TRIBOLOGY OF POLYMERIC NANOCOMPOSITES
TRIBOLOGY OF POLYMERIC NANOCOMPOSITES Editor B.J. Briscoe (U.K.) Advisory Board M.J. Adams (U.K.) J.H. Beynon (U.K.) D.V. Boger (Australia) P. Cann (U.K.) K. Friedrich (Germany) I.M. Hutchings (U.K.)
Vol. 27 Vol. 28 Vol. 29 Vol. 30 Vol. 31 Vol. 32 Vol. 33 Vol. 34 Vol. 35 Vol. 36 Vol. 37 Vol. 38 Vol. 39 Vol. 40 Vol. 41 Vol. 42 Vol. 43 Vol. 44 Vol. 45 Vol. 46 Vol. 47 Vol. 48 Vol. 49 Vol. 50 Vol. 52 Vol. 53 Vol. 54
J. Israelachvili (U.S.A.) S. Jahanmir (U.S.A.) A.A. Lubrecht (France) I.L. Singer (U.S.A.) G.W. Stachowiak (Australia)
Dissipative Processes in Tribology (Dowson et al., Editors) Coatings Tribology – Properties, Techniques and Applications in Surface Engineering (Holmberg and Matthews) Friction Surface Phenomena (Shpenkov) Lubricants and Lubrication (Dowson et al., Editors) The Third Body Concept: Interpretation of Tribological Phenomena (Dowson et al., Editors) Elastohydrodynamics – ’96: Fundamentals and Applications in Lubrication and Traction (Dowson et al., Editors) Hydrodynamic Lubrication – Bearings and Thrust Bearings (Frêne et al.) Tribology for Energy Conservation (Dowson et al., Editors) Molybdenum Disulphide Lubrication (Lansdown) Lubrication at the Frontier – The Role of the Interface and Surface Layers in the Thin Film and Boundary Regime (Dowson et al., Editors) Multilevel Methods in Lubrication (Venner and Lubrecht) Thinning Films and Tribological Interfaces (Dowson et al., Editors) Tribological Research: From Model Experiment to Industrial Problem (Dalmaz et al., Editors) Boundary and Mixed Lubrication: Science and Applications (Dowson et al., Editors) Tribological Research and Design for Engineering Systems (Dowson et al., Editors) Lubricated Wear – Science and Technology (Sethuramiah) Transient Processes in Tribology (Lubrecht, Editor) Experimental Methods in Tribology (Stachowiak and Batchelor) Tribochemistry of Lubricating Oils (Pawlak) An Intelligent System For Tribological Design In Engines (Zhang and Gui) Tribology of Elastomers (Si-Wei Zhang) Life Cycle Tribology (Dowson et al., Editors) Tribology in Electrical Environments (Briscoe, Editor) Tribology & Biophysics of Artificial Joints (Pinchuk) Tribology of Metal Cutting (Astakhov) Acoustic Emission in Friction (Baranov et al.) High Pressure Rheology for Quantitative Elastohydrodynamics (Bair)
Aims & Scope The Tribology Book Series is well established as a major and seminal archival source for definitive books on the subject of classical tribology. The scope of the Series has been widened to include other facets of the now-recognised and expanding topic of Interface Engineering. The expanded content will now include: • colloid and multiphase systems; • rheology; • colloids; • tribology and erosion; • processing systems; • machining; • interfaces and adhesion; as well as the classical tribology content which will continue to include • friction; contact damage; • lubrication; and • wear at all length scales.
TRIBOLOGY AND INTERFACE ENGINEERING SERIES, 55 EDITOR: B.J. BRISCOE
TRIBOLOGY OF POLYMERIC NANOCOMPOSITES Friction and Wear of Bulk Materials and Coatings
Klaus Friedrich Alois K. Schlarb
Amsterdam • Boston • Heidelberg • London • New York • Oxford Paris • San Diego • San Francisco • Singapore • Sydney • Tokyo
Elsevier The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, UK Radarweg 29, PO Box 211, 1000 AE Amsterdam, The Netherlands First edition 2008 Copyright © 2008 Elsevier Ltd. All rights reserved No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone (+44) (0) 1865 843830; fax (+44) (0) 1865 853333; email:
[email protected]. Alternatively you can submit your request online by visiting the Elsevier web site at http://elsevier.com/locate/permissions, and selecting Obtaining permission to use Elsevier material Notice No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-444-53155-1 ISSN: 1572-3364 For information on all Elsevier publications visit our web site at books.elsevier.com Printed and bound in Great Britain 08 09 10
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CONTENTS
FOREWORD LIST OF CONTRIBUTORS 1
Tribological applications of polymers and their composites: Past, present and future prospects Brian J. Briscoe and Sujeet K. Sinha
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1
PART I: BULK COMPOSITES WITH SPHERICAL NANOPARTICLES 2
3
4
5
The influence of nanoparticle fillers in polymer matrices on the formation and stability of transfer film during wear Shyam Bahadur and Cris J. Schwartz
17
Synergistic effects of nanoparticles and traditional tribo-fillers on sliding wear of polymeric hybrid composites Li Chang, Zhong Zhang, Lin Ye and Klaus Friedrich
35
The influence of nanoparticle fillers on the friction and wear behavior of polymer matrices Qihua Wang and Xianqiang Pei
62
Tribological behavior of polymer nanocomposites produced by dispersion of nanofillers in molten thermoplastics Stepan S. Pesetskii, Sergei P. Bogdanovich and Nikolai K. Myshkin
82
6
Sliding wear performance of epoxy-based nanocomposites Ming Qiu Zhang, Min Zhi Rong and Ying Luo
7
Wear simulation of a polymer–steel sliding pair considering temperature- and time-dependant material properties László Kónya and Károly Váradi
108
130
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PART II: USE OF CARBON NANOTUBES IN BULK COMPOSITES 8
On the friction and wear of carbon nanofiber-reinforced PEEK-based polymer composites Holger Ruckdäschel, Jan K. W. Sandler and Volker Altstädt
149
9 Wear behavior of carbon nanotube-reinforced polyethylene and epoxy composites Olaf Jacobs and Birgit Schädel
209
10
Tribology properties of carbon nanotube-reinforced composites Gong Qianming, Li Dan, Li Zhi, Yi Xiao-Su and Liang Ji
245
PART III: SCRATCH/WEAR RESISTANCE OF NANOCOMPOSITES/ COATINGS 11
Wear and wear maps of coatings Koji Kato
12
Hybridized carbon nanocomposite thin films: Synthesis, structures and tribological properties Eiji Iwamura
283
Friction and sliding wear of “nanomodified” rubbers and their coatings: Some new developments József Karger-Kocsis and Dávid Felhös
