Effect of phosphorus film on frictional properties of molybdenum dithiocarbamate

This paper describes the effects of phosphorus film on the boundary lubrication performance of molybdenum compounds. To study this, ball-on-plate-type spinning friction test was conducted using molybdenum dithiocarbamate and acid phosphate ester as additives. To form surface phosphorus films, specimens were heated in acid phosphate ester for 1, 3, and 6 h, with the results showing that the formation of the phosphorus film depends on the heating time and on the increases in concentration of phosphorus with the heating time. It was also shown that the friction coefficient in the test with the lubricant containing molybdenum dithiocarbamate, depends inversely on the heating time, with specimens heated for 3 and 6 h having lower coefficients that the specimen heated for 1 h. Energy-dispersive X-ray analysis revealed that the chemical reaction films comprising phosphorus, oxygen, and sulfur formed on the friction surfaces after the test, with the concentrations of sulfur in the film increasing with increased concentrations of phosphorus. These results suggest that the thickness of the phosphorus film affected the amount of molybdenum disulfide formed on the surface. Moreover, as more molybdenum disulfide film formed, the friction coefficient decreased.