TiO₂ nanotubes from stirred glycerol/NH₄F electrolyte: Roughness, wetting behavior and adhesion for implant applications

Titanium oxide (TiO₂) was anodically formed on titanium from non-aqueous electrolyte containing glycerol and 0.5 wt.% ammonium fluoride (NH₄F). Oxidation was carried out for 30, 60, 120 and 240 min at potentiostatic 30 V with the bath being stirred using magnetic pellet. All the conditions produced amorphous nanotubes. They had an average diameter of 50-130 nm and length in the range of 1.2-1.9 μm. The porosity was in the range of 70-80%. Stirring of the glycerol-based electrolyte has proved to be advantageous in retaining the tubular structure and providing smooth tubes even at 30 V condition. The coatings had surface roughness Ra lower than 0.5 μm, water wetting angles in the range of 58-84°. Increasing pore diameters increased the water wetting angles. All the coatings invariably showed poor tensile pull-off adhesion strengths. This poor adhesion is attributed to the stirring of the electrolyte.