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

The formation of niobium nitrides was investigated in the temperature range 1400-1800 C by reaction of niobium sheet with nitrogen (1-30 bar N2). Equilibrated samples yielded the p-T-[N]/[Nb] relationship of δ-NbN1-x: [N]/[Nb]=1.585-4.148.10-4.T-0.05176.ln(pN2)+4.984.10-5.T.ln(pN2). In diffusion couples the formation of the phases δ-NbN1-x, γ-Nb4N3±x and β-Nb2N was observed. Between δ-NbN1-x and γ-Nb4N3±x a concentration discontinuity was absent, which is due to the fact that γ-Nb4N3±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-NbN1-x. The c/a ratio of the tetragonal cell of γ-Nb4N3±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-Nb2N phase band vs. the δ-NbN1-x/γ-Nb4N3±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-NbN1-x phase and the influence of the homogeneity range on the layer growth rate. (orig.)