The lubrication effectiveness of dialkylpentasulfide in synthetic ester and its emulsion

The application of dialkylpentasulfide (DPS) in synthetic ester (pentaerythritol tetraoleate known as PETO ester) and its emulsion is studied in this paper. PETO ester blended with DPS shows good antiwear, friction reducing and extreme pressure performance, while PETO emulsion doped with DPS exhibits only excellent extreme pressure properties. The above difference originates from the tribofilm formed on the worn surface by tribochemical reaction. SEM-EDS and XANES are applied to determine the relationship between tribochemical products and lubrication effectiveness. The tribofilm is composed of FeSO4, FeS, FeS2 and FeSO3. DPS in PETO ester and PETO emulsion undergoes tribochemical reaction dominated by sulfidation and thermal oxidation respectively, resulting in distinct tribological properties. This provides an essential explanation for the improvement of tribological properties of DPS in PETO ester and emulsion, and also indicates that sulfur is not fitted as antiwear, friction-reducing additive in water based system.