EFFECTS OF 0.2Y‐9.8ZrO2 ADDITION ON THE MECHANICAL PROPERTIES AND PHASE DEVELOPMENT OF SINTERED CERAMIC PRODUCED FROM IPETUMODU CLAY

copyrightFacultyofEngineeringHunedoara,UniversityPOLITEHNICATimisoara 343 | Fascicule 4

ANNALSofFacultyEngineeringHunedoaraInternationalJournalofEngineering

TomeXII[2014]Fascicule4[November]

ISSN:15842673[CDRom,online]afreeaccessmultidisciplinarypublication oftheFacultyofEngineeringHunedoara

1,2.F.O.ARAMIDE,1,2.K.K.ALANEME,3.P.A.OLUBAMBI,1,2.J.O.BORODE

EFFECTSOF0.2Y9.8ZrO2ADDITIONONTHEMECHANICALPROPERTIESANDPHASEDEVELOPMENTOFSINTEREDCERAMICPRODUCEDFROMIPETUMODUCLAY1.Department ofMetallurgical andMaterialsEngineering, FederalUniversity ofTechnology,P.M.B. 704,Akure,NIGERIA2.AfricanMaterials Science and EngineeringNetwork, (AMSEN) a Subsidiary of Regional Initiative forScienceEducation(RISE)3. Applied Microscopy and Triboelectrochemical Research Laboratory, Department of Chemical andMetallurgicalEngineering,TshwaneUniversityofTechnology,Pretoria,SOUTHAFRICAAbstract:Theeffectsof theadditionofyttriastabilizedzirconiaandsintering temperatureson thephasesdeveloped and the mechanical properties of sintered ceramic produced from Ipetumodu clay wasinvestigated.Theclayofknownmineralogicalcompositionswas thoroughlyblendedwith2% (vol)yttria,8%(vol)zirconia.Fromthisandtherawclaystandardsampleswereprepared,firedat1200oCand1300oC.Thesinteredsampleswerethencharacterizedforthedevelopedphasesusingxraydiffractometryanalysis,microstructuralmorphologyusingultrahighresolutionfieldemissionscanningelectronmicroscope(UHRFEGSEM) and variousmechanicalproperties. Itwas observed thatprimarymullite and sillimaniteweredeveloped in thesamplesproduced.Thesamplesproducedsolely from therawclaycouldnotwithstand1300oC due to stress induced by the transformation from one silica phase to another. This phasetransformation also adversely affected themechanical properties of the sintered raw clay. The additiveimprovedonthemechanicalpropertiesofthesamplesbytheconsumptionofthesilicaphasestoformzirconphase inadditionwiththeotherphasesandtheexcesszirconia.Itwasconcludedthatthesamplefiredat1300oChavetheoptimummechanicalproperties.Keywords:yttriastabilizedzirconia;sinteringtemperatures;phasesdeveloped;clay;sinteredceramic

1.INTRODUCTIONTheword ceramics is abroadnamewhich is applicable inmanyproducts.Theypossessgoodcorrosionresistanceandabilitytopreservetheirpropertiesatahightemperatureupto1200oC.Other useful properties that make the ceramic attractive for engineering applications includeexcellenthightemperaturestrengthandcreepresistance,goodthermalandchemicalstability,lowthermalexpansioncoefficient,lowtruedensity,wearresistanceandgooddielectricproperties[16].Recently,manyresearchershavefocusedtheirinvestigationsonhowtoimprovethepropertiesof ceramics for various applications. Zum Gahr [7], demonstrated that modification of themicrostructure could reduce the frictionand thewearofalumina.Likewise in1996,he showedthatreducingthegrainsizeofaluminaandzirconiawillsignificantlydecreasethewear[8].FurthermoreDong et al [9], specified the effect of both single additives (MgO, SiO2, Fe2O3 andZrO2)and compoundadditiveson themechanicaland thermalpropertiesofaluminum titanateceramics,andfinallypointedoutthatthecompoundadditivesofMgOandFe2O3haveanexcellentimprovementonthestabilityofaluminumtitanate.Jiangetal[10],alsodemonstratedthattheroleof additives can be rationalized in terms of promotion of sintering process, formation of newphasesandinfluenceonlatticeconstantofaluminumtitanateceramics.Ebadzadeh and Ghasemi [11], observed that the addition of TiO2 to mulliteZrO2 compositesprepared fromdifferent alumina sources results into change of reaction sintering,densificationandmicrostructurewhichcanalternatelyaltertheformationtemperatureandretentionoftZrO2

