braunovic - electrical contacts - fundamentals, applications and technology

q 2006 by Taylor & Francis Group, LLC
q 2006 by Taylor & Francis Group, LLC
Preface
The multidisciplinary study of the electrical contact in modern engineering is significant, but often
neglected. The scientist and engineers who have spent their professional lives studying and
applying electrical contacts know that these components are critical to the successful operation of
all products that use electricity. In our civilization, all electricity transmission and distribution, most
control, and most information exchange depends upon the passage of electricity through an
electrical contact at least once. The failure of an electrical contact has resulted in severe
consequences, e.g., an energy collapse of a megapolis, a failure of the telephone system, and even
the crash of an airplane.
Ragnar Holm, the prominent researcher, renowned engineer, and inventor, developed the validity
of “electrical contacts” as its own technical discipline with his book Electric Contacts (1958). The
50 years following its publication have given a firm confirmation of the accuracy of his predictions
and conclusions. Since that time, however, there has been a huge increase in the application of
electrical contacts. For example, the era of the information highway and the development of the
integrated circuit have created new challenges in the use of electrical contacts. The use of electrical
contacts on the microscopic scale presents numerous problems never considered by previous
generations of researchers and engineers. The future MEMS/NEMS technology is another area
where the theory and practice of the electrical contact is of critical importance.
The purpose of the authors has been to combine the progress in research and development in the
areas of mechanical engineering and tribology, which Holm postulated to be key segments in
electrical contacts, with the new data on electrical current transfer, especially at the
micro/nanoscale.
This book complements the recent volume Electrical Contacts: Principles and Applications
(published by Marcel Dekker, 1999). It takes a practical applications approach to the subject and
presents valuable design information for practicing mechanical and electrical engineers. In fact, the
information contained here will serve as an excellent source of information not only for anyone
developing equipment that uses electricity, but for postgraduate students who are concerned about
the passage of current from one conductor to another.
The authors of this book have many years of research and practical experience. One unusual and

interesting aspect of the book’s development is that it comes through the cooperation of the
different approaches to the subject from the West and the East. They have succeeded in making the
bulk of research and engineering data equally clear for all the segments of the international
audience.
Paul G. Slade
Ithaca, New York
q 2006 by Taylor & Francis Group, LLC

The Authors
Dr. Milenko Braunovic
´
received his Dipl. Ing degree in technical physics
from the University of Belgrade, Yugoslavia, in 1962 and the M.Met. and
Ph.D. degrees in physical metallurgy from the University of Sheffield,
England in 1967 and 1969, respectively. From 1971 until 1997, he was
working at Hydro-Que
´
bec Research Institute (IREQ) as a senior member of
the scientific staff. He retired from IREQ in 1997 and established his own
scientific consulting company, MB Interface. From 1997 until 2000 he was
consulting for the Canadian Electricity Association as a technology
advisor. He is presently R&D manager with A.G.S. Taron Technologies in
Boucherville, QC, Canada.
During the last 30 years, Dr. Braunovic
´
has been responsible for the development and
management of a broad range of research projects for Hydro-Que
´
bec and the Canadian Electrical
Association in the areas of electrical power contacts, connector design and evaluation, accelerated

test methodologies, and tribology of power connections. He has also initiated and supervised the
R&D activities in the field of shape-memory alloy applications in power systems. Dr. Braunovic
´
is
the author of more than 100 papers and technical reports, including contributions to encyclopaedias
and books, in his particular areas of scientific interests. In addition, he frequently lectures at
seminars world wide and has presented a large number of papers at various international
conferences.
For his contributions to the science and practice of electrical contacts, Dr. Braunovic
´
received the
Ragnar Holm Scientific Achievement Award in 1994, and for his long-term leadership and service
to the Holm Conference on Electrical Contacts he received, in 1999, the Ralph Armington
Recognition Award. He is also a recipient of the 1994 IEEE CPMT Best Paper Award. He
successfully chaired the Fifteenth International Conference on Electrical Contacts held in Montreal
in 1990, and was a technical program chairman of the Eighteenth International Conference on
Electrical Contacts held in Chicago in 1996. He is a senior member of the Institute of Electronics
and Electrical Engineers (IEEE), the American Society for Metals (ASM), the Materials Research
Society (MRS), the Planetary Society, the American Society for Testing of Materials (ASTM), and
The Minerals, Metals & Materials Society (TMS).
Dr. Valery Konchits was born on January 3, 1949 in the city of Gomel,
Belarus. He graduated from Gomel State University in 1972. He received
his Ph.D. degree in tribology from the Kalinin Polytechnic Institute, Russia
in 1981.
In 1972, he joined the Metal-Polymer Research Institute of the National
Academy of Sciences of Belarus in Gomel. In 1993, he became the head of
the laboratory in the Tribology Department. Since 2001, Dr. Konchits has
been Deputy Director of the Metal-Polymer Research Institute.
The scientific interests of Dr. Konchits lie mainly in electrical
contacts’ friction and wear, contact phenomena at their interfaces, and

