Physical Behavior of Nanoporous Anodic Alumina Using Nanoindentation and Microhardness Tests
© to the authors 2007
Received: 14 May 2007
Accepted: 16 June 2007
Published: 19 July 2007
In this paper, the mechanical response and deformation behavior of anodic aluminum oxide (AAO) were investigated using experimental nanoindentation and Vickers hardness tests. The results showed the contact angle for the nanoporous AAO specimen was 105° and the specimen exhibited hydrophobic behavior. The hardness and the fracture strength of AAO were discussed and a three-dimensional finite element model (FEM) was also conducted to understand the nanoindentation-induced mechanism.
Anodic aluminum oxide (AAO) has attracted much attention due to its excellent physical and chemical properties. This material can be applied in the field of catalysis, chemical/biosensors, templates for self-assembly, filters, nanofluidic transistors and humidity sensors [1–3]. Owing to its low cost and easy fabrication, an anodization technique is used to synthesize nanoporous AAO.
The size of the nanopores can be controlled by the voltages applied during anodization and the modulus and the hardness of nanoporous alumina has been shown to vary with the pore size . The mechanical responses of the complicated network geometries of AAO can be simply measured by the size of couple pores, although it is difficult to predict the responses theoretically.
In this paper, we investigate the mechanical properties of nanoporous AAO using nanoindentation and microhardness tests. The mechanism and properties were determined and discussed by experimental measurement as well as finite element analysis (FEA).
Nanoporous AAO was prepared electrochemically using a two-step anodization technique to achieve an oxide film with a regularly ordered porous structure. The first anodization was carried out until the residual Al film thickness approached the desired level, then the oxides were stripped away, and subsequently a second anodization was performed until the remaining Al samples were completed anodized. A Ti sheet was used as a cathode for the anodization of the Al samples under a constant voltage. The first anodization was performed using a 0.4 M oxalic acid solution at 20 °C and 50 V for 4 h, and then the oxides were removed by immersing the samples in a mixture of 2 wt.% chromic acid and 6 wt.% phosphoric acid at a temperature of 60 °C.
The desired thickness of the AAO films was obtained by a subsequent second anodization. After the second anodization, the AAO could be widened by increasing the anodization time and concentration of the acid solution. For the pore widening process, the solution used was 0.1 M phosphoric acid solution at a temperature of 30 °C for about 1 h.
Results and Discussion
Structure and Surface Properties
Wetting and Optical Behavior
where the real contact area Areal could be defined as the contact area multiplied by the porosity of AAO. The contact area is a function of the contact depth, h.
where S is the stiffness of the test material, which can be determined from the slope of the initial unloading by evaluating the maximum load and the maximum depth, where S = dP/dh. β is a shape constant of the indenter, which is 1.034 for the Berkovich tip.
where A is the projected surface area of the residual indent, P is the force applied to the diamond and d is the average indent length of the diagonal left by the indenter. In this study, the forces were set as 50, 100, 500 and 1,000 g. The time for the initial application of force was 5 s, and then the test force was maintained for another 15 s. The calculated hardness was between 0.3 GPa and 2.0 GPa. One may see that there is almost the same of hardness with the smallest load when compared to the nanoindentation results for the smallest indentation. However, with regard to the greatest load of microhardness test and the greatest depth of nanoindentation, their differences are significant. This was due to the substrate effect rebounding from this micro-scale test, while nanoindentation was only localized. On the other hand, if the aim is to obtain the intrinsic mechanical properties of this particular sample, mechanical loads ought to be small enough to prevent any substrate effect.
FEA of Indentation
In summary, we have demonstrated the wetting, optical and mechanical behaviors of nanoporous AAO. The contact angle of nanoporous AAO is over 90°, it exhibited hydrophobic behavior and the film reflection is lower than 20% when the wavelength between 300 nm and 800 nm. Both micro to nano-scale indentations were carried out to examine their mechanical behaviors. Since the diameter of these particular pores is in general greater than normal, both the hardness and the Young’s modulus are comparatively small. FEA with an elastically deformable solid was used to model the AAO. Lower von Mises stress was found for AAO with larger pores, which resembled to experimental findings with regard to hardness and Young’s modulus.
This work was supported in part by the National Science Council of Taiwan under Grant No. NSC 95-2221-E150-066.
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