JOURNAL OF HARD MATERIALS, VOL. 1,NO. 3, 1990

FractureToughnessEvaluationof WC-Co Alloys byIndentationTesting

ROLAND SPIEGLER, SIEGFRIED SCHMADDER & LORENZ S. SIGL

147

ABSTRACT Indentationmodelsfor determiningthefracturetoughnessofbrittlesolidsarecomparedwith respectto theirapplicabilityto WC-Co cermets.An approachduetoShetty is consideredto be mostsuitable.This model is appliedto 10 WC-Co alloyscoveringtherangeof commercialhardmetals.Thetrendsin toughnessarein accordancewith known dependenceson cobaltvolumefraction and carbidegrain size. The resultsagreefavourably withfracturetoughnessdataobtainedfrom single-edge-notched-beamspecimenswith maximumdeviationsbeinglessthan 20%. Thus indentationanalysisusingShetty'sformula appearsto be a fast and reasonablyaccurateway of toughnessevaluationfor WC-basedhard metals.

1.INTRODUCTION

Cementedcarbidesare a group of sinteredmaterialswhere a hard transitionmetalcarbideof theB subgroupsIV-VI is bondedby a toughmetalbinderof theB subgroupVIII. Thus they combineappreciablehardnesswith good fractureresistance.Whilemany cermet componentsrely primarily on the material'shardness,a numberofapplication-relevantproperties,suchas strengthandwearresistance,stronglydependon the crack resistance.Hence easyaccessto the fracturetoughnessis desirableforalloy developmentand quality control. While the measurementof other interestingpropertiessuch asmicrostructuralparameters,Young's modulus,hardnessor bendingstrengthhas becomea routine matter,conventionaltoughnesstestingrequirescon-siderableeffort,in particularprecrackingof specimenshasremaineda seriousobstacle.

Consequendy,alternativemethodsfr evaluatingthe fracture toughnessfromindentationdatahavebeensought.An importantmethodis the indentationtestwhich,during the last few years, has emergedas a potential alternativeto conventionalfracture toughnessmeasurements.Its prime advantagesare that no specialsampiegeometry,large amountof specimenmaterialnor sophisticatedmeasurementtech-niques are needed.To date,however,the accuracyand reliability of conventionaltechniquesappearsunmatched,thoughmuchprogresshasbeenmadein establishingasoundbasisfor theindentationmethodby linking fracturemechanicswith indentationcracking.

R. Spiegier,Max-Planck-Institutfr Metallforschung,Institutfr Werkstoffwissenschaft,Seestrae92,D-7000 Stuttgart1, FRG; S. Schmauderis presentlyon leaveat Universityof California,MaterialsDepartment,Collegeof Engineering,SantaBarbara,CA 93106,USA; L. S. Sigl,ElektroschmelzwerkKemptenGmbH,D-8960Kempten,FRG.

148 R. SPIEGLER ET AL.

Using the lengthof cracks,which emanatefrom the cornersof Vickers hardnessindentations(seeFigure 1), to characterizethe toughnessof brittlesolidsgoesbacktothepioneeringwork of Palmqvist[1,2].A linearrelationbetweentheindentationload,P, and the sum of lengthsof thefour cracksat the cornersof the indentation,L, wasfound. The slope of the line, P/L, was defined as the crack resistance,W, and isnowadaysreferredto as the Palmqvisttoughnessfollowing a suggestionof Exner [3].Considerableeffort has been devotedto the developmentof relationswhich linkindentationparameters(such as W) to the fracturetoughnessKIc' Not surprisinglyasimplelinearrelationbetweenthePalmqvisttoughness,W,andthefracturetoughness,Klc, is not found [4]. A majorgoalpursuedduring recentyearshasbeento deve10pfracturemechanicsconceptswhich allow the calculationof fracturetoughnessfromindentationdata.For a reviewof thevarietyof differentmodelsthe readeris referredto the comparativestudiesof Ponton & Rawlings[5, 6].

Figure 1.Pa1mqvistcracksat thecornersof a Vickers indentation(500 N, alloy 6F/M).

Indentationtoughnessbasedon mediancrack modelscan deviatefrom conven-tional four-pointbendingtoughnessvaluesby up to 30% in singlephaseceramicsandglasses(e.g.Shaoboetal. [7] andSmithetal. [8]). Arecent analysisonAI203-SiC [9]also indicatedthat suchmaterialsneeda significantcalibrationof toughnessobtainedfrom indentation experiments.The present work comparesavailablemodels formeasuringthe fracturetoughnessby indentationtestingand aimsat findingthe mostsuitableapproachfor cementedcarbides.This is attemptedby comparingtheindenta-tion resultswith toughnessvaluesfrom single-edge-notched-beamspecimenswhichareknownto bevery accurate.

