A process for the production of a conductive corrosion-resistant hard metal coating

Abstract

This invention relates to the field of thermal spraying technology and discloses a preparation process for a conductive and corrosion-resistant cemented carbide coating, addressing the problem of conductive failure in existing cemented carbide coatings under corrosive environments due to preferential corrosion of the binder phase or oxide formation. This invention proposes a composite coating system based on a nickel-chromium-molybdenum ternary alloy binder phase, carbon nanotubes, and lanthanum oxide dual-effect modification, along with an optimized entire process. Ternary alloying achieves complementary enhancement of mechanical properties and corrosion resistance; carbon nanotubes construct a low-resistivity conductive network; and the interface purification effect of rare earth oxides improves coating density. The prepared coating exhibits excellent bonding strength, extremely low volume resistivity, and outstanding long-term corrosion resistance, effectively providing dual protection of physical shielding and functional conductivity under harsh operating conditions.

Description

Technical Field

[0001] This invention relates to the field of thermal spraying technology, and more specifically, to a process for preparing a conductive and corrosion-resistant hard alloy coating. Background Technology

[0002] Hard alloy coatings, due to their extremely high hardness and wear resistance, are widely used for surface protection of critical components in petrochemical, marine engineering, and energy power industries. They are typically formed by thermal spraying, where tungsten carbide (WC) particles are mixed with a metal binder phase and deposited to extend the service life of workpieces under wear conditions. However, with the expansion of emerging applications such as deep-sea exploration and electrochemical machining, higher requirements are being placed on the functionality of coatings, especially in special applications where not only wear resistance is required but also electrical signal transmission or current conduction. Traditional single-protection coatings are no longer sufficient to meet these needs. Summary of the Invention

[0003] In view of the aforementioned existing problems, the present invention is proposed.

[0004] Therefore, the present invention provides a process for preparing a conductive and corrosion-resistant cemented carbide coating, which solves the problem that existing cemented carbide coatings fail to conduct electricity in corrosive environments due to preferential corrosion of the binder phase or formation of oxides.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution: This invention provides a process for preparing a conductive and corrosion-resistant hard alloy coating, comprising the following steps: S1. Weigh out 15.0wt%~25.0wt% of nickel-chromium-molybdenum binder powder, 1.0wt%~3.0wt% of functional modifier, and the remaining tungsten carbide hard phase powder by mass percentage. S2. Add the weighed raw material powders of each component into a ball mill jar, add grinding media and process control agent and ball mill to mix, obtain a mixed slurry, spray granulate the mixed slurry, dry it and then sinter it to obtain spherical composite precursor powder. S3. Clean, degrease and roughen the surface of the workpiece substrate by sandblasting; S4. The spherical composite precursor powder is loaded into a powder feeder, and a supersonic flame spraying process is used to deposit the semi-molten particles onto the pretreated substrate surface to form a conductive and corrosion-resistant coating.

[0006] As a preferred embodiment of the preparation process of the conductive and corrosion-resistant hard alloy coating of the present invention, the mass ratio of each metal component in the binder phase powder is nickel:chromium:molybdenum = 30~40:25~35:30~40.

[0007] In a preferred embodiment of the preparation process of the conductive and corrosion-resistant hard alloy coating of the present invention, the binder phase powder accounts for 19.2 wt% of the total mass of the raw materials.

[0008] As a preferred embodiment of the preparation process of the conductive and corrosion-resistant hard alloy coating of the present invention, the functional modifier is prepared by mixing carbon nanotubes and lanthanum oxide, wherein the amount of carbon nanotubes added is 0.8wt%~1.2wt% and the amount of lanthanum oxide added is 0.4wt%~0.6wt%.

[0009] As a preferred embodiment of the preparation process of the conductive and corrosion-resistant hard alloy coating of the present invention, the mass ratio of each metal component in the binder phase powder is nickel:chromium:molybdenum = 1:1:1; the amount of carbon nanotubes added is 1.0 wt%; and the amount of lanthanum oxide added is 0.5 wt%.

[0010] As a preferred embodiment of the preparation process of the conductive and corrosion-resistant hard alloy coating of the present invention, wherein: the ball milling and mixing uses anhydrous ethanol as the process control agent, the ball-to-material ratio is (5~10):1, the ball milling speed is 250~350 r / min, and the ball milling time is 20~30 hours; The sintering process is carried out under a hydrogen protective atmosphere, with the temperature increased to 900℃~1000℃ at a heating rate of 3~5℃ / min, and held for 45~90 minutes. Hydrogen is used to reduce the oxides on the powder surface and achieve micro-area metallurgical bonding.

