and corrosion on the steel surface with increased life [160].

3.4

Protection against liquid Li and Li-Pb

In fusion reactors, blanket systems especially liquid blanket systems, are affected by severe corrosion damage due to (1) the use of Li, Li-Pb, and molten salt FeLiBe (2LiF-BeF2) as the candidate liquid breeder material, (2) the use of corrosive fluoride salts, and (3) the generation of highly corrosive HF for tritium breeding. A coating of Eu2O3, Y2O3, and AlN applied on V-4Cr-4Ti alloy shows compatibility with liquid Li blanket. On the other hand, Al2O3, Y2O3, MgO, and MgAl2O4 show corrosion prevention at 500 C against a liquid Li-Pb blanket, which is expected as Li-Pb has lower activity than Li. Double layer coating of Er2O3-Fe deposited by vacuum arc deposition showed a decrease in corrosion rate over Er2O3 coating after immersion for 1505 h at 550 C in a Li-Pb blanket [161]. Multilayer coatings of Er2O3/ZrO2 with a ZrO2 outer layer fabricated by metal-organic decomposition show better resistivity toward corrosion damage up to 1000 h at 600 C when exposed to molten Li-Pb over ZrO2/Er2O3 (Er2O3 outer layer). This is due to the reaction with zirconia and Li and the formation of degradation product Li2ZrO3 occurs at a higher temperature than LiErO2 [162].

3.5

Protection against molten salt

Pyro-chemical reprocessing is used for the reprocessing of spent metallic fuel (U-PuZr) in fast breeder reactors using LiCl-KCl molten salt. In this process, graphite is used extensively as crucibles, electrodes, and liners, whereas 304 and 316 stainless steel and

Ceramic coated surface for corrosion and wear resistance

291

copper are major structural materials. Corrosion in molten LiCl-KCl is primarily due to the presence of a very high concentration of corrosive Cl and particularly at higher temperatures (w900 C) [163]. The mechanism of corrosion in stainless steel follows selective diffusion of Cr to the surface and the formation of Cr compounds, which result in the formation of voids. Ceramic materials such as ZrO2, ZrO2eSiO2, and thermal plasma sprayed YSZ (ZrO2e8 wt% Y2O3) are often used as candidate materials for prevention of molten salt corrosion [164]. Chromium nitride (Cr2N) films have also been reported to have corrosion resistance against molten LiCleKCleLi3N systems up to 450 C [165]. CVD assisted TiC and TiN on iron-based substrate coatings show promising corrosion protection against Li-Al/molten, LiCleKCL/FeS2 used in battery cells [166]. Moreover, concentrated solar power plants are often operated at higher temperatures to increase efficiency and hence increase in production of solar electricity. Generally, molten solar salt mixtures (60 wt% NaNO3 and 40 wt% KNO3) are used to achieve higher operating temperatures from 250 C to 550 C. However, the structural materials, mostly stainless-steel components, being directly in contact with the molten salt undergo severe corrosion damage and degradation. Hence, the development of a ceramic corrosion barrier coating also needs to have absorption and emission characteristics. ZrB2SiC ceramic coating not only provides protection against corrosion but also has a high solar absorptance of 0.848 with a high thermal emissivity of 0.66 [167].

3.6

Novel functional coatings

Ceramic-based novel functional coatings, especially oxides, are widely used on metal implants to meet biomedical needs due to their efficient bonding ability with bones and soft tissues. Corrosion by human body fluids basically takes place similarly to as aqueous medium, i.e., via electrochemical reactions. The fatigue strength of the metal implants decreases with the interaction with body fluids, which enhances corrosion cracking and wear. This facilitates the release of allergenic and carcinogenic debris, such as Fe, Cr, Ni, and Ti ions. A magnetron sputtering-deposited zirconium-based ceramic coating, ZrO2 and ZrON, on stainless steel (AISI 316) substrates is one of the candidate materials for its antiwear resistance properties in human body fluid [168]. Apart from stainless steel Ti is widely used for implants owing to its excellent biocompatibility, but suffers from insufficient osteointegration, affecting implant longevity. SiO2-incorporated nanostructured titania bioceramic coatings are generally used for the protection of Ti alloys [169]. Apart from TiO2, other ceramic coatings such as Ca3(PO4)2, sodium titanate (Na2Ti3O7) and CaTiO3 coatings show promising results on Ti alloys under various biological conditions [170]. Magnetic sputtering fabricated thin TiO2 layer and TiO2/MgO double layer or plasma electrolytic oxidation deposited a-Al2O3 on a magnesium-based die-cast alloy (AZ91D) reduces implant failure by restricting bacterial colonization and increased microhardness [171,172]. MgO-MgSiO3-Mg3(PO4)2 with hydroxyapatite ceramic coating on biodegradable Mg66Zn29Ca5 microarc oxidation (MAO) shows compatibility with blood and resistance to wear corrosion [173]. Nickeletitanium orthodontic wires (NTWs) are

292

Advanced Ceramic Coatings

necessary for orthodontic treatment, but undergo corrosion damage due to the acidic environment of the buccal cavity, the presence of prophylactic agents, and the use of mouthwash solutions. Moreover, ceramic coatings used to protect NTWs from corrosion are expected to have aesthetic properties as well. For example, PVD of aluminumesilicon dioxide (AleSiO2) on the NTWs used as archwire not only acts as a biocompatible anticorrosion layer but also retains the aesthetic values of NTWs [174]. It is reported that cerium (Ce)-incorporated niobium oxide (Nb2O5) bioceramics deposited by the sol-gel technique on 316L SS substrates improve the surface roughness (Ra), achieve better hydrophilic nature, lower corrosion current density (icorr), lower corrosion potential (Ecorr) and increased breakdown potential (Ebreak) on osteoblast MG 63 cells specimens. Therefore, it is found to have increased biocompatibility, significant cell spreading, and better adhesion with 316L SS and hence superior corrosion protection. Self-healing properties of the coating depend on the incorporation of a corrosion inhibiting agent, i.e., Ce which improves corrosion protection by counterbalancing the anodic activity in the defects [175]. Self-healing is one of the interesting characteristics of the novel functional ceramic coating materials and is illustrated in detail in the next section.

3.7

Self-healing coating

Intelligent self-healing corrosion protective coatings have been studied extensively to protect metal components against the harmful action of the corrosion environment. The most important characteristic of these coatings is their ability to automatically self-heal the damaged surface by chemical means when it undergoes mechanical damage and natural degradation processes. Various metallic materials, such as steel, aluminum alloys, and magnesium alloys used in automobile parts, building structures, bridges, and home appliances, are often coated with such self-healing ceramic materials [176]. The functionality of such self-healing goes through a mechanism of removal and filling of its defects arising during usage by an increase in temperature, followed by oxidation of the coating components (Fig. 13.6). Ceramic materials, namely chromate conversion coatings, TiC/Al2O3, and Ti2AlC, are commonly used for this purpose [177]. Self-healing based active corrosion resistance aims release of self-healing active agents, namely corrosion inhibitors when the primary coating matrix barrier is damaged and the substrate is in direct contact with the corrosive environment. Sol-gel coated coatings are most likely used for the fabrication of self-healing coatings, such as ZrO2 and ZrO2-CeO2-benzotriazole ceramic-based coatings synthesized using solegel process has self-healing properties [178]. Self-healing coatings are also reported to perform well against wear damages caused by solid particle-induced wear, plastic deformation, and cutting. Multilayered coatings of different combinations of silica and swelling clay (hectorite) layers swell themselves against any damage to improve corrosion protection [179]. Nanoporous Al2O3 film with sodium benzoate (SB) as a corrosion inhibitor on aluminum provides wear resistance against scratches by a self-healing mechanism [180].

Ceramic coated surface for corrosion and wear resistance

293

Figure 13.6 Mechanism of anticorrosion protection by self-healing coating. Taken with permission from A. Yabuki, Particle-induced damage and subsequent healing of materials: erosion, corrosion and self-healing coatings, Advanced Powder Technology, 22 (2011) 303e310. 2010 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan.

3.8

Aesthetic properties

Some specific application urges not only corrosion protection but also it is necessary to have high aesthetic property of ceramic coatings. For example, innovative household ceramic tiles should provide aesthetic properties along with improved tribological properties and better durability. In this regard, Zn-Ni added Al2O3 and Zn-ZnOyttria powder coatings on mild steel are used very often in various industries, as they not only provide protection from rusting and corrosion but also have greater aesthetic values like a white, uniform matte finish [181]. Industrial PVD multicathode arc-deposited TiN and ZrN coatings provide an attractive gold like color with a smoother surface along with corrosion and wear resistance [182]. Transparent silicabased thin film protective coatings are deposited frequently on metal artifacts.

3.9

Thermal barrier coating

TBC is applied to the surface, especially in aviation engines or gas turbine parts that operate at extremely high temperatures for prolonged period of time. TBC protects the base metal or alloy surfaces from heat, oxidation, and corrosion. It was recognized that the TBC must meet the following requirements: (1) high melting points, (2) high surface emissivity, (3) low density (4) low vapor pressure (5) high thermal shock

294

Advanced Ceramic Coatings

resistance and (6) resistance to oxidation or chemical environment (7) high CTE (8) resistance to gaseous and particulate erosion and (9) low thermal conductivity [1]. When these requirements are considered, 3Al2O3-2SiO2 (mullite), Ca2SiO4, MgAl2O4, CaTiO3, ZrSiO4, ZrTiO4, NaZr2(PO4)3, and stabilized ZrO2 meet the criteria. Among these materials, stabilized ZrO2, with its low thermal conductivity and excellent thermal shock resistance is more suitable. Additionally, NaZr2(PO4)3 (NZP) is proposed considering its low CTE. The minimum thermal conductivity for complex and multicomponent materials can be expressed as [183].   1=2 2 kmin ¼ KB nm Lmin /0:87KB Ua 3 E r

(13.2)

 Where Lmin is the minimum phonon mean free path, and Ua ¼ M ðmrNA Þ, is the average volume per atom, E is the elastic modulus, and r is the density, M is the molecular mass and m is the number of atoms per molecule. NA is Avogadro’s number and E=r is proportional to the acoustic velocity. Fig. 13.7 shows the variety of materials with low conductivity [184]. TBCs contain four main layers (1) metal substrate (2) metallic bond coat (3) ceramic top coat (4) thermally grown oxide as shown in Fig. 13.8.

Figure 13.7 Minimum thermal conductivity values for materials of interest in thermal barrier coatings as calculated kmin from Eq. (13.1). Taken with permission from C.G. Levi, Emerging materials and processes for thermal barrier systems, Current Opinion in Solid State and Materials Science, 8 (2004) 77e91. Copyright 2004 Elsevier Ltd.

Ceramic coated surface for corrosion and wear resistance

295

Figure 13.8 Thermal barrier coatings applied on turbine blade. Taken with permission from F. Miranda, F. Caliari, A. Essiptchouk, G. Pertraconi, Atmospheric plasma spray processes: from micro to Nanostructures in atmospheric pressure plasma, A. Nikiforov, Z. Chen (Eds.), London, United Kingdom, 2018. Copyright Creative Commons Attribution 3.0 License.

