## 4 Conclusions  
Investigation of the microstructure-properties relationship and fracture behavior in the ZrO\({}_{2}\) ceramics stabilized with 3 and 6 mol% Y\({}_{2}\)O\({}_{3}\) (3 YSZ and 6 YSZ) and sintered at 1450 \({}^{\circ}\)C, 1500 \({}^{\circ}\)C, and 1550 \({}^{\circ}\)C for 2 h has led to the following conclusions:  
1. For the 3 YSZ ceramic, with an increase in sintering temperature from 1450 to 1550 \({}^{\circ}\)C, the porosity decreased (in particular, the pore number density decreased by 2.2 times and their area fraction by 1.4 times), and the number density of fracture voids decreased by 3.6 times. However, at 1550 \({}^{\circ}\)C, high area fractions (up to 15%) of coarse grains in the \(>\)2.4 \(\upmu\)m size range were generated, which was consistent with an increased cleavage facet size. After sintering at the intermediate temperature of 1500 \({}^{\circ}\)C, the total percentage of relatively small cleavage facets (from 2 to 10 \(\upmu\)m\({}^{2}\)) reached 90%, the cleavage fracture area was reasonably low (13%), and the flexural strength was high (800 MPa). This resulted from an optimum phases balance consisting of 60% m-ZrO\({}_{2}\) and near 20% of each t-ZrO\({}_{2}\) and c-ZrO\({}_{2}\). Therefore, we propose sintering at 1500 \({}^{\circ}\)C for 2 h as a promising technology mode for the 3 YSZ ceramic.
2. For the 6 YSZ ceramic, with an increase in sintering temperature from 1450 to 1550 \({}^{\circ}\)C the porosity decreased (in particular, the pore number density decreased by two times and their area fraction decreased by five times), the number density of fracture voids decreased by 2.4 times, the total percentage of relatively small cleavage facets (from 2 to 10 \(\upmu\)m\({}^{2}\)) reached 96%, and the cleavage area decreased to a minor value of 9%. However, the temperature of 1550 \({}^{\circ}\)C resulted in a lower fraction of coarse grains (\(>\)2.1 \(\upmu\)m), probably the m-ZrO\({}_{2}\) phase, and a significantly increased the fraction of medium t-ZrO\({}_{2}\) grains (0.9-1.5 \(\upmu\)m), as compared to 1450 \({}^{\circ}\)C for which a high fraction of small t-ZrO\({}_{2}\) grains (0.3-0.6 \(\upmu\)m) and a quite high fraction of coarse m-ZrO\({}_{2}\) grains (1.5-2.4 \(\upmu\)m) was found. Therefore, an increase in sintering temperature of the 6 YSZ ceramic above 1450 \({}^{\circ}\)C did not produce a substantial improvement in strength. This could be explained by a stronger stabilization of the crystal structure with an increased Y\({}_{2}\)O\({}_{3}\) content at 1450 \({}^{\circ}\)C leading to an \"optimum\" phase balance with \(\sim\)50% m-ZrO\({}_{2}\), 35% t-ZrO\({}_{2}\), and 15% c-ZrO\({}_{2}\).
3. 3 YSZ sintered at 1550 \({}^{\circ}\)C showed about 17% higher strength than 6 YSZ sintered at 1450 \({}^{\circ}\)C. Thus, lower concentrations of Y\({}_{2}\)O\({}_{3}\) might be positively considered from the mechanical properties point of view. However, a higher cost of energy for sintering at higher temperatures may facilitate adoption of materials with a higher Y content processed at lower temperatures. Due to the comparatively high strength, the investigated samples can find potential practical applications in manufacturing prostheses and dental implants.  
Conceptualization, V.K. and Z.D.; experiment, A.K., B.V. and O.M.; analysis, O.M. and A.K.; writing--original draft preparation, B.V., V.K. and A.K.; writing--review and editing, V.K., A.K, Z.D. and B.V.; supervision, B.V. and V.K.; funding acquisition, V.K., B.V. and Z.D. All authors have read and agreed to the published version of the manuscript.  
This study was supported by the Ministry of Education and Science of Ukraine under the project number 0122U000952, \"Development of a scientific basis for the creation of multifunctional oxide ceramic materials and coatings\".  
Not applicable.  
All the supporting and actual data are presented in the manuscript.