In-vitro Investigation of Air Plasma-Sprayed Hydroxyapatite Coatings by Diffraction Techniques

The influence exposure to simulated body fluid (SBF) has on plasma-sprayed hydroxyapatite (HAp) coatings of medical-grade Ti6A14V rods was investigated by quantifying the depth dependence of the phase composition and residual stress through the coating thickness using diffraction techniques. Chemical phase identification showed HAp existing together with its thermal decomposition products, tetra-calcium phosphate (TTCP), tri-calcium phosphate (TCP) and calcium oxide. With depth, the HAp content decreases with a corresponding increase in TTCP. The near surface stress condition comprised similar to 50 +/- 10 MPa hoop stress with the radial stress being close to zero. With depth the hoop stress decreases linearly to similar to-50 +/- 25 MPa at the substrate interface, whilst the radial stress increases with depth. Upon exposure to SBF, the coating composition reveals an increase in HAp from similar to 80.0 +/- 0.5 to similar to 86.0 +/- 0.5 wt%, accompanied by a decrease of TTCP from similar to 10 +/- 2 to similar to 6 +/- 2 wt% wt%. A change in stress state occurred within the first day of incubation; where after, with further exposure time the stress state converted back to values similar to that of the as sprayed condition.