PHASE AND MICROSTRUCTURE EVOLUTION IN ALKOXIDE-DERIVED MULLITE MGO-PARTIALLY-STABILIZED ZIRCONIA/

The microstructure and phase evolution of a mullite/MgO-partially-stab ilized-zirconia alkoxide-derived composite was studied by XRD and TEM/ EDS in samples hot pressed at 850 degrees to 1400 degrees C for variou s periods of time. Upon hot pressing, Al-Si spinel, with a composition close to that of (2:1) mullite, crystallized at a temperature as low as 850 degrees C. It converted to (3:2) mullite at temperatures above 1200 degrees C. Fine spherical metastable cubic-ZrO2 particles, unifor mly distributed in the amorphous matrix, precipitated at T approximate to 850 degrees C. They coarsened at higher temperatures and transform ed to tetragonal or monoclinic ZrO2, depending upon the grain size. Th ese primary precipitates grew to become intergranular zirconia in the final structure. The solid solution of zirconia in mullite decreased w ith increasing temperature, so that secondary zirconia particles preci pitated at higher temperatures and remained as inclusions inside mulli te grains. Irregularly shaped elongated zircon was found at T greater than or equal to 1200 degrees C. It decomposed at higher temperatures to form zirconia particles, which existed at the grain boundaries of t he final mullite. Other phases found in hot-pressed samples included M gAl2O4 spinel and sapphirine (2MgO . 2Al(2)O(3) . SiO2). For powders h eat treated in air, gamma-Al2O3 (instead of Al-Si spinel) was formed a t temperatures above 900 degrees C. The gamma --> alpha-Al2O3 phase tr ansformation occurred at T greater than or equal to 1300 degrees C, fo llowed by a rapid grain growth, In contrast with the hot-pressed sampl es, mullite in heat-treated powders crystallized only at temperatures above 1400 degrees C. Previous studies on phase evolution during press ureless heat treatment of powders were generally unindicative of phase and microstructural evolution upon hot pressing.