The thermal stability and antiwear performance of some thiophosphoric acid derivatives

Abstract

In order to study the relationship between the thermal stability and antiwear performance of metal-free phosphorus-sulphur compounds, some mono- and dithiophosphoric acid -O,O,S- triesters with specifically modified structures were synthesized and characterized by elemental analysis and 31P NMR spectroscopy. The thermal stability of these compounds was determined by TGA, while the wear-reducing properties were measured at different temperatures using the ball-on-plate test device of the SRV apparatus. The compounds evaluated show similar antiwear performance at temperatures below the temperature at which decomposition starts. Under more severe lubricating conditions at higher temperatures, the wear-reducing properties are largely dependent on the inherent thermal stability of the compounds. At the higher temperatures compounds with a low thermal stability lose their antiwear performance completely, whereas products with higher thermal stability are still active. It is highly likely that the loss in activity of the less thermally stable compounds can be attributed to additive depletion in the friction zone by general decomposition reactions on metal surfaces.