Surface andCoatingsTechnology, 68/69(1994) 427—432 427

Superlubricityof MoS2: crystalorientationmechanisms

J. M. Martina,H. Pascala,C. Donneta,Th. Le Mognea,J. L. Loubeta,Th. Epicier”aEcoleCentrale deLyon. Laboratoirede TribologieetDynamiquedesSystèmes,URACNRS855, Dpt.de Technologie desSurfaces,BP163, 69131

Ecully, FranceblnstitutNational des Sciences Appliquees deLyon, Grouped’Etudes deMetallurgiePhysiqueetde Physique desMatériaux,URA CNRS341, 69621

VilleurbanneCedex,France

Abstract

We haveinvestigatedtheorigin of theextraordinarylow friction coefficient (inthe 10-srangeoreven less)ofpureandstoichiometricsputteredMoS2coatings,in ultrahighvacuum.In theseconditions,shearstrengthsof theinterfaceas low as1 MPa weremeasured.Importantly, thetribometerwasoperatingin macroscopiccontactconditions,typically ata longtime-lengthscale.Friction-inducedorientationof (0001)basalplanesof MoS, grainsparallelto thesliding directionwasfirst verified by meansof electrondiffraction.Friction-inducedrotation of thesecrystalsaround the c axis, during intercrystallite slip in thecontact, was investigatedby highresolution transmissionelectronmicroscopyperformedon selectedwearfragments.Atomic force microscopyat atomicresolutionwas alsocarriedout on the surface insideand outsidethewearscar. Thedataindicatedthat thevanishingof thefriction force wasdue to frictional anisotropyin the interface betweennanometre-scaledomains in rotationaldisorder (intercrystalliteslippageofincommensuratesulphur-richhexagonallattices). Thetermsuperlubricitywas usedherebecauseof thezerofriction statethat couldbe theoreticallypredictedin theseconditions.Finally, themechanismsof MoS, superlubricityarethoughtto dependon thepropercombinationof thegrain size,thetwo crystalorientationeffectsandtheabsenceof contaminants.

1. Introduction analysedin detail from the point of view of the crystalstructure by Takahashi and Okada [5], using high

1.1. MechanismsofMoS2 lubrication in termsofcrystal resolutiontransmissionelectronmicroscopy(HRTEM).structure It has been shown that the hexagonal—rhombohedral

stackingtransitioncould be associatedwith S—S glide1.1.1. Basal plane orientation in the (0001)plane through(a/2) [0110], just abovetheIt has been recognizedearly that, when sputtered slip plane. Consideringthe characteristiccrystal struc-

MoS2 coatings are allowed to slide relative to each ture of MoS2, this transformationhas been shown toother, the crystallites reorient with their basalplanes occurreversiblyduring mechanicalshearingof rhombo-(within a few degrees)parallelto the directionof sliding hedral MoS2. This may provide an explanationof the[1,2]. Themain techniquethat hasbeenusedto demon- easyglide of thelamellarcompoundin termsof succes-stratethis effectis grazingangleX-raydiffraction directly sive inversetransformations,allowing slipping over rela-carried out on the worn surface.Another way is to tively largedistances.examinecleavedor wear fragmentsby electrondiffrac- Dealing withthe theoreticalaspectsof energydissipa-tion in the transmissionelectron microscope[3]. As tion in sliding crystal surfaces,Sokoloff [6] has showndemonstratedby Auger electron spectroscopyand low that,in theabsenceof dislocationmovement,intragranu-energyelectron diffraction, the remarkablelubricating lar shearof MoS2 isunlikely to occur.Theappliedstresseffect of MoS2 existsevenin the caseof films of nano- necessaryispractically equivalentto therupturestrengthmetre-scalethickness [4]. This is an indication of a of thematerial.predominantsurfaceratherthana bulk effect.

1.1.3. Intercrystalliteslip1.1.2. Intragranularshear Intercrystallite slip is an alternativeto intragranularThe easyshear in the basal planeorientation is shearto explain the lubricating effect of MoS2 in the

consideredto be mainly responsiblefor the very low steadystate regime, asrecently pointed out by Hiltonfriction coefficient of MoS2andothermetal dichalcogen- andFleischauer[7]. Intercrystalliteslip needs atransferides. Recently, as aconsequenceof the basal plane film to beformedfirst on theantagonistsurface.Sokolofforientation, the mechanism of intragranular shear [6] studied the slippageat the interface betweentwobetweensulphur-richatomic planes(S—Sglide) hasbeen incommensuratelatticesby solving theproblem of the

