Processing and Characterization of Multiphase Ceramic CompositesPart II: Triplex Composites with a Wide Sintering Temperature Range

Dong-Kyu Kim* and Waltraud M. Kriven*,**,w

Department of Materials Science and Engineering, University of Illinois at Urbana—Champaign, Urbana, Illinois 61801

Three-phase (triplex) ceramic composites with improvedthermal stabilities resulting from minimized grain growth dueto extended grain separations are introduced. Al2O3, yttriumaluminum garnet (YAG, Y3Al5O12), and ZrO2 were chemicallycompatible to make a stable triplex composite. Another stablecomposite could be made due to chemical compatibility amongAl2O3, NiAl2O4, and 3 mol% yttria-tetragonal zirconia poly-crystals (3Y-TZP). The composites of 33 vol% Al2O3–33 vol%YAG–33 vol% ZrO2, 33 vol% Al2O3–33 vol% NiAl2O4–33 vol% 3Y-TZP, and 50 vol% Al2O3–25 vol% NiAl2O4–25 vol% 3Y-TZP were fabricated using either sintering or hotpressing procedures. The sintered 33 vol% Al2O3–33 vol%YAG–33 vol% ZrO2 and 33 vol% Al2O3–33 vol% NiAl2O4–33 vol% 3Y-TZP composites demonstrated a ‘‘self accommo-dating sintering effect’’ having a wide sintering range withoutany extensive change in properties. Annealing of the 33 vol%Al2O3–33 vol% NiAl2O4–33 vol% 3Y-TZP composite at16001C for 50 h resulted in higher strength retention afterheat treatment compared with that expected from the sum ofthe strengths of constituent phases. This was attributed to amutual, grain growth retardation effect. Chemical compatibili-ties, mechanical properties, microstructures, and thermal stabil-ities of the composites were studied.

I. Introduction

CERAMICS usually have high strength, high modulus, verygood wear resistance, low thermal expansion coefficients,

and good thermal and chemical stabilities. As the grain sizesdecrease, hardness,1–3 strength,4–6 and toughness7,8 generally in-crease. Larger-grained ceramics have a higher creep rate,9 poorthermal shock resistance,10 and lower optical transparency.11,12

Ceramics with a grain size below about 1 mm demonstratesuperplastiticy.13–15 In ZrO2 ceramics, the grain size must bekept below a critical particle size to retain the tetragonal phaseat room temperature and utilize the phase transformation for itstoughening effect.16 Because of the benefits of smaller grainsizes, the design of engineered ceramics has migrated frommicrostructures to nanostructures during the last few years.The innovative properties of nanostructured ceramics are high-temperature superplasticity and improved mechanical, thermal,and wear properties.17

Grain growth is a natural phenomena occurring duringceramic processing, and final stage sintering is always accom-panied by rapid grain growth.18 Different efforts have been con-ducted to make a fine-grained, sintered ceramic. Hot pressing,19

spark plasma sintering,20 gel casting,21 two-step sintering,22 aswell as the addition of second phase particles,23 and grain

growth inhibitors24 as sintering aids have helped to producefine-grained, sintered microstructures.

Because grain growth and high-temperature creep aredependent upon diffusion in the material, suppression of diffu-sion is a prerequisite for high-temperature stability of ceramics.25

Different kinds of efforts have been conducted to fabricate ce-ramics having a minimized grain size by separating same phasegrains with different phase grains. Lee et al.26 observed thatgrain growth rates of carbide grains without any contiguouscarbides and surrounded by Co grains, were very low in theWC–Co system. El-Khozondar et al.27 showed that grain growthrates were remarkably varied in two-phase composites as thevolume fraction of each phase changed, due to the variation ofdiffusion distances for the two phases. An equivolumetricAl2O3–ZrO2 composite was fabricated and its grain growthrate was much slower than that of undoped Al2O3 due to theextended diffusion path length.28,29 The composite also showeda lower creep rate than did either of its single-phase constitu-ents.30 Kim et al.31 fabricated a 40 vol% ZrO2–30 vol% spinel–30 vol% Al2O3 composite. The composite exhibited extensivesuperplasticity without any strain hardening, because intermix-ing of different phases induced grain separation and longer in-terdiffusion distances, resulting in highly limited grain growth.

In this research, three different kinds of chemically compat-ible, triplex (three-phase) composites were fabricated by sinte-ring and hot pressing procedures and their physical,microstructual, and mechanical properties were measured. Thehigh-temperature microstructural stabilities of triplex compos-ites were compared with those of single phase materials afterhigh-temperature annealing.

