Abstract
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 (N6,6,6,14)(BEHP) and (N8,8,8,8)(BEHP) and a phosphonium silyl-sulfonate (P6,6,6,14)(SSi) were compared to a phosphonium phosphate (P6,6,6,14)(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 (P6,6,6,14)(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 (P6,6,6,14)(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.
Highlights
In many tribological applications the lubricant viscosity required to adequately protect parts from wear results in a significant amount of energy being needed to move these parts through the oil
The quaternary ammonium Ionic liquid (IL) seemed to be fully miscible in the mineral oil, the mixtures up to and including 0.10 molkg1 were clear after stirring at 70 ̋ C for 1 h and stayed clear on cooling
Wear tests conducted over a period of two months resulted in a gradual increase in wear, until the values were similar to that of neat mineral oil
Summary
In many tribological applications the lubricant viscosity required to adequately protect parts from wear results in a significant amount of energy being needed to move these parts through the oil. Two phosphonium ILs, trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate and trihexyltetradecylphosphonium bis (2,4,4-trimethylpentyl) phosphinate, were found to be fully miscible in a mineral oil and a PAO and they performed well as anti-wear additives [11,12]. The IL should contain long alkyl chains and have high ion association for miscibility in non-polar base oils Since this discovery, similar structured ILs, such as phosphonium carboxylates and a tetraoctyl phosphonium phosphate have been produced that have proven to be miscible. Using what is known about the structural requirements for an effective anti-wear additive that is oil miscible, researchers have started to design ILs for this application [13,14,15]. The miscibility in a mineral oil and subsequent wear performance of these IL additives have been compared to that of (P6,6,6,14 )(BEHP) and a traditional zinc dialkyldithiophosphate (ZDDP) anti-wear additive
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