Experimental and numerical study on wear characteristics of steel surfaces involving the tribochemistry of a fully formulated oil. Part I: Experiments

This paper studied a fully formulated oil (FFO) in the hydraulic piston pump, containing mainly zinc dialkyldithiophosphate (ZDDP), detergents, and dispersants. The results of tribo-tests show that the antiwear characteristics are different when using FFO compared to using a single additive oil such as base oil + ZDDP. A two-phase temperature-dependent wear trend was found when FFO is used. Phase (I) is in the low-temperature range, where wear increases with increasing tribofilm thickness and temperature. Phase (II) is in the high-temperature range where the tribofilm thickness increases but wear decreases as the temperature increases. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis of the tribofilm show that the most temperature-dependent compositions are phosphates and iron sulphides, where the signal intensity of phosphates increases, but that of sulphides decreases when the temperature increases. It indicates that the participation of detergent and ZDDP in the tribofilm formation increases at low and high temperatures, respectively. However, no signal shows the dispersant participating in the tribofilm formation. A hypothesis of mechanisms of reaction priority or enhanced competition among different additives at different temperatures is proposed from the iron loss perspective to explain the two-phase temperature-dependent wear mechanism. This study also provides the experimental basis for the wear modelling involving the tribochemistry of FFO in Part (II) of the research series.