Effect of Heat Treatments on HVOF Hydroxyapatite Coatings

Highly crystalline hydroxyapatite (HAp) powder was thermally sprayed onto Ti-6Al-4V substrates using the High-Velocity Oxy-Fuel (HVOF) process. Coatings were heat treated for 60 min at 700 °C to study the influence of the crystallization on chemical and mechanical properties. Characterization of the HAp coatings was carried out by Fourier Transform Infrared Spectroscopy (FTIR) and x-ray diffraction (XRD) using Rietveld analysis. The results showed that the coatings were highly crystalline (82%) and no other phases of calcium phosphate were present. Coatings were 100% crystalline after the heat treatment. Bioactivity of the coatings was investigated by immersion in Kokubo’s simulated body fluid. The dissolution/precipitation behavior was studied and the degradation of HAp coatings caused by the immersion test was studied by measuring the adhesion strength of the coatings. After immersion in SBF bond strength decreased for the as-sprayed coatings, without any thermal treatment, but it was constant for the heat-treated coatings. This phenomenon was related to the dissolution of the amorphous phase in the interface substrate-coating in the as-sprayed coatings.

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