Bond degradation at the plasma-sprayed HA coating/Ti-6AI-4V alloy interface: an in vitro study

The successful use of the plasma-sprayed HA-coated Ti-6AI-4V system requires strong adhesion between the ceramic coating and the underlying metal substrate. The aim of this study was to evaluate the bond strength at the HA coating (HAC)/Ti-6AI-4V interface, for specimens that had and had not been subjected to immersion in a pH-buffered, serum-added simulated body fluid (SBF). Moreover, coating characteristics affecting the mechanical stability after having been immersed in SBF were clarified. The results showed that bonding degradation of approximately 25-33% of the original strength was measured after immersion in SBF, and that this predominantly depended on the characteristics of the HAC and the period of immersion. Since the surface morphologies of HACs have dissolved in the SBF, it is suggested that the interlamellar structure of the HAC was weakened and, therefore, the bond strength degraded. As both the crystallinity and impurity phases of the HAC increased with immersion time, it can be concluded that the dissolution of the HAC resulting from the initial microstructure has overtaken that of the coating crystallinity and phase purity. A denser microstructure is required to ensure a satisfactory HAC/Ti-6AI-4V interface.