A method for producing a dense carbide ceramic coating

Abstract

The application discloses a preparation method of a dense carbide ceramic coating. A first carbide slurry, a second carbide slurry and the first carbide slurry are sprayed on a substrate surface in sequence to obtain a composite slurry layer, the composite slurry layer is dried, and sintering is performed to obtain the carbide ceramic coating. The solid content of the first carbide slurry is 75-83 wt%. The solid content of the second carbide slurry is 65-70 wt%. The dispersants are all mixtures of low-molecular-weight dispersants and high-molecular-weight dispersants. The application can provide the carbide ceramic coating with a thickness of greater than or equal to 40 microns and high density by synergistically optimizing the dispersant combination and gradient spraying process, and the carbide ceramic coating can be applied as a protective coating under a high-temperature corrosion environment such as silicon carbide long crystal growth and epitaxy.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of ceramic coating preparation, and particularly relates to a preparation method of a dense carbide ceramic coating. BACKGROUND

[0002] Carbide ceramic coatings (such as TaC and SiC) are widely used in high-temperature protection fields due to their high melting point, high hardness and excellent corrosion resistance. At present, the slurry sintering method has become a mainstream preparation process due to its low cost and advantage of being suitable for batch production of large-size components, but the method has the problem of insufficient coating density, especially as the coating thickness increases (> 40 microns), the porosity significantly increases, which causes the corrosion medium to easily penetrate through the pores and accelerate the failure of the coating, thereby limiting its application in high-performance fields.

[0003] In the prior art, although chemical vapor deposition (CVD) or physical vapor deposition (PVD) can be used to prepare high-density coatings, the equipment is complex and the cost is high, and it is difficult to realize mass production of large-size components. In addition, the existing carbide dispersant system easily causes particle agglomeration or bridging flocculation in high solid content slurry, which leads to insufficient density and stability of the coating, and single solid content slurry is difficult to achieve a coating with high thickness and low porosity. Therefore, there is an urgent need for a low-cost and high-efficiency preparation method to realize a high-density thick coating and meet the use requirements in high-temperature and high-corrosion environments. SUMMARY

[0004] In view of the deficiencies of the prior art, the purpose of the present application is to provide a preparation method of a dense carbide ceramic coating. The present application can provide a carbide ceramic coating with a thickness of greater than or equal to 40 microns and high density by synergistically optimizing the dispersant combination and gradient spraying process, and the coating can be used as a protective coating in high-temperature and high-corrosion environments such as silicon carbide crystal growth and epitaxy.

[0005] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0006] The preparation method of the dense carbide ceramic coating of the present application comprises the following steps: sequentially spraying a first carbide slurry, a second carbide slurry and a first carbide slurry on the surface of a substrate to obtain a composite slurry layer, drying the composite slurry layer, and sintering to obtain a carbide ceramic coating.

[0007] The solid content of the first carbide slurry is 75-83wt%;

[0008] The solid content of the second carbide slurry is 65-70wt%;

[0009] The first carbide slurry and the second carbide slurry are both composed of carbide powder, dispersant, binder, sintering aid and solvent.

[0010] The dispersants are all mixtures of low molecular weight dispersants and high molecular weight dispersants. The low molecular weight dispersants are selected from at least one of low molecular weight polyethylene glycol (PEG) with a molecular weight of 200 to 600 and Tween. The high molecular weight dispersants are selected from at least one of polyvinylpyrrolidone (PVP) with a molecular weight of 50,000 to 200,000 and high molecular weight polyethylene glycol (PEG) with a molecular weight of 10,000 to 20,000.

[0011] The slurry of this invention comprises carbide powder, dispersant, binder, sintering aid, and solvent. The binder primarily provides strength to the green body and forms nanoscale through-pores during sintering, promoting the removal of polymer pyrolysis gases. The sintering aid promotes low-temperature liquid phase formation, facilitating particle rearrangement and reducing porosity. For the dispersant used to disperse the carbide powder, this invention simultaneously incorporates both low-molecular-weight and high-molecular-weight dispersants with good compatibility. The high-molecular-weight polymer in the slurry acts as the main dispersant, requiring adsorption onto the particle surface via coordination bonds to form a long-chain steric hindrance layer, ensuring stable dispersion of most particles. However, a small number of uncaptured adsorption sites still exist. Further increasing the content of the high-molecular-weight polymer can easily lead to bridging flocculation, thereby reducing the slurry's fluidity. Furthermore, the risk of interparticle bridging and flocculation increases further with increasing solid content. In this invention, by supplementing some low molecular weight polymers as secondary dispersants to fill unadsorbed sites, the short-chain adsorption reduces direct contact between particles, further improving the stability of the slurry. The synergistic effect of high and low molecular weight polymers jointly constructs a more effective adsorption layer. For example, high molecular weight PVP is selected to form coordination bonds (such as Si…O=C, Ta…O=C, etc.) with metal sites on the particle surface through carbonyl C=O. After adsorption, the long molecular chains form a spatial protective layer to prevent most particles from agglomerating. Then, low molecular weight PVP is added, and its terminal hydroxyl groups are quickly adsorbed on the active sites not covered by PVP. The short-chain structure avoids bridging and flocculation, forming a solvation layer to enhance short-range repulsion and improve the stability of the slurry.

