Modification of physicochemical properties and bioactivity of oxide coatings formed on Ti substrates via plasma electrolytic oxidation in crystalline and amorphous calcium phosphate particle suspensions

One way to improve characteristics of oxide coatings prepared by plasma electrolytic oxidation (PEO) method in suspensions lies in variation of solid particle properties. In this study, the effect of particle crystallinity on different physicochemical properties and bioactivity of coatings prepared on Ti and Ti6Al4V substrates was studied, and repeatability of resulting characteristics was assessed depending on selected substrates. PEO was performed in direct current mode at a voltage of 350 V in electrolytes containing 0.1 M KH₂PO₄ and 100 g.L^-1 of stoichiometric crystalline hydroxyapatite or amorphous tricalcium phosphate particles with average size of ∼ 450 nm. Series of measured physical parameters included coating thickness, as well as surface wettability, roughness and topography. Surface and cross-sectional chemical composition was studied by Raman and EDX spectroscopy, while bioactivity was evaluated with simulated body fluid tests. Results showed specific influence of particle crystallinity on surface morphology and repeatable particle incorporation pattern depending on the substrate utilized. It was found that amorphous particles lead to the increased amount of incorporated calcium phosphate phases followed by improved coating bioactivity and possible explanation of this phenomenon was suggested. Variation of particle crystallinity was proposed as a new potential instrument for the adjustment of PEO coating properties.