A chemical vapor deposition hafnium carbide coating for repairing a tungsten cathode

Abstract

This invention discloses a chemical vapor deposition (CVD) hafnium carbide (HfC) coating for repairing tungsten cathodes, belonging to the field of high-temperature electrochemistry and surface engineering technology. The coating comprises a tungsten carbide gradient transition layer, discretely distributed hexagonal boron nitride (HfC) nanoislands, and a hafnium carbide deposition layer. This invention uses HfC nanoislands as templates to induce the growth of HfC nanopillars, forming strong nanomechanical interlocking. Combined with the metallurgical bonding of the WC gradient layer, this imparts the coating with superior adhesion and thermal shock resistance. The use of a nitrogen-free hafnium source ensures the low-nitrogen intrinsic high purity of the HfC coating, thereby achieving excellent resistance to high-temperature molten salt corrosion. The extremely low coverage hfC island design ensures mechanical reinforcement while maximizing the maintenance of low interfacial contact resistance in the coating system. This method has a wide process window, excellent performance, and can significantly extend the service life of tungsten cathodes under extreme operating conditions.

Description

Technical Field

[0001] This invention relates to the fields of high-temperature electrochemistry and surface engineering, and in particular to a chemical vapor deposition hafnium carbide coating for repairing tungsten cathodes. Background Technology

[0002] Due to its high melting point, high electron emission capability and low gas desorption rate, tungsten metal is often used as a key electrode material in electrochemistry, high-temperature metallurgy and nuclear energy fields, such as as a consumable cathode for molten salt electrolytic refining.

[0003] However, under harsh conditions of high temperature, strong corrosion, and thermal cycling, the surface of tungsten cathodes is highly susceptible to wear, corrosion, and cracking, leading to functional failure. Surface repair and strengthening of damaged cathodes, rather than complete replacement, is an urgent need to extend their service life and reduce costs.

[0004] Currently, protective or repair coating technologies for tungsten surfaces mainly face the following interrelated challenges: 1) Insufficient adhesion between coating and substrate: Due to the significant difference in the coefficient of thermal expansion between tungsten and most coating materials (such as refractory metal carbides and nitrides), huge thermal stress will be generated at the interface during thermal cycling, leading to coating peeling. Coatings obtained by traditional physical or chemical vapor deposition are usually columnar crystal structures, which are mostly mechanically interlocked or weakly diffused with the substrate, resulting in limited adhesion and difficulty in withstanding severe thermal shock.

[0005] 2) Poor thermal shock resistance: Weak adhesion directly leads to premature failure of the coating during rapid heating and cooling. Although existing gradient coatings or multilayer structures can alleviate thermal mismatch, they often sacrifice other properties (such as electrical conductivity), and the preparation process is complex. Under long-term thermal cycling, interfacial diffusion may form a brittle phase, becoming a new weak link.

[0006] 3) Limited resistance to high-temperature molten salt corrosion: In applications such as molten salt electrolysis, the coating needs to resist the erosion of molten salts such as fluorides and chlorides at high temperatures. The segregation of impurity elements (especially nitrogen and oxygen) at grain boundaries or defects in the coating significantly accelerates the corrosion process. However, traditional preparation methods (such as using metal-organic precursors containing amino and alkoxy groups) inevitably introduce these impurities, impairing the intrinsic corrosion resistance of the coating.

[0007] 4) Deterioration of interfacial electrical properties: As an electrode, the repaired cathode must maintain good conductivity. While introducing a buffer layer (such as boron nitride) between the coating and the substrate can improve thermal matching, materials like h-BN are insulators. If a continuous layer is formed, it will lead to a sharp increase in interfacial resistance, severely affecting electrode efficiency. To simultaneously resolve these contradictions, a completely new coating structure design and material system is needed. Summary of the Invention

[0008] This invention aims to solve the aforementioned technical challenges by constructing a tungsten carbide (WC) gradient transition layer in situ on a tungsten substrate and precisely preparing a discrete h-BN nanoisland array with ultra-low coverage on its surface as a selective growth template. A specially designed zero-aminohafnium precursor and plasma-assisted CVD are used to induce HfC to form a sparse but strong nanopillar-reinforced composite structure from bottom to top. Interfacial metallurgical strengthening and coating densification are achieved through high-temperature heat treatment, thus proposing a chemical vapor deposition hafnium carbide coating for repairing tungsten cathodes.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: A chemical vapor deposition hafnium carbide coating for repairing tungsten cathodes comprises, in sequence, a tungsten carbide (WC) gradient transition layer coated on the surface of a tungsten (W) cathode, hexagonal boron nitride (h-BN) nanoislands discretely distributed on the surface of the tungsten carbide gradient transition layer, and a hafnium carbide (HfC) deposition layer covering the surfaces of the tungsten carbide gradient transition layer and the hexagonal boron nitride nanoislands. The hafnium carbide deposition layer consists of a dense continuous hafnium carbide matrix and hafnium carbide nanopillars vertically distributed inside the continuous matrix, with the roots of the hafnium carbide nanopillars connected to hexagonal boron nitride nanoislands. The hexagonal boron nitride nanoislands have an area coverage of 2-5% on the surface of the tungsten carbide gradient transition layer, and the average size of the hexagonal boron nitride nanoislands is 20-100 nm.

[0010] The key component of this invention is the ultra-low coverage (2%-5%) discrete hexagonal boron nitride (h-BN) nanoisland array, which functions as follows: First, as a selective nucleation and growth template, it guides the self-assembly of HfC nanopillars (structure guidance role), which is the physical basis for realizing the special nanocomposite structure of the coating of this invention.

[0011] On the exposed WC surface: WC and HfC have similar crystal structures (both cubic), good chemical compatibility, and low interfacial energy. HfC precursors are easily adsorbed, decomposed, and rapidly nucleated in two dimensions and grown laterally to form a continuous thin film. On the surface of h-BN nanoislands: h-BN has a hexagonal layered structure, high chemical inertness, and extremely high interfacial energy with HfC. The nucleation barrier of HfC here is huge, making it extremely difficult to form stable crystal nuclei. At extremely low coverage (2%-5%), over 95% of the surface area is WC-friendly. During CVD deposition, HfC species preferentially and rapidly form films in the vast WC regions. However, nucleation is strongly suppressed on h-BN islands, which occupy only 2%-5% of the area. Even if a very small number of high-energy points (such as island edges) form nuclei, in order to minimize the high-energy interface with h-BN, these nuclei will adopt a three-dimensional island-like growth mode. Their lateral expansion is limited by the size and inertia of the h-BN islands, thus forcing them to grow mainly vertically upwards, eventually forming HfC nanopillars with their roots anchored to the h-BN islands.

