The embrittlement of Nicalon/alumina composites at intermediate and elevated temperatures

The strength and toughness of a 2-D woven Nicalon/Al2O3-matrix composite were measured at ambient, intermediate (800°C), and elevated (1000–1200°C) temperatures. The composites exhibited non-linear behavior over the whole temperature range but their mechanical properties were significantly degraded at 800°C and above. The in situ fiber strength and the interfacial sliding resistance were determined through quantitative microscopy techniques and they were used to predict the composite properties by means of using the appropriate micromechanical models. Comparison of the model and the experiments, together with the fractographic observations, led to the conclusion that the strength reduction was caused by localized interface oxidation at 800°C and by the degradation of the fiber strength at 1000°C and above. The decrease in the composite fracture energy was mainly induced by a transition in the fracture mode, which changed from the development of a diffuse damage zone with multiple matrix cracks at 25°C to the propagation of a single dominant crack at 800°C and above.