Thin Film Coatings: Properties, Deposition, and Applications 1032065109, 9781032065106

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Thin Film Coatings

Emerging Materials and Technologies Series Editor: Boris I. Kharissov Nanomaterials for Environmental Applications Mohamed Abou El-Fetouh Barakat and Rajeev Kumar

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Self-Healing Cementitious Materials Technologies, Evaluation Methods, and Applications Ghasan Fahim Huseien, Iman Faridmehr, and Mohammad Hajmohammadian Baghban

Thin Film Coatings: Properties, Deposition, and Applications Fredrick Madaraka Mwema, Tien-Chien Jen, and Lin Zhu For more information about this series, please visit: https://www.routledge.com/ Emerging-Materials-and-Technologies/book-series/CRCEMT

Thin Film Coatings

Properties, Deposition, and Applications

Fredrick Madaraka Mwema, Tien-Chien Jen, and Lin Zhu

First edition published 2022 by CRC Press 6000 Broken Sound Parkway NW, Suite 300, Boca Raton, FL 33487-2742 and by CRC Press 2 Park Square, Milton Park, Abingdon, Oxon, OX14 4RN © 2022 Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, LLC Reasonable efforts have been made to publish reliable data and information, but the author and ­publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material ­reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, access www.copyright. com or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. For works that are not available on CCC please contact m ­ pkbookspermissions@ tandf.co.uk Trademark notice: Product or corporate names may be trademarks or registered trademarks and are used only for identification and explanation without intent to infringe. ISBN: 978-1-032-06510-6 (hbk) ISBN: 978-1-032-06511-3 (pbk) ISBN: 978-1-003-20261-5 (ebk) DOI: 10.1201/9781003202615 Typeset in Times by codeMantra

I dedicate this work to the departed souls of my dad, late Dominic Mwema Muvengei, and my elder sister, Ms Evalyne Kasyoka Mwema. Sleep on well in peace. To my sweetheart, Joan, and sons, Robin & Victor, thank you for the love Fredrick Madaraka Mwema

Contents Preface...................................................................................................................... xv Acknowledgements.................................................................................................xvii Authors.....................................................................................................................xix Chapter 1 Introduction to Thin Films and Coatings..............................................1 1.1

Definition of Terminology..........................................................1 1.1.1 What Are Thin Film Materials?....................................1 1.1.2 What Are Thick Film Materials?..................................1 1.1.3 What Are the Differences between Thin and Thicker Film Materials.................................................1 1.1.4 Thin Film Depositions..................................................1 1.1.5 Target, Precursors, Substrate, and Coatings..................2 1.1.6 Why Is Thin Film Deposition and Coating Important?.....................................................................2 1.2 History and Early Uses of Thin Films.......................................2 1.3 Classification of Thin Film Deposition Methods.......................5 1.4 Mechanism of Thin Film Growth..............................................7 1.4.1 Frank–Van der Merwe Growth......................................7 1.4.2 Stranski–Krastanov Growth (Layer-Plus–Island).........8 1.4.3 Volmer–Weber (Isolated Island) Growth Mode............ 8 1.5 Parameters Influencing Thin Film Depositions.........................8 1.6 Properties of Thin Film Materials............................................ 11 1.7 Modern Applications of Thin Film Materials.......................... 11 1.8 Summary.................................................................................. 13 1.9 Scope of the Book.................................................................... 14 References........................................................................................... 14 Chapter 2 Methods of Thin Film Deposition....................................................... 17 2.1 Introduction.............................................................................. 17 2.2 Physical Vapour Deposition..................................................... 18 2.2.1 Sputtering....................................................................20 2.2.1.1 Direct Current (DC) and Radiofrequency (RF) Sputtering................. 22 2.2.1.2 Magnetron Sputtering..................................24 2.2.1.3 High Power Impulse Magnetron Sputtering (HIPIMS)...................................25 2.2.1.4 Reactive Sputtering......................................28 2.2.1.5 Bias Sputtering............................................28 2.2.1.6 Equipment.................................................... 29 2.2.2 Thermal Evaporation................................................... 29 vii

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2.2.2.1 Equipment.................................................... 31 Ion Plating................................................................... 31 Electron Beam Deposition.......................................... 33 Pulsed Laser Deposition.............................................. 35 Thermal Spray............................................................. 37 2.2.6.1 Flame Spray................................................. 38 2.2.6.2 Plasma Spray Deposition Technique........... 39 2.2.6.3 High Velocity Oxy-Fuels............................. 41 CVD Techniques...................................................................... 42 2.3.1 Science of CVD........................................................... 42 2.3.1.1 Steps in CVD............................................... 42 2.3.1.2 Advantages of CVD Methods in Preparation of Thin Film Technologies....... 42 2.3.2 Atmospheric Pressure CVD........................................44 2.3.2.1 Reactors in APCVD.................................... 45 2.3.2.2 Advantages of APCVD............................... 45 2.3.2.3 Limitations of APCVD................................46 2.3.2.4 APCVD Parameters....................................46 2.3.3 Low Pressure Chemical Vapour Deposition...............46 2.3.3.1 Reactors Used in LPCVD............................ 47 2.3.4 Ultrahigh Vacuum CVD............................................. 49 2.3.4.1 Operating Principles.................................... 49 2.3.5 Plasma Enhanced Chemical Vapour Deposition (PECVD)................................................... 50 2.3.5.1 Advantages of PECVD................................ 52 2.3.5.2 Disadvantages of PECVD........................... 52 2.3.5.3 Applications................................................. 52 2.3.5.4 Parameters in PECVD................................. 52 2.3.6 Sub-Atmospheric Pressure Chemical Vapour Deposition................................................................... 52 Atomic Layer Deposition......................................................... 53 2.4.1 Introduction................................................................. 53 2.4.2 Principle of Atomic Layer Deposition........................ 54 2.4.3 Thermal ALD.............................................................. 56 2.4.4 Plasma Assisted Atomic Layer Deposition................. 56 2.4.5 Photo-Assisted ALD................................................... 56 2.4.6 Metal ALD.................................................................. 57 2.4.7 Catalytic SiO2 ALD..................................................... 58 2.4.8 Attributes/Advantages of ALD Process...................... 58 2.4.9 Precursors and Materials for ALD.............................. 59 2.4.10 Applications of ALD................................................... 61 2.4.10.1 Applications in Microelectronics................. 61 2.4.10.2 Application in the Medical Field................. 61 2.4.10.3 Applications in Photovoltaics (PV)/ Solar Cells.................................................... 62

2.2.3 2.2.4 2.2.5 2.2.6

2.3

2.4

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2.4.10.4 Application in Energy Storage Systems...... 62 2.4.10.5 Application in Desalination......................... 62 2.4.10.6 Application in Catalysis............................... 62 2.4.10.7 Application in Optics................................... 63 2.5 Some Chemical Deposition Methods....................................... 63 2.5.1 Sol-Gel Technique....................................................... 63 2.5.2 Electro-Deposition...................................................... 65 2.5.3 Chemical Bath Deposition..........................................66 2.6 Summary.................................................................................. 67 References........................................................................................... 68 Chapter 3 Characterisation Techniques of Thin Films........................................ 77 3.1 3.2

Classification of Characterisation Techniques......................... 77 Structure Characterisation........................................................ 78 3.2.1 Morphology of Thin Films.......................................... 78 3.2.2 Grain Size and Crystal Analyses................................ 81 3.2.3 Thin Film Defects....................................................... 83 3.2.3.1 Surface Pre-treatments of Substrate............ 83 3.2.3.2 Thin Film Defects Formed during Deposition Processes................................... 86 3.3 Topography Characterisation................................................... 87 3.4 Fractal Theory in Thin Films...................................................90 3.5 Mechanical Characterisation....................................................92 3.5.1 Nanoindentation Tests.................................................94 3.5.2 Wear/Scratch Tests......................................................96 3.6 Chemical Characterisation..................................................... 100 3.6.1 Chemical Composition.............................................. 100 3.6.2 Corrosion Characterisation....................................... 104 3.7 Summary of Characterisation Methods................................. 106 3.8 Summary................................................................................ 106 References......................................................................................... 111 Chapter 4 Hybrid and Scalable Thin Films....................................................... 115 4.1

Introduction............................................................................ 115 4.1.1 What Is a Hybrid Material?....................................... 115 4.1.2 What Is a Hybrid Thin Film Material?..................... 115 4.1.3 What Is a Multilayer Thin Film?............................... 115 4.2 Properties of Thin Hybrid and Multilayer Thin Films........... 116 4.3 Free-­Standing and Scalable Thin Films................................. 119 4.4 Summary................................................................................ 121 References......................................................................................... 121

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Chapter 5 Bibliometric Analysis of Applications of Thin Film Materials........ 123 5.1 5.2

Introduction............................................................................ 123 Bibliometric Analyses on Thin Film Applications................ 123 5.2.1 Growth Trend over the Years.................................... 124 5.2.2 Thin Film Research by Country............................... 124 5.2.3 Applications of Thin Film Materials........................ 125 5.3 Summary................................................................................ 127 References......................................................................................... 127 Chapter 6 Thin Films for Biomedical Applications........................................... 129 6.1

Introduction to Biomaterials................................................... 129 6.1.1 Metal and Metal Alloy Biomaterials......................... 130 6.1.2 Ceramics Biomaterials.............................................. 131 6.1.3 Biopolymers.............................................................. 133 6.1.4 Composite Biomaterials............................................ 133 6.2 Thin Film Materials and Their Applications in Biomaterials....................................................................... 134 6.3 Specific Applications of Thin Films in the Biomedical Sector...................................................................................... 136 6.3.1 Hip Replacement....................................................... 136 6.3.2 Knee and Shoulder Prosthesis................................... 137 6.3.3 Neural/Brain Implants.............................................. 137 6.3.4 Protein Repellent Coatings........................................ 139 6.4 Emerging Trends on the Application of Thin Film Materials in Biomedical Field................................................ 140 6.4.1 Self-­Healing Biomaterial Coatings........................... 140 6.4.2 Development of Hybrid Biomaterial Thin Films...... 140 6.5 Summary................................................................................ 141 References......................................................................................... 141

Chapter 7 Thin Films for Surface Protection.................................................... 147 Introduction to Surface Protection......................................... 147 Thin Film Materials in Wear Protection................................ 150 Hydrophobic and Hydrophilic Thin Materials....................... 152 Thin Film Materials for Corrosion Protection....................... 155 7.4.1 Aluminium Oxide..................................................... 155 7.4.2 Titanium Oxide......................................................... 155 7.4.3 Aluminium Oxide–Titanium Oxide.......................... 156 7.4.4 Tantalum Oxide (Ta2O5)............................................ 156 7.4.5 Titanium Nitride........................................................ 157 7.5 Trend and Progress of Thin Film Technology in Surface Protection Industry................................................................. 157 7.6 Summary................................................................................ 158 References......................................................................................... 158 7.1 7.2 7.3 7.4

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Chapter 8 Thin Films for Cutting Tools............................................................ 163 8.1 8.2

Introduction to High-­Speed Machining................................. 163 Application of HSM............................................................... 163 8.2.1 Aerospace Applications............................................. 164 8.2.2 Die and Mould Manufacturing.................................. 165 8.2.3 Automotive and Other Manufacturing Industries..... 165 8.3 Advantages and Disadvantage of HSM.................................. 165 8.3.1 Disadvantages of HSM.............................................. 166 8.4 HSM Tools Materials............................................................. 166 8.5 HSM Methods and Their Applications.................................. 166 8.6 Importance of Coating Cutting Tools for Machining Processes................................................................................ 166 8.7 Coating of Cutting Tools for HSM......................................... 167 8.7.1 Classification of Coating Materials Used in Cutting Tools............................................................. 167 8.7.2 Nitrides...................................................................... 168 8.7.2.1 Titanium Nitride (TiN).............................. 168 8.7.2.2 Titanium Aluminium Nitride (TiAlN)...... 169 8.7.2.3 Chromium Nitride (CrN)........................... 170 8.7.2.4 Titanium Chromium Nitride (TiCrN) Coating....................................................... 171 8.7.2.5 Zirconium Nitride...................................... 172 8.7.2.6 Titanium Silicium Nitride.......................... 172 8.7.2.7 Titanium Aluminium Silicon Nitride (TiAlSiN)................................................... 173 8.7.2.8 Chromium Aluminium Nitride (CrAlN)...... 173 8.7.2.9 Titanium Molybdenum Nitride (TiMoN)...... 173 8.7.2.10 Boron Nitride (BN).................................... 173 8.7.3 Carbides..................................................................... 173 8.7.3.1 Titanium Carbide (TiC)............................. 174 8.7.3.2 Chromium Carbide (CrC).......................... 174 8.7.3.3 Tungsten Carbide (WC)............................. 174 8.7.4 Others........................................................................ 174 8.7.4.1 Titanium Boride (TiB2).............................. 174 8.7.4.2 Diamond Carbon....................................... 175 8.7.4.3 Molybdenum Disulphide (MoS2)............... 175 8.7.4.4 Aluminium Oxide...................................... 175 8.8 Progress in Thin Film Materials for Cutting Tool Industry... 176 8.9 Summary................................................................................ 177 References......................................................................................... 178

Chapter 9 Thin Films for Electronic, Spintronics, and Optical Applications......183 9.1 9.2

Introduction............................................................................ 183 Importance of Optics and Spintronic Technologies............... 183

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9.3

Thin Film Materials for Optic Devices.................................. 184 9.3.1 Oxide-Based Ceramic Coatings................................ 184 9.3.2 Non-Oxide Ceramic Coatings................................... 185 9.3.3 Metal-Based Thin Films........................................... 186 9.4 Thin Film Materials for Spintronic and Photonic Applications............................................................................ 186 9.5 Thin Film Materials for Microelectronic Applications.......... 188 9.6 Thin Film Materials for Nanodevices and Flexible Gadgets..................................................................... 189 9.7 Future Outlook....................................................................... 189 9.8 Summary................................................................................ 190 References......................................................................................... 191 Chapter 10 Thin Film Materials for Energy Applications................................... 195 10.1 Energy Materials and Renewable Energy Devices................ 195 10.1.1 Introduction............................................................... 195 10.1.2 Solar Cells and Photovoltaic Materials..................... 195 10.1.3 Fuel Cells................................................................... 196 10.1.4 Wind Turbines........................................................... 196 10.1.5 Nuclear Reactors....................................................... 197 10.2 Thin Film Materials for Solar Cell Device Applications....... 197 10.2.1 Introduction............................................................... 197 10.2.2 Copper Indium Selenide/Copper Gallium Selenide...... 198 10.2.3 Cadmium Telluride Thin Films................................ 198 10.2.4 Amorphous Silicon (a-Si).......................................... 198 10.2.5 Dye Sensitised Solar Cell.......................................... 199 10.2.6 Perovskite Solar Cells............................................... 199 10.3 Thin Film Materials for Nuclear Applications.......................200 10.3.1 Introduction...............................................................200 10.3.2 Detectors...................................................................200 10.3.3 Cladding.................................................................... 201 10.3.4 Insulators...................................................................202 10.4 Application of Thin Films for Fuel Cells...............................203 10.4.1 Introduction............................................................... 203 10.4.2 Low Temperature Fuel Cells.....................................204 10.5 Thin Film Materials for Wind and Hydro-Power Systems...... 205 10.5.1 Wind-Power System..................................................205 10.5.2 Hydro-Power System.................................................207 10.6 Emerging Technologies in Thin Films for Energy Materials................................................................................. 215 10.7 Summary................................................................................ 215 References......................................................................................... 216

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Chapter 11 Smart and Self-Healing Thin Film Materials................................... 221 11.1 11.2 11.3 11.4 11.5

Self-Healing Materials........................................................... 221 Smart Thin Film Materials..................................................... 221 Self-Healing Thin Film Materials and Applications.............. 221 Smart Thin Film Materials and Their Applications............... 223 The Future of Smart and Self-Healing Thin Film Materials................................................................................. 226 11.6 Summary................................................................................ 227 References......................................................................................... 227 Chapter 12 Thin Films for Antimicrobial Applications...................................... 231 12.1 Introduction............................................................................ 231 12.2 Brief Description of Microbial Characteristics...................... 231 12.3 Importance of Antimicrobial Materials in Today’s Society.................................................................................... 233 12.4 Thin Film Materials for Antimicrobial Applications............. 236 12.4.1 Silver Thin Film Coatings......................................... 236 12.4.2 TiO2 Thin Film Coatings........................................... 243 12.4.3 Chitin/Chitosan-­Based Thin Films...........................244 12.4.4 Starch Hybrid Thin Film...........................................246 12.5 The Future of Thin Film Materials for Antimicrobial Applications............................................................................246 12.6 Summary................................................................................ 250 References......................................................................................... 250 Chapter 13 High Entropy Alloy Thin Films........................................................ 257 13.1 Introduction to High Entropy Alloys...................................... 257 13.2 Importance of HEAs in the Modern Industry........................ 258 13.3 High Entropy Alloys and Thin Films..................................... 258 13.3.1 AlCoCrCuFeNi........................................................260 13.3.2 AlCoCrFeNi............................................................. 261 13.3.3 AlCoCrCuFeNiTix and AlCoCrFeNiTix.................. 261 13.3.4 AlCrFeNiMn............................................................ 261 13.3.5 AlCoCrFeNiMox and AlCoCrCuFeNiMox............... 262 13.3.6 AlCoCrFeNiNbx....................................................... 262 13.3.7 AlCoCrFeNiSix......................................................... 262 13.3.8 Alx(TiVCrMnFeCoNiCu)100−x................................... 263 13.3.9 TiNbMoMnFe.......................................................... 263 13.3.10 CoCrFeNiZrx............................................................ 263 13.3.11 NbMoTaW................................................................ 263 13.3.12 TiTaHfNbZr.............................................................264

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13.3.13 CuMoTaWV.............................................................264 13.3.14 NbSiTaTiZr.............................................................. 265 13.3.15 Other HEA Thin Films............................................ 265 13.4 Future Application and Development of Thin Film HEAs...... 265 13.4.1 Transport and Energy Sectors................................... 265 13.4.2 Gas Turbines, Rocket Nozzles, and Nuclear Plant Construction.....................................................266 13.4.3 Protective Coatings...................................................266 13.4.4 Biomedical Applications...........................................266 13.5 Summary................................................................................ 267 References......................................................................................... 267 Chapter 14 Thin Film Technology and Industry 4.0........................................... 271 14.1 The Industry 4.0..................................................................... 271 14.2 Industry 4.0 and Thin Film Industry...................................... 272 14.3 Role/Future of Thin Film Technology in Industry 4.0........... 273 14.4 Summary................................................................................ 274 References......................................................................................... 275 Chapter 15 Thin Films and 3D Printing Technology.......................................... 277 15.1 Introduction to 3D Printing Technology................................ 277 15.2 3D Printing and Thin Film Technology................................. 281 15.2.1 Surface Engineering of AM Parts............................. 281 15.2.2 Complex Structures and Patterned Thin Film Materials........................................................... 281 15.2.3 Preparation of Flexible Substrates............................. 283 15.3 Summary................................................................................ 283 References......................................................................................... 283 Index....................................................................................................................... 287

Preface Thin film materials offer attractive properties for application in various sectors and industries. As such, these materials have attracted a lot of interest from different experts and industries. In terms of research on this topic, there is an extensive literature on the subject of thin film materials. However, it is usually difficult to find a single book exploring a wide range of thin film processing techniques, properties, and applications. Most of the existing books focus on a specific application and hence limited thin film materials. As such, the motivation of developing and writing this book was to present a huge resource of thin film materials for most applications and their properties. To achieve this objective, this book consists of 15 chapters and the details of each chapter are briefly highlighted. Chapter 1 introduces thin film materials and coatings where the basic terminologies of thin film deposition and technology are defined. The chapter also briefs the readers on the historical evolution of thin film technology. Chapter 2 consists of discussions on various techniques of thin film deposition in which operating principles and conditions of specific deposition methods are provided. In Chapter 3, the various methods of characterising the properties of deposited thin film materials are illustrated. Chapter 4 discusses the hybrid and scalable thin film materials and illustrates some typical protocols for fabrication of scalable and nanostructured thin film parts. In Chapter 5, a bibliometric analysis on the applications of thin film materials is presented. The focus areas of applications demonstrated by the bibliometric analysis in Chapter 5 form the basis of discussions of the book from Chapter 6. As shown, Chapter 6 discusses thin film materials for biomedical applications, Chapter 7 discusses thin film materials for surface protection, Chapter 8 presents thin film materials for cutting tool industry, and Chapter 9 demonstrates the properties and applications of thin film materials in photonic, spintronics, and optical devices. In Chapter 10, thin film materials for energy applications are discussed, whereas in Chapter 11 thin film materials for self-healing and smart properties are presented. Chapter 12 presents thin film materials for antimicrobial applications, Chapter 13 provides the properties and applications of high-entropy alloy thin films, and Chapters 14 and 15 describe the relationship between Industry 4.0 and 3D printing, respectively. The book shall be very resourceful for students, researchers, and industry to understand the principles of thin film technologies, thin film materials for various application, and their properties. The book has also provided future outlook of the thin film deposition and, therefore, it can be used by researchers and innovative scientists when developing products for the future market.

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Fredrick Madaraka Mwema, University of Johannesburg, South Africa & Dedan Kimathi University of Technology, Nyeri, Kenya Tien-Chien Jen, University of Johannesburg, South Africa Lin Zhu, Anhui Agricultural University, P.R. China

Acknowledgements The authors of this book acknowledge family, friends, and colleagues for their encouragements and support during the development and writing of this book. Fredrick Mwema specifically acknowledges Mr Harrison Shagwira for proofreading the final manuscript of the book. He also acknowledges his postgraduate students in mechanical engineering, Dedan Kimathi University of Technology, for their support during the development of the book. Finally, he acknowledges his family, Ms Joan, Robin and Victor, for their encouragement and love during the writing of this book. Professor Tien-Chien Jen and Dr Fredrick Madaraka Mwema acknowledge the University of Johannesburg for providing the avenue through the GES 4.0 postdoctoral fellowship under which this manuscript was developed. Professor Tien-Chien Jen would also like to acknowledge the financial support from NRF.

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Authors Dr Fredrick Mwema i s a Senior Lecturer in the Department of Mechanical Engineering at Dedan Kimathi University of technology (DeKUT), where he teaches undergraduate and postgraduate courses. He is the chair of the Department of Mechanical Engineering (DeKUT) and also the director of the Center for Nanomaterials and Nanoscience Research (CNSR) at DeKUT. Dr Mwema is also a researcher at the University of Johannesburg (UJ). Dr Mwema obtained a PhD (Mechanical engineering) from the University of Johannesburg in 2019, and MSc (July 2015) and BSc (July 2011) degrees in mechanical engineering from Jomo Kenyatta University of Agriculture and Technology (JKUAT), Nairobi, Kenya. In terms of research, Dr Mwema undertakes research in the field of thin film materials, especially sputtering method, where he has published extensively in peerreviewed journals, books, and conferences. His Scopus H-index is about 11 with over 330 citations. His other research interests include material characterisation, surface engineering, manufacturing, and engineering education. Dr Mwema has guided five students to completion of their master’s degrees in mechanical engineering. Prof T.C. Jen was a faculty member at the University of Wisconsin, Milwaukee, and then joined the University of Johannesburg in August 2015. Prof Jen received his PhD in Mechanical and Aerospace Engineering from UCLA, specialising in thermal aspects of grinding. He has received several competitive grants for his research, including those from the US National Science Foundation, the US Department of Energy and the EPA. Dr Jen has brought in $3.0 million of funding for his research, and has received various awards for his research including the NSF GOALI Award. Prof Jen has recently established a Joint Research Centre with Nanjing Tech University of China on the “Sustainable Materials and Manufacturing.” Prof Jen is also the director of Manufacturing Research Centre of the University of Johannesburg. Meanwhile, SA National Research Foundation has awarded Prof Jen a NNEP (National Nano Equipment Program) grant worth of USD 1 million to acquire two state-of-the-art Atomic Layer Deposition (ALD) Tools for ultra-thin film coating. These two ALD tools will be the first in South Africa and possibly the first in Africa continent. In 2011, Prof Jen was elected as a fellow to the American Society of Mechanical Engineers (ASME), which recognised his contributions to the field of thermal science and manufacturing. As stated in the announcement of Prof Jen fellow status in the 2011 International Mechanical Engineering and Congress Exposition, “Tien-Chien Jen has made extensive contributions to the field of mechanical engineering, specifically in the area of machining processes. Examples include, but not limited to, environmentally benign machining, atomic layer deposition, cold gas dynamics spraying, fuel cells and hydrogen technology, batteries, and material processing.” Prof Jen has written a total of 198 peer-reviewed articles, including 84 peer-reviewed journal papers, published in many prestigious journals such as International Journal of Heat xix

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and Mass Transfer, ASME Journal of Heat Transfer, ASME Journal of Mechanical Design, and ASME Journal of Manufacturing Science and Engineering. He also has written many chapters in special topics book, for example, a chapter in Numerical Simulation Proton Exchange Membrane Fuel Cell, published by WIT Press, and another chapter in Application of Lattice Boltzmann Method in Fluid Flow and Heat Transfer in Computational Fluid Dynamics – Technology and Application. Dr Lin Zhu is a professor at the Department of Mechanical Engineering of Anhui Agricultural University, P. R. China. He received his PhD in Precision Machinery and Precision Instrumentation from the University of Science and Technology of China (USTC), specialising in flowing characteristics and structural analysis of a silicon-based micro-combustor. And he was ever a postdoctoral research associate in mechanical engineering at the University of Wisconsin-Milwaukee and a visiting scholar of the State University of New York at Stony Brook, respectively. The main research interests of Dr Lin Zhu are focussed on the following three aspects: (1) performances and structures of plants and animals using Mechanical Engineering Methods, (2) optimisation structures and performances of mechanical components, and (3) micro-fluids and micro-structure. He also has an interest in additive manufacturing technologies including laser-based manufacturing and fused deposition modelling. He has received several competitive grants for his research, including those from the National Science Foundation of China (NSFC). Currently, he has over 80 publications and has written three books. Dr Lin Zhu mentors several postgraduate students in the field of fluid mechanics. He is currently supervising six masters and one PhD students on liquid-solid interaction, spray mechanism, and additive manufacturing. He is a member of the Vibration Society of China.

1

Introduction to Thin Films and Coatings

1.1 DEFINITION OF TERMINOLOGY 1.1.1 What Are Thin Film Materials? Thin films are layers of materials of thickness ranging from several nanometres to a few micrometres [1]. Thin films can also be defined as layers of materials that extend along any two directions but are restricted along the third direction and described as layers of thicknesses less than 1 μm. Thin films are nanomaterials (nanometric grain structure) and exhibit properties different from bulk materials. These materials are manufactured via ‘layering’ of the smallest building blocks such as atoms, ions, or molecules.

1.1.2 What Are Thick Film Materials? Literally, thick film materials are those exhibiting thickness above 1 μm. However, compared to thin film materials, thick film materials exhibit coarse (micro- and macro level) grain structures.

1.1.3 What Are the Differences between Thin and Thicker Film Materials Thin film has a thickness in the order of 0.1 μm or smaller, while thick film is thousands of times thicker. The most important difference between these two classes of material is methods of creating them. Thin film materials are usually manufactured using sophisticated and vacuum-based methods, whereas thick films are produced through cheaper and simple methods. For example, thin film metal resistors are usually produced by atomic-based processes such as sputtering, whereas thick film metal resistors are manufactured using stencil and screen-printing methods. In terms of properties, thin films exhibit attractive active/functional properties, whereas thick films exhibit better surface protection properties.

1.1.4 Thin Film Depositions These are techniques used for producing thin film materials. Generally, most of these methods involve vacuum and high-power operating conditions. Thin films can be deposited on a surface of another material (known as substrate) through thermal evaporation, chemical reactions and deposition, sputtering, etc. [2]. Thin film deposition methods determine/influence the purity, performance, and behaviour of the prepared thin film materials.

DOI: 10.1201/9781003202615-1

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Thin Film Coatings

1.1.5 Target, Precursors, Substrate, and Coatings A target is the source material from which the thin film materials are formed through plasma-involving deposition techniques. For instance, to deposit thin film of AlN through sputtering, the target material is pure solid aluminium and nitrogen as the reactive gas. A precursor is a chemical (usually in gaseous state) that reacts with another chemical inside a reactor chamber (usually in a vacuum) to form thin film materials. In deposition methods such as chemical vapour deposition (CVD) and atomic layer deposition (ALD), the chemicals used for the production of the final stoichiometry of the thin film are known as precursors. A substrate refers to the surface on which the thin/thick film material is deposited. Usually, the thin film material is deposited on the substrate to modify its surface properties or to just support the growth of the thin film material. Coatings refer to thin/thick film materials grown on the surface of another material (substrate) with the aim of protecting the surface of that material. It can be thought of as a raincoat, which during rains human beings/animals cover themselves against the harsh rainy weather conditions! Coatings enhance surface hardness, wear protection, and corrosion resistance of the substrate material.

1.1.6 Why Is Thin Film Deposition and Coating Important? Traditionally, painting is the most used method of protection or modification of surfaces of materials or components. Similar to painting, thin film deposition and coating are important in the following ways: • Deposition of thin film materials on substrates imparts some properties which the substrate surface would otherwise not exhibit. Such properties include optical, electrical, biocompatibility, etc. • The deposition of thin film and coating protects the surfaces of the substrate from harsh conditions such as extreme heat, humidity, acidity, alkalinity, and so forth. • Thin film coating can be undertaken for decoration purposes.

1.2 HISTORY AND EARLY USES OF THIN FILMS The extensive application of coatings and thin film materials can be dated back to more than half a century. These materials have been used extensively over time in the following applications: • • • •

Manufacture of optical coatings, Hard coatings on machines and equipment, Ornamental items and devices, and Electrical appliances.

Ongoing research and development of coatings and thin film materials has evaluated their chemical and structural composition. This has led to the development of thin

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film material technologies, which focus more on improving thin film materials and determining optimal processing techniques to reduce the consumption of some toxic materials and lower energy usage during processing of these materials [2]. As mentioned earlier, coatings have found a wide range of applications in both protective and decorative fields. However, the history of coatings displays that they were predominantly used for beauty and ornamental purposes [3]. These coatings were made on walls and other ornaments to enhance the beauty and improve their functionality through artistic features and paintings. The coatings comprised edible species of natural polymers such as earth pigments and synthetic oils. The use of coatings for protective purposes can be traced back to Egypt at around 5000 BC, where pitches and balsams were used to create water-tight surfaces on sea vessels. This practise spread to other parts of the world and led to the adoption of biopaints in the Middle Age to preserve the quality of surfaces, especially those made from wood. However, this was still limited since most of the structures were not permanent and timber was readily available for replacement. This changed during the Industrial Revolution when mechanical and electrical equipment were discovered and adopted for application. As a result, the manufacturers faced the massive challenge of tackling corrosion on machine surfaces. The demand led to the high production of bio-oils-based coatings. These oils, most of which were extracted from vegetables, formed the basis for the coating technology up to the 14th century and formed a considerable part of polymer and resin technology [4]. Coating technologies evolved after the Industrial Revolution as people sought better, less expensive, and long-lasting coatings. This led to the rise and development of thin film materials, which became basis for the modern coating technology. The first chemo-mechanically made inorganic thin film could be traced back to the Egyptians, who used them for beauty purposes. This coating technology was later adopted in optical applications [5]. The coating was made of several layers of gold during the Middle Bronze Age (approximately 5000 years ago) and was deposited in tombs and the pyramids owned by royal families. These gold coatings were also found on bronze statues and other religious items in Egypt. This was prompted by the availability of the minerals along the Nile and shores of the Red Sea [6]. Some of the early uses of gold coatings have been documented [4] and examples are illustrated in Figure 1.1. Thin film coatings were initially employed manually, where artisans practised gold sheathing, which involved the creation of a thin layer of gold strips on surfaces such as wood and bronze to increase their beauty (Figure 1.1). This art was found in areas believed to be owned by noble rulers and families in ancient Egypt, such as Queen Hetepheres and Pharaoh Tutankhamun. Moche Indians further advanced the thin film coating technique at the onset of approximately 100 BC by developing the oxidation-reduction coating method. The Indians used gold to create thin coatings on copper and other metals to increase their strength and improve functionality [3]. This technology has advanced and is currently known as electroless plating [3]. The evolution of thin film technology can also be traced back to the discovery of vacuum technology in 1640 with the invention of the barometer. This technology was essential since scientists needed cleaner surfaces to carry out the deposition of thin films. Otto von Guericke came up with a third-generation vacuum system from a piston pump in 1652, which kick-started the ‘vacuum’ journey [6]. This was followed

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FIGURE 1.1  Showing gold melting and beating to form foils, leaves, or films during the ancient Egypt. Some of the uses for the gold sheath and leaves are also shown [4]. (Reused under open access licence.)

by the discovery of electrical power, with the basis on the work of Von Guericke, who developed a system to convert mechanical energy to electrical energy in 1663 [7]. The discovery led to the rise of physical vapour deposition (PVD) towards the mid-1800 century [5]. Since then, thin film technology has been primarily adopted in semiconductor technology and the manufacture of energy devices. Among the primary applications are the manufacturing of photovoltaic cells and nanoscale elements for advanced microprocessors in electronic devices [8–10]. The discovery of thin film technology led to the innovation of the photovoltaic (PV) effect by Alexandre-Edmond in 1839. The physicist made the first-ever PV cell, which led to further discoveries in the thin film field [11]. The discovery of the vacuum cleaner was further advanced by Herman Sprengel in 1865, who came up with a more efficient vacuum pump, which allowed researchers to investigate gas discharges and sputter deposition. Conclusively, considering the advancements in technology, the need for thin film technology has widened. Advances have also been made on the processing and

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development of thin film materials. Deposition (physical and chemical) has been adopted as among the significant processing method for these materials. This has increased the manufacture of thin film equipment and devices for industrial applications such as optical coatings, LEDs, hard coatings on machining tools, electrical devices, energy devices, power storage equipment, and drug delivery devices in the medical industry [12]. There lies a great future for thin film materials and ­technologies, and extensive research and development are critical.

1.3 CLASSIFICATION OF THIN FILM DEPOSITION METHODS There are so many methods of manufacturing thin film materials. These methods are classified according to the state of the ‘source’ material used in depositing thin film material. As such, these methods are classified as shown in Figure 1.2. • Gaseous state, • Solution state, and • Molten or semi-molten state methods. As shown, gaseous state deposition methods are those methods in which the source material has to be in gaseous state or has to be transformed into a gaseous state before deposition onto the substrate for condensation and formation of thin films and coatings. These methods are further classified as follows. • Physical vapour deposition, • Chemical vapour deposition, and • Ion beam–assisted deposition methods. In PVD methods, the target (source) material is evaporated or atomised and there are no chemical reactions involved for the formation of thin film coatings. Examples of PVD methods include sputtering, thermal spray, arc vapour, ion beam, pulsed laser deposition, etc. Further details of PVD and other methods are presented in Chapter 2 of this book. CVDs, on the other hand, involve chemical reactions among the chemical (gaseous) precursors to form thin film coatings. The CVD methods are classified according to their operating conditions such as thermal CVD, low pressure CVD, atmospheric pressure CVD, laser CVD, photon CVD, plasma-enhanced CVD, and ALD. Pulsed CVD/ALD/atomic layer epitaxy (ALE) is an enhanced CVD technology that is not usually classified under CVD since it overcomes nearly all the challenges of the other CVD techniques. It has the following advantages: • • • •

It is capable of forming ultrathin films through surface chemical reactions. It is suitable for conformal thin film deposition and hence high uniformity [9]. This technique has been used widely to grow hybrid nanostructures. It is very useful in semiconductor nanofabrication due to its capability to form defect-free thin films.

FIGURE 1.2  Classification of surface coating/thin film deposition techniques [11]. (Reused under open access licence.)

