Effect of Microstructure on Strength of Si 3 N 4 ‐SiC Composite System

The Si 3 N 4 ‐SiC composite system was investigated to better understand the effect of microstructure on the strength‐controlling factors, i.e. fracture energy, elastic modulus, and crack size. Silicon carbide dispersions with average particle sizes of 5, 9, and 32 μm were used to form 3 composite series within this system, each containing 0.10, 0.20, 0.30, and 0.40 vol fraction of the dispersed phase. These composites were fabricated by hot‐pressing. Fracture energy and strength values were measured for each composite. A linear relation between the elastic modulus of the two phases was assumed. The crack size was calculated for each composite using the appropriate property values. The strength behavior of the 9‐ and 32‐μm series was controlled by the crack size, which, in turn, was controlled by the particle size and volume fraction of the SiC phase. Particle size and volume fraction did not affect the crack size of the 5‐μm series, in which strength was controlled by both fracture energy and elastic modulus. Strengths measured at 1400°C and thermal conductivity measurements indicate that several of these composites are promising as high‐temperature structural materials.