304
13
271
14
Scratch resistance of protective sol-gel coatings on polymeric substrates Zhong Chen and Linda Y. L. Wu
325
15
Scratch behavior of polymeric materials Robert L. Browning, Han Jiang and Hung-Jue Sue
354
16
Wear and scratch damage in polymer nanocomposites Aravind Dasari, Zhong-Zhen Yu and Yiu-Wing Mai
374
PART IV: DEVELOPMENTS OF NANOCOMPOSITES/COATINGS FOR SPECIAL APPLICATIONS 17 Polytetrafluoroethylene matrix nanocomposites for tribological applications David L. Burris, Katherine Santos, Sarah L. Lewis, Xinxing Liu, Scott S. Perry, Thierry A. Blanchet, Linda S. Schadler and W. Gregory Sawyer
403
18
Development of nanostructured slide coatings for automotive components Andreas Gebhard, Frank Haupert and Alois K. Schlarb
439
19
Friction and wear behavior of PEEK and its composite coatings Ga Zhang, Hanlin Liao and Christian Coddet
458
Contents 20
vii
Polymer composite bearings with engineered tribo-surfaces Jayashree Bijwe, Werner Hufenbach, Klaus Kunze and Albert Langkamp
483
21 Novel nanocomposites and hybrids for lubricating coating applications Namita Roy Choudhury, Aravindaraj Govindaraj Kannan and Naba K. Dutta
501
INDEX
543
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FOREWORD
The area of tribology deals with the design, friction, wear and lubrication of interacting surfaces in relative motion. Polymer composite materials have been used increasingly for such tribological applications in recent years. Yet, by now, much of the knowledge on their tribological behavior is often empirical, and very limited predictive capability currently exists. Nevertheless, it has been attempted in several books and scientific papers of the last two decades to determine to what degree phenomena governing the friction and wear performance of polymer composites can be generalized (see e.g. refs. [1–5] and references therein). Within the past 10 years, many developments of new polymers composites have incorporated nanofillers as reinforcing agents, resulting in the term “polymeric nanocomposites”. In fact, it has been demonstrated that these fillers of very small dimension (as compared to the classical micrometersized fibers or particles) can also result in remarkable improvements in the friction and wear properties of both bulk materials and coatings. Therefore, it is the intention of this book to give a comprehensive description of polymeric nanocomposites, both as bulk materials and as thin surface coatings, and their behavior and potential use in tribological applications. The preparation techniques, friction and wear mechanisms, properties of polymeric nanocomposites, characterization, evaluation and selection methodology in addition to application examples will be described and discussed. One aim of the book is to bring together, systematically in a single volume, the state-of-the-art knowledge on the tribology of polymeric nanocomposites and coatings. This has previously been difficult to achieve and overview because the information has only been available in the form of numerous separate articles not linked logically together. More than 20 groups of authors worldwide, many of them well known in the tribology community since years, have agreed to write down their particular expertise on nanocomposites’ tribology in individual chapters. The latter cover not only different types of polymer matrices, i.e. from thermosets to thermoplastics and elastomers, but also a variety of micro- and nanofillers, from ceramic nanoparticles to carbon nanotubes, in combination with traditional tribo-fillers, such as short carbon fibers, graphite flakes and polytetrafluoroethylene (PTFE) particles. The coatings can be prepared on ceramic metallic or polymeric substrates and applied in many different applications, including automotive, aerospace and mechanical engineering. In Chapter 1 by Briscoe and Sinha, in which tribological trends for polymer composites, both traditional and nanocomposites, are presented, using data currently available in the literature, the book is structured into four main sections. The first one is dedicated to the tribology of bulk polymer
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Foreword
composites against metallic counterparts, with particular emphasis to the use of spherical nanoparticles. S. Bahadur and C. J. Schwartz report on the influence of nanoparticle fillers in polymer matrices on the formation and stability of transfer films during wear. Synergistic effects of nanoparticles and traditional fillers on the sliding wear of polymeric hybrid composites are discussed by L. Chang et al. In more detail, Q. Wang and X. Pei illustrate that the volume content, the size and the shape of the nanoparticles may exert a great influence on the friction and wear behavior of various polymer matrices. In the following chapter by Pesetskii, Bogdanovich and Myshkin, special emphasis is focused on the tribological behavior of thermoplastic nanocomposites, containing as fillers carbon nanomaterials, layered clays, metals and metal-containing compounds. Sliding wear of thermosetting nanocomposites based on an epoxy resin matrix is, on the other hand, the topic of the contribution by M. Q. Zhang and his group. In the conclusion of this section, L. Kónya and K. Váradi present some wear simulation studies of a polymer–steel sliding pair by considering temperature and time-dependent material properties. The second section of this book concentrates on the use of carbon nanotubes and nanofibers as reinforcements in bulk nanocomposites against metallic counterparts. Ruckdäschel, Sandler and Altstädt give a comprehensive overview on the friction and wear of carbon nanofiber-reinforced PEEK-based polymer composites. Based on their promising results, the performance of advanced nanocomposite hybrid materials for an intended industrial tribological application is discussed. The chapter of O. Jacobs and B. Schädel elucidates the effect of carbon nanotube reinforcement on the sliding wear of epoxy resin and of ultra high molecular weight polyethylene