Structural Characterization of Fine γ′-Fe4N Nitrides Formed by Active Screen Plasma Nitriding

The paper presents the structural characterization of γ′-Fe4N nitrides produced by active screen plasma nitriding (ASPN) processes. Experiments were performed on the Fe-Armco model material at 693, 773, and 853 K for 6 h. Investigation of the properties of the substrate was realized using scanning electron microscopy (SEM, SEM–EBSD/Kikuchi lines), energy-filtered transmission electron microscopy (TEM-EFTEM), X-ray diffraction (GID, grazing incidence diffraction, micro-XRD), and secondary ion mass spectroscopy (SIMS). Results have confirmed that the γ′-Fe4N nitrides’ structure and morphology depend considerably on the nitriding process’s plasma conditions and cooling rate. In addition to that, γ′-Fe4N nitrides’ formation can be correlated with the surface layer saturation mechanism and recombination effect. It has been shown that the γ′-Fe4N structure depends considerably on several phenomena that occur in the diffusive layer (e.g., top layer decomposition, nitrogen, and carbon atoms’ migration). Our research proves that the nitrogen concentration gradient is a driving force of nitrogen migration atoms during the recombination of γ′-Fe4N nitrides. Finally, realized processes have allowed us to optimize active screen plasma nitriding to produce a surface layer of fine nitrides.