• Journal of the Korean Crystal Growth and Crystal Technology
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• v.10 no.3
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• pp.233-238
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• 2000

t-$ZrO_2/Al_2O_3$composites having a superior biocompatability and phase stability were prepared by adding 0~4 mol% of $Eu_2O_3$and sintered for 1 h at $1600^{\circ}C$ to evaluate phase stability, chromaticity and mechanical properties of the composites. No tetragonal to monoclinic phase transformation was observed for the composites containing $Eu_2O_3$after heat treatment for 20 h at $180^{\circ}C$ under 3.5 MPa water vapor pressure condition. As $Eu_2O_3$content increased, the color of the composites was changed from a slight white ivory to a light pink. The strength and the fracture toughness of the composites containing $Eu_2O_3$were above 620 MPa and 7.6 MPa.$m^{1/2}$, respectively, when $Eu_2O_3$was added up to 3 mol%.

• Journal of the Korean Ceramic Society
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• v.21 no.2
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• pp.149-155
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• 1984

The effect of $ZrO_2$ dispersed phase on the mechanical properties in $Al_2O_3$-$ZrO_2$system has been studied. There are both metastable tetragonal phase and stable monoclinic phase of $ZrO_2$particles diespersed in Al2O3 matrix at room temperature. Metastable tetragonal $ZrO_2$ changes to the stable monoclinic structure within the stress field of the crack. And microcracks are formed by the expansion of $ZrO_2$during the tetragonal-monoclinic transformation on cooling. Therefore stress-induced phase transformation and inclusion-induced microcracking contribute to the mec-hanical properties of $Al_2O_3$-$ZrO_2$system. Sintered composites containing 10m/o $ZrO_2$ yield KiC values of 6.5MN/$m^{3/2}$ much greater than that of pure $Al_2O_3$ This increase results from microcrack extension and stress-induced phase transformation absor-bing energy by crack propagation. Flexural strength of composites is decreased considerably in comparison with pure $Al_2O_3$ This decrease results from microcrack as a crack former and higher porosity than pure $Al_2O_3$.

• Journal of the Korean Ceramic Society
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• v.37 no.4
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• pp.354-358
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• 2000

Tetragonal (t)-ZrO2/Al2O3 composites doped with Y2O3, Nb2O5, and Fe2O3 ((Y, Nb, Fe)-TZP/Al2O3) were prepared over the range containing Fe2O3 from 0.1 to 0.5 mol% with 0.1 mol% intervals to evaluate the effect of Fe2O3 addition on chromaticity, hydrothermal stability, and mechanical property of the composites. After autoclaving for 20 h at 18$0^{\circ}C$ under 3.5 MPa water vapor pressure, no tlongrightarrowm phase transformation was observed probably due to the preferential solid solubility of Fe2O3 in Al2O3, the presence of the rigid Al2O3 particles, and the inherent phase stability of (Y, Nb)-TZP. The optimized strength and the fracture toughness of the composite were 700 MPa and 8.5 MPa.m1/2, respectively, when 0.1 mol% Fe2O3 was added. The composites have shown a gradual color change from a slightly white ivory to a pale yellowish brown with increasing the Fe2O3 concentration.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.49 no.2
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• pp.92-97
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• 2000

The mechanical and electrical properties of the hot-pressed and annealed $\beta$-SiC+39vol. %ZrB2 electroconductive ceramic composites were investigated by adding 1, 2, 3wt% Al2O3+Y2O3(6:4wt%) of the liquid forming additives. In this microstructures, no reactions were observed between $\beta-SiC$ and ZrB2. The relative density is over 90.8% of the theoretical density and the porosity decreased with increasing Al2O3+Y2O3 contents. Phase analysis of the composites by XRD revealed $\alpha-SiC(6H, 4H)$, ZrB2 and $\beta-SiC$(15R). Flexural srength showed the highest of 315.5MPa for composites added with 3wt% Al2O3+Y2O3 additives as room temperature. Owing to crack deflection and crack bridging of fracture toughness mechanism, the fracture toughness showed 5.5MPa.m1/2 and 5.3MPa.m1/2 for composites added with 2wt% and 3wt% Al2O3+Y2O3 additives respectively at room temperature. The area fraction of the elongated SiC grain in the etched surface of sample showed 65% and 65.1% for composite added with 2wt% and 3wt% Al2O3+Y2O3 additives respectively. The electrical resistivity at room temperature. The electrical resistivity of the composites wall all positive temperature coefficient(PTCR) against temperature up to $700^{\circ}C$.

