• Mg2+, Na+, and CO3 2- from BHAp feedstock are found in the coatings • Secondary phases in samples can be modulated by the process parameters • Ionic substitutions occur in BHAp and in the secondary phases formed after spraying • This is the first report of bovine-derived hydroxyapatite coatings by Flame Spray • Thickness and porosity are controlled by the processing parameters’ combination This work reports the fabrication of bovine derived-hydroxyapatite (BHAp) coatings by flame spray (FS) technique. To the best of our knowledge, this study is the first to simultaneously evaluate the effects of the fuel-to-oxygen ratio (F/O) and stand-off distance (SOD) on the thermal history of BHAp particles, as well as on the resulting composition, structure, and microstructure of the coatings. The coatings showed thicknesses ranging from 26.8 to 486.7 µm, along with varying porosity levels. These also exhibited compositional changes compared to the feedstock powder, attributed to thermal decomposition processes such as dehydroxylation, decarbonation, and recarbonation. Importantly, biologically relevant ions such as Mg 2+ , Na + , and CO 3 2- were preserved in all coatings. The presence of crystalline phases in the coatings was directly related to the flame temperature, which is determined by the processing parameters. By controlling the fuel-to-oxygen ratio (F/O) and stand-off distance (SOD), the phase composition of the coatings could be effectively modulated. Specifically, lower F/O values and increased SOD favored the formation and transformation of β-Tricalcium Phosphate (β-TCP) and α-Tricalcium phosphate (α-TCP). In contrast, higher F/O ratios promoted the development of α-TCP and Tetracalcium Phosphate (TTCP), accompanied by a progressive reduction in BHAp content under certain conditions. All coatings exhibited a degree of crystallinity (DOC) greater than 50%. These findings demonstrate the feasibility of the flame spray technique to produce BHAp-based coatings while preserving the non-stoichiometric composition of the feedstock and enabling tunable microstructures through controlled processing parameters. This strategy represents a promising alternative for biomedical applications where bioactive, ion-substituted apatite is desirable.
Salazar-Martinez et al. (Sun,) studied this question.