The role of internal stress on the growth and breakdown of barrier and porous anodic oxide films

Abstract

Anodic oxidation, or anodizing, can be defined as the electrochemically-controlled growth of an oxide film on a metal. Owing to its simplicity, anodizing is used extensively in a wide range of applications. These include providing corrosion protection and an aesthetic finish to metallic surfaces, manufacturing nanoporous templates, and producing a dielectric medium in electrolytic capacitors. In all these cases, process instabilities which terminate the growth of a dense anodic oxide film, like breakdown and pore initiation, are of key importance. Internal stresses have often been suggested to be a key factor controlling these instabilities in anodic oxide films. However, no direct quantitative correlation between internal stress and either pore development or breakdown of anodic oxide films has ever been established. In this thesis, we measured the internal stress in anodic oxides in situ during their growth. By systematically varying the electrochemical growth conditions, and thereby the magnitude of the internal stress, we were able to unravel its influence on the development of porosity in anodic alumina films, and on the breakdown of anodic zirconia films. In the case of anodic alumina, energy-based stability analyses revealed that the internal stress is unlikely to be the controlling factor for pore initiation and pore spacing selection. Instead, these processes were found to be rather governed by an electrostatic energy induced surface instability. On the other hand, experimental evidence was provided that, contrary to pore initiation, pore growth in anodic alumina can be considered to be a stress-assisted viscous flow process. In this respect, our internal stress data allowed to quantify the viscosity of anodic alumina at room temperature in the presence of large ionic currents. Finally, internal stress was identified as a key factor as well for initiating breakdown in anodic zirconia films. The microstructural origin of the breakdown in that case was identified as a phase transformation allowing the zirconia to densify, the compressive internal stress in the growing anodic oxide film being the driving force for such a transformation.(FSA 3) -- UCL, 201