II. Experimental Procedures

Equivolumetric, triplex, 33 vol% Al2O3–33 vol% yttriumaluminum garnet (YAG)–33 vol% ZrO2 composite powder wassynthesized using the organic, steric entrapment method.32–48

Aluminum nitrate nonahydrate [Al(NO3)3 � 9H2O, 981%,Aldrich Chemical Inc.], zirconium (IV) propoxide [AldrichChemical Inc., Milwaukee, WI, 70 wt% solution in 1-propanol],and yttrium nitrate hexahydrate [Y(NO3)3 � 6H2O, 99%, AldrichChemical Inc.], were used as Al13, Zr14, and Y13 sources, re-spectively. They were mixed in ethanol [ethyl alcohol USP, AA-PER ALCOL and Chemical, Shelbyville, KY] for 30 min. Then,polyethylene glycol [PEG, MW5 200, Aldrich Chemical Inc.]was added as a polymeric component in solution, and the so-lution was mixed for another 50 min. The mixed solution washeated to remove the alcohol, and the alcohol-free cakes weredried overnight at 1501C. The dried, porous cakes were pulver-ized and then calcined at 10001C for 1 h. The calcined powderwas attrition milled for 1 h and then sieved through a 100-meshsieve. The final powder was dry pressed at 35 MPa, and coldisostatically pressed (CIPped) at 414 MPa. The samples weresintered at temperatures ranging between 16001 and 17501C.Sintering shrinkages of the composites were determined for thethree orthogonal directions of each pellet.

Fifty vol% Al2O3–50 vol% NiAl2O4 and 67 vol% Al2O3–33 vol% NiAl2O4 two-phase (duplex) composites were also

J. Roedel—contributing editor

This work was partially supported by Kennametal Inc., under Contract # SRA 03-430.It was also partially supported by the AFOSR under grant # FA9550-06-1-0386.

*Member, The American Ceramic Society.**Fellow, The American Ceramic Society.wAuthor to whom correspondence should be addressed. e-mail: kriven@uiuc.edu

Manuscript No. 23141. Received April 27, 2007; approved November 26, 2007.

Journal

J. Am. Ceram. Soc., 91 [3] 793–798 (2008)

DOI: 10.1111/j.1551-2916.2008.02262.x

r 2008 The American Ceramic Society

793

chemically fabricated by the organic, steric entrapment method.Appropriate amounts of aluminum nitrate nonahydrate [Al(NO3)3 � 9H2O, 981%, Aldrich Chemical Inc.] and nickel hexa-hydrate [Ni(NO3)2 � 6H2O, Aldrich Chemical Inc.] were used aschemical sources for Al13 and Ni12, respectively. The chemicalswere mixed in de-ionized water for 30 min and then 5 wt% PVA(polyvinyl alcohol, 205S, Celanese Ltd., Dallas, TX) aqueoussolution was added, followed by another 50 min of mixing. Themixture was dried at 2001 and then 4001C until all of the waterwas removed. The water-free mixture was dried overnight at1501C. The dried, porous cake was pulverized and then calcinedat 8001C for 1 h. The calcined powder was heat treated at13001C for 1 h to make a high crystallinity powder, then 1 hattrition milled, and finally sieved through a 100 mesh sieve,to obtain a homogeneous, ‘‘in situ’’, equivolumetric, two-phase,50 vol% Al2O3–50 vol% NiAl2O4 composite powder.

Appropriate amounts of 50 vol% Al2O3–50 vol% NiAl2O4

and 67 vol%Al2O3–33 vol%NiAl2O4 in situ composite powderswere mixed with calculated amounts of 3 mol% yttria-tetrago-nal zirconia polycrystals [3Y-TZP, Tosoh chemicals Inc.,Tokyo, Japan] powder by ball milling for 24 h, in order tomake 33 vol% Al2O3–33 vol% NiAl2O4–33 vol% 3Y-TZP and50 vol% Al2O3–25 vol% NiAl2O4–25 vol% 3Y-TZP compositepowder, respectively. Some of 33 vol% Al2O3–33 vol%NiAl2O4–33 vol% 3Y-TZP composite powder was dry pressedat 35 MPa, and then CIPped at 414 MPa. The final pellets weresintered between 15501 and 17501C at sintering times of 1, 3, 5,and 10 h. Some of the 33 vol% Al2O3–33 vol% NiAl2O4–33vol% 3Y-TZP and 50 vol% Al2O3–25 vol% NiAl2O4–25 vol%3Y-TZP composite powders were unidirectionally pressed intodiscs, having a diameter of approximately 6.0 cm, under a pres-sure of 35 MPa. The pressed discs were CIPped at a pressure of414 MPa. The CIPped pellets having a diameter of about 5.5 cmwere ground down to 5.1 cm in diameter to fit into the hot-pressgraphite die having a sample diameter of 5.1 cm. These com-posites were hot pressed in an Ar atmosphere under 67 MPapressure at 14501C for 1 h and 17001C for 1 h. The heating ratefor hot pressing was 501C/min and the furnace was cooled afterholding for 1 h at the targeted temperature.

The relative thermal stabilities of the three-phase compositeswere tested and compared with those of the single-phase mate-rials. Al2O3, 3Y-TZP, NiAl2O4, and the triplex 33 vol% Al2O3–33 vol% YAG–33 vol% 3Y-TZP composite were sintered at16001C for 3 h, 15501C for 1 h, 16001C for 5 h, and 15501C for3 h, respectively. They were all heat-treated at 16001C for 50 h.The average grain size was determined from at least an averageof 300 measurements.

