## 3 Results  
### Coating Characterization  
The coating microstructures in the as-sprayed and gas nitrided conditions are shown in Figure 1. The as-sprayed coating presents the typical structure of a wire arc sprayed deposit comprising of steel and oxide lamellas with embedded pores (Figure 1a,b). Small oxide particles might break out during the cross-sectional preparation, causing an overall porosity content of 3.6%.  
Figure 1c,d illustrates the gas nitrided coating after etching with Nital. At the surface, a white layer appears with an average thickness of 25 \(\upmu\)m \(\pm\) 9 \(\upmu\)m. This coating area indicates the compound layer. Undernath, the diffusion layer, also called the precipitation layer in the case of alloyed steels, formed up to the substrate. The through porosity in the arc sprayed coatings ensures the penetration and coatings enrichment by the gaseous nitriding media in the applied thermochemical treatment process.  
XRD analysis demonstrates the phase compositions of the 17 Cr steel coatings after finishing in the untreated and gas nitrided state (Figure 2). Before thermochemical treatment (finished f1), the quantitative coating's phase composition results in a mass fraction of  
Figure 1: Light microscopic cross-section images of arc sprayed 17 Cr-steel coatings: (**a,b**) as-sprayed; (**c,d**) gas nitrided after etching with Nital (EA = embedding agent, C = coating, S = substrate, CL = compound layer, DL = diffusion layer).  
63% ferrite, 14% retained austenite, and 23% magnetite (Fe\({}_{3}\)O\({}_{4}\)) using the Rietveld method. The gas nitriding leads to the formation of hexagonal \(\epsilon\)-Fe\({}_{3}\)N and face-centered cubic \(\gamma^{\prime}\)-Fe\({}_{4}\)N phases in the compound layer (finished 1 and gas nitrided). After machining, approximately 150 \(\upmu\)m coating material had been removed. According the XRD analysis (gas nitrided and finished f1), the diffusion layer consists of a mass fraction of 73% ferrite and 27% magnetite; FeN or CrN phases are not detected. However, the etched cross-section (Figure 1c,d) suggests that submicron CrN precipitations in the diffusion layer were formed [22].  
The comparison of the hardness values confirms the formation of precipitates in the diffusion layer (Table 3). The as-sprayed 17 Cr steel layer exhibits an average hardness of 461 HV0.3, which exceeds the coating's hardness values of 250 to 370 HV0.3 of thermal spray steel coatings produced with less oxidation [17; 18; 23]. After gas nitriding, the hardness of the diffusion layer averages 556 HV0.3. In Figure 3, the hardness depth profile indicates that the hardness within the layer is independent of the surface distance, not taking into account the compound layer. Additionally, the hardness was determined by nanoindentation (Table 3), which revealed a 25% harder thin compound layer than the diffusion layer. In contrast to that, the magnetite phase possesses a lower hardness than the diffusion layer; however, the precipitation-hardened and oxide areas exhibit similar hardness values on average. As a result, the hardness of the coating showed less heterogeneity in the surface hardened state, not considering the compound layer, compared to the as-sprayed state.  
Figure 2: X-ray diffraction diagrams of the arc sprayed 17 Cr steel coatings in the finished and gas nitrided state.