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

Barium carbonate (BaCo₃) is a thermodynamically stable heavy metal carbonate with many applications in producing glass, ceramic and ferroelectric materials. The material has demonstrated strong potential applications in catalysis, sensors, solid oxide fuel cells and batteries. Alternatively, Barium titanate (BaTiO₃) present a ferroelectric material with remarkable properties such as high dielectric constant, good ferroelectric properties, large electro-optic and non-linear optic coefficients. It is well known that titanium and its alloys have a unique combination of high strength to weight ratio, excellent biocompatibility and high corrosion resistance, and are widely used in many industries such as aerospace, marine and biomedical. Therefore, combining functional BaCO₃ and BaTiO₃ nanostructures and Ti is expected to achieve new engineered functional materials by integrating their desirable properties. For instance, ferroelectric BaTiO₃ films fabricated on structural materials (titanium, nickel, etc.) using pulsed laser deposition (PLD) can be used for structural health monitoring.

Researchers previously fabricated barium carbonate using various approaches and managed to produce barium titanate by simply heating TiO₂-coated BaCO₃ particles and by solid-state reaction between BaCO₃ and TiO₂. Nonetheless, a more feasible approach of producing the two is still required. To this end, Engineer Hsiao-Chien Wu, Professor Jiechao Jiang and distinguished university Professor Efstathios I. Meletis from the Department of Materials Science and Engineering at University of Texas at Arlington developed a new fabrication technique for BaCO₃ and BaTiO₃ nanostructure coatings on titanium using plasma electrolytic oxidation processing method. Their work is currently published in the research journal, Applied Surface Science.

In their approach, barium carbonate and barium titanate layers were coated on titanium by plasma electrolytic oxidation at 5 mA/cm² in 0.5M Ba (CH₃COO)₂ and 2M sodium hydroxide electrolyte. The research team then carefully looked at the coatings characteristics using a variety of advanced techniques including scanning electron microscopy, X-ray and electron diffraction, and high-resolution transmission electron microscopy.

The authors found that processing for 1h required low voltages and yielded a BaCO₃ coating along with a thin TiO₂ interlayer (~60 nm). The coating was observed to be 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. Further, the team reported that processing for 4h 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.

In summary, Professor Efstathios Meletis and his colleagues exploited the innovation of using plasma electrolytic oxidation technique to produce either barium carbonate or barium titanate layers on titanium. Results obtained showed that the BaTiO₃ was produced by transformation of pre-existing BaCO₃ layer. Further, the BaCO₃ to BaTiO₃ transformation was found to be facilitated by their epitaxial relationship. Overall, based on their findings, the Texas University research team proposed a structural model to elegantly describe the BaTiO₃ growth from the BaCO₃ lattice.