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Response of a molybdenum alloy to plasmanitridingZhang, Zhenxue; Li, Xiaoying; Dong, Hanshan

DOI:10.1016/j.ijrmhm.2018.01.014

License:Creative Commons: Attribution-NonCommercial-NoDerivs (CC BY-NC-ND)

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Citation for published version (Harvard):Zhang, Z, Li, X & Dong, H 2018, 'Response of a molybdenum alloy to plasma nitriding', International Journal ofRefractory Metals and Hard Materials, vol. 72, pp. 388-395. https://doi.org/10.1016/j.ijrmhm.2018.01.014

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1.1IntroductionWhensinteringhigh-strengthmaterialsathightemperatures,themajortechnicalproblemsassociatedwithtraditionalgraphitediesinanelectric-field-activatedsinteringtechnology(FAST)process,aretheirprematurefailureorshortlifespanrelatedtothelow

mechanicalstrengthatelevatedtemperatures.Thereforetools(dies/punches)havetobemadefromsuperalloys,refractorymetalalloysorevenceramics[1].Titanium-Zirconium-Molybdenum(TZM)alloy isagreatchoicedueto itsstabilityandstrengthatelevated

temperaturesupto1500 °C.Inaddition,TZMexhibitsgoodthermalconductivity,highelectricalconductivityandlowcoefficientofthermalexpansionwhichmakethematerialsresistanttothermalshockandcrackingarisenwhenthetoolsurfaceexperiencingcyclesof

rapidheatingandcooling[2].Aftersintering,theceramicmicro-partsneedtobepushedtoslideoutthemouldoff-siteoron-sitedependingontheproductionsystemsetupandthroughput.However,whenthesizeofthetoolsandcomponentsisscaleddowntomillimetre-

scale,solderingduetosevereadhesionbetweenthemouldandtheworkingmaterialwouldoccurandthusdemouldingbecomesamajorchallengeforMicro-FASTprocess.

InourearlierworkontheslidingtestbetweenTZMandtheceramicmaterials(i.e.Al2O3andSi3N4),itwasfoundthatthefrictionwashighandhadanadhesivecharacterbetweenTZMandAl2O3counterpartballwhichwasnotbeneficialforasmoothdemoulding,

andthussomesurfacemodificationsorcoatingswereneededtoimprovethesurfacetribologicalproperties[3].Manyattemptshavebeenmadetoimprovehot-workingtools,suchassurfacewelding,thermalspraying,electro-depositing,diffusiontreatmentsandthermal

chemicaltreatmentlikenitridingandcarburisingetc.Nagaeandhiscolleaguesnitridedpuremolybdenumat1100 °CinNH3gasfor16 hoursh,andreportedamolybdenumnitridesurfacelayerformationwhichconsistsofaγ-Mo2Nouterlayerandaβ-Mo2Ninnerlayer[4].

However,Martinez’'sworkfoundthatnonitrogenabsorptioncouldhappenunder800 °Cinmolybdenumandthereactionstartedbetween800‐and1500 °Cinagasnitridingwithforminggases(N2 + H2).HeattributedtheslownitridingofTZMtotheexistenceoftitanium,

whichisastrongnitrideformingelement[5].Itiswidelyknownthatthetitaniumalloysalsoneedahightemperature(700‐–1000 °C)tobeeffectivelynitrided[6].High-temperaturetreatmentsgenerallyledtorecrystallizationofthedeformedmolybdenum,andtheforming

gases,i.e.ammonia,embrittledthematerial[5],thereforetheresearchonnitridingofmolybdenumstalledforalongtime.Recently,itwasreportedthatmolybdenumcanbenitridedatamuchlowertemperaturewiththehelpofplasma.Jauberteauandhiscolleagues

foundthatadepositedmolybdenumlayercouldbenitridedat600 °Cinagasmixtureofargon,nitrogenandhydrogenbyusinganexpandingmicrowaveplasmareactorforanexposuretimeof20 minutesmin[7].TheyalsoclaimedthatMo–Nphasescouldbeformedin

molybdenumfilmsevenatatemperatureaslowas400 °C,althoughtheelectronbeamevaporationdepositedmolybdenumlayerwasverythin(400‐–600 nm)[8].Theirresearchesshedsomelightonplasmanitridingofmolybdenum-basedalloysatlowertemperature,

whichhasbeenwidelyreportedtoincreasethesurfacehardnessaswellasthewearandcorrosionresistanceofhot-workingtools[9].Thereforeitnecessitatesthisresearchonplasmanitridingthemolybdenumalloyatalowertemperature(<800 °C)tochallengethe

previoushightemperaturegasnitriding.

