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

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.