The detrimental aging phenomenon observed in ZrOrY203 materials, which causes tetragonal ZrOz to transform to its monoclinic structure at temperatures between 150 and 4OO0C, was investigated with respect to the gaseous aging environment and the Yzo3 and SiOz content of the material. It is shown that the aging phenomenon is caused by water vapor and that intergranular silicate glassy phases play no significant role. Transmission electron microscopy of thin foils, before and after aging, showed that the water vapor reacted with yttrium in the ZrOz to produce clusters of small (20 to 50 nm) crystallites of (Y-Y(OH)~. It is hypothesized that this reaction produces a monoclinic nucleus (depleted of Y203) on the surface of an exposed tetragonal grain. Monoclinic nuclei greater than a critical size grow spontaneously to transform the tetragonal grain. If the transformed grain is greater than a critical size, it produces a microcrack which exposes subsurface tetragonal grains to the aging phenomenon and results in catastrophic degradation. Degradation can be avoided if the grain size is less than the critical size required for microcracking.

Dispersion states of aqueous composite A1203 /ZrOz colloidal suspensions were studied by measuring particle size distribution as a function of pH. Mutual dispersion was achieved at pH values of 2.0 to 3.5. Consolidated composites formed by colloidal filtration reflected the uniformity of the colloidal state. The mean flexural strength (896 MPa) of the sintered compacts was 1.6 times that of bodies consolidated by isostatic pressing.

Large, hard ZrOz agglomerates remained in an AI2O3/ZrO2 composite suspension after inefficient ball-milling. The ZrOz agglomerates shrank away from the consolidated A1,03/Zr02 powder matrix during sintering, producing cracklike voids which were responsible for strength degradation.

Al203/30 v/o Z r O q l i 2.3 m/o Y20311 Kc = 6.9 MPa mOrganic inclusions (e.g. lint) produce irregularly shaped voids observed at fracture origins. These voids can be eliminated from a powder compact by an organic burnout followed by isopressing at room temperature prior to sintering. This procedure was demonstrated for the case of voids produced by polystyrene spheres 4 to 100 pm in diameter.

An investigation was made of the effect of Y2O3 in solid solution in ZrO2 on the phases that are formed when ZrO2 is reacted with either N2 or Si3N4. The results suggest that the Zroxynitride phase can be precluded as a reaction product when the Y2O3 content of the ZrO2 is >4 mol %. Dense Si3N4/ZrO2( + Y2O3) composites were fabricated, which did not degrade during oxidation at temperatures <1000 °C. Severe degradation was observed for composites containing the Zr-oxynitride phase. The fracture toughness of the Si3N4/ZrO2( + Y2O3) composites increased with ZrO2 content.66