production of al 2 o 3 sic nano composites by spark plasma sintering

Samples density, open and closed porosity and water absorption were estimated by Archimedes method [19].. 2 – W–H diagram of milled samples for calculation of crystalline sizes and mean

w w w e l s e v i e r e s / b s e c v

Q1

Q2

a r t i c l e i n f o

Accepted24January2017

Availableonlinexxx

Keywords:

Sintering

Hardness

Nano-composite

SPS

Strength

Density

XRD

Wear

a b s t r a c t

Inthispaper,Al2O3–SiCcompositeswereproducedbySPSattemperaturesof1600◦Cfor

weresinteredinaSPSmachine.Aftersinteringprocess,phasestudies,densificationand mechanicalpropertiesofAl2O3–SiCcompositeswereexamined.Resultsshowedthatthe specimenscontainingmicro-sizedSiChaveanimportanteffectonbulkdensity,hardness andstrength.Thehighestrelativedensity,hardnessandstrengthwere99.7%,324.6HVand

2329MPa,respectively,inAl2O3–20wt%SiCmicrocomposite.Duetoshorttimesintering,the growthwaslimitedandgrainsstillremainedinnano-meterscale

©2017SECV.PublishedbyElsevierEspa ˜na,S.L.U.Thisisanopenaccessarticleunderthe

CCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/)

Producción de nano-composites – SiC–Al2O3por spark plasma sinterizado

Palabras clave:

Sinterización

Dureza

Nano-composite

SPS

Fuerza

Densidad

DRX

Desgaste

EnestetrabajosemuestrancompuestosdeAl2O3-SiCproducidosporSPS,envacío,a1.600◦C durante10min.Paralapreparacióndemuestras,semolieronpolvosdeAl2O3durante5h conlasegundafasedemicro-y-nanopolvodeSiC.Posteriormente,estospolvosmolidos

sesinterizaronmedianteSPS.Despuésdelprocesodesinterización,serealizaronestudios

defase,densificaciónypropiedadesmecánicasdeloscompuestosdeAl2O3-SiCobtenidos Losresultadosmostraronquemicro-SiCenlasmuestrastieneunefectoimportanteensu densidadaparente,durezayresistencia.Lamayordensidadrelativa,durezayresistencia fueronrespectivamentedel99,7%,324,6HVy2.329MPaparaAl2O3 conun20%enpeso micro-SiC.Debidoalcortotiempodesinterización,elcrecimientolosgranosfuelimitadoy

semantuvieronenescalananométrica

©2017SECV.PublicadoporElsevierEspa ˜na,S.L.U.Esteesunart´ıculoOpenAccessbajola

licenciaCCBY-NC-ND(http://creativecommons.org/licenses/by-nc-nd/4.0/)

∗ Corresponding author.

E-mailaddress:m-razavi@merc.ac.ir(M.Razavi)

http://dx.doi.org/10.1016/j.bsecv.2017.01.002

0366-3175/©2017SECV.PublishedbyElsevier Espa ˜na,S.L.U.Thisisanopen accessarticleundertheCCBY-NC-NDlicense (http:// creativecommons.org/licenses/by-nc-nd/4.0/)

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Introduction

Thermalandchemicalstability,relativelyhighstrength,

ther-Q3

advan-tages,lowfracturetoughnessofthismaterialleadtolimitation

ofitsapplication.Compositesareoneofthemethodswhich

overcometothislimitation.Inthistechniquealuminamatrix

isreinforcedbyparticlesorfibersassecondaryphase,which

canbemetal,polymerorceramic.Siliconcarbide(SiC)asa

ceramicmaterial canbe one ofthe option which leads to

improvementofaluminamatrix[6–10].Niharaetal.reported

thatsinteringofAl2O3–SiCcompositewasdonesuccessfully

They found out that adding a little amount of SiC to

alu-minamatrixcanimprovemechanicalpropertiesofcomposite

significantly in comparison with non-composite materials

Theyincreasedstrengthandfracturetoughnessfrom350to

1520MPaand3.5to4.8MPam1/2,respectivelybyadding5vol.%

SiC[11]

There are different methods of sinter this composite

Non-pressureandhotpresssinteringarethemostcommon

methodofsintering forthiscomposite, but newtechnique

whichisconsiderabletodayissparkplasmasintering (SPS)

