**3. Results**

#### *3.1. Microstructure of MoSi2-ZrB2 Coating*

Figure 1 shows the XRD patterns of the surface of as-prepared MoSi2-ZrB2 coatings. It could be seen that the constituent phases of coating at different spraying powers are MoSi2, Mo5Si3, Mo and ZrB2. In the process of plasma spraying, the temperature of the plasma arc was about 10,000 ◦C [28], which is much higher than the oxidation temperature of MoSi2. Therefore, the raw materials are oxidized to form Mo5Si3, SiO2 and Mo according to the Equations (1) and (2) [29–32]. SiO2 is an amorphous phase, and its amount is relatively small; therefore, no SiO2 phase is detected in XRD patterns.

$$\text{\textbullet MoSi}\_2 + \text{\textbullet O}\_2 \rightarrow \text{Mo}\_5\text{Si}\_3 + \text{\textbullet SiO}\_2 \tag{1}$$

$$\text{MoOSi}\_2 + 2\text{O}\_2 \to \text{Mo} + 2\text{SiO}\_2 \tag{2}$$

**Figure 1.** XRD patterns of MoSi2-ZrB2 coatings.

Figure 2 shows the surface morphology of MoSi2-ZrB2 coatings. The surfaces of all spraying samples are rough. In addition, the molten zone is interwoven with the incompletely molten particles, which is a typical structure feature of plasma sprayed coatings. In the process of coating preparation, high-speed particles are heated by plasma flame, and then the molten particles impinge on the substrate to form a flat structure. The temperature of the plasma arc elevates as the spraying power increases. Therefore, the full molten area of the mz43 sample is larger than that of the mz40 sample, leading to a much smoother surface of the mz43 sample. As for the mz45 sample, the completely molten area of the mz45 sample is the largest (as shown in Figure 2c). Therefore, the mz45 sample shows a more compact surface as compared with that of the mz40 and mz43 samples.

**Figure 2.** Surface morphology of MoSi2-ZrB2 coatings: (**a**) mz40, (**b**) mz43 and (**c**) mz45.

Figure 3 shows cross-sectional morphologies of MoSi2-ZrB2 coatings. The mean thickness of the coatings of the mz40, mz43 and mz45 samples are about 122, 138 and 158 μm, respectively. As shown in Figure 3, the interface between the coating and the substrate becomes denser and more uniform as the power increases. For the mz45 sample, the interface is more compact, indicating that the mz45 sample has a better combination of the coating and substrate as compared to that of the mz40 and mz43 samples. As shown in Table 2, the main constituent phase of the mz45 sample is confirmed to be MoSi2. The elements mapping, as shown in Figure 4, reveals that the coating mainly consists of Mo, Si and O. The existence of O element may be induced from the spraying in oxygen atmosphere. Furthermore, the Vickers hardness of the coating prepared by 40, 43 and 45 Kw are measured to be 850, 924 and 979, respectively. This may be due to the better combination of the substrate and coating as the increase of the spraying power.

**Figure 3.** Cross-section morphology of MoSi2-ZrB2 coatings: (**a**) mz40, (**b**) mz43 and (**c**) mz45.



**Figure 4.** Elements mapping for the MoSi2-ZrB2 coating of the mz45 sample.

#### *3.2. High Temperature Oxidation Resistance*

Figure 5a shows the weight gain per unit area as a function of the exposure time at 1250 ◦C. The Nb-Si based alloy suffers serious oxidation with the mass gain of 205.24 mg cm<sup>−</sup><sup>2</sup> after oxidation at 1250 ◦C for 60 h and follows a linear oxidation behavior. The mass gains of the mz40, mz43 and mz45 samples were 11.81, 5.32 and 1.66 mg/cm2, respectively. Therefore, MoSi2-ZrB2 coatings could effectively improve the oxidation resistance of Nb-Si based alloy. As shown in Figure 5b, the mz40 and mz43 samples follow linear oxidation behavior, and the linear kinetic constants (g<sup>2</sup>/cm4s) of the

mz40 and mz43 sample are calculated to be 1.89 × 10−<sup>5</sup> and 7.8 × <sup>10</sup>−6, respectively, according to Equation (3) [4], where Δm is the weight change of the sample, *A* is the surface area and t is the exposure time. During oxidation, the edges of the coating are the place where stress is easily concentrated, leading to the failure of the coating. As shown in Figure 5d, the coating edges of the mz40 and mz43 samples have peeled off after oxidation, while the coating edge of mz45 sample is compact. The mz45 sample shows excellent high temperature oxidation resistance and conforms to the parabolic oxidation behavior. The parabolic kinetic constant (g<sup>2</sup>/cm4s) of the mz45 sample is calculated to be 1.27 × 10−<sup>11</sup> according to Equation (4) [17], where Δm is the weight change of the sample, and *A* is the surface area and t is the exposure time.

$$\frac{\Delta m}{A} = k\_l \mathbf{t} \tag{3}$$

$$\left(\frac{\Delta m}{A}\right)^2 = k\_\text{p}\text{t}\tag{4}$$

**Figure 5.** (**a**) Oxidation weight gain curve of Nb-Si based alloy and coatings, (**b**) oxidation weight gain curve of coatings, (**c**) representation of the weight gain versus the square root of time for mz45 oxidized in air, and (**d**) the photograph of oxidized samples.

Figure 6 shows the XRD patterns of MoSi2-ZrB2 coatings after oxidation at 1250 ◦C for 60 h. As shown in Figure 6, the oxidized MoSi2-ZrB2 coatings mainly consist of MoSi2, Mo5Si3, SiO2 and ZrSiO4. Figure 7 demonstrates the surface morphologies of MoSi2-ZrB2 coatings after oxidation at 1250 ◦C for 60 h. The surface of the mz40 sample is loose and undulate, as shown in Figure 7a. It can be observed from Figure 7b that the surface of mz43 sample is much denser. As shown in Figure 7c, the mz45 sample displays a uniform, dense and integrated surface.

**Figure 6.** XRD patterns of MoSi2-ZrB2 coatings after oxidation at 1250 ◦C for 60 h.

**Figure 7.** Surface morphology of MoSi2-ZrB2 coatings after oxidation in air at 1250 ◦C for 60 h: (**a**) mz40, (**b**) mz43 and (**c**) mz45.

Figure 8 shows the cross-sectional morphologies of MoSi2-ZrB2 coatings after oxidation at 1250 ◦C for 60 h. According to the XRD results and EDS analysis (as listed in Table 3), the coating consists of the black SiO2, the black-gray MoSi2, the gray-white Mo5Si3 and the white ZrSiO4. Moreover, some cracks and holes are observed in all coatings, and they are filled with black SiO2 phase. As shown in Figure 5d, the coating edges of the mz40 and mz43 samples are peeling off during oxidation. Therefore, the edge of the substrate (Nb-Si based alloy) exposes to the oxygen environment, leading to the worse oxidation performance of these two samples. Figure 8d shows the microstructure of the failure edge of the mz40 sample after oxidation. The oxides of the edge of mz40 sample are confirmed to be TiNbO4 and SiO2 according to EDS analysis, which is the typical oxides of Nb-Si based alloy at high temperature [17].

**Table 3.** The EDS results of constituent phase of the oxidized mz45 sample (at.%).


**Figure 8.** Cross-section morphology of MoSi2-ZrB2 coatings after oxidation in air at 1250 ◦C for 60 h: (**a**) mz40, (**b**) mz43, (**c**) mz45 and (**d**) the failure edge of the mz40 sample.
