Availableonlineatwww.sciencedirect.com
Journal of Applied Research and Technology
www.jart.ccadet.unam.mx JournalofAppliedResearchandTechnology15(2017)396–401
Original
Annealing effect on the mechanical properties of ultrafine WC–Co materials
Yigao Yuan
∗, Lanping Fu, Jianpeng Li
CollegeofMechanicalEngineering,DonghuaUniversity,201620Shanghai,China Received6July2016;accepted21March2017
Availableonline26August2017
Abstract
Indrymachining,thetool-chipinterfacetemperatureisacriticalfactoraffectingtheengineeringperformanceofWC–Cotools.Asthetemperature variationofthecuttingtoolindrymachiningissimilartothatinannealingprocess,theeffectoftool-chipinterfacetemperatureonthemechanical propertiesofcuttingtoolmaterialsindrymachiningcanbestudiedbymeansofannealingexperiment.Inthispaper,aseriesofannealingexperiments ofultrafine-grainedWC–Comaterialswereconducted,andtheeffectsoftheannealingtemperatureandholdingtimeonthemechanicalproperties ofWC–Comaterials,includingflexuralstrength,hardnessandfracturetoughness,wereinvestigated.Theresultsshowthatundertheexperimental conditions,theannealingtemperaturedominatesthemechanicalpropertiesofWC–Comaterials.Theflexuralstrengthandfracturetoughnessof theultrafine-grainedWC–Comaterialsdecreaseindifferentextentwithincreasingofannealingtemperature,whereasthehardnessdoesnotchange obviously.Themeasurementsofthe microstructuresandresidualstressesrevealedthatthedeteriorationofmechanicalpropertiesofWC–Co materialsafterannealingcanbeattributedtotherelaxationofcompressiveresidualstress.
©2017UniversidadNacionalAutónomadeMéxico,CentrodeCienciasAplicadasyDesarrolloTecnológico.Thisisanopenaccessarticleunder theCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Ultrafinecementedcarbides;Annealing;Mechanicalproperties;Residualstress
1. Introduction
Withtherapiddevelopmentofmodernmanufacturingindus- try,highspeedmachiningandprecisionmachininghavebeen usedextensivelyinstructuralpartsmanufacturing.Additionally, drymachiningwithoutcuttingfluidorwithminimalquantityof lubricant(MQL)canobviouslyreducetheharmofcoolantsto theenvironment,whichisproducedmainlyinwetmachining (Hadad&Sharbati,2016).Inviewofthe abovereasons,it is veryimportantfor cuttingtoolstohavegoodtribologicaland thermalpropertiesatseveremachiningconditions.
Inthepastdecades,theultrafine-grainedWC–Cocemented carbideswithgrainsizelessthan0.6mhavebeenwidelyused as cuttingtools in machining processes, owing totheir high hardnessandwearresistance(Jia,Fischer,&Gallois,1998;Shi, Shao,Duan,Yuan,&Lin,2005).Indrymachiningprocesses, cuttingtoolsaregenerallysubjectedtoseveremechanicaland thermalconditions,andtheheatproducedduringthemachining
∗Correspondingauthor.
E-mailaddress:yuanyg@dhu.edu.cn(Y.Yuan).
PeerReviewundertheresponsibilityofUniversidadNacionalAutónomade México.
