ZDDP (zinc dialkyldithiophosphate) is arguably the most successful antiwear additive ever
employed in crankcase engine lubricants. It was originally used as an antioxidant and shortly
afterwards recognized for its antiwear and extreme pressure properties. Unfortunately,
another critical additive polyisobutylsuccinimide-polyamine (PIBSA-PAM), which is used as
a dispersant in engine oils, is known to be antagonistic to ZDDP in terms of film formation,
friction and wear. The mechanisms of this antagonism have been widely studied, but they are
still not well understood. Furthermore, in order to protect engine exhaust catalysts from
sulphated ash, phosphorus and sulphur (SAPS) and extend drain intervals of engine lubricants,
a progressive reduction in ZDDP quantity but a growth in the use of PIBSA-PAM is required.
The aim of this study is to explore the mechanisms and practical effects of the antagonism
between ZDDP and PIBSA-PAM. Of particular interest is the impact on performance of the
ratio of ZDDP to PIBSA-PAM, as measured by P:N ratio. Since ZDDP is a very effective
antiwear additive, it produces only very low or "mild" rates of wear. To study this requires a
new way to measure mild wear behaviour of formulated oils.
Several techniques have been applied in this study to investigate the film formation, friction
and wear properties of ZDDP- and/or PIBSA-PAM-containing oils. These include a new mild
wear testing method, which is tested and developed using a range of different types of
additives.
It is found that the ratio of P:N plays a strong role in determining tribofilm formation and
friction of ZDDP/PIBSA-PAM blends. However it plays a much weaker role in determining
wear behaviour. It is found that some PIBSA-PAMs have considerable friction-reducing
properties in their own right. The results suggest that PIBSA-PAM may interfere with the
behaviour of ZDDP in several ways: by forming a ZDDP/ PIBSA-PAM complex at the metal
surfaces to reduce the local activity of ZDDP; by PIBSA-PAM partially removing the ZDDP
film; possibly also by PIBSA-PAM blocking ZDDP from metal surfaces. The newly-developed
wear testing method can be used conveniently and effectively to study mild wear
properties not just of ZDDP but of a wide range of other additives.