Influence of the barrier layer on the flexural properties of nanoporous alumina film

This study evaluated the influence that the barrier layer has on the flexural properties of anodic aluminium oxide (AAO) film. The flexural properties, including the bending strength and the bending modulus were measured using a three-point bending test. The results of the bending test revealed a significant difference in the bending strength between the two configurations of the AAO film that could be attributed to its asymmetric structure. The barrier layer was placed in a state of compression in one configuration and in a state of tension in the other. The difference in the bending strength disappeared when the barrier layer changed from a dense structure to a porous structure via chemical etching. In addition, the bending strength and the bending modulus gradually decreased as the porosity of the barrier layer increased. The bending strength decreased from 263 to 140 MPa as the porosity of the barrier layer increased to 39.7%. Meanwhile, there was a reduction of 58.5% in the bending modulus. The fractographic analysis showed that the AAO film without a dense barrier layer had a brittle fracture in which cracks were initiated at the specimen surface in a state of tension, propagated across the pores, and this finally led to structural fracture.