[6,12–15] On the base of spark plasma, which is created

byapulsed directcurrent,SPSleadstoquickincreasingof

mold’sandthepowder’stemperature.Highheatingrate,using

pressure and electricalcurrent is the specifications ofthis

technique which distinguishthis technique in comparison

withothermethod.Inadditiontoreductionofparticle’s

coars-ening,highheatingrateincreasedcondensationthroughthe

eliminationofsurfacediffusionmechanismandcreatingof

extra driving force by high temperature gradient Pressure

applyingduringtheheatingcanincreasethedrivingforceof

processandfacilitatethesinteringprocess.Electricalcurrent

cancondensethepowderinmoldbycreatingofmanysparks

betweenparticlesandcreatingofplasmaenvironment.Effect

ofplasmaonsurface’s cleannessofparticles and

improve-mentofsintering processhasbeen reportedbyresearchers

[16–21]

SynthesisofAl2O3–SiCcompositebySPShasbeen

inves-tigatedbyafewresearchers[16,17],buttheeffectofparticle

sizeonthedensification,mechanicalandwearpropertieshas

notbeenreporteduntilnow.Sointhispapersintering

pro-cessandpropertiesofAl2O3matrixreinforcedbymicroand

nano-sizedSiCwillbeexamined

Experimental

Al2O3andSiCpowdersinmicroandnano-meterscalewith

purity99.8%,99.5%and99.9%andmeanparticlesizeof1.5␮m,

10␮mand 50nm, respectivelywere usedasraw materials

Al2O3 powder withtwo sourcesofSiC(microand nano as

systems1and2,respectively)powderweremilledina

plan-etaryballmill(asahighenergyballmill)for5hindistilled

water.Balltopowderratiowas10to1inalltests.Inthe

fol-lowing,preparedpowdersweresinteredinamoldwith8cmin

diameterunderspecificconditionsaccordingtoTable1bySPS T

◦C)

Soaking time

Al2

O3

∗Systems

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machine.Thesinteringprocesswasdoneunderhighpulsed

directcurrent(1000–3500A)invacuum.Auniaxialpressureof

10MPawasusedduringthereactionandincreasedto20MPa

afterreachingtotheexpectedtemperatureandmaintained

duringholdingtime.Afterholdingtimetheuniaxialpressure

decreasedto10MPaandmaintainedduringcooling

Aftersinteringprocess,thesampleswerepolishedandcut

Inordertodetectofphasesinsample’sstructureand

eval-uateofpropertiesXRD (Siemens,30kV,25mA,CuK␣)was

used.Thecrystallitesizeandstrainwereevaluatedthrough

ScherrerandWilliamson–Hallmethodsapplyingthefollowing

equations[18]:

d= 0.9

bcos Scherrerequation

Bcos= 0.9

d +2sin Williamson-Hallequation

whereB,,,dandarethefullwidthofthepeakathalf

inten-sity(rad.),positionofpeakinthepattern(rad.),thewavelength

ofX-ray(nm),crystallinesize(nm),andmeaninternalstrain,

respectively

Samples density, open and closed porosity and water

absorption were estimated by Archimedes method [19]

Strengthofthesampleswasmeasuredthroughthreepoints

test.Fivesampleswithdimensionof3×4×45mmwere

pre-paredandaverageofstrengthreported[20].Hardnessofthe

samplewasdeterminedthroughVickersmethod.5testswere

doneforeachsampleandaverageofresultswerereported[21]

Microstructuresofmilledandsinteredsampleswerestudied

byscanningelectronmicroscope(Cambridgemodel).Finally,

wearresistanceofsampleswasdoneinordertodetermine

thewearproperties.Inthistestcompositesampleswereused

asapinandaluminawasusedasthedisc.Theforceonthe

pintipwas15.3N.Themachinewasstoppedinthedistances

of1000mandtheweightlossesofthesampleswerewiththe

accuracyof0.0001g[22]