against two different steel counterparts. It also shows that the dispersion method of the carbon nanotubes has a remarkable influence on the tribological properties. Finally, the friction and wear characteristics of randomly dispersed vs. well-aligned carbon nanotubes in two different matrices, i.e. epoxy vs. carbon, is outlined by Q. Gong et al. The third main section of this book relates to the problem of scratch/wear resistance of nanocomposites and their coatings. In the spirit of the now-classical treatment of mechanical data in the form of fracture and failure maps, K. Kato describes, in general, wear and corresponding wear maps of hard coatings. In the following, E. Iwamura introduces two types of hybridized carbon films with different nanocomposite configurations with regard to their tribological properties. In particular, the structurally modified column/inter-column films showed a high wear resistance in spite of a distinctively poor film hardness. Some new developments in the field of wear of “nanomodified” rubbers and their coatings, studied under three different test configurations, are presented in the chapter of J. Karger-Kocsis and D. Felhös. It is the intention to use these new developments in automotive seal applications. Another coating material, i.e. sol–gel coatings on polymer substrates, has been widely used in optical lenses, safety windows and flexible display panels. Z. Chen and L. Y. L. Wu review in their chapter the scratch failure modes of such coatings on polymeric substrates, the related failure mechanisms and the parametric models related to these failure modes. A new scratch testing and evaluation method is presented by R. L. Browning, H. Jiang and H.-J. Sue. The potential of this procedure is demonstrated with regard to the scratch behavior of acrylic coatings and epoxy nanocomposites. Finally, a wide survey of the existing literature on scratch and wear damage in polymer nanocomposites, followed by own results on the scratch behavior of various nanoclay and other nanoparticle-filled polymer systems, is given by A. Dasari, Z.-Z. Yu and Y.-W. Mai. In the fourth section, chapters are found which especially focus on the tribological use of naocomposites and their coating for special applications. The group of W. G. Sawyer gives a comprehensive review on the development of PTFE matrix nanocomposites for the use of these systems in moving mechanical devices. A. Gebhard, F. Haupert and A. K. Schlarb report about the application of nanostructured slide coatings for automotive components, such as engine piston skirts and polymer/metal-slide bearings. A special topic, namely the friction and wear of nanoparticle-filled PEEK and its composite coatings applied by flame spraying or painting on metallic parts, is described by G. Zhang, H. Liao and C. Coddet. In a more traditional composite-processing method, Bijwe, Hufenbach, Kunze and Langkamp report about the development of polymer composite bearings with engineered tribosurfaces. They can be used in extreme conditions of temperature (cryogenic to 300 ◦ C), vacuum to
Foreword
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high-pressure environment, especially where liquid lubrications cannot be considered. On the contrary, well-lubricating coating and thin film applications are the focus of the chapter by N. R. Choudhury, A. G. Kannan and N. Dutta. The materials are organic–inorganic hybrids, their methods of preparation, their relevance to biomaterials’ friction, their fundamental mechanics of tribology and the lubrication mechanisms of such coatings and thin films. When considering the content of this book as a whole, it becomes clear that it is primarily intended for scientists in academia and industry, who are involved or want to become involved in tribology problems, and who look for new solutions in materials’ development and for particular applications. The book will be, therefore, a reference work and a guide to practice for those who are or want to become professional in the field of polymer composites tribology. By preparing this book, we hope that we have managed to take the first step toward a systematic structure in this complex field of technology. At the same time, we believe that this is timely, but only a first attempt to cover a topic that is in the process of rapid development since the last couple of years. We are sure that many more interesting results on the tribology of nanocomposite materials will be published in the open literature in the near future. Finally, we would like to thank all the contributors who managed to include their thoughts and results in this special book. We are also grateful to many other scientists who made their contributions by taking part in the extensive peer-reviewing process. These reviewers include: G. W. Stachowiak (Australia); A. Sviridenok (Belarus); P. De Baets (Belgium); S. Y. Fu, Y. Meng, L. Zhang (China); M. E. Vigild (Denmark); A. Fischer, T. Gradt, B. Lauke, M. Scherge, K. Schulte, J. Schuster (Germany); V. Kostopoulos (Greece); J. S. Wu (Hong Kong); S. Fakirov (New Zealand); A. O. Pozdnyakov (Russia); J. Botsis (Switzerland); V. Karbhari (USA). The Editors Klaus Friedrich Alois K. Schlarb
References [1] K. Friedrich, ed., Advances in Composite Tribology, Composite Materials Series, Vol. 8 (series editor: R. B. Pipes), Elsevier, Amsterdam, The Netherlands, 1993. [2] K. Holmberg, A. Matthews, Coatings Tribology: Properties, Techniques and Applications in Surface Engineering, Tribology Series, Vol. 28 (series editor: D. Dowson), Elsevier, Amsterdam, The Netherlands, 1994. [3] G. W. Stachowiak, A. W. Batchelor, Engineering Tribology, Elsevier Butterworth-Heinemann, Oxford, UK, 2005. [4] A. Sethuramiah, Lubricated Wear: Science and Technology, Tribology Series, Vol. 42 (series editor: D. Dowson), Elsevier, Amsterdam, The Netherlands, 2003. [5] G. W. Stachowiak, A. W. Batchelor, G. B. Stachowiak, Experimental Methods in Tribology, Tribology Series, Vol. 44 (series editor: D. Dowson), Elsevier, Amsterdam, The Netherlands, 2004.
