HIGH-TEMPERATURE CORROSION AND CRACK-GROWTH OF SIC-SIC AT VARIABLE OXYGEN PARTIAL PRESSURES

Thermal gravimetric analysis (TGA) and subcritical crack growth measur ements of chemical-vapor-infiltrated SIC matrix reinforced with Nicalo n fibres and with a 1 mu m thick C fiber-matrix interface have been co nducted at 1100 degrees C over O-2-Ar mixtures ranging from 0.25% to 2 0.0% O-2. The TGA and interface recession measurements both gave linea r reaction kinetics for O-2 concentrations of 2.0% or less and a react ion order of unity. Subcritical crack growth measurements demonstrated that the crack velocity, in the stress-intensity-independent stage II regime, increases with increasing O-2/Ar ratio. Also, the transition from stage II to the stress-intensity-dependent stage III regime is sh ifted to lower stress intensities with increasing O-2/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 suc cessfully explains the subcritical crack growth characteristics.