## 3 Results and Discussion
### Wear  
The worn surface of the samples after the pin-on-disc tests is shown in Figures 10 and 11, which collect and compare the corresponding topography of the worn-out region acquired with the FV microscope. Samples were blown with air before the topographic analysis, and the wear tracks were gently wiped with a dry lint-free swab to remove any residual wear debris from the surface. The samples were not further cleaned to avoid physical or chemical damage to the carbon layer. Figure 11: Comparison of the topography acquired with the FV 3D microscope: (**a**) Bare Cu, (**b**) transferred graphene, (**c**) 3 h-sonicated graphene, (**d**) 6 h-sonicated graphene. Figure 10: Samples after pin-on-disc tests. (**a**) bare sample, (**b**) directly grown CVD graphene, (**c**) transferred CVD graphene coating, (**d**) 3 h-sonicated graphene, (**e**) 6 h-sonicated graphene. A transparent film adhered to the central part of the metal surface is noticeable on the sample with transferred CVD graphene. The sample with direct growth graphene, whose results will be presented separately in Section 3.2, featured a pattern of bright patches similar to grains visible to the naked eye, probably the contour of the copper grains after annealing. As to the counterpart, minor damages appeared on it, and the inspection under the optical microscope provided no evidence of transferred or oxidized matter on the ball. The damage was mostly limited to a local pattern of scratches on the sphere. Therefore, the wearing-out of the sphere was neglected because the related wear volume was not confidently measurable (it was limited to minimal deviations from the initial spherical shape). The average volume data are presented in detail in Figure 12, and summarized in Table 3. The result of the wear tests revealed different wear behaviours for the various types of graphene coatings. The two samples coated by self-assembled graphene performed best, and, in particular, the 6 h-sonicated graphene let the amount of removed material decrease by 36%, taking the bare Cu sample as the reference. The adhered material decreased by 10-15% on average, even though its value was slightly higher for the 2 mm and 3 mm track radii. These results  
Figure 12: (**a**) Removed material volume and (**b**) deposited or plastically displaced material volume. \begin{table}
\begin{tabular}{c c c c c} \hline
**[mm\({}^{3}\)]** & **Bare Cu** & **Transfer** & **Percolative-3 h** & **Percolative-6 h** \\ \hline Average volume of material loss & 0.0030 & 0.0056 & 0.0019 & 0.0018 \\ \hline St.Dev. of the volume of material loss & 0.0009 & 0.0022 & 0.0003 & 0.0006 \\ \hline Average volume of deposited material & 0.0024 & 0.0045 & 0.0017 & 0.0021 \\ \hline St.Dev. of the volume of deposited material & 0.0007 & 0.0014 & 0.0001 & 0.0004 \\ \hline \end{tabular}
\end{table} 
Table 3: Average value of material loss and deposited/displaced material. were remarkably different from those obtained with graphene-coated aluminium samples in a previous work by the authors [37], where no benefits in terms of wear were observed. The relative reduction of wear was substantial, yet the early graphene removal may have hidden the full potential of these coatings if they had endured for a more extended time. The wear volume was measured only at the end of each test, whilst the coating failure happened on average at 1/6 th or 1/10 th of the total sliding distance. Therefore, the benefits of these high-lubricity graphene nano-coatings might have proven even more significant if the wear volume were inspected repeatedly after shorter sliding distances or at the end of shorter tests. The performance of transferred graphene was underwhelming. The average amount of lost and transferred material was two times higher than with the bare copper sample. This evidence was surprising because it is not in line with the results of other papers in the scientific literature; for instance, the work by [PERSON] et al. [62], who observed a substantial reduction of the wear rate by transferred CVD-graphene.