A Rigaku X-ray diffractometer (Model D-Max automateddiffractometer, Rigaku/USA. Danvers, MA) was used to iden-tify phase development in the heat-treated composites and fi-nally chemical compatibilities among the different phases in themultiphase composites were determined. The microstructures ofthe sintered and hot-pressed ceramics were analyzed by scanningelectron microscopy (SEM, Model S4700, Hitachi, Osaka,Japan). The SEM images of the sintered, three-phase, 33 vol%Al2O3–33 vol% YAG–33 vol% ZrO2 composite were analyzedby computer using NIH image analysis software [developed bythe National Institutes of Health, Bethesda, MD], and theamounts of each phase in the composite were determined. Flex-ural strengths were measured by a three-point bending test in ascrew-driven machine [Model 4502, Instron Corp., Canton,MA]. The supporting span was 30 mm, the cross head speedwas 0.1 mm/min, and sample size was 3 mm (H)� 4 mm(W)� 40 mm (L). Three to five samples were tested for eachcondition. The Vickers indentation hardness was determinedusing a commercial microhardness tester (Zwick 3212 micro-hardness tester, Mark V Laboratory Inc., East Granby, CT)with the impact rate of 3 mm/min at a load of 0.8 Kg. In orderto determine the hardness value of each material, 10 Vickers in-dentations were made for each measurement reported. Thetoughness values of materials were determined by the Anstiset al.49 indentation crack measurement method. Ten to 15

measurements were averaged in order to determine thefinal toughness value.

III. Results and Discussion

The increase in separation distance between grains of the samephase in the multiphase composite could induce mutual retar-dation of grain growth, resulting in thermal stability duringprolonged service at high temperatures. Figure 1 illustrates asimple schematic explanation for grain separation in a close-packed plane of a hypothetical, multiphase composite. All thecircles of the same design represent grains of the same phase. Ifthe material is a single phase system, in which all the nearest sixgrains are of the same phase, the average grain separation dis-tance (Savg), with respect to the center grain, from the nearestsix grains of the same phase, is 2 R (R is the average radius ofthe grains). Savg in the three- and five-phase composites become3.46 R, and 4.68 R, respectively. This means that Savg in thethree- and five-phase composites increase by 1.73 and 2.34 times,respectively, as compared with that of the single-phase material.

The triplex, 33 vol% Al2O3–33 vol%YAG–33 vol% 3Y-TZPpellet heat treated at 17501C for 5 h demonstrated chemicalcompatibilities among the three phases, without formation ofany other phase, as shown in the X-ray diffraction data ofFig. 2.

The equivolumetric, triplex, 33 vol% Al2O3–33 vol% YAG–33 vol% ZrO2 composites were sintered in the temperaturerange of 16001 and 17501C, and their three-point bend strengthsresults are summarized in Table I. All of the composites sinteredat conditions between 16501C for 5 h and 17001C for 15 h hadvery similar linear sintering shrinkages in the range of 19.1%–22.2% and showed a very narrow three-point bend strength dis-tribution ranging from 299 to 352 MPa. This means that thesintering shrinkages and strengths of this triplex composite didnot change very much with sintering conditions and so thecomposites had a very wide sintering range.

The reason for this wide sintering range can be attributed to a‘‘self-accommodating sintering effect’’ among the Al2O3, ZrO2,and YAG phases. Single-phase ZrO2 sinters at a relatively lowertemperature of about 15501C, single-phase Al2O3 sinters at me-dium temperature of about 16001C, and single-phase YAG sin-ters at a relatively higher temperature about 17001C. However,during the sintering of the three-phase composite, a combinedsintering effect of each phase’s sintering behaviors occurs. At a

(a) 1 phase (b) 2 phases (c) 3 phases (S = 2 R) (S = 3.15 R) (S = 3.46 R)

(d) 4 phases (e) 5 phases (S = 4.25 R) (S = 4.68 R)

1 1

1 1 1

1 1

2 1 3 2 1

2 1

3 2 1 3

2 1 3 2 1

3 2 1 3

3

4 1 5 3 2 4 1

4 1

3 2 4 1 5 3

4 1 5 3 2 4 1

3 2 4 1 5 3

5 3 2

5

5 2

2

2

4 1 3 2 4 1

2 4 1 2

3 2 4 1 3

4 1 3 2 4

3 4 1 3

2

1

3

2 1 2 1

2

2 1

2

2 1 2 1

1

1

Fig. 1. Schematic diagrams showing variations of average grainseparations (Savg) from the center grain to the six nearest grains ofsame phase in the multiphase composite systems. (R is the average radiusof each grain).

794 Journal of the American Ceramic Society—Kim and Kriven Vol. 91, No. 3

relatively lower sintering condition of 16001C for 5 h, the ZrO2 isfully densified, while Al2O3 is only partially densified and YAGdensified only to a small extent. Even though YAG was not welldensified, the highly densified ZrO2 and Al2O3 were expected toprovide the strength for the composite. At the higher sinteringcondition of 17001C for 15 h, the YAG starts to sinter to ahigher density. As the sintering of YAG becomes more active,the grain growth of ZrO2 and Al2O3 is suppressed. Thus thegrain growth retardation of Al2O3 and ZrO2 and the higherdensity of YAG, are the main factors maintaining the strengthof the composite even after higher temperature sintering.