ISSN: 1584-2673 [CD-Rom, online]

344 | Fascicule 4

phase in these composites. Moya et al. [12] demonstrated that the microstructure of mullitezirconiaandaluminazirconiacompositescanbemodifiedbyadditiveslikeCaOandMgO.Ipetumodu isa town inOsunState, in thesouthwesternpartofNigeria.Thesizeof the town is64km2(25sqmi).ThetownistheheadquartersofIfeNorthlocalgovernmentArea.Itislocatedatanelevationof227metersabovesea leveland itspopulationamountsto164,172.Itscoordinatesare7310Nand4270E.Variousresearchershaveinvestigatedthepropertiesofthisclaydepositforindustrialapplications[1316].Butthereisnorecordoftheinvestigationoftheeffectsoftheconcernedadditivesonthemechanicalpropertiesandphasedevelopmentofthesinteredceramicproducedfromclaysourcedfromthedeposit.2.MATERIALSANDMETHODS2.1.MaterialsClaySamplesused inthisworkweresourcedfromIpetumodu,OsunState in thesouthwesternpartofNigeria.HighpurityoxidessuchaszirconiaandyttriaweresuppliedbyF.J.BODMANN&CO,L.L.C.(TheThermalSprayMaterialsandTechnologiesSource).OakmereBusinessPark,2072SussexStreet,Harvey,LA70058.2.2.RawClayPreparationTheclaysampleswerefirstsoakedinwaterforthreedaystodissolvethedeleteriousmaterialsinthemandat the same time to form slurry.The slurrieswere then sieved to removedeleteriousmaterialsandotherforeignsubstances. Thesievedslurrieswerethenallowedtosettledownforthreedaysafterwhich the clear floatingwaterwasdecanted.Thedispersed fine clays inwater(clayslurries)werethenpouredintoplasterofParis(P.O.P)mouldsandleftundisturbedforthreedaysinordertoallowtheremainingwaterpresenttodrainoutcompletely.Theresultingplasticclaymassesweresundriedandsubsequentlydriedinalaboratoryovenat110oCfor24hours.TheresultingdriedclaysampleswerecrushedandmilledinaRawwleySussexgrindertoanaverageparticlesizeof300m.2.3.ChemicalandMineralogicalCompositionofRawClaySamplesThe compounds present in the samplesweredetermined by xray fluorescence (XRF). Theresultsobtainedarepresented inTable1.Themineralogical phases present in the sampleswere also determined using xraydiffractometry (XRD). The phases present arereportedinFigure1andTable2.Thechemicalcomposition of the clay samples weredeterminedusing atomic absorption spectroscopy (AAS).The results obtained arepresented in

Table3.Ultrahighresolution fieldemissionscanningelectronmicroscope (UHRFEGSEM) equippedwith energydispersivespectroscopy(EDS)wasusedtoexaminethemicrostructureofthe clay samples.TheobservedmicrostructuresarepresentedinFigures2,3,4and5.

Table2:XRDResultoftheRawClaySamplesShowingtheQuantityofDifferentPhasesPresent

Phases Weight(%)Kaolinite 23.74Microcline 26.12

Muscovite/Illite 15.02Plagioclase/Albite 11.28

Quartz 23.84

P o s i ti o n [2 T h e ta ]

1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0

0

1 0 0

2 0 0

Al (

O H

)3

Al (

O H

)3;

Si O

2

Ti O

2

Si O

2

Ti O

2; A

l ( O

H )

3

Ti O

2; A

l ( O

H )

