Surface morphology and nanomechanical properties of tribological antiwear films derived from zinc dialkyl dithiophosphate compounds

The protection of mechanical equipment from wear is of significant economic interest. It has been estimated that up to half of a percent of the gross domestic product of industrialized countries goes to replacing mechanical components that have lost compliance due to wear. Antiwear additives are key ingredients in lubrication oils that assist in protecting components from wear during high loads. These agents form sacrificial films on metal parts that limit the adhesion between the contacting surfaces and reduce the wear rate considerably. One of the most common classes of compounds employed as an antiwear agent is zinc dialkyldithiophosphates (ZDDP).; This work will explore the formation, structure, and mechanical properties of ZDDP derived antiwear films on the nanoscale. These studies are important because the macroscopic performance of antiwear coatings is dictated by their nanoscale surface properties. As a first study, scanning force microscopy (SFM) is employed to track the formation of films formed from the thermooxidative decomposition of ZDDP on gold substrates. The SFM analysis is correlated with infrared spectroscopy to relate surface structure to chemical composition.; The morphology and mechanical strength of ZDDP tribofilms formed at the interface of sliding stainless steel contacts is also investigated. The tribofilms evolve morphologically with contact time and are characterized by distinct segregated islands at low times that transforms to a full film at longer times. The nanomechanical properties of the tribofilms are evaluated by nanoindentation analysis. It is found that the films are mechanically softer than the underlying steel substrate. SFM and nanoindentation analyses reveal that calcium sulphonate detergents promote the formation of ZDDP tribofilms and impart to them greater mechanical stability. By contrast succinimide dispersants reduce the capacity of ZDDP to form effective antiwear films.; The first application of SFM and nanoindentation to the study of automotive engine components obtained directly from vehicles is demonstrated. The wear zone on a rocker arm bridge from a diesel engine is shown to contain a tribofilm at the periphery of contact where the contact load is less. SFM images also reveal wear damage at the center of the wear zone where the contact load is expected to be higher.