PROCESSING OF ZIRCONIA AND CALCIUM ALUMINATE ......and calcium aluminate cement, as a source of calcium, is investigated in this work. The effect of important processing variables

Original papers

Ceramics – Silikáty 59 (4) 261-269 (2015) 261

PROCESSING OF ZIRCONIA AND CALCIUM ALUMINATE CEMENT MIXTURES BY SPARK PLASMA SINTERING

#Y. L. BRUNI* , G. SUAREZ*, Y. SAKKA** ,L. B. GARRIDO*, E.F. AGLIETTI*

*CETMIC (Centro de Tecnología de Recursos Minerales y Cerámica, CIC-CONICET La Plata)Camino Centenario y 506. C.C.49 (B1897ZCA) M.B. Gonnet. Buenos Aires. Argentina

**Fine Particle Processing Group, Nano Ceramics Center, National Institute for Materials Science (NIMS),1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan

#E-mail: [email protected]

Submitted June 8, 2015; accepted September 8, 2015

Keywords: Calcia stabilized ZrO2, Spark plasma sintering, High alumina cement

Spark Plasma sintering (SPS) was applied for the densification of Calcia stabilized ZrO2 based composites obtained from mixtures of pure zirconia (m-ZrO2) and calcium aluminate cement (HAC). Two commercial powders of pure zirconia were employed as reactants. One of these powders had a coarse mean particle size (d50 = 8 μm) and the other was a submicrometer sized power (d50 = 0.44 μm). Several compositions containing different proportions of HAC (5 to 30 mol. % CaO in ZrO2) were sintered by SPS at temperatures between 1200 and 1400ºC under a pressure of 100 MPa during 10 min. The effect of processing conditions on phase composition, densification, microstructure and Vickers hardness of the obtained composites was examined. SPS significantly enhanced the densification in both type of composites (relative density > 93 %) as compared to those previously produced by conventional sintering. Composites with low CaO content consisted of mixtures of c-ZrO2, (Ca0.15Zr0.85O1.85), unreacted m-ZrO2 and calcium dialuminate (CaAl4O7 or CA2). The highest hardness was determined for composites sintered at 1400ºC being related to the maximum relative density (~ 99 %). High densification of composites with 30 mol. % CaO composed by similar proportions of CaAl4O7 and c-ZrO2 were obtained even at 1200ºC but led to a slightly lower hardness. In general, the use of the finer m-ZrO2 powder contributed to increase both the c-ZrO2 content and densification of composite sintered at a relatively lower temperature. For these composites, best hardness (Hv near to 10 GPa) resulted when the microstructure consisted of a fine grained ZrO2 matrix surrounding the dispersed CaAl4O7 grains instead of large interconnection between grains of both phases existed.

INTRODUCTION

SparkPlasmaSintering(SPS)isaneffectivemethodfor the consolidation and rapid sintering of ceramics to fulldensification.Thecombinationofhighheatingrate,andtheeffectsofelectricfieldandpressureenhancethedrivingforcefordensification[1].Thehighheatingratesresult in abenefitbypassing thegrain coarsening lowtemperaturemechanisms(e.g.surfacediffusion)and,inmany cases, inhibit the grain growth.The influenceofelectriccurrentonmasstransportisattributedtoseveralmechanisms including electromigration and intensifieddiffusion in ionic conductors, electroplasticity [2] andalsotoothereffectsatmicrostructurallevelsuchastheincrease in point defect concentration, or a reduction in themobilityactivationenergy fordefects [3].PressureappliedduringSPSaffectssinteringasaresultofparticlere-arrangement and the disintegration of soft aggregates, andalsoprovidesanintrinsiccontributiontothedrivingforceonsintering[1,4]. TheSPSpresentsmanyadvantagesincomparisonwith several conventional sintering methods includingpressureless sintering, hot-pressing and others, such as lower sintering temperature and shorter holding time

