Compatibility of Ionic Liquids with an Organic Friction Modifier in Lubricating a Steel-Bronze Contact

Weimin Li^{1, 2}, Huimin Luo³, Chanaka Kumara¹, Jun Qu^1,*

¹Materials Science & Technology Division, Oak Ridge National Laboratory, USA

²State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, China

³Energy & Transportation Science Division, Oak Ridge National Laboratory, USA

INTRODUCTION: Current engine lubricants were designed for ferrous alloys and their compatibility with bronze is not well understood. Tribological performance of engine oil largely depends on the friction modifier (FM) and anti-wear (AW) in the additive package and thus it is important to understand the interactions between them^{1, 2}. Particularly, ionic liquids (ILs) have recently been developed as candidate AWs and their compatibility with FMs is not well understood. This work aims to investigate the compatibility of various oil miscible ionic liquids³ with an organic friction modifier (OFM) for a steel-bronze contact under boundary lubrication.

METHODS: Sample lubricants were prepared with a PAO 4 cSt base oil containing an IL or ZDDP together with an OFM (0.8%). Different concentrations were used for the ILs and ZDDP to control the P content at the same 800 ppm level. Boundary lubrication tests were conducted on a Plint TE 77 tribometer under a ball-on-flat contact configuration. The bronze plates were tested in sliding against an AISI 52100 steel ball. Tests were carried out under a 20 N load and an oscillation frequency of 10 Hz with a stroke of 10 mm, at 100 ^oC for 1000 m of sliding. Wear volume of the bronze plate was quantified using a Wyko NT9100 white light interferometer.

RESULTS: Figure 1 shows the friction results. The PAO+OFM showed a stable friction coefficient (COF) of 0.075. Addition of ZDDP, [P₈₈₈₈][DEHP], [P₆₆₆₁₄][BTMPP], or [P₆₆₆₁₄][C₁₇H₃₅COO] to the PAO+OFM increased the COF. In contrast, a significant reduction of COF (stabilized at around 0.03-0.035) was achieved by the combination of [N₈₈₈H][DEHP] and OFM. Results suggested that the lubricating performance is highly dependent on the AW-FM compatibility. Elongated test of the PAO containing OFM and [N₈₈₈H][DEHP] produced an extreme low steady-state friction coefficient of ~0.02 along with a rather low wear on the bronze plate (0.8×10⁷ μm³) after 10,000 m sliding. The effect of free ions is being studied to gain a better understanding of the low-friction and low-wear behavior. Surface characterization (SEM, EDS, STEM) did not detect a tribofilm on the bronze surface.

DISCUSSION: OFM is known to construct a low-friction film on the metal surface and the intermolecular H-bonding with the AW could influence the film structure either

beneficially or detrimentally. Results suggested that most AWs in this study negatively impacted the OFM film leading to higher friction. In contrast, the H-bonding between the free ions of [N₈₈₈H][DEHP] and OFM molecules may help grow a thicker and denser film with lower friction and better wear protection. Additional testing and analysis are ongoing to validate the hypothesis.

Figure 1. A: Friction coefficient traces of the PAO containing various IL-OFM combinations, B: Elongated test of the PAO containing OFM and [N₈₈₈H][DEHP].

ACKNOWLEDGEMENTS: Research sponsored by the US Department of Energy, Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office. W. Li was supported by the Visiting Scholarship from the Chinese Academy of Sciences.

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