Antiwear mechanisms of zinc dithiophosphate: a chemical hardness approach

The role of zinc polyphosphate in the antiwear mechanisms of zinc dithiophosphate (ZDDP) is investigated in the light of recently published analytical data carried out on thermal films and tribochemical films obtained from the additive. Special attention is paid to explaining the mechanisms of the polyphosphate in eliminating abrasive wear due to the presence of transition metal oxide species in boundary lubrication. A set of tribochemical reactions between the polyphosphates and the oxides is proposed on the basis of the hard and soft acids and bases (HSAB) principle. The antiwear action of ZDDP is found to be very adaptive and in severe conditions, the model predicts a layered tribofilm with the presence of polymer-like zinc metaphosphate overlying a mixed-transition-metal phosphate glass, in good agreement with recent analytical data. The role of residual sulphur in the lubricant is also envisaged and the model is in agreement with the formation of metal sulphides embedded in the phosphate matrix. The model, based on the chemical hardness concept, could be used as a basis for the prediction of interactions between ZDDP and other additives in motor oils.