The tribological chemistry of polysulfides in mineral oil and synthetic diester

Two organic polysulfides, dipropyl trisulfide (DPTS) and 3,5-diisopropyl-1,2,4-trithiolane (DIPTT) in mineral oil (MO) and synthetic diester (DE) were investigated on thermal films and tribofilms by using X-ray absorption near edge structure (XANES) spectroscopy. The results of surface analysis reveal that the thermal films formed from MO for two additives are consist of sulfate and sulfite, while the distinct composition of the films from DE implies the different sensitivity to oxygen between two compounds. As to the tribofilms, “hydrosulfite-like compound” is differentiated from sulfite and FeS_x (1 < x < 2) is detected in low energy region from DPTS/MO, as well as a lower concentration of DPTS/DE. At a high concentration in DE, the surface of sample for DPTS is full of iron sulfide. With regard to DIPTT, iron sulfite is the exclusive oxidized form of sulfur instead of “hydrosulfite-like compound” in the tribofilms, which is an evidence suggesting a mechanism involving a role played by a thiyl radical.