*3.2. RSTT against Di*ff*erent Counterbody Materials*

The additives ZDDP are highly optimized to work efficiently on metallic (mostly steel) surfaces as anti-wear agents [12,17–19]. Since in the previous experiments (shown in Section 3.1) balls made of 100Cr6 steel were used as counterbodies, the question remains open, whether the beneficial tribological effect originates from the binding of the additive molecules to the oxidized titanium alloy surface or to the surface of the steel ball. Hence, in a second set of experiments the material of the counterbody was systematically varied in order to use also non-metallic, i.e., oxide- (Al2O3) or nitride-based (Si3N4) ceramics, which feature different chemical compositions and were available in a similar surface quality as the 100Cr6 steel balls.

Figure 5 gathers the results by showing the graphs of the CoF and OM and SEM micrographs of the corresponding wear tracks on polished and fs-laser-processed Ti6Al4V surface after the RSTT in VPX oil + 0.5% RC 3180 against balls made of 100Cr6 steel [left column, (a)], Al2O3 ceramic [middle column, (b)] and Si3N4 ceramic [right column, (c)].

**Figure 5.** Tribological performance of fs-laser-processed Ti6Al4V surfaces after RSTT (normal force 1.0 N, stroke 1 mm, frequency 1 Hz, cycles 1000) in RC 3180-additivated VPX oil (VPX oil + 0.5% RC 3180) against 10 mm balls made of different materials. RSTT against 100Cr6 steel (metallic) counterbody (**a**), Al2O3 (oxide ceramic) counterbody (**b**) and Si3N4 (non-oxide ceramic) counterbody (**c**). The top row provides the coefficient of friction (CoF) as function of the number of sliding cycles. The middle row shows optical micrographs of the corresponding wear tracks after the tribological tests and the bottom row depicts detailed scanning electron microscope (SEM) micrographs taken at the centre of the wear tracks. The capitalized labels indicate tests performed on polished surfaces (**A**) or fs-laser-processed (LSFL-covered) surfaces (**B**). Common scale bars are provided at the left.

For a direct comparison, Figure 5a displays again the results obtained for the 100Cr6 steel balls previously shown in Figure 2 (left column). The CoF's measured in the fs-laser-processed regions (B, blue curves) with both ceramic counterbodies (Al2O3, Si3N4) are as small as those obtained for the metallic steel balls, see Table 3. In all cases, the LSFL endured the RSTT, see the high-resolution SEM images for the associated wear tracks (B) in Figure 5, bottom row. The CoF's recorded in the polished reference surfaces (A, red curves) always start between ~0.4–0.6. However, after less than 200 cycles the CoF's are remarkably lower for the ceramic counterbodies when compared to the metallic steel ball. Nevertheless, the averaged CoF always shows a clear reduction between the polished surfaces and the fs-laser-processed regions, although the reduction ratio is smaller for both ceramic counterbodies. The reduced CoF's on the polished surfaces when using ceramic balls compared to the steel balls are also reflected in the size of the corresponding wear tracks (compare the tracks A in the OM, middle row, Figure 5).


**Table 3.** Mean values and standard deviations of the coefficient of friction (CoF) corresponding to the entire test (RSTT in Ti6Al4V, normal force 1.0 N, stroke 1 mm, frequency 1 Hz, cycles 1000) and resulting wear volumes for different lubricants and tested surfaces [reference (polished) and fs-laser-processed (LSFL-covered)] using different counterbody materials.

The quantitative values of the averaged CoF's along with the estimated wear volumes provided in Table 3 prove that the difference in the counterbody material (chemical composition as well as hardness) and the tribological contact characteristics estimated from an elastic (Hertzian) deformation model [20] (contact pressure, contact radius, sample-ball deformation, see Table 1) are not the dominating effect in our RSTT featuring the beneficial tribological effect. Hence, it is likely that the laser-induced oxidation and the interplay with surface topography are the crucial aspects here. Inferring that a minimum oxygen-containing layer thickness and an enlarged surface area are required together with the additives ZDDP [14] to form a sufficiently thick anti-wear layer during RSTT, which prevents a direct contact of the two sliding tribological bodies. These findings are supported by Figure 6, which visualizes the values gathered in Table 3.

#### **4. Conclusions**

Low spatial frequency LIPSS (LSFL) were uniformly processed by Ti:sapphire fs-laser pulses on Ti6Al4V titanium alloy samples. The tribological performance of the surfaces [reference (polished) vs. fs-laser-processed (LSFL-covered)] was determined in linear reciprocating sliding tribological tests against 100Cr6, Al2O3 and Si3N4 balls as counterbodies in two different lubricants [VPX oil and VPX oil + 0.5% of an anti-wear (ZDDP) additive, RC 3180]. Subsequently, the wear tracks were characterized by OM, SEM, STEM, and confocal profilometry. For the specific testing conditions here, a reduction by a factor of 4–5 for the coefficient of friction and by a factor >2500 for the wear volume was observed in RC 3180-additivated VPX oil, while the tribological tests against 100Cr6 balls showed no beneficial influence of LSFL-covered surfaces with VPX oil lubrication. These new results proved the similar behaviour between VPX oil and paraffin oil (free of any additives) and between RC

3180-additivated VPX oil and commercial VP1 oil containing ZDDP as anti-wear agent. This clearly evidences our previous speculation that only the admixture of the specific ZDDP molecules account for the reproduction of the beneficial tribological behaviour on the laser-processed Ti6Al4V surfaces. Additionally, no significant influence of the counterbody material (metal vs. ceramics) was observed, thereby implying that the positive effect is mainly caused by the presence of the 2-ethylhexyl zinc dithiophosphate molecules in the lubricant along with a nanostructured and oxidized layer on the laser-processed surfaces. The interplay between the sample topography (featuring an enlarged surface area and a confinement of the lubricant) and the local chemistry in the tribological contact area—via formation of a protective surface layer on the LSFL—prevents a direct contact of the two sliding bodies, finally resulting in reduced friction and wear.

**Author Contributions:** Conceptualization, D.S. and J.B.; Methodology, J.B. and D.S.; Validation, all authors; Formal analysis, J.J.A. and N.S.; Investigation, all authors; Resources, J.B., N.S., and D.S.; Data curation, J.J.A. and N.S.; Writing—original draft preparation, J.J.A. and J.B.; Writing—review and editing, all authors; Visualization, J.J.A., N.S., and J.B.; Supervision, D.S., J.B., A.Z., I.L., and A.A.; Project administration, D.S., J.B., A.Z., and I.L.; Funding acquisition, J.J.A., A.Z., J.B. and D.S.

**Funding:** This work was supported by the ERASMUS+ program of the European Union (KA103). This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 814494 (project "i-TRIBOMAT") and grant agreement No. 665337 (project "LiNaBioFluid"). The Surface Technologies research group (Mondragon University, Faculty of Engineering) gratefully acknowledges the financial support given by the Red Guipuzcoana de Ciencia Tecnología e Innovación 2018 program through the project ASEFI (Orden Foral Número 218/2018).

**Acknowledgments:** The authors would like to thank C. Neumann (BAM 6.3) for help with profilometric sample characterizations, S. Benemann (BAM 6.1) for the SEM characterizations, R. Koter (BAM 6.4) for the laser surface processing of a sample copy, and S. Binkowski (BAM 6.3) for polishing the titanium samples. The STEM analysis was performed by I. Dörfel (BAM 5.1). The femtosecond laser processing was performed within the frame of German Science Foundation (Deutsche Forschungsgemeinschaft, DFG) project under Grant KR 3638/1-2.

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