## 2 Materials and Methods
To perform the honing, the surface was scratched in two different directions using 320 granulometry sandpaper, creating geometric shapes identical to the diamond. After the previous procedure was completed, the surface was polished with 600 and 1000 granulometry sandpaper, and the roughness was determined using a profilometer. To finish the honing, it was necessary to remove the peaks created when using the sandpaper, leaving the valleys to ensure that the ring always contains lubricating oil in its rotational movements. In this way, it was necessary to again use sandpaper 1000 granulometry (standard parameters used in CEMPRE Laboratory). The polishing of the surface is necessary to carefully observe its microstructure. For that, its cross-section is observed (cutting a small section of the ring, wetting it in acrylic resin, and drying it so that the surfaces are parallel and flat), polishing it with diamond paste (3 \(\upmu\)m). All this process is done under an optical microscope (1000\(\times\) magnification) after revealing the structure using 2% Nital etchant. Since the objective of this work was to study the tribological properties of the coatings, it was necessary to use a base lubricating oil, free of additives such as friction modifiers, for a better understanding of the real role of the coatings. In addition, although the contact between the piston ring and cylinder liner is mostly governed by hydrodynamic and mixed lubrication regimes in the internal combustion engines [20], when the piston ring hits the top dead center of the stroke, the contact is in the boundary lubrication regime where the role of the coatings matters [21]. Therefore, Polyalphaolefin (PAO) 8 base oil was chosen as the lubricating oil for this study because it is used as the base oil for some of the commercial engine oils [22]. The tests were performed using different speeds (Table 2) at 250 revolutions of the ring, which was enough to reach the stationary friction regime. For temperature control during the tests, a radiation heating system was used to heat the ring. In this way, it was possible to carry out the tests at different high temperatures in the oil as well, which were 60, 80, and 100 \({}^{\circ}\)C in this study, based on the fact that the oil temperature in the operating conditions in an engine varies from less than 60 \({}^{\circ}\)C to a maximum of 104 \({}^{\circ}\)C [23]. The viscosity-temperature curve was determined by using the viscometer (A&D, SV-10 model,  
Figure 1: Schematic diagram of the block-on-ring sliding test tribometer. Tokyo, Japan). To do that, the oil was heated up to 100 \({}^{\circ}\)C by the oven (JP-SELECTA, DIGITHEAT model, Barcelona, Spain) before starting to measure the viscosity when the oil was cooled down naturally to room temperature. The surface wettability of the films was assessed by measuring the contact angle of a drop of lubricating oil (PAO 8) on the dried and cleaned surface of the films at room temperature (25 \({}^{\circ}\)C). Following that, the surface free energy of the films was measured by substituting the values of contact angle in [PERSON]'s equation [24]. The hardness of the films was measured by nanoindentation (MicroMaterials NanoTest, Wrexham, UK) using a Berkovich diamond indenter. Sixteen hardness measurements were performed for each specimen at different points. A 10-mN load was used to ensure an indentation depth lower than 10% of the coating's thickness.