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在很大程度上还不清楚。等离子体直接渗氮的研究进展（即未经等离子体预处理），（即几十秒）与它们用于质子交换膜燃料电池的双极板的潜在用途相关的研究表明除了扩散层之外，直接表面改性可以对表面性能的形成起到关键作用。该项目的目的是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，是表面改性的新概念。后者是基于两个连续的过程：沉积一层薄的（即在氮化温度下可渗透氮扩散的）一种或多种过渡金属（如铬、钛等）层，和等离子渗氮。这项工作是基于工作假设，铬氮化物形成的奥氏体不锈钢表面上，即使在极短的时间直接离子渗氮。这些氮化物对表面性质有很大的影响。此外，假定仅当表面上的铬含量耗尽到小于13原子%时，耐腐蚀性劣化这是由于形成了氮化铬。在等离子渗氮过程中，这种消耗应该通过铬预涂层中的铬扩散到基材中来补偿。基础研究的重点是新的表面改性的基础研究相对于扩散层和扩散层系统，分别在表面附近，和表面性质。考虑到氮向基材中的扩散和等离子体渗氮的机理、铬与基材之间的扩散过程、元素的分布和相的形成以及包括氮化铬形成的表面状态，将分析总扩散层。此外，表面改性对不锈钢表面的导电性和耐腐蚀性的影响将被执行。通过评估工作假设，从而阐明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理和表面性能的基础知识的相关贡献。

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.