Role of α- and β-Si₃N₄ additions in microstructural regulation, thermal and mechanical property optimization of sintered reaction-bonded Si₃N₄ ceramics

Achieving a balance between high thermal conductivity and mechanical property remains a critical challenge for sintered reaction-bonded Si₃N₄ (SRBSN) ceramics. This work investigates the effects of α- and β-Si₃N₄ additions on nitridation behavior, microstructural evolution, and properties of SRBSN ceramics. The results reveal fundamentally different roles of α- and β-Si₃N₄ during nitridation and sintering. Moderate α-Si₃N₄ addition suppresses the α→β transformation, inducing localized grain growth and a bimodal microstructure at 20 wt%. In contrast, β-Si₃N₄ markedly promotes the α→β transformation and heterogeneous nucleation. The sample with 30 wt% β-Si₃N₄ exhibits a pronounced bimodal grain structure with 20.2 % coarse elongated grains (>6 μm), a maximum mean aspect ratio of 1.89, an ultrathin grain-boundary film (∼2.2 nm), and a nitrogen-rich intergranular phase. Consequently, a high thermal conductivity of 114 W·m⁻¹ ·K⁻¹ is achieved while maintaining a flexural strength of 791 ± 29 MPa and a fracture toughness of 9.0 ± 0.3 MPa·m 1/2 .