Anisotropic mechanical and functional properties of graphene-based alumina matrix nanocomposites

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OULOUSE

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To link to this article : DOI:10.1016/j.jeurceramsoc.2016.02.032URL

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To cite this version :

Çelik, Yasemin and Çelik, Ali and Flahaut,

Emmanuel and Suvacı, Ender Anisotropic mechanical and

functional properties of graphene-based alumina matrix

nanocomposites. (2016) Journal of the European Ceramic Society,

vol. 36 (n°8). pp. 2075-2086. ISSN 0955-2219

Any correspondence concerning this service should be sent to the repository

administrator:

staff-oatao@listes-diff.inp-toulouse.fr

Anisotropic

mechanical

and

functional

properties

of

graphene-based

alumina

matrix

nanocomposites

Y.

C¸

elik

_,

_A.

_C¸

_elik

_,

_E.

_Flahaut

_,

_E.

_Suvaci

a_{AnadoluUniversity,DepartmentofMaterialsScienceandEngineering,26480Eskisehir,Turkey}

b_{UniversitéPaulSabatier—ToulouseIII,INP,InstitutCarnotCirimat,118RoutedeNarbonne,F-31062Toulousecedex9,France} c_{CNRS,InstitutCarnotCirimat,F-31062Toulouse,France}

Keywords: Anisotropy

Grapheneplatelets/Al2O3nanocomposites

Mechanicalproperties Electricalconductivity Thermalconductivity

a

b

s

t

r

a

c

t

Grapheneplatelets(GPLs)containingAl2O3nanocomposites,whichexhibitanisotropicmicrostructure,

havebeenpreparedbysparkplasmasintering(SPS),andeffectsofthisanisotropyonmechanical, elec-tricalandthermalpropertiesofthenanocompositeshavebeeninvestigated.3vol.%GPLsadditioninto monolithicAl2O3causedfracturetoughness(Kıc)toincreaseby26.7%inthein-planedirectionandto

decreaseby17.2%inthethroughthicknessdirection.Kıcstartedtodecreaseinthein-planedirectionand

toincreaseinthethrough-thicknessdirectionwithfurtherincreaseintheGPLsamount.Theelectrical conductivityofthenanocompositesexhibitedaslightanisotropywithalowerresistivityinthein-plane direction.OrientedGPLsalsoledtoalessresistiveheatconductionpathinthein-planedirection.∼44% increaseinthein-planethermalconductivitywasachievedat600◦_{Cwith15vol.%GPLsadditionintothe}

monolithicAl2O3andthisresultedin∼52%increaseinthekin-plane/kthrough-thicknessratio.

1. Introduction

Nanocomposites,whichexhibitsuperiormechanicaland phys-icalpropertiescomparedtotheirrespectivematrixmaterials,are amongthemosttechnologicallypromisingmaterialstomeetthe worldwide demandfor high performance applicationsin many fields.Inthatrespect,developmentofnovelnanocompositeswith improved properties playsa critical role toextendtheirusein industry.

Carbon-basedfillers,especiallycarbonnanotubes(CNTs),have beenwidelyutilizedinnanocompositeresearchinordertoimprove structural and functional properties of various host materials [1–3].Graphene-based materials are alsopromising candidates as fillermaterialsin nanocompositesdue totheirunique com-binationofoutstandingmechanicalpropertiesandexceptionally highthermal and electrical conductivities,as wellas theirtwo dimensionalnatureandhighaspectratio.Studieson nanocom-posites containing graphene-based materials have beenmainly focusedonpolymermatricesandithasbeenshownthatsignificant multifunctionalpropertyenhancementsarepossibleevenatlow

∗ Correspondingauthorat:AnadoluUniversity,DepartmentofMaterialsScience andEngineering,26480Eskisehir,Turkey.

E-mailaddress:ybozkaya@anadolu.edu.tr(Y.C¸elik).

fillercontents.Recentachievements andadvancesin graphene-basedpolymermatrixcompositeshavebeenreviewedbymany authors[4,5].However, potentialof graphene-basedfillers also in ceramic-matrix nanocomposites has been realizedin recent years. Although high mechanical strength, thermal resistance and good chemicalstability of monolithic ceramicsmake them promising materials for high technology applications such as electronics, defense, aerospace and transportation, their brit-tle and electrically insulating nature limit their use in these potential applications. Wear resistant and structural materials for extreme environments, such as high temperature/pressure, nuclearradiation,andchemicals,arerequiredtobeboth strong and tough [6]. It is also challenging to shape these materials into complex geometries due to their brittle nature. Manufac-turing of complex-shaped ceramic parts is possible by electro discharge machining (EDM),if the material has a certain level ofelectricalconductivity(>0.3–1S/m)[7].Therefore,itis essen-tialtoimprove mechanical and electrical propertiesof ceramic materials,which can beobtainedby nanocompositeformation. Porwaletal.[8] haverecentlyreviewedthestateoftheartfor graphene-based ceramic matrix nanocomposites. Although sig-nificant improvements of mechanical and electrical properties of monolithic ceramics have been reported with incorporation ofgraphene-based materials,there areverylimitednumber of

studies where thermal properties of graphene-based ceramic-matrixnanocompositeshavebeeninvestigated[9,10].

GPLsgenerally havehigherthickness comparedtofew-layer (2–5layers)andmultilayer(2–10layers)graphenedueto agglom-erationand/oroverlappingofindividualsheets,buthaveaRaman spectrum differentfromthat of bulk graphite. GPLsare attrac-tivefillersinnanocompositessincetheycanbeeasilyproduced at a large scale by liquid phase exfoliation and may enable onetodevelopmultifunctionalnanocompositeswithanisotropic propertiesforawiderangeofapplicationsduetotheirunique two-dimensionalgeometry,highaspectratioandstiffness.Duetotheir relativelyhighthickness(upto100nm),GPLscanbepreferentially orientedinthematrixduringsparkplasmasintering(SPS)[9–12]. However,thenumberofstudieswhereanisotropyin graphene-basedceramicmatrixnanocompositeswasinvestigatedindetail isverylimited.Centenoetal.[11]investigatedeffectof orienta-tionofreducedgrapheneoxidesheetsonmechanicalproperties of Al2O3 matrix nanocomposites, but only for one composition

(i.e.,0.22wt.% graphene—containingAl2O3). Ramirez et al. [12]

examinedanisotropicelectricalconductivityofGPLs—containing Si3N4 nanocompositesas a functionof orientation ofthe GPLs.

Miranzoetal. [9]investigated anisotropic thermalconductivity of Si3N4 ceramics containing GPLs. Rutkowski et al. [10] have

very recently prepared Al2O3-GPLscomposites by hot isostatic

pressingand SPS,and evaluatedthecorrelationbetween mate-rialanisotropy and thermal conductivity.The authorsobserved anisotropicmicrostructureduetopreferentialorientationofGPLs inperpendiculardirectiontopressingaxisforhotpressedsamples, whilenotmuchanisotropywasobservedforthesparkplasma sin-teredcomposites.Al2O3hasbeenoneofthemostwidelyutilized

matrixmaterials;however,theinfluenceofanisotropyon mechan-icalandfunctionalpropertiesofgraphene-basedmaterials/Al2O3

nanocompositeshasnotbeenclarifiedindepth,yet.Accordingly, theresearchobjectivesofthisstudyweretoproducesparkplasma sinteredAl2O3matrixnanocompositescontainingGPLsthatwere

preparedbyliquidphaseexfoliationasafiller,andtodevelopan understandingabouteffectsofpreferentialorientationofGPLsin theAl2O3matrixonmechanical,electricalandthermalproperties

ofthesenanocomposites.

