## 1 Introduction  
Plasma nitrocarburizing (PNC) is an industrially established process applied in order to improve corrosion resistance and fatigue behavior, as well as the wear resistance of engineering materials like plain carbon or tool steels [1]. The production of thick compound layers consisting of the \(\varepsilon\)-Fe\({}_{3}\)(N,C)\({}_{1\times\mathrm{x}}\) phase, which is advantageous under corrosive and tribological conditions, is the objective [2]. In order to stabilize the \(\varepsilon\)-Fe\({}_{3}\)(N,C)\({}_{1\times\mathrm{x}}\) phase and to avoid the formation of the \(\gamma\)-Fe\({}_{4}\)N phase within the compound layer, carbon-containing gas, such as CH\({}_{4}\) is admixed to nitrogen-rich plasmas. However, in addition to the positive effect of small CH\({}_{4}\) additions on the nitriding reaction, in the case of excess carbon, the undesired cementite Fe\({}_{3}\)C phase is formed in the compound layer. Such a cementite formation within nitride layers is detrimental under tribological and fatigue loads and limits the process windows of PNC treatments using carbon-containing gases [3]. On the contrary, by using a solid-phase carbon source, [PERSON] et al. showed that on AISI 4140 steel, compound layers can form, which consist of nearly single-phase \(\varepsilon\)-Fe\({}_{3}\)(N,C)\({}_{1\times\mathrm{x}}\) without Fe\({}_{3}\)C formation [4]. The solid-phase carbon material serves as a chemical source for carbon due to plasma-surface-interactions, forming HCN and various volatile hydrocarbons, such as C\({}_{2}\)H\({}_{2}\), CH\({}_{4}\), C\({}_{2}\)H\({}_{6}\), and CH\({}_{3}\)[5]. The mechanism known as chemical sputtering is characteristic for plasma-discharged carbon materials [6]. Besides hydrocarbons and HCN, NH\({}_{3}\) is formed from the N\({}_{2}\)-H\({}_{2}\) precursor gas and CO from oxygen leakage [7]. In a comparative spectroscopic laboratory study, [PERSON] et al. [8] showed that on plasma-discharged surfaces the use of a carbon-fiber reinforced carbon (CFC) cathode increased the concentrations of carbon-containing species, such as HCN and C\({}_{2}\)H\({}_{2}\), in the resulting process gas by more than one order of magnitude, compared to a steel material with methane gas addition. As a result of this finding, the concept of using a solid-carbon source for nitrocarburizing is implemented for the active screen (AS) technology by replacing the steel screen with a carbon screen made of CFC [9]. Analogously for the upscaled active screen plasma nitriding (ASPN) process, the capability of the carbon cathode was tested. Thereby, the concentrations of highly reactive plasma products were measured as functions of various process parameters, such as the power applied to the carbon screen, nitrogen-hydrogen ratio of the feed gas, pressure, and gas flow [7]. In addition to spectroscopic evidence of the improved reactive gas composition, various material studies show the possibilities of surface modification, using austenitic stainless steels as an example. Thereby, a significantly extended process window for a successful nitrocarburizing process was revealed. In particular, the nitrogen-hydrogen ratio [10] and the bias power at the steel components to be treated [11], as well as the power applied to the AS [12], were identified as factors influencing the resulting steel surface modifications. However, it was also demonstrated that the carbon-containing fraction within the resulting process gas can be varied only within small limits by lowering the plasma power at the carbon screen [12]. In order to exploit the possibilities of the extended process window of AS plasma nitrocarburizing (ASPNC) using a carbon AS for plain carbon and low-alloy steels, further concepts of process control and related parameters influencing the process itself need to be investigated.