A stable carbide slurry, a dense carbide coating and a method for producing the same

Abstract

The application discloses a kind of steady carbide slurry, dense carbide coating and its preparation method, the present application is introduced two-dimensional nanostructure carbon source stabilizer in carbide slurry, and with corresponding oxide as the precursor of controllable carbothermic reduction, combined with fine particle ratio, dispersing agent and process optimization, realize the high stability of slurry, then the carbide slurry is coated on the surface of carbon base material, after drying, again through sintering, in the sintering process, by the two-step densification method of first exhaust, after healing, the high-density carbide coating with internal defect is prepared.

Description

Technical Field

[0001] This invention belongs to the field of coating preparation technology, specifically relating to a stable carbide slurry, a dense carbide coating and its preparation method, which is applicable to the preparation of high-temperature and corrosion-resistant coatings such as graphite-based carbides. Background Technology

[0002] Graphite-based carbides have broad application prospects in aerospace, semiconductors, and other fields. Among existing preparation methods, slurry sintering shows great potential due to its low cost, simple equipment, convenient operation, and ability to achieve uniform coating of large-area or complex-shaped substrates. However, carbide slurries are prone to rapid sedimentation during preparation, making it difficult to maintain consistent performance, especially for denser carbides (such as tantalum carbide), where slurry stability issues are more pronounced. Traditional dispersants cannot maintain the stability of high-solids-content (≥70wt%) carbide slurries for extended periods, which not only affects the uniformity of the coating but may also lead to porosity in the final coating, limiting its practical application. Therefore, there is an urgent need to develop a method to significantly improve slurry stability while ensuring high density (porosity <2%) of the coating after sintering to meet stringent application requirements. Summary of the Invention

[0003] To address the shortcomings of existing technologies, the first objective of this invention is to provide a method for preparing a stable carbide slurry. This invention achieves high stability of the slurry by introducing a two-dimensional nanostructured carbon source stabilizer into the carbide slurry and using a corresponding oxide as a precursor for controllable carbothermal reduction, combined with precise particle ratio, dispersant, and process optimization.

[0004] A second objective of this invention is to provide a stable carbide slurry prepared by the above-described preparation method.

[0005] A third objective of this invention is to provide a method for preparing a dense carbide coating.

[0006] A fourth object of the present invention is to provide a dense carbide coating prepared by the above preparation method.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] This invention discloses a method for preparing a stable carbide slurry. Graphene oxide, solvent, and dispersant are ball-milled at 500-600 rpm to obtain a dispersion. Then, carbide powder, oxide powder, and sintering aid are added to the dispersion and ball-milled at 400-480 rpm to obtain a ceramic slurry. Finally, a binder is added and ball-milled at 200-300 rpm to obtain the carbide slurry.

[0009] The dispersant is a block copolymer;

[0010] The carbides obtained by the carbothermic reduction reaction of the oxides in the oxide powder with carbon are the same as those in the carbide powder.

[0011] The carbide in the carbide powder is selected from one of tantalum carbide, hafnium carbide, and silicon carbide;

[0012] The particle size D50 of the carbide powder is ≤1.5μm; preferably 1~1.5μm.

[0013] The particle size D50 of the oxide powder is ≤3μm, preferably 2~3μm;

[0014] The carbide powder has a mass fraction of 70-75 wt% in the carbide slurry;

[0015] The oxide powder has a mass fraction of 2-5 wt% in the carbide slurry.

[0016] The method for preparing the stable carbide slurry in this invention involves first ball milling graphene oxide, solvent, and dispersant at 500-600 rpm. The high shear force of the ball milling process ensures the graphene oxide sheets are fully exfoliated and dispersed in the solvent, and the dispersant molecules are fully adsorbed onto its surface, forming a stable primary dispersion system. Then, fine-particle carbide powder, coarse-particle oxide powder, and sintering aids are added to this primary dispersion system using a medium-speed ball milling process at 400-500 rpm. High-density ceramic powder is uniformly introduced into the established stable dispersion medium to prevent violent agglomeration. Finally, a binder is added, using a milling process at 200-300 rpm. Low-speed ball milling at rpm ensures uniform mixing of the binder while avoiding damage to the already formed stable dispersion structure and excessive shear heat that could lead to solvent evaporation or premature cross-linking of the slurry. Through the above preparation method, a highly stable carbide slurry is obtained. In this invention, the principle of high stability is that the surface of the graphene oxide sheets exfoliated by high-speed ball milling is rich in oxygen-containing functional groups, such as hydroxyl and carboxyl groups, ensuring good wetting and initial dispersion in polar solvents. These sheets, with their ultra-large specific surface area and high aspect ratio, interact with each other through van der Waals forces and other interactions, overlapping and entangled to spontaneously form a continuous and flexible three-dimensional nano-network structure within the slurry. When high-density carbide and oxide particles are distributed in the above three-dimensional nano-network structure, the grid structure directly constrains and spatially restricts the free sedimentation motion of the particles, significantly reducing the sedimentation rate of the particles according to Stokes' law. In addition, the combination of fine carbide particles and coarse oxide particles optimizes the particle packing density and reduces the large-scale sedimentation space between particles, thereby fundamentally reducing the sedimentation driving force. More importantly, the dispersant, by adsorbing on the surface of powder particles, provides electrostatic repulsion or steric hindrance, effectively inhibiting the soft agglomeration of particles and ensuring that particles are uniformly dispersed in the graphene oxide network at their native size, avoiding local density unevenness and accelerated sedimentation caused by agglomeration formation.

