Abstract
Active screen plasma nitrocarburizing (ASPNC) increases the surface hardness and lifetime of austenitic stainless steel without deteriorating its corrosion resistance. Using an active screen made of carbon opens up new technological possibilities that have not been exploited to date. In this study, the effect of screen power variation without bias application on resulting concentrations of process gas species and surface modification of AISI 316L steel was studied. The concentrations of gas species (e.g., HCN, NH3, CH4, C2H2) were measured as functions of the active screen power and the feed gas composition at constant temperature using in situ infrared laser absorption spectroscopy. At constant precursor gas composition, the decrease in active screen power led to a decrease in both the concentrations of the detected molecules and the diffusion depths of nitrogen and carbon. Depending on the gas mixture, a threshold of the active screen power was found above which no changes in the expanded austenite layer thickness were measured. The use of a heating independent of the screen power offers an additional parameter for optimizing the ASPNC process in addition to changes in the feed gas composition and the bias power. In this way, an advanced process control can be established.
Highlights
Surface engineering of austenitic stainless steels has become popular to improve the tribological behavior, in particular in corrosive environments [1,2]
In order to avoid the formation of chromium nitride and carbide phases, which are detrimental to the corrosion resistance of the material, thermochemical treatments are typically performed as low temperature processes [2,7]
The present study found that the minimal concentration of 1.5 × 1015 molecules cm−3 can produce nitrocarburized layers of sufficient quality
Summary
Surface engineering of austenitic stainless steels has become popular to improve the tribological behavior, in particular in corrosive environments [1,2]. Thermochemical surface treatments, which enrich the surface layer with nitrogen and/or carbon leading to enhanced wear properties without deteriorating their excellent corrosion behavior, are of growing significance [3]. Due to the characteristic passive layer, which prevents the diffusion of elements like nitrogen and carbon, the activation of the self-passivating surface is of particular importance [4]. Plasma-assisted technologies like plasma nitriding (PN), plasma carburizing (PC), or plasma nitrocarburizing (PNC) have proven to be useful for both in situ sputtering of the passive layer and providing nitrogen and/or carbon for diffusion [5,6]. Thereby, a layer of metastable supersaturated solid solution, Coatings 2020, 10, 1112; doi:10.3390/coatings10111112 www.mdpi.com/journal/coatings
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