[0012] Furthermore, experiments revealed that when using a single low-solids content (65-72 wt%) slurry to prepare coatings, the low carbide particle concentration resulted in significant pores due to solvent evaporation during drying. As the coating thickness increased, the solvent evaporation path lengthened, leading to accumulated drying stress and further increasing porosity, ultimately making it difficult to form a dense coating. Conversely, when using a single high-solids content (75-83 wt%) slurry to prepare coatings, the high viscosity and poor flowability of the slurry made it difficult to achieve high-density packing through particle rearrangement as the coating thickness increased. Simultaneously, the solvent evaporation rate on the surface was faster, and the difference in drying rate between the surface and the substrate increased significantly with increasing thickness, ultimately resulting in more surface pores. In this invention, a three-layer gradient spraying is employed. The bottom layer uses a high-solids-content slurry to form a dense underlayer, reducing interfacial porosity. The middle layer uses a low-solids-content slurry, which utilizes its low viscosity and high fluidity to promote particle rearrangement and fill the pores in the bottom layer. The top layer uses a high-solids-content slurry to form a dense surface layer with high particle packing density. By introducing the low-solids-content slurry as the middle layer, its low viscosity and high fluidity are utilized, making it easy for particles to rearrange during spraying. This effectively fills any large pores that may exist in the bottom layer. At the same time, it acts as a stress buffer layer to balance the drying rate between the top and bottom layers. The slower drying rate allows the surface particles more time to rearrange. Combined with the high particle packing density of the high-solids-content surface layer slurry, a dense surface layer is ultimately formed.

[0013] With the synergistic effect of the slurry formulation and spraying method of this invention, a carbide ceramic coating with a thickness of ≥40μm and high density can be obtained.

[0014] In a preferred embodiment, the carbide powder is selected from at least one of tantalum carbide (TaC) powder, silicon carbide (SiC) powder, and hafnium carbide (HfC) powder.

[0015] In a preferred embodiment, the mass fraction of the dispersant in both the first carbide slurry and the second carbide slurry is 0.5-2 wt%.

[0016] In a preferred embodiment, the mass ratio of low molecular weight dispersant to high molecular weight dispersant in the dispersant is 2-5:1.

[0017] In a preferred embodiment, the Tween is selected from at least one of Tween-20 and Tween-40.

[0018] In a preferred embodiment, the polyvinylpyrrolidone is of type K30 and / or K60.

[0019] In a preferred embodiment, the binder in both the first and second carbide slurries comprises a mass fraction of 0.5-1.5 wt%. The binder primarily provides strength to the green body and forms nanoscale through-pores during sintering, promoting the removal of polymer pyrolysis gases.

[0020] In a preferred embodiment, the adhesive is selected from at least one of polyvinyl butyral (PVB) and polyvinyl alcohol (PVA).

[0021] In a preferred embodiment, the sintering aid has a mass fraction of 0.8-1.2 wt% in both the first carbide slurry and the second carbide slurry.

[0022] In a preferred embodiment, the sintering aid is selected from at least one of nickel powder (Ni), cobalt powder (Co), or molybdenum disilicide (MoSi2).

[0023] In a preferred embodiment, the solvent is a mixed solvent composed of a low-boiling-point solvent and a high-boiling-point solvent. The low-boiling-point solvent is selected from at least one of ethanol and n-butanol, and the high-boiling-point solvent is selected from at least one of dimethyl sulfoxide, benzyl alcohol, and diethylene glycol monoethyl ether. Experiments have shown that using a combination of low-boiling-point and high-boiling-point solvents can effectively reduce drying pressure.

[0024] In a further preferred embodiment, the mass ratio of the low-boiling-point solvent to the high-boiling-point solvent is 3-5:1.

[0025] In a preferred embodiment, the preparation process for both the first and second carbide slurries involves: first, ball milling the carbide powder, sintering aid solvent, and high molecular weight dispersant at 400-450 rpm for 2-3 hours; then, adding a low molecular weight dispersant and ball milling at 250-350 rpm for 1-1.5 hours; finally, adding a binder and ball milling at 150-200 rpm for 1-1.5 hours. By using this feeding method and controlling the ball milling speed, competitive adsorption of the components, especially the high and low molecular weight dispersants, on the surface of the carbide powder can be effectively avoided, ensuring that each component fully exerts its function.

