BluSkyi·Engineering Insight Article
Ionic Liquids · Tribofilms · Lubricant Additives
Ionic Liquid Additives Lose Their Anti-Wear Edge Once Oil Temperatures Rise
The best of three phosphonium-based ionic liquids matched or beat the incumbent zinc anti-wear additive on steel at 18 °C, but none kept pace at 60 °C and 80 °C.
- Source Journal
- Wear
- Lead Institution
- Tohoku University
- Published
- 8 October 2025
- Discipline
- Mechanical
- Focus Area
- Tribology, Surfaces & Wear
- Research Type
- Applied
- Licence
- CC BY 4.0
- Authors
- Jiang et al.
The Bottom Line
Phosphonium ionic liquids are increasingly seen as candidates to replace some conventional lubricant additives. This study tempers that expectation. On steel ball-on-disc contacts at 18 °C, one candidate, [P4442][DEP], delivered the lowest wear of any additive tested. At 60 °C and 80 °C none of the three liquids protected as well as zinc dialkyldithiophosphate (ZDDP). Nanoscale tests point to the reason: their protective films grow slowly and appear to need fresh wear to start.
Key Findings · At a Glance
- At 18 °C, [P4442][DEP] kept specific wear rates below 5 × 10⁻¹⁰ mm³/N·m across every film-thickness (λ) ratio tested.
- At room temperature, all three ionic liquids gave lower friction than base oil and ZDDP when λ exceeded 0.4.
- At 60 °C and 80 °C, no ionic liquid matched ZDDP's wear protection; some wore more than neat base oil.
- The fluorinated [P66614][NTf2] produced the highest wear at low λ, which the authors attribute to corrosion.
- In nanoscale sliding at 80 °C, ZDDP films reached about 80 nm; [P4442][DEP] alone among the liquids formed one, a few nanometres thick.
- Mild nanoscale sliding did not trigger ionic-liquid tribofilm growth, suggesting surface wear must first create active sites.
Opportunity
Lubricant formulators have spent two decades looking at ionic liquids, salts that are molten below 100 °C, as additives for both conventional and electric vehicles. The appeal is a combination the authors list plainly: good friction and wear performance, high thermal stability and low flammability and volatility. Phosphonium-based variants have attracted particular interest, and the recurring benchmark is ZDDP, which the authors describe as arguably the most successful agent in current engine-oil formulation.
The weak point is mechanism. The literature is split on whether ionic-liquid films form the way ZDDP films do, and, the authors note, their performance under realistic automotive conditions remains poorly understood. That leaves formulators with a promising chemistry and little guidance on when it will actually protect a surface.
The team at Tohoku University, with collaborators from four other institutions, set out to close that gap. They blended three phosphonium ionic liquids at 1 wt% into a PAO4 synthetic base oil and ran them against neat PAO and a ZDDP-treated oil. Steel ball-on-disc tests at 25 N covered sliding speeds from 0.02 to 0.15 m/s and temperatures of 18 °C, 60 °C and 80 °C, giving λ ratios from 0.868 down to 0.036, all within boundary lubrication. In-lubro atomic force microscopy (AFM) then watched films form on steel at the nanoscale, and oil-change tests probed how durable those films were.
Results
Room temperature
At 18 °C the picture favours the ionic liquids. [P4442][DEP] held specific wear rates below 5 × 10⁻¹⁰ mm³/N·m at every λ, which the authors rate as the best anti-wear performance of any sample, comparable to or better than ZDDP. Its wear tracks were the smoothest in the set, and the authors attribute the effect mainly to a robust adsorbed boundary layer rather than a thick film. Friction told a different story for each liquid. Above λ = 0.4, all three undercut both PAO and ZDDP, with the viscous, sulphur-bearing [P4444][C2C2PO2S2] lowest. At the harsher λ of 0.225, [P4442][DEP] and [P66614][NTf2] ran at about 0.1, similar to base oil, while [P4444][C2C2PO2S2] settled near 0.14. [P66614][NTf2] showed the highest wear at that condition, which the authors link to corrosive products of its fluorine-containing anion.
