Comparative Study of Hot Pressing and Spark Plasma Sintering on the Phase Transformation, Microstructure, and Properties of Si3N4 Ceramics with YF3-MgSiN2 Additives

Si3N4 ceramics with a YF3-MgSiN2 binary additive system were fabricated by hot pressing (HP) and spark plasma sintering (SPS) at 1500–1700 °C, followed by annealing at 1850 °C for 6 h. The effects of sintering route and temperature on phase transformation, microstructure evolution, thermal conductivity, and mechanical properties were systematically investigated. SPS significantly accelerated the α→β phase transformation compared with HP, and the β-Si3N4 content in SPS samples exceeded 94% at 1600 °C. After annealing, all samples were completely transformed into β-Si3N4, accompanied by obvious grain growth. Thermal conductivity was closely related to both grain size and relative density. Grain growth reduced grain-boundary phonon scattering, whereas density loss and residual porosity deteriorated heat transport. The mechanical properties were jointly governed by β-Si3N4 content, grain morphology, and porosity. Elongated β-Si3N4 grains promoted crack deflection and crack bridging, thereby improving fracture toughness, while excessive porosity reduced flexural strength. These results demonstrate that optimizing the balance between phase transformation, grain growth, and densification is essential for tailoring the thermal and mechanical performance of Si3N4 ceramics containing non-oxide sintering additives.