Atomic diffusion induced degradation in bimetallic layer coated cemented tungsten carbide

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ContentslistsavailableatScienceDirect

Corrosion

Science

jo u r n al ho m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / c o r s c i

Atomic

diffusion

induced

degradation

in

bimetallic

layer

coated

cemented

tungsten

carbide

Zirong

Peng

a

_,

_Michael

_Rohwerder

a,∗

_,

_Pyuck-Pa

_Choi

a,b

_,

_Baptiste

_Gault

a,∗

_,

Thorsten

Meiners

a

_,

_Marcel

_Friedrichs

c

_,

_Holger

_Kreilkamp

c

_,

_Fritz

_Klocke

c

_,

_Dierk

_Raabe

a

a_Max-Planck_Institut_für_{Eisenforschung}_GmbH,_Max-Planck_Str._1,₄₀₂₃₇_Düsseldorf,_Germany

b_Korea_Advanced_Institute_of_Science_and_Technology_(KAIST),_Department_of_Materials_Science_and_Engineering,_Daejeon_305-338,_Republic_of_Korea c_Fraunhofer_Institute_for_Production_Technology_(IPT),_{Steinbachstrasse}_17,₅₂₀₇₄_Aachen,_Germany

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received2December2016

Receivedinrevisedform12January2017 Accepted13January2017

Availableonline20February2017 Keywords: A.Metalcoatings A.Sputteredﬁlm B.TEM C.Oxidation C.Interfaces

a

b

s

t

r

a

c

t

Weinvestigatedthetemporaldegradationofglassmouldingdies,madeofcementedtungstencarbide coatedwithPtIronanadhesiveCrorNiinterlayer,byelectronmicroscopyandatomprobetomography. Duringtheexposuretreatmentsat630◦Cunderanoxygenpartialpressureof1.12×10−23bar,Cr(Ni) wasfoundtodiffuseoutwardsviagrainboundariesinthePtIr,alteringthesurfacemorphology.Upon dissolutionoftheinterlayer,theWCsubstratealsostarteddegrading.Extensiveinterdiffusionprocesses involvingPtIr,Cr(Ni)andWCtookplace,leadingtotheformationofintermetallicphasesandvoids, deterioratingtheadhesionofthecoating.

1. Introduction

Multilayercoatingsorthinfilmshavevariousapplicationfields. Theyarefrequentlyutilisedinengineeringcomponents,toolsand mouldsaswellasmicroelectronic,magneticandopticaldevices [1–4].Comparedtosinglelayercoatings,multilayercoatings, com-biningbeneficialpropertiesofdifferentmaterials,mayfulfilseveral practicalrequirementsatonce.Theymayalsobeusefulincases whereonlyapplyingasinglelayercoatingmaynotbesufficient. Animportantexampleistheuseofaninterfacialbondinglayer betweenthetopcoatingandthesubstratetoenhanceadhesion [5–8].However,multi-layeredstructuresarecommonly thermody-namicallyunstable.Thereisastrongdrivingforceforinterdiffusion and reaction, which will bedetrimental tothermal stability of thecoatingunlesskineticsoftheseprocessesareslow. Unfortu-nately,thetypicalfine-grainedmicrostructureofthinfilmsaswell aslargesurfaceandinterfaceareasprovidenumeroushigh diffu-sivitypaths.Togetherwiththeshortdiffusiondistancesnaturally associatedwiththincoatings,tremendouscompositionalchanges

∗ Correspondingauthors.

E-mailaddresses:[email protected](M.Rohwerder),[email protected] (B.Gault).

canariseandnewphasesaswellasvoidscanform,which will considerablydeterioratethecoatingproperties[9–12].

Here, wereportonthetemporal degradationof abimetallic layercoatingdepositedoncementedtungstencarbidesubstrate duringisothermalexposuresunderanoxygenpartialpressureof 1.12×10−23barat630◦C.The coatingconsistsofan ∼650-nm-thickPtIrtop layerand an∼20-nm-thickintermediate layerof metallicCrorNi(PtIr/CrorPtIr/Ni),whichisfrequentlyutilised in moulds for precision glass moulding (PGM) [13–15]. It is a promising and worldwide employed process for manufacturing highquality optical products withcomplex-geometries. During PGM,glassblanksareplacedinthemouldofapressingtoolandare heatedundervacuumorN2protectiveatmosphereuptotheglass formingtemperature,whichnormallyrangesfrom400to700◦C. Subsequently,highpressforces(2–20kN)areappliedforseveral minutesuntilthemouldformishomogeneouslyﬁlledwithglass. Aftercontrolledcooling,theﬁnalproductscanbedetachedfrom themould[16–18].PtIr,asanoblemetalbasedcoating,is chem-icallyinactiveanddoesnotsticktoglass.Anintermediatelayer, CrorNi,isusedtoimprovetheadhesionstrengthbetweenbase materialandtopcoating[19,20].Duetotheharshoperating con-ditionduringglasspressingwhereintensethermal,chemicaland mechanicalforcesareimposedsimultaneously,coating degrada-tiontakesplaceveryfastandbecomesasevereproblem,strongly limitingtheservicelifetimeofmoulds[21–23].Sincethe produc-http://dx.doi.org/10.1016/j.corsci.2017.01.007

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Fig.1.(a)Conﬁgurationofthedepositionchamber.(b)SketchshowingthearrangementofthepureIrandPtpieceswithinthetarget.(c)Schematicdrawingofthetopview ofthedepositionchambertoillustratethecoatingprocessofthePtIrlayer.

tioncosts are directly related to themould lifetime, designing high-performanceand high-durabilitymouldsaswell as devel-opingconceptstoimpedetheirdegradationhave becomea top priorityforthePGMindustry.Akeytoachievethisgoalisthe under-standingofthefundamental degradationmechanismsofmould materialssuchasPtIr/Cr/WCandPtIr/Ni/WC.

