*2.1. Samples Geometry and Dimensions*

For the present tribological investigations, the upper specimen consisted of a convex cylinder made of hardened 31CrMoV9 (around 700HV), while the lower specimen consisted of a disc made of hardened 42CrMo4 (around 650HV; hardness values were measured with a hardness tester from EMCO-TEST Prüfmaschinen GmbH, Kuchl, Austria) whose dimensions are shown schematically in Figure 1. For all tribological tests performed, cylinders were used as received, but disc specimens were further treated in order to evaluate the effect of a 3D surface texturing. Furthermore, the influence of an antifriction coating was also studied for the present manuscript. The four different disc specimen surface states investigated in the present work are shown schematically in Figure 2. The as-received convex cylinders had surface roughness values of *R*<sup>a</sup> = 0.12 ± 0.01 μm and *R*<sup>z</sup> = 1.68 ± 0.38 μm, and the benchmark disc samples had surface roughness values of *R*<sup>a</sup> = 0.06 ± 0.01 μm and *R*<sup>z</sup> = 0.65 ± 0.19 μm.

The antifriction coating used was applied on one side of the previously cleaned discs (blank or surface microtextured) through the coating manufacturer (Carl Bechem GmbH, Hagen, Germany). The thickness of the antifriction coating ranged between 15 and 20 μm. The choice of this coating was based on the fact that it should be especially designed for a reduction of the friction coefficient (AF320E). Some details on the antifriction coating used in the present study are listed in Table 1.

**Figure 1.** Geometries of the samples used in the present study: (**a**) cylinder (upper specimen) and (**b**) disc (lower specimen) (Note: drawings are not to scale).

**Figure 2.** Different processed disc surfaces investigated in the present study: (**a**) benchmark disc, (**b**) benchmark disc with an antifriction-coating, (**c**) 3D microtextured disc and (**d**) 3D microtextured disc with an antifriction coating.

**Table 1.** Excerpt of important properties of the antifriction coating investigated in the present study.


Disc specimens were 3D surface microtextured using a femtosecond laser; the procedure used will be explained in detail later. It is worth noting that only a small central quadratic region (16 mm × 16 mm) of one side of the disc specimens was actually microtextured as shown in Figure 2c. This 3D microtexture was chosen based on previously performed tribological investigations, in which the influence of a reduction of the effective nominal contact area through the production of microridges on elastomer pads was analyzed and reported [22]. The very promising results obtained from the aforementioned studies led the authors to produce, characterize and evaluate a further optimized surface microtexture in the present study, which consisted of a combination of 2 microridged textures perpendicular to each other, as schematically shown by the red arrows in Figure 3. Again, based on the results of previous studies [7,22], the desired dimensions of the plateaus of the 3D microtexture should range between 40 and 50 μm for the sides, between 30 and 40 μm for the height, and about 80 μm for the period in both horizontal and vertical directions. With these desired dimensions for the uppermost plateaus (which define the contact area), a calculated ratio of approximately 20% for the nominal contact area of the 3D-textures samples in comparison to the untextured (benchmark) specimens may be obtained (benchmark: nominal contact area of 100%, without taking into account its surface roughness).

**Figure 3.** Example of the desired 3D surface microtexture to be produced using an ultra-short pulsed laser on one side of disc specimens.

All specimen surfaces were characterized using a non-contact laser optical surface roughness measuring apparatus (VK-X250/260, Keyence International NV/SA, Mechelen, Belgium). The surface roughness values of the convex cylinders and benchmark disc specimens (without and with coating) were evaluated. Furthermore, the characteristics of the 3D microtexture (without and with coating), such as side dimensions, heights and periods of the different plateaus, to name a few, were measured and reported as average values.
