Principles and Applications of Tribology ebook

Contents

Cover

Tribology Series

Title Page

Copyright

Dedication

About the Author

Foreword

Series Preface

Preface to Second Edition

Preface to First Edition

Chapter 1: Introduction

1.1 Definition and History of Tribology

1.2 Industrial Significance of Tribology

1.3 Origins and Significance of Micro/Nanotribology

1.4 Organization of the Book

References

Chapter 2: Structure and Properties of Solids

2.1 Introduction

2.2 Atomic Structure, Bonding and Coordination

2.3 Crystalline Structures

2.4 Disorder in Solid Structures

2.5 Atomic Vibrations and Diffusions

2.6 Phase Diagrams

2.7 Microstructures

2.8 Elastic and Plastic Deformation, Fracture and Fatigue

2.9 Time-Dependent Viscoelastic/Viscoplastic Deformation

References

Further Reading

Chapter 3: Solid Surface Characterization

3.1 The Nature of Surfaces

3.2 Physico-Chemical Characteristics of Surface Layers

3.3 Analysis of Surface Roughness

3.4 Measurement of Surface Roughness

3.5 Closure

References

Further Reading

Chapter 4: Contact between Solid Surfaces

4.1 Introduction

4.2 Analysis of the Contacts

4.3 Measurement of the Real Area of Contact

4.4 Closure

References

Further Reading

Chapter 5: Adhesion

5.1 Introduction

5.2 Solid–Solid Contact

5.3 Liquid-Mediated Contact

5.4 Closure

References

Further Reading

Chapter 6: Friction

6.1 Introduction

6.2 Solid–Solid Contact

6.3 Liquid-Mediated Contact

6.4 Friction of Materials

6.5 Closure

References

Further Reading

Chapter 7: Interface Temperature of Sliding Surfaces

7.1 Introduction

7.2 Thermal Analysis

7.3 Interface Temperature Measurements

7.4 Closure

References

Chapter 8: Wear

8.1 Introduction

8.2 Types of Wear Mechanisms

8.3 Types of Particles Present in Wear Debris

8.4 Wear of Materials

8.5 Closure

Appendix 8.A Indentation Cracking in Brittle Materials

Appendix 8.B Analysis of Failure Data Using the Weibull Distribution

Appendix 8.C Methods for Establishing the PV Limit

References

Further Reading

Chapter 9: Fluid Film Lubrication

9.1 Introduction

9.2 Regimes of Fluid Film Lubrication

9.3 Viscous Flow and the Reynolds Equation

9.4 Hydrostatic Lubrication

9.5 Hydrodynamic Lubrication

9.6 Elastohydrodynamic Lubrication

9.7 Closure

References

Further Reading

Chapter 10: Boundary Lubrication and Lubricants

10.1 Introduction

10.2 Boundary Lubrication

10.3 Liquid Lubricants

10.4 Ionic Liquids

10.5 Greases

10.6 Closure

References

Further Reading

Chapter 11: Nanotribology

11.1 Introduction

11.2 SFA Studies

11.3 AFM/FFM Studies

11.4 Atomic-Scale Computer Simulations

11.5 Closure

References

Further Reading

Chapter 12: Friction and Wear Screening Test Methods

12.1 Introduction

12.2 Design Methodology

12.3 Typical Test Geometries

12.4 Closure

References

Further Reading

Chapter 13: Bulk Materials, Coatings, and Surface Treatments for Tribology

13.1 Introduction

13.2 Bulk Materials

13.3 Coatings and Surface Treatments

13.4 Closure

References

Further Reading

Chapter 14: Tribological Components and Applications

14.1 Introduction

14.2 Common Tribological Components

14.3 MEMS/NEMS

14.4 Material Processing

14.5 Industrial Applications

14.6 Closure

References

Further Reading

Chapter 15: Green Tribology and Biomimetics

15.1 Introduction

15.2 Green Tribology

15.3 Biomimetics

15.4 Closure

References

Further Reading

A: Units, Conversions, and Useful Relations

A.1 Fundamental Constants

A.2 Conversion of Units

A.3 Useful Relations

Index

Tribology Series

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Library of Congress Cataloging-in-Publication Data

Bhushan, Bharat, 1949– Principles and applications of tribology / Bharat Bhushan. – Second edition. pages cm Includes bibliographical references and index. ISBN 978-1-119-94454-6 (cloth) 1. Tribology. I. Title. TJ1075.B473 2013 621.8′9–dc23 2012031664

A catalogue record for this book is available from the British Library.