Result and discussion

PatternsofX-raydiffractionofmilledpowdersareillustrated

inFig.1.Asitisseeninthesepatterns,millinghasnotled

tophasetransferinrawmaterials,andidentifiedphasesare

Al2O3 and SiC with1125-071-01 and 00-002-1048reference

code,respectively.Asit stands,byincreasingofSiCphase,

intensityoftheirpeakshasbeenincreased.Crystallinesize

(d)andmeanstrain()ofmilledpowderweremeasured.The

changesofB·costo2sinareseeninFig.2.Calculationresults

arebroughtinTable2, asit isseeninthistablecrystalline

sizeofmilledpowderinallcompoundsforbothphasesare

innano-meterscale.Thecrystallinesizesofmilledsamples

(whichallofthemhavebeentreatedinasimilarway)havea

rangebetween36and40nmand17and36nmforAl2O3and

SiCphases,respectivelyandnosignificantdifferencebetween

sizes.Asitstands,sizeofphasesinthesecondsystemisfiner

thanfirstsystem,thatcanbeattributedtotworeasons:the

firstoneisusingofSiCwithnanoscaleinthesecondsystem

andthenextoneisthepresenceofmorefinerSiCparticlesin

thesecondsystematequalweightfractionwhichcanoperate

asfineballsandfacilitateofmillingprocess.Theseparticles

increasesmillingenergyandleadtocrushofparticles[23]

Counts

10n

5n

20m

15m

5m

Position [º2Theta](copper (cu))

10m

Fig 1 – X-ray patterns of milled samples (*) Al 2 O 3 , (+) SiC.

SEMimageformmilledsamplesarepresentedinFig.3.Asit

isobviousinthisfigure,millingleadtodecreasingthesizeof particlesinbothsystemsandthemeanparticlesizesarein nanometerscale.Thesizeofparticlesinthesystem contain-ingnanoSiCarelowerthanthesystemwithoutthat

X-raypatternofsinteredsampleisbroughtinFig.4,asitis seen,thereisnochangeinphasesofsinteredsamplesandin milledsample(Fig.1 Al2O3andSiCareidentifiedphases.As

itisobviousinFig.5,theonlyconsiderablepointin compar-isonofthispatternwithmilledsamplepatternisincreasing

ofpeak’sintensityanddecreasingofpeak’swidthinsintered sample.Increasingoftemperatureduringthesintering can leadtogrowthofcrystalsandreducingofmeanlatticestrain

[24,25] Thechanges ofB·cos to 2sin insintered samplesare showninFig.6.Theresultsofthesecalculationsarebrought

in Table 2 As these calculations show, although the crys-tallinesizesareinnanometerscalewiththerangeof59–66nm and30–52nmforAl2O3andSiCphases,respectivelybut sin-tering leads togrowth of crystals and decreasing ofmean

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0.005 0.01 0.015 0.02 0.025

1.4 1.2

1 0.8

0.6 0.4

2 sinθ

5m (Alumina) 10m (Alumina) 10m (Silicon carbide) 15m (Alumina) 15m (Silicon carbide) 20m (Alumina) 20m (Silicon carbide) 5n (Alumina) 10n(Alumina) 10n (Silicon carbide)

Fig 2 – W–H diagram of milled samples for calculation of crystalline sizes and mean strains of phases.

latticestrainslightly.Unlikecommonmethodsofsintering,

whichfollowextremegrowth,sinteringthoughSPShasalittle

growth,sothatthecrystalsareinnanoscaleyet.Whenthe

sintering(includingheatingandkeepingprocesses)is

com-pletedinafewminutes,thecrystalscouldbesmall[26]

The changes of sample’s thickness to time for sample

whichcontaindifferentamountofSiCincludethreezones

Atfirst,thetimeoflessthan35min,whichsamplehasbeen

heatedandhasalittleexpansion.Inthefollowingby

increas-ingthetemperature,sinteringprocessoccurredandsamples

werecontractedquicklyduringabout5minandthenchange

ofdisplacementwillbeconstant.Asitstands,thesintering processwascompletedattheendofsecondzoneand sam-plesweredense.Accordingtothechangesofsamplethickness andtemperatureversussinteringtimeplots,beginning tem-peratureofsintering(beginningofsecondzone)isdetermined ThisinformationisbroughtinTable3.Decreaseinthickness

ofthesamplesandthestartingofsecondzoneisduetothe overcomingofcontractionofsinteringonthermalexpansion

ofthesamples

Table 2 – Results of calculations of crystalline sizes and mean strain of milled and sintered samples.

