MacroscaleSuperlubricityInducedby MXene/MoS2Nanocompositeson RoughSteelSurfacesunderHigh ContactStressesAli Macknojia,AdityaAyyagari,*DarioZambrano,AndreasRosenkranz,ElenaV. Shevchenko,and DianaBerman*Cite This:ACS Nano2023,17, 2421−2430Read OnlineACCESSMetrics& MoreArticleRecommendations*sıSupportingInformationABSTRACT:Towardthe goalof achievingsuperlubricity,or near-zerofriction,in industriallyrelevantmaterialsystems,solution-processedmultilayerTi3C2Tx-MoS2blendsare spray-coatedontorough52100-gradesteelsurfacesas a solidlubricant.The tribologicalperformancewas assessedin a ball-on-diskconfigurationin a unidirectionalslidingmode.The test resultsindicatethat Ti3C2Tx-MoS2nanocompositesled to superlubriciousstates,whichhashithertobeenunreportedfor bothindividualpristinematerials,MoS2and Ti3C2Tx, undermacroscaleslidingconditions,indicatinga synergisticmechanismenablingthe superlativeperformance.The processing,structure,andpropertycorrelationwerestudiedto understandthe underlyingphenomena.Ramanspectroscopy,scanningelectronmicroscopy,and transmissionelectronmicroscopyrevealedthe formationof anin siturobusttribolayerthat wasresponsiblefor the performanceat highcontactpressures(>1.1GPa)and slidingspeeds(0.1m/s).Thisreportpresentsthe lowestfrictionobtainedby eitherMoS2or MXeneor any combinationof the two so far.KEYWORDS:superlubricity,MXenes,molybdenumdisulfide,steel,friction,wear,solidlubricationIdentifyingmaterialsand systemsthat can rendersuper-lubricityin real-worldapplicationsis an importantresearcharea in slidingmechanicalsystems.1Severalnicheapplicationscall for oil-freelubricationthat can benefitfromnearlyfrictionlessslidingon multiasperitycontacts,also knownas engineeringroughsurfaces(ERS).Graphene,2−6nano-diamonds,7,8and their combinations9,10have been showntoinducesuperlubricity.However,it must be pointedout thatthis phenomenonhas been reliablyobservedin a limitedrangeof controlledsystems:namely,with an inert counterfacelikediamond-likecarbon(DLC),usingatomicallysmoothsubstratessuch as silicawafers,or undervacuumcondi-tions.11−16Other2D materials,such as MoS2and grapheneoxide,are knownto be promisingsolid lubricantmaterials,butthe superlubricityin those has been dependenton the materialalignmentand is highlysensitiveto atmosphericconditionsand materialspurity.Martinet al.12,13,17have attributedthemechanismsin the lubricityof MoS2to intragranularshear andintercrystalliteslip, both of whichare closelyrelatedto theindividualcrystalrotation/orientation.Importantly,thelubricitypropertieshave beenattributedto (a) intrinsicmaterialpropertiesof MoS2flakesin the aforementionedReceived:September27, 2022Accepted:December20, 2022Published:January25,2023Articlewww.acsnano.org© 2023AmericanChemicalSociety2421https://doi.org/10.1021/acsnano.2c09640ACS Nano2023,17, 2421−2430
Figure1. Schematicillustrationshowingthe generationof suspensionscontainingsolidlubricantsin a nonreactivecarriermedium(ethylalcohol)nd subsequentspraycoatingontothe preheatedsubstrate,as wellas unidirectionalball-on-diskslidingexperiments.High-resolutionTEMimagesshowingthe latticespacingsof (b) MoS2as 5.75Å and (c) Ti3C2Txas 8.78Å. (d) Lightmicroscopicimageof thecoatedsteelsubstrate(e) withthe correspondingsurfaceprofileshowingthe coatingthicknessof the solidlubricantdepositedontoan AISI52100steelsubstrate.The averagecoatingthicknesswas 3.3±0.1μm, and the roughnessof the coatedsubstrateswas 450±50 nm.ACS Nanowww.acsnano.orgArticlehttps://doi.org/10.1021/acsnano.2c09640ACS Nano2023,17, 2421−24302422
