Hydroxyapatite (HAp) is known to be bioactive, i.e. able to bond to bone. This makes HAp very suitable to be applied as coatings on bone-metallic implants. In this work high velocity oxy-fuel spraying (HVOF) was used successfully for obtaining hydroxyapatite coatings on Ti-6Al-4V substrates. With optimized HVOF process parameters, coatings with similar bond strength to plasma sprayed HAp coatings, good microstructure and higher crystallinity degree than atmospheric plasma sprayed ones were obtained. As-deposited HAp coatings contains amorphous calcium phosphate (ACP) that can be crystallized by a heat-treatment of 60 minutes at 700 °C, resulting in a more stable coating when they are immersed in simulated body fluid (SBF).Coating structural characteristics of as-sprayed and post heat treated coatings were analysed with X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy analyser (EDS), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). Furthermore, in vitro tests were done in order to evaluate the coating response. Surface changes were observed for as-sprayed coating but not after a post heat treatment. Moreover, the strength of the coatings were evaluated after in vitro leaching. The high degree crystallinity of the post heat treated coating improves the adhesion between the coating and the substrate after an in vitro test in a free-protein simulated body fluids (SBF). Consequently a relation between the amorphicity, the in vitro response and mechanical degradation of the coating was found.

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