[0026] In a further preferred embodiment, the grinding balls used in the high-speed ball milling, medium-speed ball milling, and low-speed ball milling are all zirconia balls or silicon nitride balls, and the ball-to-material ratio is 1-3:1.

[0027] In a preferred embodiment, the solid particles in the first carbide slurry have a particle size D50 of 0.5-1.1 μm and a viscosity of 200-250 mPa·s, while the solid particles in the second carbide slurry have a particle size D50 of 0.5-1.1 μm and a viscosity of 100-180 mPa·s. By controlling the slurry parameters within this range, the final coating exhibits optimal performance.

[0028] In a preferred embodiment, the thickness of the first layer obtained by spraying the first carbide slurry is 30-55 μm, the thickness of the second layer obtained by spraying the second carbide slurry is 10-20 μm, and the thickness of the third layer obtained by spraying the first carbide slurry is 20-45 μm.

[0029] Experiments have shown that, based on the application requirements of the coating, when obtaining a coating of a specific thickness, controlling the thickness of each layer of the slurry spraying within the range of this invention results in the highest final coating density. However, when the thickness of the intermediate layer (low solid content) is insufficient, the solvent in the coating will still evaporate rapidly after spraying, making it difficult to effectively fill the pores of the bottom layer or balance the drying rate between the bottom layer and the surface layer, thus weakening the role of the intermediate layer in improving the coating density. When the intermediate layer is too thick, the low particle concentration of the low solid content slurry leads to a low particle packing density, which easily forms more pores after drying and sintering, reducing the overall density of the coating.

[0030] In a preferred embodiment, the spraying method is air spraying. During spraying, the air pressure is controlled at 0.2-0.4 MPa, the spray distance is 10-20 cm, the nozzle diameter of the spray gun is 0.8-1.2 mm, the spray gun moving speed is 50-200 mm / s, and the spray gun angle is 70-90° (where 90° means the spray gun angle is perpendicular to the surface to be sprayed).

[0031] Experiments have shown that the above-mentioned spraying method can better ensure a smooth coating surface and controllable and consistent thickness. If brushing is used to sequentially apply the various materials of this invention, it will lead to brush marks and uneven thickness. Uneven shrinkage stress is easily generated during drying, resulting in cracking, warping or peeling of the coating.

[0032] In a preferred embodiment, the drying process is as follows: first, the temperature is raised to 40-60℃ and held for 2-4 hours; then, the temperature is raised to 80-100℃ at a rate of 5-10℃ / min and held for 3-4 hours; finally, the temperature is raised to 140-180℃ at a rate of 1-3℃ / min and held for 6-8 hours. This drying process can remove solvents with different boiling points in a gradient, preventing excessively rapid evaporation from causing stress concentration and cracking in the green body.

[0033] In a preferred embodiment, the sintering process is as follows: heating to 250-330℃ at a heating rate of 5-7℃ / min and holding for 1-3 hours, then heating to 350-450℃ at a heating rate of 5-7℃ / min and holding for 1-2 hours, then heating to 1200-1600℃ at a heating rate of 6-10℃ / min and holding for 2-3 hours, and finally heating to 1700-2200℃ at a heating rate of 5-7℃ / min and holding for 3-5 hours, with Ar gas as the protective atmosphere and a vacuum degree of 5-7 mbar. The sintering process of this invention employs a gradient heating and holding process. In the low-temperature stage, the temperature is gradually increased and held for a sufficient time to expel the gases generated after the pyrolysis of polymers such as binders and dispersants. Then, the temperature is increased to 1200-1600℃ and held for a period of time to allow the liquid phase to fill the gaps between particles. This enhances material migration through capillary forces and promotes densification. Temperatures outside this range are not conducive to material diffusion and affect the densification of the coating.

[0034] In a preferred embodiment, the thickness of the carbide ceramic coating is ≥40μm.

[0035] Density of sintered tantalum carbide coating: The density of the sintered tantalum carbide coating was tested according to the method specified in GB / T 39688-2020.

[0036] Principles and advantages

[0037] First, this invention achieves a stable slurry through the synergistic effect of high and low molecular weight polymers. The two dispersants work together to prevent particle agglomeration and bridging flocculation, ensuring that the particles in the high solids content slurry have a high packing density, while the low solids content slurry has low viscosity and high fluidity. During the spraying process, the particles are more likely to slide and rearrange in the liquid medium, which can effectively fill the large pores that may exist in the bottom layer.

