TEM results of the MAO outer layer show that MAO-0W is composed of an ultra-thin amorphous outward layer and a poly-crystalline inward layer, however, the outer layer of MAO-4W is all amorphous. Then, the inner layer of MAO-4W is further analyzed, and the microstructure of inner layer is shown in Fig. 1. Typical concentric diffraction rings(Fig. 1F and I) and diffuse halo(Fig. 1C) confirm that there are not only crystalline regions but also amorphous regions in the inner layer, and the crystal region consists of coarse and fine grains(Fig. 1B). The poly-crystalline diffraction rings(Fig. 1F and I) in the coarse-grained region and fine-grained region are almost the same, suggesting that there is little difference in the phase composition between the coarse-grained region and the fine-grained region, the detailed identification of poly-crystalline diffraction rings in Fig. 1(F) and Fig. 1(I). In addition, even if W is incorporated into the MAO coating, the crystal structure of MAO-0W(as illustrated in Fig.2C) and MAO-4W is similar.
The disorder of intergranular atoms in the marking frame of Fig. 1(E)and(H) verifies that the amorphous phase still exists in the poly-crystalline region. Further, the elemental distributions of amorphous, coarse-grained and fine-grained regions are analyzed and EDS results are listed in Supplementary Table S1. The amorphous region of inner layer(zone I in Fig. 1B) contains more W than the outer layer(point Cin Fig.2F), and the W content in the fine-grained region (zone II in Fig. 1B) is also high, indicating that W with a relatively considerable concentration is distributed throughout the MAO-4W coating. The point A of coarse grain mainly contains O, Ti, Al as well as a small amount of Si,P and W, and the mass ratio of Al, Ti and O is close to that of Al2TiO5, so it can be considered as Al2TiO5. Whereas, point B,C and D of coarse grain are principally composed of O and Ti, the content of Al is lower and the content of Si,P and W is higher than point A, which is more similar to that of TiO2.W may replace part of Ti atoms in TiO2 crystals or locally assemble in the form of elements, in accordance with the SAED pattern of MAO-4W crystalline region.
Fig. 1. TEM results of the MAO-4W inner layer. (A) FIB slice. (B) Low-magnification TEM image of the rectangle in (A). (C) SAED pattern of the amorphous area in (B). (D) enlarged morphology of the coarse-grained region in (B). (E) High-resolution TEM (HR-TEM) image of the coarse grain. (F) SAED pattern of the coarse- grained region. Microstructure (G) and HR-TEM image (H) of the fine-grained region. (I) SAED pattern of the fine-grained region.
Elemental distribution across the interfaces of coarse-fine grain and crystal-amorphous detected by EDS is depicted in Fig. 2. Obviously, the fluctuation of curve in the coarse-grained region is sharper than that in the fine-grained region in Fig. 2(B). Similarly, the fluctuation in the fine-grained region is more remarkable than that in the amorphous region in Fig. 2(D), which is attributed to the composition difference of different crystal states. Based on the EDS results, there are more Si and P in the intergranular, and more Ti and Al in the grains, in agreement with the EDS results of MAO-0W.
Fig. 2. Elemental distribution of coarse-fine grain (A, B) and crystal-amorphous (C, D) interfaces.
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