3

Ti O

2; S

i O2

Ti O

2; S

i O2

Si O

2

IP E T U .R D

Figure1:XRayDiffractionPattern(PhaseAnalysis)of

IpetumoduClaySample

Table1:XRFSemiquantitativeanalysisoftheelementsofraw

claysamples(weight%)PhasesAl2O3SiO2Fe2O3K2OMgOCaONa2OTiO2Zr

Total

Amount(%)25.0357.4829.2261.2590.910.7630.3721.5120.10397.1

ANNALS of Faculty Engineering Hunedoara International Journal of Engineering

345 | Fascicule 4

Table3.ChemicalCompositionofRawClaySamples(%)Samples SiO2 Al2O3 Fe2O3 TiO2 CaO MgO Na2O K2O LOIRawClay 54.82 25.90 1.44 2.08 0.12 0.15 0.2 1.57 11.64

Table4.XRDResultoftheSinteredCeramicSamplesRT1,FT1andFT2ShowingtheQuantityofDifferentPhasesPresent

Samples

Qua

rtz

Hem

atite

Mullite

Silliman

ite

Pseu

dob

rook

ite

Iron

(III)

Niobium

Oxide

(1/1/4)

Rutile

Aluminium

Niobate(V)

Zircon

mZrO

2

Cristob

alite

RT1 14.1 9.26 42.51 6.63 4.63 0 0.54 0 0 0 22.3FT1 8.16 4.72 7.37 16.07 1.26 0.42 0.45 1.91 4.12 42 13.52FT2 1.96 3.96 10.97 18.38 1.12 0.78 1.39 2.53 23.19 28.8 6.92

2.4.PreliminaryTestingofSinteredRawClayTheclaysamplefromIpetumoduwasfirstmilled inahighenergyplanetaryballmillat300rev/minfor4hour to obtain average particle size of 75m.Standard dimensions test samples were producedfrom the milled clay; for phase analysis andSEM/EDS analysis and mechanical properties bycompacting inastainlesssteelmoldusinga loadof750 MPa. The green compact were then sintered(fired)at1200oCand1300oC(T1andT2respectively)heldforanhour, inanelectricfurnace.Thesampleswere designated as R The sample sintered at1200oC were subjected to various tests such asdeterminationofthephasesdevelopedinthesampledue to sintering using XRD and bulk density,apparentporosity,coldcrushingstrengthmodulusof

elasticity,andabsorbedenergy.The resultsobtainedare shown inTables4and5 forXRDandmechanicalpropertiesrespectively.

(a) (b)Figure3.TypicalSEMmicrographsofSampleRT1showingitsmorphologyandcracks(a)BackScattered

Imageand(b)SecondaryElectronImage

(a) (b)Figure4.TypicalSEMmicrographsofSampleFT1showingitsmorphology(a)BackScatteredImageand(b)

SecondaryElectronImageX=PhasewithZrO2,whileZ=phasesofsinteredclay

Figure2.TypicalSEM/EDSmicrographsof

RawClaySampleshowingthemorphologyofthemineralanditschemicalcomposition

X

Z


346 | Fascicule 4

(a) (b)Figure5.TypicalSEMmicrographs(BackScatteredImage)ofSampleFT2showingitsmorphology(a)BackScatteredImageand(b)SecondaryElectronImage.X=PhasewithZrO2,Y=Zirconphase,whileZ=phasesof

sinteredclay2.5.PowderBlendingThemasspercentofthecompositionbelowwascomputedusingEquations(1)and(2)usingthedensities in Table M2 and the individual powders volume fraction in each composition. Thepowderswereweighedperbatchof50.00gonasensitiveelectronicweighingbalancetofive(5)decimalplaces.Theindividual(batch)compositionwasthoroughlymixedinaTurbulaMixerfor18hoursataspeedof72rev/min.ThecompositionsoftheblendedsampleswereshowninTableM1.

