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RESEARCH PAPERS

Instrumented Microindentation of Nanoporous Alumina Films

[+] Author and Article Information
Ken Gall, Yiping Liu

Department of Mechanical Engineering, University of Colorado, Boulder, CO 80309

Dmitri Routkevitch

 Nanomaterials Research LLC, Longmont, CO 80501

Dudley S. Finch

Department of Mechanical Engineering, University of Colorado, Boulder, CO 80309 and  National Institute of Standards and Technology, Materials Reliability Division (853), Boulder, CO 80305

J. Eng. Mater. Technol 128(2), 225-233 (Jun 21, 2005) (9 pages) doi:10.1115/1.2172626 History: Received May 04, 2004; Revised June 21, 2005

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.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 2

Scanning electron microscope images from the open-pore surface of sample S1 at magnifications of (a)20,000× and (b)100,000×

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Figure 3

Scanning electron microscope image from the closed-pore surface of sample S1 at a magnification of 100,000×

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Figure 4

Scanning electron microscope images from the open-pore surface of samples (a) S3, (b) S5, and (c) S6 at a magnification of 100,000×

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Figure 5

Midpoint force versus deflection data for representative three-point microbending tests. The points are experimental measurements, and the solid lines represent linear fits to the data. I is the moment of inertia.

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Figure 6

Representative sharp-indentor curves of force versus depth for solid alumina and nanoporous alumina in crystalline and amorphous states

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Figure 7

Representative sharp-indentor curves of force versus depth for nanoporous alumina on surfaces with open and closed pores

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Figure 13

Plots of (a) hardness and (b) elastic modulus as a function of volume fraction of porosity for various alumina materials. The elastic solutions in Fig. 4 are based on the elastic properties of the solids alumina E and are as follows: out-of-plane elastic modulus for a honeycomb (45), Eop=fE; in-plane elastic modulus, Mori-Tanaka method (46), Eipmt=E∕(1+3f(1−f)−1); in-plane elastic modulus for a periodic honeycomb (45), Eiph=1.5(1−f)3E.

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Figure 12

Scanning electron microscope images of the corner of an indent on sample S2 at a magnification of 50,000×

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Figure 11

Scanning electron microscope images of the corner of an indent on sample S3 at magnifications of (a) 100,000× and (b) 200,000×. The sample is tilted at an angle.

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Figure 10

Scanning electron microscope images of the corner of an indent on sample S3 at magnifications of (a) 50,000× and (b) 100,000×

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Figure 9

Curves of force versus depth for selected alumina materials, obtained using a spherical indentor tip. The load-depth calculations are based on Hertzian contact (43).

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Figure 8

Representative sharp-indentor curves of force versus depth for nanoporous alumina with varying pore fractions and diameters

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Figure 1

Schematic and scanning electron microscope images of nanoporous alumina from top and side views (18)

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