Relation Between Strength, Fracture Energy, and Microstructure of Hot‐Pressed Si 3 N 4

A fracture mechanics approach was used to investigate the high strength of hot‐pressed Si 3 N 4 . Room‐temperature flexural strengths, fracture energies, and elastic moduli were determined for material fabricated from α‐ and β‐phase Si 3 N 4 powders. When the proper powder preparation technique was used, α‐phase powder resulted in a high fracture energy (69,000 ergs/cm 2 ), a high flexural strength (95,000 psi), and an elongated (fiberlike) grain morphology, whereas β‐phase powder produced a low fracture energy (16,000 ergs/cm 2 ), a relatively low strength (55,000 psi), and an equiaxed grain morphology. It was hypothesized that the high strength of Si 3 N 4 hot‐pressed from α‐phase powder results from its high fracture energy, which is attributed to the elongated grains. High‐strength Si 3 N 4 has directional properties caused, in part, by the elongated grain structure, which is oriented preferentially with respect to the hot‐pressing direction.