Plasma electrolytic oxidation of Ti-25Nb-xTa alloys in solution containing Ca and P ions

Abstract

In this study, the plasma electrolytic oxidation (PEO) of Ti-25Nb-xTa alloys in a solution containing Ca and P ions was investigated by using various experimental techniques. The Ti-25Nb-xTa ternary alloys having Ta contents (x) of 0, 3, 7, and 15 wt% were fabricated by using pure Ti. The applied voltage and treatment time of PEO were 280 V and 3 min, respectively. Calcium acetate monohydrate [Ca(CH3COO)2·H2O; 26.69 g/L] and calcium glycerophosphate (C3H7CaO6P; 4.29 g/L) were used as the electrolytes in the PEO. The surface and mechanical properties of the Ti-25Nb-xTa alloys were analyzed by optical microscopy, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, nanoindentation, atomic force microscopy, and wettability measurements. With the increase in the Ta content, the Ti-25Nb-xTa alloy exhibited martensitic and equiaxed structures, while its Vickers hardness and elastic modulus decreased. All PEO-treated samples exhibited porous surfaces with irregularly shaped pores. While the pore size of PEO-treated alloys decreased, the number of pores and thickness of the coating tended to increase with increasing Ta content. The PEO-treated alloys in the solution containing Ca and P ions exhibited surface roughnesses of 235–271 nm. The contact angle of the PEO-treated surface gradually decreased with the increase in Ta content.