Int.JournalofRefractoryMetalsandHardMaterials36(2013)38–45
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Int.JournalofRefractoryMetalsandHardMaterials
journalhomepage:www.elsevier.com/locate/IJRMHM
FunctionallygradedWC–Ti(C,N)–(Ta,Nb)C–Cohardmetals:Metallurgyandperformance
J.Glühmanna,M.Schneewei?a,H.vandenBergb,D.Kasselb,K.R?digerb,K.Dreyerb,W.Lengauerc,?
abcWestSaxonUniversityofAppliedSciencesofZwickau,D-08012Zwickau,GermanyKennametalSharedServicesGmbH,D-45145Essen,GermanyViennaUniversityofTechnology,A-1060Vienna,Austria
articleinfoabstract
Thethermodynamicandkineticaspectsoftailoringthenear-surfacezonesoffunctionallygradedhardmetals(FGHM)bynitrogenmanagementaredescribedandsummarised.Since,asaresultofthedevelopmentprocess,industrialproductionaswellasabroadusageascuttingtoolsisaimed,thefocusisputontestingofcoatedFGHMsubstrates.Testingwasperformedbyindividualin-housetests,systematictestsinanappli-cationlaboratoryaswellasinaproductionenvironment(turningofstampingpunches).
FGHMgradeswithcarbonitride-enrichedsurfacezonesprovedtooutperformothergradessigni?cantly.DiamondcoatingsonFGHMsshowexcellentadhesionaswellasabrasionresistance,actuallythebestofalltestedgrades.Incontinuousturningoperationsofsteel,both?ankandcraterwearresistancewereimprovedsigni?cantly,andthetoollifealmostdoubledinFGHMgradeswhichshowacarbonitride-enrichednear-surfacezone.
?2011ElsevierLtd.Allrightsreserved.
Articlehistory:
Received28September2011Accepted16December2011Keywords:SinteringDiffusionCutting
CarbonitrideNitrogenDiamondCoatingTestingSurface
Cementedcarbides
1.Introduction
Gradedhardmetalshavefoundtheirwayintoindustrialuse.Oneofthe?rstwascertainlytheKC850gradeofKennametalinthe1980swhichcontainedCostriations,i.e.lamellarvariationsofCoenrichmentsnearthesurfacewhichprovidedanincreaseintough-nessandhencecrackresistanceincuttingapplications.Fig.1showsacoatedinsertwiththiskindofgradient.ItcanbeseenthattheperipheryofthecuttinginsertisgraduallyenrichedwithCo,buttheoutermostsurfaceofthesubstrateisfreeofbindermetal.OnlyWCispresent,whichismostfavourablefortheadhesionofthecoating.Inaddition,adropofconcentrationofthecubiccarbides(browncolour)isvisible,buttheircontentdoesnotreachzeronearthesubstrate'ssurface.
ForapplicationinstonedrillingandmineralcuttingagradedhardmetalwasreportedbySandvik[1]forwhichaneta-phase-containingbodywassubjectedtoacarburisationtreatment.Thesurfacezoneisthentwo-fold,theouterpartiseta-freeandcontainslessCothantheinterior,whereasintheinnereta-freepartCoisenriched.
?Correspondingauthor.Tel.:+4315880116127;fax:+4315880116490.E-mailaddress:walter.lengauer@tuwien.ac.at(W.Lengauer).0263-4368/$–seefrontmatter?2011ElsevierLtd.Allrightsreserved.doi:10.1016/j.ijrmhm.2011.12.009
Inthesameapplication?eldanothergradewasintroducedbyCeratizit/HiltiquiterecentlybyjoiningtwopartsofWC–CohardmetalsofdifferentWCgrainsize[2,3].Likeotherhardmetaljoints(e.g.alsohardmetaljoinedtowardssteel),thisisnotagradedhardmetalinitsusualmeaning,sincetwohardmetalgrades,differentinWCgrainsizeandCocontent,arejoinedtogetherforthepurposeofhavingaharderendforbetterdrillingperformanceandacoarserandCo-enrichedendforbetterweldingabilitytowardsthesteelshaft.However,uponsintering/annealingsuchacouple,agradientattheinterfaceofthetwohardmetalgrades,duetodiffusionofthevari-ousconstituents,developsandwecanlearnwhichfactorsarein?uencingdiffusionandmigrationofconstituents.Indiffusion-coupletestsitcouldbeshown,thatComigration–asafunctionofdifferentcarbonactivity,CocontentandWCgrainsize–occurredoveradistanceofseveralcentimetres(!)[3].Thefactorsin?uencingdiffusioninsuchhardmetaldiffusioncouplesofvariouscombina-tionswererecentlydescribedalsoin[4,5].
