The Formation of Zinc Dithiophosphate Antiwear Films

In situ spacer layer interferometry has been used to monitor and compare the formation of both tribo- and thermal zinc dialkyldithiophosphate (ZDDP) films on steel surfaces. For the ZDDP solution studied, negligible thermal film formation takes place below a temperature of 110°C, but a ZDDP tribofilm is generated on rubbed surfaces even at room temperature. The ZDDP tribofilm formation is critically dependent on the extent of direct solid solid contact during rubbing and does not take place if the elastohydrodynamic film thickness is significantly greater than the surface roughness. Tests at different slide-roll ratios indicate that the rate of ZDDP tribofilm formation is proportional to the product of sliding distance and rubbing time. Based on the results, it is not possible to explain ZDDP tribofilm formation in terms of a response to contact pressure or flash temperature. It is likely that the tribofilm reaction is either strongly catalysed by species released during rubbing, such as soluble FeII or FeIII, or is triggered by triboelectronic processes such as exoelectron emission.