ISBN: 978-1-119-94454-6

To my wife Sudha, my son Ankur and my daughter Noopur

About the Author

Dr Bharat Bhushan received an MS in mechanical engineering from the Massachusetts Institute of Technology in 1971, an MS in mechanics and a PhD in mechanical engineering from the University of Colorado at Boulder in 1973 and 1976, respectively, an MBA from Rensselaer Polytechnic Institute at Troy, NY, in 1980, Doctor Technicae from the University of Trondheim at Trondheim, Norway, in 1990, a Doctor of Technical Sciences from the Warsaw University of Technology at Warsaw, Poland, in 1996, and Doctor Honoris Causa from the National Academy of Sciences at Gomel, Belarus, in 2000 and University of Kragujevac, Serbia, in 2011. He is a registered professional engineer.

He is presently an Ohio Eminent Scholar and The Howard D. Winbigler Professor in the College of Engineering, and the Director of the Nanoprobe Laboratory for Bio- & Nanotechnology and Biomimetics (NLB2) at the Ohio State University, Columbus, Ohio. His research interests include fundamental studies with a focus on scanning probe techniques in the interdisciplinary areas of bio/nanotribology, bio/nanomechanics and bio/nanomaterials characterization and applications to bio/nanotechnology, and biomimetics. He is an internationally recognized expert on bio/nanotribology and bio/nanomechanics using scanning probe microscopy, and is one of the most prolific authors on the subject. He is considered by some a pioneer of the tribology and mechanics of magnetic storage devices. He has authored 8 scientific books, 90+ handbook chapters, 700+ scientific papers (h-index – 57+; ISI Highly Cited in Materials Science, since 2007; ISI Top 5% Cited Authors for Journals in Chemistry since 2011), and 60+ technical reports. He has also edited 50+ books and holds 17 US and foreign patents. He is co-editor of the Springer NanoScience and Technology Series and co-editor of Microsystem Technologies. He has given more than 400 invited presentations on 6 continents and more than 200 keynote/plenary addresses at major international conferences.

Dr Bhushan is an accomplished organizer. He organized the 1st Symposium on Tribology and Mechanics of Magnetic Storage Systems in 1984 and the 1st International Symposium on Advances in Information Storage Systems in 1990, both of which are now held annually. He is the founder of an ASME Information Storage and Processing Systems Division founded in 1993 and served as the founding chair during 1993–1998. His biography has been listed in over two dozen Who's Who books including Who's Who in the World and he has received more than two dozen awards for his contributions to science and technology from professional societies, industry, and US government agencies. He is also the recipient of various international fellowships including the Alexander von Humboldt Research Prize for Senior Scientists, the Max Planck Foundation Research Award for Outstanding Foreign Scientists, and the Fulbright Senior Scholar Award. He is a foreign member of the International Academy of Engineering (Russia), the Byelorussian Academy of Engineering and Technology and the Academy of Triboengineering of Ukraine, an honorary member of the Society of Tribologists of Belarus, a fellow of ASME, IEEE, STLE, and the New York Academy of Sciences, and a member of ASEE, Sigma Xi and Tau Beta Pi.

Dr Bhushan has previously worked for Mechanical Technology Inc., Latham, NY; SKF Industries Inc., King of Prussia, PA; IBM, Tucson, AZ; and IBM Almaden Research Center, San Jose, CA. He has held visiting professorships at the University of California at Berkeley, the University of Cambridge, UK, the Technical University Vienna, Austria, the University of Paris, Orsay, ETH Zurich, and EPFL Lausanne. He is currently a visiting professor at KFUPM, Saudi Arabia, the Harbin Institute, China, the University of Kragujevac, Serbia, and the University of Southampton, UK.

Foreword

The concept of tribology was announced in 1966 in a report by the UK Department of Education and Science. It encompasses the interdisciplinary science and technology of interacting surfaces in relative motion and associated subjects and practices. It includes parts of physics, chemistry, solid mechanics, fluid mechanics, heat transfer, materials science, lubricant rheology, reliability and performance.

Although the name tribology is new, the constituent parts of tribology – encompassing friction and wear – are as old as history. The economic aspects of tribology are significant. Investigations by a number of countries arrived at figures of savings of 1.0% to 1.4% of the GNP, obtainable by the application of tribological principles, often for proportionally minimal expenditure in Research and Development.

Being an interdisciplinary area, the important aspects of tribology have been difficult to cover in a single book of interest to readers ranging from students to active researchers in academia and industry.

To prepare such a wide-ranging book on tribology, Professor Bhushan has harnessed his knowledge and experience gained in several industries and universities. He has set out to cover not only the fundamentals of friction, wear and lubrication, friction and wear test methods and industrial applications, but also includes a chapter on the field of micro/nanotribology, which may be of special interest in the light of the emergence of proximal probes and computational techniques for simulating tip–surface interactions and interface properties.

Professor Bharat Bhushan's comprehensive book is intended to serve both as a textbook for university courses as well as a reference for researchers. It is a timely addition to the literature on tribology, and I hope that it will stimulate and further the interest of tribology and be found useful by the international scientific and industrial community.

