Concurrent optimization of fracture toughness, thermal conductivity, and tribological behavior in Cf/Si3N4 composites via phase driven selection

Carbon fiber-reinforced silicon nitride (C_f/Si₃N₄) composites were fabricated by spark plasma sintering (SPS) using α-, β-, and γ-Si₃N₄ powders to clarify the influence of the initial Si₃N₄ phase on microstructural evolution and functional properties. The results show that the starting phase significantly affects densification behavior, phase transformation, and mechanical and tribological performance. The composite derived from α-Si₃N₄ achieved the highest relative density (96.53%) and exhibited an optimal balance of fracture toughness (10.87 MPa m^0.5), thermal conductivity (66 W/m K), and stable friction behavior (COF ≈ 0.46). This performance is attributed to the in-situ formation of a self-reinforced β-Si₃N₄ microstructure during the α → β phase transformation, which promotes crack deflection, crack bridging, and effective load transfer in synergy with carbon fibers. In contrast, β- and γ-Si₃N₄-based composites showed lower densification or excessive hardness associated with increased porosity and secondary phase formation. These findings demonstrate that controlling the initial Si₃N₄ phase provides an effective microstructural design strategy for developing high-performance C_f/Si₃N₄ composites for thermostructural applications such as aerospace brake discs.