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

The oxidation behavior of bidirectional silicon carbide-based composit es is studied in the temperature range from 900-1200-degrees-C under a n oxygen pressure equal to 1 kPa. The composite consists of silicon-ba sed 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 b y scanning electron microscopy on polished sections of the external an d the internal parts of the samples. Two main phenomena occur during o xidation: the carbon interlayer is gasified resulting in the formation of annular cavities around the fibers whereas the SiC-based fibers an d SiC matrix are oxidized leading to the formation of silica layers. T he in-depth oxidation of the carbon interlayer is strongly decreased a t 900-degrees-C due to a limited transport rate of oxygen. The composi te exhibits a self-healing behavior at 1200-degrees-C, since cavities are sealed by silica before total gasification of the carbon interlaye r.