Impacts of environments on nanoscale wear behavior of graphene: Edge passivation vs. substrate pinning

Being a typical layer structured two-dimensional material, graphene has a high potential as an efficient and atomically-thin solid lubricant. However, the outstanding lubricating property of graphene is found to be seriously impacted by environmental humidity, exhibiting a complicated and sometimes contradicting trend. In this work, the anti-wear performance of graphene was explored by nanoscale scratch tests under different humidities. The experimental results show that wear of graphene primarily occurs at the step edge where dangling bonds are most abundant. In general, humid environment would improve wear resistance of graphene due to dangling bond passivation by condensed water, as confirmed by molecular dynamics simulations. However, for substrate surfaces that can strongly interact with the carbon atoms at the graphene edge, a noticeably improved anti-wear performance, less dependent on humidity, can be observed due to substrate pinning effect. The revealed mechanisms successfully clarify humidity dependence of wear characteristics of graphene on various substrates. The findings also provide a general strategy for improving the performance of graphene in a broad range of mechanical applications. (C) 2018 Elsevier Ltd. All rights reserved.