Inthecurrentresearch,DCplasmanitridingofTZMalloybetween500‐and760 °Cwasdesignedtoinvestigatetheresponseofmolybdenum-basedTZMalloytothetreatmenttemperatureandtime.Thefrictionandwearbehaviourofthenitridedanduntreated

TZMsampleswerecomparedbyreciprocatingslidingtestsagainstAl2O3atroomtemperatureandunidirectionalslidingatelevatedtemperaturetosimulatethedemouldingofmicro-ceramicsinteringpartsoff-siteoron-site.

Responseofamolybdenumalloytoplasmanitriding

ZhenxueZhang⁎

zhzhxue@yahoo.com

XiaoyingLi

HanshanDong

SchoolofMetallurgyandMaterials,UniversityofBirmingham,BirminghamB152TT,UK

⁎Correspondingauthor.

Abstract

Amolybdenumalloy(TZM:0.50 wt%titanium,0.08 wt%zirconiumand0.02 wt%carbonwithbalancedmolybdenum)wasusedtoinvestigateitsresponsetoplasmanitridingintermsoflayerformationandhardeningattemperaturesbetween500and

760 °Cusingvarioustimes.Athinhardnitridecaseplusashallowdiffusionzonewasformedafterplasmanitriding.Thethicknessofnitridelayerincreasedwiththetreatmenttemperatureandtime.Thenitridedsurfaceofmolybdenumalloyhadalowcoefficient

offrictionagainsttheAl2O3counterpartballatambientatmosphereandelevatedtemperatures,andthewearresistanceofthesurfacewasgreatly improved.Molybdenumhadatendencytoreactwithnitrogentoformamixednitridephasesatthetested

temperaturerange,andtheTZMalloyhadanactivationenergyof330 kJ/mol.

Keywords:Plasmanitriding;Molybdenum;TZM;Friction;Al2O3

2.2Experimental2.1.2.1Designoftheplasmanitridingprocesses

Amolybdenum-basedTZMalloybarprovidedbyEdfaganEuropeInc.wasusedinthisstudy.Ithasadensityof10.22 g·cm‐−3andchemicalcompositionsof0.50%titanium,0.08%zirconiumand0.02%carbon(inwt%)withbalancedmolybdenum.Couponsof

Φ25 × 4.5 mmweresectionedandwetgroundwithSiCpaperdownto1200grit,followedbyprogressivepolishingwith9,6and1 μmdiamondpastetogenerateamirror-likefinish.

Plasmanitridingwascarriedoutina60 kWKlöchnerDCplasmafurnaceatapressureof4 mbar.ADCvoltagefrom300to1000 Vwasappliedbetweenthesample(cathode)andthewallofthefurnace(anode)duringtheprocess.Thetreatmenttemperature

wasmeasuredwithathermocoupleinsertedinaholeinthejig.Temperatureanddurationincreasedgraduallyuntilaclearnitridinglayerwasobserved(660/25).13batchesofsampleswereplasmanitridedwithagasmixtureof25%N2and75%H2attemperatures

rangingfrom500to760 °Cforvariedtimes.DetailedtreatmentconditionsandthecorrespondingsamplecodesarelistedinTable1.

Table1.Table1Detailedplasmanitridingconditionsandthecorrespondingsamplecodes.

alt-text:Table1

Samplecode Temperature(°C) Duration(hour)

500/5 500 5

550/10 550 10

600/20 600 20

660/25 660 25

720/12.5 720 12.5

720/25 25

720/37.5 37.5

720/50 50

720/62.5 62.5

720/75 75

760/8 760 8.33(500 minutesmin)

760/16 16.67(1000 minutesmin)

760/25 25(1500 minutesmin)

Untreated ‐– ‐–

2.2.2.2CharacteriszationofthesurfacelayersThesurfacemorphologiesandmicrostructureoftheplasmanitridedspecimenswereobservedunderscanningelectronmicroscopy(JEOL7000SEM).ThesurfaceroughnesswasassessedbyaXP-200PlusStylus3D-profilometer.Thephaseconstitutionofthe

plasmanitridedlayerswasidentifiedbyX-raydiffraction(PhilipsX'pertX-Raydiffractometer)usingaCu-Kαradiation(λ = 0.154 nm).Thechemicalcompositionsofthelayerswereanalysedusingaglowdischargespectroscopy(GDS,LECOGDS-750QDP),whichallowed

forcontinuousdepthprofilingforMoandN.Metallographicallypreparedcross-sectionsampleswithoutetchingwereusedfornano-hardnessprofilingusinganano-indenter(MicroMaterialsLtd.)underaloadof20 mN.Thesurfacemicro-hardnesswasmeasuredbya

Vickersmicrohardnesstester(MitutoyoMVK-H1)underaloadof25gf.SpecimenswereelectricallyetchedinaphosphatesolutionforlayerstructureobservationbyJEOL7000SEMequippedwithanEnergyDispersionX-ray(EDX)forthelocalisedcompositionanalysis.

