Formation of self-ordered oxide nanotubes layer on the equiatomic TiNbZrHfTa high entropy alloy and bioactivation procedure

Highlights: • Nanotubes' oxides layer was formed on TiNbZrHfTa high entropy alloy surface. • 10 V-2 h anodization parameters provided the best adhesion of nanotubes' layer. • Five cycles of alternative immersion method provided Ca and P saturation on surface. • Synergism between nanotubes and precalcification by AIM bioactivated the surface. • High corrosion properties and HAP deposition were promoted by surface modification. -- Abstract: The TiNbZrHfTa high entropy alloy was studied for its use as a base material for orthopedic implants. The alloy produced by arc melting contains only biocompatible elements and has an attractive combination of mechanical properties with a tensile strength of 1050 MPa and Young's modulus of 66 GPa. Surface modification techniques were carried out to bioactivate the alloy surface, ensuring better interaction between the implanted material and the human body. The first technique was the anodization in an organic electrolyte, containing F^- ions in different potentials and time, obtaining the best condition at 10 V potential for 2 h at 20 °C, producing an ordered and adherent nanotubular morphology to the substrate. The second technique was precalcification. Using the alternation immersion method (AIM), the saturation and nucleation of Ca-P deposits were guaranteed, providing high deposition rates of hydroxyapatite (HAP) when immersed in simulated body fluid (SBF) solution. Structural characterizations were performed by XRD and XPS. Morphological analyses were performed by SEM and chemical semi-quantitative analyses by EDX. Corrosion resistance properties were performed through polarization and impedance curves. The TiNbZrHfTa high entropy alloy proved to be a competitive and attractive candidate in the research for new compositions to replace conventional alloys, having biocompatibility, high strength, and low Young's modulus, reducing stress shielding effects and showing good bioactivity characteristics through the surface modification techniques presented here.