清华大学微纳米力学与多学科交叉研究中心（简称微纳力学中心）郑泉水教授领导的一个跨学科研究团队近期在超润滑研究方面取得重大突破。该研究成果以“微米尺度石墨中超润滑现象的观察”为题在国际顶尖物理杂志《物理评论快报》（Physical Review Letters）上发表。文章第一作者为郑泉水指导的博士生刘泽。

  A multidisciplinary research team led by Prof. Quanshui Zheng atthe Center for Nano and Micro Mechanics (CNMM) in Tsinghua University has achieved a breakthrough in superlubricity. The research result, titled “Observation of microscalesuperlubricity in graphite”, has been published in the flagship physics journal Physical Review Letters. The first author Mr. Ze Liu is a PhD student under Prof. Zheng’s supervision.

　　 固体之间的摩擦是所有物理现象中最具挑战性的问题之一。几千年来人类一直在努力控制和减小摩擦（例如车轮和车轴之间的摩擦）。据统计，全世界约1/3的一次性能源由摩擦过程消耗；工业发达国家因摩擦磨损造成的损失高达GDP的5%-7%。现代制造领域呈现出超精密化和微型化趋势，由于比表面积的增大，界面摩擦成为一个决定性的因素。

Friction between solid objects, for example between a wheel and an axle, is a challenging topic that humans havestruggled to control and minimize for thousands of years, and is one of the most challenging physics phenomena. Approximately one-third of the world's energy resources in present use are wasted overcoming friction in one form or another. In industrialized countries, the friction loss accounts for 5%-7% of GDP. With modern mechanical manufacturing developing toward high precision and miniaturization, and the resulting increase in the area-to-volume ratio of micro- and nano-mechanical systems, interfacial friction is becoming a dominant factor.

　　超润滑（superlubricity）或称结构润滑（structural lubricity）是1990年代早期由平野元久（MotohisaHirano）提出的由于晶体表面以非公度形式接触时，可能出现的界面摩擦和磨损几乎为零的现象。超润滑是解决摩擦磨损问题的根本途径，若能获得普遍应用，将是人类文明史上的一大进步。过去二十年中超润滑实验现象主要在纳米尺度以及高真空条件下实现。并且被认为在大尺度下不存在该现象。这极大的限制了超润滑的实际应用。

Superlubricity(or structural lubricity) is a phenomenon proposed by MotohisaHirano in the early 1990s, where the friction force almost vanishes between two solid surfaces. The effect occurs when there is structural incommensuration between two crystalline solid surfaces, typically due to a relative rotation of their lattices, leading to the systematic cancelling out of the friction force on the atomic scale.Superlubricity is an ideal solution to reduce friction and wear, and thus its real and wide applications would mark a big step in civilization.But in the past twenty years, superlubricitywas only observed on the nanometer scale and under high vacuum conditions. Such conditions significantly limit the practical application of superlubricity. It has even been suggested that superlubricitymay not exist on a larger size scale。                                                                                

       、

         滑移形成的分层

           Sheared

            Retracted                                                                               

   郑泉水教授的研究团队最近取得了突破性进展，他们实现了在微米尺度（1-10微米）以及大气环境下的超润滑。他们的实验基于郑泉水研究团队于几年前发现的石墨岛奇妙的自缩回现象，成果于2008年在《物理评论快报》上发表，是所有单晶材料该类现象的第一个发现。如上图所示，利用微机械臂推动微米级石墨岛（由离子蚀刻高定向热解石墨制成）可以造成石墨岛的分层现象。当释放微机械臂，在表面能的驱动下，被推出的石墨岛上层会自动缩回。

   Recently, a multidisciplinary research group led by Prof. Zheng has made an experimental breakthrough. They have demonstratedsuperlubricity in micrometer scale and under ambient conditions. Their experiments are based on a novel self-retraction phenomenonthat occurs in lithographically manufactured graphite mesas, observed for the first time in a couple years ago by Zheng’s term, and also published in Physical Review Letters in 2008. As shown in above figures, using a microprobe to shear the top of a graphite mesa, which is fabricated by using plasma ion etching high-quality samples, leads to a delamination of the graphite mesa. After releasing the shearing force by raising the microprobe, the sheared top flake will spontaneously return back to its original position, driven by the reduction in surface energy that this produces.

