Mechanisms of ZDDP—An Update

Abstract

This paper describes how research over the last 20 years has advanced our understanding of the mechanisms of action of ZDDP, especially with respect to tribofilm formation. We now know that ZDDP tribofilm formation is promoted by applied shear stress and this explains many of the features of these films. We also now recognise that ZDDP tribofilms evolve during rubbing from relatively soft, long chain polyphosphate films to much more wear-resistant, short chain phosphates. Several disadvantages of ZDDPs as lubricant additives have emerged in recent years, in particular their tendency to increase friction in thin film rubbing conditions, their promotion of micropitting wear, and accelerated wear when present together with soot contaminants in engine oils. Research has revealed the origins of all these effects. Over the last 20 years there have been growing efforts to model ZDDP tribofilm formation, both at a macro- and molecular-scale, so far with limited success. Finally, this paper outlines some aspects of ZDDP behaviour that we still do not fully understand and where further research is needed.