ESTIMATION OF CRACK CLOSURE STRESSES FOR INSITU TOUGHENED SILICON-NITRIDE WITH 8 WT-PERCENT SCANDIA

An 8-wt%-scandia silicon nitride with an elongated grain structure was fabricated. The material exhibited high fracture toughness (almost-equal-to 7 MPa.m1/2) and a rising R-curve as measured by the indentation strength technique. The "toughening" exponent m was found to be m almost-equal-to 0.1. The high fracture toughness and R-curve behavior was attributed mainly to bridging of the crack faces by the elongated grains. The crack closure (bridging) stress distribution in the wake region of the crack tip was estimated as a function of crack size from the R-curve data, with an arbitrarily assumed distribution function.