Chemo-mechanical softening during in situ nanoindentation of anodic porous alumina with anodization processing

Abstract

Simultaneous application of mechanical stresses on a material as it undergoes an electrochemical reaction can result in interesting coupling effects between the chemical and mechanical responses of the material. In this work, anodic porous alumina supported on Al is found to exhibit significant softening during in situ nanoindentation with anodization processing. Compared with ex situ nanoindentation without anodization processing, the in situ hardness measured on the alumina is found to be much lower, when the estimated maximum stress underneath the indenter is exerted on the metal/oxide (m/o) interface at the bottom of the oxide. Numerical calculation reveals that a high electric field exists across the nanometrically thin barrier layer where the electrochemical reactions mainly take place. In microindentation with a flat punch, in situ softening is also observed, but no significant difference in the deformation of the oxide and the Al substrate between in situ and ex situ cases can be observed from cross-sectional transmission electron microscopy examination. The evidence, therefore, indicates that the observed in situ softening is due to a combination of high compression stress and electric field acting near the m/o interface, and it is likely that such conditions enhance Al ionization at the m/o interface, thus causing the m/o interface to move faster into the Al substrate under the in situ condition. © 2013 AIP Publishing LLC.