[0012] If the coverage is too high (e.g., >15%), excessive h-BN regions will occupy the WC area that could be used to form a strong-bonded film, not only reducing the overall adhesion but also making the nanopillars too dense, which may affect the overall density and uniformity of the coating. A coverage of 2%-5% ensures the dominant position of the strong-bonded film while inducing the formation of HfC nanopillar reinforcement phases inside the deposited layer with minimal defects.

[0013] Second, as a nanomechanical interlocking anchor point, it achieves a dimensional enhancement of interfacial bonding force (mechanical reinforcement), which is the core mechanism for obtaining super strong bonding force.

[0014] Nanoriveting effect: Each HfC nanopillar grown from the h-BN islands is like a high-strength microfiber rivet, supported at its root by the h-BN islands, with its body penetrating the dense HfC coating above. When the coating is subjected to stresses that cause interfacial separation (such as thermal stress or external loads), these nanopillars need to be pulled out through their own shear deformation or fracture, which requires a huge amount of energy.

[0015] Synergistic effect with large-area metallurgical bonding: 95%-98% of the coating area is a direct metallurgical bond between HfC and WC, providing basic and strong adhesion. Discrete nanopillars act as additional, distributed reinforcement points, working in synergy with the metallurgical bonding area to elevate the bonding strength from the planar adhesion of traditional materials to the level of three-dimensional composite reinforcement, with the critical load Lc in scratch testing exceeding 75N.

[0016] Excessive coverage does not linearly increase the binding force. On the contrary, it may cause stress concentration between columns or affect the continuity of the film due to excessive density. A coverage of 2%-5% provides the optimal enhancement efficiency, that is, the maximum mechanical interlocking gain is obtained under the premise of introducing the weakest phase (h-BN).

[0017] Third, as a nanoscale stress buffer and redistributor, it ensures excellent thermal shock resistance (stress management function). The thermal expansion coefficients (CTE) of h-BN and HfC are very different, and they are potential stress concentration points. This invention transforms these macroscopic threats into nanoscale controllable energy dissipation centers by discretizing and nanoscale them.

[0018] Microslip of h-BN islands: Under thermal stress, tiny shear slip can occur between h-BN layers, consuming energy.

[0019] Elastic / plastic bending of nanopillars: Thermal stress causes nanopillars to bend and deform like micro-springs, absorbing strain energy.

[0020] Stress redistribution in the interface region: The discrete point structure prevents the rapid propagation of cracks along a single plane, forcing the crack front to deflect, branch, or pin, which greatly increases the energy required for crack propagation.

[0021] Continuous h-BN layers provide a monolithic, slippery weak surface, ultimately leading to catastrophic spalling. In contrast, discrete and sparse nanoisland structures distribute stress across countless independent nanoscale units for management, avoiding systemic failure and thus achieving superior durability through hundreds of thermal shock cycles.

[0022] Fourth, as a localized chemical diffusion barrier, it maintains interface stability and coating purity (chemical barrier effect), which is a chemical guarantee for ensuring long-term high-temperature stability.

[0023] During subsequent long-term service (>800℃), W, C, and Hf atoms exhibit a tendency for interdiffusion. h-BN is a known excellent high-temperature diffusion barrier material. Each h-BN nanoisland effectively blocks the direct elemental interdiffusion channel between the WC below and the HfC above; by preventing excessive interdiffusion, h-BN islands help maintain a relatively clear and pure WC / HfC interface, avoiding the formation of excessively thick, potentially brittle, complex interdiffusion regions or ternary compound layers, thus maintaining the original toughness of the interface. While continuous h-BN layers can block diffusion, they severely sacrifice conductivity. Discrete nanoislands, while playing a local diffusion-blocking role, have a negligible impact on overall conductivity due to their extremely low coverage.

[0024] Conclusion: This invention transforms the disadvantages (inertness, insulation) of h-BN materials into advantages (selective growth template, diffusion barrier) through an ultra-low coverage discrete h-BN nanoisland array with ingenious structural design. Combined with its low coverage characteristics, it ultimately synergistically gives rise to a novel coating structure with ultra-strong, ultra-tough, ultra-stable and superconducting properties.

[0025] This invention also proposes a method for preparing a chemical vapor deposition hafnium carbide coating for repairing tungsten cathodes, comprising the following steps: Step S1. Prepare a tungsten carbide gradient transition layer in situ on the surface of the damaged tungsten cathode; Step S2. Discretely distributed hexagonal boron nitride nanoislands are prepared on the surface of the tungsten carbide gradient transition layer; Step S3. Using a nitrogen-free organic hafnium compound as the hafnium source, a hydrocarbon compound as the carbon source, and hydrogen as the reducing gas and carrier gas; chemical vapor deposition is performed under plasma-assisted activation at a substrate temperature of 900-1200℃ and a reaction chamber pressure of 1-10kPa; chemical vapor deposition is performed under plasma assistance to generate a dense hafnium carbide deposition layer with internally encapsulated hafnium carbide nanopillars; taking advantage of the difference in nucleation and growth rates of hafnium carbide on the surface of tungsten carbide and hexagonal boron nitride nanoislands, a continuous hafnium carbide film is formed on the surface of tungsten carbide, and hafnium carbide nanopillars are grown in situ on the hexagonal boron nitride nanoislands. Finally, the hafnium carbide nanopillars are covered by the lateral growth of the continuous hafnium carbide film to form a top layer of homogeneous dense hafnium carbide deposition layer. Step S4. Perform high-temperature heat treatment on the dense hafnium carbide deposit to obtain the hafnium carbide coating.

[0026] Preferably, step S1 includes the following specific steps: placing the damaged tungsten cathode, which has been treated by sandblasting, chemical cleaning, and hydrogen reduction activation, in a CVD reaction chamber; introducing tungsten source precursor, carbon source gas, and hydrogen into the reaction chamber, and performing carburizing and deposition reactions at a temperature of 1000-1300℃ and a pressure of 1-10kPa for a reaction time of 1-5h; by precisely controlling the partial pressure of the carbon source gas and the reaction time, carbon atoms diffuse to the surface of the tungsten substrate and react, thereby growing a tungsten carbide gradient transition layer that forms a metallurgical bond with the tungsten substrate in situ; the gradient layer is characterized by its surface being a dense WC with a near-stoichiometric ratio, and the carbon element concentration and phase transitioning continuously and smoothly from the surface to the interior tungsten substrate.