orientations,ratherthan contactmechanicsconditions,(b)orientationof the grains,or basal plane reorientation,and (c)intragranularand intercrystallineshearin the MoS2grains.12,13,17The majorlimitationfor the advancementisthat most 2D materialscall for highlycontrolledprocessingsteps such as atomiclayer deposition,physicalor chemicalvapordeposition,or sputteringthat invariablyneed high-vacuumsystems,thus presentinglimitationsof size, geometry,and uniformity.18Slip and/orshearbetweenthe atomicplanesin layeredstructuressuch as in graphite,19graphene,20,21grapheneoxide,22MoS2,17,23−25and WS2,26−29amongothers,areprimarilyresponsiblefor lubricityin solid-statesliding.Thetribomechanicalor chemicalphenomenasuch as crumblingofthe layeredstructureunderhigh contactpressure,30reactionsbetweenthe substrateand coatingmaterialchangingitschemical/crystalstructure,31−33or materialsdegradationbyintercalationwith impurities,17oxygen,or watermolecules(also referredto as structuralwater)34are generallyattributedto the loss of lubricityover prolongedslidingand are barriersto realizingsuperiorperformance.Therefore,the searchformorereliableand tribologicallyeffectivematerialsystemscontinues.MXenesare a class of layeredmaterialsthat could potentiallyovercomethesedrawbacks,as they offer flexibilityin thestructureand compositionbeyondpreviouslytested2Dmaterialswhileprovidingthe lubricationbenefitsof thelayeredmaterials.MXeneshave beenexploredfor theirtribologicalproperties35,36as lubricantadditivesin oils37,38and organicsolvents39,40and as reinforcementsin polymer-based composites41−45and solid lubricants.46−48MXeneshavebeen shownpreviouslyto providea superlubricityregimewhenthey are depositedon an atomicallyflat siliconsubstrateandtestedagainstthe DLCsurface.48However,thereare noreportsthat have demonstrateda prolonged,sustainablesuperlubricationperformancewhenthey are used as a solid-state lubricantreduceron ERS with a steel counterface.Tobridgethis knowledgegap, we demonstratethe pathwaysforachievingsuperlubricityon ERS (Ra≈200 nm) multiasperitycontacts,using hybridTi3C2Tx-MoS2coatings.This paperreportsa simplespray-coatingprocessfor thedepositionof hybridTi3C2Tx-MoS2coatingsthat resultedin anexceptionallubricityperformanceunderdry conditionsunderhigh contactpressuresof∼1.1 GPa. The detailedcharacter-izationof the tribologicalinterfacerevealedthe originof suchsuperlubricityin the rearrangementof the 2D materialstoaccommodatethe appliedstresses.Opticalprofilometry,scanningelectronmicroscopy,transmissionelectronmicros-copy,Ramanand X-rayphotoelectronspectroscopy,andnanoindentationwere used to quantifythe propertiesanddelineatethe phenomenon.Our findingsunravelopportunitiesfor frictionlessslidingsystemsoperatingunderconditionsrelevantto industrialapplications.Figure2. (a) Coefficientof frictionbehaviorshowingthe performanceof MoS2-Ti3C2Txsolidlubricantcoatingsunderunidirectionalslidingat 0.1 m/sundervariouscontactloadsas a functionof slidingdistancein dry nitrogen.(b) Summaryof steady-statefrictionvaluesjuxtaposedwithsteel-on-steel,MoS2-on-steel,and Ti3C2Tx-on-steelreferences.(c) Coefficientof frictionmeasuredat 20 N and 0.1 m/sunderambientconditionsin contrastwiththe frictionat 20 N underdry nitrogenconditions,showingthe effectof humidityon thetribologicalperformance.(d) Coatingwearon the steelsubstrateas a functionof the normalloadafterslidingfor the samedistance.Thefrictionwas observedto decreasewithan increasein the normalload(contactpressure),withthe 20 N test conditionsurpassingthesuperlubricitythresholdby 1 orderof magnitude(0.0034).Wearrateswereobservedto decreasewithincreasingloadas was the case forfriction.ACS Nanowww.acsnano.orgArticlehttps://doi.org/10.1021/acsnano.2c09640ACS Nano2023,17, 2421−24302423
