**4. Discussion**

### *4.1. The Formation Process for 3D Structures of PEO Coating*

The surface morphologies of the PEO coatings (Figure 2) demonstrate that numerous nodules surrounding the molten-shaped products were the main feature of the coating surface. The EDS results (Figures 1e,f and 4) confirmed that nodules were Si-rich products of electrolyte deposits and that the molten-shaped products were mainly oxides of the substrate. The different morphologies and components at the surface were ascribed to various kinds of discharges in the integrated discharge model [30–32]. Additionally, a ~1 μm barrier layer that consists of dense cells was present at the aluminum/coating interface. Hill-like protrusions at the aluminum/coating interface enlarged over time. According to the surface and the aluminum/coating interface, it can be deduced that molten zones were present around the plasma discharge channels due to the high temperature (~16000 ± 3500 K [33]). The molten zone was considered to be the basic unit for the formation of the coating.

Figure 7 gives a schematic diagram of the discharge at the molten zone of local coating. When the discharge occurred, the aluminum was melted and reacted with oxygen.

$$\text{Al} \rightarrow \text{Al}^{3+} + \text{e} \tag{1}$$

$$\text{Al}^{3+} + \text{3O}^{2-} \rightarrow \text{Al}\_2\text{O}\_3 \tag{2}$$

The aluminum/coating interface was an important cooling region because of the extremely high thermal conductivity (~230 W·m−1·K−<sup>1</sup> [34]) of the aluminum substrate. The region of the molten zone near the aluminum rapidly solidified and formed a hill-like protrusion, as shown in Figure 7a. A part of the molten products was ejected along the discharge channel to the coating surface. The coating/electrolyte interface acted as a vital cooling region, and the molten products rapidly solidified. A molten-shaped product structure formed at the coating surface.

At local high temperatures of the discharges, electrolyte will evaporate, concentrate, transform, and deposit at the coating surface to form nodules consisting of electrolyte constituents [8]. Thus, nodules rich in Si elements formed around the molten-shaped products.

In general, the following transformation process takes place.

$$\text{CH}\_2\text{O} + \text{SiO}\_3^{2-} \rightarrow \text{SiO}\_2 + 2\text{OH}^- \tag{3}$$

This analysis is confirmed by Figures 2 and 4, which show that most nodules were distributed around the molten zones and contained higher levels of Si.

**Figure 7.** A schematic diagram of the discharge at the molten zone of local coating: (**a**) molten zone; (**b**) 3D structure.

Plasma discharges occurred repeatedly near the cooling region. The previously formed nodules would be broken again and incorporated into the molten products. Thus, a fresh molten-shaped product was formed after the molten zone cooling. A molten-shaped product structure of alumina surrounded by the nodules containing some electrolyte constituents was finally produced. The molten zone was generally considered to be a closed system during the cooling process. The escape of a large amount of gas was impeded, and numerous closed holes were enclosed inside the coating.
