The influence of surface roughness and high pressure torsion on the growth of anodic titania nanotubes on pure titanium

Abstract

Anodic titanium dioxide nanotube (TNT) arrays have wide applications in photocatalytic, catalysis, electronics, solar cells and biomedical implants. When TNT coatings are combined with severe plastic deformation (SPD), metal processing techniques which efficiently improve the strength of metals, a new generation of biomedical implant is made possible with both improved bulk and surface properties. This work investigated the effect of processing by high pressure torsion (HPT) and different mechanical preparations on the substrate and subsequently on the morphology of TNT layers. HPT processing was applied to refine the grain size of commercially pure titanium samples and substantially improved their strength and hardness. Subsequent anodization at 30V in 0.25wt.% NH4F for 2h to form TNT layers on sample surfaces prepared with different mechanical preparation methods was carried out. It appeared that the local roughness of the titanium surface on a microscopic level affected the TNT morphology more than the macroscopic surface roughness. For HPT-processed sample, the substrate has to be pre-treated by a mechanical preparation finer than 4000 grit for HPT to have a significant influence on TNTs. During the formation of TNT layers the oxide dissolution rate was increased for the ultrafine-grained microstructure formed due to HPT processing.