## 3 Results and Discussion
### Surface Roughness
The surface structure can also be assessed on the basis of the vector distribution of microgrooves. In the case of the burnished surface (experimental run no.9), these observations only confirm the earlier observations regarding the anisotropic structure. The density of peaks Spd is a parameter included in the group of features. It expresses the number of elevations per unit area. The Spd parameter determines the density of bumps on the tested surface. In the case of the base surface, the value of the Spd parameter was 45.9 1/mm\({}^{2}\). The burnishing process reduced this parameter. Only in experimental run no. 7, it increased slightly to Spd = 52.2 1/mm\({}^{2}\). In other cases, it ranged from 1.93 to 30.7 1/mm\({}^{2}\). The arithmetic mean peak curvature Spc is another parameter from the group of features. A smaller value of the parameter indicates that the tops of the surface are characterized by rounded shapes, while higher values indicate their \"sharp\" character. Depending on the value of the Spc parameter, various types of surface deformation may occur, such as elastic, elastic-plastic, and plastic. For the base specimen, the Spc parameter reached a value of 59.3 1/mm. This parameter increased for specimens where the burnishing force was \(P=40\) N. In other cases, the peaks of the burnished surfaces had a more rounded shape  
Figure 8: Abbott–Firestone curve (**a**) for base surface and (**b**) after burnishing \(P=60\) N, \(f=1000\) mm/min (experimental run no. 8). than the base specimen. The lowest value of the parameter \(\mathrm{Spc}=28.5\,1/\mathrm{mm}\) was obtained for experimental run no. 7, which was also characterized by the highest density of vertices. Based on the obtained test results, it can be concluded that the smallest value of the burnishing force \(P=40\) N, although it contributes to the reduction of height parameters compared to the base specimen, at the same time causes a number of surface damages in the form of furrows, scratches, and build-up. The situation is much better in the case of the other two force values, i.e., \(P=50\) N and \(P=60\) N. The reduction of the height of irregularities is then clear and stable (Sa in the range of 0.333 \(\mathrm{\SIUnitSymbolMicro m}\)-0.666 \(\mathrm{\SIUnitSymbolMicro m}\)). However, it can be noticed that the maximum smoothing of the surface was obtained in variants 6 and 9 (0.333 \(\mathrm{\SIUnitSymbolMicro m}\) and 0.359 \(\mathrm{\SIUnitSymbolMicro m}\)), i.e., corresponding to the force of \(P=50\) N and \(P=60\) N and the highest feed rate \(f=1500\) mm/min. It seems that the use of input parameters at the mentioned levels will ensure obtaining optimal GSS results. Due to the fact that there is a high statistical correlation between some roughness parameters, only the parameters carrying the most information about the process were adopted for the analysis of optimization of burnishing process parameters. In the case of height parameters between the parameters Sa, Sz, Sv, Sp, and Sq, the Pearson correlation coefficient ranges from \(r=0.90\) to \(r=0.99\); therefore, the coefficient Sa and Sku, correlated with Sa to the degree of \(r=0.14\), were selected for further analysis. The spatial parameters Str and Sal, correlated with each other in the degree of \(r=0.82\), were omitted in the further analysis due to the high value of Pearson's correlation (\(r=0.84\)) with the parameter Sa. Due to the fact that the hybrid parameters Sdr and Sdq are correlated with each other to the degree of \(r=0.99\) and that the parameter Sdr is correlated with the parameter Sa to the degree of \(r=0.98\), they were also omitted from further analysis. To optimize the parameters of the burnishing process, apart from the Sa and Sku parameters, the \(\mathrm{Spc}\) feature parameter correlated to a degree of \(r=0.72\) with the Sa parameter was selected. Table 8 presents the results of the significance analysis (ANOVA test--\(p\)-value) of the impact \(f\) and \(P\) on the \(Sa,\ Sku\), and \(Spc\) parameters.