Heat generation and distribution during friction can lead to a large amount of energy loss. Friction-related wear can lead to serious problems such as damage to surfaces, reduced performance and reduced reliability of the devices. Approximately one third of the energy consumed in vehicles is lost due to friction and wear. Friction and wear considerably affect a wide range of applications, from conventional bearings, pistons and gears to micro-scale devices, gears and bio-related devices such as implants. Lubrication is the most effective way to control friction and wear. Water, animal products and vegetable oils have been used as lubricants in civilizations dating back to 4000 years as described in ancient frescoes in Egypt. We are still taking steps to increase our ability to control friction and wear by the help of modern lubrication science.
Every day, elaborate researches are carried out on high performance lubricants, because the available lubricants are still far from perfect. Using additives in lubricants is an effective way to improve performance. Certain materials, called additives, are blended into lubricants during production to add some desirable properties, to improve the existing properties, to minimize or eliminate undesirable properties. Improved lubricants available today typically include both base oil and additives. Base oil, the main component of the lubricant, determines the primary properties of the oil. On the other hand, the additives play a key role in providing new properties or in compensating for the disadvantages of the base oil, although the amount of additives in the oil formulation is quite low. Corrosion inhibitors, viscosity modifiers, friction modifiers, anti-wear agents, extreme pressure agents, antioxidants and antifoam agents, other special additives have been developed to produce lubricants with the desired properties to suit specific applications. Due to the difficulties of the current energy crisis, the demand for high-performance lubricant additives with improved friction reduction and anti-wear properties has increased significantly.
Materials with layered structures such as graphite and MoS2 are commonly applied as solid lubricants. The two adjacent layers of these structures combine with weak van der Waals force which causes very low sliding resistance. This allows adjacent layers to easily slide on each other under shear force (i.e., lubricating effect). The atoms in the same atomic layer are bonded to the monolayer structure by covalent bonds giving high sliding modulus and high strength. In addition, when compared to other nano-structured materials, two-dimensional (2D) nanomaterials have much higher specific surface area. This ensures that they cover a larger surface area than other nanomaterials, reducing the likelihood of direct contact with the friction surfaces and lowers the amount of friction between them.
Recently, 2D nanomaterials have been extensively investigated as novel lubricant additives. Developments in surface modification technologies offer new approaches to homogeneously distributed layered structure in base liquid media. In addition, techniques for synthesizing a monolayer structure and for isolating a single layer from multilayer structures provide a better understanding of the interaction between nanoparticle surfaces and other materials. These developments have supported the further investigation of the tribological performance of 2D nanomaterials as lubricant additives.
2D nanomaterials are superior in two respects when compared to conventional organic lubricant additives such as molybdenum dithiocarbamate (MoDTC), zinc dialkyldithiophosphate (ZnDTP) and zinc dialkyldithiophosphate (ZDDP). First, they have better performance in terms of friction reduction and wear prevention. Second, their increased chemical stability offers less harmful emissions and less toxicity than organic additives, which makes them attractive for environmental sustainability.
2D nanomaterials investigated as lubricant additives can be roughly divided into three categories: Graphene group, metal dichalcogenides and others. Due to its superior mechanical, chemical and electrical properties, graphene is extensively investigated both theoretically and experimentally. Tribological properties of graphene and its derivatives as lubricant additives were first published in 2011. In addition to its functions as a lubricant additive, graphene-based nanomaterials are a useful additive in water-based environments and in the oil. With its ability to prevent contact and form a protective film on the surface, significant improvement in lubrication performance can be achieved in various base environments by adding graphene family nanomaterials.
Metal dichalcogenides are characterized by MX2 stoichiometry and rombohedral structures. The most common metal dichalcogenides -MoS2 and WS2- have long been recognized as excellent solid lubricants and lubricant additives. Other metal dichalcogenides, such as TiSe2 and NbSe2, have recently proved satisfactory performance as additive materials.
Other materials with layered structure have recently been used as lubricant additives. Newly researched 2D nanomaterials have layers of phosphate, silicate and oxide with layered structure. The tribological performance of α-ZrP, Y2O3 and its derivatives as a lubricant additive is extensively investigated in various lubrication environment such as water, oil and grease. It has been experimentally proven that these materials as additives lower the viscosity and improve the performance of the base lubricants. Unlike graphene-like nanomaterials and MoS2, these nanomaterials exhibit a very high van der Waals force between any two adjacent layers, and the exfoliation of these layered structures is less likely. With this new mechanism, 2D nanomaterials can reduce friction by modifying the oil flow.
This work aims to provide brief information about the lubrication properties and tribological performance of 2D nanomaterials, their friction reduction and wear prevention properties, and mechanisms for improving lubrication performance. This work has been adapted from the introduction chapter of “Xiao, H., Liu, S., 2D Nanomaterials as Lubricant Additive: A Review, Materials and Design 135 (2017) 319–332, https://doi.org/10.1016/j.matdes.2017.09.029”.