2.CRACK MODELS

There areessentiallythreedistinctmodesof indentationfracturein brittlesolids[10].Hertzian conecracksoccur by contactof a curved indenterto the sampiesurface.

FRACTURE TOUGHNESS EVALUATION OF WC-Co ALLOYS 149

Median ventsor lateralventscanform by contactwith a sharpindenter.Medianventsare crackswhich nueleateat the tip of the indenterandpropagateinto theunderlyingmaterial.Two typesof medianventsare distinguished:Palmqvistcracksandmediancracks (for the definition of the crack type seeFigure 2). Lateral vents are crackswhich propagateat a small angleto the surfaceand sometimescausemacroscopicchippingaroundthe indentation.They extendwith a positivecrackopeningdisplace-ment on unloadingthe indenter and tend to elose slightly on reloading.Materialswhich are very brittle, e.g. glasses,tend to form lateral vents, whereasmaterialsexhibiting some plasticity tend to form median vents. In cementedcarbidestheformationof lateralventsis restrictedto cyelicloading,especiallyfor hardgrades[10],or stresscorrosioncrackingin aggressiveatmospheres[11]. Under normal circum-stancesonly medianventsareformedin WC-Co alloys.

Palmqvist Cracks Median Cracks

Figure 2.Crackgeometriesof Palmqvistandmediancracks.

As both Palmqvist and mediancrack types are idealizedand differ more or lessfrom the actualcrackshapein cementedcarbides,modelsbasedon both cracktypeswill be takeninto consideration.

2.1 MedianCrack Models

Initial fracturemechanicsconceptswerebasedon theassumptionof halfpenny-shapedcracks. Measurementsof the indentationhalf diagonala, mediancrack length c(measuredascracktraceon thespecimensurface),indentationloadP, hardnessHandYoung's modulusE werecorrelatedwith the KIc value.From dimensionalconsidera-tions, Lawn & Fuller [12] deducedthat the ratio P/C3/2 shouldbe constant.Basedontheir work, Evans & Charles [13]deriveda widelyapplicablebut still semi-empiricalformula for the toughnesson the basisof dimensionalanalysisand empiricalcurvefitting:

150 R. SPIEGLER ET AL.

The validity of this relation is restrictedto caseswith cla>3. Evans & Charles[13] claimed that this characterizationshould enablefracture toughnessdata to beobtainedby indentationto within an accuracyof 30%.

Their expressionwas later expandedby a non-dimensionalfactor (EIH)n [14],which takes effects of the residual stressesbeneaththe hardnessindentationinto

account.Lawn etal. [15], who explicitly resolvedthe complexelastic-plasticstressfield beneaththe indentationinto a reversibleelasticstressfield componentand anirreversibleresidualstressfield component,derivedthefollowingequation:

KIc =0.028Hal/2(EI H)l/2(cI a)-3/2

The plasticzonesizewasincorporatedin their modelby usingHill's expandingcavitymodel.They showedthat theelasticcomponent,beingreversible,assumesasecondaryrole in the fractureprocess:althoughit doesenhancedownward(median) extensionduringthe loadinghalf cycle,it suppressessurface(radial)extensionto theextentthatsignificantgrowth continuesduring unloading.The residualcomponentaccordinglyprovidesthe primary driving force for the crack configurationin the final stagesofevolution,wherethe cracktendsto thehalf-pennygeometry.

Niihara etal. [16]split up theloadingrangeinto two regimes,wherePalmqvistandmediancracks, respectively,are formed.The transitionpoint was found by fittingpublisheddata.For cla>2.5 mediancracksshouldform, followingthe relation:

KIc =0.0309(EI H)215(PI c3/2)

The work of Anstis et al. [17] is basedon the resultsof Lawn et al. [15].By empiricalfitting of fracturetoughnessdataof a varietyof ceramicsthey calibratedthe scalingfactorandobtained

Laugier [18]usedananalyticalapproximationof therelativeplasticzonesizefor inputinto Hill's expandingcavity modeland obtaineda non-dimensionalfactor (EI H)213insteadof (EIH)l/2, which was introducedby Lawn et al. [15]. Laugier [18] showedthat (EI H)213gavethebetterfitting to experimentaldata.His formulahasthe form

KIc =O.OlO(EI H)2/3(PI c3/2)

2.2PalmqvistCrack Models

In the caseof Palmqvistcracksthe meanvalueof the individual crack lengths,I, isdefined,asshownin Figure 2. Warren & Matzke [19]showedempiricallyfor WC-Coalloys that KIc is proportionalto (HW)l/2, where W is the aforementionedPalmqvisttoughness.The constantof proportionalityis 0.087.A similar proportionalitywasderivedby Niihara [20] for cracklengths0.25

FRACTURE TOUGHNESS EVALUATION OF WC-Co ALLOYS 151

assumedto be accommodatedin the plastic zone. In reality apart of the hardnessimpressionvolume is takenup by some displacementof the materialto the surface(surfacepile-up). The plasticzonealsoprobablyundergoessomeelasticcompression.

