Toughening mechanisms and mechanical properties of graphene nanosheet-reinforced alumina

High-frequency induction heat sintering (HFIHS) technology was employed for fabrication of highly dense (>99.5%) graphene-reinforced alumina nanocomposites. The mixed powders were consolidated at temperatures up to 1500°C with 0.25 to 3.0wt.% exfoliated graphene nanosheets (GNS). Compared with monolithic alumina, there was grain size refinement by 46% with an associated increase in the fracture toughness (by 72%) and hardness values (7%). Electron microscopy revealed that exfoliated GNS retained their inherent planar structure against any possible chemical and/or thermal adverse effect caused by rapid sintering. The intrinsic 2-dimensional sheet morphology and flexibility of the GNS promoted formation of large Al2O3/GNS interfacial area, thus leading to a dominant reinforcing mechanism via grain anchoring. The presence of GNS at the grain boundary areas not only inhibited grain growth through pinning effect, it also modified friction traits at nanoscale level by inducing slip-stick phenomenon that increased the Al2O3/GNS interfacial strength by means of improved efficiency of graphene pull-out and crack-bridging that subsequently imparted toughness and altered the failure behavior of the composites. A possible correlation between GNS incorporation into Al2O3 matrix and the resulting mechanical properties is established through high-resolution TEM studies that indicated graphene/alumina interface formation without any presence of severe intermediate phases.