that produce comparative improvements in propertiesof resulting products [1, 5-7]. Pressure and lower sin-tering temperatures limit grain growth compared toconventional sintering, but coarsening is never totallysuppressed[8]. Previous studies reported that SPS was effectivefor enhancing density and to retain low grain size inalumina and zirconia ceramics [8-15]. In particularprevious studies show that SPS improves mechanicalproperties(flexuralstrength,Vicker´shardnessandfrac-ture toughness) aswell as ionic conductivity of 3 and8 – mol. % Y2O3stabilizedzirconiawhich,inturnaffecttheir perfomance as structural ceramics, electrolyte for SOFC, etc. These properties are strongly dependent on the microstructure and processing conditions. Thus, the optimization of sintering parameters such as heating rate,sinteringtemperatureanddwellduration,pressureapplied,etc.leadtoasuccessfulconsolidationofvariousnano sized ZrO2 ceramics[8,14]. CaO is a current stabilizerof commercially avail-able ZrO2 products and the typical additions varybetween 7.5 - 8.7 mol. % in ZrO2. Standard partially stabilized ZrO2 contains approximately 5mol.%CaOwhile the full stabilization in the cubic form requires

Bruni Y. L., Suarez G., Sakka Y., Garrido L. B., Aglietti E. F.

262 Ceramics – Silikáty 59 (4) 261-269 (2015)

approximately20mol.%[16-19].Recently,compositeswith high ZrO2 content belonging to the ZrO2–CaO–Al2O3systemwere produced from variousmixtures ofm-ZrO2andhighaluminacement(HAC)byconventional(pressureless)sintering[20].Thesecompositesattaineda relativelymoderate density up to 1500°C indicating the low sinterability of thismixture.The porous composites with open pore structure have potential appli-cationsasfilters,biomaterials,catalyst,componentsforSOFC, thermal insulating materials, etc. On the other hand, high densification is beneficial for CaO–ZrO2

based composites designed for structural, furnace and biomedicalapplicationsduetoensuretheimprovementof the mechanical properties required to the desiredapplication[17-21].Therefore,theprocessingbySPSofdense calcia stabilized ZrO2 based composites obtained frommixturesofm-ZrO2 and calcium aluminate cement, as a source of calcium, is investigated in this work.The effect of important processing variables such aschemical composition, m-ZrO2 particle size, sintering temperature(1200to1400°C)onthereaction(thatleadsto the formation of phases corresponding to the ZrO2–CaO–Al2O3 system), densification, microstructure andVicker´s hardness were examined. Crystalline phaseformationwasinvestigatedbyX-raydiffractionanalysis.

EXPERIMENTAL

Starting Materials

Twocommercialpowdersofpurezirconia(mono-clinic phase, m-ZrO2) were employed as reactants.One of these with coarse particle size (mean particlesize d50=8μm)hasanarrowparticlesizedistributionrangingbetweend90 =15μmandd10 =5μm;theotherindicatedasfinezirconia(meansized50 =0.44μm)wasa submicrometer sized powder (CZE, Saint-GobainZirPro)havingd90 =1.46μmandd10 =0.1μm;withthespecificsurfacearea5.6m2∙g–1. A commercial high alumina cement (HAC)wide-ly used in the refractory industry [22]was the secondreactant (Secar71,Kerneos,France).Themainconsti-tuents of this cement are Al2O3 and CaO, whoseapproximatechemicalcomposition(productdatasheet)indicates26.6-29.2wt.%CaOand69.8-72.2wt.%Al2O3. The mineralogical composition of HAC wasverified using XRD indicating that the major compo-nent is calcium monoaluminate (CaAl2O4 orCA)with25 wt. % of CaAl4O7 or CA2 and scarce amounts of Al2O3.ApreviousstudyreportedthatHACshowsspe-cific surface area of 1.2m2∙g-1 and consists mainly of relativelylargeparticlesrangingfrom1to63μmwith a mean particle size d50 =13μm[23].Itshouldbenotedthat the HAC has relatively coarser particles whencomparedtothoseofthetwoZrO2powdersused. From these raw materials, two series of powdermixtures were prepared with different chemical com-

