2.3.3. Characterization of Tribological Properties

For the tribological evaluation of the surface textures, an Anton Paar Tribometer (MCR301) was used. This device is a ball-on-three-plates tribometer. Figure 2 shows the experimental setup. As a first step, three samples were fixed in the sample holder with an angle of 45◦ regarding the horizontal plane. Secondly, 1 μL of the PAO was put on each sample with a μL-pipette (Eppendorf). Then, the sample holder was fixed on the device and the ball in the ball-holder was placed above the sample. The ballholder was approached to the sample which makes the oil ousted around the tribocontact. Finally, a normal force of 20 N was applied, which corresponds to 9.43 N per sample. When this force is reached, the ball started to rotate and the measurement cycle began. The measurement cycle consisted of 9 Stribeck curves, which were grouped in 3 sections. Furthermore, it consisted of three endurance parts at the beginning of the cycle and in between the Stribeck sections. Their purpose is to avoid run-in behavior. The total normal force always was 20 N and the rotation speed during the endurance parts was 20 rpm which corresponds to a sliding velocity of 0.009 m/s. The Stribeck curves were recorded using a logarithmic ramp. The normal force per sample of 9.43 N is kept constant while the ball starts to rotate and speeds up to a sliding velocity of 1.4 m/s. The COF is recorded during the

entire ramp. Within the test cycle this procedure is repeated nine times such that in the end a mean value and the standard deviation of the COF versus the sliding velocity can be determined.

**Figure 2.** Experimental setup for tribological evaluation. A side view on the left-hand side and a top view on the right-hand side shows the important components of the setup and the contact line (**4**) on the rotating ball (**3**).

## 2.3.4. Surface Characterization

A Laser Confocal Microscope (Olympus LEXT OLS4000) was used to characterize the surface morphology of the laser treated samples. For postprocessing of the images the software MountainsMap was applied.
