Double auxiliary anodes collaborate to deposit CrNx coatings on the inner wall of slender tube by built-in cylindrical cathodic-arc ion plating

Chromium nitride (CrNx) coatings demonstrate exceptional mechanical and tribological properties, making them ideal for protective applications in demanding industrial environments. However, uniform deposition of CrNx coatings on inner surfaces of slender tubes remains challenging due to confined spaces and difficulties in precise phase control. Here, we introduce a novel approach utilizing a double auxiliary anode and built-in cylindrical cathodic-arc ion plating to successfully deposit CrNx coatings inside tubes (40 mm diameter, 120 mm length). Systematic investigation of nitrogen flow rates revealed unexpected phase transformations, transitioning from mixed CrN + Cr₂N phases to single CrN phase, then reverting to mixed phases at higher flow rates. Coatings produced at 40 sccm exhibited optimal mechanical properties with maximum hardness (17.4 GPa), while minimum wear rate (2.95 × 10⁻⁶ mm³/(N·m)) was achieved at 80 sccm with pure CrN phase. Our findings demonstrate that stable deposition and precise phase control can be consistently achieved in slender tubes using this innovative technique. This advancement addresses critical stability and uniformity challenges unresolved by conventional methods, enhancing coating performance and expanding protective coating capabilities for diverse industrial applications.