The bond coat is an oxidation-resistant metallic layer that is deposited directly on top of the metal substrate. The primary purpose of the bond coat is to protect the metal substrate from oxidation and corrosion, particularly from oxygen and corrosive elements that pass through the porous ceramic top coat. Moreover, the bond coat enhances chemical bonding interaction between the metal substrate and the top ceramic layer resulting in improved adhesiveness of TBCs. Finally, the ceramic topcoat, with a notable lower thermal conductivity than the metallic substrate, causes a remarkable temperature drop across the layer by restricting the conduction heat flow. For example, in a gas turbine the metal bond coat is normally 75e125 mm thick, and the oxide layer is 250e375 mm thick. For large marine engines, the ceramic coat is much thicker, w1.5e6.25 mm. Movchan et al. [186] studied a TBC assembly where the metallic bond coat is divided into an upper oxidation-resistant alloy and a lower complaint alloy next to the substrate, with an oxide layer on top of this composite bond coat. The choice of the bond coat is as critical as the choice of the top ceramic coat. Studies showed that strengthening the bond coats with greater creep resistance improves the thermal cycle life [187]. Moreover, the coating must have good resistance to interdiffusion with the substrate surface, and the bond coat must not form brittle phases as well [1]. Moreover, the bond coat bridges the CTE mismatch between the ceramic top coat and the substrate and plays a crucial role during thermal cycling. For high-temperature protective yttria coating (CTE ¼ 8  106  Ce1). On graphite (CTE ¼ 4  106  Ce1), niobium (CTE ¼ 8  106  Ce1), and zirconia (CTE ¼ 10.6  106  Ce1), latter is often used as an interlayer bond coating to address the CTE mismatch and to enhance the performance of TBCs to more numbers of thermal cycles. The bond coat not only bridges the CTE mismatch between the ceramic top coat and the metal surface, but it also slows down oxidation of the surface at elevated temperatures. Thermally grown oxide scales on bond coatings may be affected by the thickness rate, bond coat surface roughness, hold time, and top coating composition [188]. The growth of the oxide layer throughout thermal cycling causes

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Advanced Ceramic Coatings

cracking and spallation of the top ceramic coating. For instance, oxidation of the bond coat is the sole reason behind the spall-inducing cracks within the ZrO2 at the bond coat interface. Interestingly, the occurrence of spalling can be prevented when the thermal cycle tests are conducted in an inert Ar atmosphere [1].

3.10

Cutting tools

The high chemical stability, greater hardness at high temperature, and better thermal shock resistance of advanced ceramic materials make them an affordable choice for cutting tool materials. In general, the wear mechanisms observed in ceramic cutting tools are abrasive, adhesive, tribochemical (i.e., dissolution/diffusive wear), and wear by fracture. This is expected, as during the cutting process the edge is subjected to high pressure, friction, and temperature. Thus an ideal tool meant for longer life should possess traits that include [189]: 1. Resistant to cracking and fracture 2. Resistant to plasticity 3. Good wear and chemical resistance.

It is observed that ceramic tools are more wear-resistant and chemically stable than cemented carbide and high-speed steel (HSS) tools. However, the low fracture toughness and tensile strength cause chipping and cracking under stress. Substantial efforts had been made to extend the tool’s life by reducing friction and wear. As mentioned earlier in the text, the abrasive wear resistance is related to K and H (see equation 13.1), and the incorporation of a second phase in the form of ZrO2, TiC, or SiC improves the fracture toughness, hot hardness, and thermal and mechanical shock resistance. Fig. 13.9 shows the comparison of the properties of ceramic and tungsten carbide tools [190]. It is observed that Si3N4-based ceramics are more prone to tribochemical wear when cutting iron-based materials, which is attributed to the chemical affinity between Fe and Si3N4. Thus, often Al2O3 and ZrO2 are incorporated into Si3N4 to reduce the tribochemical wear. Thus, fracture toughness and thermal shock resistance are major criteria for ceramic tools during cutting applications.

Figure 13.9 Comparison of the properties of ceramic and tungsten carbide tool.

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However, it has been shown practically that the performance of the cutting tools can be improved by depositing coatings on them. Based on the hardness, these coatings are classified as soft and hard coatings. The optimum thickness of the coating depends on the cutting modes, as when the coating layer is too thick, it may peel off owing to the stress generated. Schintlmeister et al. [191] pointed out that the effect of coating can be summarized as follows: 1. Reduction in friction, in generation of heat, and in cutting forces 2. Reduction in the diffusion between the chip and the surface of the tool 3. Prevention of galling at lower cutting speeds

In general, the task of hard coatings is in general to provide wear and thermal protection for the tool substrate. For continuous and interrupted cutting processes, temperatures may rise to as high as 1000e1200 C, then thicker (w5e20 mm) coatings with low thermal conductivity are required. Otherwise, thinner (w3e5 mm) coatings are favorable [192]. For interrupted cuts (for example, milling applications), impact strength and resistance against thermal cycling need more attention. So, coating with excellent adhesion to the base and with a very uniform grain structure is sought. At present, HSS and cemented carbide tools are coated with single or multilayer hard ceramic coating (2e25 mm) of TiN, HfN, and ZrN, and TiC, HfC, and ZrC. These coatings are found to be more resistant to tribochemical wear. It is reported that the life of coated cemented carbides is increased by three to 10 times than the uncoated ones. The main effect of the coating is to act as a diffusion barrier. This reduces the temperature generated in the cutting tips by reducing the friction. The coating is also hard, and that decreases the wear of coated carbides. Ronkainen et al. [193] studied Ti(B,N) and (Ti,Al)N ceramic coatings deposited with dc plasma-assisted PVD and rf magnetron sputtering on HSS tools. It was found that the coatings improved the tool life considerably, particularly (Ti,Al)N coatings showed excellent resistance against flank wear in steel. To extend the study further, Schintlmeister et al. [191] worked with tools that consisted of a 1.5 mm thick TiC layer adjacent to the base material, followed by a 1.5 mm titanium carbonitride layer, and a 2 mm thick TiN layer on top. The improved life of a cutting tool with such multilayer coatings can be attributed to the increased crater resistance, impact strength, and resistance to thermal stresses. Dobrza
nski and Mikuła [194] investigated the mechanical and functional properties of Al2O3-ZrO2 ceramics deposited with multilayer hard coatings (TiAlSiNeTiN, TiNeTiAlSiNe AlSiTiN, TiCN-TiN, etc.). Results showed that the Al2O3-ZrO2 ceramic deposited with hard coatings resulted in an increasing microhardness, high wear resistance, and a significant increase of the tool life. Similar observations were done by Kumar et al. [195,196]. Authors found that a thick TiAlN layer on the ceramic substrate resulted in a significant decrease in coating/substrate adhesion strength. TiAlSiN/ TiSiN/TiAlN nanocomposite coating, on the other hand, outperformed duplex coatings under each conditions due to lower thin film thickness and better coating/substrate adhesion strength. The TiAlSiN layer exhibited superior antiadhesive and antiabrasive behavior when compared to the TiAlN layer. It is also found that tool surface

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topography also influences friction and wear, and that nanoscale textures are more effective at decreasing friction than the microscale textures. Schalk et al. [192] in a very comprehensive discussion detail the deposition, growth, and properties of a variety of coating materials such as Ti1-xAlxN, Ti1-x-yAlxTayN, Cr1-xTaxN, TiCxN1-x, ZrCxN1-x, TiB2 and TiBN, among others. Deng et al. [197] observed that the WS2/ Zr-coated Al2O3/TiC cutting tools with and without nanotextures significantly improve the lubricity at the tool-chip interface. The cutting force, temperature, friction coefficient, and tool wear are reduced. As such, the nano-textured tools deposited with WS2/Zr composite soft coatings are the most effective. Kumar et al. [196] undertook continuous turning tests with the help of DLC and WC/C low friction coatings on Al2O3/TiCN-based mixed ceramic cutting tools for dry and hard turning of AISI 52,100 steel. Performance was investigated in terms of machining forces, cutting temperature and tool wear. The aim to improve the mechanical properties of these coatings will surely enhance productivity and competitiveness. Moreover, additional functionality, self-healing abilities, surface self-organization mechanisms, and thermal management, including optimum thermal conductivity based on the material of the workpiece, will provide the necessary edge to futuristic multilayer coated cutting tools. An innovative design for a future hard coating for metal-cutting applications should combine all these functionalities.

4.

Factors affecting ceramic coatings on corrosion resistance

4.1

Homogeneity, thickness, and uniformity of the coating

The Corrosion resistance performance of the protective ceramic coating layer not only depends on the smart choice of the substrate-specific coating materials and method of coating but also gets affected by the quality and uniformity of the coatings. The quality of the coating includes the homogeneity, thickness, and uniformity of the applied ceramic coating layer, which tremendously affect the linear polarization resistance (LPR). LPR is an important technique to measure rates of electrochemical corrosion and is expressed by the SterneGeary equation icorr ¼ ½ba bc =f2303ðba þ bc Þg=Rp

(13.3)

Where, icorr is the corrosion current density, Rp is the polarization resistance, and ba and bc are the anodic and cathodic Tafel slopes, respectively. Coatings with greater uniformity and homogeneity show higher Rp values and hence lower corrosion rates. For example, dip-coated yttria-doped zirconia on AISI 304 stainless steel has a higher Rp value and greater corrosion resistance with an increase in the uniformity and homogeneity of the coating. On the other hand, thicker coating has shown lower Rp values and hence faster corrosion rates [198].

Ceramic coated surface for corrosion and wear resistance

4.2

299

Surface roughness

The surface roughness of the ceramic coating possesses a remarkable effect on wear. The surface roughness of the as-sprayed/deposited coatings is usually higher (micron level) and can be reduced to submicron scale by grinding or finishing processes. Nanoscale surface roughness (less than 100 nm) cannot be achieved by conventional grinding alone and requires shape-adaptive grinding [199]. The mechanism of wear corrosion differs depending on different surface roughness. For micron-level roughness, wear debris gets entrapped between the surfaces, and wear takes place through a three-body abrasion action mechanism process, i.e., removal of the binder and displacement of the coating particles. For the submicron level of roughness, the efficacy of the three-body abrasion mechanism decreases as the entrapped wear debris particles are mainly fragmented/pulverized in nature. Hence, the removal of binder and fractured particles becomes predominant for wear. On the other hand, in nano finished coating, the predominant way of wear is plastic flow, and microcutting as the local stress is lower in the contact points. This type of wear produces fine wear debris, which further recombines to form a tribofilm on the wear track. Formation of this oxidized tribo film causes further reduction of the coating wear rate [200e202]. Ghosh et al. [203] have shown the corrosion current density (icorr) values of cermet coating (WCeCo) on low carbon steel substrates with micron level and submicron roughness were 11 and 2.2 times higher than the coatings of nanoscale roughness. The investigation further shows surface roughness parameter is directly proportional to the specific wear rate and can be expressed as follows. 3

=

Specific wear ratefSa8

(13.4)

Where, Sa is the surface roughness parameter of the coating.

4.3

Porosity

Porosity is one of the key coating features to impact strong corrosion and wearresistive properties of the coating and is generally coined as porosity-induced corrosion. Porosity defects result in a reduction in corrosion resistance and cohesion by enhancing the passive current density and lowering the stability of the passive film [204]. Dietzel et al. [205] have evaluated the effect by the ratio of the corrosion current densities of alloys and the corrosion current densities of their porous Mg(OH)2 film coatings (i0corr/i1corr) which is equal to 90 and 19 for the magnesium alloys VMD10 and IMV2, respectively, in a neutral environment (0.3% NaCl solution). Hence, this marks an existing correlation between the degree of corrosion protection and the porosity of a coating. However, authors have found not porosity but chemical composition plays a major role to determine the corrosion resistivity of the oxide-ceramic coating in acid environments (0.3% HCl).

300

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Effect of additives

The Corrosion resistance of ceramic coatings is often enhanced by doping additives in the ceramic layer. Doping of additives effectively impacts corrosion and wear protection by influencing the coating chemical composition during formation and the thickness, topography, and surface morphology of the coatings. TiCN coating with CeO2 nanoparticles additive on Ti-6Al-4V substrates shows a 14% reduction in wear corrosion volume less than that of the coatings without CeO2 under a simulated body fluid (SBF) lubrication corrosive environment [206]. Moreover, additives can alter the nature of the coatings, making them applicable in different corrosive environments. The doping of EDTA in MgO/Mg2SiO4 coating on Mge5wt% Li substrate by microarcoxidation (MAO) method results in a more uniform and thinner coating for better general corrosion resistance, while doping of Na2B4O7 results much thicker and compacter coating with better pitting corrosion resistance [207].

4.5

Effect of instrumentation

Corrosion fatigue strength depends upon the method used for the deposition or formation of the coating. When similar coating material on a particular substrate is deposited by two different methods, corrosive resistivity alters for even in an identical environment. This is due to the microstructural change of the substrate as well as the coating layer for different deposition processes. The effect of the deposition process on the coating quality includes homogeneity, thickness, uniformity and porosity. Pretreatment of the substrate for various depositional processes or deposition parameters (high temperature, pressure, effect of plasma) produce microstructural changes in the substrate and affects the performance of the coating. For example, improvement in corrosion fatigue strength was found to be 16%e23% for CVD-coated TiN film on a carbon steel substrate over PVD-coated TiN film. This is due to the decarburization of the substrate near the CVD-coated TiN layer while treatment of the substrate at high temperatures during CVD coating compared with the PVD method [208].

