## 1 Introduction  
Perfect properties of ZrO\({}_{2}\)-based ceramics (zirconia) such as high bending strength and fracture toughness, as well as their high chemical stability and biocompatibility [1; 2], allow these ceramics to be widely used in the manufacturing of various high-temperature components and units [3; 4; 5; 6; 7], as well as in chemical industry [8; 9; 10], energy generation [11; 12], and biomedical applications, in particular, for the manufacture of dental and other implants [13; 14] and prostheses [15; 16; 17]. In dentistry, zirconia ceramics are used for the manufacture of frameworks for dental prostheses, monolithic restorations [18; 19; 20; 21; 22; 23], as well as abutments for implants, inlay- and onlay-bridges, and single crowns [24]. Previously published results [1; 25; 26; 27] showed that transformation strengthening of zirconia occurs as a result of the transformation of the tetragonal (t) phase into the monoclinic (mom) one under significant stresses, especially in the crack tip vicinity, which effectively decelerates the advancing cracks. Therefore, improved mechanical properties of the ceramics can be ensured due to the tetragonal phase present in the ceramic microstructure. For stabilizing the metastable tetragonal phase at ambient temperature, zirconia ceramics are doped with the following stabilizing oxides: Y\({}_{2}\)O\({}_{3}\)[28; 29; 30], CeO\({}_{2}\)[31; 32], CaO [33], and MgO [34]. In the zirconia-based ceramics containing 5.29 wt.% Y\({}_{2}\)O\({}_{3}\) and 0.005 wt.% Al\({}_{2}\)O\({}_{3}\), non-uniform distribution of Y\({}_{2}\)O\({}_{3}\), grain growth during sintering at elevated temperatures, and significant percentage of the cubic (c) zirconia phase, formed under these conditions, were found to retard the tetragonal-monoclinic (t-m) transformation and, as a result, transformation strengthening of the material [18]. Numerous works [1; 25; 26] indicated that zirconia ceramics stabilized with 2-3 mol\(\%\) Y\({}_{2}\)O\({}_{3}\) contain a significant fraction of the tetragonal phase and, therefore, show the highest tendency towards transformation strengthening. Ceramics, the microstructure of which is formed by 0.3-0.4 \(\upmu\)m grains of the tetragonal phase, are widely utilized in dentistry [2]. In tetragonal zirconia stabilized with yttria (Y-TZP) after sintering for 2 h at 1550 \({}^{\circ}\)C, the grain size was 0.3-0.7 \(\upmu\)m [35]. In Reference [17], it was shown that the t-ZrO\({}_{2}\) phase became more stable with decreasing grain size. When the average size of grains in the microstructure decreases to values smaller than the critical one, namely 0.3 \(\upmu\)m, the t-ZrO\({}_{2}\) phase is stabilized at ambient temperature [36]. Based on the study of Y-TZP ceramics [20], variations in the sintering mode were shown to affect the material microstructure and properties. In particular, a change in the exposure time can affect the grain size. This, in turn, causes a change in strength of these ceramics. The flexural strength of ceramics sintered for 60 min at 1350 \({}^{\circ}\)C was found to be 1080 MPa, and this of ceramics sintered for 180 min at 1550 \({}^{\circ}\)C increased to 1604 MPa. In the first case, a fine microstructure with grains ranged within 0.1-0.4 \(\upmu\)m was observed. However, in the second case, the prevailing grain size was 0.5-0.8 \(\upmu\)m, with a certain percentage of much larger grains with up to 0.9-1.3 \(\upmu\)m size. Such grain growth caused corresponding changes in mechanical behavior of the material. The influence of the percentage of ZrO\({}_{2}\) on fracture toughness and strength of zirconia-tughened alumina was investigated in Reference [37]. The studied specimens comprised the \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) phase and the tetragonal and monoclinic ZrO\({}_{2}\) phases. With an increase in the total ZrO\({}_{2}\) percentage, the fraction of the t-ZrO\({}_{2}\) phase decreased. Growth of the Al\({}_{2}\)O\({}_{3}\) crystals was efficiently inhibited with the addition of 10-20% ZrO\({}_{2}\).