[0038] Secondly, the coating structure is optimized through gradient spraying process in this invention. The three-layer structure of "dense bottom layer - pore-filling middle layer - high-density surface layer" avoids the problem of pore accumulation caused by uneven particle packing in thick coatings due to the gradient optimization of particle packing density and pore distribution.

[0039] Furthermore, the stepwise drying process in this invention slows down the solvent evaporation rate, avoiding stress concentration and microcracks caused by rapid drying. The binder provides green strength during drying and facilitates the formation of nanoscale through-pores during pyrolysis, allowing for gas expulsion and reducing porosity caused by gas retention. During high-temperature heat treatment, the sintering aid forms a liquid phase at high temperatures (1200-1600℃), promoting particle rearrangement and diffusion. Liquid-phase sintering reduces the contact resistance between particles, accelerates pore closure, and forms a highly dense coating.

[0040] Compared with the prior art, the present invention has at least the following advantages:

[0041] 1. By using the synergistic dispersion effect of high molecular weight and low molecular weight dispersants, a high and low solid content slurry that meets the performance requirements is prepared, which effectively avoids interparticle bridging and flocculation while reducing the residue of unadsorbed points.

[0042] 2. By designing a gradient spraying process using slurries with high and low solid content, the pore distribution of the thick coating (>40μm) was optimized, improving the density of the coating.

[0043] 3. The process is simple, suitable for large-scale production, and significantly improves coating performance without the need for complex equipment. Attached Figure Description

[0044] Figure 1 This is a process flow diagram of the present invention.

[0045] Figure 2This is a SEM image of the tantalum carbide coating in Example 1.

[0046] Figure 3 SEM image of tantalum carbide coating for Comparative Example 1.

[0047] Figure 4 SEM image of tantalum carbide coating for Comparative Example 4. Detailed Implementation

[0048] Example 1

[0049] This embodiment provides a method for preparing a low-porosity carbide ceramic coating, the specific preparation process of which is as follows:

[0050] Step S1: Mix 100g of tantalum carbide powder (1-5μm particle size), 1.22g of nickel powder (1-3μm particle size), 1.21g of PVP (model K30), 16.8ml of ethanol, and 4.02ml of dimethyl sulfoxide evenly in a planetary ball mill. Use zirconium oxide grinding balls at a ball-to-particle ratio of 3:1 and ball mill at high speed (450 rpm). The mixture was first milled at 100 rpm for 2 hours, then 0.61 g of PEG (molecular weight 200) was added and milled at medium speed (300 rpm for 1 hour), and finally 1.22 g of PVB was added and milled at low speed (180 rpm for 1.5 hours) to obtain a high solid content (82 wt%) tantalum carbide slurry. 100 g of tantalum carbide powder with a particle size of 1-5 μm was mixed with 1.51 g of nickel powder with a particle size of 1-3 μm, 2.04 g of PVP (model K30), 46.08 ml of ethanol, and 8.7 ml of benzyl alcohol and placed in a planetary ball mill. The grinding balls were zirconium oxide, and the ball-to-material ratio was 3:1. The mixture was then milled at high speed (450 rpm for 2 hours). After adding 0.68g of PEG (molecular weight 400) and ball milling at medium speed (300 rpm, ball milling time 1.5h), 1.75g ​​of PVB was added and ball milled at low speed (180 rpm, ball milling time 1.5h) to obtain a low solid content (66wt%) tantalum carbide slurry.

[0051] Step S2: The slurry obtained in step S1 is uniformly sprayed onto a 50×50×5mm graphite sheet. The spraying parameters are: nozzle diameter 0.8mm, air pressure 0.4MPa, spray distance 18-20cm, spray gun moving speed 180mm / s, and spray gun angle 85°. The bottom layer uses a high-solids content slurry with an average single-layer thickness of 5μm and a final average coating thickness of 48μm; the middle layer uses a low-solids content slurry with an average single-layer thickness of 5μm and a final average coating thickness of 16μm; the top layer uses a high-solids content slurry with an average single-layer thickness of 5μm and a final average coating thickness of 45μm.

[0052] Step S3: Dry the sprayed part prepared in S2 according to the following process: keep it at 50℃ for 2 hours, then raise the temperature to 100℃ at 5℃ / min and keep it at 100℃ for 4 hours, and then raise the temperature to 180℃ at 3℃ / min and keep it at 180℃ for 6 hours.

[0053] Step S4, sintering: Under an Ar atmosphere, the temperature is increased to 300℃ at 6℃ / min and held for 1 hour, then increased to 350℃ at 5℃ / min and held for 2 hours, then increased to 1200℃ at 6℃ / min and held for 2 hours, then increased to 2200℃ at 6℃ / min and held for 4 hours, with a vacuum degree of 5-7 mbar; then cooled to room temperature with the furnace to obtain the final product.