[0027] Further, in step S1, the molar flow ratio of the tungsten source precursor, carbon source gas, and hydrogen is 1:0.5-5:20-100; the tungsten source precursor is tungsten hexafluoride or carbonyl tungsten, and the carbon source gas is methane, acetylene, or propylene.

[0028] Preferably, step S2 includes the following specific steps: On the surface of the tungsten carbide gradient transition layer prepared in step S1, using ammonia borane (NH3BH3) as a precursor, chemical vapor deposition is performed under hydrogen carrier gas at a temperature of 900-1100℃ and a pressure of 50-1000Pa for 5-15 minutes to form a discretely distributed array of hexagonal boron nitride (h-BN) nanoislands with an area coverage strictly controlled between 2% and 5%; the average size of the nanoislands is controlled between 20 nanometers and 100 nanometers. The volume ratio of ammonia borane to hydrogen is 1:19-99, that is, the volume fraction of the precursor is controlled between 1.0-5.0%.

[0029] Preferably, the operation process of S2 is as follows: The sample treated in step S1 is placed in a chemical vapor deposition reaction chamber. Solid ammonia borane (NH3BH3) is placed in a temperature-controlled bubbler, with the bubbler temperature maintained between 30°C and 60°C. High-purity hydrogen (H2) is used as the carrier gas, passing through the bubbler at a flow rate of 50 to 200 standard cubic centimeters per minute (sccm), carrying out the NH3BH3 vapor. Before entering the reaction chamber, this precursor gas stream is mixed with another stream of high-purity hydrogen to obtain diluted hydrogen, with a flow rate of 1000 to 5000 sccm.

[0030] Preferably, in step S3, the nitrogen-free organic hafnium compound is a hafnium complex containing one cyclopentadienyl ligand or its derivative ligand and one to three chlorine atom ligands, preferably tert-butylcyclopentadienyl hafnium trichloride or methylcyclopentadienyl hafnium trichloride; the carbon source in step S3 is selected from one or more of methane, acetylene, ethane, propane or ethylene.

[0031] Preferably, step S3 includes the following specific steps: (a) After vacuum deoxygenation, high-purity hydrogen (H2) at a flow rate of 500-2000 sccm is introduced into the reaction chamber to adjust and stabilize the pressure in the reaction chamber at 1-10 kPa. The plasma power supply is then turned on, and plasma pretreatment is performed at a temperature of 900-1200℃ for 5-15 min, with the plasma power density controlled at 0.5-3.0 W / cm². 2 ; (b) Using H2 at a flow rate of 20-100 sccm as the carrier gas to carry hafnium source vapor into the reaction chamber, a carbon source gas at a flow rate of 10-200 sccm is introduced, and the total H2 flow rate is maintained at 500-2000 sccm. Plasma treatment is carried out at a temperature of 900-1200℃ for 20-60 min, and the plasma power density is controlled at 0.5-3.0 W / cm². 2 This stage mainly generates HfC crystal nuclei, and the WC region is completely covered by the HfC thin film, while the crystal nuclei on the h-BN island are smaller in size (<50nm), ensuring that subsequent vertical growth dominates.

[0032] (c) Maintain the flow rate, temperature and plasma power density of (b) and continue processing for 70-180 min. During this stage, HfC crystal nuclei grow into nanopillars. (d) Maintain the flow rate, temperature and plasma power density of (b) and continue processing for 30-60 min. During this stage, the HfC film laterally inhibits the growth of HfC nanopillars and gradually covers the HfC nanopillars, generating a continuous HfC film with homogeneous growth at the top layer.

[0033] Preferably, the actual operation process of S3 is as follows: S301. System preparation and plasma preheating and cleaning stage: The sample treated in step S2 is placed into a chemical vapor deposition (CVD) reaction chamber equipped with a radio frequency (RF) or direct current (DC) plasma source. After sealing the reaction chamber, the vacuum system is activated to evacuate the background vacuum to ≤1.0×10⁻⁶. -3 Pa, then high-purity argon (Ar) is introduced to atmospheric pressure and evacuated again. This process is repeated 2-3 times to completely replace any residual air in the system. The heating system is then started, and the sample is heated to a deposition temperature of 900-1200℃ (preferably 1000-1100℃) at a heating rate of 5-10℃ / min. After the temperature stabilizes, high-purity hydrogen (H2) is introduced into the reaction chamber at a flow rate of 500-2000 sccm. The pressure in the reaction chamber is adjusted and stabilized at 1-10 kPa (preferably 2-5 kPa). The plasma power supply is turned on (RF frequency is typically 13.56 MHz), and the sample surface is subjected to plasma pretreatment for 5-15 min in a pure H2 atmosphere, with the plasma power density controlled at 0.5-3.0 W / cm². 2 This step aims to clean and activate the surface, especially the high-energy sites at the edges of the h-BN nanoislands; S302. Selective nucleation and initial rapid film growth stage: Keep the plasma on, load the selected nitrogen-free organic hafnium precursor into the bubbler, control the bubbler temperature at 80-100℃, use H2 at a flow rate of 20-100 sccm as the carrier gas to carry its vapor into the reaction chamber, and at the same time, introduce carbon source gas (such as CH4) at a flow rate of 10-200 sccm, and maintain the total H2 flow rate at 500-2000 sccm. During this stage (lasting approximately 20-60 minutes), selective growth is achieved by utilizing the significant nucleation barrier difference between HfC and the WC and h-BN surfaces: In the exposed WC region, which accounts for 95%-98% of the surface area, the precursor rapidly decomposes and forms high-density two-dimensional nuclei. These nuclei quickly merge laterally, forming a continuous, dense initial HfC film with a thickness of approximately 100-500 nm. In the h-BN nanoisland region, which accounts for only 2%-5% of the surface area, nucleation is strongly suppressed, with sparse, isolated three-dimensional island-like HfC initial nuclei forming only at extremely high energy sites such as their edges or defects. By the end of this stage, the WC region is completely covered by the HfC film, while the nuclei on the h-BN islands are smaller (<50 nm), ensuring that subsequent vertical growth dominates.