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The solution state deposition methods involve liquid/solution conditions for the formation of the thin film materials. In most cases, the source materials (precursors) have to be in a solution state for the deposition to take place. Examples of the solution state deposition methods include chemical bath solution, sol-gel, and electrochemical deposition methods. Generally, these methods involve chemical reactions of the chemical solutions to form precipitates as thin film deposits on substrate materials. Finally, molten or semi-molten deposition methods involve transforming the source material into a molten state before it can be deposited for thin film coating. The methods under this category include laser melting, thermal spray, and weldbased processes. The molten material is usually atomised through high pressure acceleration onto the surface of substrate material.

1.4 MECHANISM OF THIN FILM GROWTH The growth behaviour of the films is strongly dependent on the characteristics of the substrate, the thickness of the film, and the method of deposition. In general, it has been established that three steps are involved in creation of a thin film material.

i. Creation of the film’s species, which includes the interaction between the energy systems and precursors/reactants/target material to generate the film’s material, ii. Transport of the species to the surface of the substrate through the vacuum (in most cases), and iii. Growth of the species on the surface of the substrate to form thin film structures. The behaviour of the film’s species on the surface of the substrate is affected by the following parameters. i. Activation energy ii. Binding energy between the substrate and species iii. The coefficient of sticking between the substrate and the film’s species The thin film’s species may undergo several processes based on the method of deposition and other underlying conditions especially those associated with substrate and temperature of the deposition system. Regardless of the activities at the surface of the substrate, which have been documented extensively in several references, thin film formation occurs through nucleation of the species onto the substrate surface [13]. The mechanism of nucleation has been classified into three major types depending on the species– substrate interactions. These classifications are described as follows (Figure 1.3).

1.4.1 Frank–Van der Merwe Growth In this type of growth mode, there is an imbalance between adsorbate–adsorbate and adsorbate-surface interaction. The adsorbate–adsorbate interaction is stronger

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FIGURE 1.3  Showing the three thin film nucleation mechanisms.

than the adsorbate–surface interaction. The growth occurs due to the accumulation of surface atoms and coalescence on the surface, resulting in the formation of island structures.

1.4.2 Stranski–Krastanov Growth (Layer-Plus–Island) In this type of growth mode, there is a combination of both the island and layerby-layer growth mechanisms. First there is the formation of a layer, which is then ­followed by the formation of islands.

1.4.3 Volmer–Weber (Isolated Island) Growth Mode This model is opposite to the Van der Merwe growth, where the interaction between adsorbate and surface is stronger than the interaction between adsorbate and adsorbate. This interaction results into a three-dimensional layer-by-layer structure on the substrate surface.

1.5 PARAMETERS INFLUENCING THIN FILM DEPOSITIONS Thin film deposition methods are influenced by various process parameters. Although each method is unique, the parameters have been generally classified as [14,15]: • Parameters associated with material properties, • Parameters associated with equipment, and • Other/external parameters. The parameters associated with the materials properties include the following:

i. The chemical composition and properties of the target/precursors and ­substrate materials ii. The surface roughness of the substrate iii. The binding energy of the atoms in the target materials Some of the parameters associated with the deposition equipment include the following: i. The type of the deposition process and equipment used ii. The temperature of the substrate materials

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iii. The temperature of the precursors or target materials iv. The operating pressure of the deposition chamber v. The type and amount of power supplied to the deposition process vi. The flowrate of the gaseous materials inside the deposition chamber vii. The deposition times viii. The rotation speed of the substrate ix. The orientation of the substrate surface in relation to the source (precursor or target) The other/external factors affecting thin film deposition may include the expertise of the operator of the deposition system, the chemistry of the catalysts used, the properties of the doping materials, etc. It can be seen that deposition processes are influenced by multiple parameters and the choice of the correct process parameters for deposition of quality thin films is complex. For specific deposition techniques, the influence of each of these parameters is not a direct or straightforward concept. Several experimental trials and design are required for the determination of combination of deposition parameters for quality thin film formation. The influence of some of the parameters for different deposition methods are highlighted below.

i. The substrate temperature is one of the most studied parameters for thin film deposition process. The temperature on the substrate affects the diffusion, nucleation, and growth rates of thin films. The crystallinity of most thin films becomes stronger as the substrate temperature increases from room temperature [16,17]. The high temperature of the substrate results in higher energy of mobility of the nanoparticles, which leads to the conglomeration of smaller-sized grains into larger structures. The increase in sizes of the structures with temperature results in heterogenous structure and high roughness of the thin film structures. For instance, as shown in Figure 1.4, the increase in temperature from 150°C to 350°C for sputtered aluminium zinc oxide (AZO) thin films reveals an increase in surface structures, roughness, and heterogeneity according to the scanning electron microscopy (SEM) and atomic force microscopy (AFM images) [16]. ii. The type of deposition method/process is another important parameter in thin film fabrication. Some techniques/equipment can provide quality thin film materials as compared to the others. For instance, to obtain ultra-thin films, ALD is preferred over the other methods. Techniques such as thermal spray are inferior in terms of the quality of thin films as compared to sputtering and CVD methods. To achieve low temperature deposition and enhance the quality of thin films, low temperature techniques such as sputtering are used over high temperature methods such as CVD and ALD. iii. The pressure inside the deposition chamber especially for vacuum-based techniques also affects the deposition process as well as the quality of the materials produced. For instance, in sputtering, high pressure means more energy of the plasma and hence high sputtering yield. However, the increase in pressure is limited since it also increases the particles inside the

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FIGURE 1.4  SEM and AFM micrographs of sputtered AZO thin films at temperatures of (a) 150°C, (b) 250°C and (c) 350°C. The micrographs show that the sizes of the surface structures increase with the increasing substrate temperature [16]. (Reused under the Creative Commons Attribution Licence.)

deposition chamber, which may impede/block the free path of the dislodged atoms while they are travelling onto the substrate surface. The high-energy plasma ions inside the chamber can also strike the surface of the substrate causing defects on the deposited thin films. iv. The flowrate of the inert gases (argon) into the vacuum deposition chamber is associated with some effects of the thin film properties [18]. Some thin film materials prepared through sputtering have shown an increase in crystallinity with the increase in argon gas flowrate. Additionally, an increase in the working/argon gas flowrate has been shown to enhance deposition of thicker films. The flowrate of oxygen and other reactive gases in vacuum-based thin film technology is also said to influence the film thickness, crystallinity, optical transmittance, and chemistry of the deposited thin films [19]. v. The chemical properties of the precursors or targets influence the deposition, diffusion, nucleation, and formation of the structure of the thin film material. They also determine the stoichiometry and chemical behaviour of the films. The adhesion behaviour of the film materials onto the substrate is significantly dependent on the chemical properties of the precursors or target materials. The property–parameter relationship is an important aspect of investigation especially for the design of new materials and systems. The choice of establishing the relationship is usually at the discretion and experience of the user of the deposition facility.

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1.6 PROPERTIES OF THIN FILM MATERIALS Generally, thin films are nanostructured materials and as such exhibit different properties from their bulk counterpart. Some of the common properties of thin film materials are listed below.

i. Mechanical properties: As noted, thin films are nanostructured materials and therefore exhibit excellent mechanical properties [20]. They have high yield strengths and can withstand high residual stresses. They also exhibit attractive elastic and plastic characteristics which are important for various applications. Mechanical properties of these materials can be determined using nanoindentation testing equipment. ii. Optical properties: As compared to bulk materials, thin film materials exhibit superior optical properties such as reflectivity and absorption across various energies. iii. Electrical properties: Thin film materials are characterised by better electrical properties as compared to bulk materials. Contrary to bulk materials where electrical properties depend on the type of the material, thin film materials are influenced by the scaling effects. The properties of the films such as film thickness, lattice dimensions, and surface roughness influence the electrical transport in thin films. iv. Microstructural stability: Some thin film materials exhibit structural stability in various conditions such as at high temperature and cryogenic conditions. v. Surface roughness and film thickness: These are important properties of thin film materials. Generally, the surface roughness of thin films is usually very low compared to bulk materials, and it is usually in terms of microand nanodimensions. The film thickness determines the surface protection and other functional characteristics of the thin film materials. vi. Chemical compositions: The chemical compositions of thin film materials can be controlled during the deposition processes. As such, the chemical behaviour of these materials can be enhanced. vii. Surface protection properties: Thin film materials exhibit excellent mechanical and chemical properties. As such, they have good wear and corrosion resistance suitable for protecting various surfaces on deposition. However, it should be noted that the above properties are general and every deposition process seeks to achieve a specific or several performance properties. Several characterisation techniques for thin film materials are available, and some of which include microstructure, mechanical, chemical, optical, electrical, magnetic, and so many others. Some of these techniques have been described in details in Chapter 3.

1.7 MODERN APPLICATIONS OF THIN FILM MATERIALS Due to the attractive properties offered by thin film materials and coatings, their applications have evolved over history from mere decorative materials to modernday sophisticated composites and functional components. The purpose of this book

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is to discuss thin film materials, their properties, and applications. As such, most of the chapters of the book focus on specific applications of thin film materials and their properties. Here, some of the most common commercial and industrial applications of thin materials are stated.











i. Semiconductor applications: Thin films are used in the fabrication of integrated circuits (ICs) used in semiconductors for electronics devices. Thin film technology is also used in the fabrication of transistors, resistors, rectifiers, light-emitting diodes, capacitors, and so many other components for semiconductor devices. Thin film semiconductors are low cost and can be created on large areas of complex structures and geometry. Additionally, the creation of single- or multi-crystalline semiconductor structures of various complex structures and configurations can be easily achieved by adjusting the process parameters of the thin film deposition method [21]. ii. Cutting tools: To improve machining quality in the industry, cutting tools should have excellent resistance to wear, high temperature stability, high toughness, and hardness. In high-speed machining processes such as milling, cutting tools used should be coated with thin film materials to impart these properties at a lower cost. Some of the coatings used for cutting tools include AlTiN, TiN, TiCN, WC, ZrN, etc. These materials are described in Chapter 8. iii. Solar cell fabrication: The second-generation type of solar cells consists of thin layers of photovoltaic materials. There are three common thin film solar cells used today for outdoor applications, namely, cadmium telluride (CdTe), copper indium gallium selenide (CIGS), and amorphous silicon (a-Si). These materials are presented in details in Chapter 10. Additionally, thin film technology is used to fabricate semiconductors, and anti-reflecting and transparent conducting coatings on the solar cell substrates. iv. Optical devices: To improve the reflection or transmission properties of lenses or mirrors, thin film materials are usually deposited on the surfaces of these devices. The thin film coating may enhance the reflecting characteristics of mirrors for various applications such as laser mirrors where the reflectivity of up to 99.99% is desired. In anti-reflecting optical devices, thin film coatings are used to reduce unwanted reflections from the surfaces. Some of the antireflecting devices include spectacle, photographic lenses, and solar cells. v. Fuel cells: Thin film technology and materials are being used to manufacture thin film lithium ion batteries. These batteries consist of thin materials such that the batteries can be used for miniature devices. Thin film technology is also being used in fabrication of effective catalysts to improve the performance of the fuel cells. vi. Medical devices and biomaterials: In the medical sector, devices and equipment are prone to contamination by microorganisms. As such, thin film materials with antimicrobial properties are used to coat the surfaces of these devices. Silver-based coatings have been mostly used for protection of surfaces against microbial attack and several commercial coatings and paints are available. Biomaterials, used as implants, have also been manufactured through thin film technologies. Titanium is the most common

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implant material; however, its application is usually limited by poor wear and corrosion properties. The surface properties are enhanced by deposition of thin coatings of a wear-resistant material such as ceramics. Thin film materials used in the medical industries and for antimicrobial applications have been, respectively, presented in Chapters 6 and 12. vii. Data storage: Thin film technologies are used to manufacture devices for thermo mechanical data storage. Thin film Ni-Ti shape memory alloy (SMA) is one of the exciting technologies used due to recovery behaviour of the alloy. viii. Sensor fabrication: Thin film technology is used to manufacture the active layers of sensor devices for various applications. Sensory materials exhibit piezoelectric characteristics, and it has been shown that ultra-thin films (e.g. ZnO and AlN) exhibit superior piezoelectric properties suitable for sensor applications. The applications of thin film technology for both commercial and industrial purposes are inexhaustible. With the growing adoption of Industry 4.0 and digital manufacturing, the demand for miniature and high performing devices shall continue to increase. Thin film technologies shall continue to play a key role in supporting the devices Internet of Things (IoTs) and other digital platforms driving the Fourth Industrial Revolution. The fabrication of smart devices and sensors to support big data and analytics for industrial processes and optimisation shall continue to drive innovation in the thin film technology. The increased need for health monitoring for understanding and treatment of complex diseases such as cancer demand for very precise, innovative, and miniature sensor structures. The sporting industry is another area that is progressively adopting thin film structures to support the essential technologies. The use of flexible and small monitoring devices in the sport industry is growing and shall also push the growth and innovation of thin film technologies. In the medical industry, the demand for innovative materials especially for drug delivery and tissue engineering is also going to contribute significantly to the growth of thin film technology and industry. Some of the modern technologies driving the growth and demand for thin film materials and technologies are highlighted in Chapters 14 and 15.

1.8 SUMMARY In this chapter, a general introduction to thin film technology is presented. The definitions of the most common terms used in thin film technology are presented to provide a foundation for dummies of the technology. A short history of the evolution of thin film technology over time shows that Egyptians were the first to adopt the technology for the processing and utilisation of gold. The golden materials were extracted, melted, and beaten to sheaths, plates, or leaves. The leaves were used as coatings on various surfaces especially for decorative and religious purposes. The technology later evolved to other materials and different parts of the world to what has become the sophisticated thin film deposition techniques for a wide range of materials. The chapter has further illustrated the classification of the modern technologies of thin film deposition, and short descriptions of each category have been included. Also included in the chapter are the general mechanisms of thin film

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deposition, parameters influencing thin film growth, and properties of thin film materials. Finally, highlights of the modern uses of thin film technology and materials are provided. It is postulated that the increasing adoption of Industry 4.0 and digital technologies in modern society shall drive the demand, and hence the market, for thin film materials and technology in the 21st century and the future.

1.9 SCOPE OF THE BOOK The book aims to provide a huge resource of thin film materials and coatings, their deposition methods, properties, and applications. The book is divided into 15 chapters. Chapter 1 provides a general introduction of thin film materials and methods. It also provides a brief historical evolution of thin film materials and some of their applications. A brief summary of the methods of deposition, mechanism of thin film formation, and modern applications of thin materials is also discussed. Chapter 2 offers detailed literature on different methods of thin film deposition with the benefits and constraints of each method highlighted. Chapter 3 presents some of the common characterisation techniques for thin film materials. Chapter 4 presents scalable and hybrid thin film materials, their preparation protocols, general properties, and applications. A bibliometric analysis of thin film materials and applications is presented in Chapter 5. From Chapters 6 to 13, properties of thin film materials for applications such as biomedical, surface protection, cutting tools, optics and electronics, energy, self-healing and smart thin films, and microbial and high entropy alloy thin films have been discussed. Chapters 14 and 15 discuss the role and relationship between thin film and modern technologies, i.e. Industry 4.0 and 3D printing technology. The book is a very useful resource for scientists in advancing the research and applications of thin film technologies and materials.

REFERENCES

1. E. Wallin, Alumina Thin Films: From Computer Calculations to Cutting Tools. Linköping: Linkoping University, Institute of Technology, 2008. 2. P. C. Safa Kasap, Springer Handbook of Electronic and Photonic Materials. Cham: Springer International Publishing, 2017. 3. J. E. Greene, “Review article: Tracing the recorded history of thin-film sputter deposition: From the 1800s to 2017,” J. Vac. Sci. Technol. A Vacuum, Surfaces, Film., vol. 35, no. 5, p. 05C204, Sep. 2017, doi: 10.1116/1.4998940. 4. E. Darque-Ceretti, E. Felder, and M. Aucouturier, “Foil and leaf gilding on cultural artifacts; forming and adhesion,” Rev. Mater., vol. 16, no. 1, pp. 540–559, 2011, doi: 10.1590/S1517-70762011000100002. 5. H. Frey and H. R. Khan, Handbook of Thin-Film Technology. Berlin, Heidelberg: Springer, 2015. 6. A. Khadher, M. Farooqui, M. Mohsin, and G. Rabbani, “Metal oxide thin films: A mini review,” J. Adv. Sci. Res., vol. 7, no. 1, pp. 01–08, 2016. 7. J. E. Greene, “Tracing the 5000-year recorded history of inorganic thin films from ∼3000 BC to the early 1900s AD,” Appl. Phys. Rev., vol. 1, no. 4, p. 041302, Dec. 2014, doi: 10.1063/ 1.4902760. 8. J. Datta, C. Bhattacharya, and S. Bandyopadhyay, “Cathodic deposition of CdSe films from dimethyl formamide solution at optimized temperature,” Appl. Surf. Sci., vol. 253, no. 4, pp. 2289–2295, 2006, doi: 10.1016/j.apsusc.2006.04.020.

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9. T. Unold and H. W. Schock, “Nonconventional (non-silicon-based) photovoltaic materials,” Annu. Rev. Mater. Res., vol. 41, no. 1, pp. 297–321, Aug. 2011, doi: 10.1146/annurevmatsci-062910-100437. 10. A. Le Donne et al., “Study of the physical properties of ZnS thin films deposited by RF sputtering,” Mater. Sci. Semicond. Proc., vol. 71, no. July, pp. 7–11, 2017, doi: 10.1016/j. mssp.2017.06.042. 11. O. Oluwatosin Abegunde, E. Titilayo Akinlabi, O. Philip Oladijo, S. Akinlabi, and A. Uchenna Ude, “Overview of thin film deposition techniques,” AIMS Mater. Sci., vol. 6, no. 2, pp. 174–199, 2019, doi: 10.3934/matersci.2019.2.174. 12. A. Jilani, M. S. Abdel-wahab, and A. H. Hammad, “Advance deposition techniques for thin film and coating,” Mod. Technol. Creat. Thin-Film Syst. Coat., vol. 32, no. July, InTech, 2017, pp. 137–144. 13. A. C. Levi and M. Kotrla, “Theory and simulation of crystal growth,” J. Phys. Condens. Matter, vol. 9, no. 2, pp. 299–344, 1997, doi: 10.1088/0953-8984/9/2/001. 14. F. M. Mwema, E. T. Akinlabi, and O. P. Oladijo, Sputtered Thin Films: Theory and Fractal Descriptions. Boca Raton, FL: CRC Press, 2021. 15. F. M. Mwema, O. P. Oladijo, S. A. Akinlabi, and E. T. Akinlabi, “Properties of physically deposited thin aluminium film coatings: A review,” J. Alloys Compd., vol. 747, pp. 306–323, May 2018, doi: 10.1016/j.jallcom.2018.03.006. 16. Z. Ghorannevis, M. T. Hosseinnejad, M. Habibi, and P. Golmahdi, “Effect of substrate temperature on structural, morphological and optical properties of deposited Al/ ZnO films,” J. Theor. Appl. Phys., vol. 9, no. 1, pp. 33–38, Mar. 2015, doi: 10.1007/ s40094-014-0157-1. 17. F. M. Mwema, E. T. Akinlabi, O. P. Oladijo, and J. D. Majumdar, “Effect of varying low substrate temperature on sputtered aluminium films,” Mater. Res. Express, vol. 6, no. 5, p. 056404, Feb. 2019, doi: 10.1088/2053-1591/ab014a. 18. M. Al-Mansoori, S. Al-Shaibani, A. Al-Jaeedi, J. Lee, D. Choi, and F. S. Hasoon, “Effects of gas flow rate on the structure and elemental composition of tin oxide thin films deposited by RF sputtering,” AIP Adv., vol. 7, no. 12, 2017, doi: 10.1063/1.5001883. 19. C. L. Tien, H. Y. Lin, C. K. Chang, and C. J. Tang, “Effect of oxygen flow rate on the optical, electrical, and mechanical properties of DC sputtering ITO thin films,” Adv. Condens. Matter Phys., vol. 2018, 2018, doi: 10.1155/2018/2647282. 20. M. C. Rao and M. S. Shekhawat, “A brief survey on basic properties of thin films for device application,” Int. J. Mod. Phys. Conf. Ser., vol. 22, pp. 576–582, 2013, doi: 10.1142/s2010194513010696. 21. O. Sancakoglu, “Technological background and properties of thin film semiconductors,” in Phuong Pham, Pratibha Goel, Samir Kumar, and Kavita Yadav (eds.), 21st Century Surface Science - A Handbook, vol. i, no. tourism, London: IntechOpen, 2020, p. 13.

2

Methods of Thin Film Deposition

2.1  INTRODUCTION The developments in thin film technology have opened possibilities for engineering material structures at an atomic level. The technologies have the potential to fabricate materials at nano- and micro-levels and with tailored structures. As noted in Chapter 1, thin films are manufactured by layering material’s building blocks (such as ions, atoms, or molecule species) to achieve the desired properties. In this way, it is possible to manipulate the structures to the desired orientation, texture, and size for optimum performance. The thin films are usually deposited on substrates surfaces, where they act as surface coatings. Generally, surface coatings are required in a wide range of applications. These applications may include but are not limited to the following. i. Protection of erosion and corrosion of surfaces, ii. Improvement of wear, friction, and fatigue properties of various substrates, iii. To improve the electrical conductivity, and iv. To impart reflectivity, hydrophobicity on the surfaces. There exist various deposition processes for thin films. There are so many continually formulated methods used to obtain high-quality thin films for various critical areas of applications such as those stated above. A high-quality thin film can be defined by several features, including but not limited to the following. i. The films should have smooth interfaces and homogenous structures. ii. The films should have a low density of defects. iii. Their thickness and properties should be uniform throughout their microstructure [1]. iv. The properties of the films should be satisfactory for their functionality. For example, the density of thin film is an important factor of consideration because a lower film density would lead to adsorption of water vapour on exposure to the atmosphere, which would, in turn, affect the properties of the material. Achieving the above features in thin films requires an understanding of the deposition techniques and their control. This chapter presents various thin film methods, emphasising the basics, operating principles, and control for better performance. As introduced in Chapter 1, the thin film technologies can be broadly categorised as follows: DOI: 10.1201/9781003202615-2

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i. Those processes that involve transforming the source material into a gaseous state for deposition are known as gaseous state deposition techniques. ii. Those processes in which the source material should be in the liquid state for the deposition to take place are known as liquid state deposition techniques.

The detailed classification of these technologies is presented in Chapter 1 and is also readily available in different literatures [2]. In the next sections of this chapter, physical vapour deposition (PVD), chemical vapour deposition (CVD), atomic layer deposition, and other methods of thin film depositions are presented.

2.2 PHYSICAL VAPOUR DEPOSITION PVD refers to coating methods in the category belonging to vacuum coating technologies [3]. With PVD, the surface of a material is covered with solid materials such as pure metallic elements (e.g. aluminium, copper), alloys/oxides/compounds, or ceramic materials (e.g. hydroxyapatite and carbides). These techniques involve vaporisation of the solid materials from a target in the form of ions, molecules, or atoms and then transporting and depositing them on a substrate in a vacuum environment. On the substrate’s surface, diffusion, coalescence, agglomeration, nucleation, and condensation occur, resulting in the formation of protective, functional, or decorative coatings [4,5]. The PVD processes occur according to three basic steps as illustrated below (Figure 2.1).

FIGURE 2.1  Illustrating the basic steps in physical vapour deposition processes.

Methods of Thin Film Deposition



i. Deposition generation or vapour phase species: The material vapour species (atoms, ions, or molecules) are dislodged from the surface of the target. The plasma energy must be able to overcome the binding energy of the species on the target for vaporisation and dislodgement of the species from the target to take place. The material vaporisation can be obtained either through thermal heating or through sputtering. ii. Transportation or movement from the source material to the substrates: The atoms, ions, or molecules dislodged from the target surface are transported through the process of the molecular flow and the thermal scattering towards the substrate’s surface. During this process, there may occur collisions between the target materials and the gaseous/vapour species of the plasma. The desirable condition is a collision-free process for 100% efficiency of depositing the target material onto the substrate. iii. The deposition of the target material onto the surface of the substrates: The species (atoms, ions, or molecules) hits the surface of the substrates where several processes occur such as velocity loss, diffusion, coalescence, nucleation, and condensation, forming thin film structures. PVD techniques are attractive in the following ways:







i. The techniques present a means of rapid coating of quality thin films onto substrates. Therefore, it is suitable for mass production of thin films due to higher deposition rates (especially for metals) compared to other techniques. ii. The techniques can be used for a range of materials (target and substrates). For instance, through PVD techniques, it is possible to use plastic substrates since the processes can be undertaken at room temperatures. The use of temperaturesensitive substrates such as high-speed steel is possible due to the low deposition temperature. It is also suitable for metals, non-metals, and dielectrics targets. iii. The PVD techniques permit deposition at temperatures lower than the thermodynamic equilibrium, and as such it is possible to deposit new metastable thin film compounds such as TiAlN [6–8]. iv. PVD techniques result in stronger adhesion of coatings onto most substrates. This is because of the possibility of controlling the structure, density, and stoichiometry of the films due to the coating application at the atomic level. As such, they are highly durable for corrosion and scratch/wear-resistant. These coatings are therefore used to provide a barrier against damage and reduce friction [7]. v. Unlike in chemical vapour deposition (CVD), PVD does not exert stresses on the substrates. Since the CVD process is carried out at elevated temperatures, inducing stresses in the substrate and coatings is inevitable [9]. vi. A wide variety of coatings are possible through the combination of different materials. For instance, it is possible to undertake reactive PVD depositions to produce oxides and nitrides thin films. It is also possible to utilise more than one source material (targets) to produce alloy and high entropy thin films. vii. The level of impurity in thin films is reduced PVD techniques as compared to CVD and other techniques.

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Due to these attributes, PVD is used in various industries, including photovoltaic for solar cells fabrication, optics ranging from eye glasses to self-cleaning tinted windows, displays and communications, device applications like computer chips, as well as functional or decorative finishes [10,11]. They are also used in the fabrication of biomedical and tribology coatings [12,13]. This is due to their high durability, adhesion strength, high hardness, and corrosion resistance properties [9]. As introduced in Chapter 1, there are several PVD techniques, which have been discussed from Subsection 2.2.1–2.2.6.

2.2.1 Sputtering Sputtering is a PVD method that employs thermo-physical processes of releasing the atoms from the target. The material from the target is ejected through the collision of the surface by high-energy ions from the plasma. This way the target material is transformed into atomic particles which are then directed towards the substrate through a chamber in vacuum conditions [5,14]. The general principle of sputtering involves the following steps: a. Ions acceleration from the plasma onto the target (collisions of energetic ions with a target surface and the process usually leads to ejection of target atoms). b. The sputtering off of some surface atoms from the target. c. These sputtered atoms ‘flow’ across the chamber to the surface of the substrate for thin film formation to occur. At the target surface, various processes occur during sputtering, as illustrated in Figure 2.2. As shown, when the Ar+ inert gas ion hits the surface of the target at high energies and based on the magnitude of the ion energy and ratio of the ion mass to that of the target atoms, the interaction may be a complex process. Usually, when the ions reach the surface, they become neutral and the Ar+ ions may get implanted or bounce back, creating collision cascades in target atoms. Additionally, the displacement of the target atoms creates a vacancy, interstitial, and other defects on the

FIGURE 2.2  Interaction of plasma ions with the target surface during a sputtering process [5].

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target’s surface. Other ions may desorb, creating photons and losing their energies to the atom of the target, which may result in the ejection of secondary electrons [5]. The sputtering process is classified according to system operations and conditions, such as [15]:

i. DC and radiofrequency (rf) sputtering, ii. Magnetron sputtering, iii. Ion beam sputtering, iv. Reactive sputtering, v. Diode sputtering, and vi. Bias sputtering

The following are some of the advantages and disadvantages of sputtering as a thin film deposition technology.





Advantages i. Sputter deposition can sputter a wide range of materials, including alloys, compounds, or pure elements of metals and non-metals. It is, therefore, possible to generate a wide range of nanomaterials. ii. The target of the sputtering provides a stable, long-lived source of thin film species. The target’s location can be any direction (down, up, or sideways) based on the design of the sputtering system. iii. Through sputtering, the deposition of the target material can conform to the substrate surface’s shape. iv. Sputter deposition systems consist of lower radiant heating as compared to vacuum evaporation. v. There is a possibility of high utilisation of sputtered materials due to the presence of rotatable cylindrical magnetrons which confine the plasma to the surface of the substrate. As such, most of the sputtered atoms can reach the surface of the substrate with minimum loss. vi. Preparation of the surface of the substrate can easily be incorporated in situ with the deposition process. Disadvantages i. The target must be cooled because most sputtering energy goes into the target heat ii. There is a lower rate of sputter vaporisations compared to those that can be achieved by thermal vaporisation. As such, there is no efficient target material utilisation. iii. Sputtering is not energy-efficiency, and sputtering targets are expensive, although the cost of the target depends on the purity of the target. iv. The dislodging of the target atoms depends on the target material’s threshold energy. Therefore, the success of sputtering is determined by the equipment operator’s expertise in the target material’s properties. v. The bombardment of the electrons to the substrate may be high in some configurations, resulting in the substrates’ heating.

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vi. During sputtering, the target material tends to deposit onto the walls of the vacuum chamber. This leads to the formation of contaminants, which may affect the quality thin films created. In the following subheadings, the various methods of sputtering and equipment are discussed. 2.2.1.1 Direct Current (DC) and Radiofrequency (RF) Sputtering Sputtering techniques can be classified according to the source of power supplied between the cathode (target material) and anode (substrate). The most common sputtering systems utilise direct current (DC) in which the system consists of a planar electrode pair called the cathode and anode, as shown in Figure 2.3a. In radiofrequency systems, the target is connected to an alternating current (AC) induced via a radio wave, as shown in Figure 2.3b. In these systems, the radio frequencies are usually in the range of 0.5–30 MHz, with 13.56 MHz being the most accepted frequency in commercial rf sputtering systems [16]. Argon gas is usually preferred as the working gas because it has a larger mass (than helium and neon). The higher the mass of the working gas, the higher the energy of the plasma to dislodge atoms from the surface of the target. The rf sputtering systems are advantageous over the DC systems in the following ways:

i. The radio frequencies passing through the vacuum avoid charge building up on the target materials unlike in DC systems. ii. DC sputtering is limited to dielectric target materials such as aluminium Oxide, silicon oxide, and tantalum oxide. iii. DC sputtering of dielectric (non-conducting) targets can lead to charge build-up, which can damage the sputtering process and the quality of the thin films.

FIGURE 2.3  Schematic diagram: (a) direct current (DC) and (b) radio-frequency sputtering systems.

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iv. The charge build-up causes arcing into the plasma, leading to the emission of droplets. As such, arcing creates quality control problems on the thin films. v. The arcing in the plasma can completely interrupt or stop the sputtering process. The problem of arcing has been directly correlated to poor quality thin films by various researchers. For example, Hom-on et al. [17] compared the surface roughness of aluminium oxide thin films deposited via DC and rf magnetron sputtering and concluded that DC sputtering yields films of higher surface roughness. In another study, Jiang, Liu, and Feng [18] compared DC sputtering of Ti thin films with pulsed sputtering and cathodic arc evaporation techniques. Figure 2.4 shows the microstructure of the Ti thin films deposited via the three methods. As shown, the DC sputtered Ti thin films exhibit loose columnar structure across the thickness, and the surface morphology has cauliflower-like clusters with several gaps [18]. The structure of the films deposited via the other two methods is denser, and there is more adherence to the substrate as compared to the DC deposited films. The adhesion tests of the films revealed that DC sputtered Ti thin films exhibit the lowest adhesion to the substrate as compared to those deposited via cathodic arc evaporation and pulsed DC sputtering.

FIGURE 2.4  The SEM images of the cross-sections and surface of the Ti thin films deposited via (a) and (b) DC sputtering (c) and (d) Cathodic arc evaporation, and (e) and (f) pulsed DC sputtering. (Reused with permission from Elsevier [18].)

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2.2.1.2 Magnetron Sputtering This is the most common and effective form of sputtering system. These systems consist of magnets located under the target holder in such a way that one pole is located at the central axis while the second pole is formed by a ring of magnets located around the edge of the target. In this way, the magnetic fields are transverse to the electric fields on the target surfaces and therefore there is the generation of the Lorentz force. The Lorentz force ensures that the secondary electrons are constrained in cycloid trajectories near the vicinity of the target such that they do not bombard the substrates. As such, the magnets increase the plasma density near the surface of the target. Thus, the secondary electrons do not contribute to increased substrate temperature and radiation damage [19,20]. Additionally, this process involves the formation of dense plasma, which implies higher ionisation and deposition rates. Magnetron sputtering systems, therefore, find applications in large-scale/industrial production of thin film materials [5]. The systems may be DC magnetron sputtering in which the source of power is direct current or radiofrequency (rf) magnetron sputtering. Figure 2.5 is an example of a dual target rf/DC magnetron sputtering equipment manufactured by AdNaNoTek (USA). There are two main configurations of magnetron sputtering systems, viz. balanced/ conventional and unbalanced magnetrons, as shown in Figure 2.6. 2.2.1.2.1 Conventional Magnetron Sputtering In the conventional magnetron sputtering systems, there is a balance between the magnets’ inner pole and the outer ring. As such, the plasma is confined to the region around the target. The substrate can be located within or outside the plasma region (Figure 2.6a). If the substrate is positioned within the plasma, there is concurrent ion bombardment, leading to the formation of a dense structure of thin films. When the substrate is located outside the plasma region, there is no concurrent ion bombardment and the substrate is under a very low plasma density. In fact, the current drawn

FIGURE 2.5  A dual target rf/DC magnetron sputtering equipment of UHV-MSD-X model manufactured by AdNanoTek, USA (a) schematic and (b) pictorial representations (Obtained from https://www.adnano-tek.com/.)

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FIGURE 2.6  A schematic representation of (a) conventional and (b) type-1 and (c) type-2 unbalanced magnetron sputtering systems. (Reused with permission from Elsevier Ltd [21].)

may not be sufficient to modify the structure of the target atoms before the deposition to the substrate surface. In this case, coating a large substrate surface is challenging although this can be enhanced by negative biasing of the substrate to increase ion bombardment. However, it has been reported that this leads to highly defective and more stressed thin films due to the increased kinetic energy of the plasma ions. These challenges can be overcome by using unbalanced magnetron sputtering systems. 2.2.1.2.2 Unbalanced Magnetron Sputtering Systems In these systems, the strength of the inner and outer poles is not equal. When the strength of the inner pole is higher than that of the outer rings, this is known as the Type-1 unbalanced magnetron (Figure 2.6b). When the outer ring has higher strength than the inner pole, this is known as the type-2 unbalanced magnetron system (Figure 2.6c). In both cases, the magnetic field lines are not fully constrained around the target as is the case of the conventional magnetron sputtering. Some magnetic field lines leak to the substrates, which allows some secondary electrons to follow them. An extension of the plasma to the substrate surfaces leads to high ion currents flowing to the substrate. In type-1 unbalanced magnetron sputtering systems, the leaking field lines are directed to the walls of the chamber and the density of the plasma on the substrate is relatively low compared to type-2 system. The type-2 design is preferred over type-1 system because a higher density of plasma flows to the substrate. As such, the high ion currents can be harnessed from the plasma without biasing the substrate. Further details of these magnetron sputtering systems can be obtained from other literatures [21]. 2.2.1.3 High Power Impulse Magnetron Sputtering (HIPIMS) This is a recent magnetron sputtering technology that is based on a high voltage pulsed power source. The technique was developed and patented by Kouznetsov et al. in 1999 [22]. The technique uses very high voltage and pulsed power to create

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a high density plasma on the surface of the target [23]. This decreases the distance of ionisation for the sputtered species. In simple terms, there is the application of high electrical power to increase the plasma density of the sputtering process. Usually, the power is applied through short pulses with a low duty factor to avoid the use of very high electrical power. Figure 2.7 shows a schematic diagram of HIPIMS systems. The circuit consists of a capacitor bank, which is discharged through a semiconductor-based switch. In most cases, the inductance (20–50 µH) is incorporated, which acts as a chock to the circuit, ensuring the current increase is not too steep to destroy the circuit elements such as switches and capacitors. The power supplies connected to the circuit are also incorporated with the capability to suppress a transition to an arc discharge. The power provided on the target in the HIPIMS system ranges between 10 kW and 5 MW.

