A method for preparing ultra-high temperature ceramic coating based on molten salt medium ultra-high temperature metal gas source

Abstract

This invention discloses a method for preparing ultra-high temperature ceramic coatings based on an ultra-high temperature metal gas source using a molten salt medium. The method involves mixing a low-melting-point salt A, a metal fluoride, and a metal M to obtain mixture C. Mixture C is placed in a first crucible. A low-melting-point salt B and a metal M are mixed to obtain mixture D. Mixture D is placed in a second crucible. A carbon substrate is placed in a deposition furnace. The first crucible is then heated to melt mixture C, obtaining melt E. The second crucible is then heated to melt mixture D, obtaining melt F. A carrier gas is introduced into the first crucible, and the carrier gas flows through melt E to form a gas stream containing a metal gas source. This gas stream containing the metal gas source is introduced into the second crucible, where it is filtered through melt F to form a pure metal reaction gas source. Finally, the pure metal reaction gas source is introduced into the deposition furnace to deposit an ultra-high temperature ceramic coating onto the carbon substrate. This method is simple to operate and low in cost.

Description

Technical Field

[0001] This invention belongs to the field of ultra-high temperature ceramic coating technology, specifically relating to a method for preparing ultra-high temperature ceramic coatings based on an ultra-high temperature metal gas source with molten salt medium. Background Technology

[0002] Ultra-high temperature ceramics such as tantalum carbide, hafnium carbide, and zirconium carbide possess extremely high melting points (TaC: 3880°C, HfC: 3928°C, ZrC: 3532°C) and excellent thermal stability, making them key materials in aerospace, nuclear energy, and other fields. Chemical vapor deposition (CVD) is commonly used to prepare tantalum carbide, hafnium carbide, and zirconium carbide coatings. During the CVD deposition of ceramic coatings, a stable supply of metal gas is crucial to determining the coating quality. Traditional methods mainly rely on the sublimation of metal chlorides (such as tantalum pentachloride, hafnium tetrachloride, and zirconium tetrachloride) to generate a gaseous metal source through heating. However, this method has the following drawbacks: First, the raw material utilization rate is low and the cost is high. Chlorides are prone to incomplete decomposition or side reactions at high temperatures, resulting in low metal deposition efficiency. A large amount of unreacted chlorides need to be recovered and treated separately, and the gas passages are easily blocked, requiring frequent replacement of gas passages, evaporators, and other components, which increases costs. Second, the process control is difficult. The chloride sublimation process is sensitive to temperature fluctuations and requires a precision evaporator and conveying system. Otherwise, local condensation can easily block the pipes. Third, there is equipment corrosion and pollution. Chloride byproducts (such as HCl) are highly corrosive and irritating, requiring exhaust gas treatment, which increases equipment and environmental costs. In addition, existing technologies such as metal fluorides, organometallic compounds (MOCVD precursors), or electron beam evaporation can also generate metal gas sources, but they still face problems such as high technical difficulty and high investment costs. Summary of the Invention

[0003] To address the shortcomings of existing technologies, the present invention aims to provide a method for preparing ultra-high temperature ceramic coatings using an ultra-high temperature metal gas source based on molten salt media. The method of the present invention is simple to operate, low in cost, can generate various metal gas sources, obtain various ultra-high temperature ceramic coatings, and has good process applicability.

[0004] To achieve the above objectives, the present invention adopts the following technical solution:

[0005] This invention discloses a method for preparing ultra-high temperature ceramic coatings based on an ultra-high temperature metal gas source using a molten salt medium. The method involves mixing a low-melting-point salt A, a metal fluoride, and a metal M to obtain mixture C. Mixture C is placed in a first crucible. A low-melting-point salt B and a metal M are mixed to obtain mixture D. Mixture D is placed in a second crucible. A carbon substrate is placed in a deposition furnace. The first crucible is then heated to melt mixture C, obtaining melt E. The second crucible is then heated to melt mixture D, obtaining melt F. A carrier gas is introduced into the first crucible, and the carrier gas flows through melt E to form a gas stream containing a metal gas source. This gas stream containing the metal gas source is introduced into the second crucible, where it is filtered through melt F to form a pure metal reaction gas source. Finally, the pure metal reaction gas source is introduced into the deposition furnace to deposit an ultra-high temperature ceramic coating onto the surface of the carbon substrate.