[0034] S303. Directed growth of nanopillars and synergistic thickening of thin films / pillars: Maintain stable temperature, pressure, plasma power, and gas flow rates during the S302 stage, and continue deposition. This stage is the main deposition period, typically lasting 70-180 min depending on the desired total coating thickness (20-100 μm). During this stage, in the WC region already covered by the HfC film, deposition continues in a two-dimensional layered growth mode, resulting in uniform film thickening. In the h-BN island region, the existing three-dimensional island nuclei, due to the extremely high lateral growth energy barrier on the h-BN sides, are forced to preferentially grow primarily along the direction perpendicular to the substrate under the drive of minimizing surface energy, gradually evolving into distinct HfC nanopillars. Their root diameter is limited by the initial h-BN island size (20-100 nm), while their height increases approximately linearly with deposition time. As deposition continues, the physical gap between the HfC film extending laterally from the WC region and the HfC nanopillars growing vertically from the h-BN islands continuously decreases. The film not only thickens itself but also begins to laterally envelop and connect the lower and middle parts of the nanopillars.

[0035] S304. Structural Fusion and Final Densification Stage: In the final stage of deposition (the last 30-60 minutes), the laterally grown HfC film comes into contact and merges with the top and upper-middle parts of the vertically grown HfC nanopillars. At this point, mass transport occurs not only to the film surface and the nanopillar tips but also through diffusion at the formed interface. This process allows the nanopillars to be completely embedded into the dense HfC matrix, forming a continuous, three-dimensional interpenetrating composite structure. This stage ensures the metallurgical bond between the nanopillars and the substrate coating and further fills any existing micropores, achieving overall densification of the coating.

[0036] S305. Deposition Termination and Controlled Cooling Stage: After the total deposition time reaches the predetermined value, the inlet valves for the organic hafnium precursor and the carbon source gas (CH4) are first closed. While maintaining the H2 flow rate and plasma operation, a 5-10 minute purging step is performed to remove residual precursors and gaseous byproducts from the reaction chamber and piping. The plasma power supply is then turned off. Under a continuous H2 or Ar protective atmosphere, the sample is cooled to below 200°C at a controlled cooling rate of 5-20°C / min. This slow cooling process helps release internal stress in the coating. The heating system is then turned off, and the sample is removed after allowing it to cool naturally to room temperature.

[0037] This step leverages the intrinsically significant difference in nucleation barrier and growth rate between hafnium carbide on the surface of exposed tungsten carbide (high surface energy, high reactivity) and hexagonal boron nitride nanoislands (low surface energy, chemical inertness) to achieve structural self-assembly: In the exposed tungsten carbide region, which accounts for 95% to 98% of the surface area, hafnium carbide preferentially and rapidly nucleates and expands laterally in a two-dimensional layered growth pattern, forming a continuous and dense hafnium carbide film; in the hexagonal boron nitride nanoislands region, which accounts for only 2% to 5% of the surface area, hafnium carbide nucleation is strongly suppressed, and nucleation only occurs on the islands. A very small number of high-energy sites at the edge or top form three-dimensional island-like crystal nuclei, and their lateral growth is hindered, forcing them to grow preferentially mainly along the direction perpendicular to the substrate, forming the prototype of hafnium carbide nanopillars. As deposition continues, the hafnium carbide film in the tungsten carbide region continues to thicken, while the hafnium carbide nanopillars on the hexagonal boron nitride islands continue to grow taller and eventually merge with the film, self-assembling to form a unique composite coating structure: this structure uses highly dense hafnium carbide as a continuous matrix, and vertically embeds sparse but tough hafnium carbide nanopillars derived from hexagonal boron nitride nanoislands inside it.

[0038] Preferably, step S4 includes the following specific steps: in an inert protective atmosphere or vacuum, the composite coating sample obtained in step S3 is subjected to high-temperature heat treatment; the heat treatment temperature is 1300-1600℃, and the holding time is 1-3h; this process promotes the interdiffusion of tungsten, carbon, and hafnium atoms at the interface between tungsten carbide and hafnium carbide coating, forming a metallurgical bonding zone; at the same time, the hafnium carbide nanopillars inside the coating undergo moderate coarsening through the Ostwald ripening mechanism, with their diameter evolving from 50-100nm in the deposited state to 100-250nm, and the aspect ratio evolving from 5-20 to 2-8, so that the coating as a whole achieves densification and internal stress release.

[0039] The present invention also proposes a tungsten cathode repaired by the aforementioned chemical vapor deposition hafnium carbide coating preparation method for repairing tungsten cathodes, comprising a tungsten substrate and a hafnium carbide composite coating, wherein the bulk nitrogen content of hafnium carbide in the hafnium carbide composite coating is less than 0.5 atomic percentage.

[0040] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention first establishes a metallurgical bond between a WC gradient layer and a tungsten substrate, providing a first-level strong bond and thermal expansion transition. Secondly, discretely distributed h-BN nanoislands serve as inert templates, suppressing their lateral growth during subsequent HfC deposition. This encourages HfC to preferentially nucleate at the island edges and grow vertically upwards, ultimately forming HfC nanopillars rooted in the h-BN islands. These nanopillars, like anchors, are deeply embedded in the upper continuous HfC film, working together with the WC region below the film through strong chemical bonds to form a three-dimensional reinforcement system combining surface bonding and point anchoring. This structure effectively dissipates and redistributes stress generated by thermal mismatch at the interface, preventing crack initiation and propagation, and exhibits high bonding strength and thermal shock resistance.

[0041] 2. This invention uses a hafnium source without amino ligands (such as -NR2), such as (tBuCp)HfCl3. After the decomposition of amino-containing precursors, nitrogen atoms (N) enter the HfC lattice as interstitial impurities, causing severe lattice distortion and tending to accumulate at grain boundaries, greatly reducing the chemical stability of the material. In high-temperature molten salts, these nitrogen-rich grain boundaries become preferential corrosion channels. In contrast, the nitrogen-free hafnium source of this invention eliminates the introduction of nitrogen impurities at the source (bulk N content < 0.3 at.%), obtaining a high-purity HfC coating with a complete structure and clean grain boundaries, thus exhibiting extremely strong corrosion resistance.

[0042] 3. This invention controls h-BN to a discrete nano-island morphology occupying only 2%-5% of the surface area, ensuring that the vast majority of the region is a continuous conductive pathway formed by direct contact between the highly conductive HfC film and the WC gradient layer. Therefore, while obtaining the h-BN-induced nanopillar reinforcement effect, the overall conductivity of the system is almost unaffected.

