Effect of dissolved oxygen on lubricating performance of oils containing organic sulfides

To elucidate the role of dissolved oxygen in the synergistic lubrication mechanism of oils containing organic sulfides, four-ball tests were conducted under increasing-temperature or two-step constant-temperature conditions by using oils with different concentrations of dissolved oxygen. In increasing-temperature tests, high oxygen concentration with DPDS (diphenyl disulfide) and antioxidant additives exhibited superior load-carrying capacity to reactive DBDS (dibenzyl disulfide) with the same antioxidant additives. On the contrary, in two-step constant-temperature tests, the oils containing antioxidant additives alone with high concentration of oxygen showed better performance than oils containing DBDS or DPDS. These results are discussed from the viewpoint of optimum composition of surface films and cooperative action of additives and oxygen.