[0006] The low-melting-point salt A is a mixed salt composed of at least two salts;

[0007] The low-melting-point salt A has at least one more salt than the low-melting-point salt B, or the low-melting-point salt A and the low-melting-point salt B have completely different compositions.

[0008] In the melt E, the metal M is in a supersaturated state;

[0009] In the molten material F, the metal M is in a supersaturated state;

[0010] The metal M is selected from at least one of tantalum, zirconium, niobium, and hafnium.

[0011] The preparation method disclosed in this invention employs a staged gas source treatment mode. First, in a first crucible, a metal element reacts with a molten salt medium to generate a mixed gas phase containing the target metal gas source and a large amount of mixed salt vapor. Then, in a second crucible, selective purification of the gas source is performed. The composition of the molten material F in the second crucible differs from that of the mixed salt in the molten material E, but it is also a liquid molten salt containing an excess of metal. According to the principle of phase equilibrium, since the composition of the mixed salt in the second crucible differs from that in the first crucible, the saturated salt vapor generated in the first crucible is unsaturated in the mixed molten salt in the second crucible and will be rapidly captured and dissolved in the molten material in the second crucible. At the same time, since a large amount of metal element has been pre-dissolved in the second crucible, it is already saturated with the target metal gas source, so the metal gas source is hardly absorbed, thus allowing for efficient and selective passage. The gas flow through the system achieves separation of gaseous salt and gaseous metal source at the outlet, resulting in a pure metal reaction gas source. This ensures that the final ultra-high temperature ceramic coating (such as TaC, HfC, ZrC) has high chemical purity, excellent density, and outstanding high temperature performance.

[0012] In this invention, both melt E and melt F contain supersaturated metal M. In melt E, ensuring that metal M is supersaturated can maximize the efficiency of generating gaseous metal M. If it is not supersaturated, a portion of the generated metal gas source will be absorbed by the melt itself, reducing the efficiency of metal gas source generation. In melt F, the supersaturated state of metal M can prevent the metal gas source in the gas flow containing the metal gas source from being absorbed.

[0013] In a preferred embodiment, the low-melting-point salt A is selected from at least two of lithium chloride, sodium chloride, calcium chloride, potassium chloride, magnesium chloride, barium chloride, cesium chloride, lithium fluoride, magnesium fluoride, sodium fluoride, potassium fluoride, and calcium fluoride.

[0014] In a preferred embodiment, the metal fluorate is selected from at least one of potassium fluorotantalate, potassium fluorozirconate, potassium fluoroniobate, and potassium fluorohafnium fluoride. In this invention, the metal fluorate acts as a catalyst, and the elemental metal can react with non-corresponding metal fluorates to generate a gaseous source of the elemental metal.

[0015] In a preferred embodiment, the mass ratio of metal fluoride to metal M in the mixture C is 0.8-1.2:3-8, preferably 1-1.2:3-6. The amount of metal fluoride catalyst is crucial for ensuring the effective dissolution of the elemental metal in the molten salt, generating a sufficient amount of volatile precursor metal gas. Below this range, the reaction kinetics are insufficient; above this range, unnecessary waste occurs.

[0016] In a preferred embodiment, the mass ratio of the total mass of the low-melting-point salt A and the metal fluorate to the mass of metal M in the mixture C is 5-30:1, preferably 5-10:1. Maintaining this mass ratio within the aforementioned range yields optimal results. The appropriate mass ratio of the low-melting-point salt A ensures that the molten salt medium has a suitable melting point and good fluidity, providing an optimal environment for the reaction and gas transport. The mass ratio of the elemental metal to the mixed salt determines the continuity and stability of the gas supply. This ratio ensures a balance between maximizing the reaction interface and continuous replenishment.

