Effect of alternating hydrostatic pressure (AHP) on the corrosion and tribocorrosion behavior of gradient W-DLC coated WC-based cermet

Marine hydraulic components (e.g., seawater piston pumps) continuously endure coupled high hydrostatic pressure, electrochemical corrosion, and tribological degradation during prolonged operation, significantly compromising equipment stability and longevity. While WC-20Cr₃C₂-7Ni/W-DLC (WC/W-DLC) duplex coatings demonstrate improved tribocorrosion resistance for metal substrates, their long-term performance under extreme deep-sea conditions remains insufficiently characterized, particularly regarding high-pressure tribocorrosion mechanisms. In this work, the long-term corrosion and tribocorrosion properties of the coatings were evaluated using an autoclave equipped with an in-situ electrochemical monitoring system in a 20 MPa alternating hydrostatic pressure (AHP) environment. The results showed that the corrosion resistance of the WC/W-DLC coating was always superior to that of the WC-based coating during AHP cyclic immersion. It was attributed to the barrier of the structurally stable W-DLC layer against the corrosive solution and Cl-adsorption. The tribocorrosion results after AHP cycling revealed that the bonding between WC grains was weakened due to the corrosion of the Ni binder phase. Therefore, WC grains were pulled out and exfoliated during the sliding process, causing relatively severe failure of the WC-based coating. In contrast, the duplex coating, featuring a structurally stable and strongly bonded W-DLC layer, exhibited a coefficient of friction (COF) and wear rate of only 0.067 and 4.71 × 10⁻⁸ mm³/N·m after sliding for 1200 m. Additionally, the potential drop induced by sliding was only 5 mV, significantly lower than that of the WC-based coating (↓159 mV). In short, the optimized WC/W-DLC duplex coating can provide excellent and consistent surface protection for metal parts in deep-sea environments.