Characterization and analysis of microscale superlubricity graphite surface
Authors: Shi Yun-Sheng  , Liu Bing-Qi  , Yang Xing  , Dong Hua-Lai  ,   Department of Electronics and Optics Engineering, Mechanical Engineering College, Shijiazhuang 050003, China;,   Department of Precision Instruments, Tsinghua University, Beijing 100084, China;,   State Key Laboratory of Precision Measurement Technology and Instrumentation, Tsinghua University, Beijing 100084, China;,   Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, China
Published: December 2020

Superlubricity may be the ideal and final solution for friction and wear.Superlubricity on a micrometer scale based on an excellent self-retraction phenomenon has been observed and realized under ambient conditions recently.But not all of the graphite interfaces can realize superlubricity even they are incommensurate.Therefore,in-depth studies of graphite interfaces are needed to find out the factors which prevent the superlubricity for being realized.For this reason, microscopic graphite mesas are fabricated on a highly oriented pyrolytic graphite in this paper to obtain superlubricity interfaces.After poor quality graphite layers are mechanically exfoliated from the highly oriented pyrolytic graphite,a silicon dioxide film is grown on a new graphite surface by plasma-enhanced chemical vapor deposition.Then the film is coated with photoresist.Microscopic photoresist square pattern is defined by electron beam lithography and used as a mask for reactive ion etching the SiO2 and highly oriented pyrolytic graphite to define graphite mesas.The graphite interfaces are obtained by shearing the graphite mesas by tungsten tips.Some of them are super lubricative,while others are not.
To study the graphite interfaces,atomic force microscope is used to characterize the morphologies of graphite mesas.The edges of graphite contact surfaces are also tested by energy dispersive spectrometer (EDS) and X ray photoelectron spectroscopy (XPS).The morphologies of the four graphite surfaces show that the superlubricity surfaces are atomically flat while other surfaces have many defects such as steps and tears.These results are consistent with those from the stone wall model of graphite crystal structure.The results of EDS and XPS show that there are many oxygen-containing bonds at the edges of the graphite surfaces.It is found that the polycrystalline structure of the highly oriented pyrolytic graphite plays an important role in the forming process of graphite interface and can affect the quality of the graphite interface.The quality of the graphite surface will determine whether the superlubricity can be realized.Besides the inner of graphite interface,the edges of the interfaces can also hinder the superlubricity from being realized.There are a large number of induced chemical bonds and the adsorbed physical bonds adhered to the edge of the graphite contact surfaces.When these bonds are broken,the energy is required.These bonds are the origin of the resistance when the graphite mesa is sheared away from the contact surface and causes friction force when the contact surface is relatively sliding along the other contact surface even the interface is super lubricative.
The results show that the polycrystalline structure of the highly oriented pyrolytic graphite can affect the quality of the graphite interface and determine whether the superlubricity can be realized.For the destruction of bonds sticking at the interface edge requires energy,the edge of the contact surface can cause the friction force of superlubricity.It is indicated that increasing the sizes of the graphite grains is beneficial to the realization of large area superlubricity.Using high temperature annealing or other methods to reduce the adsorbed bonds of the graphite edges will also reduce the frictional resistance in the process of superlubricity.