Microstructural characterisation and stress determination in as-plasma sprayed and incubated bioconductive hydroxyapatite coatings

The aim of this research was to relate the microstructure and coating morphology to the residual stress state of plasma-sprayed hydroxyapatite (HAp) coatings. After incubation of so-called “dog bone” samples in simulated body fluid (r-SBF after Kokubo) for up to 56 days, the coating morphology was studied using scanning electron microscopy. In addition, the stress state was determined after 0, 7, 28 and 58 days incubation time using specimen curvature measurements. It was found that the incubation process changed drastically the stress state in the coatings. While a compressive stress of −48.5 MPa was measured on grit-blasted uncoated Ti6Al4V samples, the stress level increased for as-sprayed HAp coatings to −121 MPa. Incubation in r-SBF for 28 days produced a dramatic stress relaxation to yield a virtually stress-free (or slightly tensile) state while in samples incubated for 56 days a compressive stress level of −50 MPa was reestablished, owing presumably to the effect of precipitation of secondary hydroxyapatite.