Effect of Grain Growth of β‐Silicon Nitride on Strength, Weibull Modulus, and Fracture Toughness

Abstract

β‐Si 3 N 4 powder containing 1 mol% of equimolar Y 2 O 3 –Nd 2 O 3 was gas‐pressure sintered at 2000°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 β Si 3 N 4 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‐m 1/2 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 β‐Si 3 N 4 powder was shown to be effective in fostering selective grain growth for obtaining the desired composite microstructure.