Humidity-sensitive macroscopic lubrication behavior of an as-sprayed graphene oxide coating

The mechanism by which graphite exhibits humidity-sensitive lubrication performance remains disputed. Considering graphene oxide (GO) is the basic unit of the 3D layered structure and oxygen functional groups can easily interact with water molecules, this study systematically determines the influence of humidity on the friction behavior of GO. GO shows a friction behavior that completely contrasts that of reduced GO (RGO) under varying humidities due to the presence of oxygen functional groups in the former. While the friction coefficient of GO increases with increasing humidity, that of RGO decreases. Moreover, the friction coefficient of GO is reversible under alternating humidities. This is because low humidity favors the formation of an ordered layered structure on the friction interface, resulting in a low and stable friction coefficient. In high-humidity conditions, water molecules generate strong hydrogen bonding at the GO interlayer, gradually disrupting and transforming the ordered layered structure into an amorphous carbon one and increasing friction coefficient. The result of this study can help improve the understanding on the water molecule-related lubrication mechanism of graphitic materials and provide a feasible strategy for manipulating the friction behavior of GO by adjusting humidity, thereby shedding light on the development of novel environment-responsive micro/nano devices. (C) 2018 Elsevier Ltd. All rights reserved.