positions varying between 5 to 30 mol. % of CaO inZrO2. The composites were identified in this workaccordingtotheCaOmolarcontentrelativetoZrO2 and considering the particle size of zirconia powder usedas reactant.ThecompositeswerenamedasZCfor thecoarse zirconia (mean size d50 =8μm)andZCFforthefinezirconia (mean sized50 =0.44μm).Thus i.e,ZC5indicatestheZCcompositewith5mol.%CaOinZrO2 andZCF30indicatestheZCFcompositewith30mol.%CaO in ZrO2.

SPS system

Densificationofthemixedpowderswasconductedusing a SPS machine (SPS-1050, Sumitomo, Japan). Thepowderwasplacedintoagraphitediewithaninnerdiameter of 10mm. In a typical sintering experiment,1.0gofthemixedpowderwaspouredintothedie.Thedevice is composed of an outer and an inner graphitedie,withgraphitepunches;theremainingdeviceswerealso made entirely of graphite. The temperature wasmeasured accurately using an optical pyrometer focused onthediesurface.Graphitefeltwasusedtoreducetheheatlossbyradiation.Thepowderwasheatedfromroomtemperatureupto700°Cwithin5min,subsequently,upto the sintering temperature (1200, 1300, and1400°C)usingaheatingrateof300°C∙min-1.Thedwelling timewas 10 min and pressure (100 MPa) was raised justbefore the holding. Heating was conducted using asequenceconsistingof12DCpulses(40.8ms)followedby zero current for 6.8 ms. During the entire duration of theexperiments,theelectriccurrentintensitywasbelow1000A and the voltage dropbetween the cooled ramswasbelow4V.

Characterization of SPS composites

X-ray diffraction analysis (XRD) was carried outusing a Philips diffractometer model PW3020 withradiationCu-KαandNifilterintheregion°2θ10–80°.Therelativeproportionofm-ZrO2phasewasdeterminedquantitativelyfromtheXRDdiagramsusingthemethodofGarvieandNicholson[24]formixturesofstabilizedzirconia and monoclinic phase. Scanning electron microscopy (SEM, Quanta 200 FEIMK2Serie)combinedwithenergydispersiveX-rayspectroscopy was used to examine the microstructuredevelopment. The obtained results were correlated tocomposition and SPS temperature. Apparent density and porosity of the sintered samp-leswasevaluatedbyHgimmersion.TherelativedensityRDwascalculatedasaratiobetweentheapparentandtheoretical density of composite which was evaluatedconsideringthevolumefractionandthedensityofeachphase present being for m-ZrO2, c-ZrO2 and CaAl4O7

of5.82,5.55and2.88g∙cm-3,respectively.Thevolumefraction of each phase was estimated based on XRD

Processing of zirconia and calcium aluminate cement mixtures by spark plasma sintering


results and the chemical composition. For this, it was considered the relative content of c-ZrO2 and the aluminapresentwasasCA2 (CaAl4O7). Hardness(Hv)of thecompositewasevaluatedbyVicker´s indentation technique (Buehler durometer,dentamet1100,SerieTestMacroVickers)withaloadof1 Kgf during 15 s for each material on polished surfaces upto1μm.Thehardnessreportedrepresentstheaveragevalueoftenindentations.

