Growth of niobium nitrides by nitrogen-niobium reaction at high temperature

The reaction between nitrogen and bulk niobium under high purity conditions was studied in the range 1100-1900 °C at 150 kPa. Two different nitride layers are formed: an inner ? phase (Nb2N) and an outer ? phase (NbN). The latter undergoes a transformation to ?-NbN and ?-NbN during cooling down. Nitrogen dissolves in the niobium core forming ? solid solution. Both the weight gain and the thickness of the nitride layers follow the parabolic rate law, indicating that the reaction is mainly dominated by diffusional transport through the nitrides. Two parabolic kinetic stages are observed, the first before ? phase saturation and the second after saturation. Data analysis based on a multiphase, moving boundary diffusion model allows the calculation of the effective nitrogen diffusion coefficients, yielding the expressions D? = 1.11[±0.5] exp(-3.23[±0.4] × 10^5/RT) cm2 s-1 and D? = 3.27[±1.5] exp(-3.19[±0.25] × 10^5/RT) cm2 s-1.