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428 f.M. Martin et a!. / Superlubricityof MoS2

sliding of a crystal lattice whosesurfaceinteracts witha data generationand logging can be found elsewhereperiodic potential of period incommensurate withthe [11,12].lattice periodicity.Hefound that the dissipativestrength Thecollection of wear fragmentsfor HRTEM studiesis a factor io’~smallerthan for a commensuratelattice, wasmadeat the end of thetest(100 cycles)as follows:indicating a virtual vanishingof the friction force in this firstwe observedthe wearscar of thepin with anopticalcase. ShinjoandHirano [8] recentlyshowedthat incom- microscope and, approachinga holey carbon filmmensuratesurfacesare easilyproduced whentwo similar mountedon a coppergrid, weardebrissurroundingthecrystal latticeswith a certainmisfit angleare allowedto wearscarstuck naturallyonto carbon.Specialattentionslide up on eachother. In the case ofhexagonallattices, was paid to avoid rubbing the carbonfilm onto thefor example,it was pointed out that the misfit angleis wearparticles.At low magnificationin the transmission30°andthe situationwasreferredto as frictional anisot- electronmicroscope,amongthe greatamountof debris,ropy. Fromthe theoretical pointof view, it canbe shown some flake-likeelectron-transparentparticles werethat, in such asituation,“low friction trajectories”can selectedfor further investigationat high magnificationbe found along selectedcrystal directions and that, in a 400 kV acceleratingvoltagemicroscope.quantitatively, the friction force can completelyvanish For AFM imaging, we usedan Si3N4 pyramidal tip,leading to the so-calledzero friction or superlubricity with a radius of curvature of 50 nm and a stiffness[9]. Atomic-level simulations(moleculardynamic(MD) cantileverof 0.064N m’. The normal load wasset upcalculations for example) can also be usedto study to 1.0 RN and the effective contactforce wasmeasuredfriction at the atomic level. MD simulationsof friction to be 50.0 nN. The scanningfrequencywas 20 Hz.Thebetweenhydrogenateddiamond surfaces, for example, scanning system was calibrated usingon a cleavedlead to the sameconclusionthat zero friction is pos- sampleof graphite(highly orientedpyrolytic graphite),sible [10]. where the nearest atomic distance is known to be

Therefore,during MoS2 basal planefriction-induced 0.246nm.orientation,the effect of the individual crystal rotationaround the c axis is important for both intragranularshearand intercrystalliteslip. This aspect,however,has 3. Resultsand discussionnot beenhighlightedin the literature in order to explainthe ultralow friction coefficient of sputteredcoatings. 3.1. Friction coefficientand shearstrengthofMoS2

In this study, we used twocomplementary techniques Fig. 1 presentsin vacuofriction curves as afunctionthat give the atomic resolution: the crystal orientation of timefor MoS2coatings(MoS2-ECLin the following).ofMoS2grainsinthebulkwasinvestigatedbyHRTEM, 100 cycles wererun at a0.3mm s~sliding speedon aperformed on electron-transparentwear fragments. 3 mm tracklength andcorrespondingto differentnormalFurthermore,the surfaceof oriented MoS2 grains can loads.As a comparison,some friction curves obtainedbe probedby atomic force microscopy(AFM) directly in similar tribological conditionsbut with commercialinsidethe wear track. sputter-depositedMoS2coatingsoriginatingfrom other

0.04

2. Experimental details o.o~

R.f. magnetronsputteringof MoS2 wasperformedon ~ o.o~steel surfaces (AISI52100)at ambienttemperature,usinga previouslydegassedMoS2 target.As already reported ~earlier [11], the MoS2 coating can be described aspolycrystallinepuremolybdenite(stoichiometric molyb- ~denumdisulphide)with a grain size of a few nanometres 0 50 100

anda preferentialbasalplaneorientation perpendicular ~ Ito the surface(edgeorientation). ~C CS

Friction testswere performed in ultrahigh vacuum Fig. I. Evolutionof thefriction coefficientof sputteredMoS2coatings

conditions (1 nPa partial pressure)using a specially as afunction ofthe numberof cycles, inultrahighvacuumconditions.

