*4.4. Tribological Performance Test*

For tribological performance testing, the self-organizing laser textures were produced on the ring-shaped contact surface of the test specimens described above in Section 3. The test specimens were made of molybdenum alloy steel 42CrMo4 + QT which is a high-grade steel widely used in engineering purposes. Preliminary tests reveal that the microscopic surface textures produced on austenitic chromium-nickel steel 1.4301 metal plates for Raman and cross section analysis develop of similar topographical shape on 42CrMo4 + QT specimen material. The COF values given in the following represent mean values averaged over 3 individual measurements for 100 MPa surface pressure in the contact area under dry friction condition. As a reference, two non-laser treated test specimen with fine grinded surface topography were tested, showing a static COF of μmax = 0.30 and Type A frictional characteristic for 100 MPa contact surface pressure [37]. The corresponding slipping curve for the non-laser textured contact system will be given in the following plots (indicated by a red line) for reference purposes.

The self-organizing CLP texture shown in Figure 9a was made by crossover scanning a near-infrared laser beam of 10 ps pulse duration, 15 W average laser power at 1 MHz pulse repetition frequency and 3.7 J/cm2 laser peak fluence. The hatch and line distance of the raster-scanned cross grid was kept constant of 10 μm. The effective area processing rate was considerably low of 0.04 cm2/min that resulted from the high number of scan crossings applied to produce this special shape of CLPs. The maximum height *S*<sup>Z</sup> and the root mean square height *S*<sup>q</sup> as representative surface roughness values were measured of *S*<sup>Z</sup> = 21.4 μm and *S*<sup>q</sup> = 3.46 μm, respectively.

In the frictional tests, the CLP textures were tested against a counter body with a fine grinded (non-laser treated) contact area of *S*<sup>Z</sup> = 2.9 μm and *S*<sup>q</sup> = 0.28 μm average surface roughness. The recorded slipping curves presented in Figure 9d indicate a tribological characteristic according to Type A. The summary of the tribological characteristics of the CLP texture is provided in the table of Figure 9 (bottom, right) showing static COFs of μ<sup>2</sup> = 0.42 and μ<sup>20</sup> = 0.44. The maximum static COF was determined of μmax = 0.46 that is about + 53 % higher than the COF reference value of 0.30. In addition, SEM micrographs taken from the CLP texture and the contact area of the counter body after the friction test are shown in Figure 9b,c. The CLPs appeared squashed after the testing and scratches with a maximum depth of 5.6 μm can be observed on the counter body contact surface.

**Figure 9.** SEM micrographs showing the CLP texture (**a**) before and (**b**) after friction testing and (**c**) the fine grinded counter body surface after the friction test; the recorded slipping curves (**d**) and a summary of the tribological characteristic of CLPs (table bottom, right) are presented.

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

The LSFL ripples presented in Figure 10a were produced on the fine grinded specimen surface by irradiating ultrashort pulses of 400 fs pulse duration at 1.03 MHz pulse repetition frequency and 7.9 J/cm<sup>2</sup> laser peak fluence. The laser beam was raster-scanned in a single line-scan pattern where the line distance was set of 10 μm. The effective area processing rate was 18 cm2/min obtained at 10 m/s laser beam moving speed. The height of the ripples could be estimated of about 0.4 μm in Section 4.2. The LSFL ripple textured specimen were tested against another rippled surface textured with the same laser parameter set. The testing against a laser textured counter body was due to the fact that the roughness of the fine grinded surfaces was significantly larger than the ripple height and thus no significant effect on the friction performance was expected for the test of ripple textured surfaces against surfaces of larger roughness. The slipping curve recorded for the ripple versus ripple contact system reveal a clear increase of the static COF of μ<sup>2</sup> = 0.41 at the very beginning until the maximum of μmax = 0.42 was reached. However, with further displacement a rapid drop can be seen in the curve with COF of about the non-laser textured reference value of μ<sup>20</sup> = 0.29. This result for the stainless steel alloy is a bit contrary to our expectations as higher friction forces were reported for ripple textured silicon [51] as well as the hypothesis of a potential enhancement of the adhesion part of friction resultant from the lower roughness of the ripple textured surfaces. Figure 10a–c shows the LSFL textured surface before and after the friction test, the underlying fine grinded surface structure is still apparent. On both test specimens, the ripple texture smeared, or rather was pushed away during frictional testing. The recorded slipping curve in Figure 10d indicates a COF Type A.

**Figure 10.** SEM micrographs showing the LSFL ripple texture (**a**) before and (**b**) after friction testing and (**c**) the rippled counter body surface after the friction test; the recorded slipping curves (**d**) and a summary of the tribological characteristic of LSFLs (table bottom, right) are presented.
