## 3 Results and Discussion  
### W-TCP Eutectic  
A remarkable feature of the W-TCP eutectic grown by the laser floating zone technique is the possibility of obtaining glass, glass-ceramic, or crystalline microstructures depending on the growth conditions, which enables them to be used for desired application because of their different biological activity and mechanical behavior. Crystalline materials can be obtained at low growth rates of 20 mm/h. Its appearance is opaque, white, and with a tendency to crack due to high thermal gradients generated upon solidification. Vitreous cylinders can be obtained at speeds above 150 mm/h, adjusting the power of the laser beam to avoid the nucleation of crystallites in the glass. To prevent the precipitation of crystalline phases, the melt must be cooled sufficiently fast defining a minimum cooling rate. At solidification rates between 75 and 150 mm/h phosphate crystalline phases nucleate in a glassy matrix of calcium silicate producing a glass-ceramic material. Scanning electron microscope images of the samples grown at 20 mm/h are presented in Figure 1. The volume fraction of W is 0.61 and the phases are arranged in colonies attached to each other and formed by alternating lamellae. Figure 1: SEM Images of a transversal (**a**) and longitudinal (**b**) cross-section of a W-TCP rod grown by LFZ at 20 mm/h. The sample surfaces were etched with dilute acetic acid to remove the TCP phase. Figure 2 presents the microstructure of a eutectic rod grown at 100 mm/h. The images were taken at the edge of the rod, and the phases are evenly distributed throughout the sample. Due to the greater thermal gradient generated with respect to the center of the sample, the size of the phases is smaller at the edge, but the morphology is the same. EDS analysis showed that the darker phase (52.3 vol%) corresponded to calcium silicate with a small amount of phosphorus in solution (Ca/Si = 1, Ca/P = 7.38) while the lighter phase was silicon calcium phosphate (Ca/P = 2.21; Ca/Si = 3.33 and Ca/P + Si = 1.33). The results of qualitative XRD analysis of the crystalline, glass-ceramic, and glass samples are shown in Figure 3. The XRD powder patterns show the presence of apatite and pseudo-wollastonite (ps-W) for the crystalline sample and apatite for the glass-ceramic sample. No diffraction peaks were detected in the eutectic glass. Figure 3: XRD patterns of the samples grown at 20 mm/h (crystalline), 100 mm/h (glass–ceramic), and 150 mm/h (glass). Figure 2: SEM Images of a transversal (**a**) and longitudinal (**b**) cross section of a W–TCP rod grown by LFZ at 100 mm/h. Lighter phase corresponds to silicon calcium phosphate and darker phase to calcium silicate. We can conclude that at high solidification rates, the eutectic material has an amorphous structure, at lower speeds an apatite crystalline phase precipitates, and at slower speeds two crystalline phases are formed, apatite and pseudo-wollastonite. The glass-ceramic microstructure with crystalline phases of apatite in a glassy phase of calcium silicate is in good concordance with the devitrification studies of wollastonite-tricalcium phosphate eutectic glass carried out by [PERSON] et al. [13]. These authors reported that the devitrification starts with the precipitation of apatite crystals from the glass followed by the crystallization of wollastonite. Dimensionless parameter \(\gamma=\mathrm{T_{x}}/(\mathrm{T_{g}}+\mathrm{T_{I}})\) firstly proposed by [PERSON] and [PERSON] [14] was used an indicator of the glass-forming ability of the system, where \(\mathrm{T_{x}}\) is the crystallization temperature, \(\mathrm{T_{g}}\) the glass transition temperature, and \(\mathrm{T_{I}}\) the liquidus temperature.