*3.2. Friction Measurement*

In the following diagrams (Figures 3 and 5–7), the resulting COF is shown for the reference surface (black solid curve), the LBIA-structured surfaces (red curves) and the LSFL-covered surfaces (blue curves). Furthermore, the relation between the periodic surface structures and the sliding direction is indicated by the line style. For a sliding direction along the periodic grooves, dotted lines and the index "−0" are used. Dashed lines represent a perpendicular sliding movement which is also indicated by the added index "−90". The presented COF values are calcuated by the average of 50 s of steady measurement of the applied force and the induced lateral force. The shown error bars in Figures 3 and 5–7 represent the deviation of the calculated COF for a time range of 50 s.

## 3.2.1. Dry Test 100Cr6 on 100Cr6

Figure 3 shows the results of the tribological linear reciprocating ball-on-disc evaluation using a 100Cr6 disc and a 100Cr6 triboball for 50 mN and 200 mN load force. The oscillating sliding test is performed for 500 s with a stroke of 2 mm and a frequency of 1 Hz which corresponds a total track distance of 2000 mm. On the reference surface, the COF starts at values below 0.2 and rises slightly with ongoing runtime. After 300 s, the COF rises strongly and approaches a steady state after 450 s with a COF of 0.8. This COF incline can be described by a damage of the natural superficial oxide layer on the surface [32]. With increasing load force, the COF incline starts already within the first 50 s and after a total run time of 150 s, the steady state is nearly reached at a COF of approx. 0.9–1. Surfaces covered by LSFL reveal a comparable high COF directly at the beginning of the linear reciprocating movement. For the movement parallel to the LSFL orientation, the COF starts with a value of 0.65 and increases with time to a value of approx. 0.93. The perpendicular movement on LSFL show higher values for the COF over the entire test duration, starting at 0.81 and with a steady state value of 1. Contrary to the LSFL-covered surface, the LBIA structures introduce only a small COF increase at the beginning of the tribological test. For a perpendicular movement, the COF starts with 0.3 and 0.5 for 50 mN and 200 mN respectively. The break-in effect, which is defined as the initial COF modification by ongoing test duration, is visible for both load forces: For 50 mN the steady state is reached after 400 s and for 200 mN after 250 s. While LSFL-0, LSFL-90, and LBIA-90 result in a COF increase over the entire linear reciprocating ball-on-disc evaluation for both load forces, LBIA structures combined with a parallel movement (LBIA-0) result in a stable COF value at approx. 0.2 for both 50 mN and 200 mN.

**Figure 3.** Temporal evolvement of the coefficient of friction using 100Cr6 triboball without lubrication for a load force of 50 mN (**a**) and 200 mN (**b**).

The differences of the coefficient of friction for LSFL- and LBIA-covered surfaces are also visible in the microscopy image after the linear reciprocating ball-on-disc evaluation. The high COF value for laser surface texturing using LSFL implies a strong interaction between the disc and the triboball. Figure 4a shows the borderline between the initial LSFL and the friction affected area using a parallel movement and a load force of 50 mN. In the sliding track, LSFL vanished completely and big artefacts caused by a strong material adhesion and galling [33] are visible. As indicated by the small COF in

Figure 3a, the wear on the LBIA surface after the parallel movement in Figure 4b is less pronounced. The wear track is visible but the laser surface texturing is still present. Along the complete stroke, no artefacts of material adhesion and galling are visible.

**Figure 4.** Wear track after dry linear reciprocating evaluation with a load force of 50 mN and a duration of 500 s on LSFL-0 (**a**) and LBIA-0 (**b**) covered surfaces.