Codeofsample Phase Crystallinesize(nm) Meanstrain(%) Usedmethod

Milled

samples

Sintered

samples

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Fig 3 – Microstructure of milled samples for 300 min (a) 10m sample and (b) 10n sample.

Asit isseen in Table 3, inthe first system by

increas-ingofSiCto10wt.%,beginningtemperatureofsinteringhas

beenincreasedandthendecreased.SiCwithhighermelting

pointthanAl2O3,increasethesinteringtemperatureofAl2O3

Counts

10n

5n

20m

15m

5m

Position [º2Theta](copper (cu))

10m

Fig 4 – X-ray patterns of sintered samples (*) Al 2 O 3 , (+) SiC.

But increasing ofSiC which has lower thermal expansion (TEC)coefficientincomparisonwithAl2O3leadsto decreas-ing ofthermal expansioncoefficientofcomposite (melting point and thermal expansionofAl2O3 and SiC are 2072◦C and8.1×10−6/Cand2730◦Cand4.0×10−6/C,respectively)

[27].Hencetheearlyexpansionhasbeendecreasedduringthe heatingandasaresultbeginningtemperatureofcontraction

isdecreasedtoo.So,after10msample,decreaseofsintering temperatureisseen.Thistreatmentissimilarlyseenin sec-ondsystemtoo.Onlyinthissystem,overcomingofthesecond phenomenontofirstoneisquickerthanandishappenedin lessamountofSiC

The changes ofrelativedensity ofsintered samplesfor the twosystemsare presentedinTable3.As itisobvious,

in system 1, addition ofdifferent amount of SiC to Al2O3

matrixleadstocompletingofsinteringandachievementto samples withnearlyfull density.Lower sintering tempera-ture, short temperature and holdingtimes have prepare it possibletoproducenano-compositeofSiC–Al2O3tonear the-oreticaldensitywithlittlecrystalgrowth[28].Toattainment fulldensitybycommonsinteringtechnique,higher tempera-tureandsoakingtimeisneeded.Shietal.succeededtosinter

Al2O3–SiC composite with relativedensity of100% via hot presstechniqueattemperature over1700◦C.Theyreported sintering byhot pressleaded toabnormalgrowth insome samples[29]

Against,presenceofnano-sizedSiCinsystem2couldnot obtainasamplewithhighdensity.Agreatdifferencebetween particle sizeofmatrix and the reinforcementphasein the second systemdecreased packingand leadstodecreaseof finaldensity.ThereisevidencewhenaSiCasfinecomponent are addedtotheAl2O3particles,itadheretothe large par-ticlesstronglyanddelaythepenetrationoffinecomponents

tothemixtures[30].Widedistributionofparticlesinsystem

2confirmsthis point(Fig.3b).Furthermore,asitisobvious

inTable3,byincreasingtheweightpercentofSiC,densityis decreased.Thiscouldbeduetothepoorsintering property

ofSiCatexaminedtemperaturesinthispaper.Similarresult wasreportedbyotherresearchers[29,31,32]

Fig.7showstheSEMimagesforthemicrostructureofthe sintered compacts There are twodifferent phases inboth

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10n 5n

20m 15m 10m 5m

Intensity after milling (cts) 647.10 616.10 575.31 534.81 638.86 612.33 Intensity after sintering (cts) 659.99 622.22 582.11 561.88 648.64 622.74 FWHM after milling (2θ) 0.6085 0.6240 0.6331 0.6341 0.5384 0.5451 FWHM after sintering (2θ) 0.5112 0.5324 0.5347 0.5552 0.5434 0.5153

0.0000 0.1000 0.2000 0.3000 0.4000 0.5000 0.6000 0.7000

0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00

Fig 5 – The changes of intensity and full width at half maximum (FWHM) of alumina peak for milled and sintered samples (2 Â = 40–45 ◦ ).

