The influence of magnesium compounds on the properties of silicon nitride ceramics

Abstract Silicon nitride (Si ₃ N ₄ ) ceramics using four compound sintering aids Er ₂ O ₃ –MgO, Er ₂ O ₃ –MgF ₃ , Er ₂ O ₃ –Mg ₂ Si, and Er ₂ O ₃ –Mg ₃ N ₂ . The effects of binary sintering aid types and temperatures on the microstructure, mechanical properties, and thermal conductivity of Si ₃ N ₄ ceramics were investigated. The depolymerization effect of fluorine atoms introduced by MgF ₂ sintering additives on the silicate melt structure reduced the viscosity of the liquid phase. However, the existence of volatile phase leads to low density of Si ₃ N ₄ ceramics. While the additional Si and N atoms increase the concentration of solute atoms, which was conducive to sintering densification of Si ₃ N ₄ ceramics. Although Mg ₂ Si and Mg ₃ N ₂ sintering aids produced higher viscosity liquid phase compared with MgF ₂ , denser Si ₃ N ₄ ceramics were obtained at 1900°C. The Si ₃ N ₄ ceramics doped with 5 mol% Er ₂ O ₃ and 2 mol% Mg ₃ N ₂ at 1900°C showed the excellent properties: including the highest Vickers hardness (16.8 GPa), flexural strength (912 MPa), and fracture toughness (8.1 MPa m ^1/2 ), as well as the desirable thermal conductivity (83 W/(m K)).