High temperature oxidation of ceramic matrix composites

Ceramic matrix composites are developmental high temperature materials. We focus on composites with SiC matrices, SiC fibers, and BN fiber coatings. High temperature oxidation/corrosion reactions are discussed for each of these constituents. SiC forms a highly protective SiO2 scale in pure oxygen, but problems arise in complex gas mixtures. These include: SiO(g) formation, Si-O-H(g) formation, impurity enhanced SiO2 formation, and fluxing of SiO2 in molten salts and slags. Refractory oxide coatings may minimize these effects. The major issue with these composites is oxidation of the BN fiber coating. The key reactions here are borosilicate glass formation, potential gettering of oxygen by SiC, and volatilization. We conclude with an oxidation study of an actual composite, which illustrates some of these effects.