Friction and wear behavior of zinc dialkyldithiophosphate additive

A method has been developed for monitoring the film-forming properties of a ntiwear additives in rolling-sliding, lubricated contacts. This makes it po ssible to study both the kinetics of reaction plm growth and also the evolu tion of the film morphology as a function of rubbing time. The technique ha s been applied to investigate the behavior of a zinc dialkyl-dithiophosphat e (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 r ubbing 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 sl iding. This directional roughness inhibits the entrainment of fluid film in the mixed lubrication regime, increases the proportion of load supported b y solid-solid contact and consequently results in the high friction often a ssociated with the use of ZDDP additives.