AnX-rayphotoelectronspectroscopy(XPS)wasusedtoanalysetheelementalvalencesonthenitridedsurface.

2.3.2.3EvaluationofthefrictionandwearpropertiesofthesurfacelayersFrictionandwearpropertiesoftheuntreatedandtreatedsampleswereassessedviaaTE79multi-axistribologymachine(Compend2000Version2.3.1)inreciprocatingmodeatroomtemperaturewitha8 mmindiameterAl2O3ballasthecounterpartunderthe

loadsof2,5,10and20 Nrespectively.Thewholetribo-testerwasenclosedinachamberwithtransparentwallstoavoiddustandairturbulence.Thereciprocatingslidingweartestswereconductedinair,innitrogenbynitrogenpurging,indistilledwaterandinsimulated

seawatercontaining3%NaClbyfillingtheliquidintothecontainer.Thereciprocatingslidingdistancewassetto5 mm,theslidingspeedwas10 mm/s,andthecoefficientoffriction(CoF)wasrecordedfor1000 cycles.Eachreciprocatingtestwasrepeated2‐–3timesuntil

astablecurvewasobtained,andonlythepositiveside(onedirection)wasplottedasthevalueofCoFwassymmetrical.

UnidirectionalslidingweartestswereconductedusingtheCSMHTpin-on-disctribo-testerunderaloadof2 NwithanAl2O3ballof6 mmindiameterataunidirectionalslidingspeedof10 cm/sfor5000 cyclesatthreedifferenttemperatures:roomtemperature

(20 °C),300 °Cand600 °C.

TheweartracksofthesamplesweremeasuredutilisingaXP-PlusStylusProfilometerandthewearvolumeswerecalculatedaccordingly.ThemorphologyoftheweartrackswasobservedunderSEMandthecompositionofthetrackswasanalysedbyEDX.

3.3Results3.1.3.1Surfacemorphologyandroughness

Theoriginalshiningmetalliccolourofthespecimenswaschangedtovaryingbandsofgreyishafterplasmanitridingtreatmentsat720 °Corunder,andatypicaloneisshowninFigure.1a(720/25).Thesurfacecolourturnedintodarkgreywithextendedtreatment

timelike720/50and720/75.At760 °C,thespecimensurfacesweredarkandgreyandlackofmetallustreafter8.3-hourtreatmentandturnedtotallybrownishafter25-hourtreatment(Figure.1aright&Figure.1b).

Thesurfaceroughnesswasmeasuredacrossa0.5 mmlengthandthetypicalRavalueofapolisheduntreatedTZMsurfaceisabout0.01 μm.TheRavaluewasabout0.05 μmafterplasmanitridingtreatmentof660/25and720/25asseeninFigure.2.The

surfaceroughnesswasrapidlyincreasedfortreatmenttimelongerthan37.5 hat720 °C.Whentreatedat760 °Cfor25 hoursh(760/25),thesurfacebecameveryroughwithaRavalueof0.1933.Generally,longertreatmenttimeandhighertreatmenttemperatureledtoa

roughersurface.

3.2.3.2SurfacelayerstructuresCross-sectionalSEMobservationsonallthenitridedsamplesrevealedthatadensesurfacelayerwithsomespikesalongcertaindirectionswasformed(Figures.3a&b).Therehardlyanydiffusionzonecanbediscernedunderthehardenedlayer.Forthesamples

Figure1.Fig.1Surfacemorphologychangesafterplasmanitriding:(a)as-PNedsamplesand(b)sample760/25surfaceunderSEM.

alt-text:Fig.1

Figure2.Fig.2Surfaceroughnesschangesafterdifferentplasmanitridingtreatments.

alt-text:Fig.2

treatedatthehighertemperature(760/25),threelayersinthenitridecasecanbedistinguishedunderbackscatteringelectronimageasdenotedbydashedlinesinFigure.3c.Thesuperficiallayerisverythinandirregular,andthesecondandthirdlayersareinterlocked

eachotherbutwithdarkandlightcontrastrespectively.