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LIST OF CONTRIBUTORS
Volker Altstädt Universität Bayreuth, Polymer Engineering – FAN A, Universitätsstrasse, Bayreuth, Germany Shyam Bahadur Mechanical Engineering Department, Iowa State University, Ames, IA, USA. Jayashree Bijwe Industrial Tribology Machine Dynamics & Maintenance Engineering Centre (ITMMEC), Indian Institute of Technology, Delhi, Hauz Khas, New Delhi, India Thierry A. Blanchet Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute Sergei P. Bogdanovich V.A. Belyi Metal-Polymer Research Institute of National Academy of Sciences of Belarus, Gomel, Belarus Brian J. Briscoe Department of Chemical Engineering & Chemical Technology, Imperial College, London, UK Robert L. Browning Polymer Technology Center, Department of Mechanical Engineering, Texas A&M University, College Station, TX, USA David L. Burris University of Florida, Mechanical & Aerospace Engineering, Gainesville, FL, USA Li Chang Centre for Advanced Materials Technology, School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, Australia Zhong Chen School of Materials Science & Engineering, Nanyang Technological University, Singapore Namita Roy Choudhury ARC Special Research Centre for Particle and Material Interfaces, Ian Wark Research Institute, University of South Australia, Mawson Lakes, South Australia, Australia
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Christian Coddet LERMPS, University of Technology of Belfort- Montbéliard, Belfort, France Li Dan Department of Mechanical Engineering, Tsinghua University, Beijing, P.R. China Aravind Dasari Centre for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical and Mechatronic Engineering J07, The University of Sydney, Sydney, Australia Naba K. Dutta ARC Special Research Centre for Particle and Material Interfaces, Ian Wark Research Institute, University of South Australia, Mawson Lakes, South Australia, Australia David Felhös Institut für Verbundwerkstoffe GmbH (Institute for Composite Materials), University of Kaiserslautern, Kaiserslautern, Germany Klaus Friedrich Centre for Advanced Materials Technology, School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, Australia and Institut für Verbundwerkstoffe GmbH (IVW), (Institute for Composite Materials), University of Kaiserslautern, Kaiserslautern, Germany Andreas Gebhard Institut für Verbundwerkstoffe GmbH (IVW), (Institute for Composite Materials), University of Kaiserslautern, Kaiserslautern, Germany Frank Haupert Institut für Verbundwerkstoffe GmbH (Institute for Composite Materials), University of Kaiserslautern, Kaiserslautern, Germany Werner Hufenbach Technische Universitaet Dresden, Institut fuer Leichtbau und Kunststofftechnik, Dresden, Duererstr, Dresden-Johannstadt, Germany Eiji Iwamura Arakawa Chemical Industries, Ltd., Tsurumi, Tsurumi-Ku, Osaka, Japan Olaf Jacobs Fachhochschule Lübeck, Fachbereich Maschinenbau und Wirtschaftsingenieurwesen, Mönkhofer Weg, Lübeck, Germany Liang Ji Department of Mechanical Engineering, Tsinghua University, Beijing, P.R. China Han Jiang Polymer Technology Center, Department of Mechanical Engineering, Texas A&M University, College Station, TX, USA Aravindaraj Govindaraj Kannan ARC Special Research Centre for Particle and Material Interfaces, Ian Wark Research Institute, University of South Australia, Mawson Lakes, South Australia, Australia József Karger-Kocsis Institut für Verbundwerkstoffe GmbH (Institute for Composite Materials), University of Kaiserslautern, Kaiserslautern, Germany
List of Contributors
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Koji Kato Mechanical Engineering Department, College of Engineering, Nihon University, Koriyama City, Japan László Kónya Institute of Machine Design, Budapest University of Technology and Economics, Mûegyetem, Budapest, Hungary Klaus Kunze Technische Universitaet Dresden, Institut fuer Leichtbau und Kunststofftechnik, Dresden, Duererstr, Dresden-Johannstadt, Germany Albert Langkamp Technische Universitaet Dresden, Institut fuer Leichtbau und Kunststofftechnik, Dresden, Duererstr, Dresden-Johannstadt, Germany Sarah L. Lewis Materials Science and Engineering, Rensselaer Polytechnic Institute Hanlin Liao LERMPS, University of Technology of Belfort-Montbéliard, Belfort, France Xinxing Liu Materials Science and Engineering, Rensselaer Polytechnic Institute Ying Luo College of Science, South China Agriculture University, Guangzhou, P.R. China Yiu-Wing Mai Centre for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical and Mechatronic Engineering J07, The University of Sydney, Sydney, Australia Nikolai K. Myshkin V.A. Belyi Metal-Polymer Research Institute, National Academy of Sciences of Belarus, Gomel, Belarus Xianqiang Pei State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, P.R. China Scott S. Perry Materials Science and Engineering, University of Florida, Gainesville, FL, USA Stepan S. Pesetskii V.A. Belyi Metal-Polymer Research Institute of National Academy of Sciences of Belarus, Gomel, Belarus Gong Qian-Ming Department of Mechanical Engineering, Tsinghua University, Beijing, P.R. China Min Zhi Rong Materials Science Institute, Zhongshan University, Guangzhou, P.R. China Holger Ruckdäschel Universität Bayreuth, Polymer Engineering – FAN A, Universitätsstrasse, Bayreuth, Germany Jan K.W. Sandler Universität Bayreuth, Polymer Engineering – FAN A, Universitätsstrasse, Bayreuth, Germany
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Katherine Santos Materials Science and Engineering, University of Florida, Gainesville, FL, USA W. Gregory Sawyer University of Florida, Mechanical & Aerospace Engineering, Gainesville, Florida, USA Birgit Schädel Fachhochschule Lübeck, Fachbereich Maschinenbau und Wirtschaftsingenieurwesen, Mönkhofer Weg, Lübeck, Germany Linda S. Schadler Materials Science and Engineering, Rensselaer Polytechnic Institute Alois K. Schlarb Institut für Verbundwerkstoffe GmbH (Institute for Composite Materials), University of Kaiserslautern, Kaiserslautern, Germany Cris J. Schwartz Mechanical Engineering Department, Texas A&M University College Station, TX, USA Sujeet K. Sinha Department of Mechanical Engineering, National University of Singapore, Singapore Hung-Jue Sue Polymer Technology Center, Department of Mechanical Engineering, Texas A&M University, College Station, TX, USA Zhong-Zhen Yu Centre for Advanced Materials Technology (CAMT), School of Aerospace, Mechanical and Mechatronic Engineering J07, The University of Sydney, Sydney, Australia Károly Váradi Institute of Machine Design, Budapest University of Technology and Economics, Mûegyetem, Budapest, Hungary Qihua Wang State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, P.R. China Linda Y. L. Wu Singapore Institute of Manufacturing Technology, Singapore Yi Xiao-Su National Key Laboratory of Advanced Composites (LAC/BIAM), Beijing, P.R. China Lin Ye Centre for Advanced Materials Technology, School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, Sydney, Australia Ga Zhang LERMPS, University of Technology of Belfort-Montbéliard, Belfort, France Ming Qiu Zhang Materials Science Institute, Zhongshan University, Guangzhou, P.R. China Zhong Zhang National Center for Nanoscience and Technology, Beijing, P.R. China Li Zhi Department of Mechanical Engineering, Tsinghua University, Beijing, P.R. China
