Table Of ContentTribology
Tribology
Friction and Wear of Engineering
Materials
Second Edition
Ian Hutchings
University of Cambridge, UK
Philip Shipway
University of Nottingham, UK
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Preface to the second edition
Overthe25yearssincethefirsteditionwaspublishedithasbeencontinuallyinprint,
andhasattractedaninternationalreadershiptobecomeawidelyusedandwidelycited
text. In collaborating to update and extend this book, we have maintained its core
emphasis on friction and wear of materials, but have also strengthened its coverage
of the more traditional tribological topics of contact mechanics and lubrication.
Wehavedrawnheavilyonourexperienceinteachingcoursesontribologyatunder-
graduate and postgraduate level at the Universities of Cambridge and Nottingham,
in the annual Cambridge Tribology Course, and in Tutorial sessions at the biennial
International Conferences on Wear of Materials, and also as Editors-in-chief of the
journal Wear.
Readersfamiliarwiththefirsteditionwill,wehope,findtheleveloftreatmentto
be unchanged, and should find some of the new material useful and informative.
While the fundamental principles underlying tribological phenomena remain the
same,theworldinwhichengineersandscientistsoperatehaschangedsignificantly,
withanincreasedawarenessoftheenvironmentalimplicationsofthedesignanduse
of engineering systems. Tribologists have an important part to play in reducing the
global use of energy, of fossil carbon and of strategic and hazardous materials, and
alsoincontributingtohumanhealthandwellbeing.Inrefreshingthebook’scontent
wehaverevisedandupdatedallthechaptersandalsoincludedseveralnewcasestud-
iesontopicsrangingfrommagneticdatastoragetoartificialhipjointsthatillustrate,
wehope,someofthemodernengineeringapplicationsinwhichtribologicalprinciples
playvitalroles.Inresponsetodemandwehavealsoincludedquestionsforeachchap-
ter, suitable for classroom use orindividual study.
IanHutchings
Philip Shipway
October2016
Preface to the first edition
Tribology, the studyof friction,wearand lubrication, isan interdisciplinary subject
thatdrawsontheexpertiseofthephysicist,thechemistandthemechanicalengineer,
aswellasthematerialsscientistormetallurgist.Itcanthereforebeapproachedfrom
severaldifferent viewpoints.Most previous textbooks have focused ontopics intri-
bology that can be modelled with mathematical precision: contact mechanics, fluid
film lubrication and bearing design, for example. These are undoubtedly important.
Inthepresentbook,however,theemphasisismoreonanequallyimportantsubject
thatislessamenabletoprecisequantitativeanalysis:thebehaviourofmaterialsinthe
context oftribology,and inparticular friction and wear.
Thebookisintendedforfinal-yearundergraduatestudentsofthephysicalsciences
andtechnology,especiallystudentsofmechanicalengineering,materialsscienceand
metallurgy.Itshouldalsobevaluableforthosetakingpostgraduateorpost-experience
courses,andsinceitprovidesnumerousreferencestotheresearchliterature,Ihopeit
willproveausefulsourceofinitialinformationontribologyforscientistsandengi-
neersworking inindustry.
Iamwellawarethatinofferingatextbookonthetribologicalbehaviourofmate-
rials I have undertaken a daunting task, particularly in tackling the subject of wear.
ProfessorDuncanDowsonhasremarkedthatwhereasthescientificstudyoffriction
dates back some 300 years, and that of lubrication more than a century, wear has
receivedsimilarattentionforonly40years.Onecangoevenfurther,andsuggestthat
our understanding of wear mechanisms has developed most rapidly only with the
widespread use of electron microscopy and instrumental methods of microanalysis
overthepast20years.Inasubjectsoyoungandsocomplex,itisinevitablethatthere
will still be competition between theories, confusion over nomenclature and defini-
tions,andinconsistenciesbetweenexperimentalobservations.Nevertheless,thefoun-
dations of the subject now seem to be well established, and I have tried to present
them,aswellasmorerecentdevelopments,insuchawaythatthereaderwillbeable
toappreciate the future advancesthat will certainlyoccur.
Ihavelistedrecommendationsforfurtherreadingattheendofeachchapter,butthe
sourcesofthenumerousillustrations,whichIhavetriedtociteaccuratelyinthecap-
tions, should also provide valuable points of entry into the research literature. I am
very grateful to numerous authors and publishers for their permission to reproduce
copyright material.
IanHutchings
Cambridge,1992
1
Introduction
The movement of one solid surface over another is fundamentally important to the
functioning of many kinds of mechanisms, both artificial and natural. The subject
ofthisbook,tribology,isdefinedas‘thebranchofscienceandtechnologyconcerned
withinteractingsurfacesinrelativemotionandwithassociatedmatters’,andincludes
the study offriction, wear, lubrication andthe design ofbearings.
Mechanical systems in which surfaces do not slide or roll against each other are
rare,andtribologyisthereforeakeyenablingtechnologywithverywideapplication.