3.3.3.3NitrogendistributionafterplasmanitridingtreatmentThetypicalelementaldistributiontrendsofnitrogenofthesamplestreatedat720 °CagainsttreatmenttimeareshowninFigure.4.Thedepthofthenitrogenrichlayerincreasedwiththeprogressofthetreatmenttime.Fora12.5-hourtreatment,thenitrogen

contentdecreasesfasterinthenearsurfacearea(upto0.2 μmindepth)toanintermediatelevel(dashedline)andthendecreasedrapidlyagainfrom0.5to0.75 μmtozero.Fora25-hourtreatment(solidline),therearealsothreegradientsofchangeforthenitrogen

content:averyhighgradientofdescentinthenearsurfaceareaupto0.05 μm,arelativelylowslopefrom0.05‐–0.7 μm,andanotheraccelerateddeclinefrom0.7to1.6 μmtonearlynothing.Fora50-hourtreatment(dottedline),apartfromasharpdropintheverynear

surfaceregion,thenitrogencontentcomesdownalmostsmoothlyupto3 μm.OxygenhasbeendetectedinthenearsurfaceareawhichisalsoidentifiedbyEDX.Forsample760/25,nitrogencontentpeaksatthesurfaceanddecreasesgraduallyinthenearsurface

regiontoarelativelevelareato2‐–3 μmdeep,thenthereisanaccelerateddownwardtrendtillzeroatabout4 μmdepthwhichrepresentsthelevelinthesubstrate.

3.4.3.4SurfacephaseconstitutionsandchemicalinformationofthenitridedlayersTheXRDpatternsofthesamplesplasmanitridedat660,720and760 °Cfor25 hourshinagasmixtureof25%N2 + 75%H2andtheas-receivedTZMsampleareshowninFigure.5a.Thetypicalbccstructurephaseofmolybdenumwasidentifiedfortheuntreated

sample,asdenotedbyacrossmarkintheXRDpatterns.Comparedwiththeuntreatedsample,theintensitiesofthebccmolybdenumpeaksforalltheplasmanitridedsampleswerereducedwiththeelevatingtemperatureindicatinganincreasednitridelayerthickness.

TwonitridephasesofMoN(ASTM01-089-4318)andMo2N(ASTM00-025-1366)weredetectedforallthenitridedsamples.Themajorpeaksoffccγ-Mo2N(circles)canbeclearlyidentifiedforsample660/25:(111),(200)and(220)correspondingto2θ = 37.377°,43.451°

and63.108°respectively(Figure.5a).Inthemeantime,peaksofhcpδ-MoN(triangles)canalsobeidentifiedwiththestrongestpeakof(022)correspondingto2θ = 49.008°.Forsample720/25,therelativeintensitiesofγ-Mo2Nphaseespecially(111)and(220)arestronger

whilethepeaksofδ-MoNarereducedandtherelativeintensityisweakened(Figure.5a).However,forashorttimetreatmentat720 °C(720/12),afewmorepeaksofδ-MoNphasecanbeidentified,suchas(002)and(004)correspondingto2θ = 31.89and66.656°.Fora

longertreatment(sample720/75),apartfromstrongerγ-Mo2Nphase,morepeaksofδ-MoNphasecanbeidentifiedwithstrongestpeaksof(022)and(002)at2θ = 31.89and49.008°respectively.Fortreatmentsat760 °C,withtheincreasingtreatmenttime,therelative

Figure3.Fig.3Layerstructuresofplasmanitridedsamples:(a)660/25(SEIimage),(b)720/25(SEIimage)and(c)760/25(BEIimage).

alt-text:Fig.3

Figure4.Fig.4Nitrogendistributionagainstthedepthofsample760/25andsamplestreatedat720 °Cfordifferenttimes.

alt-text:Fig.4

intensitiesofδ-MoNphaseswerestrongerandmorepeakscanbeidentified,however,threemajorγ-Mo2NpeaksareallweakenedasseeninFigure.5b.Thereisnoclearmolybdenumoxidecanbeidentifiedonthesurfaceofthesamplesaftershorttreatment(760/8and

760/16),someweakpeaksofoxidescanbeidentifiedforsample760/25.

Figure.6displaystheX-rayphotoelectronspectroscopy(XPS)characterizationofplasmanitridedsamples.TheMo3d5/2peakisat228.68forthesample720/25whichisclosetothereportedMo2+bindingenergyintheMo2Nnitride[10],anditshiftsslightlyto

228.78afterthesurfacewaspolishedawayaverythinsurfacetoplayer(720/25P)indicatingastrongerbond.TheMo3d5/2peakshiftstowardsahigherbindingenergy228.98butlessthanthereported230.0forMo4+(MoN)withincreasingtreatmenttemperatureand

time(760/16and760/25P)suggestinghighvalenceMospecies>2oramixtureofvalences.Peakof231.88and231.98(720/25and720/25P)isclosetothereportMo5+bindingenergyinthenitride,whilepeakof232.08(760/16)and232.28(760/25P)ismorelikely

relatedtoMo6+(Mo2N3).PeakofN1 sis397.78atthesurfaceof720/25andincreasesslightlyto397.88atadeeperlayer(720/25P),itincreasesto397.98for760/16butdecreasesslightlyto397.18forsample760/25eventheverysurfacelayerwaspolishedaway

(760/25P)whichsuggestingthenitridewasslightlyoxidised.Therewasnopeakinthebindingenergyareaoftitaniumandzirconiumwhichsuggestingnotitanium/zirconiumnitridewasformedatthesurfaceand/orthedeeplayerofthenitridecaseoftheabovementioned

samples.

