A Novel Polishing Process for Ultra-Smooth Aluminum Surfaces via Anodizing in Sodium Metaborate

The aluminum specimens were anodized in a 0.3 M etidronic acid (293 K) at 20 Am⁻² for 30 min to form a porous alumina film on the surface. Because galvanostatic anodizing causes a relatively large voltage of approximately 190 V, a porous alumina film with a large cell size can be obtained. The anodized specimens were then immersed in a 0.20 M CrO₃/0.51 M H₃PO₄ solution to completely dissolve the porous alumina film. Figure 2 shows an SEM image of the exposed aluminum surface and the corresponding depth profile measured by AFM. Many disordered dimples with various polygonal shapes were distributed on the aluminum surface via galvanostatic anodizing and subsequent oxide dissolution, and this morphology corresponds to the shape of the barrier layer at the bottom of the porous alumina film formed by anodizing in etidronic acid. Analysis of the SEM image showed that the average cell size (dimple diameter) was approximately 460 nm. It was observed from the AFM image that elliptical hemispherical dimple structures were formed on the aluminum surface, and the arithmetic mean roughness and the maximum height difference of the dimple structures were measured to be 31.5 nm and 166.5 nm, respectively. This aluminum surface with submicron-diameter dimples that were tens of nanometers in depth was used as the reference starting material for the subsequent polishing processes.

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The dimpled aluminum specimen was immersed in 78 vol% CH₃COOH/22 vol% 70%-HClO₄ solution at 280 K and then electropolished at a constant voltage of 28 V for 5 min Fig. 3a shows the current density–time curve during the electropolishing of the dimpled aluminum specimen. The current density rapidly reached 2000 Am⁻² at the beginning of electropolishing because of the rapid dissolution of the aluminum surface. The current density then decreased to a constant value of approximately 150 Am⁻² owing to steady-state electropolishing. Figure 3b shows the SEM image and corresponding AFM depth profile of the aluminum specimen after electropolishing for 5 min. Although the submicron-scale large-dimple structures disappeared from the aluminum surface after electropolishing, small-dimple structures with an average diameter of approximately 120 nm were newly formed on the surface. Such small-dimple structures are typically formed on the aluminum surface after electropolishing in CH₃COOH/HClO₄ or C₂H₅OH/HClO₄ solution. Based on the AFM measurements, the maximum height difference and the arithmetic mean roughness of the electropolished aluminum surface were calculated to be 7.1 nm and 1.3 nm, respectively. Although a smooth aluminum surface was successfully obtained using this electropolishing process, periodic nano-scale unevenness remained on the surface.

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The dimpled aluminum specimen was potentiostatically anodized in a neutral borate solution at 298 K, and the obtained current density–time curve is shown in Fig. 4a (green line in the figure). During this anodizing process, the voltage was increased linearly up to 200 V for the first 10 min. It was then maintained at this voltage for 5 min (black dashed line). The current density increased rapidly to approximately 1 Am⁻² as the voltage initially increased and then decreased gradually with a periodic increase and decrease. This stage corresponds to the increase in barrier layer thickness with increasing applied voltage. The current density then decreased rapidly as the applied voltage reached the steady-state voltage of 200 V. Figure 4b shows an SEM image of the fracture cross section of the aluminum specimen obtained after anodizing for 15 min A thin barrier oxide film approximately 200 nm thick formed on the aluminum surface, and the surface morphology of the barrier oxide reflects the structure of the submicron-scale dimples formed on the aluminum surface before anodizing in borate. The anodized specimen was immersed in a CrO₃/H₃PO₄ solution to dissolve the barrier oxide film, and the smoothness of the exposed aluminum surface was evaluated by AFM.

