**3. Results and Discussion**

Figure 4a,b show the top view and the cross-section, respectively, of LIPSS-treated surface of the steel, imaged by SEM. Figure 4a shows the typical morphology of the related texture, characterized by the periodic alternation of parallel grooves and valleys, separated by about 600 nm. This morphology was uniformly distributed all over the processed surface, proving the accuracy and reproducibility of the method. The observation of the texture at a 52◦ tilt angle with respect to the normal revealed a second-order pattern, composed of parallel straight nanostructures perpendicular to the primary-order grooves (Figure 4b).

**Figure 4.** Top (**a**) and cross-section (**b**) view of the LIPSS (laser-induced periodic surface structures) textured surface. E-beam was 15 kV, and the magnification is 20 k.

In the FIB cross-section, the bright Pt protection layer was visible on top of the section. Just under the Pt layer, the height variation of the texture was evident. The peak-to-valley distance was measured for a large number of structures, resulting in between 120 and 300 nm. The sub-surface portion of the texture appeared uniform and compact, with no presence of void or crack, indicating that the laser effect was localized in the very outermost part of the material.

In Figure 5, the results of the tribological investigation were reported, for the patterned surface and the unpatterned flat one, used as a reference. The test was referred to one pin, but similar results were found for other similar pins (not shown). The curves represented the CoF variation in ML regime. As evident from the curves, at the lowest Stribeck number, the system was in the minimum of the ML regime and progressively increased towards the EH region (not evident from the graph). All over the investigated range, the CoF on the patterned surface was sensibly lower than on the unpatterned one; the CoF reduction went from a minimum of 10% for 4–8 <sup>×</sup> 10−<sup>7</sup> m−1, to a maximum of 25% for 5–9 <sup>×</sup> 10−<sup>7</sup> m−1. This was a very important result because the most effective reduction appeared in the most severe condition of the exploited range (namely at low Stribeck number). As already reported in many other studies on surface texturing, this could be ascribed to the reservoir effect of the grooves, which release lubricant even when the two counterparts are in contact, maintaining effective lubrication [23–25].

**Figure 5.** Stribeck curves for flat surface (open black squares) and the textured one (open red circles). Tests were performed in a box filled of oil.

The effect of LIPSS has been studied in terms of wear reduction as well. As evident from Figure 6, the wear of the disc during the Stribeck tests was less pronounced in the case of the textured surface, leading to a wear reduction of about 65%. This was probably related to the role played by the grooves as debris pocket, which limits the amount of particulates all over the interface, and so their effect as the abrasive body.

**Figure 6.** Wear of the disc after the Stribeck tests, for the textured and untextured surfaces.
