Surface modification of Ti–6Al–4V alloy using an oxygen glow-discharge plasma to suppress the elution of toxic elements into physiological environment

We present the application of a plasma-oxidation technique for modifying a the corrosion resistance of a Ti–6Al–4V bio-implant material in a physiological environment. The corrosion resistance of a plasma-oxidized Ti–6Al–4V alloy sample was evaluated by immersing the sample in a lactic acid solution and determining the amounts of eluted elements in the immersion-test solution. The amounts of toxic elements, aluminum and vanadium, eluted from the plasma-oxidized sample were compared with those from the thermally oxidized sample to demonstrate the corrosion resistance of the plasma-oxidized surface to be greatly improved. To elucidate the reasons for this improvement in corrosion resistance, the oxide layers formed on the Ti–6Al–4V alloy were characterized by X-ray photoelectron spectroscopy, which revealed that Al and V oxides were enriched at the outermost surface of the thermally oxidized sample, whereas they were less segregated at the outermost surface of the plasma-oxidized sample. Based on the atomic fractions of elements at the outermost surface of the oxide layers after the immersion test, it was deduced that the elution occurred mainly at the outermost surface of the oxide layer. Therefore, it can be deduced that the characteristic surface composition of the plasma-oxidized sample, with reduced fractions of toxic elements, played an important role in suppressing the elution of toxic elements.