EVALUATION OF THE STRENGTH AND CREEP FATIGUE BEHAVIOR OF HOT ISOSTATICALLY PRESSED SILICON-NITRIDE

The strength of a commercially available hot isostatically pressed silicon nitride was measured as a function of temperature. To evaluate long-term mechanical reliability of this material, the tensile creep and fatigue behavior was measured at 1150-degrees, 1260-degrees, and 1370-degrees-C. The stress and temperature sensitivities of the secondary (or minimum) creep strain rate were used to estimate the stress exponent and activation energy associated with the dominant creep mechanism. The fatigue characteristics were evaluated bv allowing individual creep tests to continue until specimen failure. The applicability of the four-point load geometry to the study of strength and creep behavior was also determined by conducting a limited number of flexural creep tests. The tensile fatigue data revealed two distinct failure mechanisms. At 1150-degrees-C, failure was controlled by a slow crack growth mechanism. At 1260-degrees and 1370-degrees-C, the accumulation of creep damage in the form of grain boundary cavities and cracks dominated the fatigue behavior. In this temperature regime, the fatigue life was controlled by the secondary (or minimum) creep strain rate in accordance with the Monkman-Grant relation.