Fabrication and Oxidation Resistance of a Novel MoSi2-ZrB2-Based Coating on Mo-Based Alloy

To enhance the oxidation resistance of Mo-based TZM alloy (Mo-0.5Ti-0.1Zr-0.02C, wt%), a novel MoSi2-ZrB2 composite coating was applied on the TZM substrate by a two-step process comprising the in situ reaction of Mo, Zr, and B4C to form a ZrB2-MoB pre-layer followed by pack siliconizing. The as-packed coating was composed of a multi-layer structure, consisting of a MoB diffusion layer, an MoSi2-ZrB2 inner layer, and an outer layer of mixture of MoSi2 and Al2O3. The composite coating could provide excellent oxidation-resistant protection for the TZM alloy at 1600 °C. The oxidation kinetic curve of the composite coating followed the parabolic rule, and the weight gain of the coated sample after 20 h of oxidation at 1600 °C was only 5.24 mg/cm2. During oxidation, a dense and continuous SiO2-baed oxide scale embedded with ZrO2 and ZrSiO4 particles showing high thermal stability and low oxygen permeability could be formed on the surface of the coating by oxidation of MoSi2 and ZrB2, which could hinder the inward diffusion of oxygen at high temperatures. Concurrently, the MoB inner diffusion layer played an important role in hindering the diffusion of Si inward with regard to the TZM alloy and could retard the degradation of MoSi2, which could also improve the long life of the coating.


Introduction
Molybdenum (Mo) alloys have been considered as a promising candidate for hightemperature structural materials owing to their high melting points, outstanding mechanical properties at elevated temperatures, low coefficient of thermal expansion (CTE), high thermal conductivity, and good thermal shock resistance [1][2][3].However, poor hightemperature oxidation resistance of molybdenum alloys limits their applications in an oxidizing environment at high temperatures [3][4][5].It has been recognized that oxidationresistant coatings could offer an effective approach to improve the oxidation resistance of Mo-based alloy at elevated temperatures.An effective high-temperature oxidation-resistant coating must be capable of forming a protective oxygen barrier at high temperatures.Dense silica scale provides excellent resistance to oxygen diffusion or permeation at high temperatures.Thus, great attention has been currently paid to materials based on molybdenum disilicide (MoSi 2 ) for use as a protective coating against high-temperature oxidation on Mo-based alloys, due to its high melting point (2030 • C), moderate density of 6.24 g/cm 3 , similar CTE (8.1 × 10 −6 / • C) to the Mo substrate (6.7 × 10 −6 / • C), and the regenerating of a protective SiO 2 scale on the coating surface [6][7][8].However, MoSi 2 performs limited protective performance in higher temperature applications (e.g., up to 1600 • C), mainly related to two factors: one is the decrease in the thermal stability of SiO 2 at higher temperatures, and the other is the thermal-mechanical limitations of MoSi 2 induced by the rapid interdiffusion between the coating and the substrate and the thermal stresses generated by the CTE mismatch at the coating-substrate interface [9].
Zirconium diboride (ZrB 2 ) is typical ultra-high-temperature ceramic and has been widely studied for high-temperature structural applications owing to its excellent and unique combination of high melting points exceeding 3000 • C, good thermo-chemical property, high thermal conductivity, and ablation resistance [26][27][28].As with other nonoxide ceramics, ZrB 2 will be oxidized when exposed to air at elevated temperatures and generate oxidation products of ZrO 2 and B 2 O 3 .However, since B 2 O 3 would volatilize at temperatures above 1200 • C, a pure ZrO 2 scale would be formed by oxidation of ZrB 2 at high temperatures up to 1200 • C [29].The ZrO 2 scale has a porous structure and could not act as a diffusion barrier of oxygen; thus, the single ZrB 2 coating cannot be directly applied to oxidation and ablation protection of high-temperature structural materials.Adding silicon compounds, such as MoSi 2 and SiC, is an effectively way to solve this problem.There are a large number of reports concerning the fabrication of ZrB 2 -based structural ceramics and functional coatings, such as ZrB 2 -SiC [30], ZrB 2 -MoSi 2 [31,32].However, there are hardly any studies on the ZrB 2 -doped MoSi 2 based coating on Mo-based alloy.
The lifetime of the MoSi 2 coating was limited by the Si depletion induced by the interdiffusion between the MoSi 2 coating and the refractory metals substrate.Applying a diffusion barrier between the MoSi 2 -based coating and the refractory metals substrate is another way to improve the stability and serving life of the coating.Many kinds of materials, including oxides (Al 2 O 3 diffusion barrier for the Mo-Si-B/Nb-Si system [33], Y 2 O 3 diffusion barrier for the MoSi 2 /Nb-Si system [34]), carbides (Mo 2 C diffusion barrier for the MoSi 2 /SiC-Mo 2 C/Mo system [35], SiC-glass diffusion barrier for the MoSi 2 /Nb-Si system [36]), borides (MoB diffusion barrier for the MoSi 2 /Mo system [8,37], Nb 3 B 2 -NbB 2 diffusion barrier for the Si-Mo-W-ZrB 2 -Y 2 O 3 /Nb system [38]) and silicides (TaSi 2 diffusion barrier for the MoSi 2 /Nb system [39], WSi 2 diffusion barrier for the MoSi 2 /Nb-Ti-Si system [40]) have been used as diffusion barriers between the coatings and the substrates, thereby effectively lengthening the lifetime of these coatings.Reference [25] demonstrated that the Mo 2 C barrier layer plays a key role in slowing down the diffusion of C and Si toward the inner Mo substrate at high temperatures, which led to less cracks in the surface and a thinner Mo 5 Si 3 layer at high temperatures in air and thus brought about better oxidation resistance.References [8,34] reported that the MoSi 2 /MoB coating (MoB as a diffusion barrier) possesses a longer service life compared to the single MoSi 2 coating on the Mo substrates, owing to the diffusion barrier effect of the MoB layer on the Si element diffusion into the Mo substrate.
In this work, a composite coating, composed of the MoSi 2 -ZrB 2 main layer and the MoB diffusion layer between the coating and substrate, was fabricated on the TZM alloy substrate by a two-step method, including a slurry sintering step of Mo, Zr, and B 4 C, followed by a halide-activated pack cementation process.Isothermal oxidation behavior of the composite coating was evaluated at 1600 • C in air.Microstructure and phase composition of the as-prepared and oxidized composite coating were characterized, and the antioxidant mechanism of the coating at high temperatures was also discussed.In contrast, the structure evolution and oxidation resistance of a single MoSi 2 prepared on the TZM alloy through the same two-step method were studied as well.