[0011] As a preferred embodiment of the preparation process of the conductive and corrosion-resistant hard alloy coating of the present invention, the sandblasting roughening treatment uses 46~60 mesh white corundum sand, the sandblasting pressure is 0.4~0.6MPa, and the surface roughness of the substrate is controlled between 3.0μm and 5.0μm.

[0012] As a preferred embodiment of the preparation process of the conductive and corrosion-resistant hard alloy coating of the present invention, the oxygen-fuel ratio of the supersonic flame spraying is 0.95~1.02, and the oxygen flow rate is 17800~19500m³ / h.

[0013] As a preferred embodiment of the preparation process of the conductive and corrosion-resistant hard alloy coating of the present invention, the supersonic flame spraying has a spraying distance of 300~380mm, a powder feeding rate of 30~60g / min, a carrier gas flow rate of 170~255m³ / h, and a spray gun moving speed controlled at 300~500mm / s.

[0014] The present invention also provides a conductive and corrosion-resistant hard alloy coating, which is prepared by the above method, wherein: the conductive and corrosion-resistant coating has a bonding strength ≥75MPa, a volume resistivity ≤6.0mΩ⋅cm, and a self-corrosion potential ≥-0.15V.

[0015] The beneficial effects of this invention are as follows: By constructing a composite coating system with tungsten carbide as the skeleton, nickel-chromium-molybdenum alloy as the strong and corrosion-resistant matrix, and carbon nanotubes and lanthanum oxide as functional modifiers, and combining it with a fully optimized supersonic flame spraying process, the technical challenge of balancing conductivity and corrosion resistance in traditional hard alloy coatings is effectively solved. Specifically, the nickel-chromium-molybdenum ternary alloy binder phase utilizes the synergistic effect of each component to significantly improve the coating's resistance to pitting corrosion and passivation while ensuring high bonding strength; the introduction of trace amounts of carbon nanotubes constructs a highly efficient microscopic conductive network without damaging the coating's mechanical structure, significantly reducing volume resistivity; and the interface purification and densification effects of lanthanum oxide further eliminate pore defects and block the penetration channels of corrosive media. The final prepared coating has a bonding strength >89 MPa, a volume resistivity as low as 4.86 mΩ⋅cm, and a neutral salt spray resistance time exceeding 1500 hours, maintaining stable electrical transmission and physical protection functions even under extreme corrosive environments. Attached Figure Description

[0016] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 Response surface plots for the nickel, chromium, and molybdenum ternary binder phase ratio.

[0018] Figure 2 Contour plot of the results of the screening experiment for optimizing the core parameters of the spraying process, where the upper left is the bonding strength, the upper right is the volume resistivity, and the lower left is the self-corrosion potential.

[0019] Figure 3 This is a flowchart illustrating the preparation process of a conductive and corrosion-resistant hard alloy coating. Detailed Implementation

[0020] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0021] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0022] Secondly, the term "one embodiment" or "example" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the invention. The appearance of an embodiment in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that mutually excludes other embodiments.

[0023] Example 1 This embodiment provides an experimental study on the screening of framework materials and the determination of the preparation process for a conductive and corrosion-resistant hard alloy coating. Given that the realization of coating performance depends on a deep match between the intrinsic properties of the material and the characteristics of the film-forming process, this embodiment aims to establish a robust experimental benchmark, providing a material basis and technical pathway for the subsequent construction of high-performance composite coating systems.

[0024] 1.1 Screening Experiment for Hard Phase Materials To eliminate the interference of coating porosity, oxidation and decarburization defects caused by mismatched process parameters (such as flame temperature and particle velocity) during thermal spraying on the evaluation of the material's performance, this screening stage does not directly prepare coatings. Instead, it uses the method of preparing candidate materials into powder compacts or sintered bodies with controllable density to evaluate the intrinsic properties of the pressed and sintered powders, directly examining the electrical conductivity and chemical stability of the materials at the microcrystalline structure level. In this way, the intrinsic property advantages of the materials can be extracted through de-processing methods.

[0025] Seven representative commercial powders of transition metal carbides, nitrides, and borides (with uniform particle size sieved to the 3-5 μm range to eliminate particle size effects) were selected as candidate hard phase groups. The selected materials included: chromium carbide. Titanium carbide TiC Titanium diboride tungsten carbide WC Titanium nitride TiN tantalum carbide TaC vanadium carbide VC .

