Synergistic effects between alkylphosphate-ammonium ionic liquid and alkylphenylborate as lubricant additives in rapeseed oil

The tribological properties of di(2-ethylhexyl)phosphate-di(2-ethylhexyl)ammonium ionic liquid (DOPD) and 2-(4-dodecylphenyl)−6-octadecyl-1,3,6,2-dioxazaborate (DBDB) as lubricant additives in rapeseed oil (RSO) were investigated. The results indicate that DBDB and DOPD have outstanding synergistic effects on extreme pressure, friction-reducing and antiwear properties and the synergistic effects are enhanced with the increase of applied load. SEM-EDS and XPS analysis results reveal that the outstanding synergistic effects between DBDB and DOPD originate from the good collaboration of the triboreaction products generated from DBDB and DOPD to form a stable and compact tribofilm with low shearing strength, which is composed of B2O3, BN, phosphates and/or polyphosphates, organic amine and organic and/or inorganic ammonium, as well as Fe2O3. •Alkylphosphate-ammonium ionic liquid and alkylphenylborate were prepared as lubricant additives in rapeseed oil;•DBDB and DOPD have outstanding synergistic effects on extreme pressure, friction-reducing and antiwear properties;•The composition and structure of boundary films at the worn surfaces were analyzed by SEM-EDS and XPS.•The tribofilm of the combination of DBDB and DOPD is composed of B2O3, BN, phosphates and/or polyphosphates, organic amine and organic and/or inorganic ammonium, as well as Fe2O3.