Achieving strength-toughness synergy in Si3N4 ceramics through regulation of multiple RE₂O₃ additive contents and sintering temperature

The synergistic improvement of flexural strength and fracture toughness in silicon nitride (Si 3 N 4 ) ceramics is critically dependent on their microstructural configuration. In this study, multiple rare earth oxides (RE 2 O 3 ) were used as sintering additives to construct a multi-component RE-ion liquid phase. By systematically varying the total additive content from 2 mol% to 8 mol% and the sintering temperature from 1650°C to 1750 °C, we reveal a non-monotonic, temperature-dependent optimization window. At the optimal additive content of 6 mol% and a sintering temperature of 1750°C, the multi-component RE-ion liquid phase promotes complete α→β phase transformation, enhances densification, and regulates grain growth, yielding a bimodal microstructure of fine grains interlocked with high aspect ratio β-Si 3 N 4 grains. This tailored microstructure, together with the in-situ formed crystalline high-entropy (Gd 0.2 Y 0.2 Yb 0.2 Lu 0.2 Sc 0.2 ) 2 Si 2 O 7 grain boundary phase, effectively achieves a balance between strength and toughness, with a flexural strength of 1312 ± 48 MPa and a fracture toughness of 10.36 ± 0.85 MPa·m 1/2 .