3.2.3. Lubricated Test 100Cr6 on 100Cr6

In Figure 6, the results for the lubricated tribology test using the 100Cr6 triboball are shown. Due to the lubricated friction and the adjunctive COF and wear decrease, it is possible to increase the maximum applied load to 1000 mN and the test duration to 1000 s. The oscillating sliding test is performed with a stroke of 2 mm and a frequency of 1 Hz which corresponds a total track distance of 4000 mm. Figure 6a–c shows the coefficient of friction in a 100Cr6/100Cr6 configuration for 100 mN, 500 mN, and 1000 mN respectively. The COF measurement on the polished reference surface reveals no remarkable influence of the applied load force and is stable at 0.14–0.15. For all lubricated test configurations, the initial COF changes with ongoing test duration. Contrary to the dry friction evaluation, this observed break-in effect causes a COF reduction after a specific test duration. While the applied load does not effect the COF on the reference surface, both LSFL- and LBIA-structured surfaces reveal a load dependency for the COF. For a test load of 100 mN, laser surface structuring in general introduce a COF increase. Figure 6a reveals that the highest friction is measured on the LBIA surface for a parallel movement. Starting with a COF of 0.29, the ongoing COF decrease within the test duration implies that the break-in effect, i.e., the ongoing change of the COF until it reaches a steady state, is not finished after 1000 s. For 100 mN, the influence of sliding direction is well pronounced for both LBIA and LSFL surfaces. The initial friction of LBIA-90 is 0.2 and reduces to 0.18. The COF for LSFL-covered surfaces is smaller; for a perpendicular sliding direction it starts at 0.19 and reduces to 0.16 and for a parallel movement it starts with a COF of 0.17 and decline to 0.15. With increasing load force, a remarkable effect is observed, the COF reduces on structured surfaces with increasing load force. For 500 mN and 1000 mN the COF for a parallel movement is smaller on both LBIA- and LSFL-covered surfaces. At the end of the observation time, LBIA surfaces still show an increased COF. According to the reference COF, perpendicular sliding direction on LBIA introduces a 12% increase and parallel movement a 3% increase. LSFL reveal the potential to decrease the COF. While the perpendicular movement on LSFL introduces a COF reduction of only 1%, the friction reduction for parallel movement is 7%. Figure 6c shows that increasing the load force to 1000 mN lead to a further friction reduction for parallel movement on LSFL. After the break-in (approx. 200 s), a stable COF reduction of 12% according to the reference surface can be measured. On the LBIA-structured surface, a small COF increase of 9% remains for perpendicular movement and 6% for parallel movement.

Figure 6d shows the width of the generated wear track on the disc's surface for the lubricated oscillating sliding test with a applied load of 1000 mN. On laser textured surfaces, the wear track width decreases according to the measured COF values, i.e., the highest COF (LBIA-90) leads to the highest wear width and the lowest COF (LSFL-0) leads to the lowest wear track width. The comparison of the magnitude of the wear track width and the contact diameter between the ball and the flat surface shows a pronounced difference. On LBIA-90 the wear track is more than double for a load force of 1000 mN (cf. Table 1). A friction-based temperature rise in the contact zone could lead to elastic–plastic deformation of the involved interfaces [34,35]. It is worthwhile to mention that the similar COF of the reference and the LSFL-90 surface does not result in a similar wear track width. The laser processing generates a thin oxidized passivation layer on the surface [32], which protects the surface against friction-induced damage.

**Figure 6.** Tribological evaluation using 100Cr6 triboballs under lubrication. Temporal evolvement of the coefficient for a load force of 100 mN (**a**), 500 mN (**b**), and 1000 mN (**c**). Wear track width on the 100Cr6 substrate after after 1000 s test duration using a load force of 1000 mN and 100Cr6 triboballs (**d**).

