Crystallization of hydroxyapatite during hydrothermal treatment on amorphous calcium phosphate layer coated by PEO technique

Surface modification of titanium implants is recently considered by several researchers. In this study, PEO was performed over commercially Ti-6Al-4V alloy pellets in an aqueous electrolyte containing calcium acetate (C.A.) and calcium glycerphosphate (Ca–GP) with a Ca/P molar ratio of 6.8, and applying current density of 0.212 A/cm2, frequency of 100 Hz and duty ratio of 60% for 4 min. In the next step, hydrothermal treatments were carried out for various durations and at different temperatures inside an autoclave chamber containing a NaOH solution with pH of 11.5. XRD and SEM results confirmed formation of needle-shaped HAp after all hydrothermal conditions. Maximum intensity of HAp peaks was observed for the hydrothermally treated sample at 190 °C for 15 h. Moreover, crystallinity degree of HAp was increased by increasing hydrothermal duration and temperature. According to the SEM line scan analysis of cross section of the PEO coated sample after hydrothermal treatment at 190 °C for 15 h, it seems that Ca+2 and [PO4]−3 ions from internal layer of amorphous coating diffuse to the coating surface and dissolve into the solution during hydrothermal treatment, and finally precipitate on the outer layer of coating as needle-shaped HAp.