process is critical for tool life andworkpiece surface quality (Nouari,List,Girot,&Coupard,2003).Accordingtoprevious works (Ezugwu&Wang,1997;Jiang,2005),thetemperature along the tool-chipinterface canreach ashighas 1000◦Cin cuttinghard-to-machinematerials,suchastitaniumalloysand nickelalloys.Suchahightemperaturewillresultinnotonlytool wearsuchasabrasion,adhesionanddiffusion,butalsothedete- riorationofmechanicalpropertiesofcuttingtoolmaterials.List etal.(2005)describedtoolwearmechanismindrymachining ofacommonAl-CualloywithanuncoatedWC–Cotool.They foundthat toolwearbehaviorisprimarilydependent ontool- chipinterfacetemperature.Atconventionalcuttingconditions, theinterfacetemperatureislow(150–180◦C)andtheadhesion of built-up edge(BUE)isprincipallymechanical,continuous sliding of BUEfragments betweentool and chipcauses tool wearincreasingly.Onthecontrary,atseverecuttingconditions, temperature is higher (245–310◦C) and mechanisms of tool wear involvechemicalactionanddiffusion. Kagnaya,Boher, Lambert, Lazard, and Cutard (2009) have studied the wear mechanism of WC–Co cutting tools from high-speed tribo- logicaltests.TheyfoundthatWC–Cotribologicalpinsexhibit different wear mechanisms: abrasion, adhesion,transgranular WC micro-cracking and WC/WC debonding. At low sliding
http://dx.doi.org/10.1016/j.jart.2017.03.005
1665-6423/©2017UniversidadNacionalAutónomadeMéxico,CentrodeCienciasAplicadasyDesarrolloTecnológico.Thisisanopenaccessarticleunderthe CCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).
speeds,thewearmechanismsof thepinincludeplasticdefor- mation and microcracking of WC grains, fragmentation and debondingofWCgrainsandpolishingofthepincontactsurface.
But, athigh slidingspeeds,88.5% of the mechanical energy dissipated inthe pin is transformed into thermal energy and adhesionplaysanimportantpartinthetoolwearmechanisms.
Althoughtherehasbeenconsiderableresearchreportedover thelasttwodecadesonthecorrelationbetweentool-chipinter- facetemperatureandthewearbehaviorofWC–Cotoolsindry machining,theresearchontheinfluenceoftool-chipinterface temperature onthe mechanical propertiesof WC–Co materi- als,tothebestoftheauthor’sknowledge,hasnotbeendeeply reportedinthe openliteratureuptonow.In fact,variationof temperatureof thecuttingtool indrymachining issimilarto thatinannealing process,i.e., thetemperature ofcuttingtool graduallyrisesatthebeginningofmachining,andthenremains atastabletemperaturefor aperiodof time, finallydecreases slowly to room temperature after machining, thus the effect of tool-chip interface temperature on the mechanical proper- tiesofcuttingtoolmaterialsindrymachiningcanbestudied bymeansofannealingexperimentsofWC–Comaterials.Inthe presentstudy,aseriesofannealingexperimentsoftheultrafine- grainedWC–Comaterialswereconductedtoevaluatetheeffect ofannealingtemperatureandholdingtimeonthemechanical propertiesoftheWC–Comaterials.
2. Experimental
2.1. Materialsandsamplepreparation
The materials used in this work were ultrafine-grained WC–Co cemented carbides with nominal composition of 12wt.%CoandmeanWCgrainsizeof0.4m.Theprepara- tionofsampleswascarriedoutasfollows:TheWC–12wt.%Co mixed powder was milled in alcohol for 72h in a ball mill.
Themilledpowderwasdriedat95◦Candthencoldpressedat 200MPaintogreencompactsof21mm×6.5mm×5.25mmin dimensions.Finally,thegreencompacts weresintered invac- uumat1400◦Cfor1h,andthenthehotisostaticpressing(HIP) technologywasusedtominimizepores,consideringthefactthat mechanicalpropertiesoftheWC–Comaterialsareverysensitive toporositylevels.Theopticalmicrographof polishedsurface oftheultrafine-grainedWC–ComaterialsisshowninFigure1.
AscanbeseenfromFigure1,thesampleisdenseenoughand porositiescanbenegligible.
Aftersintering,allsamplesweregroundtodimensionsusing aresin-bonded diamondstraight wheelanda M7120surface grinder,subsequentlywerepolishedwith10mdiamondpaste toachievedimensionalaccuracyaswellasgoodsurfacefinishes.
