## 3 Results and Discussion
### Technology Optimization  
We have analyzed how the key parameters under study should be changed for providing high strength of the studied ceramics. In terms of lowering porosity, the sintering temperature for the 3 YSZ ceramic should be about 1500 \({}^{\circ}\)C, as the increase in both the pore number density and area fraction by 1.54 times was found with a decrease in sintering temperature from 1500 to 1450 \({}^{\circ}\)C. In terms of decreasing the cleavage facet size, the sintering temperature of 1500 \({}^{\circ}\)C would also be beneficial, as the total percentage of relatively small cleavage facets (from 2 to 10 \(\upmu\)m\({}^{2}\)) exceeds 90% for this temperature. An increase in sintering temperature to 1550 \({}^{\circ}\)C generates unusually high area fractions (up to 15%) of coarse grains in the >2.4 \(\upmu\)m size range. This was confirmed by a slightly increased cleavage facet size compared to that for the 3 YSZ ceramic sintered at 1500 \({}^{\circ}\)C. These larger grains exhibit a monoclinic crystal structure with a remainder of the c-ZrO\({}_{2}\) or t-ZrO\({}_{2}\) phase enriched with yttrium [39; 40; 41; 42]. In spite of the highest strength observed after sintering at 1550 \({}^{\circ}\)C and the fracture growth mainly occurring along the boundaries of the fine-grained t-ZrO\({}_{2}\) agglomerates [39; 40; 42], the transgranular fracture of coarse m-ZrO\({}_{2}\) grains may deteriorate toughness. The lower sintering temperature (1500 \({}^{\circ}\)C) providing a higher fraction of relatively small cleavage facets allows this issue to be solved. In terms of lowering porosity, the sintering temperature for the 6 YSZ ceramic should be about 1550 \({}^{\circ}\)C, as the increase in the pore number density and area fraction by 2.58 and 4.8 times, respectively, were found with a decrease in sintering temperature from 1550 to 1500 \({}^{\circ}\)C. A decrease in the cleavage facet size for the 6 YSZ ceramic was also revealed after sintering at 1550 \({}^{\circ}\)C, and the total percentage of relatively small cleavage facets (from 2 to 10 \(\upmu\)m\({}^{2}\)) exceeded 96% for this condition. In all the other cases, i.e., sintering at 1450 \({}^{\circ}\)C and 1500 \({}^{\circ}\)C, the total percentage of relatively large cleavage facets increased from 20% to 29%. With respect to grain sizes, higher area fractions were observed for medium grains (of 0.9-1.5 \(\upmu\)m size) after sintering at 1550 \({}^{\circ}\)C, compared to sintering at 1500 \({}^{\circ}\)C and 1450 \({}^{\circ}\)C. According to our previous works [39; 40; 42], these are t-ZrO\({}_{2}\) grains. An increase in area fractions was revealed for larger grains, probably the m-ZrO\({}_{2}\) phase, especially from 2.4-2.7 \(\upmu\)m to \(>\)2.7 \(\upmu\)m and from 1.5-1.8 \(\upmu\)m to 2.1-2.4 \(\upmu\)m, for 6 YSZ ceramics sintered at 1500 \({}^{\circ}\)C and 1450 \({}^{\circ}\)C, respectively. Larger fractions of coarser grains correlate with the highest strength of this material. Therefore, in the cases of 1450 \({}^{\circ}\)C up to 1500 \({}^{\circ}\)C sintering temperatures, a slightly increased cleavage facet size confirms the implementation of the high energy-consuming fracture micromechanism [42] similar to that described above for the 3 YSZ ceramic sintered at 1550 \({}^{\circ}\)C. This micromechanism based on dominant fracture along the boundaries of the fine-grained t-ZrO\({}_{2}\) agglomerates, also includes cleavage fracture of the larger grains with a monoclinic crystal structure and a remainder of the c-ZrO\({}_{2}\) or t-ZrO\({}_{2}\) phase enriched with yttrium [39; 40; 42]. In contrast, the high amount of relatively small cleavage facets formed in the 6 YSZ ceramic sintered at 1550 \({}^{\circ}\)C does not provide the complete implementation of such fracture micromechanism. Therefore, the sintering modes providing high mechanical strength (800 MPa and higher) of YSZ ceramics were determined.