EVOLUTION OF STRESS FAILURE RESULTING FROM HIGH-TEMPERATURE STRESS-CORROSION CRACKING IN A HOT ISOSTATICALLY PRESSED SILICON-NITRIDE

Stress-corrosion cracking in a commercially available, hot isostatically pressed (HIPed), yttria-fluxed, silicon nitride was the prevalent mode of failure in specimens creep-ruptured at 1370 degrees C. High-temperature diffusional processes associated with oxygen were responsible for the creation of an advancing stress-corrosion front that had formed at the specimen surface and advanced radially inward. The volume of material in the wake of the stress-corrosion front possessed a high concentration of lenticular cavities at two-grain boundaries, a high concentration of multigrain junction cavities, and large amorphous ''pockets'' in other multigrain junctions that were abnormally rich in oxygen and yttrium. The combination of tensile stress and the high concentration of cavities in the nearsurface volume of the material resulted in microcrack coalescence or the formation of a planar, stress-corrosion crack. The concurrent growth of the stress-corrosion front and crack during the tensile creep-rupture tests ultimately led to stress-induced failure.