3.TEST MATERIALS AND EXPERIMENTAL PROCEDURES

The WC-Co alloysusedin this studyarelistedin Table 1.All alloysarecommercialgradesandweremanufacturedby Metallwerk Plansee,Reutte,or in the caseof alloy10 CC by Krupp Widia, Essen.The lettersF, M andC denotethecarbidegrainsizelevels fine, mediumand coarse,respectively,while the numbersdenotethe cobaltweightfraction.

Table 1.Materials usedin thepresentstudy

j(Co)a

d(WC)bHvKICeW

Alloy

(%)(um)(MPa)(MPa m1/2) (Nimm)

6 F/M

9.80.831440011.687858M

12.91.431260014.2113158C

13.22.161170015.7117209F

14.80.681370012.4810409C

15.11.831150016.04285010M

16.11.501190015.52181510C

16.52.181190016.73237510 CC

16.24.46980018.80393012 F

20.40.691270013.06153012 C

19.81.681090017.04400015 F

24.70.641160013.631895

aCobaltvolumefraction.bWC grain size.eKle measuredwith single-edge-notched-beamspecimens.

The reported hardness and Palmqvist toughnessvalues are mean values ofmeasurementsat threeor moredifferentloads.Fracturetoughnessandmicrostructuralparametersof thesealloysweremeasuredvery accuratelyaspart of a studythatdealtwith the fracturemechanismof WC-Co [22].The methodof determiningthefracturetoughnesswas a single-edge-notched-beamtechniqueusing chevron-notchedspeci-mensto getsharpprecracks,which is describedin detailin thatstudy.

The broken fracturetoughnesssampiesof Sigl & Fischmeister[22]wereusedforthe presentindentationstudy.They were first ground planewith diamondgrindingwheels(74, 30 and 1011m),then polishedwith a Technotronsystem(polishing clothMM 414, polishingliquid MM 140) until an opticallyperfectsurfacewas obtained.Planopol equipmentfrom Struerswas usedfor all procedures.Since grindingcausessurfacestresses[3] which do affect crack development,a stressfree surfacemustbeachieved.Therefore, the polished specimenswere annealedat 850C for 2 h in anevacuatedquartzglassampouleto relieveresidualstressesandto restorethe originalstressstate.All sampies werecooledat 200 K/h in order to avoid influencesof thecoolingrateon residualstresses[23,24]andcobaltgrainsize[25].After annealing,thesampieswerebriefly polishedwith 6 11mdiamondpasteto removepossiblesurfaceoxidationlayerscausedby residualoxygenin theampoule.A comparisonwith sampies,wherethe deformedsurfacelayerswereremovedby polishingaccordingto Exner [3],showedno discernibledifferencesin themeasuredcracklengths.

152 R. SPIEGLER ET AL.

Apart from the surfacestressstateotherpotentialsourcesof errorneedattention:the cracksmust be largerthan the typical dimensionsof the microstructure;lateralvents,which interactwith the mediancracksystemin manyceramics,do not form inWC-Co alloyswith cobaltcontentsasusedin thepresentstudy;time-dependentcrackgrowth,whichis found in manybrittle materials,hashardlybeenobservedin WC-Coalloys [26];thus,effectsof the testingatmosphereshouldbe negligible.

All indentationswere madewith a commercialFrank hardnesstestingmachine.The distancebetweenan indentationandtheedgesof thesampieor otherindentationswas chosento be at leastfour timesthe distancebetweentheendsof oppositecracks,in orderto avoidinteractionsaffectingthecracklengths.Dependingonthealloy,loadswithin the range of 300-2500 N were applied and at each load three or moreindentationswere made. Crack lengths were in the range of 0.25l/ a)< 1.25(1.25cI a)

FRACTURE TOUGHNESS EVALUATION OF WC-Co ALLOYS 153

E 1000.3- '"

.c.... 800Cl c::QJ...J.:.:u 6001tI '-U....'"..... 400> ere....1tIc..- 2000 e;:]U) 00 50010001500200025003000

Load

[N]

Figure 3.Surnof Palrnqvistcracklengthsversusloadfor indentationsin alloy10CC.

that Palmqvistcracksarethe only cracksobservedin reasonablytoughmaterialssuchascementedcarbidegradeswith morethan6 wt% binder.