Because of extensive contact between ZrO2 and YAG grainsin the triplex composite, yttrium (Y31) is believed to diffuse intointerstitial sites in the ZrO2 lattice and stabilize the ZrO2, re-sulting in a deficiency of YAG phase in the composite and re-duction in strength and toughness of the composite. The SEM/back scattered electron (BSE) images of the sintered compositeswere computer analyzed and the relative amounts of each phasewere quantitatively determined. The expected composition of33 vol% Al2O3–33 vol% YAG–33 vol% ZrO2 composite hadan actual composition of 39.8 vol% Al2O3–10.7 vol% YAG–49.5 vol% ZrO2 after analysis. Therefore, extra YAG was addedto the composite in order to obtain an equivolumetric, triplexmicrostructure. Table II summarizes the measured amounts ofeach phase present in the composite to which 0–70 vol% extraYAG was added. Figure 3 presents the SEM/BSE image and thecorresponding computer processed image of the 70 vol% excessYAG, triplex composite. Table II indicates that as the amountof extra YAG added to the composite increases, the relative

amounts of Al2O3 and ZrO2 decrease and the relative amount ofYAG increases. However, even after adding 70 vol% extraYAG to the composite, the composite needed more YAG tomake an actually equivolumetric composite. The sample with 90vol% excess YAG resulted in a nearly equivolumetric phasedistribution (Fig. 4). Zero vol% to 110 vol% of excess YAG wasadded to the triplex composites, and all samples were sintered at16501C for 15 h, before being mechanically tested for their three-point bend strengths. The three-point bend strength test resultslisted in Table III indicate that the strengths of the compositesdid not change very much with composition of the composites.From the results of Tables I and III, it could be said that themechanical properties of these three-phase composites wererelatively stable and independent of sintering conditions andchemical compositions.

The in situ 50 vol% Al2O3–50 vol% NiAl2O4 compositepowder was mixed with 3 mol%—tetragonal zirconia polycrytal(3Y-TZP, Tosoh Chemicals Inc., Tokyo, Japan) powder,pressed into a pellet, and then sintered at temperatures between15501 and 17501C at sintering times of 1, 3, and 5 h to make a 33vol% Al2O3–33 vol% NiAl2O4–33 vol% 3Y-TZP composite.The three phases in the 33 vol% Al2O3–33 vol% NiAl2O4–33vol% 3Y-TZP composite were chemically compatible with oneanother after heat treatment at 17001C for 5 h (Fig. 5). Table IVsummarizes the results of the mechanical tests of the sinteredcomposites. The composites showed strengths ranging between523 and 658 MPa except for the composite, which was sinteredat 17501C for 5 h. The linear sintering shrinkages ranged be-tween 13.5% and 14.2% except for the composite sintered at17501C for 5 h. The composite was assumed to be thermallydegraded after sintering at 17501C for 5 h. The composite sin-tered at 15501C for 3 h had the highest bend strength of 658MPa. A clear dependence of linear sintering shrinkage and bendstrength on sintering temperature and time could not be found.The strengths of the composites did not change very much withsintering conditions and so the composites had a very widesintering range. The reason for this could be attributed to a‘‘self-accommodating sintering effect’’ between Al2O3, NiAl2O4,and 3Y-TZP as could be seen in Table I for the 33 vol% Al2O3–33 vol% YAG–33 vol% ZrO2 composite. The SEM microstruc-tures of the composites sintered at 15501C/1 h, 16001C/1 h,16501C/10 h, and 17501C/5 h are shown in Fig. 6. As the sinte-ring time and temperature increased the grain sizes grew. Thecomposite sintered at 17501C for 5 h had an B7 mm averageequal grain sizes for all of the three different phases.

Table I. The Variation of Sintering Shrinkages and Three-Point Bend Strengths of the Al2O3–ZrO2–Y3Al5O12 Composites withSintering Conditions

Sintering

times (h) 16001C 16501C 16751C 17001C 17501C

5 Bend strength (MPa) 205712 31879 323715 323713 8474Shrinkage (%) 16.16 21.26 21.59 19.10 23.49

10 Bend strength (MPa) — 352713 29976 302711 —Shrinkage (%) — 22.04 21.97 19.61 —

15 Bend strength (MPa) — 327713 297710 319714 —Shrinkage (%) — 22.22 21.81 21.68 —

Fig. 2. The X-ray diffraction profiles showing the chemicalcompatibilities in the 33% Al2O3–33% yttrium aluminum garnet–33%ZrO2 composite heat-treated at 17501C for 5 h.

Table II. The Amount of Each Phase in the Additional YAG,Al2O3–ZrO2–YAG Composite

YAG additions Al2O3 (%) ZrO2 (%) Y3Al5O12 (%)

0% excess YAG 39.8 49.5 10.730% excess YAG 40.4 41.6 18.050% excess YAG 37.8 37.5 24.770% excess YAG 37.2 37.9 24.9

YAG, yttrium aluminum garnet.

