R-CURVE BEHAVIOR AND STABLE CRACK-GROWTH AT ELEVATED-TEMPERATURE (1500-DEGREES 1650-DEGREES-C) IN A SI3N4/SIC NANOCOMPOSITE

The crack growth resistance behavior and the stable crack growth regime of a Si3N4/SiC composite have been examined at high temperature (1500 degrees-1650 degrees C). SENB specimens were used and the load/unloading technique, with high deflection rates to ensure an elastic behavior, has been employed to estimate the crack lengths. Rising R-curves have been obtained with a maximum crack growth resistance almost twice as high as the initial value. Above the T-g of the intergranular glassy phase, the behavior changes from brittle to visco-plastic and, consequently, the fracture characteristics become strongly rate dependent. It is observed experimentally that in the enhanced ductile region the crack extension velocity during the stable crack propagation from a preexisting flaw decreases rapidly with time. This phenomenon has been tentatively attributed to dynamic crack-tip stress relaxation resulting from the rapid flow of the glassy intergranular phase in the process zone. Thus, the rheological properties of the composite appear to be of major importance to gain insight into the mechanical behavior at such elevated temperatures.