*2.3. Method of Measurement for Real Dimple Dimensions*

An additional important aspect for the determination of the influence of the dimple textures on friction are the real dimensions of the dimples. While the nominal values of the investigated textures are given in Table 1, the real values of diameter, distance and depth vary due to imperfections in the innovative industrialised TDM production process of the rubber specimens. During this process, the negative of the desired texture is applied to a mould by LST ablation. A 5-axis 100 W pico-second laser is utilised for this purpose. During the injection moulding and vulcanisation procedure, the texture is directly transferred from the mould to the rubber surface. Protrusions in the mould become dimples in the elastomer. Due to imperfections in the laser machining of the mould, both a deviation from the nominal dimple parameters and ring-shaped cavities in the peripheral areas of the dimple valleys are identifiable, see Figure 4b. These deviations mainly result from re-solidifying metal vapours on the surface of the metallic vulcanisation mould.

In order to measure and visualise the actual dimple texture dimensions, a 3D optical microscope, based on focus-variation, is used. Figure 4a shows an exemplary 4 × 4 mm surface scan of rubber specimen 6 with a nominal dimple diameter of 300 μm, a distance of 200 μm and a depth of 30 μm.

**Figure 4.** (**a**) Microscope image of rubber specimen 6, considering a measured area of 4 × 4 mm and (**b**) related height profile *h* as function of the measuring length *l*, considering the indicated line scan trough four adjacent dimples.

Figure 4b shows the corresponding line scan as height profile *h* over the measuring length *l*, which is measured through the centre of four adjacent dimples. The positions that are measured to determine the real dimple dimensions of diameter, distance, and depth are marked in the height profile. To ensure accurate measurement results of the dimple dimensions, the curvature of the contact surface of the rubber samples was subtracted, resulting in the flat surface scan depicted. Beyond this, no corrections were made to the surface scans. The dimple dimensions were measured at 16 different positions of the textured area of each rubber sample. The mean values of the 16 measurements and the corresponding dimensionless texture parameters are listed in Table 3.

**Table 3.** Real dimple dimensions, indicated by diameter, distance, and depth. The aspect ratio is the quotient of dimple depth and diameter. The textured area percentage is determined from the quotient of the textured area *Atextured* and the nominal contact area *Anominal*.

