## 2 Materials and Methods
### Characterization of the Arc Sprayed 17 Cr Steel Coatings  
The arc sprayed 17 Cr steel coatings were evaluated concerning their microstructure, phase composition, and surface properties using qualitative and quantitative methods. Cross-sections were prepared in the as-sprayed and gas nitrided conditions in order to characterize their microstructure. The etching with the alcoholic nitric acid Nital exposed the nitriding depth. The optical microscope GX51 (Olympus, Shinjuku, Japan) equipped with a SC50 camera (Olympus, Shinjuku, Japan) was used for image capturing. The phase determination was performed by X-ray diffraction (XRD) using a D8 DISCOVER diffractometer from Bruker AXS (Billerica, MA, USA). The samples were measured with a line focus in Bragg-Brentano geometry (10 mm irradiated sample length) and a Lyneye XE-T detector. Co K\(\alpha\) radiation (U: 40 kV; 1: 40 mA) was used to measure in a diffraction angle (20) range of 20\({}^{\circ}\) to 130\({}^{\circ}\) with a step size of 0.02\({}^{\circ}\) and 2.5 s/step. For quantitative phase determination, the Rietveld method was applied using the program TOPAS (Bruker AXS, Billerica, MA, USA). Additionally, the microhardness in the cross-sections (Vickers hardness HV0.3) was measured with a Wilson Tukon 1102 device ([PERSON], Uzwil, Switzerland). Average hardness values of at least five indentations, as well as hardness depth profiles, were prepared with the same load. A distance of 50 \(\upmu\)m for each indentation in combination with an alternating indentation pattern was chosen to create the hardness depth profiles. Furthermore, the hardness of the single phases was evaluated using a UNAT nano-indenter (ASMEC GmbH, Radeberg, Germany) with a Berkovich tip in the quasi-continuous stiffness measurement mode (QCSM) with a maximum load of 0.25 mN. The nanohardness values were determined at the maximum indentation depth. The quantitative surface properties were determined using a stylus instrument hommelteamic T8000 (Jenoptik AG, Jena, Germany). The roughness profiles were recorded in the direction of the feed motion with a stylus having a 5 \(\upmu\)m radius and cut-off wavelength \(\lambda\)c of 2.5 \(\upmu\)m resulting in a measured length of 12.5 mm and a traversed length of 15 mm. The selection was carried out in accordance to DIN ISO 11562. For every treatment condition, two samples with five measurements evenly distributed around the specimen perimeter were analyzed. The filtered profiles were calculated in regards to the average surface roughness \(Ra\) and the surface roughness depth \(Rz\). The surface topography before and after face turning was analyzed by laser scanning microscopy (LSM) with a Keyence VK-X200 device (Keyence, Osaka, Japan). For a detailed investigation of the coating surface after gas nitriding, additional images with a scanning electron microscope (SEM) LEO 1455 VP (Zeiss, Jena, Germany) were recorded. The LSM data were further processed with the image analyzing software MountainsMap(r) (Digital Surf, Besancon, France) to calculate the Abbott curves and valley void volume \(Vvv\) of all faced samples. The valley void volume \(Vvv\) correlates to the oil retention volume in possible tribological applications. This enabled the comparison of the proportion of the aforementioned surface defects between different post-treated surfaces. For the determination, areas of the size 750 \(\upmu\)m \(\times\) 750 \(\upmu\)m were analyzed. A form filter (polynomial 2 nd order) was applied, and the periodic kinematic roughness profile was removed from the surface using \"line orientation\". The Abbott curve of the resulting surface was characterized \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Machine** & **Cutting** & **Cutting Speed** & **Feed \(f\) (mm)** & **Depth of Cut** \\
**Parameter** & **Material** & \(\
u_{\text{C}}\) (m/min) & & \(a_{\text{P}}\) (mm) \\ \hline f1 & CBN & & 0.12 & \\ f2 & CBN & 62 & 0.17 & \(\sim\)0.05 \\ f3 & PCD & & 0.12 & \\ \hline \hline \end{tabular}
\end{table}