Theroleofnitrogenintheformationofanitride-depletedzonenearthesurfacewasrealisedinthelate1980s[6,7].Itwasrealisedthatuponnitrogenout-diffusiontitaniumdiffusesinwardssothatanitride-orcarbonitride-depletedzoneisformed.Afurtherstepinthedesignofgradedhardmetalswasdonebytheuseofnitrogen-containingatmosphereandinthemid1990snitride-enrichedzoneswerereported[8].
J.Glühmannetal./Int.JournalofRefractoryMetalsandHardMaterials36(2013)38–4539
coatingCoWCsubstratefcc
10μmFig.1.Coatedgradedhardmetal(KennametalKC850)withCoenrichmenttowardsthesurfaceinformofstriations(bright).Thecoatingiscomposedofthreesublayers.ThesurfaceitselfisfreeofCoandconsistsofWC(occasionallywithsomeface-centredcubiccarbidesappearinginbrowncolour).Thesefcccarbidesaredepletedinthesurfacezoneascomparedtothebulk.
InthelaboratoriesoftheTUViennasystematicresearchonthethermodynamicsandthekineticsofformationofgradientsinWC–Co-basedhardmetalscontainingnitrideformerssuchastitaniumcarbide,titaniumcarbonitrideandtantalum/niobiumcarbideswasstarted.Wedescribed[9]avarietyoffourdifferentgradienttypeswiththreesub-types,whichcanberealiseddependingonthecompo-sitionofthehardmetalsandthenitrogenpotentialofthesinteringatmosphere[9–11].Fig.2(top)givesthegeneraldependencyof
)eruregion of Nssin-diffusionerCN enrichedp negortin( not graded
golregion of N out-diffusionCN depletedtemperature
10μm
AB
CD
1
2
Fig.2.Top:scheme(left)ofnitrogenpressurevs.temperatureforformationofacarbonitride-enriched(topright)andacarbonitride-depletedgradient(lowerright).Ifequilibriumnitrogenpressureisadjusted(atsomeintermediatepressure)nogradi-entisformed.Thetoprightimageshowsasmoothcarbonitride-enrichedgrade,buttheenrichmentcanalsobeintensi?ed(asshowninFig.3,topface),dependingonthetimeandtitaniumcarbide/carbonitridecontentofthestartingformulation.Bottom:samples(12×12×4mm)withdifferentstartingformulations(A–D)sub-jectedtotwodifferentsinteringcycleswhichshowsthedifferentreactivityagainstthesinteringatmosphere.
formationofgradientsonthenitrogenpressure.Fora?xedstartingformulationanenrichmentofcarbonitridesclosetothesurface(“CNenriched”)athighnitrogenpressuresoccurs.Atintermediatepressure,nomacroscopicdiffusionofnitrogenisvisibleifthepres-sureisclosetotheequilibriumnitrogenpressure.Atlownitrogenpressureout-diffusionoccurs,causingaCN-depletedmicrostructurewhichshowsaWC–Cozoneattheoutermostpartofthealloy.Thecarbonitrideformed,hasamulti-componentnaturewhereallmetalconstituentsofthehardmetalexceptthebinderphaseareincorporatedtosomeextent.AlargepartisofcourseTi(C,N)butalsoTa,Nb,W,…arepresentinthefcclatticeofthiscarbonitride,designated“CN”inthefollowing.
Theoverallcompositionofahardmetalalloyhasin?uenceontheformationofgradients,i.e.whetheranalloyis?nallyCNenriched,CNdepletedordoesnotshowagradedmicrostructureatall.Thisbe-haviourisshowninFig.2(bottom)fortwodifferentsinteringcycles(1and2)andfourgradesdifferentincomposition.Alreadyfromtheouterappearanceofthesesinteredcuttinginsertsthereactivityagainstthesinteringatmosphereisvisible.Somesamplespickupnitrogen(pinktobrowncolourduetoTi(C,N)formationofdifferentcompositionandamount),andsomelosenitrogen(colouredmetallicgrey).Inadditiontotheabovementionedsevengradienttypesandsub-types,asocalled“inversegradient”wasdescribed,inwhichanenrichmentofTi(C,N)couldbeestablishedbelowa“CN-depleted”WC–Cozone(Fig.3,leftface[9])whilethetopfaceinFig.3showsaheavilyCN-enrichedmicrostructure.