2. Experimentalprocedure

2.1. GPLs/Al2O3nanocompositeproduction

Highconcentration(∼1.3mg/mL)graphene-baseddispersions werepreparedbyexfoliationofahighsurfacearea(∼175m2_/g)

nano-graphitepowder(SurfaceEnhancedFlakeGraphite(Grade 3725),kindlyprovidedbyAsburyCarbons,Inc.,USA)inisopropyl alcohol(IPA)within90minofbathsonicationfollowedby centrifu-gationat500rpmfor45min.a-Al2O3powder(TM-DAR,Taimei

ChemicalsCo.,Japan—99.99%purityand∼0.1mmaverageparticle size)wasdispersedseparatelyinIPAbymagneticstirringfor∼1hin combinationwithbathsonicationfor∼5mininevery15min.The graphene-baseddispersionwasthenincorporatedintotheAl2O3

suspensionduringmagneticstirringinrequiredamountsasto pro-vide3,5,7,9,10and15vol.%GPLsandstirredfor∼45min.The resultingGPLs/Al2O3mixtureswereball-milledinIPAat200rpm

for 3husing yttria-stabilizedZrO2 balls.The milledslurrywas

driedbyrotaryevaporatorandthengroundinanagatemortar. WelldispersedGPLs/Al2O3powderwasthenloadedintoa14mm

innerdiametergraphitedieandsinteredbysparkplasmasintering (SPS,FCTSystemeGmbH—Anlagenbau,Germany)at1250–1600◦_C

(dependingonthegraphenecontent)for5 minunderauniaxial pressureof50MPa.Thedimensionsofthesinteredsampleswere ca.14mmindiameterand8mminthickness.SPSmethodenables

rapidheatingratesandapplyingpressuresimultaneously; there-fore,itlimitsthermallyinducedstructuraldamagetothegraphene byavoidinglongprocessingtimesathightemperatures[13].Asa resultoftheappliedpressureduringSPS,GPLsarepreferentially orientedinthematrix withtheirbasalplanes perpendicularto theSPSpressingaxis,asshown inFig.1.Thedirectionparallel totheSPSpressingaxiswillbereferredtoasthrough-thickness andthedirectionperpendiculartotheSPSpressingaxiswillbe referredtoasin-planedirection,fromnowon.Mechanical, ther-malandelectricalcharacterizationsofthenanocompositeswere performedbytakingthisanisotropyintoconsideration.Samples withca.8mm×8mm×1–2mmwerecutalongboththein-plane andthrough-thicknessdirectionsforfurthercharacterization.

DensityofthenanocompositeswasmeasuredbyArchimedes methodwithwaterimmersion.Inordertodeterminetheirrelative density,thetheoreticaldensity ofthenanocompositeswas cal-culatedbythevolume-basedruleofmixturesassumingdensities of3.96g/cm3_and2.2g/cm3_forAl

2O3andGPLs,respectively.The

microstructureofthesampleswascharacterizedbyfieldemission gun—scanningelectronmicroscope(FEG-SEM,Supra50VP). Micro-RamananalysesoftheinitialGPLs,theas-prepared10vol.%GPLs containingAl2O3powderandofthe10vol.%GPLs/Al2O3

nanocom-posites(onboththrough-thicknessandin-planedirections)were performedonaRenishawInviaspectrometerusing532nmlaser excitationand100×objectivelens.Thelaserpowerwaskeptbelow 1mWinordertopreventsampledamage.50spectrawererecorded (eachoneatadifferentlocation)forthesesamplestocreate statis-ticalhistogramoftheID/IGratio.

2.2. Mechanicalcharacterization

Vickershardnesstestswereperformedbyapplyingaforceof 2kgonthepolishedsamplesurfaces.Hardnessandfracture tough-nessvaluesofthemonolithicAl2O3andthenanocompositeswere

determinedfromVickersindentations(averageofthree indenta-tions)andthecorrespondingcrack-lengthmeasurementsusingthe equationsdevelopedbyEvansandCharles[14].

2.3. Electricalcharacterization

Electricalmeasurementswerealsoperformedalongboth in-plane and through-thickness directions. The resistance of the sampleswasfirstmeasuredbySignatonesemi-automaticprobe stationconnectedtoKeithley4200semiconductor characteriza-tionsystem.Au-Pdcoatingwasappliedtothesurfacesofinterest by sputtering. MonolithicAl2O3 and nanocompositeswith 3, 5

and 7vol.% GPLsexhibited high resistance, while nanocompos-ites withhigherGPLscontents showedconductive behavior. In ordertoeliminatethepossibleeffectofsamplethicknessonthe orientationdependentconductivitymeasurements,cubicsamples (∼5mm×5mm×5mm)werecutfromthesintered nanocompos-iteswith9,10and15vol.%GPLs.Thecorrespondingsurfacesofthe

Fig.1. SchematicrepresentationoforientationofGPLsinthematrixwiththehelp ofappliedpressureduringSPSprocess.

Fig.2.FEG-SEMmicrographsoffracturesurfacesof(a)monolithicAl2O3andofnanocompositeswithGPLscontentsof(b)3vol.%,(c)5vol.%,(d)7vol.%,(e)10vol.%and(f)

15vol.%.Arrowsindicatethealignedprotrudedandpulled-outGPLs;dashedcircleshowsthepulled-outGPLs.

cubicsampleswerecoatedwithAu-Pdbeforeeachmeasurement. TheresistancevaluesweremeasuredbyAgilent4294Precision ImpedanceAnalyzerinthrough-thicknessandin-planedirections andthecorrespondingconductivityvalueswerethencalculated.

2.4. Thermalcharacterization

In-planeand through-thickness thermal diffusivity measure-mentswerecarried outfromroomtemperatureupto600◦_Cat

intervalsof∼100◦_CinN

2atmospherebylaserflashmethodusing

NetzschLFA457Microflash(USA)equipment.Threeshotswere recordedpertemperatureforeachsampleandthedatawere aver-aged.Specificheat(Cp)measurementsofthemonolithicAl2O3and

thenanocompositeswith3,7,10and15vol.%GPLscontentswere carried out bya differentialscanning calorimeter (Netzsch STA 449F3,USA) in 42–700◦_{C temperaturerange inN}

2 atmosphere

using a sapphire crystal as a reference. TheCp values atroom

temperatureandofthe5and15vol.%GPLs/Al2O3sampleswere

determinedbyextrapolation andinterpolationof themeasured data.Thecorrespondingthermalconductivity(k)valueswere cal-culatedbyusingthefollowingequation[15]:

k=˛××Cp (1)

where a and  represent the thermal diffusivity and density, respectively.