Professor H. Peter Jost President, International Tribology CouncilAngel Lodge Laboratories & WorksLondon, UKJuly, 1998

Series Preface

The first edition of Principles and Applications of Tribology was published in 2002. The second edition promises to deliver much more than the earlier version. In the past few decades, since the concept of “tribology” was introduced by Peter Jost in 1966, the industry has gone through dramatic changes. These changes were dictated by demands for new, more reliable products and to improve the quality of life. To fulfill these demands, new technologies and products have emerged. In the field of tribology, improved materials and surface treatments were developed, new lubricants were introduced and new insights into the mechanisms of contacting surfaces were gained. Nowadays, humanity is facing new challenges such as sustainability, climate change and gradual degradation of the environment. Tribology, like any other field of science, is continuously developing to stay at the forefront of the emerging technologies.

This book provides a comprehensive account of the field of tribology. The text starts with the physical and chemical characteristics of surfaces and surface contacts. It then describes the basic principles of friction, wear and lubrication mechanisms. An attractive feature of this book is its wide scope. The book content extends far beyond the more traditional approach of some tribological books that concentrate mainly on lubricants and lubrication mechanisms. In this book, the newer areas of nanotribology, green tribology and biomimetics are covered. There is even a short discussion on the experimental methods used in tribology. A long chapter is devoted to industrial components and applications relevant to tribology. Tribological constraints on new technologies such as MEMS and microfabrication are introduced. This approach demonstrates that the field of tribology is evolving and adapting to remain relevant to modern industry.

Not so long ago, tribologists were running experiments on tribometers trying to understand the mechanisms of wear and friction at a macro level. Today, they run complex experiments, aided by computer simulations, which permit insights to be gained into what is happening during contact at the molecular or atomic level. In the past few decades, a substantial knowledge base on various aspects of tribology has been built. As tribology is an interdisciplinary area of science, knowledge from chemistry, physics, material science, engineering, computational science and many others is required to provide an understanding of the phenomena that occur. The book reflects that by providing a comprehensive coverage of this important topic. It is recommended for undergraduate and postgraduate students and also for practicing engineers.

Gwidon Stachowiak University of Western Australia

Preface to Second Edition

Tribology is an important interdisciplinary field. It involves design of components with static and dynamic contacts for the required performance and reliability. The second edition of Principles and Applications of Tribology has been revised to reflect the developments in the field over the past decade.

Modern tools and techniques as well as computational modeling have allowed systematic investigations of interfacial phenomena down to atomic scales. These developments have furthered the field of nanotribology and nanomechanics and our understanding of the interface of science and technology.

The advances in micro/nanofabrication processes have led to the development of micro/nanoelectromechanical systems (MEMS/NEMS) used in various electro/mechanical, chemical, optical, and biological applications. These devices are expected to have a major impact on our lives, comparable to that of semiconductor technology, information technology, or cellular or molecular biology.

Chapters on nanotribology and introduction to nanotechnology (MEMS/NEMS) have been totally rewritten. A major addition to this new edition is the chapter on ecological or green tribology. The tribological aspects of ecological balance and of environmental and biological impacts, including tribological components, materials and surfaces that mimic nature (biomimetic surfaces) and the control of friction and wear that is important for alternative energy production, make up a novel and growing area of science and technology.

The author hopes that the second edition will be a useful addition to interface science and technology. Thanks are due to Megan BeVier for typing the manuscript.

Power point presentation of the entire book for a semester course is available from the author. Solution manual is also available from the author. Both Power point presentation and the solution manual will be shipped to those who are using the book as textbook for a class of minimum of 6 students.

Professor Bharat Bhushan Powell, OhioMay, 2012

Preface to First Edition

Tribology is the science and technology of interacting surfaces in relative motion and of related subjects and practices. Its popular English language equivalent is friction, wear and lubrication or lubrication science. The nature and consequence of the interactions that take place at the interface control its friction, wear and lubrication behavior. During these interactions, forces are transmitted, mechanical energy is converted, physical and chemical nature, including the surface topography of the interacting materials, are altered. Understanding the nature of these interactions and solving the technological problems associated with the interfacial phenomena constitute the essence of tribology.

Sliding and rolling surfaces represent the key to much of our technological society. Understanding of tribological principles is essential for the successful design of machine elements. When two nominally flat surfaces are placed in contact, surface roughness causes contact to occur at discrete contact spots and interfacial adhesion occurs. Friction is the resistance to motion that is experienced whenever one solid body moves over another. Wear is the surface damage or removal of material from one or both of two solid surfaces in a moving contact. Materials, coatings and surface treatments are used to control friction and wear. One of the most effective means of controlling friction and wear is by proper lubrication which provides smooth running and satisfactory life for machine elements. Lubricants can be liquid, solid, or gas. The role of surface roughness, the mechanisms of adhesion, friction and wear, and physical and chemical interactions between the lubricant and the interacting surfaces must be understood for optimum performance and reliability. The importance of friction and wear control cannot be overemphasized for economic reasons and long-term reliability. The savings can be substantial, and these savings can be obtained without the deployment of investment.

