## 3 Results and Discussion
### Wear
The morphological analysis in Figure 13 was carried out to investigate the wear mechanisms acting at the tribological interface. The dominant wear mechanisms appeared to change from sample to sample, and were influenced by the production method of the graphene nano-coatings. The wear tracks on bare copper and the percolative-graphene-coated samples have a rather uneven width in the circumferential direction, and feature isolated deeps and hills, suggesting the detachment of chunks of material due to local adhesion and subsequent deposition of oxidized matter inside the sliding path. This is also clearly visible in Figure 14, which shows that a dark grey third layer made of oxidized matter covered the wear tracks, and was responsible for the peculiar morphology visible in Figure 13. Therefore, tribo-oxidation activated by initial adhesion was probably the primary wear mechanism here. In the presence of graphene, this layer may have encapsulated lubricious carbon matter and protected the substrate from wear. This was not the case for the transferred CVD graphene. The wear track featured a uniform width in the circumferential direction and the presence of longitudinal grooves  
Figure 13: Analysis of the wear tracks morphology. The wear tracks of the tests performed at 3 mm radius were considered for this comparative analysis. and ridges suggested that an abrasion wear mechanism has a role here. Few traces of grey adhered matter were visible inside the wear track, and shining areas of copper were still visible by the end of the test in Figure 14f. Therefore, no transfer layer stabilised at the contact interface, and tribo-oxidation had a limited effect. Abrasion was well correlated with the friction curve recorded during the tests, which were more erratic than the others and characterised by higher scattering of the CoF. Moreover, the contribution of abrasion to wear may also justify the higher wear volume measured with this sample. Abrasion might be activated by the presence of wrinkles and ripples in the transferred graphene layer. According to [PERSON] et al., wrinkles and ripples are relatively common in graphene produced by CVD due to the negative thermal expansion coefficient of graphene. Wrinkles appear during the cooling process as the substrate shrinks while the coating dilates [63]. [PERSON] et al. [64] also pointed out that the critical phase of the CVD method is the cooling-down of the Cu-C system, since at about 1000 \({}^{\circ}\)C, a thin melted layer is likely to appear on top of the catalyst body and it may run into quenching. Such undercooling is responsible for forming a surface Cu lattice favourable to many graphene ripples, e.g., ripples formed due to cellular and dendritic solidification patterns. [PERSON] [65] proved that transfer from the copper foil to a super smooth surface (like SiO\({}_{2}\)) might make the graphene sheets flatter and less defective. However, if some ripples formed during the CVD process, they could be easily transferred to the end copper sample because the surface finish of these samples was much higher than those employed by [PERSON]. The high shear stress experienced by the wrinkles and ripples under sliding contact interactions may lead to structural changes from sp\({}^{2}\) to amorphous carbon, according to [PERSON] et al. [66], leading to harsher tribological conditions. Figure 14: (**a**) Wear scars on the steel balls (**left**) and wear tracks on the copper samples (**right**). (**a**,**b**) Bare Cu, (**c**,**d**) transferred graphene, (**e**,**f**) 3 h-sonicated graphene, (**g**,**h**) 6 h-sonicated graphene. The analysis of the roughness values measured inside the wear tracks after the tests are presented in Table 4. Interestingly, the roughness values were higher than the initial ones and correlated with the inherent wear results.