Contents
Part I: Fundamentals of Electrical Contacts ........................................................................ 1
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
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
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
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
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
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
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
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
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
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