Hydroxyapatite (HAp) coatings on prosthetic devices have been used for several years. The combination of excellent mechanical properties of metals with the osteoconductive properties of HAp make this kind of implants widely used in orthopaedic surgery. The most used coating method is plasma spraying. However, the main drawback of this technique is the low stability of the coating in a load-bearing situation after long-term implantation due to generation of an amorphous phase, other bioactive calcium phosphate phases and CaO that are more soluble in body fluids  than HAp. High Velocity Oxy-fuel Spraying (HVOF) is an alternative technique for obtaining HAp coatings onto metallic substrates due to the lower temperatures that the sprayed particles achieve. Nevertheless, the stability of HAp thermal sprayed coatings seems to be potentially weak due to the failure of the HAp/metallic interface rather than the Hap/bone interface one due to the presence of an amorphous phase in the HAp/metallic interface.
 
The aim of the present study was to obtained a high degree crystallinity of the HAp in order to improve the adhesion between the coating and the substrate after an in vitro test in simulated body fluids (SBF). Therefore, a heat treatment was done to crystallise the amorphous phase content (>18%) of the as-sprayed coatings.
 
In conclusion, the results shows that a post heat treatment of the coating allows a complete crystallisation of the amorphous calcium phosphate present in the as spayed HAp coatings and after the in vitro test the adhesion strength of the heat treated coatings was higher than for the as-sprayed ones possibly due to the low solubility of the crystalline HAp in SBF.