Despitethe differencein theactualcrackshape,theloaddependenceof thelengthsfor mediancracksaccordingto equation(2) is suitableto describethe behaviourofWC-Co, as the studiesof Laugier [18] or Shettyetal. [21] demonstrated.Lankford[30] evenclaimedthat the relationfor mediancracksdueto Evans & Charles[13] isapplicableto all crack morphologies.To compareequations(1) and (2), the simplerelationbetweenc,1 anda

c=l+a (3)

will beused.Insertingthe Palmqvisttoughness,Iv, andtherelationbetweenindenta-tion diagonalandhardnessinto equation(3) yields

C=P/4W+(P/2H)1/2 (4)

Figure 4 showsthe mean squarefits of equation(2) for mediancracksandequation(4) for Palmqvist cracks to the experimentalvalues obtainedwith alloy 10 CC.Surprisingly, both predictionsare almost identical. Therefore, both the Palmqvistmodelsandthe mediancrackmodelsmustbetakeninto considerationin a comparisonof indentationtoughnessmodels.

To find theindentationmodelbestapplicableto WC-Co, a combinationof variousexperimentalfacts will be used to develop suitable selectioncriteria. First, theformation of a steadystatebridging zone extendingnot more than 20 j,lm into thecrack wakeis a characteristicfeatureof crackingin WC-Co and is responsiblefor avirtually flat R-curve [31].The consequenceis a load-independentfracturetoughnessfor cracksin the asymptoticregime,i.e. for cracks>20 j,lm [32].Secondly,it shouldbe notedthatWC-Co is notproneto environmentallyassistedcracking[26].To assessthe applicabilityof indentationmodelsto WC-Co, our key demandis thus the loadandtime independenceof toughness.Obviouslywealsorequiretheindentationresultsto matchconventionallymeasuredtoughnessdata..

As the measured crack lengths of the studied alloys lie in the range

Median Cr'ack Fit

PalmQvist Cr'ack Fit

154 R. SPIEGLER ET AL.

600E5002-

c4000 ....1IlCQl 300E ....0

.::L

200

u coL100u

0

0

500 1000 1500 2000

Load [N]

2500 3000

Figure4. A comparisonof indentationcracklengthsand load datawith themeansquarefits ofequations(4) and (2) for Palmqvistandmediancracks,respectively(alloy 10 CC).

0.25lIa)

FRACTURE TOUGHNESS EVALUATION OF WC-Co ALLOYS 155

from further considerations.The reason for the load dependenceof toughnessisassumedto lie in the inadequatedescriptionof the Palmqvistcracksby the medianmodels.All other modelsshowno discernibleload dependence.Resultsof the crackmodelsof Laugier [18]or Niihara [20] arenot induded in the diagram.They differfrom the resultsof the Anstis et al. [17] modeljust by a constantfactor.Obviously,only the Shettyet al. [21] modelwith a toughnessvalueof 17.5MPa m1/2 is dose tothe expectedvalueof 18.8MPa ml12The modelsof Anstis etal., Laugier,Niihara etal. (mediancrack) andNiihara (Palmqvistcrack), however,yield unrealisticallyhightoughnessnumbersof 27, 33, 34 and 24.5MPa ml12ThereforeShetty'sapproachisconsideredto be mostapplicableto WC-Co alloys.

30

,Erc

25 r

Cl.. -0--0- -0- - -0-- - - - 0~cn

NIIHAAA 1981cn Q.lc:.r:: 20Cl ~0~

15 l...-.........-e- - -e- - - - -

....

u SHETTY ET AL.1985rc c-u..-0c:.... 10

050010001500200025003000

Load

[N]

Figure 6. Indentationfracturetoughnessvaluescalculatedwith thePalmqvistcrackmodelsof

Niihara [20]and Shettyetal. [21] for alloy 10CC.

Figure 7 showsthe toughnesscalculatedfrom the Shettymodelas a function ofcobaltvolumefractionand carbidegrainsize.With increasingcobaltvolumefractionthe indentationfracturetoughnessincreases.Coarse-grainedalloys are tougherthanfine-grainedalloysat constantcobaltvolumefraction.This behaviouris in agreementwith thetrendsknownfromconventionallymeasuredfracturetoughness[33].Figure8comparesthe indentation fracture toughnessaccordingto Shetty with Kic valuesmeasuredon single-edge-notched-beamspecimens.For comparison,the 1:1correlationline is added.The agreementis reasonable,thelargestrelativedeviationsbeingabout20%.

