Toward Cost-Effective Environmentally Acceptable Lubricants: Influence of Synthetic Ester and Low-Viscosity Polyalphaolefin Blend Type on Additive Performance

The pressing need to develop reliable and cost-effective Environmentally Acceptable Lubricants (EALs) is a challenge to the entire lubrication field in academia and industry. In this work, we propose blending low-viscosity polyalpholefins (PAOs) and synthetic esters as base fluids, as a strategy to formulate cost-effective EALs. Pentaerythritol polyol ester (PEPE) and 2-ethylhexyl palmitate (2-EHP) are the synthetic esters chosen. Tributyl(ethyl)phosphonium diethyl phosphate (PEP) ionic liquid, dialkyl dithiophosphate (DDP), and alkoxylated long chain ether phosphate (AEP) have been chosen as lubricant additives. Two reference lubricant additives were also tested for comparison, i.e., hexadecanoic acid (palmitic acid, C16) as friction modifier, and primary zinc dialkyl dithiophosphate (ZDDP) as anti-wear additive. Stainless steel AISI 316L disks were tested against alumina balls using a ball-on-disk tribometer to evaluate the performance of the formulated lubricants. All additives improved the performance of plain PAO, while in the case of esters competition with the additives was observed due to their high polarity. The blends performed better than the lubricants with a single base fluid. ToF–SIMS studies showed richer sulfur and phosphorus-based tribofilms for those samples lubricated with blends than with single base fluids. The blends also helped improving the solubility of the ionic additives with respect to plain PAO. Overall, the results confirmed the better performance of blends as a strategy for formulating cost-effective EALs and for improving the solubility of ionic liquids in non-polar base fluids.