Low-Temperature Processing and Mechanical Properties of Zirconia and Zirconia-Alumina Nanoceramics

J. Am. Ceram. Soc., 86 [2] 299 –304 (2003)

journal

Low-Temperature Processing and Mechanical Properties of Zirconia and Zirconia–Alumina Nanoceramics Oleg Vasylkiv,*,†,‡ Yoshio Sakka,*,† and Valeriy V. Skorokhod‡ National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki, 305-0047, Japan Institute for Materials Science, National Academy of Science of the Ukraine, Kiev, 03680, Ukraine The 1.5- to 3-mol%-Y2O3-stabilized tetragonal ZrO2 (Y-TZP) and Al2O3/Y-TZP nanocomposite ceramics with 1 to 5 wt% of alumina were produced by a colloidal technique and lowtemperature sintering. The influence of the ceramic processing conditions, resulting density, microstructure, and the alumina content on the hardness and toughness were determined. The densification of the zirconia (Y-TZP) ceramic at low temperatures was possible only when a highly uniform packing of the nanoaggregates was achieved in the green compacts. The bulk nanostructured 3-mol%-yttria-stabilized zirconia ceramic with an average grain size of 112 nm was shown to reach a hardness of 12.2 GPa and a fracture toughness of 9.3 MPa䡠m1/2. The addition of alumina allowed the sintering process to be intensified. A nanograined bulk alumina/zirconia composite ceramic with an average grain size of 94 nm was obtained, and the hardness increased to 16.2 GPa. Nanograined tetragonal zirconia ceramics with a reduced yttria-stabilizer content were shown to reach fracture toughnesses between 12.6 –14.8 MPa䡠m1/2 (2Y-TZP) and 11.9 –13.9 MPa䡠m1/2 (1.5Y-TZP). I.

However, the results have not always been consistent. For example, Y-TZP ceramics with slightly different yttria contents (2–3 mol%) showed opposite KIC values after alumina addition.25 Also, the KIC values of alumina–zirconia composites showed a dependence on the testing and evaluation techniques. Fukuhara showed that alumina addition to Y-TZP increased the hardness, but decreased KIC.31 Bhaduri et al. reported that nanocrystalline Al2O3/ZrO2 fully dense composite had average hardness and toughness values of 4.45 GPa and 8.38 MPa䡠m1/2, respectively.33 Lange reported a hardness of 15 GPa for a conventionally processed nanoceramic.34The main discrepancies in these results should be attributed to the different starting materials and fabrication routes. The very poor fracture toughness of nanocrystalline 3-mol%-yttria-stabilized zirconia ceramic was attributed by Cottom and Mayo to overstabilization of the tetragonal phase.22 In the current study we expected that the fracture toughness of the yttria-stabilized zirconia might be increased if the thermodynamic stability of the tetragonal zirconia decreased (for occurrence of the phase transformation of tetragonal to monoclinic on the introduction of the cracks during indentation). Tetragonal zirconia ceramics with less than 3 mol% of the yttria-stabilizer content were produced and examined to optimize the fracture toughness. Controlling the powder dispersion in suspensions effectively improves the sintered microstructure of a ceramic (especially nanoceramic) because the powder dispersion significantly influences the consolidated compact density and green microstructure.2 Although the preparation of a nanostructured bulk ceramic is possible only by using nanopowders, redispersion is needed to properly disperse such powders, because nanoparticles spontaneously agglomerate in suspensions.13–15 The dispersion of zirconia and alumina particles in a suspension is stabilized by electrosteric repulsion, which is controlled by adsorbing polyelectrolyte ionized onto the particle surface.14,15 This is why a colloidal technique coupled with ultrasonic redispersion was developed to prepare uniform green bodies virtually free of agglomerates, which resulted in a markedly low sintering temperature. In the current study, the influence of the sintering conditions and the ␥-alumina content in 1- to 5-wt%-alumina-doped zirconia and the lowering of the yttria-stabilizer content from 3 to 1.5 mol% Y2O3 on the mechanical properties of the bulk nanoceramic were considered. The tetragonal zirconia nanoceramics (doped and undoped with alumina) with the highest hardness and fracture toughness were produced.

