Basic research on surface modification of austenitic stainless steel sheets by means of combined coating and plasma diffusion vacuum processes

• Applicant: Professor Dr. Claus-Peter Klages
• Subject Area: Coating and Surface Technology ; Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: from 2015 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 266379143

The project deals with plasma nitriding of austenitic stainless steel sheets which is an established technology used to improve the surface mechanical properties. Still largely not understood is the influence of the diffusion nitrided layers on the corrosion resistance and surface electrical conductivity. Recent studies on the direct plasma nitriding (i.e. without plasma pre-treatment) of austenitic stainless steels for extremely short time (i.e. for a few tens of seconds) related to their potential use for bipolar plates for proton exchange membrane fuel cells suggest that beside the diffusion layer, the direct surface modification can play a key role for the formation of the surface properties.The aim of the project is a fundamental study of the influence of chromium as a main alloying element and the nitrogen on the corrosion resistance and electrical conductivity of austenitic stainless steels within a new concept for surface modification. The latter is based on two consecutive processes: deposition of a thin (i.e. permeable for the nitrogen diffusion at the nitriding temperature) layer of one or more transition metals (such as chromium, titanium, etc.) and plasma nitriding. The work is underlying the working hypothesis that chromium nitrides form on the surface of austenitic stainless steels even during extremely short-time direct plasma nitriding. These nitrides have a great influence on the surface properties. Further, it is assumed that the corrosion resistance is deteriorated only when the chromium content on the surface depletes to less than 13 at.-% due to formation of chromium nitrides. This depletion should be compensated by the chromium from the chromium pre-coating through its diffusion into the base material during the plasma nitriding. The basic research is focused on the fundamental study of the novel surface modification with respect to the diffusion layers and diffusion layer systems, respectively, nearby the surface, and the surface properties. The total diffusion layers will be analyzed taking into account the nitrogen diffusion into the base material and the mechanism of the plasma nitriding, diffusion processes between chromium and the base material, the distribution of elements and the phase formation, and the surface state including the formation of chromium nitrides. Furthermore, the influence of the surface modification on the electrical conductivity and the corrosion resistance of the stainless steel surfaces will be performed. By means of evaluating the working hypothesis and hence elucidation of the relationship between the structure and properties a relevant contribution of basic knowledge regarding the corrosion mechanisms and surface properties of the diffusion-treated austenitic stainless steels can be acquired.

• DFG Programme: Research Grants