Nitriding of stainless steel by low energy ion implantation - the nitriding kinetics

Stainless steels are known for their excellent corrosion resistance based on a native surface oxide layer and a moderate hardness leading to short life times in applications with intensive wear. Hence, surface hardening could greatly enlarge their range of applications. However, stainless steels are considered as difficult candidates for surface hardening. Nitriding by low-energy ion implantation is a promising technique. At target temperatures of about 400°C the formation of a highly nitrogen enriched layer has been observed. This layer exhibits an increased surface hardness and a reduced wear without adversely affecting the excellent corrosion resistance.
This work focuses on the nitriding kinetics which are not well understood. Experimental data has been obtained by time and depth resolved elemental analyses using elastic recoil detection (ERD) during the nitriding process. The ERD technique has been optimised for time resolved in-situ data acquisition. Fast nitrogen diffusion, an energy and current density dependence of the thickness of the nitrogen enriched layer and an influence of the surface oxide layer have been observed. The physical origin of these phenomena will be revealed and a simple model of the nitriding process taking into account the governing physical processes, i. e. sputtering, recovering of the surface from the residual gas and diffusion in the solid, will be presented.