Size effect and tribological behavior of nickel oxythiomolybdate as an additive in lubricating oil

The fine nickel oxythiomolybdate (NiMoO2S2) powder was prepared by means of aqueous solution technique, high energy ball milling, and emulsion phase transfer process. The size effect of nickel oxythiomolybdate particles on its dispersive stability in oil and tribological behavior was studied. The test results on four-ball machine indicate that the stability of NiMoO2S2 in oil increases considerably with the decrease of size. The PB values of the oil containing the additive of smaller size are also higher as compared with that of the oil containing relatively larger size additive, while the wear scar diameter of the steel ball under the lubrication of the oil containing smaller sized additive is small as well. The pin-on-disc test results indicate that in a temperature range from 20-800°C, the friction coefficient can be decreased by 70%, when the oil containing smaller size additive is used as lubricating medium. The friction-reducing and antiwear behavior of the oil containing the additive was also found to be dependent on the surface roughness of the counterpart alloy disc or glass disc.