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 (
Figure 1e,f and
Figure 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,
31,
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.
The aluminum/coating interface was an important cooling region because of the extremely high thermal conductivity (~230 W·m
−1·K
−1 [
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.
This analysis is confirmed by
Figure 2 and
Figure 4, which show that most nodules were distributed around the molten zones and contained higher levels of Si.
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.
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,
31,
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 (
Figure 2 and
Figure 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-Al
2O
3. 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 (
Figure 1b,
Figure 6f, and
Figure 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
Figure 1b and
Figure 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-Al
2O
3 and σ-Al
2O
3. 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.
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).