The effect of nano pore size and porosity on deformation behaviors of anodic aluminum oxide membranes

With unique combination of properties such as low density, high strength, thermal stability and corrosion resistance, ceramics are essential for aerospace and other applications involving extreme environments. However, current application of ceramics is limited due to their low fracture toughness. In this study, the effects of nanoporosity on the mechanical deformation behaviors, particularly quasi-plasticity, were studied by conducting nanoindentation tests on anodic aluminum oxide (AAO) membranes. AAO membranes with varying porosity (∼10-30 %), pore diameter (∼38-210 nm) and material phases (amorphous and polycrystalline) were tested using Berkovich and cube-corner tips under load up to ∼400 mN. Measurement of apparent elastic modulus and hardness, combined with post-indentation scanning electron microscopy (SEM) inspections, was performed on the AAO samples. Their mechanical behaviors were studied as a function of varying porosity, pore size and phase combinations. During nanoindentation using a cube-corner tip, shear bands in the form of collapsed or deformed pore arrays were observed around the indentation impressions, resulting in quasi-ductility when the samples have smaller pore size (∼38-61 nm) or larger porosity (∼20-30 %). This new failure mechanism induced by nanoporosity can potentially enhance ceramics' ability to dissipate energy and thus increase the fracture toughness. If such toughening is successful, nanoporous ceramics can potentially be used in structural applications in extreme environments, including aero-engines, gas turbines, protection armors, thermal protection tiles and heat exchangers.