[0054] Testing revealed that the high-solids content slurry prepared in step S1 had a particle size D50 of 0.89 μm and a viscosity of 233 mPa·s, while the low-solids content slurry had a particle size D50 of 1.0 μm and a viscosity of 112 mPa·s. After final heat treatment, the average coating thickness was 81 μm, and the density was 97.1%. Figure 2 The image shown is a SEM image of the tantalum carbide coating prepared in Example 1. The coating prepared by the gradient spraying process has good overall thickness uniformity, high coating density, and no obvious large pores, which fully meets the requirements for use.

[0055] Example 2

[0056] This embodiment provides a method for preparing a low-porosity carbide ceramic coating, the specific preparation process of which is as follows:

[0057] Step S1: Mix 100g of tantalum carbide powder (1-5μm particle size), 1.57g of cobalt powder (1-3μm particle size), 1.68g of PVP (model K60), 25.66ml of ethanol, and 6.14ml of dimethyl sulfoxide evenly in a planetary ball mill. Use zirconium oxide grinding balls at a ball-to-particle ratio of 3:1 and ball mill at high speed (400 rpm). The mixture was first milled at 100 rpm for 2 hours, then 0.42 g of PEG (molecular weight 200) was added and milled at medium speed (300 rpm for 1 hour). Finally, 1.22 g of PVB was added and milled at low speed (180 rpm for 1.5 hours) to obtain a high solid content (76 wt%) tantalum carbide slurry. 100 g of tantalum carbide powder with a particle size of 1-5 μm was mixed with 1.43 g of cobalt powder with a particle size of 1-3 μm, 1.9 g of PVP (model K60), 38.53 ml of ethanol, and 7.27 ml of benzyl alcohol. The mixture was then placed in a planetary ball mill with zirconium oxide grinding balls at a ball-to-material ratio of 3:1 and milled at high speed (450 rpm for 2 hours). After adding 0.95g of PEG (molecular weight 200) and ball milling at medium speed (300 rpm, ball milling time 1.5h), 0.85g of PVB was added and ball milled at low speed (180 rpm, ball milling time 1.5h) to obtain a low solid content (70wt%) tantalum carbide slurry.

[0058] Step S2: The slurry obtained in step S1 is uniformly sprayed onto a 50×50×5mm graphite sheet. The spraying parameters are: nozzle diameter 0.8mm, air pressure 0.3MPa, spray distance 18-20cm, spray gun moving speed 180mm / s, and spray gun angle 90°. The bottom layer uses a high-solids content slurry, with an average single-layer spray thickness of 6μm and a final average coating thickness of 45μm; the middle layer uses a low-solids content slurry, with an average single-layer spray thickness of 6μm and a final average coating thickness of 12μm; the top layer uses a high-solids content slurry, with an average single-layer spray thickness of 6μm and a final average coating thickness of 31μm.

[0059] Step S3: Dry the sprayed part prepared in S2 according to the following process: keep it at 50℃ for 2 hours, then raise the temperature to 100℃ at 5℃ / min and keep it at 100℃ for 4 hours, and then raise the temperature to 170℃ at 3℃ / min and keep it at 170℃ for 6 hours.

[0060] Step S4, sintering: Under an Ar atmosphere, the temperature is increased to 300℃ at 6℃ / min and held for 1 hour, then increased to 350℃ at 5℃ / min and held for 2 hours, then increased to 1300℃ at 6℃ / min and held for 2 hours, then increased to 2200℃ at 6℃ / min and held for 4 hours, with a vacuum degree of 5-7 mbar; then cooled to room temperature with the furnace to obtain the final product.

[0061] Testing revealed that the high-solids-content slurry prepared in step S1 had a particle size D50 of 0.81 μm and a viscosity of 209 mPa·s, while the low-solids-content slurry had a particle size D50 of 0.96 μm and a viscosity of 158 mPa·s. After final heat treatment, the average coating thickness was 63 μm, and the density was 97.8%. The coating morphology was essentially the same as in Example 1, with no cracks or defects, and the coating exhibited good density.

[0062] Example 3

[0063] This embodiment provides a method for preparing a low-porosity carbide ceramic coating, the specific preparation process of which is as follows:

[0064] Step S1: 100g of tantalum carbide powder with a particle size of 1-5μm, 1.26g of nickel powder with a particle size of 1-3μm, 1.71g of PVP (model K30), 19.04ml of ethanol, and 4.55ml of dimethyl sulfoxide are uniformly mixed and placed in a planetary ball mill. Zirconia grinding balls are used, and the ball-to-material ratio is 3:1. High-speed ball milling is performed (450 rpm, 2h). Then, 0.57g of PEG (molecular weight 200) is added and ball milling is performed at medium speed (300 rpm, 1h). Finally, 1.22g of PVB is added and ball milling is performed at low speed (180 rpm, 1.5h) to obtain a high solids content (80wt%) tantalum carbide slurry.