Introduction

T

processing of nanostructured materials is a part of an emerging and rapidly growing field referred to as nanotechnology. Study in this field emphasizes the production of bulk nanomaterials with controlled structural characteristics, properties, and technological functions.1 The tetragonal zirconia polycrystal (TZP) ceramic has attracted major attention because of the possibility of obtaining a nanograined bulk ceramic with a controllable microstructure and improved mechanical properties.3–13 The addition of one ceramic to another often produces a composite with more desirable properties than the individual components. Small quantities of alumina (Al2O3) are known to aid densification, and have recently been shown by Suzuki et al.18 to enhance tensile deformation during superplastic flow of the zirconia ceramic. The addition of alumina to yttria-stabilized tetragonal zirconia polycrystals (YTZP) has produced ceramics with improved toughness. Examples of the effect of a small alumina addition (0.375–1.5 mol% Al2O3) have been shown by Kihara et al.32 The KIC increased by 17% and 15% with the addition of 1 and 4 vol% Al2O3, respectively. Many other investigations of the Y-TZP ceramic have shown improved KIC with various alumina additions.24 –34 HE

II. N. Claussen—contributing editor

Experimental Procedure

The 3 mol% Y2O3–97 mol% ZrO2 (3Y-TZP) nanopowder preparation technique was previously described.5,6 The preparation conditions of the powders with reduced yttria content were exactly the same as described earlier.5 The temperature range of 450°– 800°C and the holding time of 0.5–2 h were used for the powder’s calcination. Phase identification was determined from the X-ray diffractometry data (XRD) (Model JDX-3500, JEOL, Tokyo, Japan). The XRD profiles were recorded using CuK␣

Manuscript No. 187137. Received February 18, 2002; approved October 8, 2002. *Member, American Ceramic Society. † National Institute for Materials Science 1-2-1, Sengen, Tsukuba, Ibaraki, 3050047, Japan. ‡ Institute for Materials Science, NASU, 3, Krzhizhanivs’kogo Str., Kiev, 03680, Ukraine.

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radiation under 30 kV and 300 mA at room temperature. Observations via transmission electron microscopy (TEM; Model JEM2000-EX, JEOL, Tokyo, Japan) operated at 200 kV were used to determine the primary crystallite size and the evolution of the aggregate morphology. Aqueous suspensions were prepared containing 18 to 20 vol% powder of the 1.5- to 3-mol%-yttria-stabilized zirconia (Y-TZP) and 1- to 5-wt%-alumina-doped 3Y-TZP by adding the dispersant (ammonium polycarboxylate (ALON A-6114), Toaghosei Co., Tokyo, Japan). The ␥-alumina powder (Al2O3 NanoTek, CI Chemical Co., Tokyo, Japan) with an average particle size of 30 nm was dispersed to the suspensions within 2 h before the zirconia powder addition. All suspensions were prepared at room temperature under the same conditions of mixing the powder with a magnetic stirrer and with ultrasonic redispersion and homogenization. The homogeneous aqueous suspensions were obtained by mixing with a magnetic stirrer for 24 h. Before consolidation, the suspensions were evacuated in a vacuum desiccator to eliminate any air bubbles. Consolidations of the suspensions by slip casting and subsequent cold isostatic pressing (CIP) at 400 MPa were applied. The green density and the final density of the sintered bodies were measured by the Archimedes method in kerosene and water, respectively. The relative density of the zirconia ceramic was based on 6.06 g/cm3, and the relative densities of the zirconia–alumina composites were calculated according to the weight percent of Al2O3 in each composite, assuming the ␣ form (d ⫽ 3.98 g/cm3). The samples were sintered under ambient pressure in air at a temperature of 1150°C and times ranging from 2 to 50 h to produce a ceramic with a range of densities and grain sizes. The samples were heated at 5°C/min to the desired temperatures, held for the prescribed times, and then furnace cooled. The samples for the Vickers indentation tests were squares with an approximate 4 mm height and 12 mm side. The surface on which the indentations were performed was previously polished with diamond paste in an ordinary metallographic polisher. The quality of the finishing was checked by optical microscopy to avoid the presence of any scratches on the surfaces before testing. Grain sizes were determined by a linear analysis of SEM micrographs of the polished and etched (1100°C for 1 h) surfaces. Hardness indentations (hardness testing machine (MVK-H2), Akashi Co., Japan) were obtained by applying the forces of both 4.9 and 9.81 N (0.5 and 1 kg masses, respectively) for a dwell time of 15 s. For each sample, 10 indentations were used to obtain the average hardness and standard deviation. The high load (98 N and 196 N ⫹ 15 s hold) indentation tests (hardness testing machine (AVK-A), Akashi Co., Japan) were performed on each sample to generate cracks, and from them, the fracture toughness values were obtained. The higher force of 196 N (20 kg mass) was applied to generate cracks on the surfaces of the Y-TZP nanoceramics with a reduced yttria content. From the two diagonal crack lengths, two fracture toughness values were obtained for each indentation. An average fracture toughness and standard deviation for each sample were computed from the total number of fracture toughness values per sample (12 values). Because zirconia cracks in a Palmqvist mode, the fracture toughness (KIC) was obtained from the expression given by Niihara et al.23 for the Palmqvist cracks in brittle materials:

冉 冊冉 冊 K IC䡠␸ H䡠a1/ 2

H E䡠␸

2/5

冉冊

⫽ 0.035

l a

⫺1/ 2

(1)

where H is the Vickers hardness, E is Young’s modulus, 2a ⫽ d is the diagonal of the indentation, ␸ is the constrain factor, and l is the crack length. The so-called Palmqvist cracks (l) begin only at the end of the diagonals of the indentation, and the criteria for such cracks are as follows: 0.25 ⱕ l/a ⱕ 2.5.23 The expression for the fracture toughness (Eq. (2)) was obtained from the above relation (Eq. (1)): K IC ⫽ 9.052 ⫻ 10⫺3䡠H3/5䡠E2/5䡠d䡠l⫺1/ 2

(2)

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A value of E ⫽ 210 GPa has been assumed for all of the ceramic samples irrespective of their compositions. In addition, the crack lengths were measured immediately after the indentation was conducted to avoid slow crack growth after removing the load.22,23 III.

Results and Discussion

(1) Powder Preparation Figure 1 shows the XRD patterns of (a) 3-mol%-Y2O3stabilized ZrO2 hydrolyzed (155°C for 10 h) and washed with water and ethanol, (b) unwashed 3Y-TZP powder (hydrolyzed at 155°C for 10 h) calcined at 450°C for 1 h, (c) hydrolytically precipitated 3Y-TZP powder washed with water and ethanol and calcined at 450°C for 1 h, (d) hydrolytically precipitated 3Y-TZP powder washed with water and ethanol and calcined at 800°C for 1 h, and (e) hydrolytically precipitated 3Y-TZP powder washed with water and ethanol, calcined at 450°C for 1 h, and subsequently sintered at 1150°C with 10 h holds. The 3-mol%-yttriadoped zirconia powder prepared by hydrolysis with subsequent washing with water and ethanol without subsequent calcination shows the wide XRD lines (Fig. 1(a)) that can be attributed to tetragonal (or cubic) zirconia. The unwashed hydrous yttriastabilized zirconia (3Y-TZP) calcined at 450°C for 1 h (Fig. 1(b)) shows only the XRD peaks of the tetragonal phase. However, a powder produced in the same way, washed with water and ethanol and subsequently calcined at 450°C for 1 h, showed the strong XRD peaks of the monoclinic phase (Fig. 1(c)). A higher amount of tetragonal zirconia polycrystals was detected for the powder hydrolytically precipitated and washed with water and ethanol, however, calcined at the higher temperature of 800°C for 1 h. Only traces of the monoclinic phase (⬃10%) were detected for this powder (Fig. 1(d)). The sintered 3Y-TZP ceramic produced by the above method (Fig. 1(e)) showed only the XRD peaks of the fully tetragonal zirconia polycrystals. This surprising result was obtained after sintering (1150°C with 10 h holds) of the sample prepared from the 3-mol%-yttria-doped zirconia powder that after calcination at 450°C for 1 h was