Anisotropic electrical conduction from heterogeneous oxidation states in plasma sprayed TiO2 coatings

Microstructural and electrical characterizations of air plasma sprayed TiO₂ coatings were carried out to investigate the details of de-oxidation during the spray process and the changes following air annealing. The coatings were found to behave as an n-type semiconductor indicating the presence of oxygen vacancies. Direct-current resistivity measurements in-plane ( r_ip) and through-thickness ( r_tt) of the coatings as a function of annealing time and temperature showed remarkably large anisotropies (= r_tt/r_ip) of up to 10⁵. Impedance spectroscopy of the specimens coupled with microstructural analysis revealed that the origin of this anisotropy lies in the heterogeneous de-oxidation and re-oxidation behavior of the coatings. Due to rapid quenching, the high temperature de-oxidation state is preserved in the splat boundaries making them more conductive than the bulk of the splat in the as-sprayed coating. Upon annealing in air, the splat boundaries get selectively oxidized due to faster surface diffusion of oxygen and become more insulating. This behavior, together with the layered morphology of plasma sprayed coatings results in anisotropy.