Elevated temperature
Heat reversed the ranking. At 80 °C and λ = 0.036, [P4442][DEP] reached a friction coefficient of 0.075 after only 5 m of sliding but climbed steadily thereafter, and its wear ended similar to neat PAO. ZDDP gave the most significant wear reduction. [P66614][NTf2] produced extremely high wear at that condition, and [P4444][C2C2PO2S2] showed a negative effect on wear at high temperature, leading the authors to judge it unsuitable for internal-combustion engines running at 70–80 °C. The authors describe the result as a critical limitation for high-temperature applications of these ionic liquids. Because the calculated flash temperature rise was small and the liquids typically decompose above 200 °C, they judge thermal decomposition less likely, and point instead to thinner oil films, removal of surface oxides and oxidative degradation.
What the nanoscale tests show
At 18 °C the AFM detected adsorbed layers for all three liquids but no solid tribofilm. At 80 °C ZDDP built pad-like films of about 80 nm maximum height; only [P4442][DEP] among the ionic liquids formed a solid-like film, a few nanometres thick and growing far more slowly. The authors conclude that "mild sliding alone is insufficient to fully activate IL tribochemistry", and suggest that wear which strips the oxide layer and exposes fresh iron may be a prerequisite. In oil-change tests, wear with [P4442][DEP] and ZDDP barely differed from the steady tests, suggesting anti-wear protection persisted after the additive was removed, although friction with [P4442][DEP] rose once the oil was changed, which the authors read as a sign of a weak adsorbed layer or film.
The Practical Applications
Engine oil formulation
For formulators weighing phosphonium ionic liquids as partial substitutes for ZDDP, the evidence here argues for caution at typical oil temperatures. At 1 wt% none of the three candidates matched ZDDP's wear protection at 60 °C or 80 °C.
Electric vehicle drive units
The authors frame their work around automotive and electric vehicle lubricants. The strong room-temperature result for [P4442][DEP] suggests value where contacts run cool, although drive-unit conditions such as electric potential were not tested.
Industrial lubricants
Machinery lubricated at or near room temperature may be the most natural fit for the best performer, given its low wear and smooth wear tracks at 18 °C. Fluorinated chemistries warrant scrutiny, since [P66614][NTf2] accelerated wear through corrosion.
Additive design
The link between adsorption kinetics, surface wear and film growth gives chemists a design target: ionic-liquid structures that react more readily with the surface, or operating conditions severe enough to activate film growth.
Assessment
The authors are explicit that further work is needed to understand these additives under the complex conditions of modern automotive drive units, including variation in load, sliding speed, temperature and electric potential. They note that the low 1 wt% concentration likely produced a sparse, discontinuous adsorption layer, and that tailored formulations will be needed before these liquids withstand automotive operating conditions. Their mechanistic account of oxide removal and fresh iron is offered as a possibility rather than a demonstrated pathway.
Readers should weigh the scope. All macroscale results come from one steel pairing, a single 25 N load, one base oil and one ionic-liquid concentration on a laboratory tribometer, not a running engine. The paper reports repeat measurements for profilometry and Raman spectroscopy, but the text does not state how many tribometer runs sit behind each wear rate, and the paper's own descriptions of room-temperature tribofilms are not fully consistent. The strategic reading is nonetheless clear. ZDDP remains the reference to beat once oil is hot, and the ionic-liquid case now rests on chemistries that build films faster.
About the Research
Conducted by Shaoli Jiang and colleagues at the Department of Mechanical Systems Engineering, Tohoku University, Japan, with researchers from Imperial College London, Ecole Centrale de Lyon, the University of Tokyo and Wuhan University of Technology. Published open access in Wear under a Creative Commons Attribution 4.0 licence.
Citation
Jiang, S., Puhan, D., Kuriyagawa, K., Yamana, T., Martin, J. M., Zhang, L., Liu, S., Adachi, K., & Murashima, M. (2026). Tribological characteristics of phosphonium-based ionic liquids: The role of adsorption layer and tribofilms. Wear, 584–585, 206382.