modified reciprocating pin-on-flat tribometer. The Comparisonof in situ elaboratedMoS2-ECL (~A, A) with MoS2tribometer is capable of measuringvery low friction coatings originating from other laboratories(~,A, •). For the

different normal loadsW, the shearstrengtht hasbeencalculatedatcoefficients (in themilhrange)even when macroscopic theendof thefriction test A, W= 1 N, t= 1.8 MPa; •, W= 1 N, t=

tribological conditionsare appliedon in situelaborated 21 MPa; A, W=1N, T~1.5MPa;A, W~lN,r=9.4MPa; A, W=pristine coatings. Detailson the calibrationprocedures, 2 N, r=0.9 MPa; •, W=l N, r=6.6 MPa.

f.M. Martin et a!. / Superlubricityof MoS2 429

laboratoriesare alsopresented.Resultsclearly indicate (Fig. 2(a)),andthe edgeof a typical wearparticlefromthat, in the steadystate regime, the average friction the friction test (Fig. 2(b)). It is very clear that thecoefficients of MoS2-ECL films are the smallest (gen- original film is mainly edgeoriented,with an azimuthalerally below 5 x 10~)and that in some cases the disorderof the c axisdirection in the coatingplane.Thetangentialforce wasscarcelymeasurable(with theequip- crystal structure is hexagonal and the grain size isment at hand, forces below2 mN correspondingto approximatelyten times the basal planedistance,i.e.friction coefficientsbelow iO~with a 2 N normal load 7 nm. As can be seenin Fig. 2(a), the MoS2crystalsare not detectable).It is interesting to examine the havemanydefects:curledup atomicplanes,dislocations,beginning of the friction curves,correspondingto the voids etc. Friction-inducedorientation of basalplanesevolution betweenthe first and four cycles approxi- in weardebris(correspondingto the absenceof the (002)mately:the MoS2-ECLcoatingsgive low friction imme- ring in thediffraction pattern)is clearly seen in Fig. 2(b).diately,exceptfor test fl which hadbeen previouslyin However,from the diffraction patternof the same area,contactwith humid air for one week. All thecommercial no recrystallizationof MoS2 is visible in thisexperimentcoatingswerein contactwith air for a long timebefore andthe grain size is practicallyunchanged.The diffrac-the tests.This indicatesthat the contaminationof the tion patternof the wear debris indicatesa disorderoffilm by the environment (water vapour and oxygen) thegrain in the (100) and (110) directions.It is notaffects only the extreme surfaceand causes invacuo evidentat thisstagewhetherthe grainsare superimposedfriction to increaseonly overthe first cycles.The differ- or simply juxtaposed.If the grain are superimposed,entiationbetweenthe coatingsclearlyappearsten cycles Moire rotation patternsare expectedin the HRTEMafterwards:our films maintainverylow friction to reach images.the millirange (so-called superlow), whereasthe other Fig. 3 representsan enlargementof the TEM imagefilms developa regular increaseto reachthe 10-2range, of the MoS2wearparticleof Fig. 2(b) andshowsatomicwhich correspondsto classicalvaluesdepictedin the resolutionin very thin regions.In theseworking condi-literature [13]. tions, the correspondencebetweenthe electron image

Shearstrengthsof the interfacefilms at the end of the andthe calculated projected potentialhasalreadybeentest were calculated fromthe values of thefriction discussedearlier [12]. Fig. 3 indicatesthe presenceofcoefficient and the hertziancontact pressure,using the characteristicMoire patternsin some areaswhere themodel developedby Singer [14]. The results are also rotation anglecanbe easilymeasuredby calculatingthereportedin Fig. 1 andshowthat thereis approximately diffractogramsof the digitized areas. Itis interestingtoone order of magnitude between the two kinds of observethat the atomiccommensurabilitybetweentwocoatings: MoS2-ECL films produce interfaceshear superimposedcrystals seems to disappearwhen thestrengthsas low as1 MPa. We deducefrom thesedata rotationangleis near30°,as predictedby the theoreticalthat the extraordinarylow friction of the films is not calculationsof Hirano et al. [9]. Near6°,for example,due to specific contact mechanicsconditions but to there are still several regions of atomic coincidencematerialpropertiesof the MoS2interfacefilm, asdetailed betweenthe basalplanes. Unfortunately, the rotationin the following section. anglescannotbe easilycalculatedif thereare morethan

two superimposedstackings.Consequently,the analysis3.2. High resolutiontransmissionelectron microscopy is limited to the edges of theparticle where there is astudyofwearfragments distribution of the misfit angles.We believethat these

