Effect of Spraying Power on Oxidation Resistance of MoSi2-ZrB2 Coating for Nb-Si Based Alloy Prepared by Atmospheric Plasma

The MoSi2-ZrB2 coatings were prepared on Nb-Si based alloy by atmospheric plasma spraying with the spraying power 40, 43 and 45 kW. The effect of spraying power on the microstructure and oxidation resistance of MoSi2-ZrB2 coating at 1250 °C were studied. The results showed that the main constituent phases of coatings were MoSi2 at all spraying power. The coating became more compact as the spraying power increased. The coating prepared at 45 kW was dense and uniform, which exhibited the best oxidation resistance due to the formation of a dense and uniform glass layer consisting of SiO2 and ZrSiO4.


Introduction
Nb-Si based alloys are considered to be one of the most promising high-temperature structural material, owing to the high melting points (1750 • C), medium density and excellent high-temperature strength [1][2][3][4][5][6][7]. However, the high-temperature oxidation resistance of Nb-Si based alloys are poor, which limits their application [8][9][10]. Adding elements such as Cr, B, Ta, Ti, Si in Nb-Si based alloys or preparing coatings on the Nb-Si based alloys can improve the oxidation resistance; however, alloying would compromise mechanical properties of alloys [11,12]. Therefore, more attention is paid to preparing coatings on Nb-Si based alloys.
MoSi 2 is a promising coating material for Nb-Si based alloy [13][14][15][16]. It can produce SiO 2 oxide film with excellent oxidation resistance at high temperatures. Besides, its thermal expansion coefficient (8.1 × 10 −6 • C −1 ) is close to that of Nb-Si based alloys (8.4 × 10 −6 • C −1 ) [17]. However, MoSi 2 would suffer pest oxidation at 400-600 • C. The addition of B can effectively avoid this disadvantage [18]. The formation of borosilicate glass at high temperatures can effectively protect MoSi 2 against pest oxidation at 400-600 • C [18]. Furthermore, borosilicate glass coating has a better self-repair ability than SiO 2 due to its lower viscosity [19][20][21]. Fu et al. prepared B 2 O 3 modified SiC-MoSi 2 coating on C/C composites by a two-step pack cementation [22]. The coating could protect C/C composites from oxidation at 1500 • C in air for more than 242 h. Pang et al. prepared a Mo-Si-B coating on Nb-Si-based alloys by spraying Mo first and then co-deposition of Si and B [23]. The mass gain was 0.92 mg/cm 2 after oxidation at 1250 • C for 100 h. However, stresses caused by CTE mismatch between the MoSi2 coating and silica can produce cracks in the oxide film if they exceed the strength of the SiO 2 .
Atmospheric plasma spraying has attracted widespread attention due to the advantages such as high spraying temperature, high deposition efficiency and precise control of the composition and thickness of the coating [24,25]. Le et al. directly deposited the oxidation-resistant coating MoSi 2 on Table 1. Detailed parameters of the sprayed MoSi 2 -ZrB 2 coating.

Isothermal Oxidation
An isothermal oxidation test was carried out in an open tube furnace, in air, at 1250 • C. Each sample was placed in a separate alumina crucible. Samples were taken from the furnace at intervals of 10, 20, 40 and 60 h, and weighed with a crucible, using a precision analytical balance (model CPA225D, Sartorius, Göttingen, Germany) with an accuracy of 0.00001 g.

Coating Characterization
Phase composition of the coating and oxidation specimens were analyzed by X-ray diffraction (XRD, CuKα-radiation, X'Pert Pro, Panalytical, Almelo, Holland) with Cu radiation. Morphology details and elemental distribution characteristics of the coated specimens were investigated by scanning electron microscope combined with energy dispersive spectroscopy (EDS) (Sigma 500, Zeiss, Oberkochen, Germany).  Figure 1 shows the XRD patterns of the surface of as-prepared MoSi 2 -ZrB 2 coatings. It could be seen that the constituent phases of coating at different spraying powers are MoSi 2 , Mo 5 Si 3 , Mo and ZrB 2 . 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 MoSi 2 . Therefore, the raw materials are oxidized to form Mo 5 Si 3 , SiO 2 and Mo according to the Equations (1) and (2) [29][30][31][32]. SiO 2 is an amorphous phase, and its amount is relatively small; therefore, no SiO 2 phase is detected in XRD patterns.

Microstructure of MoSi 2 -ZrB 2 Coating
Materials 2020, 13, x FOR PEER REVIEW 3 of 11 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][30][31][32]. SiO2 is an amorphous phase, and its amount is relatively small; therefore, no SiO2 phase is detected in XRD patterns.  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 shows the surface morphology of MoSi 2 -ZrB 2 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 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][30][31][32]. SiO2 is an amorphous phase, and its amount is relatively small; therefore, no SiO2 phase is detected in XRD patterns.  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 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 MoSi 2 . 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.

