Modelling nitrogen atom flux in post-discharge nitriding processes

Nitriding of pure iron using microwave N2–H2 post-discharges is modelled by coupling kinetics of the gas phase, surface processes and solid diffusion. Despite basic data are missing for iron and are thus estimated, the description of the interface phenomena provides a clear understanding of the material response to a given gas-phase composition. The most important aspect presented deals with the dissolution of nitrogen atoms into the iron nitride ɛ-Fe2N1−x at the topmost surface of the sample. As chemically adsorbed nitrogen atoms are involved in both the Langmuir–Hinshelwood and the Eley–Rideal loss mechanisms of N atoms into N2, their dissolution implies a subsequent decrease of the loss probability of N atoms at the surface. This is the main reason why post-discharge nitriding works at high temperature. The consumption of the nitrogen atoms at the surface of the sample limits considerably their loss by recombination. A first comparison with experimental results of nitriding at 723 K shows that the nitrogen profile along the post-discharge does not exhibit any change of slope from the Pyrex surface to the nitrided iron foil. The time evolution of the nitrogen density in the middle of the iron foil is either weak or null, showing that the loss of N atoms by solid diffusion at 723 K in iron is probably very weak with respect to other loss processes to be monitored by optical emission spectroscopy (OES).