The bonding of plasma-sprayed hydroxyapatite coatings to titanium: effect of processing, porosity and residual stress

Hydroxyapatite coatings (HACs) were plasma sprayed on titanium to explore the effect of processing [using either a fixed-holder (A-HACs) or a rotational-holder (B-HACs)], structural characteristics and mechanical state of the coatings on the bonding strength of the system. Statistics was employed for analyzing the data obtained from the experiment. It was found that A-HACs exhibit a rough surface morphology, higher crystallinity and porosity content with non-uniform porosity distribution, as well as higher residual stress. Together, these factors cause a change of fracture mode and bonding strength of the coatings. With insignificant variation of porosity in A-HACs or B-HACs themselves, the fracture mode and the bonding strength of either group of coatings were found dominantly correlated with the change of residual stress, which affects the adhesive bonding between HACs and substrate. The large difference of fracture behavior and bonding strength between B-HACs and A-HACs should not only be attributed to the difference in porosity content. The non-uniformity in porosity distribution in A-HACs is rationalized to play a major role, which significantly lowers the bonding strength through dominantly degrading the cohesive strength of the coatings.