3. Resultsanddiscussion

3.1. Microstructuredevelopment

Theexfoliatednano-graphitepowderusedinthepresentstudy ismostlycomposedoffew-layer(<5layers)graphenesheetswith alateralsizeof<1mm(∼400nminaveragediameterof equiva-lentsphericalparticleasdeterminedbydynamiclightscattering analysis)asconfirmedbyhighresolutionTEMandRaman analy-ses.However,theindividualfew-layergraphenesheetsareusually folded,scrolledandentangledeachotherduringprocessing, form-ingtheso-calledGPLsinthisstudy.

Each nanocomposite was sintered at a specific temperature dependingontheirGPLscontenttoensurethatallthe nanocom-positesexhibit ashighdensificationaspossible.Asa result,the monolithicAl2O3andthenanocompositeswerehighlydensified

withrelative densitiesof≥98.5%. Table1shows sintering tem-peraturesofthenanocompositesdependingontheGPLscontent andtheresultantgrainsizeofthenanocomposites.Sinteringof eachsampleattheiroptimumsinteringtemperatureenablesone

Table1

Sinteringtemperature,relativedensityandmeangrainsizeforthemonolithicAl2O3andthenanocomposites.

GPLscontent(vol.%) Sinteringtemperature(◦_{C) Relativedensity(%TD) Meangrainsize(mm)}

0 1250 100 2.39 3 1350 99.6 Bimodal(0.70,1.40) 5 1400 99.4 1.27 7 1450 98.7 1.31 9 1500 99.2 1.31 10 1525 98.7 1.36 15 1600 98.5 1.33 Table2

MechanicalpropertiesofGPLs/aluminananocomposites.

GPLscontent(vol.%) Hardness(GPa) KIC(through-thickness)(MPam1/2) KIC(in-plane)(MPam1/2)

0 18.4±0.86 2.9±0.06 3.0±0.14 3 16.2±0.11 2.4±0.03 3.8±0.13 5 15.1±0.27 2.6±0.06 3.6±0.15 7 13.1±0.33 2.8±0.02 3.0±0.05 9 11.8±0.12 3.0±0.11 3.1±0.06 10 11.3±0.18 3.2±0.10 2.8±0.05 15 9.8±0.19 3.2±0.06 2.6±0.05

toinvestigatetheeffectofGPLsonthemechanicalpropertiesmore clearlybyeliminatingtheeffectofgrainsizeontheseproperties. Ascouldbeexpected,thesinteringtemperatureincreasedwiththe GPLscontent.

Fig. 2 shows FEG-SEM micrographs of the fracture surfaces of sintered monolithic Al2O3 and nanocomposites. The

mono-lithicAl2O3 iscomposedofequiaxed-shapedfacetedgrainswith

∼2.4mm in size in average (determined by ImageJ software) (Fig.2a).AdditionofGPLsinhibitedgraingrowthofAl2O3resulting

inafinermicrostructure(Table1andFig.2).Thiscouldbeattributed tothe pinning effect of uniformlydistributed GPLswhich hin-dersmovementofgrainboundaries.Exceptforthenanocomposite with3vol.% GPLscontent, thefracture surfaces of thesintered nanocompositesrevealedmostlyuniformmicrostructures indicat-ingthehomogeneousdistributionofGPLsthroughoutthematrix (Fig.2).Itcanbeclearlyseenfromthesemicrographsthatsome oftheGPLsareagglomeratedandoverlappedformingflakeswith ∼50nminthickness,whilethethinneronesarelocatedaround thematrixgrainsandcannotbeeasilyobserved.ThethickGPLs arealigned inthematrix withtheirbasal planes perpendicular totheSPSpressingaxis,leadingtoananisotropicmicrostructure (Fig.2b–f); consequently,orientationdependentfracture tough-nessvalueswereobserved.3vol.%GPLscontainingnanocomposite exhibitedabimodalmicrostructurewithsomeveryfine(∼0.7mm indiameter),facetedandequiaxedmatrixgrains,aswellas rela-tivelylargergrains(∼1.4mminaverage)(Fig.2b).Thismayindicate that3vol.%GPLscontentisnotsufficienttobedistributedaround mostoftheAl2O3 grainsinthematrix;therefore,differentgrain

growthrateswereobservedin themicrostructure.HigherGPLs loadingsresultedinmuchmoreuniformmicrostructures.

3.2. Mechanicalproperties

ThefracturesurfaceofmonolithicAl2O3revealedan

intergranu-larfracturemode,whiletheGPLs/Al2O3nanocompositesexhibited

acombinationoftransgranularandintergranularfractures(Fig.2). Thetrendof theAl2O3 grainstofracturetransgranularly inthe

nanocompositesindicatestheimprovedinterfacialstrength, rel-ativetothegrainstrength.Thefracturemodeandthemechanical properties of ceramic nanocomposites strongly depend on the strengthofgrainboundaries.Thestrongboundarymayforcethe crackstodeflectintothematrixgrain,resultinginatransgranular fracture[16].Fanetal.[17]reportedthatthephenomenaof trans-granularfractureincreasesinmilledexpandedgraphitecontaining

0 1 2 3 4 5 6 7 8 9 10 11 12 1314 15 16 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 in-plane

through thickness

s s e n h g u o T er ut c ar F ( m. a P M 1/2 )

GPLs volume fraction (vol.%)

Fig.3.Through-thicknessandin-planefracturetoughnessvaluesofGPLs/Al2O3

nanocompositesasafunctionofGPLscontent.

Al2O3nanocompositescomparedtomonolithicAl2O3,suggesting

thehighstrengthofthegraphene-basedmaterial,asinagreement withthepresentstudy.Ontheotherhand,Wangetal.[18]reported that theirreduced grapheneoxide-based Al2O3 nanocomposite

exhibitedpredominantlyintergranularfracturemode.Theauthors explainedthisphenomenonbytheexistenceofresidualstressat theAl2O3grainboundariescausedbythermalexpansionmismatch

whichmayweakentheinterfaceboundaries.Thedifferences in fracturemodesobservedinvariousstudiescanbeattributedto thedifferencesingraphene-basedmaterials(intermsofthickness, aspectratio,quality,etc.)andtothedifferencesinnanocomposite productiontechniques,asalsohighlightedbyDuszaetal.[19].

Table2showsthemechanicalpropertiesofthemonolithicAl2O3

andtheGPLs/Al2O3nanocomposites.Itwasobservedthathardness

valuesdecreasedwithGPLscontent,althoughthenanocomposites haveamuchfinermicrostructureincomparisontothemonolithic Al2O3.ThiscanbeexplainedbyslidingorcleavageofGPLsunder

thein-planeandout-ofplanestresses,assuggestedbyFanetal. [20].

