Phosphonium-based ionic liquids as additives in calcium/lithium greases.

Abstract

This work studies the performance of three phosphonium-based ionic liquids ([P6,6,6,14][BEHP], [P6,6,6,14][TCM] and [P6,6,6,14][DEC]) when used separately as additives on the thermal conductivity, thermal stability and tribological properties of two non-additised greases (a lithium-based and a calcium-based grease). The main results confirmed that the [P6,6,6,14][TCM] improved the thermal stability of both greases. Only the mixtures G1 + 5%[P6,6,6,14][BEHP] and G2 + 5%[P6,6,6,14][TCM] showed a slight increase in thermal conductivity. In general, the addition of the ILs to the lithium-based grease increased the friction coefficient in both the four-ball tests and the reciprocating tests, while the friction coefficient decreased with the addition of the ILs to the calcium-based grease in the four-ball tests. The addition of the [P6,6,6,14][BEHP] and [P6,6,6,14][TCM] improved the antiwear behavior of the lithium-based grease under both tribological tests, whereas all the ionic liquids worsened the antiwear behavior of the calcium-based grease in the four-ball tests. The [P6,6,6,14][BEHP] had the best antiwear behavior due to the higher interaction of phosphorous with the metal surface. A possible interference of the calcium thickener with the ionic liquid–metal surface interaction was found, resulting in lower antiwear behavior of the mixtures with the calcium-based grease.