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 a nd carbon, preferably forming SiC, B or B,C. The best results were obt ained with the trilayer coatings, that is with, SiC as the internal la yer, boron or boron carbide, as the intermediate layer and an external SiC layer. To get a good protection in a large temperature range, fro m 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 high er than 5 mu m. Morphological characterization of oxidized samples has shown that, for intermediate oxidation temperatures, a glass was prod uced in the cracks. When the oxidation temperature was equal to or hig her than 1300 degrees C, sealing of the cracks was rarely observed, bu t the oxidation resistance remained satisfactory.