Effect of Ni Content on the Microstructure and Tribological Properties of NiTi Alloy Coatings
Plasma arc cladding was employed to fabricate NiTi coatings with varying Ni contents (60, 63, 65, 67, and 70 wt.%) on TC4 titanium alloy to enhance surface wear resistance. Microstructural characterization via SEM, OM, and XRD revealed that coatings with 60–67 wt.% Ni were dense and defect-free, whereas the 70 wt.% Ni coating exhibited through-thickness cracks. All coatings exceeded 1.2 mm in thickness and comprised a NiTi toughening phase and Ti2Ni strengthening phase. Increased dilution ratio with higher Ni content reduced actual Ni in the coating, maximizing Ti2Ni fraction (78.6%) in the 67NiTi coating, which achieved a peak hardness of 677.41 HV0.2 (2.05 times that of the TC4 substrate). Tribological testing under 5–20 N loads showed that the average wear rate of all coatings decreased significantly, following a V-shaped trend with Ni content. The 67NiTi coating exhibited the lowest wear rate (2.74×10⁻⁴ mm³/(N·m)) at 20 N, a 65% improvement over the substrate, with wear mechanisms dominated by mild abrasive and adhesive wear. These findings demonstrate that optimized Ni content in plasma-clad NiTi coatings effectively mitigates the poor wear resistance of titanium alloys, offering a viable surface engineering solution for load-bearing applications.