Load-Dependent Friction Hysteresis for Graphitic Surfaces in n-Hexadecane

Sliding-induced friction behavior of a single-asperity silica probe against few-layer (FL) graphene and bulk graphite is measured in the presence of n-hexadecane using an atomic force microscope (AFM). The load-dependent nanoscale friction measurements display friction hysteresis, i.e., higher friction forces during unloading of the contact than loading, at a given normal load. However, unlike hysteresis in friction of graphene measured in ambient, several unique trends are observed when the contact is immersed in n-hexadecane. First, the friction hysteresis is measured up to a transition load and beyond which is found negligible; second, a similar behavior is observed on bulk graphite; and third, a friction strengthening of the contact persisted up to several nanometers. Quasi-static force-separation curves identify up to four layers of n-hexadecane solvation layers on graphitic surfaces. Molecular dynamic simulations illustrate that the solvated n-hexadecane molecules within the contact carry the probe load and determine the generated contact area, affecting friction hysteresis. Further, during AFM probe sliding, instead of a pucker, a molecular pile-up of n-hexadecane, in front of the tip is observed. These findings provide new perspectives on understanding of the dissipation mechanisms of graphene that predominantly are surrounded by structured liquid molecules.