Surface Modification of NiTi shape memory alloy using HA-Ag-ZrO₂ nanocomposite coating: Structure and performance

Nickel–titanium (NiTi) shape memory alloys exhibit super elasticity, shape recovery, and favorable mechanical compatibility; however, their biomedical applications remain limited by surface corrosion, nickel release, insufficient bioactivity, and susceptibility to implant-associated infection. This study investigated the effect of dip-coated nano-hydroxyapatite (nHAp), silver nanoparticles (AgNPs), and zirconium dioxide nanoparticles (ZrO₂ NPs) coatings, applied individually and in combination, on the surface and corrosion properties of NiTi alloy. Eight conditions are evaluated: uncoated NiTi, AgNPs, nHAp, ZrO₂ NPs, AgNPs–nHAp, AgNPs–ZrO₂ NPs, nHAp–ZrO₂ NPs, and AgNPs–nHAp–ZrO₂ NPs. Coating thickness and surface characteristics are examined using optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). Functional performance is assessed through water contact angle measurements, Vickers hardness, and potentiodynamic polarization testing. Coating thickness ranged from 44.5860 to 66.0207 µm. All coated surfaces exhibited markedly improved wettability, with contact angles of 7.364–19.409°, compared with 76.533° for uncoated NiTi. The ZrO₂ NPs coating achieved the highest hardness of 521.8 HV, representing a 126.6% increase over the substrate, while the ternary AgNPs–nHAp–ZrO₂ NPs coating reached 383.2 HV. All coatings improved corrosion resistance relative to uncoated NiTi. The ternary coating exhibited the lowest corrosion current (9.082 × 10⁻⁸ A) and corrosion rate (8.893 × 10⁻⁴ mm/year), compared with 2.808 × 10⁻⁶ A and 2.745 × 10⁻² mm/year for the uncoated substrate, corresponding to a protection efficiency of 96.76% and an approximately 30.87-fold reduction in corrosion rate. The nHAp–ZrO₂ NPs and AgNPs coatings also exhibited high protection efficiencies of 95.08% and 94.94%, respectively. Overall, the AgNPs–nHAp–ZrO₂ NPs coating provided the most favorable balance of corrosion resistance, wettability, and hardness, whereas ZrO₂ NPs showed superior performance when maximum surface hardness is prioritized.