## 3 Results and Discussion
### Results of Abrasive Wear Resistance
#### 3.6.3 Surface Roughness After Abrasive Wear Testing  
Figures 14-16 show optical micrographs illustrating the surface of the countersamples and samples, and their surface topography after the wear test. Analysing the surfaces after the friction of the 3D-printed countersamples from 20 MnCr5 steel, it can be concluded that due to their significant surface roughness (Ra = 19.49 \(\upmu\)m and Rz = 86.39 \(\upmu\)m) only the highest asperities were in contact with the surfaces of the samples during friction. Many friction studies on the mechanism of adhesion in frictional contact in sheet metal forming indicate that the concept of the real contact area becomes significant when taking into account the microscopic scale of the solid body surfaces [41]. This approach is the basis for texturing the tool surface, which leads to a decrease in the amount of frictional shear stresses and as a result the total frictional force in the process [42]. Figure 14: (**a**) An optical micrograph illustrating the surface of the countersample and (**b**) its surface topography after the wear test involving the 321 steel sample; (**c**) an optical micrograph illustrating the surface of the 321 steel sample and (**d**) its surface topography after the wear test. Figure 15: (**a**) An optical micrograph illustrating the surface of the countersample and (**b**) its surface topography after the wear test involving the pure copper sample; (**c**) an optical micrograph illustrating the surface of the pure copper sample and (**d**) its surface topography after the wear test. In the friction pairs of 20 MnCr5/Inconel 625 and 20 MnCr5/321 steel, the abrasive wear of the countersample asperities dominates (Figures 14a and 17a). This results in low friction coefficients of these friction pairs (COF = 0.213 for the 20 MnCr5/Inconel 625 configuration and COF = 0.194 for the 20 MnCr5/321 steel friction pair) and the stability of the friction force throughout the friction process (Figure 11). The initial total roughness profile height of 122.435 mm did not change significantly. After the friction process of 20 MnCr5/Inconel 625 and 20 MnCr5/321 steel pairs, the total profile height of the countersamples was 125.67 mm (Figure 17a) and 116.549 mm (Figure 14a), respectively. Abrasive wear occurs when the material loss in the surface layer is caused by the separation of material particles due to micro-cutting, scratching or ploughing. The resistance to abrasive wear is dependent on the chemical composition, relative hardness (relative to the hardness of the worn material), specific contact pressures, sliding speed and many other parameters [43; 44]. In the case of the steel samples, the dominant wear feature is abrasive scratching, characterised by distinct grooves aligned with the sliding direction. These scratches suggest a micro-cutting or ploughing mechanism, typical for harder counterface interactions. For the Inconel samples, the wear traces are notably different. The surface exhibits shallower scratches and numerous grooves, but without significant material removal. This indicates a milder abrasive or surface fatigue mechanism, where deformation occurs without substantial mass loss. The presence of multiple shallow grooves may also suggest a combination of micro-ploughing and surface fatigue, possibly due to the higher hardness and oxidation resistance of Inconel, which limits adhesive interactions and material transfer. Figure 16: (**a**) An optical micrograph illustrating the surface of the countersample and (**b**) its surface topography after the wear test involving the EN AW-6061 T0 sample; (**c**) an optical micrograph illustrating the surface of the EN AW-6061 T0 sample and (**d**) its surface topography after the wear test. These differences in wear morphology help explain the lower mass loss observed for Inconel, despite visible surface damage. The wear in this case is more superficial and does not result in a significant detachment of material, unlike in the steel samples.