Investigation of admixed gas effect on plasma nitriding of AISI316L austenitic stainless steel

Abstract

The influence of discharge gas composition on the plasma nitriding of AISI316L austenitic stainless steel was investigated using a laboratory-made apparatus consisting of a DC diode and a plasma monitoring system. H2-N2 and Ar-N2 gas mixtures with various mixture ratios were introduced in the apparatus, and nitriding was performed without auxiliary substrate heating. A >4 μm-thick surface layer, consisting of an expended austenitic phase, γN, was formed on the AISI316L substrate when the optimized H2-N2 plasma was used. The thickness and maximum nitrogen content of the surface layer increased with an increase in the population of N2+ species in the plasma; the N2+ population in the plasma is considered an underlying parameter for the optimization. Conversely, the penetration of nitrogen into the AISI316L surface was scarce when Ar-N2 plasma was used, irrespective of the gas mixture ratio. Such a phenomenon indicates that Ar-N2 plasma is ineffective for nitriding. However, the effects of H2-N2 plasma, which are not limited to the surface reaction, are beneficial to facilitate nitriding.