tetragonal and monoclinic in a nearly equal proportion (Fig. 1(c)). Figure 2(a) shows the TEM micrograph of 3-mol%-yttriastabilized zirconia produced by the hydrolysis of metal chlorides and a urea aqueous solution (150°C for 10 h), washed, treated by microtip ultrasonication,5 and calcined at 450°C for 1 h. This figure shows that the primary crystallites of zirconia were bound together into the aggregates with an average aggregate size of 25 nm. The morphology of the aggregates can by characterized by an opened arrangement of the crystallites. Taking into account that the shape of the aggregates appeared to be nonuniform, the higher calcination temperature was chosen. To obtain very uniformly shaped and dense nano-aggregates of zirconia powder, and at the same time minimize the high-temperature necking of the neighboring aggregates (presintering in the contact areas), a calcination temperature of 560°C was chosen. A TEM micrograph of the resulting 3-mol%-yttria-stabilized zirconia nano-aggregates is shown in Fig. 2(b). The primary crystallites with an average size of ⬃8 nm are aggregated into uniformly shaped secondary nanoaggregates with a mean aggregate size of 20 – 40 nm. (2) Colloidal Processing and Sintering Aqueous suspensions with the minimum possible viscosity were prepared by changing the solid content and the amount of additional dispersant. A suspension’s stability for a good dispersion is the main factor in determining the resulting green microstructure. The high viscosity of the suspensions implies the rapid flocculation of the particles in the as-slip-cast suspension. However, the low viscosity of the suspensions leads to the appearance of nonuniformity and cracks in the as-slip-cast green body. Nonuniform packing in both cases leads to localized, nonhomogeneous densification during subsequent sintering. In the case of colloidal processing of nanopowders, a low enough viscosity of the suspension may be achieved by reducing the volume percent of solids in the suspensions (i.e., using the

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Fig. 1. XRD pattern of (a) 3-mol%-Y2O3-stabilized ZrO2 hydrolyzed (155°C for 10 h) and washed with water and ethanol, (b) unwashed 3Y-TZP powder (hydrolyzed at 155°C for 10 h) calcined at 450°C for 1 h, (c) hydrolytically precipitated 3Y-TZP powder washed with water and ethanol and calcined at 450°C for 1 h, (d) hydrolytically precipitated 3Y-TZP powder washed with water and ethanol and calcined at 800°C for 1 h, (e) hydrolytically precipitated 3Y-TZP powder washed with water and ethanol, calcined at 450°C for 1 h, and subsequently sintered at 1150°C with 10 h holds.

