## 3 Results and Discussion  
### Composition and Morphology of MAO Layer  
Figure 2 exhibits the phase composition results of the coating. The 7050 aluminum alloy was treated by single-phase pulse and two-phase pulse mode MAO (NaOH concentration, 4 g L\({}^{-1}\); Na\({}_{2}\)SiO\({}_{3}\) concentration, 12 g L\({}^{-1}\); NaF concentration, 6 g L\({}^{-1}\); current density of 8 A dm\({}^{-2}\); and treatment time  
Figure 1: Schematic illustration of the micro-arc oxidation (MAO) device: 1: special power supply; 2: electrolyzer; 3: workpiece to be processed; 4: agitator; and 5: cooling system. of 10 min). Figure 2c exhibits that the phase composition of the 7050 coating after MAO treatment is mainly \(\gamma\)-Al\({}_{2}\)O\({}_{3}\), as there is no obvious \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) peak in the spectrogram. This may be attributed to the fact that the MAO layer is too thin and the molten oxide cools too fast after discharge, which prevents the conversion of \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) into \(\alpha\)-Al\({}_{2}\)O\({}_{3}\). The specific surface area of \(\gamma\)-Al\({}_{2}\)O\({}_{3}\) is larger than that of \(\alpha\)-Al\({}_{2}\)O\({}_{3}\); therefore, the interface area between MAO layer and subsequent PU coating is increased, which is beneficial to the improvement of adhesion strength [25]. Figure 2a shows the SEM micrographs of the MAO layer surface, exhibiting that the surface is porous. Based on the cross-section morphology observation of the MAO ceramic layer by SEM images shown in Figure 2b, the MAO layer can be divided into a dense internal layer and an external porous layer. The porous layer has a porous structure that can form a wedge-shaped structure with PU coating to increase its adhesion; the dense internal layer is the main strengthening layer, and thus, can improve the corrosion resistance of the aluminum alloy substrate [26]. Effect of Different Concentrations of Sodium Silicate on Porosity, Thickness, and Roughness of Layers  
Sodium silicate has the most significant influence on the growth rate and morphology of the layer. Therefore, the effects of different concentrations of sodium silicate on the thickness, porosity, roughness, adhesion strength, and corrosion resistance of the MAO-PU composite coating were studied. SEM images with small magnification were selected to exclude the influence of uneven local distribution on the measurement results. Figure 3 displays the test results, indicating that with the increase of the sodium silicate concentration, the discharge channel of the MAO layer gets gradually blocked with the ejecta produced by the discharge. Moreover, the diameter of holes formed by the cooling of the discharge channel gradually increases and the porosity increases. At the same time, the increase of the layer thickness leads to the increase of stress and the appearance and increase of cracks in the MAO layer. Figure 2: Composition and micromorphology of the MAO layer. (**a**) the MAO layer surface zoom in 200\(\times\); (**b**) the MAO layer cross section zoomed in at 1000\(\times\); and (**c**) the XRD pattern of the layer under unidirectional pulse and bidirectional pulse conditions. These results showed that the addition of sodium silicate in the electrolyte is beneficial to the formation of a high-impedance film on the surface of aluminum alloy. Moreover, the increase of sodium silicate concentration can promote the arc discharge on the surface of aluminum alloy and reduce the energy consumption in the arcing process of MAO. The MAO layer was prepared at 8, 10, 12, 14, and 16 g L\({}^{-1}\) Na\({}_{2}\)SiO\({}_{3}\) concentrations. Figure 3 shows that when the concentration of sodium silicate is between 8 and 12 g L\({}^{-1}\), the roughness of the film is \(\sim\)1 \(\upmu\)m; however, the thickness is \(\sim\)10 \(\upmu\)m. The porosity factor increases gradually with the increase of sodium silicate concentration from 0.3 to 1.8%.