Vacuum tribological performance of phosphonium-based ionic liquids as lubricants and lubricant additives of multialkylated cyclopentanes

•Four kinds of tetraalkylphosphonium phosphate ionic liquids were synthesized.•PP-ILs as lubricants can eliminate the seizure-like high friction appeared in MACs.•PP-ILs as additive exhibits high friction elimination effect and excellent lubricity.•Tribological mechanism is the formation of a protective film on the worn surface.

The tribological effects of tetraalkylphosphonium phosphate ionic liquids (PP-ILs) as lubricants and lubricant additives of multialkylated cyclopentanes (MACs) was evaluated in vacuum (10−4 Pa) by a vacuum four-ball tribometer. Results showed that the transient seizure-like high friction appearing for MACs could be effectively eliminated by neat PP-ILs or by adding the PP-ILs with long alkyl chain into MACs. Confirmed by the surface analysis, the improved tribological performances could be ascribed to the high load-carrying capacity of ionic liquids and the protective films formed on the contact surfaces through the tribochemical reaction between the PP-ILs and fresh steel surfaces under the boundary lubrication.