Compatibility between Various Ionic Liquids and an Organic Friction Modifier as Lubricant Additives

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Abstract

Tribological performance of a boundary lubrication contact is largely dominated by the friction modifier (FM) and antiwear (AW) additives in the lubricant. While oil-soluble ionic liquids (ILs) have recently demonstrated promising AW functionality, their compatibility with FMs is little known and even less understood for nonferrous alloys. Here, we report the latest results for several selected ILs when used together with an organic FM (OFM) in lubricating a steel-bronze contact. Depending on the IL chemistry, either synergistic or antagonistic effects were observed. The three aprotic ILs ([P8888][DEHP], [P66614][BTMPP], and [P66614][C17H35COO]) seemed to degrade the OFM's lubricating performance. In contrast, the protic IL [N888H][DEHP] exhibited a strong synergistic effect with the OFM, yielding an ultralow steady-state friction coefficient (0.02) and a low wear rate (<10-8 mm3/(N m)), which significantly outperformed the IL or the OFM alone. Surface characterization found no chemically reacted tribofilm on the bronze worn surface. On the other hand, a unique physically adsorbed surface film as a result of interconnection between the IL and OFM molecules by hydrogen bonds is proposed on the basis of chemical analysis. Such an adsorption surface film is expected to be difficult to compress vertically but easy to shear horizontally, leading to low friction and wear.

Original languageEnglish
Pages (from-to)10711-10720
Number of pages10
JournalLangmuir
Volume34
Issue number36
DOIs
StatePublished - Sep 11 2018

Funding

Research was sponsored by the Vehicle Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy (DOE). Electron microscopy characterization was in part performed at ORNL's Center for Nanophase Materials Sciences, sponsored by the Scientific User Facilities Division, Office of DOE-BES. The support by the Chinese Academy of Sciences (CAS) during the visit of W.L. to ORNL is also appreciated. *E-mail: [email protected]. Tel: (865) 576-9304. ORCID Jun Qu: 0000-0001-9466-3179 Notes The authors declare no competing financial interest. This manuscript has been authored by UT-Battelle, LLC under Contract No. DEAC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research by the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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