A comparison of the crack growth behavior of a silicon carbide-based fiber reinforced, silicon carbide matrix composite (SiCf/SiCm) with or without boron, is presented. The lifetime of boron-enhanced composites was greater than that of the non-boron-containing composite. Transmission electron microscopy (TEM) of the boron-enhanced fracture surface indicates sealing of matrix cracks by an amorphous oxide phase in the sample tested at the lower oxygen level and nearly complete interphase removal in the sample tested at the higher oxygen level. The results indicate the dependence of the acting failure mechanisms on oxygen concentration is distinctly different for the two composites
