Tribological properties and tribochemical mechanisms of oil-soluble organo-molybdenum additives

A four-ball machine was performed to investigate the tribological performances of organo-Mo compounds (oxymolybdenum dithiophosphate and molybdenum dithiophosphate, coded as MoDDP and MoDTP, respectively) as additives in liquid paraffin. The synergetic friction-reducing and antiwear function of the two additives was also examined on the same test rig. The morphologies of the worn steel surfaces and the elemental distributions thereon under the lubrication of liquid paraffin and liquid paraffin containing different additives were observed and determined by means of scanning electron microscopy and energy dispersive spectrometry. The chemical states of some typical elements on the worn steel surface were analyzed on an X-ray photoelectron spectroscope. It was found that MoDDP and MoDTP as additives in liquid paraffin had good anti-wear and friction-reduction properties to the steel-on-steel system, which was attributed to the formation of a reaction film composed of MoS₂, FeS, and MoO₂, by way of the tribochemical reaction of the additives, and a deposited film composed of phosphate, on the worn steel surface. Moreover, the two additives had synergistic friction-reducing and antiwear effect, which was closely related to the chemical states of S on the worn steel surface.