HIGH-TEMPERATURE FRACTURE MECHANISM OF LOW-CA-DOPED SILICON-NITRIDE

High-purity Si3N4 (with 2.5 wt% glassy SiO2) doped with 0 to 450 at.ppm of Ca nas prepared as a model system to investigate the effects of grain-boundary segregants on fracture phenomenology at 1400 degrees 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 super-position 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 degrees 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 Si3N4 polycrystalline materials containing intergranular glassy-SiO2 film.