Engineering Tribology PDF

E N G I N E E R I N G T R I B O L O G Y This page is intentionally left blank ENGINEERING TRIBOLOGY F O U RT H E D I T I O N Gwidon W. Stachowiak  epartment of Mechanical Engineering D Curtin University Perth, Australia Andrew W. Batchel...

E N G I N E E R I N G T R I B O L O G Y This page is intentionally left blank ENGINEERING TRIBOLOGY F O U RT H E D I T I O N Gwidon W. Stachowiak  epartment of Mechanical Engineering D Curtin University Perth, Australia Andrew W. Batchelor Saudi Aramco Dhahran, Saudi Arabia AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORD PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO Butterworth-Heinemann is an imprint of Elsevier Butterworth-Heinemann is an imprint of Elsevier The Boulevard, Langford Lane, Kidlington, Oxford, OX5 1GB, UK 225 Wyman Street, Waltham, MA 02451, USA First edition 1993 Copyright © 2014 Elsevier Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher. 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British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-397047-3 For information on all Butterworth-Heinemann publications visit our website at books.elsevier.com Printed and bound in the United States 14 15 16 17 10 9 8 7 6 5 4 3 2 1 To the most important persons in our lives Grazyna Stachowiak Gwidon (Jr.) Stachowiak and Valli M. Batchelor Diana, Vicky & Vincent Batchelor This page is intentionally left blank  C O N T E N T S P R E F A C E XXVII A C K N O W L E D G E M E N T S XXIX 1 INTRODUCTION 1 1.1 Background 1 1.2 Meaning of tribology 2 Lubrication 3 Wear 5 1.3 Cost of friction and wear 5 1.4 Summary 9 Revision questions 9 References 10 2 PHYSICAL PROPERTIES OF LUBRICANTS 11 2.1 Introduction 11 2.2 Oil viscosity 11 Dynamic viscosity 12 Kinematic viscosity 13 2.3 Viscosity temperature relationship 13 Viscosity-temperature equations 14 Viscosity-temperature chart 14 2.4 Viscosity index 15 2.5 Viscosity pressure relationship 17 2.6 Viscosity-shear rate relationship 22 Pseudoplastic behavior 22 Thixotropic behavior 24 2.7 Viscosity measurements 24 Capillary viscometers 24 Rotational viscometers 26 · Rotating cylinder viscometer 27 · Cone on plate viscometer 29 Other viscometers 29 2.8 Viscosity of mixtures 31 VIII ENGINEERING TRIBOLOGY 2.9 Oil viscosity classification 31 SAE viscosity classification 31 ISO viscosity classification 33 2.10 Lubricant density and specific gravity 33 2.11 Thermal properties of lubricants 34 Specific heat 34 Thermal conductivity 35 Thermal diffusivity 35 2.12 Temperature characteristics of lubricants 35 Pour point and cloud point 36 Flash point and fire point 37 Volatility and evaporation 37 Oxidation stability 38 Thermal stability 39 2.13 Other lubricant characteristics 40 Surface tension 40 Neutralization number 43 Carbon residue 43 2.14 Optical properties of lubricants 43 Refractive index 43 2.15 Additive compatibility and solubility 44 Additive compatibility 44 Additive solubility 44 2.16 Lubricant impurities and contaminants 44 Water content 44 Sulphur content 45 Ash content 45 Chlorine content 45 2.17 Solubility of gases in oils 45 2.18 Summary 48 Revision questions 48 References 49 3 LUBRICANTS AND THEIR COMPOSITION 51 3.1 Introduction 51 3.2 Mineral oils 52 Sources of mineral oils 52 Manufacture of mineral oils 54 Types of mineral oils 56  CONTENTS IX · Chemical forms 56 · Sulphur content 57 · Viscosity 57 3.3 Synthetic oils 57 Manufacturing of synthetic oils 58 Hydrocarbon synthetic lubricants 59 · Polyalphaolefins 59 · Polyphenyl ethers 59 · Esters 60 · Cycloaliphatics 61 · Polyglycols 61 Silicon analogues of hydrocarbons 62 · Silicones 62 · Silahydrocarbons 62 Organohalogens 63 · Perfluoropolyethers 63 · Chlorofluorocarbons 63 · Chlorotrifluoroethylenes 64 · Perfluoropolyalkylethers 64 Cyclophosphazenes 64 3.4 New developments in synthetic lubricants 64 Ionic liquid lubricants 65 Mesogenic lubricants 66 3.5 Emulsions and aqueous lubricants 67 Manufacturing of emulsions 67 Characteristics 67 Applications 68 Polyelectrolyte lubricants 68 3.6 Greases 69 Manufacturing of greases 69 Composition 69 · Base oils 70 · Thickener 70 · Additives 71 · Fillers 71 Lubrication mechanism of greases 72 Grease characteristics 75 · Consistency of greases 75 · Mechanical stability 76 X ENGINEERING TRIBOLOGY · Drop point 77 · Oxidation stability 78 · Thermal stability 79 · Evaporation loss 79 · Grease viscosity characteristics 79 Classification of greases 81 Grease compatibility 83 Degradation of greases 83 3.7 Lubricant additives 84 Wear and friction improvers 84 · Adsorption or boundary additives 85 · Antiwear additives 86 · Extreme-pressure additives 88 Nanoparticle additives 89 Anti-oxidants 89 · Oil oxidation 89 · Oxidation inhibitors 92 Corrosion control additives 95 Contamination control additives 95 Viscosity improvers 97 Pour point depressants 98 Foam inhibitors 98 Interference between additives 99 3.8 Summary 100 Revision questions 100 References 101 4 HYDRODYNAMIC LUBRICATION 105 4.1 Introduction 105 4.2 Reynolds equation 105 Simplifying assumptions 107 Equilibrium of an element 107 Continuity of flow in a column 111 Simplifications to the Reynolds equation 113 · Unidirectional velocity approximation 113 · Steady film thickness approximation 113 · Isoviscous approximation 114 · Infinitely long bearing approximation 114 · Narrow bearing approximation 115  CONTENTS XI Bearing parameters predicted from Reynolds equation 117 · Pressure distribution 117 · Load capacity 117 · Friction force 118 · Coefficient of friction 119 · Lubricant flow 119 Summary 119 4.3 Pad bearings 120 Infinite linear pad bearing 120 · Bearing geometry 120 · Pressure distribution 121 · Load capacity 123 · Friction force 124 · Coefficient of friction 127 · Lubricant flow rate 128 Infinite Rayleigh step bearing 129 Other wedge geometries of infinite pad bearings 132 · Tapered land wedge 132 · Parabolic wedge 133 · Parallel surface bearings 134 · Spiral groove bearing 135 Bearings with surface textures 136 Finite pad bearings 137 Pivoted pad bearing 139 Inlet boundary conditions in pad bearing analysis 141 4.4 Converging-diverging wedges 143 Bearing geometry 144 Pressure distribution 144 · Full-Sommerfeld boundary condition 146 · Half-Sommerfeld boundary condition 147 · Reynolds boundary condition 149 Load capacity 150 4.5 Journal bearings 152 Evaluation of the main parameters 152 · Bearing geometry 152 · Pressure distribution 154 · Load capacity 155 · Friction force 160 · Coefficient of friction 161 XII ENGINEERING TRIBOLOGY · Lubricant flow rate 163 Practical and operational aspects of journal bearings 165 · Lubricant supply 165 · Cavitation 169 · Journal bearings with movable pads 170 · Journal bearings incorporating a Rayleigh step 171 · Oil whirl or lubricant caused vibration 171 · Rotating load 174 · Tilted shafts 176 · Partial bearings 177 · Elastic deformation of the bearing 178 · Infinitely long approximation in journal bearings 178 4.6 Thermal effects in bearings 179 Heat transfer mechanisms in bearings 180 · Conduction 180 · Convection 181 · Conducted/convected heat ratio 181 Isoviscous thermal analysis of bearings 182 · Iterative method 183 · Constant flow method 184 Non-isoviscous thermal analysis of bearings with locally varying viscosity 185 Multiple regression in bearing analysis 186 Bearing inlet temperature and thermal interaction between pads of a Michell bearing 188 4.7 Limits of hydrodynamic lubrication 189 4.8 Hydrodynamic lubrication with non-Newtonian fluids 190 Turbulence and hydrodynamic lubrication 191 Hydrodynamic lubrication with non-Newtonian lubricants 192 Inertia effects in hydrodynamics 193 Compressible fluids 194 Compressible hydrodynamic