Itshouldbenotedthatsurfaceroughness(Ra)ofaspecimenmust belessthan0.4m,oritwouldnotbeusedforthisstudy.The dimensionsofthewheelswere250mmindiameter,12mmin width,and8mminthethicknessofthediamondlayer.Grinding wascarriedonthe21mm×6.5mmsurfaceofthesamples.Prior tobeginningtheexperiments,thesamplesweregluedonasteel platewithhotwax.Then,thesteelplatewasfixedonthegrinding machineasparallelaspossiblewiththeabrasivewheeltoavoid
50 μm 100X Fig.1.Surfacemorphologyofthematerialsinvestigated.
Table1
Grindingparametersusedduringgrinding.
Wheelspeed,m/s Tablespeed (workpiece), m/min
Averagesizeofthe abrasivegrains,m
Depthofcut,
m
20 30 109 10
Table2
Annealingheattreatmentparameters.
Annealing temperature,◦C
Holdingtime,min Heatingrate,
◦C/min
Cooling method 300,500,700,900
and1100
30 5 Furnace
coolinga
900 10,20,30,40and80
aAfterholdingtheannealingtemperatureforaprescribedtime,thenturning offthepower,thesampleswerecooledslowlytoroomtemperatureinthefurnace.
avariationofdepthofcutoverthesurfaceofthesample.Water- oilemulsionwasusedasacoolantduringthegrindingprocessto avoidburningoutandthermaldamage.Thegrindingparameters wereusedlistedinTable1.
Aftergrinding,allsampleswereultrasonicallycleanedwith acetone,andthensomesampleswereannealedinavacuumsin- teringfurnace.Inthisstudy,therangeofannealingtemperatures wasselectedfrom300to1100◦C,whichisbasedonthehighest temperature of tool-chipinterfaceindrymachining ofdiffer- entmaterialsprovidedin(Ezugwu&Wang,1997;Jiang,2005;
List et al., 2005).During the annealing experiments,various parameterswereusedaslistedinTable2.
2.2. Characterizationofmechanicalproperties
The surfaces of samples before and after annealing were investigated using an X-ray microstress analyzer (LXRD, Canada) to measure the residual surface stresses by sin2ψ method.Thestressesinthesurfaceofsamplesweremeasured usingthe(301)reflectionofWCwithCuKaradiation.Thefull widthhalfmeanpeakpositionwasdeterminedforarangeofψ
angles(−25◦,−16.78◦,−5.85◦,0◦,5.85◦,16.78◦and25◦).The residualstressmeasurements wererepeatedsixtimesoneach sampletoimproveaccuracyandtoobtainthemeanvalue.
Mechanical properties of the specimens before and after annealing, including flexural strength, hardness and fracture toughness,weremeasuredandcompared.Theflexuralstrength was measured on the ground specimens by universal testing machines according to ASTM B406 standard. The hardness wasmeasuredbyVickershardnesstesterwithaloadof30kgf accordingtoASTMB294standard.Thefracturetoughnesswas obtainedbymeasuringthecracklengthgeneratedatthevertices ofVickersindentationusingtheequation(Schubert,Neumeister, Kinger,&Lux,1998):
K1C=0.0028
H×P L
1/2
(1)
whereHistheindentationhardness(N/mm2),Pistheinden- tationload(N),and
Listhesumofcracklengths(mm).At leasttenspecimensweretestedforthe measurementsofeach property,andtheaveragevaluewasfinallydetermined.
In addition, microstructure of samples before and after annealing werealsoinvestigated by scanningelectron micro- scope (SEM) and X-ray diffraction (XRD) to find whether any growth of WC grain or phase transformation of the Co phase has taken place during annealing. The average grain size of WC was measured based on the backscattered elec- tron(BSE) images of themicrostructure andIMAGEJimage analysis software (Yuan, Zhang, Ding, & Ruan, 2013). The CophasecompositionofthespecimenswasdetectedbyX-ray diffractometrywithCuKaradiation.InordertodecreaseWC phasediffractionpeakinterferencetoCo-phase X-raydiffrac- tion analysis, specimens wereetched by Murakami’s reagent (K3Fe(CN)6:KOH:H2O=1:1:10)for24hbeforeXRDexami- nation,consideringthatCocontentislowerinWC–Comaterials.
