Phase and strain mapping of a protective coating on carbon-carbon

Carbon-carbon (C/C) is used in high temperature structures. To prevent rapid oxidation of the carbon, protective coatings are applied. This paper examines such a protective coating with high energy X-ray diffraction. The coating consisted of a 0.02 mm outer layer of sodium silicate glass on a 0.2 mm layer of silicon carbide (SiC) with a 0.1 mm transition layer into the C/C of carbon fibres surrounded by a SiC matrix. It was examined in its pristine state and after heating with an oxy-acetylene torch. An area X-ray detector was used to obtain phase and strain information as a function of depth through the coating. In-plane tensile strains were observed in both a crystalline SiO2 phase and within the SiC of the transition layer. Oxidation protection was compromised by through-thickness cracks in the SiC layer caused by the large tensile cooling strains in the layer. The strains and the density of the cracking increased after heating. However, several mechanisms were identified that moderated the strains. Based on two of these mechanisms a buffer layer of graphite between the C/C and the SiC was proposed to improve oxidation resistance. The low elastic modulus of the graphite reduces the strains in the SiC coating, which reduces the crack density, which in turn improves oxidation resistance. Adding SiC fibres to the graphite and SiC was also proposed to further reduce cracking.