FracturetoughnessofthemonolithicAl2O3andthe

nanocom-posites as a function of GPLs content is plotted along both through-thicknessandin-planedirectionsinFig.3.Preferential ori-entationofGPLsthroughoutthematrixresultedinanisotropyinthe

Fig.4. FEG-SEMmicrographsofin-planecrackpaths(createdbyindentation)of(a)monolithicAl2O3andofGPLs/Al2O3nanocompositeswith(b)3vol.%,(c)5vol.%,(d)

7vol.%GPLs,(e)10vol.%,and(f)15vol.%GPLs.Themagnificationof(e)and(f)ishigherthanthatof(a)–(d).

fracturetoughnessvalues.FracturetoughnessofmonolithicAl2O3

wasalmostthesameinthein-planeandthrough-thickness direc-tions(3.0and2.9MPam1/2_{,respectively).Itincreasedby∼26.7%}

in thein-plane directionand decreased by ∼17.2% in through thickness direction with the addition of 3vol.% GPLs into the monolithic Al2O3 (Fig.3).TheFEG-SEM micrographof the

frac-turesurface of this nanocompositerevealed theprotruded and pulled-outthickGPLs,whicharealignedmostlythroughthe in-planedirection (Fig. 2).The change of thefracture modefrom intergranulartotransgranularwiththeintroductionofGPLsinto themonolithicAl2O3 isaclearindicationofimprovedinterfacial

strength;however,itisclearthatthisbondingisweakenoughto allowde-bondingattheGPLs-Al2O3interfaceinthein-plane

direc-tion.DelaminationoftheGPLsthemselvesmayalsooccur.Asa result,energythatwouldnormallycausecrackpropagationis par-tiallyexpendedbyde-bondingandshear,resultinginanincrease infracturetoughness[21].Accordingly,pull-outwassupposedto bethemaintougheningmechanismforthe3vol.%GPLscontaining Al2O3 nanocompositeinthein-planedirection.Furtherincrease

inGPLscontentstartedtodecreasethein-planefracture tough-ness(Fig.3,Table2).ThehighestGPLsloading(15vol.%)resulted ina reduction inthefracturetoughnessby∼13.3%and ∼31.6% comparedtothemonolithicAl2O3andthe3vol.%GPLscontaining

nanocomposite,respectively.Thedecreaseinthefracture tough-nesscouldbeattributedtoweakeningoftheinterfaceafteracertain amountofGPLsloadings(>3vol.%).

Fig.4a–dshowin-planecrackpathsoriginatingfromthe Vick-ersindentationsonthemonolithicAl2O3andthenanocomposites

with3,5and7vol.%GPLscontents.Thecrackpathsofthe nanocom-positesrevealedbothstraightandtortuousregionsindicatingthat thefractureisamixtureofintergranularandtransgranularmodes, asinagreementwiththefracturesurfaces.GPLsshowedahigher tendencytoagglomerateandtooverlapathigherloadings.Asa resultofthisagglomeration/overlapping,theamountoflargepores betweenthematrixgrainsandthethickGPLsincreasedresulting inweakeningoftheinterfacialbondinginthein-planedirection (Fig.4eandf).ThisisinagreementwithLiuetal.[22]whoreported thatthelargeporesarethoughttobetheoriginofthefracturesand reducethestrengthofceramiccomposites.Duszaetal.[19] pre-paredSi3N4matrixnanocompositesusingvariousgraphene-based

materials withdifferent geometry, length/width and thickness. Similarly,theyobservedthattheGPLswithlargerlateralsizeand higherthickness,andoverlappedGPLsareusuallyconnectedwith porosity,whichmayresultinaweakadhesionbondofGPLs/matrix andlowerenergydissipationduringpull-out[19].

Thedecreaseinthethrough-thicknessfracturetoughnesswith 3vol.%GPLsadditioncouldbeattributedtotheGPLs-Al2O3

inter-face which might be too strong in that direction; therefore, thepossiblepull-outorbridgingmechanismsarepreventedand the crack is forced to pass through the GPLs. Further increase in GPLs amount started to increase the fracture toughness in through-thickness direction, and the fracture toughness of the nanocomposites got higher than that of the monolithic Al2O3

at GPLsloadings of ≥9vol.%(Fig.3).Fig.5 shows theFEG-SEM micrographs of the through-thickness crack paths originating fromtheVickersindentations onthemonolithicAl2O3 andthe

nanocompositeswith3,5,7,10and15vol.%GPLscontents.The crackpathofthe5vol.%GPLs/Al2O3nanocomposite,which

exhib-itedaslightly higherfracturetoughnessthanthatofthe3vol.% GPLs/Al2O3nanocomposite,butthatisstilllowerthanthatofthe

monolithicalumina,showedadamagedGPLsinducedbycrack pen-etrationthroughit (Fig.5c).Crackdeflectionandcrackbridging wereobservedasthemaintougheningmechanismsin through-thickness direction especially at relatively low GPLs loadings (∼5–7vol.%)(Fig.5candd).IncreasingGPLscontentto≥9vol.%led toamuchmoretortuousandnarrowercrackpath(Fig.5e–f).Crack branching appeared for the15vol.% GPLscontaining nanocom-positeasa dominanttoughening mechanism (Fig.5f),resulting in∼10%and∼33%increaseinfracturetoughnesswithrespectto themonolithicAl2O3 andthe3vol.%GPLscontaining

nanocom-posite,respectively(Fig.3,Table2).Theseresultsrevealedthat themechanicalpropertiesoftheGPLs/Al2O3nanocompositesare

stronglyaffectedbytheorientationof theGPLsthroughoutthe matrix.

In contrast to relatively high improvements of fracture toughnessofceramicmaterialswithgraphene-basedmaterial rein-forcing,suchas75%and135%improvementsasreportedbyKim et al. [6] and Walkeret al. [13], respectively;lower increment hasbeenachievedinthepresentstudy.Therelativelylow frac-turetoughnessvaluescouldarisefromthetendencyoftheGPLsto

Fig.5.FEG-SEMmicrographsofthrough-thicknesscrackpaths(createdbyindentation)of(a)monolithicAl2O3andofGPLs/Al2O3nanocompositeswith(b)3vol.%,(c)5vol.%,

(d)7vol.%,(e)10vol.%and(f)15vol.%GPLs.

agglomerateandtooverlapespeciallyatrelativelyhighloadings, andfromloweraspectratiooftheGPLsusedinthepresentstudy. Theextentofthefinaltougheningstronglydependsontheaspect ratioofthegraphene-basedmaterialandhighaspectratioplatelets aregenerallyfoundtobemorebeneficialtothemechanical prop-ertiesofa composite[4].Theintrinsicmechanical propertiesof the graphene-based fillers also play an important role in their reinforcementefficiency.Kimetal.[6]producedgraphene-based aluminananocomposites bypressurelesssintering using differ-ent graphene-based materials (chemically exfoliated graphene, grapheneoxideandreduced-grapheneoxide)andcomparedthe mechanicalpropertiesofthesenanocomposites.Theyreportedthat theultra-thin(2–5nm)chemicallyexfoliatedgraphene(0.5vol.%),

whichhasthelowestdefects,providedthegreatestimprovement (∼75%)infracturetoughnesscompared tomonolithicAl2O3 [6].

Grapheneoxideandreduced-grapheneoxideshowedlittleorless enhancementoffracturetoughness(by14%and48%,respectively) due to degradedmechanical strength of the reduced-graphene oxideandthestructuraldefectsofthegrapheneoxidecomposites [6].

