Obstructed and unobstructed growth of expanded austenite layers

该研究项目的目的是研究利用钢制和碳制活性筛对奥氏体不锈钢进行等离子体辅助热化学扩散处理时，膨胀奥氏体层的阻碍和畅通生长的基本方面。重点研究具有不同表面精加工状态的AISI 316L ASS表面处理过程中最重要的挑战之一，即由不同的机制引发的必要的表面活化。这种表面活化是形成厚而均匀的膨胀奥氏体层的先决条件。有源屏蔽技术与热化学处理相结合开辟了独特的可能性，例如根据处理类型产生不同种类的反应物种，通过改变有源屏蔽等离子体功率来调节处理气氛中的物种浓度，以及在处理样品处改变偏置等离子体功率。这些独特的机会使我们能够设计不同的方案，在离子渗氮、离子渗碳和离子氮碳共渗过程中进行表面活化和扩展奥氏体层生长。系统地研究了AISI316L钢不同表面活化机制的有效性，以及N/C等离子体浓度和时间依赖处理对膨胀奥氏体层形成的影响。这涉及使用激光吸收光谱进行原位等离子体分析，以确定等离子体与活性屏幕材料表面相互作用产生的反应气体的类型和浓度。这些数据将与使用表面敏感技术产生的材料响应相关，并将深入分析因N和/或C加入到膨胀奥氏体中而导致的组织演变。结合定量分析方法的结果，对扩展奥氏体层的阻碍和畅通生长有了全面的认识。根据热化学扩散处理的类型和ASS的表面精加工条件，这一知识可以确定产生粗大而均匀的膨胀奥氏体层的最有效的表面激活机制。此外，将考虑到扩展奥氏体形成的现有理论，并考虑到向常规等离子体辅助治疗(无活性筛选)的转移，对这些发现进行评估。

The goal of the research project is studying the fundamental aspects of obstructed and unobstructed growth of expanded austenite layers during plasma-assisted thermochemical diffusion treatment of austenitic stainless steels (ASS) by utilizing steel-made and carbon-made active screens. The focus will be to study one of the most important challenges during surface treatment of AISI 316L ASS with different surface finishing states, namely the necessary surface activation initiated by different mechanisms. This surface activation is a prerequisite to form thick and uniform expanded austenite layers.Combination of active screen technology with thermochemical treatment opens up unique possibilities, such as the generation of different kind of reactive species depending on treatment type, adjustable species concentrations in the treatment atmosphere by varying the active screen plasma power, and variable bias plasma power at the treated samples. These unique opportunities enable to design different scenarios for surface activation as well as expanded austenite layer growth during plasma nitriding, plasma carburizing and plasma nitrocarburizing. The effectiveness of different surface activation mechanisms on AISI 316L and the influence of concentration- and time-dependent treatments by the N/C-supplying plasma on the formation of expanded austenite layers are systematically investigated. This involves the use of laser absorption spectroscopy for in-situ plasma analysis to determine the types and concentrations of reactive gas species generated by the interaction of plasma and active screen material surface. These data will be correlated to the resulting material response using surface sensitive techniques as well as in-depth analysis of the microstructure evolution induced by incorporation of N, and/or C, into the expanded austenite. The full understanding of obstructed and unobstructed growth of expanded austenite layers is obtained by combining the results of the quantitative analysis methods. This knowledge allows the identification of the most effective surface activation mechanism to yield thick and uniform growth of expanded austenite depending on the type of thermochemical diffusion treatment and surface finishing condition of the ASS. Furthermore, the findings will be assessed in view of extending current theories of expanded austenite formation and in view of a transfer to conventional plasma-assisted treatments (without active screen).