Deye Li

In this study, white thermal control coatings were produced on a 6061 Al alloy using plasma electrolytic oxidation (PEO). The coatings were mainly formed by incorporating K₂ZrF₆. The phase composition, microstructure, thickness, and roughness of the coatings were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), a surface roughness tester, and an eddy current thickness meter, respectively. The solar absorbance and infrared emissivity of the PEO coatings were measured using a UV–Vis–NIR spectrophotometer and FTIR spectrometer, respectively. The addition of K₂ZrF₆ to the trisodium phosphate electrolyte was found to significantly enhance the thickness of the white PEO coating on the Al alloy, with the coating thickness increasing in proportion to the concentration of K₂ZrF₆. Meanwhile, the surface roughness was observed to stabilize at a certain level as the K₂ZrF₆ concentration increased. At the same time, the addition of K₂ZrF₆ altered the growth mechanism of the coating. In the absence of K₂ZrF₆ in the electrolyte, the PEO coating on the Al alloy surface predominantly developed outwards. However, with the introduction of K₂ZrF₆, the coating’s growth mode transitioned to a combination of outward and inward growth, with the proportion of inward growth progressively increasing in proportion to the concentration of K₂ZrF₆. The addition of K₂ZrF₆ substantially enhanced the adhesion of the coating to the substrate and endowed it with exceptional thermal shock resistance, as the inward growth of the coating was facilitated by the presence of K₂ZrF₆. In addition, the phase composition of the aluminum alloy PEO coating in the electrolyte containing K₂ZrF₆ was dominated by tetragonal zirconia (t-ZrO₂) and monoclinic zirconia (m-ZrO₂). With the increase in K₂ZrF₆ concentration, the L* value of the coating increased from 71.69 to 90.53. Moreover, the coating absorbance α decreased, while the emissivity ε increased. Notably, at a K₂ZrF₆ concentration of 15 g/L, the coating exhibited the lowest absorbance (0.16) and the highest emissivity (0.72), which are attributed to the enhanced roughness resulting from the substantial increase in coating thickness caused by the addition of K₂ZrF₆, as well as the presence of ZrO₂ with higher emissivity within the coating.

Silver-doped graphite-like carbon (Ag-GLC) coatings were prepared on the surface of aluminum alloy and single-crystal silicon by magnetron sputtering under different deposition parameters. The effects of silver target current and deposition temperature, as well as of the addition of CH₄ gas flow, on the spontaneous escape behavior of silver from the GLC coatings were investigated. Furthermore, the corrosion resistance of the Ag-GLC coatings were evaluated. The results showed that the spontaneous escape phenomenon of silver could take place at the GLC coating, regardless of preparation condition. These three preparation factors all had an influence on the size, number and distribution of the escaped silver particles. However, in contrast with the silver target current and the addition of CH₄ gas flow, only the change in deposition temperature had a significant positive effect on the corrosion resistance of the Ag-GLC coatings. The Ag-GLC coating showed the best corrosion resistance when the deposition temperature was 500 °C, which was due to the fact that increasing the deposition temperature effectively reduced the number of silver particles escaping from the Ag-GLC coating.