The influences of ZrO
2 particle addition upon improvement in mechanical properties of Al
2O
3 ceramics and their degradation due to a deionized water environment under high temperature and high pressure were investigated in terms of the microstructural changes and residual strength characteristics. Sintered Al
2O
3 bodies containing, 3.3, 10.2, 21.7vol.% of 3mol% Y
2O
3-doped tetragonal ZrO
2 were made using a pressureless sintering process at 1550, 1600, 1650°C, respectively. These ceramic composites were corroded in deionized water under its equilibrium vapor pressure of 1.59MPa at 200°C. The effects of microstructure upon mechanical properties of Al
2O
3/ZrO
2 composites were studied for the specimens treated with, and not treated with water under high temperature and pressure, by residual bending strength measurement, FE-SEM with EDX spectrometry observation and XRD and FE-TEM structural analysis. The mechanical properties were found to be superior when specimens were 21.7vol.% of ZrO
2 contents and sintered at 1550°C. It was shown that the strength degradation due to the corrosion by deionized water molecule was suppressed in the case of the specimens with higher contents of ZrO
2 particles sintered at lower temperatures. The correlation between bending strength scatter and microstructural defects was discussed.
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