RESULTSAND DISCUSSIONA schematicillustrationof the coatingprocessand tribologicaltestingin unidirectionalslidingis shownin Figure1a. Thecompositionof the coatinghas been optimizedto revealthemaximumlubricitypotentialof the materials(SupportingInformation).High-resolutionTEMmicrographsof the pristineas-receivedMoS2and Ti3C2Tx(Txindicatesthe surfaceterminationsof the outer metal layers in the MXenestructure,such as O, OH, etc.) are shownin Figure1b,c, respectively.Both materialshad a microcrystallinesize (up to 500 nmlengthand width),and multiple-layer(up to 30−50layersforMoS2and up to 50−60layersfor Ti3C2Tx) structures.Thelatticeparametersof MoS2and Ti3C2Txwere about5.75 and8.78 Å, respectively,as calculatedfrom Figure1b,c. An SEMimageof the as-depositedcoatingis shownin Figure1d. Theroughnessof the coatedsubstrateswas 450±50 nm. Thecoatingthicknesswas measuredby removingthe depositedmaterialwith a sharpscribeand measuringthe step heightbetweenthe underlyingsteel substrateand the top surface.This procedurewas repeatedat multiplelocationson eachsubstrate,thus calculatinga coatingthicknessof 3.1±0.1μmas shownin Figure1e.Frictiondata for individualtest conditionsare showninFigure2, whilethe averagesteady-statefrictionvaluesareshownin Figure2b. The coefficientsof frictionof the steel-against-steel,MoS2-on-steel,and Ti3C2Tx-on-steelbench-markingtests, performedwith a 1 N load, were 0.78, 0.19, and0.2, respectively.Higherload benchmarkingwas successfulonly for the MoS2-on-steelsystemfor loads up to 5 N (FigureS1), since for all other samplesthe peak frictiontriggeredtheautocutoffon the tribometerand terminatedthe testsimmediatelyafter the break-in.Coefficientof frictiondata for hybridcompositesshowedthat frictionreducedby 89% in the 1 N test to 0.0791,by 91%in the 3 N test to 0.055,by 94% in the 5 N test to 0.047,by 2ordersof magnitudeto 0.013at 10 N, and surpassingthesuperlubricitythreshold(of 0.01) by 1 order of magnitudeat20 N with a frictionof 0.0034.Each test was repeatedat leastthreetimesto confirmthe measurements.The best testconditions,namely20 N and 0.2 m/s sliding,was run for 2 htotaling40000cycles(1.2 km), demonstratingsteady-statefrictionat 0.003,whichconfirmedthe materials’performanceto be prolongedand repeatable.Figure2c showsthe frictionresultson slidingunderhumidconditions(RH 42%),withvaluesreachingas high as 0.65. This, in comparisonwith thetest resultsfromdry nitrogensliding,clearlyshowsthedeleteriouseffectsof environmentaloxygenand/orwatervapor.Followingascertainingfrictionresponseand itsrepeatability,the wear inducedon the coatedsurfaceswasstudied,and the resultsare presentedin Figure2d (and inFigureS2). The resultsindicatea 2 ordersof magnitudereductionin the wear rate for the coatingtestedat a 20 N loadin comparisonto the bare steel vs steel slidingat 2 N and 0.1m/s. A representativesurfaceprofileof the wear track is shownin Figure3a. The depthof the wear track was about0.7−0.8μm for the 20 N 0.1 m/s test conditionand was muchshallowerfor lowerloads.Theseresultssuggestthat thecoatingstill remainedintact duringslidingand its compressionand adaptationallowedreducingthe effectof the steelsubstrateroughnesson the frictionalbehavior.The modifiedmaterialsin the wear track also exhibitedstrongadhesiontoFigure3. (a) Surfaceprofileof the weartrackof the hybridnanocompositeat 20 N. (b) SEM-EDSmappingshowingthe distributionofelementson the coatingand weartrack.