5.

Degradation

Degradation of the coating may occur because of environment, temperature, gas composition,and also due to microstructural alteration during the use of applications. It is mentioned that coatings with a high hardness value often fail due to brittle fracture associated with plastic deformation in the underlying substrate material. If the coating is subjected to rolling contact in a bearing, sliding between gear teeth, or abrasion by a rough hard counterbody, it may be removed by detachment at the interface rather than by progressive wear (see Fig. 13.10). If the interface is diffuse, the chances of peeling off the protective coating of the substrate are less. Researchers found out that a thicker coating will show a greater tendency to delaminate, as it is related to the strain energy release rate, which is proportional to coating thickness. Internal stresses introduced during the processing of

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Figure 13.10 Schematic of protective coating layer exhibiting (A) progressive wear; (B) delamination. Image taken with permission from I. Hutchings, P. Shipway, Tribology: Friction and Wear of Engineering Materials, Butterworth-Heinmann, Elsevier, Cambridge, 2017.

ceramic coatings for turbine components can lead to coating failure. The sources of such stresses include elastic anisotropy and thermal gradients introduced within the ceramic during deposition or phase transformations (see Fig. 13.11) [29,209]. Hot-section components of gas-turbine engines are made of ceramic-matrix composites (CMCs), comprising continuous fibers and matrices made of SiC. These components face active oxidation and recession in the high-pressure, high-temperature, and high-velocity gas stream. Thus, to protect SiC-based CMCs, environmental barrier coatings (EBCs) are chosen. However, the choice of materials for EBCs should be such that they bond with SiC; otherwise cracks formed at the bonding phase will destroy the protective coating and remove through high-velocity abrasive flow. The transition between the principal SiC grains and the bonding phase matches perfectly to minimize microcracking of the structure [210]. EBCs in the form of rare earth (RE) monosilicates (RESi2O5), disilicates (RE2Si2O7), and others mostly encounter volcanic ash by aircraft engines, runway debris, airborne sand, ambient dust, and/or fly ash by electricity generation, which melts at temperatures w1200 C and deposits on EBCs as a molten glassy layer. This results in the formation of reaction products with uncontrolled microstructures and undesirable properties such as hardness, CTE, and fracture toughness. Thus the crack formation starts from the weakest boundary of a glassy phase, eventually leading to the failure of the EBC [211]. The thermal expansion mismatch between ceramics (coating) and metals (substrate) is an important

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Figure 13.11 SEM images showing microstructure flaws developed during plasma spray deposition of (A) a titania coating, (B) a WC-Co coating. Degradation of a composite coating: (C) After tribological service a through-depth crack was developed below the contact area and (D) propagated through the carbide reinforcing particles resulting in their multirupture. Images Taken with permission from P.P. Psyllaki, An Introduction to wear degradation mechanisms of surface-protected metallic components, Metals 9 (2019) .

source of strain [212]. In addition, the impact of abrasive particles with different sizes, shapes, weights, and velocities forms all kinds of cracks. Cracks in oxide coatings are sealed with B2O3 or a similar sealant. For SiC and SiN, the formation of SiO2 at higher temperatures helps seal the cracks. However, at lower temperatures SiO2 is not formed, so as an alternate arrangement, boron is added to the coating to seal cracks. These coatings are basically affected by gaseous attacks and hot corrosion. It is observed by Medvedovski [210] that in surface engineering with ceramics, the use of smaller alumina particles with a higher specific surface area renders superior mechanical and wear resistance. However, such advantages become void due to impurities present in the raw materials or additives used to promote crystal growth or the presence of open porosity, as these have negative influences on wear resistance. It is also observed that in the presence of a chemically aggressive environment ceramic coatings register higher wear resistance under sliding abrasion than under impact action owing to their low fracture toughness and impact strength [210]. Similar scenarios were observed when, on some samples, the ceramic coating separated from the steel as a result of thermal shock and, on others, by the formation of an oxide layer between the steel and the ceramic layer. Incidentally, the oxide layer appeared to remain bonded to the ceramic but not to the steel. It is mentioned in the above text that multilayer

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coatings have better wear resistance and resistance to fracture than monolayer coatings, and the properties of multilayer coatings depend on the number of layers and the thickness of each layer. More layers mean interfaces that have a great effect on coating properties, energy, and stress distribution.

6. Conclusion In this article, a brief discussion about the protective ceramic coating to prevent corrosion and wear in challenging environments is detailed. A small introduction on different types of corrosion and wear is touched upon. The requirements of protective coating, especially ceramic coating, the properties of ceramic coating, material selection, and features required for such application are highlighted. The prime strategy of binding ceramic coating with metal or alloy, interfacial properties, and the influence of grain boundary and porosity on adhesion, wear, hardness, and toughness are discussed. A summary of TBC and coated cutting tools is presented. Finally, the degradation mechanism and failure of such coatings are discussed.

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[184] C.G. Levi, Emerging materials and processes for thermal barrier systems, Current Opinion in Solid State & Materials Science 8 (2004) 77e91. [185] F. Miranda, F. Caliari, A. Essiptchouk, G. Pertraconi, in: A. Nikiforov, Z. Chen (Eds.), Atmospheric Plasma Spray Processes: From Micro to Nanostructures in Atmospheric Pressure Plasma, 2018 (London, United Kingdom). [186] B.A. Movchan, I.S. Malashenko, K.Y. Yakovchuk, A.I. Rybnikov, Two- and three-layer coatings produced by deposition in vacuum for gas turbine blade protection, Surface & Coatings Technology 67 (1994) 55e63. [187] L. Ajdelsztajn, D. Hulbert, A. Mukherjee, J.M. Schoenung, Creep deformation mechanism of cryomilled NiCrAlY bond coat material, Surface and Coatings Technology 201 (2007) 9462e9467. [188] A. Avci, A.A. Eker, B. Eker, in: I. Dincer, C.O. Colpan, O. Kizilkan (Eds.), Microstructure and Oxidation Behavior of Atmospheric Plasma-Sprayed Thermal Barrier Coatings in Exergetic, Energetic and Environmental Dimensions, 2018, pp. 793e814. [189] G.W. Stachowiak, G.B. Stachowiak, Wear behavior of ceramic cutting tools, Key Engineering Materials 96 (1994) 137e164. [190] X.S. Li, I.M. Low, Ceramic cutting tools-an introduction, Engineering Materials 96 (1994) 1e18. [191] W. Schintlmeister, w. Wallgram, J. Kanz, K. Gigl, Cutting tool materials coated by chemical vapour Deposition, Wear 100 (1984) 153e169. [192] N. Schalk, M. Tkadletz, C. Mitterer, Hard coatings for cutting applications: physical vs. chemical vapor deposition and future challenges for the coatings community, Surface and Coatings Technology 429 (2022) 127949. [193] H. Ronkainen, I. Nieminen, K. Holmberg, A. Leyland, A. Matthews, B. Matthes, E. Broszeit, Evaluation of some titanium-based ceramic coatings on high speed steel cutting tools, Surface and Coatings Technology 49 (1991) 468e473. [194] L.A. Dobrza
nski, J. Mikuła, The structure and functional properties of PVD and CVD coated Al2O3-ZrO2 oxide tool ceramics, Journal of Materials Processing Technology 167 (2005) 438e446. [195] C. Kumar, S.K. Patel, Effect of duplex nanostructured TiAlSiN/TiSiN/TiAlN-TiAlN and TiAlNTiAlSiN/TiSiN/TiAlN coatings on the hard turning performance of Al2O3-TiCN ceramic cutting tools, Wear 418e419 (2019) 226e240. [196] C. Kumar, H. Majumder, A. r Khan, S.K. Patel, Applicability of DLC and WC/C low friction coatings on Al2O3/TiCN mixed ceramic cutting tools for dry machining of hardened 52100 steel, Ceramics International 46 (2020) 11889e11897. [197] Y. Xing, J. Deng, S. Li, H. Yue, R. Meng, P. Gao, Cutting performance and wear characteristics of Al2O3/TiC ceramic cutting tools with WS2/Zr soft-coatings and nanotextures in dry cutting, Wear 318 (2014) 12e26. [198] U. Angst, M. B€uchler, On the applicability of the SterneGeary relationship to determine instantaneous corrosion rates in macro-cell corrosion, Materials and Corrosion 66 (2015) 1017e1028. [199] A. Beaucamp, Y. Namba, P. Charlton, Process mechanism in shape adaptive grinding (SAG), CIRP Annals - Manufacturing Technology 64 (2015) 305e308. [200] H.J.R. Wang, Dry sliding wear in 2124 Al-SiCw/17-4 PH stainless steel systems, Wear 147 (2) (1991) 355e374. [201] R.G. Bayer, J.L. Sirico, The influence of surface roughness on wear, Wear 35 (2) (1975) 251e260. [202] S.F. Tian, L.T. Jiang, Q. Guo, G.H. Wu, Effect of surface roughness on tribological properties of TiB2/Al composites, Materials & Design 53 (2014) 129e136.

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[203] G. Ghosh, A. Sidpara, P.P. Bandyopadhyay, Understanding the role of surface roughness on the tribological performance and corrosion resistance of WC-Co coating, Surface and Coatings Technology 378 (25) (2019) 125080. [204] S.D. Zhang, J. Wu, W.B. Qi, J.Q. Wang, Effect of porosity defects on the long-term corrosion behavior of Fe-based amorphous alloy coated mild steel, Corrosion Science 110 (2016) 57e70. [205] W. Dietzel, M. Klapkiv, H. Nykyforchyn, V. Posuvailo, C. Blawert, Porosity and corrosion properties of electrolyte plasma coatings on magnesium alloys, Materials Science 40 (5) (2004) 585e590. [206] H. Wang, T. Chen, W. Cong, D. Liu, Laser cladding of Ti-based ceramic coatings on Ti6Al4V alloy: effects of CeO2 nanoparticles additive on wear performance, Coatings 9 (2) (2019) 109. [207] L. Shi, Y. Xu, K. Li, Z. Yao, S. Wu, Effect of additives on structure and corrosion resistance of ceramic coatings on MgeLi alloy by micro-arc oxidation, Current Applied Physics 10 (3) (2010) 719e723. [208] K. Shiozawal, Some affecting factors for fatigue strength of ceramics coating steel, Transactions on Engineering Sciences 17 (1997) 1743e3533. [209] P.F. Becher, R.W. Rice, C.C. Wu, R.L. Jones, Factors in the degradation of ceramic coatings for turbine alloys, Thin Solid Films 53 (1978) 225e232. [210] E. Medvedovski, Wear-resistant engineering ceramics, Wear 249 (2001) 821e828. [211] S. Mukherjee, S. Das, A. Jain, P. Ghosh, Oxidation protective coating of Y2O3Na2SixO2xþ1 composite on graphite crucible for high temperature applications, Surfaces and Interfaces 25 (2021) 101158. [212] E. Medvedovski, A. Leong, R.J. Llewellyn, Wear resistant ceramics for protection in mining and mineral processing, in: L. Collins (Ed.), Materials for Resource Recovery and Transport, The Metallurgical Society of CIM, 1998, pp. 495e510.