HRTEM is a powerful techniqueto determine the datademonstratethe existence offrictional anisotropycrystal structurechangesat the atomic scale and to betweenorientedMoS2 grains and are at theorigin ofstudythe faultsandthe dislocationsin the Mo52 struc- the extraordinarylow friction.ture. Lattice fringe HRTEM images showthe atomic As recently pointedout by Isshiki et al. [16] on WS2,scalestackingof layersthat are ingood agreementwith the calculatedimages for [0110]hexagonalMoS2 showthe calculatedsimulations.TakahashiandShiojiri [15] only (0002) lattice fringes and it is not possible tohave already performeda detailedstudy of the layer discriminate the stacking sequences.In contrast, thestructureandstackingfaults in MoS2 andWS2 powders [2110] imagescangive veryinterestinginformation onusingHRTEM. the atomic configurationandon thearrangementof the

For this HRTEM study, weardebriswere used from sMos or SM0S columns.Particularlyinterestingis thethe end of the friction test ~/, wherethe friction coeffi- alternatechevron-likeimage whosespotscorrespondtocient stabilizedaround anaveragevalueof 0.003. Fig. 2 exact atompositions. For example,a changefrom theshows electron diffraction patternsin the selectedarea hexagonalto the rhombohedralstructurewill appearinmodeperformetl in the transmissionelectronmicroscope the HRTEM image inregionswherethe samechevronsand the correspondinghigh resolutionimagesobtained are repeated:the hexagonalpacking appearsas rows offor two different specimens: theoriginal MoS2 coating right-side up and upside-downtriangles alignedon the

430 f.M. Martin et a!. / Superlubricityof MoS2

(002)

(110)

(100)

- ..~

a b9nm

Fig. 2. Friction-inducedorientationof the basalplanesof MoS2: (a) HRTEM micrograph andcorrespondingelectrondiffraction patternof thepristine coating;(b) HRTEM micrograph andelectrondiffraction patternof a flake-like wearparticle.Note the absenceof the(002) ring in thediffraction patternof thewearparticle andthefact that nopreferentialorientationeffect is observedin the other directions(100) and(110).

Fig.3. HRTEM micrographof an MoS2wear particle,showing atomic resolution.Theelectron beamis parallel to the c axis.The calculateddiffractogramson the micrograph correspondto image framesshowing characteristicMoire patterns.Zones where atomiccoincidencehasdisappearedarecorrelatedwith a rotation angleof two superimposed crystalsequalto about30°.

f.M. Martin et al. / Superlubricityof Mo52 431

(0001)layer(a zigzagstructure),whereas, in therhombo- 0.316nm+0.003nm, in agreementwith apreviousAFMhedralorder, the stacking sequenceis composedof rows work on cleavedmolybdenitesurfaces [17].The imageof triangles alignedin the same directionandthe inter- also suggeststhat a part of thecrystal(in the upperleft)face betweenthe regions directlygives information on is slightly disoriented,indicatingthe presenceof a grainthe stackingfaults. Fig. 4 shows an HRTEM imageof boundary. This strongly supports theidea that thethe edge of a curled wear particle which is oriented sliding surfacecould becomposedof amosaicof atomi-with the electron beam parallel to the basal plane. cally cleanbasal planeorientedgrains,with azimuthalAtomic resolutioncanbe observedin the S—Mo—S layer disorderaroundthe c axis.and,in some areaswith good orientation,the chevron-like structurecan be observed.This shows that somerhombohedral(3R) stackingcan be detectedand that 4. Conclusionintergranularshearin MoS2 grains might occur duringthe process.Unfortunately, the edge-orientedpristine The origin of the superlow friction of pure MoS2coatingdoesnot allow this kind of imaging andwe do coatings in ultrahigh vacuum conditions was studiednotknow whetherthis 3Rstackingis presentbefore the from thepoint of view of crystal structure.The mecha-friction process. nisms ofcrystal orientationprocesseswere investigated

by means of electron diffraction in the transmission3.3. Atomicforcemicroscopyin thewear track electronmicroscope,HRTEM and AFM. The results

An AFM picture of the pristine coated surface is are asfollows.shown in Fig. 5(a), indicating that the as-grownMoS2 (i) The pristine MoS2 film is polycrystalline andcoatingis not atomicallysmoothandthat the grain size composedof grains whosesizes lie in the nanometreis in the 5 nm range,in good agreementwith the TEM range. These grainsare mostlyedgeorientedin our case,study. Tentatively,highresolutionAFM images gave no with azimuthal disorderof the c axis within the coat-clearatomic resolutionof theterminationof theS—Mo—S ing plane.sandwichesat the surface. (ii) Friction-induced orientation of the grains was

Fig. 5(b) shows a high resolution AFM image foundboth on the rubbed surfaceand in most of theobtainedinside the wear scar and the corresponding wearfragments, with the basal planeof the crystalnumerical diffractogram. The hexagonal symmetryof structureparallel to the slidingdirection. No grain sizethe atomicarrangementpresenton the surfaceis easily increasewas observedduring this process.seen. The interatomic distancewas calculated to be (iii) Intragranularshearinside the Mo52 grain may

~ Si~ .~ . •.~ .‘..

