Influence of electrochemical anodization and thermal annealing on the morphology, hardness, and friction coefficient of TiO₂ nanotubes

Electrochemical anodization (EA) of Ti–6Al–4V alloy was performed to evaluate the effect of NH₄F concentrations (0.15–0.45 M), anodization times (5–120 min), and applied voltages (10–60 V) on surface morphology. The anodized samples were then subjected to thermal annealing (TA) at temperatures ranging from 400 to 650 °C. The main objective was to understand how the interaction between morphology and crystalline structure influences microhardness and the friction coefficient (COF), aspects that are underexplored in this material. Results indicated that TiO₂ nanotubes formed only with 0.45 M NH₄F, 30 min of anodization, and a voltage of 30 V. The best surface properties were achieved with TA at 400 °C, yielding a Vickers hardness of 4.87 GPa and a COF of 0.12. TA and the resulting crystalline phase significantly impacted the nanotube dimensions, influencing the mechanical and tribological properties of the surface.