On the mechanisms of various fretting wear modes微动磨损.pdf

On the mechanisms of various fretting wear modesM.H.Zhu,Z.R.ZhounTribology Research Institute,Traction Power State Key Laboratory,Southwest Jiaotong University,Chengdu 610031,PR Chinaa r t i c l e i n f oArticle history:Received 30 May 2010Received in revised form31 January 2011Accepted 17 February 2011Available online 9 March 2011Keywords:Fretting wearRadial frettingTorsional frettingRotational frettinga b s t r a c tAccording to relative motion directions for a ball-on-flat contact,there are four fundamental frettingwear modes,e.g.,tangential,radial,torsional and rotational modes.In this paper,the mechanisms ofthese four fundamental fretting wear modes,particularly for the later three modes,have been reviewedfrom results obtained by the authors in combination with results from literature.Some general featureshave been reported.Differences both in running and degradation behavior have been discussed in detail.Results showed that some similar laws for three fretting regimes(partial slip regime,mixed regime andslip regime),fretting maps(running condition fretting map and material response fretting map),wearand cracking mechanisms obtained from the classic mode(i.e.tangential fretting)were also identifiedand useful to characterize the other modes.Nevertheless,the occurrence of relative slip for the radialfretting,the formation of mixed regime for the torsional fretting,the evolution of surface morphology forthe rotational fretting were quite different compared to that of the classical fretting mode.&2011 Elsevier Ltd.All rights reserved.1.IntroductionFretting phenomenon was first reported in the open literature innear one century ago 1.It is caused by the relative oscillatorymovement of small amplitude which may occur between contact-ing surfaces subjected to such as mechanical vibration,cyclic loads,electromagnetic shocks,and thermal cycling 2,3.The fretting mayresult in contact wear by detachment of particles and contactfatigue by rapid crack nucleation and propagation 24.In general,according to the relative motion directions,for acontact of ball-on-flat,the fretting wear modes are composed ofthe tangential,radial,torsional and rotational modes,as noted inFig.1 5.In fact,a lot of experimental tests of the tangential or theclassical fretting wear have been frequently reported in literaturewhile the other three modes have been rarely investigated due tosome difficulties in realization of experimental rigs.For radial fretting(Fig.1(b),it is mainly induced by varyingnormal load or thermal cycling,the two surfaces remain in contactand no impact effect occurs 6.Torsional fretting can be defined asthe relative motion which is induced by reciprocating torsion underthe oscillatory vibratory environment 7,as seen in Fig.1(c).Simi-larly,the rotational fretting(Fig.1(d)is the relative motion ofreciprocating rotation under the oscillatory vibratory environment 8.Recently,the fretting wear tests of the later three modes havebeen developed in our laboratory.In this paper,as a reference,theclassical fretting wear(i.e.tangential fretting wear)behaviors havebeen reviewed in brief,and the main behavior of the other threemodes has been discussed in detail.General features and differencesin both running and degradation mechanisms for the various modeshave been summarized.This paper will not only offer an overview ofall simple fretting modes,but also be very useful to better understandthe behavior of dual and multi-directional fretting modes in practice.2.Fretting wear testersFretting wear experiments of the four modes have been investi-gated with a contact configuration of ball-on-flat.The test rigs havebeen,respectively,described in detail in previous works 68.For theall fretting modes,the precise control and realization in displacementof small amplitude are very important;however,the description offriction behavior in relation of frictional force(Ft)or torque(T)vs.relative motion vs.test time is quite different.For the classical frettingmode,the relative motion was described by the imposed displace-ment amplitude(D),while by the imposed torsional angular dis-placement amplitude(yt)for torsional fretting,and by the imposedrotational angular displacement amplitude(yr)for rotational fretting.For the radial fretting,the variation of normal load(Fn)vs.verticaldeformation(d)is used to characterize its running behavior.3.Tangential fretting or classical frettingFtD curves are the most important kinetic informationobtained from the tangential fretting wear tests.There are onlythree type curves of friction force(on the strictest interpretation,here is tangential force)vs.displacement amplitude(FtD curves)in all test results 9,as shown in Fig.2(a).For the linear FtDContents lists available at ScienceDirectjournal homepage: International0301-679X/$-see front matter&2011 Elsevier Ltd.All rights reserved.doi:10.1016/j.triboint.2011.02.010nCorresponding author:Tel.:[phone];fax:[phone].E-mail