Application of an analytical model of nitrogen diffusion in nitrided pure iron for the study of the γ'-Fe4N monolayer growth

Application of an analytical model of nitrogen diffusion in nitrided pure iron for the study of the gamma'-Fe4N monolayer growth. Gaseous nitriding is the thermochemical process by which nitrogen is introduced into the surface of pure iron between 823 and 853 K. The produced film can be subdivided into a compound layer, consisting predominantly of epsilon-Fe2-3N and/or gamma'-Fe4N phases which is responsible for the good tribological and anticorrosion properties of tire surface, and a diffusion zone where nitrogen is dissolved interstitially in the ferrite matrix, leading to improved fatigue resistance. An analytical diffusion model was applied for the gaseous nitriding of pure iron taking into account the diffusion of nitrogen through the gamma' and a phases and the thermodynamic properties of the Fe-N binary system. It is shown that the phase constitution of the nitrided layer produced during this treatment of pure iron can be predicted by the analytical model from Fick's laws, where we assume that thermodynamic equilibrium is established locally at the moving phase interface with a fixed surface content of nitrogen. The monolayer growth of gamma', imposed by the choice of an appropriate nitriding potential, has been analyzed theoretically. Results from the analytical solutions of the diffusion equations as well as the experimental results are presented and compared with each other in order to validate the model.