RESULTS AND DISCUSSION

Crystalline phases present

Figures 1a-c shows the XRD patterns of ZCcomposites (mixtures with coarse zirconia) sinteredat diffe-rent temperatures (1200, 1300 and 1400°C).TheZC5andZC15compositesconsistedofamixtureof m-ZrO2 and c-ZrO2 (Ca0.15Zr0.85O1.85.) with a smallamount of CA2 (CaAl4O7) due to the small content of HAC. Therefore the small content of CaO in these composites derived in the lowest formation of c-ZrO2 (produced by solid reaction from CaO and m-ZrO2) at 1200°C and 1300°Cbeingm-ZrO2 the main phase. Moreover,thec-ZrO2formationincreasedwithsinteringtemperatureasindicatedbythehighrelativeintensityofthecharacteristicpeaksofthisphase(2θ:30.15,34.95,50.22,59.68°)and thuswasobserved that them-ZrO2 phasedecreasednoticeablyat1400°C. ForZC30composites(Figure1c),thehighavailableamount of HAC resulted in high formation of CA2 and c-ZrO2. In fact in these composites the high amount ofCaOpresent and the sintering at 1300 and 1400°Cdetermined a complete transformation of m-ZrO2 to c-ZrO2 phase. The CA2 was the only crystalline phase formedcontainingaluminaat1400°C(neitherCA6norα-Al2O3 weredetectedbyXRD). It should be noted that themetioned phaseswereinagreement to thosepresent in thephaseequilibriumdiagramofthesystemZCA[25] Figures 2a-c shows the XRD patterns of ZCFcomposites (mixtures with fine zirconia) sintered at1200to1400°C.Thesecompositespresentedasimilarcomposition than ZC composites but showed a smallincrease in c-ZrO2 contentswith respect to thatof therespectiveZCcomposites. XRD characteristic peaks of each phases appeared slightly broader in both series of composites sintered at 1200°C indicating the presence of small crystallitesizes.RelativenarrowerpeaksinXRDpatternsforZCcomposites sintered at 1400°C in comparison to thoseof the ZCF series suggested the high crystallinity of the material. Figures3a,bshowthevariationofrelativecontentof c-ZrO2inpercentage(withrespecttototalZrO2)withthe CaO content for ZC and ZCF composites sintered at 1200-1400°C,respectively.

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Figure 1. XRD of ZC composites (produced from coarse ZrO2–HACcomposition)sinteredat1200-1400°C;a)ZC5,b)ZC15,c)ZC30.

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InZC5 andZCF5 composites sintered at 1200°C and 1300°C the small content of CaO derived in thelowest formation of c-ZrO2 (less than 20 %). But at 1400°Cinthesecompositeswasobservedanincrement ofthisphasecontent(30-40%ofc-ZrO2,respectively)that indicated the high effectiveness of the SPS tem-perature to activate the diffusional processes involvedinthereaction(betweenm-ZrO2andCaO)formixtureswithlowcontentsofCaO. InZCF30composites thestabilizationofzirconiaas c-ZrO2 phasewascomplete at all temperatures (in-clusiveat1200°C)duetothehighCaOcontent.InZC30composites the same results was determined at 1300and1400°C,whileat1200°Cthecontentofc-ZrO2 only attained 60 %. The highest stabilization of zirconia as c-ZrO2 phase onZCFcomposites inclusive at 1200°C canbeexplainedbecausethefinerZrO2 particle size increased contact area between reactant particles enhancing

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Figure 2. XRD of ZCF composites (produced from fine ZrO2–HACcomposition)sinteredat1200-1400°C;a)ZCF5,b)ZCF15,c)ZCF30.

Figure 3. Evolution of relative c-ZrO2 contentwithmol.%CaO of composites sintered at different temperatures; a) ZCcomposites (coarse ZrO2),b)ZCFcomposites(fineZrO2).

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diffusionoftheCaioninthelatticeofthem-ZrO2 The diffusionofCaionsalsoseemstobeacceleratedbythesparkplasmasintering,aswellastheparticlesizeeffect. In general the formation of c-ZrO2 phase mainly depends on CaO content. The effect of sintering tem-perature was significant only at lowest CaO content (i.e. ZC and ZCF composites those containing 5 and 15mol.%CaO).Theeffectoflowparticlesizeofzir-conia (used as starting reactant in ZCF composites) that enhancing c-ZrO2 formation had a notable influenceat the lowest sintering temperature (1200°C) as waspreviouslydiscussed.