TableM1.TheCompositionsofSamplesproducedfromblendedpowdersusedfortheExperimentalInvestigationsDesignation RawClay(Vol.%)

Zirconia(ZrO2)(Vol.%)

Yttria(Y2O3)(Vol.%)

F 90 9.8 0.2

TableM2.DensitiesofVariousComponentsused

Oxide/Clay Claya ZrO2b Y2O3bDensity(g/cm3) 1.20 5.68 5.01

IfMt=Vclcl+VZrzr+VYY (1)

whereVcl,VZr,andVYarerespectivelythevolumefractionofclay,ZrO2andY2O3,cl,ZrandYarerespectivelythevolumefractionofclay,ZrO2andY2O3,andMtisthetotalmasscontributionofallthecomponents.AndifMisthemassofeachbatch,thenthemasscontributionofeachcomponentcouldbecalculatedfrom:

Mc=t

cQc

MxMV (2)

Mcisthemasscontributionofacomponentinabatch(clayorZrO2orY2O3),Vc,ctherespectivevolumefractionanddensityofthecomponent.The resulting homogenous powdermixtureswere compacted uniaxially into standard sampledimensions for cold crushing strength,bulkdensity,porositymeasurementsand thermal shockresistance. The sampleswere compressed uniaxially using 750Mpa compaction load inside astandard stainless steel die. The resulting green compactswere fired at 1200oC (for FT1) and1300oC(forFT2) inanelectricfurnace.Thesinteredsampleswere thencharacterizedforvariousmechanicalpropertiesasdescribedbelow:aDeterminedinthelaboratorybProvidedbythesupplier2.6.Testing2.6.1.ApparentPorosityTest samples from each of the ceramic compositeswere dried for 12 hours at 110C. The dryweightofeachfiredsamplewastakenandrecordedasD.Eachsamplewasimmersedinwaterfor6hrstosoakandweighedwhilebeensuspended inair.TheweightwasrecordedasW.Finally,the specimen was weighed when immersed in water. This was recorded as S. The apparentporositywasthencalculatedfromtheexpression:

p= %)-()-(100x

SWDW (3)

TheresultsobtainedarepresentedinTable5.

Z

Y

X


347 | Fascicule 4

Table5:MechanicalPropertiesofSamplesRT1,FT1andFT2

SamplesThermalShockResistance(cycles)

ApparentPorosity(%)

BulkDensity(g/cm3)

ColdCrushingStrength(N/mm2)

AbsorbedEnergy(J)

YoungsModulusE(N/mm2)

RT1 +30 22.70 1.95 1833.3 0.4320 21363FT1 +30 17.50 1.87 4409.00 1.2919 55208FT2 +30 15.40 2.06 8447.00 4.1753 64994

2.6.2.ColdCompressionStrength,ModulusofElasticityandAbsorbedEnergyColdcompressionstrengthtestistodeterminethecompressionstrengthtofailureofeachsample,anindicationofitsprobableperformanceunderload.Thestandardceramicsamplesweredriedinan oven at a temperature of 110C, allowed to cool.The cold compression strength testswereperformedonINSTRON1195atafixedcrossheadspeedof10mmmin1.Sampleswerepreparedaccording to ASTM C13397 (ASTM C13397, 2003) and cold crushing strength, modulus ofelasticity and absorbed energy of standard and conditioned sampleswere calculated from theequation:

CCS=aOfSampleSurfaceAre

tureLoadToFrac (4)

TheresultsobtainedarepresentedinTable5.2.6.3.BulkDensityThetestspecimensweredriedat110Cfor12hourstoensuretotalwaterloss.Theirdryweightsweremeasuredandrecorded.Theywereallowedtocoolandthenimmersedinabeakerofwater.Bubbleswereobservedastheporesinthespecimenswerefilledwithwater.Theirsoakedweightsweremeasuredandrecorded.Theywerethensuspended inabeakeroneaftertheotherusingaslingandtheirrespectivesuspendedweightsweremeasuredandrecorded.Bulkdensitiesofthesampleswerecalculatedusingtheformulabelow:

bulkdensity=)-( SW

D (5)