March 2008 Processing and Characterization of Multiphase Ceramic Composites Part II 795

The three-point bend strengths of hot-pressed 50% Al2O3–25% NiAl2O4–25% 3Y-TZP and 33% Al2O3–33% NiAl2O4–33% 3Y-TZP composite are listed in Table V. Compared withthe strength of the sintered materials, shown in Table IV, thehot-pressed materials possessed higher bend strengths. The 50%Al2O3–25% NiAl2O4–25% 3Y-TZP composite had a higherstrength compared with that of 33% Al2O3–33% NiAl2O4–33% 3Y-TZP composite due to a higher fraction of Al2O3 anda lower fraction of NiAl2O4 in its composition.

The thermal stabilities of the 33 vol% Al2O3–33 vol%NiAl2O4–33 vol% 3Y-TZP triplex composite compared withthose of Al2O3, 3Y-TZP, and NiAl2O4, were investigated. Allthe materials were sintered at 16001C for 50 h. The sinteringshrinkages and bending strengths of these materials aresummarized in Table VI. The linear sintering shrinkages fordifferent materials were in the range of 13.7%–17.3%. The 3Y-TZP had surface cracks and exhibited the lowest strength of5 MPa. Al2O3 had a 339 MPa strength after a heat treatment of16001C/50 h. Al2O3 sintered at 16001C for 3 h had a 437 MPa

bend strength. 3Y-TZP sintered at 15501C for 1 h can possess1073 MPa of strength.50 This indicates that 3Y-TZP retained0.5% of its strength and Al2O3 retained 77.6% of its strengthafter heat treatment at 16001C for 50 h. The single phaseNiAl2O4, sintered at 16001C/5 h, had a bending strength of240 MPa and its strength increased to 289 MPa after heat treat-ment for 50 h at 16001C. Even though the composite containedabout 33 vol% of 3Y-TZP, which had only 5 MPa of bendingstrength after heat treatment, the 33 vol% Al2O3–33 vol%NiAl2O4–33 vol% 3Y-TZP composite had a bending strengthof 494 MPa after heat treatment. This meant that the grains of3Y-TZP could be retarded from grain growth, and the materialretained most of its strength. This kind of grain growth retar-dation would be also expected for the Al2O3 and NiAl2O4

grains. Therefore, the higher strength in the three-phase com-posite would be expected because of the mutual grain growthretardation effects due to the elongated, same phase, grain sep-arations.

IV. Summary and Conclusions

Two different kinds of chemically compatible, three-phaseceramic composites with the compositions of Al2O3–YAG–ZrO2 and Al2O3–NiAl2O4–3Y-TZP were fabricated by sinteringor hot pressing procedures.

All of the 33 vol% Al2O3–33 vol% YAG–33 vol% ZrO2

composites sintered between 16501C for 5 h and 17001C for 15 hhad similar sintering shrinkages in the range of 19.1%–22.2%and three-point bend strengths ranging from 299 to 352 MPa,resulting from a ‘‘self-accommodating sintering effect’’ amongthe Al2O3, ZrO2, and YAG phases in the triplex composite.About 90 vol% of excess YAG addition was required to makea virtually equivolumetric 33 vol% Al2O3–33 vol% YAG–33 vol% ZrO2 composites because some of the yttrium (Y13)migrated into the ZrO2 solid solution. The 33 vol% Al2O3–33 vol% NiAl2O4–33 vol% 3Y-TZP composite sintered attemperatures between 15501 and 17501C at sintering times of1, 3, 5, and 10 h also showed a ‘‘self-accommodating sintering

Fig. 4. The back scattered electron/scanning electron microscopymicrograph of three-phase ceramic composite with 90% excess yttriumaluminum garnet (YAG) and sintered at 16501C for 15 h (bright phase,ZrO2; grey phase, YAG; and dark phase, Al2O3).

Fig. 3. The back scattered electron/scanning electron microscopyimage (a) and computer processed image (b) of the 70% excess yttriumaluminum garnet (YAG) added, three-phase composite, sintered at16501C for 15 h. (bright or yellow phase, ZrO2; grey or green phase,YAG; and dark or blue phase, Al2O3).

Table III. The Three-Point Bend Strengths of the Composites with Excess YAG

0% 30% 50% 70% 80% 90% 100% 110%

Bend strength (MPa) 29774 307710 319724 317714 31473 31878 304720 317725

YAG, yttrium aluminum garnet.

796 Journal of the American Ceramic Society—Kim and Kriven Vol. 91, No. 3

effect’’. The composite sintered at 15501C for 3 h had the highestbend strength of 658MPa. The composite sintered at 17501C for5 h was thermally degraded.

The triplex, equivolumetric 33 vol% Al2O3–33 vol%NiAl2O4–33 vol% 3Y-TZP composite heat treated at 16001Cfor 50 h exhibited a higher strength compared with that expectedfrom the strengths of constituent phases, due to the mutual graingrowth retardation effect operating in the composite.