(c) Summaryof the resultsobtainedby nanoindentationanalysisinsidethe formedweartracksshowingan increasinghardnessand moduluswithslidingacrosstest loads.ACS Nanowww.acsnano.orgArticlehttps://doi.org/10.1021/acsnano.2c09640ACS Nano2023,17, 2421−24302424
the substratethat made them capableof sustaininghigh loadand shear stresses.The coatinginsidethe slidingpath remainedintactat theend of the slidingexperiments,both short-termand long-term,and did not revealthe underlyingsteel in any of the test cases.A representativeelementalmappingfor the extremetestcondition,namely20 N, is shownin Figure3b. The faint Fesignalis expectedto originatefrom the underlying,unexposedsteelsubstratedue to the depthof electronmaterialinteractions.The entirewearon the surfaceseenasdeformation/compressionof the hybridcoatingwas containedwithinthe thicknessof the deposit.The slidingpath wasbrighterthan the as-depositedcoating,indicatingthat thecoatingmay have been slightlypolishedduringsliding.To quantifythe changesin the mechanicalpropertiesof thecoatingmaterialduringslidingand to understandthemechanismsresultingin superlubricity,nanoindentationexperimentswere carriedout insidethe formedwear tracksfor all slidingconditions.The correspondingresultsare shownin Figure3c. A clear trendof increasingthe resistancetoindentationand stiffnesswas seen with increasingcontactpressure.The indentationdata are presentedup to 5 N, sinceduringthe tests performedat 10 and 20 N the depthof thewear tracksapproachesthe thicknessof the coatingand thusFigure4. Ramanspectraacquiredon (a) as-depositedhybridcoatingand (b) insidethe weartrackaftertribologicaltesting.(c) Ramanmappingon the weartrack.The datashownherecorrespondto the weartrackgeneratedfor a normalloadof 20 N.ACS Nanowww.acsnano.orgArticlehttps://doi.org/10.1021/acsnano.2c09640ACS Nano2023,17, 2421−24302425
the steel substratelargelyaffectsthe measurementsof thehardnessand the elasticmodulus.Indeed,the measuredhardnessvaluesfor 10 and 20 N wear trackswere measuredtobe 6.5 and 6.6 GPa respectively,whichis close to the 6.6 GPahardnessof the steel substrate.The observedtrendof thehardnessand elasticmodulusincreaseas a functionof the loadat whichthe tribotestswereperformedis unambiguousevidenceof a compaction-drivenincreasein hardnessandmodulusthat has improvedthe lubricity,thus preventingasteel-on-steelcontactand improvingthe respectivewearresistance.The surfaceof the tribologicalcounterbodystudiedby opticalprofilometryand scanningelectronmicroscopydoesnot indicateany pronouncedwearbut did showfirmadherenceof the transferlayer that couldnot be removedbywipingor sonication(to assessball cap diameter).Thenanoindentationresultsand wear data, from both the substrateand counterfaceball, indicatethat there is an absenceof wearloss of the steel surfaces;slidingat high contactpressuresresultedin compactionand densificationof the coatingthatresultedin a robusttribofilm,whichpreventedthe directsteel-on-steelcontact.This mechanicalphenomenonin combinationwith the shearingof the layeredmaterialswithinthe tribofilmcontributedto the sustainedsuperlubricity.The densificationphenomenonis consistentwith previousreportsby Mogneetal., wherevery dense,hard coatingswere observedto produceexceptionallubricity.13To furtherprobethe originof the observedsuperiorlubricity,structural(crystallographicand morphological)andcompositional(chemicalbonding)changesin the coatingwereevaluated.Figure4 summarizesthe spectracollectedfrom theFigure5. Bright-fieldTEMimagesof the tribolayerat (a) the 10 N test showingcompositeMXeneand MoS2overlappedflakes,(b) the 20 Nshort-termtest, and (c) the end of the long-termtest. (d) Electrondiffractionimage.