______ • , ~i.~, .~ ~.•

- •- i.’.

L...~ c. ____

__J-ET1 w138 284 m213 284 lSBT__________________ _________ ~ :~

____ ____________

_______ ‘I

— • .•,•_~

a . —. — •4j •_t*~_.

mm

Fig. 4. HRTFM micrographof theedgeof anMoS, wearparticle,with theelectronbeamnormalto thec axis.Detailsof theatomicarrangementin theS—Mo—S sandwishescan beobserved.An areawith the [2110] orientationdepictinga rhombohedralstacking(chevronsaligned in thesamedirection) is alsoshown.

432 f.M. Martin et al. / Superlubricityof MoS2

particlesalthoughit could alsooriginate from the origi-nal coating.

(iv) Frictional anisotropyduring intercrystallite slipwas clearly identified in the interface products. Thepresenceof incommensurateatomic basal planes inslippage conditions (rotation of approximately 30°betweentwo superimposedcrystals)hasbeenexperimen-tally establishedby HRTEM.

Therefore,we havedemonstratedthat the conditions308 of the superlubric state (or zero friction) can be

approachedin some places of the sliding interface.Therefore,the drasticdecreasein the friction force can

i~Lbe attributedto this phenomenon.

More work is necessaryto find the driving forces oftheseorientationmechanisms:do they dependon the

1 pm original film structure(a memory effect), or are theya inducedby friction to minimize the energydissipation

during sliding?

References

[1] E. W. Roberts,Tribol. mt., 23 (2) (1990) 95.[2] P. D. FleischauerandR. Bauer, Tribo!. Trans.,31(1988)239.

F.F.T. [3] A. Mogami andA. Okitsu, Proc. Eurotrib 85 (1), 1985, p. 45.

_______________________________________ [4] N. Takahashiand K. Okada,Proc. ASLE mt. Conf. on SolidLubrication,Denver, CO, 1978, ASLE, New York, 1978, p. 14.

[5] N. Takahashi,Wear,124 (1988)279.[6] J. B. Sokoloff, Phys.Rev.B, 42 (1) (1990) 760.

~ [7] M. R. Hilton and P. D. Fleischauer,J. Mater. Res., 5 (2)(1990) 406.

[8] K. Shinjoand M. Hirano, Surf. Sci., 283(1993) 473.[9] M. Hirano, K. Shinjo, R. Kaneko andY. Murato, Phys. Rev.

Lett., 67 (1991)2642.

[10] J. A. Harrison, C. T. White, R. J. Colton and D. W. Brenner,Ii Phys. Rev. B,46(1992)9700.

nm b [11] C.Donnet,Tb. LeMogneandJ. M. Martin, Surf. Coat. Technol.,62 (1993)406.

Fig. 5. AFM imagesof theMoS, coating:(a) low magnificationimage [12] J. M. Martin, C. Donnet andTb. Le Mogne, Phys.Rev.B, 48of the pristine MoS2coating,showing thepresenceof grains;(b) high (14) (1993)10583.resolution imageinside the wearscargiving evidence forbasal plane [13] T. Spalvins,J. Vac. Set. Technol. A,5(1987) 212.orientation andthe cleanlinessof theexposedsurface to friction. [14] I. L. Singer, in I. L.SingerandH.M. Pollock (eds.),Fundamentals

in Friction: Macroscopic and Microscopic Processes,Kluwer,Dordrecht,1992, p. 237.

[15] N. Takahashiand M. Shiojiri, Wear,167 (1993)163.occur via the hexagonal—rhombohedralphase trans- [16] T. Isshiki K. Nishio, I. Aoyagi, Y. Yabuuchi, N. Takahashi,formation. Actually, some S—Mo—S stacking with the H. Saijo andH. Shiojiri, Wear, 170 (1993) 55.rhombohedralarrangementwas observedin the wear [17] C. M. Lieberand Y. Kim, Thin Solid Films,206 (1991) 355.