Mechanical properties of highly ordered nanoporous anodic alumina membranes
Young's modulus, hardness and fracture toughness of highly ordered nanoporous alumina (PAA) were measured by nanoindentation and Vickers microindentation. A finite element model that accounts for the anisotropy and the pores was developed to extract the Young's modulus and hardness values from the experiments. The measured Young's modulus of 140 GPa was not affected by a heat treatment at 650 degreesC. However, the same heat treatment causes the hardness to increase from 5.2 to 6.3 GPa, while the fracture toughness sharply decreases from 3.4 to 0.4 MPa-m(1/2). When the indentation is made on the top surface of the membrane, nanopore collapse in shear bands rather than crack formation was observed around the indent. This suggests that the pores greatly improve the toughness of the porous alumina in the transverse direction. X-ray diffraction shows no sign of crystallization of the amorphous alumina after annealing at 650 degreesC. However, FTIR spectra show that the heat treatment significantly reduces the OH content of the amorphous structure. Thus it appears that the sharp reduction in the fracture toughness after annealing can be attributed to changes in the amorphous structure due to the loss of water.