[0017] In a preferred embodiment, the metal M in the mixture C comprises a powder state and a three-dimensional mesh skeleton state; the metal M in the three-dimensional mesh skeleton state is obtained by weaving or overlapping filamentous, rod-shaped, sheet-shaped, strip-shaped, or foil-shaped metal M, preferably by weaving filamentous metal M.

[0018] Experiments have shown that the state of metal M has a significant impact on the efficiency and effectiveness of gas generation. Powdered metal can rapidly saturate the metal phase in the molten mixture during heating, which is beneficial for the rapid generation of subsequent metal gas. The addition of a mesh structure is because powdered metal, due to its higher density than liquid molten salt, will sink to the bottom when the mixture reaches a molten state, resulting in a lack of metal in the upper part of the liquid mixture and reducing the efficiency of metal gas generation. The addition of a mesh structure allows for rapid supply of metal to the molten salt system after the metal powder settles to the bottom in the molten state, improving the efficiency of metal source generation. The synergy of these two states ensures the efficiency of gas generation throughout the entire reaction process. If only one state is used, the efficiency of gas generation will be reduced.

[0019] In a further preferred embodiment, the mass ratio of the powdered metal M to the three-dimensional mesh framework metal M is 1:1-3.

[0020] In a preferred embodiment, the low-melting-point salt B is selected from at least one of lithium chloride, sodium chloride, calcium chloride, potassium chloride, magnesium chloride, barium chloride, cesium chloride, lithium fluoride, magnesium fluoride, sodium fluoride, potassium fluoride, and calcium fluoride.

[0021] In a preferred embodiment, the mass ratio of low-melting-point salt B to metal M in the mixture D is 5-20:1, preferably 5-8:1.

[0022] In a preferred embodiment, the metal M in the mixture D comprises a powder state and a three-dimensional mesh skeleton state; the metal M in the three-dimensional mesh skeleton state is obtained by weaving or overlapping filamentous, rod-shaped, sheet-shaped, strip-shaped, or foil-shaped metal M, preferably by weaving filamentous metal M.

[0023] The metal M in mixture D is simultaneously in powder and three-dimensional network framework states. This avoids the situation where the powdered state sinks to the bottom and causes the dissolved metal in the upper molten material F to be in an unsaturated state. This would prevent the metal gas source from the first crucible from being absorbed by the molten salt in the second crucible, thus failing to achieve the desired gas source generation effect. The simultaneous use of powder and three-dimensional network framework states ensures that the metal phase in molten material F is always in a supersaturated state, thereby ensuring the purity and efficiency of the gas source.

[0024] In a further preferred embodiment, the mass ratio of the powdered metal M to the three-dimensional mesh framework metal M is 1:2-4.

[0025] In a preferred embodiment, the carbon substrate is selected from graphite, carbon fiber, and carbon-carbon composite materials.

[0026] In a preferred embodiment, the first crucible is heated to melt the mixture C to obtain the melt E, and the temperature is further increased to 700-1400℃ and held.

[0027] In a preferred embodiment, the second crucible is heated to melt the mixture D to obtain melt F, and the temperature is further increased to 700-1300℃ and held.

[0028] In practice, the first crucible and the second crucible can be placed in the same independently temperature-controlled heating furnace, or they can be placed in different heating furnaces.

[0029] In a preferred embodiment, the bottom of the first crucible is provided with a first air inlet for introducing carrier gas, and the top is provided with a first air outlet for discharging the gas flow containing the metal gas source. The bottom of the second crucible is provided with a second air inlet, which is connected to the first air outlet through a conduit to introduce the gas flow containing the metal gas source into the second crucible. The top of the second crucible is provided with an air outlet, through which the pure metal reaction gas source is introduced into the deposition furnace through a conduit.

[0030] In a preferred embodiment, the deposition temperature is controlled at 1000-1500℃, preferably 1100-1300℃.