Sample Preparation
TZM Mo-based alloy (Mo-0.5Ti-0.1Zr-0.02C,wt%) was used as substrates in the study.Long strip specimens (90 mm × 8 mm × 2 mm) were cut from the TZM alloy plate.All samples were hand-polished by SiC grit papers up to 1000 mesh and cleaned in an ultrasonic ethanol bath and then dried at 80 • C for 2 h in vacuum.
The composite coating was prepared on the TZM alloy substrates through a two-step process: (1) Firstly, two kinds of slurry were prepared.Mo powders (purity > 99.5%, 1~2 µm) and ethyl acetate (as solvent, AR) were putted into a ball milling tank, and the weight/volume ratio of Mo powders to solvent (g:mL) was 1:0.8.A small amount of nitrocellulose (as organic binder, AR) was added to the ball milling tank.The mixture was attrition milled for 6 h to obtain a Mo slurry.Concurrently, a Zr-B 4 C slurry was prepared by mixing of pure Zr powders (purity > 99%, 2-3 µm), B 4 C powders (purity > 99.5%, 2-3 µm), and ethyl acetate.The molar ratio of Zr:B 4 C was 1:1, while the weight/volume ratio of powders to solvent (g:mL) was also 1:0.8.In the same way, a small amount of nitrocellulose (AR) was added to the mixture.Then, the mixture was also attrition milled for 6 h.
Secondly, the Mo slurry was sprayed evenly on the surface of the Mo-based alloy substrate.The as-sprayed samples were dried using an infrared baking lamp and a Mo pre-layer was obtained on the substrate.Subsequently, the Zr-B 4 C slurry was sprayed evenly on the surface of the Mo pre-layer and then dried using infrared baking lamp as well.
Finally, the samples were taken into a vacuum furnace and sintered at 1700 • C for 2 h in a vacuum (<1 Pa).Thus, a Mo/Zr-B 4 C ceramic pre-layer was fabricated on the Mo-based alloy substrate.To simplify the expression, the as-ceramic pre-layer will be referred to as the "Mo/Zr-B 4 C ceramic pre-layer" in this paper.
(2) Subsequently, a conventional pack cementation process was carried out.The pack mixture consisted of Si powders (pack cementation element, 99.9% purity, 1~3 µm), Al 2 O 3 powders (inert powder, 98% purity, ~75 µm), and NaF powders (activator, 99.9% purity, 1~2 µm) with a mass ratio of 67:30:3.The pack powders were mixed up by tumbling in a ball mill for 12 h and dried at 110 • C for 6 h.The samples containing ceramic pre-layer on surface were embedded in the pack mixture and then heated to 1150 • C and held for 10 h in an argon-protected tube furnace.Finally, the sintered coating samples were furnace-cooled down to room temperature.
For comparison, a pure MoSi 2 coating on the TZM alloy was prepared through the same two-step method: (1) slurry sintering to obtain a Mo pre-layer on the TZM alloy substrate and (2) siliconizing with Si powders to generate a MoSi 2 coating on the substrate.

Oxidation Test and Characterization
Isothermal oxidation tests were performed at 1600 • C in static air using an electric furnace to investigate the oxidation resistance of both kind of coatings.Weight gains of both coatings after oxidation for different time were measured using an analytical balance with an accuracy of 10 −4 g to study the oxidation characteristics of the coatings.
The coated specimens before and after oxidation were wire cutting and acetone cleaned.Phase composition of the surface of the as-prepared and oxidized coatings was analyzed by X-ray diffraction (XRD, D/Max 2500, Cu-Ka radiation, Tokyo, Japan).The surface of the coatings before and after oxidation was obtained using scanning electron microscopy (SEM, TESCAN MIRA3, Brno, Czech Republic) coupled with an energy dispersive spectrometer (EDS) device.Since the surface of the oxidized coating was not conductive after high-temperature oxidation, it needed to be sprayed with gold before observation.The sample section was embedding by bakelite at 175 • C with the pressure of a 175 bar.After sanding (using 200 mesh, 400 mesh, 600 mesh and 800 mesh sandpaper) and polishing, the cross-sectional microstructure and elemental distribution of the as-prepared and oxidized coatings were characterized by SEM and an electron-probe micro-analyzer (EPMA, JEOL JXA-8230, Musashino, Japan) equipped with wave dispersive X-ray spectroscopy (WDS).[41][42][43].However, in our previous work, ZrO 2 proved to be beneficial to the high temperature oxidation resistance of silicide coating [44].

Results and Discussion
of a 175 bar.After sanding (using 200 mesh, 400 mesh, 600 mesh and 800 mesh san and polishing, the cross-sectional microstructure and elemental distribution of the pared and oxidized coatings were characterized by SEM and an electron-probe analyzer (EPMA, JEOL JXA-8230, Musashino, Japan) equipped with wave dispe ray spectroscopy (WDS).

