HIGH-TEMPERATURE FAILURE MECHANISMS OF HOT-PRESSED SI3N4 AND SI3N4/SI3N4-WHISKER-REINFORCED COMPOSITES

The high-temperature flexural strength of hot-pressed silicon nitride (Si3N4) and Si3N4-whisker-reinforced Si3N4-matrix composites has been measured at a crosshead speed of 1.27 mm/min and temperatures up to 1400-degrees-C in a nitrogen atmosphere. Load-displacement curves for whisker-reinforced composites showed nonelastic fracture behavior at 1400-degrees-C. In contrast, such behavior was not observed for monolithic Si3N4. Microstructures of both materials have been examined by scanning and transmission electron microscopy. The results indicate that grain-boundary sliding could be responsible for strength degradation in both monolithic Si3N4 and its whisker composites. The origin of the nonelastic failure behavior of Si3N4-whisker composite at 1400-degrees-C was not positively identified but several possibilities are discussed.