## 3 Results
### Comparison of the Surface Properties after Gas Nitriding and Facing  
The influence of the surface hardening and the machining process on the surface properties is illustrated in Figure 4 using LSM images of the sample surfaces. The finished surfaces (Figure 4a-d) are characterized by feed marks, opened pores, cracks, and a high valley void volume resulting from coating material pulled-out during the cutting process. The opened pores reflect the coating porosity, but the cracks can also be caused by either the gas nitriding or the finish-machining step. Using the same machining parameter (f1) before and after nitriding leads to a significant increase in the quantity of pulled-out coating material in the surface hardened condition (Figure 4a,b). The heterogeneous hardness distribution in the just-faced coatings promotes deformation, while the diffusion layer predominantly chips. An increase of the feed or change of the cutting material from CBN to PCD (Figure 4c,d) reduces the volume of pulled-out material. Similar findings were identified in [11]. In contrast to that, the finished and gas nitrided sample (Figure 4e,f) shows the characteristic nubby surface of the hard compound layer. Predominately, the nitrides were formed at the edge of pores and in cracks, and cover the surface nearly completely. The topography of the different treatment states recorded by laser scanning microscopy is shown in Figure 5 by means of Abbott curves. According to the diagram, a significant increase in the volume fraction of the valleys could be achieved by the process combination of gas nitriding and subsequent face turning. The greatest increase was achieved when using a CBN-tipped indexable insert at a low feed (machining parameter f1), which coincides with the parameter used for facing the as-sprayed coating. Furthermore, the curves show a good correlation with the surface images shown in Figure 4. As a result, an increase of the \(Rz\)-value can be related to an increase in pulled-out material areas (Table 4). While, for surface-hardened variants, the profile heights decreased slightly, and generally smaller topographical differences were observed, surface hardening in the finished turned state has no influence on the proportion of removed material. The grown nitrides slightly increased the profile heights. However, the ro  
Figure 3: Hardness depth profiles of the arc sprayed 17 Cr steel coating in the as-sprayed and gas nitrided state. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Hardness**} & **As-Sprayed** & **Gas Nitrided** \\ \hline Microhardness (HV0.3) & Total coating area & 461 \(\pm\) 50 & 556 \(\pm\) 114 [1] \\ \hline \multirow{3}{*}{Nanoindentation hardness (GPa)} & Compound layer (CL) & - & 10 \(\pm\) 2.5 \\  & Steel lamellas/ & \multirow{2}{*}{6 \(\pm\) 1} & \multirow{2}{*}{8 \(\pm\) 1} \\  & Diffusion layer (DL) & & \\ \cline{1-1}  & Oxide & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The hardness of the arc sprayed 17 Cr-steel coatings in the as-sprayed and gas nitrided condition. \(Rz\) are almost comparable to the finished surface in the as-sprayed condition (machining parameter f1, Table 4). Figure 4: Laser scanning microscope images of the finished surfaces of arc sprayed 17 Cr-steel coatings: (**a**) finished with machining parameter f1; (**b**) gas nitrided and finished with f1; (**c**) gas nitrided and finished with f2; (**d**) gas nitrided and finished with f3; (**e**,**f**) finished with f1 and gas nitrided with (**f**) showing an SEM image recorded with the secondary electrons (SE) detector. The determined values of the valley void volume \(Vvv\) shown in Table 5 confirm the previously described behavior regarding the proportion of pulled-out coating material. The lowest proportions are reached after machining the as-sprayed coating. After nitriding and subsequent machining, the lowest proportion of pulled-out coating particles is achieved when using a PCD-tipped indexable insert and a small feed (machining parameter f3, \(f\) = 0.12 mm).