Synergistic effects of hexagonal boron nitride nanoparticles and phosphonium ionic liquids as hybrid lubricant additives

Hybrid nanostructure combinations of ionic liquids (ILs) and hexagonal boron nitride (h-BN) nanosheets are proposed as additives to lubricants to enhance their tribological performance and their temporal stability. Trihexyltetradecylphosphonium bis(2-ethylhexyl)phosphate (IL1), tributylethylphosphonium diethylphosphate (IL2) and trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate (IL3) are the ILs (at 1 wt%) that have been combined, each separately, with h-BN nanosheets (at 0.1 wt%), were added as hybrid additives in a polyalphaolefin base lubricant, PAO 32. Some of the nanodispersions remain stable up to 240 days after preparation. Increases in both viscosity and density owing to h-BN nanoparticles and/or ILs are lower than 7% and 0.3%, respectively. The tribological performance (friction and wear) of the several developed nanolubricants were investigated using a tribometer with a ball-on-three-pin configuration and a 3D optical profiler, respectively. The results showed that the hybrid additives improve, in general, the tribological performance of the lubricant rather than being used separately, thus synergistic effects have been found. Raman spectroscopy conducted on the worm surface showed protective IL and h-BN tribofilms. PAO 32 + IL1 + h-BN is the nanolubricant that exhibits the best tribological performance of those studied in this work.The authors acknowledge Repsol for providing us the PAO 32 sample. Authors would also thank RIAIDT-USC for the use of analytical facilities, especially to Mr. Ezequiel Vázquez for his useful advice. This work was supported by the Spanish Ministry of Science, Innovation and Universities and the ERDF programme (FEDER in Spanish) through ENE2017-86425-C2-2-R project, and by the Xunta de Galicia (ED431E 2018/08, ED431D 2017/06 and GRC ED431C 2016/001). These funders also financed the acquisition of the 3D Optical Profile (UNST15-DE-3156)S