A microdebonding study of the high-temperature oxidation embrittlement of a cross-ply glass-ceramic/SiC composite

A constant cross-section specimen with adhesively bonded tabs has been used for an investigation of the high-temperature tensile behavior of a cross-plied glass-ceramic-matrix composite consisting of CAS-II reinforced with Nicalon SiC fiber. Oxidation of the exposed interfaces along matrix cracks in 0° plies lowers the composite failure strain at 800°C to the 0° ply matrix-cracking strain. Scanning electron microscopy and microdebonding analysis of the fracture surfaces indicate that the embrittlement process is the result of oxidation of the carbon-rich interphase as the matrix crack encounters 0° ply fibers, the interphase subsequently fuses with a higher bond strength and the crack grows through the fibers. Planar cracks grow inwards from the surface, covering the entire fracture surface given enough time (or sufficient strain). Degradation of the fibers does not appear to contribute to the embrittlement. Transverse plies crack at a lower strain than does the matrix in the 0° plies. However, it appears that oxygen does not enter 90° ply cracks in sufficient quantity to produce oxidation embrittlement, at least up to the 0° matrix-cracking strain. The strain to crack the 90° plies does not decrease significantly at high temperatures despite the fact that the cracks are primarily in the fiber/matrix interphase as they grow across the 90° plies.