## 3 Results and Discussion
### W-TCP Coatings on MgO-MgSZ Rods  
In the previous section, physical modifications in eutectic MgO-MgSZ cylinders have been analyzed by modifying the surface topography as well as chemical changes associated with the dissolution of one of the phases. In this section, another well-established chemical technique for surface functionalization is considered--the coating of the substrate surface to create a specific chemical environment that offers a favorable biological response. A layer of W-TCP ceramic in the eutectic composition was placed by dip coating on the surface of a MgO-MgSZ eutectic rod, sintered, and subsequently melted by means of the modified LFZ technique to integrate the coating on the eutectic rod. The power of the laser and the rod movement were adjusted to melt the entire layer with a minimum dilution with the substrate. Deliberate conditions were used to obtain a bioactive glass layer on the fully dense body inside, with macro-size pores that could favor tissue integration. In this case, air was used as growth atmosphere, and a sufficiently high solidification rate and layer thickness to prevent the escape of all the bubbles formed in the liquid pool. Figure 11 shows a longitudinal (a) and transversal (b) cross-section of a W-TCP glass-coated MgO-MgSZ eutectic rod. The coating showed a clean interface suggesting good interfacial strength, as shown in Figure 11c. Accounting that the sample was not preheated, there were no visible cracks or delamination phenomena at the coating/eutectic interface, favored by the cylindrical symmetry of heat flow. It can be observed that small dendrites of MgO-MgSZ were distributed within the darker bioactive phase, close to the interface. From a mechanical behavior point of view, the hardness and toughness increase from the coating to the substrate. The [PERSON]'s moduli of substrate and coating are very different, E\({}_{\text{MgO-MgSZ}}\) = 285 GPa [23] and E\({}_{\text{w-TCP}}\) is estimated in 110 GPa, considering the [PERSON]'s modulus of the wollastonite, a tricalcium phosphate, 130 GPa [29], and 72 GPa [30], respectively. The [PERSON]'s modulus estimated for the coating is closed to the measured in other multiphase ceramics obtained from the melt [31]. The load transfer plays an important role in the success of an implant, and this is favored when the elastic properties are similar. Optimizing the composition, structure, and microporosity of the coating, the [PERSON]'s modulus can approach the one of the bone tissues (between 12 GPa and 28 GPa for the cortical bone [32]). The bioactive layer can be tailored to match a specific tissue healing rate by changing its structure (glass or crystalline) through modification of growth conditions, as described in Section 3.1. It is also possible to add certain elements (Li, Mg, Sr, Cu, Co, Mn, B, Si) to the coating composition that can be released, playing a role in angiogenesis, osteogenesis, and osteoconduction [33; 34]. The presence of zirconia dendrites in the coating increases when the composition of the substrate is deviated to the zirconia-rich side. Usually, the microstructure of one off-eutectic composition system consists of coarse primary crystals embedded by eutectic constituent. Figure 12a shows the microstructure of an off-eutectic MgO-MgSZ. The primary phase of MgSZ (bright phase) and the eutectic constituent can be seen clearly. The globular shape of zirconia phase shows a weak-faceted growth. Figure 12b corresponds to the transversal cross-section of an off-eutectic rod covered by a solidified layer of W-TCP. A detail of the coating and interface is presented in Figure 12c. The coating was 115 \(\upmu\)m thick and was formed by a bright phase of Ca\({}_{0,17}\)Mg\({}_{0,11}\)Zr\({}_{0,72}\)O\({}_{1,60}\) and a dark phase with composition Ca/Si = 3.58 and Ca/P = 2.48, close to the theoretical W-TCP eutectic of Ca/Si = 3 and Ca/P = 2.25. Figure 11: Cross section images of deposited bioactive coating—eutectic composite. Longitudinal (**a**) and transversal (**b**) section of the coated rod.