Microstructure and mechanical properties of silicon nitride ceramics prepared by pressureless sintering with MgO–Al2O3–SiO2 as sintering additive

The microstructure and mechanical properties of pressureless sintered Si3N4 ceramics prepared both by conventional ball-milling and planetary high-energy ball-milling for a sintering additive from the MgO–Al2O3–SiO2 system were investigated by XRD, SEM, TEM, HRTEM, and EDS. For pressureless sintered Si3N4 ceramics prepared by planetary ball-milling, a flexure strength of 1.06 GPa, Vickers hardness of 14.2 GPa, and fracture toughness of 6.4 MPa·m0.5 were achieved. The microstructure of the sintered materials consists of elongated grains with almost identical size and aspect ratio that uniformly distributed throughout the body. The improved mixing efficiency by planetary ball-milling has simply reduced the particle size and improved the distribution of sintering additives, leading to a slightly improved densification and microstructural homogenization that are responsible for the promising properties of Si3N4 ceramics. In comparison with traditional ball-milling route, the planetary high-energy ball-milling is an efficient processing to produce high homogeneous microstructure and promising mechanical properties.