Microstructure, mechanical, and thermal properties of graphene and carbon nanotube-reinforced Al₂O₃ nanocomposites

The inherent brittleness of monolithic alumina (Al₂O₃) has been limiting its potential structural applications in some high performance areas, such as engine turbine components and high-temperature space materials. In the current study, effects of spark plasma sintering (SPS) parameters on the microstructure, mechanical, and thermal properties of graphene/CNTs-reinforced alumina nanocomposites were studied. In order to confirm that this was a successful process, the morphology and structure were examined by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). With the increase of graphene contents, the fracture toughness of Al₂O₃ and CNTs-Al₂O₃ increased by 38.4% and 49.5%, respectively. Novel toughening mechanisms, such as bridging, crack deflection, and pull-out and embedded graphene, have been observed to be responsible for the significant increase in toughness. The highest thermal conductivity of 23.3 and 22.4 Wm⁻¹ K⁻¹ was observed for monolithic alumina and 0.4 wt% graphene & 1 wt%CNTs/Al₂O₃ nanocomposites, respectively. The results also revealed that the increase in temperature leads to the decrease of thermal conductivity, but leads to the increase of heat capacity of monolithic Al₂O₃ and the hybrid composites.