*2.3. Tribology Test*

The tribological tests were performed in a dedicated chamber filled with 20 mL commercial motor oil Shell Elix Synthetic 5w-40. The tests consisted of building the Stribeck curves of the textured and untextured surfaces, the last one used as a reference.

To check the load range that has to be applied to fall in BL and ML regimes, some preliminary tests were performed at different loads and velocity, not reported in the manuscript. We realized that, when falling in the BL regime, wear was very severe, and the texture was completely destroyed. To avoid this, we decided to focus on ML regime, assuming that the effect in the BL regime was similar.

In the calculation of the Stribeck parameter, the value of the oil viscosity was set to 1 Pa·s for simplicity, considering that all the tests were performed with the same oil. To change the Stribeck number, two strategies can be applied, namely changing the load or changing the sliding velocity. In the present study, the load was varied from 0.5 to 7.5 N in steps of 0.5 N, keeping the sliding velocity constant at 7 mm/s. The sliding configuration was pin-on-disc, where a flat pin of 2 mm diameter was loaded on a rotating disc (Figure 2). The range of the track radius was between 8.5 and 14 mm. The pins are made of 100Cr6 steel. Each test was repeated at least 3 times, and the results were averaged, leading to the Stribeck curves shown in the Results and Discussions section. The oil was replaced every 3 tests with the new one.

**Figure 2.** Scheme of the pin-on-flat tribological configuration. The flat pin is loaded on the rotating flat disc.

After the Stribeck tests, the line cross-sections of the related wear tracks were measured by a stylus profilometer, to quantify the wear. In Figure 3, a wear-track profile is reported, by way of example. It is possible to identify the worn material (red area) and the pile-up (green area). For each wear track, 4 profiles were acquired, corresponding to the 4 cardinal points of the circular track. The corresponding wear areas were averaged, enabling the calculation of the average wear volume.

**Figure 3.** Line profile of one of the wear track after a tribo-test, by way of example. The red area represents the worn materials, while the green one is the pile-up.