higher than usual amount of dispersant—more than 70 vol% of water in the case of zirconia powder) and/or increasing the amount (i.e., increasing the weight percent) of the surfactant. The optimum solid content of the 3Y-TZP nanopowder suspension was found to be 20 vol%. Figure 3 shows the changes in the zirconia and alumina-doped zirconia aqueous suspension viscosities (at a share rate of 100 s⫺1) versus the amount of dispersant. The amount of 4.5–5 wt% dispersant was appropriate for obtaining a welldispersed suspension from the zirconia powder and from the alumina-doped (1.25, 2, 2.5, and 5 wt% of ␥-Al2O3) zirconia. At a lower or higher amount of dispersant, the suspension became too stiff for slip casting immediately or during the first 2 h. Extraction of excessive water from the green body after slip casting and especially after subsequent CIPing usually leads to the generation and expansion of a crack network into the green body volume. Slow water extraction was used for obtaining crackless green bodies of yttria-stabilized zirconia (1.5–3 mol% yttria), and alumina/Y-TZP composites. The green densities of the Y-TZP and ␥-Al2O3/3Y-TZP composites after slip casting and slip casting with subsequent CIP are shown in Table I. The green bodies with a relative density of 42– 44% were obtained by slip casting. The relatively high (for nanopowder) density of the 47– 49% was achieved after CIPing. The isothermal sintering behavior was studied at 1150°C. The isothermal sintering results for the Y-TZP and alumina-doped 3Y-TZP samples are shown in Fig. 4, where the relative densities are plotted as a function of the sintering time. At this temperature, the grain size remained in the nanoscale range.5,6 The relative density of ceramic sample prepared from the 3Y-TZP powder

reached 92% during heating (5°C/min) to the prescribed temperature, and 95% after a 2 h hold. The densification, D ⫽ 97%, was demonstrated by sintering at 1150°C for 12 h. Sintering at 1150°C with a longer hold of 30 h produced a ceramic that was 99.5% dense. The higher relative density of 94.3% of the 2Y-TZP ceramic after the nonisothermal stage of sintering and 98.6% after a 10 h hold was attributed to the better green microstructure of the 2Y-TZP samples. The addition of a small amount of ␥-alumina allowed the densification rate to be increased in comparison with the aluminafree 3Y-TZP samples. The 1.25-wt%-Al2O3-doped zirconia ceramic demonstrated 95.1% densification by nonisothermal sintering to 1150°C and 99.2% densification after a 12 h hold. A nearly full-density ceramic was obtained after a 20 h hold. The microstructure of the zirconia ceramic (3Y-TZP) sintered at 1150°C for 30 h is shown in Fig. 5. The average grain size is about 110 nm. Figure 6 shows the SEM microstructure of the 2.5 wt% Al2O3/3Y-TZP composite sintered at 1150°C for 20 h. The addition of the alumina enhanced the sintering process, shortened the sintering time (Fig. 4), and allowed the ceramic microstructure to remain in a nanoscale region. Once sintered, the ceramic samples had varied densities (92%–100% relative density) and grain sizes (60 to 160 nm) depending on the sintering parameters and composition. The alumina grain size in the sintered ceramic varied in the range of 35– 80 nm. (3) Mechanical Properties As can be seen from Fig. 7, the average hardness increased with the increasing hold times. The hardness of the 1.25 wt% alumina/

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Fig. 3. Viscosity of zirconia and zirconia–alumina suspensions versus the amount of dispersant (at a share rate of 100 s⫺1).

Table I. Relative Densities of Slip-Cast, and Slip-Cast and CIPed Powder Samples

Chemical composition

3Y-TZP 2.7Y-TZP 2Y-TZP 1.5Y-TZP 3Y-TZP ⫹ 3Y-TZP ⫹ 3Y-TZP ⫹ 3Y-TZP ⫹

1.25 wt% Al2O3 2.0 wt% Al2O3 2.5 wt% Al2O3 5.0 wt% Al2O3

Relative density of slip-cast samples (%)

Relative density of slip-cast and CIPed samples (%)

43.3 43.0 44.2 43.1 42.8 43.5 44.2 42.1

48.8 48.3 49.6 48.9 47.8 47.3 49.2 46.9

16.23 GPa was demonstrated by the 2.5-wt%-alumina-doped zirconia ceramic with a relative density of 99% and average grain size of 94 nm. The longer holds at temperature gradually increased the grain size and decreased the hardness of the alumina/zirconia ceramic (Fig. 7).

