*4.2. Growth Model of PEO Coating*

In general, mechanisms such as "dielectric breakdown", "discharge-in-pore", and "contact glow discharge electrolysis" were the mainstream views about the plasma discharges during the PEO process [35]. Various growth models were proposed based on these mechanisms to describe the formation of the PEO coating [30–32,36]. These models illustrated the origins of plasma discharges and the relationship between discharges and coating structure. However, the aluminum/coating interface and the correlations between the growth mechanism and the 3D structure were not considered in these models. Many studies have shown that the discharges become more powerful and extend as the coating thickens [30,37]. Therefore, the molten zones caused by discharges will also enlarge and last longer as the coating thickens.

According to the surface and aluminum/coating interface morphologies (Figures 2 and 6), the molten-shaped products, the nodules at the surface, and the hill-like structures at the aluminum/coating interface tended to decrease in number and increase in size as the coating thickened. Since the molten zones were the main routes by which a new coating was formed at local regions, continuous changes and overlaps of the molten zones would lead to the evolution of the coating structure. Based on the above results, a growth model is here proposed to explain the correlations between the molten zones and 3D structure evolution during the PEO process.

Figure 8 provides the growth model of the PEO coating. The formation of a dielectric film on the surface of the sample was a necessary condition for the plasma discharge. The dielectric film would be damaged once plasma discharge occurred.

In the early stage (5 min), as the discharges were weak, the previously formed anodic film had not been significantly damaged. The major crystalline phase in the coating was γ-Al2O3. The height difference of the aluminum/coating interface was lower, as demonstrated by the weak hill-like features. At this stage, obvious porosity defects were difficult to find, because the gas could easily escape from the molten zone, as shown in Figure 8a.

With the increase in oxidation time, a thin and dense barrier layer near the coating/substrate interface was clearly visible in the coating formed at 15 min (Figures 1b, 6f, and 7j). The higher height difference of the aluminum/coating interface (Figure 6b) suggested an increased depth of the oxidation of the aluminum substrate. The larger nodules at the surface (Figure 2b) indicated an increased area and a longer duration of the molten zone, which was caused by a stronger discharge. Additionally, as shown in Figures 1b and 2b, trench-like open pores were present in the center of the molten-shaped products, which was very common and usually appeared in the thinner PEO coatings on Al [38], Mg [31], and Ti [6]. A reasonable explanation for the open pores was that the amount of molten products was insufficient to complement the plasma discharge channels in the cooling process. Now, the model of the PEO coating and discharge was shown in Figure 8b. At this stage, the molten-shaped product presented a "crater"-like morphology with an open pore in the center.

As the oxidation time increased, the dielectric breakdown of the thicker coating became difficult, and the discharge events appeared to be more powerful. Longer cooling periods might occur in this situation, and there is a strong tendency for discharges in the cascade [33]. It was reasonable to consider that the long-lasting molten zones were easier to form in the later stages. As oxidation time increased, the coating tended to form more high temperature phases of α-Al2O3 and σ-Al2O3. Consequently, the larger pancake-like structures with closed center pores formed on the coating over 45 min, and the hill-like protrusions at the aluminum/coating interface were further enlarged. Figure 8c,d illustrates the model of these coatings.

**Figure 8.** A growth and 3D structure model of the PEO coating at different stages: (**a**) breakdown of dielectric film under plasma discharges; (**b**) formation of PEO coating with open pores; (**c**) initial formation of three-layer structure; and (**d**) further evolution of three-layer structure.

It has been proven that excessive gas is released during discharge [39] and that gas might be generated near the substrate/coating interface [9,15]. In this work, cracks or pores would form when high-pressure gases escaped from the molten zone. Many gas bubbles could not escape in time, which left the closed, spherical pores outside the coating. The volume shrinkage during the solidification of the molten regions and the gas expansion left large cavities near the substrate/coating interface. Different discharges occurred repeatedly at adjacent locations, which caused an overlap of molten zones, implying in turn that channels, cracks, and large cavities formed in the coating. Thus, a fine, interconnected porosity network structure was formed in the PEO coating. The porosity network caused the electrolyte to penetrate into the large cavities, and a secondary coating/electrolyte interface formed inside the coating. Further discharges were likely to initiate at the base of the cavities, so a thinner, finely porous inner-layer coating was formed, as shown in Figure 2c,d. As the coating grew, the thickness of the inner-layer continuously increased. It can be inferred that a series of reactions (evaporation, dehydration, and deposition) occurred in the large cavities. Thus, a higher proportion of electrolyte species was present at the edges of large cavities (Figure 1e,f).

#### **5. Conclusions**

The 3D structure of a PEO coating on aluminium, including the surface, the internal structure, the aluminum/coating interface, and the fracture cross-section morphology, was obtained. The PEO coating surface can be described as molten-shaped products of alumina surrounded by Si-rich nodules. A barrier layer consisting of clustered cells was present at the aluminum/coating interface. The PEO coating gradually evolved into a distinct three-layer structure as the coating thickened, including a barrier layer, an inner layer with enclosed pores, and an outer layer with a rough surface. Interestingly, obvious cavities appeared between the inner and outer layers in the thicker coatings.

PEO coatings were grown via the oxidation of aluminum and the deposition of electrolyte compounds. The oxidation of aluminum resulted in a crater- or pancake-like molten-shaped product, whereas the deposition of electrolyte compounds usually formed nodules at the coating surface.

During the PEO process, molten zones were formed around the plasma discharges. The thickening of the coating mainly depended on the forming, closing, and repeated movement of molten zones. The uneven cooling rates around the molten zones resulted in a distinction between the coating surface and the aluminum/coating interface structure. At different discharge periods, the intensity and duration of discharges determined the volume and lifetime of the molten zones, which resulted in various 3D microstructures of the PEO coating.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-6412/8/3/105/s1, Figure S1: the EDS spectrum of PEO coating formed at 60 min.

**Acknowledgments:** This work was supported by the National Natural Science Foundation of China (Grant No. 51361025) and the Natural Science Foundation of Jiangxi Province (Grant No. 20171BAB216006).

**Author Contributions:** Xiaohui Liu, Shuaixing Wang, and Xinyi Li performed the PEO treatments; Nan Du and Qing Zhao contributed reagents, materials, and analysis tools; Xiaohui Liu and Shuaixing Wang characterized the PEO coatings and wrote the paper.

**Conflicts of Interest:** The authors declare no conflict of interest.
