The effect of heat treatment on the structure and mechanical properties of anodic aluminum oxide (AAO) membranes

Nanoporous anodic aluminum oxide (AAO) membranes can be fabricated with highly controllable thickness and porosity, making them ideal for filtration applications. Use of these membranes is currently limited largely due to their size and overall fragility. The objective of this research was to improve mechanical properties of AAO membranes through use of high temperature heat treatment to induce phase transformation in the material. A repeatable two-step anodization procedure was developed for consistent sample fabrication and heat treatments were performed at 900°C and 1200°C. The pore morphology and phase composition of the as-anodized and heat-treated membranes were then observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Microhardness and nanoindentation testing were utilized to measure the mechanical behavior of the membranes before and after heat treatment.As-anodized AAO membranes were determined to be amorphous, and membranes heat-treated to 900°C and 1200°C were transformed to crystalline phases while retaining their original porous structure. Heat treatment to 900°C resulted in formation of the gamma-alumina transition phase in the skeleton regions of the membrane and nanocrystalline regions of alpha-alumina throughout the structure, while heat treatment to 1200°C completely transformed the material to the stable alpha-alumina phase. The microhardness testing showed an increase in hardness from 2.5 +/- 0.4 GPa to 4.7 +/- 1.0 GPa following the transformation from amorphous to alpha-alumina. The nanohardness results supported the microhardness values, but were not entirely reliable due to significant sources of fluctuation in the data.