Densification

Figures 4a and b show the relative density (RD)ofZCandZCFcompositessinteredat1200,1300and1400°C.Thisparameterindicatedthesinteringevolutionofthesecompositeswiththeeffectofheattreatmentandthe CaO content. In general the RD of these composites

washigher,rangedbetween92%and99%.TheseresultsindicatedtheadvantageofsinteringbySPSthatattainedhighestRDforthesecompositesincomparisonwiththoseproduced for the same compositions by conventionalsintering (pressureless) which reached a maximumRDof nearly 65% at 1400°C as shown in a previousstudy [20]. In the case of plasma sintering process,there is the direct application of pulsed electric current that rapidly heated the graphite die and can generate a dischargebetweentheparticlesofthepowder(thatcleanand activate the surfaces). In addition, high heatingratessuppressmechanismsofsurfacediffusion,therebylimitinggraingrowth,whereaspossiblyenhanceseveralmasstransfermechanismssuchasvolumediffusionandgrainboundarywhichleadtoahighdensification.Alsothe applied pressure favorably contributed to enhancedensification. Moreover recentlywas investigated the effects ofthe high temperature conductivity of zirconia in im-proving densification process during sintering by SPS.In this study commercial powders of zirconia YZS-8 mol. % Y2O3withanaverageparticle sizeof50μmweresinteredbySPSbyapplyingpressureof40MPaand a heating rate of 80°C min-1, for 20 minutes between800 - 1200°C [26]. The authors showed that sinteringat high temperatures (800°C or more) increases theconductivity of the zirconia and for this reason thepowder can be subject to a direct electric heating(through the powder). Other previous studies reportedthatthiseffectcontributestotheneckformationprocessduringSPS[27-31]. MoreoverforZCandZCFcompositesRDincrea-sed with increasing sintering temperature. This effectofthermaltreatmentonRDwasmoresignificantinZCcomposites. For ZCF composites the low particle size of thezirconia as startingpowder enhanced thedriving forceandthekineticsofthesinteringprocess.Thiseffectleadto a higher degree of densification inZCF compositesandthustheeffectofsinteringtemperatureonincreasingRDwaslesssignificantascomparedwiththatofRDofZC composites. In general the RD of ZCF composites resulted slightly higher than the RD of ZC composites. At 1400°C the effectiveness of thermal treatmentdetermined the highest RD for ZC and ZCF composites. Except for ZC30 composite containing larger amountof CA2 forwhichRDslightlydecreasedbetween1300and1400 °C.Thismaybe attributed tomicrocrackingresulting from mismatch in thermal expansion coeffi-cients of the CA2 and c-ZrO2 phases with 4.1 and10 × 10-6°C-1,respectively[32,33]. The effects of the specific parameters for the sin-tering by SPS could be observed analyzing theRD ofZC and ZCF composites sintered by SPS at 1200°Cand 1300°C. The ZC and ZCF composites containing5and15mol.%CaOattainedsimilarRDwhichwereslightly lower than thoseZC30andZCF30composites

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sinteredat the same temperatures.Thisdifferencemaybe attributed to the significant effect of the pressureapplied (100 MPa) during SPS on the larger agglomerate size for the mixture with more HAC content. Highpressure could reduce the size of agglomerates and less porosity between agglomerates of the powdermixtureenhanceddensificationofthecompositeevenat1200°C.Contrarily,adifferentbehaviorwasobservedat1400°Cin ZC composites. The ZC5 and ZC15 composites showedsimilarRDbuthigherthanthatofZC30.Thus,the contribution of pressure applied on densificationappeared to be more effective for low sinteringtemperatures.

