Transition from parabolic to linear kinetics in the high-temperature nitridation of niobium

The nitridation behavior of pure niobium was investigated in nitrogen gas o ver the pressure range of 10(-2) to 1 torr and over the temperature range 1 600-2000 degrees C. A surface layer of Nb2N formed at 1 and 10(-1) torr, ob eying parabolic kinetics for the former and linear kinetics for the latter. This transition in time dependency first manifested itself at 1 torr in no nlinear plots of ln(K-c) vs. 1/T, consistent with an adsorption-controlled reaction. An evaluation of the flux of nitrogen across the gas-metal interf ace is presented, comparing the kinetics of gas arrival based on the kineti c theory of gases with gas removal because of inward diffusion into the met al. Data from the literature are used to estimate these adsorptive and diff usive fluxes and justify the observed behavior. Nitridation of niobium at 1 0(-2) torr resulted in gas dissolution only, with no nitride layer being fo rmed.