Dependingonthedesiredgradientsitisnecessarytoadjustcarefullythestartingformulationbecauseoftheimpactofnitrogenonthephaseequilibriainthehardmetals.Inaddition,whilesintering,theshrinkagebehaviourisdependentonthenitrogenandcarbonpotential[10,11].Increasednitrogenpotentialhasaneffectsimilartoanincreaseincarbonactivity.Hence,alsoeta-phasescanformtemporarilyduringthesinteringcyclebutaredecomposedbynitro-gen(see[12],thisissue).
Thekineticforcefortheformationofbothkindsofgradientzoneswasfoundtobediffusioncontrolled(asparaboliclawforthethick-nessasafunctionoftime)[10,13],seeFig.4.ThebasicdiffusionmechanismisthesameforCN-depletedzonesandCN-enrichedzones.BothneeddiffusionofTi,W,CandN,onlythedirectionschangebetweendepletionandenrichment.ThehighergrowthrateoftheWC–Cozone(CN-depleted)inFig.4,ascomparedtotheCN-enrichedgrades,isduetothemuchhighertemperatureofWC–Coformation(CNdepletion)inthiscase.
Upontailoringaninverse-gradientmicrostructure,however,thegrowthrateofaCN-depletedWC–Cozoneisverylow,duetothelowerdiffusionvelocityatatemperaturemuchbelowthesinteringtemperature[10].Downto1200°CitremainsessentiallyunchangedinthicknessbutshowssomeWCgraingrowth.
10μmFig.3.Inversegradient:WC–Co(CNdepleted)ontopofatitaniumCN-enrichedinnerzoneontheleftfaceofacuttinginsertwithatailorededge(rounded).AtthetopfaceaCN-enrichedsurfaceisformed.Inversegradientscanbeestablishedbylowtempera-turediffusionaltreatmentofcuttinginserts.
40J.Glühmannetal./Int.JournalofRefractoryMetalsandHardMaterials36(2013)38–45
480440400360320280240200160120804000123456p(N2)=5barp(N2)=25barWC-Cofurtherincreasetheircuttingperformance.Thegradientofthesub-stratewouldthenserveasahardskeletonbelowthecoatingforbet-terplasticdeformationpropertiesofthecuttingedgeaswellasasupportwhenthecoatingiswornoffduringcuttingoperations.
Alreadyfromtheverybeginningofcommonresearchitwasclear,thatthelaboratoryconditionshavetobedesignedtobewellsuitedforupscalingtoindustryproductionlevel[14].Thismeans,thatheat-ingandcoolingratesaswellasthegassupplyhavetobewithintherangeofmassproductioninanindustrialsinteringfurnace,whichiscapabletoproduceuniformhardmetalbatchesofsomevolume.
Itwasalsoevidentrightfromthestartoftheresearch,thatexten-sivemetalcuttingtestswillhavetoplayasigni?cantroleto?tthepropergradientstructurestothebroadrangeofpossibleworkpiecematerialsandcuttingoperations.
layer thickness x2 / μm2Diffusion time t / h
Fig.4.IncreaseoflayerthicknessoftheCN-depletedWC–Cozone(at1490°C)andtheCN-enrichedzone(bothat1200°C)independenceonnitrogenpressureanddiffusiontime.ThegrowthofthedepletedzonewasinvestigatedonagradewhichcontainedalreadyaWC–Cozonewhenthetemperaturewasreached,sothatthedatapointsshowathicknessoffset.ThegrowthoftheCN-enrichedlayerisalsodependentonnitrogenpressureforagivenstartingformulation.Therelationshipshowsthattheformationofbothgradientsisdrivenbydiffusion(parabolictimelaw).
2.FGHMsubstratesforcoatings
Functionalgradienthardmetalsprovedtobeanexcellentsub-strateforavarietyofcoatings,bothCVDandPVD,includingdiamondcoatings.Fig.5showsthenitrogen-enrichedFGhardmetalsubstrateastheyoriginatefromoppositepositionsinthesinteringfurnace(topleft)andamicrostructureofthesubstrate(topright).Theexcel-lentadhesionofthediamondcoatingonaCN-enrichedFGHMsub-strateisvisibleuponcleavingthesampleandinspectingtheedge.Whilefromtheungradedsubstratethecoating?akesoff(Fig.5,
Althoughsometypesoffunctionally-gradedhardmetalsclearlyoutperformedhardmetalsofthesameoverallcompositionbutwithoutagradient[9,13–15],itwasclear,thatacoatingwouldeven
topflankbottom
10μm5μm 5μm 15μm15μmFig.5.Toprow:cuttinginserts(12×12×4mm)withCN-enrichedsurfacetakenfromtwodifferentpositionsinthefurnace(topleft),LOMcross-sectionofinsert(topright).Centrerow:diamondcoatingatthecuttingedge;centreright:delaminateddiamondlayerattheedge(arrow)aftercleavingthesample,centreleft:diamondcoatingonCN-enrichedFGHM.Thecoatingisnotdelaminatedfromthesubstrate.Bottomrow:topviewofdiamondcoatingonCN-enrichedFGHMbeforeabrasiontest(bottomleft)andnearthejetafterthetest(bottomright).Aftertestthecoatingshowednocracksinthevicinityofthejetasopposedtoungradedinserts(testsperformedbyCemeCon).
