Systematic microstructure modification effect on nanomechanical, tribology, corrosion, and biomineralization behavior by optimized anodic alumina nanotubes coated Ti–6Al–4V alloy

Abstract

Electrochemical anodization is a cost-effective surface modification method that has been used in recent years not only for creating protective layers but also for the fabrication of porosity in biomedical applications. This method allows for the optimized fabrication of self-ordered nanotubular or nanoporous oxide structures. In this study, the structural characteristics, mechanism, adhesion, scratch hardness, wettability, nanoindentation, wear resistance, corrosion behavior, and in-vitro bioactivity of aluminum/alumina (Al/Al2O3) nanotube coating on Ti–6Al–4V (Ti64) were systematically studied. As the primary layer, a thin layer of pure Al was sputtered onto a Ti64 substrate via physical vapor deposition. Consequently, a dense and uniform Al2O3 nanotubes layer was fabricated as the second layer by anodization in a 0.35 wt% NH4F electrolyte solution (90 ethylene glycol:10 water) at 60 V (constant potential) for various durations, followed by optimization of the sealing to remove the oxide layer and transform the nanoporous into nanotubes. Optimized anodizing, sealing, and annealing resulted in a homogeneous nanotube structure with a mean diameter of 85 nm and a length of 1.58 μm. Systematic improvements in the adhesion, scratch hardness, nanoindentation, and wettability were achieved in each modification process. The corrosion rate and the coefficient of friction also reached 0.7297 × 10−1 (mm year−1) and 0.208 (load of 10 N) after 1 h of anodization and 30 min sealing followed by annealing at 450 °C, respectively. When a thick apatite layer with the desired Ca/P ratio was created on the annealed Al2O3 nanotube structure, an excellent improvement in surface bioactivity was demonstrated.