Effect of Frictional Pair Materials on Tribological Behavior of Molybdenum Dithiocarbomate as an Additive

The effect of frictional pair materials on the tribological behavior of molybdenum dithiocarbomate (MoDTC) as an additive in a fully formulated petroleum based engine oil SJ/5W-30 was investigated on an Optimol-SRV test rig, by sliding Mo-sprayed coating, nitrided coating and Cr-plated coating against cast iron under the lubrication of the engine oil containing MoDTC. The worn coating and cast iron surfaces were observed with a scanning electron microscope, and the tribochemical function of the additive was examined with an X-ray photoelectron spectrometer. Results show that the tribological behavior of MoDTC is related to the type of the frictional pair materials. Namely, MoDTC decreased the friction and reduced the wear of all the three coating materials paired with cast iron. However, MoDTC exhibited the best friction-reducing and wear-resistant effect for the nitrided coating-cast iron pair. This was related to the different tribochemical function of the additive for different coating materials. In other words, the chemical state of Mo and S and the proportions of their compounds on the worn surfaces were dependent on the type of the frictional pair materials. It was thus supposed that the difference in the tribological behavior of MoDTC for different frictional pair materials was attributed to the content of MoS 2 and other wear-resistant compounds generated on the worn cast iron surfaces.