Fabrication, Geometry, and Mechanical Properties of Highly Ordered TiO2 Nanotubular Arrays

Highly ordered TiO2 nanotubular arrays have attracted increasing interest because of their exceptional physical properties and wide range of existing and potential applications. Techniques for fabrication and characterization of TiO2 nanotubular arrays have been reported extensively; however, the mechanical behavior of TiO2 nanotubular arrays has not been systematically investigated, which could be critical for their applications. In this study, we synthesized a series of highly ordered TiO2 nanotubular array films from a few to hundreds of microns in thickness by changing the anodic voltage or reaction time during anodization. The as-prepared nanotubular arrays were examined carefully using field emission scanning electron microscopy, and their mechanical properties were evaluated using a micromechanical probe and a microtribometer. It was demonstrated that the diameter and wall thickness of TiO2 nanotubes were almost independent of the anodic voltage and anodization time, whereas thicker nanotubular array films or longer nanotubes could be fabricated at higher voltage or with longer anodization time. The micromechanical probing tests demonstrated that the apparent Young's modulus, 77 value, and hardness decreased as the thickness of the nanotubular array films increased due to the densification and collapse of longer nanotubes under external force. The resistance of the nanotubular array films to scratch was evaluated by performing the microscratch tests with in situ measurement of the contact electrical resistance. To determine their resistance to sliding wear, sliding wear tests were performed in different environments. Compared to wear in air, the wear loss in water significantly decreased. The pH value of water slightly affected the wear loss of TiO2 nanotubular arrays; the results showed that the wear loss of TiO2 nanotubular arrays decreased with increasing pH from 4, through 7, to 10.