Figure5.Fig.5Phaseconstituentchangeswith(a)temperature(25 h)and(b)treatmenttime(760 °C).

alt-text:Fig.5

3.5.3.5HardnessofthesurfacelayeranddepthprofileAsshowninFigure.7a,forthesametreatmenttime(25 h),thesurfaceisharderwithincreasingtreatmenttemperature(660,720and760 °C).Atthesametreatmenttemperature(720 °C),thesurfacehardnessincreasedwiththeextendedtreatmenttime.The

nano-hardnessplottedagainstthedepthisshowninFigure.7bandahardlayercanbeseeninallsamplesafterplasmanitridingandthehigherthetreatmenttemperature,theharderthecaseandthenearsurfacezonewhichissimilartothesurfacemicrohardness

measurementasshowninFigure.4Fig.7a.

3.6.3.6TribologicalpropertiesofthesurfacelayersFigure.8showsthechangeofcoefficientoffriction(CoF)oftheplasmanitridedTZMsample(720/25)againstmovingcycleswithAl2O3counterpartballatdifferentloads(5 N,10 Nand20 N)intheaircomparedwiththeuntreatedsample(5 N).Underaloadof5 N

withanAl2O3counterpartball,thecalculatedmaximumHertziancontactpressurewasabout1300 MPafortheuntreatedTZMsample.AscanbeseeninFigure.8,theCoFoftheuntreatedTZMsamplewashighandunstableatthebeginningofthetestandthengradually

reducedtoabout0.45attheendofthetest.Fortheplasmatreatedsample(720/25),underaloadof5 N,thecalculatedmaximumHertziancontactpressureincreasedtomorethan>1400 MPaduetoahigherYoung’'smodulusandlowerPoissonratioofmolybdenum

nitrides.TheCoFrevealedaverylowvalueof0.08atthebeginningandincreasedsteadilytoaround0.25afterabout150 cycles(Figure.8).When10 Nand20 Nloadswereapplied,themaximumHertziancontactpressureincreasedtomorethan>2000 MPa,andthe

CoFsincreasedataveryslowpacetoabout0.30attheendofthetest.OveralltheCoFwasunder0.3whichwasmuchlowerthanthevaluefortheuntreatedTZMsample.Itwasfoundthatthechangeofcoefficientoffriction(CoF)showedasimilartrendforother

Figure6Fig.6X-rayphotoelectronspectroscopy(XPS)characterizationofthesurfaceofplasmanitridedsamples(720/25Pand760/25Parethesamesamplesof720/25and760/25withaslightlypolishedsurfacetoremovethesuperficiallayer).

alt-text:Fig.6

Figure7.Fig.7(a)Surfacemicro-hardnesschanges;(b)Nano-hardnessprofilealongthedepthofthesamplesafternitridingtreatment.

alt-text:Fig.7

plasmanitridedsampleswhentestedunderaloadof2 N.

Postfrictiontestweartrackswereplottedin2-DprofilesasshowninFigure.9.Aclear,deepconcaveweartrackwaswornonuntreatedsamplebyanAl2O3counterpartballunderaloadof5 N.Fortheplasmanitridedsample(720/25),theweartrackswerevery

shallowandquitesmoothevenunderaloadof20 N.SEMobservationsrevealedthatsomeflakypieceswerepeeledoffandthensmearedonthebottomoftheweartrackfortheuntreatedsamplesandpatchofdebriswasstuckonthewearsurfaceoftheAl2O3ball[3]

(Figure.8rightsideinsetpicture).However,thisflakyfeaturewasnotobservedforthetrackofplasmanitridedsample(720/25)testedunderthesameloadandhigherloads(10 Nand20 N).Sometransversecrackswereobservedatthebottomofthetracktestedunder

20 NandminorscratchmarkscanbeseenontheAl2O3counterpartball(Figure.9leftsideinsetpicture).EDXanalysisatdifferentspotsoftheweartracksrevealedthatthelightandthedarkbitinthecrackedareawashighinoxygenandnitrogenindicatingtheareawas

partiallyoxidisedduringtribo-test.

Unidirectionalpin-on-discfrictiontestsatelevatedtemperaturesofuntreatedandplasmanitridedsamplesarecomparedinFigure.10.Whentestedat300 °C,theCoFofuntreatedsampleincreasedgraduallyfrom0.65toabove0.9at2400 cyclesuntiltheendof

thetest(Figure.10).TheinitialfrictionswerelowforthePNtreatedsamples,andtheCoFincreasedrapidlyto0.57atcycle333,anditthendecreasedslowlyto0.48atcycle1067andclimbedupsteadilyandslowlyto0.67tilltheendofthetribo-test.TheCoFchanges

againstthecycleoftheotherplasmanitridedsamplesshowedasimilartrend.

