Oxidation behavior of carbon-carbon composites with a three-layer coating from room temperature to 1700°C

Oxidation tests of carbon-carbon composites with a three-layer coating were conducted in dry air from room temperature to 1700°C for 5 h. A continuous series of empirical functions relating weight change after 5 h oxidation to temperature was found to fit the test results quite well over the whole temperature range. This approach was used to interpret the different oxidation mechanisms which were rate-controlling over different temperature ranges. Below 700°C, oxidation was controlled by oxygen diffusion through coating cracks and the activation energy was 28 Kcal/mol; from 700°C to cracking temperature, the controlling stepes were complicated; below 1550°C, oxidation was controlled by extrinsic diffusion of oxygen through oxide film on the coating and the activation energy was about 125 Kcal/mol; from 1550°C to 1700°C, oxidation through intrinsic diffusion was dominant and the activation energy was about 276 Kcal/mol. The presence of W ion in the oxide film, as impurities, increased the activation energies in both intrinsic and extrinsic regions and lowered the transition temperature from intrinsic to extrinsic, thus decreased greatly the protection time of the coating, especially at high temperature.