Ceramic coatings for membranes Subhasis Pati Regional Institute of Education, Bhubaneswar, Odisha, India

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1. Introduction to ceramic membranes and their applications The objective of this chapter is to give an overview of the ceramic membranes used for high-temperature gas separation applications and their synthesis techniques. In this section, we shall discuss about the fundamentals of membrane separation, types of membranes, and the state of the art of improving membrane performance. A membrane is a barrier consisting of semipermeable material that allows specific components to pass through selectively compared to others. Membrane-based separation is considered one of the most advanced and energy-efficient processes. This technique is used in a broad range of industrial applications, such as watertreatment, separation of chemicals, medical applications, and gas separation. In gas separation, membranes are used for hydrogen separation, oxygen production, carbon capture, flue gas treatment, natural gas processing, etc. Several types of membrane materials are used for different gas separation applications. The membranes are classified into porous and nonporous membranes based on the characteristics and transport mechanisms of gases. Different gas transport mechanisms are discussed in the literature, such as viscous flow, Knudsen diffusion, molecular sieving, and solution diffusion mechanisms. The gas transport mechanism varies from porous to nonporous membranes. For the porous membranes, the transport is decided by the Knudsen constant (Kn), which is defined as the ratio between the mean free path (l) of the gas molecule and the pore diameter of the membrane (d). Kn ¼ l=d

(14.1)

Viscous flow, Knudsen transport, and molecular sieving mechanisms are followed by porous membranes. However, the solution diffusion mechanism is followed in the case of dense, nonporous membranes. Several types of membranes have been designed and developed for gas and liquid separation. Inorganic membranes (silica, alumina, zeolites, ceramics, and metallic), polymeric membranes, metal-organic frameworks, and covalent organic frameworks have been developed. Polymeric membranes are most widely used for water treatment, dialysis, and natural gas treatment. Although polymeric membranes have a lion’s share in the field of membrane separation; however, inorganic membranes are taking momentum over the past few years. Polymeric membranes suffer performance losses when applied in high temperatures (>200 C) and corrosive environments, etc. Membrane fouling is another problem faced in polymeric Advanced Ceramic Coatings. https://doi.org/10.1016/B978-0-323-99659-4.00002-4 Copyright © 2023 Elsevier Ltd. All rights reserved.

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membranes. Therefore, inorganic membranes are gaining special interest in gas and liquid separations due to their unmatched potential to be applied under harsh environments. The inorganic membranes have high thermal and mechanical stability, which allows them to be used for a long period of time without any performance failure. Such a diverse array of applications demands the facile design of novel materials and the implementation of the mass transport mechanism in those materials. The inorganic membranes can be made up of dense metallic films, perovskites, zeolites, carbon, and ceramic materials. These ceramic membranes have gained recent attention for gas separation due to their high thermal and mechanical stability and excellent corrosionresistant properties. The major applications of these ceramic membranes for gas separation includes the production of highly pure gases (H2, O2, and CO2, etc.) and their use in membrane reactors for the direct production/conversion of gases to other products by combining separation and reaction in a single unit. Moreover, ceramic membranes are used in both pure gas separation and catalytic membrane reactors. All these applications require high flux and excellent gas selectivity. It is well known that gas separation through membranes follows Fick’s law of diffusion, where the flux is inversely related to the thickness “dx” of the membrane as given in Eq. (14.2). f ¼  Dðdc = dxÞ

(14.2)

Where, D ¼ diffusivity, dc ¼ concentration gradient across the membrane. In order to increase the flux of the membranes while maintaining their mechanical stability, supported composite membranes have been developed and applied in gas separation. These membranes have reduced thickness and are coated on certain macroporous supports whose thermal conductivity matches nearly with that of the membrane material. The porosity of the support materials generally varies in the range of 50e200 nm. The material selection and coating techniques are vital in the preparation of highly stable ceramic membranes. Moreover, for the preparation of composite membranes, a three-layer approach was adopted, in which the microporous layer is sandwiched between the microporous layer and the dense membrane layer. This type of membrane is known as an asymmetric membrane and is often made up of similar materials in three different layers. The top layer in this structure acts as the separation membrane layer, and the bottom of the layered structure acts as a support for the membrane. The intermediate mesoporous layer acts as a pore regulator and helps in controlling the thickness of the top separation layer. The pore size of this layer varies between the pore of the support and the pore of the top membrane layer. Often, the membrane layer is prepared by the chemical vapor deposition (CVD) method, and the sol-gel method, dip-coating, electrophoretic deposition (EPD), etc. However, the intermediate layer is prepared via the sol-gel synthesis method with a pore-forming agent like corn starch in the casting solution. These asymmetric membranes have a thickness of a few microns and show high gas permeance. Additionally, these membranes have a low material cost due to their reduced thickness. Moreover, the supported composite membranes are more popular and widely accepted these days for gas separation applications. Further, to increase the packing density of membranes, hollow fiber supports are being used. These supports have a smooth surface and asymmetric inner

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pores. In hollow fiber composite membranes, no intermediate layer is required. These ceramic hollow fiber membranes are highly stable, and the fouling problem is eliminated by back flashing technology. The different types of membranes require special engineering designs for coating the surface. However, both inner surface and outer surface coating technologies are being developed and explored industrially. In the next section, the ceramic membranes used for different gas separation, such as hydrogen, oxygen, and CO2 separation, are described in detail, along with the materials and mechanisms of gas permeation.

2. Ceramic membranes for gas separation This section deals with different types of ceramic membranes used for gas separation. The importance of gas separation and its application to the environment are also discussed in the chapter. Mostly hydrogen production and carbon capture are the two important aspects discussed for gas separation. Hydrogen, oxygen, and CO2 permeation will be discussed in the section and further subsections. The different types of membranes used for each gas separation and the mechanism of permeation of gas through the different membrane materials are discussed in this section. The research on ceramic membranes for gas separation started way back in the early 1990s when researchers started using porous Al2O3, ZrO2, etc. for gas separation filters. However, the development of mixed oxygen and electron conductors for use of electrodes in solid oxide fuel cells brought a revolution in the field of ceramic membranes for gas separation. Latter, these mixed ionic-electronic conducting materials (MIECs) were used as membranes for O2 and H2 separation. This technology is very mature for oxygen separation and is used for several applications [1]. In the latter years, the development of novel ceramic mixed ion-electron conductive membranes was explored for hydrogen, oxygen, and CO2 permeation [2,3]. The membranes with superior proton and electron conductivity are used as electrodes in fuel cells and as membranes for the production of high-purity hydrogen. Similarly, the combination of carbonate (CO2 3 ) and oxide ion conducting material is used as CO2 permeable membranes [3]. In addition to these dense membranes, porous ceramic membranes are also widely used for the separation of hydrogen and other gases. As mentioned earlier, the difference between porous and nonporous membranes is the mechanism at which the gaseous molecules diffuse through the membrane. Since molecular diffusion is the limiting step in porous membranes, the manufacturing techniques for these materials are entirely different from that of the dense membranes. In the preceding subsection, each type of membrane and its application for different gas separations are discussed in detail, followed by the coating techniques.

2.1

Oxygen permeable ceramic membranes

Oxygen permeation in ceramic materials demands high oxygen ions and electronic conductivity. Therefore, MIEC ceramic materials are widely used for oxygen

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separation [1,4]. These are the dense membranes, and oxygen permeation through these ceramic materials takes place through the diffusion of oxygen vacancies and electronic defects. The permeated oxygen has a purity of 99.999%, which is higher than any other membrane-based separation process. Perovskite materials having a general formula of ABO3-d and good oxygen conductivity are used as oxygen-permeable membranes. In these materials, the A site is occupied by alkaline metals or rare earth metals, whereas the B site is occupied by transition metals. The combination of these materials creates oxygen vacancy in the crystal to maintain electro-neutrality, this helps in the oxygen ion (O2) mobility in the crystal. In addition to the oxygen ion mobility, the counter electron conductivity also ensures the charge neutrality in the material [5]. Thus, electron conductivity in the material is as important as oxygen ion conductivity for the development of an oxygen-permeable membrane. The main steps involved in oxygen permeation through the membranes are as follows: ➢ Physisorption of oxygen in the surface of the crystal forming surface oxygen, ➢ Dissociation surface exchange reaction on the membrane surface by interaction with the lattice vacancy, ➢ Formation of lattice oxygen and electron-hole in the crystal lattice ➢ Bulk diffusion of the lattice oxygen and electron holes from the high concentration region to the low concentration region, ➢ Recombination of the oxide ion and electron and desorption of molecular oxygen from the surface.

Fluorite and perovskite crystals having good oxygen conductivity are used in oxygen membrane separation applications. To improve the stability of these membrane materials while maintaining the oxygen permeability, both the metallic sites (A and B sites) are modified in the literature. One such example is Ba0.5Sr0.5Co0.8Fe0.2O3d (BSCF), which has very high oxygen permeability; however, the membrane has poor stability in presence of CO2 [6]. An example of a modified perovskite membrane material is BaBi0.05Co0.8Nb0.15O3d, which is found to be stable in presence of CH4 [7]. This membrane is used for the partial oxidation of methane and NOx decomposition reactions. Further, several fluorine-doped perovskite materials have been reported to have better chemical stability, one such example is La0.6Sr0.4Co0.2Fe0.8O3d (LSCF). One more important aspect of stability is mechanical stability. These inorganic membranes are brittle in nature and making self-supported thin membranes results in poor mechanical stability. Therefore, several coating techniques have been explored in the literature for the synthesis of composite membranes having asymmetric structures. The coating techniques are discussed in Section 3.

2.2

Ceramic membranes for H2 permeation

Membrane-based hydrogen separation is the most advanced and energy-efficient technique. Palladium-based dense metallic membranes are the most studied materials in this field; however, the cost of the membrane limits its industrial application. Several attempts have been made to reduce the membrane cost while maintaining hydrogen purity similar to that of the Pd membranes [8e10]. However, the porous membranes

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are not able to produce the purity of hydrogen required for polymer electrolytic membrane fuel cell applications as targeted by the United States Department of Energy. Dense ceramic proton-conducting membranes have emerged as a promising membrane material for producing high-purity hydrogen at high temperatures. SrCeO3 is the first ceramic material studied for proton-conducting applications at high temperatures. However, both proton conductivity and electron conductivity are required for the hydrogen permeable membrane materials [11]. Hydrogen doesn’t transfer as protons through the perovskite materials rather it is associated with lattice oxygen and diffuses by a quantum mechanical hooping mechanism. Moreover, proton transfer through the membrane in the form of hydroxyl ions instead of the proton. Therefore, several attempts have been made to design and synthesize ceramic materials having high proton and electron conductivity to match the hydrogen permeability of these materials same as Pd-base membranes. One of the best ways to improve the hydrogen flux is by enhancing the electronic conductivity by doping with a suitable element having low ionization potential, and the other is reducing the thickness of the membrane. Moreover, these materials are not stable in the presence of contaminating gases like CO and CO2, due to the formation of stable carbonates of alkali metals. Therefore, several porous ceramic membranes, such as ZrO2, Al2O3, SiO2, TiO2, etc., are also studied for hydrogen separation [12e14]. SiO2 is highly investigated for hydrogen separation from CO2 and CH4. These membranes are synthesized by the CVD technique, and several types of modifications have been made to improve the stability and tune the pore size of the separation layer. However, these membranes have poor hydrothermal stability due to the hydrolysis of the SieO bond in the presence of steam and the structure of the silica network that irrupts [15]. In order to improve the stability of the SiO2 membrane, Zr, Ni, Co, etc. are doped in the silica matrix [15,16]. ZrO2, TiO2, yttria stabilized zirconia (YSZ), and TiN, etc. membranes having uniform pore sizes have been developed and investigated for gas separation applications. Another obvious approach is the reduction of effective membrane thickness by synthesizing supported asymmetric membranes. These porous composite membranes consist of three different layers, such as a macroporous support, a mesoporous intermediate layer, and the top nano-porous separation layer. These asymmetric membranes are synthesized by spray coating, spin coating, tape casting, and the slurry coating method. The different types of coating methods are discussed in Section 3.

