Incorporation mechanism of ZnO nanoparticles in PEO coating on 1060 Al alloy

Herein, a ZnO-doped plasma electrolytic oxidation (PEO) composite coating was prepared on the surface of a 1060 Al alloy by adding ZnO nanoparticles (ZnO-NPs) to the composite electrolytes of alkaline sodium silicate (Na 2 SiO 3 ) and sodium hexametaphosphate ((NaPO 3 ) 6 ). To efficiently investigate the coating's microstructure, the detached PEO coating was prepared without damage via an electrochemical method. The morphology and phase composition of the PEO coating were analyzed using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), and X-ray diffraction (XRD), and an incorporation model of ZnO-NPs was established. The results showed that the addition of ZnO-NPs slightly prolonged the anodic oxidation time and increased the breakdown voltage, whereas the working voltage and final voltage decreased slightly. The XRD results indicated that although the addition of ZnO-NPs did not result in the production of Zn-containing phases, it affected the formation and transformation reactions of other phases. SEM micrographs of the fractured cross-section of detached coating and coating/substrate interface indicated that the ZnO-NPs did not enter the interface layer, nor did it participate in the formation of the cap structure at the interface. Based on the experimental results and the “cascade discharge” model, an incorporation model of ZnO-NPs in PEO has been proposed.