Synergistic Effects of Mg2Si-YH2 Composite Additives on the Microstructure and Properties of Silicon Nitride Ceramics

Sintering additives play a decisive role in the densification behavior, mechanical properties, and thermal conductivity of silicon nitride ceramics. In this study, Mg2Si and YH2 were used as sintering additives for gas pressure sintering of silicon nitride based on the synergistic mechanism of “silicide silicon extraction-hydride dehydrogenation”. The regulation rules of the additives on ceramic densification, mechanical properties, and thermal conductivity were systematically investigated. Two optimization strategies were proposed for the technical route of replacing traditional oxide additives with non-oxide systems. (i) Rare-earth hydride YH2 was used to replace traditional rare-earth oxides. It reacts with SiO2 to achieve strong deoxidation and precisely regulate the liquid phase composition. (ii) Metal silicide Mg2Si was used to replace metal oxides. It promotes the preferred growth of β-Si3N4 grains, consumes oxygen in the system, and reduces lattice defects. Mg2Si introduces Si into the liquid phase, increasing the Si/O ratio, which lowers lattice oxygen content and supports higher thermal conductivity. YH2 consumes SiO2 on the Si3N4 surface, which reduces liquid phase oxygen content and inhibits lattice oxygen incorporation, promoting a liquid phase with a high N/O ratio. Compared with traditional Y2O3, YH2 increases the Y2O3/SiO2 ratio in the liquid phase. It promotes grain growth, reduces SiO2 activity, and further improves the thermal conductivity of ceramics. Silicon nitride ceramics prepared by gas pressure sintering at 1750 °C with 3 wt.% Mg2Si and 4 wt.% YH2 composite additives exhibit the highest thermal conductivity of 87 W/(m·K), with a Vickers hardness of 14.36 GPa and a flexural strength of 643.15 MPa. This study provides an innovative idea for the preparation of high-performance silicon nitride heat dissipation substrates.