Mechanical properties of tribofilms formed by metal dithiodiphosphates

Abstract

Lubrication-based tribofilms are crucial in protecting contacting parts from wear and friction. A popular and well-studied additive commonly used in tribofilm formation is zinc dialkyldithiophosphate (ZDDP). While ZDDP has been shown to be an effective additive, the use of other metals in place of zinc may provide further benefits. Tribofilms were generated using fluids which contained DDPs neutralized with octylamine or different metals: magnesium, titanium, and zinc. Tribofilms were formed at 30 min, 60 min, and 120 min and were chemically and mechanically analyzed using EDS, AFM, and nanoindentation. Tribofilms were also analyzed based on property differences at the peak surface interactions. The metal used was found to have a significant effect on both tribofilms morphology and mechanical properties. Properties were further seen to change in the top 10% of the tribofilm surface as compared to bulk surface properties. Hardness and elastic modulus ratios were higher for the tribofilms formed by octylamine and MDDP as compared to TDDP and ZDDP tribofilms, but with the tradeoff of less tribofilm coverage of the substrate.