Low-voltage micro-arc oxidation of plastically deformed titanium in silicate-based electrolytes: microstructure, tribological properties and cytocompatibility

This paper describes the effect of using three variants of low voltages on the microstructure and functional properties of coatings produced with micro-arc oxidation (MAO) from an electrolyte based on sodium silicate on hydrostatically extruded titanium of commercial purity (cp-Ti). Application of low voltages during the MAO process together with detailed microstructure studies with scanning and transmission electron microscopes enabled a better understanding of their growth mechanisms. Characteristic layered structure of the produced coatings is formed in several steps: (1) a thin, dense amorphous zone of several tens of nm in thickness close to the substrate as well as an ultrafine-grained (UFG) zone containing fine porosity above it (conventional anodization), (2) exceeding the sparking voltage allowed the formation of coarse-grained microstructure above the UFG layer, (3) development of amorphous silica material as the voltage increases and the MAO process progresses. The MAO coatings produced at the lowest positive and negative electrical potentials were characterised by the lowest wear rate in SBF solution, i.e. 7.24 ∙ 10⁻⁶mm³/Nm due to a dense microstructure contributing to almost three times lower wear as compared with other, more porous variants of the tested coatings. The extracts harvested from all MAO samples (100 V–20 V, 125 V–25 V, and 150 V–30 V) present visibly higher cell (human osteoblasts) viability compared to the Ti-HE i.e. at ∼70 % vs ∼40 %, respectively.