Chemomechanical Behavior of a Melt-Infiltrated SiC-Si Composite at 900C

The fracture behavior of melt-infiltrated, equivolume SiC-Si composites, designed to mimic the matrix phase of an industrial ceramic matrix composite (CMC), was examined with the aim of evaluating failure mechanisms operative in different atmospheres and temperatures, and at different applied stress rates. Specimens tested in four-point-bending at 900°C in flowing oxygen-gettered argon, air, or steam-rich atmospheres exhibited higher average fracture strengths than specimens tested at 25°C. Higher mean fracture strength values were obtained for specimens tested in flowing dry air or in a steam-rich atmosphere at 900°C than for specimens tested in high-purity, oxygen-gettered argon at this temperature. The increased fracture strengths obtained in air and in steam-rich atmospheres coincided with increased specimen oxidation, and apparent oxide filling and blunting of flaws in these composites. A transition in the location of catastrophic failure, from the site of preexisting damage created by a Vickers indentation to surface flaws located elsewhere on tension surfaces, was also consistent with such apparent oxide filling/healing of the Vickers indentation-induced flaws.

Stress-rate-dependent failure was observed for melt-infiltrated SiC-Si composites tested at 900°C in oxygen-gettered argon and in a steam-rich atmosphere, with increasing average fracture strengths obtained for specimens loaded at decreasing stress rates. The apparent enhanced ductility of specimens loaded in oxygen-gettered argon at 0.111 MPa/min coincided with an increase in mean failure stress relative to specimens tested at 333 MPa/min in this atmosphere. The stress rate dependence of the mean fracture strength of specimens tested in the steam-rich atmosphere was greater than for specimens tested in oxygen-gettered argon. A transition in the location of catastrophic failure, from the site of preexisting damage created by a Vickers indentation to surface flaws located elsewhere on tension surfaces, was observed with a decreasing applied stress rate for specimens tested in the steam-rich atmosphere, but not for specimens tested in oxygen-gettered argon. Both of these latter trends were associated with the apparent oxide filling/healing of the Vickers indentation-induced flaws.