Contribution of Phosphorus to the Antiwear Performance of Zinc Dialkyldithiophosphates

Four-ball wear studies show that the antiwear (low load) performance of zinc dialkyldithiophosphates (ZDPs) can be duplicated by combining a dialkyl acid phosphate and a soluble zinc compound. Antiwear performance of acid phosphates was independent of chain length for C8 to C18 but affected by chain branching and ring substitution. Zinc compound effectiveness depended on the strength of the parent acid compared to that of the acid phosphate, and its concentration in the blend (an atomic ratio of 2P:1Zn being optimum). Zinc was the most desirable metal naphthenate tested. Base oil viscosity had little effect on additive effectiveness. Added possible inorganic ZDP reaction products exhibited no antiwear activity. At higher loads in the EP regime, ZDPs lose their antiwear effectiveness, whereas both acid phosphates and sulfur compounds can reduce wear.