Halide-Free Synthesis and Tribological Performance of Oil-Miscible Ammonium and Phosphonium-Based Ionic Liquids

TLDR

Ionic liquids offer low vapor pressure, nonflammability, high thermal stability, and excellent tribological properties, yet their practical use is limited by the need for oil miscibility, halide‑free composition, and compatibility with standard additives. The authors propose a halide‑free, oil‑miscible ionic liquid design that could enhance lubrication and reduce energy consumption. They synthesized these ILs via an economical route and assessed their performance through sliding tests on bearing steel and XPS analysis of the resulting surface films. Corrosion tests on various bearing metals in contact with the ILs and salt water confirm their suitability as real‑life lubricants.

Abstract

Due to their low vapor pressures, nonflammability, high thermal stabilities, and excellent tribological properties ionic liquids (ILs) are highly attractive lubricant base oils and additives. However, for practical applications of ILs in lubrication, two requirements are often limiting, the required miscibility with standard mineral oils (≥5 wt %) and the complete absence of corrosive halide ions in the ionic liquid. Moreover, the need for full compatibility with standard oil additives reduces the number of potential IL-based lubricant additives even further. In this contribution, an economic halide-free synthesis route to oil-miscible ionic liquids is presented, and very promising tribological properties of such ILs as base oil or additive are demonstrated. Therefore, sliding tests on bearing steel and XPS analysis of the formed surface films are shown. Corrosion test results of different bearing metals in contact with our halide-free ILs and (salt) water prove their applicability as real life lubricants. In the sustainable chemistry and engineering context, we present a halide-free design approach for ionic performance chemicals that may contribute to significant energy savings due to their enhanced lubrication properties.