The formation of metallic W and amorphous phase in the plasma electrolytic oxidation coatings on an Al alloy from tungstate-containing electrolyte

Abstract Detailed characterization of W species and amorphous material in plasma electrolytic oxidation (PEO) coatings formed in W-containing electrolyte is important to PEO study. In this paper, PEO coatings were prepared on an Al-Cu-Li alloy in a mixed electrolyte of silicate and tungstate, and the coatings were characterized by a series of advanced analysis methods. Scanning electron microscopy (SEM) shows that PEO leads to a bi-layered coating with big internal pores and the W element was enriched close to the coating/substrate interface. X-ray photoelectron spectroscopy (XPS) shows that hexavalent tungsten is the predominant species; however, weak peaks for free-state W have been found in the inner coating exposed by grinding. Focused ion beam (FIB) was used to prepare an electron-transparent sample across the coating/substrate interface for the transmission electron microscopy (TEM) characterization. TEM results show that free-state W exists in the form of nanocrystalline particles, while the amorphous material was found surrounding the large internal pores of the coating. The amorphous material consists of O, Al, W, Si and Cu, and the elements other than O have comparable atomic percentages. According to models proposed in this paper, the nanocrystalline W particles were formed by the thermite reaction between fine molten Al drops and tungsten oxides. However, amorphous material was formed due to its complex composition and the rapid cooling rate brought from the ingress of electrolyte into the big pores.