Relation Between Strength, Fracture Energy, and Microstructure of Hot‐Pressed Si3N4

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Abstract
A fracture mechanics approach was used to investigate the high strength of hot‐pressed Si3N4. Room‐temperature flexural strengths, fracture energies, and elastic moduli were determined for material fabricated from α‐ and β‐phase Si3N4 powders. When the proper powder preparation technique was used, α‐phase powder resulted in a high fracture energy (69,000 ergs/cm2), 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/cm2), a relatively low strength (55,000 psi), and an equiaxed grain morphology. It was hypothesized that the high strength of Si3N4 hot‐pressed from α‐phase powder results from its high fracture energy, which is attributed to the elongated grains. High‐strength Si3N4 has directional properties caused, in part, by the elongated grain structure, which is oriented preferentially with respect to the hot‐pressing direction.