[0017] In this invention, due to the free carbon generated by the pyrolysis of the binder in the slurry and the carbon provided by some graphene oxide, a small amount of oxide powder is added to react with it to avoid the generation of free carbon that would reduce the density.

[0018] In a preferred embodiment, the mass fraction of graphene oxide in the carbide slurry is 0.05~0.1 wt%.

[0019] Experiments have shown that the optimal performance of the final product is achieved when the amount of graphene oxide in the carbide slurry is controlled within the range of this invention. If too little graphene oxide is added, a continuous and effective network cannot be formed, and the slurry is prone to rapid sedimentation and agglomeration when left to stand. When applied to sintering, it forms submicron-level pores. However, if too much graphene oxide is added, the graphene oxide sheets may become excessively entangled, which will destroy the dispersion stability.

[0020] In a preferred embodiment, the solvent is selected from at least two of water, ethanol, and ethylene glycol. In actual operation, ultrapure water is used, and anhydrous ethanol is selected.

[0021] In a further preferred embodiment, the solvent is a mixed solvent composed of water, ethanol, and ethylene glycol, wherein the mass fraction of water in the mixed solvent is 5-10 wt%, and the mass ratio of ethanol to ethylene glycol is 2-3:1.

[0022] Among the preferred solvents, ethanol and ethylene glycol are used as the main solvents, and water is used as an auxiliary regulator to balance solubility and drying characteristics.

[0023] In a preferred embodiment, the dispersant is Pluronic F-127. When Pluronic F-127 is selected as the dispersant, Pluronic F-127 (PEO-PPO-PEO) is an amphiphilic triblock copolymer. In the water-ethanol-ethylene glycol mixed solvent system of the present invention, its hydrophilic polyethylene oxide segments (PEO) tend to extend into the polar solvent, while the hydrophobic polypropylene oxide segments (PPO) tend to adsorb onto the hydrophobic substrate of graphene oxide and the nonpolar surface of the carbide / oxide powder. This unique molecular structure allows it to be simultaneously and firmly anchored to the surfaces of graphene oxide sheets and ceramic particles. The extended PEO chains form a thick hydration layer in the solvent, generating a strong steric hindrance effect, preventing close contact and aggregation between particles and sheets.

[0024] In a preferred embodiment, the sintering aid is selected from at least one of cobalt powder, nickel powder, and molybdenum disilicide.

[0025] In a preferred embodiment, the adhesive is selected from at least one of polyvinyl butyral (PVB) and polyvinyl alcohol (PVA).

[0026] In a preferred embodiment, the high-speed ball milling time is 2-3 hours.

[0027] In a preferred embodiment, the medium-speed ball milling time is 2-3 hours, and the ball-to-material ratio is controlled to be 1-3:1 during the medium-speed ball milling process.

[0028] In a preferred embodiment, the low-speed ball milling time is 1-2 hours.

[0029] In a preferred embodiment, the carbide slurry comprises, by mass percentage, the following: 0.05-0.1 wt% graphene oxide, 70-75 wt% carbide powder, 2-5 wt% oxide powder, 0.5-3 wt% binder, 0.5-3 wt% dispersant, 0.5-2.0 wt% sintering aid, with the balance being solvent.

[0030] The present invention also provides a stable carbide slurry prepared by the above preparation method.

[0031] In a preferred embodiment, the solid phase particle size D50 of the carbide slurry is 0.8~1.5 μm, the viscosity is 150~250 mPa·s, and the pH value is 8~9. The carbide slurry provided in this invention shows no hard sedimentation or obvious phase separation after standing for 72 hours, and can recover its original rheological state with slight stirring.

[0032] In practical operation, the stability of the slurry is determined through sedimentation experiments: The prepared slurry is injected into a transparent glass sedimentation tube (25 mm inner diameter, 150 mm height), with a constant volume of slurry injected, and the initial total height of the slurry in the tube is marked as H. The sedimentation tube is placed vertically in a constant temperature (25±1°C) environment, and the sedimentation is started. At predetermined time points (12 hours, 48 ​​hours, and 72 hours), the stratification phenomenon of the slurry is observed and recorded. After sedimentation, the slurry usually separates into two layers: an upper layer of clear or slightly turbid liquid and a lower layer of suspended particles or a dense sedimentation layer. The interface height of the lower sedimentation layer (or uniform suspension layer) is measured and recorded as h. The stability of the slurry is quantitatively characterized by calculating the sedimentation volume ratio F = h / H. The closer the F value is to 1, the less sedimentation and the higher the slurry stability. The smaller the F value, the more severe the sedimentation and the worse the slurry stability.