lubrication in gas bearings 196 4.9 Reynolds equation for squeeze films 198 Pressure distribution 199 Load capacity 200 Squeeze time 201 Cavitation and squeeze films 202 Microscopic squeeze film effects between rough sliding surfaces 203 4.10 Porous bearings 203 4.11 Summary 204  CONTENTS XIII Revision questions 205 References 207 5 COMPUTATIONAL HYDRODYNAMICS 211 5.1 Introduction 211 5.2 Non-dimensionalization of the Reynolds equation 211 5.3 The Vogelpohl parameter 212 5.4 Finite difference equivalent of the Reynolds equation 214 Definition of solution domain and boundary conditions 216 Calculation of pressure field 217 Calculation of dimensionless friction force and friction coefficient 217 Numerical solution technique for Vogelpohl equation 220 5.5 Numerical analysis of hydrodynamic lubrication in idealized journal and partial arc bearings 220 Example of data from numerical analysis, the effect of shaft misalignment 221 5.6 Numerical analysis of hydrodynamic lubrication in a real bearing 226 5.6.1 Thermohydrodynamic lubrication 226 Governing equations and boundary conditions in thermohydrodynamic lubrication 227 · Governing equations in thermohydrodynamic lubrication for a one-dimensional bearing 228 · Thermohydrodynamic equations for the finite pad bearing 231 · Boundary conditions 232 Finite difference equations for thermohydrodynamic lubrication 233 Treatment of boundary conditions in thermohydrodynamic lubrication 236 Computer program for the analysis of an infinitely long pad bearing in the case of thermohydrodynamic lubrication 237 Example of the analysis of an infinitely long pad bearing in the case of thermohydrodynamic lubrication 238 5.6.2 Elastic deformations in a pad bearing 241 Computer program for the analysis of an elastically deforming one- dimensional pivoted Michell pad bearing 243 Effect of elastic deformation of the pad on load capacity and film thickness 243 5.6.3 Cavitation and film reformation in grooved journal bearings 246 Computer program for the analysis of grooved 360° journal bearings 250 Example of the analysis of a grooved 360° journal bearing 250 5.6.4 Vibrational stability in journal bearings 256 Determination of stiffness and damping coefficients 256 Computer program for the analysis of vibrational stability in a partial arc journal bearing 261 Example of the analysis of vibrational stability in a partial arc journal bearing 261 XIV ENGINEERING TRIBOLOGY 5.7 Summary 264 Revision questions 264 References 265 6 HYDROSTATIC LUBRICATION 267 6.1 Introduction 267 6.2 Hydrostatic bearing analysis 268 Flat circular hydrostatic pad bearing 268 · Pressure distribution 268 · Lubricant flow 269 · Load capacity 269 · Friction torque 270 · Friction power loss 272 Non-flat circular hydrostatic pad bearings 272 · Pressure distribution 273 · Lubricant flow 274 · Load capacity 275 · Friction torque 275 · Friction power loss 275 6.3 Generalized approach to hydrostatic bearing analysis 276 Flat circular pad bearings 276 Flat square pad bearings 276 6.4 Optimization of hydrostatic bearing design 277 Minimization of power 277 · Low-speed recessed bearings 279 · High-speed recessed bearings 279 Control of lubricant film thickness and bearing stiffness 280 · Stiffness with constant flow method 281 · Stiffness with capillary restrictors 281 · Stiffness with an orifice 283 · Stiffness with pressure sensors 284 6.5 Aerostatic bearings 285 Pressure distribution 286 Gas flow 286 Load capacity 287 Friction torque 287 Power loss 288 6.6 Hybrid bearings 288 6.7 Stability of hydrostatic and aerostatic bearings 288  CONTENTS XV 6.8 Summary 289 Revision questions 289 References 290 7 ELASTOHYDRODYNAMIC LUBRICATION 293 7.1 Introduction 293 7.2 Contact stresses 294 Simplifying assumptions to Hertz's theory 294 Stress status in static contact 295 Stress status in lubricated rolling and sliding contacts 295 7.3 Contact between two elastic spherical or spheroidal bodies 296 Geometry of contacting elastic bodies 297 · Two elastic bodies with convex surfaces in contact 298 · Two elastic bodies with one convex and one flat surface in contact 299 · Two elastic bodies with one convex and one concave surface in contact 300 Contact area, pressure, maximum deflection and position of the maximum shear stress 301 · Contact between two spheres 301 · Contact between a sphere and a plane surface 304 · Contact between two parallel cylinders 306 · Contact between two crossed cylinders with equal diameters 309 · Elliptical contact between two elastic bodies, general case 311 Total deflection 316 7.4 Elastohydrodynamic lubricating films 317 Effects contributing to the generation of elastohydrodynamic films 318 · Hydrodynamic film formation 318 · Modification of film geometry by elastic deformation 318 · Transformation of lubricant viscosity and rheology under pressure 319 Approximate solution of Reynolds equation with simultaneous elastic deformation and viscosity rise 319 Pressure distribution in elastohydrodynamic films 323 Elastohydrodynamic film thickness formulae 324 Effects of the non-dimensional parameters on EHL contact pressures and film profiles 325 · Effect of the speed parameter 325 · Effect of the materials parameter 326 · Effect of the load parameter 326 · Effect of the ellipticity parameter 327 Lubrication regimes in EHL - film thickness formulae 328 XVI ENGINEERING TRIBOLOGY · Isoviscous-rigid 329 · Piezoviscous-rigid 330 · Isoviscous-elastic 330 · Piezoviscous-elastic 330 Identification of the lubrication regime 331 Elastohydrodynamic film thickness measurements 331 7.5 Micro-elastohydrodynamic lubrication and mixed or partial EHL 334 Partial or mixed EHL 335 Micro-elastohydrodynamic lubrication 337 7.6 Surface temperature at the conjunction between contacting solids and its effect on EHL 339 Calculation of surface conjunction temperature 340 · Flash temperature in circular contacts 343 · Flash temperature in square contacts 343 · Flash temperature in line contacts 346 True flash temperature rise 347 Frictional temperature rise of lubricated contacts 351 Mechanism of heat transfer within the EHL film 353 Effect of surface films on conjunction temperatures 354 Measurements of surface temperature in the EHL contacts 354 7.7 Traction and EHL 356 A simplified analysis of traction in the EHL contact 359 Non-Newtonian lubricant rheology and EHL 361 EHL between meshing gear wheels 363 7.8 Summary 365 Revision questions 365 References 367 8 BOUNDARY AND EXTREME PRESSURE LUBRICATION 371 8.1 Introduction 371 8.2 Low temperature - low load lubrication mechanisms 373 8.3 Low temperature - high load lubrication mechanisms 374 Model of adsorption on sliding surfaces 375 · Physisorption 376 · Chemisorption 378 · Influence of the molecular structure of the lubricant on adsorption lubrication 380 · Influence of oxygen and water 383 · Dynamic nature of adsorption under sliding conditions 385  CONTENTS XVII · Mixed lubrication and scuffing 387 · Metallurgical effects 394 · Interaction between surfactant and carrier fluid 395 8.4 High temperature - medium load lubrication mechanisms 395 Chain matching 395 Thick films of soapy or amorphous material 398 · Soap layers 398 · Amorphous layers 400 8.5 High temperature - high load lubrication mechanisms 404 Model of lubrication by sacrificial films 404 Additive reactivity and its effect on lubrication 405 Nascent metallic surfaces and accelerated film formation 408 Influence of oxygen and water on the lubrication mechanism by sacrificial films 410 Mechanism of lubrication by milder EP additives 413 Function of active elements other than sulphur 414 Lubrication with two active elements 415 Temperature distress 417 Speed limitations of sacrificial film