where:D=Weightofdriedspecimen,S=Weightofdriedspecimensuspendedinwater,andW=Weightofsoakedspecimensuspendedinair.TheresultsofthisarepresentedinTable5.2.6.4.ThermalShockResistanceEachsampleoftheclay/sawdustblendwasplacedinanelectricallyheatedfurnacetoattainthetesttemperatureof1000Cforover3hours.Eachsamplewasthenwithdrawnfromthefurnaceandheldfor10minutes.Theprocedurewasrepeateduntilanappearanceofacrackwasvisible.Thenumberofcyclesnecessarytocauseacrackwasrecordedforeachofthesamplesandtakenasameasureofitsthermalshockresistance.2.7.Analysis2.7.1.QualitativeandQuantitativeXRDThesampleswerepreparedforXRDanalysisusingabackloadingpreparationmethod.Theywereanalysed using a PANalytical XPert Pro powder diffractometerwith XCelerator detector andvariable divergence and receiving slits with Fe filtered CoK radiation. The phases wereidentifiedusingXPertHighscoreplussoftware.Graphicalrepresentationsofthequalitativeresultfollowbelow.Therelativephaseamounts(weights%)wereestimatedusingtheRietveldmethod(Autoquan Program). Amorphous phases, if presentwere not taken into consideration in thequantification.TheresultsobtainedarepresentedinFigure2andTables2and4.2.7.2.ScanningElectronMicroscopy Morphologyandmicroanalysisof theclayandcompositesamplesweredeterminedusingultrahigh resolution field emission scanning electron microscope (UHRFEGSEM) equipped withenergydispersive spectroscopy (EDS).Thepulverized clay samples/sintered ceramic compositesampleswerepreviouslygoldcoated.Thesampleswerestudiedusingultrahighresolutionfieldemission scanning electron microscope (UHRFEGSEM) equipped with energy dispersivespectroscopy (EDS). Particle images were obtained with a secondary electron detector. The


348 | Fascicule 4

SEM/EDSmicrographsofthepowderclayandthevarioussinteredceramicsamplesareshowninFigures2to5.2.7.3.ChemicalAnalysis(a)XRayFluorescence(XRF)ThemajorelementsweredeterminedbyXrayfluorescencewithanARL9800XPspectrometer.Thepulverizedclaysamplesweremixedwithlithiumtetraborateforchemicalanalysis.Theignitionlosswasmeasuredbycalcinationsat1000C.TheXRFresultisshowninTable1.(b)AtomicAbsorptionSpectroscopy(AAS)Thechemicalanalysisoftheclaysampleswasalsocarried outusing atomic absorption spectrophotometry (AAS)methodwith SpectraAA 220FSMachine.ThepercentagecompositionsofthevariousconstituentswerethenrecordedinTable3.3.RESULTSANDDISCUSSION3.1.PhaseDevelopmentandMechanicalPropertiesoftheSinteredRawIpetumoduClayTables4and5shows respectively thephasesdeveloped in thesamplesRT1/1,FT1/1andFT2/1andthemechanicalpropertiesofsamplesRT1/1,FT1/1andFT2/1.Figure3showsthemicrographsofthesampleRT1/1.3.1.1.PhaseDevelopmentintheSinteredRawIpetumoduClayFromTable4thephasesdevelopedintheceramicsampleproducedfromtherawIpetumoduclayandsinteredat1200oC(RT1/1)areclearlyshown.ComparingthiswiththeXRDresultsshowninTable2thetotalsilica(quartz)intheclayis23.84%,butinthefiredsample,thetotalsilica(quartzandcristobalite)is36.4%.Theexcess/extrasilicacomesfromthemullitizationprocess.Moreover,itisobservedthatthesampleRT1/1containsmulliteandsillimaniteaslistedabove,comparingthesewithTable2,itisobservedthattheclaysamplecontains23.73%kaoliniteandabove37%feldspar.Whenkaolinite is fired, the following transformationsoccur in itsstructure:Thedehydrationofkaolinite completes by ~150oC, followed by dehydoxylation at ~420660C and its structuralbreakdown occurs in the temperature range ~800900oC, depending upon the particle size andamount and type of the impurities present. The kaolinite in the Ipetumodu clay (Table 2)containing 23.84% quartz, 15.02%muscovite/illite (and about 37% feldspars) dehydroxylates tometakaoliniteat~420660Cdependingonthetypeandamountofimpuritiespresent[17,18,19]via

Si2Al2O5(OH)4420660CAl2O3*2SiO2+2H2O(6)which involves the combination of two OH groups to formH2O and oxygenwhich remainsincorporated inmetakolin.Atabout900C,metakaolinitedecomposestoamorphousSiO2andAl2O3typespinelvia

