The selective oxidation behaviour of WC–Co cemented carbides during the early oxidation stage

Letter

TheselectiveoxidationbehaviourofWC–Cocementedcarbidesduring

theearlyoxidation

stage

LiyongChena,DanqingYia,⇑,BinWanga,HuiqunLiua,ChunpingWua,XiangHuanga,HuihuiLia,

YuehongGaob

aSchoolofMaterialsScienceandEngineering,CentralSouthUniversity,Changsha410083,People’sRepublicofChinabZhuzhouCementedCarbideCuttingToolsCompanyLimited,Zhuzhou412007,People’sRepublicofChina

articleinfo

Articlehistory:Received4February2015Accepted18February2015Availableonline2March2015

Keywords:A.CeramicmatrixcompositeA.CobaltC.SelectiveoxidationB.EPMAB.AFMabstract

TheoxidationexperimentalofWC–Cocementedcarbideswascarriedoutat500°Cinair.Selectiveoxida-tionphenomenonofCophaseinWC–CocementedcarbideswasobservedandprovedbyEPMAandAFM.Atfirst,theOcontentinCophaseincreasesquickly,andthentendstobestableafteroxidationof20min.Insharpcontrast,nochangeofOcontentinWCphasewasobserved.Moreover,itwasfoundthattheoxidescalesthicknessofCophaseincreasedparabolicallywithoxidationtime.Andthermodynamicandkineticanalysisofoxidationwasdone.Ó2015ElsevierLtd.Allrightsreserved.

1.Introduction

Cementedcarbidesbelongtoaclassofhard,wear-resistant,

refractorymaterialsinwhichhardcarbideparticles(WC,TiC,

TaC,NbC,etc.)arebondedwithasoftandductilemetalbinder

(Co,Ni,Fe,etc.).Itisknowntoallthattheyareusedinawiderange

ofapplications,suchasmetalcutting,mining,construction,rock

drilling,metalforming,structuralcomponentsandwearparts.

However,theyareoftenexposedtovariouscorrosiveenviron-

ments,suchasacid,alkali,salt,elevatedtemperature,whichwill

resultinthedegradationofmechanicalpropertyofcementedcar-

bidesproducts,suchashardness,strengthandwearresistance,and

finallywillresultinthedecreasingoftheirservicelifetime.For

instance,theoxidationofWC–Cocementedcarbidescuttingtools

atelevatedtemperatureisoneofthekeyfactorsindetermining

thelifetimeofcementedcarbidesproducts.Asamatteroffact,

theirservicelifetimeisgreatlydeterminedbytheiroxidation.

Thereasonsareasfollows:firstofall,oxidefilmswithporous

andcrackswillformduringtheoxidation.Theseoxidefilmsare

veryeasytoflakeoff,andwilldefinitelydecreasewearresistance

[1–10].Additionally,ithasbeenprovedthatthestrengthofthe

cementedcarbidesisweakenedbecauseofsurfaceoxidation

[4–6,11–13].Thus,theinvestigationofoxidationofcementedcarbidesissignificanttothedesignandapplicationsofthese

materials.

Asfortheoxidationofcementedcarbides,manyresultshave

beenreported[1–3,7–10,13–17].Kineticanalysis,themacro-,

andmicro-structureandthephaseconstituteofoxidescalesare

themostimportantaspectsoftheinvestigationofoxidation.

Attainingthecurvesofmassgainorthethicknessoftheoxide

scalesvs.oxidationtimebyisothermalmethodandcalculating

theapparentactivationenergy(Ea)byisothermalornon-isother-

malmethodarethemajortaskofkineticanalysis.Barbattietal.

[7]havereportedthatthelinearlaworparaboliclawofmassgain

vs.oxidationtimewasinfluencedbytemperature.Eawasinflu-

encedbymanyfactors,suchasconsistofcementedcarbidesand

oxidationtemperature[2,3,8,9].ThereportedvaluesofEafor

WC–Cocementedcarbideswiththesamechemicalconstituents

areverydifferent.Forinstance,theEaofWC–15Co(inwt%)was

reportedtobe190kJ/molbyAristizabaletal.[9]and

234.1kJ/molbyVoitovichetal.[15],respectively.Thegreatdiffer-

enceofEamaybecausedbyinstrumentsand/orexperimental

methods.

Fromthepreviousliterature,weknowthattheoxidescales

formedonaWC–CosubstratemainlyconsistofWO3and

CoWO4,andasmallamountofCo2O3andCo3O4[1–3,7–10,13].

