High-temperature reactive phase formation in the Nb-N system

Abstract The formation of niobium nitrides was investigated in the temperature range 1400–1800°C by reaction of niobium sheet with nitrogen (1–30 bar N 2 ). Equilibrated samples yielded the p − T −[N]/[Nb] relationship of δ-NbN 1− x : [N]/[Nb]=1.585−4.148·10 −4 · T −0.05176·ln( p N2 )+4.984·10 −5 · T ·ln( p N2 ). In diffusion couples the formation of the phases δ-NbN 1− x , γ-Nb 4 N 3± x and β-Nb 2 N was observed. Between δ-NbN 1− x and γ-Nb 4 N 3± x a concentration discontinuity was absent, which is due to the fact that γ-Nb 4 N 3± x is not stable in the investigated temperature range but forms upon cooling. This is visible by the typical hatched microstructure at the nitrogen-poor boundary of δ-NbN 1− x . The c / a ratio of the tetragonal cell of γ-Nb 4 N 3± x increases with increasing nitrogen content. The homogeneity ranges of phases were determined from the diffusion bands by wavelength-dispersive EPMA. A portion of the phase diagram is presented. The thickness ratio of the β-Nb 2 N phase band vs. the δ-NbN 1− x /γ-Nb 4 N 3± x phase band was found to be dependent on the applied nitrogen pressure. This is due to the different surface concentration of nitrogen in the δ-NbN 1− x phase and the influence of the homogeneity range on the layer growth rate.