Functionally graded oxygen-containing coating on CP-titanium for bio-applications: characterization, biocompatibility and tribocorrosion behavior

Tribocorrosion of titanium alloys improves by thermal oxidation which results in formation of an oxygen diffusion layer (ODL). However, the formation of ODL decreases the biocompatibility of titanium. The main goal of this paper was to remove this drawback by generation of a more bio-active layer of porous anodic oxide on ODL. Effects of normal load and sliding distance on the tribocorrosion behavior of anodic oxide layers produced by an anodizing process on an ODL were investigated. The ODL layer was formed on CP-Ti by thermal oxidation process in an atmospheric furnace at 850 °C followed by the anodizing process performed at voltages of 150 and 175 V in sulfuric acid solution. Cell viability tests revealed higher biocompatibility for the anodized samples compared to the ODL and titanium. Tribocorrosion tests were carried out under normal loads of 0.5, 1 and 1.5 N at sliding distances of 5–300 m in a phosphate saline buffer solution (PBS) using a pin-on-disk tribometer. The results showed that the anodized layers were durable enough to resist tribological stresses at sliding distances of 5 and 25 m. Further increase in the sliding distance, however, resulted in local damages on the oxide layers followed by a complete removal at a sliding distance of 300 m, especially under the high normal loads of 1 and 1.5 N. The ODL showed the lowest tribocorrosion rate followed by the sample anodized at 150 V and 175 V. Highest hardness and elastic modulus were obtained for the ODL using nano-indentation tests.