Progressinunderstandingthesurfaceinteractionsthatlieattheheartoftribologyhas
demanded the skills of mechanical engineers, materials scientists, physicists and
chemists;inthecontextofbio-tribologywhichconcernslivingorganisms,thedisci-
plinebecomesevenwiderandincludesthemedicalandbiologicalsciences.Advances
in tribology have underpinned much of the world’s engineering progress. Its reach
extends to all aspects of modern technology, in the broadest sense of the word, in
whichsurfacesmoveagainsteachother,fromtransportandpowergenerationtomed-
ical engineering,foodscience and cosmetics.
Although the subject of tribology ismuch wider than a study of friction, friction
does play a central role in the performance of many mechanical systems. In some
cases,lowfrictionisdesirableandevenessential.Thesatisfactoryoperationofjoints,
forexample,whetherhingesondoors,humanhipjointsorbridgesupports,demandsa
low friction force. Work done in overcoming friction in bearings and gears in
machines is dissipated as heat, and by reducing friction we can achieve an increase
in overall efficiency. But low friction is not necessarily beneficial in all cases. In
brakesandclutches, adequateandcontrolledfrictionisessentialtodissipatekinetic
energyandtransfertorque;highfrictionissimilarlydesirablebetweenavehicletyre
andtheroadsurface,justasitisbetweenthehumanfootandthegroundforwalking.
Wheneversurfacesmoveovereachother,wearwilloccur:damagetooneorboth
surfaces, generally involving progressive loss of material. Sometimes the wear is
imperceptiblyslight,butitcanalsobeextremelyrapid.Inmostcases,wearisdetri-
mental, perhaps causing increased clearances between the moving components,
unwanted freedom of movement and loss of precision. It often leads to vibration,
toincreasedmechanical loading andyet morerapidwear,and sometimes tofatigue
failure. The loss by wear of relatively small amounts of material can be enough to
cause complete failure of large and complex machines, and there are unfortunately
caseswheretherootcausesofmajorengineeringdisasterscanbetracedbacktowear.
Highwearrates,however,aresometimesdesirable.Grindingandpolishing,forexam-
ple,arewidelyusedmanufacturingprocessesthatemployweartoremovematerialin
acontrolledmanner,andaninitialsmallamountofwearisoftenanticipatedandeven
welcomedduringthe ‘running-in’processinsomekinds ofmachinery.
Akeymethodofreducingfriction,andoftenalsowear,istolubricatethesystemin
someway,andthestudyoflubricationisverycloselyrelatedtothatoffriction and
Tribology.http://dx.doi.org/10.1016/B978-0-08-100910-9.00001-5
©2017IanHutchingsandPhilipShipway.PublishedbyElsevierLtd.Allrightsreserved.
2 Tribology
Table1.1 Methodsbywhichfinancialsavingscouldbemadethroughimprovedtribological
practiceinUKindustry.Theproportionsrepresentproportionsofthetotalannual
saving(whichwasestimatedat£515millionat1965prices)(informationfromHMSO,
1966.Lubrication(Tribology)EducationandResearch.UKDepartmentofEducation
andScience)
Savingsinmaintenanceandreplacementcosts 45%
Savingsinlossesresultingfrombreakdowns 22%
Savingsininvestmentthroughincreasedlifeofplant 20%
Reductioninenergyconsumptionthroughlowerfriction 5%
Savingsininvestmentthroughgreateravailabilityandhigherefficiency 4%
Reductioninmanpower 2%
Savingsinlubricantcosts 2%
wear.Evenwhenanartificiallubricantisnotadded toasystem,componentsofthe
surroundingatmosphere(especiallytheoxygenandwatervapourintheair)oftenplay
asimilarroleandmustbeconsideredinanystudyoftheinteractionofthesurfaces.
The word ‘tribology’ (from the Greek: τριβος¼rubbing or attrition) was first
coined in 1966 by a UK government committee, although friction, lubrication and
wear had been studied for many years before then and have a long and fascinating
history. That committee also made an estimate of the savings that could be made
by UK industry if known tribological principles were widely applied. Similar exer-
ciseshavesubsequentlybeencarriedoutinseveralothercountries.Theirconclusions
are all in broad agreement: that at least 1% of the gross domestic product might be
saved with minimal further investment in research, and that the potential for even
larger savings might exist with further research. The savings arise from several
sources.TheoriginalsurveyidentifiedthesavingslistedinTable1.1.Thereduction
inenergyconsumptionthroughreducedfrictionwas,inmoneyterms,ratherasmall
componentofthetotal;savingsinmaintenanceandreplacementcosts,inlossescon-
sequentialuponbreakdowns,andininvestmentsthroughtheincreasedlifeofmachin-
ery were at that time much more important. A subsequent survey in the UK, taking
accountoftherelativeincreaseinthecostofenergysince1966,focusedonthesav-
ings in energy that can be made by improved tribological design, and identified the
kinds ofsavings shown inTable 1.2.
Substantial advances in tribological understanding and in the application of that
knowledgehaveoccurredsincetheword‘tribology’wasfirstdefined.Wehaveonly
tolookattheadvancesinroadandairtransport,powergeneration,informationtech-
nology,manufacturingprocessesandmedicalengineeringtoseeimportantexamples.
