Structural evaluation, preliminary in vitro stability and electrochemical behavior of apatite coatings on Ti6Al4V substrates

Medical-grade alloys, such as Ti-6Al-4V, have been used for fixation of fractured bone and for the total replacement of defective bone. Their bioactivity could be improved by applying a bone-like apatite layer onto their surfaces. This, in turn, enhances their integration with the surrounding tissues upon implantation. In addition, the presence of a bioactive bone-like coating minimizes the likelihood of corrosion. Various methods are known for the formation of apatite coating onto Ti-6Al-4V, among which sputtering has shown its promise as a simple direct method. In the current work, a sputtering technique was used to develop a 300 nm-thick bone-like apatite layer onto Ti-6Al-4V. Structural composition, integrity and morphology of the as-coated and thermally treated coatings were investigated. Coated substrates were further evaluated after soaking them in a simulated body fluid (SBF) for up to 14 days. Results showed the formation of an amorphous apatite layer onto the alloy, that was further shown to partially crystallize upon heat treatment. As a result of SBF treatment, the apatite layer was found to remodel through a dissolution-precipitation mechanism due to its amorphous and non-stoichiometric nature, forming a smooth layer with better homogeneity and decreased surface roughness. Electrochemical analysis of the coated alloys showed the enhanced corrosion protection of the alloy surfaces by coating them with apatite. In addition, pre-grinding of the alloy surfaces before the formation of the coating was also found to improve the corrosion inhibition of the alloy surfaces in aqueous media.