Figure8.Fig.8CoefficientoffrictionforuntreatedandPNtreated(720/25)samplesmeasuredunderdifferentloads.

alt-text:Fig.8

Figure9.Fig.9WeartracksfortheuntreatedandPNtreated(720/25)samplesatdifferentloadsandtheSEMimagesoftheweartrackofuntreatedsampleunderaloadof5 NandthescarontheAl2O3ball,andtheweartrackofthePNtreatedsample(720/25)underaloadof20 NandthescarontheAl2O3ball.

alt-text:Fig.9

Interestingly,whentestedat600 °C,apartfromtheshortrunning-inprocess,theCoFsofPNtreatedanduntreatedsampleswereallfluctuatingatarelativelylowvalueof0.5duringthewholetribo-testcycles(Figure.10).SEMobservationsonthetestedtrack

revealedthatthesurfaceofthesamplewerealloxidisedat600 °Cduetothetestrigrunintheambientenvironment.Thewearrateofplasmatreatedsampleat600 °Cwascalculated,anditwasabout0.0452 × 10−3 mm3/mwhichwasslightlyhigherthanthatofthe

untreatedsampleof0.0258 mm3/masreportedearlier[3].

4.4DiscussionAratherthinnitridecasewithalimiteddiffusionzonewasformedonthesurfaceofthemolybdenum-basedTZMafterplasmanitridinginamixtureof25%N2and75%H2incomparisonwithothermaterialslikestainlesssteelandthetreatmenttemperatureisat

660 °Candabove[11,12].Thisissimilartotheplasmanitridingoftitaniumalloyswhichistypicallycarriedoutinthetemperaturerangeof700–1100 °Cfor6–80 hourshinanitrogen-containingmedium[6].

4.1.4.1Thenitridingmechanismofmolybdenum-basedTZMalloyThethicknessof thenitride layerwasmeasuredunderSEMincorrelationto thenitrogenprofilesmeasuredbyGDOES.Fora fixedtreatment timeof25 hoursh, the thicknessofnitride layer increasedwith theelevated temperatures.Fora fixed treatment

temperatureof720or760 °C,nitridedlayergrewsteadilywiththetreatmenttime.ThesquarethicknessincreasesnearlylinearlywiththetreatmenttimeasshowninFigure.11.Basedonthis,theactivationenergyforthenitrogendiffusioninthismolybdenum-basedTZM

alloycanbecalculatedas330 kJ/mol.However,ithasbeenreportedthattheactivationenergyisbetween100‐and200 kJ/molforoutgassingofnitrogeninanitrogensaturatedmolybdenumattemperaturesbetween1050‐and1950 °Cinanultrahighvacuum[13].This

meansthatitneedsahighenergytoactivatethenitridingofTZMalloy.

DirectreactionsofthemetalwithnitrogengasemployveryhightemperaturestocleavethestrongN–

N triple bond. Earlier reports suggested that there was hardly any nitrogen absorption for molybdenum alloys under 800 °C in a gas nitriding [4,5]. The use of NH3 instead of N2 as reactive gas is not easy to control and often leads to a mixture of phases.

Moreover,thedecompositionofNH3isendothermicbutitcanbereplacedbyN2–H2gasmixtures,whichcircumventtheproblemofheattransfer[14].SomeresearchersuseddifferentapproacheslikeanexpandingmicrowaveplasmareactortoformaMo–

Ncompoundwithdefectsat600 °CorunderinagasmixtureofAr–25%N2–30%H2[7,8].

Figure10.Fig.10CoefficientoffrictioncomparisonofuntreatedandPNtreated(720/25)samplesunderelevatedtemperaturesof300 °Cand600 °C.

alt-text:Fig.10

Figure11.Fig.11Squarethicknessagainstnitridingtimeat720 °Cand760 °C.

alt-text:Fig.11

Intheplasmanitridingprocess,nitrogenionsandradicalssuchasN+,N2+,aswellasfastneutralnitrogenmoleculesplayasignificantroleinnitrogentransfer,andthepresenceofhydrogeninthenitridingmediumresultsintheformationofH+,NH+,andNH2+

radicalsthathaveacatalyticeffectonnitridingkineticsthroughincreaseddiffusionofnitrogenintothemolybdenumalloybyreducingtheremainingpassiveoxideandcarbidelayers.Thisoccurseitherbygeneratingactivenitrogenatomsonthecathodicpartsurfaceorby

attachingtoatomssputteredfromthesurfaceandredepositedbackonthecathodeintheformofnitrides[15].Givenplentyofenergy,i.e.,atthetemperatureof600 °Corabove,molybdenumwouldreactwiththeactivatednitrogenionstoformmolybdenumnitrides.

