Phase Evolution in Silicon Carbide-Whisker-Reinforced Mullite/Zirconia Composite during Long-Term Oxidation at 1000° to 1350°C

A composite consisting of 30 wt% SiC whiskers and a mullite‐based matrix (mullite–32.4 wt% ZrO2–2.2 wt% MgO) was isothermally exposed in air at 1000°–1350°C, for up to 1000 h. Microstructural evolution in the oxidized samples was investigated using X‐ray diffractometry and analytical transmission electron microscopy. Amorphous SiO2, formed through the oxidation of SiC whiskers, was devitrified into cristobalite at T≥ 1200°C and into quartz at 1000°C. At T≥ 1200°C, the reaction between ZrO2 and SiO2 resulted in zircon, and prismatic secondary mullite grains were formed via a solution–reprecipitation mechanism in severely oxidized regions. Ternary compounds, such as sapphirine and cordierite, also were found after long‐term exposure at T≥ 1200°C.