ABSTRACT
Secondary-ion mass spectrometry (SIMS) is an attractive method for elemental depth profiling for nitrided stainless steel. However, if taken at face value, the results suggest migration of Mo and Cr from the nitrogen-rich layer. More plausibly, the results indicate the presence of significant matrix effects, and this conclusion casts doubt on the interpretation of all the SIMS depth profiles, including that of nitrogen. We explored this question by examining standard samples prepared by nitrogen-ion implantation and comparing assays and depth profiles from SIMS, energy-dispersed X-ray microanalysis (EDX), heavy-ion elastic-recoil detection analysis (HI-ERDA) and X-ray photoelectron spectroscopy (XPS). We conclude that there are negligible matrix effects in the SIMS sputter yields of nitrogen and carbon, but that the yields of MoCs+ and CrCs+ ions are significantly affected by the concentration of nitrogen. Thus, the shape of the nitrogen and carbon depth profiles from SIMS can be taken at face value. HI-ERDA gives depth profiles of the same quality as SIMS, but is limited to a relatively thin nitrogen-containing layer. Results from EDX, HI-ERDA and XPS show that the relative concentrations averaged over the treated layer of the main metals in the alloy are the same as in the bulk material. Detailed examination of the peak positions and widths in XPS shows a clear chemical association between the nitrogen in the treated layer and Cr and Mo, and probably also an N–Fe association, but none between nitrogen and Ni. These results support a model of the kinetics of nitriding in austenitic stainless steel.
