TRANSIENT CREEP-BEHAVIOR OF HOT ISOSTATICALLY PRESSED SILICON-NITRIDE

Transient creep is shown to dominate the high-temperature behaviour of a grade of hot isostatically pressed silicon nitride containing only 4 wt% Y2O3 as a sintering aid. Contributing factors to transient creep are discussed and it is concluded that the most likely cause of long-term transient creep in the present study is intergranular sliding and interlocking of silicon nitride grains. In early stages of creep, devitrification of the intergranular phase, and intergranular flow of that phase may also contribute to the transient creep process. The occurrence of transient creep precluded the determination of an activation energy on the as-received material. However, after creep in the temperature range 1330-1430-degrees-C for times exceeding approximately 1100 h, an apparent activation energy of almost-equal-to 1260 kJ mol-1 was measured. It is suggested that the apparent activation energy for creep is determined by the mobility and concentration of diffusing species in the intergranular glassy phase. The time-to-rupture was found to be a power function of the minimum strain rate, independent of applied stress or temperature. Hence, creep-rupture behaviour followed a Monkman-Grant relation. A strain rate exponent of -1.12 was determined.