   微纳力学中心的研究团队根据表面能驱动力和接触面积估算的摩擦剪应力上界（0.02－0.04MPa）远远小于目前公认的超润滑摩擦剪应力（0.1－1.0MPa）。当有意识的利用微机械臂旋转移出的石墨岛顶层，他们发现在特定的角度下，自回复现象完全消失。这些自锁角度成6次面内对称，和石墨的面内晶格对称性完全吻合。实验而且证实在自锁情况下，摩擦剪应力极限提高了3个量级（约为0.1GPa）。

The CNMM teamcouldestimate the upper bound of the friction shear stress by comparing with the self-retraction force and the known contact area: 0.02-0.04MPa. This is far smaller than the widely accepted threshold for superlubricity, a friction shear stress of 0.1-1.0MPa. When deliberately rotating the top flakeusing the microprobe, the researchers found that at certain angles the self-retraction completely disappeared (a so-called lock-in effect, where the atomic lattices match up commensurately at the interface). These lock-in angles exhibit a clear 6-fold symmetry, which is consistent with the crystal symmetry of HOPG in the basal plane. Additionally, in the lock-in states, experimentally measured values of the friction shear stress are increased over three orders of magnitude, up to 0.1GPa.

   这说明石墨岛的自缩回现象是超润滑的直接体现，清楚地展现了微米尺度的超润滑可以出现。特别值得注意的是该超润滑现象可以在大气环境中实现，并且具有极好的重复性。

These results provide compelling evidencethat the self-retraction motion isa direct manifestation ofstructural superlubricityon the micrometer scale. A particularly significant observation, for the consideration of future practical applications,is that this structural superlubricity can be achieved under ambient conditions and exhibits excellent reproducibility.

　　该研究成果最近被著名的学术网站physicsworld.com头条报道(http://physicsworld.com/cws/article/news/2012/apr/05/nanomachines-could-benefit-from-superlubricity)。在该报道中，超润滑的奠基者平野元久教授如此评论“这是在超润滑现象超越纳米尺度所迈出的一大步(Big Advance)。它可以促进超润滑现象在机械工程实际中的应用”。以色列Tel Aviv大学的Michael Urbakh教授认为“这是一个非常有趣和令人鼓舞的研究工作，可以推动超润滑领域和更广泛意义上的摩擦研究领域的突破性进展。这项工作开辟了制备石墨润滑剂来提高润滑性能的新方向。”

This research has been headlined as the top news on April 5 in the popular website physicsworld.com (http://physicsworld.com/cws/article/news/2012/apr/05/nanomachines-could-benefit-from-superlubricity). In the report, the original discoverer of thesuperlubricity phenomenon, Prof. Hirano,commented “This is a big advance beyond the nanometre-scale superlubricity experiment. It could lead to implementing superlubricity ［as a］ lubricant for future practical use in mechanical engineering, including ［devices for］ saving energy.” Another specialist in superlubricity, Prof. Michael Urbakh of Tel Aviv University in Israel commented “I think this is very interesting and promising work, which could lead to a breakthrough in the field of superlubricity and more generally in the control of friction properties. This work may open a new way for preparation of graphite lubricants with improved lubrication properties.”

　　这项研究是与澳大利亚莫纳什大学刘哲高级讲师、清华大学微纳力学中心的程曜教授和弗朗索瓦.格雷（Francois Grey）教授等研究小组合作完成的，由国家自然科学基金委员会、国家重大研究发展计划、863计划资助。

This research work is done in collaboration with Dr. Zhe Liu of Monash University in Australia, and Profs. Yao Cheng and Francois within CNMM. This project is supported by the NSFC, the National Basic Research Program of China, and the National 863 Project.