CHAPTER 1
Tribological applications of polymers and their composites: Past, present and future prospects Brian J. Briscoe and Sujeet K. Sinha
Contents 1.1 1.2 1.3 1.4 1.5 1.6
Introduction Classical works on polymer tribology Tribology of polymer composites Tribology of polymer nanocomposites Future prospects Final remarks References
1 2 6 7 11 12 13
Abstract This chapter presents a brief account of the current state-of-the-art in the area of the tribology of polymers and their composites. The classical explanation of friction based upon the “two-term model” is presented. Further, important factors affecting friction and wear of polymers from the design and materials selection perspectives are described in detail. Tribological trends for polymer composites, both traditional and nanocomposites, are presented using data currently available in the literature. Finally, based on our current understanding of this field, we have speculated upon some future trends and directions in the area of polymer tribology. Our assessment is naturally very subjective and selective given the vast potential for research growth in this field. This review is not meant to be exhaustive, and hence readers will naturally need to refer other chapters presented in this book for a much more detailed knowledge of the area of polymer tribology in general and tribology of nanocomposites in particular.
1.1 Introduction Polymers play an important part in materials and mechanical engineering, not just for their ease in manufacturing and low unit cost, but also for their potentially excellent tribological performance in engineered forms [1]. In the pristine or bulk form, only a few of the polymers would satisfy most of the tribological requirements, however, in the composite and hybrid forms, polymers often have an advantage over other materials such as metals and ceramics. Polymer tribology, as a research field, is now well-mature given that roughly 50 plus years have seen publication of numerous research articles and reports dealing with a variety of tribological phenomena on a considerably large number of polymers, in bulk, composite and hybrid forms. Tribological applications of polymers include gears, a range of bearings, bearing cages, artificial human joint bearing surfaces, bearing materials for space applications including coatings, tires, shoe soles, automobile brake pads, non-stick frying pans, floorings and various types of surfaces for optimum tactile properties such as fibers. The list is growing. For example, in the new area of micro-electromechanical systems (MEMS), polymers (such as PMMA and PDMS)
1
2
Tribology of Polymeric Nanocomposites
are gaining popularity as structural materials over the widely used material, Si [2]. Often, Si is modified by a suitable polymeric film in order to enhance frictional, anti-wear or anti-stiction properties [3]. Similar to the bulk mechanical responses, the tribological characteristics of polymers are greatly influenced by the effects of temperature, relative speed of the interacting surfaces, normal load and the environment. Therefore, to deal with these effects and for better control of the responses, polymers are modified by adding appropriate fillers to suit a particular application. Thus, they are invariably used in composite or, at best, in blended form for an optimum combination of mainly friction and wear performances. Also, pragmatically fillers may be less expensive than the polymer matrix. The composition of the filler materials, often a closely guarded secret of the manufacturer, is both science and art, for the final performance may depend upon the delicately balanced recipe of the matrix and filler materials. However, the past many years of research in the area of polymer tribology in various laboratories have shed much light into the mechanisms of friction and wear. This has somewhat eased the work of materials selection for any particular tribological application. This chapter on the tribology of nanocomposite, an area which is still in its infancy, would endeavor to set the background of the research in polymer tribology. We will refer to the term “polymer” for synthetic organic solid in pristine form, with some additives but no fillers aimed at modifying mechanical properties. The word “composite” would be used when one or more than one filler has been added to a base polymer with the aim of drastically changing mechanical and tribological properties. “Nanocomposite” will mean a composite in which at least one filler material has one of its dimensions in the range of a few to several nanometers. The chapter would review some of the past, but now classical, works when much of the mechanisms of friction and wear for general polymers were studied and these explanations have stood the test of the time. The early works led to the area of polymer composites where polymers were reinforced with particles and/or short or long fibers. Often, the use of the filler materials has followed two trends that mainly reflect the actual function that the fillers are expected to perform. This type of work on the design of multi-phase tribological materials continues mainly aimed at improving an existing formulation, or, using a new polymer matrix or novel filler. The present trend is expected to extend into the future but with much more refinement in materials and process selections. For example, the use of nano-sized particle or fibers coupled with chemical enhancement of the interactions between the filler and the matrix seems to produce better tribological performance. Also, there have been some very recent attempts on utilizing some unique properties of polymers, often mimicking the biological systems in one way or the other, which has opened up new possibility of using polymers in tribological applications. One example of this is the polymer brush that can be used as a boundary lubricant. This trend will definitely continue into the future with great promises for solving new tribological issues in micro, nano and bio systems.
