Additive Interactions and Their Effect on the Performance of a Zinc Dialkyl Dithiophosphate

Four-ball wear studies have demonstrated that the antiwear performance of a commercial zinc dialkyl dithiophosphate (ZDP) antiwear agent can be adversely affected by the presence of some friction modifiers, EP agents, oxidation inhibitors, and detergent-dispersants. To explain these observations, ZDP decomposition studies were conducted with these other additives present. None of the measured ZDP decomposition characteristics completely explain the observed changes in ZDP antiwear performance. Complex formation in the bulk oil and competition for the surface seem to be involved.