Lubrication Mechanism of Phosphonium Phosphate Ionic Liquid in Nanoscale Single‐Asperity Sliding Contacts

Abstract

While phosphonium phosphate ionic liquids (ILs) have been evaluated as additives for engine oils owing to their excellent physico‐chemical properties, miscibility with hydrocarbon fluids, and promising tribological properties, their lubrication mechanism is still not established. Here, atomic force microscopy (AFM) nanotribological experiments are performed using diamond‐like carbon‐coated silicon tips sliding on air‐oxidized steel in neat trihexyltetradecylphosphonium bis(2‐ethylhexyl)phosphate IL. The AFM results indicate a reduction in friction only after the removal of the native oxide layer from steel. Laterally resolved analyses of the steel surface chemistry reveal a higher concentration of bis(2‐ethylhexyl)phosphate ions adsorbed on regions where the native oxide is mechanically removed together with a change in surface electrostatic potential. These surface modifications are proposed to be induced by a change in adsorption configuration of bis(2‐ethylhexyl)phosphate anions on metallic iron compared to their configuration on iron oxide together with a reduction of surface roughness, which lead to the formation of a densely packed, lubricious boundary layer only on metallic iron.

In situ nanoscale evaluation of pressure-induced changes in structural morphology of phosphonium phosphate ionic liquid at single-asperity contacts

“…It is also critical to highlight that, while the absence of any stress-assisted surface reaction in the experiments presented herein is in agreement with our previous work, 68 the formation of the surface layer shown in Fig. 2 upon sliding at 5.5 AE 0.3 GPa contrasts with the results obtained from AFM experiments carried out at a higher applied pressure (7.3 AE 0.4 GPa) in the same previous study.…”

“… 64 )). The difference between our work and previously published studies can be ascribed to the completely different contact conditions employed in the experiments: while the low sliding speed of the AFM tip resulted in a negligible temperature rise (≪1 °C) at the contact, the high sliding speeds of multi-asperity, macroscale sliding contacts could drastically increase the contact temperature (as high as 140 °C), 68 thus accelerating any surface mechano-chemical reaction of the [P 6,6,6,14 ][DEHP] IL. In light of this, the variations in topographic AFM images observed in the present study can be attributed to a pressure-induced change in structural morphology of the [P 6,6,6,14 ][DEHP] molecules.…”

“…Despite this difference in the evolution of the surface topography upon sliding at elevated pressures, a decrease in nanoscale friction was measured in the area slid at high loads in both scenarios (5.5 ± 0.3 GPa and 7.3 ± 0.4 GPa (ref. 68 )), which indicates that different mechanisms underpin the measured friction reduction: in the present study, the decrease in friction in the area scanned at 5.5 ± 0.3 GPa can be ascribed to the pressure-induced variation in structure of [P 6,6,6,14 ][DEHP] IL, whereas in our previous work the friction reduction was shown to be due to the adsorption of phosphate ions on the mechanically smoothened metallic iron surface that leads to the formation of a densely-packed, lubricious boundary film. 68 This finding sheds new light on the dependence of the lubrication mechanism of phosphonium phosphate ILs on applied normal pressure, as it demonstrates that pressure-induced changes in structural morphology of phosphonium phosphate ILs can control the nanoscale friction response through the formation of a well-defined interfacial layer that is mechanically stable up to a critical applied normal pressure, above which wear occurs and the surface adsorption of phosphate ions on freshly-exposed metallic iron surfaces dictates the friction response at the nanoscale.…”

“…The synthetic procedure for [P 6,6,6,14 ][DEHP] IL is reported in ref. 68 . The water content of this IL was 17 139 ± 1549 ppm at room temperature (22 ± 1 °C) as determined by a Brinkman 831 Karl Fischer coulometer before tests.…”

A review on tribological performance of ionic liquids as additives to bio lubricants

“…85 Although, ZDDP is being used as lubricant additives from decades, however, it generates sulfur and zinc in large amounts that choke the exhaust system of an engine. 86 According to recent literature, ionic liquids have emerged as promising lubricant additive due to their variety of functionalities. 87 Ionic liquids form a boundary lubrication layer as a result of molecular reactions occurring at the area of contact, due to lubrication regime shifts from boundary to mixed and elastohydrodynamic lubrication.…”