systems,i.e.,darkandlightphase.AsitisseenfromFig.4and

asitwasdiscussedearlier,XRDpatternimpliedthattherewas

noreactionbetweentherawmaterials.SoAl2O3andSiCare

onlyphasesinsinteredsamples.Accordingtotheseimages,

SiCwithlowermassabsorptioncoefficientthanAl2O3aslight

anddarkphases,respectivelyweredispersedinthematrix

[33,34].AsitwascalculatedporositybyArchimedesmethod,

porositycanbeseeninmicrostructuralimagesofsamples

Theamountandsizesofporosityinsystem2ishigherthan system1

Flexural strength and hardness ofsintered samples are illustratedinFig.8.Asitstands,insystem1withadensity closetoeachother,theflexuralstrengthofsamplesincreased

by increasing the amount of SiC particles up to 10% and after that adding moreSiC could not increasethe flexural strength.WhentheweightpercentofSiCwasmorethan10,

0.005 0.007 0.009 0.011 0.013 0.015 0.017 0.019 0.021 0.023 0.025

1.4 1.2

1 0.8

0.6 0.4

2 sinθ

5m (Alumina) 10m (Alumina) 10m (Silicon Carbide) 15m (Alumina) 15m (Silicon Carbide) 20m (Alumina) 20m (Silicon Carbide) 5n (Alumina) 10n(Alumina) 10n (Silicon Carbide)

Fig 6 – W–H diagram of sintered samples for calculation of crystalline sizes and mean strains of phases.

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Fig 7 – Microstructure of sintered samples (a) 10m sample and (b) 10n sample.

Table 3 – Temperature of sintering with physical

properties of sintered samples.

Codeof

sample

Temperature

ofsintering (◦C)

Relative density(%)

Amount porosity(%)

Water absorption (%)

asuitabledistributionofthis phasecannotbeseen [32,35]

ExistenceofhardSiCparticleswithafinestructurecouldbe

improvethemechanicalproperties[36].Furthermore,dueto

residualstressfromthe mismatchofTECbetweenSiCand

Al2O3, matrix isunder thecompressive stress during cool-ing.So,existenceofSiCinthematrixofAl2O3increasesthe strength[37].AddinghardSiCasreinforcementphasetothe

Al2O3 matrix could increasethe hardness numbers signifi-cantly(hardnessofSiCandAl2O3are1175HVand2800HV)

[27].Sinceinsystem2achievementstosampleswithfull den-sitywerenothappening,flexuralstrengthandhardnesswere weakerthanspecimensoffirstsystem.Theporosityeffectson themechanicalpropertiesofceramicmaterialsmeaningfully

[29] Lossofweightinthesampleafterwearresistanceisseenin

Fig.9.DuetohighhardnessinsamplewhichcontainhardSiC, wearresistanceofthemareincreased.Duringdrysliding,the SiCparticlesdonoteasilycomeoutinthedebrisbecauseof theirreasonablygoodbondingwiththematrix.Bydecreasing theparticlesize,bondingtakesplacebetterandthewear resis-tanceincreases.Furthermore,theformationofoxidelayeron

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300

250

200

150

100

50

0

2500

1500 2000

1000

500

0

Flexural strength (Mpa)

Hardness (HV)

Fig 8 – The changes of flexural strength and hardness of sintered samples.

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242

243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259

2.95 3 3.05 3.1 3.15 3.2 3.25

0.5 1 1.5 2 2.5 3 3.5

20 15

10 5

0

Weight percent of SiC System 1 System 2

Fig 9 – The changes of weight of sintered samples during wear test.

thewearsurfaceofthecompositereducedwearresistance

Theseoxidesconsistofveryfinewithsizesofabout10–100nm

which havebeen compacted ontothe composite’s surface,

preexistingsurfaceoxidelayers,orcompressedcoarsewear

debris[38].Becauseoflowerdensityofsamplesinthesecond

system,wearpropertiesofthesesamplesarelowerthanthe

firstsystem

Conclusion

Al2O3–SiCcompositeswerepreparedsuccessfullybySPSwith

relativedensityof100%.Thecompositeswithdenser

struc-ture have higher flexuralstrength 293.1MPa and 2329 HV

ofthehighesthardnessandflexuralstrengthwereobtained

fromthesamplesreinforcedby10 and20wt.%SiC,

respec-tively.ByincreasingtheamountofSiC,flexuralstrengthwas

improvedfirstandthendecreasesbecauseofabad

distribu-tionofsecondedphaseinthematrix.Duetolowerdensity

insamplescontainingnano-sizedSiC,mechanicalproperties

wereweakerthanspecimensofthefirstsystem

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