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

Silicon nitride (Si3N4) ceramics using four compound sintering aids Er2O3-MgO, Er2O3-MgF3, Er2O3-Mg2Si, and Er2O3-Mg3N2. The effects of binary sintering aid types and temperatures on the microstructure, mechanical properties, and thermal conductivity of Si3N4 ceramics were investigated. The depolymerization effect of fluorine atoms introduced by MgF2 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 Si3N4 ceramics. While the additional Si and N atoms increase the concentration of solute atoms, which was conducive to sintering densification of Si3N4 ceramics. Although Mg2Si and Mg3N2 sintering aids produced higher viscosity liquid phase compared with MgF2, denser Si3N4 ceramics were obtained at 1900 degrees C. The Si3N4 ceramics doped with 5 mol% Er2O3 and 2 mol% Mg3N2 at 1900 degrees C showed the excellent properties: including the highest Vickers hardness (16.8 GPa), flexural strength (912 MPa), and fracture toughness (8.1 MPa m1/2), as well as the desirable thermal conductivity (83 W/(m K)).