(e) Schematicof an in-operandomechanismconsistingof coatingdensificationand reorientationthat resultedin superlubricity.ACS Nanowww.acsnano.orgArticlehttps://doi.org/10.1021/acsnano.2c09640ACS Nano2023,17, 2421−24302426
(centerof) wear track formedduringsuperlubricslidingunder20 N test conditionsin comparisonto a representativeRamanspectrumacquiredon the as-depositedhybridcoating.Agradualtransitionin the Ramansignalis observedacrosstheregionsof the slidingpath. The tribotestsinducedchangesinthe characteristicpeaksof the as-deposited,pristineTi3C2Tx-MoS2nanocompositeat 284 (Ti3C2Tx), 377 (E12gandTi3C2Tx), 406 (A1g), 598 (E12g+ LA), and 822 cm−1(2A1g),while the intensityof the peakslocatedat 379 and 406 cm−1remainedunchanged.The intensitiesof the E12gand A1gpeakswere highercomparedto thoseof the as-depositedcoating,indicatingthat the structureof MoS2experienceda basal planereorientationof the basalplanesundershearingandcompressiveloads.The MXenecomponentof the nano-compositedid not produceany resolvablespectralsignal in theslidingpath due to signal-drowningfrom the MoS2. A faintpeak at 820 cm−1was noticedin the as-depositedcoating,whichwas observedto be absentinsidethe wear track,andnone of the spectrahad 159 or 666 cm−1peaks.Thesethreepeaks,namely,159, 666, and 820 cm−1, correspondto MoO349and may indicatea completeabsenceof oxidation/oxideformationduringthe wear testing.2D maps of all the majorRamanpeaks(Figure4c) indicatethe consistencyof thecoatinginsidethe wear track. Interestingly,an analysisof thesteel counterpartafter sliding(FigureS3) indicatestransferofthe coatingto the ball side to ensureeasiershearingofmaterialswith no signs of the steel substratewear. In starkcontrast,the Ramananalysisof the wear track producedin ahumidenvironment(FigureS4) showsno coatingpresenceinsidethe wear track after the sliding.The X-rayphotoelectronspectra(presentedin FigureS5)for the as-depositedcoatingand insidethe wear track formedduringsuperlubricslidingsummarizethe changesin the Mo3d,Ti2p, and S2ppeak regions.The peak deconvolutionof theMo3dregionindicatesa decreaseof the Mo6+oxidationstatewith sliding.This corroboratesthe resultsobtainedby Ramanspectroscopyand affirmsthe understandingthat there is aneliminationof MoO3. The formationof MoO3is a knownmechanismof danglingbond saturationin MoS249−52whencrystaldegradationor disruptionoccursduringslidingand hasbeenextensivelydocumentedto deterioratethe lubricityperformance.25,30−33This bringsup the questionas to whyan indicationof oxide was observedin the startingmaterials/pristinecoatingand not in the tribolayer.This couldbeattributedto two plausiblesources.On the one hand,theexposureto ambientconditionsand adsorptionof oxygenmayhave led to the oxidationof MoS2crystallitesat defectsites oredges.On the other hand,it may be tracedback to potentialoxidationfrom the residualoxygenfrom the ethanol-baseddepositionprocedure.Continuouspurgingof nitrogenat lowdew-pointtemperaturecombinedwith