## 3 Results and Discussion  
Figure 3 shows the X-ray diffraction pattern of the multilayer gradient coatings, demonstrating that their crystal structure consists entirely of non-transformable (metastable) tetragonal phase of zirconia (JCPDS: # 01-081-1545). According to the X-ray diffraction pattern of YSZ powder (Figure 1), the original material contained both monoclinic and tetragonal phases of zirconia. However, during the detonation spraying process, a phase transition from the monoclinic phase to the tetragonal phase occurs (Figure 3), which is confirmed using X-ray phase analysis. This transition is caused by the high heating  
temperature of the particles during the spraying process and their subsequent rapid cooling during deposition on the substrate, which leads to the stabilization of the tetragonal phase in the coating structure. The phase behavior of the material is determined by extreme temperatures exceeding 2000 \({}^{\circ}\)C, which is significantly higher than the stability limit of the monoclinic phase. The monoclinic phase of ZrO\({}_{2}\), which is stable at room temperature, loses its stability when high temperatures are reached. Under detonation spraying conditions, where the particle temperature can instantly increase to more than 2000-3000 \({}^{\circ}\)C, the atoms in the crystal lattice begin to exhibit greater mobility, which contributes to the destruction of the energy barriers holding the monoclinic phase. As a result, Zr and O atoms begin to redistribute into a more symmetrical and stable tetragonal structure, which is more stable at high temperatures. Another key factor ensuring the stabilization of the tetragonal phase is the rapid cooling of ZrO\({}_{2}\) particles that occurs after their impact on the substrate. This cooling occurs several times faster than under conventional heat treatment conditions, which prevents the material from returning to the monoclinic state. As a result, the tetragonal phase is \"frozen\", which, despite its thermodynamic instability at low temperatures, remains stable at room temperature. The tetragonal phase of ZrO\({}_{2}\) is characterized by high symmetry and a less distorted crystal lattice compared to the monoclinic phase. These features give the material increased crack resistance and durability, especially in conditions of heat-protective coatings, where the material experiences both high temperatures and mechanical loads. X-ray diffraction patterns of the coatings reveal differences in the intensity of phase reflections corresponding to the tetragonal phase of ZrO\({}_{2}\). In sample No. 1, the peaks of the tetragonal phase have a higher intensity, which may indicate its higher concentration in the crystal structure. In sample No. 2, on the contrary, a decrease in the intensity of these peaks is observed, especially in the intervals 20 \(\approx\) 33-35\({}^{\circ}\), 49-52\({}^{\circ}\), and around 60\({}^{\circ}\). This weakening may be associated with the increased porosity of sample No. 2, which affects the number and structure of defects in the material, as well as its diffraction characteristics. The analysis of the ratio of the crystal lattice parameters (c/a\(\surd\)2) allows us to distinguish between the t-ZrO\({}_{2}\) and t'-ZrO\({}_{2}\) phases, since the degree of tetragonality directly reflects their structural features. As shown in a number of studies [44], the non-transformable t'-ZrO\({}_{2}\) phase is characterized by a reduced tetragonality value (about 1.010 and below), whereas for the transformable t-ZrO\({}_{2}\) phase, this parameter is higher. Table 3 presents the tetragonality values obtained for the studied multilayer gradient coatings, which indicate the predominant content of the t'-ZrO\({}_{2}\) phase (the values are 1.008-1.009). Figure 3: X-ray diffraction patterns of multilayer gradient coatings. Figure 4 shows the cross-sectional microstructure of multilayer gradient coatings obtained using SEM.