微米级超润滑石墨接触面的表征与分析∗

摘要

超润滑可能是解决摩擦磨损问题的理想方案。目前已经能够在大气环境下实现基于石墨的微米尺度超润滑，但石墨接触面在超润滑实现过程中的影响还需要深入研究。为此，本文用电子束曝光及反应离子刻蚀方法在高定向热解石墨上加工出微米尺度的氧化硅/石墨方台结构，并用钨针尖推开方台的上部获得超润滑的石墨接触面。然后用原子力显微镜对多个石墨接触面进行了形貌表征，并使用能谱仪及X射线光电子能谱对石墨接触面的边缘进行测试。研究发现，高定向热解石墨的多晶结构在接触面的形成过程中有重要影响，能够决定接触面的质量进而决定超润滑能否实现。石墨接触面的边缘存在大量加工中引入的化学键及在大气中吸附的物理键，这些键是推开石墨方台形成接触面时阻力的来源，并在接触面发生相对滑动时表现为摩擦力。本文通过对具有微米尺寸的超润滑石墨接触面进行研究，明确了接触面内部及边缘影响超润滑实现的规律，对大面积超润滑的实现及应用能够提供有益的帮助。%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.