[0033] The present invention also provides a method for preparing a dense carbide coating, wherein a carbide slurry is coated on the surface of a carbon substrate, dried, and then sintered to obtain the coating.

[0034] The sintering process is as follows: first, hold at 350~450℃ for 2~4 hours, then hold at 1200~1600℃ for 4~6 hours, and finally hold at 2000~2200℃ for 3~5 hours.

[0035] In this invention, during the sintering process, at the low-temperature stage (350~450℃), organic matter is slowly removed, and graphene oxide maintains its structure to form a stable framework. At the medium-temperature stage (1200℃~1600℃), graphene oxide loses oxygen-containing functional groups during heating, transforming into reduced graphene oxide (rGO). The rGO sheets (especially the edge active sites) in close contact with the oxide particles serve as a high-quality nano-carbon source, undergoing a mild and uniform in-situ carbothermic reduction reaction with oxides (such as Ta2O5) (Ta2O5 + 7C → 2TaC + 5CO↑). This reaction is not a violent explosion, but rather generates a layer of highly active nanoscale tantalum carbide grains in situ on and around the oxide particles. The unconsumed rGO sheets form an interconnected nanoscale interface network, where CO gas preferentially diffuses and escapes mildly and continuously along the low-energy interfaces between these rGO / ceramic particles, avoiding macroscopic porosity caused by bubble bursts. Within this temperature range, the added metals (Co or Ni) and other sintering aids begin to melt or form a low-melting-point eutectic phase, producing a small amount of liquid phase. This liquid phase rapidly wets and encapsulates the solid particles under capillary force, initiating mass transport through a dissolution-precipitation mechanism, driving particle rearrangement, significantly enhancing the sintering driving force, and beginning to fill the micropores left by the decomposition of organic matter. As the temperature further increases (>1600℃), the nano-TaC grains surrounding the oxide continue to drive the carbothermic reduction reaction until the oxide is completely converted. The released CO gas is also discharged through the rGO channel, thereby eliminating oxide residue and ensuring a pure-phase composition of the coating. Nano-TaC grains bridge micron-sized tantalum carbide particles through surface diffusion and grain boundary migration, further squeezing the temporary space occupied by rGO sheets. Some of the rGO is encapsulated by the grown grains, becoming a nano-reinforcing phase within the grains or at the grain boundaries, improving the toughness of the coating. A small amount of rGO may remain after pyrolysis, leaving submicron closed pores, but these are quickly healed as the liquid phase fills them. At the same time, the remaining rGO pins the grain boundaries, inhibiting abnormal grain growth, thereby obtaining a dense carbide coating through sintering.

[0036] Experiments have shown that the stable carbide slurry provided by this invention is crucial for obtaining a dense carbide coating. When there is too much graphene oxide (>0.15 wt%), its large specific surface area and abundant functional groups will cause excessive adsorption of solvents and dispersants, resulting in excessively high slurry viscosity, a sharp decrease in fluidity, and even premature formation of an irreversible gel-like structure, making it difficult to spray. During drying, microcracks will be introduced prematurely due to uneven internal stress. During final sintering, excess graphene (rGO) cannot be completely consumed or replaced, resulting in a large number of dispersed fine closed pores. When there is too little graphene oxide (<0.05 wt%), a continuous and effective network cannot be formed. The slurry is prone to rapid sedimentation and agglomeration during standing, resulting in submicron-level pores during final sintering. When there is too much oxide (> 5 wt%), the excess reactants cause a violent and concentrated carbothermal reduction reaction, instantly generating a large amount of gas (CO). The generation rate of these gases far exceeds the conduction capacity of the graphene oxide network, causing the gases to accumulate and merge locally, ultimately forming micron-sized macropores. When there is too little oxide (< 2 wt%), a large amount of free carbon generated from the pyrolysis of the binder in the slurry, as well as some carbon provided by the graphene oxide, remains due to the lack of sufficient reaction interfaces. This residual carbon is the main reason for the final formation of a large number of fine pores.

[0037] In a preferred embodiment, the coating is selected from one of brushing, dipping, or spraying.

[0038] In a further preferred embodiment, the coating is performed by air spraying. During air spraying, the nozzle diameter is controlled to be 0.8~1.2mm, the air pressure to be 0.2~0.4MPa, the spray distance to be 10~20cm, the spray gun moving speed to be 50~200mm / s, and the spray gun angle to be 70~90°, where 90° means the spray gun angle is perpendicular to the surface to be sprayed. Using spraying ensures the uniformity of the coating thickness and reduces shrinkage stress.