Advantages of HIPIMS i. One of the attractive features of the technique is its simplicity and can be easily modified for large-scale applications. All one needs is the supply of HIPIMS power.

FIGURE 2.7  A schematic demonstration of a HIPIMS system (Reused with permission from ACS Publications from reference [24].)

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FIGURE 2.8  The cross-sections of high resolution TEM of AlN thin films deposited on Si(100) through (a) DC magnetron sputtering and (b) HIPIMS methods. (Reused from [25] with permission from Elsevier.)







ii. The HIPIMS produces high-pulsed voltage conditions that result in ionisation of a large percentage of the target without overheating, leading to dense and droplet-free films formation. For instance, Aissa et al. [25] reported that AlN thin films deposited via HIPIMS in Si (100) substrate exhibited a dense and continuous growth at the film–substrate interface, as shown in Figure 2.8. iii. The HIPIMS method produces thin films with higher nanohardness as compared to other sputtering techniques. iv. Because of the capability of this method to obtain dense and smooth thin films, it is possible to deposit thin films with quality crystalline properties and low residual stresses. v. The films deposited via HIPIMS have higher adhesion to complex substrates. This is due to the high voltage generated at short durations of time and higher ionisation velocity of the target materials. As such, thin films deposited via HIPIMS can find applications as coatings for high-speed machining as demonstrated by Kumar et al. [26] for TiAlN thin films. vi. Ehiasarian et al. [13] compared the wear behaviour of CrN thin films deposited via HIPIMS and other techniques as shown in Figure 2.9. It can be demonstrated from the figure that HIPIMS CrN films exhibit the lowest coefficient value of wear and relatively low friction.

Disadvantages of HIPIMS i. As indicated, HIPIMS requires higher power as compared to conventional sputtering and other deposition techniques. The energy needed for equivalent deposition rates compared to DC magnetron sputtering is in the range of 30%–80%. ii. Arcing on the target of the HIPIMS system is another drawback. There are two types of arcs: light and heavy arcs. The light arcs are minor problems to the process since they look like sparks on the target. The heavy arcs appear like plasma spreading from the target into the plasma, and they tend to cause overheating of the cathode target. This leads to the production of micro-droplets from the target, resulting in non-uniform thin film deposition on the substrate surface.

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FIGURE 2.9  Wear coefficients of CrN thin films prepared via different methods. (Reused from [13] with permission from Elsevier.)

2.2.1.4 Reactive Sputtering In cases where oxides and nitrides thin films are desired, a reactive sputtering process is applied. In this case, a reactive gas is introduced into the plasma in the vacuum chamber during the sputtering process [14]. The reactive gas gets activated by the plasma reacting with target atoms inside a vacuum chamber or at the surface of the substrate to form a compound, which is mostly an oxide or nitride. When depositing oxide materials such as alumina, oxygen is used as the reactive gas, whereas when depositing nitride thin films, nitrogen gas is introduced as the reactive gas. The process involves a complex interaction of the reactive gas and target parameters. Therefore, proper choice of each parameter for both materials is necessary for quality thin films such as oxide, nitride, carbides, or a mixture of three. The thin film composition is determined by the control of amounts of reactive and inert gases within the sputtering chamber. Generally, the quantity of reactive gas used is smaller than the inert gas; however, by changing the ratio of the two gases, thin film materials ranging from a resistor, insulator, to a semiconductor can be deposited. The process is a very versatile technique in depositing thin films for dielectrics, insulations, and semiconductor industry applications. Some of the thin film films obtained via reactive sputtering include Ta3N2, AlN, Al2O3, TaO, TiO2, SiO2, etc. 2.2.1.5 Bias Sputtering In this form of sputtering, the substrate is connected to a small negative potential prior to deposition so that ion bombardment occurs on the substrate for purposes of priming (cleaning) or during sputtering to continue the sputtering of the previously

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deposited material. This process is also known as protective sputtering, back sputtering, or reverse sputtering. In a typical set-up of bias sputtering, the target will be at a stronger negative charge than the substrate such that when sputtering starts, the substrate surface will also be sputtered. The substrate sputtering ensures that the surface is cleaned off any imperfections and due to its low negative potential, the dislodged target atoms get deposited on the (substrate) surface [27]. This produces advantages such as re-sputtering of loosely bonded film material, thereby improving the surface adhesion of the films, low-energy ion implantation, desorption of gases, conformal coverage of the contoured surface, or modification of a large number of film properties, and improvement of the fill density and reactivity of the materials [28]. The bias depends on the material of substrate and target. For example, RF bias is recommended for non-conductive substrates. 2.2.1.6 Equipment Generally, the equipment for the sputtering process consists of a number of components, namely vacuum chamber, pumping system, sputter sources, vacuum gauges, power supply, substrate holder, and gas flow supply [29]. A schematic diagram illustrating the general features of sputtering equipment is shown in Figure 2.10. The sputtering target is the source of the sputtering material and could be a metal, an alloy, or a ceramic. The sputtering gas is an inert gas, and it is mostly argon gas due to its high mass as compared to helium. It is responsible for forming plasma and acts as a carrier gas for the sputtered atoms to the substrate surface. The substrate is the material on which the target atoms (sputtered materials) are deposited to form a thin film. The substrate holder is the means by which the substrate on which the thin film is to be deposited is held. Vacuum gauges are used to measure the vacuum conditions, and the conditions can be altered appropriately to achieve the desired quality of sputtering. A plasma at higher pressure and energy is used to dislodge the atoms from the surface of the target. For dislodgement of the atoms to occur, the plasma energy must overcome the binding energy of the target material atoms. A higher working pressure inside the vacuum chamber produces better step coverage due to more random angled delivery of the atoms onto the substrate surface. The excess energy of the ions also aids in increasing the surface mobility of the atoms on the substrate, thereby enhancing diffusion and thin film formation [30].

2.2.2 Thermal Evaporation Thermal evaporation is the simplest method in PVD for thin film deposition. It is a technique used for a vast variety of target and substrate materials [31,32]. The process involves heating the target material with a resistive heater until it transforms into a gas phase within the vacuum chamber. The continuous heating of the target ensures that the vapour atoms are transported through a vacuum to the surface of the substrate where they get deposited and condense to solid films. The resistive heater is connected to a large direct current and at chamber pressure below 10 −4 to ensure all the materials vaporising as neutral atoms both in the solid and liquid phases of the material are transported to the substrate surface [32].

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FIGURE 2.10  General features of a sputtering system equipment. A typical equipment consists of the (a) deposition chamber, (b) human interface control system, (c) sample holding, and (d) target holding systems. It also consists of the power supply, the vacuum system, and gas supply systems.

At low vacuum pressure, the present gas molecules strike the substrate and interact with the target atoms to cause contamination of the deposited thin films. In most cases, materials that vaporise below 1500°C can be used as targets in the thermal evaporations process (most thermal evaporation techniques can operate between 1000°C and 2000°C). The target is usually placed on a hot surface (resistive material) that is heated by passing a current through it. Some of the heating elements used in the thermal evaporation process are carbon, molybdenum, tantalum, tungsten/ wolfram, and BN/TiB2 composite ceramic. The resistive elements occur in different configurations including basket, boat, crucible, and wire configurations [3]. To obtain a uniform desired thickness of the deposited films, the substrate has to be rotated in such a way that each point on the substrate should receive almost the same amount of vapour material during the deposition. The position of the substrate in relation to the target is an important aspect to control for effective attainment deposition of the thin film material on the substrate.

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Advantages of Thermal Evaporation i. High deposition rates. ii. The process is simple as compared to other techniques such as sputtering. iii. The cost of thermal evaporation equipment is low. iv. Evaporation provides a means for higher flexibility in the variety of the materials to be deposited. Disadvantage i. Thermal evaporation is not suitable for multicomponent target materials due to differences in melting points and vapour pressures. Some materials may evaporate before others and therefore the formation of the thin films from such materials cannot be possible. ii. Unlike Sputtering, the evaporation process can easily contaminate the target material through reactions of the target heating elements and the target material. As a result, the stoichiometry of the material is strongly influenced by this method, and consequently, the films that are created by evaporation are not very practicable for optoelectronic applications [33].

2.2.2.1 Equipment Figure 2.11 shows that a typical thermal evaporation system constitutes of a substrate and its holder, target material, the heating elements, and a vacuum pump. Cryogenic pumps are commonly used to create a vacuum within the deposition chamber although other forms of pumps are also used. In thermal evaporation, the vacuum is created through roughing and backing processes. A mechanical rotary pump can also be used to evacuate the chamber through the roughing line from atmospheric pressure to a certain vacuum level after which a diffusion pump (high vacuum pump) can be utilised as a primary pump to maintain the pressure in the chamber to less than 5 × 10 –2 millibar. As mentioned, the heating elements on which the target material is heated can be made from carbon, molybdenum, tantalum, tungsten/wolfram, and BN/TiB2 composite ceramic materials.

2.2.3 Ion Plating Vacuum coating processes take place in three stages: (1) the generation or production of the material to be deposited, (2) transportation of the material through a vacuum to a substrate, and (3) condensation of the generated material onto the growing film on a substrate. The vacuum in these processes maintains a mean free path that reduces gaseous contamination and enhances the deposition efficiency of the generated material. Ion plating is a vacuum form of the PVD process in which there is a concurrent bombardment of substrates and target material by energetic particles of the size of an atom. Ion plating was first outlined in 1963 where it was used to enhance film adhesion to improve surface coverage [34]. Since then, the technique has been improved and is finding application in corrosion protection, tribology, and electrical contacts. In ion plating, atomic sized energy particles are utilised in the bombardment of a substrate and the target material [35]. This bombardment of the substrate before deposition aids in cleaning the surface through a sputtering process. The bombardment

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FIGURE 2.11  Thermal evaporation equipment [32].

during deposition aims to enhance adhesion, densify the films, drive the chemical reactions, and improve the quality of the deposited thin films. The depositing atoms are usually ions of an inert gas and can come from various sources such as sputtering, vacuum evaporation, arc vaporisation, or a combination of various other sources such as that employed in the formation of titanium carbide where sputtered titanium and carbon condenses and reacts [36]. The bombardment should be continuous throughout the thin film deposition process to maintain atomically clean surfaces of the substrates. The essential features of an ion plating apparatus include a vacuum chamber and a pumping system, an inert gas inlet whose flow is regulated by a needle valve, and an insulated high-tension electrode where the specimen is mounted. An electron beam gun, boat, or resistance heated filament is used as a vapour source. Ion plating can be undertaken either in plasma or in vacuum conditions and hence the names plasma ion plating and vacuum ion plating, respectively (Figure 2.12). Ion plating involves three main stages of thin film deposition: (1) surface preparation in which contaminants such as oxide layers which may prevent ion adhesion are removed, (2) the deposited materials then undergo nucleation, surface coverage, and interface formation, and (3) the film growth and formation [36,37]. As outlined by [38], a typical ion plating coating sequence involves attaching the specimen to the electrode, where the filament is loaded with the coating material and pressure decreases to near-vacuum pressure levels. The chamber is then filled with argon gas, while the vacuum pressure is maintained via the vacuum pumps. The bias

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FIGURE 2.12  Plasma-based and vacuum-based ion plating deposition techniques. It can be seen that plasma-based method uses cathodic arc vaporisation, whereas vacuum-based method uses thermal evaporation and an ion gun [36].

voltage is then connected to the specimen holder and held at a voltage of about 5 kV, which causes a glow discharge between the specimen and the earthed parts, leading to the bombardment of the specimen by high-energy argon ions. The ion bombardment cleans the surface through sputtering. The source of vapour is therefore energised, vapourising the coating material into deposits on the surface of the specimen. The use of electron beam gun as the source of energy for target material vaporisation has been the most useful development of ion plating, in that it has brought a possibility of provision of coatings from metals of high melting points such as tungsten and molybdenum [38]. Moreover, the deposition rate is faster, and thick coatings can be produced with the aid of an electron beam gun as opposed to a resistance heated source. Ion plating can also be used to deposit thin films of hard ceramic films by incorporating a reactive gas into the deposition chamber. Thin films of tungsten carbide, titanium carbide, and titanium nitride have been generated by introducing methane or nitrogen in the coating chamber. Ion plating provides a powerful film deposition methodology, which involves periodic deposition and bombardment, and as such, it can be used on thermally sensitive substrates. There are several proven applications of ion plating in existence and are awaiting commercial exploitation.

2.2.4 Electron Beam Deposition The electron beam PVD process is a widely used technique in a variety of areas such as turbine aerofoil blade coatings and works with the principle of thermionic emission [31]. Thermionic emission leads to the generation of electrons from a heated cathode; these electrons are shaped into beams and are then accelerated towards the anode surface. These beams are accelerated at almost half of the speed of light, meaning that these beams have an enormous amount of energy.

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The focussed electron beam process utilises the decomposition principle of volatile precursors by a focussed beam in a vacuum environment to create nanostructures [39]. Electron beam deposition utilises electron beams produced from an electron source in a vacuum to heat and evaporate by irradiating a target material so that the evaporated material forms a thin film on a substrate material. Electron beam evaporation differs from thermal evaporation, in that in thermal evaporation, electrical current is used to heat the source material to melt and evaporate, whereas in electron beam evaporation, a stream of electrons is focussed onto the source material which heats, melts, and evaporates. An electron beam evaporator has two main components (Figure 2.13): an electron gun that produces the beam of electrons and the crucible where the source material is contained. The crucible may be made of ceramic or tungsten materials depending on the source material to be evaporated. The electron gun contains a filament that is the source of the electrons and magnets are used to focus the electrons onto the source material for melting and evaporation. The power level is controlled by adjusting the filament current for purposes such as differences in melting points of various materials. It usually consists of an in-built film thickness metre that monitors the thickness of the layer formed in real time. A vacuum pump evacuates the deposition chamber to a pressure lower than 10 –6 Torr. Since only a small area of the source material is heated, common types of electron beam deposition apparatus can hold up to four different source materials, leading to the likelihood of achieving up to four different layers on a material in a single deposition process. In this way, different crucibles can be mounted onto the electron gun and different source materials loaded onto the crucibles, and therefore the direction of the electron beam could be altered from one crucible to another or the crucibles simply rotated and focussed onto the electron beam path. This is one of the strengths of electron beam deposition in comparison to other PVD methods. Also, this method

FIGURE 2.13  Components of a typical electron beam evaporator [39].

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achieves a faster deposition rate and thicker deposited thin films. As such, the electron beam evaporation process yields a higher deposition process of 0.1–100 μm/min at lower substrate temperatures [40]. Electron beam deposition has, however, a few limitations such as inefficient heating due to the direct heating by the electron beam, non-uniform evaporation rate due to filament degradation, and certain materials being not well suitable for this method. More research should therefore be done to increase the applicability of this method of PVD.

2.2.5 Pulsed Laser Deposition Pulsed laser deposition (PLD) is a technique in which a laser with a high-power density and narrow frequency bandwidth is used as the source of heat to vapourise the source material and it involves coating thin films on surfaces in cycles. This process is therefore inherently discrete in time, and the thickness of the coating can be controlled by expressing it as a function of the number of cycles [41]. In most cases, films deposited via PLD have a thickness range of 10–500 nm [42]. Due to the use of high-power laser, very high temperatures are involved and the method is suitable for the growth of thin films of a wide range of materials. In operation, the high-power pulsed laser beam is usually directed towards the target material where it causes decomposition of the material. This leads to vaporisation and the formation of a plasma plume, which is then deposited on the surface of a substrate material [41]. PLD applies the concept of laser ablation, whereby as the laser beam hits the target material, the energy of the laser is transferred as electronic excitation and then converted to mechanical, chemical, and thermal energy, resulting in ablation, plasma formation, and target material constituent’s evaporation (Figure 2.14) [41]. The evaporated material is carried within the chamber in the form of molecule

FIGURE 2.14  A schematic illustration of a typical pulsed laser deposition equipment [41]. (Reused with permission from Elsevier Ltd.)

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ions, atoms, electrons, globules, and clusters that get heated to specific temperatures depending on the growth properties of the deposited material. In some cases, larger particles and molten globules of materials can be ejected from the target. When these larger particles are deposited onto the substrate, thin films rather than nano films are formed. Therefore, materials deposited on the substrate form a uniform film with desired roughness, thickness, and physical composition. PLD can be used for the deposition of a variety of materials; in fact, due to the versatility of the PLD technique, there is almost no restriction on the source (target) materials that can be used. It is mostly used where other deposition methods cannot be utilised and is used to manufacture nanotubes, nanopowders, and quantum dots. It can also be used to synthesise hybrid metal organics, biomaterials, and polymers. It has been used to deposit highly crystalline V2O5 and V6O13 thin films which have potential for applications in energy storage devices [43]. Molybdenum trioxide (MoO3) thin films have also been deposited via the PLD method for photochromic applications [44]. Thin films of MoS2 have been prepared via PLD for low-power and high mobility transistor applications [45]. The deposition process is carried out either in a low-pressure environment or in a vacuum that is achieved with the aid of vacuum pumps located next to the deposition chamber. The properties of the deposited film are thus tuned by a gas environment of the desired pressure and composition. There are five significant aspects of PLD which affect the deposition of the target material. These aspects include laser absorption by the target material, laser ablation and plasma formation, plasma dynamics, material deposition on the substrate, and growth of the required film [41]. These aspects require careful consideration to achieve high quality film coatings. The film properties such as crystallinity, composition, thickness, roughness, conductivity, optical properties, and mechanical properties depend on various deposition parameters such as temperature, target material, target-sample distance, laser wavelength, pulse length and rate, and radiation intensity. A larger distance between the target material and substrate leads to less material deposition, in that the movement time of the particles is increased; therefore, more particles will have been deviated away from the substrate. Also, using a laser with short pulses results in electrostatic ablation where the electrons holding the material together get excited too fast, leading to an explosion of small amounts of particles from the target surface. If longer pulses are used, most of the impinging pulse will be absorbed by the plasma plume. Film deposition is also affected by the substrate temperature, in that films deposited at higher temperature are crystalline, while those deposited at lower temperature substrates are amorphous. As such, the PLD process can be carried out in various gas environments and pressures, in that these parameters affect the film growth rate and crystallinity. To improve on the crystalline properties of the thin films, PLD thin films can be annealed after deposition. Metal oxides deposition is carried out in the presence of oxygen gas to ensure that enough amounts of oxygen bond to the metal. In these cases, the reactive gas (oxygen) and the metal ratio are important parameters to consider for the stoichiometry of the deposited thin films. PLD has since become an important technique for novel material fabrications despite the general impression that the growth mechanism differs partially from the continuous chemical and physical deposition techniques resulting from pulsed deposition [46–48].

37

Methods of Thin Film Deposition

2.2.6 Thermal Spray Thermal spraying is a process of thin film technology where metals, cermet, ceramics, and polymers are deposited onto the substrate layer by layer to form film thicknesses ranging between 0.1 and 10 mm [49]. The method is applicable to nearly all materials as long as they can melt and become plastic. The materials (metals, ceramics, or polymers) known as the feedstocks are fed in a device that heats them above their melting points. The coating molten material is then propelled by the stream impacting on the surface to be coated [50]. The particles form ‘splats’ on the substrate surface that interlock as it flattens, cools, and solidifies. It can also overlap and build up to form the coating. In thermal deposition, the coating does not fuse with the substrate, contrary to the other PVD methods, but a mechanical bond is formed between the coating and the substrate. The strength of the bond depends on the nature of the substrate surface [51]. To enhance the formation of the mechanical bonding, the surface of the substrate should be cleaned and roughened through sandblasting. Besides the acceleration and heating of the molten material to form droplets sprays, chemical reactions, e.g., metal alloy oxidation, could also result during heating, leading to a better coating. Splat formation is key in the coating process since the rapid quenching characteristics lead to a quasi-stable fine microstructure formation [51]. Thermal spray methods have been used over the years across different engineering sectors to protect and rehabilitate engineering components [52]. In applications such as aerospace, automotive, and machine tools components such as shafts are exposed to wear, cavitation, corrosion, and oxidation, which degrades their surface properties. This leads to an extended service life of high-performance components and substantial savings in cost through reductions of downtime and production costs. There are several thermal spray techniques, which are classified according to the energy of the source, feedstock, gun type, and deposition condition. The energy supplied should melt and accelerate the feedstock material droplets onto the surface of the substrate. Thermal spray deposition techniques can be classified into conventional combustion spraying, HVOF spraying, flame spraying, and plasma spraying. The various conditions of operation of these techniques are summarised in Table 2.1.

TABLE 2.1 Classification of Thermal Spray Techniques and Their Operating Conditions of Temperature and Substrate Temperature Type Detonation gun Plasma jet (HVOF) Plasma spraying Wire arc Flame spraying

Flame of Plasma Exit Substrate Particle Impact Porosity Adhesion Temperature (°C) Temperature (°C) Velocity (m/s) (%) Strength 3000

20–150

800–1000

0.1–1

2500–3100 5500–8300 4000–6000 2500–3000

500–700 700–1000 500–800 500–700

500–800 200–600 240 30–180

1–10 1–10 10–20 10–30

Extremely high Very high Very high High Low

38

Thin Film Coatings

The following are the common benefits of thermal spray techniques for film depositions:

i. It can be used to deposit films from a wide range of materials, as long as they can melt and become plastic to be transferred to the surface of the substrate. ii. It can be used to deposit thick films which are used in the protection and repair of engineering components. iii. The bonding between the substrate and coating is mechanical rather than metallurgical, and therefore it is suitable for coating substrates with dissimilar metallurgy to the coatings. iv. The substrate can be coated with a coating material with a high melting point. This is important since it improves the temperature properties of the substrates. v. The coating process does not involve preheating or post-treatment of the coating and therefore makes the process low cost.

2.2.6.1 Flame Spray This is one of the earliest modes of thermal spray deposition technique which uses wire, rods, or powder as feedstocks. The feedstock material is continuously fed into a hot flame where it melts and is then accelerated towards the surface of the substrate for deposition. The flame spray process uses gases to produce heat and kinetic for melting and accelerating particles towards the material to be coated. Oxy-acetylene torches (oxygen mixed with acetylene fuel) are used to generate combustion of high temperatures ranging between 3000°C and 3300°C although the temperature obtained at the gun depends on the oxygen–fuel ratio. Other fuels which can be used include propane, propylene, hydrogen, and ethane [53]. Based on the feedstock, there are two types of flame spray systems (Figure 2.15): i. Powder feed and ii. Wire feed flame spray As shown, flame spray equipment typically consists of the following parts:

i. Oxygen and fuel (gas) supply system; ii. Supply of the compressed air; iii. Gas hoses and regulators for oxygen, air, and fuel; iv. Rotametres; v. Flame spray torch; and vi. Feedstock delivery mechanism

The projected molten material impacts on the surface of the substrate to form a mechanical bond. This is followed by fusion treatment to form a dense and wellbonded coating. Some of the materials deposited via flame spray include titania, alumina, Ni, and Co-based alloys, etc. [53].

Methods of Thin Film Deposition

39

FIGURE 2.15  Illustrating (a) powder feed and (b) wire feed flame spray techniques. (Reused from [53] with permission from Elsevier Ltd.)

2.2.6.2 Plasma Spray Deposition Technique This type of thermal spray technique is where ionised conductive gas (called plasma) melts and propels powdered material onto a substrate surface. The gun used has two electrodes, a cathode (made of tungsten) which is surrounded by an anode (made of copper), as shown in Figure 2.16. The electrodes are insulated from one another and cooled with water. Typically, a plasma deposition tool device consists of a controlling unit, cooling water and power supply unit, and auxiliary units [54]. Inert gas like argon or nitrogen

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Thin Film Coatings

FIGURE 2.16  Illustrating the various parts of a plasma spray technique [54]. (Reused with permission from Elsevier Ltd.)

with the addition of helium or hydrogen is injected into the annular space between the anode and the cathode. It is then struck with a direct current power source. This results in high temperature and partially conductive plasma expanding through the anode at a high velocity as a very hot plasma jet. Powdered material to be deposited is injected into the plasma where it is melted and projected towards the surface to be coated (substrate). Core temperature in plasma spray is above 15,000°C, which is sufficient to melt most metallic and ceramic materials. As such, plasma spray can be used to deposit metals, ceramics, glasses, and polymers. Materials are introduced to the plasma spray in size ranges of 5–50 μm for oxides and 50–100 μm for lower melting-points metal powders. Powder of sizes 0.5 1

1000

30 Å

>10,000

20 Å

Electrons

2 Å

0.01 Å

100 Å

Electrons

3 Å

3 Å

300 Å

Diameters Thickness

Spatial Resolution (μm)

Electrons

Emitted Beam (Particle)

STM (scanning tunnelling microscopy) Distance (1~2 Å) Tunnelling current (1~10 nA) ICP-AES (inductively coupled plasma Plasma Photons atomic emission spectroscopy) EXAFS (extended X-ray absorption X-rays X-rays fine structure)

LEED (low-energy electron Electrons diffraction) TEM (transmission electron Electrons microscopy) SEM (scanning electron microscopy) Electrons

Techniques

Incident Beam (Particle)

TABLE 3.5 (Continued) A Summary of Methods for Surface Analysis of Thin Film Materials [55]

10

0.01~1

±0.1

Detection Limits Accuracy (Atomic ppm) (%) Elements

Other Features

Quantitative elemental composition Fine structure nondestructive

Surface/interface structure Surface topography Conductor Surface topography conductor Bias (1 mV~1 V)

110 Thin Film Coatings

Characterisation Techniques of Thin Films

111

REFERENCES





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4

Hybrid and Scalable Thin Films

4.1 INTRODUCTION 4.1.1 What Is a Hybrid Material? Hybrid materials are combinations of two or more different materials, with each material exhibiting distinct characteristics from each other. In most cases, a hybrid material consists of an organic and inorganic material. The constituent materials provide complimentary properties to each other and hence superior performance of the hybrid material. There are two classes of hybrid materials (class I and class II), and the classification is based on the interactions between the inorganic and organic species in the hybrid material. Class I consists of materials with weak bonding between the constituents, e.g. van der Waals and hydrogen bonding. In class II hybrid materials, there are strong interactions among the inorganic-­organic bonds, e.g. covalent bonding [1].

4.1.2 What Is a Hybrid Thin Film Material? A thin film consisting of organic–inorganic materials is considered to be a hybrid thin film. These films consist of various functional components and therefore exhibit superior properties and characteristics. An example of such coatings is graphene oxide-­zirconia dioxide/epoxy thin film coating for corrosion protection. These films have superior characteristics over the traditional films, since they harness the properties of both organic and inorganic materials [2].

4.1.3 What Is a Multilayer Thin Film? These are thin film materials consisting of more than one layer of thin films. Usually, a multilayer film consists of several monolayers of different materials stacked together, as shown in Figure 4.1. The figure shows a thin film consisting of four monolayers of different materials. The multilayering in thin films helps in enhancing the adhesion between the substrate and the top monolayer. It also helps in enhancing mechanical strength of the thin films for protective applications such as against corrosion and wear degradation. In semiconductor and optical applications, multilayered thin films are related to higher quantum yield, higher photoluminescence efficiency, improved optical properties, and increased half-­life times.

DOI: 10.1201/9781003202615-4

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FIGURE 4.1  A multilayered thin film consisting of four monolayers of different materials deposited on a substrate.

The multilayering process can be used to produce hybrid thin film materials, in which organic and inorganic materials are deposited alternatively as monolayers. The type of interaction at the interfaces of the layers can determine the class of hybrid materials produced. For example, using physical vapour deposition or chemical vapour deposition methods, metal/ceramic oxides layers can be deposited to create hybrid-­multilayered thin film materials.

4.2 PROPERTIES OF THIN HYBRID AND MULTILAYER THIN FILMS As mentioned, hybrid and multilayer thin film materials exhibit superior properties over the traditional thin films (those films consisting of a single material type or layer) due to the fact that they consist of more than one type of material. As such, they harness the composite properties of the constituent materials. To illustrate properties and examples of hybrid thin films, Table 4.1 is extracted from the paper by Malucelli published in 2016 in the open access Journal of Coatings [2]. The coatings in this case were prepared using dual-­cure processes. There are several multilayer thin films which have various applications. These films can be deposited through several methods, including sol-­gel, sputtering, spray coating, pulsed laser deposition, atomic layer deposition, etc. Examples of multilayer thin film materials include TiO2/VO2, three-­layer TiO2/VO2, five-­layer TiO2/ VO2, epoxy resin/SiO2, WO3/Ag/WO3, TiO2/Au/TiO2, TiO2/NiCrOx/Ti/Ag/NiCr/ Si3N4, etc. [3]. Other films include Al/Ni [4], (CrN/CrAlN)/(CrAlN/VN), and (CrN/ TiAlN)/(TiAlN/VN) [5]. These films exhibit superior properties and can be used for extreme condition applications such as high temperatures, and high wear and corrosion conditions. They can also be used at instances where special properties, such as self-­cleaning and solar absorbance, are desired [3,6].

3-­Glycidyloxypropyltrimethoxysilane; zirconium (IV) n-­propoxide; tetraethoxysilane; dimethyldiethoxysilane n-­alkyltrimethoxysilane precursors

3-­(Trimethoxysilyl)propyl methacrylate

Tetraethoxysilane

Tetraethoxysilane; 3-­acryloxypropyl-­trimethoxysilane 3-­(Glycidyloxypropyl)trimethoxysilane [2–3,4-­Epoxycyclohexylethyl] trimethoxysilane

3-­Triethoxysilylpropylamine; Tetraethoxysilane

Methacrylic acid

Diglycidyl ether of bisphenol A

Perfluoropolyether ethoxysilane-­terminated

Ethoxysilyl-­modified hyperbranched aliphatic-­ aromatic polyesters

Hexafunctional aliphatic polyester-­acrylate

2-­Hydroxyethyl acrylate; 1,6-­hexanediol diacrylate; bisphenol A epoxy acrylate (containing 25 wt.%; tripropyleneglycol diacrylate)

Diglycidyl ether bisphenol A

Methacryloxymethyltriethoxysilane

Inorganic Precursors

Hexanediol diacrylate; aliphatic polyester urethane diacrylate; trifunctional polyester acrylate; fluoroacrylate

Organic Part

TABLE 4.1 Typical Hybrid Thin Film Materials Prepared Using Dual-­Cure Processes

Protective coatings

Transparent protective coatings for flammable thermoplastic substrates Dental restoration/ adhesion

Applications

• High transparency; • Enhanced thermal and mechanical properties

• Improved wear resistance of steel substrates; • Improved mechanical properties

• Improved scratch resistance

(Continued)

Protective coatings

Protective coatings

Protective coatings

Hydro and oil repellent finishing treatment for fabrics (cotton and polyester) • Improved thermo-­mechanical behaviour; Toughened protective coatings • Increased surface hardness; • High transparency • Hydrorepellency and oleorepellency

• Enhanced corrosion protection

• Enhanced mechanical properties; • High thermal stability

• Increased flame retardancy; • Hydrophobicity; • Good optical properties

Properties

Hybrid and Scalable Thin Films 117

Trimethoxypropyl silane methacrylate; 3-­mercaptopropyltriethoxysilane

3-­Mercaptopropyltrimethoxysilane

3-­Isocyanatopropyltriethoxysilane

Tetraethoxysilane

3-­Isocyanatopropyltriethoxysilane; tetraethoxysilane

3-­Trimethoxysilylpropyl methacrylate; titanium (IV) butoxide

Trimethylolpropane trimethacrylate; polydimethylsiloxane

2,2-­Diallylbisphenol A, trimethylolpropane tris(3-­mercaptopropionate)

Bisphenol-­S epoxy dimethacrylate

Bisphenol A ethoxylatediacrylate

Bisphenol A epoxy resin

2-­Hydroxyethylmethacrylate; oligodimethacrylate based on poly(ethylene glycol); 3-­(acryloyloxy)-­2-­hydroxy-­propyl methacrylate

Source: Used under Open Access licence from Malucelli [2]. The properties and applications of the films are also shown.

Methacryloxypopyltrimethoxysilane; tetraethoxysilane

Inorganic Precursors

Hexanediol diacrylate; acrylated soybean oil

Organic Part

TABLE 4.1 (Continued) Typical Hybrid Thin Film Materials Prepared Using Dual-­Cure Processes Hydrophobicity; Improved mechanical properties; Enhanced thermal stability; Enhanced flame retardancy

• Increased thermal stability; • Improved tensile strength; • Efficiency in removing phenolic compounds from water

• Improved thermal stability; • Enhanced impact strength and dynamic-­mechanical behaviour

• Enhanced scratch resistance; • High transparency

• Enhancements of hardness and gloss; • Improved thermal stability

• Improved thermal stability; • Enhanced flame retardancy; • Hydrophobicity

• Water repellency; • High transpirability

• • • •

Properties

Protective photocatalytic coatings

Protective coatings

Light-­emitting coatings

Protective coatings

Protective coatings

Cultural heritage protection

Protective coatings

Applications

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4.3 FREE-­STANDING AND SCALABLE THIN FILMS In some applications, there may be a need to produce free-­standing thin film materials, i.e. without having the films supported on a substrate material. A free-­standing thin film can also be defined as a thin film that is supported on a non-­continuous substrate such as a hoop rather than a metallic, glass, or silicon surface [7]. These films may find application in fabrication of sensors, micro-­electromechanical systems (MEMS), tissue engineering, and filtration. The general protocol for preparation of free-­standing thin films is described in Figure 4.2. The most critical step is the removal of the thin film from the substrate surface. The ‘lift off-­float on’ is the most used method. In this method, the thin film substrate systems are immersed in a suitable solution in a bath. The solution causes swelling of the film, and there is an instantaneous delamination of the thin film from the surface. The films float on the solution and can be captured using a hoop or an annular ring (Figure 4.2). Another method for removal of thin film involves the use of a sacrificial interlayer between the film and the silicon/glass substrate (Figure 4.3). The interlayer can then be eroded away in a solvent separating the thin film and the substrate. The use of a sacrificial interlayer can, however, degrade the mechanical and chemical properties of the free-­standing thin films. The free-­standing thin films have the potential for manufacturing of scalable high strength sheets for various applications. A protocol which maybe adopted especially in fabrication of high strength hybrid/multilayer nanostructured thin sheets with potential of replacing bulk sheets in some applications is illustrated in Figure 4.4.

FIGURE 4.2  General steps of fabricating a free-­standing thin film [7]. As shown, the process involves four steps. The first step involves treatment of the surfaces of the substrates (glass or silicon wafer). The second step involves deposition of the thin film material using any suitable technique. The third step involves immersing the thin film–substrate system gently into a solution which lifts off the thin film from the surface of the substrate, and the last step involves using a hoop or annular ring to capture the peeled thin films from the solution.

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FIGURE 4.3  Illustrating the use of sacrificial interlayer method for preparation of free-­ standing thin films.

FIGURE 4.4  A protocol for scalability of thin film materials.

As shown, three steps can be adopted for fabrication of scalable hybrid thin film materials. First, free-­standing thin films are fabricated followed by interlaying the thin films, and then formation of strong bonds among the interfaces. The last two steps can be carried out concurrently, and several methods can be used to carry out those steps. One of the methods which have the potential for this application is the

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accumulative roll bonding (ARB) process of the free-­standing thin films. In this process, the films can be stacked together and then severely deformed under rolling until strong bonds are obtained at the interfaces. Another method which can be adopted is the use of adhesive chemicals, which can react with the interfaces to form strong bonds between two adjacent thin film materials.

