## 3 Results
### Ultrastructural Analysis of the Surface Layer  
The CaP layer formed on the LAB-processed substrate was approximately 1-2 \(\upmu\)m thick and contained submicron-sized pores. Cross-sectional TEM observations (Figure 6a) revealed the presence of a micron-thick layer with a rugged surface between the LAB-processed (30 min) substrate (bottom dark region) and the top W coat (upper black region) that was applied for surface protection for the FIB processing. The micron-scale rugged structure of the layer surface in the cross-sectional TEM image (Figure 6a) corresponds to the micrograin-like structure in the top-view SEM image (lower right image in Figure 2a). The micron-thick layer on the substrate was composed mainly of CaP, as revealed by the cross-sectional STEM-EDX results (Figure 7). STEM-EDX elemental maps (Figure 7b,c) showed that Ca (red), P (green) and O (dark blue) constituted the matrix of the micron-thick layer over the dense Co\(-\)Cr alloy substrate composed of Co (purple), Cr (light blue) and Mo (brown). Within the CaP layer, irregularly shaped submicron-sized pores were present (Figures 6a and 7a-c). According to the STEM-EDX spectrum (Figure 7d), the CaP layer also contained Cr and Co, although the distribution of Co in the CaP layer was not apparent in the elemental map (Figure 7b). The peak intensity of Cr with respect to that of Co in the STEM-EDX spectra (Figure 7d) was higher in the CaP layer than in the substrate. This observation was confirmed by quantitative STEM-EDX analysis at five different regions; the Cr/Co elemental ratio of the CaP layer was \(2.0\pm 0.2\), whereas that of the substrate was \(0.5\pm 0.0\). These results suggest that a Cr-rich compound different from the base Co\(-\)Cr alloy substrate was formed and contained in the CaP layer.  
The CaP layer contained a large number of CoCr\({}_{2}\)O\({}_{4}\) crystalline nanoparticles dispersed throughout the layer. The magnified TEM images of the CaP layer cross-section (Figure 6c,d) showed nanoparticles with diameters of a few tens of nanometers dispersed within the matrix layer. These nanoparticles have a crystalline structure according to the contrast in the dark-field TEM image (Figure 6d). Debye-Scherrer rings ascribed to CoCr\({}_{2}\)O\({}_{4}\) were observed in the selected area electron diffraction (SAED) pattern (Figure 8b) of the CaP layer (circled area in Figure 8a). A broad ring (marked as *) with a d-spacing of approximately 0.29 nm is ascribed to ACP [20]. Figure 8c shows a high-resolution TEM (HRTEM) image obtained from a nanoparticle, which could be ascribed to crystalline CoCr\({}_{2}\)O\({}_{4}\) through analysis  
Figure 6: (**a–c**) Bright-field and (**d**) dark-field transmission electron microscopy (TEM) images of the ultra-thin cross-section of the CaP layer formed on the LAB-processed (30 min) Co\(-\)Cr alloy substrate. The W top coat in (**a**) was prepared during focused ion beam (FIB) processing. The image in (**c**) corresponds to that in (**d**).  
of the fast Fourier transform (FFT) image (Figure 8d). From these results, the TF-XRD results (Figure 3) and the STEM-EDX results (Cr/Co elemental ratio = 2.0 \(\pm\) 0.2), the nanoparticles dispersed in the CaP layer were identified as CoCr\({}_{2}\)O\({}_{4}\).