[0039] In a preferred embodiment, the drying process is as follows: first, the temperature is raised to 50-70°C and held for 4-6 hours; then, the temperature is raised to 80-100°C at a rate of 5-10°C / min and held for 3-4 hours; finally, the temperature is raised to 140-180°C at a rate of 1-3°C / min and held for 6-8 hours. The carbide slurry used preferably employs a combination of three solvents with significantly different boiling points: water (boiling point 100°C), ethanol (boiling point 78°C), and ethylene glycol (boiling point 197°C). This creates a gradient evaporation effect during the drying stage after slurry coating. The low-boiling-point solvent evaporates first, allowing the slurry to rapidly and initially solidify, fixing the particle network framework. The high-boiling-point solvent, acting as a wet film leveling agent, evaporates slowly last. During this process, it effectively alleviates the capillary pressure caused by rapid solvent evaporation, preventing microcracks or structural collapse of the coating due to stress concentration in the early stages of drying.

[0040] In a preferred embodiment, the sintering process is as follows: the temperature is increased to 350-450℃ at a heating rate of 5-7℃ / min and held for 2-4 hours; then the temperature is increased to 1200-1600℃ at a heating rate of 6-10℃ / min and held for 4-6 hours; finally, the temperature is increased to 2000-2200℃ at a heating rate of 5-7℃ / min and held for 3-5 hours.

[0041] In a preferred embodiment, the sintering is carried out under an Ar atmosphere, with the vacuum level controlled at 5-7 mbar.

[0042] The present invention also provides a dense carbide coating prepared by the above preparation method, wherein the porosity of the carbide coating is <2%.

[0043] In this invention, the density of the sintered carbide coating was tested according to the method specified in GB / T 39688-2020.

[0044] Principles and advantages

[0045] The stable carbide slurry provided by this invention features graphene oxide sheets with rich oxygen-containing functional groups, such as hydroxyl and carboxyl groups, ensuring good wetting and initial dispersion in polar solvents. These sheets, with their ultra-large specific surface area and high aspect ratio, interact with each other through van der Waals forces, spontaneously forming a continuous, flexible three-dimensional nanonetwork structure within the slurry. When high-density carbide and oxide particles are distributed within this three-dimensional network, the mesh structure directly constrains and spatially restricts the free sedimentation motion of the particles, significantly reducing the sedimentation rate according to Stokes' law. Furthermore, the combination of fine carbide particles and coarse oxide particles optimizes the particle packing density, reduces the large-scale sedimentation space between particles, and fundamentally reduces the sedimentation driving force. More importantly, the dispersant, by adsorbing onto the surface of the powder particles, provides electrostatic repulsion or steric hindrance, effectively inhibiting the soft agglomeration of the particles. This ensures that the particles are uniformly dispersed at their native size within the graphene oxide network, avoiding localized density inhomogeneities and accelerated sedimentation caused by agglomerate formation. When Pluronic F-127 was chosen as the dispersant in the experiment, Pluronic F-127 (PEO-PPO-PEO) is an amphiphilic triblock copolymer. In the water-ethanol-ethylene glycol mixed solvent system of this invention, its hydrophilic polyethylene oxide segments (PEO) tend to extend into the polar solvent, while the hydrophobic polypropylene oxide segments (PPO) tend to adsorb onto the hydrophobic substrate of the graphene oxide and the nonpolar surface of the carbide / oxide powder. This unique molecular structure allows it to be simultaneously and firmly anchored on the surfaces of both graphene oxide sheets and ceramic particles. The extended PEO chains form a thick hydration layer in the solvent, generating a strong steric hindrance effect that prevents close contact and aggregation between particles and lamellae.