3. Resultsanddiscussion
3.1. Effectofannealingtemperature
Cementedcarbidesaredifficulttomachinebecauseoftheir extremehardness,sogrindingwithdiamondwheelstodateis stillthe mostimportantmachiningmethodintheindustryfor variousWC–Comaterials.Ingrinding,duetothe mechanical interactionsofdiamondabrasiveswiththecarbidematerials,sig- nificantresidualstress,namely,grinding-induceresidualstress orgrindingresidualstresswasgeneratednearthegroundsurface ofsample.Theexistingworkhasshownthatthegrinding-induce residualstressinthegroundcarbidematerialsiscompressivein WCandtensileinCo(Hegeman,DeHosson,&DeWith,2001;
Yinetal.,2004),anditcanberelievedeffectivelybyanneal- ingheattreatment(Yang,Odén,Johansson-Jõesaar,&Llanes, 2014).Relaxationdegreeofresidualstressescanbedefinedas therelaxationrate,anditcanbeexpressedas(Yuan&Xu,2012):
σrelr =σbr−σar
σrb ×100% (2)
whereσrelr istherelaxationrateofresidualstresses,σbr,σar are themagnitudesofresidualstressesingroundsurfacebeforeand aftertreated,respectively.
Afterannealedat300,500,700,900and1100◦Cwiththe holding time of 30min respectively, the variations of relax- ation rate of residualstresses, flexural strength, hardnessand fracture toughness of ultrafine-grained WC–Co materials are showninFigure2.Itcanbeclearlyseenthattheannealingtem- peratureshaveasignificantinfluenceontherelaxationrate of residual stressesandflexural strength of theultrafine-grained WC–Comaterials.Relaxationrateofresidualstressesincreases almostlinearlywithincreasingofannealingtemperaturesfrom 300 to 900◦C, then decreases suddenly at temperature over 900◦C (as shown in Fig. 2a). By contrast, flexural strength oftheultrafine-grainedWC–Comaterialsdecreasesobviously withincreasingofannealingtemperaturesfrom300to900◦C.
Onlywhenannealingtemperatureexceeds900◦C,willflexural strength rebound slightlybut still lowerthan that of samples untreated(asshowninFig.2b).
Itcanalsobeseenthatthefracturetoughnessofthesamples afterannealingdecreasedslightlycomparedwiththatofsamples untreated,butitvariesslightlywithannealingtemperature(as showninFig.2c).AstohardnessofWC–Comaterials,itseems thattheannealingtemperatureshavenoobviousinfluence.The measurements of the hardness showthat thereis onlyalittle difference betweenthe samplesof beforeandafter annealing (asshowninFig.2d).
3.2. Effectofholdingtime
Thegroundsampleswereannealedat900◦Cwiththeholding timeof10,20,30,40and80minrespectively.Figure3displays the variations of relaxation rate of residual stresses, flexural strength, hardnessandfracture toughness ofultrafine-grained WC–Comaterialswiththeholdingtime.AsseenfromFigure3, although thereareslight decreaseinthe flexuralstrengthand fracturetoughnessoftheannealedsamplesincomparisonwith thatofsamplesuntreated,noobviousdifferencewereobserved between the different holdingtimes. Also, the measurements show that the holding time has no obvious influence on the residualstressesandhardnessoftheultrafine-grainedWC–Co materials.Theseresultssuggestthatannealingtemperaturedur- ingannealingprocessmaydominatethemechanicalproperties ofWC–Comaterials,insteadofholdingtime.
3.3. Microstructure
Figure4showstheX-raydiffractionpatternsofthesamples beforeandafterannealing.ComparingwiththeX-raypatternsof theuntreatedsample,itcanbeclearlyseenthatthecharacteristic diffractionpeaksoftheannealedoneshavenoobviouschange, whether atannealing temperature900◦Cor at1100◦C,indi- catingnophasetransformationofCofrom␣→occursduring annealingprogress.
