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

The kinetics and the mechanism of the reaction between Si3N4 and Nb have been investigated under a N2 or an Ar atmosphere at temperatures from 1373 to 1673 K. Using a Si3N4-Nb powder mixture, the reaction rates were determined with a thermobalance, and the reaction products were examined by X-ray diffraction.Under the N2 atmosphere, the mass gain was caused by the production of the niobium nitrides. With increasing temperature, the nitrides changed in the following order: Nb4.62N2.14, Nb2N, Nb4N3, NbN0.95, and NbN. It is considered that Nb is not reacted with Si3N4 but with N2.Under the Ar atmosphere, the reaction products were both the nitrides (Nb4.62N2.144, Nb2N and NbN) and the silicides (Nb5Si3 and NbSi2). At higher temperatures and on prolonged heating, the niobium nitrides disappeared as a result of the interaction with Si3N4. Above 1523 K, the production of niobium silicides resulted in the mass loss. The initial rate followed a linear rate law, and the activation energy was 320 kJ/mol. An interfacial reaction may be considered as the rate-determining step. At a late stage of the reaction, the kinetics followed a parabolic rate law, and the activation energy was 507 kJ/mol. The reaction rate is probably controlled by a solid state diffusion through the reaction layer.