4.4 SUMMARY In this chapter, a brief discussion on hybrid and multilayer thin film materials is presented. The meaning of the terms ‘hybrid materials’, ‘hybrid thin films’, and ‘multilayer materials’ is clearly explained. Then, properties of hybrid and multilayer thin film materials have been described with some examples of such materials provided. These materials exhibit superior properties and hence are suitable for advanced applications as compared to single-­layer thin film materials. Some of the applications of these materials are also mentioned. Additionally, an interesting description of the protocols for preparation of free-­standing thin film materials has been presented. Finally, the procedure for fabrication of scalable thin film hybrid and multilayer materials has been explained. It is important to note that this is a developing area in thin film technology and can be exploited for the development of ultra-­high strength materials for advanced applications.

REFERENCES





1. R. Sharafudeen, “Smart hybrid coatings for corrosion protection applications,” in Advances in Smart Coatings and Thin Films for Future Industrial and Biomedical Engineering Applications, Abdel Salam Hamdy Makhlouf and Nedal Yusuf Abu-­ Thabit, Eds. New York: Elsevier, 2020, pp. 289–306. 2. G. Malucelli, “Hybrid organic/inorganic coatings through dual-­cure processes: State of the art and perspectives,” Coatings, vol. 6, no. 1, p. 10, Mar. 2016, doi: 10.3390/ coatings6010010. 3. C. Garlisi et al., “Multilayer thin film structures for multifunctional glass: Self-­cleaning, antireflective and energy-­saving properties,” Appl. Energy, vol. 264, no. February, p. 114697, Apr. 2020, doi: 10.1016/j.apenergy.2020.114697. 4. A. S. Ramos, S. Simões, L. Maj, J. Morgiel, and M. T. Vieira, “Effect of deposition parameters on the reactivity of Al/Ni multilayer thin films,” Coatings, vol. 10, no. 8, p. 721, Jul. 2020, doi: 10.3390/coatings10080721. 5. A. Wilczek, J. Morgiel, Ł. Rogal, W. Maziarz, and J. Smolik, “Microstructure and wear of (CrN/CrAlN)/(CrAlN/VN) and (CrN/TiAlN)/(TiAlN/VN) coatings for molds used in high pressure casting of aluminum,” Coatings, vol. 10, no. 3, p. 261, Mar. 2020, doi: 10.3390/coatings10030261. 6. M. Muralidhar Singh et al., “Evaluation of multilayer thin film coatings for solar thermal applications,” Arab. J. Sci. Eng., vol. 44, no. 9, pp. 7789–7797, Sep. 2019, doi: 10.1007/s13369-­019-­03904-­9. 7. M. Stadermann et al., “Fabrication of large-­a rea free-­standing ultrathin polymer films,” J. Vis. Exp., vol. 2015, no. 100, pp. 1–7, Jun. 2015, doi: 10.3791/52832.

5

Bibliometric Analysis of Applications of Thin Film Materials

5.1 INTRODUCTION According to Zhang and Koutra [1], a bibliometric analysis can be defined as, “…a statistical evaluation of published journal papers, books, or other scientific articles, etc. and it is an effectual way to measure the influence of publications, scholars, or institutions in the scientific community”. Bibliometric analysis is used to study the trends in research outputs in various topics over the years and across different regions of the world [2]. In this chapter, bibliometric analysis of thin film applications is demonstrated to enlighten and justify the significant applications of thin film materials. The aim of the analysis is to identify the recent progress and focus on the applications of thin film materials in the industry. The bibliometric analysis presented here is based on the Web of Science (WoS) database and results on publication over the years and publication by country/region, and keyword analyses are then presented.

5.2 BIBLIOMETRIC ANALYSES ON THIN FILM APPLICATIONS The bibliometric analyses were based on WoS core collection database. The WoS is a reliable source of peer-reviewed and quality articles and data on thin film materials and technology. It has a wide collection of journals and reports across various disciplines, including engineering, medicine, bio- and natural sciences, and social sciences [3]. The WoS database was accessed on 24 October 2021 through the University of Johannesburg (South Africa) online library services. Only journal articles (data-based and reviews) were considered, whereas the other document types such as book chapters, conferences, retracted publications, and news articles were excluded. The articles considered were published between 1980 and 2021.The search expression used in the WoS database was ‘“thin film applications.’ The search query was used for all the fields in the WoS database. The publications over the years and distribution across the countries were analysed and discussed. Keyword analysis was also carried out using VOSviewer software created by Leiden University, from which network visualisation of keyword relationship was generated [4]. The keyword analysis was used to identify the key areas of application of thin film materials in 2020–2021 period.

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5.2.1 Growth Trend over the Years Based on the search criteria, 103,696 journal articles were obtained from a wide range of WoS categories, with Materials Science Multidisciplinary and Applied Physics categories leading in terms of the highest number of publications over the years. Figure 5.1 shows the number of research articles published between the years 1980 and 2020. It also shows the cumulative number of publications within the same period. As shown, the number of publications has increased exponentially within this period. The applications and demand for thin film materials across various industries can be attributed to the growth in publication trends. The market for thin film materials has grown from $9.9 billion in 2014 to about $11.3 billion in 2021, which is about a compound annual growth rate (CAGR) of 3.0%. By 2026 the CAGR is projected to grow by over 4% [5]. Some of the applications driving the thin film market include solar photovoltaic cells, electronics (printed circuit boards, capacitors, sensors), and so many others [6].

5.2.2 Thin Film Research by Country Table 5.1 shows the top ten countries publishing highly on applications of thin film materials. As shown, China and USA are the first two publishers in this topic. Germany, Japan, South Korea, and India follow closely. France, England, Taiwan, and Italy close the list of the top ten in that order. As expected, the top ten countries are among the world most developed economies and their manufacturing industries are progressive in terms of adoption of Industry 4.0 technologies such as additive manufacturing, augmented/virtual reality, Internet of Things (IoT), data analytics, the cloud, and cybersecurity. As discussed in Chapter 14,

FIGURE 5.1  Trends in publication on application of thin film materials between 1980 and 2020.

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TABLE 5.1 The Distribution of Publications on Application of Thin Film Materials as Per Regions/Countries Number 1 2 3 4 5 6 7 8 9 10

Country

Number of Articles

% of Total Articles

Peoples R China USA Germany Japan South Korea India France England Taiwan Italy

21,213 20,030 8963 8679 8653 8031 5553 4428 4097 3607

20.457 19.316 8.644 8.370 8.345 7.745 5.355 4.270 3.951 3.478

Fourth Industrial Revolution is significantly supported by thin film technologies and materials, and as such, it drives the market for thin film materials. For instance, to control and monitor industrial processes, miniaturised sensors and control systems are required, which can be fabricated through thin film materials and technologies. The countries listed in Table 5.1 are known to host some of the leading semiconductor industries which utilise a huge percentage of thin film materials. For instance, the Chinese semiconductor industry is known for Fairchild semiconductor, ST microelectronics, Infineon Technologies, and NXP semiconductors. Japan is known for Mitsubishi Electric, Denso, Fuji Electric, New Japan Radio, and Hitachi, whereas Germany has Infineon Technologies, Carl Zeiss, and Siltronic AG. South Korea is associated with DI Corporation, Hana Micron, Samsung Electronics, and Telechips semiconductor industries. United States of America hosts several semiconductor industries including Intel Corporation, Texas Instruments, Micron Technology, Analogue Devices, Microchip Technology, Skyworks Solutions, etc.

5.2.3 Applications of Thin Film Materials Bibliometric studies can be used to analyse keywords to identify the key/focus areas of specific technologies [7]. In this section, keyword analysis is used to identify the major applications of thin film materials in 2020 and 2021. The VOSviewer software is used to study the interrelationship and co-occurrence of keywords on the search query. From the WoS 5984 journal articles were filtered for 2020 and 2021. A thresholding was defined by setting the minimum occurrence per keyword as 25, and out of 20,305 keywords, only 122 keywords met the condition. Figure 5.2 shows the network visualisation of the relationship among the keywords. It shows the co-occurrence and interrelationships among the keywords. As shown, there were five clusters differentiated in terms of colours of the co-occurrence map. Each keyword in a specific cluster is represented by a sphere, and the size of the sphere indicates the significance of the keyword. As shown (Figure 5.2), the most

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FIGURE 5.2  Co-occurrence of keywords on the applications of thin film materials.

FIGURE 5.3  Cluster analysis of the keywords from the co-occurrence map shown in Figure 5.2.

significant words are nanoparticles, optical properties, fabrication, nanocomposites, electrical properties, solar cells, temperature, photodetector, electronic structure, magnetic properties, silicon, water, adsorption, surfaces, electrodes, desalination, nanofiltration, MEMS, semiconductor, energy, and coatings. These words are associated with the most common applications of thin film materials in the 21st century. From the co-occurrence map of the keywords, the cluster analysis of the keywords is represented in Figure 5.3. As shown, the cluster analysis provides the most common applications of thin film materials based on the publications in 2020 and 2021. These applications include the following: i. Optics and light-related devices ii. Electrical and microelectronics devices

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iii. Energy such as solar cells and fuel cells iv. Magnetic and spintronic devices v. Surface coatings/protection vi. Water applications such as desalination and filtration vii. Nanoparticles for various applications such as fabrication of scalable sheets and nanocomposites

These applications involve specific thin film materials, for instance, silicon in the semiconductor applications, aluminium/gold/copper/silver in electrical conductivity, and so forth [8]. In the subsequent chapters, thin film materials and their properties for above specific applications are described.

5.3 SUMMARY In this chapter, bibliometric analysis on thin film applications based on the WoS database is presented. The aim of the analysis is to derive the current trends on the applications of thin film materials and technologies so as to inform the subsequent discussions of this book. The growth trend in research and publication of thin film applications over the period spanning between 1980 and 2020 is shown to have been on the increasing trend. The research and applications of thin film materials as per country over the years show that China and USA are two of the top ten countries in the area. The market of thin film in these countries is driven by the presence of leading semiconductor industries and the increasing adoption of Industry 4.0 by those countries. Based on the keyword analysis using the VOSviewer software on 2020 and 2021 publications, the most important applications of thin film materials and technologies in the modern society are identified. Some of these applications include semiconductors, optics, magnetic, electrical, surface coating/protection, etc. In the subsequent chapters of this book, these areas of applications and their corresponding thin film materials are discussed.

REFERENCES

1. J. Zhang, J. Zhang, and S. Koutra, “Bibliometric analysis,” in Handbook of Disease Burdens and Quality of Life Measures, Victor R. Preedy and Ronald R. Watson, Eds. New York: Springer, 2010, pp. 4155–4155. 2. N. Donthu, S. Kumar, D. Mukherjee, N. Pandey, and W. M. Lim, “How to conduct a bibliometric analysis: An overview and guidelines,” J. Bus. Res., vol. 133, no. March, pp. 285–296, 2021, doi: 10.1016/j.jbusres.2021.04.070. 3. Y. Chen et al., “A bibliometric analysis for the research on laser processing based on web of science,” J. Laser Appl., vol. 32, no. 2, p. 022001, 2020, doi: 10.2351/1.5097739. 4. N. J. van Eck and L. Waltman, “Software survey: VOSviewer, a computer program for bibliometric mapping,” Scientometrics, vol. 84, no. 2, pp. 523–538, 2010, doi: 10.1007/ s11192-009-0146-3. 5. T.S. Senthil and C.R. Kalaiselvi, “New materials for thin film solar cells,” in Coatings and Thin-Film Technologies, A. P.-T. Jaime and A. G. A. Bernal, Eds. London: IntechOpen, 2019, p. 13. 6. F. M. Mwema, E. T. Akinlabi, and O. P. Oladijo, Sputtered Thin Films: Theory and Fractal Descriptions. Boca Raton, FL: CRC Press, 2021.

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7. J. Nyika, F. M. Mwema, R. M. Mahamood, E. T. Akinlabi, and T. Jen, “A five-year scientometric analysis of the environmental effects of 3D printing,” Adv. Mater. Process. Technol., pp. 1–11, Jul. 2021, doi: 10.1080/2374068X.2021.1945267. 8. F. M. Mwema, O. P. Oladijo, S. A. Akinlabi, and E. T. Akinlabi, “Properties of physically deposited thin aluminium film coatings: A review,” J. Alloys Compd., vol. 747, pp. 306–323, May 2018, doi: 10.1016/j.jallcom.2018.03.006.

6

Thin Films for Biomedical Applications

6.1 INTRODUCTION TO BIOMATERIALS As recently documented by Sankar et al. [1], the most accepted definition of the term ‘biomaterial’, which was provided by Williams in 1987, is “… a material intended to interface with biological systems to evaluate, treat, augment or replace any tissue, organ or function of the body”. These materials are specially engineered to interact well with biological substances for mainly therapeutic, medical, or diagnostic use. Biomaterials are non-­living, and the study of their interaction with biological systems has evolved to be defined as biocompatibility [1]. Biomaterials in most cases are derived from either synthesised or natural materials to suit their medical uses for supporting, enhancing, and replacing broken biological parts [2]. The synthesis involves a step by step procedure while making use of chemical or physical processes to modify the properties of the materials (which could be polymers, metals, composites, or ceramics) to enhance their application as biomaterials [3]. Regardless of the origin (whether natural or synthetic), the modification must ensure the biocompatibility of the biomaterials so that they can be used to replace living parts such as lungs, heart, and liver, among others [4]. Biocompatibility is a fundamental determinant for the application of a material in the human/animal body tissues [5]. Biocompatibility of the biomaterials to the living parts of the body can be related to their performance under various chemical and physical conditions. Therefore, these materials should be able to withstand the conditions of the body and should integrate with the tissues without any adverse effects. The biomaterials are said to have historically originated from ancient Egypt where the use of sutures has been documented. The documentation of the use of dental implants by the Mayan people in 600 AD and artificial heart in 1881 are indicators of historical biomaterials [6]. The most commonly used biocompatible materials are polymeric and plastic composites due to their suitable mechanical properties; these materials are easy to process and tune their properties. It is important to note that the use of these and any other kind of materials for biomedical applications requires a series of tests for certification by the relevant government and international bodies. Today, various materials have been utilised as biomaterials including ceramics, metals, composites, and hybrid materials. Recently hydrogel materials that are mostly used during surgical operations have been developed to aid in surgical operations. Such hydrogels can be inserted into the abdomen during a surgical operation to prevent the complications related to the tissue clogging or sticking together. Consequently, researchers have extensively formulated a combination of hydrogen

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and polymers in drug administration and medical devices like contact lenses among others. In their application, the biomaterials have been used in various key operational or functional status of the body such as replacement of the damaged or deceased parts (artificial hip joint, kidney), healing process (in sutures, bone plate, etc.), functional improvement (cardiac pacemakers), intraocular lens, correction of cosmetic problems (plastic surgery), and uses in treatment and diagnosis equipment such as catheters and probes. Biomaterials can be classified based on their original material of synthesis as follows: i. Metal and metal alloy biomaterials, ii. Polymer biomaterials, iii. Composite biomaterials, and iv. Ceramic biomaterials.

6.1.1 Metal and Metal Alloy Biomaterials Most metals are used as biomaterials due to their good properties in terms of thermal conductivity, electrical conductivity, and mechanical properties. The common types of metal and metal alloys used are cobalt-­based alloys, titanium and its alloys, tantalum-­based alloys, stainless steel, magnesium alloys, zinc alloys, and iron alloys, among others [7]. Biomaterials such as zinc, magnesium, and iron alloys have been described as biodegradable materials while the others as permanent biomaterials [7]. The properties of these materials are described in Table 6.1.

TABLE 6.1 Showing Attributes and Applications of Various Biomaterials [8] Materials

Advantages

Stainless steel

Low cost, easily available, acceptable biocompatibility

Cobalt-­based alloys

Better wear and corrosion resistance, fatigue strength

Ti and Ti-­based Biocompatible, corrosion alloys resistance, fatigue strength, low modulus, light weight Magnesium-­ based alloys

Biocompatible, biodegradable, low Young’s modulus

Disadvantages High modulus, Low corrosion resistance, Allergic reaction High modulus, Expensive, Biologically toxic Low wear resistance

Low corrosion resistance, hydrogen evolution during degradation

Source: Reused with permission from Elsevier Ltd.

Application Temporary devices, plates, and screws Total hip replacement, bone plates, and wires Total joint replacement, fracture fixation elements Biodegradable orthopaedic devices, bone pins, and plates

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Metallic biomaterials are preferred over other materials due to the following reasons: i. They have higher fatigue resistance due to their ductile nature. ii. They exhibit higher strength as compared to polymers. iii. They have been extensively exploited for fabrication through different technologies including plastic deformation, machining, etc. iv. Metals have generally better wear resistance as compared to other materials. v. They are easier to sterilise. The disadvantages of metals compared to other biomaterials are that they have high elastic modulus and high metal ion sensitivity, are prone to corrosion attach, and have high toxicity at low wetting conditions. In terms of their application, most metal and metal alloys are used in implants and orthopaedics applications. Stainless steel is used predominantly as a trial implant material due to its inferior biocompatibility, corrosion resistance, and osseointegration. It is also used to fabricate 3D dental implants using selective laser sintering [7]. Titanium alloys have been widely used as bone implants due to their excellent biocompatibility and high strength-­to-­weight ratio, and one such alloy is Ti-­Mg as described in a mini-­review by Rajesh Verma in 2020 [9]. The common titanium alloys used as surgical implants are American Society for Testing and Materials (ASTM) F 136/F 1472 (Ti6Al4V), and ASTM F1295 (Ti6Al7Nb) [8,10]. These alloys are used to manufacture femoral, acetabula and tibia for total knee and hip arthroplasty [8]. However, titanium and its alloys have relatively higher elastic modulus that may cause stress shielding, leading to periodontal bone loss [11]. Cobalt-­based alloys have been demonstrated to exhibit high wear resistance and are therefore suitable where frictional contact is involved such as at the hip joins where there is direct contact between femoral head and plate. An example of such alloy is Co-­Ni-­Cr-­Mo, which has been reported to exhibit high strength although it is limited by the presence of Ni, which is toxic to the human body. Co-­Cr alloys have been employed for dental and cardiac applications due to their high biocompatibility, wear resistance, durability, excellent corrosion performance, and high mechanical strength [12]. Usually, most metallic biomaterials are limited by their surface properties such that during their interaction with the biological systems, their properties and performance degrade. As such, surface treatments are usually employed to enhance their surfaces for better biocompatibility, strength, and corrosion [13]. One of the techniques for improving the surface properties of these biomaterials is thin film coating. The surface is coated with a thin layer of a material to enhance its surface characteristics. In Sections 6.2 and 6.3 the role of thin film technology and various thin film materials for biomedical applications are presented in detail.

6.1.2 Ceramics Biomaterials Ceramics have high biocompatibility and mechanical strength, making them suitable for biomaterials. Examples of ceramic materials used as biomaterials are tri-­calcium

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phosphate, metal oxides (Al2O3 and SiO2), porous, and apatite ceramics materials. Figure 6.1 shows the classification of ceramic materials used as biomaterials. As shown, ceramic biomaterials can be classified into three broad categories: (1) bioinert, (2) bioresorbable, and (3) bioactive materials. Bioinert ceramics are those biomaterials that do not regenerate but help in holding the tissues for healing and include oxides and nitrides, bioresorbable ceramics include the calcium phosphate– based materials, whereas the bioactive ceramics include glasses/glass ceramics [14]. Bioactive ceramics are those biomaterials that promote the formation of bone at their surfaces, and they form direct bonds with the host tissues. Ceramics biomaterials’ advantage over other biomaterials is that they possess very higher compression strength, high polishability, and high wear and corrosion resistance. Additionally, ceramic biomaterials, such as hydroxyapatite, have similar physio-­chemical properties to some biological parts such as the bone [15]. On the contrary, some of the setbacks of bio-­ceramics are high Young’s modulus, brittleness, and difficulty in processing. It, therefore, poses a challenge to produce biomedical parts and materials from ceramics than it is for metallic or polymeric material. In terms of their application example, Al2O3 is used in orthopaedic and dental implants due to its high mechanical strength, low coefficient of friction, and resistance to abrasion [15]. Zirconia (mostly stabilised with oxides) is used in dentistry to produce dental pins, crowns, bridges, veneers, and orthodontic brackets due to its tooth-­like appearance and excellent mechanical strength. Compared to alumina, zirconia ceramics have higher fracture toughness, higher flexural rigidity, and lower modulus of elasticity.

FIGURE 6.1  Classification of bio-­ceramic materials [14]. (Reused with permission from Elsevier Ltd.)

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Bioactive ceramics include calcium phosphate and glass-­based ceramics. Some of the calcium phosphates used as biomaterials include tetracalcium phosphate (TTCP), hydroxyapatite (HA), tricalcium phosphate (TCP), octacalcium phosphate (OCP), brushite, tetracalcium dihydrogen phosphate (TDHP), etc. The most commonly used bioactive ceramic for bone replacement is HA because of its chemical similarity to bone and teeth. It has a chemical formula of Ca10(PO4)6(OH)2 and has a Ca/P molar ratio of 1.67. These calcium phosphates are used in bone and teeth replacement and tend to dissolve during bone regeneration. Bioactive glass® is another ceramic biomaterial that has close structural similarity to the bone and can bond directly with tissues. The bioactive glasses have a higher rate of reactivity and bonding with the tissues although their applications are limited by the low fracture toughness.

6.1.3 Biopolymers Biopolymers are made from micro molecules of biobased materials which can either be protein (silk, collagen, and elastin), starch, nuclei, or polysaccharides. These materials are formed naturally during the growth cycle of living organisms. Biopolymer materials are biodegradable, and the monomeric units are usually sugars, amino acids, and nucleotides. They are classified as starch-­based, sugar-­based, cellulose-­ based, and synthetic materials [16]. Biopolymers have the following advantages compared to the other biomaterials: ease in processing, better physical and mechanical properties, ease of surface modification, and biodegradable [17]. However, these materials have several limitations which include the following: highly leachable and easily absorb water and protein, experience a considerable amount of surface contamination, have high wear rate and breakdown. Additionally, biopolymer materials are difficult to sterilise. In their application, these polymeric materials are used to make disposable medical supplies, dental materials, implants, prosthetic materials, dressing, and polymeric drug delivery systems, among others. Polyhydroxyalkanoates (PHAs) such as poly(lactic) acid (PLA) are resorbable biomaterials that find applications as adhesion barriers, bone graft substitutes, medical scaffolding (for treatment of bone defects and injuries), and valves to guide repair of tissues [18].

6.1.4 Composite Biomaterials A composite consists of two or more distinct constituent materials or phases, with each material offering unique properties to the composite for better performance [19]. In biomaterials, each of the constituent materials must be biocompatible with the body tissues. Composite biomaterials can be classified as biodegradable and non-­biodegradable. Biodegradable composites tend to dissolve into the system as the healing process occurs. For instance, degradable polymeric matrices combined with non-­resorbable glass or carbon fibres have been developed through injection moulding. These composites have been used as screws, plates, and wires for fracture fixation [20,21]. Other degradable composites include polycaprolactone (PCL)/PLA fibre for scaffolds for bone regeneration, PCL/HA particulate composite for bone replacement,

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and aliphatic polyester/nanoparticles of HA, SiO2, MgO, etc. PCL/HA micro-­ and macro-­porous composites have been developed for bone tissue applications [22–24]. A detailed review of degradable polymer-­ceramic biomaterials was recently presented by Alizadeh-­Osgouei [25], and it was shown that most of these biomaterials find applications in bone tissue engineering, bone graft substitutes, drug delivery, interference screws, hard and soft tissue engineering, wound healing, and bone fracture fixation. The non-­biodegradable composites have been mostly manufactured from thermoset polymers such as carbon/glass reinforced poly(sulfone) (PS), polyetherimide (PEI), and polymethyl methacrylate (PMMA) [20]. These polymers have attractive mechanical stability and are suitable for hard tissue treatment. Lopes et al. [26] prepared bioactive glass reinforced PMMA and investigated the effect of filler proportion on the biocompatibility properties of the composite. The composite was reported to be potentially bioactive and can find application in bone fixation. TiMoCu/PMMA composite prepared via immersion of porous TiMoCu into the solution of PMMA resulted in a high strength composite for biomedical application [27]. PMMA/modified multi-­walled carbon-­nanotube nanocomposites for biomedical applications were developed by Ayanoğlu and Doğa [28]. The development of composite biomaterials is attracting a lot of attention among researchers, and the focus is currently on the most effective techniques of producing these materials. Modern technologies such as 3D printing or additive manufacturing are extensively being explored for potential application in the production of composites [25]. Other methods of manufacturing of these composites include extrusion moulding, injection moulding, chemical co-­precipitation, co-­extrusion, solvent casting, sputtering, thermally induced phase separation, selective laser sintering, air-­jet spinning, solvent casting, and so many others.

6.2 THIN FILM MATERIALS AND THEIR APPLICATIONS IN BIOMATERIALS Thin film technology finds application in surface modification of materials for biomedical applications; it basically involves coating a surface of a bulk material with a nanometric thickness of a biocompatible material. It is practically difficult to find a material with optimal surface properties and physical and chemical characteristics to match the desired functionality [29]. Therefore, there has been a vast use of the improved and coated surfaces and consequently, several deposition technologies have been introduced and implemented to deposit the thin films for biomedical applications. As described in Chapter 2, there are several methods of thin film deposition and each has its own advantages and limitations. As such, the choice of the technique to employ for a specific biomaterial coating depends on the following: i. The properties of the material for thin films coating, ii. The properties of the substrate material, iii. The level of purity desired for the deposited thin films, iv. The coating thickness required, and v. The size of the substrate being used for the thin film deposition.

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The final thin film coating and its performance depend on the thin film deposition process and the conditions of deposition employed. For instance, the sputtering process is mostly dependent on the chemical properties of the discharge gas, the target material, the target distance from the discharged gas, and the pressure of the deposition process. As such, the correct combination of process parameters is key to achieving quality thin films for biomedical applications. Thin film materials have become an interesting field of research and application as far as the biomedical field and biomaterials are concerned. Examples of thin film materials or coatings that have found wide application in biomedical applications are compounds of carbon (diamond-­like carbon (DLC) and tetrahedral carbon), chromium nitride, silicon carbide and titanium (titanium alumina nitride, titania, titanium carbide, titanium niobium nitride), zirconium nitride, ceramics such as HA, bioactive glass, etc., [30]. The use of thin films in the medical and biomedical field has grown over the years, and it is safe to say that it is still in the development stages and there is room for more work and research. The practise is to deposit a biocompatible thin film (such as those stated above) onto the surface of a material with less or no biocompatible properties. During the deposition of thin films for biomedical applications, the following considerations should be put into account [31]:

i. The corrosion resistance of the film material: The deposited thin film material should improve the corrosion resistance of the bulk substrate in biological systems. ii. The wear and abrasion resistance of the thin film material: The deposited thin films should have higher resistance to wear than the substrate to ensure it protects it against abrasion failure during operation in the biological system. iii. Adhesion of the thin film material: The thin film material should have excellent adhesion with the bulk material to avoid delamination/failure of the coatings during operation in the human body. iv. Biocompatibility of the thin films: The deposited film material should have superior compatibility as compared to the substrate to avoid inflammation and infections in the human systems. v. Thermal stability of the films: The deposited thin film materials should be thermally stable to withstand thermal changes inside biological systems. vi. Proper mechanical properties: The thin film material should have proper elastic modulus and hardness for safe performance inside the body tissues. Besides manufacturing thin films for body implants, other applications of thin film technology in the biomedical field include but are not limited to the following:

i. Oncology research and treatment, ii. Delivery of drugs in the human body, iii. Dialysis, iv. Manufacture of membranes that are gas permeable, v. Biosensors (self-­healing and self-­cleaning), and vi. Blood oxygenation.

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There has been a widespread issue of resistance of bacteria to painkillers and antibiotics, hence raising the need to come up with and develop thin films that will help to prevent and mitigate eventualities such as proliferation or the adhesion of bacteria [32,33]. Paradoxically, the manufacture of thin films that can allow biofilms to be formed in the body has attracted keen interest from researchers. In the next subsection, the specific applications of thin films in the biomedical field are discussed.

6.3 SPECIFIC APPLICATIONS OF THIN FILMS IN THE BIOMEDICAL SECTOR 6.3.1 Hip Replacement The number of hip replacements has increased considerably over the past couple of years and is expected to continue rising due to increasing cases of osteoarthritis, ankylosing spondylitis, and injuries [34]. The hip replacement procedure is carried out by a surgeon, who will replace the patient’s diseased, worn out, or damaged joints. This replacement can be a full or a partial replacement, called hip arthroplasty. A full hip replacement will involve the replacement of the hip socket (acetabulum) and the femoral head, while a partial hip replacement procedure only involves the replacement of the femoral head. In this operation, the artificial joint is called a prosthesis and is usually made up of a bulk material coated with thin films. The replacement ensures the hip operates normally and eases the patient’s pain from the broken, ill, or wounded hip. The femoral head can be replaced with materials coated with thin films of either a metal, ceramic, or polymer. Some of the thin film metals that are used include cobalt-­ chromium, austenitic stainless steels, and cobalt-­chromium-­molybdenum alloys. The bulk materials (substrate) to be used should undergo various machining processes to ensure they are of the required size and shape before the appropriate coating is deposited on their surfaces. The surfaces of the substrate should also be polished before coating to avoid injury and pain to the patient. The use of ceramic coating materials in the femoral head lowers the surface roughness, coefficients of friction, and wear compared to metal-­coated femoral head. In contrast, however, the ceramic-­coated implants tend to be more brittle, which may pose a risk of failure. Thin film coatings for the femoral head should exhibit high hardness, high resistance to wear, high toughness, and low friction coefficient [35]. It is important to note that there may be several combinations of materials in the implant, i.e., the prosthetic to have improved physical properties. Some of the paired materials include a metal on a metal, a ceramic on a ceramic, and a metal and/or a ceramic on a polyethylene or a cross-­linked polyethylene [20]. For each combination, there will be advantages and disadvantages which will be exhibited. The cons can be reduced by using a thin film coating that is resistant to wear, corrosion, friction, and fatigue [35]. There are, therefore, so many examples of thin film coatings which have been created for artificial femoral head.

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Li et al. manufactured and investigated the vitro and in vivo performance of a-­C/ a-­C:Ti nanomultilayer coated Ti6Al4V artificial femoral head [36]. The thin films were deposited through magnetron sputtering method. Zirconia coatings toughened with alumina and other elements have been used as coatings for femoral head applications due to their bioinert characteristics [37–39]. TiN thin films are also used in coating the heads of artificial femoral implants [40]. Thin films of tetrahedral DLC deposited through pulsed laser deposition have been reported for artificial hip joints [41]. DLC thin films deposited on Co-­Cr substrate have also been evaluated for hip joint implants [42]. In conclusion, metal-­on-­metal hip implants are preferred to ceramics and polymers due to their low roughness, high wear resistance, and low chance of dislocation from its implantation position on the tissues. The most used substrate metals for hip joint replacement are stainless steel, cobalt-­based alloys, and titanium alloys.

6.3.2 Knee and Shoulder Prosthesis A knee or shoulder replacement surgery is usually carried out fully (or a total knee replacement) or partially. The knee and shoulder replacement or arthroplasty aims to treat a variety of degenerative and traumatic diseases or disorders of the shoulder. Total hip joint replacement is one of the most utilised treatments in the US today [43]. This kind of operation basically involves the replacement of a damaged knee or shoulder with a metal or plastic component that has previously been machined and thus shaped to allow for mobility in these joints. The main problem with prosthetic joints is that they usually experience wear and may even corrode after a long time of operation inside the biological tissues. Consequently, the patient may experience tissue inflammations, loosening of the implants, and even in extreme cases, osteolysis (which is the resorption of bone matrices by the osteoclasts). Due to the material sliding against each other at the shoulder and knee joints, the softer material will have abrasion and wear. The material may hence come in contact with body fluids leading to corrosion. As such, these prostheses should be coated with thin film materials that protect them against abrasion, wear, and corrosion. Thin films of DLC, Co-­Cr, and Ti-­based and ceramic-­ based alloys are suitable for thin film protection and can be used to coat prostheses for knee and shoulder implants.

6.3.3 Neural/Brain Implants Neural implants are devices that are surgically inserted into the nervous system to heal or assist its operation. Neurons communicate through electrical signals, and the implant devices are usually in the form of an electrode. With such implant devices, the researchers can hack into the operation of the nervous system of a human body and observe the patterns by which the neural circuit communicates to each other. As such, the development of electrodes for neural implants is on the rise due to its efficacy in treating a number of neural and traumatic disorders. Some of the specific

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applications of neural-­electrodes in modern medicine include but not limited to the following:

i. Deep brain stimulation (DBS): This is a widely accepted method in which an electrode is implanted on a specific site in the brain to treat neurological and psychiatric diseases such as Parkinson’s disease [44]. DBS reduces tremors and involuntary motor contraction for dystonia patients, and epileptic episodes, and is used to treat neuropathic pain. ii. Spinal cord simulation (SCS): This technique is used to treat diseases such as back and leg pains. The device consists of electrodes that are connected to a pulse generator and programmer. The electrodes are inserted into the body through an open surgery [45]. It is used to treat other conditions such as chronic pains arising from failed back surgery syndrome and angina pectoris. However, the application of the above electrodes is limited by the response of nervous tissues at the injury site [46]. The right combination of materials should be used for the electrodes to avoid inflammation and failure of the implants. The electrode device should be biocompatible and durable to protect the tissues from injury as well as allow the device to operate without failure. The correct choice of the implant material is crucial in mitigating body reactions and degradation of the structures through corrosion. Additionally, the electrode material’s electrical conductivity, chemical behaviour, and mechanical properties should be considered for the satisfactory performance of the implant devices [47]. The most commonly used materials for these devices are silicon and polymers as the substrates, whereas metals, conductive polymers, and CNT are used as electrode site materials [48]. Usually, the electrodes are designed as micro-­wires composed of metals such as gold, stainless steel, and tungsten. The wires are coated with insulators, and the most common insulating materials include glasses, Teflon, and resins. These insulators are deposited as thin film coatings on the micro-­wires using techniques such as physical vapour deposition (PVD), chemical vapour deposition (CVD), dip coating/chemical bath, and electro-­deposition [49]. Thin film piezoelectric Lead zirconate titanate (PZT) for cochlear implants was developed by Ilik and others [50] in which the PECVD method was employed. To improve on the stability of glass coverslips used as substrates for neuronal cell cultures, Piedade et al. [51] deposited a thin film of polyimide for a permanent modification of the substrates for better performance in biomedical applications. Several electrode designs have been developed for various neural implants as highlighted by Oldroyd and Malliaras [52]. Some of these include Si-­based substrates with conductive hydrogel coating, Si coating and Pt-­Ir as the micro-­wire, polyimide-­based substrates with gold as the wire, and poly (3, 4-­ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) as the coating or SiC as the coating. Thin film materials are also used in neuroscience to assist in the delivery of drugs to the neural system, cranial pressure evaluation, and monitoring. The major inhibitor of the implementation of thin film materials in the neurological field is the absence of mechanical and physical compatibility of the particular neuro tissue and the thin

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film material that has been used as the implant. The choice of thin film material for the specific application must therefore consider their biocompatibility, mechanical strength, and corrosion behaviour.

6.3.4 Protein Repellent Coatings There has been the need to come up with biomedical films that can have protein adsorption resistance. The reasons for preventing protein adsorption on the surfaces of biomaterials are listed below [53].

i. The adsorption of proteins on a particular biomaterial can influence cellular activities at the interface of the implant and the body tissues involved. ii. It leads to a series of chemical reactions that may lead to inflammatory/ immunological responses and blood clotting (coagulation) around the tissues. iii. Some blood protein factors tend to be activated by the presence of surfaces or films that carry negatively charged materials such as some polymers, polyvinyl chloride, and glass. iv. In the event macrophages adhere to neutrophils, an attack may occur on the biomaterial surface by some enzymes which could be detrimental in the long-­r un performance of the biomaterial. v. The protein adsorption and attack may eventually lead to the need for repeat procedures on the infected implants.

