Effects of heating rate on microstructure and property of sintered reaction bonded silicon nitrides

We prepared sintered reaction-bonded silicon nitride (SRBSN) ceramics using yttria and magnesia as sintering additives and evaluated the effects of heating rates of nitridation on their microstructure, bending strength, fracture toughness, and thermal conductivity of the specimens post-sintered at 1850 °C for 6 h. The heating rate strongly affected the microstructure of the nitrided compacts obtained by heating at 1400 °C for 4 h. A too-high heating rate (e.g., 10 °C/min) could result in the formation of Si droplets on the surface due to melting of Si particles at the high local temperature caused by the exothermic nitridation reaction. While the heating rate was sufficiently low, the pores inside the nitrided compacts were small because the solid Si particles fully reacted with nitrogen gas to form tiny Si3N4 particles at relatively lower temperatures. The difference in the amount and structures of the pores in the nitrided body affected the densification of the nitrided body and the shape of β-Si3N4 grains formed during post-sintering, thereby leading to variations in the mechanical and thermal properties.