RELATION BETWEEN STRENGTH, MICROSTRUCTURE, AND GRAIN-BRIDGING CHARACTERISTICS IN IN-SITU REINFORCED SILICON-NITRIDE

The grain size of in situ Si3N4 is varied, and its effects on strength-flaw size relations are related to the behavior of a bridging zone behind the crack tip. The bridging-zone properties are calculated from a Dugdale model assuming that the bridging zone has a constant bridging stress (p*) and length (D-b) at the moment of the critical fracture. The results show that as grain size increases, p* decreases while D-b and the critical bridging zone opening (u*) first increase and then decrease, resulting in a maximum for short-crack fracture toughness at an intermediate grain size. The initial increase of u* and D-b with grain size is attributed to an increase in debonding length, while the decrease of p* is attributed to a decrease in strength for bridging grains due to a statistical effect which also causes D-b and u* to drop in the large-grain regime. Implications on microstructure design are discussed.