*3.4. Aluminum/Coating Interface Morphology of PEO Coating*

To understand the structure evolution of the coatings during the PEO process, the aluminum/coating interface structures were obtained after the coatings were detached from the samples. Figure 6 shows the 3D and SEM images of the aluminum/coating interface at different oxidation times. The structure of the aluminum/coating interface was greatly different from the outer surface and the internal structure of the PEO coating. The aluminum/coating interface was irregular and uneven, and appeared as "hill"-like features.

**Figure 6.** (**a**–**d**) 3D color maps, (**e**–**h**) SEM image, and (**i**–**l**) magnified SEM images of aluminum/coating interface for PEO coatings formed at (**a**,**e**,**i**) 5 min, (**b**,**f**,**j**) 15 min, (**c**,**g**,**k**) 45 min, and (**d**,**h**,**l**) 60 min.

The 3D images (Figure 6a–d) displayed that the height difference at the aluminum/coating interface raised as the oxidation time increased. This transformation indicated that the "hills" had grown with the development of the PEO process. The sites of the hill-like protrusions reflected the depression locations in the aluminum substrate. Thus, the depth of oxidation of the aluminum substrate increased with the growth of the coating.

A similarly increasing trend could be observed in the corresponding SEM images shown in Figure 6e–h. With the increase in oxidation time, the volume of the concave and convex regions at the interface increased and the boundary became smoother. According to the magnification images in Figure 6i–l, the aluminum/coating interface structure was more compact than the surface and internal structure of the PEO coating. The aluminum/coating interface was formed by a high density of cells within the cluster, and almost no pores or cracks were present. Moreover, the diameter of the cells always maintained a constant size of ~1 μm with the increase in treatment time, although the volume of the "hill"-like projections increased.

It can be inferred that the inward growth of the PEO coating was carried out such that the aluminum transformed into oxide in many valley-shaped pools as oxidation time increased. Both Figures 1 and 6 indicate that the aluminum/coating interface was a dense layer composed of many small cells within clusters embedded into the aluminum substrate.
