High‐Temperature Fracture Mechanism of Low‐Ca‐Doped Silicon Nitride

High‐purity Si 3 N 4 (with 2.5 wt% glassy SiO 2 ) doped with 0 to 450 at.ppm of Ca was prepared as a model system to investigate the effects of grain‐boundary segregants on fracture phenomenology at 1400°C. Subcritical crack‐growth (SCG) resistance as well as creep resistance was degraded significantly by the presence of a small amount of Ca. The internal friction of the doped materials exhibited the superposition of a grain‐boundary relaxation peak and a high‐temperature background, and the apparent viscosity of the grain‐boundary film was determined from the peak. Based on these experimental data, the fracture mechanism at 1400°C was divided into three regions: “brittle,” SCG, and creep failure as a function of both external strain rate and Ca concentration, C Ca . From the investigation of the C Ca dependence of the critical strain rate for the transition from “brittle” to SCG fractures, the SCG phenomenon is suggested to be triggered by small‐scale, grain‐boundary sliding. The C Ca dependence of “steady‐state” creep rate was far from the theoretical dependence of diffusional creep via a solution‐precipitation mechanism. The discrepancy was interpreted to be due to the presence of an impurity‐insensitive creep component. This component may correspond to the lowest limit of the tensile creep rate in Si 3 N 4 polycrystalline materials containing intergranular glassy‐SiO 2 film.