LAYER-GROWTH KINETICS ON GASEOUS NITRIDING OF PURE IRON - EVALUATION OF DIFFUSION-COEFFICIENTS FOR NITROGEN IN IRON NITRIDES

Models were derived for monolayer and bilayer growth into a substrate in which diffusion of the solute governs the growth kinetics, as in gas-solid reactions, for example. In the models, the composition dependence: of the solute diffusivity in the phases constituting the layers was accounted for by appropriate definition of an effective diffusion coefficient for a (sub)layer. This effective diffusion coefficient is the intrinsic diffusion coefficient weighted over the composition range of the (sub)layer. The models were applied for analyzing the growth kinetics of a gamma'-Fe4N1-x monolayer on an alpha-Fe substrate and the growth kinetics of an epsilon-Fe2N1-x/gamma'-Fe4N1-x bilayer on an alpha-Fe substrate, as observed by gaseous nitriding in an NH3/H-2-gas mixture at 843 K. The kinetics of layer development and the evolution of the microstructure were investigated by means of thermogravimetry, layer-thickness measurements, light microscopy, and electron probe X-ray microanalysis (EPMA). The effective and self-diffusion coefficients were determined for each of the nitride layers. The composition dependence of the intrinsic (and effective) diffusion coefficients was established. Re-evaluating literature data for diffusion in gamma'-Fe4N1-x, on the basis of the present model, it followed that the previous and present data are consistent. The activation energy for diffusion of nitrogen in gamma'-Fe4N1-x was determined from the temperature dependence of the self-diffusion coefficient. The self-diffusion coefficient for nitrogen in epsilon-Fe2N1-x was significantly larger than that for gamma'-Fe4N1-x. This was explained qualitatively, considering the possible mechanisms for interstitial diffusion of nitrogen atoms in the close-packed iron lattices of the epsilon and gamma' iron nitrides.