Current Development of Solid Superlubricity at the Microscale

Superlubricity, which refers to a state of extremely low frictional resistance, is the ultimate goal pursued by the development of lubrication technology. It can significantly reduce and even eliminate the wear at sliding interfaces and inhibit the energy dissipation induced by friction, effectively extending the life of moving components. The studies of superlubricity are thus of both fundamental and engineering importance, as the frontier of tribology. On the one hand, all frictional dissipation and wear are macroscopic manifestations of microscopic frictional actions at the sliding interfaces, so the understanding and realization of superlubricity at micro/nano-scale is the basis to restrain or even eliminate friction dissipation and wear failure at macroscopic scale. On the other hand, with the continuous improvements in manufacturing and processing technologies, miniaturisation and miniaturisation have led to an increase in the specific surface area of moving parts, which makes ultra-precision mechanical equipment and electronic devices face a more severe challenge of microscopic frictional wear and tear. Hence the understanding and realization of superlubricity at micro/nano-scale is also driven by the development of modern manufacturing miniaturization and nanotechnology revolution. From the early experience summaries of life and production and the classical Amonton’s law, to the modern doctrines of interface adhesion and mechanical engagement, to current tribological theory at the atomic or molecular scale, the understandings of friction have always improved with the progresses of science and technology. Although these understandings of friction vary from one period to another, it has never avoided the view that all frictional behaviors are always accompanied by kinetic or mechanical energy consumption, i.e., friction is the mechanical manifestation of the energy dissipation occurring in interface sliding. Precisely, the existence of sliding energy barriers is the intrinsic cause of friction. In view of this, this paper will focus on the idea on how to reduce the sliding potential barriers and reduce frictional dissipation, introduce the development and status quo of mainstream theories of solid superlubricity, discuss the general strategy for achieving micro-nano-scale solid superlubricity, and review the principles and methods of typical solid superlubricity in academia, etc. Firstly, the proposal, development and applications of structural superlubricity are introduced; secondly, the underlying principles and applications of continuous sliding ultra-low friction behaviors are also discussed; in additions, the concept of pressure-induced superlubricity proposed by the author's team in recent years is described, focusing on the research progress of pressure-induced superlubricity in terms of phenomenological discovery, basic principles, experimental observation methods and its possible foundations and applications. Finally, several aspects of superlubricity research that may need to be strengthened are proposed. It is hoped to enrich the academic community's understanding of the fundamental issues, scientific significance and its application value, elucidate the microscopic mechanism and realization strategy, point out the challenges and development direction of solid superlubricity, and make the studies of solid superlubricity moving forward from fundamental science to engineering applications through our current review.