Microstructure and Phase Composition of Ceramic Pre-Layer
Figure 1 displays the XRD pattern of the surface of the sintered ceramic p According to the XRD pattern, the as-fabricated ceramic pre-layer was composed and MoB.Carbide (e.g., ZrC, Mo2C, MoC) was not obtained in the sintered samp diffraction peaks belonging to ZrB2 were shifted toward larger angles, which ar tially attributed to dissolution of C in the ZrB2 lattice.No initial component suc Mo, or B4C was detected in the sintered samples, indicating sufficient reactions b Zr, Mo, and B4C.The presence of ZrO2 in the sintered pre-layer could be attribute oxide impurities on the surface of the strating powders of Zr and B4C.Meanwhile B2O3 would react and generate ZrO2 from 5Zr + 2B2O3 = 3ZrO2 + 2ZrB2 [41][42][43].H in our previous work, ZrO2 proved to be beneficial to the high temperature oxida sistance of silicide coating [44].In order to further investigate the phase morphology and elemental distrib the ceramic pre-layer, the typical microstructure and element mapping of the surfa phology of the ceramic pre-layer are given in Figure 2a-f.As shown in Figure 2a, t ing had a relatively loose surface and showed an island-like morphology.There w merous pores around the island-like clusters.However, no visual crack was pre the surface of the sintered ceramic pre-layer.Figure 2b shows an enlarged view o in Figure 2a.It is clear that those island-like clusters became intermingled with eac which could offer a good interlocking of the coating against peeling off. Figure 2c the element mapping of the surface of the sintered ceramic pre-layer in Figure 2 be seen that the pre-layer mainly contained Zr, Mo, B, and C elements.Given t pattern in Figure 1 and the element mapping in Figure 2c-f, we concluded that the like clusters were composed of ZrB2, in which some of the MoB phases were dis uniformly.
XRD pattern of the surface of the pre-fabricated ceramic layer.
In order to further investigate the phase morphology and elemental distribution in the ceramic pre-layer, the typical microstructure and element mapping of the surface morphology of the ceramic pre-layer are given in Figure 2a-f.As shown in Figure 2a, the coating had a relatively loose surface and showed an island-like morphology.There were numerous pores around the island-like clusters.However, no visual crack was present on the surface of the sintered ceramic pre-layer.Figure 2b shows an enlarged view of area A in Figure 2a.It is clear that those island-like clusters became intermingled with each other, which could offer a good interlocking of the coating against peeling off. Figure 2c-f show the element mapping of the surface of the sintered ceramic pre-layer in Figure 2b.It can be seen that the pre-layer mainly contained Zr, Mo, B, and C elements.Given the XRD pattern in Figure 1 and the element mapping in Figure 2c-f, we concluded that the island-like clusters were composed of ZrB 2, in which some of the MoB phases were distributed uniformly.Figure 3a shows the cross-sectional microstructure of the ceramic pre-layer.From a cross-sectional perspective, the ceramic pre-layer represented a loose but interconnected morphology, which had 100~110 μm of thickness.No visible crack was found in the cross section either.Figure 3b-d show the element mapping of the cross section of the pre-layer.The ceramic pre-layer regions in Figure 3a match well with Zr-rich regions in Figure 3b.There was no obvious diffusion layer between the ceramic pre-layer and the substrate.It can be seen from Figure 3c,d that the MoB phases were distributed uniformly throughout the depth, concurrently, although B was likely to diffuse into the Mo-based alloy substrate besides in the pre-layer.Figure 3a shows the cross-sectional microstructure of the ceramic pre-layer.From a cross-sectional perspective, the ceramic pre-layer represented a loose but interconnected morphology, which had 100~110 µm of thickness.No visible crack was found in the cross section either.Figure 3b-d show the element mapping of the cross section of the pre-layer.The ceramic pre-layer regions in Figure 3a match well with Zr-rich regions in Figure 3b.There was no obvious diffusion layer between the ceramic pre-layer and the substrate.It can be seen from Figure 3c,d that the MoB phases were distributed uniformly throughout the depth, concurrently, although B was likely to diffuse into the Mo-based alloy substrate besides in the pre-layer.For the Zr-B4C system, when sintering at temperatures above 900 °C, B4C could react with metals (Zr and Mo) to generate corresponding borides and carbides, and the main reaction products usually include ZrB2 and ZrC [45,46].The possible reactions for the Zr-B4C system are listed as Reactions ( 1)-( 4).For a certain reaction, it could proceed positively only when the Gibbs free energy ΔG < 0. The standard Gibbs free energy at a certain temperature T could be calculated, which is given in Figure 4.It is clear that Reactions (1)-( 4) are all thermodynamically favorable in the calculated temperature range.The standard Gibbs free energy of Reaction (1) was more negative than that of Reaction (2).On basis of the principle of minimum free enthalpy, the Reaction (1) was more likely to occur.During the solid reaction process of the Zr-B4C system, Zr would firstly react with B4C to generate ZrB2 and amorphous carbon (C), and then amorphous carbon reacted with residual Zr to form ZrC by Reaction (4) [35].Rehman S.S. et al. [36] also proved the formation of ZrB2 and element C during spark plasma sintering of B4C and ZrH2 mixture powders.However, in the present work, the sintered ceramic pre-layer was mainly composed of ZrB2 and MoB, and no ZrC, MoC, or Mo2C phase was detected by XRD according to Figure 2. It is supposed that the formation of ZrC from the in situ reaction of Zr and B4C is related to the reaction temperature and the ratio of Zr/B4C.For one thing, the sintering temperature in this work was 1700 °C.Wu W. et al. [39] reported the reaction behavior of ZrH2 and B4C at a temperatures range from 900 to 1600 °C.They revealed that ZrH2 decomposed to Zr and H2(g) at 900 °C.At a temperature range from 900 °C to 1000 °C, Zr would react with B4C to form ZrC and ZrB2, and when the temperature increased to 1200 °C and 1400 °C, the XRD peak intensity of ZrC decreased, whereas the ZrB2 peak intensity increased.Upon For the Zr-B 4 C system, when sintering at temperatures above 900 • C, B 4 C could react with metals (Zr and Mo) to generate corresponding borides and carbides, and the main reaction products usually include ZrB 2 and ZrC [45,46].The possible reactions for the Zr-B 4 C system are listed as Reactions ( 1)-( 4).For a certain reaction, it could proceed positively only when the Gibbs free energy ∆G < 0. The standard Gibbs free energy at a certain temperature T could be calculated, which is given in Figure 4.It is clear that Reactions (1)-( 4) are all thermodynamically favorable in the calculated temperature range.The standard Gibbs free energy of Reaction (1) was more negative than that of Reaction (2).On basis of the principle of minimum free enthalpy, the Reaction (1) was more likely to occur.During the solid reaction process of the Zr-B 4 C system, Zr would firstly react with B 4 C to generate ZrB 2 and amorphous carbon (C), and then amorphous carbon reacted with residual Zr to form ZrC by Reaction (4) [35].Rehman S.S. et al. [36] also proved the formation of ZrB 2 and element C during spark plasma sintering of B 4 C and ZrH 2 mixture powders.However, in the present work, the sintered ceramic pre-layer was mainly composed of ZrB 2 and MoB, and no ZrC, MoC, or Mo 2 C phase was detected by XRD according to Figure 2. It is supposed that the formation of ZrC from the in situ reaction of Zr and B 4 C is related to the reaction temperature and the ratio of Zr/B 4 C.For one thing, the sintering temperature in this work was 1700 • C. Wu W. et al. [39] reported the reaction behavior of ZrH 2 and B 4 C at a temperatures range from 900 to 1600 • C.They revealed that ZrH 2 decomposed to Zr and H 2 (g) at 900 • C. At a temperature range from 900 • C to 1000 • C, Zr would react with B 4 C to form ZrC and ZrB 2 , and when the temperature increased to 1200 • C and 1400 • C, the XRD peak intensity of ZrC decreased, whereas the ZrB 2 peak intensity increased.Upon a heat treatment at 1600 • C, the ZrC phase completely disappeared and only ZrB 2 was detected.The occurrence of chemical reactions between ZrB 2 and ZrC at temperatures above 1500 • C has also been reported, indicating the carbon in ZrC would be substituted by boron with a further increase in temperature [47].Moreover, Chen H.A. et al. [42] reported the formation mechanism of ZrB 2 -ZrC-B 4 C ceramics from the in situ reaction of Zr-B 4 C; according to their analysis, since the formation of ZrB 2 was a priority and rapid, and the free energy of the ZrC formation was relatively higher than that of ZrB 2 , the formation of ZrC depended on the content of Zr.No ZrC was obtained below 50 vol.%Zr content.In this work, the molar ratio Zr/B 4 C was 1:1 (~38 vol.%Zr), and due to the lack of Zr, amorphous carbon could not react with Zr to generate ZrC and existed in the pre-layer in a solid solution.The atomic radius of the C atom (0.77 Å) was smaller than that of the B atom (0.97 Å), and the presence of the interstitial C atom in the ZrB 2 lattice caused a change in the lattice constants, which led to the ZrB 2 diffraction peaks shifting toward larger angles in the XRD pattern, as shown in Figure 1.
Materials 2023, 16, x FOR PEER REVIEW 7 a heat treatment at 1600 °C, the ZrC phase completely disappeared and only ZrB2 detected.The occurrence of chemical reactions between ZrB2 and ZrC at temperat above 1500 °C has also been reported, indicating the carbon in ZrC would be substit by boron with a further increase in temperature [47].Moreover, Chen H.A. et al. [42 ported the formation mechanism of ZrB2-ZrC-B4C ceramics from the in situ reaction o B4C; according to their analysis, since the formation of ZrB2 was a priority and rapid the free energy of the ZrC formation was relatively higher than that of ZrB2, the forma of ZrC depended on the content of Zr.No ZrC was obtained below 50 vol.%Zr con In this work, the molar ratio Zr/B4C was 1:1 (~38 vol.%Zr), and due to the lack o amorphous carbon could not react with Zr to generate ZrC and existed in the pre-lay a solid solution.The atomic radius of the C atom (0.77 Å) was smaller than that of t atom (0.97 Å), and the presence of the interstitial C atom in the ZrB2 lattice caused a ch in the lattice constants, which led to the ZrB2 diffraction peaks shifting toward large gles in the XRD pattern, as shown in Figure 1.For the Mo/Zr-B4C system, besides Reactions (1)-( 4), the potential chemical reac are listed as (5)- (8).The standard Gibbs free energy for Reactions (5)-( 8) at a certain perature T has also been calculated and shown in Figure 4. Similarly, the standard G free energy of Reaction (5) was more negative than that of Reaction (6), implying the ority formation of MoB rather than Mo2C, which is consistent with the XRD result in ure 1.For the Mo/Zr-B 4 C system, besides Reactions (1)-( 4), the potential chemical reactions are listed as (5)- (8).The standard Gibbs free energy for Reactions (5)-( 8) at a certain temperature T has also been calculated and shown in Figure 4. Similarly, the standard Gibbs free energy of Reaction (5) was more negative than that of Reaction ( 6), implying the priority formation of MoB rather than Mo 2 C, which is consistent with the XRD result in The ceramic pre-layer samples were pack siliconized further to obtain the MoSi 2 -based composite coating.Figure 5a displays the XRD pattern of the surface of the composite coating.The XRD pattern shows the peaks of MoSi 2 (JCPDS No. 80-0544) and ZrB 2 (JCPDS No. 89-3930).The diffraction peaks belonging to MoSi 2 and ZrB 2 were both shifted toward the larger angles.According to the above analysis, the X-ray diffraction peaks of the ZrB 2 phase with the solution of C shifted toward the larger angles.It is supposed that C or/and B dissolved in the MoSi 2 phase.Since the atomic radius of the B and C atoms are smaller than that of Si atom (1.18 Å), the peak of MoSi 2 also shifted toward lager angles.The ZrO 2 phase in the sintered ceramic pre-layer was not detected in the as-packed coating probably because of low content.It is noted that the MoB phase in the sintered ceramic pre-layer was also absent in the as-packed coating due to the chemical reaction with Si to form MoSi 2 , which will be discussed below.It can be seen from Figure 5b that only MoSi 2 (JCPDS No. #80-0544) was detected for the MoSi 2 coating.
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Microstructure and Phase Composition of the Coating
The ceramic pre-layer samples were pack siliconized further to obtain the MoSi2based composite coating.Figure 5a displays the XRD pattern of the surface of the composite coating.The XRD pattern shows the peaks of MoSi2 (JCPDS No. 80-0544) and ZrB2 (JCPDS No. 89-3930).The diffraction peaks belonging to MoSi2 and ZrB2 were both shifted toward the larger angles.According to the above analysis, the X-ray diffraction peaks of the ZrB2 phase with the solution of C shifted toward the larger angles.It is supposed that C or/and B dissolved in the MoSi2 phase.Since the atomic radius of the B and C atoms are smaller than that of Si atom (1.18 Å), the peak of MoSi2 also shifted toward lager angles.The ZrO2 phase in the sintered ceramic pre-layer was not detected in the aspacked coating probably because of low content.It is noted that the MoB phase in the sintered ceramic pre-layer was also absent in the as-packed coating due to the chemical reaction with Si to form MoSi2, which will be discussed below.It can be seen from Figure 5b that only MoSi2 (JCPDS No. #80-0544) was detected for the MoSi2 coating.Figure 6a-c show the surface morphology of the as-prepared MoSi2-ZrB2 coating.Element mapping of the surface of the coating in Figure 6b is displayed in Figure 6d-i.As shown in Figure 6a, the surface of the coating was more compact compared to the ceramic pre-layer in Figure 2a.However, the coating surface was relatively loose and rough.Island-shape clusters were dispersed uniformly on the entire coating surface.An enlarged view of area B in Figure 6a was presented in Figure 6b.As shown in the picture, the average diameter of these clusters was approximately 20 μm, and they were closely adhered to the other clusters around them.Some flaky particles were inlaying in the clusters, and were supposed to be Al2O3 according to element mapping in Figure 6h,i, which was originated from the pack mixture during pack cementation.Al2O3 has been proved to be beneficial to the high-temperature oxidation resistance of the MoSi2 coating in our previous work [18,19].Nevertheless, Al2O3 was not detected by XRD in Figure 5 due to low content.An enlarged view of area C in Figure 6b is given in Figure 6c.Combined with Figure 6dg, it is clear that a number of the ZrB2 particles with size of about 1~3 μm were distributed on the surface of the MoSi2 clusters, which could be confirmed by XRD in Figure 6 and element mapping in Figure 6f,g.Figure 6a-c show the surface morphology of the as-prepared MoSi 2 -ZrB 2 coating.Element mapping of the surface of the coating in Figure 6b is displayed in Figure 6d-i.As shown in Figure 6a, the surface of the coating was more compact compared to the ceramic pre-layer in Figure 2a.However, the coating surface was relatively loose and rough.Island-shape clusters were dispersed uniformly on the entire coating surface.An enlarged view of area B in Figure 6a was presented in Figure 6b.As shown in the picture, the average diameter of these clusters was approximately 20 µm, and they were closely adhered to the other clusters around them.Some flaky particles were inlaying in the clusters, and were supposed to be Al 2 O 3 according to element mapping in Figure 6h,i, which was originated from the pack mixture during pack cementation.Al 2 O 3 has been proved to be beneficial to the high-temperature oxidation resistance of the MoSi 2 coating in our previous work [18,19].Nevertheless, Al 2 O 3 was not detected by XRD in Figure 5 due to low content.An enlarged view of area C in Figure 6b is given in Figure 6c.Combined with Figure 6d-g, it is clear that a number of the ZrB 2 particles with size of about 1~3 µm were distributed on the surface of the MoSi 2 clusters, which could be confirmed by XRD in Figure 6 and element mapping in Figure 6f,g.Figure 7a show the cross-sectional microstructure of the as-prepared coating.The coating displayed a multi-layer structure, consisting of an outer layer (layer I in Figure 7a), an inner layer (layer II in Figure 7a) and an interdiffusion layer (layer III in Figure 7a) from outside to inside, with thicknesses of about 20~24 μm, 57~67 μm and 5~7 μm, respectively.WDS analysis revealed that the composition of Spot 1 in Figure 7a is 31.45Mo-66.09Si-0.83Zr-0.53B-0.67Al-0.43O(at.%), and it was identified to be MoSi2.The composition of Spot 2 was similar to that of Spot 1, which was composed of MoSi2 as well.Black phases (Spot 3) were observed inlaid in the outer layer and were determined to be Al2O3 by WDS analysis.The diffusion layer (layer III) between the coating and substrate mainly contained the Mo and B elements and was inferred to be MoB since the ratio of Mo and B was ~1:1 by WDS analysis.Figure 7b-f  Figure 7a show the cross-sectional microstructure of the as-prepared coating.The coating displayed a multi-layer structure, consisting of an outer layer (layer I in Figure 7a), an inner layer (layer II in Figure 7a) and an interdiffusion layer (layer III in Figure 7a) from outside to inside, with thicknesses of about 20~24 µm, 57~67 µm and 5~7 µm, respectively.WDS analysis revealed that the composition of Spot 1 in Figure 7a is 31.45Mo-66.09Si-0.83Zr-0.53B-0.67Al-0.43O(at.%), and it was identified to be MoSi 2 .The composition of Spot 2 was similar to that of Spot 1, which was composed of MoSi 2 as well.Black phases (Spot 3) were observed inlaid in the outer layer and were determined to be Al 2 O 3 by WDS analysis.The diffusion layer (layer III) between the coating and substrate mainly contained the Mo and B elements and was inferred to be MoB since the ratio of Mo and B was ~1:1 by WDS analysis.Figure 7b-f show the element mapping of the cross section of the as-prepared composite coating, which further confirms the phase distribution and element composition of the coating.It can be seen that the Si elements were mainly distributed in inner layer (layer II) and outer layer (layer I), while the diffusion layer (layer III) was rich in B and poor in Si.The Zr elements were mainly distributed in the inner layer.Combined with XRD pattern and EPMA analysis, it is concluded that the outer layer (layer I) consisted of MoSi 2 and Al 2 O 3 , and the inner layer (layer II) comprised a mixture of MoSi 2 and ZrB 2 , and the diffusion layer (layer III) was MoB.
Microcracks were found at the interface between the MoB diffusion layer and MoSi 2 layer, which was expected to form during the cooling down from the sintering temperature to room temperature, mainly due to the difference of the coefficient of thermal expansion (CTE) between and MoB and MoSi 2 .Thermal stress resulting from the CTE mismatch between the interface could cause cracks in the coating if they reached or exceeded a critical value of the strength of MoSi 2 .The thermally induced stress on the cooling can be approximated calculated by the equation as follow [48].
where σ therm is the thermal induced stress, E is the Young's modulus, ∆T is the temperature difference, α is the CTE, υ is the Poisson's ratio, and the subscripts C and S represent the coating and substrate.The thermal stresses developed in the MoSi 2 coating are 1041 MPa and 441 MPa for Mo and MoB, respectively, which definitely exceed the tensile strength of MoSi 2 (275 MPa) [18,48,49].
During siliconization, Si would react with NaF to generate active Si atoms via catalysis of NaF, which was favorable for the formation of the MoSi 2 -based coating due to the lower activation energy of the Si diffusion [50].The possible reactions during siliconization are listed in Reactions ( 10)-( 14): 2Si 3Si + 5Mo = Mo 5 Si 3 (11) 2Si + Mo = MoSi 2 (12) 7Si + Mo 5 Si 3 = 5MoSi 2 (13) B + Mo = MoB (14) Figure 8 shows the values of the Gibbs free energy for the Reactions ( 10)- (14).All the Reactions are thermodynamically favorable in the calculated temperature range.During the siliconization process, active Si started to deposit on the surface of the samples and then diffused into the ceramic pre-layer and reacted with the ceramic pre-layer to form silicides by Reaction (10).Because the structure of the ceramic pre-layer was loose, the active Si could diffuse through the pre-layer and react with the TZM Mo-based substrate to generate Mo-silicides by Reactions ( 11)-( 13).The sintering temperature was far below the melting point of ZrB 2 , and there was no chemical reaction between ZrB 2 and Si at the sintering temperature.According to the XRD patterns in Figures 1 and 5a, the ZrB 2 particles were restrained from the ceramic pre-layer to the as-packed coating.Note that a MoB diffusion layer (layer III in Figure 7a) was formed between the MoSi 2 layer (layer II in Figure 7a) and the TZM substrate.During siliconization, the consumption of MoB in the ceramic pre-layer to MoSi 2 by Reaction (10) released B. The diffusion of B in the coating is much faster than that of Si; thus, the excess B would diffuse toward the coating-substrate interface and reacted with Mo at the interface to form a thin MoB diffusion layer between the coating and substrate by Reaction (14).According to the above analysis, the schematic diagram of the coating preparation is shown in Figure 9.

