## 3 Results and Discussion
### Surface Structuring of MgO-MgSZ Eutectic by Laser Remelting and Phase Dissolution in SBF  
In this section, we explore two approaches of surface structuring MgO-MgSZ eutectic: surface laser remelting and dissolution of one of the eutectic phases by immersion of the material in SBF. The strength of a eutectic ceramic can be improved by minimizing the structural dimensions, in terms of lamellar or fiber spacing. Laser remelting consists in a surface solidification technique with high temperature gradients that significantly increases the nucleation rates, resulting in refined feature structures and hopefully with improved  
Figure 7: FESEM micrographs of transverse cross-sections of samples grown at 150 mm/h (a,b), 300 mm/h (c,d), and 750 mm/h (e,f), in the center and in the edge, respectively. mechanical properties. In our case, we used the laser floating zone set up creating a superficial molten zone that is translated along the eutectic rod. The penetration of the laser-treated layer in the rod can be feasibly controlled by adjusting solidification rate by the moving laser radiation, and the laser power. The fully eutectic microstructures with lamellar and nano-sized morphology can be observed in Figure 8. Their lamellar spacing was about 1 \(\upmu\)m in as-solidified eutectic (noted as AS) and 150 nm in the laser remelted layer (noted as LRL), indicating that laser remelting can significantly refine the structure and provides a promising increase in mechanical strength. The lamellar structure in AS is anisotropic and ordered, and in LRL the lamellae were organized in cells oriented in the laser direction. No phase segregation, crack, or porosity were found on the surface and inside of the sample. In Figure 9, a cross-section of a lamellar MgO-MgSZ eutectic rod after soaking in SBF at 37 \({}^{\circ}\)C for 4 weeks is shown. The microanalyses performed by EDS confirmed that the composition of the layer in contact with the liquid is Mg\({}_{0.13}\)Zr\({}_{0.86}\)O\({}_{1.73}\), and the composition in the inner zone corresponds to the eutectic MgO-MgSZ. The liquid has penetrated about 80 microns into the sample, dissolving the MgO phase and generating an MgSZ skeleton whose sheets tend to collapse. This last fact hinders the possibility of achieving a surface with a controlled porous topography. Better control can be achieved with the use of eutectic samples that have been subsequently remelted. The fast dissolution of MgO phase in a physiological environment has already been described by [PERSON] et al. [9] and used to generate in situ porous scaffolds. In this case, the amount of released magnesium ions corresponds to the difference in magnesium composition in zones 1 and 2 of Figure 8.  
The cylinders that have been remelted on their surface showed the same behavior, but the smaller surface pore size allowed the topography to be maintained once the MgO was dissolved in the medium, as shown in Figure 10. In practice, the control of the melt volume is carried out by regulating the laser power and the translation speed of the cylinder, which also determines the phase spacing. Figure 10 corresponds to the cross section of a cylinder treated with a laser at three different powers moving the rod at 100 mm/h. The depth of the resulting layers was 2, 8.5, and 14 \(\upmu\)m for laser powers of 15, 25, and 30 W, respectively. The phase size of the treated layer determines the topography and the size and distribution of pores on the resulting surface after immersion in SBF. Figure 8: Microstructure of the longitudinal cross section of the as-solidified MgO–MgSZ eutectic grown at 50 mm/h and subsequently remelted with laser at 100 mm/h and rotation of 50 rpm. Except for the dissolution of MgO with the consequent generation of a porosity of about 28%, no change in the surface of the sample was observed. The release of Mg ions and the inert character of zirconia do not favor the formation of a HAp layer on the surface of the sample.