Microstructure

Themicrostructure of the compositeswere analy-zed by SEM-EDX. Dense SEM microstructure of the ZC5compositessinteredat1200and1400°C(Figures

5a, b) consisted of a continuousmatrix of ZrO2 (light gray area) identified by EDX as the principal volumephasewithdispersedCA2 grains (dark gray areas). The CA2 as isolated grains (sizes ranging between 10 and50μm)weredistributedthroughoutthematrix.TheCA2 wastheonlyphasecontainingaluminapresentinthesecomposites(Figures1and2).At1400°C,thedispersedCA2 grains appeared smaller as a result of more c-ZrO2 stabilizationwithtemperature(Figure3). Figures5canddshowthatgrainsizeofremainingCA2 reduced notablyforZCFcomposites.At1400°C,enhancement of Ca ion diffusion from the CA2 phase promoted the c-ZrO2 formation, and finer particles ofZrO2favoredthereactivitybetweentheZrO2 and Ca ion. Figures 6a and b show that a coarsermicrostruc-ture resulted with increasing CaO content from 5 to 30mol.%, due to the increase in the amount of CA2 relative to ZrO2. These microstructures were charac-terizedby interconnectedgrainsof twophaseswithout

Figure5.SEMmicrographsofcompositeswith5mol.%CaO;a)andb)ZC(coarseZrO2)sinteredat1200and1400°C,respectively;c)andd):ZCF(fineZrO2)sinteredat1200and1400°C,respectively.

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distinguishingacontinuousmatrix.On theotherhand,themicrostructureofZCF30composites(Figures7a,b)consistedofafinezirconiamatrixsurroundingthedis-persed CA2 grainsofvariablesizes.Themicrostructureofthesecompositeswassimilarevenataftersinteringathigh temperatures .

Hardness

Figures8aandbshowthehardnessevolutionwithtemperatureforZCandZCFcomposites,respectively. The hardness of ZC composites sintered between1200and1400°Cvariedbetween7.8and9.4GPa.Thisresultwasinagreementwiththatofpreviouslyreportedfor 8 - 16 mol. % calcia doped zirconia. Such ceramics achievedRDhigherthan90%viamicrowavesinteringat1585°C1handexhibitedVickershardnessof8-9GPa[17].

Although aminor difference in hardness betweenZC and ZCF composites existed, a different trend canbe noted when this property was correlated with themicrostructure and porosity. Most of ZC composites presented a moderate hardness near to 9 GPa. In general, the best hardness agreed well with high densificationof composites. The lowest hardness about 8 GPa cor-responded to theZC15compositeafterSPSat1200°Candalso to theZC30compositesinteredat1400°C.Inthefirstcase,thedecreaseinhardnessmaybeassociatedwiththerelativelyhighporosityofthecomposite(~7%). Contrarily, low hardness of ZC30 composite having alowerporositywasattributed to thesignificantamountof CA2relativetoc-ZrO2 in this composites. There is a difference in hardness (at zero porosity) of each crys-talline phases present in the sintered composites being 10, 12 , and 11 GPa for m-ZrO2, c-ZrO2 and CA2, respec-tively[18,34,35].Thus,reductioninhardnesswithCA2

Figure6.SEMmicrographsofZC30composites(coarseZrO2) sinteredatdifferenttemperatures;a)1200°C;b)1400°C.

Figure7.SEMmicrographsofZCF30composites(fineZrO2) sinteredatdifferenttemperatures;a)1200°C,b)1400°C.