[0043] 4. This invention significantly reduces the deposition reaction temperature by activating the precursor with plasma assistance, avoiding overheating damage to the tungsten substrate and promoting selective growth kinetics on the h-BN template. Simultaneously, stable process parameters ensure uniform and dense growth of the nanopillars and thin film structures. Within the range of 2%-5% h-BN coverage, 900-1200℃ HfC deposition temperature, and 1300-1600℃ heat treatment temperature, the nanopillar composite structure and high-purity dense coating can be stably reproduced, both yielding coatings with excellent performance.

[0044] 5. In summary, discrete h-BN nanoislands and a nitrogen-free hafnium source are both necessary and sufficient conditions, and neither can be omitted. Without h-BN islands, there is no interlocking of nanopillars, resulting in mediocre performance; using amino-containing precursors leads to a collapse in corrosion resistance; and replacing h-BN with a continuous film results in the loss of conductivity. This invention organically combines discrete h-BN nanoislands, a nitrogen-free hafnium source, and plasma-assisted activation. Through the synergy of structural innovation and material purification, it achieves simultaneous optimization of four major properties: bonding strength, thermal shock resistance, corrosion resistance, and low resistance, thus meeting the requirements for long-term and reliable service of tungsten cathodes in extreme environments. Attached Figure Description

[0045] Figure 1 These are SEM images of the substrate surface obtained in steps S1-S4 of Embodiment 1 of the present invention, wherein... Figure 1 (a) is a polycrystalline surface diagram of the WC gradient transition layer. Figure 1 (b) shows the discrete distribution of h-BN nanoislands. Figure 1 (c) is a microscopic image of the surface of S3 after 40 minutes of initial deposition. Figure 1 (d) is a surface microstructure image of S3 after 125 min of continued deposition. Figure 1 (e) is a microscopic image of the surface deposited in S3 45 min later. Figure 1 (f) is a microscopic image of the surface after S4 heat treatment. Detailed Implementation

[0046] To make the objectives, technical solutions, and advantages of this invention clearer, the technical effects of this invention are described in detail below with reference to systematic embodiments and comparative examples, and its underlying mechanism is revealed through data analysis. All experiments followed uniform pretreatment and testing standards.

[0047] I. Unified Testing Standards and Methods: All experiments used cylindrical pure tungsten cathode specimens from the same batch, with similar surface damage morphologies, measuring 20 mm in diameter and 5 mm in thickness. Before repair, all specimens underwent a completely identical pretreatment process: sandblasting, alkaline ultrasonic cleaning, acid immersion, repeated ultrasonic cleaning with deionized water, and drying, followed by reduction treatment at 1000℃ in a flowing hydrogen atmosphere for 1 hour. Performance testing adopted the following unified standards: Coating adhesion test: The critical load Lc (N) at which the coating fails (peeling or cracking) is recorded using the scratch method (ISO 20502:2005).

[0048] Thermal shock resistance test: The sample was placed in a muffle furnace at 1100℃ and held for 10 min, then rapidly quenched in deionized water at room temperature, constituting one cycle. After each cycle, the coating was examined using an optical microscope, and the first occurrence of macroscopic failure (peeling area > 1 mm) was recorded. 2The number of cycles (or through-cracks).

[0049] Coating composition analysis: Secondary ion mass spectrometry (SIMS) was used for depth profiling to quantitatively analyze the percentage content (at.%) of nitrogen atoms in the bulk phase of the coating (far from the interface).

[0050] Molten salt corrosion resistance test: The coating was statically immersed in LiF-NdF3 (mass ratio 1:1) molten salt at 900℃ for 150h. The average thickness loss (μm) was calculated by measuring the coating cross-sectional thickness at the same point before and after immersion.

[0051] Interface electrical performance testing: A modified four-point probe method was used to measure and calculate the interfacial contact resistance per unit area (mΩ·cm) between the coating and the tungsten substrate. 2 ).

[0052] Microstructure characterization: The surface morphology of the coating was observed using scanning electron microscopy (SEM).

[0053] II. Effects of experimental series A.hBN and nitrogen-containing hafnium sources on coating performance: This series of experiments aims to demonstrate that the HfC nanopillar composite structure induced by discrete h-BN nanoislands and the use of a nitrogen-free hafnium source are the core and necessary conditions for achieving the superior performance of this invention.

[0054] Example 1 (Complete Optimal Solution): S1 (Preparation of WC gradient layer): Using WF6 as the tungsten source and CH4 as the carbon source, the reaction was carried out at 1200℃ and 5kPa for 3h to obtain a WC gradient layer with a thickness of about 15μm that is metallurgically bonded to the matrix.

[0055] S2 (Preparation of h-BN nanoislands): Using ammonia borane (NH3BH3) as a precursor, a discrete h-BN nanoisland array with an average diameter of about 60 nm and an area coverage of about 3.5% was obtained by reacting at 1000 °C and 200 Pa for 7 min.

[0056] S3 (HfC composite coating deposition): using (tBuCp)HfCl3 as the hafnium source and CH4 as the carbon source, deposition was carried out for 3.5 h under the assistance of 1050℃, 3kPa, and 2kW radio frequency plasma, divided into three stages: 40 min for the early stage, 125 min for the middle stage, and 45 min for the late stage. S4 (Heat Treatment): Heat treatment at 1500℃ for 2 hours in flowing high-purity argon gas.

[0057] from Figure 1 The SEM images clearly show the deposition process, in which a continuous film forms in the WC region, sparse nuclei form in the h-BN island region, and eventually grow into nanopillars through self-assembly. Figure 1(a) is a polycrystalline surface diagram of the WC gradient transition layer, which conforms to the size of WC grains of 0.5-2μm and is uniform and flat overall; Figure 1 (b) shows the discrete distribution of h-BN nanoislands, with a size of approximately 35-50 nm, and exposed WC substrate between the islands; Figure 1 (c) is a surface microscopic image of S3 after 40 min of initial deposition. The substrate is a continuous film (HfC dense layer in WC region), and a small number of protrusions appear at the edge of BN island. Figure 1 (d) is a surface microstructure image of S3 after 125 min of continued deposition, which includes vertically growing protrusions (HfC nanopillars) with bottom dimensions corresponding to BN islands and sparse distribution; Figure 1 (e) shows the surface microstructure after 45 min of S3 deposition, where the substrate HfC film is denser, without nanopillars, and the surface exhibits a continuous HfC film state. Figure 1 (f) shows the surface microstructure after S4 heat treatment, where the particles are more tightly bound and the grain boundaries are less blurred.

