## 4 Discussion  
From the XRD patterns (Figure 1), it was observed that at a higher concentration of ZrO2, the peaks associated with zirconia also increased by around 28-32\({}^{\circ}\) (20 value), which corresponded to the overlapping of the HAp-Zirconia peaks. Furthermore, no intermediate peaks were observed, which confirmed that either HAp does not decompose or that no interfacial reactions occurred between HAp and zirconia. Therefore, it was decided that the composition used for preparing the HAp-Zirconia composite (HZ1 sample) under hydrothermal conditions was the optimized condition. Therefore, only HZ1 was used further for the morphological, thermal, mechanical, and cytotoxicity analyses. The formation of bulk agglomerates was observed in the FESEM analysis due to the composite's primary nucleation (Figure 2c) [37]. However, under higher magnification, the sample surface was observed to have an attachment of small-sized freely dispersed particles due to secondary nucleation. According to the TEM results in Figure 2d, the size of the HZ1 composite powder was observed to be in the 15-20 nm range, which confirmed the formation of nanocomposites. HAp has an oval and rounded hexagonal morphology, and zirconia has a spherical morphology. The SAED pattern shown in Figure 2f confirmed the corresponding planes of the HAp (211) and zirconia (311) phases, as discussed in the XRD analysis, which reaffirmed the formation of the HAp-Zirconia composites. A proper distribution of zirconia on the surface of HAp was observed in the elemental mapping results (Figure 2g-k). This showed that the majority of the HAp served as the matrix and that trace amounts of zirconia served as the reinforcement. Further, the EDS spectrum (Figure 2l) revealed the ideal percentage of zirconia (<10%) added to the HAp matrix which prevented Hap's decomposition at higher temperatures [38]. The N2 absorption-desorption results (Figure 4) revealed the Type II isotherm that causes capillary condensation. The surface area, pore volume, and pore diameter were calculated as 19.319 m\({}^{2}\)/g, 0.030152 cm\({}^{3}\)/g, and 14.27 nm, respectively. This indicates that the multilayer adsorption of the prepared composites limits the high uptake of P/P0 [39]. However, the moderate surface area of the composite powders might help with molecular attraction for biological applications [40]. According to the TG/DSC results (Figure 3), a relative weight loss of 10% at around 30-100 \({}^{\circ}\)C was observed, which corresponded to the vaporization of the adsorbed water [27]. The second stage of weight loss of up to 15% was attributed to the removal of hydroxyl groups adsorbing on the surface of the zirconia [41]. The third stage of weight loss (up to 10%) corresponded to the release of chemically bound water and the breakdown of the organic matrix and carbonates; moreover, the phase transition of zirconia from the amorphous to the tetragonal phase occurs in this temperature region [42; 43; 44]. When zirconia was added, the exothermic peak associated with the thermal conductivity of HAp, which typically appeared at about 400 \({}^{\circ}\)C, was diminished [21]. The endothermic peak observed at 103 \({}^{\circ}\)C corresponded to moisture evaporation, while the exothermic peak observed at 290 \({}^{\circ}\)C was associated with the decomposition and degradation of organic residues. Additionally, the endothermic peak associated with the crystallization of HAp was obtained at a temperature of 517 \({}^{\circ}\)C. The sharpness of the endothermic peak indicated a high degree of crystallinity [45]. Furthermore, the results confirmed a respectable thermal stability for the as-prepared HAp-Zirconia composite. From the nanoindentation study (Figure 5), for the HAp sample, the obtained hardness was 0.8 GPa, which increased to 0.9 GPa with a Young's modulus of 8.19 GPa in the HAp-Zirconia composite sample. The results illustrated that the addition of zirconia increased the mechanical strength of the HAp matrix [30].