Effects of the amount of magnesia on mechanical, thermal, and electrical properties of silicon nitride ceramics

Silicon nitride (Si3N4) ceramics were prepared using various amounts of magnesia (MgO) and a fixed amount of yttria as sintering additives, and their effects on the microstructures and four essential properties were evaluated: bending strength, fracture toughness, thermal conductivity, and dielectric breakdown strength (DBS). The microstructure varied with the MgO content; a smaller amount promoted grain growth, while an excess amount formed pores. The properties varied along with these structural changes. It was revealed that the four properties coexist at a high level in the range of 4–7 mol% of MgO. In addition, the large amount of MgO enhanced the electric conductivity of grain boundary and decreased the DBS. It is speculated that tips of multigrain junctions composed of a sufficiently large amount of glass modifier might work as the severe defects for the dielectric breakdown.