Al2O3.2SiO2900CAl2O3+2SiO2 (7)Al2O3typespinelandSiO2recrystallizeintomulliteattemperaturesabove1100Cvia

3Al2O3+2SiO2>1100C3Al2O3.2SiO2 (8)Srikrishna,etal[20],reportedtheformationofasinglephase(withcompositionclosetomullite)andexcessSiO2at900C.Thus the formationofmullitebeginsat temperatures>900Cand theprocesscontinuestill1000CFromtheTable4,itisobservedthattheamountofmullitedevelopedintheRT1is42.51%.Thisisnotunconnectedwith thepresenceofhematiteand rutile in the raw clayononehandand thepresenceoffeldsparontheotherhand.DifferentauthorshavedemonstratedthatmullitizationcanbeincreasedbycatalyticionssuchasFe3+andTi4+[21,22].ThesemetallicionshelpinmulliteformationbyreplacingtheAl3+ionsintheglassstructureduringfiring.ThepresenceofsmallamountsofFe3+andTi4+inkaolinmodifiesthechemical compositionof the ceramicbodies and therefore the sinteringbehaviourwhich in thecaseofporcelainischaracterizedbymullitization.ItcanbeobservedfromTable3thatthesinteredsampleRT1contains4.63%pseudobrookiteand0.54%rutilewhichareoxidesofironandtitanium.Thephenomenonofmullitization isalso influencedby thegradeof feldsparwhichparticularly


349 | Fascicule 4

affectstheformationofsecondarymullite.Thequalityoffeldsparisdeterminedbytheamountoforthoclase and albite. Nowadays, mixed feldspars are considered suitable fluxing agents forporcelain manufacture since they develop a very viscous liquid phase that embeds the newforming crystals and part of the residual crystals present in themicrostructure, enhancing thedensification process [23, 24].However, from the SEM images of the sample RT1, themulliteneedles couldnotbe seen (Figure2).Themullite in theRT1as seen theTable3 is theprimarymulliteasexplainedinthefollowingparagraphs.Mulliteformedfromkaolinclayaloneistermedprimarymullitewhereas that formed from reactionwith an alkali flux is termed secondary. Invitrifiedtriaxialporcelainsoftheclayfeldsparflintcomposition,mullitecrystalsoftwoorthreedistinctivemorphologieshavebeenreported[2531].Apartfromtheshape,thecrystalscanalsobedifferentiatedbytheirsizeandaspectratio[31],andbythechemicalcompositionthatchangesfromthenonstoichiometric2:1ofprimarymullitetothestoichiometric 3:2 of secondaymullite [29] on heating to temperatures higher than 1400C intriaxialcompositions.Primary mullite of composition close to 2:1, forms only from clay relicts of mostly kaolinite(Al2O3.2SiO2.2H2O)at11001150Cas small crystalsof