Inpreviousstudies,thereliabilityandstabilityofdiverse metal-licmultilayerthinfilmshavebeeninvestigated,e.g.Cu/Sn [24], Au/Ni/Cr[25],Cu/Ta[26],Pt/Ti-W[27],MoRu/Cr/Ti[28],PtIr/Ni [23,29], and PtIr/Cr[15,23], bymeans ofa variety of analytical techniquessuchasX-raydiffraction(XRD),Rutherford backscat-tering spectroscopy (RBS), Auger electron spectroscopy (AES), energy-dispersiveX-rayspectroscopy(EDS),X-rayphotoelectron spectroscopy(XPS)andsecondaryionmassspectroscopy(SIMS) [29–32].However,thespatialresolutionanddetectionsensitivity ofthesetechniqueswasnotsufficienttorevealthediffusionand segregationphenomenaatgrainboundaries,andlimitedinsights weregainedonthedegradationmechanisms.Atomprobe tomog-raphy(APT)isauniquetechnique,whichhasthecapabilitytoreveal three-dimensional(3D)chemicalinformationat(sub-)nanometre scalewithhighsensitivity(intherangeoftensofpart-per-million) and which hasbeensuccessfully appliedto thin filmmaterials [33–38].Therefore,inthis workweappliedAPTtostudy diffu-sionand degradationphenomenainPtIr/Cr/WCandPtIr/Ni/WC. In addition to APT, we also use scanning electron microscopy (SEM)and(scanning)transmissionelectronmicroscopy((S)TEM) formicrostructuralanalysisandSTEM-EDSandAESforchemical analysis.

2. Materialandmethods

2.1. Specimenpreparation

The basematerial was nano-grain sized cementedtungsten carbidecontainingaround0.3wt.%Cobinder (CTN01L,Ceratizit S.A). Before coating, bulk substrate samples were grinded and polished,makingtheirsurfaceroughness Rasmallerthan5nm. Subsequently, the samples were cleaned in an ultrasonic bath (SurTec089/131, SurTecInternationalGmbH). The coating pro-cesswasaccomplishedinamagnetronsputteringunit(CemeCon CC800/9).Theconﬁgurationofthedepositionchamberisshownin Fig.1(a).Beforecoating,aplasmaetchingprocesswascarriedoutto cleanandactivatethesubstratesurfacetoensureagoodadhesion. Subsequently,aCrorNilayerofabout20nminthicknessandan ∼650-nm-thickPtIrtopcoatingweredepositedsequentially.The detailedparametersofthecoatingprocessarelistedinTable1.The

PtIrtargetusedinthisstudyishome-made,andisnotaPtIralloy butconsistsofthreepiecesofpurePtstackedwithninepiecesof pureIr,asshowninFig.1(b).

2.2. Exposuretreatment

Anovellaboratoryset-upwasusedtoinducedegradationofthe coating,withwhichtheexposureconditionse.g.time,atmosphere andtemperaturecanbeaccuratelycontrolled[39].Mimickingthe realPGMprocess,thetreatmenttemperaturewassettobe630◦C andthefurnaceatmospherewasselectedas97.5%N2-2.5%H2with thedewpoint (DP)of+20◦C. TheDPismanipulatedbyforcing thegasmixturethroughcolumnswhichareﬁlledwithwater.By adjustingtheirtemperature,desireddewpointscanbeobtained. Undertheseconditions,theresultingoxygenpartialpressurewas calculatedtobeapproximately1.12×10−23bar.Suchalowoxygen partialpressureisdesirable forcontrollingcoatingdegradation. Thereby,thesuccessivestagesofthedegradationprocesscanbe accessedsimplyby adjusting theexposuretime.Moreover,the partialpressureofoxygeniskepthighenoughtooxidiseCrbut notNi,sotheeffectsofoxidationondegradationcanbeseparated andstudiedindividually.DuringtherealPGMprocesses,the oxy-genpartialpressuresinthePGMmachinewereestimatedtolie intherange of10−6bar,much higherthanthepartialpressure weappliedinthisstudy.Inrealprocessingsystems,it is deter-minedbytheleakagerateofthesystemandthermodynamically notwelldeﬁned.TheH2O/H2equilibriumconditionsusedinthis studyprovidefullthermodynamiccontrol.Inthiswork,resultswill beshownforthePtIr/Cr/WCsystemintheas-fabricatedstate,as wellasafter6h,18h,and168hexposureat630◦Cunderthe above-describedcondition(PO2≈1.12×10−23bar)inordertoillustrate theevolutionofdegradation.AsforthePtIr/Ni/WCsystem,only representativeresultsforthe48h-exposedspecimenwillbeshown, sincetheobserveddegradationmechanismisverysimilartothat inthePtIr/Cr/WCsystem.Itisworthtomentionthatalthoughin ordertobeabletoresolvethesuccessivestagesofthe degrada-tion,especiallytheearlystages,thepartialpressureofoxygenin theatmosphereusedhereismuchlowerthanthatintheactual processing,wedonotexpectaneffectonthedegradation mecha-nisms.Wealsoanalysedsamplesobtainedfromatechnicalset-up andfoundthesamedegradationfeatures. Furthermore,thefact thatthedegradationmechanismsobservedinthePtIr/Cr/WCand PtIr/Ni/WCsystemsareverysimilarisalsoaproofthatthe degra-dationmechanismisnotstronglyaffectedbytheoxygenactivity.

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Table1

Detailedparametersforlayerdepositionprocess.

Etching Interlayerdeposition PtIrlayerdeposition

Target – Chromiumornickel(99.9%) Platinum(99.99%)andiridium(99.9%)

Heatingpower/kW 0 2 2 Argonpressure/mPa 390 200 200 Biasvoltage/V(f=240kHz) 650 60 60 Cathodepower/W(f=50kHz) – 125 250 Tablerevolution/rpm 2.5 2.5 2.5 Time/min 15 4 40 Thickness/nm – ∼20 ∼650

Fig.2. SketchtoillustratethepositionandorientationofAPTand(S)TEMspecimens.