The recent emergence and proliferation of proximal probes, in particular,tip-based microscopies (the scanning tunneling microscope and the atomic force microscope) and the surface force apparatus, and of computational techniques for simulating tip–surface interactions and interfacial properties, have allowed systematic investigations of interfacial problems with high resolution, as well as ways and means for modifying and manipulating nanoscale structures. These advances provide the impetus for research aimed at developing a fundamental understanding of the nature and consequences of the interactions between materials on the atomic scale, and they guide the rational design of material for technological applications. In short, they have led to the appearance of the new field of micro/nanotribology, which pertains to experimental and theoretical investigations of interfacial processes on scales ranging from the atomic and molecular to the microscale. Micro/nanotribological studies are valuable for a fundamental understanding of interfacial phenomena to provide a bridge between science and engineering.

There is a concern that some of today's engineering and applied science students may not be learning enough about the fundamentals of tribology. No single, widely-accepted textbook exists for a comprehensive course on tribology. Books to date are generally based on the authors' own expertise in narrow aspects of tribology. A broad-based textbook is needed. The purpose of this book is to present the principles of tribology and the tribological understanding of most common industrial applications. The book is based on the author's broad experience in research and teaching in the area of tribology, mechanics and materials science for more than thirty years. Emphasis is on the contemporary knowledge of tribology, and includes the emerging field of micro/nanotribology. The book integrates the knowledge of tribology from mechanical engineering, mechanics and a materials science points of view. Organization of the book is straightforward. The first part of the book starts with the principles of tribology and prepares the students to understand the tribology of industrial applications. The principles of tribology follow with materials, coatings and surface treatments for tribology. Chapter 15 describes the tribological components and applications.

The book is intended for three types of readers: (1) senior undergraduate and graduate students of tribology and design; (2) research workers who are active or intend to become active in this field; and (3) practicing engineers who have encountered a tribology problem and hope to solve it as expeditiously as possible. The book should serve as an excellent text for one or two semester graduate courses in tribology as well as for a senior level undergraduate course of mechanical engineering, materials science or applied physics. For a first or one semester course on introduction to tribology and industrial applications the following sections may be included: Chapter 1, 3.1, 3.2, 3.3, 3.4.1, 3.4.2.4, 3.4.2.6, 3.4.3.2, 3.4.7, 3.4.8, 3.5, 4.1, 4.2.1, 4.2.3, 4.3.1.2, 4.3.3, 4.4, 5.1, 5.2, 5.4, 6.1, 6.2.1 to 6.2.6, 6.3, condensed 6.4, 6.5, 7.1, 7.2.1, 7.2.3, 7.3.1, 7.4, 8.1, 8.2, 8.3, condensed 8.4, 8.5, 9.1, 9.2, 9.3.1, 9.3.2.5, 9.5.2, 9.6.1, 9.6.2, 9.6.3, 9.7, 10.1, 10.2, 10.5, 11.1, 11.3, 11.5, 12.2, 12.3.1, 12.4, and 14.2. For a second semester course on materials, friction and wear of materials, and industrial applications, the following sections may be included: Chapter 2, short reviews of the following sections: 3.3, 3.4.1, 3.4.2.6, 3.4.3.2, 4.2.3.1, 4.2.3.2, 4.2.3.4, 4.3.1.2 and 6.2, 6.4, 6.5, short reviews of 8.2 and 8.3, 8.4, 8.5, 8.A, 8.B, 8.C, 9.1, 9.2, Chapter 10, Chapter 12, Chapter 13, and Chapter 14.

I wish to thank all of my former and present colleagues and students who have contributed to my learning of tribology. I was introduced to the field of tribology via a graduate course in Tribology in Fall 1970 by Profs. Brandon G. Rightmyer and Ernest Rabinowicz at Massachusetts Institute of Technology. I learnt a great deal from Prof. Nathan H. Cook, my M.S. thesis supervisor. My real learning started at R& D Division of Mechanical Technology Inc., Latham, New York, under the guidance of Dr. Donald F. Wilcock, Dr. Jed A. Walowit and Mr. Stanley Gray, and at Technology Services Division of SKF Industries Inc., King of Prussia, Pennsylvania, under the guidance of Dr. Tibor Tallian. I benefited immensely from the help of many colleagues at the General Products Division of IBM Corporation, Tucson, Arizona, and at the Almaden Research Center of IBM Corporate Research Division, San Jose, California. Dr. Kailash C. Joshi helped me in establishing myself at IBM Tucson and Dr. Barry H. Schechtman mentored me at IBM Almaden, San Jose, and helped me immensely. Prof. Bernard H. Hamrock at The Ohio State University has provided nice companionship. Since 1991, I have presented many graduate and undergraduate tribology courses at The Ohio State University as well as many on-site short tribology courses in the U.S. and overseas. The book is based on the class notes used for various courses taught by me.