electrophysical diagnostic methods of friction. He is the author of more
than 80 papers and holds 10 patents. He is also the co-author of a monograph in Russian, “Tribology
of electrical contacts” (authors: Konchits V.V., Meshkov V.V., Myshkin N.K., 1986, Minsk).
q 2006 by Taylor & Francis Group, LLC
Prof. Nikolai Myshkin was born on May 12, 1948 in the city of Ivanovo,
Russia. He graduated from the Power Engineering Institute in 1971 with a
degree in electromechanics. Has received his Ph.D. from the Institute for
Problems in Mechanics of the Russian Academy of Sciences in 1977. The
same year, he joined the Metal-Polymer Research Institute in Gomel where
since 1990 he has been Head of the Tribology Department. He has also
been the director of MPRI since 2002. He earned his Dr.Sc. degree in
tribology in 1985 and became a full professor of materials science in 1991.
He was elected as a correspondent member of the Belarus National
Academy of Sciences in 2004.
He received the USSR National Award for Young Scientists in 1983, the
Award for Best Research given by the Belarus National Academy of Sciences in 1993, and the
Award of the Russian Government in Science and Technology in 2004.
The scientific interests of Prof. Myshkin lie mainly in the characterization at micro and nanoscale
surfaces, the contact mechanics of solids, wear monitoring, electric phenomena in friction,
tribotesting equipment, and aerospace engineering.
He has authored or co-authored more than 180 papers and 60 patents. He is a co-author of the
Tribology Handbook (Russian edition 1979, English translation 1982), monographs Physics,
Chemistry and Mechanics of Boundary Lubrication (1979), Tribology of Electric Contacts (1986),
Acoustic and Electric Methods in Tribology (Russian edition 1987, English translation 1990),
Structure and Wear Resistance of Surface Layers (1991), Textbook in Materials Science (1989),
Magnetic Fluids in Machinery (1993), the English textbook Introduction to Tribology (1997), and
Tribology: Principles and Applications (2002).
Prof. Myshkin is chairman of the Belarus Tribology Society and vice-president of the
International Tribology Council. He is also assistant editor-in-chief of the Journal of Friction and
Wear, and a member of editorial boards of Tribology International, Tribology Letters, Industrial

Lubrication and Tribology, and the International Journal of Applied Mechanics and Engineering.
q 2006 by Taylor & Francis Group, LLC
Acknowledgments
In the preparation of the book, the authors have used a large number of published materials, either in
the form of papers in referenced journals, or from the websites of different companies and
organizations. In both cases, proper permissions for using these materials have been obtained. In
many instances, the authors obtained the required information directly from the authors of the
papers or from the company authorities.
The authors are indebted to Dr. Paul Slade for writing the preface of the book. Special thanks go
to Dr. Daniel Gagnon of Hydro-Que
´
bec Research Institute (IREQ) in Varennes, QC, Canada for
providing essential reference material and fruitful discussions concerning certain topics of power
connections.
The authors are grateful to Dr. Mark. I. Petrokovets for fruitful discussion and his help in
preparation of Chapters 2, 3 and 5. We also thank Dr. Denis Tkachuk for his valuable assistance in
preparation of the manuscript.
Acknowledgement is made to the many individuals and company authorities for permission to
use the original material and, in particular, to modify the original figures to maintain the uniformity
of graphic presentation throughout the book. The following is a list of these individuals and
company authorities.
Prof. George M. Pharr, Department of Materials Science and Engineering, University of
Tennessee, Knoxville, USA for providing the papers on nanoindenation testing methods and
instrumentation and allowing modification of some of the figures appearing in these papers.
Prof. Doris Kuhlmann-Wilsdorf, Department of Materials Science and Engineering, University
of Virginia, Charlottesville, USA for permission to use the information on fiber-brushes and
dislocation nature of the processes occurring during friction.
Dr. Roland Timsit of Timron Scientific Consulting, Inc., Toronto, Canada for permission to use
the relevant material from his papers and publications.
Dr. Robert Malucci of Molex, Inc., Lisle, IL, USA, for permission to use the relevant material

from his papers and publications and modify some of the figures from his original publications cited
in this book.
Dr. Bill Abbott of Batelle, USA for helpful suggestions and discussions regarding the problems
of corrosion in electrical and electronic connections.
Dr. Sophie Noel, Laboratoire de Ge
´
nie Electrique, Supe
´
lec, Gif sur Yvette, France for helpful
discussions concerning the lubrication of electrical contacts and permission to use some of data
from the publications cited in this book
Dr. Magne Runde of the Norwegian University of Science and Technology, Norway, for helpful
discussions concerning the problem of electromigration in electrical contacts.
Prof. Zoran Djuric and Milos Frantlovic of the Center for Microelectronics Technologies and
Single Crystals, MTM, University of Belgrade, Serbia and Montenegro for providing the
information on the wireless temperature monitoring system.
Dr. Bella Chudnovsky of Square D, USA, for helpful discussions concerning the whisker
formation in electrical contacts and for permission to use the information on the On-Line Wireless
Temperature Monitoring System for LV, MV electrical equipment fond on the company web site
().
Prof. L.K.J. Vandamme of the Department of Electrical Engineering, Eindhoven University of
Technology, The Netherlands for providing and allowing the use of reference materials concerning
the noise in electrical connections.
Mr. Larry Smith of USi, Armonk, NY, USA, for permitting the use of the images and
descriptions of the Power-donut unit found on the company web site ().
q 2006 by Taylor & Francis Group, LLC
Dr. Young-kook (Ryan) Yoo, Director of Global Sales and Marketing of PSIA Corp. Sungnam
462-120, Korea, for permission to use descriptions of different surface analytical equipment as
posted on the company web site (.).
Dr. G. Palumbo of Integran Technologies, Inc., Toronto, Canada for providing the information

on the grain size effects in nanocrystalline materials ().
Mr. J. Renowden of Transpower New Zealand, for providing the information concerning the field
applications of the microohmeter Ohmstik on power lines ().
Mr. J. Lebold of Boldstarinfrared, Canada for permission to use the infrared images from the
company web site ().
R.N. Wurzbach of Maintenance Reliability Group (MRG), York, PA, USA for permission to use
description of the web-based cost benefit analysis method for predictive maintenance
().
ndb Technologie, Inc., Que
´
bec, Canada for permission to use the information about the
microohmeters found on their web site ().
In addition, the authors would like to acknowledge the courtesy of the following companies for
allowing the use of the information found on their respective websites: Omega Madge Tech., Inc.,
(), FLIR Systems (http://www.flirthermography.com), Mikron Infrared,
Inc. (), Electrophysics Corp. (), Infrared
Solution, Inc. (), Elwood Corp. (),
Sensolink Corp., ().
Lastly, it is a pleasure to acknowledge and express our gratitude to Mrs. K. Braunovic
´
for her
generous hospitality shown to the authors during the preparation of the book manuscript.
q 2006 by Taylor & Francis Group, LLC
Introduction
This book provides detailed analytical models, state-of-the-art techniques, methodologies and tools
used to assess and maintain the reliability of a broad class of moving and permanent electrical
contacts in many technological devices, such as automotive and aerospace components, high- and
low-power contact joints, sliding and breaking contacts, electronic and control apparatus, and
electromechanical systems. It provides a comprehensive outline of the tribological behavior of
electrical contacts that is rarely discussed in the existing literature; these are problems of