Oxidation Behavior of the Coating
Figure 10a shows the isothermal oxidation kinetic curves of the coating at 1600 °C.The composite coating performed excellent oxidation resistance and could offer effective anti-oxidation protection for the TZM alloy over 20 h at 1600 °C.The weight gain of the oxidized coating was plotted as a function of oxidation time.As seen in Figure 10a, the coating exhibited continuous weight gain with increasing oxidation time.The mass gain of the sample increased rapidly and approximately linear until the value reached 2.16 mg/cm 2 after 2 h of oxidation.The overall weight gain of the coating was 5.24 mg/cm 2 after 20 h of oxidation.The approximate linear relationship between the oxidation time and the square of the mass gain in Figure 10b implied that the mass gain of the coating obeyed a parabolic law.The parabolic rate constant could be calculated by Equation ( 15) [51]: where △ m is the mass change of the coated sample (mg), A is the superficial area (cm 2 ) and t is the oxidation time (h), and K P is the oxidation rate constant (mg 2 •cm −4 •h −1 ).The oxidation parabolic constant K P , which corresponds to the slope of the curve, was approximately 1.2811 mg 2 •cm −4 •h −1 .It is reported in previous works [18,19] that the weight gain of the coating is likely to relate to the formation of an oxide scale on the coating surface during oxidation.Chemical reactions of the coating with oxygen were conducted to form an SiO2-based oxides scale (containing e.g., SiO2, ZrO2, B2O3) when exposed to high-temperature oxidizing environments.However, in the initial stage, the oxide scale formed on the coating surface was incomplete and discontinuous, which could not completely cover the substrate, and the coating phases were still exposed to air and could react with oxygen.Consequently, rapid oxidation was observed in the initial stage.When a compact protective oxide scale was formed on the surface of the coating, the oxidation rate was under the control of the permeation of oxygen through the oxide scale; subsequently, the mass gain of the coatings slowed down.