2.3

Membranes for CO2 separation

CO2 capture and utilization is a global problem and needs to be addressed on an urgent basis. Direct CO2 capture and conversion from the flue gas is achieved using membrane technology. Membrane separation can be used as a potential solution to CO2 capture. Ceramic-based membranes are utilized for high-temperature CO2 separation directly from the hot flue gas. For this, mixed ion-electron conducting ceramic membrane, materials have been developed and tested. These materials have both CO3 ion conductivity and O2 ion conductivity [3]. Recently, dual-phase molten carbonate membranes have been developed for high-temperature CO2 separation. The membrane consists of highly oxygen ion, and electronic conductive ceramic solid phase and alkali

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or alkaline earth metal carbonate inserted inside the solid matrix as liquid phase. The molten carbonate membrane conducts carbonate ions and the cooperative effect between the solid phase and the molten phase results in the permeation of CO2 through the dense membrane. The materials having high oxygen ion conductivity, like Sm0.15Ce0.85O1.925, Bi1.5Y0.3Sm0.2O3d, LSCF, Y2O3eZrO2:YSZ, and La0.85Ce0.1Ga0.3Fe0.65Al0.05-O3d are used as solid matrix and a eutectic mixture of carbonates of alkali and alkaline earth metals are infiltrated in the molten state inside the solid as carbonate conducting materials [3,17,18]. The combination of the two ion conduction results in CO2 permeation. In addition to the dense membranes, microporous ceramic membranes such as Al2O3, Si3N4, metal oxides, etc. are being explored for CO2 separation. The gas separation in these porous membranes generally follows molecular sieving and the Knudsen diffusion mechanism. The main focus is to fabricate a homogenous and defect-free membrane having superior gas separation ability. This is achieved by selecting a suitable membrane material and using a proper deposition technique. The most common process for the fabrication of the membranes is via the dip coating method. These techniques are the most convenient ways of the thin layer fabrication process, which requires very low operation costs.

3.

Coating techniques used for fabrication of ceramic membranes

This section describes the different types of coating techniques used for ceramic membrane synthesis. Each type of deposition technique and the optimization parameters are described in different subsections. Starting from material selection, the slurry preparation to sintering processes are described in detail. Examples of the membranes synthesized for individual gas separation are also given for a better understanding. Membranes with different shapes, such as disc-shaped, tubular, hollow fiber, etc. are synthesized and used for gas separation applications. Disc-shaped, tubular, and hollow fiber ceramic membranes are widely used for high-temperature gas separation applications [11,19]. As discussed in the previous sections, the flux is inversely related to the thickness of the membrane. Therefore, a very thin membrane is needed of the hour to achieve high gas permeation. Deposition of the ultrathin membrane while maintaining its mechanical stability is achieved by coating the ceramic materials over different porous substrates. However, some thin, self-supported membranes are also reported to have very high gas permeability. These kinds of membranes are developed by coating the ceramic materials over the porous ceramic substrate using different coating methods. Moreover, thin film coating techniques have become popular in recent years due to their wide range of applicability in the development of ultrathin membranes and electrodes for fuel cell applications. The different types of coating techniques adopted for the synthesis of ceramic membranes decide the membrane morphology and separation properties. The coating techniques such as dip-coating, tape casting, sol-gel synthesis, EPD, CVD, magnetron sputtering, and spray pyrolysis are widely used for the fabrication of composite ceramic membranes [20e22]. Each

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type of technique with detailed coating parameters is discussed in detail in the next section.

3.1

Dip-coating

Dip coating is one of the most cost-effective and convenient ways to deposition of ceramic materials. It has been used long back for the deposition of perovskite materials and porous substrates. However, in 1984, the first paper on sol-gel-derived ceramic membranes was published by Zhu and coworkers [23]. Later stage this process was largely used for the preparation of microporous/mesoporous ceramic membranes for gas separation. In the dip-coating method, the porous support is coated with ceramic particles usually dispersed in a liquid medium or a sol. The process involves repeated immersion and controlled removal of the porous support inside the ceramic suspension, followed by drying and finally sintering. After dipping in the solution, a thin wet layer of ceramic material is coated over the substrate, and after drying the volatile liquid (dispersing solvent) is evaporated. Ethanol is a commonly used dispersant liquid; however, aqueous medium such as DI water is also used for the application [24]. A schematic representation of the dip coating process is shown in Fig. 14.1. During the dipping and withdrawal process, a thin layer of particles gets deposited on the substrate surface. The overall process is repeated to get a uniform and defect-free membrane layer. During the immersion process, the particles are deposited on the substrate surface by a capillary action mechanism. The particles tend to deposit on the holes of the substrate due to capillary force during the dipping process, and the deposition is mainly due to the capillary filtration effect. Whereas, a dragging force is applied on the particles by the substrate during the withdrawal process. In this process, the tangential flow of the suspension fluid results in removing weekly adhering particles and dragging them to the pinhole sites. The overall process leads to the formation of an adhering layer on the porous substrate. However, it is difficult to fine-tune ultrathin membrane deposition by the dip-coating process. Heat treatment is carried out for a strong membrane-support interaction. The heat treatment causes the membrane layer

Figure 14.1 Steps involved in the dip-coating process.

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to interact with the support surface, and a hard layer is coated over the porous substrate. In this process, the ceramic particles are sintered and form a thin layer over the surface. The drying and sintering are done in a controlled manner to prevent crack formation in the membrane. During the heat treatment process, the ceramic particles are sintered to form a dense layer. This process creates a decrease in the volume of the deposited layer, for which cracks are generated on the membrane. However, controlled and programmed heating in a stepwise manner prevents the sudden collapse in volume. Thus, defect-free membranes are synthesized in the process. Supports of all shapes (disc, tubular, plate, etc.) are easily coated by this process. The morphology of the membrane depends upon two major factors: (a) dispersion of the particles in the medium to form a stable suspension, and (b) uniformity in pore distribution of the support. Usually, smaller particle size and homogenous dispersion of the particles in the medium help in uniform membrane thickness and defect-free membrane synthesis [11]. In this process, a very thin membrane of thickness ranging from a few microns to hundreds of microns can be synthesized. The formation of a homogenous suspension depends upon the particle size of the ceramic powder. The difference in density between the dispersing medium and the particles results in the sedimentation of the powders in the bottom by gravitational force. However, different techniques are adopted to decrease the particle size, which forms a stable suspension. The Brownian motion of the small particles overcomes the gravitational force and restricts the sedimentation. Changing the dispersion medium has also proven to form a stable suspension. Ethanol is often used as a solvent for preparing the suspension; however, some other solvents, like 1-methoxy-2-propanol, have shown better suspension stability than the former. Another method to prepare a stable suspension is by adding suitable surfactants, which electrochemically stabilize the suspension. La1xSrxCoO3y (LSCO-x) is a perovskite structured material, known to have mixed ionic conductive properties and can be used as an oxygen-permeable membrane. 15e60 mm La0.2Sr0.8Fe0.8Ta0.2O3d membrane prepared by dip coating was tested for oxygenpermeation by Ref. [25]. A commercial polymeric-based surfactant was used along with ethanol to prepare a good-quality suspension. The membrane showed an oxygen flux of w8.7 mL/cm2/min at 1000 C. Several asymmetric membranes are synthesized by the dip-coating method. For such types of membranes, pore distribution of the microporous layer plays an important role. In order to control the pore distribution and pore size, pore-forming reagents such as corn starch, ethyl cellulose, carbon fiber, activated carbon, etc. are used. Corn starch is most widely used for this application. Hong et al. reported the synthesis of a 20-mm-thin La0.2Sr0.8CoO3y (LSCO-80) membrane by the dip coating method over a porous MgO support [26]. Corn starch was used as a pore-forming agent for such applications. This method has been proven to be the easiest and cheaper way of deposition of membranes, and is a widely adopted technique.

3.2

Tape casting

Tape casting is the process of coating ceramic thin sheets from a ceramic-binder slurry over a support material. In this process, a thin layer of ceramic material is coated over a

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surface, followed by drying and sintering to get the deposited layer. Proper care is taken during drying to prevent crack formation on the deposited membrane surface. This process is popularly used in the manufacture of piezoelectric materials for capacitors and electrode materials for fuel cell applications. The original work was started in the 1940s by Ref. [27] for the manufacture of capacitors. However, tape casting is also used to fabricate mixed-ion electronic conductor membranes on various asymmetric supports for the application of gas-permeable membranes. This process offers ease in the fabrication of a wide range of thicknesses (10 to 1 mm) of composite and self-supported membranes. The technique is also applied as a continuous process for the development of self-supported membranes for large-scale applications. Moreover, four main steps are involved in the fabrication of ceramic membranes by the tape casting technique: (a) preparation of ceramic powders, (b) synthesis of the slurry, (c) coating the slurry over porous support by using a desired doctor blade gap, followed by (d) drying and sintering of the tape to get a defect-free membrane. The stepwise preparation method of the ceramic membrane is schematically illustrated in Fig. 14.2. For this particular application, ceramic materials are usually perovskitebased MIECs. These materials are prepared by the solid-state route and the gel combustion method. Initially, these powders are ball milled or ground to get the desired particle, size followed by mixing with proper solvents and solvent-compatible binders to form a uniform slurry, also known as the tape. The solvent and the binder help the powder to spread in a uniform manner over the surface, and some surfactants are also added to control the morphology of the tape surface. The tape casting technique is used to prepare the MIEC membranes, either selfsupported or composite membranes, used in the high-temperature hydrogen/oxygen separation application. The first step in this process is the synthesis of powders for the preparation of the ceramic-binder slurry. The step is very critical, and precise care needs to be taken to achieve dense, defect-free, and uniform-quality tape. The slurry needs to be a highly homogenous and well-dispersed sample with low viscosity. The percentage of solid loading in the slurry is also a critical parameter, which controls the viscosity and the shear thinning behavior. In order to achieve a perfect slurry, the

Figure 14.2 Schematic representation of the tape-casting process.

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ratio of solid to solvent and the amount of binder are optimized. The powder, along with the binder, dispersant, and plasticizer is ball milled to prepare a perfect tape slurry. The ball milling time is adjusted depending upon the powder, nature of the binder, and interaction between the binder and the dispersant. The dispersant is usually added to the ceramic powder to control the deflocculation of the materials. Deflocculation is a process in which the viscosity of the slurry is reduced without the addition of water. Generally, the dispersant, which is the electrolyte-sourcing materials, is added to the slurry to make it uniform and pourable consistency. The dispersant/deflocculants create a repulsive interaction in the ceramic matrix and make it slip over the surface without increasing the specific gravity. Which then helps it to dry quickly and properly without imparting much shrinkage in the tape. It is worth mentioning that, the ceramic slurry has a specific gravity of 1.7e1.85. One of the examples of a dispersant is triethanolamine, which enhances the dispersibility of a proton-conducting material, CaZr0.9In0.1O3d by preventing the agglomeration of the materials. Polyvinyl pyrrolidone is another kind of material used as a dispersant or binder in many applications. This material is used as a dispersant for the synthesis of barium zirconate/cerate (BaCexZr1xyYyO3-d) as a proton-conducting material. In addition to the solvent and dispersant, the binder and plasticizer are also used for holding the ceramic particles together after the tape casting and drying processes. The binder-to-plasticizer ratio should be maintained for achieving better tape quality after drying. For preparing a slurry of CaZr0.9In0.1O3d, PVB was found to be a suitable binder, and around 6e9 wt.% PVB was found to be the optimized quantity. BSCF, and LSCF are wellknown materials for their high oxygen permeation properties. Serra and coworkers prepared a 30 mm LSCF membrane by tape sequential casting method [21]. The sequential casting method is generally adopted for asymmetric membrane synthesis, where the support layer and the membrane are the same material. However, the support layer has a definite pore size, and the membrane is a dense layer. For such applications, the porous support layer is formed by adding a pore-forming agent such as corn starch or rice starch to the slurry. Whereas, the membrane slurry does not contain any poreforming agents [21,28]. The pore-forming agents burn off and create well-defined pores in the support layer during the calcination/sintering processes. This type of asymmetric membrane has advantages over other membranes in terms of similar thermal expansion of support and membrane, and homogenous chemical composition throughout the membrane. Due to the same thermal expansion of the support and the membrane, there are no delamination issues in these membranes. The asymmetric membranes are also explored for hydrogen permeation applications. Montaleone et al. reported the synthesis of asymmetric perovskite-based 20 mm BaCe0.65Zr0.20 Y0.15O3d-Gd0.2Ce0.8O2d (BCZY-GDC) membrane by tape casting method and evaluated its hydrogen permeation properties [29]. Maintaining the phase purity of these membranes is important to preserve the electron and proton conductivity of the material. Therefore, both the sintering temperature and the environment in which the process is done need to be optimized. Tape casting is used to prepare membranes of different shapes and sizes. Using the technique, a dense tubular proton-conducting SrCe0.9Eu0.1O3-d membrane was synthesized by Yoon and coworkers [30]. The tubular membranes are prepared by preparing the tape followed by rolling it over the substrate.

