T. Hansson et al., ELEVATED-TEMPERATURE FRACTURE-BEHAVIOR OF MONOLITHIC AND SICW-REINFORCED SILICON-NITRIDE UNDER QUASI-STATIC LOADS, Materials science & engineering. A, Structural materials: properties, microstructure and processing, 209(1-2), 1996, pp. 137-148
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.