[0065] 100g of tantalum carbide powder with a particle size of 1-5μm, 1.51g of nickel powder with a particle size of 1-3μm, 2.04g of PVP (model K30), 46.08ml of ethanol, and 8.7ml of benzyl alcohol were uniformly mixed and placed in a planetary ball mill. Zirconia was used as the grinding ball, and the ball-to-material ratio was 2:1. The mixture was ball-milled at high speed (400 rpm for 3 hours). Then, 0.68g of PEG (molecular weight 400) was added and the mixture was ball-milled at medium speed (300 rpm for 1.5 hours). Finally, 1.75g ​​of PVB was added and the mixture was ball-milled at low speed (180 rpm for 1.5 hours) to obtain a low solids content (68wt%) tantalum carbide slurry.

[0066] Step S2: The slurry obtained in step S1 is uniformly sprayed onto a 50×50×5mm graphite sheet. The spraying parameters are: nozzle diameter 0.8mm, air pressure 0.3MPa, spray distance 18-20cm, spray gun moving speed 180mm / s, and spray gun angle 85°. The bottom layer uses a high-solids content slurry with an average single-layer thickness of 5μm and a final average coating thickness of 32μm; the middle layer uses a low-solids content slurry with an average single-layer thickness of 5μm and a final average coating thickness of 10μm; the top layer uses a high-solids content slurry with an average single-layer thickness of 5μm and a final average coating thickness of 20μm.

[0067] Step S3: Dry the sprayed part prepared in S2 according to the following process: keep it at 50℃ for 2 hours, then raise the temperature to 100℃ at 5℃ / min and keep it at 100℃ for 4 hours, and then raise the temperature to 180℃ at 3℃ / min and keep it at 180℃ for 6 hours.

[0068] Step S4: Under an Ar atmosphere, heat to 300℃ at 6℃ / min and hold for 1 hour, then heat to 350℃ at 5℃ / min and hold for 2 hours, then heat to 1200℃ at 6℃ / min and hold for 2 hours, then heat to 2200℃ at 6℃ / min and hold for 4 hours, with a vacuum degree of 5-7 mbar; then cool to room temperature with the furnace to obtain the final product.

[0069] Testing revealed that the high-solids-content slurry prepared in step S1 had a particle size D50 of 1.02 μm and a viscosity of 212 mPa·s, while the low-solids-content slurry had a particle size D50 of 0.95 μm and a viscosity of 140 mPa·s. After final heat treatment, the average coating thickness was 48 μm, and the density was 97.7%. The coating morphology was essentially the same as in Example 1, with no cracks or defects, and the coating exhibited good density.

[0070] Comparative Example 1

[0071] This embodiment provides a method for preparing a low-porosity carbide ceramic coating, the specific preparation process of which is as follows:

[0072] Step S1: 100g of tantalum carbide powder with a particle size of 1-5μm, 1.22g of nickel powder with a particle size of 1-3μm, 1.21g of PVP (model K30), 16.8ml of ethanol, and 4.02ml of dimethyl sulfoxide are mixed evenly and placed in a planetary ball mill. Zirconia grinding balls are used, and the ball-to-material ratio is 3:1. High-speed ball milling is performed (450 rpm, 2h). Then, 0.61g of PEG (molecular weight 200) is added and ball milling is performed at medium speed (300 rpm, 1h). Finally, 1.22g of PVB is added and ball milling is performed at low speed (180 rpm, 1.5h) to obtain a high solids content (82wt%) tantalum carbide slurry.

[0073] Step S2: The high-solids content material obtained in step S1 is uniformly sprayed onto a 50×50×5mm graphite sheet. The spraying parameters are: nozzle diameter 0.8mm, air pressure 0.4MPa, spray distance 18-20cm, spray gun moving speed 150-180mm / s, and spray gun angle 90°. The thickness of a single layer is 5-7μm, and the final average thickness is 82μm.

[0074] Step S3: Dry the sprayed part prepared in S2 according to the following process: keep it at 50℃ for 2 hours, then raise the temperature to 100℃ at 5℃ / min and keep it at 100℃ for 4 hours, and then raise the temperature to 160℃ at 3℃ / min and keep it at 160℃ for 5 hours.

