Instrumented Microindentation of Nanoporous Alumina Films

We examine the mechanical behavior of anodic alumina thin films with organized nanometer-scale porosity. The cylindrical pores in the alumina film are arranged perpendicular to the film thickness in a near-perfect triangular lattice. The films used in this work had pore diameters ranging from 35 to $75nm$⁠, and volume fractions ranging from 10% to 45%. Films with both amorphous and crystalline structures were considered. Mechanical properties of the thin films were studied using an instrumented indentor to measure the force-depth response of the films during indentation or the force-deflection response of micromachined beams in bending. The films showed increasing hardness/modulus with a decrease in pore volume fraction or transformation from amorphous to a polycrystalline alpha-alumina phase. The asymmetric films show higher hardness and modulus on their barrier side (with closed pores) relative to their open pore side. The force-depth response, measured with a spherical ball indentor, demonstrates fairly good agreement with an elastic Hertzian contact solution. The force-depth response, measured with a sharp Vickers indentor, shows an elastoplastic response. Microcracking at the corners of sharp indentations was not observed in amorphous nanoporous films, and rarely in harder, crystalline nanoporous films. High-resolution scanning electron microscopy revealed a collapse of the nanoporous structure beneath the indentor tip during sharp indentation. The results are discussed in light of continuum-based models for the elastic properties of porous solids. In general, the models are not capable of predicting the change in modulus of the films, given pore volume fraction and the properties of bulk crystalline alumina.