Thermal oxidation is a promising technique to improve the tribological properties of Ti6Al4V. Herein, a single-step process consisting of oxidation in air, a two-step process with an additional solid-state oxide layer reduction step under vacuum, and a three-step process with an appended final oxidation step in air are applied to Ti6Al4V. The oxide layer adhesion after the three-step process is improved compared with the single-step process. This improved adhesion is not due to a different nature of the oxide layers because oxide layers obtained during the single-step and three-step process show a similar morphology and composition. Instead, it is ascribed to the presence of an optimized oxygen diffusion zone with two distinct regions: a gradual decrease in oxygen concentration from the maximum possible oxygen concentration at the oxide-substrate interface until a depth of 20 μm followed by a near-linear decrease until a depth of about 85 μm. Both regions are also visible in the correlated microhardness-depth profile.

The Ti-6Al-4V alloy is extensively used in aerospace, automotive and biomaterial applications. In the aerospace industry, the service temperature of Ti-6Al-4V is currently limited to 350 °C due to its insufficient oxidation resistance. Oxidation at higher temperatures causes the formation of a fast-growing oxide scale and an oxygen-enriched subsurface layer, which is known as the “alpha-case.” Additionally, the effect of water vapor on the oxidation behavior is critical. In the present study, the oxidation behavior of Ti-6Al-4V in dry air and air containing 10 vol.% H2O at 500, 600 and 700 °C for up to 500 h has been investigated. The main focus of this study is the examination of the different oxide scale morphologies along with the oxygen enrichment in the subsurface zone. It has been observed that spallation of the oxide scale is more severe in a water vapor-containing environment. In dry air, the oxide morphology shows the typical layered TiO2/Al2O3 structure after exposure at 700 °C for 300 h, while Al2O3 precipitates are present in the outermost part of the TiO2 scale when oxidized in wet air. This indicates that the solubility and diffusivity of Al3+ ions in TiO2 are influenced by water vapor. In addition, the extent of oxygen enrichment in the subsurface zone (alpha-case) as a function of temperature and time is determined by nanoindentation profiles. It was shown that in contrast to the scale formation, the alpha-case thickness is not affected by the presence of water vapor in the atmosphere.

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