The Relation of Antiwear Function to Thermal Stability and Structure for Metal: O,O-Dialkylphosphorodithioates

Abstract

The antiwear characteristics of pure metal O,O-dialkylphosphorodithioates in n-hexadecane are determined for steel-on-steel and copper-on-steel systems under mild conditions of sliding. Their effectiveness is found to be directly related to their rate of thermal decomposition; and more thermally stable structures exhibited better antiwear characteristics. This demonstrates that differences in wear are due to differences in chemical, not adhesive, wear. Because thermal stability is systematically dependent upon alkyl group structure and metal cation size, it is concluded that neither the alkyl group nor the metal play a direct role in the effectiveness of the additive in reducing wear. The nature of the surface reaction leading to chemical wear is not known.