*2.1. SLIPS Fabrication*

The nanostructured pseudoboehmite AlOOH coating was obtained by deposition of an alumina suspension synthesized via a previously reported sol-gel route [25]. Aluminum samples (Al 1050 alloy) were used as substrates; prior to deposition, they were cleaned with an ultrasonic bath in ethanol for 5 min and dried in air. The synthesized alumina nanoparticles were deposited on the substrates via dip-coating in controlled conditions, followed by treatment with boiling water. The complete procedure for the coating fabrication is reported elsewhere [26]. Pseudoboehmite-coated samples were observed by Scanning Electron Microscopy (FESEM Gemini Columns SIGMA, Carl Zeiss Microscopy GmbH, Oberkochen, Germany).

In order to increase the affinity between the nanostructured, porous coating and the infused lubricant, the former was chemically modified depending on the nature of the latter. When n-hexadecane was used as lubricant, alkyl chains were grafted to pseudoboehmite by immersion in a hexadecyltrimethoxysilane (≥85%, Merck, Darmstadt, Germany) solution in ethanol (6 wt.%) for 2 min, followed by drying at 80 ◦C for 1 h and annealing at 170 ◦C for 5 min. Otherwise, when Perfluoropolyethers (PFPE) were used as lubricants, fluorinated alkyl chains were grafted to the nanostructured AlOOH by immersion in a commercial solution of fluoroalkylsilane in isopropyl alcohol (Protectosil SC200, Evonik, Essen, Germany) for 2 min, followed by drying in air and annealing at 150 ◦C for 30 min.

Finally, lubricants were infused in the nanostructured layer by brushing until a continuous, shiny layer covered the sample surface. In order to remove excess oil, the samples were held vertical overnight. Five different lubricants were used: four commercial perfluoropolyethers PFPE with increasing viscosity (Krytox GPL 100, 103, 105, and 107, Chemours, Geneva, Switzerland), or n-hexadecane (≥99%, Merck). The resulting SLIPS were labeled as K100, K103, K105, K107, and HEX, respectively.
