On the occurrence of dual diffusion layers during plasma-assisted nitriding of austenitic stainless steel

This paper is devoted to the investigation of the formation of dual diffusion layers during low-temperature (T < 420 °C) plasma-assisted nitriding (PAN) of austenitic stainless steels. New experiments are presented and compared with previous published results. It is shown that a cleaning pre-treatment in Ar–H2 before PAN produces a double-layer structure. Two distinct layers with well-defined interface are observed by SEM. SNMS depth profile measurements show that the inner layer is due to a carbon solid solution phase (γC) located at the interface with the bulk substrate and the outer γN layer. It is shown that carbon contamination is responsible for the formation of this γC sublayer. Carbon contamination is observed only when an in situ cleaning procedure in Ar–H2 is performed before a nitriding treatment. A discussion on the origin of this carbon contamination is presented and it is also explained why it occurs in Ar–H2 discharges and not in N2–H2 discharges.