Oxygen penetration into silicon carbide ceramics during oxidation

The penetration of oxygen into polycrystalline silicon carbide ceramics, in advance of the oxide/substrate interface, during oxidation for 1–100 hrs at 1200–1400°C was studied using SIMS and TEM techniques. Fully dense hot pressed ceramics containing aluminum additives, with and without an oxide grain boundary phase and CVD silucon carbide exhibited sharp interfaces. Sintered silicon carbides with boron and carbon additives (～ 97% dense) and aluminum carbide additive (～ 90% dense) exhibited a region of oxygen penetration ～10–15 μm in depth beneath the oxides scale, the depth of which was insensitive to the time and temperature of oxidation. The amorphous oxide phase in this zone was located at three and four grain junctions but the two grain junctions were unaffected in this zone by oxidation. This oxygen affected region, which is responsible for the slow crack growth susceptibility of these ceramics after oxydation, results from gaseous oxygen penetration along interconnected or nearly interconnected pores and oxidation of impurity laden channels and SiC surfaces. The depth of penetration is presumably limited by closure of the channels by the oxidation products.