On fatigue damage and small-crack growth behavior of silicon nitride under cyclic thermal-shock loading

The relationship between the cyclic crack growth rate and the maximum stress intensity factor was determined for polycrystalline silicon nitride under repeated thermal-shock loading conditions. For comparison purposes, the rate of fatigue-crack growth was also determined under load cycling conditions. It was observed that at the same nominal K-max value the crack growth rate under thermal-shock conditions was several orders of magnitude faster than under mechanical-fatigue loading. During quenching, a thermal gradient developed and macroscopic thermal stresses were produced in the singular stress field ahead of the crack. Also during the quench, microscopic tensile stresses developed within the grain boundary glassy phase as the result of a difference in the coefficient of thermal expansion between the silicon nitride and the glassy phase. The resulting combination of stresses resulted in the much greater rate of crack growth under cyclic thermal-shock conditions as compared to mechanical-fatigue conditions.