Tribological performance of three fatty acid anion-based ionic liquids (FAILs) used as lubricant additive.

Abstract

This paper studies the tribological behavior of three fatty acid anion-based ionic liquids: methyltrioctylammonium octanoate ([N8881][C8:0]), methyltrioctylammonium laurate ([N8881][C12:0]) and methyltrioctylammonium palmitate ([N8881][C16:0]) used as additive at 0.5, 1 and 2 wt% in an ester base oil. The tribological experiments were performed in two different tribometers: a Bruker UMT-3 using a reciprocating “ball-on-disc” configuration for pure sliding tests and a Mini Traction Machine (MTM) for rolling/sliding tests. After sliding tests, the wear scar was analyzed by scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) and X-ray photoelectron spectroscopy (XPS). Sliding tests results showed both friction and wear reduction of the mixtures with respect to the ester only at 25 °C. The worse tribological behavior at 100 °C could be attributed to the moderate thermal stability of these ionic liquids. Under rolling/sliding conditions, samples displayed similar antifriction and ECR behavior than the base oil at high speeds, changing to a higher friction and lower ECR at low speeds and increasing temperatures. EDX analysis showed mainly the steel disc elements. The XPS oxygen signal showed higher amount of C[dbnd]O bond on the worn surface after reciprocating tests at 25 °C, which could be attributable to IL-surface interaction and can explain the better wear reduction performance.