## 3 Results and Discussion
### Friction
Interestingly, very similar values were reported in previous research works, as summarized in Table 2. Even with different substrates and environmental conditions, other researchers have recorded friction curves and average friction values with extraordinary similarities to those reported in this research, featuring initial low friction followed by a gradual increase up to the typical CoF of the substrate. Therefore, the value of about 0.17-0.2 should be regarded as the characteristic friction coefficient for the graphene-steel pair regardless of the substrate nature and the characteristic scale of the phenomenon (either microscale or macroscale interaction). Figure 8: Friction curves from pin-on-disc tests on the graphene-coated samples. Friction curves plotted against the number of rotation cycles allows appreciating the high repeatability of the results despite the different wear track radii. The difference between the average CoF value related to Part 2 and the overall average value of the test is almost null with transfer graphene and 3 h-sonicated self-assembled graphene, because Part 1 lasted very briefly. On the contrary, the 6 h-sonicated self-assembled graphene lasted at the interface long enough to produce a significant 11% reduction of the overall friction coefficient compared to Part 2. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Authors** & **Ref.** & **CoF** & **Testing Conditions** \\ \hline [PERSON] et al. & [26] & 0.22 & Pin-on-disc; graphene-coated and textured M2 steel against stainless steel; 0.5 N load; Testing in ambient air; solution-processed graphene coating; \\ \hline [PERSON] et al. & [25] & 0.19 & Pin-on-disc; graphene-coated 440C steel against 440C steel; 2 N load; Testing in ambient air, in ethanol bath and with the addition of droplets of graphene solution; solution-processed graphene coating; \\ \hline [PERSON] et al. & [24] & 0.16-0.23 & Pin-on-disc; graphene-coated 440C steel against 440C steel; 1 N load; testing in N\({}_{2}\) and H\({}_{2}\); CVD graphene and solution-processed graphene coating; \\ \hline [PERSON] et al. & [55] & 0.2 & Pin-on-disc; graphene-coated steel against steel; 1 N to 5 N load; Testing in dry Nitrogen; solution-processed graphene coating; \\ \hline [PERSON] et al. & [23] & 0.2 & Pin-on-disc; graphene-coated copper against stainless steel; 20 mN load; Testing in ambient air; CVD graphene; \\ \hline [PERSON] et al. & [22] & 0.19-0.4 & Flat-on-flat; graphene-coated bronze against AISIS2100 steel; 10 N to 30 N load; Testing in ambient air; CVD graphene; \\ \hline [PERSON] et al. & [38] & 0.15-0.25 & Pin-on-disc; graphene-coated aluminum against steel; 5 N load; Testing in ambient air; CVD graphene and self-assembled graphene \\ \hline [PERSON] et al. & [40] & 0.15-0.2 & Pin-on-disc; graphene-coated steel against steel; 5 N load; Testing in ambient air; CVD graphene \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Coefficient of friction of graphene coating reported in previous literature. Figure 9: Average CoF values measured in Part 1 and Part 2. The error bars were determined as three times the standard deviation of the average CoF of the three tests carried out with each sample (Lower part). Average lifetime of the coatings determined by the cycles accumulated at the time the transition from Part 1 to Part 2 started. However, when the overall average friction values obtained with the graphene-coated samples are compared to that of bare copper (i.e., comparison of the solid orange columns in Figure 9), some reduction in the CoF was always observed. It was equal to 5% with transferred CVD graphene, and 10% and 31% for 3 h-sonicate dan d 6 h-sonicated self-assembled graphene, respectively. This suggests that graphene was able to provide a beneficial effect to the steel-copper friction pair even after the failure of the nano-coatings.