Ionic liquids (ILs) have emerged as potential lubricants in 2001. Subsequently, there has been tremendous research interest in ILs from the tribology society since their discovery as novel synthetic lubricating materials. This also expands the research area of ILs. Consistent with the requirement of searching for alternative and eco-friendly lubricants, IL lubrication will experience further development in the coming years. Herein, we review the research progress of IL lubricants. Generally, the tribological properties of IL lubricants as lubricating oils, additives and thin films are reviewed in detail and their lubrication mechanisms discussed. Considering their actual applications, the flexible design of ILs allows the synthesis of task-specific and tribologically interesting ILs to overcome the drawbacks of the application of ILs, such as high cost, poor compatibility with traditional oils, thermal oxidization and corrosion. Nowadays, increasing research is focused on halogen-free ILs, green ILs, synthesis-free ILs and functional ILs. In addition to their macroscopic properties, the nanoscopic performance of ILs on a small scale and in small gaps is also important in revealing their tribological mechanisms. It has been shown that when sliding surfaces are compressed, in comparison with a less polar molecular lubricant, ion pairs resist "squeeze out" due to the strong interaction between the ions of ILs and oppositely charged surfaces, resulting in a film that remains in place at higher shear forces. Thus, the lubricity of ILs can be externally controlled in situ by applying electric potentials. In summary, ILs demonstrate sufficient design versatility as a type of model lubricant for meeting the requirements of mechanical engineering. Accordingly, their perspectives and future development are discussed in this review.

Ionic liquid lubricants: when chemistry meets tribology

Cutting fluid corrosion inhibitors from inorganic to organic: Progress and applications

Review of the toxic effects of ionic liquids

Towards superior lubricity and anticorrosion performances of proton-type ionic liquids additives for water-based lubricating fluids

Water-based lubricants (WBLs) have been at the forefront of recent research, due to the abundant availability of water at a low cost. However, in metallic tribo-systems, WBLs often exhibit poor performance compared to petroleum-based lubricants. Research and development indicate that nano-additives improve the lubrication performance of water. Some of these additives could be categorized as solid nanoparticles, ionic liquids, and bio-based oils. These additives improve the tribological properties and help to reduce friction, wear, and corrosion. This review explored different water-based lubricant additives and summarized their properties and performances. Viscosity, density, wettability, and solubility are discussed to determine the viability of using water-based nano-lubricants compared to petroleum-based lubricants for reducing friction and wear in machining. Water-based liquid lubricants also have environmental benefits over petroleum-based lubricants. Further research is needed to understand and optimize water-based lubrication for tribological systems completely.

Water-Based Lubricants: Development, Properties, and Performances

In situ preparation of multifunctional additives in water

Taking 1,4-naphthalene dicarboxylic acid (1,4-ND), 2,3-naphthalene dicarboxylic acid (2,3-ND) and tetrabutyl ammonium hydroxide (N4444OH), tetrabutyl phosphonium hydroxide (P4444OH) as raw materials, water-based lubricant additives tetrabutyl ammonium 1,4-naphthalene dicarboxylate (1,4-NDN4444), tetrabutyl phosphonium 1,4-naphthalene dicarboxylate (1,4-NDP4444), tetrabutyl ammonium 2,3-naphthalene dicarboxylate (2,3-NDN4444), and tetrabutyl phosphonium 2,3-naphthalene dicarboxylate (2,3-NDP4444) were prepared in aqueous solution. The additives are not only simple to prepare, but also have good solubility in water. More importantly, they can significantly improve the friction reducing and anti-wear performances of water. On account of the mechanism analysis, it is estimated that the introduction of aromatic ring and more carboxyl groups to the additives play an important role for them to form stable lubrication protective films during the friction process and then to achieve excellent lubrication performance on steel/steel, copper/steel, and aluminum/steel friction pairs. Remarkably, the performances of 2,3-NDN4444 and 2,3-NDP4444 are superior to those of 1,4-NDN4444 and 1,4-NDP4444, which is ascribed to their stronger adsorption on the metal surfaces contributed by the two adsorption targets (two –COO− groups). For 1,4-NDN4444 and 1,4-NDP4444, there is only one –COO− group that can be simultaneously adsorbed on the metal surfaces from the perspective of space configuration. It weakens the adsorption and decreases the effectiveness of the lubrication protective films resulting in the reduced friction reducing and anti-wear properties of 1,4-NDN4444 and 1,4-NDP4444. Water-based lubricant additives tetrabutyl ammonium 1,4-naphthalene dicarboxylate (1,4-NDN4444), tetrabutyl phosphonium 1,4-naphthalene dicarboxylate (1,4-NDP4444), tetrabutyl ammonium 2,3-naphthalene dicarboxylate (2,3-NDN4444) and tetrabutyl phosphonium 2,3-naphthalene dicarboxylate (2,3-NDP4444) were prepared in aqueous solution. The additives are not only simple to prepare, but also have good solubility in water. More importantly, they can significantly improve the friction reducing and anti-wear performances of water. Evolution of the friction coefficients (a) and wear volume losses (b) for different samples on steel/steel friction pairs.

Facile Preparation and Tribological Properties of Water-Based Naphthalene Dicarboxylate Ionic Liquid Lubricating Additives

Naphthoate based lubricating oil with high oxidation stability and lubricity

The depletion of petroleum resources and the intensification of environmental problems have necessitated the development of renewable products from bio-based chemicals instead of petroleum resources. Herein, a new kind of ester lubricating oil, isooctyl furan dicarboxylate (isooctyl-FD), was prepared from bio-based 2, 5-furandicarboxylic acid. The structure of isooctyl-FD was evaluated using nuclear magnetic resonance imaging and high-resolution mass spectroscopy. Its physicochemical and tribological properties including thermal and oxidation stabilities, flash point and pour point, viscosity and viscosity index, and friction-reducing and anti-wear properties were systematically evaluated. The results show that isooctyl-FD has comparable thermal and oxidation stability to the synthetic ester lubricating oil, isooctyl sebacate (isooctyl-S). Its friction-reducing and anti-wear properties are superior to isooctyl-S; however, its viscosity-temperature and low-temperature properties are inferior to isooctyl-S.

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Xin Du

Papers

In situ preparation of multifunctional additives in water

Facile Preparation and Tribological Properties of Water-Based Naphthalene Dicarboxylate Ionic Liquid Lubricating Additives

Naphthoate based lubricating oil with high oxidation stability and lubricity

Lubricating properties of ester oil prepared from bio-based 2, 5-furandicarboxylic acid

A mild and one-step synthesis of 2,8-dioxabicyclo [3.3.1] nonane derivatives via classical Knoevenagel condensation