mechanism 418 Triboemission from worn surfaces 418 8.6 Boundary and EP lubrication of non-metallic surfaces 420 8.7 Vapor phase and gas lubrication 420 8.8 Summary 421 Revision questions 421 References 422 9 SOLID LUBRICATION AND SURFACE TREATMENTS 429 9.1 Introduction 429 9.2 Lubrication by solids 429 9.2.1 Lubrication by lamellar solids 430 Friction and wear characteristics of lamellar solids 433 · Graphite and molybdenum disulphide (MoS2) 433 · Carbon-based materials other than graphite 437 · Minor lamellar solid lubricants 438 9.2.2 Reduction of friction by soft metallic films 439 Reduction of friction by metal oxides at high temperatures or high speeds 440 9.2.3 Solid lubricant mixtures and composites 441 9.2.4 Deposition methods of solid lubricants 441 Traditional methods of solid lubricant deposition 442 XVIII ENGINEERING TRIBOLOGY Modern methods of solid lubricant deposition 442 Solid lubricants as additives to oils and polymers 444 9.3 Wear resistant coatings and surface treatments 445 9.3.1 Techniques of producing wear resistant coatings 446 Coating techniques dependent on vacuum or gas at very low pressure 446 · Physical vapor deposition (PVD) 447 · Chemical vapor deposition (CVD) 449 · Physical-chemical vapor deposition 450 · Ion implantation 451 Coating processes requiring localized sources of intense heat 451 · Surface welding 452 · Thermal spraying 452 · Laser surface hardening and alloying 454 Coating processes based on deposition in the solid state 456 Miscellaneous coating processes 456 9.3.2 Application of coatings and surface treatments in wear and friction control 458 General characteristics of wear resistant coatings 458 Current trends in coating technology 462 · Nitride, carbide and carbonitride coatings 462 · Diamond-like carbon (DLC) coatings 464 · Carbon-based composite coatings 465 · Multilayer coatings 465 · Nano-engineered coatings 466 · Thick coatings 466 · Other coatings 467 9.4 Summary 467 Revision questions 467 References 468 10 FUNDAMENTALS OF CONTACT BETWEEN SOLIDS 475 10.1 Introduction 475 10.2 Surfaces of solids 475 Surfaces at a nanoscale 476 Surface topography 477 Characterization of surface topography by statistical parameters 480 Multiscale characterization of surface topography 482 · Surface data presentation 484 · Characterization of surface topography by Fourier transform 484  CONTENTS XIX · Characterization of surface topography by wavelets 485 · Characterization of surface topography by fractals 486 · Characterization of surface topography by combination of wavelets and fractals 488 Recent developments in multiscale surface characterization 489 · Directional Fractal Signature (DFS) methods 489 · Partitioned Iterated Function System (PIFS) 491 Optimum surface roughness 492 10.3 Contact between solids 493 Model of contact between solids based on statistical parameters of rough surfaces 495 Model of contact between solids based on the fractal geometry of rough surfaces 498 Limitations of continuum theory in contact mechanics 500 Effect of sliding on contact between solid surfaces 500 Out-of-contact time 501 10.4 Friction and wear 502 Onset of sliding and mechanism of stick-slip 504 Structural differences between static and sliding contacts 507 Running-in phenomena 509 Effects of reversal of sliding direction 510 Friction and other contact phenomena in rolling 510 Concentration of frictional heat at the asperity contacts 513 Effect of temperature on friction and wear 515 Triboelectrification of sliding contacts 515 Wear between surfaces of solids 515 10.5 Summary 518 Revision questions 518 References 519 11 ABRASIVE, EROSIVE AND CAVITATION WEAR 525 11.1 Introduction 525 11.2 Abrasive wear 525 Mechanisms of abrasive wear 526 Modes of abrasive wear 528 Analytical models of abrasive wear 529 Abrasivity of particles 536 · Particle hardness effect 536 · Particle size effect 538 · Particle shape effect 540 XX ENGINEERING TRIBOLOGY Abrasive wear resistance of materials 542 · Abrasive wear resistance of steels 544 · Abrasive wear resistance of polymers and rubbers 547 · Abrasive wear resistance of ceramics 548 Effect of temperature on abrasive wear 548 Effect of moisture on abrasive wear 550 Control of abrasive wear 551 11.3 Erosive wear 551 Mechanisms of erosive wear 552 Effect of impingement angle and impact speed on erosive wear rate 552 Effect of particle shape, hardness, size and flux rates on erosive wear rate 555 Erosive wear by liquid 556 Effect of temperature on erosive wear 558 Effect of erosion media on erosive wear 559 Erosive wear resistance of materials 562 · Erosive wear resistance of steels 564 · Erosive wear resistance of polymers 564 · Erosive wear of ceramics and cermets 566 11.4 Cavitation wear 567 Mechanism of cavitation wear 567 Cavitation wear resistance of materials 568 11.5 Summary 571 Revision questions 571 References 572 12 ADHESION AND ADHESIVE WEAR 577 12.1 Introduction 577 12.2 Mechanism of adhesion 577 Metal-metal adhesion 577 Metal-polymer adhesion 580 Metal-ceramic adhesion 581 Polymer-polymer and ceramic-ceramic adhesion 581 Effects of adhesion between wearing surfaces 582 · Friction due to adhesion 582 · Junction growth between contacting asperities as a cause of extreme friction 583 · Seizure and scuffing 586 · Asperity deformation and formation of wear particles 586 · Transfer films 588  CONTENTS XXI 12.3 Control of the adhesive wear 592 Contaminant layers formed due to surface oxidation and bulk impurities 593 Lubricants 593 Favourable combinations of sliding materials 594 12.4 Summary 594 Revision questions 594 References 595 13 CORROSIVE AND OXIDATIVE WEAR 597 13.1 Introduction 597 13.2 Corrosive wear 597 Transition between corrosive and adhesive wear 603 Synergism between corrosive and abrasive wear 605 Tribocorrosion studies 606 Tribochemical polishing 606 13.3 Oxidative wear 607 Kinetics of oxide film growth on metals at high and low temperatures 608 · Oxidative wear at high sliding speeds 609 · Oxidative wear at low sliding speeds 611 · Oxidative wear at high temperature and stress 612 · Oxidative wear at low temperature applications 614 · Transition between oxidative and adhesive wear 614 · Oxidative wear under lubricated conditions 614 Means of controlling corrosive and oxidative wear 615 13.4 Summary 616 Revision questions 616 References 617 14 FATIGUE WEAR 621 14.1 Introduction 621 14.2 Fatigue wear during sliding 622 Surface crack initiated fatigue wear 623 Subsurface crack initiated fatigue wear 626 Effect of lubrication on fatigue wear during sliding 627 Plastic ratchetting 628 14.3 Fatigue wear during rolling 629 Causes of contact fatigue 630 · Asperity contact during EHL and the role of debris in the lubricant in contact fatigue 630 XXII ENGINEERING TRIBOLOGY · Material imperfections 631 · Plastic deformation in wheel-rail contacts 631 Self-propagating nature of contact fatigue cracks 632 Subsurface and surface modes of contact fatigue 633 Effect of lubricant on contact fatigue 636 Hydraulic pressure crack propagation 636 Chemical effects of lubricant additives, oxygen and water on contact fatigue 637 Materials effect on contact fatigue 639 Influence of operating conditions on rolling wear and contact fatigue 640 14.4 Means of controlling fatigue wear 641 14.5 Summary 641 Revision questions 641 References 642 15 FRETTING AND MINOR WEAR MECHANISMS 647 15.1 Introduction 647 15.2 Fretting wear 648 Microscopic movements within the contact under applied loads 648 · Elastic model for fretting contacts 648 · Elasto-plastic model for fretting contacts 650 Fretting regimes 651 Effect of amplitude and debris retention on fretting wear 652 Environmental effects on fretting wear 654 Effects of