[0075] Step S4: Under an Ar atmosphere, heat to 300℃ at 6℃ / min and hold for 1 hour, then heat to 350℃ at 5℃ / min and hold for 2 hours, then heat to 1200℃ at 6℃ / min and hold for 2 hours, then heat to 2200℃ at 6℃ / min and hold for 4 hours, with a vacuum degree of 5-7 mbar; then cool to room temperature with the furnace to obtain the final product.

[0076] Testing revealed that the slurry prepared in step S1 had a particle size D50 of 0.89 μm, a viscosity of 233 mPa·s, and an average coating thickness of 60 μm and a density of 93.1% after final heat treatment. Figure 3 The image shown is a SEM image of the tantalum carbide coating prepared in Comparative Example 1. The coating bottom is relatively dense, while the surface layer has many pores. This indicates that as the coating thickness increases, the difference in drying rate between the surface and the bottom layer amplifies, stress accumulation becomes more significant, and the surface layer is prone to forming pores or cracks.

[0077] Comparative Example 2

[0078] Comparative Example 2 is identical to Comparative Example 1 in all other conditions except that, in step S2, when preparing the coating on the graphite substrate, the final average thickness of the sprayed coating is 58 μm.

[0079] After testing, the average coating thickness after final heat treatment was 47 μm, and the density was 94.6%. The coating morphology was basically consistent with that of Comparative Example 1, with a denser bottom and more pores on the surface. This indicates that when using a single high-solids content slurry to prepare a coating with a thickness exceeding 40 μm, the surface density is insufficient.

[0080] Comparative Example 3

[0081] Comparative Example 3 is identical to Example 2 in all other conditions except that: the bottom layer uses a high-solids content slurry with an average single-layer spray thickness of 5 μm and a final average coating thickness of 45 μm; the middle layer uses a low-solids content slurry with an average single-layer spray thickness of 5 μm and a final average coating thickness of 30 μm; and the top layer uses a high-solids content slurry with an average single-layer spray thickness of 5 μm and a final average coating thickness of 20 μm.

[0082] The tests showed that the average coating thickness after heat treatment was 61 μm and the density was 92.2%. The coating contained a large number of pores. When the intermediate layer was thicker, the solvent content in the coating was higher, which easily led to the formation of more pores after drying and sintering, reducing the overall density of the coating. This further illustrates that the thickness of the intermediate layer has a significant impact on the final density of the coating.

[0083] Comparative Example 4

[0084] Comparative Example 4 is identical to Example 1 under all other conditions except that in step S1, when preparing carbide slurries with high and low solid content, only 1.21g of PVP (model K30) is added as a dispersant in the high solid content (82wt%) tantalum carbide slurry, and no low molecular weight PEG (200) is added; in the low solid content (66wt%) tantalum carbide slurry, only 2.04g of PVP (model K30) is added as a dispersant, and no low molecular weight PEG (400) is added.

[0085] Testing revealed that the high-solids content slurry prepared in step S1 had a particle size D50 of 1.1 μm and a viscosity of 310 mPa·s, while the low-solids content slurry had a particle size D50 of 1.4 μm and a viscosity of 222 mPa·s. After final heat treatment, the average coating thickness was 80 μm, and the density was 92.6%. According to... Figure 4 The image shown is a SEM image of the tantalum carbide coating prepared in Comparative Example 4. After using a single high molecular weight dispersant, the particle size and viscosity of the slurry with both solid contents increased. This was mainly because the particles were prone to agglomeration and failed to achieve good dispersion, resulting in insufficient slurry fluidity. Ultimately, the coating had low density after sintering, and large pores were widely distributed in the coating. This made it easy for cracks to form and propagate during use, which could not meet the application requirements.

[0086] The coating properties obtained by the embodiments and comparative examples of the present invention are shown in Table 1:

[0087]

[0088] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any form or substance. It should be noted that those skilled in the art can make various improvements and additions without departing from the method of the present invention, and these improvements and additions should also be considered within the scope of protection of the present invention. Any modifications, alterations, and equivalent changes made by those skilled in the art based on the above-disclosed technical content without departing from the spirit and scope of the present invention are equivalent embodiments of the present invention. Furthermore, any modifications, alterations, and evolutions made to the above embodiments based on the essential technology of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a dense carbide ceramic coating, characterized in that: A composite slurry layer is obtained by sequentially spraying a first carbide slurry, a second carbide slurry, and another first carbide slurry onto the surface of a substrate. The composite slurry layer is then dried and sintered to obtain a carbide ceramic coating. The solid content of the first carbide slurry is 75-83 wt%; The solid content of the second carbide slurry is 65-70 wt%; Both the first carbide slurry and the second carbide slurry are composed of carbide powder, dispersant, binder, sintering aid, and solvent; The carbide powder is selected from at least one of tantalum carbide powder, silicon carbide powder, and hafnium carbide powder; The dispersants are all mixtures of low molecular weight dispersants and high molecular weight dispersants. The low molecular weight dispersants are selected from at least one of low molecular weight polyethylene glycol with a molecular weight of 200 to 600 and Tween. The high molecular weight dispersants are selected from at least one of polyvinylpyrrolidone with a molecular weight of 50,000 to 200,000 and high molecular weight polyethylene glycol with a molecular weight of 10,000 to 20,000.

