Friction reduction efficiency of organic Mo-containing FM additives associated to ZDDP for steel and carbon-based contacts

The lubricating properties of two different organomolybdenum additives combined with ZDDP were evaluated for different tribopairs involving carbon-based coatings, hydrogenated DLC and nanocrystalline diamond NCD. Their performances were investigated using reciprocating linear tribometer and compared with the traditional additivation MoDTC/ZDDP. The highly sulfurised moly-trimer additive and organic molybdate ester additive displayed good friction reducing properties in comparison with traditional additivation MoDTC for all tribopairs tested, especially organic molybdate ester. XPS and ToF-SIMS surface analyses performed on the tribofilms formed with these different organomolybdenum additives showed the formation of MoS2/MoS2−xOx species exhibiting a solid lubricant character, but preferentially on steel surfaces. HRTEM/EDS analyses confirmed the presence of MoS2/MoS2−xOx sheets embedded inside an oxygen-rich amorphous matrix. This is different from the traditional additivation that leads to the polyphosphate/phosphate glass matrix. Organic molybdate ester is a sulfur- and phosphorus-free additive that appears particularly interesting for achieving low friction while minimizing the S content of the lubricant.