1.2 Classical Works on Polymer Tribology 1.2.1 Friction The earliest works on polymer tribology probably started with the sliding friction studies on rubbers and elastomers [4, 5]. Further work on other polymers (thermosets and thermoplastics) led to the development of the two-term model of friction [6]. The two-term model proposes that the frictional force is a consequence of the interfacial and the cohesive works done on the surface of the polymer material. This is assuming that the counterface is sufficiently hard in comparison to the polymer-mating surface and undergoes only mild or no elastic deformation. Fig. 1.1 shows a schematic diagram of the energy dissipation processes in the two-term model [7]. The interfacial frictional work is the result of adhesive interactions and the extent of this component obviously depends upon factors such as the hardness of the polymer, molecular structure, glass transition temperature and crystallinity of the polymer, surface roughness of the counterface and
Tribological Applications of Polymers and Their Composites
3
Velocity Normal load Hard surface
Polymer Interfacial zone Rigid asperity
Cohesive zone
Fig. 1.1. Two-term model of friction and wear processes. Total friction force is the sum of the forces required for interfacial and cohesive energy dissipations. Likewise, the distinction between interfacial and cohesive wear processes arises from the extent of deformation in the softer material (usually polymer) by rigid asperity of the counterface. For interfacial wear, the frictional energy is dissipated mainly by adhesive interaction while for cohesive wear the energy is dissipated by adhesive and abrasive (sub-surface) interactions [7] (with publisher’s permission).
chemical/electrostatic interactions between the counterface and the polymer. For example, an elastomeric solid, which has its glass transition temperature below the room temperature and hence very soft, would have very high adhesive component leading to high friction. Beyond interfacial work is the contribution of the cohesive term, which is a result of the plowing actions of the asperities of the harder counterface into the polymer. The energy required for the plowing action will depend primarily upon the tensile strength and the elongation before fracture (or toughness) of the polymer and the geometric parameters (height and the cutting angle) of the asperities on the counterface. The elastic hysteresis is another factor generally associated with the cohesive term for polymers that show large visco-elastic strains, such as in the case of rubbers and elastomers [8]. Further, both the interfacial and the cohesive works would be dependent upon the prevailing interface and ambient temperatures, and the rate of relative velocity as these factors would in turn modify the polymer’s other materials’ parameters. Pressure has some effect on the interfacial friction as normal contact pressure tends to modify the shear strength of the interface layer by a relation given as [9] τ = τo + α p
(1.1)
The implication of the above relation is that as the contact pressure increases, the shear stress would increase linearly leading to high friction. Eq. (1.1), as simple as it may look in the form, hides the very complex nature of polymer. Also, it does not include the temperature and the shear rate effects on the shear stress. In a normal sliding experiment, it is non-trivial to separate the two terms (interfacial and cohesive) and therefore most of the data available in the literature generally include a combined effect. Often, the practice among experimentalists is to fix all other parameters and vary one parameter to study its effect on the overall friction coefficient for a polymer. Looking at the published data one can easily deduce that depending upon other factors, the friction is greatly influenced by the class of polymers viz. elastomers, thermosets and thermoplastics (semi-crystalline and amorphous). Semi-crystalline linear thermoplastic would give lowest coefficient of friction whereas elastomers and rubbers show large values. This is because of the molecular architecture of the linear polymers that helps molecules stretch easily in the direction of shear giving least frictional resistance. Table 1.1 provides some typical values of the coefficient of friction for pristine or virgin polymers.
4
Tribology of Polymeric Nanocomposites Table 1.1. Friction coefficient of few polymers when slid against a steel disk counterface (surface roughness, Ra = 1.34 μm). Corresponding specific wear rates and the pressure (P) × velocity (V ) values are also presented. Polymer
Coefficient of Friction
Specific Wear Rate (×10−6 mm3 /Nm)
PV Value (Pa m/s)
PMMA PEEK UHMWPE POM Epoxy
0.48 0.32 0.19 0.32 0.45
1315.9 31.7 15.5 168.2 3506.6
145,560 149,690 187,138 149,690 153,997
Wear classification for polymers
Generic scaling approach
Two-term interacting model: • Cohesive wear • Interfacial wear
Phenomenological approach
Origin of wear process model: • Abrasive wear • Adhesive wear • Transfer wear • Chemical wear • Fatigue wear • Fretting wear • Erosion • Delamination wear
Material response approach
Polymer class model: • Elastomers • Thermosets • Glassy polymers • Semi-crystalline polymers
Fig. 1.2. Simplified approach to classification of the wear of polymers [7] (with publisher’s permission).
1.2.2 Wear The inevitable consequence of friction in a sliding contact is wear. Wear of polymers is a complex process and the explanation of the wear mechanism can be most efficiently given if we follow one of the three systems of classification. Depending on the classification, wear of a polymer sliding against a hard counterface may be termed as interfacial, cohesive, abrasive, adhesive, chemical wear, etc. Fig. 1.2 describes the classification of polymer wear [7]. It is to be noted that, similar to the case of friction, polymer wear is also greatly influenced by the type (elastomer, amorphous, semi-crystalline) of the polymer. Of particular importance are the properties such as the elastic modulus, tensile strength and the percentage elongation at failure (toughness), which changes drastically as we move from one type of polymer to another. Usually, high tensile strength coupled with a high elongation at failure promotes wear resistance in a polymer. Therefore, given all other factors remain constant, some of the linear thermoplastic polymers with semi-crystalline microstructure perform far better in wear resistance than thermosets or amorphous thermoplastics. These observations are in line with the idea that for polymers, surface hardness is not a controlling factor for wear resistance. In fact, high hardness of a polymer may be harmful for wear resistance in dry sliding against hard counterface as hardness normally comes with low toughness for polymers. High extents of elongation at failure of a polymer means that the shear stress in a sliding event can be drastically reduced due to extensive plastic deformation of the polymer within a very thin layer close to the interface. This interfacial layer accommodates almost all of the energy dissipation processes and thus the bulk of the polymer undergoes minimal deformation or wear.