[0026] Each powder group was pressed into an insulating mold under a constant pressure of 20 MPa, and the volume resistivity of the compacted powder was measured using the four-probe method. This was used to characterize the upper limit of the material's ability to form a conductive network in a packed state.

[0027] Equal masses of powder were immersed in a 10% sulfuric acid solution for 48 hours (simulating an acidic service environment). After cleaning and drying, the percentage of mass loss was calculated to quantify the intrinsic inertness of the material against chemical corrosion.

[0028] Each powder group was cold-pressed under the same pressure (e.g., 200 MPa), and its saturated oil absorption in standard oil was measured. Based on the principles of powder metallurgy, under the same molding pressure, the oil absorption rate of the compact indirectly reflects the density of particle packing and pore connectivity. A lower oil absorption rate means a stronger filling capacity of the powder particles after deformation under stress, indicating that it is easier to form a dense, low-porosity coating structure during subsequent processing, thus providing better physical corrosion barrier capabilities. Differential thermal analysis (DTA / TG) was used to determine the temperature at which the material experiences a significant weight gain in an air atmosphere. This metric is used to assess the material's survivability in subsequent thermal processing environments. Excessively low oxidation initiation temperatures can cause the material to form an oxide scale (typically an insulator) during flight, disrupting the electrical conductivity between particles.

[0029] The average value was calculated after multiple measurements, and the test results are shown in the table below: in, It has the lowest volume resistivity, but its high corrosion weight loss rate and low oxidation initiation temperature indicate that it has serious chemical stability defects. and TaC It performs best in terms of corrosion resistance and thermal stability, but Excessively high resistivity cannot meet the requirements for constructing efficient conductive networks, while TaC The relatively high oil absorption rate of the preform reveals its relative inadequacy in terms of densification and film-forming potential. WC Maintaining second only to While achieving excellent electrical conductivity, it also achieves chemical stability on par with the corrosion-resistant group, and ensures optimal physical densification in subsequent processes with the lowest oil absorption rate in the pressed blank, thus achieving the best balance between electrical conductivity and corrosion resistance.

[0030] 1.2 Coating preparation process screening experiment The final performance of a coating depends not only on the material properties but also on the preparation process. Samples are prepared under optimal operating conditions to match the process to the best raw material morphology according to industry standards.

[0031] T1 is a high-purity, dense... WC-Co ( Co Physical vapor deposition (PVD) was performed using a conventional metal binder phase alloy target as raw material via magnetron sputtering; T2 utilized WF , Gas-phase precursors are deposited by chemical vapor deposition (CVD) on a high-temperature substrate surface; T3 employs... WC-Co Core-spun filament ( Co With package WCPowder coating utilizes an electric arc to melt the filament and then sprays it onto compressed air; T4 employs a high-energy fiber laser and a coaxial powder feeding nozzle, combined with powder particles of 45-105μm. WC-Co Laser cladding is performed using mixed powders; T5 utilizes a transfer plasma arc as a heat source and employs coarse particles (60-180μm). WC-Co Thick-layer welding with solder powder, i.e., plasma-transferred arc welding (PTA); T6 cold spraying, using high-pressure cold spraying equipment, in conjunction with customized... NiCr Cover WC Composite powder deposition is achieved through supersonic airflow-driven plastic deformation of solid particles; T7 atmospheric plasma spraying (APS) utilizes a high-energy plasma spraying system in conjunction with conventional agglomerated sintered state. WC-Co Powder; T8 uses an HVOF system with liquid fuel (such as aviation kerosene) and high-pressure oxygen for combustion, combined with micro-spherical particles (15-45μm). WC-Co The powder is sprayed with high-velocity oxygen fuel (HVOF).

[0032] The encapsulated coated sample was placed in a 3.5wt% solution. NaCl In solution, potentiodynamic polarization curves were tested using an electrochemical workstation. Lower electrochemical self-corrosion current densities indicated a denser coating, making it more difficult for corrosive media to penetrate the coating and reach the substrate, and also indicating better corrosion resistance of the coating itself. The coating was prepared on an insulating substrate surface, and its volume resistivity was measured using the four-probe method. According to ASTM C633, the adhesion between the coating and the substrate was tested using the adhesive tensile test to evaluate the coating's resistance to peeling during subsequent processing or service. X-ray diffraction (XRD) was used to obtain the diffraction pattern of the coating, and semi-quantitative analysis was performed to calculate... WC The integral area of ​​the main peak and all tungsten-containing phases in the coating (including WC , C , and W The ratio of the total integral area of ​​the diffraction peaks is used to quantify the effective conductive phase during the process. WC The phase structure retention rate of resistivity difference was determined to reveal the microscopic phase transition mechanism.