2.3. Specimencharacterisation

BothAPTand(S)TEMspecimenswerefabricatedusingadual

beam focused-ion-beam (FIB) instrument (FEI Helios Nanolab

600/600i)andthein-situlift-outmethod[40,41].Asillustratedin

Fig.2,theAPTspecimenswerepreparedwithanalignment per-pendiculartothesamplesurface,sothattheanalysisdirectionis paralleltothecoatinggrowthdirection.Forthe(S)TEM measure-ments,bothcross-sectional(perpendiculartothesamplesurface)

andplan-view(paralleltothesamplesurface)lamellaewere pre-pared.Toremoveregionsseverelydamagedbythehigh-energy (30kV)GaionbeamduringAPTspecimensand(S)TEMlamellae fabrication,afinalcleaningprocedurewascarriedoutat2kVand 28pA.APTmeasurementswereconductedusingaLEAPTM_3000X HR(CamecaInstruments)instrument,equippedwithareflectron lens.Laserpulsingmode(532nmwavelength,∼10␮mspotsize) wasappliedatapulserepetitionrateof250kHzandapulseenergy of0.8 nJ.Thespecimenbasetemperaturewaskeptat 60Kand thetargetdetectionratewassettobe0.2%(2ionsdetectedevery 1000pulses).Theseanalysisparameterswereoptimised accord-ingtoapreviouswork[42].(S)TEMimageswereacquiredusing aJEOLJEM-2200FSmicroscopewith200kVaccelerating voltage (Figs.3(b),5(b),6(a),10(a),11(b),12(a),13(a),15(a),17) STEM-EDS were performed using a probe-correctedFEI Titan Themis 60–300S/TEM with300kVacceleratingvoltageusinga Super-X windowlessEDS detector.Correspondinglyahigh-angleannular darkfield (HAADF)imagewastakenfromthemeasuredregion usingaFischioneHAADFdetector(FischioneInstruments)(Fig.8). AESmeasurementswereconductedwithaJEOLJAMP-9500F sys-temequippedwitha hemisphericalanalyser.Thefieldemission electrongunoperatedatanaccelerationvoltageof25keVanda

Fig.3. Representativecharacterisationresultsobtainedfromtheas-fabricatedPtIr/Cr/WCspecimen:(a)SurfaceSEMimage,(b)cross-sectionalBFSTEMimage,and(c)1D concentrationproﬁleandacorrespondingclippedatommapacrosstheCrlayer.Forclarity,onlyIr(blue),Cr(cyan),O(magenta)andWions(orange)areshown.The13at.% Oiso-concentrationsurfacesrepresentthesmallCroxidesfoundinsidetheCrlayer.(Forinterpretationofthereferencestocolourinthisﬁgurelegend,thereaderisreferred tothewebversionofthisarticle.)

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beamcurrentof10nA.AESpeaksofPtMNN,IrMNNandCrLMM wererecordedandused.

3. Results

3.1. PtIr/Cr/WCspecimen 3.1.1. As-fabricatedstate

The surface SEM and cross-sectional bright-field (BF) STEM micrographsoftheas-fabricatedPtIr/Cr/WCsampleareshownin Fig.3(a)and(b),respectively.ThePtIrandCrlayersaswellasthe WCsubstratecanbeclearly identifiedintheSTEMimage.They exhibitdistinctlydifferentmicrostructures.Ascommonlyobserved forsputteredfilms,thePtIrlayerconsistsofveryfinecolumnar grains,growingalongthedepositiondirection.Individualgrains canbedistinguishedbydifferencesincontrast,wherethegrain sizesrangefrom50to100nm.ThethicknessoftheCrlayeris uni-form.BothPtIr/CrandCr/WCinterfacesappearflatandsharp.No voidsorcrackscanbeobserved.APTanalysesofthePtIrlayershow thattheoverallatomicratiobetweenIrandPtisaround2,matching thetargetvalue.However,periodicIr-andPt-richsublayerswere observed(seeFigs.3(c)and14(b)),whichcanbeattributedtothe combinationofthePtIrtargetand substraterotationduringthe deposition.AsillustratedinFig.1,thenon-uniformarrangement ofthePtandIrpiecesinthePtIrtargetcanleadtoan inhomo-geneousdistributionofPtand Irionsintheplasmaflux.When thesubstrateisdirectlyfacingthetarget,thecontentofPtwillbe slightlyhigher,while,whenitmovesaway,Ircontentwillincrease. Withtherotation of thesubstrate, this concentration variation willappearperiodically.Similarphenomenahavebeenreported inseveralpreviousstudies[43–45].Theatomicmapandthe1D concentrationprofileacrosstheCrlayerobtainedfromAPT mea-surement(Fig.3(c))demonstratethatCrdiffusesneitherintothe PtIrlayernorintotheWCsubstrate.Around10at.%Irand8at.%Pt arefoundtobesegregatedattheCr/WCinterface,indicatingthat duringdeposition,IrandPtrapidlydiffusedthroughtheCrlayer, buttheirpenetrationintotheWCsubstratewasrestricted.There isalsoaround10at.%WsegregatedattheCr/WCinterface,which maybeattributedtothelossofcarbonduringtheplasma activa-tionpriortocoating.WithintheCrlayer,somesmall,dispersed Cr-oxideparticlesarefound.SinceCrisafairlyactivemetal,itis difficulttoachievevacuumconditionsinsidethedeposition cham-bergoodenoughtoavoiditsoxidation.Therefore,duringsputter deposition,someCratomswillreactwithoxygenfromthe resid-ualgasandformCroxides.TheAPTanalysesoftheWCsubstrate indicatethatCandWatomsdistributehomogeneouslywhileCo, asthebinderphase,ismainlysegregatedatWCgrainboundaries (notshown).

3.1.2. After6hexposureat630◦Cunder1.12×10−23barO2 partialpressure

Comparingthe6h-exposedstatewiththeas-fabricatedstate, noobviousdifferencescanbefoundbetweenboth surfaceSEM andcross-sectionalSTEMimages(notshownhere).Thesideview atommapoftheCrlayer(Fig.4(a))alsoconﬁrmsthatnomassive diffusiontookplace.ThePtIrcoating,theCrinterlayerandtheWC substratearewellseparated.BoththePtIr/CrandCr/WCinterfaces arestillsharpandﬂat.However,asshowninFig.4(b),atthe bot-tomofthePtIrlayer,Crisobservedtosegregatetoseveralplanar features,whicharelikelytobegrainboundaries.However,the seg-regatedamountisrelativelylimited(∼1.5-3.5at.%Cr)andaround 30nmawayfromtheoriginalCrlayer,nofurthersegregationcan bedetected(Fig.4(c)).