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Figure 5a shows an SEM image and the corresponding AFM depth profile of the exposed aluminum surface obtained via anodizing in borate and subsequent oxide dissolution. Compared to the SEM image of the starting dimpled surface shown in Fig. 2, the ridges of the dimpled aluminum structure were more difficult to distinguish after anodizing and oxide dissolution, and the height difference obtained from the AFM image decreased slightly because of the preferential oxide growth at the ridges and consequent smoothing. However, unevenness measuring approximately 100 nm remained on the aluminum surface owing to the incomplete smoothing of the interface between the oxide and the aluminum substrate by the thin barrier oxide formation. On the other hand, because excess anodizing in a neutral borate solution causes the oxide breakdown phenomenon, ^29–32 it is difficult to form thicker barrier oxide films for the smoothing of the oxide/aluminum interface by a one-time anodizing process. Therefore, we repeated the processes of anodizing in borate and oxide dissolution to obtain a smoother surface (Figs. 5b and 5c). The unevenness of the aluminum surface was gradually lost by repeated anodizing and oxide dissolution, and a relatively smooth aluminum surface was obtained after repeating the processes three times. Notably, the tens of nanometers-diameter small-dimple structures shown in Fig. 3 were not observed on the surface after repeating these processes, although relatively large-scale unevenness remained.

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Figure 6 summarizes the changes in the maximum height difference and arithmetic mean roughness with the number of anodizing/oxide dissolution processes. The maximum height difference of the dimpled aluminum surface (166.5 nm) gradually decreased as the number of successive processes increased, reaching 55.4 nm by the first process, 20.5 nm by the second process, and 11.4 nm by the third process. Similarly, the arithmetic mean roughness (31.5 nm) gradually decreased to 11.2 nm by the first process, to 5.2 nm by the second process, and to 2.3 nm by the third process. Thus, a relatively smooth aluminum surface was successfully obtained by repeating these processes three times. However, the values obtained by the third process were still larger than those of the electropolished aluminum surface (the blue dashed line in the figure). In addition, considering that the method consists of complicated multi-step processes, this technique may be unsuitable for the advanced smoothing of submicron-scale dimpled aluminum surfaces.

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Anodizing in NaBO₂ at relatively higher voltages causes the formation of a unique porous alumina film with an extremely smooth interface between the barrier layer and the aluminum substrate; thus, we considered that this anodizing process can be applied to the ultra-smoothing of the aluminum surface. Because the smoothness of the interface between the aluminum substrate and anodic oxide increases with the anodizing voltage, ³⁴ the aluminum specimens were anodized at the higher voltage of 200 V. Figure 7 shows the current density–time curve while anodizing aluminum in a 0.3 M NaBO₂ solution at 298 K. Here, the method of applying the voltage was the same as that for the dimpled aluminum specimen potentiostatically anodized in a neutral borate solution at 298 K (Fig. 4a). The current density increased rapidly to approximately 8 Am⁻² in the initial stage of increasing voltage and then decreased gradually with a periodic increase and decrease. Although the shape of this current change was similar to that obtained by anodizing in borate (Fig. 4a), the current density was 6–8 times larger. In addition, the current density was maintained at a higher value of 4–6 Am⁻² after the steady-state voltage of 200 V was reached. Therefore, it is expected that continuous growth of the anodic oxide film occurs at this stage, which is different from anodizing in borate.

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Figures 8a and 8b show SEM images of the fracture cross section of the dimpled specimen anodized for 15 min and 40 min, respectively. A porous alumina film consisting of a smooth inner barrier layer 260 nm thick and an outer porous layer 1.4 μm thick with extremely narrow pores was formed on the aluminum substrate. The dimpled structure originating from the shape of starting materials can be observed on the top surface of the porous layer. Although the curved pore growth in the initial stage reflected the shape of this dimple structure, the pores soon became vertically arranged. The interpore distance after the applied voltage reached 200 V (approximately 35 nm) was significantly smaller than that obtained by typical anodizing processes (200 V × 2.5 nmV⁻¹ = 500 nm). Notably, the inner barrier layer possessed an extremely smooth morphology without the typical hemispherical structure. The thickness of the outer porous layer increased to approximately 3.4 μm after further anodizing for 40 min (Fig. 8b), and the morphology of the inner barrier layer was smooth. In other words, the outer porous layer became thicker after anodizing in NaBO₂, whereas the thickness and smoothness of the inner barrier layer was maintained without oxide breakdown. Therefore, further smoothing of the aluminum surface can be expected by extending the anodizing process.

