Obstructed and unobstructed growth of expanded austenite layers

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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).