The influence of ion energy on the nitriding behaviour of austenitic stainless steel

Recent work on austenitic stainless steels has indicated that low energy, high current density nitrogen implantation can result in nitrided layers several micrometers in depth and of considerable hardness. This work was initiated to examine the effects of ion energy during ion implantation at elevated temperatures. Three nitriding methods were considered: plasma immersion ion implantation (PI3), radio frequency (r.f.) plasma nitriding and ion beam nitriding. The structure and nitrogen profile of austenitic stainless steel were examined after the different treatments by a range of analytical techniques including glancing angle X-ray diffraction (GAXD), ultra microhardness indentations (UMIS), glow discharge optical spectroscopy (GDOS) and nuclear reaction analysis (NRA). It was found that similar surface structures can be formed by PI3 treatments at high energies and ion beam nitriding at low energies. The resultant microstructures, as determined by GAXD, consist primarily of an expanded austenite layer. It appears that the adherent oxide film present on stainless steel must be either removed by sputtering, at low ion energies, or passed through by implantation, at high energies. Subsequent diffusion at elevated temperatures allows the formation of a nitrided layer several micrometers thick in both cases