Fig.4.3DatommapsobtainedfromthenearCrlayerregionofthePtIr/Cr/WC specimenafter6hexposureat630◦Cunder1.12×10−23barO2partialpressure:(a)

Sideview,(b)topviewofthesliceIImarkedin(a),and(c)topviewofthesliceI markedin(a).Forclarity,onlyIr(blue),Cr(cyan),O(magenta)andWions(orange) areshown,ifanyofthemaredetected.(Forinterpretationofthereferencestocolour inthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

Fig.5(a)isanSEMimageacquiredfromthesurfaceofthe 18h-exposedspecimen.Comparedtotheas-fabricatedstate(Fig.3(a)), thesurfacemorphologyhaschanged.Fig.5(b)isaBFSTEMimage obtained froma cross-sectional TEMlamella. Even though the Cr layer can still be distinguished, as indicated by the bright-nesschange,itsupperregionseemstobedissolved,resultingina wavyPtIr/Crinterface.However,theCr/WCinterfaceisstillintact andflat.Fig.6(a)showsarepresentative3Datommapobtained fromthe Crlayerregionand a corresponding magnified cross-sectionalBFSTEMimage.Tobetterillustratethedistributionof eachspecies,blue-to-redrainbowcolourconcentrationmaps(blue indicateslowestconcentrationandredindicateshighest concen-tration)areshowninFig.6(b)anda1Dconcentrationprofilefrom thePtIrlayertotheWCsubstrateisplottedinFig.6(c).Boththe APTandSTEM resultsrevealtheformationofan∼50-nm-thick interdiffusionzonebetweenthePtIrandCrlayerwhichexhibits aninhomogeneouschemicalcomposition.TheCrcontent progres-sivelydecreaseswhiletheIrcontentincreasestowardsthePtIr layer.TheCroxidesformedduringdepositioncanstillbedetected, which seemsstable and remain immobile, thereby providing a markerofthepositionoftheoriginalCrlayer.AsdisplayedinFig.6 (c),thisintermixedregionisfoundtoformonthebasisofthe out-warddiffusionofCr.Therearestillsomenon-uniform,interrupted fragmentsoftheCrlayerleftneartheCr/WCinterface.Fig.7(a)and (b)aresideandtopview3DatommapsobtainedfromthePtIrlayer, respectively. Comparedto the6h-exposed state,larger amount ofCr(∼2.6–5.9at.%Cr)segregatingatthese grain-boundary-like-features.Fig.8showsaSTEM-EDSmapandacorrespondingHAADF STEMimageofaplan-viewTEMlamella,fromwhichthegrainsize andgrainstructure ofthePtIrlayercanberevealed.SinceCris exclusivelydetectedatgrainboundaries,itcanbeconfirmedthat thefeatureswhereCrisseentosegregateintheAPTdatasetsare

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Fig.5.SurfaceSEM(a)andcross-sectionalBFSTEMimage(b)ofthePtIr/Cr/WCspecimenafter18hexposureat630◦_C_under_1.12_×₁₀−23_bar_O₂_partial_pressure.

Fig.6.RepresentativecharacterisationresultsobtainedfromthenearCrlayerregionofthePtIr/Cr/WCspecimenafter18hexposureat630◦Cunder1.12×10−23barO2

partialpressure:(a)A3Datommaptogetherwithamagnifiedcross-sectionalBFSTEMimage;(b)correspondingblue-to-redrainbowcolourconcentrationmapsofIr,Cr, CrOandWionswiththecolourscalethatblueindicatesthelowestconcentrationandredindicatesthehighestconcentration;(c)1DconcentrationprofilefromthePtIr layertotheWCsubstrate.TheinterdiffusionregionandundissolvedCrlayercanbeobserved.Forcomparison,theoriginallocationsofthePtIrandCrlayersaswellasthe WCsubstrateareshown.Atthediffusionfront,the∼20-nm-thicklayerwithalmostconstantcompositionissupposedtobeCr-richintermetalliccompound(IMC).(For interpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)

actuallygrainboundaries.Infact,thesegregationofCratPtIrgrain boundariesgivesevidenceoftheshortcircuitdiffusionprocess.No noticeableamountofoxygenoranyoxideparticlesaredetectedat thegrainboundaries.

Fig.9showstheAPTanalysisresultfromthesurfaceregion. Here,discontinuousnanometre-sizedCroxideislandsarefound. Fig.9(b)illustratestopviewblue-to-redrainbowcolour concen-trationmapsofCrfromdifferentdepthofthedatasetshownin Fig.9(a).Bluecolourindicates0at.%Crandredindicates3.5at.%

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Fig.7.3DatommapsobtainedfromthePtIrlayerofthePtIr/Cr/WCspecimenafter 18hexposureat630◦_C_under_1.12_×₁₀−23_bar_O₂_partial_pressure:_(a)_Side_view

(forclarity,Ptionsarenotshown);(b)topviewoftheregionmarkedbythe semi-transparentcubein(a).HereCrionsaredenotedwithbiggerdotstohighlighttheir segregationatgrainboundaries.

Cr.Asmarkedbythewhitearrow(Fig.9(b)III),underneaththe surface,there isagrainboundarydecoratedwithCratoms.The CratomsdiffuseviaPtIrgrainboundariesandthengetincontact withoxygenatthepositionswheregrainboundariesintersectwith thesurface.However,asFig.9(b)IandIIreveal,thenucleationofCr oxidesisnotrestrictedtothegrainboundaryarea(indicatedbythe whitedashedlines),implyingthatmetallicCratomsalsodiffused laterallyonthesurfacebeforereactingwithoxygen.Fig.10(a)isa cross-sectionalTEMimageofthesurfaceregionathigher magni-ﬁcation,inwhichthethinbrightestlayerisconsideredtobethe approximately1-nm-thickCroxide.Atthisstage,thesurfaceisnot fullycoveredbyCroxides.Fig.10(b)showstheAESsurfacemapsof Cr,IrandPt.ConsistentwiththeAPTandTEMobservations, inho-mogeneousCrsignalsweredetected.SinceAESisahighlysurface sensitivetechnique,theextensiveIrandPtsignalsconﬁrmthatthe surfaceCroxidesarethinnerthan2nm.