Figure5revealsthemicrostructuresoftheultrafine-grained WC–Co materials before and after annealing, in which the whitecrystalsaretheWCphase,andtheblackcontrastregions
Relaxation rate of residual stress,%
80
60
40
20
0
0 200
200 200
400 200
400 400
600 400
600 600
800 600
800 800
1,000 800
1,000 1,000
1,000 1,200
1,200 1,200
1,200 Annealing temperature,°C
Annealing temperature,°C Annealing temperature,°C
Annealing temperature,°C
Holding time:30 min Holding time:30 min
Holding time:30 min
Holding time:30 min
Fracture toughness, MPa.m1/2
12
10
8
6
4
2
0 0
a
d b
c
5,000
4,000
3,000
2,000
2,000
1,000
0
0 0
0 1,800 1,600 1,400 1,200 1,000 800 600 400 200
Flexural strength, MPa Hardness, HV
Fig.2. VariationofmechanicalpropertiesofWC–Comaterialswithannealingtemperatures.(a)Residualstress,(b)flexuralstrength,(c)fracturetoughnessand(d) hardness.
100
100
100 100
100 80
80
80 80
80 60
60
60 60
60 40
40
40 40
40 20
20
20 20
20 0
0 0
0 12
10
8
6
4
2
2,000
200 400 600 800 1,000 1,200 1,400 1,600 1,800
1,000 2,000 3,000 4,000 5,000
0
0 0
0 Annealing temperature: 900°C
Annealing temperature: 900°C
Annealing temperature: 900°C
Annealing temperature: 900°C
Relaxation rate of residual stress,% Fracture toughness, MPa.m1/2
Holding time, min
Holding time, min
Holding time, min
Holding time, min
Flexural strength, MPa Hardness, HV
a
b d
c
Fig.3.MechanicalpropertiesofWC–Comaterialsversusholdingtime.(a)Residualstress,(b)flexuralstrength,(c)fracturetoughnessand(d)hardness.
α - Co(201)
Intensity (cps)
untreated annealed at 900°C annealed at 1100°C
120 100 80 60 40 20
30 32 34 36 38 40 42 44 46 48 50 52 54 56 2Theta(deg)
58 60 62 64 66 68 70
Fig.4.X-raydiffractionspectrumofultrafinecementedcarbides.
Table3
MeasurementresultsofWCgrainsizesinsamples.
Samples AveragegrainsizeofWC(m)
Beforeannealing 0.465
Afterannealing
Annealingtemperature:900◦C,holding time:80min
0.497
Annealingtemperature:1100◦C,holding time:80min
0.526
betweentheWCgrainsarethecobaltbinderphase.Themea- surementresultsofWCgrainsizesinsamplesbeforeandafter annealingarelistedinTable3.ItcanbeseenthattheWCgrain sizesoftheannealedsamplesarelargerthanthatoftheuntreated ones,butthedifferenceisquitesmall.ItillustratesthattheWC grainshaveatendencytogrowupduringannealingprocess,but thegrowthrateisverysmall.
Ingeneral,themechanicalpropertiesoftheWC–Comateri- alsareprimarilydependentontheirmicrostructuralparameters, i.e.,cobalt(Co)contentandWCgrainsize.WithsmallerWC particle size atfixed Co content,WC–Co has ahigher hard- ness,higherflexuralstrengthandlowertoughness.Conversely, WC–Co would havea higher toughness,lower hardness and lowerflexuralstrength(Akhtar,Humail,Askari,Tian,&Guo, 2007; Okamoto, Nakazono, Otsuka, Shimoitani, & Takada, 2005).Accordingtothemeasurementsofmicrostructureofthe samples before and after annealing, and taking into account thestabilityofCocontentinWC–Comaterialsduringvacuum annealing,theflexuralstrengthandhardnessofultrafine-grained WC–Comaterialsafterannealingshoulddecreaseslightly,while thefracturetoughnessshouldincreaseslightlyduetotheslight growthofWCgrainsizes.However,ourexperimentsshowthat afterannealingprocess,theflexuralstrengthofultrafine-grained WC–Comaterialsdecreasessignificantlyandthefracturetough- nessdecreasesslightly,thisisnotinaccordancewiththetheories mentionedabove,indicatingthatsomeotherfactorsmayplay important role in affecting the mechanical properties of the WC–Comaterialsduringannealing.
