Tribo-mechanical characterization of rough, porous and bioactive Ti anodic layers

Rough and porous titanium oxide layers, which are important features for improving the osseointegration of Ti implants with bone tissues, are obtained through the technique of anodic oxidation. The thicknesses of such coatings are typically in the order of micrometers, and their mechanical characterization can be assessed by instrumented indentation, provided that the composite nature of the surface is considered. Titania anodic layers were produced on Ti under galvanostatic mode using Ca–P-based electrolytes (a mixture of (CH3COO)2Ca⋅H2O and NaH2PO4⋅2H2O), employing current densities (JJ) of 150 mA/cm2 and 300 mA/cm2. The structure and morphology were characterized by X-ray diffraction (XRD), scanning electron microscopy with electron dispersive X-ray spectroscopy (SEM/EDS), and profilometry, and the chemical features were characterized by X-ray photoelectron spectroscopy (XPS). TiO2 layers presented the crystalline phases rutile and anatase, and incorporation of Ca and P presented as a calcium phosphate compound. The porosity, roughness, and thickness increased with JJ. Analytical methods were employed to obtain the modified layers’ elastic modulus and hardness from instrumented indentation data, deducting the substrate and roughness effects. The elastic moduli were about 40 GPa for both values of JJ, which are similar to the values for human bones (10–40 GPa). The hardness decreased with indentation load, varying from 5 GPa at the near surface to 1 GPa at the layer–substrate interface. Such hardness behavior is a consequence of the surface brittleness under normal loading. Additional scratch tests using an acute tip indicated that the layer integrity under shear forces was 220 mN (J=150 mA/cm2) and 280 mN (J=300 mA/cm2). TiO2 layers produced with both current densities presented good results for in vitro bioactivity tests using simulated body fluid (SBF) solution, which can be attributed to a combined effect of the microstructure, layer porosity, and hydroxyl radicals in plenty at the near surface.

Research highlights
► Ca- and P-containing titania layers were produced on Ti, which were bioactive in SBF.
► The titania layers presented a calcium phosphate phase, and OH in plenty at the surface.
► The elastic modulus of the layers was around 40 GPa.
► The layers presented brittleness under normal and tangential loading.