Comparison of sliding speed dependency of friction between steel surfaces lubricated with several ZnDTPs with different hydrocarbon moieties

This article describes the friction characteristics of zinc dialkyldithiophosphates (ZnDTPs) with different hydrocarbon moieties. A newly developed tribometer was used for the precise measurement of friction by alteration of the sliding speed from ultra-low to moderate. Four types of ZnDTPs (sec-CX6, sec-C6, prim-C8, and prim-C12) were used as additives to measure friction between steel surfaces. Several phosphorus compounds were used as references. Auger electron spectroscopy analysis and micro-FT-IR reflection adsorption spectroscopy analysis were employed to elucidate the chemical composition and chemical structure of tribochemically reacted films on steel surfaces. The results demonstrated that a remarkable difference between friction characteristics was observed among four ZnDTPs with different hydrocarbon moieties and that friction decreased with increasing alkyl chain length of ZnDTPs at lower speeds. These results accounted for the observation that the friction characteristics of ZnDTPs were influenced by their respective hydrocarbon moieties.