Fig. 5. X-ray diffraction profiles indicating compatibility among thethree phases of 33 vol% Al2O3–33 vol% NiAl2O4–33 vol% 3 mol%yttria-tetragonal zirconia polycrystals composite sintered at 17001Cfor 5 h.

(a) 1550°C/1h

(c) 1650°C/10h

(b) 1600°C/1h

(d) 1700°C/5h

Fig. 6. Scanning electron microscopy micrographs of 33 vol% Al2O3–33 vol% NiAl2O4–33 vol% 3 mol% yttria-tetragonal zirconia polycrystalscomposite sintered under different conditions. (dark grains, Al2O3; grey grains, NiAl2O4; and white grains, ZrO2).

Table IV. The Variations of Sintering Shrinkages and BendStrengths of 33 vol% Al2O3–33 vol% NiAl2O4–33 vol%

3Y-TZP Composites with Sintering Conditions

Sintering

temperature (1C)

Sintering

time (h)

Shrinkage

(%)

Bend strength

(MPa)

1550 1 13.5 5907123 14.0 6587165 14.0 609735

1600 1 14.0 5407343 14.0 6167255 14.0 626717

1650 1 14.1 6117253 14.1 5937295 14.0 57475810 13.9 608752

1700 5 14.2 5237111750 5 12.8 11773

3Y-TZP, 3 mol% yttria-tetragonal zirconia polycrystals.

Table V. The Three-Point Bend Strengths of Hot-PressedComposites

Sample compositions 14501C/1 h 17001C/1 h

50% Al2O3–25% NiAl2O4–25%3Y-TZP

679739 85975

33% Al2O3–33% NiAl2O4– 33%3Y-TZP

532737 794714

3Y-TZP, 3 mol% yttria-tetragonal zirconia polycrystals.

March 2008 Processing and Characterization of Multiphase Ceramic Composites Part II 797

Acknowledgments

Use of the facilities in the Center for Microanalysis of Materials, University ofIllinois at Urbana-Champaign which is partially supported by the US Departmentof Energy under grant DEFG02-91-ER45439 are gratefully acknowledged.

References

1A. Krell and P. Blank, ‘‘Grain Size Dependence of Hardness in Dense Submi-crometer Alumina,’’ J. Am. Ceram. Soc., 78 [14] 1118–20 (1995).

2R. W. Rice, C. C. Wu, and F. Borchelt, ‘‘Hardness-Grain Size Relations inCeramics,’’ J. Am. Ceram. Soc., 77 [10] 2539–53 (1994).

3T. Tani, Y. Miyamoto, M. Koizumi, and M. Shimada, ‘‘Grain Size Depen-dencies of Vickers Microhardness and Fracture Toughness in Al2O3 and Y2O3

Ceramics,’’ Ceram. Int., 12, 33–7 (1986).4J. Seidel, N. Claussen, and J. Rodel, ‘‘Reliability of Alumina Ceramics: Effect

of Grain Size,’’ J. Eur. Ceram. Soc., 15, 395–404 (1995).5P. Chantikul, S. J. Bennison, and B. R. Lawn, ‘‘Role of Grain Size in the

Strength and R-Curve Properties of Alumina,’’ J. Am. Ceram. Soc., 73, 2419–27(1990).

6R.W. Rice, ‘‘Strength/Grain Size Effects in Ceramics,’’ Proc. Brit. Ceram. Soc.,20, 205–57 (1972).

7B. Mussler, M. V. Swain, and N. Claussen, ‘‘Dependence of Fracture Tough-ness of Alumina on Grain Size and Test Technique,’’ J. Am. Ceram. Soc., 65 [11]566–72 (1982).

8L. Simpson, ‘‘Effect of Microstructure onMeasurements of Fracture Energy ofAl2O3,’’ J. Am. Ceram. Soc., 56, 7–11 (1973).

9Y. Estrin, G. Gottstein, and L. S. Shvindlerman, ‘‘Diffusion Controlled Creepin Nanocrystalline Materials Under Grain Growth,’’ Scripta Mater., 50, 993–7(2004).

10X. Q. You, T. Z. Si, N. Liu, P. P. Ren, Y. D. Xu, and J. P. Feng, ‘‘Effect ofGrain Size on Thermal Shock Resistance of Al2O3–TiC Ceramics,’’ Ceram. Int.,31, 33–8 (2005).

11R. Apetz and M. P. B. van Bruggen, ‘‘Transparent Alumina: A Light-scattering Model,’’ J. Am. Ceram. Soc., 86 [3] 480–6 (2003).

12K. Hayashi, O. Kobayashi, S. Toyoda, and K. Morinaga, ‘‘Transmission Op-tical Porperties of Polycrystalline Alumina with Submicron Grains,’’Mater. Trans.JIM, 32, 1024–9 (1991).

13X. Xu, T. Nishimura, N. Hirosaki, R. Xie, Y. Yamamoto, and H. Tanaka,‘‘Effect of Sintering Additives on Superplastic Deformation of Nano-Sized b-Silicon Nitride Ceramics,’’ J. Am. Ceram. Soc., 89 [5] 1745–7 (2006).

