*5.2. Tribological Performance*

Two different surface textures were investigated regarding their dry friction performance, a line pattern and a cross pattern consisting of 2 perpendicularly crossed lines. The line pattern in Figure 12 was made on the 42CrMo4 + QT specimen contact surface by irradiating 10 ps pulses at 10 MHz and 200 W average laser power and respective 5.0 J/cm<sup>2</sup> laser peak fluence. The lateral distance between the lines in the raster-scanned pattern was 20 μm, the lateral pulse distance within a line was 1 μm. The effective area processing rate for this line pattern textured with a single pass was 34 cm2/min.

**Figure 12.** *Cont*.

**Figure 12.** SEM micrographs showing the line pattern (**a**) before and (**b**) after friction testing and (**c**) the counter body surface after the friction test; the recorded slipping curves (**d**) and a summary of the tribological characteristic of the line pattern texture (table bottom, right) are presented.

In Figure 12a, a high number of molten and re-solidified surface features can be seen following the line-by-line raster scan pattern with a spatial distance of 20 μm. The reason for material melting was the high optical energy deposition onto the material resulting from the high average laser power irradiated at high pulse overlaps within a line. The average surface roughness of the laser textured contact area was measured of *S*<sup>Z</sup> = 12.8 μm, *S*<sup>a</sup> = 1.43 μm and *S*<sup>q</sup> = 1.76 μm. After friction testing, the roughness values have been found almost unchanged of *S*<sup>Z</sup> = 11.6 μm, *S*<sup>a</sup> = 1.57 μm and *S*<sup>q</sup> = 1.87 μm. The SEM inspection of Figure 12b shows slightly compressed features after the friction test while the surface of the counter body appeared corrugated in Figure 12c. A clear indication for this surface corrugation is also given by the surface roughness values where *S*<sup>Z</sup> increased from 2.9 μm to 4.9 μm that was measured before and after the friction test. From the slipping curves presented in Figure 12 d, the static COF was determined of μmax = 0.56 for the line pattern that is about + 87% higher than the reference COF of the non-laser textured contact system. The frictional characteristic of the slipping curve follows Type B.

Aside from the pulse and line distance which were set of 1.5 μm and 40 μm, the laser settings chosen for the line pattern were also applied for the fabrication of the cross pattern shown in Figure 13a. For two scan passes in total the area processing rate was 43 cm2/min. The characteristic topography parameters of the surface features were measured of *S*<sup>Z</sup> = 14.8 μm, *S*<sup>a</sup> = 1.65 μm and *S*<sup>q</sup> = 2.05 μm. After friction testing, the tips seem to be compressed in Figure 13b but the surface roughness parameter were measured of similar dimension as before of *S*<sup>Z</sup> = 13.2 μm, *S*<sup>a</sup> = 1.83 μm and *S*<sup>q</sup> = 2.24 μm. The counter body surface appeared plastically deformed, see Figure 13. The maximum static COF was derived from the Type B slipping curve of μmax = 0.66 that is about + 120% higher than the COF reference value.

**Figure 13.** *Cont*.

*Lubricants* **2020**, *8*, 33

**Figure 13.** SEM micrographs showing the cross pattern made with 2 scan passes (**a**) before and (**b**) after friction testing and (**c**) the counter body surface after the friction test; the recorded slipping curves (**d**) and a summary of the tribological characteristic of the cross pattern texture are presented (table bottom, right).

By further scanning the laser beam, high-aspect ratio surface topographies could be produced. The deterministic surface features presented in Figure 14 have been achieved only by increasing the number of scan passes from 2 to 10. A considerably rougher surface topography was achieved that was measured of *S*<sup>Z</sup> = 47.3 μm, *S*<sup>a</sup> = 8.55 μm and *S*<sup>q</sup> = 9.85 μm. While the processing rate significantly decreased to 9 cm2/min resulting from the higher number of scan passes, only a little higher static COF was determined from the recorded slipping curves of μ<sup>20</sup> = 0.68 or rather + 127% increase compared to the reference contact. The maximum COF, however, was of μmax = 0.86 implying a substantial COF increase up to + 186%.

**Figure 14.** SEM micrographs showing the cross pattern made with 10 scan passes (**a**) before and (**b**) after friction testing and (**c**) the counter body surface after the friction test; the recorded slipping curves (**d**) and a summary of the tribological characteristic of the cross pattern texture are presented (table bottom, right).