addresses:,(Z.R.Zhou).Tribology International 44(2011)13781388Fig.1.Four simple fretting motion modes under the contact of ball-on-flat.Fig.2.Outline for tangential wear:medium carbon steel flat against 52100 bearing steel ball.M.H.Zhu,Z.R.Zhou/Tribology International 44(2011)137813881379curves,their contact conditions are in agreement with theMedlins solution 10,which implied that the contact center issticking and the micro-slip occurs at the contact edge,corre-sponding to a fretting running status of partial slip(PS).Duringthese loops,the imposed displacement is coordinated by elasticdeformation of contact interfaces.When the FtD curves open tothe elliptic,however,the relative motion is coordinated by elasto-plastic deformation although the fretting running status is stillthe partial slip.As the displacement amplitude reaches highenough,the FtD curves all turn to the parallelogrammatic,whichcorresponds a fretting running status to gross slip(GS)in wholecontact zone.The concept of fretting map,which defined threefretting regimesstick regime,mixed stickslip regime and grossslip regime,was firstly proposed by Vingsbo and S oderberg 11.Actually,these three regimes just corresponded to the three FtDcurves according to the analyses by Vingsbo and S oderberg.So,itcan say that the stick regime,mixed stickslip regime and grossslip regime are the different description for the linear,elliptic andparallelogram FtD curve,respectively.From the viewpoint ofrelative motion,to sum up,there are only two types of runningstatus,i.e.PS(sticking at the contact center)and GS(no sticking atthe contact center and slip occurred in whole contact zone).During an actual fretting process,the evolution of the FtDcurves is complex and uncertain.Nevertheless,a new three-fretting-regime definition method was given by Zhou and Vincent4,12,13 and Zhou et al.14,15 different from the frettingregimes defined by Vingsbo and S oderberg.As shown in Fig.2(b),usually under lower displacement amplitudes,the partial slipregime(PSR)is the fretting loops in the running status of PS allthe time.Due to the different deformation behavior of differentmaterials,the FtD curves can be changed between the linear andelliptic cycles.Under higher displacement amplitudes,the totalcycles are all in shape of parallelograms,it is considered that thefretting runs in the slip regime(SR).For the medium displace-ment amplitudes,the fretting running status changed from the GSto PS,then finally to GS 9.The process accompanied with thevariation of fretting running status is defined as in the mixedregime(MR).According to a great deal of experimental results fordifferent materials,in brief,the necessary condition of MR is thatthe fretting running status must be transformed between the PSand GS.In PSR,the friction coefficients(the ratios of tangentialforce to normal load,Ft/Fn,strictly)are always in a lower levelwithout obvious fluctuation in the steady-state stage.In SR,thefriction coefficients in the steady-state stage usually present ahigher level.However,in MR,the highest peak of frictioncoefficient often occurs before the steady-state stage due to thestrong plastic deformation between the contact interfaces 9.The running behaviors and damage mechanisms of tangentialfretting are strongly depend upon the fretting regimes and materi-als.As an example,the fretting wear of a contact of medium carbonsteel(LZ50 steel)flat against 52100 bearing steel ball is discussed(inthe subsequent section,this counter-pair is main discussion object).In PSR,the scar present a typical morphology of annularity,wherethe micro-slip occurs at the contact edge and only slight ploughingtraces with little oxidation can be detected(Fig.2(c).Thus,the wearmechanism is main slight abrasive wear.In MR,some strong plasticdeformation flow traces generate at the contact center,and detach-ment of particles and oxidative debris can be found at the edge area(Fig.2(c).The fretting wear mechanisms in MR are main abrasivewear,oxidative wear and delamination.Similar to the MR,thefretting wear mechanisms in SR are main abrasive wear,oxidativewear and delamination with covered a thick debris layer on thescars(Fig.2(c).Cross section of the wear scars exhibit important degradationinformationforvariedfrettingregimes.Themaindamagemechanism is cracking(fretting induced fatigue,FIF)in MR,andis material losing(fretting induced wear,FIW)in SR.It is a resultof competition between local contact fatigue and wears 4,1214.In fact,during the competition of FIW and FIF,the velocity ofcracking is usually higher than that of wear in MR;finally,thefretting cracks propagate to the inner of the base alloy.However,a reverse result is obtained in SR,i.e.the FIW becomes the winnerof the competition,while a deep pit usually forms to present a Utype profile.As mentioned above,two fretting maps(i.e.running conditionfretting map(RCFM)and material response fretting map(MRFM)established by Zhou and Vincent 4,12,13 and Zhouet al.14,15 can be used to reveal the laws