Oxidation Behavior of the Coating
Figure 10a shows the isothermal oxidation kinetic curves of the coating at 1600 • C. The composite coating performed excellent oxidation resistance and could offer effective antioxidation protection for the TZM alloy over 20 h at 1600 • C. The weight gain of the oxidized coating was plotted as a function of oxidation time.As seen in Figure 10a, the coating exhibited continuous weight gain with increasing oxidation time.The mass gain of the sample increased rapidly and approximately linear until the value reached 2.16 mg/cm 2 after 2 h of oxidation.The overall weight gain of the coating was 5.24 mg/cm 2 after 20 h of oxidation.The approximate linear relationship between the oxidation time and the square of the mass gain in Figure 10b implied that the mass gain of the coating obeyed a parabolic law.The parabolic rate constant could be calculated by Equation ( 15) [51]: where m is the mass change of the coated sample (mg), A is the superficial area (cm 2 ) and t is the oxidation time (h), and K P is the oxidation rate constant (mg 2 •cm −4 •h −1 ).The oxidation parabolic constant K P , which corresponds to the slope of the curve, was approximately 1.2811 mg

Phase Composition of the Oxidized Coating
Figure 11a shows the XRD pattern of the surface of the MoSi2-ZrB2 coating after 2 h of oxidation.The characteristic diffraction peaks of SiO2 (JCPDS No. 70-2538, 70-2539), ZrO2 (JCPDS No. 79-1796), and Mo5Si3 (JCPDS NO.65-2783) were detected.There was a broad hump in the pattern in Figure 11a, which could be assigned to amorphous SiO2.The XRD result in Figure 11a indicates the oxidation of MoSi2 and ZrB2 to form Mo5Si3, SiO2, and ZrO2.The XRD pattern of the surface of the single MoSi2 coating after 2 h of oxidation is given in Figure 11b.The result indicates that the phases of the MoSi2 coating after 2 h of It is reported in previous works [18,19] that the weight gain of the coating is likely to relate to the formation of an oxide scale on the coating surface during oxidation.Chemical reactions of the coating with oxygen were conducted to form an SiO 2 -based oxides scale (containing e.g., SiO 2 , ZrO 2 , B 2 O 3 ) when exposed to high-temperature oxidizing environments.However, in the initial stage, the oxide scale formed on the coating surface was incomplete and discontinuous, which could not completely cover the substrate, and the coating phases were still exposed to air and could react with oxygen.Consequently, rapid oxidation was observed in the initial stage.When a compact protective oxide scale was formed on the surface of the coating, the oxidation rate was under the control of the permeation of oxygen through the oxide scale; subsequently, the mass gain of the coatings slowed down.

Phase Composition of the Oxidized Coating
Figure 11a shows the XRD pattern of the surface of the MoSi 2 -ZrB 2 coating after 2 h of oxidation.The characteristic diffraction peaks of SiO 2 (JCPDS No. 70-2538, 70-2539), ZrO 2 (JCPDS No. 79-1796), and Mo 5 Si 3 (JCPDS NO.65-2783) were detected.There was a broad hump in the pattern in Figure 11a, which could be assigned to amorphous SiO 2 .The XRD result in Figure 11a indicates the oxidation of MoSi 2 and ZrB 2 to form Mo 5 Si 3 , SiO 2 , and ZrO 2 .The XRD pattern of the surface of the single MoSi 2 coating after 2 h of oxidation is given in Figure 11b.The result indicates that the phases of the MoSi 2 coating after 2 h of oxidation was comprised of Mo 5 Si 3 (JCPDS NO.65-2783), MoSi 2 (JCPDS NO.80-0544) and SiO 2 (JCPDS NO.70-2538).There was also a broad hump in the pattern of single MoSi 2 after 2 h of oxidation, which was amorphous SiO 2 as well.Since the oxide scale formed on the surface of the MoSi 2 coating after oxidation at 1600 • C for 2 h was relatively thin, the MoSi 2 phase in the residual coating beneath the scale was also detected.

