*2.2. Focused Ion Beam (FIB) Analysis*

Each disc was patterned to have three concentric textured areas with different radius. In each circle, two tribo-tests were performed at most. Similarly, the tribo-tests on the flat surface were made in the textured-free area of the discs.

FIB was employed to explore local properties, allowing to remove and observe trenches of the surface without introducing relevant additional damage, and thus to detect sub-surface micro-structure (buried cracking, defects, contaminations, etc.) [19–21]. Such characterization was performed by using a dual-beam system (FEI StrataTM DB235), combining a high-resolution FIB column equipped with a Ga Liquid Metal Ion Source (LMIS) and a Secondary Electron Microscope (SEM) column equipped with Schottky Field Emission Gun (SFEG) electron source. We obtained perpendicular "micro-cross sections" of samples using a FIB (e-beam = 30 KeV) as a micro-machining miller, setting a 1 nA ion beam current for the first rough trench, and a 300 pA ion beam current for final polishing. Thus, a step-by-step milling procedure was carried out to extract tranches of surface: preliminarily, to prevent the topmost material from mechanical intermixing upon contact with energetic ions [22], the surface sample was capped by a thin platinum layer (1 μm thickness). A Pt-shield had been grown, starting from a gas precursor and using a 300 pA ion beam current that assisted the local deposition. Finally, by tilting the sample holder, we obtained images of discovered walls collecting secondary electrons generated by electron column as a primary beam; in particular, we used electron column as a primary beam for non-destructive secondary electron analysis (e-beam = 15 KeV).