tribochemicalreactionsand reorientationmightbe responsiblefor the observeddecrease/vanishingof the oxygen.Additionally,Ti, whichmaybe releasedfrom Ti3C2Tx duringsliding,is a knownoxygenabsorberand its presencemay acceleratethe removalof theadsorbedoxygenfrom MoS2.51,52It is hypothesizedthat thesynergisticmechanismwhereindeleteriouseffectsare elimi-natedby local transferof oxygen/structuralwaterfrom oneconstituentphaseto anothereffectivelyresultedin vanishingfrictionfor unlimitedlubricity.To explainthe phenomenonbyquantifyingmicrostructuralchanges,smallscrapsof thetribolayerwere mountedonto coppergrids for transmissionelectronmicroscopy.The bright-fieldTEMimagesandelectrondiffractionpatternsare shownin Figure5.The TEM micrographsdepictedin Figure5a show the twomaterialswith distinctlatticespacingsoverlappingeach otherwith MoS2flakesextendingbeyondthe phasemixture,delineatedwith the black dottedline. Similarmicrostructuralfeaturescan be seen in Figure5b,c, respectively,whereindarkermultilayerregionswith both MoS2and Ti3C2Txwereinterspersedin largerMoS2blankets,as shownwith the whiteand red dottedlines,respectively.The threebright-fieldimages,albeitfrom differenttest and load conditions,retainsignificantsimilaritiesto that of the pristine,as-receivedmaterialsas shownin Figure1, indicatingno flake-/crystal-level deformationof latticedisruption.Figure5d (and FigureS6) showsa representativeelectrondiffraction(ED)imagefrom the phasemixture,showingbrightdiffractionspots fromboth crystalsand a completeabsenceof halos,smearing,andformationof continuousor discreterings acrossmultipleanalysisregions.This confirmsthat the phasesretainedtheirhigh degreeof crystallinitywith no significantdegradationofMoS2or Ti3C2Txwhatsoever.Moreover,therewas noformationof amorphousphasesthat are typicallyobservedtoform in high-load,high-speedslidingof 2D materials.26,30The data from white light interferometry,scanningelectronmicroscopywith energydispersiveX-rayspectroscopy,nano-indentation,Ramanspectroscopy,and X-rayphotoelectronspectroscopy,supportedby bright-fieldtransmissionelectronmicroscopyand electrondiffractionimaging,convergeto thefollowingobservations:a robusttribolayerwas formedandcontinuallyhardened(drivenby compactionand densifica-tion) duringsliding.The tribolayerwas thick and resilienttoaccommodatedeformationfrom high contactpressureandpreventeddirect(steel-on-steel)contactbetweenthe slidingtribopairs.By virtueof slidingundera dry nitrogenatmosphere,and the relativeaffinityof Ti to oxygen,thedeleteriouseffectsof oxygenor structuralwaterintercalationon the lubricitybehaviorof MoS2and Ti3C2Txcouldbecompletelycircumventedto provideultralow,stable,prolongedsuperlubricity.An increasedsignalstrengthfrom the basalplanesof MoS2, whichindicatesthe reorientationof the MoS2crystallitesin the tribolayer,and Ti3C2Txform the structuraltruss that facilitatedhithertounseenexceptionallubricityperformanceunderdry conditions.The respectivemechanismsare illustratedin the schematicsshownin Figure5e.The observedlubricityresultsweresignificantlylowercomparedto those of previouslyreportedcoatingsdepositedusing similartechnologiessuch as burnishedor (magnetron,rf) sputteredMoS2underdry, ambient,and humidslidingconditions.The resultsalso outperformcompositeandchameleoncoatings,MoS2dopedor blendedwith othermaterialssuch as gold, silica,nickel,graphene,and grapheneoxide,MXenes,and MXeneblendsand even in comparisontooil- or liquid-basedlubricationwith