## 3 Results and Discussion
### Characterization of the Wear Surfaces  
The wear mechanisms were studied by analysing the wear track surfaces using SEM and 3D optical profilometry. Although the type of GNP did not seem to have a great influence on the wear behaviour (mass loss, wear rate and friction coefficient), the analysis of the worn surfaces indicates that the wear mechanism was very different. Figure 4: Mass loss (**a**), wear rate (**b**) and friction coefficient (**c**) of epoxy/GNPs nanocomposites with different contents and types of GNPs. Figure 5 shows the track width value and profilometer images of the wear track for different nanocomposites. As depth values of wear tracks were very low, the discussion was based on track wideness. For both types of graphene, at higher GNPs contents (5 wt.% and 8 wt.%), width and roughness of the wear surfaces decrease considerably compared to epoxy resin (Figure 5b). In the case of materials with 8 wt.% of graphene, the wear track width was reduced by approximately reduced with respect to that of the neat resin (from 1194 \(\upmu\)m to 520-540 \(\upmu\)m). In the case of materials containing 8 wt.% f-GNPs, despite the 1000 m track made by the alumina pin on material, the wear track generated on the filled resin was very small in terms of depth. In fact, the track depth could not be evaluated, as it was in the same range as the part surface roughness, whereas for neat resin, the track depth was in the range 75-100 \(\upmu\)m. The material with 8 wt.% GNP behaved as if the pin slid on the material (Figure 5e). However, for low percentages of graphene (2 wt.% and 3 wt.%), the value of wear track width of materials with non-functionalised GNPs was similar to neat resin and remained almost constant. On the other hand, it can be observed that the wear track width was lower than the reference one, even at low contents when f-GNPs were used (Figure 5d). According to this result, the damage or  
Figure 5: Wear track width (**a**) and 2D Profilometer images of wear track of neat epoxy (**b**), and epoxy with 3 wt.% non-functionalised GNPs (**c**), 3 wt.% amino-functionalised GNPs (**d**) and 8 wt.% amino-functionalised GNPs (**e**). wear made by the pin on the surface was less for graphene functionalised with the amino group. This effect has been observed in previous studies that were carried out on nanocomposites with carbon nanotubes (MWCNTs). In this study, for nanocomposites with nonfunctionalised MWCNTs, the wear track width was not strongly affected by the increase of MWCNT content. However, for nanocomposite with amine functionalised MWCNTs, the track width decreased considerably, even at the lowest percentages (2 wt.%) [18]. This improvement of the interface, which leads to a strongly attached resin layer covering the GNPs when they are amine-functionalised, has been already reported for the same epoxy/GNP system used for the current research. Thus, it is the effect found in the debris generated when amine-functionalised GNPs were used [37]. Moreover, the lower thickness and lateral size could better enhance this interface between the resin and the f-GNPs. Wear surfaces have been also analysed by scanning electron microscopy to evaluate the wear mechanisms (Figures 6 and 7). In general, the wear damage decreased when GNPs were added to the epoxy resin for both types of GNPs. However, the morphology of the wear surfaces depended on the percentage of graphene. For low contents, they were similar for the two types of graphene, but at higher percentages, they were very different. In the case of the epoxy resin (Figure 6a,c) and the nanocomposite with 2 wt.% of graphene (Figure 6b,c), the observed wear track had higher width and roughness, indicating mainly abrasive and adhesive wear mechanisms. In these materials, two different wear zones could be distinguished. In the first wear zone, the material was torn out during the test (abrasive wear mechanism), and in the other wear zone, the removed material from the track (debris) was crushed during ball movement (adhesive wear mechanism).