14Y. Yoshizawa and T. Sakuma, ‘‘High-Temperature Deformation and GrainGrowth in Fine-Grained Zirconia,’’ Eng. Frac. Mech., 40, 847–54 (1991).

15Y.Maehara and T. G. Langdon, ‘‘Superplasticity in Ceramics,’’ J.Mater. Sci.,25, 2275–86 (1990).

16Y. Zhou, T. C. Lei, and Y. X. Lu, ‘‘Grain Growth and Phase Separation ofZrO2–Y2O3 Ceramics Annealed at High Temperature,’’ Ceram. Int., 18, 237–42(1992).

17A. Bellosi, D. Sciti, and S. Guicciardi, ‘‘Synergy and Competition in Nano-and Micro-Design of Structural Ceramics,’’ J. Eur. Ceram. Soc., 24, 3295–302(2004).

18Z. P. Xie, J. W. Lu, Y. Huang, and Y. B. Cheng, ‘‘Influence of a-AluminaSeeds on the Morphology of Grain Growth in Alumina Ceramics From BayerAluminium Hydroxide,’’ Mater. Lett., 57 [16/17] 2501–8 (2003).

19L. A. Xue, Y. Chen, E. Gilbart, and R. J. Brook, ‘‘The Kinetics of Hot-Press-ing for Undoped and Donor-Doped BaTiO3 Ceramics,’’ J. Mater. Sci., 25, 1423–8(1990).

20Z. A. Munir, U. Anselmi-Tamburini, and M. Ohyanagi, ‘‘The Effect of Elec-tric Field and Pressure on the Synthesis and Consolidation of Materials: A Reviewof the Spark Plasma Sintering Method,’’ J. Mater. Sci., 41, 763–77 (2006).

21A. Krell, P. Blank, H. Ma, T. Hutzler, andM. Nebelung, ‘‘Processing of High-Density Submicrometer Al2O3 for New Applications,’’ J. Am. Ceram. Soc., 86 [4]546–53 (2003).

22X. H. Wang, X. Y. Deng, H. Bai, H. Zhou, W. Qu, L. T. Li, and I. W. Chen,‘‘Two-Step Sintering of Ceramics with Constant Grain-Size, II: BaTiO3 and Ni–Cu–Zn Ferrite,’’ J. Am. Ceram. Soc., 89 [2] 438–43 (2006).

23J. Lin and H. Lu, ‘‘Grain Growth Inhibition and MechanicalProperty Enhancement by Adding ZrO2 to Al2O3 Matrix,’’ Ceram. Int., 14,251–8 (1988).

24L. A. Xue, X. Wu, and I. W. Chen, ‘‘Superplastic Alumina Ceramics withGrain Growth Inhibitors,’’ J. Am. Ceram. Soc., 74 [4] 842–5 (1991).

25H. Haneda, ‘‘Role of Diffusion Phenomena in the Processing of Ceramics,’’J. Ceram. Soc. Japan., 111 [7] 439–47 (2003).

26I. C. Lee, H. Hondo, and T. Sakuma, ‘‘Nonuniform Carbide Grain GrowthDuring High-Temperature Compressive Deformation in WC-13wt% Co,’’ ScriptaMatall., 28, 97–102 (1993).

27R. El-Khozondar, H. El-Khozondar, G. Gottstein, and A. Rollet, ‘‘Micro-structural Simulation of Grain Growth in Two-Phase Polycrystalline Materials,’’Egypt J. Solids, 29 [1] 35–47 (2006).

28J. D. French, M. P. Harmer, H. M. Chan, and G. A. Miller, ‘‘Coarsening-Resistant Dual-Phase Interpenetrating Microstructures,’’ J. Am. Ceram. Soc., 73[8] 2508–10 (1990).

29S. J. Lee and W. M. Kriven, ‘‘A Submicron-Scale Duplex Zirconia andAlumina Composite by Polymer Complexation Processing,’’ Cer. Eng. Sci.Proc., 20 [3] 69–76 (1999).

30J. D. French, J. Zhao, M. P. Harmer, H. M. Chan, and G. A. Miller, ‘‘Creepof Duplex Microstructures,’’ J. Am. Ceram. Soc., 77 [11] 2857–65 (1994).

31B. N. Kim, K. Hiraga, K. Morita, and Y. Sakka, ‘‘A High-Strain-RateSuperplastic Ceramic,’’ Nature, 413, 288–91 (2001).

32M. A. Gulgun and W. M. Kriven, ‘‘A Simple Solution-Polymerization Routefor Oxide Powder Synthesis,’’ Ceram. Trans., 62, 57–66 (1995).

33S. J. Lee and W. M. Kriven, ‘‘Crystallization and Densification of Nano-Size,Amorphous Cordierite Powder Prepared by a Solution-Polymerization Route,’’J. Am. Ceram. Soc., 81 [10] 2605–12 (1998).

