Microstructural Studies in Alkoxide‐Derived Mullite/Zirconia/Silicon Carbide‐Whisker Composites

Abstract

Mullite composites toughened with ZrO2 (with or without a MgO or Y2O3 stabilizer) and/or SiC whiskers (SiC(w)) were fabricated by hot‐pressing powders prepared from Al, Si, Zr, and Mg(Y) alkoxide precursors by a sol–gel process. Micro‐structures were studied by using XRD. SEM, and analytical STEM. Pure mullite samples contained prismatic, preferentially oriented mullite grains. However, the addition of ZrO2, as well as the hot‐pressing temperature, affected the morphology and grain size in the composites; a fine, uniform, equiaxed microstructure was obtained. The effect of SiC(W) was less pronounced than that of ZrO2. Glassy phases were present in mullite and mullite/SiC(W) composites, but were rarely observed in Al2O3‐rich or ZrO2‐containing samples. The formation of zircon due to the reaction between ZrO2 and SiO2 and the considerable solid solution of SiO2 in ZrO2 prevented the formation of the glassy phase, whereas the reaction between Al2O3 and MgO in MgO‐containing samples formed a spinel phase and also deprived the ZrO2 phase of the stabilizer. Intergranular ZrO2 particles were either monoclinic or tetragonal, depending on size and stabilizer content; small intragranular ZrO2 inclusions were usually tetragonal in structure.