AccordingtoMo-Nphasediagram,β-Mo2Niseasytoformatalowertemperature(i.e.400 °C),andγ-Mo2Nismorelikelytoformathighertemperature[16].AsseeninFigure.4,thenitrogencontentattheinterfaceofnitridecaseandsubstratei.e.reactionfrontishigher

thanthatinstoichiometricMo2N(6.82mass%),whichsuggestingγ-Mo2N(JCPD00‐–025-1366)wouldbeeasiertoforminitiallyasshowninFigure.5.WithfurthernitrogenwastakenintotheMo2Nlayer,theN/Moratiocanbeashighas1/3(Figure.4)whichexceedsthe

nitrogencontentinMoN(12.74mass%),thusMoNwaslikelytobeformedinthenearsurfaceregion.XPSanalysissuggestedthatMo2N3canalsobeformed(Figure.6).Therefore,theremightbemoreMoN/Mo2N3 formedinthenear-surfaceregion(highernitrogen

content)andmoreMo2Nformedneartotheinterface(lowernitrogencontent)whichisreflectedfromthenitrogendistributionasseeninFigure.4a&b.ThisisinagreementwithKazmanli’'sreportthatincreasingthenitrogenpressurecausesatransitionfromMotoMo2N

andthenMoN[17].

Whenthetreatmenttemperaturewasraisedto760 °C,threesub-layers(760/25)canbedistinguishedinthenitridecaseunderbackscatteringelectronimageasseeninFigure.3c.Thetopsuperficiallayerissupposedtobeoxidelayerashighoxygencontentcan

alsobetracedinGDOESanalysis.TheN1 speakdecreasesslightlyintheXPSanalysisforsample760/25eventheverysurfacelayerwaspolished(760/25P)whichindicatingthemolybdenumnitrideswasslightlyoxidised(Figure.6).However,thisoxidelayerishardly

discernibleforsamplestreatedatalowertemperature(720/660 °C).Thesublayerunderoxideisofhighnitrogenconcentration(Figure.4a)whichisdeemedasMoN.Facecentredcubicδ-MoNphasehasbeenidentifiedinallthesamplesbuttherelativeintensityincreases

withtemperatureandtreatmenttimeincrease(Figure.5),whichsuggestsincreasedthicknessofthenitridelayer.ThesublayerinterfacedwiththesubstrateisbelievedtobeMo2Nasnitrogencontentdecreased.Asharpdropofnano-hardnessacrosstheinterfaceinFigure.

7bandnoclearvisiblediffusionzoneunderthenitridelayer(Figure.3)suggeststhatashallowandlimiteddiffusionzonewasformedasthenitrogenhasaverylowsolubilityinmolybdenumanditsalloy(1.08atom%atroomtemperature)[16].Itiswell-knownthatdiffusion

ofnitrogenismuchfasteralonggrainboundariesthanthroughthebulkgrainsi.e.short-circuitdiffusion.Hence,nitrogenwouldpreferentiallydiffusealonggrainboundariesthereforesomenitridescanbefoundinamuchdeeperdepthasindicatedinFigure.3.Oncethe

denseandcompactnitridelayerwasformed,itwouldformabarrierfornitrogenintakeanddiffusionthusslowedthegrowthofthenitridelayer,andthishasbeenfoundintheplasmanitridingofothermetallicmaterials.

4.2.4.2LowfrictionofnitridedTZMsurfaceforalonglifetoolThemolybdenumnitridephaseshavedifferentlatticestructuresandparameters,whichaffectthepropertiesofthecoating.WithintheMo–Nsystem,thestoichiometrichexagonalcompoundδ-MoNisahigh-hardnessmaterialwithverylowcompressibility[18].

Kazmanlietal.[17]reportedthatMoN(hexagonal)coatingsexhibitedagreaterhardness(5000 kg/mm2)comparedwithMo2N(f.c.c.)coating(3500 kg/mm2)whichisinagreementwithJehn’'sandKhojer’'sfindings[19–20].Inthisresearch,thethickerΜοΝincreasedwith

treatmenttemperatureandtimewasmorelikelyresponsiblefortheincreasedsurfacehardnessandthenitridecase.Itisreportedthatincreasingtheannealingtemperaturecausesanincreaseofsurfaceroughnesswhichisinagreementwithcurrentfindingasshownin

