High temperature corrosion and crack growth of SiCSiC at variable oxygen partial pressures

Thermal gravimetric analysis (TGA) and subcritical crack growth measurements of chemical-vapor-infiltrated SiC matrix reinforced with Nicalon fibres and with a 1 μm thick C fiber-matrix interface have been conducted at 1100 °C over O2Ar mixtures ranging from 0.25% to 20.0% O2. The TGA and interface recession measurements both gave linear reaction kinetics for O2 concentrations of 2.0% or less and a reaction order of unity. Subcritical crack growth measurements demonstrated that the crack velocity, in the stress-intensity-independent stage II regime, increases with increasing O2/Ar ratio. Also, the transition from stage II to the stress-intensity-dependent stage III regime is shifted to lower stress intensities with increasing O2/Ar ratio. A time-dependent crack growth model that incorporates creep of the bridging SiC fibers and the removal of the C interfacial layer by oxidation successfully explains the subcritical crack growth characteristics.