ROLE OF OXIDATION IN THE TIME-DEPENDENT FAILURE BEHAVIOR OF HOT ISOSTATICALLY PRESSED SILICON-NITRIDE AT 1370-DEGREES-C

Dynamic fatigue studies were conducted on a hot isostatically pressed silicon nitride in ambient air and inert (argon or nitrogen) environme nts using four-point flexure at 1370-degrees-C. Specimens tested in am bient air exhibited a stressing rate dependence with decreased flexure strength with decreased stressing rates. All fracture surfaces of spe cimens tested in ambient air possessed a sweeping stress-oxidation dam age zone that originated at the tensile side of each bend bar. In addi tion to this stress-oxidation damage, creep damage (e.g., cavitation) was concurrently observed in the specimens tested at the slower stress ing rates, which appeared to further weaken the material. However, tes ts conducted in argon or nitrogen revealed flexure strength to be inde pendent of the stressing rate. Creep damage was present at the slower stressing rates, but no stress-oxidation damage was evident similar to that observed on the specimens tested in ambient air. By decoupling t he effects of oxidation and creep, it was evident that the former cont ributed to the formation of a detrimental stress-oxidation damage zone which significantly reduced the strength of this material at 1370-deg rees-C.