considerable interest for researchers and engineers.
Focusing on the main mechanical and electrical problems in connections with the field
applications and the relationship between structure and properties, this volume provides a well-
balanced treatment of the mechanics and the materials science of electrical contacts, while not
neglecting the importance of their design, development, and manufacturing. The book provides a
complete introduction to electric conduction across a contacting interface as a function of surface
topography, load, and physical-mechanical properties of materials, and the interrelation of
electrical performance with friction and wear; it takes into account material properties and lubricant
effects. Consideration is given to the deleterious effects of different degradation mechanisms, such
as stress relaxation/creep, fretting, differential thermal expansion, and the formation of
intermetallics, as well as their impacts on operating costs, safety, network reliability, power
quality. Various palliative measures to improve the reliability and serviceability of electrical
contact at macro-, micro-, and nano-levels are also discussed.
This book diminishes a large gap between engineering practice widely utilizing empirically
found methods for designing and optimizing the contact characteristics and theory relating to
tribological and electromechanical characteristics of the contacts. The main trends in the practical
solutions of the tribological problems in electrical contacts are discussed in terms of contact design,
research and development of contact materials, coatings and lubricants and the examples of
practical applications in various fields are given throughout the book.
Covering a wide range of references, tables of contact materials, coatings and lubrication
properties, as well as various testing procedures used to evaluate these properties, the book will be
an indispensable practical tool for professional, research, design and development engineers. The
book (or parts of it) can be used not only as a reference, but also as a textbook for advanced graduate
students and undergraduates, as it develops the subject from its foundations and contains problems
and solutions for each chapter.
q 2006 by Taylor & Francis Group, LLC

Table of Contents
Part I
Fundamentals of Electrical Contacts 1

Milenko Braunovic
´
, Valery V. Konchits, and Nikolai K. Myshkin
Chapter 1
Introduction to Electrical Contacts 3
1.1 Introduction 3
1.2 Summary of Basic Features 6
Chapter 2
Contact Mechanics 9
2.1 Surface of Solids 9
2.2 Surface Topography 11
2.3 Modern Techniques of Measuring Surface Parameters 17
2.4 Contact of Smooth Surfaces 21
2.4.1 Plastic and Elastoplastic Contacts 23
2.5 Contact between Rough Surfaces 27
2.5.1 Greenwood–Williamson Model 27
2.5.2 Multilevel Model 29
2.5.3 Transition from Elastic to Plastic Contact 33
Chapter 3
Tribology 35
3.1 Friction 35
3.1.1 Laws of Friction 35
3.1.2 Real Contact Area 38
3.1.3 Interfacial Bonds (Adhesion Component of Friction) 38
3.1.4 Deformation at Friction 41
3.1.5 Friction as a Function of Operating Conditions 42
3.1.6 The Preliminary Displacement 44
3.1.7 Stick-Slip Motion 46
3.2 Wear 47
3.2.1 Stages of Wear 48

3.2.2 Simple Model of Wear 48
3.2.3 Basic Mechanisms of Wear 50
3.2.4 Abrasive Wear 52
3.2.5 Adhesive Wear 56
3.2.6 Prow Formation 57
3.2.7 Fatigue Wear 57
3.2.8 Corrosive Wear 59
3.2.9 Fretting Wear 59
3.2.10 Delamination 62
3.2.11 Erosion 64
3.2.12 Combined Wear Modes 64
3.3 Lubrication 65
3.4 Current Trends in Tribology 67
q 2006 by Taylor & Francis Group, LLC
Chapter 4
Contact Materials 71
4.1 Metallic Contact Materials 71
4.1.1 Properties of Contact Materials 71
4.1.1.1 Copper 71
4.1.1.2 Aluminum 75
4.1.1.3 Silver 76
4.1.1.4 Platinum 78
4.1.1.5 Palladium 78
4.1.1.6 Gold 79
4.1.1.7 Rhodium 79
4.1.1.8 Tungsten 79
4.1.1.9 Nickel 80
4.1.2 Metals and Alloys for Heavy- and Medium-Duty Contacts 80
4.1.3 Metals and Alloys for Light-Duty Contacts 83
4.1.4 Materials for Liquid-Metal Contacts 85

4.1.5 Spring Contact Materials 87
4.1.6 Shape-Memory Alloys and Their Applications in Electrical Contacts 88
4.2 Coatings for Electrical Contacts 89
4.2.1 Basic Requirements 89
4.2.2 Surface Engineering Technologies 91
4.2.2.1 Surface Segregation 92
4.2.2.2 Ion Implantation 94
4.2.2.3 Electroplating 94
4.2.2.4 Electroless Plating 97
4.2.2.5 Cladding 97
4.2.2.6 Chemical Deposition 99
4.2.2.7 Plating by Swabbing 99
4.2.2.8 Physical Vapor Deposition Technology 99
4.2.2.9 Electro-Spark Deposition (ESD) 100
4.2.2.10 Intermediate Sublayers 101
4.2.2.11 Multilayered Contacts 101
4.2.3 Coating Materials 102
4.2.3.1 Coatings for Power Connectors (Copper and Aluminum Joints) 102
4.2.3.2 Coatings for Electronic/Electrical Applications 104
4.3 Composite Contact Materials 111
4.3.1 Composite Materials for Contacts of Commutating Apparatuses 111
4.3.2 Self-Lubricating Composites for Sliding Contacts 118
4.4 Nanostructured Materials 125
4.4.1 “Bulk” Properties Nanomaterials 127
4.4.2 Mechanical Properties 127
4.4.3 Electrical Properties 131
4.4.4 Magnetic Properties 136
4.4.4.1 Giant Magnetoresistance (GMR) 136
4.4.4.2 Ballistic Magnetoresistance (BMR) 138
4.4.5 Nanotubes 140

