REACTIVITY OF SILICON-CARBIDE AND CARBON WITH OXYGEN IN THERMOSTRUCTURAL COMPOSITES

The oxidation behavior of bidirectional silicon carbide-based composites is studied in the temperature range from 900-1200-degrees-C under an oxygen pressure equal to 1 kPa. The composite consists of silicon-based fibers, separated from the SiC matrix by a pyrolytic carbon layer (SiC/C/SiC). Oxidation was carried out by a volumetric technique with a mass spectrometer for the analysis of the gaseous phase. The change in morphology of the composite after exposure to oxygen was followed by scanning electron microscopy on polished sections of the external and the internal parts of the samples. Two main phenomena occur during oxidation: the carbon interlayer is gasified resulting in the formation of annular cavities around the fibers whereas the SiC-based fibers and SiC matrix are oxidized leading to the formation of silica layers. The in-depth oxidation of the carbon interlayer is strongly decreased at 900-degrees-C due to a limited transport rate of oxygen. The composite exhibits a self-healing behavior at 1200-degrees-C, since cavities are sealed by silica before total gasification of the carbon interlayer.