Mechanisms of Enhanced Wear and Corrosion Resistance in MAO/Magnetron-Sputtered DLC Composite Coatings on AZ31 Magnesium Alloy

To improve the wear and corrosion resistance of AZ31 magnesium alloy, micro-arc oxidation/diamond-like carbon (MAO/DLC) composite coatings were fabricated, consisting of a porous MAO layer and a DLC overlayer deposited by magnetron sputtering for 30, 60, and 90 min. The effects of sputtering time on microstructure, surface properties, tribological behavior, and corrosion resistance were systematically investigated. Increasing deposition time progressively sealed MAO-induced pores and densified the surface, reducing the roughness ( Ra ) from 217 nm to 79.5 nm and increasing the water contact angle from ∼20° to ∼81°. Structural analyses confirmed an amorphous DLC layer with enhanced ordering and increased sp 2 fraction. Correspondingly, the critical load increased from 4.99 N to 14.18 N, the friction coefficient decreased from 0.364 to 0.096, and the wear loss was reduced from 0.92 mg to 0.41 mg. Electrochemical tests showed a significant improvement in corrosion resistance, with Icorr decreasing from 9.76×10 -5 to 6.67×10 -6 A·cm -2 and R p increasing from 569 to 2678 Ω·cm 2 . Notably, although overall corrosion resistance improves with deposition time, impedance analysis reveals that corrosion control capability reaches an optimum at 60 min and then declines, as indicated by the reappearance of Warburg impedance. This behavior is attributed to the combined effects of excessive thickness-induced defects and sp 2 -dominated conductive pathways. Therefore, the performance evolution of the MAO/DLC system is governed by a synergistic yet competitive interplay between thickness-induced pore sealing and sp 2 -related conductive effects.