TheWO3phasehasratherlowresistancetooxidationbecauseit

containsnumerousporesandcrackswhichmaybecomeachannel

forthediffusionofO2andCO2.Comparatively,theCoWO4phaseis

denserthanWO3,andhasbetterresistancetotheoxidation.The

http://dx.doi.org/10.1016/j.corsci.2015.02.0330010-938X/Ó2015ElsevierLtd.Allrights

reserved.⇑Correspondingauthor.Tel.:+8673188830263;fax:+8673188836320.E-mailaddress:yioffice@csu.edu.cn(D.

Yi).CorrosionScience94(2015)1–5

ContentslistsavailableatScienceDirect

CorrosionScience

journalhomepage:www.elsevier.com/locate

/

c

orscipreviousresearcheshaveindicatedthattheoxidationresistanceof

cementedcarbideswasimprovedbytheincreaseofCocontentand

theadditionofcubiccarbides,suchasTiCand/or(Ta,Nb)C

[1–3,7,9,14].However,theoxidationresistancewasdecreased

whenCobinderwaspartlysubstitutedbyNibinderbecauseof

thedecreaseofcomplexoxide(Co,Ni)WO4content[15].

However,theaboveresearchesweremainlyaboutthelater

oxidationstageofcementedcarbidesthattheoxidescaleshad

alreadycoveredthesurfaceofthesamples.Thephenomenarelated

totheearlyoxidationstage,suchasselectiveoxidationandoxida-

tionlocation(intra-grain,grainboundaryorphaseboundary),are

notreportedinanypaper.Andactuallyselectiveoxidationisa

commonphenomenoninalloys,suchasiron-basedalloy

[18–20],cobalt-basedalloy[21,22],steel[23–25],andsoon.Asa

compositematerial,WC–Cocementedcarbidesconsistofatleast

twophases,andhavethepotentialforselectiveoxidation.

Additionally,theresultsofthermodynamiccalculationofthe

oxidationofWCandCoindicatethattheoxidationofWCiseasier

thantheoxidationofCo.However,inourexploratoryexperiment

thecontraryphenomenawerefound,andthepreliminaryresults

showedthatCowasfirstlyoxidisedandWCdidnot.

Therefore,basedontheresultsofexploratoryexperiments,a

detailedstudyoftheselectiveoxidationofWC–Cocementedcar-

bidesduringtheearlyoxidationstagewascarriedoutinthispaper.

ThreeapproacheswereusedtoprovethattheoxidationofCo

phaseispriortotheoxidationofWCphase.Thefirstapproachis

torevealthechangesoftheoxygendistributionatthesameloca-

tionofasamplewiththeincreasingoxidationtimebyelement

mapanalysisofelectronprobeX-raymicro-analyzer(EPMA).The

secondoneistomeasurethecontentchangeofW,C,CoandO,

whichisparticularlyimportant,ofWCphaseandCophasewith

theincreasingoxidationtimebyquantitativeanalysisofEPMA.

Andthethirdapproachistomeasurethethicknessofoxidescales

withtheincreasingoxidationtimebyatomicforcemicroscope

(AFM).

2.Materialsandexperimental

WC–15Co(inwt%)cementedcarbideswereprovidedby

ZhuzhouCementedCarbideGroupCompanyLimited.Themostof

WCgrainsofsampleare6–8lm.Thesizeofspecimensis

5mmÂ5mmÂ10mm.Inordertoobtainasmoothsurfacebefore

oxidation,oneofthe5mmÂ10mmsurfacewasgroundandpol-

ished.Allspecimenswerewetgroundusing40lmand20lmdia-

monddiscs.Afterthat,theywouldbepolishedusing9lmand

1lmwater-baseddiamondsuspension.

Sampleswereoxidisedinairat500°Cbetween5and150min

inamuffle(±2°C).Fourteensampleswereusedinthispaper.One

ofthemwasoxidisedfordifferenttimes.Whiletheotherthirteen

sampleswereseparatelyoxidisedfor0,5,10,15,...,55,60min.

Theelementmappingandquantitativecompositionanalysisof

samplesweredonebyEPMA(JXA-8230)withtungstenfilament.

TheEPMAwasoperatedunderthefollowingsettings:accelerating

voltageis15kV;probecurrent,dwelltimeandstepsizeareabout

11.3nA,70msand93.8nmforelementmapping,10.1nA,50ms

and50nmforquantitativecompositionanalysis.Thethickness

ofoxidescaleswasmeasuredbytappingmodeAFM(multimode

V)usingsilicontip.Scanareasare20lmÂ20lmand

10lmÂ10lmand512linesperimagewerescannedwith512

samplesperline.

