Strength of self-organized TiO2 nanotube arrays

Self-organized titanium oxide (TiO2) nanotube arrays were formed by an anodic oxidization technique. Nanoindentation experiments were conducted using three different indenters (i.e. flat punch, cone and triangular pyramid) to explore the mechanics governing the strength of the TiO2 nanotubes. The force–displacement responses were strongly dependent on the tip geometry and size of the nanotubes. A stress-based fracture model considering graded, porous and discrete structures of the nanotube arrays is proposed, and fracture stress was estimated based on the model. The experimental fracture stresses obtained using different indenters were almost the same, regardless of the indenter shape, which indicates the validity of the model. The evaluated compressive strength of the TiO2 ligament was approximately 3 GPa, and no clear size effects were observed for the 55–110 nm diameter nanotubes.