Oxidation through coating cracks of SiC-protected carbon/carbon

The oxidation of SiC-protected carbon/carbon through machined slots and naturally occurring craze cracks in the SiC was studied. The slot and crack geometries were characterized, and the subsurface oxidation of the carbon/carbon substrate at temperatures of 1000 to 1300 °C in air was assessed using weight change, X-ray computed tomography, and optical microscopy of sections. Rate constants were derived from these measurements and compared with a two-step diffusion control model of carbon oxidation. This model included gas phase diffusion in the channel through the SiC coating and in the growing oxidation cavity within the substrate. Oxidation measurements from sections were the most accurate and also agreed well with the model. Oxidation measurements from weight loss were higher than the model which is likely due to other contributions to net weight loss besides oxidation.