34S. J. Lee and W. M. Kriven, ‘‘Preparation of Ceramic Powders by a Solution-Polymerization Route Employing PVA Solution,’’ Cer. Eng. Sci. Proc., 19

[4] 469–76 (1998).35M. A. Gulgun, M. H. Nguyen, and W. M. Kriven, ‘‘Polymerized Organic–

Inorganic Synthesis of Mixed Oxides,’’ J. Am. Ceram. Soc., 82 [3] 556–60(1999).

36M. H. Nguyen, S. J. Lee, and W. M. Kriven, ‘‘Synthesis of Oxide Powders viaa Polymeric Steric Entrapment Precursor Route,’’ J. Mater. Res., 14 [8] 3417–26(1999).

37W. M. Kriven, S. J. Lee, M. A. Gulgun, M. H. Nguyen, and D. K. Kim,‘‘Synthesis of Oxide Powders via Polymeric Steric Entrapment,’’ (invited reviewpaper), in Innovative Processing/Synth.: Ceram. Glass Composites III, Ceram.Trans., 108, 99–110 (2000).

38E. A. Benson, S. J. Lee, and W. M. Kriven, ‘‘Preparation of Portland CementComponents by PVA Solution Polymerization,’’ J. Am. Ceram. Soc., 82 [8]2049–55 (1999).

39S. J. Lee, M. D. Biegalski, and W. D. Kriven, ‘‘Barium Titanate and BariumOrthotitanate Powders Through an Ethylene Glycol Polymerization Route,’’Ceram. Eng. Sci. Proc., 20 [3] 11–8 (1999).

40S. J. Lee, M. D. Biegalski, and W. M. Kriven, ‘‘Powder Synthesis of BariumTitanate and Barium Orthotitanate via an Ethylene Glycol PolymerizationRoute,’’ J. Mater. Res., 14 [7] 3001–6 (1999).

41M. A. Gulgun, W. M. Kriven, and M. H. Nguyen, ‘‘Processes for PreparingMixed-Oxide Powders’’; U. S. Pat. No. 6482387, November 19 (2002).

42S. J. Lee, Ch. H. Lee, and W. M. Kriven, ‘‘Synthesis of Low-Firing AnorthitePowder by the Steric Entrapment Route,’’ Ceram. Eng. Sci. Proc., 23 [3] 33–40(2002).

43B. R. Rosczyk, W. M. Kriven, and T. O. Mason, ‘‘Solid Oxide Fuel CellMaterials Synthesized by an Organic Steric Entrapment Method,’’ Ceram. Eng.Sci. Proc., 24 [3] 287–92 (2003).

44S. J. Lee and W. M. Kriven, ‘‘Synthesis and Hydration Study of PortlandCement Components by an Organic Steric Entrapment Method,’’ Mater. Struct.,38 [275] 87–92 (2005).

45W. M. Kriven and B. R. Rosczyk, ‘‘SOFC Powder Synthesis by the OrganicSteric Entrapment Method’’; pp. 381–94 in Fuel Cell Technologies: State and Per-spectives, Edited by N. Sammes, A. Smirnova and O. Vasylyvev. Kluver AcademicPublishers, Dordrecht, The Netherlands, 2005.

46D. Jia, D.-K. Kim, and W. M. Kriven, ‘‘Sintering Behavior of Gehlenite. PartI. Self-forming, Macro-/Mesoporous Gehlenite–Pore Forming Mechanism,Microstructure, Mechanical and Physical Properties,’’ J Am. Ceram. Soc., 90 [6]1760–73 (2007).

47Ch.-H. Jung, S.-J. Lee, W. M. Kriven, J.-Y. Park, andW.-S. Ryu, ‘‘A PolymerSolution Technique for the Synthesis of Nano-Sized Li2TiO3 Ceramic BreederPowders,’’ J. Nucl. Mater., 37 [3] 194–8 (2008).

48B. R. Rosczyk, S.-J. Lee, and W. M. Kriven, ‘‘Synthesis of NanocrystallineTitanate Powders via an Ethylene Glycol, Steric Entrapment Route,’’ J. Am.Ceram. Soc., submitted (2007).

49G. R. Antis, P. Chantikul, B. R. Lawn, and D. B. Marshall, ‘‘A CriticalEvaluation of Indentation Techniques for Measuring Fracture Toughness: I,Direct Crack Measurements,’’ J. Am. Ceram. Soc., 64 [9] 533–8 (1981).

50D. K. Kim and W. M. Kriven, ‘‘Oxide Laminated Composites with Alumi-num Phosphate (AlPO4) and Alumina Platelets as Crack Deflecting Materials,’’Composites Part B: Eng., 37 [6] 509–14 (2006). &

Table VI. The Variations of Shrinkages and Bend Strengthsof Different Materials Sintered at 16001C for 50 h

Sample compositions Shrinkage (%) Bend strength (MPa)

A16-SG Al2O3 15.9 33975Tosoh 3Y-TZP ZrO2 17.3 571NiAl2O4 13.7 289731Al2O3–NiAl2O4–3Y-TZP 13.8 494741

3Y-TZP, 3 mol% yttria-tetragonal zirconia polycrystals.

798 Journal of the American Ceramic Society—Kim and Kriven Vol. 91, No. 3