[0033] The test results are as follows: Among them, PVD has excellent corrosion resistance but extremely low deposition efficiency; cold spraying has excellent conductivity but extremely poor bonding strength; CVD, PTA and laser cladding have extremely high metallurgical bonding strength, but CVD is affected by heat input, while PTA and laser cladding have low retention rates and high resistivity, making it difficult to meet the requirements of precision conductivity; after comprehensive consideration, supersonic flame spraying, which has no obvious shortcomings in overall performance parameters, was finally selected as the final coating preparation process.

[0034] Example 2 Reference Figure 1 These are two embodiments of the present invention. This embodiment provides a screening and optimization experiment for a binder phase composition system. As the toughening matrix of a cemented carbide coating, the binder phase not only performs the physical functions of binding hard particles and relieving thermal stress, but also acts as a chemical barrier to protect the internal skeleton and maintain the overall integrity of the coating when corrosive media penetrate. This embodiment aims to select a binder phase system from a variety of candidate metal matrices that can form the best interfacial bond with the WC skeleton and has excellent passivation ability in harsh corrosive environments, thereby overcoming the shortcomings of a single hard phase in terms of density and toughness.

[0035] 2.1 Screening experiment of binder phase matrix composition Based on the physical properties (such as density, wetting angle, and melt flowability) of each metal binder phase in the cemented carbide system and industrial application experience, an optimal mass ratio was set for each group to maximize its intrinsic properties. Powder preparation employed an industrial-grade wet ball milling, spray drying, and vacuum sintering process. The raw material powders of each group were placed in a ball mill jar at a predetermined ratio and mixed using anhydrous ethanol as the medium for 24 hours of high-energy ball milling. Subsequently, they underwent spray granulation and vacuum pre-sintering (sintering temperature set at 0.7-0.8 times the melting point of each metal) to prepare composite thermal spraying precursor powders with concentrated particle size distribution, good sphericity, and no phase decomposition. The final product powder mass of each group was uniformly controlled to 1 kg.

[0036] The specific grouping and proportioning scheme is as follows: G1 adds 15wt% iron (Fe); G2 adds 12wt% cobalt (Co); G3 adds 20wt% nickel (Ni); G4 copper (Cu) and G5 chromium (Cr) are both added at 25wt% to compensate for the low strength of copper and the brittleness of chromium; G6 adds 15wt% titanium (Ti); G8 adds 12wt% aluminum (Al); G10 adds 15wt% manganese (Mn); and for the refractory or easily oxidized G7 molybdenum (Mo), G9 niobium (Nb) and G11 vanadium (V), the addition is uniformly limited to 10wt% to avoid the risks of fusion difficulties and structural defects.

[0037] While retaining bonding strength and volume resistivity as fundamental mechanical and electrical indicators, the testing system introduces two key quantitative parameters for density and corrosion resistance. First, the neutral salt spray corrosion life test records the exposure time from the first substrate rust point (red rust) on the coating surface, quantifying the actual protective effectiveness of each metal substrate under chloride ion corrosion, thus eliminating materials with insufficient chemical stability. Second, the introduction of self-corrosion potential monitoring not only reflects the thermodynamic stability of the material but also serves as a sensitive probe for coating density. When the coating has penetrating pores (such as incomplete melting of refractory metals), the electrolyte will penetrate and contact the substrate, causing a significant negative shift in the measured mixed potential (tending towards -0.6V for carbon steel substrates). Conversely, a dense coating exhibits a characteristic high potential, enabling a quantitative assessment of coating film quality.

[0038] The test results are as follows: Among the various alloys, titanium exhibits the best corrosion resistance, but its extremely high resistivity leads to conductive failure; copper has the best conductivity, but its poor bonding strength causes the coating to easily peel off; cobalt has the best bonding strength, but its salt spray resistance time is extremely short, failing to meet corrosion protection requirements. Nickel, chromium, and molybdenum all possess excellent overall performance and exhibit excellent complementarity. Ultimately, a ternary alloy system of nickel, chromium, and molybdenum was selected as the binder phase, achieving a comprehensive balance of performance through alloying.

