Microstructure of BaCO3 and BaTiO3 coatings produced on titanium by plasma electrolytic oxidation

Abstract

BaCO3 and BaTiO3 layers were coated on titanium by plasma electrolytic oxidation at 5 mA/cm2 in 0.5 M Ba(CH3COO)2 and 2 M NaOH electrolyte. Coatings were investigated by scanning electron microscopy, X-ray and electron diffraction, and high-resolution transmission electron microscopy (HRTEM). Processing for 1 h required low voltages producing a BaCO3 coating along with a thin TiO2 interlayer (~60 nm). The coating is composed of orthorhombic BaCO3 nanorods that were vertically oriented at the bottom layer close to Ti substrate and horizontally oriented in the top layer close to the surface. Processing for 4 h required higher voltages resulting in generation of microarc discharges and discharge channel formation. These conditions produced a tetragonal BaTiO3 coating along with a broader TiO2 interlayer at the Ti interface composed of rutile and brookite nanostructures. It is suggested that the discharge channels promote the oxidation of the substrate along with the migration of titanium ions to the coating facilitating the BaCO3 to BaTiO3 transformation. The HRTEM evidence suggests that the latter transformation occurs via an epitaxial relationship existing between the BaTiO3 and BaCO3 crystal structures. Based on the present evidence, a structural model is proposed to describe the BaTiO3 growth from the BaCO3 lattice.