PHENOMENOLOGY OF FRACTURE IN HOT-PRESSED SILICON-NITRIDE

Fracture phenomenology in hot-pressed silicon nitride has been studied fractographically as a function of flaw size, temperature and loading rate. Surface cracks of controlled size were introduced using the microhardness indentation technique. At room temperature, the fracture stress was found to depend on initial crack size according to the Griffith relationship and extrapolation of the data indicated that inherent processing flaws of the order of 12 to 24 μm are strength-controlling in virgin material. Using a simplified Griffith approach, the fracture surface energy, γ, at 20° C for hot-pressed Si3 N4 is about 22 000 erg cm-2. Two mechanistic regimes were manifest in the temperature dependence of the fracture stress. A mixed mode of fracture consisting of transcrystalline and intergranular crack propagation occurred up to 1100° C; at 1200° C and above, subcritical crack growth (SCG) occurred intergranularly and the extent of SCG increased with increasing temperature. Similarly, the extent of SCG decreased with increasing loading rate. © 1979 Chapman and Hall Ltd.