超滑与摩擦起源的探索

Investigation on the origin of friction and superlubricity

摩擦消耗掉全世界约1/3的一次能源,磨损致使大约80%的机器零部件失效,每年因摩擦、磨损造成的经济损失约占国内生产总值(gross domestic product,GDP)的2%~7%.对于中国这样的制造大国,摩擦、磨损造成的损失占比较高.仅以5%计算,2019年我国因摩擦、磨损造成的损失就可能达4.95万亿元.因此,如何减少摩擦、降低磨损是大家广为关注的问题,也是影响人类社会走向绿色、环保、有效利用资源和能源的关键因素之一.探索摩擦起源,实现超滑状态成为摩擦学研究的重大使命.超滑就是将摩擦系数呈数个数量级的降低,同时伴随磨损率呈数量级下降的一种新技术.本文重点介绍了我们课题组近期在超滑和微观摩擦能量耗散机制方面的研究进展,同时也对国内外相关研究进行了简要评述.

Friction consumes about a third of the world’s primary energy,wear causes about 80%of machine parts to fail,and the economic losses caused by friction and wear account for about 2%–7%of its gross domestic product(GDP)for different countries every year.For large manufacturing countries such as China,the loss caused by friction and wear is relatively high.If we only calculate by 5%,the loss caused by friction and wear in China in 2019 may reach 4.95 trillion RMB.Therefore,how to reduce friction and wear is a widely concerned problem,and it is also one of the key factors that affect the human society to go green,environmental protection,and effective use of resources and energy.In present paper,advancements in the research of the origin of friction and in superlubricity have been reviewed,in which most is from my group.In the part of the origin of friction,the energy dissipation mechanism of micro-friction has been introduced firstly.The works on such area by molecular dynamics simulation showed that the friction generation process will be accompanied by the energy dissipation of phonons,electrons and acoustic-electric coupling modes.The dissipative form of sliding kinetic energy is mainly that the sliding process produces lattice vibration,which makes the energy irreversibly dissipated in the form of phonons.However,the phonon dissipation measurements of the tribo-process have not been reported at present and all these results need to be proved.Our group developed a friction energy dissipation measurement system,which can realize the measurement of various emission of friction process,phonon and electron dissipation process,molecular orientation change,micro friction force and ultra-low friction coefficient.By such system,it is found that the defects will trap neutral excitons and cause the excitons to dissipate energy faster,and also can reduce the exciton diffusion length significantly.Superlubricity is a new technique that reduces the friction coefficient by several orders of magnitude,accompanied by orders of magnitude decrease in wear rate.It is divided into three sides,one is solid superlubricity,and another is liquid superlubricity,and then is the combine superlubricity of liquids and solids.In the part of solid superlubricity,the introduction of the concept of superlubricity and the latest research progress on the superlubricity properties of 2 D materials,such as molybdenum disulfide,boron nitride,graphite,and graphene,are introduced.The lowest friction coefficient for 2 D materials about 0.00002 has been gotten in my group.In the amorphous carbon system we achieve superlubricity state with friction coefficient about 0.0001 under low temperature(-110°C)and high pressure(>2 GPa).In the field of liquid superlubricity,it has developed very rapidly in the past 20 years.We conclude three mechanisms for liquid superlubricity:Fluid hydrodynamic effects,double layer force and hydration force.Sometimes one mechanism can lead to superlubricity,and sometimes it requires the joint action of two or three mechanisms.Supported by these three mechanisms,the liquid superlubricity bearing capacity has been improved from about 10 MPa 20 years ago to more than300 MPa;the system composed of water agent has been extended to solutions(acid,alkali,salt),alcohol-based liquids,oilbased system etc.;and the materials of friction pair are extended from mica,ceramics to sapphire,metal,DLC(diamondlike carbon)film,etc.In addition,recently 2 D materials as additives have been added to liquid to form a liquid-solid combined superlubricity state in my group and the bearing pressure has greatly increased to the GPa level,which is a big step forward for the application of liquid superlubricity.Therefore,the time has come for the engineering application of superlubricity,and the concept of"superlubricitive engineering"has been proposed.