Investigating the Fracture Resistance of Bioactive Glass Coatings on Metallic Implants

Bioactive glasses have been used experimentally as coatings for medical implants because of their good osseointegration properties and ability to inhibit bacterial proliferation. However, the available literature lacks quantitative studies for characterizing their mechanical properties. This research postulates two fracture mechanics testing methodologies that facilitate measuring the nearly pure mode I (opening) and mode II (shearing) critical strain energy release rate (GIC, GIIC) of the coating/substrate system. Using these methodologies, the effects of coating thickness, glass composition and degradation on the GIC and GIIC of the system were evaluated. The developed mode I testing methodology was applied on a silicate bioactive glass/Ti6Al4V substrate system and it was found that increasing the coating thickness from 90 to 390 μm, decreased the measured GIC of the system significantly, from 6.2 to 2.5 J/m2. This decrease was found to be due to the increase in the residual stresses in the thicker coatings. The mode I testing methodology was then applied on two series of silica-based and borate-based glass coating, with increasing amounts of TiO2 incorporated, and it was observed that an increase in the content of TiO2 in the glasses resulted in an increase in the GIC for both the bulk glass and for the coating/substrate system. The borate-based series was found to have a closer CTE to the substrate compared to the silica counterpart, suggesting that use of such glasses as coatings can minimize the chances of delamination and cracking. Incorporating SrCO3 in a series of borate bioactive glass coating also proved to significantly increase the GIC and GIIC of the system. In order to study the effect of degradation, the borate bioactive glass coatings on Ti6Al4V substrates were immersed in deionized water for different time periods, dried and tested. It was found that after 17% weight loss of the glass, the GIC and GIIC of the coating/substrate system for all compositions decreased by at least 80%.