## 3 Results
### Surface Modification of ARMCO\({}^{\circledR}\) Iron  
Figure 4a presents the resulting concentration-depth profiles of N, C, and O as determined by GDOES. The incorporation of oxygen into the material is independent of the admixture of oxygen-containing precursors within the margin of error. Consequently, oxygen was not preferably introduced into the surface despite its higher concentration in the vacuum reactor. However, oxygen had an indirect influence on the layer modification, since the nitrogen and carbon profiles were significantly changed by the addition of oxygen. The maximum concentration of nitrogen reached up to approx. 20 wt% and was not significantly affected by the O\({}_{2}\) addition. However, the depth profile changed, and more nitrogen diffused into the surface with the admixture of O\({}_{2}\) in the feed gas. Thus, the diffusion depth of nitrogen increased from approximately 21 \(\upmu\)m to approximately 28 \(\upmu\)m, resulting in a significantly thicker compound layer. At the same distance from the surface, a higher concentration of nitrogen was present for 6% O\({}_{2}\). No significant increase in the local nitrogen concentration at 3% O\({}_{2}\) was detected. In contrast, the carbon content integrated over the depth was almost identical in all cases. However, the local carbon maximum was reduced from 0.5 wt% to 0.4 and 0.3 wt% and the area of carbon enrichment was pushed further towards the bulk material. In Figure 4b nickel-plated cross sections of the resulting surface layers with different O\({}_{2}\) concentrations are presented. The layers consisted mainly of a sandwich-like structure with columnar \(\varepsilon\)-Fe\({}_{3}\)(N,C)\({}_{1+x}\) grains and a thin layer of \(\gamma^{\prime}\)-Fe\({}_{4}\)N at the interspace of the compound layer to the bulk caused by decreased local nitrogen concentrations. From the micrographs, it can be concluded that the compound layer thickness as well as the porosity was increased by the addition of oxygen. The phase formation as a function of the oxygen content was studied with X-ray diffraction. In Figure 5a, the X-ray diffraction profiles of the untreated ARMCO(r) iron and treated specimens are given. Figure 3: Monitored concentrations of (**a**) HCN, (**b**) NH\({}_{3}\), (**c**) C\({}_{2}\)H\({}_{2}\), and (**d**) CH\({}_{4}\) during the treatment in the industrial scale ASPN reactor with (red) and without (black) the admixture of 6% O\({}_{2}\) in the N\({}_{2}\)-H\({}_{2}\) feed gas. \(T=540\)\({}^{\circ}\)C, N\({}_{2}+\) O\({}_{2}=90\%\), H\({}_{2}=10\%\), \(P_{\text{AS}}=6.0\) kW. The compound layer on top of bcc \(\alpha\)-Fe consists of \(\gamma^{\prime}\)-Fe\({}_{4}\)N (cubic, space group Pm\(\overline{3}\)m) and a derivative of the \(\varepsilon\)-Fe\({}_{3}\)N\({}_{1+x}\) phase (hexagonal, space group P6\(\overline{3}\)22). It is remarkable to note, that the peaks of \(\varepsilon\)-Fe\({}_{3}\)N\({}_{1+x}\) in Figure 5a (except for 00l lines) are atypically broadened. The atypical broadening of the peaks results from the overlapping of a number of \(\varepsilon\)-Fe\({}_{3}\)(N,C)\({}_{1+x}\) peaks being slightly mutually displaced. The displacement is a result of changes in the \(a\) lattice parameter induced by the solution of C in \(\varepsilon\)-Fe\({}_{3}\)N\({}_{1+x}\) to form \(\varepsilon\)-Fe\({}_{3}\)(N,C)\({}_{1+x}\)[28] and the total interstitial content (C + N) [29]. It is assumed that the total (C + N) interstitial content provides the largest effect for the lattice parameter expansion [28]. A detailed investigation of the \(\varepsilon\)-Fe\({}_{3}\)(N,C)\({}_{1+x}\) peak profiles by means of a detailed line-profile analysis, in order to extract lattice parameter distributions, is beyond the scope of this report. Instead, Rietveld refinement was used to estimate the average phase composition of the compound layer.