Nitride nucleation and growth during plasma and post-discharge nitriding

Early stages during plasma and post-discharge nitriding of ferrous materials were examined. A microstructural characterisation comparing features of the nitrides produced on the surface by the two methods was carried out. Nitride growth kinetics was found to depend on the density of excited species adsorbed on the surface. Based on these observations, a mechanism for the formation of γ′-Fe4N1−x and ε-Fe2–3N was proposed and linked to a diffusion model developed to calculate the evolution of the surface nitrogen concentration. The model predicts a lower bound for the nucleation time for these nitrides. The microstructural observations indicate that after nucleation, the ensuing growth of a compact nitride layer starts with an abundant precipitation along grain boundaries, followed by thickening and branching in the case of the γ′-Fe4N1−x phase. For the case of ion nitriding, growth kinetics is also affected by the sputtering of the surface due to ion impingement.