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

该项目涉及奥氏体不锈钢板的等离子渗氮，这是一种用于改善表面机械性能的成熟技术。扩散氮化层对耐蚀性和表面导电性的影响仍未得到充分的了解。最近对奥氏体不锈钢进行极短时间（即几十秒）直接等离子体氮化（即无需等离子体预处理）的研究表明，除了扩散层外，直接表面改性对表面性能的形成起着关键作用。该项目的目的是在一种新的表面改性概念下，对铬作为主要合金元素和氮对奥氏体不锈钢的耐腐蚀性和导电性的影响进行基础研究。后者基于两个连续的过程：沉积一种或多种过渡金属（如铬、钛等）的薄层（即氮化温度下氮扩散的渗透性）和等离子体氮化。这项工作是基于工作假设，即即使在极短的时间内直接等离子体氮化，氮化铬也会在奥氏体不锈钢表面形成。这些氮化物对表面性能有很大的影响。此外，假设只有当表面的铬含量消耗到低于13at时，耐蚀性才会恶化。-%由于氮化铬的形成。在等离子体氮化过程中，铬预涂层中的铬通过扩散进入基材来补偿这种损耗。基础研究主要从表面附近扩散层和扩散层体系、表面性质等方面对新型表面改性进行了基础研究。总扩散层的分析将考虑到氮扩散到基材和等离子体氮化的机制，铬与基材之间的扩散过程，元素的分布和相的形成，以及表面状态，包括氮化铬的形成。进一步研究了表面改性对不锈钢表面导电性和耐腐蚀性的影响。通过对工作假设的评估，从而阐明了结构与性能之间的关系，可以获得有关扩散处理奥氏体不锈钢的腐蚀机制和表面性能的基本知识。

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.