Morphology, composition and electrochemical properties of bioactive-TiO2/HA on CP-Ti and Ti6Al4V substrates fabricated by alkali treatment of hybrid plasma electrolytic oxidation process (estimation of porosity from EIS results)

Ti6-Al-4V and pure titanium substrates were subjected to two-step plasma electrolytic oxidation (hybrid PEO coating) and alkali treatment. Electrochemical properties of the hybrid PEO coatings were studied in SBF solution and it was revealed that electrochemical behavior of the samples were correlated to discharge behavior of second step and its effect on compactness of first layer. If discharges are intensified corrosion performance of the existing layer deteriorates while lowering of discharge intensity of second step will result in modification of existing layer and hence improves corrosion performance. Electrochemical and microstructural tests were carried out after alkali treatment and it was found that after alkali-treatment the surface became nano-flaky, phase composition of oxide layer was not changed, and the chemical composition was slightly changed due to dissolution in NaOH solution. After 2 weeks of immersion, surface of samples were fully covered by HA. XPS and FTIR studies confirmed that hydroxyl groups were formed on the surface of oxide film which found to be responsible for enhanced bioactivity of the samples. Superior bioactivity of PEO oxide film on CP-Ti samples were correlated to presence of anatase and its lower point of zero charge, compared to rutile phase. It was also revealed that the surficial porosities favored formation of HA nuclei and hence improve bioactivity. A novel graphical method is developed for estimation of porosity of PEO oxide films, which helps to predict bioactivity behavior of oxide films. It was found that the area under graph of Nyquist admittance plot is inversely proportional to the porosity of HPEOs. It was revealed that the first peak at high-frequency range of electrolyte resistance corrected Bode-Phase plots can be considered as the upper limit of frequency for the proposed graphical method. The results of estimations of graphical method show good compatibility with image analysis method.