**3. Results and Discussion**

The tribological performance of the laser-processed surfaces was characterized by means of RSTT in two different lubricants (Section 3.1) and for three different counterbody materials (Section 3.2). Previous works under identical test conditions [10,11] disclosed no significant dependence on the CoF/wear results regarding the sliding direction relative to the orientation of the LIPSS for a metallic counterbody. Thus, the current tribological tests were always carried out in the direction perpendicular to the ablation lines and additionally on a non-irradiated or polished surface (without LIPSS) as a reference. Note that due to an update of the RSTT tribometer, the data acquisition rate was improved. Hence, the CoF results (for paraffin oil and VP1 oil) published in 2014 in Ref. [10] and displayed in the following section for comparison exhibit 10 times less data points.

## *3.1. Reciprocating Sliding Tribological Tests (RSTT) against 100Cr6 in Di*ff*erent Lubricants*

In a first set of experiments the VPX base oil was used in RSTT under identical testing conditions previously used for paraffin oil [10]. This choice of lubricant allows us to take benefit of the anti-oxidation and temperature stabilizing components contained in VPX oil, which are not present in paraffin oil. Figure 1 compiles these new RSTT results (left column) and compares them to the previous results in paraffin oil (right column).

**Figure 1.** Tribological performance of fs-laser-processed Ti6Al4V titanium alloy after reciprocating sliding tribological tests (RSTT) against a 100Cr6 steel ball (normal force 1.0 N, stroke 1 mm, frequency 1 Hz, cycles 1000). The top row exhibits the coefficient of friction (CoF) as function of the number of sliding cycles in VPX oil (**a**) and paraffin oil (**d**). The middle row displays optical micrographs of the corresponding wear tracks after the tribological tests (**b**,**e**). The bottom row shows detailed scanning electron microscope (SEM) micrographs taken at the center (**c**) or at the edge (**f**) of the wear tracks. The data for the RSTT in paraffin oil are taken from [10]. The capitalized labels indicate tests performed on polished surfaces (**A**) or fs-laser-processed (low spatial frequency laser-induced periodic surface structures (LSFL)-covered) surfaces (**B**). Common scale bars are provided at the left.

Figure 1a presents the coefficient of friction as a function of the number of sliding cycles in VPX oil for the measurements made on the polished reference surface (red curve, A) and for tests performed in the fs-laser-processed area covered by LSFL (blue curve, B). The CoF acquired on the polished surface varies around 0.4 ± 0.1 through the entire test. In contrast, the CoF measured in the laser-processed region starts at a significantly lower value of ~0.12 ± 0.02 before it sharply rises after ~80 cycles for paraffin oil and ~100 cycles for VPX oil to a similar level as seen for the reference curve. This sudden rise of the CoF is indicative of the starting damage of the surface when the protecting laser-induced oxide layer along with deeper lying material gets removed and the LSFL have no influence on the CoF behaviour anymore. This is in line with the visual inspections of the corresponding wear tracks shown in Figure 1b,c, where a severe surface damage is seen for both the polished and the laser-processed surfaces tested in VPX oil. The detailed SEM images further confirm that the LIPSS have not endured the tribological tests (compare Figure 1c, A, B). The mean values and corresponding standard deviations of the CoF and the wear volumes, quantified as 1070 <sup>×</sup> 10−<sup>6</sup> mm3 (reference) and ~840 <sup>×</sup> 10−<sup>6</sup> mm3 (LSFL), are listed in Table 2. These new results obtained with VPX oil as lubricant are in line with our previous results reported for paraffin oil (compare with the right column of Figure 1).

**Table 2.** Mean values and standard deviations of the coefficient of friction (CoF) corresponding to the entire test (RSTT in Ti6Al4V against 100Cr6 ball, 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)]. The data for the RSTT in VP1 oil and in paraffin oil are taken from [10], wear volume n.a.—not available.


In order to elucidate the role of the specific additive RC 3180, identical RSTT were performed in VPX oil + 0.5% RC 3180. These results are compared to the ones previously accomplished for the fully formulated commercial engine oil Castrol VP1, including the anti-wear additive zinc dialkyl dithiophosphate (ZDDP) [12]. Figure 2 collects the RSTT results obtained for the additivated VPX oil (left column) and our previous results for VP1 oil (right column).

Figure 2a displays the CoF vs. the number of sliding cycles in RC 3180-additivated VPX oil for the RSTT on the polished reference surface (red curve, A) along with the results obtained on the LSFL-covered surface (blue curve, B). Similar to the behaviour seen previously for VPX oil in Figure 1, the CoF recorded on the polished reference surface varies around 0.45 ± 0.15. However, the CoF recorded in the LSFL-covered region stays rather constant around ~0.1 ± 0.05 through the entire test duration, with some additional spiked features between ~200 and ~600 cycles. The latter are probably caused by worn particles intermittently present in the tribological contact area. The remarkable difference in the CoF between the polished and the LSFL-covered Ti-alloy surfaces is also clearly visible in the wear tracks visualized after the RSTT by optical microscopy (OM) (Figure 2b) and SEM (Figure 2c). The SEM images confirm that the LIPSS widely endured the tribological tests (compare A, B in Figure 2c). Very different wear volumes were found, estimated as ~1040 <sup>×</sup> 10−<sup>6</sup> mm3 (reference) and ~0.35 <sup>×</sup> <sup>10</sup>−<sup>6</sup> mm<sup>3</sup> (LSFL), see Table 2. The results obtained with RC 3180-additivated VPX oil are again very similar to our previous results reported for the fully formulated VP1 oil (compare with the right column of Figure 2). Note that the use of RC 3180-additivated VPX oil results in a lower average CoF-level than that of VP1 oil, supposedly affected by improved data acquisition rate of the tribometer. Moreover, the periods of the partly worn LSFL in wear track B of Figure 2c are somewhat smaller than

the ones for the corresponding track B presented in Figure 2f. This arises from somewhat differently laser fluences locally accumulated through the line processing with a Gaussian laser beam.