Phase Composition of the Oxidized Coating
Figure 11a shows the XRD pattern of the surface of the MoSi2-ZrB2 coating after 2 h of oxidation.The characteristic diffraction peaks of SiO2 (JCPDS No. 70-2538, 70-2539), ZrO2 (JCPDS No. 79-1796), and Mo5Si3 (JCPDS NO.65-2783) were detected.There was a broad hump in the pattern in Figure 11a, which could be assigned to amorphous SiO2.The XRD result in Figure 11a indicates the oxidation of MoSi2 and ZrB2 to form Mo5Si3, SiO2, and ZrO2.The XRD pattern of the surface of the single MoSi2 coating after 2 h of oxidation is given in Figure 11b.The result indicates that the phases of the MoSi2 coating after 2 h of oxidation was comprised of Mo5Si3 (JCPDS NO.65-2783), MoSi2 (JCPDS NO.80-0544) and SiO2 (JCPDS NO.70-2538).There was also a broad hump in the pattern of single MoSi2 after 2 h of oxidation, which was amorphous SiO2 as well.Since the oxide scale formed on the surface of the MoSi2 coating after oxidation at 1600 °C for 2 h was relatively thin, the MoSi2 phase in the residual coating beneath the scale was also detected.C for 2 h, respectively.Both coatings after oxidation exhibited a compact and smooth surface compared to that of the as-prepared coating.A glassy oxide scale embedded with grayish-white particles (Spot 1) was formed on the surface of the MoSi 2 -ZrB 2 composite coating after oxidation, as shown in Figure 12b.However, a compact glassy oxide scale without dopants was observed on the surface of MoSi 2 , as shown in Figure 12a.Figure 12c-f display the element mapping of the surface of the MoSi 2 -ZrB 2 coating after 2 h of oxidation.It can be seen that the glassy phases were rich in Si and O (SiO 2 -based glassy oxide), while the grayish-white particles were rich in Zr and O.The Zr-rich regions in Figure 12d match well with the grayish-white particles regions in Figure 12b.WDS analysis combined with XRD results in Figure 11 confirmed that the grayish-white particle was ZrO 2 and the glassy oxide phase was SiO 2 .It has been widely reported that the existence of ZrO 2 on the SiO 2 -based oxide scale was beneficial to improve the anti-oxidation property of the oxide scale, attributing to its excellent thermal stability, low oxygen permeability, and the enhancement of thermal mismatch between the oxide and coating layers [20].B was not detected in the oxide scale by WDS, mainly due to volatilization at 1600 • C (above boiling point of B 2 O 3 ).
coating.A glassy oxide scale embedded with grayish-white particles (Spot 1) was formed on the surface of the MoSi2-ZrB2 composite coating after oxidation, as shown in Figure 12b.However, a compact glassy oxide scale without dopants was observed on the surface of MoSi2, as shown in Figure 12a.Figure 12c-f display the element mapping of the surface of the MoSi2-ZrB2 coating after 2 h of oxidation.It can be seen that the glassy phases were rich in Si and O (SiO2-based glassy oxide), while the grayish-white particles were rich in Zr and O.The Zr-rich regions in Figure 12d match well with the grayish-white particles regions in Figure 12b.WDS analysis combined with XRD results in Figure 11 confirmed that the grayish-white particle was ZrO2 and the glassy oxide phase was SiO2.It has been widely reported that the existence of ZrO2 on the SiO2-based oxide scale was beneficial to improve the anti-oxidation property of the oxide scale, attributing to its excellent thermal stability, low oxygen permeability, and the enhancement of thermal mismatch between the oxide and coating layers [20].B was not detected in the oxide scale by WDS, mainly due to volatilization at 1600 °C (above boiling point of B2O3).13a that a five-layer structure (marked as layer I, II, III, IV, V, respectively, from outside to inside) was developed in the cross section of the MoSi 2 -ZrB 2 coating after oxidation.Combined with XRD and WDS analysis, the five-layer structure was identified to be composed of a dense SiO 2 -based oxide scale with dispersed ZrO 2 white particles (layer I), a Mo 5 Si 3 diffusion layer between the coating and the oxide scale (layer II), a MoSi 2 layer (layer III), a Mo 5 Si 3 diffusion layer beneath the MoSi 2 layer (layer IV) and a MoB layer between the coating and substrate (layer V), with a thickness of about 11.0 µm, 8.22 µm, 28.8 µm, 18.5 µm, and 11.5 µm, respectively.According to Figure 13a and the element mappling in Figure 14c,d, some ZrO 2 white particles distributed on the top of the oxide scale, which is in agreement with the surface observation in Figure 11b.Aggregation tends to take place among the ZrO 2 particles.The formation of the outer Mo 5 Si 3 layer (layer II in Figure 13a) was attributed to the oxidation of MoSi 2 , whereas the formation of the inner Mo 5 Si 3 layer (layer IV in Figure 13a) was derived from inward diffusion of Si from the coating to the substate [18].
distributed on the top of the oxide scale, which is in agreement with the surface observation in Figure 11b.Aggregation tends to take place among the ZrO2 particles.The formation of the outer Mo5Si3 layer (layer II in Figure 13a) was attributed to the oxidation of MoSi2, whereas the formation of the inner Mo5Si3 layer (layer IV in Figure 13a) was derived from inward diffusion of Si from the coating to the substate [18].Cracks were found in the MoSi 2 layer and Mo 5 Si 3 layer in the oxidized MoSi 2 -ZrB 2 coating.The formation of the cracks was attributed to thermal stresses induced by the CTE mismatch at the coating-substrate or coating-oxide scale interface.However, no obvious oxygen diffusion into the substrate through the cracks was observed, which indicated that the cracks were sealed by the glassy oxides.The cracks developed into through-thickness cracks perpendicular to the substrate in the MoSi 2 layer (layer III in Figure 13a) and stopped growing or transformed into transverse cracks in the Mo 5 Si 3 layer (layer IV in Figure 13a), as shown in Figure 12a.A similar phenomenon was observed for the single-MoSi 2 coating, which indicates that the Mo 5 Si 3 diffusion layer beneath the MoSi 2 layer played an important role in hindering the crack propagation toward the substrate for both coatings.Cracks were found in the MoSi2 layer and Mo5Si3 layer in the oxidized MoSi2-ZrB2 coating.The formation of the cracks was attributed to thermal stresses induced by the CTE mismatch at the coating-substrate or coating-oxide scale interface.However, no obvious oxygen diffusion into the substrate through the cracks was observed, which indicated that the cracks were sealed by the glassy oxides.The cracks developed into through-thickness cracks perpendicular to the substrate in the MoSi2 layer (layer III in Figure 13a) and stopped growing or transformed into transverse cracks in the Mo5Si3 layer (layer IV in Figure 13a), as shown in Figure 12a.A similar phenomenon was observed for the single-MoSi2 coating, which indicates that the Mo5Si3 diffusion layer beneath the MoSi2 layer played an important role in hindering the crack propagation toward the substrate for both coatings.
Similarly, as shown in Figure 13b, a four-layer structure (marked as layer I', II', III', IV', respectively, from outside to inside) was observed in the oxidized MoSi2 coating: a thin SiO2 oxide scale (layer I'), a Mo5Si3 diffusion layer between the coating and oxide scale (layer II'), a MoSi2 layer (layer III'), and a Mo5Si3 diffusion layer beneath MoSi2 layer (layer IV'), with a thickness of 8.2 μm, 10.0 μm, 15.8 μm, and 24.8 μm, respectively.
It is noteworthy to contrastively analyze the thicknesses of the Mo5Si3 diffusion layers between the coating and the substrate (layer IV in Figure 13a   Similarly, as shown in Figure 13b, a four-layer structure (marked as layer I', II', III', IV', respectively, from outside to inside) was observed in the oxidized MoSi 2 coating: a thin SiO 2 oxide scale (layer I'), a Mo 5 Si 3 diffusion layer between the coating and oxide scale (layer II'), a MoSi 2 layer (layer III'), and a Mo 5 Si 3 diffusion layer beneath MoSi 2 layer (layer IV'), with a thickness of 8.2 µm, 10.0 µm, 15.8 µm, and 24.8 µm, respectively.
It is noteworthy to contrastively analyze the thicknesses of the Mo 5 Si 3 diffusion layers between the coating and the substrate (layer IV in Figure 13a  It is clear that, after 2 h of oxidation, the thickness of the Mo 5 Si 3 diffusion layer between the coating and the substrate for the MoSi 2 -ZrB 2 coating was larger than that for the MoSi 2 coating while the thickness of the residual MoSi 2 layer for the MoSi 2 -ZrB 2 coating was smaller than that for the MoSi 2 coating.It can be deduced that the MoB layer played an important role in preventing the diffusion of Si from the coating to the substrate. Figure 15 shows the surface and the cross-sectional microstructure of the MoSi 2 -ZrB 2 coating after oxidation for a longer time (10 h) at 1600 • C. It can be seen that the crosssectional microstructure of the composite coating after 10 h of oxidation was composed of five layers, which was similar to that after 2 h oxidation.Figure 16 shows the XRD pattern of the composite coating after 10 h of oxidation.The marks show the positions of the peaks corresponding to Mo 5 Si 3 , SiO 2 , ZrO 2 , ZrSiO 4 , and Al 2 O 3 .The formation of ZrSiO 4 demonstrated the chemical reaction between the formed SiO 2 and ZrO 2 .Moreover, combined with WDS, we concluded that the phase composition of each layer in the cross section of 10 h was also the same with that of 2 h, although the thickness differed.The thickness of SiO 2 -based oxide scale (layer I in Figure 15) reduced to ~6.8 µm, while the Mo 5 Si 3 diffusion layer (layer IV in Figure 15) thickened to ~113.6 µm.Compared to the MoSi 2 -based coating without the MoB diffusion barrier in our previous works [18,19], it is worth mentioning in this work that even after 10 h of oxidation, the MoSi 2 layer was not completely converted to Mo 5 Si 3 , which could still provide the reservoir of Si for the formation of the compact SiO 2 -based glass.It is inferred that the MoB diffusion barrier played an important role in restraining the depletion of MoSi 2 by hindering the Si diffusion toward the substrate, which proved to be beneficial to the long life of the coating.It also can be seen that the composite coating was retained intact and the SiO 2 -based glassy oxide scale was still protective since no through-thickness crack was formed in the oxide scale, or the coating and the oxide scale was well adhered to the coating.
sectional microstructure of the composite coating after 10 h of oxidation was composed of five layers, which was similar to that after 2 h oxidation.Figure 16 shows the XRD pattern of the composite coating after 10 h of oxidation.The marks show the positions of the peaks corresponding to Mo5Si3, SiO2, ZrO2, ZrSiO4, and Al2O3.The formation of ZrSiO4 demonstrated the chemical reaction between the formed SiO2 and ZrO2.Moreover, combined with WDS, we concluded that the phase composition of each layer in the cross section of 10 h was also the same with that of 2 h, although the thickness differed.The thickness of SiO2-based oxide scale (layer I in Figure 15) reduced to ~6.8 μm, while the Mo5Si3 diffusion layer (layer IV in Figure 15) thickened to ~113.6 μm.Compared to the MoSi2-based coating without the MoB diffusion barrier in our previous works [18,19], it is worth mentioning in this work that even after 10 h of oxidation, the MoSi2 layer was not completely converted to Mo5Si3, which could still provide the reservoir of Si for the formation of the compact SiO2-based glass.It is inferred that the MoB diffusion barrier played an important role in restraining the depletion of MoSi2 by hindering the Si diffusion toward the substrate, which proved to be beneficial to the long life of the coating.It also can be seen that the composite coating was retained intact and the SiO2-based glassy oxide scale was still protective since no through-thickness crack was formed in the oxide scale, or the coating and the oxide scale was well adhered to the coating.SiO2-based oxide scale (layer I in Figure 15) reduced to ~6.8 μm, while the Mo5S layer (layer IV in Figure 15) thickened to ~113.6 μm.Compared to the MoSi2-ba without the MoB diffusion barrier in our previous works [18,19], it is worth in this work that even after 10 h of oxidation, the MoSi2 layer was not com verted to Mo5Si3, which could still provide the reservoir of Si for the formation pact SiO2-based glass.It is inferred that the MoB diffusion barrier played an role in restraining the depletion of MoSi2 by hindering the Si diffusion towa strate, which proved to be beneficial to the long life of the coating.It also can the composite coating was retained intact and the SiO2-based glassy oxide sc protective since no through-thickness crack was formed in the oxide scale, or and the oxide scale was well adhered to the coating.16)-( 20) are possible to conduct when the MoSi 2 -ZrB 2 coating samples were exposed to an oxidizing environment at 1600 • C. As mentioned in previous work [18,19], the standard Gibbs free energy (based on one mol oxygen) of Reaction ( 16) is more negative than that of Reaction (17).Thus, the oxidation of MoSi 2 to form Mo 5 Si 3 and SiO 2 by Reaction ( 16) was dominant.As a result, a SiO 2 -based oxide scale was formed on the coating surface; simultaneously, a thin Mo 5 Si 3 layer was generated between the MoSi 2 -ZrB 2 layer and the oxide scale due to the selective oxidation of Si.Meanwhile, the oxidation of ZrB 2 could produce ZrO 2 and B 2 O 3 by Reaction (18).Since B 2 O 3 would volatilize rapidly at the oxidation test temperature (1600 • C), the oxide scale was composed of ZrO 2 and SiO 2 .With the increase in the oxidation time, ZrO 2 and SiO 2 would react to generate ZrSiO 4 by Reaction (19), and a compact glassy SiO 2 -ZrO 2 -ZrSiO 4 oxide scale would form and completely cover the coating surface.The pores and pits in the coating could be filled; SiO 2 , ZrO 2 , and ZrSiO 4 all have low oxygen diffusion coefficients and the inward diffusion of oxygen could be effectively suppressed [52].It has been reported that the formation of a composite SiO 2 -based oxide scale embedded with the thermal stable ZrO 2 and ZrSiO 4 is believed to enhance the high-temperature oxidation resistance of the MoSi 2 -based coating.SiO 2 -ZrO 2 -ZrSiO 4 could develop a special glassceramic skeleton structure on the surface of the coating and possessed the peculiarities of the stress tolerance of a glass scale and the structural stability of the ceramic phase, in which the ZrO 2 and ZrSiO 4 particles have a pinning effect on the SiO 2 glassy oxide.Mo 5 Si 3 would be generated beneath the MoSi 2 layer, which was derived from the inward diffusion of Si (MoSi 2 ) from the coating to the substrate according to Reaction (20).At the same time, the MoB layer formed in the as-packed coating could retard the degradation of MoSi 2 into Mo 5 Si 3 low-Si silicide by hindering the inward diffusion of Si from the coating to the substrate, which is beneficial to prolonging the service life of MoSi 2 -based coating at high temperatures. 5/7MoSi

