CRACK HEALING IN A SILICON-NITRIDE CERAMIC

A detailed study has been undertaken on crack healing at high temperat ures in a silicon nitride containing 10 wt% additives in order to iden tify the dominant mechanism responsible for the phenomenon. Fracture t oughness increased with annealing time and the crack growth rate decre ased until arrest with increasing testing time. Differentiation betwee n possible operating mechanisms was obtained using critical experiment s involving detailed compliance measurements, crack wake removal, and crack reinitiation tests and a comprehensive TEM study of healed crack s. It was found that crack healing was not uniform in the crack wake. When the original crack path was either transgranular or intergranular , healing was associated with the appearance of a thin layer of silica glass due to the oxidation of Si3N4 grains. But when the crack went t hrough multigrain junctions, the farmer crack path was completely obli terated and replaced by a new, crystalline phase formed by diffusion o f the preexisting glass phase, It is concluded that the increased crac k growth resistance and fracture toughness at high temperature is attr ibutable to the partial recovery of the original strength from the cra ck segments at multigrain junctions due to vitreous phase flow and sub sequent crystallization.