TEM observations of SiC‐SiC composites with a carbon interphase layer annealed in air at high temperatures

SUMMARY SiC–SiC composites with a carbon interphase layer have been annealed in air at high temperature (800–1400°C) in order to determine the oxidation mechanism. Transmission electron microscopy and high‐resolution electron microscopy observations have been performed on thin slices of oxidized specimens. Observations show that, with oxidation above 800°C, the carbon interphase can be replaced totally by a thick silica layer, in agreement with previous work. However, in many cases the modified interphase is heterogeneous. The turbostratic carbon structure disappears and the carbon interphase layer is partially replaced by an amorphous silica layer found preferentially at the contact zone between the fibres and the carbon layer. A very thin silica layer can also be observed on the matrix side at the matrix‐fibre interfaces. Some decohesion appears in areas where carbon is still present. For short‐duration exposures, decohesion occurs between the fibres and the carbon interphase layer.