Oxidation and nitridation of niobium in air above 1150°C

When niobium is heated in static air, layers of oxides and nitrides form on the surface, depending on the temperature. At 1150–1500°C, a layer of solid Nb₂O₅ forms with linear growth kinetics. At 1500–1900°C, a layer of Nb₂N forms under a two-phase layer of liquid oxide+solid NbO₂. The two-phase oxide layer has linear growth kinetics, while the Nb₂N layer has parabolic kinetics. Above 1900°C, successive layers of Nb₂N, NbN, and liquid oxide form, with both nitrides exhibiting parabolic growth kinetics. The activation energies of layer growth are as follows: Q=105 kJ/mol for Nb₂O₅, 120 kJ/mol for the two-phase oxide layer, and 220 kJ/mol for Nb₂N (Q for liquid oxide and NbN layer formation was not determined). Above 1500°C, the nitride layers are somewhat protective, in that they slow the oxide formation rate and prevent ignition.