As such, protein adsorption is undesirable especially for biomedical equipment such as orthopaedic and surgical tools. A special focus has been on tetrahedral carbon, titanium carbide, titanium nitride, and carbon (DLC). It is important to mention that titanium carbide and titanium nitride usually undergo PVD to coat them on materials used as biomedical implants and equipment. They are extensively being used in the manufacture of medical equipment, for example, scalpel blades and orthopaedic saws. For these applications, titanium nitrides are preferred because they exhibit higher hardness almost four times that of cobalt alloys [54]. The surface roughness is also considerably lower. Its high rate of wettability with body fluids – synovial fluids – makes it the best for coating in such materials. The high scratch resistance also helps to prevent abrasion and any kind of damage or wear as a result of large loading capacities, and eventually the wear reduction is reduced. The thin film coatings made of titanium nitride also protect patients with the implants from any allergic reactions and any bacterial proliferation. The use of carbon coating films in the biomedical field has helped to provide conditions that are favourable for some biological processes to take place. The growth of some body cells such as macrophages, fibroblasts, and osteoclasts without unusual inflammations has been enabled upon using these thin film [55]. Therefore, the films help reduce the adhesion of platelets to each other, reduce and eliminate thrombogenicity, and improve the compatibility of the implants. The doping of these carbon films with nitrogen compounds improves their biocompatibility.

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Other thin film materials for protein adsorption include TiO2 [56], poly(dime­ thylsiloxane) [57], and titanium modified carbon [58].

6.4 EMERGING TRENDS ON THE APPLICATION OF THIN FILM MATERIALS IN BIOMEDICAL FIELD With the increasing demand for advanced materials in the biomedical sector, the research community and industry continuously develop new methods and materials for tissue engineering applications. The focus is on developing materials with better cellular attachment and proliferation of the body tissues [59]. There is a tendency to evaluate the various thin film preparation methods for manufacturing biocompatible thin films. The technologies showing a lot of attractions in the biomedical sector are layer-­by-­layer assembly, electrophoretic deposition, chemical vapour deposition, the sol-­gel process, dip-­coating, and pulsed laser deposition. Scientists are exploring the potential combination of these methods to undertake thin film deposition for superior and hybrid implant materials. Next, advancements of thin film deposition in the medical fields are illustrated.

6.4.1 Self-­Healing Biomaterial Coatings There is increasing attention to developing thin films which can cure or restore to their original state when impacted by conflict or diseases. In this way, the biomaterials’ life is elongated and their performance is enhanced in their applications as implants. There are so many self-­healing biomaterials ranging from ceramic, polymeric, and metallic systems available today. Shabani, Daraeinejad, and Ghofrani have detailed existing self-­healing polymeric biomaterials [60]. Some of the thin film polymeric materials include poly(glycerol sebacate) (PGS-­U) [61,62], poly(2-­ hydroxyethyl methacrylate)-­based materials, a multilayer self-­healing material consisting of MoS2 nanosheets, β-­CD-­modified poly(ethylenimine), and adamantine-­ modified poly(acrylic acid) (PAA), etc. [63,64].

6.4.2 Development of Hybrid Biomaterial Thin Films As stated earlier, each of the classes of biomaterials (ceramics, polymers, metals, and composites) has its advantages and limitations. Most biomaterials consist of a pure metal, a metallic alloy, a ceramic, or a polymer. Combining more than one class of materials would result in a superior biomaterial. There are efforts to develop biomaterial thin films consisting of a biocompatible metal/alloy, polymers, and ceramics [65]. Liao et al. [66] developed a hybrid biomaterial consisting of a pure titanium substrate, and thin film coatings of ceramic and polymers; the biomaterial exhibited improved corrosion resistance as compared to pure Ti biomaterial. Sasireka et al. [67] fabricated a Ti6Al4V/TiO2-­SiO2/polymer hybrid biomaterial; they demonstrated that the new biomaterial exhibits higher biocompatibility and can be used as an orthopaedic implant. Ti-­Mo/Ca(H2PO2)2) and (CaSiO3)/PLGA hybrid biomaterial was developed by Kazek-­Kęsik et al. [68], and it was shown that the biomaterial had better corrosion protection compared to Ti-­Mo alloy. A biomaterial consisting of

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Ti6Al4V substrate and multilayer films of PCL and HA was developed by Yusoff et al. [69]. Magnesium/ceramic/polymer biomaterials have also been developed [70]. In the future, thin film deposition methods can be explored to deposit free-­ standing thin films of ceramics, polymers, metals/alloys, and composites to prepare the hybrid nanostructured biomaterials. The free-­standing thin films can then be stacked together through severe plastic deformation techniques such as high pressure torsion and accumulative roll bonding. In this way, the severe plastic deformation causes nanostructuring as well as bonding between the multilayers of the films. Such thin film biomaterials will have superior properties and performance.

6.5 SUMMARY In this chapter, thin films for biomedical applications have been described. Biomaterials can be manufactured from the four classes of materials, i.e. ceramics, polymers, composites, and metal/metal alloys. Thin film technology can be explored as a surface engineering process to modify the surfaces of bulk biomaterials for better biocompatibility. The choice of the thin film material for biomedical applications is based on its corrosion, wear, adhesion, and biocompatibility characteristics. Other properties to consider include thermal stability, mechanical strength, and fracture toughness. Some of the specific applications illustrated in this chapter include hip implants, knee and shoulder prosthesis, neural implants, and protein repellent coatings. Through thin film technology, it is possible to develop novel drug delivery systems for target body parts. The development of electrodes to monitor the nervous system’s performance during some specific treatment has been possible through thin film technologies. In future, there will be the development of self-­healing and hybrid thin film coatings for superior performance and durability.

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41. A. S. Loir et al., “Deposition of tetrahedral diamond-­like carbon thin films by femtosecond laser ablation for applications of hip joints,” Thin Solid Films, vol. 453–454, pp. 531–536, 2004, doi: 10.1016/j.tsf.2003.11.135. 42. R. Murakami, D. Yonekura, D. Hagihara, Y. H. Kim, and K. Hirose, “Tribological properties of artificial hip joint material coated with DLC thin film in the simulated body environment,” Key Eng. Mater., vol. 297–300, pp. 1724–1729, Nov. 2005, doi: 10.4028/www.scientific.net/KEM.297-­300.1724. 43. E. R. Wagner, K. X. Farley, I. Higgins, J. M. Wilson, C. A. Daly, and M. B. Gottschalk, “The incidence of shoulder arthroplasty: Rise and future projections compared with hip and knee arthroplasty,” J. Shoulder Elb. Surg., vol. 29, no. 12, pp. 2601–2609, Dec. 2020, doi: 10.1016/j.jse.2020.03.049. 44. V. A. Coenen, F. Amtage, J. Volkmann, and T. E. Schläpfer, “Deep brain stimulation in neurological and psychiatric disorders,” Dtsch. Aerzteblatt Online, vol. 112, no. 31–32, pp. 519–526, Aug. 2015, doi: 10.3238/arztebl.2015.0519. 45. J. J. Song, “Present and potential use of spinal cord stimulation to control chronic pain,” Pain Physician, vol. 3;17, no. 3;5, pp. 234–246, May 2014, doi: 10.36076/ ppj.2014/17/234. 46. M. Gulino, D. Kim, S. Pané, S. D. Santos, and A. P. Pêgo, “Tissue response to neural implants: The use of model systems toward new design solutions of implantable microelectrodes,” Front. Neurosci., vol. 13, no. Jul, pp. 1–24, 2019, doi: 10.3389/ fnins.2019.00689. 47. D. F. Williams, “On the mechanisms of biocompatibility,” Biomaterials, vol. 29, no. 20, pp. 2941–2953, 2008, doi: 10.1016/j.biomaterials.2008.04.023. 48. R. Samba, T. Herrmann, and G. Zeck, “PEDOT–CNT coated electrodes stimulate retinal neurons at low voltage amplitudes and low charge densities,” J. Neural Eng., vol. 12, no. 1, p. 016014, Feb. 2015, doi: 10.1088/1741-­2560/12/1/016014. 49. GA Bartholomew, “Method and apparatus for coating metal strip and wire,” 1962. USA patent. 50. B. İlik, A. Koyuncuoğlu, Ö. Şardan-­Sukas, and H. Külah, “Thin film piezoelectric acoustic transducer for fully implantable cochlear implants,” Sensors Actuators, A Phys., vol. 280, pp. 38–46, 2018, doi: 10.1016/j.sna.2018.07.020. 51. A. P. Piedade, C. Veneza, and C. B. Duarte, “Polyamide 6.6 thin films with distinct ratios of the main chemical groups: Influence in the primary neuronal cell culture,” Appl. Surf. Sci., vol. 490, no. March, pp. 30–37, 2019, doi: 10.1016/j.apsusc.2019. 06.066. 52. P. Oldroyd and G. G. Malliaras, “Achieving long-­term stability of thin-­film electrodes for neurostimulation,” Acta Biomater., vol. 1, no. 1, pp. 1–6, May 2021, doi: 10.1016/j. actbio.2021.05.004. 53. D. R. Schmidt, H. Waldeck, and W. J. Kao, Biological Interactions on Materials Surfaces. New York, NY: Springer US, 2009. 54. W. Y. Wu et al., “Bioapplication of TiN thin films deposited using high power impulse magnetron sputtering,” Surf. Coatings Technol., vol. 362, no. August 2018, pp. 167–175, 2019, doi: 10.1016/j.surfcoat.2019.01.106. 55. G. Socol et al., “Biocompatible nanocrystalline octacalcium phosphate thin films obtained by pulsed laser deposition,” Biomaterials, vol. 25, no. 13, pp. 2539–2545, Jun. 2004, doi: 10.1016/j.biomaterials.2003.09.044. 56. J. L. Wehmeyer, R. Synowicki, R. Bizios, and C. D. García, “Dynamic adsorption of albumin on nanostructured TiO 2 thin films,” Mater. Sci. Eng. C, vol. 30, no. 2, pp. 277–282, Jan. 2010, doi: 10.1016/j.msec.2009.11.002. 57. K. Y. Chumbimuni-­Torres et al., “Adsorption of proteins to thin-­films of PDMS and its effect on the adhesion of human endothelial cells,” RSC Adv., vol. 1, no. 4, p. 706, 2011, doi: 10.1039/c1ra00198a.

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58. A. Wesełucha-­Birczyńska, E. Stodolak-­Zych, W. Piś, E. Długoń, A. Benko, and M. Błażewicz, “A model of adsorption of albumin on the implant surface titanium and titanium modified carbon coatings (MWCNT-­EPD). 2D correlation analysis,” J. Mol. Struct., vol. 1124, pp. 61–70, Nov. 2016, doi: 10.1016/j.molstruc.2016.04.050. 59. T. Fernandez, N. Rogers, and J. D. Whittle, “Thin film coatings for stem cell technologies,” in Thin Film Coatings for Biomaterials and Biomedical Applications, Hans J. Griesser, Ed. Sawston: Elsevier, 2016, pp. 197–223. 60. I. Shabani, Z. Daraeinejad, and R. Ghofrani, “Self-­healing polymers for biomedical applications,” in Reference Module in Materials Science and Materials Engineering, Abdul-­Ghani Olabi, Ed. New York: Elsevier, 2021, pp. 167–207. 61. Y. Wu, L. Wang, X. Zhao, S. Hou, B. Guo, and P. X. Ma, “Self-­healing supramolecular bioelastomers with shape memory property as a multifunctional platform for biomedical applications via modular assembly,” Biomaterials, vol. 104, pp. 18–31, Oct. 2016, doi: 10.1016/j.biomaterials.2016.07.011. 62. L. Vogt, F. Ruther, S. Salehi, and A. R. Boccaccini, “Poly(Glycerol Sebacate) in biomedical applications—A review of the recent literature,” Adv. Healthc. Mater., vol. 10, no. 9, p. 2002026, May 2021, doi: 10.1002/adhm.202002026. 63. B. Guo and R. Yu, “Self-­healing biomaterials based on polymeric systems,” in Self-­ Healing Polymer-­Based Systems, Sabu Thomas and Anu Surendran, Eds. 2020. New York: Elsevier. 64. Y. Han, X. Wu, X. Zhang, and C. Lu, “Self-­healing, highly sensitive electronic sensors enabled by metal–ligand coordination and hierarchical structure design,” ACS Appl. Mater. Interfaces, vol. 9, no. 23, pp. 20106–20114, Jun. 2017, doi: 10.1021/ acsami.7b05204. 65. A. Santos-­Coquillat et al., “Hybrid functionalized coatings on metallic biomaterials for tissue engineering,” Surf. Coatings Technol., vol. 422, no. July, p. 127508, Sep. 2021, doi: 10.1016/j.surfcoat.2021.127508. 66. S. C. Liao, C. T. Chang, C. Y. Chen, C. H. Lee, and W. L. Lin, “Functionalization of pure titanium MAO coatings by surface modifications for biomedical applications,” Surf. Coatings Technol., vol. 394, no. December 2019, p. 125812, 2020, doi: 10.1016/j. surfcoat.2020.125812. 67. A. Sasireka, R. Rajendran, P. Priya, and V. Raj, “Ciprofloxacin-­loaded ceramic/polymer composite coatings on Ti with improved antibacterial and corrosion resistance properties for orthopedic applications,” ChemistrySelect, vol. 4, no. 4, pp. 1166–1175, Jan. 2019, doi: 10.1002/slct.201803769. 68. A. Kazek-­Kęsik et al., “Hybrid oxide-­polymer layer formed on Ti-­15Mo alloy surface enhancing antibacterial and osseointegration functions,” Surf. Coatings Technol., vol. 302, pp. 158–165, Sep. 2016, doi: 10.1016/j.surfcoat.2016.05.073. 69. M. F. Mohd Yusoff, M. R. Abdul Kadir, N. Iqbal, M. A. Hassan, and R. Hussain, “Dipcoating of poly (ε-­caprolactone)/hydroxyapatite composite coating on Ti6Al4V for enhanced corrosion protection,” Surf. Coatings Technol., vol. 245, pp. 102–107, Apr. 2014, doi: 10.1016/j.surfcoat.2014.02.048. 70. M. Muñoz et al., “PLA deposition on surface treated magnesium alloy: Adhesion, toughness and corrosion behaviour,” Surf. Coatings Technol., vol. 388, no. December 2019, p. 125593, Apr. 2020, doi: 10.1016/j.surfcoat.2020.125593.

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Thin Films for Surface Protection

7.1 INTRODUCTION TO SURFACE PROTECTION As discussed earlier by Strafford and Subramanian [1], the emergence of surface engineering as a discipline is driven by the realisation of the importance of a surface to the functionality of an engineering component. The discipline, which encompasses physics, engineering, and material science, has also been driven by the various developments in treatment and coating technologies. It is the surface of the component that comes into direct contact (in most cases) with the extreme conditions of the operating environment. It is from the surface where failure of the component usually initiates in the form of a crack, pit, abrasion, porosity, etc. [2]. Thus, a need to have a surface with enhanced properties such as wear resistance, corrosion resistance, and reduced friction is important for the performance of an engineering part. Surface protection is a crucial aspect of tribology that involves the study of the science of the interaction of surfaces with operating conditions. Surface engineering is the process of modifying a surface of a component to induce enhanced properties to the component. There are two main categories of surface engineering: (1) surface modification and (2) surface coating. Surface engineering processes basically modify the microstructure and composition of the surface of a component, thereby improving the various surface-dependent engineering ­properties [3]. Surface coating technologies entail deposition of layers of semi-molten, molten, chemical, or vapourised material onto the surface of an engineering component. This results in enhanced surface functions that would otherwise be achieved by processing the bulk material composition [4]. The coatings can be achieved through various processes such as chemical vapour deposition (CVD), physical vapour deposition (PVD), chemical bath, and any other methods as discussed in Chapter 2. The coating processes tend to alter the chemical composition of the surface of the component. On the other hand, surface modification entails various high energy treatments destined to change or alter the microstructure of the near surface of the components. This can be achieved through processes such as case hardening and heat treatments such as carburising, nitriding, and carbonitriding treatments [5]. Surface protection is a very crucial technology in engineering. Various surface engineering applications for surface protection have been explored and documented in various literature [6]. Some of these applications are in the fields of aeronautical and transport industries, sports technology, petroleum and chemical industries, mining, food, and electronics. Also, various modern cutting applications require protection of the cutting surface with a thin coating to protect against abrasion wear during machining. Such tools can find applications in high-speed cutting, dry cutting, and DOI: 10.1201/9781003202615-7

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machining of very hard materials such as non-ferrous metals, super-alloys, and other advanced materials [7–9]. The thin coating on the surface of the cutting tool point enhances friction reduction and wear resistance [9]. Thin film materials for cutting tools are discussed in details in Chapter 8 of this book. Surface protection has a significant large contribution in the transport sector, which, by the year 1998, was utilising 6% of the total costs of manufacturing of engine and transmission system [10]. The application of surface protection through nanotechnology in the transport sector is endless [11]. The main parts in the transport sector requiring surface protection include those of vehicle components, power units, and permanent structures. In automotive systems, several parts require surface engineering for better performance and durability (Figure 7.1). For instance, surface coatings protect the power units from wear and erosion which is catalysed by conditions of high temperature and fluid flow. Components such as brakes (Figure 7.2) and suspension are protected with a thermally sprayed coating to enhance wear resistance, thereby increasing their operational life and service [12]. Engine parts such as piston and piston bores (Figure 7.2) should be surface engineered to protect them from extreme conditions of high temperature and pressure. Various exposed surfaces such as wheel arches and bumpers are coated with epoxy-based polymer to enhance corrosion and abrasion resistance. Polymer coatings also help to reduce noise levels in engineering components. Moreover, various fixed structures such as oil rigs and bridges are coated to reduce salt water corrosion and sand abrasion [4]. Component surface protection in the aerospace industry has been practised over the past five decades [11]. Surface coated gas turbines exhibit high temperature

FIGURE 7.1  Showing various parts of an automotive system that require surface engineering for enhanced performance [12]. (Reused with permission from Elsevier Ltd.)

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FIGURE 7.2  Specific auto parts which require surface coating for protection. (Obtained for free from pinterest.com.)

strength, bearing strength, and corrosion resistance properties [13]. Also, various thermal sprayed coatings are applied to a wide range of aircraft components to protect the parts against atmospheric corrosion. Aircraft gears and ball bearings are also coated through magnetron sputtering to protect them from damage arising from extreme temperatures and impact [14]. Surface engineering through nanocoating offers several benefits to the aerospace industry as depicted by Figure 7.3.

FIGURE 7.3  Showing benefits of nanocoatings to the aerospace industry. (Obtained under open access from [15].)

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In the sports industry, titanium oxide surface is used on various racing car components to enhance performance [16]. However, the application of this material is still under exploration and will be a crucial surface coating material in the 21st century. Golf club surfaces are also coated with tin and carbide coatings to reduce friction. Nanomaterials are finding applications in manufacturing comfortable sports fabrics that are air permeable, water-resistant, and self-cleaning. Sports shoes fabricated from nanomaterials exhibit water repellent and antibacterial properties [17]. Nanomaterials are used to manufacture tennis rackets and also used on floors, walls, and glasses of sports stadiums. The incorporation of nanopowders into polyurethane can improve the rebound resilience of running tracks. Therefore, surface protection of engineering components is a crucial technology that increases the service life and enhances the functionality of engineering components and thus has a vast area of application. More studies and exploration are ongoing to improve the various methods of surface protection of components for improved performance. In this Chapter, thin film materials for surface protection are discussed.

7.2 THIN FILM MATERIALS IN WEAR PROTECTION Thin film technology has a great impact on modern manufacturing and can be considered as the backbone of advances in the application of surface engineering in the wear protection industry. As stated in Chapter 1, a thin film is a layer of material whose thickness ranges from several nanometres to a few micrometres [18]. The structure of thin film can be either polycrystalline or amorphous depending on the nature of the film material and the conditions of preparation. Thin film deposition can be achieved through a wide range of deposition techniques such as PVD and CVD methods [19]. Thin film technology has resulted in the creation of materials with superior strength and wear resistance for applications in areas of constant abrasive wear and high frictional forces. According to Rabinowicz’s principle of abrasive wear when two (harder and softer) materials are in contact, abrasive wear occurs due to the harder material eroding away the softer material [20]. Therefore, to prevent abrasive wear on a given material, the softer material is made from a harder material in comparison to the contact material. This is applied in the tooling sector where the cutting tool needs to be made from a harder material than the workpiece to prevent abrasive wear on the tool surface. Abrasive wear of components is undesirable, and surface engineering is key in preventing its occurrence. In fact, wear-driven failure results in loss of dimension and thus the functionality of a component. With the increased need for machining and manufacturing, tool development has gained popularity. To enhance the performance of cutting tools, they can be reinforced through a localised surface coating of the cutting edge. With developments in machine tools and machining methods, thin protective coating has gained popularity for coating of tools [21]. These machining operations involve high abrasion and temperature generation, and as such, thin film materials used should be able to withstand those conditions. In high-speed cutting operations, such as high-speed milling, and machining of high hardness materials such as titanium and other ferrous metals and alloys, the cutting edges of the various tools used are usually coated with thin film of

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abrasive wear-resistant coatings such as diamond and Titanium nitride. Some of the thin film coatings of cutting tools are TiAlN [22], TiCN/Al2O3 coating on 4340 steel tool [23], AlTiN [24,25], TiAlCr, WC-Co [26], nitrides of Cr, Mo, Zr, Nb, and Al [27]. In the automotive and aerospace industry, power transmission elements utilising gears dissipate enormous amounts of power and are exposed to high temperatures. In high stress applications such as the engine or gearbox, there are regions where metal movement against another metal forces away lubricating oil, resulting in the build-up of heat, pressure, and friction. This extreme pressure, heat, and friction lead to wear, breakage, or welding together of the components. There is thus a need to reduce the energy consumption of these elements in addition to higher speeds, torque, and lower weight and noise levels [28]. For this reason, gears in power transmission systems are coated with thin films of various materials to enhance wear protection and increase service life. Some of the coatings for gear applications include nano-TiC [29], Mn3(PO4)2 [30], multilayer MoS2, WC-Co-Cr [31], etc. [32]. Chromium nitride thin films can be deposited onto acrylonitrile butadiene styrene (ABS) substrate to enhance wear resistance. Since ABS is among the most highly consumed polymers, manufacturers must manufacture parts or components that are wear-resistant using ABS. Sukwisute, Sakdanuphab, and Sakulkalavek [33] studied the wear resistance improvement of ABS by coating with chromium nitride and established that for a deposition made by DC magnetron sputtering process, various factors such as sputtering power influence the crystal size, crystal orientation, hardness, and elasticity of the deposited thin film wear material. Surface coating is also carried out on mechanical fasteners. Fasteners are components used to fix two or more components together. They include nuts, bolts, screws, studs, and tapping screws and are made from alloy steel or carbon steel. Fasteners used in high temperature and pressure applications are coated with a thin film of wear-resistant material to enhance the wear resistance of the fastener and therefore increase its service life [34]. Some of the films used for such applications include polytetrafluoroethylene (PTFE), TiN, and MoS2. With the increasing use of magnesium alloys due to desirable properties such as low densities and high specific strength in areas such as engine block and power train components, there is a need to improve the wear resistance properties of magnesium alloys [35]. To prevent wear, various surface engineering technologies are used. Ceramic coatings are applied to magnesium alloys with the aid of laser surface engineering [36]. Therefore, with the greater need to increase the service life of various components operating at harsh conditions, surface coatings provide a means to achieve wear resistance properties onto a material without affecting the component functionality. In general, coatings for wear protection should exhibit the following properties:

1. They should be able to withstand high temperature conditions. 2. They should possess good strength. 3. They should exhibit chemical inertness. 4. They should have attractive thermal fatigue. 5. They should have low coefficient of friction. 6. They should be affordable.

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Thin film coatings for wear protection can be classified into various groups, namely, transition metal group, post-transition metal group, reactive non-metal group, ­lanthanides group, and hybrid group [37]. Some of the examples of transition metal coatings for wear protection include TiN, TiB2, Ni-W2 composite, Cr, CrC, CrN, TiCN, Stellite, etc. Examples of post-transition metal and reactive non-metal coating include Al2O3, alumina-yttria-benzotriazole, N-DLC, PtRuN-DLC, diamond, etc. Examples of lanthanides coatings include those based on cerium, lanthanum oxide, praseodymium, gadolinium, neodymium, samarium, Yttrium, etc. Some of the hybrid coatings include (TiB+TiC)/Ti64, Ni-P-Ag-Al2O3, TiN/MoN, TiN/ZrN, TiN/NbN, TiN/TaN, CrN/AlN multilayer, TiCN+TiC+TiCN+Al2O3+TiN, etc.

7.3 HYDROPHOBIC AND HYDROPHILIC THIN MATERIALS For the past few decades, various researches have been carried out aiming at mimicking various aspects of nature through induction of non-wettability to wettable surfaces and vice versa [38]. A most notable natural anti-wetting surface is the lotus leaf which has a contact angle greater than 150° and has self-cleaning properties leading to water repulsion. Anti-wetting and anti-staining properties of surfaces have since become a major interest in surface engineering due to the desire to increase the applicability of various materials in a wide range of operations. That is, a component could be effectively made from a wettable surface material, the surface coated with a non-wettable thin film, and the component used in a humid environment efficiently. In this respect, there are three classes of thin film coatings, hydrophobic, hydrophilic, and superhydrophobic, and can be differentiated by the water contact angle as shown in Figure 7.4.

FIGURE 7.4  Illustrating the water contact angles for different surfaces (a) hydrophilic, (b) hydrophobic and (c) superhydrophobic [39]. (Copyright 2021, Elsevier.)

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A hydrophobic surface is a surface that repels water and other solvents, and its water contact angle is usually above 90°. Various topographical elements at microor nanoscale and lower surface energy are actuated by the non-adhesiveness [39]. Hydrophobic thin film materials have attracted a lot of interest and applications in the gadget industry with typical areas in electrical and electronic gadgets, microcontrollers, and microvalves. In electronic gadgets, components made from hydrophobic thin film surfaces are waterproof and can be effectively used in humid environments or underwater. For instance, watches and smartphones can be conveniently used even on exposure to water or humid conditions such as when it rains or during winter. Such gadgets can be effectively used in the sports industry, especially in water sports. This has resulted in an increase in the areas of operations for such devices. Digital cameras can now be used to take pictures and videos of aquatic life underwater conveniently. Various microcontrollers and microvalves are coated with hydrophobic thin film materials to make them waterproof, enhancing their functionality in aquatic/moist conditions. Therefore, hydrophobic thin film technology has ultimately brought about a huge change in the gadget industry, as it has enhanced gadget functionality in the presence of water or moisture, decreased dirt retention, enhanced their self-cleanability, improved corrosion resistance, and also increased service life due to reduced wettability and fouling. Some of the hydrophobic coatings include Cu2O [40], AZO thin films [41], and polymer-based coatings [42,43]. Whereas hydrophobic thin films repel water, hydrophilic thin films attract and absorb water and moisture. As stated earlier, hydrophobic surfaces have a contact angle greater than 90°, whereas hydrophilic surface has a contact angle less than 90°. A contact angle refers to the angle between the solid–liquid interface and liquidvapour interface and is dependent on the water droplet volume and the inertial force imposed by gravity. A hydrophobic surface has poor adhesiveness to water, poor wettability, and low solid surface free energy. A hydrophilic surface, on the other hand, has good adhesiveness to water, good wettability, and high solid surface free energy [44]. Superhydrophobic surfaces are highly water repellent, whereas superhydrophilic are highly water absorbers. These two concepts are applied to the coated thin films depending on the desired operating condition [43]. Hydrophobic thin films have since experienced applications in a variety of non-wettability and anti-staining operations. Hydrophobic and superhydrophobic films have unique properties such as being waterproof, corrosion-resistant, and stable against various biofouling pollutants [45]. For instance, as reported by reference [46] superhydrophobic electrodes are used to enhance electrolytic battery performance. Silicon electrodes that have hydrophobic thin film surfaces effectively separate active electrodes from liquid electrolytes developing a high interfacial capacitance acting as a high storage centre. Moreover, hydrophobic thin films can also be used as charge storage devices as outlined in reference [47]. Hydrophobic thin films can also be used to coat microvalves. Lu et al. [48] developed a hydrophobic fish bone–shaped valve that lowered protein folding characteristics and reduced protein binding onto the surfaces. Reference [49] has also discussed the application of hydrophobic thin films in microvalve structures. In both cases, plasma treatments in which anisotropic oxygen followed by C4F8 are deposited on surfaces to create nanotextured patches of hydrophobic coatings.

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Hydrophobic thin films are also coated onto textiles to enhance non-wetting and anti-fouling properties. Superhydrophobic textiles are required in many applications due to their effective water-repellency capability [50]. Moreover, various kinds of hydrophobic fabrics and fibres are used to separate water from oil or other solvents as outlined by reference [51]. The oil was strained out of the water by pouring the solution over a fabric in this method. The fabric was hydrophobic and thus resisted the passage of water. The fabric could also be dipped in an oil–water solution where it would absorb the oil and repel water. Hydrophobic thin films are also used as protective coatings in applications such as solar cells, mirrors, car windshields, and window glasses where transparency and anti-reflectibility are required. A superhydrophobic car windshield is required to enhance visibility in the event of rain or in humid environment. Hydrophobic thin films can also be used to coat components that are susceptible to corrosion to enhance their corrosion protection. For instance, the protection of copper substrate against corrosion can be achieved by coating zinc oxide and polydimethylsiloxane layers [52]. Although hydrophilic thin films absorb water, they have an immense application in the separation of solvents. For instance, a hydrophilic-coated surface is dipped in an oil–water mixture to separate water from oil. Water is thus absorbed and hence separated from the oil. As outlined in reference [50], hydrophilic thin films/fabrics exhibit several benefits over microporous materials. Some of these advantages include the following:

i. The preparation of microporous materials is complex and involves several processes and equipment unlike in hydrophilic coatings, which can be applied by conventional solvent coating equipment. ii. The microporous materials on fabrics can become contaminated through dirt, particles, and oils. In addition, micropores can become enlarged or smaller when the fabric substrate is stretched, which affects their waterproofing properties. On the contrary, hydrophilic films/coatings are not affected by the cleaning and stretching of fabrics and are durable. iii. Hydrophilic coatings such as polyurethane exhibit excellent adhesion on fabric substrates and have better gloss and resistance to water and solvents as compared to microporous materials. iv. Hydrophilic films do not cause leaks on fabrics, unlike microporous materials which can cause leaks on the fabrics. The other benefits of hydrophilic films include high strength, toughness, resistance to several chemicals and solvents, antimicrobial resistance, and excellent odour barrier characteristics. Although thin films’ superhydrophobic, superhydrophilic, and anti-fouling properties have found an immense breakthrough in various applications, a longer-lasting surface with hydrophobic or hydrophilic properties is difficult to maintain. The surfaces have shown deterioration with time, and therefore more research is necessary to create hydrophobic and hydrophilic thin films with longer service life.

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7.4 THIN FILM MATERIALS FOR CORROSION PROTECTION Thin film technology incorporates the use of various materials based on their properties to achieve the purpose of the coating performance as protective layers. The protection achieved includes corrosion resistance, wear and tear resistance that leads to improved durability of the substrate surfaces [37]. Materials for thin film coatings should exhibit inert chemical behaviour, good strength, and excellent thermal stability [53]. These properties ensure that the substrate material is protected against degradation by the corrosive environment. Corrosion manifests through the deterioration, damage, or destruction of a material surface due to its reaction with the environment. The corrosive environment may be composed of chemicals such as acids or bases. Corrosion can be detrimental and costly. As such, surfaces of components utilised in various fields should be protected against corrosion degradation. Thin film coating technology has served this purpose at length with extensive research on materials for corrosion protection. There are, therefore, several thin film materials used for corrosion protection. Some of these materials include aluminium oxide, titanium oxide, polymers, TiN, TiAlN, AlCrN, diamond-like carbon (DLC), tungsten disulfide (WS2), and Stellite.

7.4.1 Aluminium Oxide Aluminium oxide is an amphoteric oxide, which means that it exhibits both acidic and basic properties [34]. It has a monophasic structure and is characterised by the highest state of oxidation. As such, it has excellent hydrodynamic stability, chemical inactivity, and excellent corrosion resistance. Other properties include good stiffness and hardness, and resistance to wear and abrasion. However, the material exhibits poor flexural strength and fracture toughness. Aluminium oxide is an attractive protective layer due to its high impedance and low dissolution into chemical solutions such as NaCl. It is resistant to most acids and base media. Besides being chemically stable at high temperatures, it has a high refractive index, and good mechanical strength [54]. Aluminium oxide thin film can be deposited at a temperature of 150°C–400°C using the atomic layer deposition (ALD) method on stainless steel substrate to enhance its corrosion protection behaviour [55]. Al2O3/TiO2 prepared through ALD has been shown to be effective in protecting copper substrates from NaCl corrosion [56]. Due to its chemical inertness, aluminium oxide thin films promote osteointegration since it does not induce any biological or chemical activities at the interface with the biological systems.

7.4.2 Titanium Oxide Titania (TiO2) exhibits high hardness, wear-resistance, and anti-corrosion properties. Titanium oxide thin films are stable and can withstand the action of various corrosive media. Its corrosion resistance is enhanced by the low density of porosity in the structure [57]. It has excellent adherent properties onto substrates, thereby enhancing its corrosion prevention. However, it is susceptible to attack by fluorides and

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hydrogen peroxide [58]. It also exhibits excellent chemical resistance and biocompatibility and is therefore suitable for medical coatings and implants. It is non-toxic and has excellent optical and electrical properties. It finds applications in anti-bacterial protection, water purification, solar cells, and many other fields [59]. Similar to aluminium oxide, titanium oxide thin films are grown at a temperature ranging from 150°C to 400°C using ALD and related CVD processes. The film thickness is usually between 170 and 300 nm. Growing of the film at lower temperature results in the formation of an amorphous layer, whereas deposition at very high temperatures results in the formation of a polycrystalline layer. At temperatures above 300°C, a rutile structure is formed. It is therefore vivid that the structure of the thin films/layers of titanium oxide generally depends on the formation’s­ temperature [60]. Mostly, TiO2 can occur in three crystalline forms, namely anatase, rutile, and brokite [61]. The rutile structure, however, is too coarse and therefore corrosion solution can easily penetrate. It is hence advisable to create the films at low temperatures for better corrosion resistance of the material. The use of PVD techniques such as magnetron sputtering is attractive in the formation of the films at lower temperatures compared to the CVD processes.

7.4.3 Aluminium Oxide–Titanium Oxide To overcome the challenges of the above thin films’ materials, a hybrid (or a multilayer) composed of aluminium oxide and titanium oxide thin films can be used as a protective layer on surfaces. In this combination, the aluminium oxide provides a grain-free boundary dense structure, whereas titanium oxide inherently offers chemical stability [55]. As discussed in several literatures, aluminium oxide exhibits less porosity as compared to titanium oxide, which is usually susceptible to the formation of porous structures during nucleation [62]. Therefore, in multilayer or hybrid coatings, each material compliments one another in terms of their properties.