Fig. 11(a) is an SEM picture taken from the surface of the 168h-exposedPtIr/Cr/WCspecimen.Comparedtotheas-fabricated (Fig.3(a))andthe18h-exposedspecimens(Fig.5(a)),the 168h-exposedspecimenexhibitsadifferentsurfacemorphology.TheBF STEMimageofa cross-sectionalTEMlamella(Fig.11(b))shows

Fig.9.(a)3DatommapofthenearsurfaceregionofthePtIr/Cr/WCspecimenafter 18hexposureat630◦Cunder1.12×10−23barO2partialpressure.TinyCroxides

canbefoundonthesurface.(b)Topviewblue-to-redrainbowcolourconcentration mapsofCrfromdifferentdepthofthedatasetshownin(a),whereblueindicates 0at.%Crandredindicates3.5at.%Cr.ThewhitedashedlinesinIandIIdepictthe positionofthegrainboundarymarkedwiththewhitearrowinIII.(Forinterpretation ofthereferencestocolourinthisﬁgurelegend,thereaderisreferredtotheweb versionofthisarticle.)

thattheoriginalCrlayerhasbeencompletelydissolvedandvoids haveformed.TheCr/WCinterfacealsobecomes rough.Another noticeablefeatureisthesurfaceoxidescale,whichcanbeseen moreclearlyinthemagnifiedBFSTEMimageshowninFig.12(a). After168h,largeramountsofCratomshavereachedthetop sur-faceandcontributedtothedevelopmentofathickeroxidescale, whichisidentifiedtobeCr2O3byAES(Fig.12(b)).Besidesthese surfaceoxides,noothernewlyformedoxidesnoranymeasurable amountofoxygenaredetectedinsidethestructure(Fig.12(c)).In addition,unlikethe18h-exposedstate,notonlyCratoms,butalso WandCoatomsarefoundatgrainboundariesandtriple junc-tionsofthePtIrlayer,whichisastrongindicationthatWCis,at thisstage,alsoinvolvedinthedegradationprocess.Fig.13shows amagnifiedcross-sectionalHAADFSTEMimageoftheinterfacial regiontogetherwithrepresentativeAPTresultsfromthisregion: (b)thebottompartofthePtIrlayer,(c)thePtIr/Crinterface,and (d)theCr/WCinterface.Asmentionedabove,theembeddedsmall Cr-oxideparticlescanbeusedasatraceroftheoriginalpositionof theCrlayer.Fig.13(d)illustratesthattheremainingCrispresent onlyinaverylimitedamount.Non-uniformdamageoftheWC sub-strateoccursattheCr/WCinterface,wherethedegradationfeatures

Fig.8.(a)STEMHAADFimageofaplan-viewTEMlamellafabricatedfromthePtIr/Cr/WCspecimenafter18hexposureat630◦_C_under_1.12_×₁₀−23_bar_O₂_partial_pressure.

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Fig.10. (a)Magniﬁedcross-sectionalTEMimageofthenearsurfaceregionofthePtIr/Cr/WCspecimenafter18hexposureat630◦_C_under_1.12_×₁₀−23_bar_O₂_partial_pressure.

Athin,discontinuoussurfaceoxidescalecanbeobserved.(b)AESsurfaceelementalmapsshowingthedistributionofCr,IrandPtrespectively.Inaddition,theSEMimage ofthemeasuredareaisalsoshown.

Fig.11.SurfaceSEM(a)andcross-sectionalBFSTEMimage(b)ofthePtIr/Cr/WCspecimenafter168hexposureat630◦_C_under_1.12_×₁₀−23_bar_O₂_partial_pressure.

alonggrainboundariesaresevere.Theinterdiffusionzoneformed betweenPtIrandCrafter18hexposureevolvedfurtherintoa mix-tureofsmallCr-richandW-richregions(Fig.13(b)).Additionally,in thebottompartofthePtIrlayer,someCr-richparticlesarefoundat thegrainboundariesandWatomssegregateattheparticle/matrix interfaces.

3.2. PtIr/Ni/WCspecimen

ThesurfaceSEMandcross-sectionalSTEMimages(notshown here) of the as-fabricated PtIr/Ni/WC specimen reveal similar featuresasthemicrostructureoftheas-fabricatedPtIr/Cr/WC spec-imen. The three different regions, PtIr layer, Ni layer and WC substratecanbeclearlydistinguishedandbothPtIr/NiandNi/WC interfacesareﬂat,sharpandfreeofvoids.However,asillustratedin Fig.14,somedifferenceswerediscoveredbyAPTanalysis.Firstly, therearenoembeddedNioxideswithintheNilayer,whichcanbe attributedtothehigheroxidationresistanceofNicomparedtoCr. Secondly,evenintheas-fabricatedstate,outwarddiffusionofNi alongPtIrgrainboundarieshasalreadytakenplace,buttheextent issmallandthediffusionlengthisbelow50nm.

Fig.15summarisestherepresentativecharacterisationresults ofthePtIr/Ni/WCspecimenafter48hexposureat630◦Cunder 1.12×10−23barO2partialpressure.Thecross-sectionalBFSTEM image(Fig.15(a))showsthatthePtIr/Niinterfaceisblurredandthe originalNilayerseemstoalreadydisappear.Fig.15(b)exhibitsthe APTresultobtainedfromthenearsurfaceregionofthespecimen.Ni atomsratherthanNioxidesarefoundtoaccumulateatthesurface. WithinthePtIrlayer,theenrichmentofNiatomsatgrain bound-ariesisobvious(Fig.15(c)),revealingthattheoutwarddiffusionof Niisalsosupportedbytheshortcircuitdiffusionmechanism.The 1Dconcentrationproﬁleacrossthegrainboundary(Fig.15(c)) indi-catesthatatthisstage,onlyNiatomsaresegregatedatthegrain boundaries.Fig.15(d)summarisestheresultsobtainedfromAPT analysisofthenearNilayerregion.SimilartothePtIr/Cr/WC sam-ple,aninterdiffusionregionformed,whichcontainsNi-richand W-richzones.Eventhoughfromthecross-sectionalSTEMimage, noobviousdamagesarevisibleat theNi/WCinterface,theAPT resultsclearlysuggestthatthedegradationofWCalreadystarted. Howeveratsuchrelativelyshortexposuretime,elementsfromthe WCsubstratehavenotyetpenetratedintothePtIrlayer.

