Mode I Fracture Toughness of a Small‐Grained Silicon Nitride: Orientation, Temperature, and Crack Length Effects

Abstract

The Mode I fracture toughness (KIC) of a small‐grained Si3N4 was determined as a function of hot‐pressing orientation, temperature, testing atmosphere, and crack length using the single‐edge precracked beam method. The diameter of the Si3N4 grains was <0.4 µm, with aspect ratios of 2–8. KIC at 25°C was 6.6 ± 0.2 and 5.9 ± 0.1 MPa·m1/2 for the T–S and T–L orientations, respectively. This difference was attributed to the amount of elongated grains in the plane of crack growth. For both orientations, a continual decrease in KIC was observed through 1200°C, to ∼4.1 MPa·m1/2, before increasing rapidly to 7.5–8 MPa·m1/2 at 1300°C. The decrease in KIC through 1200°C was a result of grain‐boundary glassy phase softening. At 1300°C, reorientation of elongated grains in the direction of the applied load was suggested to explain the large increase in KIC. Crack healing was observed in specimens annealed in air. No R‐curve behavior was observed for crack lengths as short as 300 µm at either 25° or 1000°C.