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Tape casting is a robust technique for membrane synthesis of any shape and size. Further research needs to be done to control the thickness of the deposited layer and prepare an ultrathin membrane having a thickness of around 1e5 mm.

3.3

Sol-gel synthesis

The sol-gel process is a chemical method for the synthesis of ceramic membranes used for both gas and liquid separation. The work on sol-gel synthesis started in the year 1921 and is used for the synthesis of metal oxides/ceramic, and perovskite-based materials. The overall process can be divided into three steps: (a) formation of the sol, which is a polymeric or colloidal particulate, (b) gel formation by the establishment of a 3D network among the polymeric chains, and (c) heat treatment followed by sintering. The sol-gel synthesis method is widely adopted for the synthesis of porous ceramic membranes used for both gas and liquid separation. In this method, the metal-organic precursors, also known as the alkoxides or the metal salts (chlorides, nitrates, etc.), are reacted via a polymeric or colloidal route. The materials formed in this route are oxides or nitrides having a high surface area. The sol formed in the preliminary step later forms a 3D network-type structure (gel). The gel is dried and calcined to form the ceramic structure. The reaction is often catalyzed by an acid or base, which controls the morphology of the membrane surface. The reactions taking place in this process are hydrolysis and condensation reactions. Therefore, the kinetics of this reaction decides the particulate size in the sol. The particulate size in the sol further decides the morphology of the membrane. A smaller particle size creates a membrane layer with low porosity and small pores, whereas if the particulate size is bigger, the layer will formed have high porosity and a larger open pore structure. For asymmetric membrane synthesis, a larger particle size is needed for the support and the intermediate layer synthesis; however, finer particle size is needed for the top denser membrane layer. Therefore, the selection of proper precursors, additives, and catalysts and the tuning of the reaction conditions are crucial for controlling the pore size of the membrane. In a broader perspective, the porous membranes used for gas separation applications are made up of SiO2, Al2O3, ZrO2, YSZ, and the hybrid of any two combinations of these materials [12,14]. For the preparation of the ZrO2 membrane, nitric acid is used as a catalyst, and the particulate size is controlled by controlling the amount of HNO3. Further, the aging time also affects the particulate size. Aging for a longer time at a higher temperature than room temperature increases the size of the particulates in the gel. HNO3 is also used as a peptizing agent along with zirconium n-propoxide and propanol in the ZrO2 membrane synthesis to prepare a stable sol [12]. Similarly, porous silica membranes used for hydrogen separation applications are prepared by using tetraethylorthosilicate (TEOS) and bridged silsesquioxane as precursors [31]. Microporous zirconia membranes prepared by the sol-gel technique have shown high hydrogen flux and H2/CO2 selectivity, well above the Knudsen value [32]. These porous ceramic membranes have shown high hydrothermal stability and have been used to separate hydrogen from water gas shift reactions. However, due to the porous nature of the membrane, ultrahigh gas selectivity cannot be achieved using these membranes. The sol-gel synthesis technique is one of the most widely

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accepted technologies for the synthesis of micro and nanofiltration membranes. But this technique is not explored vastly for dense membrane synthesis.

3.4

Electrophoretic deposition

Electrophoretic deposition is a technique for the deposition of thin ceramic films in which the charged particles in a colloidal solution get deposited on the surface by the application of an electric field. This process was first patented in 1933 for the deposition of ThO2 on a Pt electrode. The advantages of this process are that it requires simple instrumentation, is less time-consuming, and can be used for deposition over any shape of conductive substrates [33e35]. The film thickness is also controlled very precisely using the EPD method by controlling the deposition time and applied electric potential. The technique is based on the principle of the deposition of charged particles suspended in any solvent (forming a colloidal solution) over a conductive substrate of opposite charge by applying a DC voltage. The rate of deposition and the morphology of the film depend upon several parameters, including the suspension parameters, the conductivity of the substrate, applied current, time of deposition, drying conditions, and time of sintering. In the suspension parameters, the particle size is an important parameter to deposit a uniform membrane layer. For a defect-free and homogenous layer, the particles need to be homogeneously distributed throughout the suspension. If the suspension has larger particles, then they tend to settle down due to gravitational force. However, when the particles are homogeneously distributed, the electric motion of the particles is higher than that of gravitational settling motion. It has been reported that a 1e20 micron of particle size is an ideal range for homogenous ceramic layer deposition [36]. Moreover, an optimum size of particle should be chosen depending on the viscosity of the suspension and applied potential difference to get a homogenous and uniform membrane. The deposition time is vital for the deposition, and it has been observed that the rate of deposition at a constant applied electric field decreases with the increase in time [37,38]. The observation is well in agreement with the fact that with the increase in time, an insulating ceramic layer is deposited on the electrode surface, which prevents the rate of deposition. Several mechanisms are proposed for the EPD of the ceramic layer. One of the convincing mechanisms proposed by Bouyer and Foissy [39] is based on the two-step deposition process. The first step involves the movement of the charged particles towards the electrode. This process is dependent on the applied field strength, charge of the particle, and viscosity of the colloidal suspension. The second step is the deposition of the charged particles on the substrate by agglomeration and the subsequentloss of charge upon contact with the electrode. Grillon et al. proposed a mechanism for single-layer deposition in which they stated that the charged particle would move to the electrode and become static after deposition [40]. Also, the charge particle will be neutralized in contact with the electrode. Conventionally, the EPD is only possible in the case of conductive surfaces; however, some reports have also shown that this process can also be used to deposit a ceramic layer over nonconductive surfaces. In the case of a nonconducting surface, the membrane can be deposited after a pretreatment to make it conductive. Therefore, some surfaces are made conductive by coating Pt or graphite. Further, it is worth mentioning

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that EPD doesn’t necessarily require a conductive surface if the substrate is porous. In the case of porous substrates, the solvent fills the pores and makes them conductive for the deposition process. However, the substrate must have a minimum critical porosity for such an application. This behavior is highly desired for the synthesis of a composite membrane. Different perovskite-based membranes have been synthesized by the EPD method and used for oxygen separation. BSCF is one of such commonly used materials for high-temperature oxygen separation [41]. This material has good electrical conductivity at room temperature; hence, the EPD technique is easily applied. K. Ishii et al. have reported the fabrication of an asymmetric BSCF membrane using a dense membrane layer and porous substrate of the same BSCF material [41]. A sequential deposition approach was adopted by the same authors to deposit a multilayer membrane in which one of the layers is a dense membrane. Moreover, the multi-layer membrane showed 1.5 times the oxygen permeability compared to the single-layer membrane. Multilayer membranes are composed of different porous and dense membrane layers, and these membranes have improved mechanical stability. However, for the deposition of such membranes, several suspension parameters need to be optimized. For such membranes, one extra pore-forming reagent is added to deposit a porous layer. Therefore, the particle size of the base material and the ratio between the pore-forming reagent and the base material is optimized in the sequential EPD method. The method has gained considerable interest in recent years for the deposition of nanocomposite membrane layers. This process is most widely used for the fabrication of electrodes for solid oxide fuel cell applications. Moreover, the deposition technique is also used for the synthesis of hydrogen-permeable membranes [42]. NieLa1.95Ca0.05Zr2O7d membrane synthesized by the EPD method showed hydrogen permeability at high temperatures. The membrane was deposited using a nonaqueous solvent medium (isopropanol) along with iodine and acetylacetone as dispersants. As mentioned earlier, choosing a proper solvent is required for the deposition of a defect-free membrane. Aqueous solvents are generally used as the medium for deposition. However, during the EPD process, electrolysis of water results in the emission of H2 and O2 gases at the electrodes. The generation of gas bubbles creates defects in membrane synthesis. Therefore, a nonaqueous medium with a dielectric constant of 12e25 needs to be selected for good-quality membrane synthesis [36]. This EPD process is highly used for perovskite-based membranes, mostly used for O2 and H2 purification.

3.5

Chemical vapor deposition

In addition to perovskite and metal oxide membranes, several other types of membranes made up of SiO2, Al2O3, zeolites, and other carbides and nitrides are being explored for gas separation membranes. SiO2 and Al2O3 are the first classes of porous membranes used for gas separation. Zeolites, due to their preferential absorption of CO2 on the cavities of zeolites are used for the CO2 separation from natural gas. Similarly, MFI and SAPO-34 are highly studied for hydrogen separation applications. The downstream gas from a hydrogen production plant mostly contains some water vapors. Therefore, the membranes used for hydrogen separation need to have high

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hydrothermal stability. But the porous silica and some of the zeolites have poor hydrothermal stability. Therefore, a metal oxide (ZrO2, CeO2, etc.) is coupled with these membranes to improve their stability. Additionally, a new class of nano-crystalline metal nitride membranes is also explored for the gas separation application. These types of membranes are often deposited by using CVD technique. CVD is the process of deposition of a thin solid layer over a substrate by using a gas-phase or solid-gas chemical reaction at a high temperature. The choice of CVD process is done by considering various factors such as the nature of reactants (solid or liquid), the shape of the substrate, and the purity of the deposit. In the case of membrane fabrication, the purity of the deposit is taken care and high-purity materials are deposited. For this, highpurity and easily vaporizable starting materials are chosen, and the carrier gas is also cleaned. The precursors used in this deposition process are generally in a solid or liquid state. These reactants are evaporated or sublimated depending upon the requirement and transported to the reactor by a carrier gas. Therefore, the prime requirement of the process is that the starting materials should have a low boiling point and can be easily vaporized. Further, the reaction taking place in the gas phase needs to produce the solid membrane and gaseous byproducts. This eliminates the deposition of any kind of impurity in the membrane. The carrier gas is saturated with the reacting species by cooling down its temperature. The deposition of the membrane takes place in the solid-gas interface. During the CVD, the substrate is heated in a reactor. If the reactor walls are also heated along with the substrate, the deposition takes place on the walls also. Therefore, to eliminate this and create a defect-free membrane, the walls of the CVD reactor are maintained in a cooler state. This deposition process covers the solid substrate, and the growth of the layer is easily controlled by controlling the reaction rate. Decomposition/pyrolysis reactions and redox reactions are the two major kinds of reactions used in the CVD process for membrane fabrication. Although the deposition produces fine and thin membranes; however, several parameters are optimized to obtain a good adhesion property. Surface contamination of the substrate influences the adhesion of the membrane; therefore, the substrate is subjected to degassing, pickling, etching, etc. before the deposition process. The flow rate of the reactants also plays an important role in the deposition process. A laminar flow is mostly used for the reactants, and high flow rates are mostly avoided due to reduced coating efficiency. Usually, the first nucleation is the most important parameter during membrane deposition, which decides the grain size and defect-free membrane synthesis. Therefore, the surface roughness and saturation value of nucleation are controlled for dense membrane synthesis. Control of these parameters results in the production of ultrathin membranes for gas separation applications. As discussed previously, metal oxide-doped SiO2 has high hydrothermal stability compared to the SiO2 membranes used for H2/CO2 separation applications. ZrO2, Al2O3, and TiO2-modified SiO2 membranes are deposited by the CVD technique and have been explored for hydrogen separation. The hydrothermal stability of these membranes has been evaluated in the literature under H2/CO2 separation conditions. A 50% SiO2eZrO2 membrane was developed by Choi et al. using the CVD technique and showed good hydrothermal stability for 105 h [16]. The deposition of ZrO2 and SiO2 is done using an organometallic compound of zirconium (zirconium (IV) tert-butoxide)

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and TEOS precursors, respectively. An inert gas is used as the carrier gas to carry the vaporized gaseous reactants to the coating substrate. The molar ratio of the reactants is also controlled by varying the flow rate of the carrier gas associated with the particular reactant. However, depending on the reaction of deposition, the gas may vary. Mahdi and coworkers prepared Al2O3-modified SiO2 membranes of different compositions by varying the flow rate of the reactants during the CVD process [13]. The modification of SiO2 with Al2O3 and ZrO2 by the CVD process has proven to enhance the hydrothermal stability of the membranes. Several modifications have been done in the instrumentation and reaction point of view to deposit tubular membranes and control the thickness of the deposit. Moreover, the CVD process has proven to be one of the best deposition tools for ultrathin membrane synthesis.