temperature and lubricants on fretting 658 Effect of materials properties and surface finish on fretting 660 Fretting fatigue 660 Practical examples of fretting 663 Means of controlling fretting 664 15.3 Melting wear 665 15.4 Wear due to electrical discharges and passage of electric current across a contact 668 15.5 Diffusive wear 669 15.6 Impact wear 670 15.7 Summary 672 Revision questions 672 References 673 16 WEAR OF NON-METALLIC MATERIALS 679 16.1 Introduction 679  CONTENTS XXIII 16.2 Tribology of polymers 679 Sliding wear of polymers, transfer layers on a harder counterface 681 Influence of counterface roughness, hardness and material type on transfer films and associated wear and friction of polymers 682 · Counterface hardness 683 · Counterface roughness 683 · Counterface surface energy 686 PV limit 686 Influence of temperature on polymer wear and friction 687 · Limit on frictional temperature rise imposed by surface melting 688 · Effect of high frictional temperatures and sliding speeds on wear 691 · Effects of reduced temperature on sliding of polymers 692 · Combined effect of high surface roughness and elevated contact temperature on wear 693 Fatigue wear of polymers and long term wear kinetics 694 Effect of sliding mode and strain reversal 695 Visco-elasticity and the rubbery state 696 Friction and wear in the rubbery state 697 · Schallamach waves 697 · Visco-elasticity and friction of rubbers 698 · Wear mechanisms particular to rubbery solids 699 Effect of lubricant, corrosive agents and microstructure on wear and friction of polymers 700 · Effects of lubricants 700 · Effects of corrosive agents 701 · Effect of oxidizing and biochemical reagents 702 · Effects of polymer microstructure 703 16.3 Tribology of polymer composites 705 Polymer blends 705 Fibre-reinforced polymers 705 · Chopped fibre reinforced polymers 706 · Unidirectional and woven fibre reinforcements 706 · Modelling of wear of fibre-reinforced polymers 709 Powder composites 709 16.4 Wear and friction of ceramics 711 Unlubricated wear and friction of ceramic-ceramic contacts 713 · Dry friction and wear of ceramics at room temperature 713 · Dry friction and wear of ceramics at elevated temperatures 715 · Friction and wear of ceramics in the presence of water or humid air 716 XXIV ENGINEERING TRIBOLOGY · Wear modelling of ceramics 716 · Dry wear and friction characteristics of individual ceramics 719 Lubricated wear and friction of ceramic-ceramic contacts 720 · Liquid lubrication 720 · Solid lubricants and vapor phase lubrication 721 Wear and friction of ceramics against metallic materials 723 Wear and friction of ceramics against polymers 725 Wear and friction of ceramic matrix composites 726 16.5 Summary 727 Revision questions 727 References 729 17 FUTURE DIRECTIONS IN TRIBOLOGY 735 17.1 Introduction 735 17.2 Biotribology 735 Biotribology of living tissues and organisms 736 Biotribology of artificial materials in close contact with living tissues 738 17.3 Environmental implications of tribology 741 17.4 Nanotribology - basic concepts 742 Relevance to tribology 744 Nanolubrication and specialized materials for nanotribology 745 · Vapor phase lubrication for MEMS and NEMS 746 · Nanolubrication with carbon-based molecules 746 · Self-assembled dual layers 747 17.5 Summary 747 Revision questions 748 References 748 APPENDIX 751 Introduction 751 A.1 User-friendly interface 752 A.2 Program ‘VISCOSITY’ 754 Program description 756 List of variables 756 A.3 Program ‘SIMPLE’ 756 Program description 758 List of variables 759 A.4 Program ‘PARTIAL’ 761 Program description 763  CONTENTS XXV List of variables 767 A.5 Program ‘THERMAL’ 769 Program description 774 List of variables 777 A.6 Program ‘DEFLECTION’ 781 Program description 784 List of variables 787 A.7 Program ‘GROOVE’ 789 Program description 795 List of variables 801 A.8 Program ‘STABILITY’ 804 Program description 807 List of variables 808 A.9 PROGRAM ‘SPIKE’ 809 Program description 813 List of variables 817 INDEX 823 This page is intentionally left blank  P R E F A C E Some 20 years ago we decided to write a textbook on ‘Engineering Tribology’ and to our pleasant surprise a publisher supported the idea. Students had requested a book suitable for the study of tribology and the existing textbooks were either too specialized or too literal in content. Many books provided exhaustive reviews of friction and wear data, while others contained detailed descriptions of the lubrication and wear problems occurring in machinery. A book which presents the concepts of tribology in terms useful to engineering students and engineers was, however, lacking. In the books published at the time, the basic models of friction and wear were not explained adequately. As a result, more sophisticated concepts could not be well understood. The interdisciplinary nature of tribology, with knowledge drawn from different disciplines such as mechanical engineering, materials science, chemistry and physics, leads to a general tendency for the chemist to describe in detail, for example, lubricant additives, the mechanical engineer to discuss, for example, pad or journal bearings and so on, with no overall guide to the subject. In this book, the interaction between these different fields of knowledge to achieve the final result, the control of friction and wear, is emphasized. The interdisciplinary view of tribology was largely developed by Professor Alastair Cameron about four decades ago and this approach has proved to be the most successful way of analysing friction and wear problems. In many cases tribology is viewed as an inaccessible subject which does not produce useful answers. In this ‘Engineering Tribology’ book we try to redress this problem. Rutherford's maxim, that ‘any good scientific theory is explainable to the average barmaid’, is applied in this book with various concepts explained in the simplest possible terms with supporting illustrations. Now it is time to write the 4th edition with amendments in the same style as the earlier versions. The previous editions were received very favourably by the reviewers and especially by the students. In the 4th edition of ‘Engineering Tribology’ we aim to update the contents of the third edition while maintaining its style. In this edition, a number of extra topics have been included to make the book more comprehensive and up to date. The listings of literature citations have been extended to include recent findings from tribology research. Extra diagrams have also been included where it was found that the readability and comprehension of the original text could be improved. Computer programs used in the numerical analysis have been upgraded to the current version of MATLAB. A new computer program ‘SPIKE’ for calculating the grit particle angularity has also been added. To provide opportunities for active learning, a series of revision questions are provided at the end of each chapter. This should be helpful in assessing the understanding for both students and lecturers. Despite all these changes, the purpose of writing ‘Engineering Tribology’ remains XXVIII ENGINEERING TRIBOLOGY the same, i.e., to provide a reader-friendly and comprehensive introduction to the subject of tribology and its implications for engineering. This edition, like the previous editions, is intended for final year under-graduate and post- graduate students in mechanical engineering and professional engineers. The subject matter of the book is also relevant to materials engineering, applied chemistry, physics and bioengineering courses. Gwidon W. Stachowiak Andrew W. Batchelor  A C K N O W L E D G E M E N T S Any book depends on the efforts of many different