[0031] Principles and advantages

[0032] Traditional CVD relies on the physical sublimation of metal halides (chlorides). This is a physical process, facing inherent challenges such as difficulty in controlling sublimation, easy condensation, and highly corrosive byproducts. The core of this invention lies in the in-situ generation of an active metal gas source through a chemical reaction in a molten salt medium. In particular, by utilizing the catalytic reaction of "elemental metal + metal fluoride," the generation of the gas source is transformed from physical sublimation to chemical dissolution. This not only circumvents all the drawbacks of chloride sublimation but also achieves a gentler and more controllable adjustment of the gas source generation rate through chemical reaction kinetics.

[0033] This invention designs and constructs a "dual-crucible synergistic purification and gas source transfer system." It departs from the traditional "single reactor" approach, proposing a staged gas source treatment mode. Through structural design, this system achieves the unique effect of selectively purifying the target metal gas source from complex mixed vapors, fundamentally avoiding the problem of salt vapor co-deposition and coating contamination. First stage (first crucible): Gas source dissolution and generation. Here, the elemental metal reacts with the molten salt medium to generate a mixed gas phase containing the target metal gas source and a large amount of salt vapor. Second stage (second crucible): Selective purification of the gas source. In this invention, the second crucible contains a different low-melting-point salt system than the first crucible, but is a liquid molten salt containing an excess of metal. According to the phase equilibrium principle, the salt vapor entering the second crucible is in an unsaturated state in this liquid salt and is rapidly captured and dissolved in the liquid phase. Meanwhile, since a large amount of elemental metal has been pre-dissolved in the second crucible, saturating it with the target metal gas source, the metal gas source is hardly absorbed, thus passing through efficiently and selectively. The gas flow through the system achieves separation of gaseous salt and gaseous metal source at the outlet, resulting in a pure metal reaction gas source. This invention eliminates the possibility of volatile salt impurities being carried into the deposition furnace at the source of the reaction gas source, avoiding problems such as coating looseness, decreased purity, and performance deterioration caused by salt vapor co-deposition. This ensures that the final ultra-high temperature ceramic coating (such as TaC, HfC, ZrC) has high chemical purity, excellent density, and outstanding high-temperature performance.

[0034] Furthermore, the metal in this invention adopts a combination of powder form and three-dimensional mesh framework, which efficiently solves the engineering problem of reactant sedimentation caused by the density difference between solid and liquid in molten salt system. This design takes into account both the initial reaction rate (the powder dissolves rapidly in the molten salt when the mixed salt becomes liquid, so that the metal phase quickly reaches saturation) and the long-term stable supply of reactants (the mesh structure prevents sedimentation and allows for continuous release).

[0035] In this invention, there is no need to use large amounts of metal chlorides that are easily hydrolyzed and produce HCl byproducts. Instead, a relatively stable molten salt system is used, which avoids the generation of highly corrosive byproducts from the source, greatly reducing the corrosion resistance requirements of equipment and the cost of exhaust gas treatment. In addition, the reaction is carried out in a closed system, and the metal element reacts continuously in the molten salt, which can achieve a metal conversion efficiency far higher than that of the chloride sublimation method, solving the core pain point of "high raw material utilization rate and low cost" in the background technology. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the process in this invention. In the diagram, 1-melt E, 2-melt F, 3-first crucible, 4-second crucible, 5-conduit, and 6-carbon substrate.

[0037] Figure 2 The graphite in Example 1 has a tantalum carbide coating deposited on it.

[0038] Figure 3 This is a SEM image of the tantalum carbide coating in Example 1.

[0039] Figure 4 The image shows a comparison between carbon fibers without a deposited coating and carbon fibers with a deposited tantalum carbide coating in Example 2.

[0040] Figure 5 This is a SEM image of the fracture surface of carbon fiber with ZrC coating deposited in Example 3.

[0041] Figure 6 SEM image of the tantalum carbide coating prepared by deposition for Comparative Example 1.

