Morphology and Nanomechanical Properties of ZDDP Antiwear Films as a Function of Tribological Contact Time

We have employed scanning force microscopy (SFM) and nanoindentation analysis to track the evolution of tribologically generated antiwear films derived from zinc dialkyldithiophosphate (ZDDP) as a function of rubbing time. The SFM images reveal that film morphology evolves with time through a growth mechanism consisting of three stages. In the first stage nucleation on active sites at the steel surface leads to the growth of distinct segregated islands. In the second stage the islands coalesce causing the film to spread over a larger fraction of the surface. In the final stage continuous rubbing induces the large islands to divide into smaller, densely packed structures. In contrast to the observed morphological changes, rubbing time does not strongly influence the nanomechanical properties of the films. This highlights the importance of film morphology in determining the effectiveness of ZDDP antiwear films. We also observe large variation in both the morphology and nanomechanical properties that are likely due to the heterogeneity in contact pressure at the pin-sample interface of the wear rig.