R-CURVES OF AN YTTRIA-DOPED AND ALUMINA-DOPED HOT-PRESSED SILICON-NITRIDE CERAMIC AT 1200-DEGREES-C AND ROOM-TEMPERATURE

An energy approach has been utilized to measure the R-curves of an Y2O 3-Al2O3-doped hot-pressed silicon nitride ceramic at 1200 degrees C in an argon atmosphere in three-point bending. In order to evaluate the R-curves at 1200 degrees C, a low constant displacement rate of delta = 5 mu m min(-1) was applied in cyclic loading to obtain the cyclic lo ading/unloading-displacement curves during controlled-crack propagatio n. Propagated crack lengths were measured directly by a microscope and they were compared to compliance-calculated crack lengths. After digi tizing the cyclic load-displacement and crack length-displacement curv es, crack-resistance parameters, R-curves and K-curves, were calculate d by computer. At 1200 degrees C this material behaved non-elastically and the crack parameters, obtained here, represent the non-elastic on es. For comparison, at room temperature, continuous loading was applie d to obtain the load-displacement curves. At room temperature, linear- elastic fracture mechanics behaviour was observed.