[0042] Figure 7 The image shows a SEM image of the fracture cross-section of the carbon fiber deposited in Comparative Example 2. Detailed Implementation

[0043] Example 1

[0044] Preparation of tantalum carbide coating on graphite surface:

[0045] Weigh out 1000g of sodium chloride (99.5% purity), 1274g of potassium chloride (99.5% purity), 100g of tantalum powder (99.9%, 100 mesh), 200g of tantalum wire woven network structure (99.9%, 0.5mm diameter), and 50g of potassium fluorotantalate (99.1% purity). Mix them thoroughly and place them into the first graphite crucible. Place the first crucible into heating furnace a. Weigh out 1200g of sodium chloride (99.5% purity), 50g of tantalum powder (99.9%, 100 mesh), and 150g of tantalum wire woven network structure (99.9%, 0.5mm diameter) and place them into the second crucible. Place the second crucible into heating furnace b. Place the graphite part to be coated in the deposition furnace. Heating furnace a is started and heated to 1300℃, while heating furnace b is started and heated to 1200℃. Argon gas is introduced into the first crucible as a carrier gas. The carrier gas vapor stream generated in the first crucible is introduced into the second crucible through a conduit. The filtered vapor stream from the second crucible is introduced into the deposition furnace through a conduit. The deposition furnace is heated to 1200℃, and the pressure inside the deposition furnace is maintained at 700-800 mbar by argon gas. The temperature is held for 0.5 hours. After the holding time is completed, the heating furnaces a, b, and the deposition furnace are turned off in sequence. The furnace is cooled to room temperature to obtain a graphite part with a tantalum carbide coating.

[0046] Figure 2The figure shows a tantalum carbide coated graphite part prepared by deposition in Example 1. As can be seen from the figure, a golden tantalum carbide coating is formed on the graphite surface. The coating area is about 280 mm in diameter. The edge of the graphite part is the obscured part. This area is used as a comparison. No coating is deposited in this area. It is still black graphite. Figure 3 The image shows a SEM image of the deposited tantalum carbide coating. It can be seen that the tantalum carbide coating is dense, with closely packed grains and relatively uniform grain size. Furthermore, ICP-MS was used to analyze its impurity content. Table 1 shows the impurity element content of the deposited tantalum carbide coating. As can be seen from Table 1, even considering the maximum amount of impurities, the purity is still above 99.994%.

[0047] .

[0048] Example 2

[0049] Preparation of tantalum carbide coating on carbon fiber surface:

[0050] Weigh out 1500g of lithium chloride, sodium chloride, and potassium chloride in a molar ratio of 0.55:0.09:0.36; weigh out 60g of potassium fluorotantalate; weigh out 50g of tantalum powder; and weigh out 150g of tantalum wire woven network structure. Mix them thoroughly and place them into a graphite crucible. Place the first crucible into heating furnace a. Weigh out 1300g of lithium chloride, 50g of tantalum powder, and 160g of tantalum wire woven network structure and place them into a second crucible. Place the second crucible into heating furnace b. Place the carbon fibers to be deposited into the deposition furnace. Heating furnace a is started and heated to 700°C. Simultaneously, heating furnace b is started and heated to 700°C. Argon gas is introduced into the molten salt in the first crucible as a carrier gas. The carrier gas vapor stream generated in the first crucible is introduced into the molten salt in the second crucible through a conduit. The filtered vapor stream from the second crucible is introduced into the deposition furnace through a conduit. The deposition furnace is heated to 1500°C, and the pressure inside the deposition furnace is maintained at 400 mbar by argon gas. The temperature is held for 0.5 hours. After the holding time is completed, the heating of heating furnace a, heating furnace b and deposition furnace are turned off in sequence. The furnace is cooled to room temperature, and the sample is taken out to obtain carbon fiber with tantalum carbide coating. Figure 4 The image shows a comparison between uncoated carbon fibers and fibers coated with tantalum carbide. As can be seen from the image, after coating, the fiber surface exhibits the typical gold color of tantalum carbide, while the uncoated carbon fiber is black.

