Characterization of surface modified α-iron by nitrogen plasma immersion ion implantation: a microstructural study

This paper discusses the morphology of pure iron surfaces modified by nitrogen plasma immersion ion implantation and compares them to similar surfaces treated with conventional plasma nitriding. Analysis of the samples was performed with glancing angle X-ray diffraction and traditional Bragg–Brentano geometry X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and depth profiling using X-ray photoelectron spectroscopy in combination with sputtering. The structures formed both by thermal nitriding and plasma ion implantation correlate very well with the Fe–N phase diagram, as the surface temperature and nitrogen concentration are decisive factors in developing specific crystalline phases. Only at lower temperatures, where chemical absorption and thermal diffusion effects are strongly limited, does the distribution of implanted nitrogen become substantially of the non-equilibrium type, and can almost be freely tailored. At low temperatures, however, the nitrided layer becomes extremely shallow (defined almost solely by the ballistic ion model), thereby limiting the applicability of this technology for iron-based materials. Hence, unless specific ferrous alloy materials are chosen which promote nitride formation and diffusion, e.g. chromium and iron-chromium alloys, the niche for nitrogen plasma ion implantation into ferrous materials seems to be limited to those cases where surface nitriding is desired, but where exposure of the workpiece to high temperature is forbidden.