Tribological Applications of Polymers and Their Composites
5
3.0E 02 M
Wear rate (mm3/Nm)
2.5E 02 K
2.0E 02
L 1.5E 02 J 1.0E 02 GH
I 5.0E 03
F E D
0.0E 00
A C B
0.0
0.1
0.2
0.3
0.4
1 1/(Se) (MPa )
Fig. 1.3. A plot of wear rate (mm3 /mm/kg) as a function of the reciprocal of the product of ultimate tensile stress and elongation to fracture [1]. The data are taken from literature. A , poly(ethylene); B, Nylon 66; C, PTFE; D, poly(propene); E, high-density poly(ethylene); F, acetal; G, poly(carbonate); H, poly(propylene); I, poly(ethyleneterephthalate glycol); J, poly(vinyl chloride); K, PMMA; L, poly(styrene); M, PMMA (refer to ref. [1] for the sources of the data; with publisher’s permission).
Frictional heat generated at the interface is the major impediment to high wear life of the polymer. Many classical and recent works suggest that the wear rate of polymers slid against metal counterface in abrasive wear condition may be given by a simple proportionality as [10, 11] Wsp · ∝
1 Se
(1.2)
The above equation, often described as the Ratner–Lancaster correlation, is supported by data obtained by several researchers (ref. [1] provides a summary of published data for the above relation, see Fig. 1.3). Eq. (1.2) is applicable across one type of polymer class such as semi-crystalline thermoplastics. We do not have data to compare this rule for other types of polymers. Because of the low friction and high wear resistance, many of the thermoplastics can be used in tribological applications without any reinforcement and notable among them are the UHMWPE and PEEK. UHMWPE has found extensive usage as bearing material for artificial human joints because of its excellent biocompatibility and wear resistance. PEEK, which is a high-temperature polymer, tends to show low wear rate but the coefficient of friction can be relatively high (∼0.3). PEEK is now a popular polymer as matrix for some new composites with the aim of formulating wear-resistant materials. Nylons are other tribological materials that show low friction and low wear. PTFE, a linear fluorocarbon, normally shows very low friction coefficient but, relative to many other thermoplastics, high wear rate due to its unique characteristic of slippage in the crystalline formation of the molecular bond structure. Due to low friction property, PTFE is a good solid lubricant if used in composite form. Amorphous thermoplastics such as PMMA and PS do not perform very well in a wear test. They show high coefficients of friction and high wear rates. Thermosetting polymers, though posses high hardness and strength among polymers, show very high wear rate and high coefficient of friction because of very low elongation at failure values. Thermosets are normally used in the form of composites as fiber strengthening can drastically reduce wear. Fiber strengthening sometimes improves the material’s resistance to sub-surface crack initiation and propagation giving reduced plowing by the counterface asperities or fatigue cracking. Interface friction can also be optimized by adding a suitable percentage of a solid lubricant. This trend has led to much research in recent times on producing composite or hybrid materials for optimum wear and friction control using epoxy or phenolic resins as the matrix [12].
6
Tribology of Polymeric Nanocomposites
1.3 Tribology of Polymer Composites Except for probably only UHMWPE and to some extent nylons, no other polymer is currently being used in its pristine form for a tribological application. The reason is that no polymer can provide a reasonable low working wear rate with optimum coefficient of friction required. Hence, there is a need to modify most polymers by a suitable filler that can reduce the wear rate and, depending upon the design requirement, either increase or decrease the coefficient of friction. Such a need was realized quite early on [13] and this trend has continued. The second component or the filler can perform a variety of roles depending upon the choice of the matrix and the filler materials. Some of these roles are strengthening of the matrix (high load-carrying capacity), improvement in the sub-surface crack arresting ability (better toughness), lubricating effect at the interface by decreased shear stress and the enhancement of the thermal conductivity of the polymer. The entire aspects of the tribology of polymer composites can be quite complex so as to defy any economic classification. Therefore, a simple but efficient way to handle this topic is to classify the composites according to the role of the filler material in the composite, by modifying the bulk or the interface [14].
1.3.1 Bulk modification: “Hard and strong” fillers in a “softer” matrix A self-lubricating polymer, such as PTFE, can be made wear resistant by strengthening the bulk with hard or strong filler material such as particles of ceramics/metals or a suitable strong fiber (carbon, aramid or glass fibers). The function of the filler here is to strengthen the polymer matrix and thus increase the load-bearing capacity of the composite. The coefficient of friction remains low or increases marginally but the wear resistance can be increased up to an order of magnitude. The disadvantage of using fillers (especially the particulate type) is that the composite material may become somewhat less tough in comparison to the pristine polymer and thus encourage wear by fatigue, however, this can be avoided by proper optimization of the mechanical and tribological properties. Strengthening by fibers, usually oriented normal to the sliding interface, has shown better result in terms of load-bearing capacity and toughness. A fiber that is non-abrading to the counterface, such as aramid and carbon fibers, is even more beneficial as it promotes the formation of a tenacious and thin transfer film on the counterface that can help in reducing the wear of the composite after a short running-in period. Examples of a composite where hard and strong filler has been added to a softer matrix are the PTFE/GF and Nylon 11/GF systems as shown in Fig. 1.4 [1]. As we can see, the coefficient of friction for each case has increased slightly and there is considerable gain in the wear resistance as a result of fiber strengthening.
