Oxygen depth profiling by resonant backscattering and glow discharge optical emission spectroscopy of Ti-6Al-4V alloy oxidized by ion implantation and plasma based treatment

Abstract

Oxygen depth profiling by means of 16O(α,α)16O backscattering and glow discharge optical emission spectroscopy (GDOES) was investigated in two different sets of Ti-6Al-4V samples. The first set was made of Ti-6Al-4V samples implanted at room temperature and 550°C with 50 and 150keV O+ ions at fluences ranging from 1.5×1017 to 6.0×1017ions/cm2. The second set consisted of Ti-6Al-4V samples treated at 550°C for 7h and 24h under low pressure (8Pa) oxygen, eventually with RF plasma activation. These results are part of a wider investigation on Ti-6Al-4V motivated by recent publications which have shown that an oxide layer can enhance hydrogen absorption and can then promote Ti-6Al-4V alloys as efficient hydrogen storage materials.The results obtained by the two characterization techniques were compared and discussed, enabling to adjust the dependence to the oxygen concentration of the sputtering rates to be used in the time-to-depth transformation required in GDOES analysis. Considering the low thickness of oxidized alloy, usual procedures employed in GDOES depth calculation were indeed not adapted. Once calibrated thanks to the resonant RBS, GDOES can then be easily employed as fast characterization of oxidized and/or hydrogenated surface of Ti-6Al-4V.The obtained results show that the oxygen content into the surface oxidized layer slightly increases in samples implanted at higher fluence and higher temperature. However the overall oxidized layer thickness (<200nm) remains within the projected ion depth range and is not significantly increased by thermal diffusion at 550°C. Taken into account the initial oxide layer, the incorporated oxygen quantity mainly corresponds to the implanted fluence but it can be slightly higher with 550°C implantation, indicating a slight additional oxidation by residual oxygen or surface contamination. The oxygen penetrations and contents in samples oxidized by thermally activated diffusion treatments were more pronounced compared to implanted samples, especially with plasma assistance. A highly oxidized layer (>50 at.%) of about 100nm was obtained after 7h of treatment. X-ray diffraction revealed the presence of rutile TiO2 phase only after a long duration (24h) thermally assisted treatment whereas oxygen incorporation mainly results in a c-axis expanded hexagonal titanium phase in all the other samples.