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

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)).