2. The method for preparing a dense carbide ceramic coating according to claim 1, characterized in that: In both the first and second carbide slurries, the mass fraction of the dispersant is 0.5-2 wt%. In the dispersant, the mass ratio of low molecular weight dispersant to high molecular weight dispersant is 2-5:1; The Tween is selected from at least one of Tween-20 and Tween-40; The polyvinylpyrrolidone is of type K30 and / or K60; In both the first and second carbide slurries, the mass fraction of the binder is 0.5-1.5 wt%. The adhesive is selected from at least one of polyvinyl butyral and polyvinyl alcohol; In both the first and second carbide slurries, the mass fraction of the sintering aid is 0.8-1.2 wt%. The sintering aid is selected from at least one of nickel powder, cobalt powder, and molybdenum disilicide.

3. The method for preparing a dense carbide ceramic coating according to claim 1, characterized in that: The solvent is a mixed solvent composed of a low-boiling-point solvent and a high-boiling-point solvent. The low-boiling-point solvent is selected from at least one of ethanol and n-butanol, and the high-boiling-point solvent is selected from at least one of dimethyl sulfoxide, benzyl alcohol, and diethylene glycol monoethyl ether. The mass ratio of the low-boiling-point solvent to the high-boiling-point solvent is 3-5:

1.

4. The method for preparing a dense carbide ceramic coating according to claim 1, characterized in that: The preparation process of the first carbide slurry and the second carbide slurry is as follows: carbide powder, sintering aid solvent, and high molecular weight dispersant are first ball-milled at a speed of 400-450 rpm for 2-3 hours, then low molecular weight dispersant is added and ball-milled at a speed of 250-350 rpm for 1-1.5 hours, and finally binder is added and ball-milled at a speed of 150-200 rpm for 1-1.5 hours. The grinding balls used in the high-speed, medium-speed, and low-speed ball milling processes are all zirconia balls or silicon nitride balls, with a ball-to-material ratio of 1-3:

1.

5. The method for preparing a dense carbide ceramic coating according to claim 1, characterized in that: In the first carbide slurry, the particle size D50 of the solid particles is 0.5-1.1 μm and the viscosity is 200-250 mPa·s. In the second carbide slurry, the particle size D50 of the solid particles is 0.5-1.1 μm and the viscosity is 100-180 mPa·s.

6. The method for preparing a dense carbide ceramic coating according to claim 1, characterized in that: In the composite slurry layer, the thickness of the first layer obtained by spraying the first carbide slurry is 30-55 μm, the thickness of the second layer obtained by spraying the second carbide slurry is 10-20 μm, and the thickness of the third layer obtained by spraying the first carbide slurry is 20-45 μm.

7. The method for preparing a dense carbide ceramic coating according to claim 1, characterized in that: The spraying method is air spraying. During spraying, the air pressure is controlled at 0.2-0.4MPa, the spray distance is 10-20cm, the nozzle diameter of the spray gun is 0.8-1.2mm, the spray gun moving speed is 50-200mm / s, and the spray gun angle is 70-90°.

8. The method for preparing a dense carbide ceramic coating according to claim 1, characterized in that: The drying process is as follows: first, heat the temperature to 40-60℃ and hold for 2-4 hours; then, heat the temperature to 80-100℃ at a rate of 5-10℃ / min and hold for 3-4 hours; finally, heat the temperature to 140-180℃ at a rate of 1-3℃ / min and hold for 6-8 hours.

9. The method for preparing a dense carbide ceramic coating according to claim 1, characterized in that: The sintering process is as follows: the temperature is increased to 250-330℃ at a heating rate of 5-7℃ / min and held for 1-3 hours; then the temperature is increased to 350-450℃ at a heating rate of 5-7℃ / min and held for 1-2 hours; then the temperature is increased to 1200-1600℃ at a heating rate of 6-10℃ / min and held for 2-3 hours; finally, the temperature is increased to 1700-2200℃ at a heating rate of 5-7℃ / min and held for 3-5 hours. The protective atmosphere is Ar gas and the vacuum degree is 5-7 mbar.

10. The method for preparing a dense carbide ceramic coating according to claim 1, characterized in that: The thickness of the carbide ceramic coating is ≥40μm.

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