J.Glühmannetal./Int.JournalofRefractoryMetalsandHardMaterials36(2013)38–4541
centreright),theCN-enrichedgradeshowsnotracesofdelaminationofthediamondcoating(Fig.5,centreleft).ThepicturesofthebottomrowshowtheexcellentwearperformanceofadiamondcoatedFGHMuponexposaltoanabrasionjet(SiCparticles,5barjetpres-sure).Fig.5,bottomleftshowsthediamondsurfaceoftheuntreatedcoating.Fig.5,bottomrightshowsthesurfaceexposedtothejet,withoutanycrackingor?akingoff.
InFig.6imagesofthetracesoftheappliedabrasivejetsontherakefaceofanSDFT1204AEFNmetalcuttinginsertaregiven.AtthediamondcoatedCN-enrichedsubstrate(top)onlytracesarevisibleonastillintactlayer,butonthediamond-coatedungradedsubstrate(bottom)cratersintothesubstrateareformedatevenshorterexpo-suretimesthanappliedtotheCN-enrichedgrade.
Itturnedout,thatthediamondcoatingsontheCN-enrichedsur-facezonewerethemostwearresistantofalltestedgrades.
Interestingly,pre-treatmentslikesubstrateetchingandpeeningwerenotfavourableforgradedhardmetals,mostprobablybecausethegradedzoneisdisturbedbythetreatments.Asaconsequencethepre-treatmentofgradedsubstratesbeforecoatingshouldbeavoided,savingtimeandproductioncosts.
Fig.7showsCVD-coatedfunctional-gradienthardmetals,withaCN-depletedzone(bottom)andaCN-enrichedzone(top).Inthelatter,thecompositionofTiN,formedintheoutermostpartofthehardmetalbygradientsintering,isverysimilartothe?rstTiNlayer(bondinglayer)ofthecoating.OntopoftheTiNlayer,aTi(C,N)layerof5μmthickness(greycolour)wasdepos-itedfollowedbyathinTiClayerandanAl2O3/ZrO2/TiO2compos-itelayerofabout5μmthickness,whichappearsblackinthemicrostructure.
10μm10μmFig.7.Crosssectionsofcarbonitride-enriched(top)andcarbonitride-depleted(bottom)cuttinginserts,coatedwithaTiN–Ti(C,N)–TiC–Al2O3/ZrO2/TiO2compositecoating.CuttingtestsofthesegradesareshowninFig.8.
5mm 500μm 5mm 500μm Fig.6.Resultsofabrasiontests(SiCparticles,5barjetpressureontherakefaceofanSDFT1204AEFNmetalcuttinginsert);left:opticalimageofhardmetalinserts,right:closerviewofexposedsurface(tiltsample);top:diamond-coatedCN-enrichedFGHMafter120stestingtime,onlytracesofimpactsarevisible;bottom:diamond-coatedungradedhardmetalofsameCocontentandgrainsizeafter100softestingtime,clearremovaloflayer(crater)visible.TheCN-enrichedFGHMclearlyoutperformstheungradedsample(testsperformedbyCemeCon).
42J.Glühmannetal./Int.JournalofRefractoryMetalsandHardMaterials36(2013)38–45
3.Cuttingtests3.1.Laboratoryscale
InFig.8theresultsofcuttingtestsareshownforungradedsub-strates(HM),gradedsubstrates(FGHM),aswellascoatedtypesofbothsubstrates(cHMandcFGHM).TheleftsidecontainsdataofCN-depletedFGHMgrades,therightsidedataonCN-enrichedFGHMgrades,whereasforthelatterthecuttingspeedwassigni?-cantlyincreased.Itcanbeseenthatthe?ankwear(Fig.8,topleft)issubstantiallyreducedforacoatedcFGHMwithaCN-depletedsurfacezone,whereasthecratering(bottomleft)isalmostidenticalforcoatedcHMandcoatedCN-depletedcFGHM.ThelatterisduetothereducedamountofTi(C,N)inthesurfacezoneoftheFGHMsubstrate(Fig.7,bottom).Ifthecarbonitridesareenrichedinthenear-surfacezone(rightcolumn),alsothedepthofcraterissubstan-tiallyreduced(Fig.8,bottomright)[8].ThisshowsclearlythelargeimpactofCNenrichmentonthecraterwear.