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Fig.12.RepresentativecharacterisationresultsobtainedfromthenearsurfaceregionofthePtIr/Cr/WCspecimenafter168hexposureat630◦_C_under_1.12_×₁₀−23_bar_O₂

partialpressure:(a)cross-sectionalBFSTEMimage;(b)AESpointanalysisresult(differentialspectrum)showingthatthesurfaceoxidesareCr2O3;(c)3Datommaps.Left:

sideview;right:topviewofthebottom30nmsliceofthedatasetshownontheleft,asmarkedbythesemi-transparentcube.ThesegregationofCr,WandCoatomsatthe PtIrgrainboundariesaredetected.

4. Discussion

ThedegradationevolutioninthePtIr/Cr/WCspecimendeduced fromtheexperimentalobservationsissketchedinFig.16.Similar phenomenawereobservedinthecaseofthePtIr/Ni/WCspecimen. TheonlymajordifferenceisthatwhenNiatomsdiffusethroughthe PtIrlayerandreachthesurface,theyarenotoxidised.Accordingto thermodynamics,seee.g.theEllinghamdiagram[46],at630◦C,the criticaloxygenpartialpressuretooxidiseNiis∼10−17_bar,_which issubstantially higherthantheoneappliedinourexperiments (∼1.12×10−23bar).

4.1. Outwarddiffusionoftheinterlayer

TheoutwardtransportandsegregationofCrorNiatomsalong thegrainboundariesofthePtIrlayerseemstoindicatetheonset ofthe degradationprocess. For thePtIr/Cr/WC specimen,these processestakeplaceslowlyatthebeginning.After6hexposure, onlyasmallamountofCrisdetectedatthePtIrgrainboundaries, withinadiffusiondistanceofapproximately30nm.Comparedto Cr,Nifollowsthesametransportmechanismbutatahigher dif-fusionrate.Alreadyduringcoatingdeposition,Niatomspenetrate intothePtIrlayer.Fromathermodynamicpointofview,the

gra-dientinchemical potentials oractivitiesofindividual elements acrosstheinterfacedrivestheirdiffusion.Thisgradientinchemical potential,togetherwiththeelementaldiffusivity,determinestheir diffusionbehaviour.Typically,diffusionalonggrainboundariesis 4–8ordersofmagnitudefasterthantransportwithingrain inte-riors[47],therefore,atrelativelyshorttimeandlowtemperature (typeB-diffusionregime[48,49]),grainboundariesarethemain pathwaysfortheoutwarddiffusionofatomsfromtheinterlayer. GrainboundarydiffusiondataforCrandNiinPtIrarenot avail-ableinliterature.Accordingtoourresultsoftheas-fabricatedand earlydegradationstates,itislikelythatNidiffusesfasterthanCr. However,after48h,asshowninFig.17,thewholesurfaceofthe PtIr/Cr/WCsampleiscoveredbyanoxidescalewithathickness intherangeof15–120nm.WhileforthePtIr/Ni/WCsample,even after48h,thequantityofNiatomsaccumulatingonthesurfaceis stillsmall(Fig.15(a)),whichseemstocontradictourprevious infer-encethatNidiffusesmorerapidlythanCr.Ithasbeenreportedin previousstudiesthatsurfaceoxidationcanstronglyaffectinternal materialtransport.Thefirstdiffusionstepofsolutesdecoratingthe PtIrgrainboundariesisdrivenbytheGibbsadsorptionisotherm,i.e. theinterfaceenergygetsreducedbysolutedecoration.Thiscreates adiffusionfluxalongthegrainboundariestowardsthesurface.As thediffusingspeciesreachthefilmsurface,theoxidationprocess

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Fig.13.Representativecharacterisation resultsobtained from thenear inter-face regionof thePtIr/Cr/WCspecimenafter 168hexposure at630◦_C_under

1.12×10−23_bar_O₂_partial_pressure:_(a)_{cross-sectional}_HAADF_STEM_image._(b)_On

theleft:3DatommapofthebottompartofthePtIrlayerwithiso-concentration sur-faceof1.2at.%W(orange);ontheright:correspondingblue-to-redrainbowcolour concentrationmapsofCrandW.(c)Ontheleft:3DatommapofthePtIr/Cr interfa-cialregion;ontheright:correspondingblue-to-redrainbowcolourconcentration mapsofIr,Cr,WandCrO.(d)Ontheleft:3DatommapoftheCr/WCinterfacial region;ontheright:correspondingblue-to-redrainbowcolourconcentrationmap ofIr,Cr,CrOandW.Inallconcentrationmaps,blueindicatesthelowest concentra-tionandredindicatesthehighestconcentration.Forclarity,inallatommaps,onlyIr (blue),Cr(cyan),W(orange)ionsandCroxideparticles(magenta)areshown,ifany ofthemaredetected.(Forinterpretationofthereferencestocolourinthisﬁgure legend,thereaderisreferredtothewebversionofthisarticle.)

Fig.14.(a)3DatommapofthenearNilayerregionoftheas-fabricatedPtIr/Ni/WC specimen.Forclarity,onlyIr(blue),Ni(green)andW(orange)ionsareshown.(b) 1DconcentrationproﬁleacrosstheNilayer.(Forinterpretationofthereferencesto colourinthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

startsremovingsolutesfromthematerial,particularlyfromthe decoratedgrainboundariesbecausetheygettrappedintheoxides. Thiseffect reducestheirsolute chemical potentialonthegrain boundaries,hencecreatingachemicalpotentialgradienttowards thesurface,promotingfurtheroutbounddiffusion[31,50–54].Then theoutwardtransportationofthediffusingelementbecomesthe slower,rate-controllingstep[55].AsforthePtIr/Ni/WCsample, theexposureatmosphere isnon-oxidising.Thereisno substan-tialattractiveforcetofurtherpromotethespreadingandlateral diffusionofNiatomsontothePtIrsurface,especiallyonceaﬁrst surfacecoverageisreached.Undersuchconditions,Niatomswill bemostly trappedalong thegrainboundariesof thePtIrlayer. Therefore,asillustratedinFig.15(c),signiﬁcantenrichmentsofNi (∼30at.%)withbroadsegregationzones(>10nm)arefrequently foundatnumerousPtIrgrainboundaries.Whentheyaresaturated, therewillbenofurtherthermodynamicdrivingforceforfurther segregation.

