## 3 Results and Discussion
### Strength and Analysis of Fracture Surface
Based on the cleavage facet size distribution analysis for 3 YSZ ceramic (Figure 10a), it was revealed that with a decrease in sintering temperature from 1550 through 1500 to 1450 \({}^{\circ}\)C: (i) fraction of cleavage facets in the 2-4 \(\upmu\)m\({}^{2}\) size range tends to increase; (ii) in all the other size ranges, no unambiguous variation of the fractions was found. That is, a distinct difference was found for 3 YSZ ceramic sintered at 1500 \({}^{\circ}\)C, as it exhibits two average levels of the fractions, namely 18-26% for the size ranges of 2-4 \(\upmu\)m\({}^{2}\), 4-6 \(\upmu\)m\({}^{2}\), 6-8 \(\upmu\)m\({}^{2}\), and 8-10 \(\upmu\)m\({}^{2}\) (relatively small cleavage facets) and about 3% for the size ranges of 10-12 \(\upmu\)m\({}^{2}\), 12-14 \(\upmu\)m\({}^{2}\), and >14 \(\upmu\)m\({}^{2}\) (relatively large cleavage facets). With an increase in the sintering temperature to 1550 \({}^{\circ}\)C, a shift in the fractions of cleavage facets towards larger cleavage facets is observed. Similar behavior with a shift in the fractions of cleavage facets towards larger cleavage facets was revealed when decreasing the sintering temperature to 1450 \({}^{\circ}\)C. However, a detailed analysis of fracture surfaces of tested specimens showed that at the lowest temperature of 1450 \({}^{\circ}\)C, fracture had occurred along the boundaries of separate clusters (Figure 8c). Therefore, the identified in this case cleavage facets cannot be associated with \"classic\" transgranular fracture, as no recrystallized zirconia grain was found. A gradual decrease in fractions of cleavage facets (from 31.5% to 16%) with increasing the cleavage facet size (from 2-4 \(\upmu\)m\({}^{2}\) to 8-10 \(\upmu\)m\({}^{2}\)) was revealed for the 6 YSZ ceramic sintered at 1550 \({}^{\circ}\)C (Figure 10b). Moreover, a very small percentage (less than 6%) was found for larger cleavage facets. In the cases of sintering temperatures of 1450 \({}^{\circ}\)C and 1500 \({}^{\circ}\)C, the total fraction of large cleavage facets of 10-12 \(\upmu\)m\({}^{2}\) and >14 \(\upmu\)m\({}^{2}\) was much higher (from 20% to about 29%). Figure 9: Size distributions of fracture voids for (a) 3 YSZ and (b) 6 YSZ ceramics sintered at three temperatures. In short summary, with a decrease in sintering temperature from 1550 through 1500 to 1450 \({}^{\circ}\)C:  
* The average size of cleavage facets (corresponding to the fracture by the transgranular micromechanism) decreased by 11% in 3 YSZ ceramic; however, it increased by 21% in the 6 YSZ material (Table 1);
* Cleavage area decreased by 8.5 times in the 3 YSZ ceramic; however, it increased by 44% in the 6 YSZ material;
* No unambiguous variations of the fractions of cleavage facet distribution were found in the 3 YSZ and 6 YSZ ceramics with a decrease in sintering temperature from 1550 through 1500 to 1450 \({}^{\circ}\)C.  
Generally, among the parameters used in this work for characterizing microstructure, the \"effective\" ones should quantitatively correlate the microstructural constituents to mechanical properties of the YSZ ceramics:  
* The most distinct differences for the pore number density and area fraction in the 3 YSZ ceramic (both are increasing by a similar value of \(1.54=0.0724/0.0471\) and \(1.54=4.0/2.6\) times) were found with a decrease in sintering temperature from 1500 to 1450 \({}^{\circ}\)C. In the 6 YSZ ceramic, these parameters varied more (increased by 0.0516/0.02 = 2.58 and 2.4/0.5 = 4.8 times, respectively) with a decrease in sintering temperature from 1550 to 1500 \({}^{\circ}\)C (Table 1);
* The pore number density and area fraction of the 3 YSZ ceramic sintered at a temperature of 1550 \({}^{\circ}\)C are 0.0328/0.02 = 1.64 and 2.9/0.5 = 5.8 times higher, respectively, than those of the 6 YSZ ceramic sintered at the same temperature (Table 1);  
Figure 10: Cleavage facet size distributions for (**a**) 3 YSZ and (**b**) 6 YSZ ceramics sintered at three temperatures.