## 3 Preparation and Characteristics of High-Absorption Ceramic Coating  
The broadband and high-absorptivity ceramic coating materials were prepared by liquid phase plasma electrolytic oxidation technology (LPPEO). LPPEO is a new type of surface coating preparation technology [35]. The basic principle is that the anode parts of valve metals or their alloys such as aluminum, magnesium, titanium, and zirconium are placed in a specific electrolyte system. By adjusting the external power supply, the spark discharge occurs on the anode surface. Under the action of instantaneous high tempera ture and high pressure in the micro area, the thermal diffusion and electric discharge are excited and enhanced on the anode surface. As a result, the oxide ceramic coating firmly bonded with the substrate is grown on the anode surface in situ. The surface absorption performance of ceramic coating is directly related to coating thickness, surface roughness, material microstructure, and composition. Previous studies have shown that these parameters are mainly affected by the power mode, electrolyte composition, power parameters, and oxidation time of PEO technology [36; 37; 38]. Therefore, the above factors affecting the surface absorption properties of ceramic coatings were investigated in this paper. Firstly, the preparation process of the ceramic coating on the aluminum alloy substrate was studied from the design of a high-absorption ceramic coating and the selection of an electrolyte system. The ceramic coating with high absorptivity was prepared by using silicate and phosphate as main salts, zinc sulfate as a coloring salt, and sodium hydroxide. Then, the concentration of electrolyte composition was optimized by orthogonal test. Taking the infrared absorption rate of the coating as the performance parameter, the influence of electrolyte component concentration on the absorption rate of the coating was analyzed to optimize the concentration of each component in electrolyte. The experimental results show that the main factors affecting the absorption rate were the concentrations of ZnSO4 and NaOH, followed by silicate and phosphate. Secondly, the effects of current density, power frequency, power duty cycle, and oxidation time on the surface absorption properties of ceramic coatings were studied. The results show that, when the current density was 3.75 A/dm2, the frequency was 200 Hz, the duty cycle was 20%, and the oxidation time was 20 min, the coating thickness and surface roughness met the performance requirements. At the same time, the absorption rate of ceramic coating on aluminum alloy was highest. The average absorption rate of the ceramic coating was 95.4% in the range of 0.2-16 um. The ceramic coating samples on aluminum alloy substrate were prepared under the above conditions. The macrostructure, 350\(\times\) microstructure, and 1000\(\times\) microstructure features on the ceramic coating surface were observed using a charge coupled device (CCD) camera, optical microscope, and scanning electron microscope (SEM), respectively. The results are shown in Figure 2. The surface absorptivities of the ceramic coating and the aluminum alloy without coating in the visible and infrared bands (0.2-16 um) were measured using an absorptivity tester, and the measurement results are shown in Figure 3. It can be seen from Figure 2 that the surface of the high-absorption ceramic coating presented a very dense porous structure. It could absorb a wide range of absorption from visible to long-wave infrared light, mainly because the radiation in different wavelengths would be absorbed by a large number of micro holes on the surface of the coating, and the photons entering the holes would transfer the energy to the coating material, thus achieving a broad absorption band and high absorptivity of photon energy. Lastly, the average surface thickness and surface roughness of the coating under the optimal experimental parameters were obtained using a coating thickness meter and atomic force microscope (AFM).