## 3 Results and Discussion
### Characterization of the Wear Surfaces
This adhesive wear caused by the crushing of the removed material was more evident at the edges of the wear track where the material crushed had accumulated due to cyclic alumina ball movement in the wear test. This wear mechanism is the same as that observed in epoxy/MWCNT nanocomposites [18]. However, at higher percentages of GNPs (5 wt.% and 8 wt.%), wear surfaces were different depending on the type of graphene, indicating that the wear mechanism was different (Figure 7). For the nanocomposites with non-functionalised graphene, an increment of GNP content up to 5 wt.% and 8 wt.% resulted in a wear track that was almost imperceptible (Figure 7a,b). For these materials, the wear track had no roughness, almost no tearing of material took place during test, and even the initial surface preparation of the sample was distinguished. This observation confirms that, in the case of the wear process, the wear process is not a  
Figure 6: SEM images of worn surfaces of neat epoxy (**a**,**c**) and 2 wt.% of non-functionalised GNPs (**b**) and 2 wt.% amino functionalised GNPs (**d**) nanocomposites. of the epoxy resin filled with 5-8 wt.% of non-functionalised GNPs, the graphene acts as a lubricant inside of the epoxy resin. The lubricating effect of the graphene inside the resin favoured the pin sliding and, consequently, almost no wear track was observed and no debris detached during the test. In the case of nanocomposites with amino-functionalised GNPs, the surface was less smooth and debris were deposited (Figure 7c,d), while these debris were not observed in samples containing non-functionalised GNPs. In nanocomposites with amino-functionalised GNPs, a film was formed between the pin and surface, which acted as a lubricant. Other studies on the mechanism by which graphene acts as a lubricant and reduces friction between surfaces have shown that it was associated with the formation of a graphene uniform layer within the area of track wear [10]. This difference in the wear mechanism, according to the type of graphene, was mainly observed at higher magnification (Figure 8). For low percentages of GNPs, material with non-functionalised GNPs had a rough surface, indicating an abrasive wear similar to that of the epoxy resin (Figure 8a). On the other hand, for amino-functionalised graphene, it was observed that for the same percentage, the surface was smoother and there was less abrasive wear. Although the percentage of graphene was also low, debris particles (GNPs + epoxy) detached and adhered to the surface, acting as a third body, which was the lubricant (Figure 8b). At higher percentages (8 wt.%), the difference in wear mechanism was even greater. The material surface of non-functionalised graphene was practically smooth, that is, there was almost no wear track and no material was detached on the surface (Figure 8c). However, in the wear surface of the nanocomposite with amino-functionalised GNPs, material detached during the test, which acted as a lubricating layer (Figure 8d). The debris deposited on the surface are shown in Figure 8e,f. It can be observed the debris was a mixture of epoxy resin and graphene because the bonding of this functionalised graphene with the resin was higher, thus they detached together. This stronger interface between f-GNP and the epoxy resin resulted in the presence of nanocomposite-based debris instead of single GNPs. As explained when discussing the friction rate, the lower size of the f-GNPs when compared to non-functionalised ones and/or the chemical interaction between amine groups and epoxy monomer resulted in stronger interfaces that promoted the detachment of resin and f-GNPs together. Figure 7: SEM images of worn surfaces of nanocomposites with 5 wt.% (**a**) and 8 wt.% (**b**) of non-functionalised GNPs and 5 wt.% (**c**) and 8 wt.% (**d**) of amino-functionalised GNPs.