Synthesis of Composite Powders of the Si3N4–NbN System

Abstract

Ceramic composites based on silicon nitride belong to the most promising materials for producing wear-resistant products operated under extreme conditions [1]-[3]. The present work is devoted to the investigation of conditions for preparing Si3N4–20 vol.% NbN highly disperse composite powders by in situ solid-state synthesis from Si3N4–Nb reaction mixture and by the traditional method of mechanical mixing of Si3N4 and NbN constituent compounds. The Si3N4–NbN and Si3N4–Nb reaction mixtures were mixed in a Pulverizette 6 planetary mill. Synthesis products were investigated by the XRD and SEM methods. The particle-size distribution of the composite powders was determined by the laser diffraction method. The behavior of reaction powder mixture of stoichiometric compositions calculated for the synthesis of higher niobium nitride in the temperature range 800–1400 °С in vacuum by the reaction 4Nb + Si3N4 was investigated [2]. Based on the established regularities, we developed a combined two-stage technological synthesis process of Si3N4–NbN powder, including vacuum annealing at 1000 °С and subsequent nitriding at 1300 °С, which enabled us to obtain Si3N4–20 vol.% NbN powder consisting of multiphase agglomerates with a mean size ≤3 µm and a nanodisperse fraction with a mean particle size of 200 nm. For the preparation of Si3N4–20 vol.% NbN composite powder by the traditional mechanical mixing method, β-Si3N4 and δ-NbN powders synthesized under laboratory conditions were used. The mean particle size of the obtained composite powder is ≤10 µm. We established advantages of the method of in situ solid-state synthesis of Si3N4–NbN powders, which guarantees obtaining fine-grained powder with a mean particle size ≤3 µm and a uniform distribution of nitride phases in a single technological cycle.