Electrochemical deposition mechanism of calcium phosphate coating in dilute Ca–P electrolyte system

In this work, the electrochemical deposition behavior of calcium phosphate coating from an aqueous electrolyte containing very dilute calcium and phosphorus species (Ca–P) was studied. The effects of three process parameters, i.e. temperature, current density and duration, were systematically investigated and the underlying mechanism was thoroughly analyzed. It was observed that the coating is mainly composed of hydroxyapatite (HA) in a wide range of temperature and current densities. The temperature had a significant effect on the deposition velocity. An apparent activation energy of 174.9 kJ mol−1 was subsequently derived, indicating the mass-transfer control mechanism for the coating formation. The current density was identified to be an important parameter for structure controllability. The results of DR-FTIR/Raman spectroscopic studies of the initial deposition phase strongly suggested that the HA coating was instantaneously and directly precipitated on the substrate; neither induction period nor precursor was detected in this dilute Ca–P electrolyte system. Finally, a phase diagram of the Ca–P electrolyte system was constructed, which offered a thermodynamic reason for the direct single-phase HA precipitation observed only in this system, but not in conventional concentrated systems.