MoS 2 film formation and friction properties of molybdenum phosphate additive in combined condition with ZnDTP

This article describes experimental studies on low friction film formation of molybdenum phosphate (MoP) in combination with zinc dialkyldithiophosphate (ZnDTP). The spinning type friction tests were conducted, and in the test procedure the test using ZnDTP containing oil was conducted following the test using MoP oil. The results showed that after switching to ZnDTP, the friction coefficient steeply decreased and reached to approximately 0.06 in the case of 1353 ppm of sulfur concentration, and that minimum friction coefficient depended on the concentration of sulfur in the oil with ZnDTP. Moreover, it is shown that the friction coefficient increased gradually, and that the coefficient at 1 h after switching to ZnDTP corresponded to that in ZnDTP alone. Raman spectroscopy demonstrated that, at 5 min after switching to ZnDTP, the MoS2 film was formed on the contact surface. Also, EDX analysis results indicated that at 1 h after switching to ZnDTP, the film derived from ZnDTP was composed of phosphorus, oxygen, and sulfur. These results suggest the film formation mechanism of MoS2: molybdenum oxide, such as FeMoO4, formed in the test of MoP, then was transformed to MoS2 by sulfur or sulfur containing compound from ZnDTP decomposition.