4. Summary and future perspective Different methods have been explored for the synthesis of asymmetric membranes for CO2 capture, O2 separation, and hydrogen production applications. The coating techniques are mature enough for the synthesis of thin-layer membranes. Numerous kinds of research have been done on the development of new materials, novel designs, and scaling up the membrane synthesis procedure. The permeance and stability of the membranes have been significantly increased by the development of novel materials and designs in membrane synthesis. In the case of oxygen and hydrogen-permeable membranes, a reasonable flux value has been obtained by developing novel materials having high MIEC values. However, these membranes are not stable in the presence of contaminants like CO2 and H2S. Thus, materials having good MIEC and high stability are the need of the hour. The ceramic materials are hard and brittle in nature. So, the development of a very thin membrane decreases the mechanical stability of the membrane. Moreover, the coating techniques have provided a new light to such developments, and ultrathin membranes of a few micrometer thicknesses have been synthesized. However, the techniques are only limited to the synthesis and testing of membranes on a smaller scale. Most of the researchers have studied the deposition of the membrane materials on a disc-shaped substrate, which is much easier and applicable to only lab-scale gas permeation testing. Since these membranes are applied at high-temperatures, developing a disc membrane permeation setup is always difficult for such applications. These membranes are hard and brittle in nature; therefore metallic gaskets cannot be used to seal the membranes properly. Another disadvantage is that most of the high-temperature sealants fail to seal the joints at such high temperatures (>700 C). Therefore, innovation and development are required for the synthesis of robust and large-scale tubular asymmetric membranes for smooth and easy operations. Tape casting and EPD are the most widely used technologies for MIEC membranes used for gas separation. But deposition of ultrathin membrane over asymmetric tubular support is difficult by tape casting method. Moreover, the EPD and solgel synthesis methods should be explored thoroughly for controlling the thickness and developing homogenous, dense membranes.

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[28] S. Baumann, J.M. Serra, M.P. Lobera, S. Escolastico, F. Schulze-K€ uppers, W.A. Meulenberg, Ultrahigh oxygen permeation flux through supported Ba0.5Sr0.5Co0.8Fe0.2O3d membranes, Journal of Membrane Science 377 (1e2) (2011) 198e205, https://doi.org/10.1016/j.memsci.2011.04.050. [29] D. Montaleone, E. Mercadelli, S. Escolastico, A. Gondolini, J.M. Serra, A. Sanson, Allceramic asymmetric membranes with superior hydrogen permeation, Journal of Materials Chemistry A 6 (32) (2018) 15718e15727, https://doi.org/10.1039/c8ta04764b. [30] H. Yoon, S.J. Song, T. Oh, J. Li, K.L. Duncan, E.D. Wachsman, Fabrication of thin-film SrCe0.9Eu0.1O3d hydrogen separation membranes on Ni-SrCeO3 porous tubular supports, Journal of the American Ceramic Society 92 (8) (2009) 1849e1852, https://doi.org/ 10.1111/j.1551-2916.2009.03103.x. [31] H. Qi, Preparation of composite microporous silica membranes using TEOS and 1, 2bis(triethoxysilyl)ethane as precursors for gas separation, Chinese Journal of Chemical Engineering 19 (3) (2011) 404e409, https://doi.org/10.1016/S1004-9541(09)60228-7. [32] L. Li, H. Qi, Gas separation using sol-gel derived microporous zirconia membranes with high hydrothermal stability, Chinese Journal of Chemical Engineering 23 (8) (2015) 1300e1306, https://doi.org/10.1016/j.cjche.2015.05.005. [33] P. Sarkar, D. De, T. Uchikochi, L. Besra, Electrophoretic Deposition (EPD): Fundamentals and Novel Applications in Fabrication of Advanced Ceramic Microstructures, Springer Science and Business Media LLC, 2011, pp. 181e215, https://doi.org/10.1007/978-14419-9730-2_5. [34] I. Corni, M.P. Ryan, A.R. Boccaccini, Electrophoretic deposition: from traditional ceramics to nanotechnology, Journal of the European Ceramic Society 28 (7) (2008) 1353e1367, https://doi.org/10.1016/j.jeurceramsoc.2007.12.011. [35] Y. Fukada, N. Nagarajan, W. Mekky, Y. Bao, H.S. Kim, P.S. Nicholson, Electrophoretic depositiondmechanisms, myths and materials, Journal of Materials Science 39 (3) (2004) 787e801, https://doi.org/10.1023/B:JMSC.0000012906.70457.df. [36] L. Besra, M. Liu, A review on fundamentals and applications of electrophoretic deposition (EPD), Progress in Materials Science 52 (1) (2007) 1e61, https://doi.org/10.1016/ j.pmatsci.2006.07.001. [37] R.N. Basu, C.A. Randall, M.J. Mayo, Fabrication of dense zirconia electrolyte films for tubular solid oxide fuel cells by electrophoretic deposition, Journal of the American Ceramic Society 84 (1) (2001) 33e40, https://doi.org/10.1111/j.1151-2916.2001.tb00604.x. [38] Y.C. Wang, I.C. Leu, M.H. Hon, Kinetics of electrophoretic deposition for nanocrystalline zinc oxide coatings, Journal of the American Ceramic Society 87 (1) (2004) 84e88, https:// doi.org/10.1111/j.1551-2916.2004.00084.x. [39] F. Bouyer, A. Foissy, Electrophoretic deposition of silicon carbide, Journal of the American Ceramic Society 82 (8) (1999) 2001e2010, https://doi.org/10.1111/j.11512916.1999.tb02032.x. [40] F. Grillon, D. Fayeulle, M. Jeandin, Quantitative image analysis of electrophoretic coatings, Journal of Materials Science Letters 11 (5) (1992) 272e275, https://doi.org/10.1007/ BF00729410. [41] K. Ishii, C. Matsunaga, K. Kobayashi, A.J. Stevenson, C. Tardivat, T. Uchikoshi, Fabrication of BSCF-based mixed ionic-electronic conducting membrane by electrophoretic deposition for oxygen separation application, Journal of the European Ceramic Society 39 (16) (2019) 5292e5297, https://doi.org/10.1016/j.jeurceramsoc.2019.07.051. [42] D. Das, Q.A. Islam, R.N. Basu, Electrophoretic deposition kinetics and characterization of NieLa1.95Ca0.05Zr2O7d particulate thin films, Journal of the American Ceramic Society 99 (9) (2016) 2937e2946, https://doi.org/10.1111/jace.14319.

Thermal barrier ceramic coatings 1,2

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Mojtaba Najafizadeh , Mehran Ghasempour-Mouziraji , Morteza Hosseinzadeh 4 , Ehsan Marzban Shirkharkolaei 5 , Mansoor Bozorg 6 and Pasquale Cavaliere 2 1 The State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, China; 2Department of Innovation Engineering, University of Salento, Lecce, Italy; 3TEMA - Centre for Mechanical Technology and Automation, University of Aveiro, Portugal; 4Department of Engineering, Ayatollah Amoli Branch, Islamic Azad University, Amol, Iran; 5Department of Mechanical Engineering, Isfahan University of Technology, Isfahan, Iran; 6Faculty of Chemical and Materials Engineering, Shahrood University of Technology, Shahrood, Iran

1. Introduction The efficiency of turbines is related to the possibility of experiencing high temperatures during service. One of the most employed solutions to increase the in-service temperature is to employ metallic components coated through thermal spray (TS) processing. This technology allows for the production of the so-called thermal barrier coatings (TBCs) that are mainly employed to dissipate huge heat fluxes [1]. A TBC is designed to retain a given thermal gradient between the substrate and the coating surface, therefore increasing the service temperature range and the component efficiency [2]. So, the overall efficiency of the turbines; however, can be increased by lowering the thermal conductivity of the coating materials, with the result of obtaining higher operating temperature ranges. It is also demonstrated that the utilization of the TBCs significantly decreased the hightemperature corrosion inconvenience by limiting the effect of the high-temperature gas corrosion against the outside of the turbine’s sharp edges, as shown in Fig. 15.1 [3]. TBCs are typically composed of a metallic substrate (generally a superalloy), a metallic bond coat in the middle, and a top ceramic coating, as shown in Fig. 15.2 [4]. The metallic bond coating is used to reduce the thermal expansion gradient between the substrate and the ceramic coating [5]. Technologies such as thermal and plasma spraying, as well as electron beam vapor deposition (EBPVD), are employed for depositing the ceramic coating. The properties of the TBCs change as a function of the deposition method. The most usually employed materials for the bond coat are NiCoCrAlY, NiCrAlY, and NiAl. Ceramic coating materials are yttria-stabilized zirconia (YSZ), alumina (Al2O3), and other ceramic-based compounds. The performance of the TBCs can also be improved by using postspray processing [6]. This chapter presents the most recent developments in TBC materials and deposition techniques. As a matter of fact, high-temperature performances of the TBCs are presented.

Advanced Ceramic Coatings. https://doi.org/10.1016/B978-0-323-99659-4.00010-3 Copyright © 2023 Elsevier Ltd. All rights reserved.

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Figure 15.1 The development history of thermal barrier coatings [3].

Figure 15.2 Schematic diagram of thermal barrier coating [4].

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2. Structure of the thermal barrier coatings The TBC usually has different layers; a ceramic top layer, a metallic bond coat, a substrate, and thermally grown oxide (TGO) (Fig. 15.2). These coatings have the main function to insulate the substrate from environmental and heat effects. Therefore, in order to reduce the metal temperature, coatings must possess low thermal conductivity. The thermal expansion coefficient between the ceramic top coat and the metal substrate is normally mediated by the bond coat. The bond coats are used for providing the adhesion between the ceramic top coat and the metal substrate. Normally, MCrAlYbased matrials for the bond coats are used as oxidation layers. The MCrAlY (M ¼ Ni, Coor Ni and Co) bond coats applied on the superalloys for protecting the substrate from oxidation have been commonly applied since the 1970s [7]. The interface of a ceramic top coat on superalloy metallic substrate is TGO. TGO is the oxide layer that forms during the thermal cycle. The materials used as substrate are usually nickel-based superalloys normally employed in high-temperature applications. The different layers of the TBCs have different thermal, physical, and mechanical properties, and they are mostly affected by load and thermal conditions [8]. The TBCs are used in different components of the turbine such, as blades, combustor, fixed combustion guide vanes in high-pressure sections, and nozzles, to obtain higher operation temperatures. These coatings have numerous advantages for the turbine system, such as improving efficiency, reducing air cooling, decreasing emission, and increasing the thrust-to-weight ratio. The systems of TBC have metal substrate properties such as stability, continuity, toughness, and additional properties provided by the ceramic material.

2.1

Ceramic top coat

The characterizations of the TBCs, when exposed to the flow of heat gases, are low thermal conductivity, high thermal gradient, and a closer thermal deployment ratio with the substrate. The top of the TBC consists of the ceramic layer, which is responsible for the protection of the substrate from the hot gases and aggressive atmosphere. In this way, the metallic substrate is protected from the environment and from thermal stresses. The ceramic top layer can be selected from materials with low thermal conductivity and optimum thermal expansion coefficient for reducing surface temperature. The thickness of the ceramic top layer is directly related to the amount of heat transferred all over the substrate. Nevertheless, it is demonstrated that as the top layer thickness increases the adhesion to the bond coat decreases. In addition, the bond coat thickness can be directly related to the increase in residual stresses in all the TBC. TBCs with the ceramic top layer of YSZ can lead to a reduction of the surface temperature by at least between 50 and 80 C, when the thickness of the ceramic layer is about 300 mm. The metallic surface temperature during operations can be reduced to about 170e200 C [9,10].

