Stress–corrosion cracking by indentation techniques of a glass coating on Ti6Al4V for biomedical applications

The fixation of bone replacement implants to the hosting tissue can be improved if the implants have a bioactive surface that can precipitate hydroxyapatite in vivo. Titanium alloys, despite their desirable mechanical and nontoxic properties, are not bioactive and do not bond directly to the bone. One of the ways to change a bioinert metallic surface such as a titanium alloy is to coat it with a bioactive material. This work presents the microstructural and stress–corrosion cracking characterization of two glass coatings on Ti6Al4V with different SiO2 contents (61% and 64%). These coatings belong to the SiO2–CaO–MgO–Na2O–K2O–P2O5 system and they were obtained by a simple enamelling technique. They will be used as the first layer of a bioactive multilayer system which will have an outer layer with a lower SiO2 content in order to ensure the surface bioactivity. Microstructural characterization performed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) shows that the coating porosity is clearly influenced by the firing time because of the longer extension of the reaction between Ti and SiO2. The X-ray diffraction (XRD) integration method shows that the amount of crystalline phase (2.4CaO 0.6Na2O P2O5) percentage is between 3 vol.% and 16 vol.%. After acid etching, a microstructure with clear boundaries is observed which is the result of the sintered glass particles separation. Stress–corrosion cracking was evaluated using Vickers and Hertzian (spherical) indentation, showing that both coatings are sensitive to subcritical crack growth, and that the coating with the lower silica content is more sensitive to stress–corrosion cracking. These two results are related with the larger residual stresses due to the thermal expansion mismatch. Finally, the stress–corrosion ring cracking behavior by Hertzian indentation is rationalized from the linear-elastic fracture mechanics framework.