Comparative Study of Hot Pressing and Spark Plasma Sintering on the Phase Transformation, Microstructure, and...

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.