2.3. Surface Roughness of Counterparts and Samples

The surface roughness of the counterparts and tool steel samples was evaluated by AFM. The measurements were carried out at five different areas. The tip with a radius of 7 nm measured the 3D topography of the surfaces. The main roughness parameter was used (arithmetical mean height (Sa)). This parameter expands the profile (line roughness) parameter Ra three dimensionally. It represents the arithmetic mean of the absolute ordinate Z (x, y) within the evaluation area. It provides stable results since the parameter is not significantly influenced by scratches, contamination, and measurement noise. Since a single parameter is not sufficient to adequately determine surface roughness and surface topography, other roughness parameters such as RMS roughness Rq (Root Mean Square), skewness Ssk, and kurtosis Sku have also been determined. According to References [28,29], skewness is the dominant parameter that affects tribological properties. The friction is lower than expected.

A graphical representation of the 3D topography of surface is shown in Figure 6, where the different surface roughness of the balls is clearly visible. This different result of the surface roughness is mainly due to a different kind of material as well as the manufacturing process. The surface roughness of the experimental samples is shown in Figure 7. The resulting roughness values of individual samples before tribological experiments are also listed in Table 5. Samples had similar values. Only X37CrMoV5-1 steel had a higher value where Sa = 501.6 nm. This higher roughness is due to the lowest hardness of the experimental material. Other roughness parameters are also very similar and, thus, their effect on the resulting tribological behavior is difficult to predict.

3D topography of the counterparts measured by AFM, (a) ZrO₂, and (b) X46Cr13.

Surface roughness of experimental samples, (a) X153CrMoV12, (b) X37CrMoV5-1, and (c) X45NiCrMo4.