Load-Dependent Friction Hysteresis on Graphene.
Nanoscale friction often exhibits hysteresis when load is increased (loading) and then decreased (unloading) and is manifested as larger friction measured during unloading compared to loading for a given load. In this work, the origins of load-dependent friction hysteresis were explored through atomic force microscopy (AFM) experiments of a silicon tip sliding on chemical vapor deposited graphene in air, and molecular dynamics simulations of a model AFM tip on graphene, mimicking both vacuum and humid air environmental conditions. It was found that only simulations with water at the tip-graphene contact reproduced the experimentally observed hysteresis. The mechanisms underlying this friction hysteresis were then investigated in the simulations by varying the graphene-water interaction strength. The size of the water-graphene interface exhibited hysteresis trends consistent with the friction, while measures of other previously proposed mechanisms, such as out-of-plane deformation of the graphene film and irreversible reorganization of the water molecules at the shearing interface, were less correlated to the friction hysteresis. The relationship between the size of the sliding interface and friction observed in the simulations was explained in terms of the varying contact angles in front of and behind the sliding tip, which were larger during loading than unloading.
Department of Mechanical Engineering, University of California Merced , 5200 North Lake Road, Merced, California 95343, United States.
Zhijiang
Z
Ye
Department of Mechanical and Manufacturing Engineering, University of Calgary , 40 Research Place NW, Calgary, Alberta T2L 1Y6, Canada.
Philip
P
Egberts
Physics Division, Center for Integrated Nanostructure Physics, Sungkyunkwan University , Suwon 440-746, South Korea.
Gang Hee
GH
Han
A T Charlie
AT
Johnson
Robert W
RW
Carpick
Department of Mechanical Engineering, University of California Merced , 5200 North Lake Road, Merced, California 95343, United States.
Ashlie
A
Martini
atomic force microscopy; chemical vapor deposition; graphene; molecular dynamics simulations; nanotribology