On the Mechanism of ZDDP Antiwear Film Formation

Abstract

Zinc dialkyldithiophosphate additives are used to control wear and inhibit oxidation in almost all engine oils as well as many other types of lubricant. They limit wear primarily by forming a thick, protective, phosphate glass-based tribofilm on rubbing surfaces. This film formation can occur at low temperatures and is relatively indifferent to the chemical nature of the substrate. There has been considerable debate as to what drives ZDDP tribofilm formation, why it occurs only on surfaces that experience sliding and whether film formation is controlled primarily by temperature, pressure, triboemission or some other factor. This paper describes a novel approach to the problem by studying the formation of ZDDP films in full film EHD conditions from two lubricants having very different EHD friction properties. This shows that ZDDP film formation does not require solid-solid rubbing contact but is driven simply by applied shear stress, in accord with a stress-promoted thermal activation model. The shear stress present in a high-pressure contact can reduce the thermal activation energy for ZDDP by at least half, greatly increasing the reaction rate. This mechanism explains the origins of many practically important features of ZDDP films; their topography, their thickness and the conditions under which they form. The insights that this study provides should prove valuable both in optimising ZDDP structure and in modelling ZDDP antiwear behaviour. The findings also highlight the importance of mechanochemistry to the behaviour of lubricant additives in general.

Stress-augmented thermal activation: Tribology feels the force

“…This problem has recently been addressed by Zhang and Spikes using the additive ZDDP [135]. The underlying problem is that the rubbing contact of asperities is an extremely severe and generally poorly defined process during which it is difficult to know or control the conditions and their effects.…”

“…If it were shear stress then, assuming a friction coefficient of 0.1, the shear activation volume would be 38 Å 3 . Zhang and Spikes [135] determined an activation volume value for ZDDP of 180 Å 3 . It is, of course, extremely unlikely that the two ZDDPs studied by Gosvami and Zhang were the same.…”

“…It should also be noted that although shear stress has been subsequently been shown to drive ZDDP tribofilm formation [135] this does not, of course, mean there might not also be a pressure activation term, and further work needs to be done to establish this. This would be very difficult to do in boundary lubrication conditions where normal pressure and shear stress are coupled, but should be feasible using Zhang and Spikes EHD approach [135], where they can be varied independently by changing the solvent friction.…”

“…The problem then becomes to develop experimental ways of distinguishing between mechanisms. This was done in the field of tribochemistry where a designed experimental approach showed that shear stress (and not many alternative drivers) almost certainly controls ZDDP reaction rate [135]. But this does not mean that all tribochemical reactions are driven by stress.…”

Generation of wear particles in lubricated contcats during running-in

“…Although it is widely accepted that the tribo-film is formed through a tribochemical reaction within the contact, the pathway of the reaction is not known [33,99].…”