Surface Engineering of NiTi via Ta₂O₅ ALD: Stability Assessment of Barrier Properties against Exposure to Simulated...

Tantalum pentoxide (Ta₂O₅) coatings deposited via atomic layer deposition (ALD) have emerged as promising protective barriers for NiTi shape memory alloys in biomedical applications. This study investigates the influence of 800 ALD cycles on the structural, tribological, and electrochemical stability of NiTi substrates during long-term exposure to physiological environments. A conformal, amorphous Ta₂O₅ layer with a thickness of ∼65 nm was achieved, effectively sealing the substrate topography. Our findings reveal that while the coating initially maintains a hydrophilic character, prolonged incubation (60 days) in PBS solution triggers a transition to a hydrophobic state of 104 ± 3°, driven by the accumulation of Ca- and P-mineralized deposits. Tribological assessments demonstrate that the Ta₂O₅ overlayer enhances wear resistance and load-bearing capacity, despite localized microdelamination observed postexposure. Most notably, the ALD coating acts as a robust diffusion barrier, reducing the corrosion current density to 7.60 × 10^-8 A/cm² and maintaining a high polarization resistance even after 60 days of incubation. These results highlight the efficacy of Ta₂O₅ overlayers in providing long-term biostability and corrosion protection for NiTi-based implants in demanding physiological conditions.