Formation mechanism of toxic-element-free oxide layer on Ti-6Al-4V alloy in d.c. glow discharge plasma with pure oxygen gas

The purpose of this study is to elucidate the formation mechanism of a toxic-element-free oxide layer on a Ti-6Al-4V alloy by using d.c. glow discharge plasma in pure oxygen atmosphere, which has been found in our previous study. The surface treatment was conducted with monitoring emission spectra irradiated from the plasma to identify the excited species, which revealed that three kinds of excited oxygen species were primarily present there. Our investigation has reported that the oxygen plasma enabled toxic elements, such as Al and V, to be largely removed from the outermost surface of the alloy; in this case, this effect would be attributed to preferential sputtering caused by positively-charged plasma species such as O⁺ and O₂⁺ which were accelerated toward the sample surface by hundreds of volts. To investigate the effect of the plasma condition on characteristics of the resultant surface layer, this study characterized the oxide layers formed on the alloy under three different conditions by using X-ray photoelectron spectroscopy. Whereas a deficit in the toxic elements on the sample surface was observed regardless of the plasma conditions, there was a significant difference in the growth rate of the oxide layers.