Influence of the alkyl group of zinc dialkyldithiophosphate on the frictional characteristics of molybdenum dialkyldithiocarbamate under sliding conditions

The frictional characteristics of oils containing molybdenum dialkyldithiocarbamate (MoDTC) together with zinc dialkyldithiophosphate (ZnDTP) were studied with a reciprocating roller-on-disk tester. At a Zn concentration of 1000 ppm there was an optimum Mo concentration for friction reduction depending upon the alkyl group of ZnDTP. The friction was affected significantly by ZnDTP type at low Mo/Zn ratios while the extent of the effect of ZnDTP type on friction became small at high Mo/Zn ratios. For a combination of MoDTC and ZnDTP with primary C8 alkyl group, the preferential formation of a film containing products derived from ZnDTP was delayed appreciably due to competitive adsorption of MoDTC onto the rubbing surface. However, this contributed to a more effective friction reduction because the ratio of products from ZnDTP producing a high friction in the outermost surface film became lower during the initial period of the run.