*3.2. Surface Morphology and Chemical Composition of PEO Coating*

Figure 2 shows the SEM and 3D images at the surface of PEO coatings prepared at various oxidation times. As shown in Figure 2a–d, a large number of micropores and cracks were present at the surface of PEO coatings. Moreover, there were two categories for all coating surfaces. One is a loose nodule, and the other is a molten-shaped product with open or sealed micropores in the center. The morphology and size of the molten-shaped products changed with the increase in oxidation time. At 15 min, the molten-shaped products presented as craters with an open pore in the center. After 45 min, the molten-shaped products looked like pancakes with a sealed pore in the center. Additionally, 3D images displayed the evolution of the height difference at the coating surface (Figure 2e–h), indicating that nodules also enlarged during the PEO process.

In the initial stage (5 min), the PEO coating was compact and flat. As shown in Figure 2a,e, many fine nodules were present at the coating surface, but the molten-shaped product was not obvious. At 15 min, a large number of striped and sub-circular pores appeared at the PEO coating surface, and a group of nodules were present around these pores, as shown in Figure 2b. The 3D image (Figure 2f) also shows many obvious peaks and valleys. However, the surface morphology of the PEO coatings that formed at 45–60 min was obviously different from that of the thinner coatings. As shown in Figure 2c,d, pancake-like structures with a central pore were the main feature of these coatings, while larger nodules were also present. In addition, the height difference at the coating surface increased slightly as the oxidation increased from 45 to 60 min, which mainly resulted from the changes in nodules.

The elemental content of the PEO coatings were investigated by EDS analysis marked in Figure 2, as presented in Table 1. All coatings were composed of O, Al, and Si. Al in the coatings originated from the aluminum substrate, whereas Si was from the electrolyte. A small P peak was also detected (as shown in Figure S1), this phosphorus might result from the residual electrolyte. With the increase in oxidation time, the surface content of coatings had no obvious change. The EDS result was affected by the aluminum substrate, because the penetration depth of the X-ray was ~3 μm under the present conditions. Thus, the aluminum content in the thin coating formed in 5 min was unusually high.

**Figure 2.** (**a**–**d**) Surface SEM (back-scattered electron mode) and (**e**–**h**) 3D color maps of PEO coatings formed at (**a**,**e**) 5 min, (**b**,**f**) 15 min, (**c**,**g**) 45 min, and (**d**,**h**) 60 min.

**Table 1.** Surface EDS analysis of PEO coatings formed at different times marked in Figure 2 (at.%).


Figure 3 shows the XRD patterns of PEO coatings prepared at different times. Both the powder sample and PEO coating sample with the substrate formed at 60 min were used for the XRD test. As shown in Figure 3d,e, the only difference between the two was that the diffraction peak of Al was present in the coating sample with the substrate. In order to reduce the effect of the aluminum substrate on the XRD results, free-standing coatings were ground to powders for the XRD tests. During the initial stages (5–15 min), the major crystalline phase in the coating was γ-Al2O3. The coatings formed over a longer period (45–60 min) showed a presence of γ-Al2O3, α-Al2O3, and σ-Al2O3.

In addition, the element distribution at the surface of the typical coating (60 min) are shown in Figure 4. As shown in Figure 4a, the surface of the PEO coating formed at 60 min was dominated by the compact pancake-like structures that were surrounded by loose nodules. EDS mappings show that Al and O were present in most regions of the coating, but the distribution of Al in the nodules was relatively low. Si and P were the main components of the nodules. EDS point analyses (Figure 4f) also revealed significant difference between the nodules and pancake-like structures.

Based on the above analysis, it can be deduced that the oxidation of aluminum and the deposition of electrolyte composition contributed to the growth of PEO coatings [28,29] in the silicate-phosphate electrolyte. The molten-shaped products resulted from the oxidation of aluminum, whereas the nodules were caused by the deposition of electrolyte compounds. Interestingly, the molten-shaped products were always surrounded by the nodules. Combined with the EDS and XRD results, the surface structure of PEO coating could be described as a molten-shaped structure of alumina surrounded by Si-rich nodules.

**Figure 3.** XRD patterns of PEO coating powder samples formed at (**a**) 5 min, (**b**) 15 min, (**c**) 45 min, and (**d**) 60 min. (**e**) XRD pattern of the PEO coating sample with a substrate formed at 60 min.

**Figure 4.** (**a**) Surface SEM images (secondary electron mode) of the PEO coating formed at 60 min. EDS mappings of (**b**) O, (**c**) Al, (**d**) Si, and (**e**) P. (**f**) Semi-quantitative analysis of element content at different locations marked in Figure 4a.
