Percolative Electrical Conductivity of Platy Alumina/Few-layer Graphene Multilayered Composites

Abstract

It is of critical importance to improve toughness, strength, and wear-resistance together for the development of advanced structural materials. Herein, we report on the synthesis of unoxidized graphene/alumina composite materials having enhanced toughness, strength, and wear-resistance by a low-cost and environmentally benign pressure-less-sintering process. The wear resistance of the composites was increased by one order of magnitude even under high normal load condition (25 N) as a result of a tribological effect of graphene along with enhanced fracture toughness (K IC ) and flexural strength (s f ) of the composites by ,75% (5.60 MPa?m 1/2 ) and ,25% (430 MPa), respectively, compared with those of pure Al 2 O 3 . Furthermore, we found that only a small fraction of ultra-thin graphene (0.25-0.5 vol%, platelet thickness of 2-5 nm) was enough to reinforce the composite. In contrast to unoxidized graphene, graphene oxide (G-O) and reduced graphene oxide (rG-O) showed little or less enhancement of fracture toughness due to the degraded mechanical strength of rG-O and the structural defects of the G-O composites.A tomically thin graphene is one of the strongest materials; therefore, it is promising as a toughening/ strengthening agent in ceramic-based structural materials 1 . Smaller nano-scale graphene, compared to conventional whisker/fiber reinforcement, would induce smaller flaws, potentially resulting in higher strength/toughness 2 . Considering the enormous surface area of graphene (e.g. single-layer graphene ,2630 m 2 / g), surprisingly small amounts of graphene would be enough to satisfy complete coverage of precursor nanoparticles (less than 1.0 vol%). In the case of carbon nanotube (CNT), it has been reported that higher reinforcement concentration (1 , 10 vol%) was generally required for the toughening and strengthening of CNT/ceramic composites [2][3][4][5][6][7][8] . Moreover, graphene is a good candidate for solid lubrication that reduces the friction force between contact surfaces at micro-and nano-scale while protecting the coated surface [9][10] . A nanometer-thick surface layer of hard, strong, and lubricating graphene on ceramic grains can lead to a significant improvement in contact-damage resistance, such as wear resistance due to a tribological effect of 2-dimensional (2D) graphene. However, previous studies on graphene tribology have focused on nano-scale friction and wear behavior (normal load ,250 mN) [9][10][11] . On the other hand, micro-/macro-scale tribological studies of graphene have remained relatively unexplored, but there is an increasing need to utilize graphene's full potential for diverse tribological applications. There has been no report on the friction and wear behavior of ceramic-based graphene composites in micro-and macro-scale under high normal load (.20 N), which is important for contact-mechanical applications (e.g. bearing, valves, nozzles, armour, prostheses) and protective coating applications.Structural materials for extreme environments (e.g. high temperature/press...