4.4.6 Thermal Stability 142
4.4.7 Characterization Techniques for Nanostructured Materials 143
4.4.7.1 Nanoindentation 143
4.4.7.2 Scanning Probe Microscopes 144
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Chapter 5
Current and Heat Transfer across the Contact Interface 149
5.1 Contact Resistance 149
5.1.1 Circular and Noncircular a-Spots 149
5.1.2 Effect of Signal Frequency 154
5.1.3 Size Effects, Nanocontacts 157
5.1.4 Effect of Surface Films 160
5.1.5 Effect of Contact Geometry 166
5.1.6 Conductivity of Rough Contact 172
5.2 Interfacial Heating 180
5.2.1 Principles of Heat Conduction Theory 181
5.2.2 Simple Problems of Heat Conduction Theory 183
5.2.3 Contact Spots Heated by Electrical Current 188
5.2.3.1 Film-Free Metal Contact 188
5.2.3.2 Heating of Contact Spots Having Surface Films 190
5.2.3.3 Field Intensity in the Contact Clearance with
Tunnel-Conductive Films 194
5.2.4 Formulation of Heat Problem with Friction 195
5.2.5 Flash Temperature of Electrical Contact 198
5.2.6 Thermal Instability of Friction Contact 200
5.2.6.1 Thermoelastic Instability 201
5.2.6.2 Instability Caused by Temperature-Dependent
Coefficient of Friction 202
5.2.6.2 Instability Related to Friction Mode Variation 202
Chapter 6

Reliability Issues in Electrical Contacts 205
6.1 Significance of Electrical Contacts Reliability 205
6.2 Electrical Contact Requirements 206
6.3 Factors Affecting Reliability 206
6.4 Connection Degradation Mechanisms 208
6.4.1 Contact Area 209
6.4.2 Oxidation 211
6.4.3 Corrosion 212
6.4.4 Fretting 214
6.4.4.1 Mechanisms of Fretting 217
6.4.4.2 Factors Affecting Fretting 219
6.4.4.3 Fretting in Electrical Contacts 219
6.4.4.4 Contact Load 221
6.4.4.5 Frequency of Motion 223
6.4.4.6 Slip Amplitude 224
6.4.4.7 Relative Humidity 224
6.4.4.8 Temperature 226
6.4.4.9 Effect of Current 226
6.4.4.10 Surface Finish 228
6.4.4.11 Hardness 229
6.4.4.12 Metal Oxide 230
6.4.4.13 Coefficient of Friction 230
6.4.4.14 Electrochemical Factor 230
6.4.5 Intermetallic Compounds 230
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6.4.5.1 Effect of Electrical Current 232
6.4.6 Electromigration 237
6.4.7 Stress Relaxation and Creep 240
6.4.7.1 Nature of the Effect of Electric Current 241
6.4.7.2 Effect of Electric Current on Stress Relaxation 242

6.4.8 Thermal Expansion 247
6.5 Impact of Connection Degradation 248
6.5.1 Prognostic Model for Contact Remaining Life 250
6.5.2 Economical Consequences of Contact Deterioration 256
6.5.3 Power Quality 258
Part II
Applications of Electrical Contacts 261
Milenko Braunovic
´
, Valery V. Konchits, and Nikolai K. Myshkin
Chapter 7
Power Connections 263
7.1 Types of Power Connectors 263
7.2 Design Features and Degradation Mechanisms 263
7.2.1 Bolted Connectors 263
7.2.1.1 Fretting in Bolted Connectors 269
7.2.1.2 Fretting in Aluminum Connections 271
7.2.1.3 Intermetallics 272
7.2.1.4 Creep and Stress Relaxation 275
7.2.2 Bus-Stab Contacts 276
7.2.3 Compression Connectors 279
7.2.3.1 Degradation Mechanisms in Compression Connectors 281
7.2.3.2 Corrosion 282
7.2.3.3 Fretting in Compression Connectors 283
7.2.4 Mechanical Connectors 284
7.2.4.1 Binding-Head Screw Connectors 285
7.2.4.2 Insulation Piercing Connectors 289
7.2.4.3 Wedge Connectors 289
7.2.5 Welded Connectors 290
7.3 Mitigating Measures 292

7.3.1 Contact Area–Connector Design 292
7.3.2 Contact Pressure 294
7.3.3 Surface Preparation 296
7.3.4 Mechanical Contact Devices 297
7.3.4.1 Retightening 300
7.3.4.2 Bimetallic Inserts 301
7.3.4.3 Transition Washers 301
7.3.4.4 Multilam Contact Elements 302
7.3.4.5 Shape-Memory Alloy Mechanical Devices 302
7.3.4.6 Self-Repairing Joints 303
7.3.5 Lubrication: Contact Aid Compounds 304
7.4 Installation Procedures 306
q 2006 by Taylor & Francis Group, LLC
Chapter 8
Electronic Connections 309
8.1 Types of Electronic Connections 309
8.2 Materials for Electronic Connections 309
8.2.1 Solder Materials 310
8.2.2 Lead-Free Solders 312
8.2.2.1 Tin 312
8.2.2.2 Tin–Silver 312
8.2.2.3 Tin–Silver–Bismuth 313
8.2.2.4 Tin–Silver–Copper 313
8.2.2.5 Tin–Silver–Copper–Antimony 314
8.2.2.6 Tin–Silver–Antimony 314
8.2.2.7 Tin–Bismuth 314
8.2.2.8 Tin–Copper 315
8.2.2.9 Tin–Indium 315
8.2.2.10 Tin–Indium–Silver 316
8.2.2.11 Tin–Zinc 316