Fig. 2. (a) TEM micrograph of 3Y-TZP primary crystallite aggregation (urea hydrolysis at 150°C for 10 h, calcinations at 450°C for 1 h). (b) TEM micrograph of 3Y-TZP dense nanoaggregates.

zirconia composites reached the maximum value at the average grain size of 105 nm (24 h hold) and relative density of 99.8%. The average hardness of the 2.5 wt% alumina/zirconia composites reached a maximum value of 16.2 GPa at an average grain size of 94 nm (15 h hold) and relative density of 99.2%. Longer holds at temperature allowed the relative density to increase and actually reach the full density (Fig. 4); however, the average grain size also increased (Fig. 8) and the average hardness of such ceramics gradually decreased (Figs. 7 and 8). The highest hardness of the 3Y-TZP ceramic was only 11.7 GPa (average grain size 112 nm, relative density 99.8%). We attributed such phenomena to the gradual decreasing of the average grain size because of the shortening of the hold time necessary for densification of the alumina-doped zirconia ceramics. The highest average hardness of

Fig. 4.

Presureless sintering at a constant temperature of 1150°C.

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Fig. 5.

Processing and Mechanical Properties of Zirconia and Zirconia–Alumina Nanoceramics

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SEM micrograph of 3Y-TZP ceramic sintered at 1150°C for 30 h.

The Vickers hardness versus relative density for the fully tetragonal yttria-stabilized zirconia ceramic with a 1.5 to 3 mol% yttria content is shown in Fig. 9. As can be seen from Fig. 9, the hardness increased with density, and after a maximum, decreased. Only the 2Y-TZP ceramic reached the highest average hardness at full density. The fully dense 2Y-TZP ceramic demonstrated an average hardness of 13.5 GPa. The highest average hardness of 13.65 GPa was demonstrated by the 2.7Y-TZP ceramic (99.2% dense). The relation between hardness and relative density is consistent with previous results (Fig. 8). There is a controversy regarding the values of the fracture toughness in the literature. Recently, Cottom and Mayo showed that without phase transformation toughening, the fracture toughness is in the range of 2.25 to 4.25 MPa䡠m1/2, a value comparable to brittle ceramics.22 However, there was a problem with their data analysis. The crack geometry was the Palmqvist type at the loads they used; however, instead of using an equation related to the Palmqvist geometry, they used an equation for the median crack geometry. This should result in an error even though the trend in toughness should be valid. Lange34 reported a toughness value of 6.73 MPa䡠m1/2. It should be noted that this value was based on a transformation-toughened composite, i.e., one that occurred during crack propagation. Fracture toughness vs yttria-stabilizer content for the fully tetragonal zirconia ceramics and 2.5 wt% Al2O3/Y-TZP composites is shown in Fig. 11. The average value of 8.62 MPa䡠m1/2 was

Fig. 6. SEM micrograph of 2.5 wt% alumina/3Y-TZP ceramic sintered at 1150°C for 20 h.

Fig. 7. Dependence of Vickers hardness on the hold time during sintering at 1150°C.

measured and calculated for the 3Y-TZP ceramic. A load smaller than 20 kg did not reveal any significant cracks. The significant increasing of KIC with the decreasing yttria-stabilizer content can be seen. The fracture toughness reached an average value of 13.5 MPa䡠m1/2 with decreasing Y2O3 content to 2 mol%. Compared with this result, the 2.5 wt% alumina/3Y-TZP composite ceramic showed a maximum average fracture toughness value of 7.86 MPa䡠m1/2, and reached a maximum at 2 mol% yttria content. However, the fracture toughness did not exceed a value of 10.3 MPa䡠m1/2 (2.5 wt% Al2O3/1.5Y-TZP ceramic). IV.

Conclusion

We showed that the dense yttria-stabilized (1.5–3 mol% Y2O3) tetragonal zirconia and 1.25- to 5-wt%-alumina-doped zirconia bulk nanoceramics were obtained by a colloidal technique and low-temperature sintering at 1150°C.

Fig. 8. Dependence of Vickers hardness on the relative densities of ceramic specimens.

