Effect of Mo content on the corrosion and tribocorrosion behavior of CoCrFeNiMox HEA coatings in simulated seawater

This study investigates the influence of Mo content on the adhesion, electrochemical, and tribocorrosion behavior of CoCrFeNiMox (x = 0–20 at.%) high-entropy alloy (HEA) coatings deposited by high power impulse magnetron sputtering (HiPIMS) on AISI 304 stainless steel. Electrochemical analyses demonstrate that the HEA coatings exhibit superior corrosion resistance compared to stainless steel, with corrosion current densities reduced by approximately one order of magnitude in 3.5 wt% NaCl solution, although no clear dependence on Mo content is observed. In contrast, tribocorrosion performance strongly depends on composition: coatings with low Mo contents (0–5 at.%) exhibit the lowest specific wear rates (45–61 × 10−6 mm3/(N·m)), outperforming AISI 304 (108 × 10−6 mm3/(N·m)), whereas Mo-rich coatings (>10 at.%) undergo premature wear-through and delamination. This trend is further supported by scratch test results, which indicate a reduction in adhesion toughness for Mo contents above ~10 at.%, with the critical load for coating failure decreasing from ~34 N (Mo10) to ~14 N (Mo15). This behavior is likely caused by the increased brittleness associated with the formation of a hard body-centered cubic (BCC) phase within the face-centered cubic (FCC) matrix at high Mo contents. Overall, these results highlight the need for careful compositional optimization of Mo-doped HEA coatings, with low-Mo compositions providing the best balance between corrosion resistance, adhesion, and tribocorrosion performance. This combination of properties makes low-Mo CoCrFeNi-based HEA coatings promising candidates for the surface protection of stainless-steel components exposed to chloride-containing environments and simultaneous mechanical wear.