[0051] Example 3

[0052] Preparation of zirconium carbide coating on carbon fiber surface:

[0053] Weigh out 1500g of sodium chloride and sodium fluoride in a molar ratio of 0.36:0.65, along with 30g of potassium fluorotantalate, 30g of potassium fluorozirconate, 50g of zirconium powder, and 150g of zirconium wire woven network structure. Mix thoroughly and place the mixture into a graphite crucible (first crucible). Place the first crucible into heating furnace a. Weigh out 1300g of sodium chloride, 50g of zirconium powder, and 160g of tantalum wire woven network structure into a second crucible. Place the second crucible into heating furnace b. Place the carbon fibers to be deposited into the deposition furnace. Heating furnace a is started and heated to 700℃, while heating furnace b is started and heated to 700℃ simultaneously. Argon gas is introduced into the molten salt in the first crucible as a carrier gas. The carrier gas vapor stream generated in the first crucible is introduced into the molten salt in the second crucible through a conduit. The filtered vapor stream from the second crucible is introduced into the deposition furnace through a conduit. The deposition furnace is heated to 1400℃, and the pressure inside the deposition furnace is maintained at 300 mbar by argon gas. The temperature is held for 0.5 hours. After the holding time is completed, the heating of heating furnace a, heating furnace b, and the deposition furnace are turned off in sequence. The furnace is then cooled to room temperature, and the sample is removed to obtain carbon fiber with a zirconium carbide coating. Figure 5 The image shows a cross-sectional SEM image of a carbon fiber coated with zirconium carbide. As can be seen from the image, the carbon fiber surface is covered with a thin layer of zirconium carbide (white), and the interior is a carbon core (black).

[0054] Comparative Example 1

[0055] Preparation of tantalum carbide coating on graphite surface:

[0056] The difference between this comparative example and Example 1 is that there is no second crucible and heating furnace b; instead, the molten salt metal vapor generated in the first crucible in heating furnace a is directly introduced into the deposition furnace. Figure 6 The image shown is a SEM image of the deposited coating. It can be seen from the image that the deposited coating is discontinuous and has exposed graphite.

[0057] Comparative Example 2

[0058] Preparation of tantalum carbide coating on carbon fiber surface:

[0059] The difference between this comparative example and Example 2 is that only 50g of tantalum powder was added to the second crucible, while the other conditions remained the same. Figure 7 The image shows a SEM image of the cross-section of the deposited carbon fiber. It can be seen from the image that the carbon fiber surface has almost no coating. This is because when the gas flow passes through the second crucible, a large number of metal elements are dissolved by the molten salt in the second crucible, and the metal content entering the deposition furnace is relatively small. Therefore, no continuous coating is formed on the carbon fiber surface.

Claims

1. A method for preparing ultra-high temperature ceramic coatings using an ultra-high temperature metal gas source based on molten salt medium, characterized in that: A mixture C is obtained by mixing a low-melting-point salt A, a metal fluoride, and a metal M. Mixture C is placed in a first crucible. A mixture D is obtained by mixing a low-melting-point salt B and a metal M. Mixture D is placed in a second crucible. A carbon substrate is placed in a deposition furnace. The first crucible is then heated to melt mixture C to obtain a melt E. The second crucible is then heated to melt mixture D to obtain a melt F. A carrier gas is introduced into the first crucible. The carrier gas flows through the melt E to form a gas flow containing a metal source. The gas flow containing the metal source is introduced into the second crucible. The gas flow containing the metal source is filtered through the melt F in the second crucible to form a pure metal reaction gas source. Finally, the pure metal reaction gas source is introduced into the deposition furnace to deposit an ultra-high temperature ceramic coating on the surface of the carbon substrate. The low-melting-point salt A is a mixed salt composed of at least two salts; The low-melting-point salt A has at least one more salt than the low-melting-point salt B, or the low-melting-point salt A and the low-melting-point salt B have completely different compositions. In the melt E, the metal M is in a supersaturated state; In the molten material F, the metal M is in a supersaturated state; The metal M is selected from at least one of tantalum, zirconium, niobium, and hafnium.

