COMPARISON OF THE CREEP AND CREEP-RUPTURE PERFORMANCE OF 2 HIPED SILICON-NITRIDE CERAMICS

Measurements of the tensile creep and creep rupture behavior were used to evaluate the long-term mechanical reliability of a commercially av ailable and a developmental hot isostatically pressed (HIPed) silicon nitride. Measurements were conducted at 1260-degrees and 1370-degrees- C utilizing button-head tensile specimens. The stress and temperature sensitivities of the secondary creep rates were used to estimate the s tress exponent and activation energy associated with the dominant cree p mechanism. The stress and temperature dependencies of creep rupture life were determined by continuing individual creep tests to specimen failure. Creep deformation in both materials was associated with cavit ation at multigrain junctions. Two-grain cavitation was also observed in the commercial material. Failure in both materials resulted from th e evolution of an extensive damage zone. The failure times were unique ly related to the creep rates, suggesting that the zone growth was con strained by the bulk creep response. The fact that the creep and creep rupture behaviors of the developmental silicon nitride were significa ntly improved compared to those of the commercial material was attribu ted to the absence of cavitation along two-grain junctions in the deve lopmental material.