Mechanism of High Temperature Reaction between Si₃N₄ and Nb Powders

Abstract

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: Nb_4.62N_2.14, Nb₂N, Nb₄N₃, NbN_0.95, 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 (Nb_4.62N_2.144, 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.