Figure.2[20].Clearly,thedenseandcompactnitridelayerwithhigherhardnessisresponsibleforthereductionofthecoefficientoffrictionandwearvolume(Figure.8-10).AsimilarCoFof0.2foranarc-PVDdepositedMo-NcoatingonHSSagainstanaluminacounterpart

ballunderaloadof7 NwasreportedbyUrgenet.al.[21].ThewearofAl2O3againstuntreatedTZMisoftypicaladhesivefeaturebutisofabrasivewearwhenagainstplasmanitrideTZMsample.TheCoFsoftheplasmanitridedsamplearelowerat300 °Cthanthatof

theuntreatedsamplebuttheyarequitesimilarat600 °C.Molybdenumoxidisesatatemperatureabove400 °C,andMoO3isanoxygendeficientphaseknownasMagnéliphasewhichhaslubricationeffectatelevatedtemperatures[22].Molybdenumnitridescanalsobe

oxidisedat600 °C,andthelubricatedoxidesgivetheplasmanitridedsurfaceasimilarfrictioncurveasthatofuntreatedsample.Avacuumorinertgasprotectedenvironmentisneededtofurtherassessthetribologicalpropertiesofplasmanitridedsampleatelevated

temperature.

TZMwasproposedtobeusedastoolmaterialinelectric-field-activatedsinteringofceramicslikeAl2O3etc.Therelativelylowcoefficientoffrictionofthenitridedsurfaceenablesthedemouldingofthesinteringpartoff-site(roomtemperatrue)oron-site(elevated

temperatures).Currently,wehaveusedtheplasmanitridedTZMpunches(Figure.12)inthesinteringofAl2O3micro-partswhichhasshownamuch-extendedlifeandacceptablelowfrictionindemouldingoverapeergraphitepunchatroomtemperature.

Figure12.Fig.12AsmachinedandplasmanitridedTZMpunches.

5.5ConclusionsTheresponseofmolybdenum-basedTZMintermsoflayerformationandhardeningtoplasmanitridingattemperaturesbetween500and760 °Chasbeeninvestigated.Afterplasmanitriding,athinnitridecasecomposingofmixedphasesofMo2NandMoNplus

ashallowdiffusionzonewasformed.

• Thethicknessofplasmanitridelayerincreasedwiththetreatmenttemperatureandtime,andtheTZMalloyhasanactivationenergyof330 kJ/molinthetreatmenttemperaturerange.

• Molybdenumhasatendencytoreactwithnitrogentofirstformγ-Mo2Nandthenδ-MoNduringtheplasmanitridingtreatment.

• Thesurfacehardnessofmolybdenum-basedTZMalloywassignificantlyincreasedafterplasmanitridingtreatmentwhichisalsorelatedtothetreatmenttemperatureandtime.

• Thecoefficientoffrictionofthemolybdenum-basedTZMagainstAl2O3ballwasreducedsignificantlyduetotheformationofathinnitridelayerregardlessoftheloadsandmediumused.Thewearresistanceoftheplasmanitridingtreatedsurfacewasgreatlyimproved.AlowerCoFof

plasmanitridedTZMagainstAl2O3ballwasalsoobservedatelevatedtemperature.

AcknowledgementTheresearchwasfundedbytheEUFP7Micro-FASTproject(ContractNoGA608720).TheauthorswouldalsoliketothankAINinSpainforcarryingoutthehightemperaturetribologicaltestandDr.HaitaoYeandDr.BaoguiShiatAston

UniversityinXPSanalysis.

References[1]S.P.S.P.Chakraborty,StudiesonthedevelopmentofTZMalloybyaluminothermiccoreductionprocessandformationofprotectivecoatingoverthealloybyplasmaspraytechnique,Int.J.Refract.Met.HardMater.29,2011,623–630,

https://doi.org/10.1016/j.ijrmhm.2011.04.010.

[2]J.A.J.A.Shields, (titleofthereferenceshouldbeadded:Applicationsofmolybdenummetalanditsalloys,)InternationalMolybdenumAssociation(IMOA),www.imoa.info,2013.

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alt-text:Fig.12

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Highlights

• TZMMolybdenumalloycanbeeffectivelyplasmanitridedattemperaturesbetween660‐and760 °C.

• Thethinnitridedsurfacehasalowcoefficientoffrictionofabout0.2atambientatmosphereagainsttheAl2O3ball.

• LowfrictionofnitridedTZMsurfaceisbeneficialinthedemouldingofelectric-field-activatedsinteringofceramics.

Ane-mailaddressismandatoryforcorrespondingauthors;however,privatee-mailaddresses(e.g.,gmail,hotmail,yahoo)shouldnotbeincluded.Pleasecheckifanalternativeacademic/institutionale-mailaddresscanbeprovidedforthe

correspondingauthor/s.

Answer:z.zhang.1@bham.ac.uk

Query:

DuplicateFig.11wasrenumberedtoFig.12includingthecitation.Pleasecheckandconfirmiftheactiondoneiscorrect.

Answer:Yes