The accuracyof the indentationfracturetoughnessmeasurementis controlledbythe errorsin the hardnessHand Palmqvist toughnessW. Hardnesscanbe measuredvery exactly even with only a few indentations.The standarddeviation for threeindentationseachat three loads is only 0.7% on average.In the caseof Palmqvistindentationsthe precisionof the toughnessis stronglyaffectedby theWC grainsize:fine- or medium-grainedalloysexhibita standarddeviationfor W of the orderof 2%,while for coarse-grainedalloys the deviationapproaches8%.As Kic varieswith WJI2,the effectis small;the standarddeviationof theKic valueis 1% for fine- andmedium-grainedalloys,4% for coarse-grainedalloys.The scatterin measuringtheindentation

WC GRAIN SIZE

Fine

oMedium L Coarse 156 R. SPIEGLER ET AL.

305

25E 10a.~ 20Ul UlQ.Ic::.c 15CI ::J0I- 10.... U10t...u... 5"Cc::..... 0

0

5 10 15 20 25 30

Coba 1t Content (Vo1-%)

Figure 7.The dependenceofthe indentationtoughnessaccordingto theShettyetal. [21]modelof WC-Co alloyson cobaltcontentandcarbidegrain size.

fracturetoughnessis, therefore,considerablyless than the fit with conventionalKicvalues.The deviationsare thus deemedto be due to simplificationsin the Shettymodel ratherthan to experimentalerrors. For practicalwork, however,thesedevia-tions arecertainlytolerable.

WC GRAIN SIZE

Fine

o Medium

Coarse

30

~

25E lOa.~ 20lJ) lJ) 15QJ

C.c:Cl::J 10

0I-....

5u

lOt...I.J..

00

5 10 15 20 25 30

Ind. Fracture Toughness [MPamV~

Figure 8. Correlation of fracture toughnessvalues for WC-Co alloys,measuredwith single-edge-notched-beamspecimensand indentationfracturetoughnessvaluesaccordingto theShetty

etal. [21] model.

FRACTURE TOUGHNESS EVALUATION OF WC-Co ALLOYS 157

5.CONCLUSION

Our conclusionis that Palmqvistcrackmodelsare muchmore applicableto WC-Cocermetsthan are the median crack approaches.This view is supportedby crackprofiling of commercialWC-Co alloys which has brought clear evidencefor theexistenceof Palmqvistcracksunder loadingconditionscomparablewith thoseof thepresentstudy.Not surprisingly,our studydemonstratesthatanymedian-typeindenta-tion modeleitherdoesnot supplyload-independentfracturt::toughnessdataor largelyoverestimatesthe fracture resistanceof WC-Co alloys. Consequentlywe regardmediancrackmodelsinappropriateto describeindentationcrackingin WC-Co.

In general,Palmqvist crack modelsdo satisfythe criterion of load independentfracturetoughness.This result is expectedin the light of the metallographieevidencediscussedabove.Nevertheless,somePalmqvist-typemodelsyieldpooragreementwithabsolutevalues of toughnessobtain~dfrom single-edge-notched-beamexperiments.Only Shetty'sformuladeviatesfrom conventionallymeasuredKic valuesby lessthan20%.It is thereforeconsideredsuperiorto the othermodelsdiscussedhere.

Shetty'sapproachallows a quick extractionof fairly accuratefracturetoughnessnumbersof WC-Co cermetsfrom only a few indentations.It is thereforeanattractivetool for the screeningof variousgradesof cementedcarbides.This featurecouldbeofspecialinterestto the hard metalindustrywhich so far hasbeenlackinga cheapandreliablemethodof assessingKlc in routinecontrol.However,carehasto be takensofar as extrapolationof the Shettymodel to other cermetmaterialsis concerned.Asresultson ceramiesin the toughnessrangebelow 8 MPa milz indicate,the reducedfracture resistanceof materialslike TiC- TiN-Ni might lead to a transition fromPalmqvist-typeto median-typecracks which would invalidate the Shetty model.Addressingthe questionof the applicabilityof indentationmodelsto cermetslesstoughthanWC-Co will be the subjectof forthcomingresearch.

ACKNOWLEDGEMENTS

S. Schmauderappreciatesthe financialsupportthrougha scholarshipsuppliedby theJapan Society for the Promotion of Science (JSPS). The authors are grateful toMetallwerk Plansee, Reutte, Austria, and Krupp Widia, Essen, for supplying thesampies.Discussionwith ProfessorExner led to improvementsof thispaper.

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