people and this book is no exception. Firstly, we would like to thank Dr Grazyna Stachowiak for very detailed research, review of technical material, proof-reading, many constructive discussions, SEM micrographs and preparation of index; Dr Mike Hamblin for the concept development of the Spike Parameter Quadratic Fit and Dr Marcin Wolski for converting it into the MATLAB; Professor Motohiro Kaneta for useful comments on elastohydrodynamic lubrication; Professor Emily Brodsky from the University of California Santa Cruz and Dr Amir Sagy from Geological Survey of Israel for the photo of the fault line (Figure 10.25) and a helpful discussion on rock faults; Gosia Wlodarczak-Sarnecka for the design of the book cover; Longin Sarnecki for the cover photo. We would like to thank the School of Mechanical and Chemical Engineering, University of Western Australia, and the Department of Mechanical Engineering, Curtin University in Western Australia for their help during the preparation of the manuscript. The support of Saudi Aramco is also gratefully acknowledged. Finally, we would like to thank the following publishers for granting us permission to reproduce the figures listed below: Figure 9.7: Society of Tribologists and Lubrication Engineers. From Tribology Transactions, Vol. 31, 1988, pp. 214-227. Figures 13.5 and 13.11: Japanese Society of Tribologists. From Journal of Japan Society of Lubrication Engineers, Vol. 31, 1986, pp. 883-888 and Vol. 28, 1983, pp. 53-56, respectively. Figures 14.2 and 15.2: Royal Society of London. From Proceedings of the Royal Society of London, Vol. 394, 1984, pp. 161-181 and Vol. 230, 1955, pp. 531-548, respectively. Figure 16.6: The American Society of Mechanical Engineers. From Transactions of the ASME, Journal of Lubrication Technology, Vol. 101, 1979, pp. 212-219. Figures 11.41 and 16.22 were previously published in Wear, Vol. 113, 1986, pp. 305-322 and Vol. 17, 1971, pp. 301-312, respectively. This page is intentionally left blank 1 I N T R O D U C T I O N 1.1 BACKGROUND Tribology in a traditional form has been in existence since the beginning of recorded history. There are many well documented examples of how early civilizations developed bearings and low friction surfaces. The scientific study of tribology also has a long history, and many of the basic laws of friction, such as the proportionality between normal force and limiting friction force, are thought to have been developed by Leonardo da Vinci in the late 15th century. However, the understanding of friction and wear languished in the doldrums for several centuries with only fanciful concepts to explain the underlying mechanisms. For example, it was proposed by Amontons in 1699 that surfaces were covered by small spheres and that the friction coefficient was a result of the angle of contact between the spheres of contacting surfaces. A reasonable value of friction coefficient close to 0.3 was therefore found by assuming that motion was always to the top of the spheres. The relatively low priority of tribology at that time meant that nobody really bothered to question what would happen when motion between the spheres was in a downwards direction. Unlike thermodynamics, where fallacious concepts like ‘phlogiston’ were rapidly disproved by energetic researchers such as Lavoisier in the late 18th century, relatively little understanding of tribology was gained until 1886 with the publication of Osborne Reynolds' classical paper on hydrodynamic lubrication. Reynolds proved that hydrodynamic pressure of liquid entrained between sliding surfaces was sufficient to prevent contact between the surfaces even at very low sliding speeds. His research had immediate practical application and led to the removal of an oil hole from the load line of railway axle bearings. The oil, instead of being drained away by the hole, was now able to generate a hydrodynamic film and much lower friction resulted. The work of Reynolds initiated countless other research efforts aimed at improving the interaction between two contacting surfaces, and the efforts continue to this day. As a result, journal bearings are now designed to high levels of sophistication. Wear and the fundamentals of friction are far more complex problems, the experimental investigation of which is dependent on advanced instrumentation such as scanning electron microscopy and atomic force microscopy. Therefore, it has only recently been possible to study these processes on a microscopic scale where a true understanding of their nature can be found. Tribology is therefore a very new field of science, most of the knowledge being gained after the Second World War. In comparison, many basic engineering subjects, e.g., thermodynamics, mechanics and plasticity, are relatively old and well established. Tribology is still in an imperfect state and subject to some controversy which has impeded the diffusion 2 ENGINEERING TRIBOLOGY of information to technologists in general. The need for information is nevertheless critical; even simple facts such as the type of lubricant that can be used in a particular application, or preventing the contamination of oil by water must be fully understood by an engineer. Therefore this book is devoted to the fundamental principles of engineering tribology. 1.2 MEANING OF TRIBOLOGY Tribology, which focuses on friction, wear and lubrication of interacting surfaces in relative motion, is a new field of science defined in 1967 by a committee of the Organization for Economic Cooperation and Development. ‘Tribology’ is derived from the Greek word ‘tribos’ meaning rubbing or sliding. After an initial period of scepticism, as is inevitable for any newly introduced word or concept, the word ‘tribology’ has gained gradual acceptance. However, as the word ‘tribology’ is relatively new, its meaning is still unclear to the wider community and humorous comparisons with tribes or tribolites tend to persist as soon as the word ‘tribology’ is mentioned. Wear is the major cause of material wastage and loss of mechanical performance and any reduction in wear can result in considerable savings. Friction is a principal cause of wear and energy dissipation and savings can be made by improved friction control. It is estimated that one-third of the world's energy resources in present use is needed to overcome friction in one form or another. Lubrication is an effective means of controlling wear and reducing friction. Tribology is a field of science which applies an operational analysis to problems of great economic significance such as reliability, maintenance and wear of technical equipment ranging from household appliances to spacecraft. The question is why ‘the interacting surfaces in relative motion’ (which essentially means rolling, sliding, normal approach or separation of surfaces) are so important to our economy and why they affect our standard of living. The answer is that surface interaction dictates or controls the functioning of practically every device developed by man. Everything that man makes wears out, almost always as a result of relative motion between surfaces. An analysis of machine break-downs shows that in the majority of cases failures and stoppages are associated with interacting moving parts such as gears, bearings, couplings, sealings, cams, clutches, etc. The majority of problems accounted for are tribological. Our human body also contains interacting surfaces, e.g., human joints, which are subjected to lubrication and wear. Despite our detailed knowledge covering many disciplines, the lubrication of human joints is still far from fully understood. Tribology affects our lives to a much greater degree than is commonly