ThedefectsmayoriginateintrinsicallyfromtheinitialGPLsor canbeformedduringprocessingofthenanocomposites.Raman spectroscopywasusedtoevaluatethequalityoftheGPLsinthe nanocompositesbeforeandafterSPSprocessinordertocompare withtheinitialGPLs.

Fig.6.Ramanspectraofthe(a)InitialGPLs,(b)10vol.%GPLs/Al2O3powder(before

SPS),(c)10vol.%GPLs/Al2O3nanocompositeinthrough-thicknessdirection,and(d)

10vol.%GPLs/Al2O3nanocompositeinin-planedirection.Thespectraare

normal-izedtotheG-band.

3.3. Ramanmeasurements

Fig.6 shows Ramanspectraof the initialGPLs, as-prepared 10vol.% GPLs-containing Al2O3 powder (beforeSPS) and ofthe

10vol.%GPLs/Al2O3nanocompositebothinthrough-thicknessand

in-planedirections,andTable3givesasummaryofthemeasured

Ramancharacteristics.Eachspectrumshows aG-band,whichis relatedtothestretchingof theC Cbondingraphitic materials andis commontoallsp2_{-bondedcarbonsystems,theso-called}

disorder/defect-inducedDandD′_{-bands,andthesecondorder}

2D-bandwhichisattributedtoasecond-order processrelatedtoa phononneartheKpointingrapheneandactivatedbydouble res-onanceprocess[23](Fig.6).Theshapeofthe2D-bandofallthe samplesisdistinctlydifferentfromthatofgraphitewhichconsists oftwopeaks[23].Thisindicatesthepresenceoffew-layer(2–5 lay-ers)and/ormulti-layer(5–10layers)grapheneflakesbothinthe initialgraphene-basedmaterialandinthenanocomposites.Itwas alsoobservedthattheG-bandoftheas-preparedGPLs/Al2O3

pow-derandthesinteredGPLs/Al2O3nanocompositewasblue-shifted

by∼7cm−1_and−10cm−1_{,respectively,accompaniedbya}

band-width broadening in comparison tothat of initial GPLs(Fig.6, Table3).Theblue-shiftandbroadeningoftheG-bandfrequency andwidth,respectively,observedinthenanocompositescanbe attributedtochargedopinginducedbytheAl2O3matrix[24,25].It

hasbeenshownthattheG-bandpositionupshiftsforbothelectron andholedoping[25].

The Raman spectra of the as-prepared 10vol.% GPLs/Al2O3

powder andof thesintered10vol.% GPLs/Al2O3 nanocomposite

revealedanincreaseintheintensityoftheD′_{-band(at∼1620cm}−1₎

compared tothatoftheinitialGPLsindicatingan incrementin theamountofdefects(Fig.6).TheintensityratiooftheD-band toG-band(ID/IG)isgenerallyusedtocharacterizedefectcontent

quantitatively[23].Fig.7showsstatisticalhistogramsoftheID/IG

ratiofortheinitialGPLs,10vol.%GPLscontainingAl2O3 powder

beforesinteringandthesintered10vol.%GPLscontainingAl2O3

Fig.7. StatisticalhistogramoftheD-bandintensitytoG-bandintensityratios(ID/IG)derivedfrom50Ramanspectra.(a)InitialGPLs,(b)10vol.%GPLs/Al2O3powder(before

SPS),(c)10vol.%GPLs/Al2O3nanocompositeinthrough-thicknessdirection,and(d)10vol.%GPLs/Al2O3nanocompositeinin-planedirection.Thedistributioncurvesindicate

Table3

RamanfeaturesoftheGPLs,10vol.%GPLscontainingAl2O3powderandofthesintered10vol.%GPLs/Al2O3nanocompositerecordedforbothin-plane(⊥)and

through-thickness(//)directions.Thedataaretheaverageof50spectra.

GPLs(initial) 10%GPLs/Al2O3(beforeSPS) 10%GPLs/Al2O3(//) 10%GPLs/Al2O3(⊥)

G-band v(cm−1_{) 1574 1581 1584 1584}

G-bandFWHM(cm−1_{) 25 28 29 30}

ID/IG 0.24±0.01 0.50±0.02 0.52±0.02 0.60±0.02

nanocompositebothinthrough-thicknessandin-planedirections. WhiletheID/IGratiooftheinitialGPLsrangedfrom∼0.05to∼0.55

withameanvalueof∼0.24,thisratiovariedfrom∼0.2to∼0.8with ameanvalueof∼0.5forthe10vol.%GPLs/Al2O3powder(Fig.7aand

b).ThesignificantincreaseintheID/IGratioindicatesthatthe

pow-derpreparationprocess(i.e.,ballmilling)introducedsomedefects intoGPLs.SinteringofthispowderdidnotaltertheID/IGratiomuch

(ameanvalue of0.52 inthrough-thicknessdirection) revealing thattheSPSprocessdoesnotdamageGPLs,asinagreementwith Miranzoetal.[9].However,theID/IGratioofthenanocomposite

wasslightlyhigherforthein-planedirection,whichrangedfrom ∼0.3to∼1withameanvalueof∼0.6,thanthatofthe through-thicknessdirection(Fig.7candd).HigherID/IGratiointhein-plane

directionarisesfromthepresenceofmoreflakeedgesinthat direc-tion,confirmingtheanisotropicstructureofthenanocomposites. Centenoetal.[11]observedasimilarorientationinfluenceonthe Ramanspectraoftheirreducedgrapheneoxide/Al2O3

nanocom-posites;however,theID/IGratiosoftheirnanocompositesaremuch

higher(∼1.13forthein-planedirectionand∼0.83forthe through-thicknessdirection).Itshouldbealsonotedthatnocorrelationwas observedbetweenthegrapheneorientationinthe nanocompos-itesandRamansignalintensity,incontrasttotheobservationsof Centenoetal.[11].

3.4. Electricalproperties

Theelectricalconductivityofcomposites,whichareformedby additionofaconductivefillerintoaninsulatingmaterial,follows a power-lawnearthepercolationthreshold[26,27]and canbe expressedbytheclassicalpercolationtheoryas:

el(c)=0(ϕ−ϕc)tcforϕ>ϕc (2)

whereel(c)istheconductivityofthecomposite,0isa

parame-terdependingontheelectricalconductivityofthefillermaterial, tc is the critical exponent, and ϕ and ϕc are the volume

frac-tionandthecriticalvolumefraction(percolationthreshold)ofthe fillermaterial,respectively.Thecriticalexponentisuniversal,with mostwidelyacceptedvaluesof1.3and1.94fortwo-dimensional and three-dimensionalpercolatingsystems,respectively [27].It dependsonlyonthetypeofpercolationmodelandonthe dimen-sionalityof thesystem[28]. ϕc depends onthefillergeometry,

dispersion,andnatureoftheconductionbetweenparticles. There-fore,findingvalues oftc and ϕc enables onetounderstandthe

natureofparticledispersionsandpercolationprocesses[29].These valuescanbedeterminedbyfittingoftheexperimentaldatatothe percolationmodel.