[0039] 2.2 Response Surface Optimization of Nickel, Chromium, and Molybdenum Ternary Binder Phase Ratio Given that the contribution weights of nickel, chromium, and molybdenum differ across performance dimensions and exhibit significant interactive coupling effects, a simple linear superposition cannot accurately pinpoint the global optimal solution for the ternary system. This section introduces the triangular phase diagram centroid method, aiming to quantify the performance contribution weights of each element, calculate the theoretically optimal centroid coordinates of each individual performance index in the ternary component space, and then determine the final equilibrium ratio through geometric fusion.

[0040] To standardize the experimental design, a normalized coordinate system based on a part-weight system was first established. Based on the industrial application limits of each component in the cemented carbide system (i.e., the maximum amount added without compromising structural integrity), the baseline part-weight definitions for each component were set: 10 baseline parts (i.e., 1 part Ni corresponds to 2.8 wt%) for the nickel component (upper limit 28 wt%), 10 baseline parts (i.e., 1 part Cr corresponds to 2.2 wt%) for the chromium component (upper limit 22 wt%), and 10 baseline parts (i.e., 1 part Mo corresponds to 1.2 wt%) for the molybdenum component (upper limit 12 wt%). Under this system, any ternary ratio can be expressed as Ni:Cr:Mo = a:b:c (where a+b+c = 10). Coatings were prepared using all 66 integer proportion characteristic points within the ternary composition triangle, and the salt spray resistance time, bonding strength, self-corrosion potential, and volume resistivity of each group of samples were tested.

[0041] Test results are as follows Figure 1 As shown, by constructing a continuous response surface model of each performance index in the ternary component space and solving the partial differential equations of each surface, the vertex coordinates (component ratio Ni:Cr:Mo) when each individual performance reaches its theoretical extreme value are calculated: the coordinates of the corrosion-resistant point are (2.38:3.25:4.37); the coordinates of the conductive point are (2.13:2.49:5.38); the coordinates of the mechanical point are (2.40:2.54:5.06); and the coordinates of the steady-state point are (1.61:3.14:5.25).

[0042] Considering the inherent physical correlation between self-corrosion potential (steady-state point) and salt spray resistance time (corrosion resistance point) in chemical stability evaluation, but the former characterizes thermodynamic tendency while the latter characterizes kinetic process, a weighted fusion of 0.6 for the steady-state point and 0.4 for the corrosion resistance point is performed to obtain a single chemical stability pole. Subsequently, this pole is calculated by geometrically averaging with the mechanical point and the electrical point with equal weights, ultimately yielding the globally theoretically optimal ratio coordinates (2.12:2.83:5.05) that balances mechanical strength, electrical conductivity, and chemical inertness.

[0043] Substituting this part ratio into the unit mass equivalent (Ni=2.80, Cr=2.20, Mo=1.20) to convert it into an actual mass ratio, corresponding to Ni:Cr:Mo=5.94:6.23:6.06. To meet the requirements of batching accuracy and error tolerance for industrial production, this parameter is optimized and rounded to 1:1:1 as the mixing ratio of the final binder phase formulation.

[0044] 2.3 Screening Experiment for the Amount of Composite Binder Phase Added After determining the optimal mass ratio of the nickel, chromium, and molybdenum ternary binder phases, it is necessary to further investigate the influence of their total volume percentage in the coating system on macroscopic properties. Given the industrial application limits of each metal component, and considering that the molybdenum component (upper limit 12 wt%) accounts for approximately 1 / 3 of the ternary formulation, the upper limit of the total addition amount of the composite binder phase is set at 36 wt% by reverse calculation based on the molybdenum limit. Based on this, eight mass fraction gradients (5 wt% intervals) from 0 wt% to 35 wt% were established, and composite powders were prepared and sprayed to form the coatings. The density (self-corrosion potential), mechanical strength (adhesion force), and conductivity (resistivity) of the coatings under different binder phase contents were examined to determine the optimal filling threshold that balances the skeletal support effect and the matrix toughening effect. The test results are as follows: Among them, the coating performance first increases and then decreases with the increase of binder content. The available addition amount of composite binder is 15-25wt%. After data fitting, the optimal addition amount is 19.23wt%. Considering industrial production, 20wt% is selected as the final optimal addition amount.

[0045] Example 3 This embodiment provides a synergistic modification and final formulation selection experiment for functional additives. After establishing a basic coating system with tungsten carbide as the skeleton and nickel-chromium-molybdenum alloy as the binder phase, in order to further improve the conductivity and density parameters of the finished product, additives that can achieve qualitative changes in performance with trace additions were screened, thereby completing the final formulation of the high-performance conductive and corrosion-resistant coating.

