Fracture Toughness and High‐Temperature Slow Crack Growth inSiC

The subcritical growth of macroscopic cracks in hot‐pressed and in sintered SiC was examined at 900° to 1100°C in oxygen partial pressures of 10 ⁻⁴ and 10 ⁻⁸ atm. The K ₁ ‐V data for the hot‐pressed SiC were generated using the double torsion specimen in the incremental displacement rate mode. These data, in addition to earlier results, included three distinct regions in the plot of log V vs log K ₁ ,: a low slope region, N∼20, of substantial slow crack growth which was insensitive to testing temperature and environment; a plateau region which shifted to higher velocities with increasing temperature; and a region of very high slope with K values near K _IC The results may be described qualitatively by viscous flow of the gram‐boundary phase. No slow crack growth could be detected in the sintered SiC when tested hi the environments described.