realized. For example, long before the deliberate control of friction and wear was first promoted, human beings and animals were instinctively modifying friction and wear as it affected their own bodies. It is common knowledge that the human skin becomes sweaty as a response to stress or fear. It has only recently been discovered that sweating on the palms of hands or soles of feet of humans and dogs, but not rabbits, has the ability to raise friction between the palms or feet and a solid surface. In other words, when an animal or human senses danger, sweating occurs to promote either rapid flight from the scene of danger, or else the ability to firmly hold a weapon or climb the nearest tree. A general result or observation derived from innumerable experiments and theories is that tribology comprises the study of: · the characteristics of films of intervening material between contacting bodies, and · the consequences of either film failure or absence of a film which are usually manifested by severe friction and wear. Film formation between any pair of sliding objects is a natural phenomenon which can occur without human intervention. Film formation might be the fundamental mechanism  INTRODUCTION 3 preventing extremely high shear rates at the interface between two rigid sliding objects. Non- mechanical sliding systems provide many examples of this film formation. For example, studies of the movement between adjacent geological plates on the surface of the earth reveal that a thin layer of fragmented rock and water forms between opposing rock masses. Chemical reactions between rock and water, initiated by prevailing high temperatures (about 600°C) and pressures (about 100 [MPa]), are believed to improve the lubricating function of the material in this layer. Laboratory tests of model faults reveal that sliding initiates the formation of a self-sliding layer of fragmented rock at the interface with solid rock. A pair of self-sealing layers attached to both rock masses prevent the leakage of water necessary for the lubricating action of the inner layer of fragmented rock and water. Although the thickness of the intervening layer of fragmented rock is believed to be between 1 and 100 [m] , this thickness is insignificant when compared to the extent of geological plates and these layers can be classified as ‘films’. Sliding on a geological scale is therefore controlled by the properties of these ‘lubricating films’, and this suggests a fundamental similarity between all forms of sliding, whether on the massive geological scale or on the microscopic scale of sliding between erythrocytes and capillaries. The question is, why do such films form and persist? A possible reason is that a thin film is mechanically stable, i.e., it is very difficult to completely expel such a film by squeezing between two objects. It is not difficult to squeeze out some of the film but its complete removal is virtually impossible. Although sliding is destructive to these films, i.e., wear occurs, it also facilitates their replenishment by entrainment of a ‘lubricant’ or else by the formation of fresh film material from wear particles. Film formation between solid objects is intrinsic to sliding and other forms of relative motion, and the study and application of these films for human benefits are the raison d'être of tribology. Sometimes, however, the formation of lubricating films between moving solid objects is not desirable. For example, autumn leaves on railway lines form low friction films that can cause wheel skidding and disrupt the motion of trains. The low friction film is generated by pectin (a plant protein) and cellulose that are released by the leaves when they are crushed by railway wheels. The pectin reacts with iron to form a gel, which combined with the cellulose is believed to form a thick viscous film, apparently capable of reducing the friction coefficient from 0.2 (pure water) to 0.05. For example, sycamore leaves were found to quickly form a tenacious visible layer on rolling steel wheels. This layer reduced the friction coefficient to 0.05 and persisted for more than 1,000 cycles of rolling contact (with 1% slip). The friction reduction in trains is undesirable and the irrigation of the rolling contact with a paste or ‘friction modifier’ consisting of an inorganic gelling agent, a stabilizer, water, sand grains and stainless steel particles was found to restore friction coefficients to useful levels. In simple terms it appears that the practical objective of tribology is to minimize the two main disadvantages of solid-to-solid contact: friction and wear, but this is not always the case. In some situations, as illustrated in Figure 1.1, minimizing friction and maximizing wear or minimizing wear and maximizing friction or maximizing both friction and wear is desirable. For example, reduction of wear but not friction is desirable in brakes and lubricated clutches, reduction of friction but not wear is desirable in pencils, increase in both friction and wear is desirable in erasers. Lubrication Thin low-shear-strength layers of gas, liquid and solid are interposed between two surfaces in order to improve the smoothness of movement of one surface over another and to prevent damage. These layers of material separate contacting solid bodies and are usually very thin and often difficult to observe. In general, the thicknesses of these films range from 1 to 100 [µm], although thinner and thicker films can also be found. Knowledge that is related to enhancing or diagnosing the effectiveness of these films in preventing damage in solid contacts is 4 ENGINEERING TRIBOLOGY commonly known as ‘lubrication’. Although there are no restrictions on the type of material required to form a lubricating film, as gas, liquid and certain solids are all effective, the material type does influence the limits of film effectiveness. For example, a gaseous film is suitable for low contact stress while solid films are usually applied to slow sliding speed contacts. Detailed analysis of gaseous or liquid films is usually termed ‘hydrodynamic lubrication’ while lubrication by solids is termed ‘solid lubrication’. A specialized form of hydrodynamic lubrication involving physical interaction between the contacting bodies and the liquid lubricant is termed ‘elastohydrodynamic lubrication’ and is of considerable practical significance. Another form of lubrication involves the chemical interactions between contacting bodies and the liquid lubricant and is termed ‘boundary and extreme- pressure lubrication’. In the absence of any films, the only reliable means of ensuring relative movement is to maintain, by external force fields, a small distance of separation between the opposing surfaces. This, for example, can be achieved by the application of magnetic forces, which is the operating principle of magnetic levitation or ‘maglev’. Magnetic levitation is, however, a highly specialized technology that is still at the experimental stage. A form of lubrication that operates by the same principle, i.e., forcible separation of the contacting bodies involving an external energy source, is ‘hydrostatic lubrication’ where a pressurized liquid or gaseous lubricant is forced into the space between contacting bodies. Bearings Brakes Clutches Gears Clamps Tyres Cams Shoes Slideways Frictional heating (e.g. initiation Free-sliding mechanical interfaces of fire by prehistoric people) etc. etc. Lubrication Wear resistant Surface coatings materials Minimum wear Maximum friction Minimum friction WEAR & FRICTION Maximum wear Sacrificial Enhancement of materials adhesion Pencils Deposition of solid lubricants Erasers by sliding contacts Friction surfacing FIGURE 1.1 Practical objectives of tribology. Liquid lubrication is a technological nuisance since filters, pumps and cooling systems are required to maintain the performance of the lubricant over a period of time. There are also environmental issues associated with the disposal of the used lubricants. Therefore ‘solid lubrication’ and ‘surface coatings’ are the subject of intense research.  