Thermochemical oxidation of commercially pure titanium; controlled formation of robust white titanium oxide layers for biomedical applications

This study addresses controlled formation of white rutile surface layers on commercially pure (CP) titanium by gaseous thermochemical oxidation. The formed oxide layers were investigated with light optical microscopy (LOM), scanning electron microscopy (SEM), X-ray diffraction (XRD), glow discharge optical emission spectroscopy (GDOES), transmission electron microscopy - energy-dispersive X-ray spectroscopy (TEM-EDXs), spectrophotometry, thermogravimetric analysis (TGA), and Vickers micro-indentation. The oxidation response of CP titanium in different single gas systems, O₂, N₂O, or CO_2, at temperatures ranging from 750 °C to 1000 °C, showed that the formed oxide scales exhibit oxide stratification, irrespective of the applied gas. Additionally, a two-step oxidation process was found to result in controlled growth of robust, dense, adherent white titanium oxide layers. The two-step process entails a first oxidation step in an atmosphere of CO/CO₂ at 750 °C; a second oxidation step is performed in N₂/N₂O at 650 °C. The oxidation in a CO/CO₂ atmosphere results in the incorporation of carbon in the forming oxide layer. TEM-EDXs analysis after oxidation in CO/CO₂ revealed that carbon resides in thin interlayers between slightly stratified 200-300 nm layers of rutile. This unique “composite” oxide layer containing carbon was found to be robust and densely adhering to the substrate. After the second oxidation step in N₂O carbon was “retracted” (oxidized) from the oxide‑carbon composite layer, resulting in an aesthetically pleasing, adherent white oxide layer. Finally, this two-step process, leading to robust white oxide layer formation, is show-cased on a biomedical demonstrator part for dental applications.