[0046] 3.1 Screening Experiment of Conductive Modifier Although the optimized nickel-chromium-molybdenum binder phase already possesses a good conductive foundation, the coating resistivity is still mainly limited by the interfacial contact resistance between the hard phase and the metallic phase, as well as the barrier effect of a trace oxide layer. This experiment aims to screen for a highly efficient conductive filler with an extremely low percolation threshold, which can be dispersed in the interparticle gaps to construct a long-range electron transport network, thereby bridging the conductive dead zones that are oxidized or physically isolated.

[0047] 3.1.1 Coarse sieving test of conductive modifier Multiple candidate additives representing different dimensions of carbon-based materials, metals, and conductive ceramics were selected. Based on the density, morphology, and percolation threshold theory of each material, a targeted addition amount was set for each group to construct an effective conductive network while minimizing damage to the coating structure. The formulation calculation adopted the matrix total amount normalization method, that is, keeping the relative ratio of WC skeleton (80wt%) to nickel-chromium-molybdenum binder phase (20wt%) constant, using this as the base, and adding a specific proportion of modifier.

[0048] Specific groupings and addition amounts: D1 contains 0.8 wt% multi-walled carbon nanotubes; D2 contains 0.3 wt% few-layer graphene; D3 contains 2.0 wt% lanthanum hexaboride (LaB6); D4 contains 5.0 wt% micron-sized silver powder; D5 contains 1.5 wt% conductive carbon black; D6 contains 3.0 wt% niobium carbide (NbC).

[0049] The test parameters focus on the balance between the efficiency of conductive network construction and the integrity of the coating structure. The volume resistivity is tested to assess the extent of conductivity improvement, while the bonding strength and self-corrosion potential are monitored to warn of potential agglomeration defects or interfacial wetting barriers caused by additives.

[0050] The test results are as follows: Among them, micron-sized silver has the best conductivity, but the bonding strength is severely weakened due to metal loss, while graphene suffers from deterioration in density due to the two-dimensional shielding effect; carbon nanotubes have a slightly lower conductivity improvement than the former, but they do not have a significant negative impact on the mechanical structure and chemical stability of the coating while effectively constructing a conductive network, thus achieving the best balance of comprehensive performance. Therefore, it is preferred as a conductive modifier.

[0051] 3.1.2 Screening experiment for the amount of carbon nanotubes added After determining carbon nanotubes as the optimal conductive modifier, it is necessary to further investigate the nonlinear effect of their addition amount on the microstructure and macroscopic properties of the coating. Ten concentration gradients (0.2 wt% to 2.0 wt% intervals) were set up to examine the percolation behavior of different concentrations of carbon nanotubes in the composite coating and their potential interference with the bonding strength. The test results are as follows. Among the various parameters observed, the volume resistivity decreased stepwise with increasing carbon nanotube content, reaching a significant low point near the percolation threshold at 1.0–1.2 wt%. Further increases in content could slightly reduce resistivity, but the bonding strength and corrosion resistance began to drop sharply, indicating that excessive carbon nanotubes caused severe aggregation and interfacial defects. Considering both conductivity and structural safety, the optimal carbon nanotube content was ultimately determined to be 1.0 wt%.

[0052] 3.2 Screening Experiment of Densification Modifier To further eliminate unavoidable trace oxidation and porosity defects during the spraying process, a highly active oxygen scavenger is introduced to purify the grain boundaries, and the fluxing effect is utilized to promote the wetting and spreading of the binder phase on the tungsten carbide surface. The screening logic focuses on the balance between interface purification efficiency and side effect control. An ideal modifier should significantly reduce the oxide content in the coating and increase the density, while avoiding abnormal increases in resistivity or embrittlement of mechanical properties due to its own residue or the aggregation of reaction products.

[0053] 3.2.1 Screening Experiment for Modifier Types The experiment selected various candidate materials, including rare earth oxides, traditional slagging agents, and metal deoxidizers. The addition amount was uniformly set at 0.6 wt% (trace doping), and mixed into the final composite powder determined in section 3.1.2. Lanthanum oxide (…) was among the materials used. ), cerium oxide ( ), Yttrium oxide ), anhydrous borax ( Bo ), calcium fluoride ( ), micron aluminum powder ( Al ), silicon powder ( Si ).

[0054] The integral area ratio of oxide peaks (such as NiO / Cr2O3 / WO3) to the main WC peak is calculated using XRD patterns. The lower the ratio, the more thorough the deoxygenation. The self-corrosion potential (characterizing density), bonding strength (characterizing interfacial bonding), and volume resistivity (monitoring insulation side effects) are also tested.

