## 1 Introduction  
High-strength aluminum alloys (2A12) are widely used as structural materials in spacecrafts to reduce their structural weight [1, 2]. However, the equilibrium temperature of a spacecraft's surface is high under solar radiation due to the low emissivity of these alloys. Coating these surfaces with a layer of low-absorption, high-emission thermal control coating can reduce the absorption of solar radiation, increase surface thermal radiation, and reduce the thermal equilibrium temperature of the spacecraft, thus ensuring that the spacecraft and its various instruments and equipment can maintain a normal operating temperature range [3, 4, 5]. At present, the common thermal control coatings (TCCs) for aluminum surfaces are mainly divided into the white paint type [6, 7, 8, 9], the second surface mirror type [10, 11, 12, 13], and the electrochemical ceramic type [14, 15, 16]. The white paint type is mainly composed of white powders (e.g., ZnO and TiO\({}_{2}\)) and binders such as epoxies, acrylics, silicones, and silicates, which are deposited on the substrate surface by coating or air-spray technology and cured to obtain a thermal control coating [8, 9]. White paint coating offers the advantages of a simple preparation process, high efficiency, and easy repair. However, the degradation of the binder in an ultra-vacuum and alternating hot and cold environments leads to the hardening of the coating, which reduces the bonding properties of the coating to the substrate. The secondary surface mirror-type thermal control coatings, also known as optical solar reflectors (OSR), are usually made by depositing SiO\({}_{2}\) thin films on metal substrates by vacuum vapor deposition [11; 12]. OSR coatings have both high IR emissivity and low solar absorption and have shown excellent stability in practical applications. However, they are expensive and cannot be deposited on the surface of large, complex structural parts. In addition, secondary surface mirror-type coatings have poor wear resistance and, therefore, are prone to wear and even peeling in the harsh environment of space, resulting in degraded coating performance [13]. Ceramic thermal control coating is an aluminum oxide coating formed on the surface of an aluminum alloy by anodic oxidation or plasma electrolytic oxidation (PEO) [15; 16]. In recent years, researchers have achieved the modulation of coatings' microstructure and composition by regulating the electrolyte composition and optimizing the reaction's electrical parameters, thus improving coatings' properties. However, this coating has poor high-temperature resistance and still needs to be improved in terms of its solar absorbance performance. In summary, due to limitations in terms of preparation technology and material properties, the mechanical properties of thermal control coatings still need to be improved. Compared with the latter materials and processes, plasma-spraying technology has a high deposition rate and can deposit a variety of materials, from metals to ceramics, on substrates of any geometry and of different sizes and offers a strong binding force with the substrate [17; 18; 19]. As is commonly known, plasma-sprayed alumina coatings are widely used in a variety of fields due to their excellent physical and chemical properties [20; 21; 22; 23]. Recently, studies have shown that coatings prepared by spray-granulated nano-alumina powder via atomization exhibit better mechanical properties than conventional coatings due to their unique microstructure [24; 25]. As shown in Figure 1a, during the spraying process, the melting state of the granulated Al\({}_{2}\)O\({}_{3}\) nanoparticles can be divided into three stages, namely, completely melted, partially melted, and unmelted, and these stages depend on their sizes and trajectory in the plasma jet [26; 27].