An estimated one-half to one-third of the world’s energy is consumed by friction and wear. Lubrication aims to effectively reduce friction, wear and energy consumption [1
]. However, conventional lubricants are limited by their service life and dependence on contact conditions [3
]. Therefore, scholars have continuously sought for high-performance and environmentally friendly lubricants [4
]. In recent years, carbon nanomaterials, such as fullerenes and carbon nanotubes, have been adopted as additives to water and lubricating oils, but have not been widely used in the friction industry because of their high cost [5
Graphene was discovered in 2004 by physicists Andre Geim and Konstantin Novoselov [6
]. Graphene is a new type of carbon nanomaterial with a hexagonal honeycomb structure composed of single-layer carbon atoms. The thickness of monolayer graphene is only 0.335 nm, which is the thinnest nanomaterial in the world [7
]. Its mechanical strength, thermal conductivity, conductivity, and other properties are excellent [6
]. Two-dimensional materials, such as boron nitride (BN) and molybdenum disulfide (MoS2
), were also discovered subsequently [9
]. Given their excellent performance, 2D materials hold great application potential in various fields including microelectronic devices, sensors, catalysts, batteries, biomedicine, and composite materials [10
]. Scholars found that the phenomenon of superlubricity exists in layered 2D materials, such as MoS2
and BN [24
], and they experimentally investigated the excellent anti-friction properties of 2D materials [30
]. Hence, further studies showed that 2D materials can easily enter friction surfaces because of the ultra-thin layer structure and extremely low shear strength between the layers; these characteristics help prevent the direct contact of the friction surface, and decrease the coefficient of friction [37
]. At present, the lubrication mechanisms of 2D materials are mainly as follows [43
Film formation mechanism. On the one hand, 2D materials become quickly adsorbed on a friction surface to form a physical adsorption film, or deposited on a friction surface to form a deposited film. On the other hand, they can also react chemically on the friction surface to create a chemical reaction film, thereby enhancing the wear resistance of the friction pair surface.
Self-healing mechanism. Two-dimensional materials can fill a concave area on a friction surface to smoothen it (Figure 1
). Gulzar et al. [44
] investigated the self-healing effect of nanomaterials. Nanoparticles deposit on interacting surfaces and compensate for the mass lost, thereby reducing wear and tear.
Ball bearing mechanism. Two-dimensional materials disperse at the contact surface to form ‘class bearings’ and transform sliding friction into rolling friction, thereby showing excellent anti-friction performance (Figure 2
Two-dimensional materials not only exhibit excellent anti-friction and anti-wear properties, but also have reasonable preparation costs in the field of lubrication. Thus, 2D materials are selected as high-performance and environmentally friendly lubricants [45
Liquid-phase lubrication is a type of lubrication system with many varieties in the industrial field. This type of lubrication system is effective for high-speed and high-load conditions in the industry because of the advantages of forming a special protective film, low friction, and low energy consumption. Liquid-phase lubrication can generally be divided into oil-based lubrication and water-based lubrication, and it can convert external friction between two moving surfaces into internal friction between protective films, thereby separating the two moving surfaces and reducing friction. In general, liquid-phase lubrication is the most widely used in traditional industrial fields such as construction machinery, steel, and automobiles. The advantages are that not only can the friction and heat of the equipment be reduced, but also its safety and reliability can be improved.
Graphite is a common standard for lubrication. However, when used in liquid-phase solution, there are many disadvantages limiting its lubrication properties. First, it is hardly possible to disperse steadily in liquid-phase solution. Second, it weakens the fluidity and reduces the lubrication performance of lubricants. Third, graphite is barely deposited on the rubbing surface, which confirms that graphite has difficulty entering the contact area and, consequently, forming a continuous protective film. By contrast, graphene can be stabilized by being modified with a surfactant or by chemical modification. Graphene easily forms protective deposited films to prevent the rubbing surfaces from coming into direct contact and thereby improves the entirely tribological behavior of the oil. Finally, during the sliding process, graphene with higher exfoliation easily forms to lamellar a protective film [61
]. Thus, graphene can be transported into the friction zone by the liquid flow. Two-dimensional materials can be used in a liquid-phase lubrication system. For instance, Song et al. [62
] investigated the lubricating properties of graphene oxide (GO) as a water-based lubricant additive. Under a load of 60 N, the water-lubricated friction coefficient of GO with a mass fraction of 1% was 0.127. GO can easily enter the two contact surfaces and form a friction film on the contact surface to prevent direct contact of the steel balls. Chen et al. [63
] synthesized oil-soluble ultra-thin MoS2
sheets by the solvothermal method. These sheets effectively control the wear when the rated load reaches about 1 GPa. The mechanism of lubrication involves ultra-thin MoS2
sheets easily entering and adhering to the contact surface; such adherence helps smoothen the sliding interface. In addition, different 2D materials possess unique characteristics. For example, boron nitride maintains good lubricating properties at high temperatures [64
]. Nanometer-scale ultra-thin MoS2
can greatly improve the extreme pressure performance of lubricating oil [65
]. Therefore, the application of 2D materials as additives for liquid-phase lubrication can further increase the effective performance by improving the lubrication characteristics, which hold a broad prospect.
This review describes in detail the research results and advances in the use of 2D materials as additives to liquid-phase lubrication, such as lubricating oil and water lubrication systems, in terms of experimental content and lubricating properties, lubrication mechanism and influencing factors in recent years. Finally, the review summarizes the prospects and challenges amongst 2D materials in this field and proposes a series of feasible measures.