[0055] The test results are as follows: Among them, aluminum and silicon have the best deoxygenation effect, but the reaction products are insulating and embrittle the coating; lanthanum oxide effectively removes oxygen, while significantly improving wettability and bonding strength, and the interface purification effect makes its resistivity decrease instead of increase. Lanthanum oxide was finally selected as the densification modifier.

[0056] 3.2.2 Screening experiment for the amount of lanthanum oxide added After determining lanthanum oxide as the optimal densification modifier, its addition amount needs to be precisely controlled to balance the contradiction between interface purification and the introduction of the insulating phase. Ten gradients (0.1 wt% intervals) from 0.1 wt% to 1.0 wt% were set up to investigate the influence of trace rare earth oxides on the microstructure and macroscopic properties of the coating.

[0057] The test results are as follows: Among the various additives, the oxide content of the coating continuously decreased with increasing addition, and the density and bonding strength reached their peaks in the range of 0.4~0.6wt%. Simultaneously, due to the interface cleansing effect, the resistivity also decreased to its lowest level. Above 0.7wt%, excessive insulating rare earth phases accumulated at the grain boundaries. Although the deoxygenation effect was slightly improved, it hindered the conductive pathways and led to increased brittleness, causing the performance to decline. Ultimately, the optimal addition amount of lanthanum oxide was determined to be 0.5wt%.

[0058] Example 4 Reference Figure 2 This is the fourth embodiment of the present invention, which provides an experiment for optimizing process parameters and controlling quality throughout the entire process. After establishing the final formulation of tungsten carbide framework + nickel-chromium-molybdenum ternary binder + carbon nanotubes and lanthanum oxide dual-effect modifier, precise control of the entire process parameters from powder state to deposition is required to transform the theoretical potential of the material into actual coating performance. By eliminating physical defects and chemical damage during the process, the final delivery indicators of coating density, adhesion, and conductivity are achieved.

[0059] 4.1 Screening of composite powder pretreatment processes Although the composite powder after ball milling meets the composition requirements, its loose density, sphericity, and internal bonding state can affect the pulsation stability and particle flight trajectory of supersonic flame spraying powder. This section aims to screen for an optimal pretreatment process that can ensure excellent powder flowability, prevent component segregation, and pre-eliminate organic residues.

[0060] The experiment divided the same batch of ball-milled composite slurry into 5 groups, and performed different drying and curing processes on each group to prepare feed powder suitable for thermal spraying: Y1 (Mechanical Mixing Direct Injection): After rotary evaporation drying, the slurry undergoes only simple mechanical crushing and sieving (-45 + 15 μm), without granulation or sintering, retaining the original irregular particle morphology; Y2 (freeze-drying): The slurry is frozen at -50℃ and then vacuum sublimated to obtain a powder with a loose, porous, sponge-like structure; Y3 (Spray granulation + hydrogen micro-sintering): After the slurry is centrifugally spray-dried into pellets, it is micro-sintered at 950°C under hydrogen protection. The binder is removed by reducing atmosphere and the particles are moderately densified. Y4 (Radio Frequency Plasma Spheroidization): Powder is fed into a high-temperature radio frequency plasma torch, and the instantaneous high temperature causes the particle surface to melt and condense into a spherical shape. Y5 (vacuum sintering and crushing): The slurry is dried, pressed into blocks, and sintered in a vacuum at 1200℃ into dense blocks, which are then crushed by jaw crusher and shaped and screened by ball milling.

[0061] Hall flow rate is used to determine whether the powder feeder is clogged or pulsating; coating adhesion strength and self-corrosion potential are used to evaluate the film quality after powder deposition in different states; oxygen increment is used to warn of oxidation risks during pretreatment.

[0062] Among them, plasma spheroidization has the best flow rate, but it suffers from severe oxidation at high temperatures; sintering and crushing has the best self-corrosion potential, but its poor fluidity makes it impossible to industrialize powder feeding pulsation; hydrogen micro-sintering has excellent overall performance and achieves the best balance between process stability and coating quality, so it is used as the final pretreatment process.

