Deposition of layered bioceramic hydroxyapatite/TiO2 coatings on titanium alloys using a hybrid technique of micro-arc oxidation and electrophoresis

Titanium alloys have been used with some success in several bioimplant applications. However, they can suffer certain disadvantages, such as poor osteoinductive properties and low corrosive-wear resistance. Attempts to overcome the first of these drawbacks have involved coating the metal with the bioceramic material hydroxyapatite (HA), a primary component of bone and a very good osteoinductor. Since TiO2 coatings are also known to be effective as chemical barriers against the in-vivo release of metal ions from the implants, a double layer HA-TiO2 coating on titanium alloys with HA as the top layer and a dense TiO2 film as the inner layer should possess a very good combination of bioactivity, chemical stability and mechanical integrity. This paper describes efforts to improve implant biocompatibility and durability by applying a hybrid treatment of micro-are discharge oxidation (MDO) and electrophoretic deposition. The most common structural titanium alloy (Ti-6Al-4V) was used as the substrate material. A phosphate salt solution and an HA powder aqueous suspension were used as the electrolyte for micro-arc oxidation and the solution for HA electrophoretic deposition, respectively. It is shown that a relatively thick and hard TiO2 coating can be produced by anodic micro-are oxidation of titanium, and an HA coating incorporated on top of the TiO2 layer can simultaneously be formed using a combination of plasma electrolysis and electrophoresis, with the suspension held at high values of pH. X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) have been used to investigate the microstructure and morphology of the coatings. The adhesive strength between the coating and substrate has been assessed using scratch adhesion testing. The corrosion resistance of the specimens was examined using potentiodynamic tests in a buffered physiological solution. The results indicate that a hybrid combination of micro-are oxidation and electrophoretic deposition can provide a phase-pure HA top layer and anticorrosive TiO2 interlayer, which should show good mechanical and biochemical stability in the corrosive environment of the human body. (C) 2000 Elsevier Science S.A. All rights reserved.