The Effect of Additive Chemical Structure on the Tribofilms Derived from Varying Molybdenum-Sulfur Chemistries

Molybdenum disulfide (MoS₂) is an effective friction modifier that can be formed on surfaces from oil-soluble lubricant additives. Different additive chemistries can be used to form MoS₂ on a surface. The tribofilms formed from three different molybdenum additives (MoDTC Dimer, MoDTC Trimer, and molybdate ester) were studied in additive monoblends and fully formulated systems. The resulting tribofilms were then characterized by Raman spectroscopic spatial mapping, XPS, and FIB-TEM. The distribution of MoS₂ on the surface was much more sparse for the molybdate ester than the other additives. No crystalline molybdenum oxides were observed by Raman spectroscopy, but their presence was inferred from XPS analysis. XPS analysis showed very similar distributions of Mo oxidation states from each additive, such that the chemical nature of the films formed from all of the additives is likely similar. Each of the additive tribofilms was observed to have MoS₃ vibrations in Raman and persulfide XPS peaks associated with amorphous MoS₃, as such this species is presented as a common frictional decomposition product for all the additives. The MoDTC trimer is more able to produce this amorphous species on the contacting surfaces due to its structural similarities to the co-ordination polymer MoS₃.