Abstract
In order to investigate the effect of elevated temperature on the crack propagation behavior of a sintered silicon nitride, crack propagation tests were carried out under square wave conditions where the test frequency f was varied from 1Hz to 0.01Hz and the test temperature ranged from 1073K to 1273K. At 1073K, the crack propagation rate increased as the frequency increased. This behavior is the same as that observed at room temperature. Above 1173K, the effect of frequency on the crack propagation rate almost disappeared, and the difference between the crack propagation rates under static load and cyclic load was not observed. The time-dependent crack propagation behavior was observed over the whole K-range. On the specimen tested above 1173K, a glassy oxide and dislocated grain boundary were observed. The thermal activation process seemed to change at about 1073K from the process related to stress corrosion cracking by H2O to the process related to oxidation and viscous flow. From the above discussion, we concluded that the slow crack growth in the temperature range from room temperature to about 1073K was dominated by the stress corrosion cracking by H2O, and oxidation and viscous flow accelerated the time-dependent fracture above 1173K.
