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

The formation of niobium nitrides was investigated in the temperature range 1400-1800 degrees C by reaction of niobium sheet with nitrogen (1-30 bar N-2). Equilibrated samples yielded the p-T-[N]/[Nb] relationship of delta-NbN1-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 delta-NbN1-x, gamma-Nb4N3+/-x and beta-Nb2N was observed. Between delta-NbN1-x and gamma-Nb4N3+/-x a concentration discontinuity was absent, which is due to the fact that gamma-Nb(4)N3(+/-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 delta-NbN1-x. The c/a ratio of the tetragonal cell of gamma-Nb(4)N3(+/-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 beta-Nb2N phase band vs. the delta-NbN1-x/gamma-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 delta-NbN1-x phase and the influence of the homogeneity range on the layer growth rate. (C) 1998 Elsevier Science S.A.