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content may be explained by both the relatively lowhardness of this phase and the differentmicrostructureofcomposites.Incomparison,Figures5and7showthatthe increase in CA2 amount changed the microstructure of composites as compared to that of ceramics havinglow HAC contents. The microstructure of low HACcompositeswascharacterizedby thedispersionof fewand small CA2 grains in a dense ZrO2matrix.Whereasthe microstructure of ZC30 sample containing similarproportion of c-ZrO2 and CA2phaseswascharacterizedby large interconnecting grains of both phases (i.e. withoutdistinguishingonecontinuousphaseasamatrix).Inaddition,thepresenceofsomemicrocracksexplainedthe reduction in hardness of this sample. Hardness of the ZCF ceramics resulted similar and approximated to9 - 10GPa (Figure8b).The slightlylowerhardness(9GPa)corresponded toceramicswith5 and 15mol.%CaO (i.e. low cement compositions)sinteredat1200and1300°CandtoZCF30compositesinteredat1300and1400°C.Inthefirstcase,thelower

hardness was due to porosity above 4 % which wascomparativelyhigher than thatof theothercomposites(2%).ThelowerhardnessofZCF30compositessinteredat1300and1400°CdidnotcorrelatewithporositybutitwasprobablyduetothechangesinthemicrostructurewithincreasingCA2 amount. In general, ZCF materials exhibited low porosity and finer grain sizes andconsequentlyreachedhigherhardnessinrelationtothatof the ZC composites.

CONCLUSIONS

Dense Calcia stabilized zirconia – CA2 (CaAl4O7) composites were produced by SPS from m-ZrO2 and calcium aluminate cement (HAC)mixtures in a range of 1200 - 1400°C.According to the high efficiency ofthis sintering technique, composites exhibited a highdensity(93-99%oftheoreticaldensity)regardlessofthe sintering temperature and composition. Moreover,thecompositeswithhighCaOcontentsinteredbySPSexhibited high densification even at 1200°C indicatingthe effectiveness of this technique in comparisonwiththe conventional sintering. Furthermore, the SPSprocess derived in a strong activation of diffusionprocesses involved in the reaction of CaO-stabilizedzirconia. Both high densification and complete c-ZrO2 formation occurred at a relatively lower temperaturethanthatrequiredbytheconventionalsintering.ForthecompositeswithlowCaOcontent,thec-ZrO2 formation increasedwithincreasingsinteringtemperature.Also,thereduction of m-ZrO2 particle size promoted the c-ZrO2 formation (even by sintering at low temperatures), butdidnothaveasignificantinfluenceonthedensificationdegree of composites. ThemaximumVickershardness(10GPa)attainedat 1400°C for the composites with low CaO content,probablyduetothecombinedeffectoflowporosityandSEMmicrostructureoffineZrO2 matrixwithsmallCA2

grains dispersed.While for compositeswith highCaOcontent sintered at the same temperature, the relativelower hardness near of 9 GPa was attributed to thecoarser microstructure formed by large interconnecting grains of CaAl4O7 and c-ZrO2 phases.

REFERENCES

1. MunirZ.A.,QuachDV.,OhyanagiM.:J.Am.Ceram.Soc.94, 1 (2011).

2. Olevsky E. A., Froyen L.: J. Am. Ceram. Soc. 92, 122 (2009).

3. Langer J.,HoffmannM.,GuillonO.: J.Am.Ceram.Soc.94,131(2011).

4. Munir Z. A., Anselmi-Tamburini U., Ohyanagi M.: J. Mat. Sci. 41,763(2006).

5. Omori M.: Mat. Sci. Eng. A 287,183(2000).

7.0

7.5

8.0

8.5

9.0

9.5

10.0

10.5

11.0

Vick

ers

Har

dnes

s (G

Pa)

5 10 15 20 25 30

ZCF

1200 °C1300 °C1400 °C

CaO in ZrO2 (mol %)

7.0

7.5

8.0

8.5

9.0

9.5

10.0

10.5

11.0

Vick

ers

Har

dnes

s (G

Pa)

5 10 15 20 25 30

ZC 1200 °C1300 °C1400 °C

CaO in ZrO2 (mol %)

Figure 8. Hardness of composites with 5-30 mol. % CaOsintered at 1200-1400° C; a) ZC composites (coarse ZrO2),b)ZCFcomposites(fineZrO2).

b) ZCF

a) ZC



6. Orru R., Licheri R., Locci A.M., Cincotti A., Cao G.: Mat. Sci. Eng. R: Reports 63,127(2009).

7. NygrenM.,ShenZ.:SolidStateSciences5,125(2003).8. Schwarz S., Guillon O.: J. Europ. Ceram. Soc. 33, 637

(2013).9. Shen Z., Johnsson M., Zhao Z., Nygren M.: J. Am. Ceram.