3.Resultsanddiscussion

Fig.1(a)showsthebackscatterelectron(BSE)imageofWC–

15Cosample,inwhichthedarkareasrepresentCophasecontain-

ingsmallamountsofdissolvedWandC,andthegreyareasrepresentWCphasecontainingaverysmallamountofdissolved

Co.ElementmapanalysesbyEPMAwereexecutedintheregion

displayedinFig.1(a).Inordertoensuretheelementmapanalyses

byEPMAareatthesimilarsameregion,thesamplewasmarkedby

Vickersindentationonthepolishedsurfacebeforeoxidation.The

averageOcontentsofmappedregionsvs.oxidationtimesareplot-

tedinFig.1(b).ItshouldbenotedthattheresultsofOmappingare

qualitativeratherthanquantitative.Beforeoxidation,theaverage

Ocontentis4%.HighOcontentcomesfromchemicalandphysical

adsorptionofoxygeninair,andfewoxideparticlesadheredtothe

polishedsurface.TheaverageOcontentis10%,17%,20%,25%after

oxidisingfor5,35,90,150min,respectively.Thecurveofaverage

Ocontentvs.oxidationtimeroughlyfollowsaparabola.From0to

5min,theaverageOcontentincreasesrapidly;from5to35min,

thegrowthratebecomeslower;after35min,theincreasingof

averageOcontentbecomesslow.Fig.1(c)–(f)showstheOdistri-

butionsafteroxidisingfor0,5,35,150min,respectively.From

Fig.1(c)–(f),itisfoundthatOcontentinCophasegoesupwith

theincreasingoxidationtime.Incontrast,OcontentinWCphase

hasscarcelychangedwiththeincreasingoxidationtime.

FromFig.1,itisseenthatonlyCophasewasoxidisedat500°C.

Inordertofurtherconfirmtheaboveresult,sampleswereoxidised

at500°Cfrom0to60minandthenquantitativelyanalysedby

EPMAtoascertaintheelementscontent.Foreachtimeinterval,5

pointsofCophaseand5pointsofWCphaseweremeasured

respectively.

Fig.2showsthevariationoftheW,C,CoandOcontentinthe

CophaseandtheWCphasewithoxidationtime.Thereislittle

changeofCcontentinCophase.AndtheWcontentinCophase

fluctuatesaround9%withoutobviouschange.However,Coand

Ocontentvariesgreatlywithoxidationtime.Within20min,Co

contentfirstlygoesdownquicklywiththeincreasingoxidation

time;over20min,Cocontentgoesdownslowlyandtendstobe

constant.ThechangeofOcontentinCophaseiscontrarytothe

changeofCocontent.Firstly,itincreasesquicklyandthentends

tobestable.ItiscertainthatCophasewasoxidisedat500°C.In

Fig.2(b),theW,C,CoandOcontentofWCphaseallfluctuated

aroundaconstantvalue.Ocontentisaboutzero.Itisimpliedthat

WCphasewasnotoxidisedat500°C.Tosumuptheaboveresults,

WC–Cocementedcarbideshavetheselectiveoxidationcharacter-

isticat500°C.

TheselectiveoxidationofCophaseinWC–Cocementedcar-

bidesresultedinthechangeofsurfacetopographycharacterised

bythethickeningoftheoxidescalesofCophase.AFMisavery

powerfultechniqueforprovidinghigh-resolutionthree-dimen-

sional(3-D)surfacetopographyonthesubmicronscalehasbeen

usedasanalternativeorsupplementarytooltoothertechniques

forstudyingtheselectiveoxidationbehaviourofsomematerials

duringtheearlyoxidationstage[26].Inthisresearchwealsoused

AFMtomeasurethevariationofsurfacetopographieswithoxida-

tiontime.Fig.3(a)showsthethicknessvariationofoxidescalesof

Cophasewithoxidationtimeat500°C.Eachdataisanaverageof

20randommeasuredvalues.Beforeoxidation,theheightofWC

phaseis31±5nmhigherthanCophase.However,after5min,

theoxidescalesthicknessofCophaseincreasedto93±6nm.

ThissuggestedthattheCophasehasbeenoxidised.Andthen,

theoxidescalesthicknessofCophaseincreasedfrom141±9nm

to215±14nmcorrespondingtooxidisingfor15minand

35min.After35min,theincreasingoftheoxidescalesthickness

ofCophaseisslow.AFMtopographiesfor0,5,15,35,60min

oxidationareshowninFig.3(b)–(f),respectively.Theselective

oxidationofCophaseisclearlyseenfromFig.3.

TheoxidescalesthicknessofpureCoinpreviousliteratureare

775nm(temperature500°C,time20min,P(O2)/Ph=0.1)[27],

440nm(467°C,20min,P(O2)/Ph=0.21)[28],360nm(505°C,

19min,P(O2)/Ph=1)[29]and500nm(500°C,15min,P(O2)/2L.Chenetal./CorrosionScience94(2015)1–5