Optimization of Si3N4 fiber and BN interface for high mechanical properties and thermal-shock resistance of Si3N4/BN composites

This work demonstrates a strategic approach to enhance the thermomechanical performance of Si₃N₄/BN composites through interface optimization and control of powder particles. By adding the MgO sintering additive at BN interfaces coupled with 0.5 μm Si₃N₄ fiber precursor, we achieved full densification with mechanical properties, attaining a density of 3.19 g·cm^-3, the remarkable flexural strength of 627.36 MPa, and fracture toughness of 13.48 MPa·m^1/2. The composites exhibit exceptional thermal shock resistance, retaining 586.19 MPa (93.44% retention) after 1000 ℃ thermal shocking, facilitated by in situ formation of a continuous SiO₂ glass barrier that effectively prevents oxidation. Intriguingly, the residual strength shows a trend of first decreasing and then increasing with the increase of ΔT. At ΔT=1500 ℃, the composite maintains the residual strength of 469.46 MPa (74.83% retention), outperforming conventional monolithic ceramics through a self-healing mechanism enabled by viscous flow of the glass phase. Microstructural evolution analysis coupled with fracture morphology establishes direct correlations between the interface, mechanical properties, and thermal-shock resistance. These findings provide fundamental insights into designing composites with excellent thermal-shock resistance for ultrahigh-temperature structural applications.