Conventionally, carboxylic acid salts are widely applied as anti-wear additives for water–glycol (W/G) hydraulic fluids. Here, 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-disc tribology tests show that CN-3 as a single lubricant additive cause poor lubrication under severe condition like a high load of 125 N. Fortunately, the combination of CN-3 and a water soluble heterocyclic compound (COMB) lead to the outstanding antiwear and friction-reducing properties. The possible reason is ascribed to the formation of more stable tribofilm containing FeS2 and carbon structures according to 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 physic-chemical properties and meets the specification of ISO 12922 requirement.
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