Study of Zinc Dialkyldithiophosphate Antiwear Film Formation and Removal Processes, Part II: Kinetic Model

Zinc dialkyldithiophosphate (ZDDP) film thickness measurements made using in situ ultrathin-film interferometry and described in Part I of this two-part paper (Fujita, et al. [1 Fujita, H., Glovnea, R. P. and Spikes, H. A. 2005. "Study of Zinc Dialkyldithiophosphate Anti wear Film Formation and Removal Processes. Part I: Experimental,". Tribol. Trans., 48: 558–566. [CSA][Taylor & Francis Online], [Web of Science ®] , [Google Scholar]]), have been used to develop and test kinetic models of antiwear film formation and removal. The main component of ZDDP film formation involves the gradual coverage of the surfaces by thick, discrete islands of film material. This process can be modeled by combining a simple coverage model in which the rate of film formation is proportional to the fraction of surface not yet covered, with an induction period. The process of film removal can be modeled by assuming that the rate of film loss is proportional to the fourth power of the coverage or film thickness. The combination of these film formation and removal rate equations is able to predict the complex, transient maximum film-forming behavior of secondary ZDDP as well as the process of film formation by primary ZDDP and the removal of antiwear film by dispersant additive.