## 3 Results and Discussion
### Direct Growth CVD  
As to direct growth CVD, the specimen disclosed different tribological behaviour against the steel sphere. Figure 15a compares the friction curve of the direct growth sample with the other friction curves of bare copper and graphene-coated copper samples. High friction was measured at the beginning of the test, with a peak of 1.4 at the onset of sliding. A gradual decrease of the CoF followed, and stabilisation at a value slightly lower than the other curves of the diagram. The friction curves in Figure 15b suggest that an additional abrasive effect worked at the interface at the beginning of the test, which was detrimental to the tribological system. The amount of material removed and displaced was also distinctly and dramatically higher than the uncoated sample. The wear volume calculations are presented in Figure 16 and Table 5. No previous investigation about samples prepared by direct growth CVD reported a similar result. \begin{table}
\begin{tabular}{c c c c c c} \hline  & & **Bare Cu** & **Transfer** & **Percolative-3 h** & **Percolative-6 h** \\ \hline Initial roughness value & \(Sa\) [\(\mathrm{\SIUnitSymbolMicro m}\)] & 0.071 & 0.236 & 0.098 & 0.096 \\ \hline Roughness inside & \(Sa\) [\(\mathrm{\SIUnitSymbolMicro m}\)] & 0.671 & 0.957 & 0.508 & 0.525 \\ wear tracks & \(Sa\) [\(\mathrm{\SIUnitSymbolMicro m}\)] & 0.897 & 1.514 & 0.641 & 0.701 \\ \hline Wear track width & [\(\mathrm{\SIUnitSymbolMicro m}\)] & 349 & 300 & 271 & 231 \\ \hline \end{tabular}
\end{table} 
Table 4: Average roughness value inside wear tracks and track width. Figure 15: (a) Friction curves were measured with the sample treated by direct growth CVD and compared to the other samples at the median radius of 3 mm. (b) Set of friction curves measured with the direct growth sample. If hard amorphous carbon formed due to anomalous activation of the carbon nucleation sites, as hypothesised, it is likely that a two-body abrasive effect played a role. Abrasion would justify the peak of the CoF in the very first cycles of the test. During the subsequent downfall of the CoF, the globular structure may have detached and turned into hard debris rolling at the interface. Hard globules may have mixed with copper oxides and strengthened the third layer covering part of the wear track. Some oxidized matter is visible in the middle of the wear in Figure 17b. This scenario would explain, on the one hand, why significant material loss continued up to the end of the test, and, on the other hand, would justify the slightly lower friction coefficient observed in the steady-state part of the tests compared to the other samples. Figure 17 shows that the wear track was large and deep with no longitudinal scratches. Many recesses distributed along the track are shown in Figure 17c, together with chunks of material ripped off the samples (Figure 17a). These pieces of evidence suggest that after the transition of the CoF, the dominant wear mechanism evolved from dominant abrasion to a mix of adhesion, abrasion, and tribo-oxidation. The wear tracks of the direct growth sample also feature the largest amount of plastically displaced material forming hills at the borders of the tracks, which explains why the wear scar on the ball has high ellipticity in Figure 17a. This effect might be related to the substrate softening due to annealing at high temperatures (1000 \({}^{\circ}\)C) during the CVD treatment. Figure 16: (**a**) Removed material volume and (**b**) deposited or plastically displaced material volume for the direct growth sample.