[0058] Depend on Figure 1 It is evident that the WC gradient layer provides strong metallurgical bonding with the substrate and a first-order thermal expansion transition; the discrete h-BN islands act as templates to induce the formation of HfC nanopillars, providing robust nanomechanical interlocking; and the nitrogen-free hafnium source ensures the intrinsic high purity of the coating (low nitrogen content), thus achieving superior adhesion, thermal shock resistance, and corrosion resistance. The low interfacial resistance is attributed to the extremely low coverage of h-BN (3.52%), which allows the conductive pathway to dominate.

[0059] Comparative Example A1 (without h-BN nanoisland layer): Process differences: The only difference from Example 1 is that step S2 is completely omitted, and no h-BN layer is deposited. All other steps and parameters are exactly the same as in Example 1.

[0060] Comparative Example A2 (continuous h-BN thin film layer): Process difference: Step S2 is modified to deposit a continuous, dense h-BN film. Specifically, in the same equipment, the NH3BH3 deposition time is extended to 20 min to obtain a continuous h-BN film with a thickness of approximately 100 nm. Other steps are exactly the same as in Example 1.

[0061] Comparative Example A3 (using an amino-containing organic hafnium precursor): Process differences: The hafnium precursor in step S3 was replaced with the commonly used tetrakis(dimethylamino)hafnium (Hf(NMe2)4). To accommodate its decomposition characteristics, the deposition temperature was adjusted to 950°C. All other steps were identical to those in Example 1.

[0062] Table 1. Effects of hBN and nitrogen-containing hafnium source on coating performance As shown in Table 1, Comparative Example A1 directly demonstrates that h-BN nanoislands are an absolute prerequisite for forming the nanopillar-reinforced structure. Without h-BN islands, HfC is uniformly deposited on the WC surface, forming a traditional columnar crystalline coating with weak bonding. Its bonding strength (41 N) is only about 48% of that of Example 1 (85 N), and its thermal shock resistance life is less than 1 / 7. The data clearly show that the mechanical interlocking of the nanopillars induced by h-BN is the core physical mechanism by which this invention achieves ultra-strong bonding strength and thermal shock resistance. Although its corrosion resistance is acceptable (thanks to the absence of nitrogen contamination), its overall performance is completely unable to meet the requirements of extreme working conditions.

[0063] Comparative Example A2 reveals the critical importance of discrete design. While a continuous h-BN film can provide some stress buffering, its inherently weak interlayer bonding makes it a weak interface in the coating, resulting in poor adhesion. More importantly, h-BN is an insulator, and the continuous film leads to catastrophic high resistance, causing the repaired cathode to completely lose its electrode function. The increased corrosion loss is due to the lack of strong bonding between HfC and the WC substrate, and the potential presence of microcracks at the interface that become corrosion channels. This result contrasts sharply with Example 1 (low resistance, strong bonding), strongly demonstrating that designing h-BN as a low-coverage discrete island, rather than a continuous film, is the only effective way to simultaneously resolve the contradictions between adhesion, conductivity, and long-term reliability.

[0064] Comparative Example A3 most intuitively demonstrates the extreme necessity of using a nitrogen-free hafnium source. Nitrogen atoms remaining after the decomposition of amino ligands enter the HfC lattice as interstitial impurities, causing severe lattice distortion and segregation at grain boundaries, significantly reducing the intrinsic corrosion resistance and bonding strength of the coating. A nitrogen content as high as 6.5 at.% directly leads to catastrophic failure of corrosion resistance (loss > 15 μm vs. 1.7 μm in Example 1). This result conclusively demonstrates that nitrogen impurities are a deadly poison for the performance of HfC coatings in high-temperature molten salt environments. Therefore, the present invention specifies the use of amino-ligand-free hafnium precursors (such as (tBuCp)HfCl3), a creative choice to ensure the long-term protective capability of the coating from the material source.

[0065] III. Experimental Series B. Influence of Process Parameter Range on Coating Performance: This series of experiments aims to systematically verify that the range of key process parameters (h-BN coverage, deposition temperature, heat treatment temperature) protected in the claims of this invention are not only effective, but also necessary conditions for achieving excellent performance.

[0066] Example 2 (Verification of Low h-BN Coverage): Process differences: In step S2, the deposition conditions were adjusted to 900℃, 50Pa, and deposition for 5 min to obtain h-BN nanoislands with a coverage of approximately 2.05% and an average diameter of 10-35 nm. The other steps were exactly the same as in Example 1.

[0067] Example 3 (High h-BN Coverage Validation): Process differences: In step S2, the deposition conditions were adjusted to 1100℃, 800Pa, and deposition for 10 min to obtain h-BN nanoislands with an area coverage of approximately 4.97% and an average diameter of 60-90 nm. Other steps were the same as in Example 1.

[0068] Example 4 (Verification of Low HfC Deposition Temperature): Process differences: In step S3, the deposition temperature was adjusted to 950°C, and (MeCp)HfCl3 was used as a precursor to accommodate the slightly lower decomposition temperature. Simultaneously, the plasma power was slightly increased to 2.2 kW to compensate for reactivity. Other steps were the same as in Example 1.

[0069] Example 5 (Verification of High HfC Deposition Temperature): Process differences: In step S3, the deposition temperature is increased to 1150°C. All other parameters are the same as in Example 1.

[0070] Example 6 (Verification of Low-Temperature Heat Treatment): Process differences: In step S4, the heat treatment temperature is adjusted to 1400℃ and the holding time is shortened to 1 hour. Other steps are the same as in Example 1.

[0071] Example 7 (High-Temperature Heat Treatment Verification): Process differences: In step S4, the heat treatment conditions are intensified to be held at 1550°C for 3 hours, and slow cooling at 1°C / min is used through the 1300-1100°C range. Other steps are the same as in Example 1.

[0072] Table 2. Influence of process parameter range on coating performance As shown in Table 2, Example 2 demonstrates that even with an extremely low coverage of 2%, it still functions effectively. Despite the low nanopillar density, the mechanical interlocking effect of the nanopillars remains significant, with performance far exceeding Comparative Example A1 (without nanopillars). Its binding force (78 N), while slightly lower than that of Example 1 (85 N), is still at an extremely high level. This indicates that 2% is the critical lower limit for the effectiveness of the present invention; below this value, the enhancement effect may be insufficient.