1.3.2 Interface Modification: “Soft” and “lubricating” fillers in a “hard and strong” matrix This type of composite utilizes the low shear strength and self-lubricating properties of the filler to reduce the coefficient of friction and, as a result, wear and frictional heating is drastically reduced. The main requirement is the availability of the filler at the interface in sufficient amount such that a reduction in the coefficient of friction and an increase in the wear resistance can be realized. The disadvantage of this type of composite is obviously the reduction in the strength and load-carrying capacity of the material in the composite form. Hence, adding this type of filler beyond a certain percentage by volume or by weight would be counterproductive for tribological performance due to a drastic decrease in the bulk strength. Several researches have focused on finding an optimum ratio of the filler and the matrix to achieve maximum wear resistance [1]. PTFE and graphite in a variety of polymer matrices have been tried with good results; popular among matrices are epoxy, phenolic and PEEK. For both types of composites, the properties of the transfer film formed on the counterface will define whether the composite can have low wear rate or not. A strongly adhering and tenacious, yet lubricating, transfer film would reduce wear after the formation of the film during the running-in period.
Tribological Applications of Polymers and Their Composites
7
1.0E 01 1.0E 00
(i) PTFE
1.0E 02
(vii) Nylon1120.7GF
1.0E 03
(vi) Nylon115.6GF
0.4
(vi) Nylon11
1.0E 04 (v) PTFE15%CF
0.5
(iv) PTFE25%GF
1.0E 05
(iii) PTFE25%GF
0.6
(ii) PTFE15%GF
1.0E 06
0.3 0.2
Coefficient of friction
Specific wear rate (mm3/Nm)
1.0E 07
0.1
Fig. 1.4. The effect of fiber addition on the specific wear rates of a few polymers [1]. The rectangular bar chart indicates specific wear rates (units on the left of the graph) and vertical arrows indicate the coefficient of friction (units on the right of the graph). Test conditions are: (i) 440C steel ball (diameter = 9 mm) sliding on polymer specimen, normal load = 5 N, v = 0.1 m/s, roughness of polymer surface Ra = 400 nm, 30% humidity; (ii) test conditions same as for (i); (iii) test conditions same as for (i); (iv) reciprocating-pin-steel plate apparatus, counterface roughness Ra = 0.051 μm, N2 environment; (v) test conditions same as for (iv); (vi) pin-on-steel (AISI02 quench hardened) disk apparatus, counterface roughness Ra = 0.11 μm, p = 0.66 MPa, v = 1 m/s; (vii) test conditions same as for (vi); (viii) test conditions same as for (vi). Note: Refer to ref. [1] for the sources of the data; data for (iii) and (iv) are for the same formulation of the composite but the values are different as they have been taken from different research works; with publisher’s permission).
Bulky and thick film has the tendency to detach itself from the counterface, which may increase the wear rate due to a continuous film formation and detachment mechanism (transfer wear) as well as promoting thermal effects.
1.4 Tribology of Polymer Nanocomposites The use of nano-particles in polymers for tribology performance enhancement started around mid1990s and this area has become quite promising for the future as newer nanomaterials are being economically and routinely fabricated. In most of the cases, a polymer nanocomposite relies for its better mechanical properties on the extremely high interface area between the filler (nano-particles or nano-fibers) and the matrix (a polymer). High interface leads to a better bonding between the two phases and hence better strength and toughness properties over unfilled polymer or traditional polymer composites. For all polymer/nano-particle systems, there will be an optimum amount of the nanoparticles beyond which there will be a reduction in the toughness as the stiffness and strength increase. Table 1.2 summarizes friction and wear results of polymer nanocomposites with data taken from the published literatures [15–44]. There are mainly two types of polymer nanocomposites that have been tested for tribological performance. One type is where ceramic nano-particles, mainly metal and some non-metal oxides, have been added with the aim to improve load-bearing capacity and wear resistance of the material against the counterface. Examples of polymer nanocomposite systems of this type include fillers such as SiO2 , SiC, ZnO, TiO2 , Al2 O3 , Si3 N4 and CuO in polymer matrices such as epoxy, PEEK, PTFE and PPS. The specific wear rates of these nanocomposites have been reported in the range 10–100 times lower than the specific wear rates of the polymers without fillers when optimum weight percent of the nano-particles is introduced. The optimum composition depends upon the systems and is high for PTFE system where the nano-particles are required for the mechanical strength while PTFE still promotes the reduction of the coefficient of friction. For this system, there is no change in the coefficient of friction after adding nano-particles. For epoxy, PEEK and PPS polymers, the role of nano-particles is to increase the load-bearing capacity of the material and thus the actual contact area is reduced leading to lower frictional stress for the nanocomposite. Also, the presence of
Table 1.2. A summary of the tribological results on polymer nanocomposites. Serial Matrix Filler Number Material
Size of Filler Material (nm)
Specific Wear Rate (×10−6 mm3 /Nm) Equipment Used, Counterface Material, With Lowest With Lowest Without With Change Without With Change Load/Pressure Used, COF Wear Filler Filler (%) Filler Filler (%) Sliding Velocity, Remarks (if any) Optimum Filler Content
Coefficient of Friction
1
PTFE
ZnO
50
15 wt%
15 wt%
0.202
0.209
+3.4
1125.3
13
−98.8
2
PTFE
Al2 O3
40
20 wt%
20 wt%
0.152
0.219
+44.1
715
1.2
−99.8
3
PTFE
CNT
20–30
30 vol.%
20 vol.%
0.2
0.17
−15.0
800
2–3
−99.6
4
PTFE
Nano-attapulgite
10–25
5 wt%
5 wt%
0.22
0.2
−9.1
625.8
31.2
−95
PTFE
2M acid treated Attapulgite
10–25
5 wt%
5 wt%
0.22
0.2
−9.1
625.8
4.9
−99.2
5
Epoxy TiO2
10
7 wt%
3 wt%
0.54
0.4
−25.9
6
Epoxy TiO2
300
–
4 vol.%
–
–
−25.9
40
14
−65
7
Epoxy Al2 O3
13
–
2 vol.%
–
–
−25.9
5.9
3.9
−33.9
8
Epoxy Si3 N4
