R-curves of an yttria- and alumina-doped hot-pressed silicon nitride ceramic at 1200°C and room temperature

Abstract

An energy approach has been utilized to measure theR-curves of an Y2O3~A⪿3-doped hot-pressed silicon nitride ceramic at 1200‡C in an argon atmosphere in three-point bending. In order to evaluate theR-curves at 1200‡C, a low constant displacement rate of\(\dot \delta \)=5 Μm min−1 was applied in cyclic loading to obtain the cyclic loading/ unloading-displacement curves during controlled-crack propagation. Propagated crack lengths were measured directly by a microscope and they were compared to compliance-calculated crack lengths. After digitizing the cyclic load-displacement and crack length-displacement curves, crack-resistance parameters,R-curves andK-curves, were calculated by computer. At 1200‡C this material behaved non-elastically and the crack parameters, obtained here, represent the non-elastic ones. For comparison, at room temperature, continuous loading was applied to obtain the load-displacement curves. At room temperature, linear-elastic fracture mechanics behaviour was observed.