MXene1or MoS2additivesas illustratedin the Ashbyplotshownin Figure6.22,23,30−33,36,46,47,53−57CONCLUSIONSThis studyreportedsuperlubriciousbehaviorobtainedfor acombinationof Ti3C2Txand MoS2. The frictionresultsshownhere have been observedto be the lowestvaluesreportedinthe publishedliteraturedespitetestingat very high contactpressures,exceeding1 GPa. This was explainedbasedonextensivemechanical,microstructural,and molecular-bondingACS Nanowww.acsnano.orgArticlehttps://doi.org/10.1021/acsnano.2c09640ACS Nano2023,17, 2421−24302427
featuresof MoS2and MXenes,originatingfromthereorientationof MoS2basalplanesand a MXenetrusssupportingthe tribolayerall the while retainingthe crystallo-graphicintegrityof the two materialsat high contactpressuresand slidingat high velocities.This templateof materialsdevelopmentis hopedto acceleratethe adoptionof solidlubricantsonto roughsurfacesand translateinto commercialapplicationsas well as developmaterialcombinationswitheven betterproperties.METHODSMaterials.To generateaccordion-likemultilayerTi3C2Txnano-sheets,2 g of MAX-Ti3AlC2in powderform (ForsmanScientificCo.Ltd., Beijing,China)was treatedwith 40% hydrofluoricacid. Theetchingsteps consistedof magneticstirringat 60 rpm at 35°C for 24h. The suspensionwas centrifugedat 3500 rpm for 5 min, and theresidualproductwas collected.The final pH of 6 was adjustedbyseveralwashingcycleswith deionizedwater.Subsequently,thesuspensionwas vacuum-filteredand freeze-driedfor 24 h at−60°Cand a pressureof <30 Pa. MultilayermicrocrystallineMoS2flakeswere purchasedfrom GrapheneSupermarket,NY, USA.CoatingPreparation.The coatingswerepreparedusingmultilayerTi3C2Txand MoS2suspendedin ethyl alcohol.For theseblends,6 mg of MoS2was mixedwith 6 mL of ethanoland 25μL of aTi3C2Txsuspension(concentration70 mg/mL).The solutionwasmixedthoroughlyto createa multilayerTi3C2Tx-MoS2blend.Thepreparedsolutionwas sonicatedfor 5 min, such that the solutionwasthoroughlymixedand readyfor deposition.This solid lubricantalcoholsuspensionwas then transferredinto the holdingcup of apneumaticspraydepositionapparatus.Dry nitrogenwas used topreventany oxidationor deleteriousintercalationduringspraycoating.The 50 mm in diametertest couponswere made of hardenedand temperedAISI 52100havinga hardnessof 59±1 HRCandroughnessRaof 250±50 nm. The substratewas heatedto 90±10°C to acceleratethe evaporationof the carriersolution,instanta-neouslydepositingthe containedsolidsonto the substrates.Specialcare was taken not to changethe propertiesof the base steel throughtemperingby limitingthe thermalcycle to less than 20 min andmaintainingsubtemperingtemperatures.TribologicalAnalysis.Followingthe preparationof substrates,the frictionand wear responsewere evaluatedundera dry nitrogenatmosphere(−44°C dew point undercontinuouspurging)using anAntonPaar tribometerunderunidirectionalslidingagainstAISI52100steel counterfacespheres(6 mm diameterwith a hardnessof57 HRCandRa= 60±2 nm). A schematicof creatingthe suspension,spray coating,and tribologicaltests is shownin Figure1. Tests werecarriedout at 1 N (Hertziancontactpressure∼480 MPa),3 N (∼690MPa),5 N (∼820MPa),10 N (∼1000MPa),and 20 N (∼1130MPa)with slidingat 0.2 m/s linear speedsand run at the same lineardistance.Endurancetests were conductedfor 2 h (equalto a distanceof 1.2 km or 40k cycles)at promisingtest parametersto assessthelongevityof the hybridcoatings.For comparisonand benchmarking,steel-against-steeltests were run at 20 N underdry