INTRODUCTION 5 The principal limitations of, in particular, liquid lubricants are the loss of load carrying capacity at high temperature and degradation in service. The performance of the lubricant depends on its composition and its physical and chemical characteristics. From the practical engineering viewpoint, prediction of lubricating film characteristics is extremely important. Although such predictions are possible, there always remains a certain degree of empiricism in the analysis of film characteristics. Prediction methods for liquid or gaseous films involve at the elementary level hydrodynamic, hydrostatic and elastohydrodynamic lubrication. For more sophisticated analyses ‘computational methods’ must be used. There is still, however, no analytical method for determining the limits of solid films. Wear Film failure impairs the relative movement between solid bodies and inevitably causes damage to the contacting surfaces. The consequence of film failure is severe wear. Wear in these circumstances is the result of adhesion between contacting bodies and is termed ‘adhesive wear’. When the intervening films are partially effective then milder forms of wear occur and these are often initiated by fatigue processes due to repetitive stresses under either sliding or rolling. These milder forms of wear can therefore be termed ‘fatigue wear’. On the other hand, if the film material consists of hard particles or merely flows against one body without providing support against another body then a form of wear, which sometimes can be very rapid, known as ‘abrasive wear’ occurs. Two other associated forms of wear are ‘erosive wear’ (due to impacting particles) and ‘cavitation wear’ which is caused by fast flowing liquids. In some practical situations the film material is formed by chemical attack of either contacting body and while this may provide some lubrication, significant wear is virtually inevitable. This form of wear is known as ‘corrosive wear’ and when atmospheric oxygen is the corroding agent, then ‘oxidative wear’ is said to occur. When the amplitude of movement between contacting bodies is restricted to, for example, a few micrometres, the film material is trapped within the contact and may eventually become destructive. Under these conditions ‘fretting wear’ may result. There are also many other forms or mechanisms of wear. Almost any interaction between solid bodies will cause wear. Typical examples are ‘impact wear‘ caused by impact between two solids, ‘melting wear’ occurring when the contact loads and speeds are sufficiently high to allow for the surface layers of the solid to melt and ‘diffusive wear’ occurring at high interface temperatures. This dependence of wear on various operating conditions can be summarized in a flowchart shown in Figure 1.2. 1.3 COST OF FRICTION AND WEAR The enormous cost of tribological deficiencies to any national economy is mostly caused by the large amount of energy and material losses occurring simultaneously on virtually every mechanical device in operation. When reviewed on the basis of a single machine, the losses are small. However, when the same loss is repeated on perhaps a million machines of a similar type, then the costs become very large. For example, about two hundred years ago, Jacobs Rowe suggested that application of the rolling element bearing to carriages could halve the number of horses required for all the carriages and carts in the United Kingdom. Since the estimated national total number of horses involved in this form of transportation was at that time about 40,000, the potential saving in horse-care costs was about one million pounds per annum at early 18th century prices [1,4]. In more contemporary times the simple analysis reveals that supplying all the worm gear drives in the United States with a lubricant that allows a relative increase of 5% in the mechanical efficiency compared to a conventional mineral oil would result in savings of 6 ENGINEERING TRIBOLOGY about US$0.6 billion per annum. The reasoning is that there are 3 million worm gears operating in the U.S.A. with an average power rating of about 7.5 [KW]. The annual national savings of energy would be 9.8 billion kilowatt-hours and the corresponding value of this energy is 0.6 billion US$ at an electricity cost of 0.06 US$ per kilowatt-hour. Is load high enough to prevent No No wear hydrodynamic lubrication (or EHL)? Yes Do the abrasives impact Are abrasives present in large quantities? the worn surface? No Yes No Cavitational Yes Does fluid cavitate on wear worn surface? Erosive wear Abrasive wear Corrosive Yes Is there a corrosive fluid? wear Is a corrosive fluid also present? Melting Yes Are sliding speeds very high, Yes Yes wear causing surface melting? Corrosive- Corrosive- erosive wear abrasive wear Yes Is the amplitude of sliding very Fretting small, i.e. µm in scale? Oxidative Yes Does the wear occur at high wear temperatures in air or oxygen? Are the wear particles large and chunky Yes Frictional seizures, and/or is friction high with a large variability? adhesive wear Is wear a gradual steady process with Yes Fatigue generation of flat lamellar particles? based wear No Is impact Fatigue + oxidative Bad luck! involved? impact wear FIGURE 1.2 Flowchart illustrating the relationship between operating conditions and type of wear. These examples suggest that a form of ‘tribology equation’ can be used to obtain a simple estimate of either costs or benefits from existing or improved tribological practice. Such an equation can be summarized as: Total Tribological Cost/Saving = Sum of Individual Machine Cost/Saving × Number of Machines A major issue which impedes the application of the ‘tribology equation’ is that the number of individual costs or potential savings in friction and wear are extremely large, while the value of each cost or saving is relatively small (apart from the exceptional cases). This means that  INTRODUCTION 7 care must be taken with accurate accounting of machinery or system operating costs before the true cost of wear and friction can be adequately estimated. Despite the difficulties, many attempts have been made in the past to calculate the tribological cost. In 1966, it was estimated by Peter Jost that by the application of the basic principles of tribology, the economy of U.K. could save approximately £515 million per annum at 1965 values. A more recent review of the original estimation put the energy savings by the application of tribological principles and practices between £468 to £700 million per annum. A similar report published in West Germany in 1976 revealed that the economic losses caused by friction and wear cost about 10 billion DM per annum, at 1975 values, which was equivalent to 1% of the Gross National Product. About 50% of these losses were due to abrasive wear. In the U.S.A. it has been estimated that about 11% of total annual energy can be saved in the four major areas of transportation, turbo machinery, power generation and industrial processes through progress in tribology. For example, tribological improvements in cars alone can save about 18.6% of total annual energy consumed by cars in the U.S.A., which is equivalent to about 14.3 billion US$ per annum. It has been estimated that, on