[0073] Example 3 demonstrates the effectiveness of the 5% coverage upper limit. At this coverage, the nanopillar density increases, the reinforcement effect is sufficient, and the thermal shock resistance is excellent. A slight increase in interfacial resistivity (from 1.0 to 1.2 mΩ·cm) is observed. 2 The study indicated a slight effect of increased coverage on conductivity, but it still fully meets electrode requirements. This result suggests that 5% is a reasonable upper limit for providing a sufficient enhancement effect while maintaining excellent conductivity.

[0074] Example 4 successfully verified the feasibility of cryogenic deposition at 900°C with plasma assistance. This indicates that the process window of the present invention has a sufficient width to significantly reduce thermal damage to the substrate (compared to conventional >1400°C processes). The performance data are close to the center point (Example 1), demonstrating the reliability of the cryogenic process and supporting the rationality of the 900°C lower limit in the claims.

[0075] Example 5 demonstrates that higher temperatures promote surface atomic migration, resulting in more complete coating crystallization and higher density, thereby achieving optimal corrosion resistance and adhesion. This result proves that the upper limit of the deposition temperature of 1200°C is effective, at which high-quality coatings can still be obtained. However, it should be noted that this temperature is close to the recrystallization-sensitive region of tungsten, and in practice, the central temperature range is preferred.

[0076] Example 6 shows that heat treatment at 1400℃ / 1h can achieve basic interface strengthening and coating densification, but the effect is not optimal. Both adhesion and corrosion resistance are lower than in Example 1 (1500℃ / 2h), indicating insufficient interfacial diffusion and stabilization of the nanopillar structure. This verifies that 1300℃ is the effective lower limit of the heat treatment scheme, but a higher heat treatment intensity is required to obtain optimal performance.

[0077] Example 6 demonstrates that high-temperature, long-duration heat treatment promotes more complete interfacial interdiffusion, resulting in a wider and stronger metallurgical bonding zone. Simultaneously, the slow cooling process effectively releases internal stress. The nanopillars become shorter and more stable through Ostwald curing. These factors work synergistically to achieve the theoretically optimal overall performance of the coating. This result strongly supports the rationality and superiority of the 1600°C heat treatment upper limit stated in the claims.

[0078] IV. Experimental Series C. Impact of Key Process Elements: This series of experiments further verifies the necessity of plasma, the importance of process stability, and explores the impact of out-of-range parameters.

[0079] Example 8 (using DC plasma): Process Difference: In step S3, a direct current (DC) plasma source is used instead of a radio frequency (RF) plasma, and the power density is adjusted to 1.2 W / cm². 2To obtain a similar plasma intensity. All other parameters are the same as in Example 1.

[0080] Comparative Example C1 (without plasma-assisted deposition): Process differences: In step S3, the plasma was not activated at all. An attempt was made to deposit at 1050°C, but the precursor decomposition efficiency was found to be extremely low, resulting in almost no coating growth. Therefore, the deposition temperature had to be increased to 1300°C to achieve a considerable deposition rate. Other steps were adjusted according to the parameters in Example 1.

[0081] Comparative Example C2 (process parameters fluctuated drastically during the later deposition stage): Process differences: During the last 45 minutes of step S3, significant process fluctuations were introduced: the temperature fluctuated periodically between 1000℃ and 1100℃ (±50℃), the pressure changed irregularly between 2-5 kPa (fluctuation of approximately ±30%), and the flow rates of each gas also fluctuated within ±20% of their set values. The total deposition time remained unchanged.

[0082] Comparative Example C3 (ultra-high coverage of h-BN nanoislands): Process differences: In step S2, by extending the deposition time and increasing the precursor concentration, the area coverage of h-BN nanoislands is increased to approximately 15%, significantly exceeding the preferred range of 2%-5% of this invention. Other steps are the same as in Example 1.

[0083] Table 3. Influence of key process factors on coating performance As shown in Table 3, Example 8 demonstrates that the plasma-assisted requirements of the present invention are not limited to RF type; DC plasma is equally effective. Both can generate highly reactive species, reduce reaction activation energy, and achieve low-temperature deposition and selective growth. This broadens the range of applicable devices for the present invention and enhances its practicality.

[0084] Comparative Example C1 demonstrates the crucial role of plasma assistance. Without plasma activation, extremely high temperatures (1300°C) are required for deposition, which severely damages the tungsten substrate. More importantly, high temperatures cannot reproduce the kinetic conditions required for selective growth, making it impossible to form the nanopillar composite structure and resulting in poor coating quality. Its performance is significantly inferior to that of Example 1 (85 N, >120 cycles), clearly demonstrating that plasma is an indispensable technical means to achieve the dual advantages of low temperature and self-assembly.

[0085] Comparative Example C2 reveals the importance of process stability for obtaining high-performance, uniform coatings. Drastic fluctuations in parameters disrupt the steady-state conditions required for uniform growth of nanopillars and dense film deposition, leading to increased structural defects and decreased interface quality. Its performance was significantly lower than that of Example 1, which was carried out under stable conditions. This, conversely, underscores the necessity of maintaining stable process parameters emphasized in the preferred embodiment of this invention.

[0086] Comparative Example C3 demonstrates that when the h-BN coverage is too high (15%), although a nanopillar structure can still be formed, it brings two negative effects: 1) the strongly bonded exposed WC area is excessively squeezed, leading to a decrease in overall bonding strength; 2) the excessive proportion of the insulating phase (h-BN) leads to an increase in the overall resistance of the coating system. Its performance is comprehensively lower than that of Examples 1-3 with coverage within the range of 2-5%, especially the conductivity is significantly degraded. This result clearly shows that 2%-5% is the performance optimization range of the present invention. Beyond this range, the performance will experience a systematic decline, thus proving the inventiveness and rationality of the range of protection parameters claimed in the present invention.

[0087] Through the above three series of 11 groups (Examples 1-8, Comparative Examples A1-A3, Comparative Examples C1-C3) systematic and rigorous comparative experiments and data analysis, the following conclusive conclusions can be drawn: Example 1 (complete scheme) exhibits unprecedented comprehensive performance: Lc > 80 N, thermal shock > 120 cycles, low corrosion loss, and low electrical resistance. Comparative Examples A1-A3 demonstrate through subtractive experiments that the discrete h-BN nanoisland structure and the nitrogen-free hafnium source are both necessary and sufficient conditions for obtaining these properties; neither can be omitted.

