## 1 Introduction
This, in turn, improved mechanical properties of the materials. According to [38], both the fracture toughness and strength should be determined for a complete characterization of mechanical behavior of zirconia-based ceramics. In our earlier works [39; 40], the mechanical behavior of YSZ ceramics stabilized by 3-5 mol\(\%\) Y\({}_{2}\)O\({}_{3}\) was investigated with respect to the material composition and phase balance. The highest fracture toughness was revealed for 5 YSZ ceramics sintered for 2 h at 1450 \({}^{\circ}\)C. For this ceramic material, the correlation between fracture toughness and a percentage of the t-ZrO\({}_{2}\) phase was found [39]. In the work [40], mechanical properties of YSZ ceramics doped with 6-8 mol\(\%\) Y\({}_{2}\)O\({}_{3}\) were investigated in relation to the sintering mode and phase balance. 7 YSZ ceramics sintered at 1600 \({}^{\circ}\)C for 2 h exhibited the highest crack growth resistance. This corresponds to the previously published results [41]. In the work [42], the effect of the addition of 3 to 8 mol\(\%\) Y\({}_{2}\)O\({}_{3}\) to ZrO\({}_{2}\) on microstructure and mechanical behavior of YSZ ceramics sintered for 2 h in argon at 1550 \({}^{\circ}\)C has been investigated. The correlations between the yttria percentage, grain size distribution, phase balance, and strength were analyzed. It was revealed that 7 YSZ ceramic had the highest strength. Such excellent characteristics are provided by m-ZrO\({}_{2}\) and t-ZrO\({}_{2}\) fine-grained microstructure. A number of works [18; 43; 44] indicated that the sintering temperature of zirconia-based ceramics can significantly affect their porosity, grain size, and the resulting mechanical properties. In Reference [43], the zirconia compacts containing 0 mol\(\%\) Y\({}_{2}\)O\({}_{3}\) exhibited a completely m-ZrO\({}_{2}\) structure already after pressing; although for ceramics with 1.5 mol\(\%\) Y\({}_{2}\)O\({}_{3}\), a completely t-ZrO\({}_{2}\) structure was obtained only after sintering at 1100 \({}^{\circ}\)C. For zirconia ceramics containing 8 mol\(\%\) Y\({}_{2}\)O\({}_{3}\), a completely c-ZrO\({}_{2}\) structure was obtained during pressing. After pressing, pores with a prevailing diameter of 10 nm were observedin all the compacts, and the maximum pore diameter reached 12.5 nm. Powder compacts containing 0 mol% Y\({}_{2}\)O\({}_{3}\) (only the m-ZrO\({}_{2}\) phase) exhibited an increase in the pore diameter when the temperature rose to about 600 \({}^{\circ}\)C. For the higher temperatures (up to 1100 \({}^{\circ}\)C), significant grain growth and decreased porosity were observed, and sample compaction occurred. Similarly, powder compacts containing 1.5 mol% Y\({}_{2}\)O\({}_{3}\) and 8 mol% Y\({}_{2}\)O\({}_{3}\) showed an increase in the pore diameter when the sintering temperature increases to about 600 \({}^{\circ}\)C. For the higher temperatures (up to 1100 \({}^{\circ}\)C), slight growth of grains, as well as gradual closing of pores and significant densification of the sample, was observed. In Reference [44], the powder compacts of ZrO\({}_{2}\)-3 mol% Y\({}_{2}\)O\({}_{3}\) were characterized by a uniform distribution of pores with a diameter of less than 8 nm. After sintering (1070 \({}^{\circ}\)C, 2 h), very high-density ceramics (grain size of about 80 nm) exhibiting defect-free microstructure were obtained. The absence of defects was explained by a fairly low sintering temperature. The aim of this study was to detect correlations between the sintering temperature, yttria percentage, phase composition, microstructure features, and mechanical characteristics of YSZ ceramics. While our previous works on YSZ ceramics had mainly been focused on the effect of yttria percentages (3-5 mol%, 6-8 mol%), this one, in contrast to them, explores changes in the microstructure of YSZ ceramics with a gradual increase in sintering temperature. Thus, this work became crucial in determining optimal sintering mode for providing high mechanical strength of material.