Besides microstructuralparameters, the natureandmagni- tudeofresidualsurfacestresshavealsosignificanteffectsonthe mechanicalpropertiesofWC–Comaterials.ByinhibitingWC particle pull-out and by introducingstresses that closecrack tips, compressive surface stresses can retard crack initiation
det BASED
det BASED
det BASED
High vaccum vacMode
vacMode High vacuum
vacMode High vacuum
HV 15.00kV
HV 15.00kV
HV 15.00kV
5 μm 1
5 μm 2
5 μm 3
c b a
mag 10 000 x mag 10 000 x mag 10 000 x
Fig.5.SEMbackscatteredelectronimagesofthematerialsinvestigated.(a) Untreated,(b)annealedat900◦C,80minand(c)annealedat1100◦C,80min.
and propagation, and thus increasing the fracture toughness of WC–Co materials (Lambropoulos, 1991). Meanwhile, a compressiveresidualstresscanalsobeappliedtoimprovethe strengthofgroundspecimens,sincethetotalstrengthScanbe estimatedbyXu,Jahanmir,andIves(1997):
S=SG+Sσ (3)
whereSGisthestrengthof thematerialgoverned byintrinsic flaws only, and Sσ is the extra stress that must be added to compensateforcompressiveresidualstress.
From Figure 2a, the relaxation rate of residual stresses increases almost linearly with increasing of annealing tem- peraturesfrom 300 to900◦C, that is tosay, the magnitudes of surface compressive residual stress of annealed samples decreasequicklywithincreasingofannealingtemperaturesfrom 300to900◦Ccomparewiththatofsamplesuntreated.Accord- ing to formula (3), the flexural strength of ultrafine-grained WC–Co materials after annealing will decrease significantly with increasing of annealing temperatures, which is just the measuredresultshowninFigure2b.Relaxationrateofresidual stresses of WC–Co materials decreases obviously at anneal- ingtemperatureabove900◦Cmayattributetoresidualthermal stress,inducedbylargertemperaturegradientuponcoolingfrom annealingtemperaturebacktoroomtemperature.Accordingly, the flexural strength of WC–Co materials after annealing at 1100◦Creboundsslightly.Similarly,duetorelaxationofcom- pressive residual stresses, the fracture toughness of WC–Co materialsafterannealingdecreasesslightly.
TherelaxationprocessofgrindingresidualstressinWC–Co alloyswouldbecompletedinashortperiodoftime,forthesize ofsampleswassmall.Asaresult,holdingtimewasfoundtohave littleinfluenceontherelaxationrateofresidualstresses.Accord- ingly,themechanicalpropertiesofultrafineWC–Comaterials afterannealinghavenoobviouschangeunderdifferentholding timeconditions.
4. Conclusion
Inthisstudy, theeffectof annealingheattreatment onthe mechanical properties of ultrafine-grained WC–Co materials was examined through a series of annealing experiments. It wasfound that under thecondition of this study,the anneal- ing temperature plays an important part on the mechanical properties of WC–Co materials, but the influence of hold- ingtimeisrelativelysmall.Theflexuralstrengthandfracture toughnessofthe ultrafine-grainedWC–Comaterialsdecrease in different extent withincreasing of annealing temperature, and the hardness almost remain unchanged during anneal- ing.
From the investigation of microstructures and residual stresses of WC–Co materials,it can be found that the dete- rioration of mechanical properties of WC–Co materials after annealingtreatmentcanbeattributedtotherelaxationofcom- pressiveresidualstress.
Conflictofinterest
Theauthorshavenoconflictsofinteresttodeclare.
Acknowledgement
ThisworkissupportedbyShanghaiNaturalScienceFoun- dation(No.13ZR1401300).
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