Effectiveness of surface activation induced by different methods during afterglow plasma nitrocarburizing of AISI 316L

Abstract

The current study addresses one of the most important challenges during thermochemical diffusion treatment of austenitic stainless steel with different surface finishing conditions, namely the necessary surface de-passivation initiated by different types of in-situ surface activations. Therefore, the effectiveness of three types of in-situ surface activations applied to polished and to ground surfaces followed by an identical afterglow plasma nitrocarburizing treatment were conducted on AISI 316L austenitic stainless steel. Utilizing a recently developed modified hot-wall reactor with a separately plasma-activated electrode, flexible treatment design with different surface activation types was enabled. Accordingly, treatments were conducted by applying the plasma at an electrode made of graphite bars as well as an electrode made of steel bars with methane addition. Besides, the reactor was further equipped with a laser-based absorption spectroscopy sensor for real-time monitoring and measurement of the gas composition resulting from the discharge at the electrodes during surface treatments. The effectiveness of each applied surface activation combined with a subsequent plasma nitrocarburizing treatment was then evaluated based on the nitrogen and carbon composition-depth profiles. The results highlight that the effectiveness of a surface activation depends on the nature of the applied activation in correlation with the samples' surface finishing conditions. It is revealed that polished surfaces can be activated more effectively than ground surfaces by applying the same activation method.