2. The method for preparing ultra-high temperature ceramic coatings based on an ultra-high temperature metal gas source using molten salt medium according to claim 1, characterized in that: The low-melting-point salt A is selected from at least two of lithium chloride, sodium chloride, calcium chloride, potassium chloride, magnesium chloride, barium chloride, cesium chloride, lithium fluoride, magnesium fluoride, sodium fluoride, potassium fluoride, and calcium fluoride. The metal fluoroate is selected from at least one of potassium fluorotantalate, potassium fluorozirconate, potassium fluoroniobate, and potassium fluorohafniumate.

3. The method for preparing ultra-high temperature ceramic coatings based on an ultra-high temperature metal gas source using molten salt medium according to claim 1, characterized in that: In the mixture C, the mass ratio of metal fluoride to metal M is 0.8-1.2:3-8; In the mixture C, the total mass ratio of the low-melting-point salt A and the metal fluorate to the mass ratio of the metal M is 5-30:

1.

4. The method for preparing ultra-high temperature ceramic coatings based on an ultra-high temperature metal gas source using molten salt medium according to claim 1, characterized in that: The metal M in the mixture C includes a powder state and a three-dimensional mesh skeleton state; the metal M in the three-dimensional mesh skeleton state is obtained by weaving or overlapping filamentary, rod-shaped, sheet-shaped, strip-shaped, and foil-shaped metal M. The mass ratio of the powdered metal M to the three-dimensional mesh framework metal M is 1:1-3.

5. The method for preparing ultra-high temperature ceramic coatings based on an ultra-high temperature metal gas source using molten salt medium according to claim 1, characterized in that: The low-melting-point salt B is selected from at least one of lithium chloride, sodium chloride, calcium chloride, potassium chloride, magnesium chloride, barium chloride, cesium chloride, lithium fluoride, magnesium fluoride, sodium fluoride, potassium fluoride, and calcium fluoride. In the mixture D, the mass ratio of low-melting-point salt B to metal M is 5-20:

1.

6. The method for preparing ultra-high temperature ceramic coatings based on an ultra-high temperature metal gas source using molten salt medium according to claim 1, characterized in that: The metal M in the mixture D includes a powder state and a three-dimensional mesh skeleton state; the metal M in the three-dimensional mesh skeleton state is obtained by weaving or overlapping filamentary, rod-shaped, sheet-shaped, strip-shaped, and foil-shaped metal M. The mass ratio of the powdered metal M to the three-dimensional mesh framework metal M is 1:2-4.

7. The method for preparing ultra-high temperature ceramic coatings based on an ultra-high temperature metal gas source using molten salt medium according to claim 1, characterized in that: The carbon substrate is selected from graphite, carbon fiber, and carbon-carbon composite materials.

8. The method for preparing ultra-high temperature ceramic coatings based on an ultra-high temperature metal gas source using molten salt medium according to claim 1, characterized in that: The first crucible is heated to melt the mixture C to obtain the melt E, and the temperature is further increased to 700-1400℃ and held. The second crucible is heated to melt the mixture D to obtain melt F, and the temperature is further increased to 700-1300℃ and held.

9. The method for preparing ultra-high temperature ceramic coatings based on an ultra-high temperature metal gas source using molten salt medium according to claim 1, characterized in that: The first crucible has a first air inlet at the bottom for introducing carrier gas and a first air outlet at the top for discharging gas flow containing metal gas. The second crucible has a second air inlet at the bottom, which is connected to the first air outlet via a conduit to introduce gas flow containing metal gas into the second crucible. The second crucible has an air outlet at the top, through which pure metal reaction gas is introduced into the deposition furnace via a conduit.

10. The method for preparing ultra-high temperature ceramic coatings based on an ultra-high temperature metal gas source using molten salt medium according to claim 1, characterized in that: During the deposition process, the deposition temperature is controlled at 1000-1500℃.

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