Ionic liquids are salts that melt below about 100 °C, and many are liquid at room temperature. They consist of a bulky organic cation, commonly imidazolium, ammonium, pyridinium or phosphonium, paired with an inorganic or organic anion. Because the ions are large and asymmetric, the lattice energy is low and the salt remains fluid, with negligible vapour pressure, wide liquid range and good thermal stability. These properties made ionic liquids attractive first as solvents for synthesis and catalysis (Welton, 1999), and the number of possible cation and anion combinations encouraged the idea that a liquid could be designed for each task.

Their use in tribology dates from the report by Ye et al. (2001) that imidazolium tetrafluoroborate salts lubricated a range of metal and ceramic pairs effectively. A large body of work followed, summarised in several reviews (Zhou et al., 2009; Minami, 2009; Bermúdez et al., 2009; Somers et al., 2013). Much of the early research treated ionic liquids as neat lubricants or base fluids. That approach has a clear place where volatility cannot be tolerated, as in vacuum, but for general lubrication it runs into the price of the fluids and their uncertain behaviour towards common engineering metals.

Using an ionic liquid as an additive changes the proposition. At a treat rate of a few per cent or less the cost penalty is contained, the bulk properties remain those of a familiar base oil, and the ionic liquid has only to perform at the surface. Zhou and Qu (2017) reviewed this line of work, and later surveys have placed it within the wider chemistry of the field (Cai et al., 2020). The present article concentrates on two matters within it: the mechanisms by which ionic liquid additives form protective films, and the compatibility problems that stand between laboratory results and formulated products.

This is a narrative review of the published literature and not a systematic one. It relies on the principal review articles in the field and on the primary papers most often cited for particular findings, selected for their relevance to additive use in hydrocarbon and synthetic base oils. Ionic liquids as neat lubricants, as additives to water or to greases, and as thin films in micro-devices are referred to only where they explain behaviour in oil. The review reports mechanisms and qualitative findings and does not tabulate friction or wear values, since those depend closely on the test method and are not comparable between laboratories.

The first generation of tribological ionic liquids paired alkylimidazolium cations with tetrafluoroborate or hexafluorophosphate anions. They gave low friction and wear in bench tests, but both anions hydrolyse in the presence of moisture, releasing hydrogen fluoride (Swatloski et al., 2003), and corrosion of steel and other metals was observed (Bermúdez et al., 2009). Attention moved to more hydrolytically stable fluorinated anions such as bis(trifluoromethylsulfonyl)imide and then to halogen-free anions based on phosphates, sulfonates, carboxylates and borates (Somers et al., 2013). Additives for the ionic liquids themselves were also explored to control corrosion and wear (Kamimura et al., 2007).

Early additive studies used polar base fluids or accepted that the ionic liquid would be present as a dispersion. Jiménez et al. (2006) examined alkylimidazolium salts both neat and as additives in steel and aluminium contacts and found that small additions could reduce friction and wear relative to the base oil alone. The obstacle to wider use was solubility: the common imidazolium salts are essentially immiscible with non-polar hydrocarbon oils. A step forward came with phosphonium cations carrying long alkyl chains paired with organophosphate anions, which were shown to be fully miscible with hydrocarbon base oils and non-corrosive towards the test metals (Yu et al., 2012; Qu et al., 2012). Most subsequent work on engine and gear oil applications has built on this family and its relatives (Xiao, 2017).

Explanations of how ionic liquid additives protect rubbing surfaces fall into three groups, shown in Figure 1: physical adsorption of ions, tribochemical reaction to form an inorganic film, and interaction with other surface-active additives. The three are not alternatives. Adsorption brings the additive to the surface, reaction converts part of it to a durable film, and co-additives alter both steps.

Whether an ionic liquid can serve as a practical additive depends on more than its anti-wear action. Table 1 lists the main compatibility issues raised in the literature, each discussed below.

Three limitations recur in the literature. First, most mechanistic work has been done with one ionic liquid in one base oil on steel, in short bench tests. Fully formulated oils, aged oils, and contacts involving coatings, aluminium alloys or copper alloys are much less well covered. Second, film formation has largely been inferred from analysis of surfaces after the test. In situ observation of film growth and removal, of the kind that clarified ZDDP behaviour, would show how quickly ionic liquid films form and how they respond to changes in load and temperature. Third, the adsorbed state of ion pairs in non-polar media is poorly characterised, which leaves the link between molecular structure and surface activity largely empirical.

Progress on compatibility calls for evidence of a different kind from that which established the anti-wear effect: long-duration oxidation and deposit tests, seal and corrosion tests, storage stability, and engine or rig trials with complete formulations. Halogen-free and phosphorus-free chemistries deserve particular attention because they sidestep the two most obvious regulatory and corrosion concerns.

Ionic liquids can function as anti-wear and friction-reducing additives in lubricating oils, and the mechanism is understood in outline: polar ion pairs concentrate at the rubbing surface, and the anion reacts under boundary contact to give a thin inorganic film whose composition reflects the anion chemistry. Oil-miscible phosphonium salts overcame the solubility barrier that confined earlier ionic liquids to polar fluids, and reported synergy with ZDDP suggests a route to lower-phosphorus formulations. The open questions are mostly about compatibility. Hydrolytic stability, corrosion, behaviour in complete additive packages, environmental acceptability and cost have each been addressed for particular compounds, but not yet with the breadth that the lubricant industry expects of a new additive class. Until that evidence exists, ionic liquid additives are best regarded as a promising and partly proven option.

To be declared by the author(s) before publication.

To be declared by the author(s) before publication.