Microscopic Mechanisms of Oxidation in SiC‐Whisker‐Reinforced Mullite/ZrO ₂ Matrix Composites

Abstract

The oxidation of SiC whiskers, contained in alkoxide‐derived mullite‐based matrices and exposed in air at 1000–1350°C for up to 1000 h, has been studied by analytical TEM, high‐resolution SEM, and XRD. Silicon carbide whiskers were effectively protected from oxidation when embedded in a pure mullite matrix, but oxidized considerably when embedded in mullite/ZrO 2 matrices. The oxidation mechanisms varied with matrix composition and exposure temperature. At 1350°C the amorphous layer first crystallized as cristobalite, then gradually incorporated alumina. At later times, the mullite portion of the mullite/ZrO 2 matrix dissolved extensively into the layer. Also, the zirconia particles reacted with silica to form zircon. At 1200°C less extensive interdiffusion and chemical reaction occurred, and the silica layer devitrified into cristobalite and quartz. At 1000°C no interdiffusion or chemical reaction was seen, and the silica layer tended to devitrify into quartz. The thickness of the oxide layer around a SiC whisker in a particular matrix depended on the morphology and composition of grains abutting it or adjacent to it.