8.2.2.12 Tin–Zinc–Silver 316
8.2.2.13 Tin–Zinc–Silver–Aluminum–Gallium 317
8.3 Degradation Mechanisms in Electronic Connections 317
8.3.1 Porosity 319
8.3.2 Corrosion/Contamination 322
8.3.2.1 Pore Corrosion 322
8.3.2.2 Creep Corrosion 323
8.3.2.3 Tarnishing 324
8.3.3 Fretting 327
8.3.4 Frictional Polymerization 334
8.3.5 Intermetallic Compounds 336
8.3.6 Creep and Stress Relaxation 348
8.3.7 Electromigration 353
8.3.8 Whiskers 357
8.4 Mitigating Measures 361
8.4.1 Effect of Coating 361
8.4.1.1 Gold Coatings 361
8.4.1.2 Palladium and Palladium Alloys 362
8.4.1.3 Tin Coatings 364
8.4.1.4 Nickel and Nickel-Base Alloys 364
8.4.2 Effect of Lubrication 364
Chapter 9
Sliding Contacts 369
9.1 Tribology of Electrical Contacts 369
9.1.1 Interrelation of Friction and Electrical Processes 370
9.1.2 Role of Boundary Films 371
9.1.3 Main Means of Improving Reliability of Sliding Contacts 371
9.1.4 Tribophysical Aspects in the Development of Sliding Contacts 373
9.2 Dry Metal Contacts 376
9.2.1 Low-Current Contacts 376

9.2.1.1 Effects of Low Current and Electrical Field on Friction 377
9.2.1.2 Effect of Interfacial Shear 378
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9.2.1.3 Adhesion, Transfer, Wear Debris Formation, and
Surface Transformation 380
9.2.2 High-Current Contacts 386
9.2.2.1 Effects of Electrical Current on Tribological Behavior 386
9.2.2.2 Influence of Electric Fields 390
9.2.2.3 Effect of Velocity 392
9.2.2.4 Effect of Material Combination of Contacting Members 393
9.2.2.5 Electroplastic Effect in Sliding Contact 394
9.2.2.6 Friction and Current Transfer in Metal Fiber Brush Contacts 396
9.2.3 Stability of the Contact Resistance. Electrical Noise 400
9.2.3.1 Contact Noise in Closed Connections 400
9.2.3.2 Electrical Noise in Sliding Contacts 402
9.3 Lubricated Metal Contacts 414
9.3.1 Introduction. Lubrication Factors 414
9.3.2 Electrical Properties of Lubricating Boundary Layers 415
9.3.3 Conductivity of Lubricated Contacts 419
9.3.3.1 Effect of Lubricant on Conductivity near the Contact Spots 419
9.3.3.2 Effect of Lubricant on Conductivity of Contact Spots 420
9.3.3.3 Experimental Studies of Electric Conductivity
of Lubricated Contacts 427
9.3.3.4 Contact Resistance between Very Smooth Lubricated Surfaces 430
9.3.3.5 Temperature Dependencies of Contact Conductivity 431
9.3.4 Lubrication Factors in Sliding Contacts 433
9.3.4.1 Effect of Lubricant Origin 434
9.3.4.2 Lubricant Durability 435
9.3.4.3 Tribochemical Aspects of Lubrication 438
9.3.4.4 Effect of Velocity in Light-Current Contacts 441

9.3.4.5 Effects of Lubricant Contact Properties 442
9.3.4.6 Current Passage and Friction in High-Current
Lubricated Contacts 444
9.3.5 Lubricants for Electrical Contacts 449
9.3.5.1 Lubricants for Sliding Electric Switch Contacts 450
9.3.5.2 Lubricants for Sliding Contacts of Sensors 451
9.3.5.3 Selection of Contact Lubricants 454
9.4 Composite Contacts 454
9.4.1 Effect of Intermediate Layers on Electrical Characteristics 455
9.4.1.1 Structure and Electrical Properties of Intermediate Films 456
9.4.1.2 Mechanism of Current Passage through the Contact with
Intermediate Films 460
9.4.1.3 Influence of Polarity on Conductivity in
Composite–Metal Contact 467
9.4.2 The “Lubricating” Effect of Electrical Current 471
9.4.2.1 Effect of Current on Friction Characteristics 471
9.4.2.2 Mechanism of the “Lubricating” Action of the Electric Current 473
9.4.2.3 Effect of Brush Material on Friction Behavior with
Electric Current 477
9.4.3 Electrical Wear 479
9.4.3.1 Wear of Currentless Contacts 479
9.4.3.2 Effect of Current on Wear 480
9.4.3.3 Factors Leading to Electrical Wear in the
Absence of Sparking 483
q 2006 by Taylor & Francis Group, LLC
9.4.3.4 Influence of the Electric Field in the Clearance 489
9.4.3.5 Wear with Sparking and Arcing 491
9.4.3.6 Some Ways to Reduce Electrical Wear 493
Part III
Diagnostic and Monitoring Technologies 495

