## 4 Discussion  
The feasibility of an oxygen admixture to an N\({}_{2}\)-H\({}_{2}\) plasma in an ASPN process using an CFC active screen was investigated. Additions of O\({}_{2}\) and CO\({}_{2}\) significantly reduced the concentrations of hydrocarbons and CN-species, but increased the CO, CO\({}_{2}\), and H\({}_{2}\)O concentrations. Furthermore, the NH\({}_{3}\) concentration was increased since more sputtered carbon formed CO. As a result, more unbound hydrogen and nitrogen were freely available in the plasma to form NH\({}_{3}\). At very high oxygen concentrations, however, the hydrogen was oxidized to H\({}_{2}\)O and, therefore, no longer available for the formation of NH\({}_{3}\), which, in turn, decreased the concentration in the process.  
Furthermore, long-term in situ monitoring has shown negligible trends in the concentrations of major reaction products over the typical duration of ASPNC treatments. Without oxygen addition, it was found that there was a non-negligible oxygen content in the reactor, which is always present during plasma processes due to the use of technical gases, leakage and the contamination of surfaces. The predominant oxidation product was CO.  
As the admixture of oxygen-containing species impacted the concentration of reactive species in the process, we further studied its effect on the material treatment results. The addition of oxygen progressively lowered the concentration of HCN and C\({}_{2}\)H\({}_{2}\); however, the formed species (CO, CO\({}_{2}\)) are known to facilitate carburization of metal surfaces in conventional PNC.  
During the material treatment of ARMCO\({}^{\circledR}\) iron with an admixture of 3% and 6% O\({}_{2}\), respectively, no excessive oxygen enrichment was detected in the nitrided layer. In contrast, the nitrogen content in the compound layer increased significantly, which increased both the concentration and the diffusion depth, resulting in a thicker compound layer and higher porosity. The integral carbon content over the depth was almost identical in all cases. However, the maximum carbon concentration in the compound layer (0.5 wt%) was lowered (0.4 and 0.3 wt%) and shifted further towards the base material in the case of the O\({}_{2}\) admixture. Both effects could ensure that the potential formation of cemite in plain carbon or low-alloyed steels is reduced by increased nitrogen and reduced carbon activity. Future in-depth optical emission spectroscopy investigations are required to gain more insights on this.  
The compound layer on ARMCO\({}^{\circledR}\) iron consisted mainly of \(\varepsilon\)-Fe\({}_{3}\)(N,C)\({}_{1+\chi}\) and small fractions of \(\gamma^{\prime}\)-Fe\({}_{4}\)N and was not significantly changed with regards to the phase composition.  
Compared to conventional plasma nitriding, where the formation of oxide layers above a critical oxygen concentration hinders nitrogen and carbon diffusion, high oxygen contents in the reaction gas of ASPNC processes operating a solid carbon precursor, such as CFC, do not negatively affect the material treatment. Rather, the positive changes on the plasma chemistry are to be emphasized. The reason could be due to the high carbon concentration and species concentration in the plasma. Oxygen is preferentially bound as CO and, thus, obviously does not tend to form an oxide layer. However, other reactive species, such as HCN, could ensure that a newly formed oxide layer is removed directly, so that a thick and continuous oxide layer cannot generate.