My special thanks go to my wife Sudha, my son Ankur and my daughter Noopur, who have been very forebearing during the years when I spent long days and nights in conducting the research and keeping up with the literature and preparation of this book. They provided the lubrication necessary to minimize friction and wear at home. Kathy Tucker patiently typed and retyped the manuscript for this book.

Professor Bharat Bhushan Powell, OhioJune, 1998

1

Introduction

In this introductory chapter, the definition and history of tribology and their industrial significance are described, followed by the origins and significance of an emerging field of micro/nanotribology. The last section presents the organization of the book.

1.1 Definition and History of Tribology

The word tribology was first reported in a landmark report by Jost (1966). The word is derived from the Greek word tribos, meaning rubbing, so the literal translation would be “the science of rubbing.” Its popular English language equivalent is friction and wear or lubrication science, alternatively used. The latter term is hardly all-inclusive. Dictionaries define tribology as the science and technology of interacting surfaces in relative motion and of related subjects and practices. Tribology is the art of applying operational analysis to problems of great economic significance, namely, reliability, maintenance, and wear of technical equipment, ranging from spacecraft to household appliances. Surface interactions in a tribological interface are highly complex, and their understanding requires knowledge of various disciplines, including physics, chemistry, applied mathematics, solid mechanics, fluid mechanics, thermodynamics, heat transfer, materials science, rheology, lubrication, machine design, performance, and reliability.

It is only the name tribology that is relatively new, because interest in the constituent parts of tribology is older than recorded history (Dowson, 1998). It is known that drills made during the Paleolithic period for drilling holes or producing fire were fitted with bearings made from antlers or bones, and potters' wheels or stones for grinding cereals, etc., clearly had a requirement for some form of bearings (Davidson, 1957). A ball thrust bearing dated about AD 40 was found in Lake Nimi near Rome.

Records show the use of wheels from 3500 BC, which illustrates our ancestors' concern with reducing friction in translationary motion. Figure 1.1.1 shows a two-wheeled harvest cart with studded wheels, circa 1338 AD. The transportation of large stone building blocks and monuments required the know-how of frictional devices and lubricants, such as water-lubricated sleds. Figure 1.1.2 illustrates the use of a sledge to transport a heavy statue by the Egyptians, circa 1880 BC (Layard, 1853). In this transportation, 172 slaves are being used to drag a large statue weighing about 600 kN along a wooden track. One man, standing on the sledge supporting the statue, is seen pouring a liquid (most likely water) into the path of motion; perhaps he was one of the earliest lubrication engineers. Dowson (1998) has estimated that each man exerted a pull of about 800 N. On this basis, the total effort, which must at least equal the friction force, becomes 172 × 800 N. Thus, the coefficient of friction is about 0.23. A tomb in Egypt that was dated as from several thousand years BC provides the evidence of use of lubricants. A chariot in this tomb still contained some of the original animal-fat lubricant in its wheel bearings.

During and after the Roman Empire, military engineers rose to prominence by devising both war machinery and methods of fortification, using tribological principles. It was the Renaissance engineer–artist Leonardo da Vinci (1452–1519), celebrated in his day for his genius in military construction as well as for his painting and sculpture, who first postulated a scientific approach to friction. Da Vinci deduced the rules governing the motion of a rectangular block sliding over a flat surface. He introduced, for the first time, the concept of the coefficient of friction as the ratio of the friction force to normal load. His work had no historical influence, however, because his notebooks remained unpublished for hundreds of years. In 1699, the French physicist Guillaume Amontons rediscovered the rules of friction after he studied dry sliding between two flat surfaces (Amontons, 1699). First, the friction force that resists sliding at an interface is directly proportional to the normal load. Second, the amount of friction force does not depend on the apparent area of contact. These observations were verified by the French physicist Charles-Augustin Coulomb (better known for his work on electrostatics [Coulomb, 1785]). He added a third law that the friction force is independent of velocity once motion starts. He also made a clear distinction between static friction and kinetic friction.

Many other developments occurred during the 1500s, particularly in the use of improved bearing materials. In 1684, Robert Hooke suggested the combination of steel shafts and bell-metal bushes would be preferable to wood shod with iron for wheel bearings. Further developments were associated with the growth of industrialization in the latter part of the eighteenth century. Early developments in the petroleum industry started in Scotland, Canada, and the United States in the 1850s (Parish, 1935; Dowson, 1998).

