Elevated temperature fracture behavior of monolithic and SiCw-reinforced silicon nitride under quasi-static loads

Fracture behavior of a monolithic and SiCw-reinforced Si3N4 over the 1000-1550 degrees C temperature range with monotonically increasing loads was investigated. Peaks in the fracture initiation toughness, K-c, were found to occur owing to the brittle-to-ductile transition in the fracture behavior. This transition is associated with the onset of viscous deformation of the secondary intergranular amorphous phase present in the microstructure and the concomitant relaxation of the crack-tip stresses. The brittle-to-ductile transition temperature (BDTT) depends on the loading rate. Damage accumulation in terms of nucleation and growth of cavities ahead of the crack-tip promote stable crack growth at temperatures higher than the BDTT. The length of the subcritical crack increases with increasing temperature. Experiments involving the introduction of stable cracks at elevated temperatures followed by room-temperature fracture toughness testing reveal that shielding owing to the crack-wake bridging increases the apparent fracture resistance above the BDTT. Micromechanisms of the brittle-to-ductile transition and subcritical crack growth at elevated temperatures in the Si3N4 ceramics were discussed.