Milenko Braunovic
´
, Valery V. Konchits, and Nikolai K. Myshkin
Chapter 10
Electrical Methods in Tribology 497
10.1 Surface Characterization 497
10.2 Diagnosis of Contact Area and Friction Regimes 503
10.2.1 Formation of Contact Area 503
10.2.2 Control of Sliding Contact with the Presence of Oxide Films 508
10.2.3 Experimental Study of Metallic Contact Spots Formation 509
10.3 Evaluation of Tribological Performance of Materials and Lubricants 511
10.3.1 Evaluation of Load-Bearing Capacity and Lubricity of Surface Films 511
10.3.2 Estimation of Lubricant Interlayer Shear Strength under Imperfect
Lubrication 515
10.3.3 Evaluation of Thermal Stability of Materials and Lubricants
by Electrical Methods 517
10.3.4 Control of Surface Coatings and Films 519
10.3.5 Novel Systems for Measuring and Analysis of Contact Characteristics 521
10.3.5.1 Method of “Triboscopy” 523
Chapter 11
Monitoring Technologies 529
11.1 Thermal Measurements 530
11.1.1 Infrared Thermography 532
11.1.2 Basic Features of Infrared Thermography 532
11.1.3 Types of Infrared Thermal Systems 534
11.1.4 SME Temperature Indicators 538
11.1.5 Temperature Stickers (Labels) 540
11.1.6 Remote Temperature Sensors 541
11.2 Resistance Measurements 542
11.3 Monitoring Contact Load (Pressure) 545

11.4 Ultrasonic Measurements 546
11.5 Wireless Monitoring 548
11.6 Cost Benefits of Monitoring and Diagnostic Techniques 552
Appendix 1: Methods of Description of Rough Surface 555
Appendix 2: Shape-Memory Materials 565
Appendix 3: Electrical Contact Tables 585
References 599
q 2006 by Taylor & Francis Group, LLC

1
Introduction to Electrical
Contacts
1.1 INTRODUCTION
An electrical contact is defined as the interface between the current-carrying members of electri-
cal/electronic devices that assure the continuity of electric circuit, and the unit containing the
interface. The current-carrying members in contact, often made of solids, are called contact
members or contact parts. The contact members connected to the positive and negative circuit
clamps are called the anode and cathode, respectively.
Electrical contacts provide electrical connection and often perform other functions. The
primary purpose of an electrical connection is to allow the uninterrupted passage of electrical
current across the contact interface. It is clear that this can only be achieved if a good metal-
to-metal contact is established. The processes occurring in the contact zone are complex and not
fully explained within the limits of present knowledge. Although the nature of these processes may
differ, they are all governed by the same fundamental phenomena, the most important being the
degradation of the contacting interface and the associated changes in contact resistance, load,
temperature, and other parameters of a multipoint contact.
Electrical contacts can be classified according to their nature, surface geometry, kinematics,
design and technology features, current load, application, and by others means.
1–3
In general,

electrical contacts can be divided into two basic categories: stationary and moving. Figure 1.1
represents the most general classification of electrical contacts according to contact kinematics,
functionality, and design features.
In stationary contacts, contact members are connected rigidly or elastically to the stationary
unit of a device to provide the permanent joint. Stationary contacts are divided into nonseparable or
all-metal (welded, soldered, and glued), and clamped (bolted, screwed, and wrapped). Nonsepar-
able (permanent) joints have a high mechanical strength and provide the stable electrical contact
with a low transition resistance. A nonseparable joint is often formed within one contact member.
For example, in commutating devices, only materials with a complex composition and arc-resistant
working layers are used as the contact members. They are made by contact welding, soldering,
coating, deposition, electrospark alloying, and mechanical methods of joining.
Clamped contacts are made by mechanically joining conductors directly with bolts or screws or
using intermediate parts, specifically, clamps. These contacts may be assembled or disassembled
without damaging the joint integrity. The simplest case of a clamped contact is the joint of two
massive conductors with flat contact surfaces, such as busbars. A more complex joint configuration
is a contact comprising several conductors, such as joints of a multistrand wire and clamp that are
used for joining wire conductors in transmission lines.
The nature of clamped and all-metal contacts is different. This is because in the all-metal
contacts there is no physical interface between conductors, whereas in clamped contacts the inter-
face is controlled by the contact pressure and the ability of the material to undergo plastic
3
q 2006 by Taylor & Francis Group, LLC
deformation. The lower the specific resistance and hardness of a material, the higher its corrosion
resistance and, consequently, the lower the contact transition resistance. For this reason, contact
surfaces are usually covered with soft, corrosion-resistant materials such as tin, silver, cadmium, or
similar materials. Different surface cleaning techniques are often used to improve the
joint connectability.
In moving contacts, at least one contact member is rigidly or elastically connected to the
moving unit of a device. Depending on their operating conditions, these contacts are divided
into two categories: commutating and sliding. Commutating contacts intermittently control the

electric circuit. They fall into two categories: separable (various plug connectors, circuit breakers)
and breaking. The latter are used for a periodical closing and opening of an electrical circuit, such as
in different switches, contactors, relays, and similar devices. Because of differences in breaking
power, current, and voltage, there is a great variety of breaking contacts. The breaking contacts can
be classified as light-, medium- and heavy-duty:
† Light-duty contacts carry very low currents, operate at voltages up to 250 V, and display no
appreciable arc-related electrical wear. The successful operation of these devices depends
mainly on maintaining relatively low and stable contact resistance and also on the selec-
tion of the contact materials. The factors that must be taken into account are tendency to
oxidize (tarnish); presence of dirt, dust or other contaminants on the contact surface; and
contact design (form, size, contact pressure, and finish). Light-duty contacts are intended
for use in instrument controls, general automation, radio and data communication, and
telecommunication systems.
† Medium-duty contacts carry appreciably higher currents (see 5A above) and operate at
voltages up to 1000 V. For this group, electrical wear is of prime importance. The factors
governing contact material selection to meet the very severe operating conditions
include tendency to welding, material transfer, and erosion (pitting). Applications of
medium-duty contacts are control devices for industrial, domestic, and distribution
network applications.
Electric contacts
Stationary Moving
Sliding
Commutating
Binding
Brush Slider Trolley Separable Relay Breaking
Soldered
welded
bonded
Current-
carrying