Fig.8showstheelectricalconductivityofthemonolithicAl2O3

andtheGPLs/Al2O3nanocompositesinthein-planeandthrough

thicknessdirectionsasafunctionoftheGPLscontent.ϕc,tc and

0 parametersweredeterminedforbothin-planeand

through-thicknessdirections by fitting theexperimental datato Eq. (2) (thered solidlinesin Fig.8).Thefittingparametersare shown in Table4. Thelog-log plots of versus (ϕ−ϕc) shownin the

inset of Fig. 8 reveal linear relationships indicating a good fit (R2 _{is 0.992and 0.998 for the in-plane and through-thickness}

directions,respectively).Fig.8shows thatthemonolithic Al2O3

andthenanocompositeswithGPLscontentsupto7vol.%

exhib-Fig.8. In-plane and through-thickness electricalconductivities ofGPLs/Al2O3

nanocompositesatroomtemperature.ThesolidlinesarefittingstoEq.(2).Inset isthedouble-logarithmicplotofelectricalconductivityversus(ϕ−ϕc),showinga

linearrelationship(R2_{is0.992and0.998forthein-planeandthrough-thickness}

directions,respectively).

Table4

FittingparametersoftheelectricalconductivitydatadependingontheGPLscontent determinedbothinthein-planeandthrough-thicknessdirectionsbyfittingthe experimentaldatatotheclassicalpercolationtheory(Eq.(2)).

0 ϕc tc AdjR2

In-plane 0.343±0.56 7.1±1.36 1.97±0.62 0.9993 Through-thickness 0.360±0.18 7.5±0.46 1.60±0.2 0.9997

itedinsulatingbehaviorwithelectricalconductivitiesintherange of ∼10−10_–10−8_{S/m. When theGPLs amountwas increasedto}

9vol.%,theelectricalconductivityincreasedsharplyby∼9orders of magnitude compared to themonolithic Al2O3 leading to an

electricallyconductivenanocompositewith1.42and0.74S/m con-ductivityvaluesinthein-planeandthrough-thicknessdirections, respectively. This increase is attributed toformation of a con-ductivenetworkbyinterconnectedGPLsresultinginanelectrical percolation.The fittingsof theexperimentaldatagave percola-tion threshold (ϕc) of ∼7.1±1.36 and ∼7.5±0.46vol.% for the

in-planeandthrough-thicknessdirections,respectively,revealing thatpreferentialorientationofGPLshasnotaffectedthe percola-tionthresholdmuch,asinagreementwithRamirezetal.[12]who reportedsimilarϕcvaluestothoseobservedinthepresentstudy.

ItshouldbenotedthatGPLsloadingshigherthanthepercolation thresholdextendedtheimprovementoftheelectricalconductivity (Fig.8).Thisphenomenonisinagreementwithpreviousstudies [11,17,30]andcouldbeattributedtoanincreaseinthenumberof interconnectionsbetweenGPLswithincreasingGPLsamount.

Even though there is an obvious preferential orientation of GPLs throughout the matrix as it was confirmed by the SEM micrographs andRaman analyses, theelectrical conductivityof thenanocompositesexhibited aslightanisotropydependingon the orientation of GPLswith a slightly lower resistivity in the in-planedirection.Thein-planeconductivityofthe

nanocompos-itesis∼2–3×oftheconductivityinthrough-thicknessdirection. The loweranisotropy than expectedcould beattributedtothe presence of somemisaligned/rotated GPLswith respect tothe alignmentplane(in-planedirection)ofmostoftheGPLswhich orientedduringSPSprocess,assuggestedbyRamirezetal.[12], whoreportedin-planetothrough-thicknesselectrical conductiv-ityratioof10–25fortheirGPLs/Si3N4composites.Moreover,the

electricalconductivityof acomposite canbeimprovedbyfiller materialeitherthroughestablishinganewconductivepathinthe matrixorthroughincreasingthecross areaoftheformedpath, whichisthethicknessofgrapheneflakesincaseofgraphene-based nanocomposites[30].Athighgraphene-basedmaterialloadings, theprobabilityofagglomerationandoverlappingincreases result-inginanincrementin thethicknessof grapheneflakes [30].In thiscase,secondmechanismbecomesdominantandthe electri-calconductivityincreasesinthethrough-thicknessdirection,as wellasinthein-planedirection.Thisphenomenon isalso sup-posed tobeeffective inthe relativelylow anisotropy observed fortheelectricalconductivityoftheGPLs/Al2O3nanocomposites.

Thinfew-layergrapheneflakes,whichlocateatthegrain bound-ariesandaroundthematrixgrains,couldalsoaffecttheanisotropy intheelectricalconductivity.Thefittingoftheexperimentaldata yieldedtcvaluesof1.97±0.62and1.60±0.2forthein-planeand

through-thicknessdirections,respectively.Thetcvaluedetermined

forthein-planedirectionisinexcellentfitwiththeexpectedvalue (∼1.94)forthree-dimensionalpercolatingsystemsindicatingthe three-dimensionalnetworkofGPLsinthenanocompositesabove thepercolationthreshold.Thisresultisinagreementwiththe rel-ativelylowanisotropyinelectricalconductivityandwithsimilar ϕcvaluesobservedinbothdirections.Thelowertcvalueobserved

forthethrough-thicknessdirectionincomparisontothein-plane directioncouldbeattributedtoapercolationwhichtakesplacein anetworkwithmore‘deadarms’orweaklyconnectedpartsthana classicalrandomnetwork[1,3]ortoaquasi-two-dimensional net-workofGPLswithacombinationoftwo-andthree-dimensional organizations. Fan et al. [17] reported tc value of 1.54 for the

GPLs/Al2O3compositesandattributedthelowvalueoftctosome

preferentialorientationofGPLsinthein-planedirection.However, theauthorsdidnotmakeanyorientationdependentmeasurements [17].Ramirezetal.[12]estimatedtc=0.89andtc=2.05forthe

in-planeandthrough-thicknesselectricalconductivitydataoftheir GPLs/Si3N4 composites andattributed theobservationof larger

tc exponentforthethrough-thicknessdirectioncomparedtothe

in-planedirectiontoabroaderrangeofinter-particleconnectivity. Themaximumelectricalconductivitiesachievedinthepresent study are ∼20.1 and ∼9.1S/m for the 15vol.% GPLs/Al2O3

nanocompositeinthein-planeandthrough-thicknessdirections, respectively(Fig.8).Althoughthesevaluesare sufficientlyhigh for EDMprocess, theyare much lower than the one reported by Fanet al.[17],who achieved5709S/m electrical conductiv-ityforthesameamountofgraphene-basedmaterialintheAl2O3

matrix.Thisdifferencecanbeattributedtothepreferential ori-entation of graphene flakes throughout the matrix which may raisethepercolationthreshold[4],andalsotodifferent charac-teristicsofthegraphene-basedmaterialsusedforthecomposite production, such as lower lateral size and aspect ratio which may affect the percolation threshold and electrical conductiv-ity.Fan etal.[17]used ballmillingtogrind expandedgraphite withAl2O3 and obtainedgraphene-based material withmostly

∼2.5–20nm in thickness; however, theydid not give informa-tion aboutthe lateralsize of theseflakes. The GPLsutilized in the present studyare small in lateralsize (mostly<1mm) and it is known that smaller graphene flakes result in more junc-tionsandconsequentlyinlowerconductivityduetotheeffectof inter-flakejunctionresistances[31,32].Moreover,higheramount of GPLsis required toform a conductive network when flakes

