Surface modification of severe plastically deformed ultrafine grained pure titanium by plasma electrolytic oxidation

Severe plastic deformation is the best method for processing ultrafine grained (UFG) high strength commercially pure titanium (CP Ti) without any toxic and harmful elements for biomedical implants. Besides, because of the vital importance of the surface bioactivity of a medical implant, this paper studies the effect of Plasma electrolytic oxidation (PEO) process of UFG CP Ti processed by equal channel angular pressing (ECAP). The aqueous electrolyte chosen for PEO process was prepared by mixing 0.15 M calcium acetate hydrate and 0.075 M sodium hypophosphite hydrate at a ratio of 1:1 wt%. The results showed that in PEO-coated coarse-grained (CG) CP Ti, the dominant components are oxygen and titanium, while the two principal elements in the coating of UFG CP Ti are oxygen and calcium. It was revealed from EDS and X-ray diffractometry analysis that the more HA and higher content of Ca and P is formed on the UFG Ti coated sample in comparison with those on coarse-grained Ti coated. The overall Ca/P ratio in the layer was determined as 1.65 and 1.70 for the cases of CG CP Ti and UFG CP Ti, respectively. Also, a few numbers of microcracks were obtained on the PEO-coated UFG CP Ti sample compared to those in PEO-coated CG CP Ti. The microhardness of PEO-coated UFG CP Ti was 3275 MPa which was noticeably higher than the microhardness of UFG CP Ti. The electrochemical impedance spectroscopy (EIS) tests were carried out at room temperature using Ringer's solution. EIS test results indicated that the corrosion resistance of PEO-coated UFG CP Ti was greater than non-coated CP Ti and UFG CP Ti. Furthermore, the PEO-coated UFG CP Ti showed more protection against corrosion compared to PEO-coated CG CP Ti. Finally, it is very promising that UFG Ti with PEO surface modification would be a suitable candidate for replacement of Ti-6Al-4V alloy implants containing toxic elements of AI and V. (C) 2017 Elsevier B.V. All rights reserved.