Analysis of Tribofilms Formed by Energy-Saving Hydraulic Fluids

To develop energy-saving hydraulic fluids, power consumption tests of a vane pump have been carried out for sample fluids containing three types of additives [i.e., tricresyl phosphate (TCP), alkylated dithiophosphate (DTP), and oleic acid (OA)] at low concentrations. The films formed with the sample fluids on sliding surfaces (referred to as tribofilms) have been analyzed using transmission electron microscope and time-of-flight secondary ion mass spectrometry. The sample fluid containing only TCP forms the thickest tribofilm among the tested fluids, having a thickness of about 230 nm. Adding DTP to the sample fluid inhibits the development of the tribofilm by TCP, leading to a thin tribofilm having a thickness of about 40 nm; it shows the worst power consumption among the tested fluids. The combined usage of TCP, DTP, and OA shows the best power consumption among the tested fluids; phosphate compounds including iron eccentrically located at the tribofilm surface provide an interface having low shear strength, even though the tribofilm is not very thick (e.g., about 70 nm).