Plasma Electrolytic Oxidation Coatings of a 6061 Al Alloy in an Electrolyte with the Addition of K(2)ZrF(6)

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 K(2)ZrF(6). 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 K(2)ZrF(6) 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 K(2)ZrF(6). Meanwhile, the surface roughness was observed to stabilize at a certain level as the K(2)ZrF(6) concentration increased. At the same time, the addition of K(2)ZrF(6) altered the growth mechanism of the coating. In the absence of K(2)ZrF(6) in the electrolyte, the PEO coating on the Al alloy surface predominantly developed outwards. However, with the introduction of K(2)ZrF(6), 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 K(2)ZrF(6). The addition of K(2)ZrF(6) 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 K(2)ZrF(6). In addition, the phase composition of the aluminum alloy PEO coating in the electrolyte containing K(2)ZrF(6) was dominated by tetragonal zirconia (t-ZrO(2)) and monoclinic zirconia (m-ZrO(2)). With the increase in K(2)ZrF(6) 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 K(2)ZrF(6) 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 K(2)ZrF(6), as well as the presence of ZrO(2) with higher emissivity within the coating.