Though essential laws of viscous flow were postulated by Sir Isaac Newton in 1668, scientific understanding of lubricated bearing operations did not occur until the end of the nineteenth century. Indeed, the beginning of our understanding of the principle of hydrodynamic lubrication was made possible by the experimental studies of Beauchamp Tower (1884) and the theoretical interpretations of Osborne Reynolds (1886) and related work by N.P. Petroff (1883). Since then, developments in hydrodynamic bearing theory and practice have been extremely rapid in meeting the demand for reliable bearings in new machinery.

Wear is a much younger subject than friction and bearing development, and it was initiated on a largely empirical basis. Scientific studies of wear scarcely developed until the mid-twentieth century. Ragnar Holm made one of the earliest substantial contributions to the study of wear (Holm, 1946).

In the West, the Industrial Revolution (1750–1850) is recognized as the period of rapid and impressive development of the machinery of production. The use of steam power and the subsequent development of the railways in the 1830s, automobiles in the early 1900s and aircraft in the 1940s led to the need for reliable machine components. Since the beginning of the twentieth century, from enormous industrial growth leading to demand for better tribology, knowledge in all areas of tribology has expanded tremendously (Holm, 1946; Bowden and Tabor, 1950, 1964; Bhushan, 1996, 2001a; Bhushan and Gupta, 1997; Nosonovsky and Bhushan, 2012).

1.2 Industrial Significance of Tribology

Tribology is crucial to modern machinery which uses sliding and rolling surfaces. Examples of productive friction are brakes, clutches, driving wheels on trains and automobiles, bolts, and nuts. Examples of productive wear are writing with a pencil, machining, polishing, and shaving. Examples of unproductive friction and wear are internal combustion and aircraft engines, gears, cams, bearings, and seals.

According to some estimates, losses resulting from ignorance of tribology amount in the United States to about 4% of its gross national product (or about $200 billion dollars per year in 1966), and approximately one-third of the world's energy resources in present use appears as friction in one form or another. Thus, the importance of friction reduction and wear control cannot be overemphasized for economic reasons and long-term reliability. According to Jost (1966, 1976), savings of about 1% of gross national product of an industrial nation can be realized by better tribological practices. According to recent studies, expected savings are to be of the order of 50 times the research costs. The savings are both substantial and significant, and these savings can be obtained without the deployment of large capital investment.

The purpose of research in tribology is understandably the minimization and elimination of losses resulting from friction and wear at all levels of technology where the rubbing of surfaces is involved. Research in tribology leads to greater plant efficiency, better performance, fewer breakdowns, and significant savings.

Since the 1800s, tribology has been important in numerous industrial applications requiring relative motion, for example, railroads, automobiles, aircrafts, and the manufacturing process of machine components. Some of the tribological machine components used in these applications include bearings, seals, gears and metal cutting (Bhushan, 2001a). Since the 1980s, other applications have included magnetic storage devices, and micro/nanoelectromechanical systems (MEMS/NEMS) as well as biomedical and beauty care products (Bhushan, 1996, 1998, 1999, 2000, 2001a, 2001b, 2010a, 2010b, 2011, 2012b). In the 2000s, bioinspired structures and materials, some of which are eco-friendly, have been developed and exploited for various applications (Nosonovsky and Bhushan, 2008, 2012; Bhushan, 2012a).

Tribology is not only important to heavy industry, it also affects our day-to-day life. For example, writing is a tribological process. Writing is accomplished by the controlled transfer of lead (pencil) or ink (pen) to the paper. During writing with a pencil there should be good adhesion between the lead and the paper so that a small quantity of lead transfers to the paper, and the lead should have adequate toughness/hardness so that it does not fracture/break. The objective when shaving is to remove hair from the body as efficiently as possible with minimum discomfort to the skin. Shaving cream is used as a lubricant to minimize friction between the razor and the skin. Friction is helpful during walking and driving. Without adequate friction, we would slip and a car would skid! Tribology is also important in sports. For example, a low friction between the skis and the ice is desirable during skiing. Fabric fibers should have low friction when touched by human skin.

Body joints need to be lubricated for low friction and low wear to avoid osteoarthritis and joint replacement. The surface layer of cartilage present in the joint provides the bearing surface and is lubricated with a joint fluid consisting of lubricin, hyaluronic acid (HA) and lipid. Hair conditioner coats hair in order to repair hair damage and lubricate it. It contains silicone and fatty alcohols. Low friction and adhesion provide a smooth feel in wet and dry environments, reduce friction between hair fibers during shaking and bouncing, and provide easy combing and styling. Skin creams and lotions are used to reduce friction between the fingers and body skin. Saliva and other mucous biofluids lubricate and facilitate the transport of food and soft liquids through the body. The saliva in the mouth interacts with food and influences the taste–mouth feel.