Conclusions
In this work, ZrB2 particles were introduced to the MoSi2 coating on the TZM alloy by the in situ reaction of Zr and B4C to form a novel MoSi2-ZrB2 composite coating by a two-step process, including slurry sintering of Mo-Zr-B4C followed by pack siliconizing

Conclusions
In this work, ZrB 2 particles were introduced to the MoSi 2 coating on the TZM alloy by the in situ reaction of Zr and B 4 C to form a novel MoSi 2 -ZrB 2 composite coating by a two-step process, including slurry sintering of Mo-Zr-B 4 C followed by pack siliconizing.A ceramic pre-layer composed of ZrB 2 and MoB was obtained on the TZM substrate by the sintering of Mo, Zr, and B 4 C.After pack siliconizing, the coating was converted into a multi-layer structure, consisting of a MoB inner layer and a MoSi 2 -ZrB 2 outer layer.The oxidation experiments revealed that the composite coating could provide excellent oxidation-resistant protection for the TZM alloy substrate.The oxidation kinetic curve of the composite coating followed the parabolic rule, and the weight gain of the coated sample after 20 h of oxidation at 1600 • C was only 5.24 mg/cm 2 .When exposed to hightemperature oxidizing environments, the coating rapidly generated a dense and continuous SiO 2 -ZrO 2 -ZrSiO 4 oxide scale with high thermal stability and low oxygen permeability, which hindered the inward diffusion of oxygen at high temperatures.Concurrently, the MoB inner diffusion layer acted as a diffusion barrier and could hinder the diffusion of Si inward with regard to the TZM alloy substrate and improve the long life of the coating.The excellent oxidation resistance of the coating was mainly attributed to the formation of a compact SiO 2 -ZrO 2 -ZrSiO 4 composite oxide scale with low oxygen permeability and high thermally stability on the coating surface and the diffusion barrier effect of the MoB layer, which hindered the degradation of MoSi 2 .

3 . 1 .
Figure 1 displays the XRD pattern of the surface of the sintered ceramic pre-layer.According to the XRD pattern, the as-fabricated ceramic pre-layer was composed of ZrB 2 and MoB.Carbide (e.g., ZrC, Mo 2 C, MoC) was not obtained in the sintered samples.The diffraction peaks belonging to ZrB 2 were shifted toward larger angles, which are potentially attributed to dissolution of C in the ZrB 2 lattice.No initial component such as Zr, Mo, or B 4 C was detected in the sintered samples, indicating sufficient reactions between Zr, Mo, and B 4 C.The presence of ZrO 2 in the sintered pre-layer could be attributed to the oxide impurities on the surface of the strating powders of Zr and B 4 C.Meanwhile, Zr and B 2 O 3 would react and generate ZrO 2 from 5Zr + 2B 2 O 3 = 3ZrO 2 + 2ZrB 2[41][42][43].However, in our previous work, ZrO 2 proved to be beneficial to the high temperature oxidation resistance of silicide coating[44].

