Fig. 1 shows that the surface oxides of the composite coatings are Al2O3, Fe2O3, Fe3O4 at 450 °C, while at 650 and 800 °C, the surface oxides are A12O3, Fe2O3, Fe3O4 and FeO. This is because the oxide of Fe is Fe2O3, Fe3O4 at 570 °C.
Fig. 1 XRD patterns of Fe-A1/WC composite coating surface oxidized at 450 °C(a), 650 "C(b) and 800 °C(c)
Fig. 2 shows the backscattered electron SEM images and EDS spectra of Fe-A1/WC composite coating surface oxidized at 450 °C and 800 °C. It is showed that the chemical composition results in the difference of the color in different areas. The atomic number is greater, the color is lighter. The Fe content is high in the white area and A1 element is rich in the black area. Fig. 2 also shows that the surface oxides distribute unevenly and alumine and iron oxide overlap each other.
Fig. 2 Backscattered electron SEM images and EDS spectra of Fe-A1/WC composite coatings surface oxidized at 450 °C and 800 °C (a)--SEM image of coating surface at 450 °C ; (b)--EDS spectrum of A area; (c)--EDS spectrum of B area; (d)--EDS spectrum of C area; (e)--SEM image of coating surface at 800 "C ; (f)--EDS spectrum of D area;
(g)--EDS spectrum of E area; (h)--EDS spectrum of F area
Fig. 3 shows SEM images and EDS spectra of Fe-A1/WC composite coating cross section oxidized at 450 and 650 °C. With the change of the color from black to white, the Fe element content increases and the A1 element content decreases gradually. The character of arc sprayed coating that composition distribute s unevenly induces the distribution asymmetry of oxides. Alumina and iron oxide coexists in A, B, C and D areas, but in different areas the proportion is different.
In the Fe-At/WC composite coating, A1 and Fe are chief elements that react with O element. The reaction Gibbs free energy of A1 and O is more negative than that of Fe and O. According to the oxidation thermodynamic criterion , A1 reacts with O more easily and creates compact Al2O3 film. While the excessive Fe produces the iron oxide. The metal oxidation theories indicate that the moving direction of O and metal atoms in oxidation film have three kinds: 1) O passes inward through oxidation film; 2) metal atoms pass outward through oxidation film; 3) two mentioned kinds of phemomena occur at the same time. A large number of porosities in the arc sprayed coating results show that the oxidation process of Fe-A1/WC composite coating include two stages. First stage, O enters coating through the porosities and generates oxide(because of the existence of porosities, the contact area of metal surface and O is bigger), the reaction speed at beginning is great, with the increase of O content, the oxidation film becomes thick to surely critical value, so that the porosities are blocked, which hinders the diffusion of O toward the coating and cuts off the reaction of metal and O. Second stage, because of oxidation driving force, diffusion of Al atom in the coating subsruface toward the coating sruface is the leading factor that controls the coating further oxidation, which induces the enrichment of Al in the coating surface and the poverty of A1 in the coating subsurface (Fig. 3). Al atoms that have moved to the coating surface react with O and the protective A1203 film that prevents the coating further oxidation is produced, which is unanimous with the conclusion in the document .
Fig. 3 SEM images and EDS spectra of Fe-A1/WC composite coating cross section oxidized at 450 °C and 650 °C (a)--SEM image of coating cross section at 450 °C ; (b)--EDS spectrum of A area~ (c)--EDS spectrum o{ B area; (d)--SEM image of coating cross section at 650 "(2 ; (e)--EDS spectrum of C area; ({)--EDS spectrum of D area
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