Hydrogenated amorphous carbon (a-C:H) is capable of providing a near-frictionless lubrication state when rubbed in dry sliding contacts. Nevertheless, the mechanisms governing superlubricity in a-C:H are still not well comprehended, mainly due to the lack of spatially resolved structural information of the buried contact surface. Here, we present structural analysis of the carbonaceous sliding interfaces at the atomic scale in two superlubricious solid lubricants, a-C:H and Si-doped a-C:H (a-C:H:Si), by probing the contact area using state-of-the-art scanning electron transmission microscopy and electron energy-loss spectroscopy. The results emphasize the diversity of superlubricity mechanisms in a-C:Hs. They suggest that the occurrence of a superlubricious state is generally dependent on the formation of interfacial nanostructures, mainly a tribolayer, by different carbon rehybridization pathways. The evolution of such anti-friction nanostructures highly depends on the contact mechanics and the counterpart material. These findings enable a more effective manipulation of superlubricity and developments of new carbon lubricants with robust lubrication properties.

Superlubricity, the state of ultralow friction between two sliding surfaces, has become a frontier subject in tribology. Here, a state-of-the-art review of the phenomena and mechanisms of liquid superlubricity are presented based on our ten-year research, to unlock the secrets behind liquid superlubricity, a major approach to achieve superlubricity. An overview of the discovery of liquid superlubricity materials is presented from five different categories, including water and acid-based solutions, hydrated materials, ionic liquids (ILs), two-dimensional (2D) materials as lubricant additives, and oil-based lubricants, to show the hydrodynamic and hydration contributions to liquid superlubricity. The review also discusses four methods to further expand superlubricity by solving the challenge of lubricants that have a high load-carrying capacity with a low shear resistance, including enhancing the hydration contribution by strengthening the hydration strength of lubricants, designing friction surfaces with higher negative surface charge densities, simultaneously combining hydration and hydrodynamic contribution, and using 2D materials (e.g., graphene and black phosphorus) to separate the contact of asperities. Furthermore, uniform mechanisms of liquid superlubricity have been summarized for different liquid lubricants at the boundary, mixed, and hydrodynamic lubrication regimes. To the best of our knowledge, almost all the immense progresses of the exciting topic, superlubricity, since the first theoretical prediction in the early 1990s, focus on uniform superlubricity mechanisms. This review aims to guide the research direction of liquid superlubricity in the future and to further expand liquid superlubricity, whether in a theoretical research or engineering applications, ultimately enabling a sustainable state of ultra-low friction and ultra-low wear as well as transformative improvements in the efficiency of mechanical systems and human bodies.

Two-dimensional MoS₂ nansheets (2D MoS₂) with enhanced tribological properties were successfully fabricated with the aid of oleylamine.