Figure 1 .
Figure 1.XRD pattern of the surface of the pre-fabricated ceramic layer.

Figure 2 .
Figure 2. (a) surface morphology, (b) enlarged view of area A in (a), (c-f) element mapping of the surface of the ceramic pre-layer in (b).

Figure 2 .
Figure 2. (a) surface morphology, (b) enlarged view of area A in (a), (c-f) element mapping of the surface of the ceramic pre-layer in (b).

22 Figure 3 .
Figure 3. (a) microstructure and (b-d) element mapping of the cross-sectional microstructure of the ceramic pre-layer.

Figure 3 .
Figure 3. (a) microstructure and (b-d) element mapping of the cross-sectional microstructure of the ceramic pre-layer.

Figure 5 .
Figure 5. XRD pattern of the surface of (a) the composite coating and (b) MoSi2 coating.

Figure 5 .
Figure 5. XRD pattern of the surface of (a) the composite coating and (b) MoSi 2 coating.

Figure 6 .
Figure 6.(a-c) Surface morphology, (d-i) element mapping of the surface of the MoSi2-ZrB2 coating.

Figure 6 .
Figure7ashow the cross-sectional microstructure of the as-prepared coating.The coating displayed a multi-layer structure, consisting of an outer layer (layer I in Figure7a), an inner layer (layer II in Figure7a) and an interdiffusion layer (layer III in Figure7a) from outside to inside, with thicknesses of about 20~24 μm, 57~67 μm and 5~7 μm, respectively.WDS analysis revealed that the composition of Spot 1 in Figure7ais 31.45Mo-66.09Si-0.83Zr-0.53B-0.67Al-0.43O(at.%), and it was identified to be MoSi2.The composition of Spot 2 was similar to that of Spot 1, which was composed of MoSi2 as well.Black phases (Spot 3) were observed inlaid in the outer layer and were determined to be Al2O3 by WDS analysis.The diffusion layer (layer III) between the coating and substrate mainly contained the Mo and B elements and was inferred to be MoB since the ratio of Mo and B was ~1:1 by WDS analysis.Figure7b-fshow the element mapping of the cross section of the as-prepared composite coating, which further confirms the phase distribution and

Figure 7 .
Figure 7. (a) cross-sectional microstructure and (b-f) element mapping of the as-packed composite coating.

Figure 10 .
Figure 10.(a) The weight gain and (b) the square of the wight change of the MoSi2-ZrB2 coating as a function of duration time.

Figure 10 .
Figure 10.(a) The weight gain and (b) the square of the wight change of the MoSi 2 -ZrB 2 coating as a function of duration time.

Figure 10 .
Figure 10.(a) The weight gain and (b) the square of the wight change of the MoSi2-ZrB2 coating as a function of duration time.

Figure 11 .
Figure 11.XRD pattern of the surface of (a) MoSi2-ZrB2 and (b) MoSi2 coating after 2 h of oxidation at 1600 °C.

3. 4 .
Figure12a,b reveal the surface morphology of the single MoSi 2 coating and MoSi 2 -ZrB 2 composite coating after oxidation at 1600 • C for 2 h, respectively.Both coatings after oxidation exhibited a compact and smooth surface compared to that of the as-prepared coating.A glassy oxide scale embedded with grayish-white particles (Spot 1) was formed on the surface of the MoSi 2 -ZrB 2 composite coating after oxidation, as shown in Figure12b.However, a compact glassy oxide scale without dopants was observed on the surface of MoSi 2 , as shown in Figure12a.Figure12c-f display the element mapping of the surface of the MoSi 2 -ZrB 2 coating after 2 h of oxidation.It can be seen that the glassy phases were rich in Si and O (SiO 2 -based glassy oxide), while the grayish-white particles were rich in Zr and O.The Zr-rich regions in Figure12dmatch well with the grayish-white particles regions in Figure12b.WDS analysis combined with XRD results in Figure11 confirmed

Figure 12 .
Figure 12.Surface morphology of (a) MoSi2 coating and (b) MoSi2-ZrB2 coating after 2 h of oxidation and (c-f) element mapping of the surface of the MoSi2-ZrB2 coating after 2 h of oxidation in Figure 9b.

Figure 12 .
Figure 12.Surface morphology of (a) MoSi 2 coating and (b) MoSi 2 -ZrB 2 coating after 2 h of oxidation (c-f) element mapping of the surface of the MoSi 2 -ZrB 2 coating after 2 h of oxidation in Figure 9b.3.4.2.Cross-Sectional Microstructure Figure 13a,b show the cross-sectional microstructure of the MoSi 2 -ZrB 2 coating and MoSi 2 coating after 2 h of oxidation at 1600 • C. It can be seen that both coatings exhibited a multi-layer structure.It can be seen in Figure13athat a five-layer structure (marked as layer I, II, III, IV, V, respectively, from outside to inside) was developed in the cross section of the MoSi 2 -ZrB 2 coating after oxidation.Combined with XRD and WDS analysis, the five-layer structure was identified to be composed of a dense SiO 2 -based oxide scale with dispersed ZrO 2 white particles (layer I), a Mo 5 Si 3 diffusion layer between the coating and the oxide scale (layer II), a MoSi 2 layer (layer III), a Mo 5 Si 3 diffusion layer beneath the MoSi 2 layer (layer IV) and a MoB layer between the coating and substrate (layer V), with a thickness of about 11.0 µm, 8.22 µm, 28.8 µm, 18.5 µm, and 11.5 µm, respectively.According to Figure13aand the element mappling in Figure14c,d, some ZrO 2 white particles distributed on the top of the oxide scale, which is in agreement with the surface observation in Figure11b.Aggregation tends to take place among the ZrO 2 particles.The formation of the outer Mo 5 Si 3 layer (layer II in Figure13a) was attributed to the oxidation of MoSi 2 , whereas the formation of the inner Mo 5 Si 3 layer (layer IV in Figure13a) was derived from inward diffusion of Si from the coating to the substate[18].
about 18.5 μm in thickness, layer IV' in Figure 13b about 24.8 μm in thickness) and the residual MoSi2 layers (layer III in Figure 13a about 28.8 μm in thickness, layer III' in Figure 13b about 15.8 μm in thickness).It is clear that, after 2 h of oxidation, the thickness of the Mo5Si3 diffusion layer between the coating and the substrate for the MoSi2-ZrB2 coating was larger than that for the MoSi2 coating while the thickness of the residual MoSi2 layer for the MoSi2-ZrB2 coating
about 18.5 µm in thickness, layer IV' in Figure 13b about 24.8 µm in thickness) and the residual MoSi 2 layers (layer III in Figure 13a about 28.8 µm in thickness, layer III' in Figure 13b about 15.8 µm in thickness).

Figure 16 .
Figure 16.XRD of the surface of the MoSi2-ZrB2 coating after 10 h oxidation.

Figure 16 .
Figure 16.XRD of the surface of the MoSi 2 -ZrB 2 coating after 10 h oxidation.

3. 5 .
Figure 17 displays the schematic oxidation mechanism of the MoSi 2 -ZrB 2 coating and single MoSi 2 coating at 1600 • C in air.Reactions (16)-(20) are possible to conduct when the MoSi 2 -ZrB 2 coating samples were exposed to an oxidizing environment at 1600 • C. As mentioned in previous work[18,19], the standard Gibbs free energy (based on one mol oxygen) of Reaction (16) is more negative than that of Reaction(17).Thus, the oxidation of MoSi 2 to form Mo 5 Si 3 and SiO 2 by Reaction (16) was dominant.As a result, a SiO 2 -based