Effect of electrical-discharging on formation of nanoporous biocompatible layer on titanium

In this study, we performed an electrical discharge machining (EDM), by which a recast layer was formed on a titanium surface. Subsequently, an α-phase and a γ-TiH-(γ-hydride)-phase were formed on the recast layer by electrical-discharging. Nano-(δ + γ) hydrides play important roles in the formation of nanostructural oxide layers. Electrical-discharging not only generates a nanostructural recast layer but also converts the alloy surface into a nanostructured oxide surface, which increases the alloy biocompatibility. A γ-hydride microstructure was also formed on the recast layer following electrical-discharging. The microstructure had a tetragonal structure with lattice constant a = 0.421 nm. In the recast layer, a transition, α → (α + δ) → (δ + γ) → γ, occurred during electrical-discharging. This result has never been previously reported. The recast layer that contains nanophases was dissolved during electrical-discharging; by this process, electrical-discharging for a short duration yields nanoporous TiO₂. Hence, electric discharging for a short duration leads to the production of nanostructures as well as bioactive titanium.