1.3 Origins and Significance of Micro/Nanotribology

At most interfaces of technological relevance, contact occurs at numerous asperities. Consequently, the importance of investigating a single asperity contact in studies of the fundamental tribological and mechanical properties of surfaces has long been recognized. The recent emergence and proliferation of proximal probes, in particular, tip-based microscopies (the scanning tunneling microscope and the atomic force microscope) and of computational techniques for simulating tip–surface interactions and interfacial properties, have allowed systematic investigations of interfacial problems with high resolution as well as ways and means of modifying and manipulating nanoscale structures. These advances have led to the development of the new field of microtribology, nanotribology, molecular tribology, or atomic-scale tribology (Bhushan et al., 1995; Bhushan, 1997, 1999, 2001b, 2010a, 2011). This field is concerned with experimental and theoretical investigations of processes ranging from atomic and molecular scales to microscales, occurring during adhesion, friction, wear, and thin-film lubrication at sliding surfaces.

The differences between the conventional or macrotribology and micro/nanotribology are contrasted in Figure 1.1.3. In macrotribology, tests are conducted on components with relatively large mass under heavily loaded conditions. In these tests, wear is inevitable and the bulk properties of mating components dominate the tribological performance. In micro/nanotribology, measurements are made on components, at least one of the mating components, with relatively small mass under lightly loaded conditions. In this situation, negligible wear occurs and the surface properties dominate the tribological performance.

Micro/nanotribological studies are needed to develop a fundamental understanding of interfacial phenomena on a small scale and to study interfacial phenomena in micro- and nano structures used in magnetic storage systems, micro/nanoelectromechanical systems (MEMS/NEMS) and other industrial applications. The components used in microstructures and nanostructures are very light (on the order of few micrograms) and operate under very light loads (on the order of a few micrograms to a few milligrams). As a result, friction and wear (on a nanoscale) of lightly-loaded micro/nano components are highly dependent on the surface interactions (few atomic layers). These structures are generally lubricated with molecularly thin films. Microtribological and nanotribological techniques are ideal ways to study the friction and wear processes of micro- and nanostructures. Although micro/nanotribological studies are critical to study microstructures and nanostructures, these studies are also valuable in the fundamental understanding of interfacial phenomena in macrostructures to provide a bridge between science and engineering.

The scanning tunneling microscope, the atomic force and friction force microscopes and the surface force apparatus are widely used for micro/nanotribological studies (Bhushan et al., 1995; Bhushan, 1997, 1999). To give a historical perspective of the field, the scanning tunneling microscope (STM) developed by Doctors Gerd Binnig and Heinrich Rohrer and their colleagues in 1981 at the IBM Zurich Research Laboratory, the Forschungslabor, is the first instrument capable of directly obtaining three-dimensional (3D) images of solid surfaces with atomic resolution (Binnig et al., 1982). STMs can only be used to study surfaces which are electrically conductive to some degree. Based on their design of the STM, in 1985, Binnig et al. (1986, 1987) developed an atomic force microscope (AFM) to measure ultrasmall forces (less than 1 μN) present between the AFM tip surface and the sample surface. AFMs can be used in the measurement of all engineering surfaces which may be either electrically conducting or insulating. AFM has become a popular surface profiler for topographic measurements on the microscale to nanoscale. AFMs modified to measure both normal and friction forces, generally called friction force microscopes (FFMs) or lateral force microscopes (LFMs), are used to measure friction on the microscale and nanoscale. AFMs are also used for the studies of adhesion, scratching, wear, lubrication, surface temperatures, and for the measurement of elastic/plastic mechanical properties (such as indentation hardness and modulus of elasticity). Surface force apparatuses (SFAs), first developed in 1969, are used to study both static and dynamic properties of the molecularly thin liquid films sandwiched between two molecularly smooth surfaces (Tabor and Winterton, 1969; Bhushan, 1999).

Meanwhile, significant progress in understanding the fundamental nature of bonding and interactions in materials, combined with advances in computer-based modeling and simulation methods, have allowed theoretical studies of complex interfacial phenomena with high resolution in space and time (Bhushan, 1999, 2001b, 2011). Such simulations provide insights into the atomic-scale energetics, structure, dynamics, thermodynamics, transport and rheological aspects of tribological processes. Furthermore, these theoretical approaches guide the interpretation of experimental data and the design of new experiments, and enable the prediction of new phenomena based on atomistic principles.

