Copper oleate–EP synergy in low-viscosity lubricants: antiwear comparison with commercial oils

One of the current trends in gear lubricants is the reduction of oil viscosity to minimize hydrodynamic drag and, consequently, energy losses. However, the use of lower-viscosity lubricants may shift the lubrication regime towards boundary lubrication, in which friction and wear are higher. Therefore, an improvement of the current additives is needed to reduce the friction and to protect surfaces against different wear mechanisms. Previous study showed a synergistic effect on wear protection between the friction modifier copper oleate and sulfur- and phosphorus-based EP additives, due to the formation of a combined FePO4 and Cu2O tribofilm. 

The main objective of the present study is to compare the friction and anti-wear performance of the low viscosity lubricant based on copper oleate end EP additives (BO+CO+SP) with other higher viscosity gear oils. The fully formulated commercial oils selected for comparison were a mineral ISO VG 150 lubricant (MIN150) and an ISO VG320 lubricant of mineral (MIN320) and synthetic (PAO320) origin. To do that, the lubricants were tested using a ball-on-disc configuration in SRVIII tribometer under reciprocal sliding conditions according to ASTM D6425, typically used to characterize lubricants that contain EP additives. 

From friction point of view, the BO+CO+SP formulation exhibited a performance comparable to that of the other lubricants, despite having a lower oil film thickness. The wear volume measured indicated that the AW performance of the BO+CO+SP was only overcome by MIN320, showing the potential of the Cu-S-P synergy. In addition, the wear scar on each disc was analyzed using SEM/EDX confirming the presence of S and/or P in the tribofilms generated by commercial fully formulated gear oils. However, in the case of BO+CO+SP, also Cu was detected in the tribofilm, evidencing the interaction of the copper oleate with metal surfaces.