**1. Introduction**

Plasma electrolytic oxidation (PEO) is a surface-modification technique for producing ceramic coatings on light metals and their alloys (such as aluminum, magnesium, and titanium) [1,2]. PEO coating is considered to be amongst the most promising protective coatings for application in a wide range of industry sectors because of its high microhardness and its good wear and corrosion resistance [3,4]. However, the long-term protection performance of a PEO coating is limited by its high porosity. Researchers have different opinions on whether the pores in the coating extend to the substrate [5,6]. Hence, the three-dimensional (3D) structure and growth mechanism of PEO coatings need to be studied.

In general, most information about the structure of PEO coatings is obtained from the conventional surface and polished cross section. The coating surface is porous and coarse, consisting of pancake-like structures with a central hole [7]. PEO coatings are divided into three layers, i.e., an outer loose layer, an inner dense layer, and the barrier layer near the substrate [8,9]. A free-standing coating detached from the substrate can be used to obtain more information about the PEO coating structure, e.g., the structure of the coating/substrate interface. Some researchers have tried to use chemical solutions to detach the coating from the aluminum substrate. However, chemical dissolution in NaOH

may dissolve alumina coating [10,11], and chemical dissolution in CuCl2 may lead to a copper cover at the coating surface [12]. Recently, free-standing coatings have been obtained via dissolving the coated aluminum with an electrochemical method [13,14]. Moreover, 3D information about the porosity of PEO coating structures has been obtained by X-ray computed tomography [6,15] and the resin replica method [16]. Additional information on the 3D structure of PEO coatings, especially the evolution of the 3D structure during the PEO process, is needed for a deeper understanding of the PEO mechanism.

For the growth of PEO coatings, the most commonly accepted mechanism is attributed to an outward–inward growth mechanism [17–21]. The presence of inward growth has been confirmed by 18O element labeling [22], which is regarded as a process of repetitive breakdown and passivation of the barrier layer at the coating/substrate interface [13]. Additionally, the outward growth of PEO coatings has been shown by analyzing the elemental distribution in PEO coatings prepared on a substrate of Ti covering Al [19]. The outward growth of coatings is usually considered a process of ejecting molten oxide [18,23,24]. Another theory is that the outward growth of PEO coatings occurs because the outer layer expands outwards under a squeezing effect owing to a thickening barrier layer [25]. Further studies of the PEO mechanism are limited by a lack of understanding of the coating structure.

In this work, PEO coatings were prepared on 1060 aluminum alloy in the silicate-phosphate electrolyte. The free-standing coating was obtained by the dissolution of substrate using an electrochemical method. The 3D structure of the PEO coating was analyzed using a field emission gun SEM (FE-SEM) and energy dispersive spectroscopy (EDS) by layer-by-layer thinning. The 3D structure of the coating, including the surface, the internal structure, the aluminum/coating interface, and the fracture cross-section structure, was studied in detail. Based on the above results, a growth model of PEO coatings is proposed.
