In-Situ Formation of MoS₂ and WS₂ Tribofilms by the Synergy Between Transition Metal Oxide Nanoparticles and Sulphur-Containing Oil Additives

This works investigates the in-situ formation of MoS₂ and WS₂ tribofilms by the synergy between transition metal oxide nanoparticles and conventional sulphur-containing anti-wear and extreme pressure additives. The formation of these low friction tribofilms can be obtained under reciprocating sliding contact and under extreme pressure conditions, as evidenced using X-ray photoelectron spectroscopy. Under reciprocating sliding conditions, the synergy between transition metal oxide nanoparticles and the ZDDP leads to coefficients of friction around 0.06 before they rise as consequence of oxidation. The synergy is more outstanding in extreme pressure conditions, particularly for MoO₃ nanotubes combined with extreme pressure additive. This combination outperforms base oil mixtures containing EP additive or MoS₂ nanotubes. While MoS₂ nanotubes build superb extreme pressure tribofilms containing iron and molybdenum oxides and sulphides, MoO₃ nanotubes are able to build similar tribofilms that can continuously re-sulphurize in the presence of the extreme pressure additive. Despite having a similar chemistry, MoO₃ nanotubes are observed to sulphurize more easily when compared to WO₃ nanoparticles. The work highlights the tribological potential of these nanoparticles otherwise typically used as precursors for the synthesis of transition metal dichalcogenide nanoparticles.