## 4 Discussion
The surface segregation of alloys is a well-known phenomenon caused by the difference in surface energy between the components of the alloy and the heat of solution of the minority element into the bulk of the majority component [26]. In the LAB process, the Cr-rich layer segregated to the surface was oxidized into Cr\({}_{2}\)O\({}_{3}\) (Figures 3 and 9), most likely due to the incorporation of oxygen atoms from water in the CP solution. In contrast, the underlying Co-rich (Cr-deficient) alloy layer was not oxidized into cobalt oxides (Figure 9). This can be attributed to the disconnection between the Co-rich alloy layer and water (by the upper Cr-rich layer), as well as the higher ionization potential of Co (7.88 eV) than that of Cr (6.77 eV). Ultrastructural analysis revealed that the CaP layer formed on the LAB-processed Co-Cr alloy substrate was composed mainly of ACP and loaded with a large number of CoCr\({}_{2}\)O\({}_{4}\) nanoparticles. A putative mechanism for the formation of this nanocomposite layer can be described as follows. The CP solution is a metastable supersaturated CaP solution that remains transparent without inducing homogeneous CaP nucleation for a certain period. In such CaP solutions, nano-sized prenucleation CaP clusters can be present similar to solutes [27; 28]. In the LAB process, these CaP nanoclusters may become densified to form post-critical amorphous nuclei at the substrate surface heated, modified and activated by laser irradiation. The ACP nuclei formed on the substrate surface grow further with irradiation time up to 30 min by the incorporation of CaP clusters and ions in the CP solution. While these reactions, CoCr\({}_{2}\)O\({}_{4}\) nanoparticles are formed via laser ablation of the Co-Cr alloy substrate. Nanoparticle formation via laser ablation of target materials has been reported elsewhere [29]. It has also been reported that Co-Cr binary clusters along with Cr\({}^{+}\) and Co\({}^{+}\) clusters are formed via laser ablation of a Co-Cr target [30]. These clusters may be involved in the formation of the CoCr\({}_{2}\)O\({}_{4}\) nanoparticles in the LAB process. The CoCr\({}_{2}\)O\({}_{4}\) nanoparticles which were formed from the LAB-processed substrate were coprecipitated with CaP on the surface during the LAB process, which results in the formation of a nanocomposite layer that consists of a CaP matrix and dispersed CoCr\({}_{2}\)O\({}_{4}\) nanoparticles. In previous studies using other substrates, CaP layers formed on the LAB-processed (30 min) substrates contained crystalline CaP compounds, i.e., apatite and/or OCP [14; 18; 19]. This is natural considering the thermodynamic stability of CaP phases in neutral aqueous solution, i.e., initially formed non-crystalline CaP transforms spontaneously into the most stable crystalline apatite through transient CaP phases such as OCP [23; 31]. However, in the case of the Co\(-\)Cr alloy substrate, the CaP layer formed on the surface retained the amorphous structure even after 30 min of the LAB process (Figures 3 and 8). Furthermore, this CaP layer contained many micron-sized, irregularly shaped pores (Figures 6 and 7). These were new phenomena specific to the Co\(-\)Cr alloy substrate found in this study. The mechanism underlying these phenomena is not clear. However, we consider that the CoCr\({}_{2}\)O\({}_{4}\) nanoparticles immobilized within the CaP layer contributed to these phenomena by acting as a light-absorbing agent. According to previous reports, light-absorbing agents (metallic ions [32] and carbon [33]) immobilized on/in CaP particles dispersed in an aqueous solution absorb pulsed laser light, thereby heating the CaP matrix above the melting point [15]. In pulse intervals, the melted CaP droplets are quenched by the surrounding solution and solidified while retaining their amorphous structure. Such a laser process (known as a pulsed laser melting in liquid process [34]) has been used to produce a variety of submicron-sized spheres.