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ORNL team shows using ionic liquid as lubricant additive improves gear performance

Researchers at Oak Ridge National Laboratory have shown that a certain class of ionic liquids (ILs), when mixed with commercially available oils, can make gears run more efficiently with less noise and better durability. Results were published in ACS Applied Materials & Interfaces.

Oil-soluble ionic liquids (ILs) have recently been demonstrated as effective lubricant additives of friction reduction and wear protection for sliding contacts. However, their functionality in mitigating rolling contact fatigue (RCF) is little known. Because of the distinct surface damage modes, different types of surface protective additives often are used in lubricants for sliding and rolling contacts. Therefore, the lubricating characteristics and mechanisms of ILs learned in sliding contacts from the earlier work may not be translatable to rolling contacts.

—Stump et al.

ORNL’s team tested three different oil-soluble ionic liquids on steel under various temperatures, loads and speeds: phosphonium-phosphate, ammonium-phosphate, and phosphonium-carboxylate.

They found that adding a small percentage of a phosphonium-phosphate ionic liquid into a gear oil can effectively reduce friction, wear loss, cracking, surface damage and associated vibration noises.



Top: ORNL researchers used a low-viscosity base oil mixed with a small amount of phosphonium-phosphate ionic oil to test the lubricant’s performance on steel.
Bottom:They found the ionic oil mix yielded less surface damage, shown here, compared to lubrication with commercial gear oil. Credit: Jun Qu/Oak Ridge National Laboratory, U.S. Dept. of Energy

… the best-performing phosphonium-phosphate IL at 2% addition made a low-viscosity base oil significantly outperform a more viscous commercial gear oil in reducing the RCF surface damage and associated vibration noise. This IL generated a thicker, smoother, and more homogeneous tribofilm compared with commercial additives, which is likely responsible for the superior RCF protection. Results here suggest good potential for using appropriate IL additives to allow the use of low-viscosity gear and axle fluids for improved efficiency and durability.

—Stump et al.

The current trend in lubrication is chasing better fuel economy by using less viscous lubricants. This ionic liquid resulted in oils that perform well with less viscosity, generating a thicker, smoother and more uniform film for superior surface protection.

—ORNL’s Jun Qu


  • Benjamin C. Stump, Yan Zhou, Huimin Luo, Donovan N. Leonard, Michael B. Viola, and Jun Qu (2019) “New Functionality of Ionic Liquids as Lubricant Additives: Mitigating Rolling Contact Fatigue” ACS Applied Materials & Interfaces 11 (33), 30484-30492 doi: 10.1021/acsami.9b10001



Lower friction sounds like a good thing, as long as it is affordable and non-toxic.

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