TEMPERATURE-DEPENDENCE OF THE OXIDATION RESISTANCE OF SIC COATED CARBON-CARBON COMPOSITE

The oxidation of SiC coated carbon/carbon composite (C/C) is studied a nd correlated to the surface stress conditions. Based on the calculati on, the thermal stress induced from the difference in coefficient of t hermal expansion (CTE) between the C/C substrate and the SiC coating i s as large as a few hundred MPa when the variation of processing tempe rature is more than 1200-degrees-C, which is enough for the occurrence of microcracks in the SiC coating layer. The analysis of stress distr ibution reveals that a lower reaction sintering temperature and a thic ker SiC coating layer are preferred to avoid the occurrence of microcr acks. But cracking seems inevitable if the reaction sintering of the S iC layer is processed above 1400-degrees-C to produce a SiC coating fo llowed by cooling to room temperature. An in situ measurement of the o xidation weight loss of coated C/C demonstrates that the abnormal vari ation of oxidation rate with respect to the test temperature is consis tent with the prediction of closure of the stress-induced microcracks in the SiC coatings.