350 | Fascicule 4

ZrO2(s)+SiO2(s)ZrSiO4(s) (9)Thisaccountsfor thereduction in thequartz,cristobaliteandmZrO2phasesandthe increase inthezirconphaseasstatedabove.Thechangesobservedintheotherphasescouldbeexplainedasduetothecontributionoftheproductofthesolidstatereactionofequation(9)intheweight(%)ofthe sample. From theTable 3, the totalweight percent consumed as a result of the solid statereaction is26%but theweightpercent increaseobserved in thezircon is just19.07%.There isadeficitof6.93%whichwillleadtochangeinthepercentagecontentsofotherphases.3.3.MechanicalPropertiesoftheSinteredSamplesFT1andFT2Table5showsthemechanicalpropertiesofthesinteredsamples.Itisobservedthattheapparentporosity of the sample at 1200oC (FT1), is 17.50%.As the sintering temperature is increased to1300oC(FT2),theapparentporosityreducedto15.40%.Thiscouldbeexplainedthatincreaseinthesinteringtemperatureallowedformoredensificationtotakeplacetherebyallowingmoreporestobefilledbytheliquidphasepresentattheconcernedtemperaturetherebyreducingtheporosityofthesample.Itisalsoobservedthatthebulkdensityofthesampleat1200oC(FT1),is1.87g/cm3,asthesinteringtemperatureisincreasedto1300oC(FT2),theapparentporosityreducedto2.06g/cm3.Thisbehaviorisalsoduetothereducedporosityofthesampleasexplainedabovewhichleadtoincrease in theamountofmatter in the sampleperunitvolume.Furthermore, it isobservedat1200oC(FT1)thatthecoldcrushingstrength(CCS),absorbedenergyandtheYoungsmodulusarerespectively 4409N/mm2, 1.2919 J and 55208N/mm2.Moreover, as the sintering temperature isincreasedto1300oC(FT2),thecoldcrushingstrength(CCS)increasedto8447N/mm2,theabsorbedenergyalso increased to4.1753Jsimilarly, theYoungsmodulusalso increased to64994N/mm2.Thereasonforthisisthatthebulkdensityofthesampleincreasedwiththesinteringtemperature(consequent to reduction in the apparent porosity of the samples with increased sinteringtemperature),whichmeansthattheremorematterinthesampletobeartheappliedload[13,19,36].Similarly,fromFigures4(b)and5(b),itisclearlyobservedthatthecracksobservedinsampleFT1(Figure4a)duetothephasetransformationfromonesilicaphasetoanother,werecompletelysealedoffbythezirconphaseinsampleFT2(Figure5b).Thisisalsoresponsiblefortheimprovedmechanical property of FT2 when compared with FT1. The thermal shock resistances of thesampleswereobserved tobemore than30cycles.This isdue to the fact that thesampleswereearliersinteredathighertemperaturethanthetemperatureatwhichthetestwascarriedout.3.4.ComparisonoftheMechanicalPropertiesofSinteredSampleRT1/1withFT1/1andFT2/1Comparing themechanicalpropertiesofsampleRT1/1with thoseof theFT1/1andFT2/1, itwasobserved that theadditionof10%volumeofyttria stabilizedzirconia improved themechanicalpropertiesof the sample. It isnoted fromTable4 that sampleRT1/1 contains14.1%quartzand22.3%cristobalite,FT1/1contains8.16%quartzand13.52%cristobalitewhileFT2/1contains1.96%quartzand6.92%cristobalite.Thesephaseshavemarkeddifferences in their thermalexpansioncoefficients and the glass matrix. During cooling from the sintering temperature to roomtemperature, themarked differences in thermal expansion results in internal residual stresseswhichadverselyaffectsthestrengthofthesample[37].SampleRT1/1hasthehighestamountofquartzandcristobalite, followedbyFT1/1while the leastamountof thementionedphaseswereobserved inFT2/1.ThisexplainedwhythesampleRT1/1hasvery lowstrength,whencomparedwiththeothertwosamplesandwhythestrengthofFT1/1isabouthalfofthatofFT2/1.ThisisalsothereasonwhythesampleRT2/1cracked/failedandcouldnotbefurtherworkupon.4.CONCLUSIONItcanbeconcludedthat:9 DuetophasetransformationofthesilicacontentoftheIpetumoduclayfromonesilicaphaseto

anotherduringsintering,themechanicalpropertiesoftheceramicproducedfromitsinteredat1200oCisverylowandcouldnotwithstandafiringtemperatureof1300oC;


351 | Fascicule 4

9 Additionof2% (vol)yttria,and8% (vol)zirconia resulted intodepletionof thesilicaphases,formationofzirconphaseandexcesszirconia inadditionwithotherphases improvedon themechanicalpropertiesofthesamplesfiredat1200oCand1300oC

9 Theoptimummechanicalpropertiesisobtainedinthesamplesfiredat1300oCAcknowledgementsTheauthorswishtoacknowledgethefollowingorganizationsfortheirsupport.TheyareRegionalInitiativeinScienceEducation(RISE),ScienceInitiativeGroup(SIG)andAfricanMaterialsScienceandEngineeringNetwork(AMSEN).Reference[1.] Aksay,I.A;Dabbs,D.MandSarikaya,M.Mulliteforstructural,electronic,andopticalapplication.J.

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