The effect of grain boundary phase characteristics on the crack deflection behavior in a silicon nitride material

Abstract

Silicon nitride is a promising structural ceramic material due to its inherent properties such as strength, thermal shock resistance and high temperature creep and oxidation resistance. Recent attempts to densify monolithic and composite materials without the use of additives have been very successful. Tanaka et al. densified different commercial silicon nitride materials by hot isostatic pressing without the use of additives, where the intrinsic SiO{sub 2} acted as the liquid phase sintering aid. Symons et al. have developed a laser-synthesized high purity amorphous Si{sub 2}N{sub 4} powder with a very fine particle size, and a controllable oxygen content. This powder was also densified without the use of additives. More recently, monolithic (not containing whiskers or fibers) silicon nitride materials have been developed with fracture toughness values as high as 10--12 Mpa.m{sup 0.5}. These are the so-called self- (or in-situ) reinforced materials reported by Pyzik et al. and Li and Yamanis. The reason for the extremely high fracture toughness values has been attributed to the presence of large, elongated {beta}-Si{sub 3}N{sub 4} grains, which activate various toughening mechanisms, such as crack deflection, crack bridging and grain pull-out. However, it was correctly pointed out that little is known about the effects ofmore » the grain boundary phase and its physical properties on the interfacial debonding/intergranular fracture behavior and fracture toughness in silicon nitride materials. This report is a brief description of a hypothesis developed to explain the large differences in fracture toughness between high purity (additive-free) Si{sub 3}N{sub 4} and conventionally sintered Si{sub 3}N{sub 4} materials.« less