Abstract
Load-dependent friction hysteresis is an intriguing phenomenon that occurs in many materials, where the friction measured during unloading is larger than that measured during loading for a given normal load. However, the mechanism underlying this behavior is still not well understood. In this work, temperature-controlled friction force microscopy was utilized to explore the origin of friction hysteresis on exfoliated monolayer graphene. The experimental observations show that environmental adsorbates from ambient air play an important role in the load dependence of friction. Specifically, the existence of environmental adsorbates between the tip and graphene surface gives rise to an enhanced tip-graphene adhesion force, which leads to a positive friction hysteresis where the friction force is larger during unloading than during loading. In contrast to positive friction hysteresis, a negative friction hysteresis where the friction force is smaller during unloading than during loading is observed through the removal of the environmental adsorbates upon in situ annealing. It is proposed that the measured friction hysteresis originates from the hysteresis in the contact area caused by environmental adsorbates between the tip and graphene. These findings provide a revised understanding of the friction hysteresis in monolayer graphene in terms of environmental adsorbates.
Highlights
A comprehensive and in-depth understanding of the frictional properties of two-dimensional (2D) materials, such as graphene, MoS2, and h-BN, is of great significance for the development of practical 2D-material-based lubricants and coatings for friction reduction and wear protection applications [1,2,3,4,5,6]
The influence of environmental absorbates on the friction hysteresis of exfoliated monolayer graphene supported on a SiO2/Si substrate was explored through temperature-controlled friction force microscopy (FFM)
The transition from positive friction hysteresis to negative friction hysteresis through the removal of the environmental adsorbates was observed
Summary
A comprehensive and in-depth understanding of the frictional properties of two-dimensional (2D) materials, such as graphene, MoS2, and h-BN, is of great significance for the development of practical 2D-material-based lubricants and coatings for friction reduction and wear protection applications [1,2,3,4,5,6]. It has been reported that nanoscale friction on 2D materials can be strongly enhanced by chemical modification of the surface [15, 16]. Another interesting frictional property of 2D materials is the loading history dependence of friction [11, 17,18,19]. It was found that the friction force on graphene measured during unloading was significantly larger than that measured during loading for a given load [18, 19] To explain this loaddependent friction hysteresis phenomenon, different mechanisms have been proposed.
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