7.4.4 Tantalum Oxide (Ta2O5) Tantalum oxide is an oxide of element tantalum whose atomic number is 73. It is a transition metal oxide that exhibits attractive electrochemical reversibility. It is passive and cannot be attacked by hydrofluoric acid and fluoride ions in acidic solutions. It has an attractive bioactive response and is stable when in contact with biological tissues [63]. It also exhibits high stability at elevated temperatures. These properties give the thin film layer excellent protection of the substrate against corrosion. They have been used for corrosion protection on SiC and Si3N4. On aluminium substrates, these oxide coatings have been illustrated to reduce pitting corrosion in sodium chloride solution [64]. These oxides are used as implants due to their biocompatibility properties [63]. They are also used in the fabrication of field-effect transistors (FETs) because it exhibits low drift and high sensitivity. The material is also used in the fabrication of pH sensing electrodes [65]. It has excellent antireflective properties and is therefore used in optical and photovoltaic gadgets such as solar cells.

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7.4.5 Titanium Nitride Titanium nitride (TiN) is a ceramic material that is characterised by high hardness and high chemical and thermal stability. They have high corrosion, wear, and heat resistance [66]. TiN–based thin film coating for protection is usually applied using PVD methods, such as sputtering and ion plating, and CVD methods [66]. These coatings are used as implants for hip, shoulder, and knee due to their resistance to scratch, enhanced wettability, and physiologically inert [67]. The coatings are not carcinogenic and do not induce toxic effects on exposure to biological tissues.

7.5 TREND AND PROGRESS OF THIN FILM TECHNOLOGY IN SURFACE PROTECTION INDUSTRY The production of thin films as a means of surface protection and for other purposes is on the rise in the 21st century, with the motivation being its role in modern science, technology, and industrial processes. The focus of research is on producing high-quality thin films for enhanced surface protection of materials and components. There is an increasing focus on developing binary, ternary, and multicomponent thin films for better corrosion and wear protection of materials and components. Some of these films include TiC, TiAg, TiCAl, TiCAg [68], Mo-B-C, Mo-Fe-B [69,70], BCN [71], etc. On the same line of thought, several multilayer thin film coatings for wear and corrosion protection have been developed and more are being produced. The protection industry is quickly adopting thin film protection over other technologies due to the vast advantages that come with the use of thin film technology as a means of protection against corrosion. The former dependence on painting as a means of protection for vehicles and equipment subjected to harsh environmental conditions is soon being replaced completely with the application of thin film coatings. This is already being applied in the military, marine, and aerospace industries in which the machines and equipment in use are subjected to harsh conditions in which corrosion is likely to occur. Traditionally, zinc has been used in the marine industry to provide cathodic protection to marine vessels. However, the advancement of thin film technologies will now enable the coating of marine vessels with protective thin film inert materials such as ceramics and polymeric films which offer stronger and durable protection against corrosion. The growth in automotive and other manufacturing industries has seen a rise in the need for hard coatings. Since hard coatings produced by PVD and CVD provide durability and less need for maintenance, it is postulated that research and development of these technologies for hard coatings shall continue to grow. There will be a focus on developing eco-friendly and self-healing thin films for hard coating industry due to pressure from several legislative provisions that prohibit toxic materials such as Cr-based thin films and coatings [72]. The laser market is expected to grow from USD 11.7 bn in 2020 to USD 17.6 bn by 2025 (www.marketsandmarkets.com). This is due to its increased importance in healthcare and other sectors. The laser market is a significant driver of reflective dielectric and protective coatings. Hence, it is clear that the industry market for thin film coatings will rapidly grow and so is the thin film protection industry. More

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technologies are coming up with the vast research aimed at improving coatings and coating technology. Thin film materials for laser applications should provide both optical and protective properties, and, therefore, there will be growing development of new materials for these applications in future. In conclusion, it is clear that the trend and progress of thin film technology in the surface protection industry is on the growing trend. This trend is a signal that thin film technology that provides nanomaterials for the protection industry offers better performance than the other technologies. As such, thin film technology will dominate the surface protection industry in the near future with the development of more new thin film coatings for the industry. The vast advantages of thin film protection, which range from efficient and durable protection to the addition of aesthetic value to the coated products, are a clear show of the fact that thin film coating and protection is going to revolutionise the protection industry.

7.6 SUMMARY In this chapter, thin film coatings for surface protection have been discussed. The coatings for the protection industry should possess high strength, hardness, chemical inertness, thermal stability, and low coefficient of friction. Thin film coating technologies have been extensively used to produce coatings for enhancing the wear and corrosion resistance of materials and components. In fact, thin film technology has been significant for surface engineering in modern times. The chapter has discussed some of the thin film materials for wear and corrosion protection. The key aspect to note is that the protection industry is growing very fast with the advancing technologies in various fields. The need for high performance parts in aeronautical, sports, and automotive industries demands thin film coatings with superior properties of wear and corrosion behaviour. The protection industry also demands hydrophobic and hydrophilic coatings and some of these coatings have been discussed herein. The sports industry, especially water sports, requires waterproof gadgets (e.g. watches); these gadgets are coated with hydrophobic thin film coatings to enable them to repel water for effective performance.

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Thin Films for Cutting Tools

8.1 INTRODUCTION TO HIGH-­SPEED MACHINING The main focus of this chapter is to discuss high-­speed machining (HSM) technology, various methods of HSM, and their applications. The importance of coating of cutting tools for machining processes and the various thin film materials used in the coating of cutting tools for HSM and other machining operations in the modern industry and state-­of-­the art progress in thin film materials for the cutting tool industry are presented. In recent years, significant advances have been made in machining technology, with a great aim to increase machining efficiency and accuracy, and improve the quality of the work workpiece, while minimising machining cost and time [1,2]. The machine tool made for HSM does not only require high spindle speeds but should also operate at high accuracy, high feed rates, and high loading conditions [3]. HSM techniques are widely applied in the manufacturing of aerospace parts, automotive components, dies, other precision components, household appliances, and micromachining [1]. Initially, the HSM machining techniques were developed for machining aircraft and missile aluminium components. However, due to the advancements of HSM technology, researchers, engineers, and industrialists have developed HSM tools that can machine hard materials, nano features, as well as three-­dimensional geometries with high precision and accuracy [1,4]. HSM processes are characterised by the following features: • • • • • •

It enhances machine tool utilisation. It increases the quality of the component. It reduces workpiece inventory. It lowers manufacturing costs. It decreases machining lead time. It shortens the process chain.

8.2 APPLICATION OF HSM HSM finds applications in many industries, with most of them being the automotive, aircraft, and die and mould manufacturing industries where it is said to increase accuracy and reduce machining time while reducing cost [5]. In die and mould manufacturing, HSM machining produces surfaces and cavities of very close tolerances and accuracy. Some of the specific applications of HSM are discussed next.

DOI: 10.1201/9781003202615-8

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8.2.1 Aerospace Applications High accuracy and good finish are required for components in the aerospace industry. This means that the machining process should not impart defects on the part. Materials such as titanium and nickel alloys are used in the manufacture of aerospace engine parts. Titanium exhibits good corrosion resistance and low density even at high performance temperatures of the aerospace engine. However, the application of titanium alloys is limited by the fact that it is a hard-­to-­cut alloy [6]. Several properties make titanium alloys hard-­to-­cut materials, some of which include the following [7]: • The alloy has low heat transfer characteristics and therefore causes accumulation of heat around the tool–work interface. • There is the generation of high thermal stresses at the cutting edge of the tool. • Due to high temperature generation, there is clogging of the tool surface leading to poor surface finish. • There is the occurrence of material diffusion, leading to wear of the tool. • There are high cutting forces involved due to the hard nature of the alloy, which leads to chattering and eventual failure of the tool. Other aero-­engine materials such as hardened steels and structural ceramic encounter a serious challenge during machining because of their properties such as hardness, strength, wear, and chemical resistance. Therefore, these materials are hard to machine and pose a challenge during manufacturing components because of the high stresses and temperatures developed at the tool–work interface during machining. These materials exhibit poor heat conduction; this leads to the accumulation of high temperatures on the surface of the parts being machined. The high temperatures accelerate the tool wear, consequently lowering the tool life and increasing the cost of the machining process. In general, these materials (for aerospace engine applications) have the following characteristics [8]: • They have a low thermal conductivity. • They undergo rapid work hardening during machining due to their austenitic matrix. • They retain their strength even at elevated temperatures. • They tend to cause build-­up edges (BUE) and stick (weld) onto the surfaces of the cutting tools. • They react with cutting tools under the atmospheric temperature and humidity. These properties result in rapid wear of flank, crater, and nose notching during machining. Due to the above-­mentioned challenges, HSM technology is the most crucial technique in the aerospace industry for machining of alloys such as aluminium 7075 alloys [9], Inconel 718 [10], and Ti6Al4V [11]. The HSM techniques overcome

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the challenges faced by conventional machining techniques of these materials. The ability to choose the cutting parameters such as feed rate and cutting speed, while ensuring low cost and maintaining high precession and high quality, makes HSM technology crucial in the aviation industry [12]. The use of coated tools during HSM of titanium and other aerospace materials has been shown to overcome the challenges associated with the conventional machining methods of such materials [13].

8.2.2 Die and Mould Manufacturing HSM is applied in making dies and moulds for injection and blow moulding processes. HSM is suitable because the dies and moulds are always small in size and it becomes economical to perform all the operations at once. Also, some of the moulds have deep cavities which require a good machining approach that can be achieved in HSM. The HSM is adopted in mould and die machining because it offers a quality surface finish, reduced machining time, and enhanced tool life [14]. The hardened AISI H13 tool steel, AISI 4340, alloy steel, and cast iron moulds have been machined via HSM [7,15], and it is possible to obtain moulds with lower surface roughness.

8.2.3 Automotive and Other Manufacturing Industries HSM has found application in the production of automotive components [16]. This has been motivated by the reduction of cost, which is a result of application of HSM. Reduction of parts cost in automotive plays a great role in lowering the overall cost of a vehicle [17]. It can also be applied in the production of computer parts and medical devices. Generally, HSM is applied in the areas which require high material removal rate, low machining temperatures, increased/high cutting speeds, and lower cutting time.

8.3 ADVANTAGES AND DISADVANTAGES OF HSM The main advantages of using HSM in the industry are: • Higher producibility In these techniques, there are higher feed rates and high materials removal rates. The processes also machine hard surfaces to give the desired quality of surface finish. This technique also eliminates the need for manual grinding or other finishing processes, which leads to shortening of process time while significantly improving process productivity. • High-­quality surface Low amounts of forces are used in the machining process, making the workpiece be at a lower temperature, meaning low defects during machining and high accuracy of machining.

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• Substantial cycle time reduction The high labour forces applied in the cutting of hard materials may result in a significant reduction of efficiencies and increased stress. HSM performs hard material cutting easily within a short duration of time at a higher efficiency.

8.3.1 Disadvantages of HSM • Due to high accelerations, fast start and stop of spindles leads to fast wearing of the guideways, spindle bearing, and ball screw, causing an increased maintenance cost. • HSM requires skilled personnel who can programme the machining equipment and operate it. • HSM technique requires proper preparation in terms of process preparation and planning as well as taking precautions and ensuring the safety of both the workpiece and operator. • HSM processes require special machine tools.

8.4 HSM TOOLS MATERIALS The materials for HSM tools should possess properties that can withstand operating conditions such as temperature and applied forces. Some of these materials include cemented carbides (including other materials coated with carbides as they have improved hardness), polycrystalline diamond, ceramics, and polycrystalline cubic boron nitride.

8.5 HSM METHODS AND THEIR APPLICATIONS There are several HSM techniques. The key techniques used in HSM are: • • • • • •

Trochoidal Machining Plunge Roughing Radial Chip Thinning Side Steps Cornering Smart Machining

The readers are referred to reference [1] for a detailed description of HSM processes.

8.6 IMPORTANCE OF COATING CUTTING TOOLS FOR MACHINING PROCESSES It is common knowledge that cutting tools should be harder than the material being machined. Also, due to the high heat evolved during the machining process, cutting tools are subjected to very high temperatures that cause thermal stress on the object [18]. The high temperatures may lead to wear of the tool, leading to a poor-­quality

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surface finish of the machined surface. These are some of the inefficiencies of cutting materials that are the subject of coating. Thus, coating is important in improving the properties of the cutting tool material and enhancing their protection against the extreme conditions of wear during machining. Roles of thin film coating on cutting materials are listed as follows: • To improve hardness: Cutting materials such as steel may tend to be softer for effective application as a cutting tool material. High machining (operation) temperatures can contribute to the softening of the cutting tool material. In such cases, it is essential to coat the cutting tool with a material that improves the hardness and increases the lifespan of the tool. Tungsten carbide (WC) is an example of a coating for such application due to its high hardness [19–22]. • To improve tribological properties: Cutting involves abrasive contact between a tool and workpiece and as such, high friction is involved. During machining, tools undergo wear due to the high friction. Coating materials act sacrificially to protect the cutting tool material from wear and tear. Coatings are also used to improve the lubricating properties of the cutting tool that prevent debris build-­up at the cutting edge [21]. • To improve thermal properties: High thermal conductive materials are used to improve the thermal conductivity of the cutting tool material. These materials essentially act to draw heat away from the cutting edge, thus preventing softening as in the case of steel and other metals and alloys [23–25]. • To improve chemical stability: Cutting tools material may react with the surface of the material being machined or even with the cutting coolants. This may cause chemical corrosion of the tool, hence reducing the life of the tool. Chemically inert coatings such as titanium boride are employed to induce chemical stability. As long as the coating is intact, the cutting material is protected from corrosion of the machining conditions [21,23,26]. • Other roles of coatings in cutting tools include improving fatigue resistance, fracture strength, compressive strength, stiffness deformation resistance, and bending strength.

8.7 COATING OF CUTTING TOOLS FOR HSM 8.7.1 Classification of Coating Materials Used in Cutting Tools a. Single-­layer coatings These coatings consist of a single layer of thin material. The most common method used in establishing these coatings is physical vapour deposition (PVD) [21,23]. b. Multiple-­layer coatings These coatings consist of two or more materials layered on top of each other. The chosen materials are usually meant to improve the pressure adaptability of the cutting tool. Usually, the mid-­layer is added to improve

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surface adhesion between the substrate material and the coatings. The plasticity of these materials improves the resistance to the spread of cracks from one layer to the other [21]. c. Super-­lattices Super-­lattices are layered coatings at the nanoscale level. These materials are known to greatly improve the hardness and strength of the cutting tool. The very thin layers prevent the formation of dislocations, while the difference in the layers’ elasticity prevents the dislocations movement from one layer to another. The only downside of these coatings is that they have high failure rates if not layered in the correct order [21]. d. Nanocomposites These are coatings in the nanoscale level and consist of one or two crystalline phases. They are of two different types, which include those that have mixed phases and those with a combination of two phases. The phases may be made of two crystalline materials or a crystalline and an amorphous material [24,26,27]. Oxides, carbides, borides, and silicones are used in making nanocomposite coating materials [19,28]. Coatings made for high-­speed cutting tools are chosen to complement the properties of the cutting tool material. These coating materials can generally be classified according to their material constituents which are grouped as nitrides, carbides, and oxides. Other special cases include molybdenum, disulphide, and diamond.

8.7.2 Nitrides These are metallic compounds that possess nitrogen ions as an anion in the structure. Most nitrides are chemically stable with very high oxidation resistance. Some of the nitrides finding applications for HSM cutting tools include the following. 8.7.2.1 Titanium Nitride (TiN) This is the most common coating used in cutting tools. It is a choice material due to its high wear resistance. TiN is a ceramic material and exhibits the following properties: • It has a hardness ranging between 2400 and 2600 HV. • TiN is inert and stable and does not oxidise until up to a temperature of 850°F. • It is biocompatible and does not react with tissues, body fluids, and bones. • The deposition of TiN coatings is desirable at low temperature conditions. • It has a dark gold appearance. • It has good adhesion on most substrates, which makes it suitable for various cutting tools. It has been shown that these coatings should be around 7 µm thick or lower since thicker coatings crack easily. Generally, the thickness of TiN coatings for cutting tools should be optimised due to the following reasons [29]:

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• Optimised coating layer ensures efficient machining. • Optimised coating layer provides high wear resistance. • Optimised coating layer reduces cost and increases productivity during the machining process. • Optimised coating extends tool life for better surface finish. TiN provides resistance to wear and a reduced coefficient of friction to the cutting tools. The surface protection for the cutting tool by the TiN thin films is offered through the “… unique combination of sufficient adhesion to the substrate, high hot hardness, high wear resistance and its ability to improve contact conditions at the cutting edge …” of the tool [30]. This coating can be used for a wide range of substrate materials from steel, tungsten carbide, cast iron, copper alloy tools, and so forth. The only limitation to this type of coating is that it is only applicable to tools whose operational temperatures are below 600°C [26]. At temperatures above this limit, the material starts to decompose. Furthermore, only low temperatures can allow the TiN coating to be applied onto the substrate since low temperature procedures such as PVD are used. The chemical vapour deposition (CVD) is limited due to its high temperature requirements. TiN also exhibits high chemical stability due to its corrosion resistance, high oxidation resistance, and high wear resistance. However, it has a low friction coefficient and hence low lubricity [21]. The advantages of TiN-­coated tools are listed as follows [31]: • • • • • • • •

It reduces tool consumption per the machining process. It reduces the machining forces by up to 40%. These tools reduce the punch pressures by up to 30%. Due to slow wear characteristics, there is reduced tool regrinding by a factor of up to 10. They reduce the cost of production by up to 30% per machining process. They reduce the unnecessary manpower and machine costs. These tools provide the capability to undertake HSM and machine hard-­to-­ cut materials such as super-­alloys, etc. The tools reduce the consumption of lubricant and hence the cost associated with it.

These coatings have been used for a variety of tools including high-­speed steel (HSS) inserts for milling, TiN-­carbide end milling tools, M2, T15, tool steels, carbide tools, etc. It is used for punching tools, injection moulds, and so many other related applications [32]. To overcome some of the limitations of the TiN coatings, several ternary coatings of TiN have been developed. Some of these include TiCuN, TiZrN, TiCuN, TiAlN, TiMoN, TiVN, TiBN, TiSiN, and so many others. 8.7.2.2 Titanium Aluminium Nitride (TiAlN) Titanium aluminium nitride (TiAlN) is a chemical compound composed of titanium, aluminium, and nitrogen at a specified ratio. It is a ternary form of nitride in which Al is added to enhance the oxidation resistance of the binary TiN. The TiN undergoes

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oxidation at 450°C–500°C, whereas the addition of Al increases its oxidation resistance to 800°C. This is attributed to the formation of alumina (Al2O3) which adheres very well onto the substrates enhancing the wear characteristics of the face-­centred cubic-­structured TiN. The material has a very high heat resistance and is applicable to high temperature operations. However, at temperatures higher than 900°C, this material decomposes to oxides of aluminium and nitrogen. It also exhibits high values of hardness at high temperatures. TiAlN is also applied using the PVD method and consists of multiple layers. The best coating thickness for tools with this coating is about 1–4 μm. Its mechanical properties, however, largely depend on the compound’s element mole ratio rather than the type of coating on the material [33]. These coatings exhibit the following properties. • • • •

Excellent oxidation resistance and very suitable for HSM High chemical stability Excellent tribological properties Attractive mechanical properties

They are used for silicon nitride inserts for lathe turning for cast iron cutting, and machining of hard materials such as steels, titanium alloys, bronze, brass, and plastic. It can be used for HSS twist drills for applications in hard materials. In such applications, it offers the HSS tool protection against high temperature and reduction of the oxidation process. TiAlN-­coated tools are very suitable for dry machining due to their ability to withstand high temperatures. In comparison to TiN-­coated tools, TiAlN-­coated tools offer the following benefits: • TiAlN coating offers up to ten times longer the service of the tool, whereas the TiN coating offers up to four times. • TiAlN is suitable for very high cutting speeds, whereas TiN coating is suitable for normal machining speeds. • TiAlN can machine harder materials than TiN (materials with over 1100 N/mm2). • TiAlN-­coated tools are suitable for dry machining and cooling is not mandatory, whereas TiN-­coated tools require cooling. • The hardness of TiAlN is higher than that of the TiN. • TiAlN can withstand higher working temperatures (800°C) than TiN which operate at 600°C. Due to these advantages, TiAlN-­coated tools are suitable for milling and drilling of die steels, high temperature alloys, titanium, nickel, high strength (chromium) steels, etc. [34,35]. Table 8.1 shows some of the properties of TiAlN thin films for cutting tools as reported in published literature. 8.7.2.3 Chromium Nitride (CrN) Chromium nitride (CrN) coatings are binary coatings of chromium and nitride and are mostly produced through PVD methods in single layers. The CrN is an interstitial

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TABLE 8.1 Properties of Some of the TiAlN Thin Film Coatings for Cutting Tool Applications Method of TiAlN Deposition

Substrate/Tool

Elastic Modulus (GPa)/Hardness

Adhesion (N)

–

1.847 38 –

Cathodic arc PVD Magnetron sputtering Sputter ion plating

Cemented carbide AISI D2 steel Cemented carbide

Magnetron sputtering Magnetron sputtering Magnetron sputtering

Cemented carbide High-­speed steel (HSS) Tool steel/steel

380/22 632/30 –­/30

Cathodic arc vapor deposition Dc reactive magnetron sputtering Unbalanced magnetron sputtering Arc evaporation Triode magnetron sputtering

M2 steel

–­/27.29

COF = 0.18 COF = 0.70 Wear rate = 2.0 × 10−16 m3/Nm –­ –­ Wear rate = 0.5 ×  10−5 m3/Nm COF = 0.25–0.5 COF = 0.075–0.093

–­/21.5–23.5

–­

60

–­ /22 337/33.8 177–282/19–32

–­ –­ –­ –­

50 60–100 –­ 36–48

Cemented carbide, HSS M2 HSS Tool steel WC/Co cutting inserts AISI OI Tool steel

460.3/ 35.72 236.2/13.2 –­/31.86

Tribological Properties

–­ 42 48.6

–­

Source: Adapted from Ref. [36]. The methods of deposition and substrates are shown.

compound in which the N atoms are intertwined in the octahedral spaces of the FCC lattice structure of Cr [37]. These coatings have fewer dislocations compared to pure chromium coatings. They also exhibit high adhesion to their substrates, which improves their performance without cracking and flaking. The CrN is known for its high corrosion resistance and hardness. It is also known for high temperature corrosion resistance. It offers higher performance efficiencies at high temperatures compared to TiN. They also have low friction and high wear resistance. However, CrN coatings have low compressive strength than TiN and thus cannot be used in high compression applications [19,37,38]. 8.7.2.4 Titanium Chromium Nitride (TiCrN) Coating These are hybrid or ternary coatings of TiN and CrN; they are TiN coatings in which the Cr element has been introduced. Compared to the binary coatings of TiN and CrN coatings, TiCrN coatings have higher hardness, low coefficient of friction, and resistance to oxidation. In terms of production, magnetron sputtering has been shown to be the most effective technology for preparing these coatings due to its ability to produce uniform and high adhesion coatings. Their hardness may vary between

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TABLE 8.2 Some of the Applications and Properties of TiCrN Coatings for Cutting Tools Method of TiAlN Deposition Cathodic vacuum arc evaporation Arc ion plating Reactive magnetron co-­sputtering Cathodic arc evaporation Reactive ion plating PVD system

Substrate/Tool HSS, cemented carbides HSS SKD61 Cemented carbide HSS

Elastic Modulus (GPa)/Hardness

Tribological Properties

Adhesion (N)

425/37

–­

–­

490–600/30–40 –­/8–14.6

–­ –­

–­ –­

–­/25–28 –­/20

–­ –­

–­ 32.2

Source: Adapted from Ref. [36].

30 and 40. Some of the examples of TiCrN coatings for cutting tools are published in scientific literature and their corresponding properties are shown in Table 8.2. 8.7.2.5 Zirconium Nitride Zirconium nitride (ZrN) coatings are nanostructures obtained through PVD methods such as magnetron sputtering, cathodic arc, and ion plasma vacuum-­arc deposition. Usually, the thickness of the films is up to 20 nm since the films can be used in microelectronics for conduction. The nanostructure ZrN possess a hardness of up to 31.5 GPa, excellent oxidation resistance, and high corrosion resistance. It also possesses high wear resistance and is a favourable choice of coating for tools for machining aluminium and titanium alloys [19]. Compared to TiN, ZrN exhibits higher hot hardness, melting point, and corrosion resistance [39]. However, it is usually challenging to obtain defect-­free ZrN thin films, and therefore some modifications of the microstructure can be carried out through the addition of Si [40]. The addition of Si into the ZrN structure results in the formation of a crystalline (ZrN) phase and an amorphous (Si3N4) phase. ZrN-­coated tools have been shown to exhibit various advantages over TiN-­coated tools for various drilling operations [39]. 8.7.2.6 Titanium Silicium Nitride Titanium silicium nitride (TiSiN) is used in coating hardened steel materials. It has a very high hardness, which makes it a favourable coating material for cutting tools for aluminium materials [36]. The coating is a ternary alloy, which overcomes the challenges of oxidation exhibited by TiN binary coating. The presence of Si in this coating enhances the hardness and resistance against abrasion. As compared to TiN binary coatings, TiSiN coatings exhibit high hardness, tribology, oxidation resistance, and wear resistance. The optimum content of 7%–12% silicon into TiSiN provides the maximum mechanical and tribological properties of the coatings [36]. It is noted, however, that Si content increases the friction coefficient of the coating.

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The Si also reduces the grain size considerably and influences the preferential orientation of the crystallographic structure of the films. These coatings increase the life of the cutting tools by up to seven times. The coatings have been successfully deposited through PVD techniques on M2 tool steel, AISI M42 tool steel, AISI M2 HSS, cemented carbide, and HSS [41]. 8.7.2.7 Titanium Aluminium Silicon Nitride (TiAlSiN) This material is commonly used in coating hardened steel and other carbide tools. It is preferred over TiSiN due to its relatively high abrasive wear resistance. The introduction of Al into the structure of TiSiN enhances ductility and lowers the friction coefficient of the coating. 8.7.2.8 Chromium Aluminium Nitride (CrAlN) These coatings are also used in coating tool steels and high-­speed steels and are a ternary version of binary CrN thin films. These coatings are made up of chromium, aluminium, and nitrogen atoms in a given molar ratio. They have high hardness and toughness. The addition of Al into the CrN coatings enhances the hardness properties and wear resistance behaviour [42]. In comparison to CrN, these coatings exhibit lower thermal conductivity, and better tribological and hardness properties. When CrAlN is exposed to high temperatures, it forms both Al2O3 and Cr2O3 which adheres to their surface and protects them from further oxidation [42]. The coatings have an oxidation temperature of up to 900°C. This makes CrAlN coatings superior for cutting tool applications. 8.7.2.9 Titanium Molybdenum Nitride (TiMoN) This is a ternary transition coating of TiN in which Mo element has been introduced. The addition of Mo into the TiN enhances the formation of oxide layers on the surface of the coating, which acts as a lubricant on abrasion and therefore lowers the coefficient of friction of these coatings [36]. The coatings have been mostly deposited via PVD methods. The coatings have been used on hardened M42 tool steel, stainless steel, and other steels for tribological applications. 8.7.2.10 Boron Nitride (BN) BN coatings can be prepared by various deposition techniques such as ion beam– assisted method in which boratine is used as the target. These coatings are chemically inert at very elevated temperatures of up to 1500°C. They exhibit high thermal stability and excellent mechanical behaviour and are very suitable for harsh condition applications. They are used in coating nickel and graphene type tools that are employed in high temperature operations. The film prevents the oxidation of these materials at high operating temperatures [43].

8.7.3 Carbides Carbides are salts composed of metals combined with carbon atoms. They have a general characteristic of high hardness values, high melting point, and good electrical and heat conductors and heat resistance. Some of the carbides used as coatings for cutting tool applications are discussed below.

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8.7.3.1 Titanium Carbide (TiC) TiC is a hard-­refractory ceramic coating material for machining steel. It has high abrasive resistance, high heat resistance due to its high thermal conductivity, and chemical stability at high temperatures [44]. It is only attacked by HNO3, HF, and halogen. It oxidises slowly in the air at 800°C and therefore can withstand extreme conditions of machining. 8.7.3.2 Chromium Carbide (CrC) CrC just like its nitride equivalent is used in high temperature operations. It has high hardness, and excellent adhesive and abrasive wear resistance. The hardness of CrC thin films varies between 3.6 and 48 GPa depending on the method and processes of deposition [45]. It has been shown that the chromium carbide coating improves the tool hardness and wear resistance by close to 40% [46]. It has excellent resistance to oxidation and corrosion. It exhibits a low coefficient of friction and is therefore suitable for high abrasion conditions. The application of these coatings on working tools has greatly increased commercially, and they are currently being used in cores and mounds for die-­casting of aluminium-­based alloys. These coatings are also used on dies for plastic injection moulding. It has been used to coat cemented carbide tools for turning, drilling, and various operations [47]. 8.7.3.3 Tungsten Carbide (WC) Cemented tungsten carbide coating can be deposited onto its substrate using both the CVD and PVD methods. It is employed mostly due to its high hardness characteristic on substrates such as steel and other ceramic cutting tools. It is usually used as the base coating layer in a hierarchical deposition. Cobalt is used as a binding agent in tungsten carbide coatings. Due to its high friction and poor thermal conductivity, it is layered with other materials which complement the friction and thermal properties. Multilayered WC coatings together with other materials coatings have higher efficiencies in increasing mechanical and tribological properties as compared to single-­ layer coatings [48]. Tungsten carbide thin films exhibit the following properties [49]: • • • •

High melting point of 2870°C Very high modulus of elasticity 700 GN/m2 Low coefficient of friction High thermal and chemical stability (up to 4000°C)

Due to these properties, WC is used in the coating of cemented carbide tools for steel cutting. In such applications, the WC is bonded to the tool surfaces through a ceramic matrix such as SiC.

8.7.4 Others 8.7.4.1 Titanium Boride (TiB2) TiB2 is a ceramic compound that is known for its high hardness, which makes it suitable in the coating industry for cutting tools. It improves the wear resistance of cutting tools and also, due to its oxidation resistance, prevents corrosion of the tool

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material via oxidation [50]. The coating has a high melting point and is therefore stable in high temperature applications. 8.7.4.2 Diamond Carbon Diamond cutting machines and tools are expensive. However, due to their applicability in high temperature and strength conditions, they are adopted in making coatings for tools to be used in such conditions. They have high hardness, excellent thermal conductivity, chemical stability, high wear resistance, and low friction. Diamond coatings are developed usually through CVD methods, with hot filament and plasma-­based CVD being the most dominant [51]. They are mainly applicable on tungsten carbide materials used in high precision machining and also in very high-­speed cutting operations like drilling. The challenge with diamond coating is the poor adhesion to various substrates, and as such several treatments on surfaces to improve the adhesion are used [52]. Some of these treatments include microwave oxidation, the reaction in alkaline solution, and cleaning by ultrasonic treatment in acid solution [53]. It can be used to coat cemented carbide tools using hot filament CVD [54,55], and tungsten carbide tools using surface-­wave plasma-­enhanced CVD and microwave plasma CVD [56,57]. During diamond coating on WC-­Co tools, there is usually diffusion of the cobalt resulting in graphitisation at the diamond–tool interface. This can cause delamination of the diamond coating during machining of hard materials. As such, doping boron into the coating enhances the adhesion of diamond onto the WC-­Co substrate, thereby enhancing its performance. 8.7.4.3 Molybdenum Disulphide (MoS2) MoS2 has a very low friction coefficient and is known as a self-­lubricating material. When employed in coating cutting metals, its role generally is to increase lubricity on WC coatings on steel cutting tools [58]. The MoS2 coatings are created using the PVD technique, which, besides creating the coating, reduce its sensitivity to moisture during application [43]. The MoS2 can be combined with PVD coatings to improve their lubricity and wear-­resistant properties [59]. For instance, it can be combined with Ti to form Ti-­ MoS2 coatings through PVD on TiB2-­coated inserts for dry machining of aluminium alloys [60]. In such applications, the MoS2 reduces the brittleness and enhances the wear resistance of the TiB2-­coated inserts of carbide end mill tools. Coatings consisting of Ti-­MoS2 exhibit good mechanical and tribological properties for effective applications in machine cutting tools [61]. These films (Ti-­MoS2) have also been used for coating tool steels (SKD 11 tool) [62]. The MoS2 can also be used for coating tool steels for cutting industries; reference [63] deposited the coatings on tool steel substrate through thermal evaporation and the film exhibited goods adherence and wear properties for tool applications. Other MoS2 used in tool steel/cutting tool coating include CrN-­MoS2 [64], and so forth. 8.7.4.4 Aluminium Oxide Aluminium oxide is a refractory material used to coat machining cast iron and steel. It is the only oxide used as a coating material. Studies have shown that multilayer aluminium oxide on cutting tools improves their wear and toughness behaviour. The

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coating also imparts good finishes on the tools’ surface. Aluminium oxide thin film is one of the few coatings that are developed on their substrate using CVD technique due to their high heat resistance [65]. Through CVD and chemical methods, alumina coatings can be deposited on cemented carbide, tungsten carbide [66], YT5 (WC-­ 10Co-­5%TiC) tool [67], etc.

8.8 PROGRESS IN THIN FILM MATERIALS FOR CUTTING TOOL INDUSTRY The desire to achieve higher precision and productivity has led to continual advancement in the coating technologies for cutting tools and the materials used in the process [68]. The advancements are aiming to attain efficient coating characteristics by achieving the desired properties. Some of these properties include the following: • • • • •

Thermal stability Corrosion resistance Wear resistance High chemical stability Property stability at higher temperatures

The purpose of thin film coating on tools is to alter both the physical and chemical properties of the tool, especially at the tool–work interface, where the tool interacts with the workpiece. The desire to achieve the desired characteristics has led to more innovation and technological advancement in thin film coating materials. In the 1990s studies showed that hard materials which were mostly single metal carbides, nitrides, oxides, and borides could not achieve the required chemical and mechanical properties for cutting tool thin film coating [69]. During that time compound materials were found to combine the advantages of each material, making their coating more useful. Also, in 1990 the utilisation of hard ceramic PVD coating to HSS was the most successful development. Additionally, the development of ceramic composites, Sialons, and PCBN found application not only in the coating of cutting tools but also in other thin film coating applications [70]. Currently, most studies are focussing on developing new and advanced materials for cutting tools coating and coating thickness for both single and multiple layer thin films [48]. For the cutting tool coating materials, the two main important properties being investigated include high hardness and high toughness which are ideal for the effective performance of a cutting tool. It is usually difficult to develop coatings that perfectly contain all required ideal cutting tool properties. Currently one of the significant thin film coating materials being used in a variety of applications is the ternary films of binary titanium nitride (TiN). TiN is known for its good wear resistance. The hard thin film coating produced by TiN is extensively used nowadays on cutting tools due to the better machining performance and ability to increase the life of the cutting tool. The hardness of TiN is relatively lower. As such, to overcome the limitations of TiN, currently, there is growing interest in developing ternary coatings of TiN. Some of these coatings include TiVN, TiMoN, TiCrN,

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FIGURE 8.1  Description of appearance of different types of coating applied on a cutting tool. (Reused under Creative Common CC BY licence [18].)

TiZrN, etc. [36]. There is currently lots of research and development of ternary coatings of TiN for better performance of cutting tools in HSM. One of the crucial advancements today for thin film coating for cutting tools is the use of multilayer, nanolayer, and nanocomposite coating where different materials are deposited layer by layer or as a composite material to achieve the desired coating characteristics (Figure 8.1) [28,71]. The development and adoption of nanocomposite as the cutting tool coating material is growing at a fast rate [71]. The process involves depositing alternatively borides, carbides, and nitrides on the surface of the tool material. Some of the composite nanocoatings include TiC/TiB2, TiN/TiB2 (which are hard/hard coatings), and B4C/W, SiC/Al (hard/soft materials combinations) [71]. Currently, more studies about nanocomposite and nanolayer coating materials are being carried out to develop durable thin film coatings on cutting tools for high efficiencies in machining processes. With materials technology advancements such as processing and engineering materials, there are thousands of material choices for coating cutting tools. Therefore, when searching for cutting tool thin film coating material it is necessary for the engineer(s) involved to consider the advantages of the available materials, their disadvantages, as well as the cost of their application.

