Oxidation of BN-coated SiC fibers in ceramic matrix composites

Thermodynamic calculations were performed to analyze the simultaneous oxidation of BN and SiC. The results show that, with limited amounts of oxygen present, the formation of SiO2 should occur prior to the formation of B2O3. This agrees with experimental observations of oxidation in glass-ceramic matrix composites with BN-coated SIC fibers, where a solid SiO2 reaction product containing little or no boron has been observed, The thermodynamic calculations suggest that this will occur when the amount of oxygen available is restricted. One possible explanation for this behavior is that SiO2 formation near the external surfaces of the composite closes off cracks or pores, such that vapor phase O-2 diffusion into the composite occurs only for a limited time, This indicates that BN-coated SiC fibers will not always oxidize to form significant amounts of a low-melting, borosilicate glass.