[0046] In a highly stable carbide slurry, a highly dispersed and stable suspension system was obtained through the particle size distribution of fine carbide and coarse oxide particles, and the synergistic effect of graphene oxide and dispersant. After coating, gradient drying effectively mitigated drying stress and prevented microcrack formation through the evaporation of solvents with varying boiling points. During sintering, at low temperatures (350-450℃), organic matter was slowly removed, and graphene oxide maintained its structure, forming a stable framework. At medium temperatures (1200℃-1600℃), graphene oxide lost oxygen-containing functional groups during heating, transforming into reduced graphene oxide (rGO). The rGO sheets (especially the edge active sites) in close contact with the oxide particles act as a high-quality nanocarbon source, undergoing a mild and uniform in-situ carbothermic reduction reaction with oxides (such as Ta₂O₅) (Ta₂O₅ + 7C → 2TaC + 5CO↑). The reaction is not a violent explosion, but rather the in-situ formation of a layer of highly active nanoscale tantalum carbide grains on and around the oxide particles. The unconsumed rGO sheets form an interconnected nanoscale interface network. CO gas preferentially diffuses and escapes gently and continuously along the low-energy interfaces between these rGO / ceramic particles, avoiding macroscopic pores caused by bubble bursts. In this temperature range, the added metals (Co or Ni) and other sintering aids begin to melt or form a low-melting-point eutectic phase, producing a small amount of liquid phase. This liquid phase rapidly wets and encapsulates the solid particles under capillary forces, initiating mass transport through a dissolution-precipitation mechanism, driving particle rearrangement, significantly enhancing the sintering driving force, and beginning to fill the micropores left by the decomposition of organic matter. As the temperature further increases (>1600℃), the nanoscale TaC grains surrounding the oxide continue to drive the carbothermic reduction reaction until the oxide is completely transformed. The released CO gas also exits through the rGO channels, thereby eliminating oxide residues and ensuring the pure phase composition of the coating. Nano-TaC grains bridge micron-sized tantalum carbide particles through surface diffusion and grain boundary migration, further encroaching on the temporary space occupied by rGO sheets. Some of the rGO is encapsulated by the grown grains, becoming a nano-reinforcing phase within the grains or at the grain boundaries, thus improving the coating's toughness. A small amount of rGO may remain after pyrolysis, leaving submicron-sized closed pores, but these quickly heal with liquid phase filling. Simultaneously, the remaining rGO pins the grain boundaries, inhibiting abnormal grain growth.

[0047] Furthermore, the gradient heating of this invention achieves a two-step densification process of "gas degassing first, then healing." In the intermediate temperature stage, this is achieved through a layered network constructed by the carbon source within the coating. This network not only acts as a uniform carbon source to undergo a mild carbothermic reduction reaction with the oxide, but more importantly, the nanoscale interconnected interface it forms provides a low-resistance diffusion channel for the reacting gas, ensuring that the gas is discharged in a mild and orderly manner, thereby avoiding macroscopic porosity defects caused by concentrated gas escape. In the later stage of high-temperature sintering, the process is mainly driven by nano-carbide grains generated in situ during the reaction. These highly active nano-grains strongly bridge the surrounding micron-sized master particles, providing a huge sintering driving force. At the same time, the liquid phase formed by the sintering aid acts as a high-speed material transport channel, synergistically filling the submicron space left after the gas is discharged, ultimately achieving the densification of the coating.

[0048] Compared with the prior art, the present invention has at least the following advantages:

[0049] A high-solids-content carbide slurry with long-term suspension stability was prepared by utilizing the synergistic stabilization mechanism of graphene oxide three-dimensional network and block copolymer.

[0050] By using highly stable carbide slurry and co-designing a two-step densification method of "venting first and then healing", a highly dense carbide coating with no internal defects and uniform distribution was prepared.

[0051] By defining the optimal quantitative ratio of carbon source and oxide, a stable process suitable for large-scale production was established. Attached Figure Description

[0052] Figure 1 This is a process flow diagram of the present invention.

[0053] Figure 2 These are comparative photographs of the carbide slurries prepared in Example 1 and Comparative Example 1 after a certain settling time. Figure 2 (a) is a comparative photograph showing the state of carbide slurry "1#" prepared in Example 1 and carbide slurry "2#" prepared in Comparative Example 1 after standing at room temperature for 12 hours. Figure 2 (b) is a comparison photo of the carbide slurry "1#" prepared in Example 1 and the carbide slurry "2#" prepared in Comparative Example 1 after standing at room temperature for 12 hours.

[0054] Figure 3 This is an SEM image of the surface of the green blank after step S3 in Example 1.

[0055] Figure 4 This is a cross-sectional scanning electron microscope (SEM) image of the tantalum carbide coating finally prepared in Example 1.

[0056] Figure 5The image shows a cross-sectional scanning electron microscope (SEM) image of the tantalum carbide coating finally prepared for Comparative Example 1.

[0057] Figure 6 The image shows a cross-sectional scanning electron microscope (SEM) image of the tantalum carbide coating finally prepared for Comparative Example 2. Detailed Implementation

[0058] Example 1

[0059] This embodiment provides a method for preparing a highly stable carbide slurry and its dense coating. The specific preparation process is as follows:

[0060] In step S1, 0.12g of graphene oxide, 25g of ethanol, 12.5g of ethylene glycol, 2.38g of ultrapure water, and 1.2g of Pluronic F-127 were added to a zirconia ball mill jar and ball-milled at 550 rpm for 2.5 hours for pre-dispersion. Then, 150g of tantalum carbide powder with a D50 of 1.5μm, 6g of tantalum oxide with a D50 of 2.5μm, and 3.5g of nickel powder were added, and the ball milling speed was adjusted to 400 rpm for 2.5 hours. Finally, 1.8g of PVB was added to the slurry, and the speed was reduced to 250 rpm for 1.5 hours to obtain a stable slurry.

