THE OXIDATION BEHAVIOR OF 2-DIMENSIONAL AND 3-DIMENSIONAL C/SIC THERMOSTRUCTURAL MATERIALS PROTECTED BY CHEMICAL-VAPOR-DEPOSITION POLYLAYERS COATINGS

The oxidation behaviour of two- and three-dimensional C/SiC protected by a chemical-vapour-deposition (CVD) ceramic coating was studied. The elements used to achieve the surface protection were silicon, boron and carbon, preferably forming SiC, B or B,C. The best results were obtained with the trilayer coatings, that is with, SiC as the internal layer, boron or boron carbide, as the intermediate layer and an external SiC layer. To get a good protection in a large temperature range, from 450 to 1500 degrees C, the total thickness of the trilayers must be higher than 160 mu m and the intermediate layer thickness must be higher than 5 mu m. Morphological characterization of oxidized samples has shown that, for intermediate oxidation temperatures, a glass was produced in the cracks. When the oxidation temperature was equal to or higher than 1300 degrees C, sealing of the cracks was rarely observed, but the oxidation resistance remained satisfactory.