Surface Texture Effect on Luster of Anodized Aluminum

Anodizing is an electrolytic passivation process commonly applied to convert aluminum to aluminum oxide. It can modify the surface texture and changes the microstructure of the metal near the surface. Such an anodized aluminum oxide layer is generally much stronger and more adherent than most types of paint and metal plating. It can significantly enhance corrosion and wear resistance and improve cosmetic effects of the products. Anodized aluminum is widely used on electronic devices and consumer products, such as cell phones, cameras, mp3 players and many others.

The appearance of a product plays an important role in influencing consumer product choice. It communicates aesthetic and functional characteristics and makes an impression of the product quality. Roughness and texture of the anodized aluminum surface is critical in the final product appearance. A better understanding of the effect of surface texture, consistency and directional patterns on the product appearance allows optimizing the processing and control measures to obtain the best result. Quantifiable, precise and reliable surface inspection of anodized aluminum is needed to control surface parameters for endless applications of anodized aluminum. The NANOVEA 3D Non-Contact Profilometers utilize chromatic confocal technology with unique capability to precisely measure the sample surface.

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Sample 1 is more lustrous compared to Sample 2, making it more visually appealing and thus seemingly better quality. In order to understand the cause of such effect, the surfaces of Samples 1 and 2 under the optical microscope are compared in Fig. 2. Sample 1 appears to reflect more light under the same condition.

The false color view and roughness of the two anodized aluminum samples are compared in Fig. 3. Sample 1 shows a relatively coarser surface texture with a higher roughness Sa of 3.94 µm, compared to an Sa of 2.65 µm for Sample 2. Such an observation seemingly contradicts common sense, where a smoother surface (with a lower roughness value) usually possesses higher reflectivity.

In order to further investigate the effect of the surface texture on the luster of the anodized aluminum, a more detailed 3D profile evaluation of these two anodized aluminum samples is performed and discussed in the following section. The 3D view and 2D profile of the two samples at a higher magnification are displayed in Fig. 4. It can be observed that both Sample 1 and Sample 2 have a rough surface. However, Sample 1 shows plateaus of larger sizes and flatter surface, which reflect more light compared to the small asperities on Sample 2. 2D profile analysis confirms this conclusion: although Sample 1 possesses a rougher surface due to the deeper pores between the plateaus, the plateaus show smoother micro texture.

Grain size analysis of the anodized aluminum samples is compared in Fig. 5. Each grain here represents a plateau on the sample surface. It can be observed that the plateaus on Sample 1 have an average size of ~28 µm, compared to ~19 µm for Sample 2. The larger plateaus exhibit better reflectivity, in agreement with the above 3D analysis.

In this application, we have showcased that the NANOVEA ST400 3D Non-Contact Profilometer is an ideal tool for analyzing and comparing the surface finish of anodized aluminum samples. The size of the plateaus on the sample surface plays an important role in the reflectivity of the anodized aluminum. The surface texture of larger plateaus is preferable to provide a better luster. The high resolution scan and comprehensive analysis tools including 3D and grain size analyses by the NANOVEA profilometer enable comprehensive and quantitative evaluation of the surface finish of metal samples, which is critical in assessing the quality and optimizing the appearance of products.

The data shown here represents only a portion of the calculations available in the analysis software. NANOVEA Profilometers measure virtually any surface in fields including Semiconductor, Microelectronics, Solar, Fiber Optics, Automotive, Aerospace, Metallurgy, Machining, Coatings, Pharmaceutical, Biomedical, Environmental and many others.