Butfurtherdevelopmentwillundoubtedlyoccur,anditismoreimportantthaneverthat
anunderstandingoftheprinciplesoftribologyformspartoftheeducationofeveryengi-
neerandappliedscientist.Thereisnowanincreasing,andoverdue,awarenessofthe
imperativetoconservetheresourcesofourplanet,includingfossilfuelsandmaterials.In
reducingtheenvironmentalimpactofhumanactivity,weneednotonlytoreducetheatmo-
sphericreleaseoffossilcarbonandgenerateenergyinsustainableways,butalsotouse
Introduction 3
Table1.2 Methodsbywhichsavingsofenergycouldbemadethroughimproved
tribologicalpracticeinUKindustry(informationfromJost,H.P.,Schofield,J.,1981.Proc.
Inst.Mech.Engrs.195,151–195)
Directsavingsofenergy
Primary: savingofenergydissipatedbyfriction
Secondary: savingofenergyneededtofabricatereplacementparts
Tertiary: savingofenergycontentofmaterialsforreplacementparts
Indirectsavingsofenergy
Savingsconsequentialondirectsavings,e.g.,inplantneededtocompensateforfrictional
losses
materials,whetherincapitalequipmentorinconsumablessuchaslubricants,inasustain-
able and environmentally-friendly manner. Not only should the correct application
of tribological principles lead to longer-lasting, more energy- and resource-efficient
products,buttherearealsosituationswheretheirneglectcanleadtounacceptableperfor-
mance, to costly product recalls with significant reputational damage for the supplier,
andinextremecasestofailuresthatleadtohumaninjuryorevenlossoflife.
Inthefollowingchaptersweshallexplorevariousaspectsoftribology.Westartby
examiningthetopographyofsurfacesandthewayinwhichtheyinteractwhenplacedin
contact(Chapter2).Theoriginsoffriction,andthefrictionalresponsesofmetalsand
non-metalsarediscussednext(Chapter3).Lubricantsandlubricationformthesubject
ofChapter4,andwearisthetopicofChapters5and6.Inthesetwochaptersthedis-
tinctionismadebetweenwearthatoccurswhentworelativelysmoothsurfacesslide
over each other, either lubricated or unlubricated (i.e., sliding wear, covered in
Chapter5), and wearinvolving hard particles(i.e.,abrasive orerosive wear, covered
in Chapter 6). The terms ‘sliding wear’, ‘abrasion’ and ‘erosion’ are not intended to
describemechanismsofwear, butformbroaderclassifications.Aswe shallsee later,
manydifferentmechanismscanbeinvolvedinwearanditwouldbesimplistictoattempt
tolistthemhere.TheimportanttopicofsurfaceengineeringisreviewedinChapter7,and
waysinwhich tribologicalprinciplescan betakeninto accountinthedesignprocess
are discussed in Chapter 8. The book concludes with a selection of case studies in
Chapter9thatdemonstratetheapplicationoftheprinciplesdevelopedintheearlierchap-
ters:bearings,automotivetribology,tribology inmanufacturing,naturaland artificial
hip joints, and magnetic data storage. Background material on indentation hardness,
corrosionandthedefinitionofwearrateiscoveredinthreeappendices.
SOURCES AND FURTHER READING
ASTM,2015.StandardTerminologyRelatingtoWearandErosion,StandardG40.
Dowson,D.,1998.HistoryofTribology,seconded.Wiley.
HMSO,1966.Lubrication(Tribology)EducationandResearch.UKDepartmentofEducation
andScience.
4 Tribology
Hutchings, I.M., 2016. A lot more than lubrication. Ingenia 66, (http://www.ingenia.org.uk/
Ingenia/Articles/1009).
Jost,H.P.,1989.Tribology—originandfuture.Wear136,1–18.
Nosonovsky,M.,Bhushan,B.,2010.Greentribology:principles,researchareasandchallenges.
Phil.Trans.Roy.Soc.A368,4677–4694.
Introduction 5
QUESTIONS FOR CHAPTER 1
(Thesegeneralquestionscanbeattemptedatanypoint,butasstudents’understanding
and knowledge oftribology deepens,they should producefuller answers.)
Question 1.1
Useanysourcesavailabletoyou,includingtheInternet,toresearchandwriteabout
the following:
(a) examplesofadvancesinengineeringoverthepast50yearsthatcanbeattributed
tobetter understanding anduse oftribological principles;
(b) examplesoffailuresinengineeringsystems(whichcanincludehouseholdprod-
ucts,powergenerationsystems,airorlandtransport,andmedicaldevices)dueto
tribological causes.
In the case of (b), explain how better understanding or application of tribological
knowledge would have avoidedthe failures or mitigated their consequences.
Question 1.2
Useanysourcesavailabletoyou,includingtheInternet,toresearchandwriteabout
the following:
(a) the role oftribology inenergy conservation;
(b) the roleof tribology inmaterialsconservation;
(c) tribology and safety.
