Sub-surface modifications induced by nitrogen ion implantation in stainless steel (SS316L). Correlation between microstructure and nanoindentation results

Abstract

The surface of SS316L was modified through nitrogen ion implantation at high fluences. Multiple energy ion implantations at decreasing energy (180, 100, 50 and 20 keV) were performed on the same samples in order to obtain nearly flat depth-profiles over a thickness of about 250 nm. The doses were chosen with help of a Monte Carlo simulation method (TRIM90) in order to obtain three different nitrogen atomic concentrations: 20%, 30% and 40%. The SIMS measurements show that the nitrogen profiles are nearly flat with a significant nitrogen diffusion toward the bulk in case of highest doses, which increases the thickness of the nitrided layers and limits the concentration to a saturation value. The grazing angle X-ray diffraction experiments show that for the 20% N sample, the austenitic unit cells are strongly expanded and form a nitrogen solid solution of γ-structure. For the 30% and 40% samples, besides the solid solution, one part of the austenite is transformed in iron nitride ε-Fe 2–3N and another part in an α-Fe phase. The nanoindentation study shows that the formation of the solid solution induces an important increase in hardness from 5.5 GPa for the non-implanted sample to 15 GPa for the 20% N implanted one (values at 50 nm depth). In the 30% and 40% N samples, the hardness remains about the same as for the 20% N implanted sample, but over a higher thickness, which confirms the nitrogen diffusion towards the bulk.