EFFECT OF GRAIN-GROWTH OF BETA-SILICON NITRIDE ON STRENGTH, WEIBULL MODULUS, AND FRACTURE-TOUGHNESS

Beta-Si3N4 powder containing 1 mol% of equimolar Y2O3Nd2O3 was gas-pressure sintered at 2000-degrees-C for 2 h (SN2), 4 h (SN4), and 8 h (SN8) in 30-MPa nitrogen gas. These materials had a microstructure of ''in-situ composites'' as a result of exaggerated grain growth of some beta-Si3N4 grains during firing. Growth of elongated grains was controlled by the sintering time, so that the desired microstructures were obtained. SN2 had a Weibull modulus as high as 53 because of the uniform size and spatial distribution of its large grains. SN4 had a fracture toughness of 10.3 MPa.m1/2 because of toughening provided by the bridging of elongated grains, whereas SN8 showed a lower fracture toughness, possibly caused by extensive microcracking resulting from excessively large grains. Gas-pressure sintering of beta-Si3N4 powder was shown to be effective in fostering selective grain growth for obtaining the desired composite microstructure.