## 3 Experimental Results  
The surface condition of the ceramic layers is shown in Figure 2a-c for all three cases of initially mixed powders. It can be seen that compact coatings were obtained, but also with characteristic pores from the atmospheric jet spraying technique. No significant differences were observed between the coatings. In the second case, when alumina-blended powders with 25% YSZ alumina were used for spraying, a slightly higher number of pores on the surface was observed, as shown in Figure 2b. The chemical analysis of the three ceramic coatings provided a qualitative identification of the deposited coating components (Zr, Y, O and Hf), as well as the identification of a small percentage of Fe (Figure 3--energy spectrum). The identification of the Hf element is attributed to zirconium, which is naturally a companion element and is more difficult to completely remove from the YSZ. The porous nature of the plasma-sprayed ceramic coatings and their partial bonding are confirmed by the appearance of the Fe element on the coated surface. The presence of Fe on the surface may also be due to partial melting of the steel substrate surface during deposition and its mixing with the ceramic material. The chemical elements showed a homogeneous distribution with no YSZ or Al agglomerates. The presence of iron is only confirmed in two or three areas on the surface, as shown in Figure 3.  
Figure 3: Energy dispersive spectrum of the coating surface (the same for all three samples) and the mapping of the main elements. Figure 2: SEM images of (**a**) sample 1, (**b**) sample 2 and (**c**) sample 3. The values given in Table 1 were obtained as the average of three surface determinations. The standard deviations of the elements showed a relatively small variation in the chemical composition of the ceramic layer. Large variations in the chemical composition, as well as the presence of pores in the ceramic layers, can affect the characteristics of the layers obtained, primarily the different mechanical properties, but also the corrosion resistance as a result of a different behaviour in these areas when in contact with an electrolyte solution. Roughness profiles of samples after APS deposition confirm the presence and spread of pores in the composed ceramic coatings [20]. An analysis of the Fe percentage on the surface can provide an indication of the ceramic layers' porosity and their ability to cover the metal substrate. Increasing the percentage of YSZ in the pre-powder mix reduced these uncovered areas by almost half the percentage of Fe identified on the surface. The ICDD PDF4+ 2022 database was used for qualitative phase analysis. Qualitative phase analysis indicated the presence of the following polycrystalline phases: \(\alpha\)-alumina (rhombohedral), gamma-alumina (cubic) and ZrO\({}_{2}\) (tetragonal). The main peaks identified are shown in Figure 4.  
The main identified compounds are a stable rhombohedral phase of alumina and a cubic form of alumina (\(\alpha\)- and \(\gamma\)-Al\({}_{2}\)O\({}_{3}\)), together with a tetragonal metastable phase of zirconia. A resonance in the occurrence of these phases is based on the extremely high solidification rate of the coatings (\(\sim\)106 K/s), characteristic of atmospheric plasma sputtering treatment. The results are in agreement with other reported works [25; 26]. Part of the \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) phase transforms into a solid solution of Al atoms that transition to ZrO\({}_{2}\) during the atmospheric plasma sputtering process, with aggregation of some Al atoms at grain boundaries forming the amorphous phase [27; 28].