[0061] Step S2: Select a flat graphite block (50mm × 50mm × 5mm), use an air spray gun with a 0.8 mm nozzle, and spray the graphite substrate uniformly under the conditions of air pressure 0.3MPa, spray distance 15 cm, spray gun moving speed 100 mm / s, and spray gun angle perpendicular (90°) to the substrate. The average thickness of the sprayed coating is 87μm.

[0062] Step S3: Place the sprayed sample in a forced-air drying oven and perform gradient drying according to the following procedure: increase the temperature to 60°C at 2°C / min and hold for 5 hours; increase the temperature to 90°C at 8°C / min and hold for 3.5 hours; increase the temperature to 160°C at 2°C / min and hold for 7 hours. After drying, the coating surface should be smooth and free of defects such as cracks and peeling.

[0063] Step S4: Under an Ar atmosphere, heat to 400°C at 6°C / min and hold for 3 hours; then heat to 1400°C at 8°C / min and hold for 5 hours; finally, heat to 2100°C at 6°C / min and hold for 4 hours, with a vacuum of 5-7 mbar; then cool to room temperature in the furnace to obtain the final product.

[0064] The slurry prepared in step S1 was tested and found to have a particle size D50 of 1.4 μm, a viscosity of 220 mPa·s (25°C), and a pH of 8.5. A portion of the slurry prepared in step S1 was injected into a transparent glass settling tube and marked "1#". Figure 2 The images show a comparison of the slurry's state after standing at room temperature for 12 hours and 72 hours, as described in this embodiment. It can be observed that after standing for 72 hours, the slurry's sedimentation volume ratio is 93%, with only a very thin clear liquid layer appearing at the top. No hard sediment is formed, and the slurry maintains a uniform suspension state. Slight shaking restores its initial fluidity, fully meeting the stability requirements of subsequent spraying processes. Figure 3 Microscopic images (corresponding to the surface of the green body after step S3) show that tantalum carbide particles (darker contrast) and tantalum oxide particles (whitish contrast) are uniformly dispersed in the green body, without significant hard agglomeration. This lays the foundation for obtaining a uniform microstructure during subsequent sintering. Figure 4 The image shown is a cross-sectional scanning electron microscope (SEM) image of the tantalum carbide coating finally prepared in Example 1. The coating exhibits good overall thickness uniformity, with an average thickness of approximately 63 μm. The interface between the coating and the graphite substrate is continuous, with no visible cracks, peeling, or other macroscopic defects. The coating grains exhibit a near-equiaxed crystal morphology, with dense intergranular packing and very few pores. The density of the coating, determined by the Archimedes displacement method, is as high as 98.3%.

[0065] Example 2

[0066] This embodiment provides a method for preparing a highly stable carbide slurry and its dense coating. The specific preparation process is as follows:

[0067] In step S1, 0.10g of graphene oxide, 22g of ethanol, 11g of ethylene glycol, 2.2g of ultrapure water, and 1.0g of Pluronic F-127 were added to a zirconia ball mill jar and pre-dispersed by ball milling at 500 rpm for 2.0 hours. Then, 145g of tantalum carbide powder with a D50 of 1.5μm, 8g of tantalum oxide with a D50 of 2.5μm, and 3.0g of cobalt powder were added, and the ball milling speed was adjusted to 450 rpm for 3.0 hours. Finally, 1.5g of polyvinyl alcohol (PVA) was added to the slurry, and the speed was reduced to 200 rpm for 1.0 hour to obtain a stable slurry.

[0068] Step S2: Select a flat graphite substrate (50mm × 50mm × 5mm), use an air spray gun with a 1.2mm nozzle, and spray the graphite substrate uniformly under the conditions of air pressure 0.25 MPa, spray distance 18 cm, spray gun moving speed 80 mm / s, and spray gun angle perpendicular (90°) to the substrate. Control the average thickness of the wet coating after spraying to be about 82μm.

[0069] Step S3: Place the sprayed sample in a forced-air drying oven and perform gradient drying according to the following procedure: increase the temperature to 55°C at 2°C / min and hold for 4 hours; increase the temperature to 85°C at 10°C / min and hold for 4 hours; increase the temperature to 150°C at 1°C / min and hold for 8 hours. After drying, the coating surface should be smooth and free of defects such as cracks and peeling.

[0070] Step S4: Under an Ar atmosphere, heat to 380°C at 7°C / min and hold for 3 hours; then heat to 1400°C at 8°C / min and hold for 5 hours; finally, heat to 2100°C at 6°C / min and hold for 5 hours, with a vacuum of 5-7 mbar; then cool to room temperature in the furnace to obtain the final product.

