Effect of Ni Content on the Microstructure and Tribological Properties of NiTi Alloy Coatings

• • Coatings with 60–67 wt.% Ni are crack-free and dense, but 70 wt.% Ni induces through-thickness cracks, defining a critical upper limit for Ni content to avoid catastrophic failure in service. • • The 67NiTi coating achieves a hardness of 677.41 HV0.2 (2.05× substrate) due to a maximum Ti2Ni volume fraction of 78.6%, directly correlating with a 65% reduction in wear rate at 20 N (2.74×10⁻⁴ mm³/(N·m)), validating dilution-driven second-phase strengthening. • • Wear rate follows a V-shaped dependence on Ni content, with the minimum at 67 wt.% Ni; this non-monotonic behavior necessitates precise compositional control to avoid the inferior wear performance of both lower (60–65 wt.%) and higher (70 wt.%) Ni coatings. • • The 67NiTi coating exhibits mild abrasive and adhesive wear at 20 N, indicating stable tribological performance under high loads, which is critical for extending the lifespan of TC4 components in aerospace and biomedical implants.