1.4 Organization of the Book

The friction, wear and the lubrication behavior of interfaces are very dependent upon the surface material, the shape of mating surfaces and the operating environment. A surface film may change the physical and chemical properties of the first few atomic layers of material through interaction with the environment. The structure and properties of solids are discussed in Chapter 2, followed by solid surface characterization in Chapter 3. Chapter 3 includes a discussion of the nature of surfaces, the physico-chemical characteristics of solid surfaces, the statistical analysis of surface roughness, and the methods of characterization of solid surfaces. Chapter 4 is devoted to the elastic and plastic real area of contacts that occur when two solid surfaces are placed in contact. Statistical and numerical analyses and measurement techniques are presented. Chapter 5 covers various adhesion mechanisms in dry and wet conditions. Various analytical and numerical models to predict liquid-mediated adhesion are described. When the two surfaces in contact slide or roll against each other, friction is encountered, thus, various friction mechanisms, the physical and chemical properties that control friction, and the typical friction data of materials are discussed in Chapter 6. Chapter 7 is devoted to the interface temperatures generated from the dissipation of the frictional energy input. Analysis and measurement techniques for interface temperatures and the impact of the temperature rise on an interface performance are discussed.

Repeated sliding or rolling results in wear. In Chapter 8, various wear mechanisms, types of particles present in wear debris, and representative data for various materials of engineering interest are presented. Chapter 9 reviews the various regimes of lubrication, the theories of hydrostatic, hydrodynamic and elastohydrodynamic lubrication and various designs of bearings. In Chapter 10, mechanisms of boundary lubrication, the description of various liquid lubricants and additives and greases are presented. In Chapter 11, various experimental techniques and molecular dynamics computer simulation techniques used for micro/nanotribological studies and the state of the art and their applications are described and relevant data are presented. In Chapter 12, the design methodology and typical test geometries for friction and wear test methods are described.

In Chapter 13, bulk materials, coatings and surface treatments used for tribological applications are described. Coating deposition and surface treatment techniques are also described. In Chapter 14, descriptions, relevant wear mechanisms and commonly used materials for standard tribological components, microcomponents, material processing and industrial applications are presented. In Chapter 15, the fields of green tribology and biomimetics are introduced and various examples in each field are presented.

References

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Bhushan, B. (1996), Tribology and Mechanics of Magnetic Storage Devices, second edition, Springer-Verlag, New York.

Bhushan, B. (1997), Micro/Nanotribology and its Applications, NATO ASI Series E: Applied Sciences, Vol. 330, Kluwer Academic Publishers, Dordrecht, the Netherlands.

Bhushan, B. (1998), Tribology Issues and Opportunities in MEMS, Kluwer Academic Publishers, Dordrecht, the Netherlands.

Bhushan, B. (1999), Handbook of Micro/Nanotribology, second edition, CRC Press, Boca Raton, Florida.

Bhushan, B. (2000), Mechanics and Reliability of Flexible Magnetic Media, second edition, Springer-Verlag, New York.

Bhushan, B. (2001a), Modern Tribology Handbook, Vol. 1: Principles of Tribology; vol. 2: Materials, Coatings, and Industrial Applications, CRC Press, Boca Raton, Florida.

Bhushan, B. (2001b), Fundamentals of Tribology and Bridging the Gap between the Macro- and Micro/Nanoscales, NATO Science Series II: Mathematics, Physics and Chemistry, vol. 10, Kluwer Academic Publishers, Dordrecht, the Netherlands.

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Bhushan, B. (2012b), “Nanotribological and Nanomechanical Properties of Skin with and without Cream Treatment Using Atomic Force Microscopy and Nanoindentation” (Invited Feature Article), Journal of Colloid and Interface Science 367, 1–33.

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2

Structure and Properties of Solids

2.1 Introduction

The internal structure of materials comprises atoms associated with their neighbors in molecules, crystals and microstructures. The properties and performance of materials depend on their internal structure. The properties, including deformation and fracture, affect the friction and wear behavior, during motion against another material.

In this chapter, first, the atomic structure, bonding and coordination are considered, followed by long-range patterns of atomic order which identify crystalline and noncrystalline (amorphous) structures, disorder in solid structures, atomic vibrations and diffusions, phase diagrams and microstructures. Next, deformation, fracture and fatigue of materials and their associated mechanical properties are discussed.

2.2 Atomic Structure, Bonding and Coordination

All solids consist of atoms or molecules. Interatomic bonds exist in all solids. Different bonding patterns lead to molecular structures or to extended, three-dimensional structures. To visualize these structures, one needs to examine the role of the valence electrons on the primary bonds – covalent, ionic and metallic – and on the secondary bonds, which affect interatomic distances and atomic coordination. Bonding patterns affect physical and chemical properties. For example, strong bonds lead to shorter interatomic distances, high moduli of elasticity, hardness, strength and melting temperatures and low coefficients of thermal expansion.