Boundary lubricating films: formation and lubrication mechanism

Boundary lubricating films are critical in effective lubrication of engine components. Their formation mechanism is not fully understood due to the lack of understanding of how molecular structures of the base oils and the antiwear additives work together. This paper examines, the base oil molecular structures and their influence on the reaction rates leading to the formation of high molecular weight organometallic compounds. Three base oils have been separated into saturates, aromatics, and polar molecules by liquid column chromatography and these fractions are studied in a micro-oxidation test. Organometallic compounds were detected by a coupled gel permeation chromatograph-atomic absorption spectroscopy. Polar molecules show the highest organometallic compound formation rate while saturates the lowest. Analysis on films from worn samples and engine components confirmed the presence of similar organometallic compounds. When combined with reaction films from antiwear additive such as zinc dialkyl dithiophosphate (ZDDP), the soft organometallic compounds provide an easily shearable sacrificial layer and the inorganic phosphate glassy structure provides the load carrying capacity. The resultant boundary film is stronger and durable. This understanding provides a design guideline for future boundary films.