Exploring the Interactive Effects of ZDDP, Metal Sulfides, and Carbon Nanotubes on the Lubrication Performance of a PAO Oil

Abstract This study investigates the frictional behavior and wear protection capacity of polyalphaolephin (PAO) oils additivated with combinations of zinc dialkyldithiophosphate (ZDDP), metal sulfides, and carbon nanotubes (CNTs) on steel surfaces. Various oil formulations, comprising PAO base oil with differing proportions of ZDDP, metal sulfides, and CNTs, were utilized to lubricate steel-steel contacts in block-on-ring tests. Both the blocks and rings were made of SAE 52100 bearing steel. The testing conditions selected for all the tests were 1 GPa of Hertzian contact stress, 1 m/s of sliding speed, and a total sliding distance of 3,600 m. Surface analysis was performed by means of SEM-EDS, laser confocal microscopy, and Raman microspectrometry. Results indicated that combining metal sulfides or CNTs with ZDDP notably reduced friction compared to ZDDP-only oil, with reductions ranging from 24 to 60%, depending on the formulation. However, wear rates varied considerably among formulations. Those containing ZDDP and Bi2S3 exhibited significant wear rate increases of 160 to 180% compared to ZDDP-only oil. Optimal wear protection for steel surfaces was achieved with ZDDP + CNTs and ZDDP + MoS2 combinations, resulting in wear rate reductions of 19 to 14% respectively.