Effect of β-Si3N4 content on microstructure, mechanical properties, and thermal conductivity anisotropy in hot-pressed Si3N4 ceramics

Silicon nitride (Si 3 N 4 ) ceramics fabricated via hot-pressing sintering exhibit pronounced anisotropy in their properties, which restricts their practical applications. To clarify underlying mechanism responsible for this anisotropy, this study investigates the effect of β-Si 3 N 4 content on the anisotropy of microstructure, mechanical properties, and thermal conductivity of hot-pressed Si 3 N 4 ceramics. Results indicate that preferential growth of rod-like β-Si 3 N 4 along sintering pressure direction, together with intrinsic property differences between its c -axis and a -axis, induces the anisotropy in both microstructure and material properties of Si 3 N 4 ceramics. Among investigated samples, Si 3 N 4 ceramic consisting primarily of elongated rod-like β-Si 3 N 4 and small amount of residual α-Si 3 N 4 , exhibits highest degree of microstructural texture and most pronounced anisotropy in mechanical properties and thermal conductivity. Fracture toughness and flexural strength of Si 3 N 4 ceramics are higher in direction perpendicular to sintering pressure while thermal conductivity is higher in direction parallel to sintering pressure. Consequently, it is difficult to simultaneously optimize mechanical properties and thermal conductivity in same direction.