Investigation on the origin of friction and superlubricity

Abstract

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 2D materials, such as molybdenum disulfide, boron nitride, graphite, and graphene, are introduced. The lowest friction coefficient for 2D 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 than 300 MPa; the system composed of water agent has been extended to solutions (acid, alkali, salt), alcohol-based liquids, oil-based system etc.; and the materials of friction pair are extended from mica, ceramics to sapphire, metal, DLC (diamond-like carbon) film, etc. In addition, recently 2D 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.