## 3 Results and Discussion
### Results of Abrasive Wear Resistance  
#### 3.6.1 Coefficient of Friction  
The tribological tests presented in Figures 11-13 were conducted under a normal load of 50 N, a sliding speed of 136 rpm, and a total sliding distance of 100 m. Figure 11 shows a comparison of the variation in friction force during wear tests. For Inconel 625 and 321 steel sheets, the friction force during the entire test was very stable at about 10 N. The differences in friction force behaviour observed in Figure 11 are attributed to the distinct wear mechanisms involved in each material pair. For Inconel 625 and 321 steel, the dominant mechanism was abrasive wear, which resulted in stable friction force values throughout the test. In contrast, the 20 MnCr5/EN AW-6061 T0 and 20 MnCr5/pure copper pairs exhibited more complex tribological behaviour. In these cases, additional phenomena such as adhesion, plastic deformation, material transfer, and oxidation contributed to increased and fluctuating friction forces. These effects are associated with the formation and rupture of adhesive junctions, which require additional force to overcome, leading to higher average friction values and reduced stability.  
Among the materials tested, these sheets are characterised by a relatively high yield stress and ultimate tensile strength (Table 3).  
Stable friction force values during the wear test indicate no wear phenomena in the friction pair. The average COFs for the 20 MnCr5/Inconel 625 and 20 MnCr5/321 steel friction pairs were COF = 0.213 and COF = 0.194, respectively. Different behaviour was observed during testing pure copper and EN AW-6061 T0 sheet metals.  
The highest friction force during the entire test was recorded for the 20 MnCr5/EN AW-6061 T0 friction pair. As is commonly known, aluminium and aluminium alloy sheets are susceptible to galling due to increased plasticity and, therefore, the susceptibility of the surface asperities to flattening under the influence of load. This causes galling by pick-up on forming tools and subsequent scoring [34]. In addition, the brittle nature of the oxide layers, under the influence of load, causes the oxide layer to crack and exposes the pure metal [35]. The die surface topography is the dominant factor controlling galling in forming [36]. The average COF of the 20 MnCr5/EN AW-6061 T0 friction pair was 1.234, which means that the friction force was greater than the normal force. Pure copper can be prone to galling during metal forming operations due to its low yield stress (Table 3) and tendency to adhere to tooling. The friction force in the wear test of the 20 MnCr5/pure copper friction pair was about 40% lower than in the contact of EN AW-6061 T0 sheet with the 20 MnCr5 steel countersample. The average COF during the testing of the pure copper sheet was 0.813. In the case of the 20 MnCr5/EN AW-6061 T0 and 20 MnCr5/pure copper friction pairs, a running-in stage can be observed, associated with an initial increase  
Figure 13: Percentage mass (**a**) and mass loss (**b**) of samples after roller-block testing.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & **Yield Stress, MPa** &
\begin{tabular}{c} **Ultimate Tensile** \\ **Strength, MPa** \\ \end{tabular} & **Reference** \\ \hline EN AW-6061 T0 & 65 & 115 & [22] \\ Inconel 625 & 528.3 & 958.8 & [22] \\
321 steel & 240 & 620 & [32] \\ pure copper & 33.3 & 210 & [33] \\ \hline \hline \end{tabular}
\end{table}
Table 3: Basic mechanical properties of test sheets.  
in the friction force and a subsequent decrease to stabilisation at a certain level. During the running-in period, a mutual adaptation of the tribological characteristics of contacting surfaces occurs [37]. In this period the topography of surfaces changes with time before reaching the steady wear regime [38].