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Recent technological development of utilizing an active screen made of solid carbon for plasma-assisted thermochemical diffusion treatments opens up new possibilities for control over the in situ generated treatment environment to guarantee reproducible treatment conditions and material responses. Until now, the investigations of active-screen plasma nitrocarburizing (ASPNC) using an active screen manufactured from solid carbon focused on the influence of a single treatment parameter variation on the material response. In this systematic study, experiments were conducted to vary the H2-N2 feed-gas composition while varying the bias plasma power. The experiments served to better understand a simultaneous variation in the mentioned parameters on the resulting treatment environment and material response during ASPNC of AISI 316L austenitic stainless steel. Therefore, nitriding and carburizing effects in the expanded austenite layer can be obtained. It is shown that an increased nitriding effect, i.e., nitrogen diffusion depth and content, was achieved in case of biased conditions and for H2-N2 feed-gas compositions with higher N2 amounts. On the contrary, an increased carburizing effect, i.e., carbon diffusion depth and content, was achieved in nonbiased conditions, independent from the H2-N2 feed-gas composition. [ABSTRACT FROM AUTHOR]

The effect of a controlled oxygen admixture to a plasma nitrocarburizing process using active screen technology and an active screen made of carbon was investigated to control the carburizing potential within the plasma-assisted process. Laser absorption spectroscopy was used to determine the resulting process gas composition at different levels of oxygen admixture using O2 and CO2, respectively, as well as the long-term trends of the concentration of major reaction products over the duration of a material treatment of ARMCO® iron. The short-term studies of the resulting process gas composition, as a function of oxygen addition to the process feed gases N2 and H2, showed that a stepwise increase in oxygen addition led to the formation of oxygen-containing species, such as CO, CO2, and H2O, and to a significant decrease in the concentrations of hydrocarbons and HCN. Despite increased oxygen concentration within the process gas, no oxygen enrichment was observed in the compound layer of ARMCO® iron; however, the diffusion depth of nitrogen and carbon increased significantly. Increasing the local nitrogen concentration changed the stoichiometry of the ε-Fe3(N,C)1+x phase in the compound layer and opens up additional degrees of freedom for improved process control. [ABSTRACT FROM AUTHOR]

Low-temperature plasma nitrocarburizing treatments are applied to improve the surface properties of austenitic stainless steels by forming an expanded austenite layer without impairing the excellent corrosion resistance of the steel. Here, low-temperature active screen plasma nitrocarburizing (ASPNC) was investigated in an industrial-scale cold-wall reactor to compare the effects of two active screen materials: (i) a steel active screen with the addition of methane as a gaseous carbon-containing precursor and (ii) an active screen made of carbon-fibre-reinforced carbon (CFC) as a solid carbon precursor. By using both active screen materials, ASPNC treatments at variable plasma conditions were conducted using AISI 316L. Moreover, insight into the plasma-chemical composition of the H2-N2 plasma for both active screen materials was gained by laser absorption spectroscopy (LAS) combined with optical emission spectroscopy (OES). It was found that, in the case of a CFC active screen in a biased condition, the thickness of the nitrogen-expanded austenite layer increased, while the thickness of the carbon-expanded austenite layer decreased compared to the non-biased condition, in which the nitrogen- and carbon-expanded austenite layers had comparable thicknesses. Furthermore, the crucial role of biasing the workload to produce a thick and homogeneous expanded austenite layer by using a steel active screen was validated. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract Plasma nitrocarburizing based on active screen technology using a carbon-fiber reinforced carbon active screen was applied in an industrial-scale unit for thermochemical surface treatment of austenitic stainless steel. This concept is based on the use of a solid-carbon-source for the generation of highly reactive process gases directly in the active screen plasma. In this work, plasma nitrocarburizing of AISI 316L stainless steel was performed by means of the variation of the precursor gases H 2 and N 2 in the range of 0% N 2 up to 100% N 2 without the use of any additional carbon bearing gas. For a set of H 2 -N 2 gas mixtures, the resulting reaction gas was monitored using infrared laser absorption spectroscopy (IRLAS). The four main stable species HCN, CH 4 , NH 3 and C 2 H 2 were detected. A detailed analysis of the surface microstructure resulting for each specific H 2 -N 2 precursor gas mixture was performed. This included glow discharge optical emission spectroscopy (GDOES), optical microscopy, micro hardness measurements, as well as X-ray diffraction analysis. The concentrations of hydrocarbons CH 4 and C 2 H 2 were most abundant in case of pure hydrogen plasma, and a carbon-expanded austenite layer with hardness values up to 600 HK0.01 and smooth hardness gradient resulted. The admixture of N 2 to the precursor gas significantly increased the concentrations of HCN and NH 3. Due to this, a duplex structure of nitrogen-expanded austenite γ N and carbon-expanded austenite γ C formed. With increasing content of N 2 up to 50% in the precursor gas, the resulting layer thickness increased and the hardness reached values up to 1300 HK0.01. At strong nitrogen excess in the precursor gas, the nitrogen concentration in the expanded austenite significantly increased, the Fe 2–3 (N, C) phase was formed, and simultaneously the layer thickness decreased. Structure and properties of the expanded austenite layer significantly changed by only varying the H 2 -N 2 ratio of the precursor gas mixture. Highlights • Plasma nitrocarburizing with carbon fiber reinforced carbon as solid carbon source. • Interaction of H 2 -N 2 plasma and carbon generates highly reactive species in-situ. • Reactive species provide surface activation, nitrogen and carbon uptake into steel. • H 2 -N 2 ratio variation in feed gas significantly effects N- and C-gradients in layer. • Infrared laser absorption spectroscopy for reaction gas monitoring [ABSTRACT FROM AUTHOR]