4.2. Formationofintermetallicphaseandevolutionofthe interfacialregion

Sincethereisnosubstantialdifferenceintheinternal degra-dation processes between the PtIr/Cr/WC and the PtIr/Ni/WC specimen,inthis and thefollowingsubsections,thePtIr/Cr/WC systemwillbetaken asrepresentative.After168hexposureat 630◦C under1.12×10−23barO2 partialpressure,Cr-rich parti-clesappearatPtIrgrainboundaries(Fig.13(b))aswellasinside theinterdiffusionregion(Fig.13(c)).APTanalysisrevealsthatthe chemical compositionsof alltheseCr-rich particlesare similar, namelyapproximately56at.%Ir,17at.%Ptand27at.%Cr. Stud-ies aboutthe Pt-Ir-Cr ternary system are rare in theliterature andknowingthechemicalcompositionoftheprecipitatesaloneis insufficienttodrawanunequivocalconclusionabouttheassociated reactionandphasetransformationpathways.Thecompositional similarityobservedisnonethelessastrongindicationthatallthese particlesbelongtothesametypeofintermetallicphase.Moreover, after168hexposure,localequilibriabetweenPtIrandthenewly formedintermetalliccompound particles aremostlikely estab-lished,which meansthatthis intermetallicphase, denotedhere as(Pt,Ir)3Cr,isthefirststableintermetallicphasefromthe PtIr-richsidetotheCr-richside,yet,withcomparablyhighPtIrcontent andlowCrcontent.After18h,volumediffusioninducedchanges becomesubstantial.Awell-definedinterdiffusionzone between thePtIrandCrlayerappears(Fig.6)andCrisproventobethe dominantdiffusingspecies.However,surprisingly,theCrcontent withinthewholeregionismuchhigherthan27at.%.Additionally, asnotedfromtheconcentrationprofile,inthediffusionfront,there isan∼20-nm-thickthinlayerwithalmostconstantcomposition (markedinFig.6(c)),namelyapproximately43at.%Ir,17at.%Pt and40at.%Cr.Itisthusverylikelythatanothertypeof intermetal-licphasehasformed,denotedhereas(Pt,Ir)3Cr2,whichshouldbe closertotheCr-richsideinthephasediagram.

The evolution fromthe Cr-richto thePtIr-rich intermetallic compoundcanbeexplainedbythefollowingmechanism.Firstly, theintermetallicphasewiththelowesteffectiveinterfacial reac-tionbarrier,e.g.(Pt,Ir)3Cr2,nucleatesatthePtIr/Crinterface.With ongoingexposure,theCrinterlayeriscontinuouslyconsumedby thegrowthoftheseintermetalliccompoundnucleiaswellasthe formationofsurfaceoxidescale.WhentheoriginalCrlayerhas beenfullyconsumed,theCractivitywilldropandtheformation of(Pt,Ir)3Cr2willstop.Subsequently,thechemicalpotential gradi-entacrossthePtIr/(Pt,Ir)3Cr2interfacewilldrivePtIrtodiffuseinto (Pt,Ir)3Cr2 andCrmaydiffuseout.Afterashorttransitionperiod, thesecondintermetallicphase,i.e.(Pt,Ir)3Cr,withhigherPtIr con-tentwillformandgrow.Correspondingly,(Pt,Ir)3Cr2 willshrink andevendisappear.Thisphenomenon,termedassupply

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limita-Fig.15.RepresentativeanalysisresultsobtainedfromthePtIr/Ni/WCspecimenafter48hexposureat630◦_C_under_1.12_×₁₀−23_bar_O₂_partial_pressure:_(a)_{cross-sectional}

BFSTEMimage.(b)3Datommapofthesurfaceregion.(c)Ontheleft:3DatommapofthePtIrlayerwithiso-concentrationsurfaceof15at.%Ni(green);ontheright:1D concentrationproﬁlewithinthecylinderhighlightedintheatommapalongthedirectionmarkedwiththeredarrow.(d)ontheleft:3DatommapofthenearNilayer region;ontheright:correspondingblue-to-redrainbowcolourconcentrationmapsofIr,Ni,andWwiththecolourscalethatblueindicatesthelowestconcentrationand redindicatesthehighestconcentration.Forclarity,inallatommaps,onlyIr(blue),Ni(green)andW(orange)ionsareshown,ifanyofthemaredetected.(Forinterpretation ofthereferencestocolourinthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

Fig.16. SchematicillustrationoftheevolutionofdegradationofthePtIr/Cr/WCsystemduringisothermalexposureat630◦Cunderawell-controlledatmosphere(N2-2.5%H2

with+20◦Cdewpoint,PO2≈1.12×10−23bar).

Fig.17.Cross-sectionalBFSTEMimageofthenearsurfaceregionofthePtIr/Cr/WCspecimenafter48hexposureat630◦_C_under_1.12_×₁₀−23_bar_O₂_partial_pressure._The

edgeofthesurfaceoxidescaleishighlightedusingreddottedline.(Forinterpretationofthereferencestocolourinthisﬁgurelegend,thereaderisreferredtotheweb versionofthisarticle.)

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Fig.18. APTresulted1Dconcentrationproﬁleandacorrespondingatommapacross theinterfacebetweenaCr-richandaW-richintermetalliccompound(IMC).For clarity,onlyCr(cyan)andWions(orange)areshown.(Forinterpretationofthe referencestocolourinthisﬁgurelegend,thereaderisreferredtothewebversion ofthisarticle.)

tioninducedsecondphaseformation,hasbeenreportedinvarious previousstudies[11,56–63].

Atalaterstage,theWCsubstratealsogetsinvolvedinthe reac-tiveinterdiffusionprocess,givingrisetotheformationofaW-rich intermetalliccompound(Fig.13).Fig.18showsa1Dconcentration proﬁlefromaCr-richtoaW-richparticle.ThesolubilityofCrinthe W-richparticlesisfoundtobeverylimited,implyingthatCratoms havebeensubstitutedbyWatomstoformaW-richphase. Actu-ally,theCr-richintermetallicphaseistheproductofthereaction betweenPtIrandCrlayer.AfterthefullconsumptionoftheCrlayer, WCbecomesthenewendmember.Forthissystem,i.e.PtIr-WC, theCr-richintermetallicsshouldbeunstablephases,whichwill begraduallyreplacedbythermodynamicallymorestablephases, suchastheobservedW-richphase.Inadditiontotheintermetallic compounds,achainofnanometre-sizedvoidsarealsoformedin theinterfacialregion(Fig.13).Thisphenomenonisattributedto theKirkendalleffect,whereanimbalanceinmasstransportdueto differentdiffusionratescreatesfreevolumeandhencevoidsare formed.Morespeciﬁc,inthepresentcasewhereCristhe dominat-ingdiffusingspecies,duringitsoutwarddiffusion,vacanciesmove intheoppositedirection.Theseextravacanciesmaycondensate furtherintovoids[9,64–66],whichmayplayanimportantrolein theonsetofdegradationoftheWCsubstrate.