Fig.9.SpecificheatvaluesofthemonolithicAl2O3andtheGPLs/Al2O3

nanocom-posites(a)experimentalvalues.

withasmallerlateralsizeareused.Recently,Fanetal.[30] pro-ducedfew-layergraphene(<5nm)/Al2O3nanocompositesbyspark

plasmasinteringofgrapheneoxide/Al2O3hybridspreparedby

col-loidalprocessingwithasimultaneousreductionofGO.Theauthors achievedapercolationthresholdaslowas0.38vol.%andobtaineda conductivityof1038.15S/mbyincreasingthegraphenecontentto 2.35vol.%[30].Theyattributedthislowerpercolationto homoge-neousdispersionofverythinfew-layergrapheneinthematrix,high qualityoftheaspreparedfew-layergrapheneandtobetter con-tactbetweenconductivenanoparticles[30].IfcomparedwithCNT containingsystems,Ruletal.[1]preparedSWNT-MgAl2O4

com-positeswithahomogeneousdistributionofSWNTsbetweenmatrix grainsbyin-situcatalyticchemicalvapordepositionmethod.They investigatedtheelectricalconductivityofthecompositeswith0.23 and24.5vol.%CNTcontentandreportedapercolationthreshold of0.64vol.%andaconductivityof0.4–850S/mdependingonthe CNTcontent[1].Zhanetal.[33]reportedelectricalconductivityof 3345S/mfor15vol.%SWNTcontainingAl2O3nanocomposite.

3.5. Thermalproperties

Thermal properties of theGPLs/Al2O3 nanocompositeswere

investigatedasafunctionoftemperature,graphenecontentand orientationofGPLsinthematrix.Fig.9showstheCpvaluesofthe

monolithicAl2O3andtheGPLs/Al2O3nanocompositesasa

func-tionoftemperaturedeterminedbyDSCmeasurements.TheCpof

allthesamplesincreasedwithtemperature(Fig.9).Heatis gener-allystoredbyphononsandfreeelectronsofamaterial;however, for graphiteand graphene,phononsdominate thespecificheat atallpracticaltemperatures(>1K),andthephononspecificheat increaseswithtemperature[34,35].Fig.9revealsthattheCp

val-uesincreasewithgrapheneaddition,asinagreementwithMiranzo etal.[9].SimilarbehaviorwasalsoreportedbyKumarietal.[36]for theCNT-Al2O3nanocompositesystems,theheatcapacityofwhich

ismuchhigherthanthatofthemonolithicAl2O3.

The thermal conductivity of the monolithic Al2O3 and the

GPLs/Al2O3 nanocompositesdecreasedwithincreasing

tempera-ture both in the in-planeand thethrough-thickness directions (Fig.10).Thisbehavioris characteristicofcrystallinesolidsand isattributedtophonon-phononUmklappscattering,whichmakes majorcontributiontothermalconductivityathightemperaturesas reducingthephononmeanfreepath[36–38].Inthrough-thickness direction, the monolithic Al2O3 exhibited higher thermal

con-ductivitythanthatoftheGPLs/Al2O3nanocompositeswithinthe

Fig.10.Through-thickness(a)andin-plane(b)thermalconductivitiesofthe mono-lithicAl2O3andtheGPLs/Al2O3nanocompositesasafunctionoftemperature.

decreasedwithincreasingGPLsamount(Fig.10a).Thedecrease inthethermalconductivityofmonolithicAl2O3withGPLs

addi-tioncouldbeattributedmainlytointerfacialthermalresistance betweenGPLsandAl2O3 grains[37–39].Althoughgraphenehas

extremelyhighintrinsicthermalconductivityinitssuspendedform (∼5000Wm−1_K−1_{at roomtemperature)[40],thefinal thermal}

propertiesofitspotentialapplications,such asnanocomposites, arestronglyaffectedbytheinterfacialthermalbarrier.Interfacial thermalresistance,alsoknownasthermalboundaryresistance,at theinterfaceofgraphenewithothermaterials,hasanon-zerovalue evenattheperfectinterfacesowingtodifferencesinthephonon densityofstates[38].ThiseffectisknownasKapitzaresistance [41].Theactualthermalboundaryresistanceisusuallyhigherthan theKapitzaresistanceowingtointerfaceimperfections.Graphene thermalcouplingtoothermaterialsdependonthesurface rough-ness,presenceorabsenceofsuspendedregionsingraphenelayers, and methods of graphene preparation [38]. Thermalexpansion coefficientmismatchbetweentheAl2O3matrixandthegraphene

plateletsmighthavecausedathermalstressleadingtoaseparation attheinterfaceforminggaps.Thisleadstoanincreaseinthe con-tactresistanceandadecreaseintheeffectiveheatdissipation[42]. Moreover,asshowninFig.3,GPLsadditionintomonolithicAl2O3

ledtoamuchfinermicrostructure;consequently,theamountof grainboundariesand interfaces,whichactasscatteringregions forphononsleadingtoareductioninlatticethermalconductivity, increased.Interfacialthermalresistancedecreaseswith

tempera-turefollowingatypicaltrendforKapitzaresistance[38].Thiscould bethereasonofthereduceddifferencewithinthethermal conduc-tivityvaluesofthemonolithicAl2O3andthenanocompositeswith

increasingtemperatureinthrough-thicknessdirection(Fig.10a). ThecontactsbetweenGPLs,thedefectswithinGPLsandthe pres-enceofthebendedGPLsattheAl2O3grainboundariesalsolimit

thethermal transportin thethrough-thicknessdirection[9]. In thein-planedirection,thethermalconductivityoftheGPLs/Al2O3

nanocompositeswereslightlylowerthanthatofthemonolithic Al2O3atroomtemperature;however,theyshowedanincreasing

trendwithGPLscontent(Fig.10b).Athighertemperatures,these values gotclosertoor even exceededthethermalconductivity values of themonolithic Al2O3 depending onthe volume

frac-tionofGPLs,whichcouldbeattributedtoadecreaseininterfacial thermalresistanceathightemperatures.Thethermal conductiv-ity curvesof themonolithic Al2O3 and the15vol.% GPLs/Al2O3

nanocompositecoincidedat100◦_{C,andabovethattemperature}

thethermalconductivityofthe15vol.%GPLs/Al2O3

nanocompos-itegothigherthanthatofthemonolithicAl2O3andthedifference

betweenthemincreasedwithtemperature(Fig.10b).It is very clearthatGPLsformalessresistiveheatconductionpathinthe in-planedirectionasexpected,sincethein-plane(paralleltobasal plane)thermalconductivityofa graphenesheetismuchhigher (over100-fold)thanthatofgraphitecrystalsalongthec-axis (per-pendiculartobasalplane)[43].Thisresultisinagreementwith Miranzoetal.[9],whostudiedthethermalconductionofSi3N4

compositeswithdifferenttypesofcarbonnanostructures(CNTs andGPLs)andinvestigatedtheeffectofnanostructureorientation withrespecttoheatflux,testingtemperatureanda/bSi3N4phase