[0071] The slurry prepared in step S1 had a particle size D50 of 1.41 μm, a viscosity of 210 mPa·s (25°C), and a pH of 8.8. After standing at room temperature for 72 hours, the slurry exhibited similar behavior to Example 1, with a sedimentation volume ratio of 92.6%, only a very thin clear liquid appearing at the top, and no hard sediment, demonstrating excellent slurry stability. The particles in the green coating were uniformly dispersed, with no significant hard agglomerates. The final sintered coating showed good adhesion to the substrate, an average thickness of approximately 60 μm, a dense structure, a density of 98.1%, and low porosity.

[0072] Example 3

[0073] This embodiment provides a method for preparing a highly stable carbide slurry and its dense coating. The specific preparation process is as follows:

[0074] In step S1, 0.12g of graphene oxide, 28g of ethanol, 9.3g of ethylene glycol, 1.86g of ultrapure water, and 1.5g of Pluronic F-127 were added to a zirconia ball mill jar and ball-milled at 600 rpm for 3 hours for pre-dispersion. Then, 150g of tantalum carbide powder with a D50 of 1.5μm, 6g of tantalum oxide with a D50 of 2.5μm, and 2.5g of cobalt powder were added, and the ball milling speed was adjusted to 400 rpm for 2.5 hours. Finally, 2.2g of PVB was added to the slurry, and the speed was reduced to 250 rpm for 1.5 hours to obtain a stable slurry.

[0075] Step S2: Select a flat graphite block (50mm × 50mm × 5mm), use an air spray gun with a 0.8 mm nozzle, and spray the graphite substrate uniformly under the conditions of air pressure 0.3MPa, spray distance 15 cm, spray gun moving speed 100 mm / s, and spray gun angle perpendicular (90°) to the substrate. The average thickness of the sprayed coating is 90μm.

[0076] Step S3: Place the sprayed sample in a forced-air drying oven and perform gradient drying according to the following procedure: increase the temperature to 60°C at 2°C / min and hold for 5 hours; increase the temperature to 90°C at 8°C / min and hold for 3.5 hours; increase the temperature to 160°C at 2°C / min and hold for 7 hours. After drying, the coating surface should be smooth and free of defects such as cracks and peeling.

[0077] Step S4: Under an Ar atmosphere, heat to 400°C at 6°C / min and hold for 3 hours; then heat to 1400°C at 8°C / min and hold for 5 hours; finally, heat to 2200°C at 6°C / min and hold for 4 hours, with a vacuum of 5-7 mbar; then cool to room temperature in the furnace to obtain the final product.

[0078] The slurry prepared in step S1 had a particle size D50 of 1.5 μm, a viscosity of 250 mPa·s (25°C), and a pH of 8.2. After standing for 72 hours, the slurry exhibited a sedimentation volume ratio of 94.1%, maintaining good stability. The final sintered coating had an average thickness of approximately 64 μm, a dense structure, and strong adhesion to the substrate. The coating density was measured to be 98.0%.

[0079] Comparative Example 1

[0080] Comparative Example 1 is identical to Example 1 in all other conditions except that graphene oxide and tantalum oxide are not added in step S1.

[0081] The slurry prepared in this comparative example had a particle size D50 of 1.5 μm, a viscosity of 270 mPa·s (25°C), and a pH of 8.3, as tested. A portion of the slurry prepared in Comparative Example 1 was injected into a transparent glass settling tube and labeled "2#". Figure 2 As shown, after standing at room temperature for 72 hours, the slurry exhibited severe sedimentation, with a turbid suspension on top and a dense sediment at the bottom, resulting in a sedimentation volume ratio of only 28.6%. The slurry's stability was significantly worse than that of Example 1. Figure 5 The image shown is a cross-sectional scanning electron microscope (SEM) image of the tantalum carbide coating obtained in this comparative example. The average coating thickness is approximately 66 μm, but the microstructure contains numerous micron-sized irregular pores, indicating a loose structure. The Archimedes displacement method determined the coating density to be only 90.1%, indicating poor compactness. This suggests a lack of sufficient graphene oxide, preventing the formation of a continuous and effective network. The slurry is prone to rapid sedimentation and agglomeration during standing, ultimately resulting in submicron-sized pores during sintering.

[0082] Comparative Example 2

[0083] Comparative Example 1 is identical to Example 1 in all other conditions except that an excess of graphene oxide (0.21 g) is added in step S1.

[0084] Testing revealed that the slurry prepared in step S1 had a particle size D50 of 2.0 μm, a viscosity of 405 mPa·s (25°C), and a pH of 8.3. The excessively high viscosity resulted in poor slurry flowability, and during the settling process, excessive graphene oxide sheets might become overly entangled, thus compromising dispersion stability. After 72 hours of settling, its sedimentation volume ratio was 57%, indicating poor stability as well. The final sintered coating had an average thickness of approximately 65 μm. Figure 6 As shown, the coating contains a large number of fine pores. The coating density was measured to be 86.5%, indicating poor compactness.

