Reinforcement of aluminum and aluminum alloys with graphene has been intensively practiced by researchers in the past dozen years. The role of Al₄C₃, which could be produced unintentionally or purposefully during the composite production, was controversial until it was found that nano-sized carbides were beneficial for strengthening the composites. aluminum-graphene-nano-sized Al₄C₃ composites were produced by us using the powder metallurgical method and subsequent annealing. The microstructure was investigated using light microscopy (LM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM), and high resolution transmission electron microscopy HRTEM. Nano-sized carbides were found at the interface aluminum-graphene. The formation of a chemical bond between aluminum and graphene during annealing was proved. Lower values of the microhardness and strength characteristics of the composites after extrusion and subsequent annealing during which nano-sized carbides are formed were found in comparison with those obtained after extrusion. It could be supposed that the annealing processes contribute more to the reduction in microhardness and strength characteristics than nano-sized carbides contribute to its increase. The presence of a strong chemical bond between the graphene and the aluminum is manifested in the failure pattern, which is characterized by graphene nano-platelets and nano-sized carbides fracture and semi-pulled out or semi-slipped Al₄C₃ from the matrix.

The study investigates the effects of graphene nanoplatelets (GNPs) on the tribological properties of aluminum-based nanocomposites, both annealed after extrusion and non-annealed. It also examines the role of nanosized Al₄C₃ (aluminum carbide), which forms in the annealed Al/GNPs nanocomposite, on the tribological performance of the nanocomposites. The nanocomposites were fabricated using the powder metallurgy method. The microstructure of the composite materials was characterized using SEM, EDS, XRD and TEM techniques. The coefficient of friction (CF) and mass wear of the composites were measured using a pin-on-disk test under dry sliding friction conditions. The results showed that adding GNPs increased the coefficient of friction (CF) of the nanocomposites by up to 44% at 0.1 wt.% GNP, but the CF decreased by 15% at 1.1 wt.% GNP. The optimal concentration of GNPs for minimizing the CF and mass wear of Al-based nanocomposites was 0.1 wt.%. Additionally, the presence of Al₄C₃ in the annealed Al/GNP nanocomposite had a positive effect on the CF at low GNP concentrations, with a 38% increases at 0.1 wt.% GNP, but this effect diminished as the GNP concentration increased. The study also found that the mass wear of the nanocomposites increased with the GNP concentration, with a 46% increase in the mass wear from 0.1 wt.% GNP to 0.5 wt.% GNP and a 202% increase from 0.1 wt.% GNP to 1.1 wt.% GNP. The presence of Al₄C₃ also affected the mass wear, with the effect diminishing as the GNP concentration increased. The study observed an increase in the mass wear with the increase in the GNP concentrations, but the mass wear of the annealed Al/GNPs with 1.1 wt.% GNP and Al₄C₃ was 52% lower than the Al composite with 1.1 wt.%. Overall, this study provides insights into the role of GNPs and Al₄C₃ on the tribological performance of aluminum-based nanocomposites.

The microstructure and mechanical properties of GNP-reinforced aluminum composites obtained by powder metallurgy and hot extrusion (at 400 °C, 500 °C, and annealing at 3 h at 610 °C), were investigated. It was found that: (i) depending on the processing applied, the composites showed an increase in yield strength (YS) and ultimate strength (US) of up to 283%, and 78%, respectively; (ii) depending on the size of the ex situ GNP and in situ Al₄C₃ reinforcements, two fracture mechanisms are observed: ductile and brittle–ductile; (iii) annealing for 3 h at 610 °C did not improve the mechanical properties; (iv) the plot of YS vs. the volume fraction of the GNP introduced showed a peculiar pattern not been reported so far. Theoretical analysis of the results showed: (1) the major contributor to the YS increase is the Hall–Petch mechanism; (2) the reinforcements contribution to YS, complements that of Hall–Petch; (3) the main contributor to the composite strength is GNP; (4) a critical size of the reinforcement exists, 1.43 nm, at which the YS is maximal, 260 MPa; (5) the increase in the processing temperature and time leads to Ostwald ripening and increase of Al₄C₃ size and deterioration of mechanical properties.