The surface structure of antibacterial coatings plays an important role in biological behavior, and the microstructure and composition of the outer layer are first studied, as shown in Fig. 1. TEM images show obvious morphological differences between MAO-0W and MAO-4W, some dark areas are observed in the gray background of Fig. 1(A), while the gray-scale in Fig. 1(F) looks very uniform. Further, the SAED analysis in Fig. 1(C) confirms that the dark areas are poly-crystalline structure, and the SAED pattern of dim halos in Fig. 1(B) and(G) verifies that the gray background region is amorphous. These results indicate that the bilayer microstructure of MAO-0W is composed of an ultra- thin amorphous outward layer (～2 um) and a poly-crystalline inwardlayer. Unexpectedly, the outer layer of MAO-4W is all amorphous, and the formation of amorphous outer layer is ascribed to the ultra-fast cooling rate. The temperature of MAO electrolyte is kept below 30℃, whereas high temperature caused by plasma discharge during the MAO process leads to a great temperature difference at the solid-liquid interface [30]. Besides, it is clear that the micro-holes in Fig. 1(A) and (F) correspond to the channels left by plasma discharge.
The SAED pattern in Fig. 1(C) exhibits typical concentric diffraction rings with some bright diffraction spots embedded in them, which suggests the dark areas are poly-crystalline. The interplanar spacings calculated from the SAED pattern(Table 1) match well(110),(101),(211) and(002) crystallographic planes of TiO2,(023),(132) and (043) crystallographic planes of Al2TiOs,(041) crystallographic plane of Al2O3, respectively, in agreement with the XRD results in the present work. To observe elemental distribution in the amorphous and poly-crystalline areas, EDS analysis of points A,B(Fig. 1D) and C(Fig. 1F) is listed in Supplementary Table S1(Supporting Information). The results demonstrate the composition of amorphous and crystalline phases is different in MAO-0W. The amorphous phase contains more Si and P, while the crystal is mainly composed of Ti, Al, and extremelylimited Si and P. Thus, it means that most of the Si and P exist in amorphous phase. Further, the elemental content of amorphous C point in MAO-4W confirms this result. It should be noted that the EDS curves across the interface in Fig. 1(E) fluctuate drastically in the polycrystalline area, which is caused by the intercrystalline gaps. There are more Ti and Al in the grains and more Si and P in the gaps, which implies that the intergranular region is amorphous. Therefore, it can be reasonably deduced that the amorphous phase exists in the whole inner layer of MAO.
Table 1
Detailed identification of the diffraction rings corresponding to SAED in Fig. 2 (C).
Fig. 1. TEM results of the MAO outer layer. (A) FIB slice of MAO-0W. (B, C) SAED pattern of the amorphous and crystalline area in (A). (D) enlarged morphology of crystal area in (A). (E) EDS analysis of a selected line L. (F) FIB slice of MAO-4W, (G) SAED pattern of (F).
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