ratio.TheauthorsreportedthattheadditionofbothCNTsandGPLs reducedthethermalconductivityinthethrough-thickness direc-tion,andtheyobtainedasignificantimprovementinthein-plane thermal conductivityfor plateletsadditionup to40Wm−1_K−1_,

twicethethermalconductivityoftheSi3N4matrix[9].Rutkowski

etal.[10]reportedthattheyobtainedorientatedGPLsin perpen-diculardirectiontopressingaxisinhotpressedsamples,whilenot muchanisotropywasachievedincompositespreparedbySPS.This couldberelatedtotherelativelylowpressure(35MPa)thatthey appliedduringSPSprocess.Duetothelackofanisotropy,the ther-malconductivityinperpendiculardirectiontopressingaxiswas lowerthanthatofmonolithicaluminaandalsolowerthan val-uesmeasuredinpressingdirection(forthesameGPLsamount) forGPLscontents<10wt%(∼16.8vol.%).Intheonlystudywhich investigatedboththein-planeandthroughthicknessthermal con-ductivityofCNTscontainingceramicnanocomposites,Zhanand Mukherjee [37] observedthat incorporation of single-wall CNT ropesdidnotchangethein-planethermaldiffusivityoftheAl2O3,

whileitdecreasedthethermaldiffusivityinthethrough-thickness direction.

Fig.11ashowsthein-planethermalconductivityvaluesofthe nanocompositesasafunctionofGPLsvolumefractionat600◦_C.

From3vol.%GPLs,thethermalconductivityincreasedalmost lin-earlywithgraphenecontentwithoutshowinganyclearthermal percolationthreshold,inagreementwiththeobservationsofShahil andBalandinfor themultilayergraphene-epoxycomposite sys-tems[44].∼44%increaseinthein-planethermalconductivityat 600◦_{Cwasachievedwith15vol.%GPLsadditionintothe}

mono-lithicAl2O3(Fig.11a).Thedifferencebetweenthethermalandthe

electricaltransportbehaviorsmainlyarisesfromdifferencesin con-ductivityratiosof fillertomatrix[39].Theeffectiveconduction pathisthroughthefillermaterialincaseofelectrical conductiv-ity;however,heatcanalsobetransmittedthroughthematrix[39], indicatingthatthermalconductivityisabulkproperty,while elec-tricalconductivityisalineproperty.Theanisotropybetweenthe in-planeandthrough-thicknessthermalconductivitiesincreased withGPLsamount.Thisanisotropyincreasearisesfromdecrement

Fig.11. (a)In-planethermalconductivityofGPLs/Al2O3nanocompositesat600◦Casafunctionofgraphenecontent(vol.%),(b)in-planetothrough-thicknessthermal

conductivityratioat600◦_{CfortheGPLs/Al}

2O3nanocompositesdependingontheGPLscontent(vol.%). ofthrough-thicknessthermalconductivityandimprovementof

in-planethermalconductivitysimultaneouslywithincreasingGPLs content.Theintrinsicanisotropyinthermalexpansioncoefficient andthermalconductivityofgraphenesheets[43]isexpectedto beeffectiveintheless resistiveheatdissipationin thein-plane direction.Fig.11bshowsthein-planetothrough-thickness ther-malconductivityratio(kin-plane/kthrough-thickness)at600◦Cforthe

GPLs/Al2O3nanocompositesdependingontheGPLscontent.∼52%

increaseinthekin-plane/kthrough-thicknessratiowasobservedforthe

15vol.%GPLs/Al2O3 nanocompositeincomparisontothe

mono-lithic Al2O3 at 600◦C (Fig.11b).Similar kin-plane/kthrough-thickness

ratios were also observed for the room temperature thermal conductivityvalues.Thermo-gravimetricanalysisofthe15vol.% GPLs/Al2O3nanocompositewasperformedbyheatingitinairup

to1000◦_{Cwithaheatingrateof10}◦_{C/mininordertoinvestigate}

itsstabilityinair,anditwasobservedthatthesampleisstableup to∼700◦_C.

Thehigherin-planethermalconductivitycanbebeneficialfor dissipationofheatfromonedirection.Theimprovementinhigh temperaturethermalconductivitycanbeadvantageousto mini-mizeheataccumulationinmaterialduringapplications,suchas cuttingtools,wherethematerialisexposedtohighloadsathigh temperatures.

4. Conclusions

GPLs containing Al2O3 nanocomposites with anisotropic

mechanical,thermalandelectricalpropertiesduetopreferential orientationofGPLsthroughoutthematrixwerepreparedbySPS.

3vol.% GPLs addition into monolithic Al2O3 resulted in an

increaseinfracturetoughnessby∼26.7%inthein-planedirection andadecreaseby∼17.2%inthroughthicknessdirectiondepending ontheinterfacestrengthbetweenGPLsandmatrixgrains.Pull-out isthemaintougheningmechanisminthein-planedirectionforthis nanocomposite.FurtherincreaseinGPLscontentdecreasedthe in-planefracturetoughnessduetoweakeningoftheinterfaceasa resultofagglomeration/overlappingofGPLs,whileincreasingitin thethrough-thicknessdirectionasaresultofcrackbridgingand crackdeflection mechanisms.Crackbranchingappearedat high GPLsloadingsasadominanttougheningmechanism,especiallyfor the15vol.%GPLscontainingnanocompositeresultingin∼10%and ∼33%increaseinfracturetoughnessinthrough-thicknessdirection comparedtothemonolithicAl2O3andthe3vol.%GPLscontaining

Al2O3,respectively.Thetougheningmechanismsobservedinthe

GPLs/Al2O3 nanocompositesdependingonGPLsorientationand

GPLscontentaresummarizedinFig.12.

Fig. 12.Summary of the suggested toughening mechanisms in GPLs/Al2O3

nanocompositesdependingonGPLscontent.

Theelectricalconductivityofthenanocompositesexhibiteda slightanisotropy witha lower resistivityin thein-plane direc-tion.Anelectricalpercolationthresholdwasobservedat∼7.1and ∼7.5vol.%GPLscontentsforthein-planeandthrough-thickness directions,respectively.Theelectricalconductivityvaluesofthe 15vol.%GPLscontainingAl2O3nanocompositeare20.1and9.1S/m

in the in-plane and through-thickness directions, respectively, whicharesufficientlyhighforEDMprocess.

Oriented GPLs also led to a less resistive heat conduction pathinthein-planedirection.Thethermalconductivityvaluesof nanocompositesinthein-planedirectiongothigherthanthatof themonolithic Al2O3 at hightemperatures(>100◦C), especially

for highGPLs loadings.The anisotropy in thermal conductivity increasedwithGPLsamount.∼44%increaseinthein-plane ther-malconductivitywasachievedat600◦_{Cwith15vol.%GPLsaddition}

intothemonolithicAl2O3andthisresultedin∼52%increaseinthe

k_in-plane/kthrough-thicknessratio.

Acknowledgment

ThefinancialsupportforthisstudybyAnadoluUniversity Sci-entificResearchProjectsCommission(undertheprojectnumbers of1110F155and1101F005)isgratefullyacknowledged.

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