Bovine-derived hydroxyapatite coatings deposited by high-velocity oxygen-fuel and atmospheric plasma spray processes: A comparative study

Bovine-Derived Hydroxyapatite (BHAp) is Ca-deficient natural hydroxyapatite with several ions substitutions that play a crucial role in its biomimetic behavior. Natural quantities of ions such as Mg2+ and CO32- increase the bioactivity and the biological performance of BHAp compared to stoichiometric hydroxyapatite (HAp). In this contribution, BHAp powder was obtained from two years old bovines and thermally sprayed by Atmospheric Plasma Spray (APS) and High-Velocity Oxygen Fuel (HVOF) on 304L stainless steel substrates. BHAp coatings morphology and structure were analyzed by scanning electron microscopy (SEM), Fourier Transformed Infra-Red (FTIR) spectroscopy and Rietveld refinements of the coatings grazing incident X-ray diffraction (GIXRD) patterns. The bioactivity of the coatings was evaluated following the modifications of its structure and surface morphology after immersion in simulated body fluid (SBF) during 3, 5 and 10 days, by SEM and GIXRD. The Ca, P and Mg concentration in SBF was also measured by inductively coupled plasma optical emission spectrometry (ICP-OPS). The obtained BHAp powder was highly crystalline and mainly a B-type HAp. The main differences between BHAp and HAp are CO32- and the Mg2+ contents, which lead to the formation of dolomite in the crystalline content (48.75 wt%) during HVOF spraying. APS coating did not exhibit any secondary phases in its crystalline content. The typical polyhedral grain apatite layer was evident after 3 days of immersion in SBF for both coatings. However, SEM micrographs of HVOF coatings show delamination after 5 days of immersion. Thus, the dolomite phase is detrimental for coating stability and bioactivity.