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Fig. 9. Vickers hardness versus relative density for fully tetragonal Y-TZP ceramics with 1.5 to 3 mol% yttria content.

Fig. 10. Fracture toughness versus yttria-stabilizer content for fully tetragonal Y-TZP ceramics, and 2.5 wt% Al2O3/Y-TZP composites.

The average hardness and toughness of the 2.5-wt%-aluminadoped 3Y-TZP bulk ceramic were 16.23 GPa and 7.86 MPa䡠m1/2, respectively. The yttria-stabilized tetragonal zirconia ceramic with a higher fracture toughness of 12.6 –14.7 MPa䡠m1/2 was obtained by reducing the yttria-stabilizer content from 3 to 2–1.5 mol%. However, the average hardness of such ceramics was 13.2–13.6 GPa. The 2.5-wt%-Al2O3-doped tetragonal zirconia ceramic with a 1.5–3 mol% Y2O3 stabilizer content demonstrated the significant reduction of KIC when compared with an alumina-free ceramic. References 1

H. Gleiter, “Nanostructured Materials: Basic Concept and Microstructure,” Acta Mater., 48, 1–29 (2000). 2 F. F. Lange, “Powder Processing Science and Technology for Increased Reliability,” J. Am. Ceram. Soc., 72 [1] 3–15 (1989).

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O. Vasylkiv and Y. Sakka, “Nonisothermal Synthesis of Yttria-Stabilized Zirconia Nanopowder through Oxalate Processing: I, Peculiarities of Y-Zr Oxalate Synthesis and Its Decomposition,” J. Am. Ceram. Soc., 83 [9] 2196 –202 (2000). 4 O. Vasylkiv, Y. Sakka, and H. Borodians’ka, “Nonisothermal Synthesis of Yttria-Stabilized Zirconia Nanopowder through Oxalate Processing: II, Morphology Manipulation,” J. Am. Ceram. Soc., 84 [11] 2484 – 88 (2001). 5 O. Vasylkiv and Y. Sakka, “Synthesis and Colloidal Processing of Zirconia Nanopowder,” J. Am. Ceram. Soc., 84 [11] 2489 –94 (2001). 6 O. Vasylkiv, Y. Sakka, and K. Hiraga, “Chemical Synthesis and Sintering of Zirconia-Based Nanopowders,” Ceram. Trans., 112, 11–16 (2001). 7 O. Vasylkiv and Y. Sakka, “Synthesis and Sintering of Zirconia Nanopowder by Nonisothermal Decomposition from Hydroxide,” J. Ceram. Soc. Jpn., 109, 500 –505 (2001). 8 O. Vasylkiv and Y. Sakka, “Hydrothermal Synthesis of Nanosize ZrO2 Powder, Its Characterization and Colloidal Processing,” Stud. Surf. Sci. Catal., 32, 233–36 (2001). 9 O. Vasylkiv and Y. Sakka, “Hydroxide Synthesis, Colloidal Processing and Sintering of Nanosize 3Y-TZP Powder,” Scr. Mater., 44, 2219 –23 (2001). 10 Y. Sakka, K. Ozava, T. Uchikoshi, and K. Hiraga, “Colloidal Processing and Ionic Conductivity of Fine-Grained Cupric-Oxide-Doped Tetragonal Zirconia,” J. Am. Ceram. Soc., 84 [9] 2129 –31 (2001). 11 Y. Sakka and K. Hiraga, “Preparation Methods and Superplastic Properties of Fine-Grained Zirconia and Alumina Based Ceramics,” Nippon Kagaku Kaishi, 1999 [8] 497–508 (1999). 12 Y. Leong, P. Scales, T. Healy, and D. Boger, “Effect of Particle Size on Colloidal Zirconia Rheology at the Isoelectric Point,” J. Am. Ceram. Soc., 78, 2209 –14 (1995). 13 T. Suzuki, Y. Sakka, K. Nakano, and K. 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