busses
Current
pickoffs of
electrical
and welding
machines
Rheostats,
potentio-
meters
code
senders
Current
pickoffs of
cranes and
transport
Plug
connectors
and circuit
breakers
Operate under conditions of friction and wear
FIGURE 1.1 Classification of electrical contacts.
Electrical Contacts: Fundamentals, Applications and Technology4
q 2006 by Taylor & Francis Group, LLC
† Heavy-duty contacts carry very high currents (tens of kA) and operate atvery high voltages
(hundreds of kV). The most common types of these connectors are contactors, starters, and
circuit breakers.
In sliding contacts, the contacting parts of the conductors slide over each other without separ-
ation. Current passage through the contact zone is accompanied by physical phenomena (electrical,
electromechanical, and thermal) that produce changes in the state (characteristics) of surface layers
of the contacting members that differ when operating without current (see Figure 1.2). The severity

of the processes occurring at contact interface depends on the magnitude and character of the
current passing through the contact, the applied voltage, operating conditions, and contact
materials.
4,5
The physical processes occurring in the contact zone of sliding contacts can be
classified as follows:
† Sliding contacts with a heavy electrical contact load are contacts whereby currents or
voltages are commutated, inducing mechanical, thermal or electrical effects including
sparking and arcing. These effects thus produce changes in the state (properties) of the
contact members. The necessary condition of such an operating regime is that the voltage
across disclosed contacts exceeds the minimal electric arc voltage for the materials used;
† Sliding contacts with a moderate electrical contact load are contacts where mechanical,
thermal or electrical effects, excluding sparking and arcing, change the state of the mated
surfaces. The voltage across opened contacts is between the softening voltage and the
minimal electric arc voltage for the material used;
† Sliding contacts with a low electrical contact load are contacts where no additional
physical phenomena and changes are induced in the state of the mated surfaces. In
this case the voltage across open contacts is less than the softening voltage.
The most important and widely used types of sliding contacts include contacts of electrical
machines, current pick-offs of transport and lifting machines, and of radio-electronic devices,
and control and automatic systems. As a rule, sliding contacts for electrical and transportation
machines are intended to commutate currents of a moderate and high intensity while those for
radio-electronic devices and control and automatic systems are usually low-current level contacts.
Physical effects
Thermal
ElectricalElectromechanical
Electroplastic effect Sparking and arcing
Softening/melting of
surface layers
Electrodynamic

repulsion
Fritting
Electrotransport
Effect of electrical
field on oxidation
FIGURE 1.2 Possible effects on the passage of electrical current through the interface.
Introduction to Electrical Contacts 5
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For electrical machines, there are two types of sliding electrical contacts: brush-collector and
brush-collector ring, in which the brushes with different polarities slide over one friction track
(brush-collector) or different rings (brush-collector ring).
1,6
Collectors are commonly made of
electrolytic copper or copper with small additions of cadmium, silver, magnesium, zirconium, or
tellurium. Collector rings are made of copper alloys with zinc, lead, and aluminum and, in some
cases (very high peripheral velocities), of ferrous metals and their alloys. Brush materials are based
mainly on multicomponent composites of graphite, soot, copper, and coke powders.
7
Sliding contacts are very important components in many devices used in automatic, teleme-
chanical, and communication equipment, such as different potentiometers serving as
electromechanical sensors.
8,9
Their design is wide ranging and, despite their low material consump-
tion, they are expensive parts of machines and devices due to an extensive use of noble metals in
their production. From a mechanical viewpoint, the operating conditions of low-current sliding
contacts are quite favorable because sliding velocities are low and loads on the contact members are
light; thus, as a rule, these devices are protected against harmful environment factors.
1.2 SUMMARY OF BASIC FEATURES
It has been established that real surfaces are not flat but comprise many asperities.
1

Therefore, when
contact is made between two metals, surface asperities of the contacting members will penetrate the
natural oxide and other surface contaminant films, establishing localized metallic contacts and,
thus, conducting paths. As the force increases, the number and the area of these small metal–metal
contact spots will increase as a result of the rupturing of the oxide film and extrusion of metal
through the ruptures.
These spots, termed a-spots, are small cold welds providing the only conducting paths for the
transfer of electrical current. A direct consequence of this is a porous contact where infiltrating
oxygen and other corrosive gases can enter to react with the exposed metal and reduce the metallic
contact areas. This will eventually lead to disappearance of the electrical contact, although the
mechanical contact between the oxidized surfaces may still be preserved. The real contact area A
r
is
only a fraction of the apparent contact area A
a
, as illustrated in Figure 1.3.
R
m
2a
Conductor resistance
Apparent (nominal) contact area
Real contact area
Load-bearing area
Quasi-metallic contact area
Conducting contact area (a-spots)
Constriction resistance
Diameter of a-spot
R
c
R

m
R
c
a
F
FIGURE 1.3 Schematic of current constriction and real contact area.
Electrical Contacts: Fundamentals, Applications and Technology6
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The relationship between the applied normal load F
c
, hardness of the metal, H, and the apparent
contact area, A
a
, is given by the following expression:
F
c
Z xHA
a
: (1.1)
The hardness, H, in this expression represents a measure of the ability of a metal to resist
deformation due to point loading; x is the pressure factor and depends on the amount of deformation
of the asperities and is equal to 1 in most practical contact systems. On the other hand, Holm
1
has
shown that hardness is related to the yield stress (s
y
) by the following expression:
H Z 3s
y
: (1.2)

