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

The strength and toughness of a 2-D woven Nicalon/Al2O3-matrix composite we re measured at ambient, intermediate (800 degrees C), and elevated (1000-12 00 degrees C) temperatures. The composites exhibited non-linear behavior ov er the whole temperature range but their mechanical properties were signifi cantly degraded at 800 degrees C and above. The in situ fiber strength and the interfacial sliding resistance were determined through quantitative mic roscopy techniques and they were used to predict the composite properties b y means of using the appropriate micromechanical models. Comparison of the model and the experiments, together with the fractographic observations, le d to the conclusion that the strength reduction was caused by localized int erface oxidation at 800 degrees C and by the degradation of the fiber stren gth at 1000 degrees C and above. The decrease in the composite fracture ene rgy was mainly induced by a transition in the fracture mode, which changed from the development of a diffuse damage zone with multiple matrix cracks a t 25 degrees C to the propagation of a single dominant crack at 800 degrees C and above. (C) 2000 Elsevier Science Ltd. All rights reserved.