8.9 SUMMARY High-­speed cutting process subjects the tools to high temperatures, forces, and pressures. For the tool to perform efficiently and durably, thin film materials are used as coatings on the cutting tools. Many thin film materials can be used to improve tool properties for higher performance efficiencies. As discussed in this chapter, these materials include nitrides, carbides, and others. These materials generally exhibit high hardness, strength, wear and corrosion resistance, chemical and thermal stability, and low coefficient of friction properties. The properties of some of the specific thin film materials suitable for application as coatings for cutting tools have been discussed in the chapter. Additionally, some of the examples of applications of specific coatings have been illustrated. It is noted that selecting the right material for the coating is important. As the manufacturing industry expands and the demand for complex and high-­performance products continues to expand, the need for precision cutting tools shall consequently grow. As such, there is a necessity to develop advanced thin film coatings for cutting tools. In this chapter, ternary, multilayer, and nanocomposite thin films have been illustrated as the

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potential future of the subject. The chapter is very useful for the cutting tool industrialists and engineers on the development and selection of coated cutting tools for various machining operations.

REFERENCES



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34. P. Jindal, A.. Santhanam, U. Schleinkofer, and A.. Shuster, “Performance of PVD TiN, TiCN, and TiAlN coated cemented carbide tools in turning,” Int. J. Refract. Met. Hard Mater., vol. 17, no. 1–3, pp. 163–170, May 1999, doi: 10.1016/S0263-­4368(99) 00008-­6. 35. P. Li et al., “Microstructure, mechanical and thermal properties of TiAlN/CrAlN multilayer coatings,” Int. J. Refract. Met. Hard Mater., vol. 40, pp. 51–57, Sep. 2013, doi: 10.1016/j.ijrmhm.2013.01.020. 36. M. Ghufran, G. M. Uddin, S. M. Arafat, M. Jawad, and A. Rehman, “Development and tribo-­mechanical properties of functional ternary nitride coatings: Applications-­based comprehensive review,” Proc. Inst. Mech. Eng. Part J J. Eng. Tribol., vol. 235, no. 1, pp. 196–232, 2021, doi: 10.1177/1350650120933412. 37. M. A. Gharavi et al., “Synthesis and characterization of single-­phase epitaxial Cr2N thin films by reactive magnetron sputtering,” J. Mater. Sci., vol. 54, no. 2, pp. 1434– 1442, 2019, doi: 10.1007/s10853-­018-­2914-­z. 38. C. S. Rao and K. S. Prasad, “Advances in plasma arc welding : A review,” J. Mech. Eng. Technol., vol. 4, no. 1, pp. 35–60, 2012. 39. J. Menghani, K. B. Pai, and M. K. Totlani, “Corrosion and wear behaviour of ZrN thin films,” Tribol. -­ Mater. Surfaces Interfaces, vol. 5, no. 3, pp. 122–128, 2011, doi: 10.1179/1751584X11Y.0000000013. 40. F. G. R. Freitas et al., “Structural and mechanical properties of Zr-­Si-­N thin films prepared by reactive magnetron sputtering,” Mater. Res., vol. 18, pp. 30–34, 2015, doi: 10.1590/1516-­1439.336214. 41. C. Y. Wang, Y. X. Xie, Z. Qin, H. S. Lin, Y. H. Yuan, and Q. M. Wang, “Wear and breakage of TiAlN-­ and TiSiN-­coated carbide tools during high-­speed milling of hardened steel,” Wear, vol. 336–337, pp. 29–42, 2015, doi: 10.1016/j.wear.2015. 04.018. 42. C. Y. Yu, S. B. Wang, T. B. Li, and Z. X. Zhang, “Tribological behaviour of CrAlN coatings at 600°C,” Surf. Eng., vol. 29, no. 4. pp. 318–321, 2013, doi: 10.1179/174329 4412Y.0000000100. 43. V. Sharma and P. M. Pandey, “Comparative study of turning of 4340 hardened steel with hybrid textured self-­lubricating cutting inserts,” Mater. Manuf. Process., vol. 31, no. 14, pp. 1904–1916, 2016, doi: 10.1080/10426914.2015.1127951. 44. F. Santerre, M. A. El Khakani, M. Chaker, and J. P. Dodelet, “Properties of TiC thin films grown by pulsed laser deposition,” Appl. Surf. Sci., vol. 148, no. 1, pp. 24–33, 1999, doi: 10.1016/S0169-­4332(99)00139-­7. 45. Z.-­L. Li, Y.-­Y. Chen, C.-­J. Wang, and J.-­W. Lee, “Comparison of chromium carbide thin films grown by different power supply systems,” Surf. Coatings Technol., vol. 353, no. Jul., pp. 329–338, Nov. 2018, doi: 10.1016/j.surfcoat.2018.07.107. 46. V. Singh, R. Diaz, K. Balani, A. Agarwal, and S. Seal, “Chromium carbide-­CNT nanocomposites with enhanced mechanical properties,” Acta Mater., vol. 57, no. 2, pp. 335– 344, 2009, doi: 10.1016/j.actamat.2008.09.023. 47. Y. L. Su, T. H. Liu, C. T. Su, S. H. Yao, W. H. Kao, and K. W. Cheng, “Wear of CrC-­ coated carbide tools in dry machining,” J. Mater. Process. Technol., vol. 171, no. 1, pp. 108–117, Jan. 2006, doi: 10.1016/j.jmatprotec.2005.06.050. 48. N. Atiqah Badaluddin, W. W. Fathul Hakim Zamri, M. Faiz Md Din, I. Fadhlina Mohamed, and J. A. Ghani, “Coatings of cutting tools and their contribution to improve mechanical properties: A brief review,” Int. J. Appl. Eng. Res., vol. 13, no. 14, pp. 11653–11664, 2018, [Online]. Available: http://www.ripublication.com. 49. R. R. Phiri, O. P. Oladijo, and E. T. Akinlabi, “Tungsten carbide thin films review: Effect of deposition parameters on film microstructure and properties,” Procedia Manuf., vol. 35, pp. 522–528, 2019, doi: 10.1016/j.promfg.2019.05.074.

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50. B. Sivakumar, R. Singh, and L. C. Pathak, “Corrosion behavior of titanium boride composite coating fabricated on commercially pure titanium in Ringer’s solution for bioimplant applications,” Mater. Sci. Eng. C, vol. 48, pp. 243–255, 2015, doi: 10.1016/j. msec.2014.12.002. 51. J. E. Butler, Y. A. Mankelevich, A. Cheesman, J. Ma, and M. N. R. Ashfold, “Understanding the chemical vapor deposition of diamond: Recent progress,” J. Phys. Condens. Matter, vol. 21, no. 36, 2009, doi: 10.1088/0953-­8984/21/36/364201. 52. E. Uhlmann, D. Hinzmann, W. Reimers, and K. Böttcher, “Multilayer structure dependent performance behaviour of CVD diamond thin film drilling tools during CFRP machining,” Procedia CIRP, vol. 87, pp. 360–365, 2020, doi: 10.1016/j. procir.2020.02.089. 53. M. Chen, X.. Jian, F.. Sun, B. Hu, and X.. Liu, “Development of diamond-­coated drills and their cutting performance,” J. Mater. Process. Technol., vol. 129, no. 1–3, pp. 81–85, Oct. 2002, doi: 10.1016/S0924-­0136(02)00580-­0. 54. E. Uhlmann and J. Koenig, “Analysis of the manufacturing chain of CVD diamond coated shaft type cutting tools,” Prod. Eng., vol. 4, no. 2, pp. 211–220, 2010, doi: 10.1007/s11740-­010-­0219-­4. 55. M. J. Jackson, L. J. Hyde, W. Ahmed, H. Sein, and R. P. Flaxman, “Diamond-­coated cutting tools for biomedical applications,” J. Mater. Eng. Perform., vol. 13, no. 4, pp. 421–430, 2004, doi: 10.1361/1059949041848819. 56. J. S. Kim et al., “Cutting performance of tungsten carbide tools coated with diamond thin films after etching for various times,” Mod. Phys. Lett. B, vol. 32, no. 20, p. 1850236, Jul. 2018, doi: 10.1142/S0217984918502366. 57. K. Shibuki, K. Sasaki, M. Yagi, T. Suzuki, and Y. Ikuhara, “Diamond coating on WC-­ Co and WC for cutting tools,” Surf. Coatings Technol., vol. 68–69, no. C, pp. 369–373, 1994, doi: 10.1016/0257-­8972(94)90187-­2. 58. N. M. Renevier, J. Hamphire, V. C. Fox, J. Witts, T. Allen, and D. G. Teer, “Advantages of using self-­lubricating, hard, wear-­resistant MoS2-­based coatings,” Surf. Coatings Technol., vol. 142–144, pp. 67–77, 2001, doi: 10.1016/S0257-­8972(01)01108-­2. 59. S. Paskvale, M. Remškar, and M. Čekada, “Tribological performance of TiN, TiAlN and CrN hard coatings lubricated by MoS2 nanotubes in Polyalphaolefin oil,” Wear, vol. 352–353, pp. 72–78, 2016, doi: 10.1016/j.wear.2016.01.020. 60. T. L. Brzezinka et al., “Hybrid Ti-­MoS2 coatings for dry machining of aluminium alloys,” Coatings, vol. 7, no. 9, pp. 1–13, 2017, doi: 10.3390/coatings7090149. 61. R. I. Amaro, R. C. Martins, J. O. Seabra, N. M. Renevier, and D. G. Teer, “Molybdenum disulphide/titanium low friction coating for gears application,” Tribol. Int., vol. 38, no. 4, pp. 423–434, Apr. 2005, doi: 10.1016/j.triboint.2004.09.003. 62. S. K. Kim, Y. H. Ahn, and K. H. Kim, “MoS2-­Ti composite coatings on tool steel by d.c. magnetron sputtering,” Surf. Coatings Technol., vol. 169–170, pp. 428–432, Jun. 2003, doi: 10.1016/S0257-­8972(03)00181-­6. 63. S. Sivarajan and R. Padmanabhan, “Characterization of thermally evaporated MoS2 thin film coatings,” Mater. Today Proc., vol. 3, no. 6, pp. 2532–2536, 2016, doi: 10.1016/j. matpr.2016.04.172. 64. S. K. Kim and B. C. Cha, “Deposition of CrN-­MoS2 thin films by D.C. magnetron sputtering,” Surf. Coatings Technol., vol. 188–189, no. 1-­3 SPEC.ISS., pp. 174–178, 2004, doi: 10.1016/j.surfcoat.2004.08.013. 65. E. Bahceci, “Characterization of atomic layer deposition coated cutting tools by X-­ray photoelectron spectroscopy and examination of cutting performance,” Spectrosc. Lett., vol. 52, no. 8, pp. 456–461, 2019, doi: 10.1080/00387010.2019.1660901. 66. B. A. Rezende et al., “Characterization of ceramics coatings processed by sol-­gel for cutting tools,” Coatings, vol. 9, no. 11, pp. 1–12, 2019, doi: 10.3390/coatings9110755.

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67. S. Tang, P. Liu, Z. Su, Y. Lei, Q. Liu, and D. Liu, “Preparation and cutting performance of nano-­scaled Al2O3-­coated micro-­textured cutting tool prepared by atomic layer deposition,” High Temp. Mater. Process., vol. 40, no. 1, pp. 77–86, Apr. 2021, doi: 10.1515/htmp-­2021-­0 021. 68. K. Bobzin, “High-­performance coatings for cutting tools,” CIRP J. Manuf. Sci. Technol., vol. 18, no. 2017, pp. 1–9, Aug. 2017, doi: 10.1016/j.cirpj.2016.11.004. 69. H. Holleck, “Material selection for hard coatings,” J. Vac. Sci. Technol. A Vacuum, Surfaces, Film., vol. 4, no. 6, pp. 2661–2669, 1986, doi: 10.1116/1.573700. 70. B. Mills, “Recent developments in cutting tool materials,” J. Mater. Process. Technol., vol. 56, no. 1–4, pp. 16–23, Jan. 1996, doi: 10.1016/0924-­0136(95)01816-­6. 71. D. Sidorenko, P. Loginov, L. Mishnaevsky, and E. Levashov, “Nanocomposites for machining tools,” Materials (Basel)., vol. 10, no. 10, pp. 1–19, 2017, doi: 10.3390/ ma10101171.

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Thin Films for Electronic, Spintronics, and Optical Applications

9.1 INTRODUCTION With the advancement in technology and smart lifestyles, the demand for small devices with huge data storage and faster transmission capabilities is growing very fast. For example, hard disk drive (HDD) utilising spin quantum number was fetching a market revenue of over $US 11 bn in 2018, which means now it fetches over $US 11 bn. Spintronics, also known as the spin electronics, has been defined as, “… the study of the intrinsic spin of electrons and its associated magnetic moment…” (https://byjus.com/physics/spintronics/). As stated by Saad Mabrouk Yakout in his power feature in the Journal of Superconductivity and Novel Magnetism “spintronics has the ability to combine the main functions of the modern semiconductor microelectronics and magnetic storage devices in single chip” [1]. The conventional electronic devices cannot be scaled down below 1 nm due to the quantum effects; however, through spintronics technology, it is possible to fabricate nanodevices with very data storage and transmission capability. In this chapter, thin film materials for spintronics, optics, and electronic devices are presented, since the three subjects are related due to the demand for small gadgets. The chapter begins by presenting the importance of optics and spintronics technologies in the modern society followed by discussion of various thin film materials for microelectronic devices. Then, thin films for optics, spintronics, and photonics are presented. The applications of thin film technology in nanodevices and flexible gadgets fabrication are also presented. Lastly, it discusses the emerging trends of thin film applications in optics and microelectronic devices.

9.2 IMPORTANCE OF OPTICS AND SPINTRONIC TECHNOLOGIES Due to technological advancement and lifestyle changes, there have been a great change and emergence of new technologies aiming to produce better products to meet these advancements and solve various societal/lifestyle problems. Some of these technologies include optics and spintronic technologies. Optics and spintronics technologies are very useful in electronic and telecommunication industries. Optics refer to technologies related to vision, transmission/reflection of light, and signals. Examples of application of this technology are in the transmission of television and telephone signals [2]. The most crucial importance of these technologies is the safety and speed at which the data or information is transmitted. One of the most important DOI: 10.1201/9781003202615-9

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applications of optics is the fibre optic technology in optical communications. Fibre optic communication involves transmission of information as pulses of light through strands of fibre made of glass or plastic over long distances. The introduction of fibre optics has revolutionised the communication industry. It is applied in television, the internet, computer networking, telephone, etc. [3]. The advantages of fibre optics include high speed of transmission of data, transmission of data and signals for long distances, thinner and lighter than copper cables, and reliable since the fibre is immune weather conditions. Additionally, they are not affected by electromagnetic interference since they carry no electric current. Optic technology therefore ensures secure, high speed, and reliable data transmission [4]. Spintronics is a new technology in electronics which harnesses both the spin and charge of electrons to carry and store information for device functionality. The technology combines the functions of the modern semiconductor microelectronics and magnetic storage devices in a single chip. The importance of the spintronics technology includes providing high speed, high power lasers, lower threshold current, high-density logic, low power, high electronic memory devices, and optoelectronic devices [1]. The technology has made it possible to develop nanoelectronics (small devices), which largely depends on the growth of spintronic materials. So far, spintronics is showing significant progress and many advantages leading to the development of new spintronics materials and becoming a promising field in memory and data storage requirements. Additionally, more interest has been developed in this area because of the capability to develop various new devices that combine sensor, logic, and storage applications. With this technology, small devices with huge storage and faster transmission of data can be developed. Material plays an important role in spintronics and optics. The application of high-performance materials, mainly thin film materials and nanomaterials in these areas, is inevitable.

9.3 THIN FILM MATERIALS FOR OPTIC DEVICES Thin film technology finds an array of applications in the field of optics. Optical thin film coatings are deposited on substrate materials such as broadband pass filters to impart properties such as antireflection, high reflection, combination, and splitting of light beams. These films are used in different areas such as the production of optical instruments for scientific studies, construction, lasers, medicine, astronomy, and synchrotron radiation and emission-absorption spectroscopy [5,6]. In optical coatings, the thickness, structure, and size of the grain and their distribution in the structure are essential in influencing the optical behaviour of a material. Thin film materials used in optics can be grouped into oxides, fluorides, polymers, and metals.

9.3.1 Oxide-Based Ceramic Coatings Thin metal oxide coatings in the range of few nanometres have varied characteristics when subjected to electromagnetic radiation. These characteristics are determined by the interaction of the oxide with the light beams. The most common oxides that

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have been used in the market are silicon oxide and titanium dioxide. Other additional oxides like niobium oxide, aluminium oxide, and tantalum oxide combine to form multiple coating layers for anti-reflective surfaces. These oxides have different optical properties; for example, niobium penta-oxide (Nb2O5) thin films exhibit higher refractive index than tantalum oxide (Ta2O5) and they are transparent in the wavelength range from 380 nm to 9 μm [7]. This is due to their transparency in wide range of wavelengths and high refractive index. Multiple layers are preferred in optical application because a single layer coating can be effective in a narrow range. The thickness of the coating layers should be equal to a quarter of the wavelength of the fraction of the electromagnetic spectrum wavelength [8]. Transparent conductive oxides such as indium oxide are employed in manufacture of display systems. Indium oxide transmits power across liquid crystal displays. The use and development of these films have enhanced the size of display systems. Metal oxide thin films with an extensive optical band gap offer exceptional optical transmittance in the visible and near-infrared zones. Titanium dioxide and silicon oxide have exceptional optical transmittance attributed to the low refractive index (as low as 1.05), which is almost close to the refractive air index for visible light spectrum [6]. Metal oxide coatings are also used for colour filtration and correction in optics. These special coatings are called dichroic filters and are a combination of several materials with high refractive indices. Film thickness is an important property for such films as it determines their performance [9,10]. Such films are employed for aesthetic purposes and in currency detection. The angle of incidence of the light will affect the absorbance and refraction of different light in the visible light spectrum to generate the intended colour [9]. Besides the optical characteristics, metal oxide thin films are utilised to impart thermal and tribological properties such as the abrasion resistance essential in lens development, and hydrophobicity [8]. TiO2 thin films are applied for shielding of very bulky integrated circuits and different optical elements due to their beneficial insulating properties. TiO/SiO2 multilayers have found applications in creating hydrophobic surfaces on optical elements and windscreens in vehicles. These films have increased the performance of solar cells during humid conditions [5]. Deck et al. [11] researched on the thermal properties of Al2O3 and SiO2 thin films for laser applications. The high corrosion resistance of some of these oxides makes them suitable as anti-corrosion films with silicon dioxide on silver-plated mirrors as one of such oxide films.

9.3.2 Non-Oxide Ceramic Coatings The main materials in this category are metallic fluorides, sulphides, and nitrides. These materials are blended with other materials like pure metals and oxide films. Some of these coatings are used to improve water-corrosion resistance like Cerium fluoride. Flourionidate fibres used in laser communication and amplification are nanomaterials that consist of a series of metal fluorides ZrF4-BaF2-LaF3-AlF3-NaF. They have higher transparency than silica fibres in the infrared range of the electromagnetic spectrum [12].

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9.3.3 Metal-Based Thin Films Metal-based materials create thin films that induce anti-reflective properties on a surface, and develop transparent conductors and highly reflective surfaces. These films are mainly deposited using physical deposition techniques, with sputtering being the most common technique employed. Ferrous oxides have found a crucial area in optical modulators and beam deflectors due to their non-centrosymmetric crystalline structure with spontaneous polarisation that can be manipulated using an electric field. The electric field–dependent response to light coupled with the refractive index of these materials when used as single layers on a substrate has high potential in the solar industry aside from being utilised as modulators [13]. Metal films of gold and silver in the 9 nm range have transparent properties and are used as anti-reflective films. These films are used to develop anti-reflective lenses and protective spectacles. They are also used to manufacture glasses for application in aviation, buildings (to shield against infrared thermal radiation), and camera lenses. Thicker metal films are used on glass for the manufacture of mirrors. Thin silver coatings applied on glass are used in making household and high reflectance astronomical telescopes’ mirrors. Aluminium films are used to manufacture ultraviolet reflecting mirrors. These metals also form dichroic thin film filters whose characteristics depend on the wavelength of the light and film thickness. Metallic thin films are also used in the manufacture of transparent conductors. A silver thin film sandwiched between titanium oxide films has high transparency with high luminous transmittance. These films have high electrochemical stability and electrical conductivity. They are used in electro-chromic devices such as inflexible displays [14,15].

9.4 THIN FILM MATERIALS FOR SPINTRONIC AND PHOTONIC APPLICATIONS Spintronics (spin-based electronics) is a field of nanotechnology that deals with spin-dependent properties of an electron instead of the charge-dependent properties. Spintronic devices find applications in information storage, processing, and transmission; this is possible through manipulation of spin degree of freedom of electrons. A good example is the HDD [15]. Photonics (photon-based electronics), on the other hand, is a technology field that is concerned with the generation, properties, transmission, and application of photons (particles of light). In photonics, light is turned into electronics and digital signals. Just as the electronics uses electrons, photonics uses photons [16]. Spintronic and photonic devices have low energy consumption, fast operation speed at room temperature, and high integration density [17]. Thin films materials for use in spintronic and photonic devices must contain properties suitable for these applications and are normally deposited on substrates that do not exhibit the spintronic/photonic properties. One such substrates are non-magnetic substrates. For this case, thin film magnetic garnets (i.e. iron garnets) are used. These films are typically insulators characterised by high-level magnetic and optical features, such as high

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Curie temperature (temperature at which magnetic materials undergo a sharp change in their magnetic properties), highly tunable bulk perpendicular magnetic anisotropy (PMA), ferromagnetic nature with low net moment, extremely low Gilbert damping (a non-linear spin relaxation constant that controls the rate at which magnetisation spins reach equilibrium), strong magneto-optic effects, and optical non-reciprocity and low optical loss [17]. These features of magnetic garnets make them suitable for applications in the spintronic and photonic devices. These garnets are artificial layers with general formula of X3Fe5O12, where X can be Y, Bi, Tm, Yb, Er, Gd, Tb, Dy, Ho, Sm, Eu, or other rare-earth elements [17,18]. Iron garnets possess superior properties in many aspects as compared to magnetic metals or other magnetic insulators, particularly for applications in spintronic and photonic devices. Yttrium Iron Garnet (YIG) thin films with ultra-low damping (α) on gadolinium gallium garnet (GGG) substrates and PMA rareearth iron garnet, doped-YIG, and pure-YIG thin films on appropriate non-magnetic garnet substrates can be fabricated using liquid phase epitaxy (LPE), pulsed laser deposition (PLD), and magnetron sputtering. YIG is a magnetic garnet, possessing the lowest α value so far on the earth among all discovered or synthesised materials. YIG thin films can be prepared using thin film deposition techniques such as LPE, PLD, and RF magnetron sputtering. Thin films of antiferromagnetic oxides such as Cr2O3 and ∝-Fe2O3 have been deposited through radio-frequency magnetron sputtering for spintronic applications [19]. These films are attractive due to the following properties: • They have ultrafast spin dynamics. • They are characterised by long spin diffusion length. • They are immune to large magnetic fields. Organic semiconductors can also be used as thin film materials for spintronics applications due to their high spin life-time, distinct electro-optical, and tunable chemical and physical properties [20]. For example, iron phthalocyanine (FePc) and manganese phthalocyanine (MnPc) blended with fluorinated cobalt phthalocyanine (F16CoPc) and tetracyanoquinodimethane (TCNQ) can be used for spintronics applications [20]. Some of the spintronics devices fabricated from organic semiconductors include the following [21]:

i. Spin memory devices: Organic semiconductor possessing spintronic properties such as tris(8-hydroxyquinolinato) aluminium (Alq3) has been used. ii. Spin photo-response devices: Due to the optical properties of organic semiconductor thin films, they can be used as sensors in spintronic devices. An example of such material is F16CuPc. iii. Spin photovoltaic devices: C60 can be used for this application due to its dual character of high electron mobility and photovoltaic properties. Additionally, the material has excellent spin transport characteristics. iv. Spin organic light emitting diode (OLED): Tris(8-hydroxyquinolinato) aluminium, Alq3 (Al(C9H6NO)3), is one of the used organic material for spin OLED.

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9.5 THIN FILM MATERIALS FOR MICROELECTRONIC APPLICATIONS Microelectronics relates to the study and manufacture (or microfabrication) of very small electronic components. Thin films of various types are used for various purposes in this field. Conducting thin films are used to form interconnections in chips and other microelectronic products, while dielectric thin films are applied to provide electrical insulations [22]. The electrical properties of the thin film materials highly depend on the method of deposition, chemistry, and purity of the target. Some of the applications of thin film materials in microelectronic devices are discussed below. • Conducting films: Thin film materials which are good conductors of electricity have found applications in microelectronics as the interconnection between microelectronic components and chips. Metallic materials can be applied when the thin film is deposited to act as a conductor in electronics. In modern semiconductor devices, the sputtering deposition method is used to deposit thin metal films. The choice of metal applied depends on particular properties such as thermal, ease of processing, and adhesion. Some of the metallic materials used in these applications include gold, aluminium, silver, and copper [23]. Other thin film materials include Cr-Au and NiCr-Au, Ti-Pd(Pt)-Au, FeCrAl-Cu-Au, and transparent oxides (SnO2, ITO, ZnO, etc.). • Protective coating: Thin films are usually used in the protection of electronic devices and components in microelectronics. An example is the titanium nitride (TiN) films used in microelectronic chips to separate aluminium lines used for electrical conduction and insulator (SiO2) and therefore suppress the formation of Al2O3. In such a case, TiN thin films are known as microelectronic barrier layers [24]. Organic thin films can be used to protect devices’ external attacks such as water, high temperature, moisture, corrosion, and so forth. Examples of such materials include hydrophobic and superhydrophobic organic thin films [25]. • Insulators: Thin film material has found application in microelectronics as insulators. The materials used for this purpose possess thermal insulation properties due to their lower thermal conductivity and may also reduce corrosion risks on the devices. Polymeric thin films are some of the most common insulating materials for microelectronic devices [26]. In microelectronic applications thin film materials are used to fabricate transistors, capacitors, sensors, batteries, energy devices, actuators, and coatings. For thin film electronics, Ni-Cr alloys (for resistors), Ta2O5 (for capacitors), and Cr-SiO cermets (for resistors) thin films have been traditionally used [27]. Thin films for resistor applications include Cu-Ni, Cr, Ta, TaN, Cermets, RuO2, etc. The advantage of application of these thin film materials in electronics has, over the years, brought a significant reduction of device sizes and improvement of their performances. The technology has greatly contributed to the development of flexible conductors, wearable devices, and health monitoring electronic devices.

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9.6 THIN FILM MATERIALS FOR NANODEVICES AND FLEXIBLE GADGETS As the material sizes reduce to micro/nanoscale levels, mechanical, chemical, and electrical properties are enhanced due to higher surface area to volume ratio and quantum confinements. With the increasing demand for high performance devices and real-time transfer of information in the 21st century, very small electronic devices with ultrahigh density and speed have evolved [28]. These devices, known as nanodevices, can find applications in various fields. The emergence of such devices has led to the introduction of nanowires and nanodots through various deposition techniques including atomic layer deposition, supercritical fluid deposition (SCFD), selective epitaxial growth (SEG), and nanohybrid process utilising self-assembled [29]. Thin film transistor (TFT) used for medical sensors is one of the developments in nanodevices. TFT sensors can be used for measurement of temperatures ranging between 28°C and 40°C. These conditions are suitable for human body and other medical applications [30]. Organic thin film materials can be used for TFT fabrication, and some of them include polyacetylene, polythiophene, poly(3-hexylthiophene), and polythienylenevinylene or organic molecules like pentacene and α-ω-dihexyl-quaterthiophene [31]. Various thin film materials have been developed for nanodevice application; for example, diamond-like carbon (DLC) and amorphous-silicon carbide thin films are used for making nanoimprint lithography. Mechanical and structural properties of these thin films are enhanced by physical vapour deposition (PVD) [32]. With the increasing complexity and multi-functionality of smart devices, there has been a growing need of stretchable, foldable, and compressible gadgets. Through thin film materials, a number of devices have been developed. Fabrication of such devices is through high temperature deposition of thin film materials on flexible substrates and e­ pitaxial-lift-off from rigid substrate [33]. Some of the flexible substrates used are Hastelloy (C-276), stainless steel, and polycrystalline Ni alloy. To deposit thin films of epitaxial flexible oxide materials on various substrates, a buffer layer is added between the substrates and thin films. Epitaxial thin films of oxides are light and highly flexible and thus suitable for applications in wearable electronics such as detectors, sensors, and electronic skins. Table 9.1 shows some of the epitaxial thin films deposited on flexile substrates. Other flexible (oxide) thin films include Pb(Zr0.53Ti0.47)O3, SrRuO3, Pb(Zr,Ti)O3, NdNiO3, La0.67, Sr0.33MnO3, ITO, YSZ, BaTiO3, AZO, etc. [33].

9.7 FUTURE OUTLOOK The demand for high speed data storage and transfer devices is steadily increasing. The demand for spintronics, photonics, flexible, and smart devices will double in the next few years. This is driven by the Fourth Industrial Revolution (Industry 4.0). Industry 4.0 dictates for adoption of digital technologies such as additive manufacturing and virtual/augmented reality, among others. There will be increased use of flexible thin film materials for sensor applications in various sectors such as health

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TABLE 9.1 Some of the Epitaxial Thin Film Oxide Materials for Flexible Devices, Their Metal Substrates, Corresponding Buffer Layers, and Methods of Deposition [33] Oxide

Buffer Layers

γ-Al2O3

IBAD-MgO(∼10 nm)

γ-Al2O3

Homo-MgO(∼30 nm)/ IBAD-MgO(∼10 nm) PLD-MgO(∼50 nm)

γ-Al2O3 Pb(Zr,Ti)O3

YBa2Cu3O7 − δ Ce0.9Zr0.1O2 − y Y2O3

LMO/homoepitaxial-MgO/IBAD-MgO/ amorphous-Y2O3 La0.7Sr0.3MnO3/IBAD-TiN LMO/homoepitaxial-MgO/IBAD-MgO/ amorphous-Y2O3/amorphous-Al2O3 LaMnO3/IBAD-TiN YSZ (001) –

Cu2O

STO/CeO2/YSZ/Y2O3/NiW

(BiFeO3)0.5:(BiMnO3)0.5 BaTiO3

La2Zr2O7

–

(K,Na)NbO3

–

Substrate Hastelloy (C-276) alloy Hastelloy (C-276) alloy Biaxially textured Ni-3at.% W(Ni-W) Hastelloy (C-276) alloy Hastelloy alloy Polycrystalline Ni alloy Hastelloy alloy Stainless steel (SS) Biaxially textured Ni–5%W Biaxially textured Ni–5%W Biaxially-textured Ni–5%W Cube-textured Ni–5%W

Method of Deposition PLD-IBAD PLD-IBAD PLD PLD-IBAD PLD-IBAD PLD-IBAD PLD-IBAD PLD-IBAD Reactive sputtering Reactive sputtering CSD CSD

Source: Reused from Elsevier Ltd under Creative Commons licence.

and industrial processes. The demand for wearable devices is on the rise and shall push the market for flexible thin film materials. With the advancement in smart technologies for livelihoods, researchers and industry shall focus on developing new and low-cost thin film materials for applications in spintronics, nanoelectronics, optics, and flexible gadgets. One of the focus areas of development will be on hybrid and organic thin film materials for these areas to fabricate small and low-energy-consuming gadgets and technologies. The development of smart and self-healing thin film materials for these applications shall also be a future focus for optics, microelectronics, and spintronics applications.

9.8 SUMMARY This chapter presents a discussion on thin film materials for microelectronics, optics, spintronics, and flexible thin film gadgets. The thin film materials for optical, spintronics, microelectronics, and flexible devices have been presented. The importance

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of spintronics and optics in the modern industry is described. Optics and spintronics technologies are very useful in electronic and telecommunication industries. Spintronic devices find applications in information storage, processing, and transmission; this is possible through manipulation of spin degree of freedom of electrons. Thin film materials for optical applications can be classified as oxide-based and non-oxide-based ceramics and metal-based thin films. In microelectronics, thin film materials are used for conductivity and interconnects of components, insulators, and protective coatings. The application of thin film materials for flexible gadgets is a huge opportunity for the development of thin film market. Various thin films can be used for flexible gadgets, and epitaxial oxide thin films deposited on flexible substrates via a buffer layer are some of the examples. Finally, a future outlook of thin film materials for these applications has been presented.

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12. H. Ebendorff-Heidepriem et al., “Fluoride glass microstructured optical fibre with large mode area and mid-infrared transmission,” in OECC/ACOFT 2008- Joint Conference of the Opto-Electronics and Communications Conference and the Australian Conference on Optical Fibre Technology, Jul. 2008, vol. 33, no. 23, pp. 1–2, doi: 10.1109/ OECCACOFT.2008.4610317. 13. D. Sando, Y. Yang, C. Paillard, B. Dkhil, L. Bellaiche, and V. Nagarajan, “Epitaxial ferroelectric oxide thin films for optical applications,” Appl. Phys. Rev., vol. 5, no. 4, p. 041108, Dec. 2018, doi: 10.1063/1.5046559. 14. C. G. Granqvist, “Electrochromics for smart windows: Oxide-based thin films and devices,” Thin Solid Films, vol. 564, no. 4, pp. 1–38, Aug. 2014, doi: 10.1016/j.tsf.2014.02.002. 15. I. Žutić, J. Fabian, and S. Das Sarma, “Spintronics: Fundamentals and applications,” Reviews of Modern Physics, vol. 76, no. 2. pp. 323–410, Apr. 23, 2004, doi: 10.1103/ RevModPhys.76.323. 16. M. S. Rider, S. J. Palmer, S. R. Pocock, X. Xiao, P. Arroyo Huidobro, and V. Giannini, “A perspective on topological nanophotonics: Current status and future challenges,” J. Appl. Phys., vol. 125, no. 12, p. 120901, Mar. 2019, doi: 10.1063/1.5086433. 17. Y. Yang, T. Liu, L. Bi, and L. Deng, “Recent advances in development of magnetic garnet thin films for applications in spintronics and photonics,” J. Alloys Compd., vol. 860, p. 158235, Apr. 2021, doi: 10.1016/j.jallcom.2020.158235. 18. V. Sharma and B. K. Kuanr, “Magnetic and crystallographic properties of rare-earth substituted yttrium-iron garnet,” J. Alloys Compd., vol. 748, pp. 591–600, Jun. 2018, doi: 10.1016/j.jallcom.2018.03.086. 19. S. A. Siddiqui, D. Hong, J. E. Pearson, and A. Hoffmann, “Antiferromagnetic oxide thin films for spintronic applications,” Coatings, vol. 11, no. 7, p. 786, Jun. 2021, doi: 10.3390/ coatings11070786. 20. P. Robaschik, Organic Semiconductor Thin Films for Spintronic Applications. London: Imperial College, 2018. 21. Y. Zhang, L. Guo, X. Zhu, and X. Sun, “The application of organic semiconductor materials in spintronics,” Front. Chem., vol. 8, no. October, pp. 1–8, Oct. 2020, doi: 10.3389/fchem.2020.589207. 22. D. T. Read and A. A. Volinsky, “Thin films for microelectronics and photonics: Physics, mechanics, characterization, and reliability,” in Micro- and Opto-Electronic Materials and Structures: Physics, Mechanics, Design, Reliability, Packaging, E. Suhir, Y. C. Lee, and C. P. Wong, Eds. Boston, MA: Springer US, 2007, pp. A135–A180. 23. S. Asgary et al., “Magnetron sputtering technique for analyzing the influence of RF sputtering power on microstructural surface morphology of aluminum thin films deposited on SiO2/Si substrates,” Appl. Phys. A, vol. 127, no. 10, p. 752, Oct. 2021, doi: 10.1007/s00339-021-04892-0. 24. Y. F. Hu et al., “A study of titanium nitride diffusion barriers between aluminium and silicon by X-ray absorption spectroscopy: The Si, Ti and N results,” J. Synchrotron Radiat., vol. 8, no. 2, pp. 860–862, Mar. 2001, doi: 10.1107/S0909049500018252. 25. Y. Aoki, “Superhydrophobic coating fabricated by electrophoretic deposition using polydimethylsiloxane-based organic–inorganic hybrid materials and ceramic powders,” Mol. Cryst. Liq. Cryst., vol. 704, no. 1, pp. 10–16, Jun. 2020, doi: 10.1080/15421406. 2020.1741795. 26. S. Cataldo and B. Pignataro, “Polymeric thin films for organic electronics: Properties and adaptive structures,” Materials (Basel)., vol. 6, no. 3, pp. 1159–1190, Mar. 2013, doi: 10.3390/ma6031159. 27. D. A. McLean, “Thin films in microelectronics,” Thin Solid Films, vol. 8, no. 1, pp. 1–17, Jul. 1971, doi: 10.1016/0040-6090(71)90092-7.

