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 ga s-solid reactions, for example. In the models, the composition depende nce: of the solute diffusivity in the phases constituting the layers w as accounted for by appropriate definition of an effective diffusion c oefficient for a (sub)layer. This effective diffusion coefficient is t he intrinsic diffusion coefficient weighted over the composition range of the (sub)layer. The models were applied for analyzing the growth k inetics 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 al pha-Fe substrate, as observed by gaseous nitriding in an NH3/H-2-gas m ixture at 843 K. The kinetics of layer development and the evolution o f the microstructure were investigated by means of thermogravimetry, l ayer-thickness measurements, light microscopy, and electron probe X-ra y microanalysis (EPMA). The effective and self-diffusion coefficients were determined for each of the nitride layers. The composition depend ence of the intrinsic (and effective) diffusion coefficients was estab lished. 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 ni trogen in gamma'-Fe4N1-x was determined from the temperature dependenc e of the self-diffusion coefficient. The self-diffusion coefficient fo r nitrogen in epsilon-Fe2N1-x was significantly larger than that for g amma'-Fe4N1-x. This was explained qualitatively, considering the possi ble mechanisms for interstitial diffusion of nitrogen atoms in the clo se-packed iron lattices of the epsilon and gamma' iron nitrides.