of the runningbehavior and damage mechanism of the tangential fretting,asseen in Fig.2(d).4.Radial frettingDuring the all radial fretting tests,in order to avoid an impacteffect,the minimum value of cyclic imposed normal loads must bepositive(above 0 N)6,1618,which induced a sticking zonealways existed.Because of the sticking at the contact center,themicro-slip only occurs at the edge area in shape of annularity(Fig.3).The main kinetics information obtained from the radialfretting tests is the curve of alterative normal load vs.indentationdisplacement(FD curve),as seen in Fig.4.There are only two typesof FD curves,i.e.the linear(closed)and elliptic FD curves whichcorrespond with the elastic deformation and elasto-plastic defor-mation between the contact interfaces,respectively 6,1618.Obviously,for the radial fretting,it always runs in PSR.When the contact counter-pairs materials of radial fretting arecompletely same(for example:52100 steel against 52100 steel),no any damage can be detected.It is consistent absolutely withthe theory analysis by Johnson 19.Therefore,the requirement ofmicro-slip occurrence for radial fretting is that the materials ofcounter-bodies must be different.Some micro-slip traces for thecontact of medium carbon steel against 52100 steel are shown inFig.5(a).For metal materials,such as a medium carbon steel in Fig.5(b),the detachment of particles usually forms at the sites of the secondphases,and the damage mechanism of radial fretting is maindelamiantion(contact fatigue)6,16.However,different frommetal materials,the damage of radial fretting for dental ceramicspresents a more homogeneous distribution under lower imposedFig.3.Typical optical morphology of radial fretting scar for a dental ceramics(Cerec Vita Mark II),Fmax200 N,Fmin50 N,and N105cycles.M.H.Zhu,Z.R.Zhou/Tribology International 44(2011)137813881380load level;and some cracking early forms at the outer region of themicro-slipzonewhentheimposedloadexceedsacriticalvalue 17,18.So,the damage mechanism transforms form delami-nation to the combination of delamination and cracking 17,18.5.Torsional frettingThe friction forces vs.displacement amplitude loops(FtDcurves)in tangential fretting have provided a rich informationof friction interface.However,for torsional fretting mode,therelative reciprocating motion is replaced from the linear to rotary.Accordingly,the friction interface information of torsional contactis characterized by the curves of friction torque(T)vs.angulardisplacement amplitude(y).Fig.6 displays the evolution of theTycurves as function of the number of cycles 20.According tothe all test results of torsional fretting 7,2123,it can besummarized that there are only three types of kinetic curvestoo,i.e.linear,elliptic and parallelogram loops,which correspondto the PS under elastic deformation condition,PS under elasto-plastic deformation condition and GS,respectively.To compare with torsional and tangential fretting,it is foundthat the relative displacement of torsional fretting increases withthe increase in the contact radius even in the condition of GS;however,it is constant for tangential fretting.It consequentiallyresults in the different characteristics of fretting running regimefor torsional fretting.In PSR,the width of the annular micro-slipzone is never changed with the increase of the number of cycles.In MR,however,when the number of cycles increases to a certainvalue,the width of the annular micro-slip zone reduces graduallyuntil the central sticking zone disappears(Fig.7).Therefore,different from the other fretting modes,the MR of torsionalfretting must be determined by the combination of Tycurvesand wear scar morphologies.According to the torsional frettingtests of the LZ50 steel against 52100 steel,the RCFM can beestablished as shown in Fig.8.The variation of the friction torques as function of the numberof cycles for torsional fretting is composed of three stages(running-in stage,ascent stage,and steady stage)7.Similar tothe tangential mode,the highest friction torque level presents inMR due to the plastic deformation between the contact interfaces,the friction torques always hold in a lower level in PSR 7,21.Itindicates that the plastic deformation plays a very important rolein the MR.In PSR,the wear scars appear slight ploughing and detachment(Fig.9(a)accompanied with a very slight oxidation detected byEDX and XPS 20.With the increase of the angular displacementamplitude,the extents of the detachment and plastic flowdevelop gradually(Fig.10)accompanied with the increase oftribo-oxidation.Similar to the other fretting modes,the tribo-oxidation is strongly dependent upon the humidity of the envir-onment and the water in the atmosphere plays very crucial roleduring the oxidation processes 22,23.In MR(Fig.9(b),the wearmechanisms can be generalized as abrasive wear,oxidative wearFig.4.FD curves of radial fretting for a contact of 1045 medium carbon steel against 52100 steel:Fmax400 N,and N105cycles.Fig.5.SEM morphologies