Spatiotemporal Instabilities in Memristive Ceramic Films Grown by Plasma Electrolytic Oxidation

The formation of anodic oxide layers under conditions of dielectric breakdown is at the core of advanced technology for the high-performance coating of light alloys. However, little is known about fundamental mechanisms governing collective microdischarge behavior during high-voltage anodizing. We used in-operando voltammetric diagnostics of the process in combination with isotope analysis and multiphysics simulation to reveal transient processes occurring in the interfacial barrier region of anodic alumina grown under pulsed bipolar polarization. This region was shown to represent a memristive structure with a quasi-capacitive response governed by rearrangements of the active zone under anodic polarization. The rearrangements are governed by the diffusion of hydrogen intercalated into amorphous alumina under the preceding cathodic bias. Kinetics of hydrogen intercalation/deintercalation play a crucial role in the development of instabilities responsible for discharge transition from stochastic filamented patterns to laminated diffuse structures. Subsequent self-organization into scanning waves is influenced by thermally induced diffusion of hydrogen in the lateral direction of the film, where concentration gradients are initially absent. This opens opportunities for the development of energy-efficient and versatile coating processes to address future manufacturing needs.