The results, shown in Table 1.1, express the real contact area as a percentage of the apparent
contact area, A
a
, at various normal loads. It should be noted, however, that the real contact area
calculated in this manner includes the load-bearing area which is covered with the oxide film and is
not, therefore, a dependable path for transfer of electrical current. Therefore, the conducting contact
area will be only a small fraction of the calculated real contact area, generally considered to be
much smaller than 1%.
It should be pointed out that the electrical interface of an a-spot is far different from the single
circular contact spot. Current passing across a contact interface is constricted to flow through
a-spots. Hence, the electrical resistance of the contact due to this constricted flow of current is
called constriction resistance and is related to the basic properties of metals such as hardness and
electrical resistivity. Holm
1
has shown that the constriction resistance for a single a-spot can be
expressed as
R
s
Z ðr
1
Cr
2
Þ=4a; (1.3)
where r
1
and r
2
are resistivities of the contacting metals, and a is the radius of the metal-to-metal
contact area. If the two contacting metals are the same, then the constriction resistance becomes
R

s
Z r=2a: (1.4)
Because the metals are not clean, the passage of electric current may be affected by thin oxide,
sulphide, and other inorganic films usually present on metal surfaces. Consequently, the total
contact resistance of a joint is a sum of the constriction resistance (R
s
) and the resistance
TABLE 1.1
Effect of Normal Load on Real Area of Contact for Clean Surfaces
Real Contact Area/Apparent Contact Area (A
r
/A
a
) (%)
Alloy
a
/Applied Load 10 N 100 N 1000 N
Al (H-19) 0.01 0.1 1.0
Al (H-0) 0.05 0.5 5.0
AlC0.75% MgC0.15% Fe (H-19) 0.01 0.1 1.0
AlC0.75% MgC0.15% Fe (H-0) 0.02 0.2 2.0
Cu (H-0) 0.008 0.08 0.8
a
(H-0), fully annealed; (H-19), fully hardened.
Introduction to Electrical Contacts 7
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of the film (R
f
)
R

c
Z R
s
CR
f
R
f
Z s=pa
2
;
(1.5)
where s is the resistance per area of the film. Both tunnelling and fritting are considered operative
mechanisms for the current transfer across the film. In most practical applications, the contribution
of these films to the total contact resistance is of minor importance because the contact spots are
usually created by the mechanical rupture of surface films.
The contact resistance is the most important and universal characteristic of all electrical
contacts and is always taken into account as an integral part of the overall circuit resistance of a
device. Therefore, although it is significantly smaller as compared with the overall circuit resist-
ance, the changes in the contact resistance can cause significant malfunctions of the device. This is
because the contact resistance can vary significantly with the changes in the real contact area,
contact pressure variations, resistive film nonuniformity, and other factors. This results in large
voltage increases, thus making the fine adjustment or good operation of devices difficult. For
instance, the instability and high values of contact resistance are especially noticeable in bulk
DC potentiometers, whose resistive members are relatively thick and have a high
specific resistance.
There are many parameters that can be used to assess the operating efficiency of electrical
contacts. Among these parameters, perhaps the most important are electric (the transition voltage
drop, commutation noise, erosion resistance), tribological (the wear resistance and friction coeffi-
cient) and chemical (corrosion resistance). In the following chapters, detailed analyses of the
factors affecting the properties and performance of electrical contacts will be given.

Electrical Contacts: Fundamentals, Applications and Technology8
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2
Contact Mechanics
2.1 SURFACE OF SOLIDS
The features of a solid surface as a physical object are governed by its spatial arrangement as a
boundary between two phases.
10
The atoms and molecules belonging to the surface have fewer
neighbors than those in the bulk. This simple fact has far-reaching consequences for geometry and
physics of a surface: the interactions between its atoms and their neighbors vary, distorting the force
field that penetrates to the depth of several interatomic distances. Given this fact, the excess of
energy to surface energy appears; consequently, the surface interacts with the environment. This
process is termed adsorption. There are physical and chemical types of adsorption.
Physical adsorption is characterized by the van der Waals interactions between the adsorbate
and the solid surface. As a rule, its energy of the interaction is below 20 kJ/mol. The polymolecular
films adsorbed on the surface are removed relatively easily.
The chemical adsorption energy is quite high (80–400 kJ/mol), usually producing a monolayer
on the surface that is hard to remove, even through the use of elevated temperatures. In addition,
chemical reactions between the surface and the active elements, such as oxidation in the environ-
ment, should be remembered. Unlike the case for chemisoption, these reactions result in a bulk
phase on the surface.
The environment exerts very different effects on a solid surface.
10
In the 1920s, A. Joffe
demonstrated that halide crystals, e.g., NaCl, that are brittle in dry air, become ductile in a moist
atmosphere and show an increase in strength. Joffe ascribed this effect to a water film on the solid
surface, assuming that the water heals surface microcracks. This circumstance holds significance in
tribological behavior of materials for which Joffe’s effect takes place. For example, aluminum
oxide is sensitive to water vapor, and high-strength steel exposed in pure hydrogen is sensitive to a

small concentration of oxygen. An attack of some active environmental species on the solid surface
of metals or nonmetals may change the mechanical behavior of surface layers because of the
wedging, or Rebinder, effect (Figure 2.1). This phenomenon was first observed by P. Rebinder.
As a rule, the species are of organic origin (fatty oxides, alcohols, soaps, etc.) and are present
in lubricants.
The adsorbed film may have the opposite effect, producing surface hardening. This hardening
occurs in, for example, oxides on certain metals (the Roscoe effect). Hence, the surface adsorptivity
produces a fine boundary layer with a structure and behavior differing from those of the surface
layer of the solid. Figure 2.2 shows schematically that the structure of the boundary layer is quite
intricate. The appearance of each sublayer depends upon the conditions of fabrication of a part. The
layers may mutually penetrate one another through the system of microcracks.
The boundary layer may be in a diversity of physical states, ranging from nearly gaseous to
solid crystalline. Both the basic parameters (temperature and pressure) and the pattern of
interactions with the solid phase determine its state. The mechanical behavior of boundary
layers demonstrates a wide spectrum of properties ranging from viscous and viscoelastic behavior
to perfect elasticity.
9
q 2006 by Taylor & Francis Group, LLC