TENSILE CREEP AND CREEP-RUPTURE BEHAVIOR OF MONOLITHIC AND SIC-WHISKER-REINFORCED SILICON-NITRIDE CERAMICS

The tensile creep and creep rupture behavior of silicon nitride was investigated at 1200-degrees to 1350-degrees-C using hot-pressed materials with and without SiC whiskers. Stable steady-state creep was observed under low applied stresses at 1200-degrees-C. Accelerated creep regimes, which were absent below 1300-degrees-C, were identified above that temperature. The appearance of accelerated creep at the higher temperatures is attributable to formation of microcracks throughout a specimen. The whisker-reinforced material exhibited better creep resistance than the monolith at 1200-degrees-C; however, the superiority disappeared above 1300-degrees-C. Considerably high values, 3 to 5, were obtained for the creep exponent in the overall temperature range. The exponent tended to decrease with decreasing applied stress at 1200-degrees-C. The primary creep mechanism was considered cavitation-enhanced creep. Specimen lifetimes followed the Monkman-Grant relationship except for fractures with large accelerated creep regimes. The creep rupture behavior is discussed in association with cavity formation and crack coalescence.