Tribological Properties of Different Carboxylates in Water–Glycol Hydraulic Fluid

Conventionally, carboxylic acid salts are widely applied as antiwear additives for water–glycol (W/G) hydraulic fluids. For this study, a Mini Traction Machine (MTM) was employed to evaluate the friction properties of four carboxylates in W/G fluid. The results indicate that undecylenate salt (CN-3) exhibits the best antifriction capability and the friction coefficient nearly remains stable when its concentration increases from 3 wt% to 6 wt%. However, ball-on-disk tribology tests show that CN-3 as a single lubricant additive causes poor lubrication under severe conditions like a high load of 125 N. Fortunately, the combination of CN-3 and a water-soluble heterocyclic compound (COMB) led to outstanding antiwear and friction-reducing properties. A possible reason is the formation of a more stable tribofilm containing FeS2 and carbon structures, according to x-ray photoelectron spectroscopy (XPS) analysis of the worn surfaces. Furthermore, a vane pump test was conducted to confirm the results of the simulating tests. The W/G hydraulic fluid formulation based on COMB as the antiwear package performs well in the pump test, giving rise to only slight pump wear loss. The results of the pump test and the simulating test are consistent with each other. Meanwhile, the W/G hydraulic fluid possesses excellent other physicochemical properties and meets the specification of ISO 12922.