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28. X. Zhang, M. Gao, L. Tong, and K. Cai, “Polypyrrole/nylon membrane composite film for ultra-flexible all-solid supercapacitor,” J. Mater., vol. 6, no. 2, pp. 339–347, Jun. 2020, doi: 10.1016/j.jmat.2019.11.004. 29. H. Kim, H.-B.-R. Lee, W.-H. Kim, S.-J. Park, and I.C. Hwang, “Nanomaterials fabrication using advanced thin film deposition and nanohybrid process,” in 2009 IEEE Nanotechnology Materials and Devices Conference, Jun. 2009, pp. 3–4, doi: 10.1109/ NMDC.2009.5167578. 30. Y. Hu, L.-Q. Guo, C. Huo, M. Dai, T. Webster, and J. Ding, “Transparent nano thin-film transistors for medical sensors, OLED and display applications,” Int. J. Nanomedicine, vol. Volume 15, pp. 3597–3603, May 2020, doi: 10.2147/IJN.S228940. 31. T. K. Ghosh, A. Dhawan, and J. F. Muth, “Formation of electrical circuits in textile structures,” in Intelligent Textiles and Clothing, H.R. Mattila, Ed. Sawston: Elsevier, 2006, pp. 239–282. 32. M. Bossard, J. Boussey, B. Le Drogoff, and M. Chaker, “Alternative nano-structured thin-film materials used as durable thermal nanoimprint lithography templates,” Nanotechnology, vol. 27, no. 7, p. 075302, Feb. 2016, doi: 10.1088/0957-4484/27/7/075302. 33. W. Liu and H. Wang, “Flexible oxide epitaxial thin films for wearable electronics: Fabrication, physical properties, and applications,” J. Mater., vol. 6, no. 2, pp. 385–396, Jun. 2020, doi: 10.1016/j.jmat.2019.12.006.

10

Thin Film Materials for Energy Applications

10.1 ENERGY MATERIALS AND RENEWABLE ENERGY DEVICES 10.1.1 Introduction The advancement in technology and industrialisation has brought about the challenge of diminishing fossil fuels, and the pollution emanating from these fossil fuels has led to climatic changes and global warming [1]. These problems have led to the growing adoption and use of sustainable and clean energy materials and devices. Energy materials are so used majorly in the storage of green energy. Green energy has been realised through the discovery of bioenergy, wind power, the invention of photovoltaic devices, etc. Energy materials have been tailored to meet essential and functional component requirements. For a material to be used in energy device construction, it must have any of the following properties: magnetic, corrosion resistance, glow, and many more which are specific to different applications [2]. The energy materials and renewable energy devices are discussed in detail in the following section.

10.1.2 Solar Cells and Photovoltaic Materials Solar energy is among the sources of clean energy nowadays. The conversion of sunlight into electrical energy is attained through the use of solar cells. There are different types of photovoltaic (PV) solar cells such as silicon solar cells, compound mono-crystalline solar cells (such as gallium arsenide (GaAs)), compound poly-­ crystalline solar cells (such as CdTe and copper indium gallium selenide (CIGS) thin film PV), organic and hybrid solar cells, and tandem solar cells [3–5]. Silicon solar cells utilise silicon as the main semiconductor material. Silicon has been used for a very long time as a photovoltaic material [6]. Silicon solar cells contribute a majority of the PV cells in the market, accounting for about 85% of the world’s solar market in 2011. Initially, crystalline silicon cells were used, but due to their shortage as of 2009, amorphous silicon became popular and thin film technology was also adopted [7]. However, recently researchers have carried extensive studies in the usage of other semiconductor materials to overcome the challenge of availability of raw materials and device efficiency. Compound mono-crystalline solar cells use GaAs as a semiconductor material. It is similar to silicon although GaAs can absorb light more efficiently when very thin layers than that of silicon are used. GaAs solar cells can attain an efficiency of between 25% and 30%. Compound poly-crystalline solar cells are of two types, namely (1) solar cell using CdTe thin film as a semiconductor, and (2) solar cell using CIGS semiconductor DOI: 10.1201/9781003202615-10

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material. Organic and hybrid solar cells are made of PV-conjugated polymers that can be single- or double-layer devices [3]. Tandem solar cells are third-generation solar cells, and they are designed to have two solar cells overlapping inside and working as one.

10.1.3 Fuel Cells Fuel cells are devices that generate electrical energy from chemical energy without combustion. Hydrogen is used as the fuel in these cells, and the resulting products of chemical conversion are usually electricity, heat, and water. These cells generally exhibit higher efficiencies than combustion engines and can transform the chemical energy in the fuel (hydrogen) directly into electrical energy with efficiency ranges of 40%–80% [8]. Fuel cell performance and stability rely heavily on the materials making up its components. The major components of a fuel cell are the electrode and electrolyte. Basically, a suitable electrode is that which can be manufactured in three stable phases to be thermodynamically stable and electrocatalytically active. On the other hand, a good electrolyte is based on its thermodynamic stability and operating temperature. There are four basic types of fuel cells, classified according to the type of electrolyte.

i. The first type is known as the alkaline fuel cell (AFC). This type is used for military applications and in aerospace. The electrodes are composed of a catalyst such as Pt or Au/Pt on carbon/Teflon gas layer. Ni has also been used lately because of its performance and economy. Electrolyte is composed of a solution of KOH and NaOH [8]. ii. The second type is known as polymeric electrolyte membrane fuel cell (PEMFC) which utilises membrane technology. The electrode is a nanostructure Pt on a gas diffusion layer. This layer is made of porous carbon mixed with Nafion, a solid conducting polymer. iii. The third type is known as phosphoric acid fuel cell (PAFC), a fuel cell commonly used in power plants and generators. The electrode is made up of graphite with Pt catalyst and concentrated phosphoric acid as the electrolyte [8]. iv. Finally, the other type of a fuel cell is called molten carbonate fuel cell (MCFC). It is suitable for use in high power distribution. The electrodes are usually made up of Ni and molten carbonate of Li-K as the electrolyte.

10.1.4 Wind Turbines Another renewable energy device is a wind turbine. A wind turbine is a system that converts wind kinetic energy into mechanical energy. The design of the turbines usually depends on the performance requirements and application. There are three types of wind turbines: (1) horizontal axis wind turbine, (2) horizontal rotor/multiblade wind turbine, and (3) DARRIEUS or vertical axis wind turbine [9]. A greater percentage of applications of wind turbines is in electrical power generation. In these applications, the choice of materials for blade construction is very important. The first working turbine was constructed and used in the US in 1941. The blades were

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made of steel and they failed after a short period. The second one, Gedser turbine, worked for 11 years without breakdown and it was made of composite material [10]. Therefore, the current design involves the use of composite material to build blades, whereas metals are used in building towers and generators. The composites are of different types: glass and carbon fibres, aramid and basalt fibres, hybrid composites, and natural fibres. Natural fibres are used to make small capacity turbines in developing countries and are usually locally available materials [10].

10.1.5 Nuclear Reactors Nuclear energy is a result of atomic fission or fusion. Each atom except hydrogen has got protons and neutrons in their nucleus held in place by nuclear forces. These are responsible for the atomic masses of elements. Elements (except Fe-56) usually tend to either gain or lose mass naturally or through induction. The mass gain or loss is accompanied by the release of nuclear energy which can be utilised to generate electricity. The density of the energy released is very immense, and if a nuclear reactor is not designed properly, it may lead to disastrous accidents [11]. Safety features considered in the design of nuclear reactors include chemical inertness to coolant and cladding, high melting point, good thermal conductivity, mechanical strength, etc. [12]. The key components of a nuclear reactor are fuel, cladding tube, moderator, control rod, coolant, reactor pressure vessel/pressure tubes, steam generator, and containment. Water is used as a moderator and a coolant to serve the purpose of retarding the fast reaction. Graphite and beryllium are used as reflectors to prevent neutron leakage from the pressure vessel. Low alloy steel is a material that is used in the construction of reactor pressure vessels. Fuel cladding is achieved by the use of UO2 ceramic pellets [11].

10.2 THIN FILM MATERIALS FOR SOLAR CELL DEVICE APPLICATIONS 10.2.1 Introduction The studies on solar and PV cells have been extensive since 1950s. Since then, crystalline silicon was the only material used to convert solar energy to electrical energy. The research focus on optimisation of materials and their respective efficiencies on the solar cells led to the adoption of other technologies, a notable one being thin film technology [13]. A thin film material is a surface coating formed on another material to enhance or introduce certain desired material properties [14]. The thickness of such coatings ranges from nanometrs to micrometres. Properties that are modified by introducing thin films on bulk materials include electrical; mechanical (e.g., abrasion resistance); chemical; and optical properties. Thin film technology has become so attractive in the production of solar cells since it is cheap and a small amount of target material is needed. Even though efficiencies of thin film solar cells can be low, more research is being done on avoiding this problem [15]. Some of the thin film materials for solar cell applications are discussed below.

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10.2.2 Copper Indium Selenide/Copper Gallium Selenide CIGS is a semiconductor of solid solution of copper indium selenide and copper gallium selenide with the compound formula Cu(In,Ga)Se2 [14,16]. Solar panels made from these thin film materials are usually highly flexible, since CIGS can be formed on a flexible substrate [14]. Given that CIGS has a very high absorption coefficient (more than 105/cm for 1.5 eV), extreme thin film material is required compared to other semiconductor materials. The benefits of CIGS include low material usage, simple manufacturing procedure, and high radiation resistance. The CIGS efficiency is about 22.8%, which can be compared to that of crystalline silicon [17]. In the past, CIGS thin films were formed through high temperature deposition methods, but improvements have been made so much such that low temperature deposition techniques are now used. This has led to a better performance close to that of polysilicon solar panels. The performance of CIGS PV cells has been boosted by the introduction of molybdenum (Mo) as a back material. Molybdenum is characterised by very high relative stability at high processing temperatures and good conductivity [15].

10.2.3 Cadmium Telluride Thin Films Cadmium telluride (CdTe) is a semiconductor material with high mobility and simple controllability of n- to p-type conductivity, and thus it may be used in semiconductors like diodes and/or transistors [18]. It was first prepared by Margottet, a French chemist, through the reaction of cadmium and tellurium at high temperatures. The result was CdTe crystalline material. Initially, CdTe was used as a pigment (up to 1946). However, it was discovered that incomplete phosphors found in Cd thin films were highly photosensitive to the range of up to β- and γ-radiations. Due to the ease of CdTe fabrication in thin film, it become possible to produce proof γ-ray and X-ray detectors. In 1948, the first CdTe photocells were produced. Thin film CdTe photovoltaic devices are usually grown on superstate material of glass type, and in most cases, barium-silicate glass [19]. CdTe PV cells technology has grown extensively compared to other technologies, especially to crystalline silicon solar cells. This has been attributed to lower production costs [14]. In addition, CdTe material is a very good absorber of incident light up to 1 µm from the cell surface due to its high optical absorption coefficient [13]. Its energy gap is suitable for PV energy conversion. A typical CdTe film forms a p-n heterojunction, whereby the CdTe is the p-type layer while the n-type layer is cadmium sulphide (CdS). CdTe layer forms through closed-space sublimation, while the CdS layer is formed by chemical bath deposition [14]. Like CIGS, CdTe thin films can be formed on flexible substrate materials.

10.2.4 Amorphous Silicon (a-Si) Liquid crystal display (LCD) thin film transistors and solar cells employ the use of amorphous (non-crystalline), a-Si. The difference between amorphous silicon and monocrystalline silicon is on the atomic bond structure. In monocrystalline silicon,

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the atoms form a tetrahedral regular structure, while in a-Si the structure is noncontinuous with a random network of hanging bonds. The semiconductor properties of a-Si are derived from these hanging bonds [14]. Like the earlier discussed thin film materials, a-Si thin films can be formed on flexible surfaces such as glass, plastic, and even metals. A-Si thin films can be formed in several ways including chemical vapour deposition (CVD), catalytic CVD, or even sputtering.

10.2.5 Dye Sensitised Solar Cell Dye sensitised solar cell (DSSC) was developed by Professor M. Grätzel in 1991 [20]. Sunlight is collected on its surface by dye adsorbed on a nanocrystalline semiconductor such as TiO2. It is so distinctive that it is the only flexible and transparent cell [21]. It is a PV device that is known to be a cheap material and offers moderate efficiency. The DSSC is made up of four elements, namely, transparent conducting electrodes and counter-conducting electrodes, nanostructured wide bandgap semiconducting thin layer, the sensitiser, and electrolyte [22]. The working principle of a DSSC is somehow like that of photosynthesis. In this case, incident photons are captured by the photo-sensitiser and then absorbed on the anode thin layer of TiO2. Liquid electrolytes are responsible for the transfer of ions to the electrode, thus forming a continuous circuit [7,15]. This material is so versatile and efficient, since the solar cell can reach an efficiency of up to 12.3%. TiO2 thin film can be deposited through a doctor blade technique or spin coating method. In the doctor blade technique (sometimes referred to as tape casting), a well-mixed ceramic particles slurry is introduced on the surface of a substrate. A blade moving at a constant on the surface of substrate results in deposition of a gellayer TiO2 on drying. On the other hand, the spin coating method involves coating by centrifugal force. The solution is deposited by rotating (spinning) the blade at high speed.

10.2.6 Perovskite Solar Cells Perovskite solar cells (PSCs) are made up of perovskite compound (named after L.A. Perovski, a Russian mineralogist), which was discovered in 1839 by Gustav Rose. The PSCs technology is solid-state sensitised technology stemming from dye-sensitised Gratzel PV solar cells [23,24]. The first specific crystal was found in CaTiO3(ABX3). A hybrid organic-inorganic material was formed by replacing a cation A with organic cations like CH 3 NH 3+ , C2 H 5 NH 3+ , and HC(NH2)+. On the other hand, cation B was replaced by divalent cations Pb+2 , Sn +2 , or Cu +2 . X represents halides such as Cl−, Br−, and I−. The difference between PSCs and DSSCs is in the excitation; i.e., PSCs do not need external force for excitations, while DSSCs require external force for excitation [15]. PSCs thin films can be produced through the drop casting method. In this process, controlled impingements of solutions are dropped on a substrate where they spread on the surface forming a non-uniform layer. This method is considered to be more efficient than spray coating [15]. Table 10.1 summarises some of the materials used for thin film solar cell fabrication.

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TABLE 10.1 Thin Film Materials for Solar Cell Applications Solar Cell Materials

Method of Deposition

Properties

Indium (III) sulphide (In2S3) thin films deposited on antimony trisulfide (Sb2S3) solar cells Antimony selenide thin films

Low In2S3 is used as electron transport layers for solar cells. temperature It has high electron mobility (~ 17.6 cm2/V·s), wide band gap (2.4 eV), and high thermal and chemical spray stability. pyrolysis It is environmentally clean. The coating increases the photoelectric efficiency (PCE) of the Sb2S3 thin film solar cell to ~4.03%. Vapour The material has high absorption coefficient. transport It has suitable bandgap. deposition When prepared with cadmium sulphide/antimony selenide for solar cells, a power conversion of 7.6% is achieved. Tin mono-sulphide Vapour It is used as an absorber for thin film solar cells SnS/ (SnS) transport CdS). deposition When intrinsic ZnO is deposited on these solar cells, power conversion efficiency of 3.5% is achieved. Pulsed laser They are thermally stable. Sulfurised deposition They have high power conversion efficiency. Cu2ZnSnS4 thin films They have excellent optoelectronic properties. Cu2FeSn(S0.8Se0.2)4 Thermal The films have a band gap of 1.49 eV. thin film evaporation They have an absorption coefficient of 1.38 × 104 cm−1. materials method

Reference Cui et al. [25]

Wen et al. [26]

Kumar et al. [27]

Hu et al. [28] Tripathi et al. [29]

10.3 THIN FILM MATERIALS FOR NUCLEAR APPLICATIONS 10.3.1 Introduction Thin film material is a layer that is formed on the surface of another bulk material (substrate) to enhance some desired physical and chemical properties. It is formed through either physical (PVD) or chemical (CVD) deposition processes [30]. Future generation of nuclear reactors needs to be designed with engineered materials that meet specific properties such as chemical structure, anisotropy, physical strength, electrical conductivity, and good response to mechanical and thermal stress [31]. Thin films in nuclear reactors are used as protective coatings in the containment structures of a nuclear reactor [32].

10.3.2 Detectors The desire for safety in using special nuclear materials (SNMs) has attracted research into the applicability of thin film detectors. These devices comprise the sensor and amplification circuit. Amplification of the sensor signal is necessary for proper processing. The system that has been used widely involves the use of X-rays, employing

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the use of thin film transistors (TFTs) while the sensors are passive pixel sensors (PPSs) [33]. Since SNMs have isotopes, it is possible to use the indirect method of detection in nuclear reactors. In this method, the fast-moving neutrons are thermalised, absorbed, and converted in the sensor to generate a signal that can be amplified and processed. Thin film-based active pixel sensors like indium gallium zinc oxide (IGZO) and a-Si TFTs have been widely used in signal amplification, although the challenge has arisen in amplifying signal generated from the neutron capture. An alternative and better method has been discovered in polysilicon technology [33].

10.3.3 Cladding The critical requirement or concern in the design of the current nuclear reactors has been radiation hardness. A study done on the Fukushima accident which occurred in March 2011 revealed that the devastating effects of the accident could have been averted if proper cladding were included in the design of the reactors. The nuclear reactor cores melted due to cooling system failure since the tsunami that occurred had caused a loss of power to the cooling system [34]. It was concluded that a stronger cladding on the inner cores could have retarded the melting of the cores, thereby minimising the accident’s effects [35]. Researches have birthed the realisation of this objective by using high entropy thin film (HEATFs) alloys as opposed to earlier bulk alloys [36]. These alloy materials are a mixture of at least five elements in nearly equal quantities. The simplest cladding is the use of SiC in zirconium-steam reaction, as shown in Figure 10.1. Without cladding, the zircaloy tubes are corroded due to the formation of cracks of outer monoclinic and inner tetragonal structures of ZrO2 which is porous to oxygen. With the introduction of SiC coating, corrosion is prevented because SiC is highly resistant to corrosion and chemical reactions at high temperatures [35]. HEATFs are thought to be stabilised by high entropy of mixing so many elements at equiatomic quantities which favour the solid solution phase rather than the intermetallic phase [37]. It has been found out that thin film alloys are much stronger compared to bulk manufactured alloys due to the nanostructure of the thin films. Refractory NbTaMoW has been found to be the best HEATF that forms a very hard coating and can be used in the extreme conditions of nuclear reactions. This thin film has been developed through sputter deposition methods [36]. Chromium has also been used to produce a thin film cladding on zirconium to prevent oxidation at high steam temperatures in the reactors. Physical vapour deposition has been used in this process. Microstructural studies have shown that there is the formation of Cr2O3 coating which acts as an oxygen diffusion barrier [38]. This alloy thin film is the most economical and easiest to produce. Accident-tolerant films (ATFs) like Cr2AlC has also been developed to protect Zr claddings. These thin film alloys are formed through High power impulse magnetron sputtering (HiPIMS). The resulting thin film coating is annealed at a high temperature to produce a very stable coating [39]. A study that was conducted by Qingsong Chen et al. showed that FeCrMnNi HEA could also be used as ATF since it possesses very good radiation hardness. They also researched on CrCuFeMoNi deposited through magnetron sputtering, and they discovered that it is superior in hardness and high

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FIGURE 10.1  A schematic view of zircaloy tube with cladding [35].

temperature corrosion resistance due to Cr2O3 and FeCr2O4 layer. This makes it suitable for use as ATF in nuclear reactors [40]. HfNbTiVZr is another HEA researched by Stefan Fritze et al. [41], and thin films were deposited through the DC-magnetron sputtering method [41]. With this alloy, it is possible to obtain different microstructure thin films in the form of amorphous, single-phase BCC, or dual-phase which possess different mechanical properties.

10.3.4 Insulators Insulation is very important in reactor containment building (RCB). Thermal and anti-sweat insulation materials that were used previously were of the filler types such as fibreglass, calcium silicate, expanded perlite, and mineral wool fibre [42]. SiC has been proposed to be a component of nuclear reactors since it has excellent structural properties and is highly stable under harsh environments [35]. Other properties associated with SiC are high hardness and stiffness, excellent thermal conductivity, steady mechanical strength at high temperatures, minimal thermal expansion coefficient, small neutron capture, irradiation stability, and lower density compared to other ceramic materials. As a layer, SiC is used in the tri-­structural isotropic (TRISO) fuels and zircaloy. It is also applied as an insulator in the separators and flow channel inserts and sandwiched between the tritium breeding and neutron multiplier Li-Pb blanket modules [35]. TRISO fuels coated with SiC are

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FIGURE 10.2  Cross-view of TRISO coatings deposited on kernel [35].

highly suitable to be used in nuclear reactors since they can withstand extremely high temperatures which imply an extended core lifetime in the reactor. The pictorial view of the SiC insulated TRISO fuels and zircaloy is shown in Figure 10.2. The formation technique of SiC coatings is principally high temperature CVD. It has the advantages of producing a uniform thin film of high quality and high density and nanocrystalline structure. Other methods such as thermal/plasma spraying also do exist at atmospheric pressure. This method cannot produce stronger coatings of the same quality as thermal CVD. The sputtering process can also be used, but it is expensive. The sol-gel method produces low-density coating; hence, it is not suitable for their preparation. A blended process known as plasma-enhanced CVD can be quite attractive since deposition can be achieved at lower pressures and this reduces unwanted gas-phase reactions while at the same time improving the uniformity of the coating [35].

10.4 APPLICATION OF THIN FILMS FOR FUEL CELLS 10.4.1 Introduction A fuel cell is an equipment that produces electrical energy from the electrochemical combination of hydrogen and oxygen in an electrolyte; NaOH and KOH are some of the electrolytes used. They have a higher efficiency since they directly convert chemical energy to electrical energy, producing more energy from the same quantity of fuel. It can be regarded as green energy since it only emits water and heat (when hydrogen and oxygen are the reactants) as a waste product in the process [43].

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However, the by-products could vary depending on the fuel used, for example, carbon dioxide or hydrocarbons. Fuel cells can be categorised into PEMFC, phosphoric fuel cell, AFC, and solid oxide fuel cell, just to mention a few [44].

10.4.2 Low Temperature Fuel Cells Since fuel cells generally entail reactions at high temperatures, high performance materials are essential for producing the device parts. This is achieved by the application of thin film deposition to modify the initial material properties. Solid oxide fuel cell is a type of cell made of tight solid electrolyte oxides that conduct ions. At the cathode side, oxygen is reduced producing electrons, while at the anode, oxygen ions react forming compounds depending on the fuel used, e.g., H2O [45]. To optimise the performance of the fuel cell, material properties such as microstructures, transport properties, and exchange kinetics should be improved. This also reduces the operating temperature and hence lowers the cost. Zirconium oxide was historically used as an electrolyte but has been overtaken by yttrium stabilised zirconia (YSZ), providing good conductivity at temperatures above 800°C. Cathode and anode performances are also improved with the use of strontium-doped lanthanum (LSM) and Ni-YSM cermet for former and later, respectively. However, it was noted that operational temperature is lowered close to 850°C to attain an optimal operational point (in terms of efficiency, lifetime, and cost). Thin film deposition technology is vital in attaining this intermediate temperature. While operating at considerably low temperatures, losses that could result are avoided using higher ionic conductivity and thinner solid electrolytes membranes with improved electrodes. Thinner electrolytes should be optimum to avoid cross mixing of reactants. A highly porous, defect-free, and dense layer of about 50 µm is deposited on the electrolyte substrate. Commonly used mechanisms to produce highquality films are sol-gel and physical vapour depositions such as sputtering [46]. Anodes made from ceramics (or oxides) doped with elements like LaCrO3 have also been exploited to replace Ni and Cu. Additionally, cathodes fabricated in a layered manner, with the help of thin film technology, have also been considered [47]. Yoon Ho Lee in his paper [48] also exploited thin film solid oxide fuel cells (TFSOFCs) operating under 600°C. This is possible with the use of anisotropic etching of Si and nanofabrication forming of thin electrolytes that enable the TF-SOFCs to operate in temperatures of 300°C–500°C. Fabrication steps of thin film solid oxide fuel cell (TFSOFC) are described as follows [48]: i. A reactive ion cuts double polished silicon ii. Deposition of Si2N4 layers on both sides of the Si substrate. iii. Electrolyte is then deposited on the silicon nitride. iv. The bottom side Si2N4 is removed by KOH and the top Si2Ni4 is removed by reactive ion etching. v. Thin film electrolyte is then finally deposited on both sides.

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Direct alcohol fuel cells and solid AFCs utilise electrodes made up of porous catalysts. CVD is being exploited to produce nanostructured porous carbon thin fibres used for low temperature fuel cells as a replacement. Electro-deposition of salts is done to obtain catalyst coating on the carbon thin fibres. Two categories of nanostructured carbon supports are used, namely nanotubes and thin films. These efficient catalysts produced reduce fabrication costs while lowering the operational temperatures of the fuel cells. For the electrodes, plasma sputtering has been used to deposit platinum catalyst alternatively with a supporting metal to modify the seeding and morphology of the catalyst. This improves the performance of the fuel cell with less catalyst deposition/ loading [49]. Due to its high activity, a nanostructured column electrode produced through sputtering has been considered for low-temperature SOFCs. Proper control of the contact angle and rotational speed of the substrate increase the performance of the anode (made from Ni in this case) and hence increase the cell’s power density [50]. According to experimental results conducted in 1997 by Alan F. Jankowski, conventional electrodes could be replaced with ceramic electrodes synthesised through vacuum deposition [51]. Ni-coated zirconia powder was produced, hence reducing metallic contents by 10% without affecting conductivity. This electrode exhibited a coefficient of thermal expansion almost equal to that of the electrolyte, eliminating thermal-cycle-induced failure on the SOFC. To meet the dense, oxygen-ion conductor requirements of the porous electron conductor, and for compatibility, thin film electrolytes were also fabricated. Physical vapour deposition, especially sputtering, was preferred. In conclusion, with the increase in population, energy demand has been sky-­ rocketing. This has necessitated the need for cleaner and cheaper energy sources. Fuel cells have offered an alternative to this, and hence the need for low temperature fuel cells like TFSOFCs. Low temperature fuel cells are designed to reduce operating temperature and cost without compromising on their performance. This has been facilitated by the use of thin film technology to modify the components of the galvanic cell. Thinner solid oxide electrolytes, improved electrodes, and use of ceramic electrodes have reduced these fuel cells’ operational cost and temperature.

10.5 THIN FILM MATERIALS FOR WIND AND HYDRO-POWER SYSTEMS 10.5.1 Wind-Power System Wind-power generation is a form of green and renewable source of energy. With the increasing demand for power worldwide, standing at 282,587 MW in 2013, wind power has been developed and supported to supplement conventional power sources like hydroelectricity and fossil sources. Wind power generation is achieved with the use of a wind turbine coupled with an alternator. Therefore, a turbine is one of the main components of a wind-power generator [52]. Besides the required toughness, strength, and flexibility, other desirable properties for a wind turbine are high wear resistance and high fatigue strength (which is

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due to friction and pitting). In operation, the turbine blades are usually exposed to very high impacts from objects such as raindrops, atmospheric particles, sand, and hail. The impact causes erosion damage at the edge of the blades which eventually degrades the power output and life of the wind-generation system. In addition, the blades are exposed to the following conditions: • • • • • • •

Temperature ranging between −50°C and 70°C Biological contamination such as blood and faeces from birds and insects Lightning and mechanical erosion Ice conditions UV radiation from sunlight Chemical attacks from various chemicals Air salinity

As such, the surface coating of the blades is important and coating technologies including some thin film methods can be used as surface engineering processes. Nitriding or plasma nitriding has been exploited for surface engineering of turbine blades. E-IONIT OX, a combination of nitriding, carburising, plasma activation, and oxidation process, has been used. The process occurs at 450°C–500°C temperatures and results in a coating ranging from 1 μm to a maximum of 0.8 mm thickness. The process results in coatings that are as hard as case-hardened surfaces. The coating exhibits a white appearance, high ductility, high hardness, and resistance to wear and pitting. The coatings also exhibit high stiffness and fatigue strength resulting in an improved component’s lifetime. E CARBO WCH, a commercial amorphous carbon coating, is another essential application for wind power plant. It protects the components against tribological wear and tribo-oxidation. This process takes place at a temperature of about 200°C through physical vapour deposition such as the sputtering process. A typical coating of 1–5 μm is formed due to the condensation of metallic materials combined with nitrogen. This is applied for hydraulics, gears, pneumatic, and other transmission components [53]. Thin film technologies are used to deposit coatings, which improve the hardness and durability of the turbine components like the blades, gears, and shafts bearing. These components normally operate under high loading and always under metalon-metal contact [54]. For instance, the sol-gel process (described in Chapter 2) is a ­chemical-based process that can be used to manufacture thin film coatings for wind turbine blades without compromising the weight requirements of the wind turbine. The advantage of using thin film technology over other coating methods is its capability to create thin coatings, which adds negligible weight to the blades [55]. Most coatings used for blade protection are polymer-based such as unsaturated polyester, epoxy, polyurethane, polyvinylidene fluoride, and acrylic [56,57]. Through sol-gel process, coatings reinforced with carbon nanotube and graphene have been produced. Some of these include graphene-modified polyurethane, carbon nanotube (CNT) reinforced resins, SiC-reinforced resin coatings, multi-walled carbon nanotube epoxy coatings, etc. [58]. The other area of application of thin film technology in wind energy is on monitoring. It is important to monitor the performance of the mechanical and electrical

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systems of the wind turbine for efficient power generation and maintenance. In most cases, the turbines are manufactured with fibre-reinforced polymeric materials which are prone to damages such as cracks and debonding. These damages should be monitored to ensure the corrective measures are carried out in time. As such, thin film sensors are embedded in the wind turbine blades to detect any damages in the system [59]. Thin film technologies can be used to produce piezoelectric materials that can generate energy from small vibrations of wind. Thin film technologies such as PVD, ALD, and CVD have been used to synthesise various piezoelectric thin film materials for energy harvesting. Some of these materials include zirconate titanate (PZT), PZT [pb(Zr,Ti)O3], ZnO, AlN, lead magnesium niobate-lead titanate (PMN-PT), barium titanate (BaTiO3), zinc stannate (ZnSnO3), biomaterials (e.g. polydimethylsiloxane, PDMS), and so many other thin film materials [60,61].

10.5.2 Hydro-Power System Hydropower system is the main source of renewable energy for generating electricity in most parts of the world. The turbine and its components form the basic units used in the conversion of the kinetic energy of water to mechanical and finally electrical energy. Corrosion and erosion affect the blades profile and surface texture, leading to efficiency loss. This can be controlled by the use of thin film coating methods such as the thermal/cold spray technique. Through thermal/cold spray coating of hydropower turbine blades, the corrosion and erosion effects are reduced considerably by up to 50%, which increases the lifetime of the turbine components by up to three times. For instance, coatings of Inconel 625 and CrC-NiCr deposited on stainless steel 316 for hydropower applications enhance erosion protection of the base metal [62]. High velocity flame sprayed Ni-TiO2 and Ni-TiO2-Al2O3 coatings have been demonstrated to offer excellent protection to hydro turbine steel structures under slurry erosion [63]. Coating of turbine surfaces (through thin films) maximises resistance for hydroabrasive erosion and thus longevity of the turbine life. High velocity oxide fuel (HVOF) coatings such as Cr3C2–25NiCr and WCCoCr have shown good performance in protecting turbine steels under slurry erosion conditions [64]. In slurry erosive conditions, the erosion is caused by hard minerals such as water sediments which cause erosion on the surfaces of the turbine blades. Nickel-based tungsten carbide coatings are extensively used in these applications since they increase the turbine components’ surface ductility, which improves resistance to erosion [65]. Superhydrophobic coating materials are attractive for friction reduction on water turbines. It is said that these materials increase the turbine efficiency by 4% [66]. These coatings are self-cleaning and highly resistant to corrosion and exhibit antiicing properties. Coating materials of oxides, carbides, nitrides, and soft non-metallic materials such as epoxy, nylon, and polyurethane are very applicable in hydropower turbines. Tungsten carbide-based coatings, for instance, exhibit dense and defect-free microstructure, high hardness, and resistance to erosion. Several materials and nanomaterials for surface coating of turbines against erosion and corrosion have been developed and some of them are summarised in Table 10.2.

CVD

Detonation gun

TiC/TiN and TiC

stellite-6, Cr3C2-NiCr and WC-Co-Cr

Inconel-718

Stellite-6

MS

CA6NM

SMAW

APS

Flame sprayed

Cemented carbide Hot rolled 21Cr-4Ni-N steel

MS

OFP and HVOF

LSA

Colmonoy 88 and Stellite 6 powders

Ni-1(95% Ni and 5% Al), Ni-2(Ni, Cr, Mo, Ti), WC/Co-Ni and lastly Cr Ni-Cr‐Si‐B

HVOF

Technique Used

Aluminium bronze

Coating Material

CA6NM

AISI 1020 carbon steel and cast nickel aluminium bronze 13Cr–4Ni

Substrate

(Continued)

Minimum erosion losses were observed at a 30° impingement angle in WC-Co-Cr coating. In stellite-6 coating, the maximum porosity concentration was noted. Erosion damage was maximum at 90° impingement angle. Stellite-6 micro hardness was 1098 HV. As the thickness of coating increased, porosity decreased and an increase in hardness increased the erosion resistance. As compared to uncoated, the coated specimen showed better resistance against erosion. Stellite 6 slurry erosion resistance was higher for all impact angles than CA6NM steel. Micro-cutting and ploughing were the dominant wear mechanisms in slurry erosion.

Macroscopically coating deposit appeared to be good without cracks. The surface hardness of Colmonoy 88 and Stellite 6 compared to substrate material increases 1.2–2 times. A linear relationship was observed between slurry erosive wear rate and angle of impingement. The material removal mechanism appeared to be a microstructure and hardness function. By applying thermal spray coatings, resistance of steel can be enhanced up to 16 times. HVOF sprayed WC/Co-Ni showed better resistance against erosion. Micro-cutting and micro ploughing were the main mechanisms of material removal. Resistance improved marginally as a result of coatings. Mechanism of wear was pitting, ploughing, and indentation. TiC layer showed better resistance against erosion.

Coating erosion occurred by cutting wear, plastic deformation, and crack propagation. Smaller particle impacts dominated the cutting process. The erosion rate of HVOF Al bronze coating was 40 times