Oxidation and nitridation of niobium in air above 1,150 C

When niobium is heated in static air, layers of oxides and nitrides form on the surface, depending on the temperature. At 1,150-1,500C, a layer of solid Nb2O5 forms with linear growth kinetics. At 1,500-1,900C, 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 1,900C, 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 1,500C, the nitride layers are somewhat protective, in that they slow the oxide formation rate and prevent ignition