Towards phosphorus free ionic liquid anti-wear lubricant additives

Anthony E. Somers, Ruhamah Yunis, Michel B. Armand, Jennifer M. Pringle, Douglas R. MacFarlane, Maria Forsyth

Research output: Contribution to journalArticleResearchpeer-review

9 Citations (Scopus)


The development of improved anti-wear additives would enable the use of lower viscosity oils that would lead to improved efficiency. Ionic liquids have the potential to be this type of new anti-wear additive. However, currently the best performing ionic liquids that are miscible in non-polar base oils, the phosphonium phosphates, contain phosphorus on both the cation and anion. Manufacturers are seeking to reduce the presence of phosphorus in oils. Here, as a first step towards phosphorus-free anti-wear additives, we have investigated ionic liquids similar to the phosphonium phosphates but having either a phosphorus-free cation or anion. Two quaternary ammonium phosphates (N 6,6,6,14 )(BEHP) and (N 8,8,8,8 )(BEHP) and a phosphonium silyl-sulfonate (P 6,6,6,14 )(SSi) were compared to a phosphonium phosphate (P 6,6,6,14 )(BEHP) and a traditional zinc dithiophosphate (ZDDP) as anti-wear additives in mineral oil. The change from a phosphonium to a quaternary ammonium cation drastically reduced the miscibility of the Ionic liquid (IL) in the oil, while the change to a smaller silicon containing anion also resulted in limited miscibility. For the pin-on-disk wear test conditions used here none of the ionic liquids outperformed the ZDDP except the (P 6,6,6,14 )(BEHP) at a relatively high loading of 0.10 mol·kg -1 (approximately 8 wt%). At a more moderate loading of 0.025 mol·kg -1 the (P 6,6,6,14 )(SSi) was the best performing ionic liquid by a significant amount, reducing the wear to 44% of the neat mineral oil, while the ZDDP reduced the wear to 25% of the mineral oil value. Electron microscopy and energy dispersive X-ray spectroscopy showed that the presence of a silicon containing tribofilm was responsible for this protective behaviour, suggesting that silicon containing ionic liquids should be further investigated as anti-wear additives for oils.

Original languageEnglish
Article number22
Number of pages12
Issue number2
Publication statusPublished - 1 Jun 2016


  • Anti-wear additive
  • Boundary lubricant
  • Ionic liquid
  • Miscibility
  • Phosphorus free
  • Wear test

Cite this