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 1 000-1550 degrees C temperature range with monotonically increasing loa ds 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 vis cous deformation of the secondary intergranular amorphous phase presen t in the microstructure and the concomitant relaxation of the crack-ti p stresses. The brittle-to-ductile transition temperature (BDTT) depen ds 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 c rack increases with increasing temperature. Experiments involving the introduction of stable cracks at elevated temperatures followed by roo m-temperature fracture toughness testing reveal that shielding owing t o the crack-wake bridging increases the apparent fracture resistance a bove the BDTT. Micromechanisms of the brittle-to-ductile transition an d subcritical crack growth at elevated temperatures in the Si3N4 ceram ics were discussed.