## 4 Conclusions  
In this work, the tribological performance of graphene-coated copper was investigated. Four methods to synthesize 2D carbon coatings by either transfer or self-assembly of flakes were compared. The nature and quality of the graphene structures thus obtained were investigated through FESEM analysis and compared to Raman spectra to confirm the defectiveness of the layer, and the uniformity of substrate coverage. The results of the macroscale tribological tests against a steel sphere under low load and low sliding speed revealed that graphene coatings could serve as a solid lubricant while reducing friction and wear of copper. However, the duration of the graphene coatings at the sliding interface was limited because of the limited adhesion to the substrate. * Self-assembled graphene nano-coatings lasted longer with duration up to 1/6 th of the total sliding distance of the test in the case of 6 h-sonicated graphene;
* The CoF dropped from 0.6-0.7 to 0.15-0.25 in the presence of graphene, with a maximum reduction by 78% in the case of 6 h-sonicated graphene. However, this beneficial effect endured as long as the carbon nano-sheet withstood the tearing-off effect of the counter body sliding against it;
* 6 h-sonicated graphene also produced a beneficial 31% reduction of the overall average friction value of the test, despite the short lifetime of the nano-coating;
* The reduction in the average wear volume of copper was remarkable in the case of self-assembled graphene coatings (UGFs). The reduction was about 36% and 40% for the 3 h-sonicated and 6 h-sonicated coatings compared to the bare copper, respectively;
* Transferred CVD graphene showed no benefits on friction and produced a detrimental increase of wear volume, which was never reported in the previous literature. * The morphological analysis of wear tracks suggested that abrasion had a role in the presence of the transferred graphene layer, while tribo-oxidation dominated in the presence of self-assembled graphene;
* The deposition of direct growth graphene on copper samples failed due to issues likely related to the use of a bulk catalytic body rather than a thin copper foil. Direct growth of CVD graphene on copper samples could not be compared to transferred CVD graphene. In the end, the results of this research suggested that:  
1. Graphene synthesised through different production methods may contribute to activate different wear mechanisms at the sliding interface;  
Figure 17: Morphological analysis of the wear track after the tribological test with the sample treated by direct growth CVD. (**a**) chunks of material ripped off the samples, (**b**) 3D measurement, (**c**) wear track measurement. 2. Graphene obtained from the self-assembly of GNPs is the most promising graphene production method for tribological applications in industry, despite its defective and uneven carbon structure;
3. Ultra-thin, high-purity, and low-defect structures like transferred CVD graphene are of little interest for macroscale friction and wear applications where gross sliding dominates. It turned out to be overly thin and weak;
4. The main benefits of self-assembled graphene are its increased thickness and the ability to promote a lubricious third layer at the steel-copper interface;
5. The synthesis of the direct growth of CVD graphene on bulk metal substrates, which is a promising way to reduce the cost of CVD graphene, needs further investigation to optimize the deposition process route. Self-assembled graphene coatings are easy to apply, and cost-effective compared to CVD graphene. Nevertheless, the critical aspect for their extensive use in tribological applications is poor adhesion, which limits their endurance against a sliding counter body. Previous research on graphene at the nanoscale [67] has proved that the weak bonding to the substrate cannot prevent ripples and puckers in the out-of-plane direction from arising in front of the contact.