**Figure 2.** Tribological performance of fs-laser-processed Ti6Al4V titanium alloy after RSTT against a 100Cr6 steel ball (normal force 1.0 N, stroke 1 mm, frequency 1 Hz, cycles 1000). The top row exhibits the coefficient of friction (CoF) as function of the number of sliding cycles in RC 3180-additivated VPX oil (VPX oil + 0.5% RC 3180) (**a**) and VP1 oil (**d**). The middle row displays optical micrographs of the corresponding wear tracks after the tribological tests (**b**,**e**). The bottom row shows detailed SEM micrographs taken at the center of the wear tracks (**c**,**f**). The data for the RSTT in VP1 oil are taken from [10]. 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.

Comparing the findings presented in Figures 1 and 2 for VPX oil and RC 3180-additivated VPX oil provides a direct evidence that the beneficial tribological performance featured at the fs-laser-processed surfaces covered with LSFL is caused by the anti-wear additive RC 3180 and not by their topographical characteristics. This is also seen in Figure 3, which visualizes the values compiled in Table 2. However, this improved tribological performance is likely promoted by the laser-induced graded oxide layer as it is not present for the polished Ti-alloy surface. While the roughened fs-laser-induced oxide layer on the LSFL consists mainly of amorphous TiO2 and extends ~200 nm into depth, the native oxide layer (<10 nm thick) [13] on the smooth polished surface is too thin to interact efficiently with the 2-ethylhexyl zinc dithiophosphate molecules. It is also interesting to note that compared to the fully formulated/additivated VP1 oil, this beneficial reduction in CoF and wear can already be achieved by solely using RC 3180.

It is worth noting that the initial values of the CoF in the LSFL-covered regions are all comparable at a level of 0.10–0.15 indicating the presence of a protecting oxide layer during the first hundred sliding cycles (see Figure 1a,d and Figure 2a,d). For the lubricants without anti-wear additives (paraffin oil, VPX oil), the oxide layer is worn, while with such additives (VP1 oil, VPX oil + 0.5% RC 3180) the laser-induced oxide layer endures the entire tribological test.

For a deeper characterization of a wear track obtained with an anti-wear additive, we extended our previous STEM analysis [14] by adding spatially information on the elemental contribution underneath the laser-irradiated and worn surface. The corresponding wear track was obtained in the same processed LSFL area as previously shown in Figure 2d,e(B),f(B), but it was tribologically tested parallel to the ablation lines, i.e., perpendicular to the LSFL, with otherwise unchanged RSTT conditions. Figure 4a shows an optical micrograph of the worn surface after an RSTT on LSFL-covered titanium alloy surfaces in VP1 oil. Note that the dark vertical lines arise from the line wise laser processing, while the bright lines in the centre correspond to the wear track. From this tribologically tested surface, a FIB lamella was prepared. The location it was taken from is marked in green in Figure 4b. A cross-sectional STEM image was taken in bright field mode from this lamella, which is presented in Figure 4c. The wear track is marked with a black double arrow. The depth of the zone affected by the laser structuring process and tribological testing can be estimated to be less than 500 nm. This zone is due to the local chemistry in the tribological contact area that creates a protective surface layer in the LSFL. It prevents a direct contact between the two sliding bodies, resulting in reduced friction and wear (Figure 2). The STEM image clearly shows the grain structure of the polycrystalline Ti6Al4V alloy. In all images, the protective Pt layer is marked, which consists of two sublayers. Figure 4d is a magnified detail of Figure 4c, of which elemental maps (EDX) were established for Fe (Figure 4e), Ti (Figure 4f), and Zn (Figure 4g). Signals in the top part of these images are reflections of the Pt protective

layer. Close to the Ti surface, material transfer (Fe) of the steel counterbody is visible (Figure 4e), as well as Zn (Figure 4g) of the ZDDP additive. Figure 4h shows a superposition of the STEM image (Figure 4d) with the elemental maps of Fe, Ti, and Zn. It reveals that within the wear track, small pits have formed, which contain more or less loose debris. In these locations, Zn could be found, which is an indicator for the presence of ZDDP.

**Figure 4.** Characterization of a wear track on LSFL-covered Ti4Al6V after RSTT [identical processed LSFL area as previously shown in Figure 2e(B), but tribologically tested parallel to the ablation lines (perpendicular to the LSFL) with otherwise unchanged RSTT conditions]. (**a**) optical micrograph. (**b**) Focused ion beam (FIB) image with respective lamella and magnified detail. (**c**) STEM cross-sectional image of this lamella. (**d**) magnified detail of (**c**), of which energy-dispersive X-ray (EDX) spectroscopy elemental maps were taken for Fe (**e**), Ti (**f**) and Zn (**g**), (**h**) is an overlay of (**d**–**g**). Images (**a**–**c**) are taken from [14].