Soc. 85, 1921 (2002). 10.ChakravartyD.,BysakhS.,MuraleedharanK.,RaoT.N.,

Sundaresan R.: J. Am. Ceram. Soc. 91,203(2008).11.AmanY.,GarnierV.,DjuradoE.:J.Am.Ceram.Soc.94,

2828 (2011).12.DemuynckM.,ErauwJ.-P,VanderBiestO.,DelannayF.,

Cambier F.: J. Europ. Ceram. Soc. 32,1957(2012).13.Gurt Santanach J., Weibel A., Estournès C., Yang Q.,

Laurent Ch., Peigney A.: Acta Mater. 59, 1400 (2011).14.LiW.,GaoL.:J.Europ.Ceram.Soc.20, 2441 (2000).15. Bernard-Granger G., Guizard Ch.: Acta Mater. 55, 3493

(2007).16. Hannink R. H. J.: J. Am. Ceram. Soc. 83, 461 (2000).17.MuccilloR.,BuissaNettoR.C.,MuccilloE.N.S.:Mat.Lett.

49,197(2001).18. Nath S., Sinha N., Basu B.: Ceram. Intern. 34, 1509 (2008).19. Durrani S.K., Akhtar J., Ahmad M., Hussain M.A.: Mat.

Chem. Phys. 100,324(2006).20. Bruni Y.L., Garrido L.B., Aglietti E.F.: Ceram. Intern. 38,

4237(2012).

21. Dudek M.: Mat. Res. Bull. 44,1879(2009).22. Parker K.M., Sharp J.H.: Trans. J. Br. Ceram. Soc. 81,35

(1982). 23.BraulioM.A.L.,MorbioliG.G.,MilanezD.H.,Pandolfelli

V.C.:Ceram.Inter.37, 215 (2011).24.Garvie R., Nicholson P.S.: J. Am. Ceram. Soc. 55, 303

(1972).25. Muromura T., Hinatsu Y.: Mat. Res. Bull. 21, 61 (1986).26.Rajeswari K., Buchi SureshM., Hareesh U.S., Srinivasa

Rao Y., Das D., Johnson R.: Ceram. Int. 37,3557(2011).27.ChenX.J.,KhorK.A.,ChanS.H.,YuL.G.:Mater.Sci.Eng.

A 341,43(2003).28. Demirsky D., Borodianska H., Grasso S., Sakka Y.,

VasylkivO.:Scr.Mater.65,683(2011).29. Frei J.M., Andelmi-Tamburini U., Munir Z.: J. Appl. Phys.

101,114(2007).30.LiuJX.,SongX.,ZhangJ.: J.Am.Ceram.Soc.89, 494

(2006).31.QuachD.V.,Avila-ParedesH.,KimS.,MartinM.,Munir

Z.A.: Acta Mater. 58, 5022 (2010).32.Skinner G.B., Johnston H.L.: J. Chem. Phys. 21, 1383

(1953).33.SuzukiY.,OhjiT.:Ceram.Int.30,57(2004).34.AsmiD.,LowI.M.:J.Mat.Sci.Lett.17,1735(1998).35.GritznerG.,StegerP.:J.Eur.Ceram.Soc.12,461(1993).

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PROCESSING OF ZIRCONIA AND CALCIUM ALUMINATE ......and calcium aluminate cement, as a source of calcium, is investigated in this work. The effect of important processing variables