#### 3.2.4. Lubricated Test Tungsten Carbide on 100Cr6

Figure 7 summarizes the results of the lubricated tribological linear reciprocating ball-on-disc evaluation using a 100Cr6 disc and a tungsten carbide triboball for 100 mN, 500 mN, and 1000 mN load force. Comparable to the results for lubricated 100Cr6/100Cr6 experiments, the COF of the polished reference surface remains nearly unaffected over the test duration. After a short break-in, the COF is stable between 0.13 and 0.14. The slight increase of the COF with increasing load force is attributed to the decreasing film thickness of the lubricant between the WC ball and the polished disc surface [36]. For the smallest investigated load of 100 mN, all laser textured surfaces introduce a COF larger (15%–20%) than the reference surface. With increasing load, both laser textured surfaces are characterized by decreasing COF. According to the research of Ben-David et al. [37] or Ma et al. [38], the nonlinear increase of the degree of surface contact with increasing contact pressure of a rough surface is responsible for this effect. Again, the COF of LBIA surfaces stays above the reference surface. LSFL-covered surfaces introduce a COF decrease for an applied load of 500 mN and 1000 mN (c.f. Figure 7b,c). In both load cases, the direction dependency on LSFL vanishes and a COF reduction of 2% and 9% for 500 mN and 1000 mN can be measured, respectively. The COF for LBIA-0 increases with increasing load force. This behaviour is in contrast to all other lasers structures surfaces in this study, and can be expalined by numerical simulations by Zhu and Wang [34,39], who studied the lubricant film thickness between a tribo ball and a surface with linear surface modulations similiar to

those of the LBIA surface. Sliding along the linear features effects a decrease of the film thickness of the lubricant. On the cusps of the LBIA features, however, little or no lubricant film will be spread. Due to the steady contact during sliding, the lubricant locally leaks out leading to an increased friction. The sliding movement transverse to the line shaped structures (LBIA-90) provides more resitance to the lubricant, causing an increase of the average lubrication film thickness between the ball and the rough surface [34,39].

**Figure 7.** Tribological evaluation using tungsten carbide triboballs in a lubricated enviroment. Temporal evolution of the coefficient for a load force of 100 mN (**a**), 500 mN (**b**), and 1000 mN (**c**). Wear track width on the 100Cr6 substrate after after 1000 s test duration using a load force of 1000 mN and tungsten carbide triboballs (**d**).

The increased and decreased friction caused by LBIA and LSFL also affects the generated wear track width shown in in Figure 7d. As mentioned above, the increased friction on LBIA leads to a temperature increase and thus to an elastic–plastic surface deformation. Figure 7d also reveals that the COF reduction on LSFL-covered surfaces leads to a wear reduction on the 100Cr6 disc. According to the reference surface, a reduction wear track width of up to 42% is possible using LSFL and parallel movement.

## **4. Conclusions**

This research work focused on femtosecond laser surface texturing on 100Cr6 steel using two different methods for the generation of periodic surface structures. Both methods produce a linear pattern on the surface. The LSFL approach fabricates a periodic surface pattern with a spatial period of ≈900 nm and a modulation depth of ≈200 nm. Using the LBIA technique, a surface modulation with a period of 1.5 μm and a depth of ≈1.6 μm is generated. To compare the potential of friction modification, a tribological linear reciprocating ball-on-disc evaluation is performed using 100Cr6 steel and tungsten carbide (WC) triboballs. The effects of sliding direction and load force are evaluated against a polished reference surface for dry and lubricated conditions. Table 3 summarizes the observed effects with respect to the reference surface, highlighting the potential fields of applications for LBIA and LSFL.

**Table 3.** Summarized modification possibilities with respect to a polished reference surface indicating no influence (0), increase (+), strong increase (++), decrease (-), and strong decrease (- -) of the coefficient of friction.


Although LBIA and LSFL generate surface structures with similar periodicity, the overall tribological behaviours are different. For the tribological analysis without lubrication, LSFL causes a COF increase for both the 100Cr6/100Cr6 and 100Cr6/tungsten carbide combinations. The perpendicular movement on LBIA structures generates COF comparable to the reference surface. Parallel sliding direction on LBIA surfaces decreases the friction remarkably for both 100Cr6 and tungsten carbide triboballs. Comparing the lubricated friction behaviour of LSFL and LBIA shows the possibility of COF reduction using LSFL-covered surfaces up to 12% for 100Cr6/100Cr6 and up to 9% for 100Cr6/tungsten carbide configuration. An additional benefit of LSFL-covered surfaces in the introduced test scenario is the reduced wear track width. In contrast to the COF reduction introduced by LSFL for lubricated friction, LBIA lead to a COF increase for both 100Cr6 and tungsten carbide triboalls under lubricated test conditions.

**Author Contributions:** Conceptualization, S.R. and K.B.; Methodology, S.R., K.B., S.S. and J.-H.K.-W.; Software, S.R.; Validation, K.B. and F.K.; Formal analysis, S.R.; Investigation, S.R., K.B. and F.K.; Resources, P.S. and R.H.; Writing—original draft preparation, S.R., K.B. and F.K.; Writing—review and editing, S.R., F.K., P.S., J.-H.K.-W., C.E. and R.H.; Visualization, S.R., K.B. and F.K.; Supervision, P.S., C.E. and R.H.; Funding acquisition, P.S. and R.H.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
