Friction and Wear Behaviour of Zinc Dialkyldithiophosphate Additive

A method has been developed for monitoring the film-forming properties of antiwear additives in rolling-sliding, lubricated contacts. This makes it possible to study both the kinetics of reaction film growth and also the evolution of the film morphology as a function of rubbing time. The technique has been applied to investigate the behavior of a zinc dialkyl-dithiophosphate (ZDDP) additive solution and to correlate this with simultaneous friction and wear measurements.

The results show that ZDDP forms a thick, solid-like, reaction film in the rubbing tracks, with negligible film growth outside of the track. This film is extremely effective in preventing metal-metal contact. However the film is unevenly-distributed, with its roughness oriented in the direction of sliding. This directional roughness inhibits the entrainment of fluid film in the mixed lubrication regime, increases the proportion of load supported by solid-solid contact and consequently results in the high friction often associated with the use of ZDDP additives.

Keywords: Antiwear Additives, Zinc Dialkyldithiophosphate, Friction, Wear, Optical Interferometry, Spacer Layer