Friction Behaviour of ZDDP Films in the Mixed, Boundary/EHD Regime

Recent work in the literature has suggested that the poor performance in the ASTM Sequence VI fuel efficiency test may result from an inappropriate balance of ZDDP antiwear agents.

This paper describes a study of the friction behavior of ZDDPs.

Two recently-developed techniques have been employed to chart the variation of film thickness and friction of ZDDP-containing oils from the EHD through to the boundary lubrication regimes.

It is shown that some secondary ZDDP additives give significantly higher friction coefficient in the mixed EHD/boundary regime at temperatures above 80 °C. This behavior is very similar to that seen using formulated ASTM Sequence VI reference oils.

Film thickness measurements show that the onset of high friction seen with the ZDDPs correlates with the formation of a solid-like reaction film in the lubricated contact.