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. [ABSTRACT FROM AUTHOR]