338

2.2

Advanced Ceramic Coatings

Bond coat

The bond coat is the layer interposed between the bond coat and the substrate is generally realized by employing MCrAlY (M ¼ Ni, Co, Ni, and Co). The adhesion of the ceramic top layer and substrate is largely increased through the deposition of the bond coat. In addition, this layer can contribute to protecting the substrate from hot corrosion and oxidation. A protective TGO realized with the a-Al2O3 compound provides bond coat by reservoir Al diffusing and protecting the TBC without reacting with that. The bond coat should work at the operating temperature to optimize cooling air form the substrate without reacting and melting. The temperature of the bond coat should be lower than the 1150 C. Bond coats are usually produced from metal alloys that can support TGOs. With the addition of rare elements to the bond coat such as Y and Hf ( l/2 p / Far field wave b. If distance r < l/2 p / Near field wave

In the case of a far field, the EM wave is a plane wave, that is, the electric and magnetic fields (E and H) are perpendicular to each other. EtH

(17.3)

Thus, wave impedance (Z) is defined by ratio of electric field intensity and magnetic field intensity, Z¼

jEj jHj

where jEj is electric field’s amplitude jHj is magnetic field’s amplitude For air, the wave impedance ðZo Þ is constant, given by, Zo ¼ 377U

(17.4)

Flexible ceramics for EMI shielding applications

Therefore, Zo ¼

385

qffiffiffiffiffiffiffiffiffiffi jum sþjuε; m ¼ m0 mr

Where s is total conductivity u is 2pf ; f is frequency of EM wave m0 is permeability in air ¼ 4p  107 H/m and mr is relative permeability Hence, Zo ¼

rffiffiffiffiffi m0 ¼ 377U ε0

(17.5)

where ε0 is permittivity in air ¼ 8:85  1012 F/m In the case of a near field, the EM wave is curved, thus electric field E is not perpendicular to the magnetic field H. Therefore, wave impedance Zo is not constant. 8 1 < Zo > 377U and decreases with r Thus; Zo ¼ 2pf3r: Zo < 377U and increases with r

2.2

(17.6)

Reflection shielding effectiveness

The major EMI shielding mechanism is reflection. It occurs due to an impedance discontinuity at the interface. Thus, reflection loss ðS:ER Þ can be defined as the relative impedance matching between the surface shielding material and the EM wave see Fig. 17.4. The reflection loss is given by [3]: S:ER ¼ 20 log

Zo 4Zin

(17.7)

Where Zo is wave impedance in air ¼ 377 U qffiffiffiffiffiffiffiffiffiffi jum Zin is impedance of shield ¼ sþjuε Eq. (17.7) can be rewritten as, S:ER ¼ 39:5 þ 10 log

s um

(17.8)

Where s is total conductivity u is 2pf ; f is frequency of wave m is relative permeability. From the above equation, we can see that the reflection shielding effectiveness depends on total conductivity (s) and relative permeability (m); that is, S:ER f

s m

(17.9)

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Advanced Ceramic Coatings

Thus, for a material to have good reflection loss, it should be conductive to interact with the incident EM wave.

2.3

Absorption shielding effectiveness

Another mechanism that contributes to the EMI shielding efficiency is absorption loss ðS:EA Þ. When an EM wave hits the surface of the shield material, a part of the wave will be transmitted. The intensity of the wave rapidly drops as it propagates through the shield, and the energy is dissipated as heat due to absorption. The characteristic of shield material with higher absorption is determined by the thickness of the material. Therefore, S:EA ¼ 20 log e d=s ¼ 8:87 d

pffiffiffiffiffiffiffiffiffiffiffi f pms

(17.10)

Where d is thickness of material. f is frequency of wave. m is relative permeability. s is total conductivity Thus, it is clear that S.EA is a function of d, s, m, that is, S:EA fdsm

2.4

(17.11)

Multiple reflection loss

Internal reflections cause multiple internal reflection loss ðS:EM Þ. Internal reflections occur when a part of an EM wave is rereflected within the shield material. It is observed in cases of thinner material, whereas for thicker material, absorption loss dominates.  2d  S:EM ¼ 20 log 1  e =d

(17.12)

Where d is thickness of material. d is skin depth The thickness below the exterior surface at which the incident wave attenuates to 1/e of its original value is referred to as the skin depth. d ¼ ðfpmsÞ1=2

(17.13)

Hence, the qualities of a good EMI shield are. (i) It should be conductive, that is, it should have mobile charges that would interact with EM waves. (ii) There should be the presence of both electric and magnetic dipole, along with a large thickness for better absorption loss. (iii) Lastly, the presence of a large surface area would affect multiple reflection losses.

Flexible ceramics for EMI shielding applications

2.5

387

Working

For a better understanding of how actually EMI shielding works, see Fig. 17.5. Fig. 17.5 shows a schematic illustration of hybrid carbon black (CB)/ferrite composite. When an EM signal enters the shielding material. The following mechanism occurs. (i) Due to the presence of ferrite, the dielectric polarization occurs which affects ε’ of the shield. (ii) Due to the presence of CB and ferrite, they together provide an interface leading to interfacial polarization, which affects losses in material and high conductivity. This leads to the accumulation of charges at the interface that contributes to EMI shielding performance.

3. Measurement methods for EMI shielding The four most frequent test techniques for determining EMI shielding effectiveness of the shields are as follows [5]: (i) (ii) (iii) (iv)

Open field approach Shielded box approach Shielded room approach Coaxial line transmission approach

3.1

Open field approach

The open field test approach is intended to assess the practical shielding efficiency of finished electrical items. It is usually carried out in an open area devoid of any metallic equipment. This test determines the number of radiated emissions from the final

EI ER Carbon Black

NBR Dipole Polarization

Interfacial Polarization

MnZn Ferrite

Eddy Current Loss

ET Multiple Scattering

Figure 17.5 Illustration of EMI shielding in carbon black/ferrite composite [4].

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Advanced Ceramic Coatings

product. A number of antennae mounted at various distances from the devices are used to measure the radiated field strength and conductive emissions. As shown in Fig. 17.6, the equipment is installed 30 m away from the receiver side, and then the radiated emissions are measured. The conducted emissions transferred are also captured in the same approach. Due to variances in the collection of data, the approach cannot evaluate the effectiveness of any single product and is subjected to large variations.

3.2

Shielded box approach

For comparing different shield materials, the shielded box approach is used. Fig. 17.6 shows the assembly of this approach. It consists of a metallic box fitted with a shield specimen on the EM wave incident side, and the transmitting antenna is fixed outside the box. The receiving antenna is mounted to receive the strength of signals from both with and without the sample fitted. The shielding efficiency is determined by the ratio of signals. The disadvantage of this procedure is that it is challenging to attain appropriate contact between the shield specimen and the metal. Another issue is that the approach works well for frequencies less than 50 MHz.

3.3

Shielded room approach

An illustration of the shielded room approach is shown in Fig. 17.6. It is the most advanced approach created to overcome the limits of the shielded box approach. The underlying idea is similar to the shielded box concept, although the individual electronic parts are enclosed in a separate protected room to prevent EMI. Furthermore, sensors are put in one room and samples in other. The size of this anechoic chamber is often on a scale of 2.5 m2. When compared to the shielded box approach,

Figure 17.6 EMI shielding effectiveness measurement methods [5].

Flexible ceramics for EMI shielding applications

389

the frequency across the chamber is uniformly attributing to valid findings that may be achieved dramatically expanded, and data consistency is markedly enhanced.

3.4

Coaxial line transmission approach

This approach makes use of a vector network analyzer (VNA). In this, it consists of a coaxial calibrated waveguide in which the shield specimen is placed, and the waveguide is connected to two ports, S1 and S2, of VNA, where S1 is for incident waves and S2 are for reflected waves see Fig. 17.7. After that, with the help of VNA the S-parameters S11, S22, S21, and S12 are measured. Here, S11: reflection coefficient at port S1 S22 : reflection coefficient at port S2 S21 : transmission coefficient through port S2 S12 : transmission coefficient through port S1 Using these parameters, the absorption coefficient, reflection coefficient, and transmission coefficient can be calculated using the following equations: 1  S211 S:EA ¼ 10 log S212

!

1  S222 ¼ 10 log S221

Figure 17.7 Vector network analyzer method.

! (17.14)

390

Advanced Ceramic Coatings

! 1 S:ER ¼ 10 log 1  S211

! 1 ¼ 10 log 1  S222

! 1 S:Et ¼ 10 log 2 S12

(17.15)

! 1 ¼ 10 log 2 S21

(17.16)

Where T is transmittance.

4.

Materials used for EMI shielding

With modernization, dealing with electronics is also exponentially increasing. With the use of electronic gadgets, EMI has become a critical concern, as these EM waves have a negative impact on both gadget performance and human health. Hence, it has become necessary to avoid EMI. This is done by decreasing the penetration of EM waves from electronic devices. We must utilize a shield that blocks an EM wave from the device into be released into the surrounding environment [1,6]. Many types of shields are now in use, and their ability to address these issues is being investigated. From a theoretical analysis of total shielding efficiency, it can be brought to light that a shielding material should equitably possess electrical conductivity (s), permittivity (ε), permeability (m), dielectric loss (ε”), and magnetic loss (m”)[7]. Till now metals were recommended for use as EMI shield materials due to their high electrical conductivity which attributes to SEA, and SER. But it suffered from a few limitations, as discussed in the introduction section. This had drawn scholars’ attention away from metal in pursuit of fresh possibilities. Ferrites and inorganic materials are also studied. For EMI absorber applications one of the most important aspects is lightweight. In this section, we will be discussing briefly different types of polymer-based composites for EMI shielding.

4.1

Polymer based composites

One of the best options for EMI shielding materials is polymer composites, which have a substantial advantage over comparable materials in terms of price, density, and production simplicity. In terms of shielding, the polymer composites shield EM radiation through absorption rather than reflection, which proves to be a critical feature in the case when a device needs to safeguard itself as well as prevent reflected emissions from meddling with the operation of other equipment in the vicinity. Polymer-based composites are basically polymer matrix composites formed by the addition of other polymers with continuous and noncontinuous fillers/reinforcements for better shield performance. Polymers can either be insulating or intrinsically conducting. Both of these polymers are suitable candidates for EMI shielding. But in the case of insulating polymers (IPs), such as polystyrene (PS), polyvinylidene

Flexible ceramics for EMI shielding applications

391

fluoride, polypropylene (PP), polymethylmethacrylate, poly (vinyl alcohol) , polyethylene (PE), polyvinylpyrrolidone, and epoxy, we need to add conductive fillers to increase its conductivity. On the other hand, polymers like polyaniline (PANI), polypyrrole (PPY), and poly (3,4eethylene dioxythiophene) possess conductivity and can also be used.

4.1.1

Insulating polymers

IPs are grouped into thermoplastics and thermosets with inherent thermal-mechanical characteristics. Because thermoplastic IPs are thermally plastic, i.e., they soften when heated. The most prevalent polymers are thermoplastics like PE, PP, and PS. Because of their high machinability and mechanical qualities, these polymers are very appealing to a variety of technical applications. Furthermore, engineering plastics such as polyamide (PA), polyacetal, polyphenylene sulfide) (PPS), and poly (ethylene terephthalate) (PET) provide enhanced thermal and chemical stability as well as high mechanical strength for a specific purpose [6]. Thermosetting polymers are the choice of materials for packaging due to their high mechanical strength, and dimensional and shape stability [6]. These IPs are not preferred for EMI shielding applications as they have relatively low EMI SE values due to poor electrical conductivity. See Table 17.1 for some of the IPs and their EMI SE values.

4.1.2

Intrinsically conducting polymers

The electrical conductivity of ICPs is low, with a large band gap. So, by doping its conductivity can be increased. Doping creates charge carriers in polymers, leading to the formation of extremely doped polymers, and also lowers the energy barrier. As a result, in a doped polymer, the electron transitions swiftly from the valence to the conduction band. This excited electron promotes molecular delocalization, allowing mobile electrons to conduct. It is observed that the one-directional polymer chain interacts with free radicals as a result of dissociation; this conduction is known as hopping conduction through quasi-1-D transport [6]. Some of the ICPs are PANI [6], polythiophene (PTP) [12], and PPy [6].

Table 17.1 EMI shielding performance of some popular insulating polymers.

Polymer Polyethylene Polystyrene Polyamide (nylon) Epoxy (bisphenol A)

Electrical conductivity (s) SmL1

Shielding efficiency (dB)

References

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