[0088] Examples 2-7 systematically verified the effectiveness of all key process parameter ranges in the claims. Within these ranges—h-BN coverage of 2-5%, HfC deposition temperature of 900-1200℃, and heat treatment temperature of 1300-1600℃—the invention could be successfully implemented with excellent performance, demonstrating the universality and necessity of this range. Examples 2, 3, and 6 showed slightly inferior but effective performance, indicating the optimal range.

[0089] Example 8 and Comparative Examples C1 and C2 demonstrate that plasma-assisted and stable process control are key guarantees for achieving repeatable, high-quality deposition. Comparative Example C3 shows that outside the preferred parameter range of the present invention, performance exhibits a predictable and systematic decline.

[0090] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A chemical vapor deposition hafnium carbide coating for repairing tungsten cathodes, characterized in that, It sequentially includes a tungsten carbide gradient transition layer coated on the surface of a tungsten cathode, hexagonal boron nitride nanoislands discretely distributed on the surface of the tungsten carbide gradient transition layer, and a hafnium carbide deposition layer covering the surfaces of the tungsten carbide gradient transition layer and the hexagonal boron nitride nanoislands. The hafnium carbide deposition layer consists of a dense continuous hafnium carbide matrix and hafnium carbide nanopillars vertically distributed inside the continuous matrix, with the roots of the hafnium carbide nanopillars connected to hexagonal boron nitride nanoislands. The hexagonal boron nitride nanoislands have an area coverage of 2-5% on the surface of the tungsten carbide gradient transition layer, and the average size of the hexagonal boron nitride nanoislands is 20-100 nm.

2. The method for preparing a chemical vapor deposition hafnium carbide coating for repairing tungsten cathodes as described in claim 1, characterized in that, Includes the following steps: Step S1. Prepare a tungsten carbide gradient transition layer in situ on the surface of the damaged tungsten cathode; Step S2. Discretely distributed hexagonal boron nitride nanoislands are prepared on the surface of the tungsten carbide gradient transition layer; Step S3. Using a nitrogen-free organic hafnium compound as the hafnium source, a hydrocarbon compound as the carbon source, and hydrogen as the reducing gas and carrier gas, chemical vapor deposition is carried out under plasma-assisted activation at a substrate temperature of 900-1200℃ and a reaction chamber pressure of 1-10kPa. Step S4. Perform high-temperature heat treatment on the dense hafnium carbide deposit to obtain the hafnium carbide coating.

3. The method for preparing a chemical vapor deposition hafnium carbide coating for repairing tungsten cathodes according to claim 2, characterized in that, Step S1 includes the following specific steps: placing the damaged tungsten cathode, which has been treated by sandblasting, chemical cleaning and hydrogen reduction activation, into the CVD reaction chamber; introducing tungsten source precursor, carbon source gas and hydrogen into the reaction chamber, and carrying out carburizing and deposition reactions at a temperature of 1000-1300℃ and a pressure of 1-10kPa for a reaction time of 1-5h.

4. The method for preparing a chemical vapor deposition hafnium carbide coating for repairing tungsten cathodes according to claim 3, characterized in that, In step S1, the molar flow ratio of tungsten source precursor, carbon source gas and hydrogen is 1:0.5-5:20-100; the tungsten source precursor is tungsten hexafluoride or carbonyl tungsten, and the carbon source gas is methane, acetylene or propylene.

5. The method for preparing a chemical vapor deposition hafnium carbide coating for repairing tungsten cathodes according to claim 2, characterized in that, Step S2 includes the following specific steps: on the surface of the tungsten carbide gradient transition layer prepared in step S1, using ammonia borane as a precursor, chemical vapor deposition is performed for 5-15 minutes under hydrogen carrier gas at a temperature of 900-1100℃ and a pressure of 50-1000Pa, wherein the volume ratio of ammonia borane to hydrogen is 1:19-99.

6. The method for preparing a chemical vapor deposition hafnium carbide coating for repairing tungsten cathodes according to claim 2, characterized in that, In step S3, the nitrogen-free organic hafnium compound is tert-butylcyclopentadienyl hafnium trichloride or methylcyclopentadienyl hafnium trichloride; the carbon source in step S3 is selected from one or more of methane, acetylene, ethane, propane or ethylene.

7. The method for preparing a chemical vapor deposition hafnium carbide coating for repairing tungsten cathodes according to claim 2, characterized in that, Step S3 includes the following specific steps: (a) After vacuum deoxygenation, high-purity hydrogen gas at a flow rate of 500-2000 sccm is introduced into the reaction chamber. The pressure in the reaction chamber is adjusted and stabilized at 1-10 kPa. The plasma power supply is turned on, and plasma pretreatment is carried out at a temperature of 900-1200℃ for 5-15 min. The plasma power density is controlled at 0.5-3.0 W / cm². 2 ; (b) Using H2 at a flow rate of 20-100 sccm as the carrier gas to carry hafnium source vapor into the reaction chamber, a carbon source gas at a flow rate of 10-200 sccm is introduced, and the total H2 flow rate is maintained at 500-2000 sccm. Plasma treatment is carried out at a temperature of 900-1200℃ for 20-60 min, and the plasma power density is controlled at 0.5-3.0 W / cm². 2 ; (c) Maintain the flow rate, temperature and plasma power density of (b) and continue processing for 70-180 min; (d) Maintain the flow rate, temperature and plasma power density of (b) and continue processing for 30-60 minutes.

8. The method for preparing a chemical vapor deposition hafnium carbide coating for repairing tungsten cathodes according to claim 2, characterized in that, Step S4 includes the following specific steps: the composite coating sample obtained in step S3 is subjected to high-temperature heat treatment in an inert protective atmosphere or vacuum; the heat treatment temperature is 1300-1600℃ and the holding time is 1-3h.

9. A tungsten cathode repaired by a chemical vapor deposition hafnium carbide coating preparation method for repairing tungsten cathodes according to any one of claims 2 to 8, comprising a tungsten substrate and a hafnium carbide composite coating, wherein the bulk nitrogen content of hafnium carbide in the hafnium carbide composite coating is less than 0.5 atomic percentages.

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