Effect of CaF ₂ on microstructure and mechanical properties of pressureless sintered Si ₃N ₄/BN composites

Si ₃N ₄/BN composites with h-BN of 25% in volume were fabricated via pressureless sintering with micron-sized α-Si ₃N ₄ and h-BN powders as starting materials and CaF ₂, Al ₂O ₃ and Y ₂O ₃ as sintering additives. The effect of CaF ₂ content on the mechanical properties of Si ₃N ₄/BN composites was investigated, and the phase composition and microstructure of the Si ₃N ₄/BN composites were analyzed by X-ray diffraction and scanning electron microscopy. The results show that as CaF ₂ content increases, the porosity of Si ₃N ₄/BN composites increases, and its shrinkage and relative density decrease. When the amount of CaF ₂ is 2% in mass, the Si ₃N ₄/BN composite has the highest flexural strength of 145.5 MPa at room-temperature. A moderate amount of CaF ₂ in Y ₂O ₃-Al ₂O ₃ sintering aids can destroy the card-house structure during the pressureless sintering process of Si ₃N ₄/BN composites, causing the fracture of micron-sized h-BN particles into nano-sized BN segments.