Mechanism of High Temperature Reaction between Si<SUB>3</SUB>N<SUB>4</SUB> and Cr Powders

The kinetics and the mechanism of the reaction of Si3N4 with Cr have been investigated. Using powdered Si3N4-Cr mixture, the reaction rate was determined by means of a thermo-balance in an Ar gas stream at temperatures from 1273 to 1573 K. The reaction products were examined by X-ray diffraction.The reaction of Si3N4 with Cr took place after heating to about 1250 K. Cr2N and Cr3Si were produced below 1373 K. Above 1473 K, Cr2N disappeared and Cr3Si changed to CrSi via Cr5Si3. The initial rate was described by the linear rate law. The activation energy was 260 kJ/mol. A chemical process may be considered as the rate-determining step. When the Cr particles were covered with the reaction product layer, the kinetics obeyed the parabolic rate law. The activation energy was 365 kJ/mol. The reaction rate is probably controlled by the diffusion through the reaction layer.