Inordertostudythewearbehaviourandmechanismsinmoredepth,asystematicseriesofinvestigationswasperformedinaspeciallyequippedtestinglaboratoryattheUniversityofZwickau.Fortheseinvestigationshardmetalswithanoverallcompositionof80wt.%WC/8wt.%Co/12wt.%cubiccarbides,withcarbonitride-depletedzonesoftwodifferentthicknessesaswellasonenitride-enrichedzone,wereproducedonanindustrialscale.Allofthemwerecoatedwithanidentical(Ti,Al)NPVDlayer.AsareferenceanungradedTi(C,N)coatedsubstrateofacomparablecompositionwasused.
Fig.9showsthenearsurfacemicrostructuresofthesealloys.Detailedmaterialanalysisofthegradientzonewasperformedin
0.7ordertomeasurethehardness(HV0.2),thecarbonitride,andtheCocontent(Fig.10).TheabsenceofcarbonitridesintheCN-depletedFGHMs(Fig.9,bottom)reachesdowntoadepthofabout20μmpar-allelwithanenrichmentofthecobaltbinderphase.Thisisassociateddirectlywithadecreaseinhardness.Withtheincreaseofcarboni-tridestowardsthebulk,thehardnessincreasesagaintothecorehardnesslevel.Inthistypeofgradienttheconceptofincreasednear-surfacetoughnesshasbeenimplemented[10].
Incontrast,bycarbonitrideenrichmentnearthesurface(Fig.9,middle),anincreaseofhardnessandalowcobaltcontentisestab-lished.Theconceptofahardandabrasion-resistantnear-surfacezonewasrealisedinthisFGHMgrade[10].
Inordertodeterminetheperformancepotentialandtolearnmoreabouttheirspeci?cwearmechanisms,thetwoFGHMgrades(CN-depletedandCN-enriched,seeFig.9)weretestedinturningappli-cationsincomparisontotheidenticalbutungradedvariant.
CuttinginsertsofgeometryCNMG120412-5werecoatedwithaconventionalPVDcoatingandtestedatcontinuoustransversalturn-ing(Fig.11).Heattreatedsteel1.7225wasusedasaworkpiecematerial.Allcuttingexperimentsweredoneatconstantconditions(vc=160m/min,f=0.3mm/rev,ap=2.5mm).DetailsofthetestingconditionscanbetakenfromTable1.
ThemeasuredtoollifeofthedifferentvariantsiscomparedinFig.12.Atthecontinuousturningprocessthevariantswithcarboni-tridedepletion(“CNdepleted”)showslightlylowertoollifethanthestandardungradedcuttingtool.
Incontrast,theFGHMgradewithacarbonitride-enrichedsur-facezone(“CNenriched”)showsastrongincreaseintoollife.Theperformancegainis32%incomparisontothereferencegrade(“notgraded”).
0.70.6
CN enrichedflank wear land VBmax / mm0.60.5FGHMc HMHM0.50.4
0.40.30.20.10180CN depletedFGHMCN depletedc FGHMHM0.30.20.10180CN enrichedc HMc FGHM010203040500246810121416HM150150HMc HMdepth of crater / μm1209060300FGHMCN depleted12090c HMc FGHM4060300CN enrichedFGHMCN enrichedc FGHMCN depleted0102030500246810121416cutting time / mincutting time / min
Fig.8.Resultsofcuttingtests(continuousdryturning)withuncoated(HM,FGHM)andcoated(cHM,cFGHM)inserts.Diagramsontheleft:FGHMandcFGHMareofacarbonitride-depletedgrade;diagramsontheright:FGHMandcFGHMareofacarbonitride-enrichedsurfacegrade.Increasedcuttingspeedontheright.ForcorrespondingmicrostructuresseeFig.7.The?ankwear(top)islowerforbothcFGHMtypes,whereasforthecarbonitride-enrichedgradealsothecraterwear(bottom)canbesubstantiallydecreasedascomparedtocoatedungradedhardmetal(cHM).Left:vc=225m/min,f=0.2mm/rev,ap=2.0mm;right:vc=315m/min,f=0.2mm/rev,ap=2.0mm.