A cyclic process for the nitridation of Si powder

The presence of coarse Si particles contributes a hindrance to achieving fully reacted Si3N4 by a reaction-bonded silicon nitride (RBSN) technique for use in real net-shape parts, such as turbo-charger rotors. A new process is described, in which the nitridation rate is enhanced by cyclic heating and the cyclic and isothermal heating processes were compared. When the nitridation was conducted at various temperatures and times (1370-1420 °C for times up to 60 h), the reaction rate of the cyclic process was three times faster than that of the isothermal reaction. This can be attributed to the cracking of the reacted Si3N4 shell during nitridation, which exposes unreacted liquid Si in the core. Cracking was found to occur by the internal stress resulting from the volume mismatch between Si3N4 and liquid Si.