Microstructure of MoSi2-ZrB2 Coating
Materials 2020, 13, x FOR PEER REVIEW 4 of 11 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 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 −2 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/cm 2 , 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 2 /cm 4 s) of the mz40 and mz43 sample are calculated to be 1.89 × 10 −5 and 7.8 × 10 −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   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 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 −2 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/cm 2 , 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 2 /cm 4 s) of the mz40 and mz43 sample are calculated to be 1.89 × 10 −5 and 7.8 × 10 −6 , respectively, according to  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 −2 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/cm 2 , respectively. Therefore, MoSi 2 -ZrB 2 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 2 /cm 4 s) of the Materials 2020, 13, 5060 5 of 10 mz40 and mz43 sample are calculated to be 1.89 × 10 −5 and 7.8 × 10 −6 , respectively, according to Equation (3) [4], where ∆m 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 2 /cm 4 s) of the mz45 sample is calculated to be 1.27 × 10 −11 according to Equation (4) [17], where ∆m is the weight change of the sample, and A is the surface area and t is the exposure time.

High Temperature Oxidation Resistance
∆m Materials 2020, 13, x FOR PEER REVIEW 5 of 11 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 2 /cm 4 s) of the mz45 sample is calculated to be 1.27 × 10 −11 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.  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 shows the XRD patterns of MoSi 2 -ZrB 2 coatings after oxidation at 1250 • C for 60 h. As shown in Figure 6, the oxidized MoSi 2 -ZrB 2 coatings mainly consist of MoSi 2 , Mo 5 Si 3 , SiO 2 and ZrSiO 4 . Figure 7 demonstrates the surface morphologies of MoSi 2 -ZrB 2 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. Materials 2020, 13, x FOR PEER REVIEW 6 of 11   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].    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].  Figure 8 shows the cross-sectional morphologies of MoSi 2 -ZrB 2 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 SiO 2 , the black-gray MoSi 2 , the gray-white Mo 5 Si 3 and the white ZrSiO 4 . Moreover, some cracks and holes are observed in all coatings, and they are filled with black SiO 2 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 TiNbO 4 and SiO 2 according to EDS analysis, which is the typical oxides of Nb-Si based alloy at high temperature [17].

Effect 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.

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

Effect 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.

Oxidation Mechanism of MoSi 2 -ZrB 2 Coatings
As shown in Figure 8, the oxidized MoSi 2 -ZrB 2 coatings mainly consist of MoSi 2 , Mo 5 Si 3 , SiO 2 and ZrSiO 4 . The excellent oxidation resistance of the mz45 sample is due to the formation of dense SiO 2 glass layer on the surface, leading to a lower diffusion rate of oxygen. The existence of Mo 5 Si 3 phase is due to the oxidation of MoSi 2 phase according to the oxidation reaction (Equation (1)) [30,31]; the formation of SiO 2 phase is due to the oxidation of silicides, such as MoSi 2 and Mo 5 Si 3 according to Equations (5) and (6) [30][31][32][33]; ZrB 2 are oxidized to form ZrO 2 and B 2 O 3 according to Equation (7). The ZrSiO 4 phase is the result of reaction between the SiO 2 and ZrO 2 according to Equation (8). Owing to the volatilization of MoO 3 and B 2 O 3 at high temperature, the MoO 3 phase and B 2 O 3 phase cannot be observed in the coating. Although the oxidation protective phase SiO 2 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.
In order to explain the oxidation mechanism of MoSi 2 -ZrB 2 coating, the oxidation process is shown in Figure 9, which is similar to that of MoSi 2 -based composite coating on Nb alloy at 1500 • C [25]. MoSi 2 and ZrB 2 are oxidized to form SiO 2 and ZrO 2 , respectively. After that, the silica glass covers the surface of the coating and heals the cracks and holes. As oxidation continued, the ZrSiO 4 is produced by the reaction of dispersive ZrO 2 and SiO 2 , which could minimize the CTE difference between silica and MoSi 2 .
Materials 2020, 13, x FOR PEER REVIEW 8 of 11 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][31][32][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.
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 difference between silica and MoSi2.

Conclusions
The effect of different spraying power on the microstructure and oxidation resistance of the MoSi2-ZrB2 coatings was investigated.
1. The microstructure of MoSi2-ZrB2 coatings prepared by atmospheric plasma spraying mainly consisted of MoSi2. The higher spraying power could produce a much more compact coating. 2. The MoSi2-ZrB2 coating prepared under 45 Kw showed the best oxidation resistance with the mass gain of 1.66 mg cm -2 after oxidation at 1250 °C for 60 h. However, the MoSi2-ZrB2 coatings prepared under 40 and 43 Kw showed worse oxidation resistance, due to the bad combination between coating and substrate at the lower spraying power. 3. The excellent anti-oxidation protection of mz45 sample was mainly due to the formation of a silica glass layer, leading to a low diffusion rate of oxygen.

Conclusions
The effect of different spraying power on the microstructure and oxidation resistance of the MoSi 2 -ZrB 2 coatings was investigated.

1.
The microstructure of MoSi 2 -ZrB 2 coatings prepared by atmospheric plasma spraying mainly consisted of MoSi 2 . The higher spraying power could produce a much more compact coating.

2.
The MoSi 2 -ZrB 2 coating prepared under 45 Kw showed the best oxidation resistance with the mass gain of 1.66 mg cm -2 after oxidation at 1250 • C for 60 h. However, the MoSi 2 -ZrB 2 coatings prepared under 40 and 43 Kw showed worse oxidation resistance, due to the bad combination between coating and substrate at the lower spraying power.

3.
The excellent anti-oxidation protection of mz45 sample was mainly due to the formation of a silica glass layer, leading to a low diffusion rate of oxygen.