• Proceedings of the Materials Research Society of Korea Conference
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• 2004.05a
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• pp.47-47
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• 2004

• Proceedings of the KIEE Conference
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• 1998.11c
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• pp.847-849
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• 1998

The effect of $Al_{2}O_{3}$ additives on the microstructure, mechanical and electrical properties of ${\beta}$-SiC+39vol.%$ZrB_2$ electroconductive ceramic composites by pressureless sintering were investigated. The ${\beta}$-SiC+39vol.%$ZrB_2$ ceramic composites were pressureless sintered by adding 4, 8, 12wt.% $Al_{2}O_{3}$ powder as a liquid forming additives at $1950^{\circ}C$ for 1h. Phase analysis of composites by XRD revealed mostly of $\alpha$-SiC(6H), $ZrB_2$ and weakly $\alpha$-SiC(4H), $\beta$-SiC(15R) phase. The relative density of composites was lowered by gaseous products of the result of reaction between $\beta$-SiC and $Al_{2}O_{3}$ therefore, porosity was increased with increased $Al_{2}O_{3}$ contents. The fracture toughness of composites was decreased with increased $Al_{2}O_{3}$ contents, and showed the maximum value of $1.4197MPa{\cdot}m^{1/2}$ for composite added with 4wt.% $Al_{2}O_{3}$ additives. The electrical resistivity of ${\beta}$-SiC+39vol.%$ZrB_2$ electroconductive ceramic composite was increased with increased $Al_{2}O_{3}$ contents, and showed positive temperature coefficient resistance (PTCR) in the temperature from $25^{\circ}C$ to $700^{\circ}C$.

• Proceedings of the Korean Ceranic Society Conference
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• 2002.10a
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• pp.27.1-27
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• 2002

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.48 no.2
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• pp.92-97
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• 1999

The mechanical and electrical properties of the hot-pressed and annealed $\beta-Sic$+39vol.%$ZrB_2$ electroconductive ceramic composites were investigated as a function of the liquid forming additives of $Al_2O_3+Y_2O_3(6:4wt%)$. In this microstructures, no reactions and elongated $\alpha$-SiC grains with equiaxed $ZrB_2$, gains were observed between $\beta-SiC$ and $ZrB_2$, and the relative density was over 97.6% of the theoretical density. Phase analysis of the composites by XRD revealedmostly of $\alpha$-SiC(6H, 4H), $ZrB_2$, and weakly $\beta-SiC$(15R) phase. The fracture toughness decreased with increasing $Al_2O_3+Y_2O_3$ contents and showed the highest of $6.37MPa.m^{\fraction ane-half}$ for composite added with 4wt% $Al_2O_3+Y_2O_3$ additives at room temperature. The electrical resistivity increased with increasing $Al_2O_3+Y_2O_3$contents and showed the lowest of $1.51\times10^{-4}\Omega.cm$ for composite added with $Al_2O_3+Y_2O_3$ additives at $25^{\circ}C$. This reason is the increasing tendency of pore formation according to amount of liquid forming additives $Al_2O_3+Y_2O_3$. The electrical resistivity of the composites was all positive temperature coefficient resistance(PTCR) against temperature up to $700^{\circ}C$.

• Korean Journal of Materials Research
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• v.23 no.9
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• pp.531-536
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• 2013

This paper investigates the effect of prolonged high temperature exposure on concentric laminated $Al_2O_3-ZrO_2$ composites. An ultrafine scale microstructure with a cellular 7 layer concentric lamination with unidirectional alignment was fabricated by a multi-pass extrusion method. Each laminate in the microstructure was $2-3{\mu}m$ thick. An alternate lamina was composed of 75%$Al_2O_3$-(25%m-$ZrO_2$) and t-$ZrO_2$ ceramics. The composite was sintered at $1500^{\circ}C$ and subjected to $1450^{\circ}C$ temperature for 24 hours to 72 hours. We investigated the effect of long time high temperature exposure on the generation of residual stress and grain growth and their effect on the overall stability of the composites. The residual stress development and its subsequent effect on the microstructure with the edge cracking behavior mechanism were investigated. The residual stress in the concentric laminated microstructure causes extensive micro cracks in the t-$ZrO_2$ layer, despite the very thin laminate thickness. The material properties like Vickers hardness and fracture toughness were measured and evaluated along with the microstructure of the composites with prolonged high temperature exposure.

• Journal of the Korean Ceramic Society
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• v.27 no.2
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• pp.201-210
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• 1990

The mullite-15v/o ZrO2 composites were prepared by dispersing ZrO2-3m/o Y2O3 powders into the mullite matrix in order to improve the mechanical properties of the mullite. The densification and retention of t-ZrO2 in the matrix of synthetic mullite were also investigated. From IR spectroscopic analysis, the obtained amorphous SiO2-Al2O3 powder was observed to have Si-O-Al chemical bond in its structure which might result in the homogeneous mullite composition. The lattice parameter of the mullite powder calcined above 130$0^{\circ}C$ (a0=7.5468$\AA$) is nearly close to the value of stoichiometric mullite (71.8wt% Al2O3, a0=7.5456$\AA$). The sintering behavior, microstructure, flexural strength and fracture toughness of the mullite and mullite-15v/o ZrO2 composites have been studied. The mullite-15v/o ZrO2(+3m/o Y2O3) ceramics with relative densities of 96% were obtained when sintered at 1$600^{\circ}C$. The flexural strength and fractrue toughness of the composites sintered at 1$600^{\circ}C$(calcination temperature of mullite powders ; 125$0^{\circ}C$) had maximum values of 307MPa and 2.50MPa.m1/2, respectively. The fracture toughness improvement in the mullite-ZrO2 cmoposite is assumed to be resulted from the combined effect of the stress-induced phase transformation of tetragonal ZrO2 and the crack deflection due to microcracking by the monoclinic ZrO2 formation.