In situ impedance spectroscopy of the plasma electrolytic oxidation process for deposition of Ca- and P-containing coatings on Ti

Pulsed bipolar plasma electrolytic oxidation (PEO) processes carried out under potentiostatic control in electrolytes containing calcium acetate and sodium phosphate with Ca:P ratio of 2 were investigated by in situ impedance spectroscopy in order to achieve insights in the coating formation mechanisms and identify suitable means for real-time control of essential coating characteristics and properties. The coating thickness, morphology, chemical and phase compositions have been studied by non-destructive eddy current techniques, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction analysis. The results demonstrate successful deposition of about 25 μm thick porous surface layers composed of amorphous calcium phosphates along with crystalline hydroxyapatite, tricalcium phosphate perovskite and titania phases. It was established that the growth of the PEO is controlled by anodic process of titanium oxidation, whereas precipitation of calcium phosphate containing compounds occurred via chemical mechanisms. Plasma discharge was responsible for partial crystallisation and decomposition of the deposited compounds. Characteristic responses related to anodic oxidation of Ti and processes associated with plasma discharge were identified in the impedance spectra of the PEO process. Kinetic parameters of charge transfer under conditions of high-voltage electrolysis have been evaluated and discussed.