The structural design and oxidation resistance of high fracture toughness Si3N4/BN composites for elevated temperature application

Si 3 N 4 /BN composites were fabricated via hot-pressing sintering, achieving outstanding mechanical properties with flexural strength of 623.41 MPa and fracture toughness of 10.22 MPa·m 1/2 - through structural optimization. Key factors contributing to these properties included the use of finer Si 3 N 4 powder, unidirectional fiber alignment, and an optimized fiber diameter of 0.95 mm, which enhanced densification. The residual strength of samples subjected to oxidation testing at 1000-1400 °C for 1-64 h was measured. Notably, samples oxidized at 1000 °C for 4 h exhibited a 5.04% increase in flexural strength compared to their room-temperature strength, attributed to SiO 2 healing the microcracks. At 1200 °C, needlelike Y 2 Si 2 O 7 crystals formed, while higher temperatures (up to 1400 °C) and prolonged oxidation led to their dissolution and the precipitation of platelike aluminosilicates. These microstructural transformations were found to critically influence the residual strength, highlighting the intricate relationship between oxidation conditions, phase evolution, and mechanical properties in Si 3 N 4 /BN composites.