average, 28% of the total energy available from the hydrocarbon fuel is dissipated as friction inside the engine and transmission of a passenger car. This is more than the 21.5% of energy needed to propel the car under typical driving conditions. In addition the aerodynamic drag consumes about 10% of the fuel energy, depending on the speed of the car. The frictional losses are reflected in increased fuel consumption and related cost. Understanding the problems of tribology economics is of extreme importance to an engineer. For example, in pneumatic transportation of material through pipes, the erosive wear at bends can be up to 50 times more than in straight sections. Apparently non-abrasive materials such as sugar cane and wood chips can actually cause abrasive wear. Many tribological failures are associated with bearings. Simple bearing failures on modern generator sets in the U.S.A. cost about US$25,000 per day, while to replace a £200,000 bearing in a single point mooring on a North Sea Oil Rig a contingency budget of about £1 million is necessary. In addition there are some production losses which are very costly. The total cost of wear for a single US naval aircraft has been estimated to be US$243 per flight hour. About 1,000 megatonnes of material is excavated in Australia. Much of this is material waste which must be handled in order to retrieve metalliferous ores or coal. The cost of wear is around 2% of the saleable product. The annual production by a large iron ore mining company might be as high as 40 megatonnes involving a direct cost through the replacement of wearing parts of A$6 million per annum at 1977 values [15,16]. Wind turbines are gaining popularity across the world as an alternative energy source. One of the problems is that contamination of the lubricant by water and dust particles occurs frequently. Current designs use an epicyclic gearbox allowing the generator to rotate rapidly while the blades and the rotor rotate slowly. The gear-teeth are vulnerable to pitting. The extreme tooth loads, due to the high torques including shock loads, cause subsurface pitting with the nucleation sites close to the region of highest shear stress in the Hertzian contact (between the gear teeth). A primary issue is the cost and difficulty of maintenance since the wind turbines are mounted on tall towers and located in remote locations. As a result, the bearings and gearboxes of wind turbines end up with much shorter service lifetimes than the desired 20–25 years. Recent study of wind turbines in UK demonstrate that they show signs of wearing out just after 12 years, i.e. half-time of their anticipated work-life. Thus, the improvements in tribological practice are urgently needed to ensure reliable operation of wind turbines and acceptable costs. The economics of tribology are of such gigantic proportions that tribological programmes have been established by industry and governments in many countries throughout the world. The analysis of the causes of friction and wear can have direct commercial 8 ENGINEERING TRIBOLOGY implications, even in terms of who bears the cost of excessive wear or friction. For example, in one instance of a gas turbine that suffered excessive damage to its first-stage blades, detailed analysis of the cause of wear helped determine whether the owner or the insurance company would pay for the damage. As soon as the extent of economic losses due to friction and wear became clear, researchers and engineers rejected many of the traditional limitations to mechanical performance and have found, or are looking for, new materials and lubricants to overcome these limits. Some of the envisaged improvements are so radical that the whole technology and economics of the product may change. A classic example is the adiabatic engine. The principle behind this development is to remove the oil and the lubricating system and use a dry, high temperature self-lubricating material. If the engine can operate adiabatically at high temperatures, heat previously removed by the now obsolete radiator can be turned to mechanical work. As a result, a fuel-efficient, lightweight engine might be built which will lead to considerable savings in fuels, oils and vehicle production costs. A fuel-efficient engine is vital in reducing transportation and agricultural costs and therefore is a very important research and development task. New ventures, even if they involve mostly conventional technologies, such as mining and processing of oil sands, can impose arduous conditions on equipment and necessitate new wear-resistant materials. Oil sands slurries are capable of wearing out a high chromium white cast iron pump impeller after only 3 months of service. It is thus necessary to find new hard yet tough materials for better wear resistance and extended service life for the equipment. Other examples of tribological innovations include surface-treated cutters for sheep shearing, surface-hardened soil engaging tools, polyethylene pipes for coal slurries and ion implanted titanium alloys for orthopaedic endoprostheses. Whenever wear and friction limit the function or durability of a device or appliance, there is a scope for tribology to offer some improvement. In general terms, wear can effectively be controlled by selecting materials with specific properties as illustrated in Figure 1.3. However, more detailed information on wear mechanisms and wear control is given in Chapters 11–16. Wear mechanism Critical materials property Abrasive Erosive Cavitation Corrosive Fretting Adhesive Melting Fatigue Hardness Toughness Fatigue resistance Inertness High melting point Heterogeneous microstructure Non-metallic character Important Fretting in air for metals Marginal Homogeneous microstructure inhibits electrochemical corrosion and, with it, Unfavourable most forms of corrosive wear FIGURE 1.3 General materials selection guide for wear control.  INTRODUCTION 9 1.4 SUMMARY Although the study of friction and wear caught the attention of many eminent scientists during the course of the past few centuries, consistent and sustained scientific investigation into friction and wear is a relatively recent phenomenon. Tribology is therefore a comparatively young science where rigorous analytical concepts have not yet been established to provide a clear guide to the complex characteristics of wear and friction. Much of the tribological research is applied or commercially oriented and already a wide range of wear resistant or friction reducing materials exist. The concept of developing special materials and coatings to overcome friction and wear problems is becoming a reality. Most analytical models and experimental knowledge of tribology have been completed in the past few decades, and some time in the future our understanding of the mechanisms of friction and wear may be radically changed and improved. The bewildering range of experimental data and theories compiled so far have helped to create an impression that tribology, although undoubtedly important, is somehow mysterious and not readily applicable to engineering problems. Tribology cannot, however, be ignored as many governments and private studies have consistently concluded that the cost of friction and wear imposes a severe burden on industrialized countries. Part of the difficulty in controlling friction and wear is that the total cost in terms of energy and material wastage is spread over every type of industry. Although to the average engineer the cost of friction and wear may appear small, when the same costs are totalled for an entire country a very large loss of resources becomes apparent. The widely distributed incidence of trib