nitrogen.Characterization.Opticalprofilometrywas used to measurethecoatingthicknessprior to the onsetof experimentsas well as therespectivewearloss on the tribopairsafterthe tests.Themorphologiesof the as-receivednanosheets,coatedsubstrates,andformedtribolayerwere studiedusing scanningelectronmicroscopy(FEIQuanta200 SEMequippedwith energy-dispersiveX-rayspectroscopy(EDS)at 5 kV beamvoltage),transmissionelectronmicroscopy(JEOL2000F),and opticalmicroscopy(Leicaopticalmicroscope).Chemicalchangesin the coatingswere recordedusingRamanspectroscopy(RenishawRamansystemwith a green 532 nmwavelengthlaser)and X-rayphotoelectronspectroscopy(PHIVersaprobe,Al Kαradiation).The mechanicalresponsewas recordedusing a KLA iNanonanoindenterwith a diamondBerkovichtip at10000mN load in a continuoussensingmode.A FilmetricsProfilm3DOpticalProfilometerfrom KLA Instruments(Milpitas,California)was used to generatethe 2D and 3D surfaceprofilesofsamplesurfaces.ASSOCIATEDCONTENTData AvailabilityStatementThe authorsconfirmthat the data supportingthe findingsofthis study are availablewithinthe article.*sıSupportingInformationThe SupportingInformationis availablefree of chargeathttps://pubs.acs.org/doi/10.1021/acsnano.2c09640.The coatingoptimizationprocedureand materialanalysis,frictionanalysisof the single-componentcoatings,opticalimagesof wear tracks,characterizationof the counterface,Ramananalysisof the weartrackformedduringslidingin A humidenvironment,and XPSand ED analyses(PDF)AUTHORINFORMATIONCorrespondingAuthorsDianaBerman−Departmentof MaterialsScienceandEngineering,TheUniversityof NorthTexas,Denton,Texas76203,UnitedStates;orcid.org/0000-0002-9320-9772;Email:Diana.Berman@unt.eduAdityaAyyagari−Departmentof MaterialsScienceandEngineering,TheUniversityof NorthTexas,Denton,Texas76203,UnitedStates;Email:Aditya.Ayyagari@unt.eduAuthorsAli Macknojia−Departmentof MaterialsScienceandEngineering,TheUniversityof NorthTexas,Denton,Texas76203,UnitedStatesDarioZambrano−Departmentof ChemicalEngineering,BiotechnologyandMaterials(FCFM),Universityof Chile,Santiago8370456,Chile;orcid.org/0000-0002-3484-4784AndreasRosenkranz−Departmentof ChemicalEngineering,BiotechnologyandMaterials(FCFM),Universityof Chile,Santiago8370456,Chile;orcid.org/0000-0002-9006-1127Figure6. Ashbyplotshowingfrictionon theyaxis and contactpressureon thexaxis for variousMoS2and MXenematerialsunderpristineandblended/dopedconditionsjuxtaposedwithcurrentresults.ACS Nanowww.acsnano.orgArticlehttps://doi.org/10.1021/acsnano.2c09640ACS Nano2023,17, 2421−24302428
ElenaV. Shevchenko−Centerfor NanoscaleMaterials,ArgonneNationalLaboratory,Argonne,Illinois60439,UnitedStates;Departmentof ChemistryandJamesFrankInstitute,Universityof Chicago,Chicago,Illinois60637,UnitedStates;orcid.org/0000-0002-5565-2060Completecontactinformationis availableat:https://pubs.acs.org/10.1021/acsnano.2c09640NotesThe authorsdeclareno competingfinancialinterest.ACKNOWLEDGMENTSThe authorsacknowledgethe supportof this work by theNationalScienceFoundation(NSF)(AwardNo. 2018132).A.R. acknowledgesthe financialsupportgiven by ANID-Chilewithinthe projectsFondequipEQM190057,FondecytRegular1220331,and FondecytPostdoctorado3220165.Workperformedat the Centerfor NanoscaleMaterials,a U.S.Departmentof EnergyOfficeof ScienceUser Facility,wassupportedby the U.S. DOE,Officeof Basic EnergySciences,underContractNo. DE-AC02-06CH11357.REFERENCES(1) Ayyagari,A.; Alam,K. I.; Berman,D.; Erdemir,A. 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