## 3 Results and Discussion
### Oxidation Kinetics
#### 3.2.2 Scanning Electron Microscopy (SEM)  
The morphologies of coated samples before and after oxidation for various periods of time in water at a supercritical temperature are displayed by SEM examination.  
Figure 5a-d shows the SEM images at magnifications of 10 kx and 50 kx obtained for NiCrAlY-coated samples before and after 720 h, 1440 h, and 2160 h of oxidation in water at 550 \({}^{\circ}\)C and 25 Mpa.  
To measure the thickness of the deposited layer, SEM in cross-section analyses were also carried out (Figure 6a-d).  
From the SEM micrographs shown in Figure 6a, we can see that the deposition of an initial layer of NiCrAlY is not very continuous, and its cross-section thickness, measured by SEM, was approximately 1.48 \(\upmu\)m. The deposition layer is granular, without visible defects (pores, cracks).  
With an increasing oxidation time, a slight increase in layer thickness is observed from 1.48 \(\upmu\)m to 2.02 \(\upmu\)m (the case of samples coated and oxidized 2160 h), which means that a thin, compact, and fairly uniform oxide layer has been deposited on the sample surfaces (see Figures 5b-d and 6b-d). A similar behavior, with comparable values of the oxide layer, was also obtained by other authors who studied the behavior at 1100 \({}^{\circ}\)C of some NiCrAlY coatings obtained by different methods, such as cathode arc evaporation [40], magnetron sputtering [40; 41], hybrid arc/sputtering deposition [40], arc ion plating [42; 43], or spraying with high-velocity oxygen fuel [44]. In principle, based on the literature data, the deposited oxide layer is Cr\({}_{2}\)O\({}_{3}\), which, being extremely thin, did not allow for measurement with the electronic microscope. Therefore, the layer thickness values determined by SEM in the  
Figure 5: SEM micrographs at magnifications of 10 kx and 50 kx obtained for coated samples before and after oxidation in water at high temperatures for (**a**) 0 h; (**b**) 720 h; (**c**) 1440 h; and (**d**) 2160 h.  
cross-section represent the thickness of the NiCrAlY layer deposited by EB-PVD and the thicknesses of the oxide formed following oxidation.  
Even if the test conditions are not like those used in this study, it can be assumed that the mechanism of the oxidation process is comparable to that proposed in other works from the literature [40; 42; 45] and that it is composed of a rapid growth stage and stable oxidation stage. Fast oxidation is the first stage, in which the chromium and aluminum elements from the coating are oxidized rapidly under the influence of high partial pressure of oxygen, forming Al\({}_{2}\)O\({}_{3}\) and Cr\({}_{2}\)O\({}_{3}\), as demonstrated by XRD tests.  
\[4(Al,\ Cr)+3O_{2}=2(Al,\ Cr)_{2}O_{3} \tag{5}\]  
The development of the oxide layer significantly reduced the oxygen partial pressure at the coated surface. The literature demonstrates that Gibbs free energy predicts that the elements inside of the coating will oxidize in the following order: Y, Al, Cr, and Ni. As a result, the major reaction that occurs at the interface between the oxide layer and the coating can be identified as the following reaction:  
\[4 Al+3O_{2}=2 Al_{2}O_{3} \tag{6}\]  
Consequently, the oxide layer that developed on the surface of the coating at the end of the first oxidation step could be divided into the following two independent layers: an Al\({}_{2}\)O\({}_{3}\) underlying layer and a mixed oxide layer on the surface. Following the formation of the protective oxide layer on the surface, the initial coating reached a stable oxidation state, during which the underlying Al\({}_{2}\)O\({}_{3}\) layer primarily caused the thickness to increase as the oxide layer gradually grew. The material may have anti-corrosive qualities due to the aluminum oxide layer. Figure 7 shows the oxidation process of NiCrAlY coatings.  
Figure 6: Layer thicknesses at magnifications of 10 kx and 50 kx were obtained for coated samples before and after oxidation in water at high temperatures for (**a**) 0 h; (**b**) 720 h; (**c**) 1440 h; and (**d**) 2160 h.