Controllable friction and wear of nitrided steel under the lubrication of [DMIm]PF6/PC solution via electrochemical potential

• Nitrided AISI 4340 steel shows a low friction at potentials negative of −0.6 V.
• Nitrided steel exhibits a higher wear resistance at −1 V than that at OCP.
• Corrosion wear at +1 V is milder for nitrided steel than that for the unnitrided.
• Nitrided steel has a higher hardness and corrosion resistance than the unnitrided.

The friction and wear properties of the nitrided steel at different electrochemical potentials of the rubbed surface have maintained interesting and still an unsolved problem. This study systematically investigated the friction and wear behavior under different potentials for the nitrided AISI 4340 low alloy steel lubricated by propylene carbonate with an additive of imidazolium ionic liquid. A nitrided layer ~170 nm thick, composed of nitrogen expanded austenite (γN) phase, nitrogen-containing martensite (α′) phase and chromium nitride (Cr2N) precipitate, was formed onto the steel substrate via plasma source ion nitriding. The nitrided layer exhibited a higher hardness and corrosion resistance than the steel substrate. It was found that the friction and wear for the nitrided sample could be controlled via electrochemical potential. A stable low friction coefficient, 0.16, was obtained within the potential range of −1 V to −0.6 V, and almost no wear appeared at −1 V for the nitrided sample. The potential controlled friction was due to the modulation of the adsorption of the lubricant ions ([DMIm]+ cations or [PF6]− anions) by the potential, and the potential controlled wear was due to the adjustment of the anti-wear ability of the lubricant by the potential. The saturated adsorption of [DMIm]+ cations at −1 V led to the improved anti-wear ability compared to that at OCP, and the anodic corrosion of the rubbed sample surface was responsible for the degraded anti-wear ability at +1 V. Also, the nitrided sample exhibited a much milder wear than the un-nitrided sample at each tested potential, ascribed to the higher hardness and corrosion resistance of the nitrided sample compared to those of the un-nitrided sample.