Synthesis of Si3N4–NbN Composite Powders

Abstract

Technology for in situ synthesis of superfine Si₃N₄–NbN composite powders without the need for subsequent milling via solid-state interaction in the Si₃N₄–Nb reaction mixture was developed for the production of nitride ceramics by spark plasma sintering and hot pressing. The regularities of solid-state interaction during vacuum heat treatment of the β-Si₃N₄–27.4 wt.% Nb reaction powder mixture, calculated for the synthesis of higher niobium nitride by the 4Nb + Si₃N₄ = 4NbN + 3Si reaction, were analyzed. The interaction of the mixture components was studied under isothermal holding for 1 h at 1000, 1100, 1200, 1300, and 1400°C. Solid-state interaction with Si₃N₄ was found to occur at 1000°C, resulting in the formation of a nitrogen solid solution in niobium (α-Nb). At 1100°C, the formation of lower nitride Nb₂N and lower silicide Nb₅Si₃ was observed. An increase in the temperature to 1200 and 1300°C led to a greater amount of Nb₅Si₃, whereas the amount of Nb₂N hardly changed. At 1400°C, the product contained a mixture of γ-Nb₅Si₃ and NbSi₂ silicides, while the lower nitride was absent. Thermodynamic calculations confirmed that the formation of higher niobium nitrides under these vacuum heat treatment conditions was thermodynamically unfavorable. Based on the established structural and phase regularities of solid-state interaction in the Si₃N₄–Nb mixture, a two-stage synthesis process was developed. This process was implemented in a single cycle consisting of vacuum heat treatment at 1000°C, followed by nitriding at 1200 and 1300°C. Nitriding at 1300°C yielded a powder composed of Si3N4 and a mixture likely containing three higher niobium nitrides of different polymorphic modifications. Using the developed synthesis process, experimental batches of Si₃N₄–20 vol.% NbN and Si₃N₄–10 vol.% NbN composite powders were produced. Analysis of the experimental batches showed that all synthesized powders possessed the required phase composition and were superfine. The particle size of the Si₃N₄–20 vol.% NbN powder ranged from 400 nm to 9 μm and that of the Si₃N₄–10 vol.% NbN powder ranged from 1.5 to 5 μm.