[0085] Comparative Example 3

[0086] Comparative Example 1 is identical to Example 1 in all other conditions except that tantalum oxide is not added.

[0087] Testing revealed that the slurry prepared in step S1 had a particle size D50 of 1.2 μm, a viscosity of 205 mPa·s (25°C), and a pH of 8.2. After standing for 72 hours, the slurry exhibited a sedimentation volume ratio of 91.1%, demonstrating good stability. The final sintered coating had an average thickness of approximately 65 μm, with numerous fine pores dispersed throughout, similar to the microstructure of Comparative Example 2. The coating density was measured to be 85.1%, indicating poor compactness. This is attributed to the presence of free carbon generated from the pyrolysis of the binder and some carbon provided by graphene oxide, which remained in large quantities due to a lack of sufficient reaction interfaces, ultimately forming numerous fine pores.

[0088] The properties of the carbide slurry and carbide coating prepared in the embodiments and comparative examples of the present invention are shown in Table 1:

[0089]

[0090] 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 steady-state carbide slurry, characterized in that: Graphene oxide, solvent, and dispersant are ball-milled at 500-600 rpm to obtain a dispersion. Then, carbide powder, oxide powder, and sintering aid are added to the dispersion and ball-milled at 400-480 rpm to obtain a ceramic slurry. Finally, a binder is added and ball-milled at 200-300 rpm to obtain a carbide slurry. The mass fraction of graphene oxide in the carbide slurry is 0.05~0.1 wt%; The dispersant is a block copolymer; The carbides obtained by the carbothermic reduction reaction of the oxides in the oxide powder with carbon are the same as those in the carbide powder. The carbide in the carbide powder is selected from one of tantalum carbide, hafnium carbide, and silicon carbide. The particle size D50 of the carbide powder is ≤1.5μm; The particle size D50 of the oxide powder is ≤3μm; The mass fraction of the carbide powder in the carbide slurry is 70-75 wt%. The oxide powder has a mass fraction of 2-5 wt% in the carbide slurry.

2. The method for preparing a steady-state carbide slurry according to claim 1, characterized in that: The solvent is selected from at least two of water, ethanol, and ethylene glycol; The dispersant is Pluronic F-127; The sintering aid is selected from at least one of cobalt powder, nickel powder, and molybdenum disilicide.

3. The method for preparing a steady-state carbide slurry according to claim 2, characterized in that: The solvent is a mixed solvent composed of water, ethanol, and ethylene glycol, wherein the mass fraction of water in the mixed solvent is 5-10 wt%, and the mass ratio of ethanol to ethylene glycol is 2-3:

1. The dispersant is Pluronic F-127.

4. The method for preparing a steady-state carbide slurry according to claim 1, characterized in that: The high-speed ball milling time is 2-3 hours; The medium-speed ball milling time is 2-3 hours, and the ball-to-material ratio is controlled at 1-3:1 during the medium-speed ball milling process. The low-speed ball milling time is 1-2 hours.

5. A method for preparing a steady-state carbide slurry according to claim 1 or 2, characterized in that: The carbide slurry comprises, by mass percentage, the following: 0.05-0.1 wt% graphene oxide, 70-75 wt% carbide powder, 2-5 wt% oxide powder, 0.5-3 wt% binder, 0.5-3 wt% dispersant, 0.5-2.0 wt% sintering aid, with the balance being solvent.

6. A stable carbide slurry prepared by the preparation method according to any one of claims 1-5, characterized in that: The carbide slurry has a solid particle size D50 of 0.8~1.5 μm, a viscosity of 150~250 mPa·s, and a pH value of 8~9.

7. A method for preparing a dense carbide coating, characterized in that: The carbide slurry of claim 6 is coated onto the surface of a carbon substrate, dried, and then sintered to obtain the final product. The sintering process is as follows: first, hold at 350~450℃ for 2~4 hours, then hold at 1200~1600℃ for 4~6 hours, and finally hold at 2000~2200℃ for 3~5 hours.

8. The method for preparing a dense carbide coating according to claim 7, characterized in that: The coating is selected from one of brush coating, dip coating or spray coating; The drying process is as follows: first, the temperature is raised to 50~70℃ and held for 4~6 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.

9. The method for preparing a dense carbide coating according to claim 7, characterized in that: The sintering process is as follows: the temperature is increased to 350-450℃ at a heating rate of 5-7℃ / min and held for 2-4 hours; then the temperature is increased to 1200-1600℃ at a heating rate of 6-10℃ / min and held for 4-6 hours; finally, the temperature is increased to 2000-2200℃ at a heating rate of 5-7℃ / min and held for 3-5 hours. The sintering is carried out under the protection of Ar atmosphere, with the vacuum degree controlled at 5~7 mbar.

10. A dense carbide coating prepared by the preparation method according to any one of claims 7-9, characterized in that: The porosity of the carbide coating is <2%.

Images