**4. Discussion**

#### *4.1. E*ff*ect of Spraying Powder on the Microstructure of Coatings*

As shown in Figure 2, the surface of the mz45 sample is more compact due to the completely molten area of the mz45 sample is the largest. In the process of spraying, the temperature of the plasma arc elevates as the spraying power increases. The plasma arc of 40 kW possesses a lower temperature. Therefore, the surface of the mz40 sample is rough consisting of many incompletely molten particles and holes, which is mainly due to the accumulation of incompletely molten particles and the escape of gas by-products during the spraying process. In addition, the incompletely molten powders are difficult to adhere to the substrate surface, which reduces the spraying efficiency, resulting in the thinnest thickness of the coating.

When the spraying power increases to 43 kW, the increasing temperature increases the melting ratio of spraying powder. These spraying powders can be riveted together with the substrate when they hit the surface of the substrate. Thus, the completely molten area increases in this sample. However, a few holes still exist at the interface between the coating and the substrate, suggesting that the spraying powders cannot be fully spread at 43 kW. When the power increases to 45 kW, completely molten particles can be uniformly distributed on the surface of the substrate, and the thickness of the coating increases. In addition, no obvious cracks and holes were observed at the interface between the coating and the substrate, indicating the good bonding of the mz45 sample. It could be concluded that the higher spraying power could produce a much more compact coating.

#### *4.2. Oxidation Mechanism of MoSi2-ZrB2 Coatings*

As shown in Figure 8, the oxidized MoSi2-ZrB2 coatings mainly consist of MoSi2, Mo5Si3, SiO2 and ZrSiO4. The excellent oxidation resistance of the mz45 sample is due to the formation of dense SiO2 glass layer on the surface, leading to a lower diffusion rate of oxygen. The existence of Mo5Si3 phase is due to the oxidation of MoSi2 phase according to the oxidation reaction (Equation (1)) [30,31]; the formation of SiO2 phase is due to the oxidation of silicides, such as MoSi2 and Mo5Si3 according to Equations (5) and (6) [30–33]; ZrB2 are oxidized to form ZrO2 and B2O3 according to Equation (7). The ZrSiO4 phase is the result of reaction between the SiO2 and ZrO2 according to Equation (8). Owing to the volatilization of MoO3 and B2O3 at high temperature, the MoO3 phase and B2O3 phase cannot be observed in the coating. Although the oxidation protective phase SiO2 is formed in the mz40 and mz43 samples, these two samples exhibit worse oxidation due to the bad combination between coating and substrate at lower spraying power.

$$2\text{ MoSi}\_2 + 7\text{O}\_2 \to 2\text{MoO}\_3 + 4\text{SiO}\_2\tag{5}$$

$$2\text{ Mo}\_5\text{Si}\_3 + 2\text{1O}\_2 \to 10\text{MoO}\_3 + 6\text{SiO}\_2\tag{6}$$

$$2\text{ZrB}\_2 + 5\text{O}\_2 \rightarrow 2\text{ZrO}\_2 + 2\text{B}\_2\text{O}\_3\tag{7}$$

$$\text{ZrO}\_2 + \text{SiO}\_2 \to \text{ZrSiO}\_4 \tag{8}$$

Moreover, it can be found that ZrSiO4 distributed in the coating, as shown in Figure 8c. Dissolving a certain amount of zirconium oxide in amorphous silica scale could enhance its oxidation resistance [27]. Zr-based oxides have higher melting temperature, of which ZrO2 is 2715 ◦C, ZrSiO4 is 2550 ◦C, and pure SiO2 is 1650 ◦C [34–36]. Therefore, the dispersion of ZrSiO4 in SiO2 glass could increase the melting temperature of the silica. Furthermore, the CTEs of ZrO2 (10.5 × 10−<sup>6</sup> ◦C−1) and ZrSiO4 (4.9 × 10−<sup>6</sup> ◦C−1) are larger than that of SiO2 (0.55 × 10−<sup>6</sup> ◦C−1) [37–40]. Therefore, the formation of ZrSiO4 could increase the CTE of silica. As a result, the di fference of CTE between silica and MoSi2 could minimize, reducing the internal stress of the coating.

In order to explain the oxidation mechanism of MoSi2-ZrB2 coating, the oxidation process is shown in Figure 9, which is similar to that of MoSi2-based composite coating on Nb alloy at 1500 ◦C [25]. MoSi2 and ZrB2 are oxidized to form SiO2 and ZrO2, respectively. After that, the silica glass covers the surface of the coating and heals the cracks and holes. As oxidation continued, the ZrSiO4 is produced by the reaction of dispersive ZrO2 and SiO2, which could minimize the CTE di fference between silica and MoSi2.

**Figure 9.** Oxidation process of MoSi2-ZrB2 coating at 1250 ◦C in air.