4.3. Dissolutionofthetungstencarbide

ThedissolutionofWCoccurringduringthelaterstagesof degra-dationfirstrequiresthedecompositionofthecompound,especially considering that only W atoms, rather than WC entities, were involvedinthereactiveinterdiffusionprocess.SincenoCatoms werefoundwithinthesurfacecoating,apossibilityisthatWCfirst reactedwithoxygen,resultingin elementaltungstenaswellas gaseousspeciessuchasCOandCO2,whichformedattheexpense ofthefreecarbonatoms,andfinallyleakedintotheatmosphere [67,68].Asforthematerialwestudiedhere,WChasnodirect con-tactwithair,thepre-requisiteisthatoxygenpenetratethrough thecoating.HoweveraccordingtotheAPTresults,nosubstantial amountsofoxygenweredetected(Fig.7(b),Fig.12(c)).Apossible reasoncouldbethatafterexposure,specimensarecooleddownto

roomtemperatureslowly(coolingrate∼5◦_C/min)._During_such_a slowcoolingprocedure,gasesincludingO2,COandCO2mayescape forinstanceviadiffusingalongthegrainboundaries.Another pos-sibilityisthatthesegaseousphasesmaybetrappedintheobserved voidsattheinterfacialregionandpromotefurthervoidnucleation andgrowth[69,70].

APTreliesonthedetectionofindividualionswhichare suc-cessivelyfieldevaporatedfromasharp,needle-shapedspecimen andidentificationofthemusingtime-of-flight(TOF)mass spec-trometry.Quantitativechemicalinformationisnormallyobtained throughcountingtheionswithinthepeaksoftheresultingmass spectrum,whichisahistogramshowingthenumberofthedetected ionsasafunctionofmass-to-chargeratio,i.e.TOFvalue[71–73]. Therefore,theaccuracyoftheresultswillbeinfluencedbyerrors forexampleinpeakrecognition,peakdeconvolution,mass rang-ingandbackgroundsubtraction[74,75].Insomecases,minorpeaks maybeburiedunderbackgroundduetotheirlowintensity.Inthis study,thepossibleimportantsimpleormolecularionspeciesthat containoxygenareO+_,_O

2+,CrO2+,CrO+,CrO22+,CrO2+andCrO3+. Theoverallnoiselevelatthesepositionsisevaluatedtofallinto therangeof0.18–0.84at.%,dependingonthedataset.Therefore, iftheoxygencontentisbelow_∼1at.%,itwouldnotnecessarilybe properlyquantiﬁed.Indeed,sinceCrisnotfoundtobeoxidised internallyatleastuptothis stage,theactivityofoxygenwithin thematerialinteriorshouldbebelowthecriticalpartialpressure of Croxidation. AccordingtotheEllingham diagram, at630◦C, thecriticaloxygenpartialpressureforCroxidationis∼10−34_bar (Cr→Cr2O3),andforWoxidationis∼2.2×10−25bar(W→WO2) [46].IfCrhasnotbeenoxidisedinternally,itisnotpossibleforWto getoxidised.However,ifcoatingspallationtakesplace,andtheWC substrateisindirectcontactwithair,orincasethatthemetallic tungstenatomsdiffusetothesurface,wherehigheroxygenpartial pressureprevails,oxidationoftungstenwilltakeplace.

5. Conclusion

We investigatedthestability and degradationevolution ofa bimetalliclayercoating(PtIr/CrandPtIr/Ni)oncementedtungsten carbidesubstrateduringisothermalexposuretreatmentat630◦C underawell-controlledatmosphere(N2-2.5%H2with+20◦Cdew point,PO2≈1.12×10−23bar).Fromourexperimentalresults,the followingconclusionscanbedrawn:

(1)The transportand migration ofdifferentatomic speciescan be traced by APT.Combining theAPT results withdetailed microstructureanalysisusingadvanced electronmicroscopy suchasSTEMprovidesadditionallynewinsightsintothe degra-dationprocessinmultilayerthinﬁlmsorcoatings.

(2)Bothsystemsstudied,i.e.PtIr/Cr/WCandPtIr/Ni/WC,are unsta-bleundertheexposureconditions.CrorNiwereobservedto diffusethroughPtIrgrainboundariestothesurface,whereCr gotoxidisedselectively,whileNiremainedmetallic.Internally, all threeregions, i.e.PtIr,Cr(Ni)and WC,wereinvolved in complexreactiveinterdiffusionprocesses,givingriseto inter-metallicparticlesandvoidsattheinterfacialregion.

(3)ThenanocrystallinecolumnargrainstructureofthePtIrcoating isakeyfactorlimitingthestabilityofthesystem,asitprovides numerousfastandshortdiffusionpathways.

(4)BycomparingthePtIr/Cr/WCtothePtIr/Ni/WCsamples,itis possibletoinferthatsurfaceoxidationpromotesthe replenish-ingdiffusionofsoluteCrtowardssurfaceatthelaterexposure stages. Initially, for both systems, segregation to the grain boundaries is thedominating drivingforce forthe outward diffusion,whichoccursfasterforNithanforCr.

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Acknowledgements

Theauthorsaregratefulfortheﬁnancialsupportfromthe Ini-tialWearproject, fundedby theMax-Planck-Gesellschaft(MPG) andtheFraunhofer-Gesellschaft.TheexposuretreatmentsandAES measurementswere conductedby AlexandraVogel, we greatly appreciateher help. Theauthors arethankful to UweTezins & AndreasSturmfortheirsupportoftheAPT&FIBfacilitiesatMPIE. AleksanderKostka(Ruhr-UniversitätBochum),MinjieLai,Volker Kreeand ChristianLiebscher arethankedfor theirvitalhelp in (S)TEMexperiments.

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