## 3 Results and Discussion  
### The Microstructure and Phase of the Coatings  
Figure 4 shows the XRD patterns of the NC and CC. The metastable \(\upgamma\)-Al\({}_{2}\)O\({}_{3}\) phase is the dominant phase of the alumina coatings. This is due to the molten Al\({}_{2}\)O\({}_{3}\) droplets dropping onto the cold substrate and cooling rapidly (about 10\({}^{6}\) K/s [31]) during the plasma-spraying process. Compared with \(\upalpha\)-Al\({}_{2}\)O\({}_{3}\), metastable \(\upgamma\)-Al\({}_{2}\)O\({}_{3}\) with lower critical free energy is more likely to nucleate during rapid solidification. In addition, to calculate the content of the \(\upalpha\)-Al\({}_{2}\)O\({}_{3}\) phase in the coatings, a semi-quantitative analysis was carried out by following Equation (1) for NC and CC [32]. \[C_{\upalpha}=\frac{I(104)+I(113)+I(116)}{I(104)+I(113)+I(116)+I(311)+I(400)+I( 440)} \tag{1}\]  
where \(C_{\upalpha}\) represents the content of the \(\upalpha\)-Al\({}_{2}\)O\({}_{3}\) phase and \(I\) denotes the area of the peak. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Composition** & **Si** & **Cu** & **Mg** & **Zn** & **Mn** & **Ti** & **Ni** & **Fe** & **Al** \\ \hline Percentage, \(w/w\) & \(\leq\)0.5 & 3.8–4.9 & 1.2–1.8 & \(\leq\)0.3 & 0.3–0.9 & \(\leq\)0.15 & \(\leq\)0.1 & \(\leq\)0.5 & Bal \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical compositions of the 2A12 aluminum. \begin{table}
\begin{tabular}{c c c} \hline
**Parameters** & **NiCrCoAlY** & **Al\({}_{2}\)O\({}_{3}\)** & **Unit** \\ \hline Plasma gas (Primary-Argon) & 50 & 41 & NLPM \\ Carrier gas (Secondary-Hydrogen) & 9 & 14 & NLPM \\ Arc current & 530 & 580 & A \\ Arc voltage & 65 & 71 & V \\ Spray distance & 130 & 120 & mm \\ Powder feed rate & 30 & 20 & g/min \\ \hline \end{tabular}
\end{table} 
Table 2: Process parameters of plasma spraying. The results show that the \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) phase in NC (21.4%) is much higher than that in CC (12.7%). It was reported that the \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) phase in these coatings is mainly derived from unmelted particles during spraying [33]. Compared with the micron-sized alumina powder, the granulated nano-alumina powder produced via atomization has a lower coefficient of thermal conductivity [24]. Therefore, the coating formed by the nano-alumina powder contains more unfused particles compared to the micron powder formed under the same spraying parameters, resulting in increased \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) phase content. Figure 5a,c show the surface morphology of CC and NC, respectively. From these figures, it is evident that the coatings exhibit lamellae structures of different shapes and sizes. In the spraying process, the molten droplets contact the substrate and spread out, leading to the formation of a lamellar morphology. As shown in Figure 5a, the lamellar area of the CC is larger compared to the NC, which is due to the different melting degrees of the powder [34]. The nanostructured coatings exhibit a two-state distributed microstructure, which consists of both fully melted lamellae and the partially melted structure of particles. This is similar to the coatings reported in the literature [34]. Further, the CC's surface has more cracks due to tensile stress exceeding the material's fracture strength during solidification [35], as shown in Figure 5c. However, in the nanostructured alumina coatings, partially melted zones can act as barriers for crack propagation but also cause greater porosity, as shown in Figure 5d. This effect has been extensively reported in the APS literature [36]. Figure 4: XRD pattern of Al\({}_{2}\)O\({}_{3}\) coating. Figure 5: SEM images of (**a,b**) micro Al\({}_{2}\)O\({}_{3}\) coating (CC) and (**c,d**) nano Al\({}_{2}\)O\({}_{3}\) coating (NC). Figure 6a,b depict the surface three-dimensional morphology of the CC and NC, respectively.