Morphological evolution of films formed from thermooxidative decomposition of ZDDP

Zinc dialkydithiophosphate (ZDDP) materials are important antiwear/antioxidant additives in most modern lubrication formulations. The current understanding of the formation of antiwear films from ZDDP involves a tribochemical and thermooxidative component. We examine here the changes in the topography and structure of films formed by the thermooxidative decomposition of ZDDP in base oil. We probe the evolution in topography with scanning force microscopy (SFM) as a function of oil heating time and observe changes on both a mesoscopic and sub-micron scale. These morphological changes are compared with infrared spectra to correlate the chemical composition of thermal films to their topography.