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The specimens anodized in NaBO₂ for 15, 20, and 40 min were immersed in a CrO₃/H₃PO₄ solution to dissolve the porous alumina film, and the SEM images and corresponding AFM depth profiles of the exposed aluminum surface are shown in Fig. 9. As compared to the relatively uneven surfaces formed by anodizing in borate shown in Fig. 5, an extremely smooth aluminum surface with uniform image contrast was successfully obtained by short-term anodizing for 15 min. Such short-term smoothing was caused by the formation of a thick, porous alumina film with a bottom flat barrier layer and stable oxide film thickening without oxide breakdown during anodizing in NaBO₂. It can be seen that anodizing for an additional 20 min and 40 min resulted in additional smoothing. Figure 10 shows the changes in the maximum height difference and arithmetic mean roughness with anodizing time as calculated by AFM measurements. In the figure, the values obtained by electropolishing (Fig. 3) and the three successive processes of anodizing in borate and oxide dissolution (Fig. 6) are represented by blue and green dashed lines, respectively. The maximum height difference of the dimpled aluminum surface (166.5 nm) drastically decreased to approximately 2.5 nm by anodizing in NaBO₂ for 15 min; the value decreased with further anodizing, and an extremely small value of 2.2 nm was successfully obtained after anodizing for 40 min. Similarly, the arithmetic mean roughness (31.5 nm) decreased to approximately 0.5 nm by short-term anodizing for 15 min, and the value further decreased to 0.4 nm after anodizing for 40 min. These height differences and mean roughness values are much lower than those obtained by a typical electropolishing process, and an extremely flat aluminum surface was obtained owing to the continuous anodic oxide growth.

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An ultra-smoothing method for aluminum surfaces with an arithmetic mean roughness of less than 1 nm was thus successfully developed via anodizing in NaBO₂ and subsequent oxide dissolution. This simple, one-time anodizing and dissolution method allowed for the fabrication of an ultra-smooth aluminum surface with a minimum roughness of 0.4 nm. Although such a smooth surface may also be obtained by the successive process of electropolishing, barrier oxide formation, and oxide dissolution, this process consists of many steps and is complicated. We used rolled aluminum plates as the starting materials in this investigation, thus macroscopic irregularities still remained on the aluminum surface after the pre-treatment of short-term electropolishing. It is difficult to remove such macroscopic, large irregularities from the surface by our smoothing method. Therefore, conventional mechanical polishing is appropriate for the smoothing of macroscopic, large irregularities, although small irregularities remain on the surface after the mechanical polishing processes. Our technique consisting of anodizing in NaBO₂ and oxide dissolution may be useful for the ultra-smoothing of aluminum surfaces with nano- and submicron-scale irregularities. For example, because the nano-level irregularities left on the electropolished surface have a significant impact on the nanostructure fabrication process and adhesion behavior, the fabrication of an ultra-smooth surface is required for the understanding and controlling of these performances. ^26–28,36 Highly reflective aluminum surfaces without nano-level irregularities can be employed for reflective mirrors in optical applications. ³⁷ We believe that our ultra-smoothing method is useful for these surface finishing processes.

Our smoothing method is based on anodizing in an alkaline NaBO₂ solution, and this anodizing causes the formation of extremely smooth inner barrier layer at the bottom of the porous alumina film. The growth behavior of such barrier layer during anodizing in NaBO₂ is greatly different from that in many typical acidic electrolyte solutions. Recently, we found that similar smoothing of the barrier layer occurs during anodizing in alkaline phosphate solutions such as disodium hydrogen phosphate, sodium diphosphate, and trisodium phosphate. ³⁵ Thus, it is considered that the barrier layer smoothing is caused by anodizing in alkaline solutions. Thompson and Hebert reported that a hemispherical barrier layer at the bottom of porous alumina film formed in acidic solutions growth by the compressive stress and subsequent alumina flow during anodizing. ^38–42 Therefore, it is expected that field-assisted dissolution without such flow occurs during anodizing in alkaline solutions, thus the flat barrier layer is formed on the aluminum surface. Further investigations of the presence or absence of oxide flow during anodizing in acidic and alkaline solutions are required to understand deeply the reasons.