The Mechanism of Friction Reduction of Sulfurized Oxymolybdenum Di-(2-ethylhexyl)-phosphorodithioate under Boundary Lubrication

In Falex tests, it is shown that the oil-soluble sulfurized Oxymolybdenum di-(2-ethlhexyl)-phosphorodithioate (MoDTP) has excellent friction-reducing behavior. The possible mechanism is investigated by means of surface analysis of the tested Falex pin specimens, using surface profilometer, SEM, AES, XPS, and laser Raman spectrophotometer. When MoDTP is used as an additive, the asperity tips of the rubbing surface are worn away gradually and many microterraces occur on the surfaces, so that the rubbing surface becomes smoother. Boundary lubrication films are formed in situ on the tops of those microterraces. The films are mainly composed of MoS2 but they contain some phosphorous compounds. Only when the real contact pressure between the rubbing surfaces is lower than the critical pressure can the films be maintained dynamically on the rubbing surfaces and thereby be effective for friction reduction.