[0063] 4.2 Optimization and Screening Experiment of Core Parameters for Spraying Process This experiment aims to transform pretreated high-quality powder into a high-performance solid coating. The key lies in achieving a precise match between the thermal and kinetic energy of the flame and the powder properties. The experiment focuses on optimizing the oxygen-fuel ratio and total oxygen flow rate, which have the most significant impact on the melting state and flight velocity of the particles. Under the premise of fixed kerosene flow rate, powder delivery rate, and spraying distance, five gradients of oxygen-fuel ratio from 0.8 (fuel-rich flame) to 1.2 (oxygen-rich flame) and five gradients of oxygen flow rate from 1800 SCFH to 2600 SCFH (SCFH was selected for ease of plotting) were set. Coatings were prepared through full-factor cross-experimentation, focusing on investigating the influence of different flame energy levels on self-corrosion potential, bonding strength, and phase structure stability.

[0064] Test results are as follows Figure 2 As shown, due to the small sample size, the contour lines have a noticeable angle, which is normal. Regarding the oxygen-fuel ratio, as the ratio increases, the flame temperature rises, promoting full melting of the binder phase. Initially, the coating's bonding strength and density show an upward trend. However, when the ratio exceeds 1.05 and enters the oxygen-rich region, the high-temperature oxidizing atmosphere causes the WC skeleton to oxidize. serious Decarbonization (generating) C / With high oxygen-fuel ratios (<0.9), resistivity deteriorates exponentially and coating brittleness increases significantly. Conversely, excessively low oxygen-fuel ratios lead to insufficient enthalpy, resulting in underdeveloped powder, porous coating, and consequently, changes in self-corrosion potential. Regarding oxygen flow rate, increased flow rate significantly imparts higher kinetic energy to particles, enhancing the compaction effect and continuously reducing porosity. However, excessively high flow rates (>2400 SCFH) excessively shorten the residence time of particles in the high-temperature zone, causing large particles to collide with the matrix before softening, resulting in elastic rebound or weak bonding deposition, thus reducing deposition efficiency and interlayer bonding strength.

[0065] Taking all factors into consideration, the oxygen-fuel ratio is controlled at 0.95-1.02 (slightly rich in fuel to near stoichiometric ratio) to balance melting and oxidation inhibition, and the oxygen flow rate is controlled at 2100-2300 SCFH (approximately 17800~19500 m³ / h) to balance density and binding force.

[0066] Example 5 Reference Figure 3 This is the fifth embodiment of the present invention. This embodiment provides a method for preparing a conductive and corrosion-resistant hard alloy coating, including the following steps: S1. Raw material ratio: Weigh out 19.2 wt% nickel-chromium-molybdenum binder powder, 1.5 wt% functional modifier, and the remaining 79.3 wt% tungsten carbide hard phase powder by mass percentage. The mass ratio of each metal component within the binder phase is nickel:chromium:molybdenum = 1:1:1; the functional modifier consists of 1.0 wt% carbon nanotubes and 0.5 wt% lanthanum oxide.

[0067] S2. Preparation of composite precursor powder: The weighed raw material powders of each component were added to a ball mill jar. Anhydrous ethanol was used as the process control agent, the ball-to-material ratio was 8:1, the ball milling speed was 300 r / min, and the mixture was ball-milled for 24 hours to obtain a mixed slurry. The mixed slurry was centrifuged and spray-granulated, dried, and then placed in a tube furnace. Under a hydrogen protective atmosphere, the temperature was raised to 950℃ at 4℃ / min and held for 60 minutes for reduction micro-sintering treatment to obtain composite precursor powder with good sphericity and extremely low oxygen content.

[0068] S3. Substrate pretreatment: 316L stainless steel is selected as the substrate. Its surface is cleaned and degreased, and roughened by sandblasting with 60-mesh white corundum sand at a sandblasting pressure of 0.5MPa. The surface roughness Ra value of the substrate is controlled between 3.5μm and 4.5μm.

[0069] S4. Coating Deposition: The prepared spherical composite precursor powder is loaded into the HVOF powder feeder and deposited using a supersonic flame spraying process. The oxygen-fuel ratio is 1.00, the oxygen flow rate is 18700 m³ / h, the spraying distance is 350 mm, the powder feed rate is 45 g / min, the carrier gas (nitrogen) flow rate is 195 m³ / h, and the spray gun moving speed is 400 mm / s.

[0070] Testing revealed that the prepared coating exhibited a uniform and dense microstructure, with no obvious pores or interlayer cracks, and its bonding strength was >80MPa (the coating remained intact even after the adhesive layer fractured); the volume resistivity was 4.86mΩ⋅cm; the neutral salt spray resistance time was >1500 hours (no red rust); and the self-corrosion potential was -0.128V.

[0071] In summary, by constructing a composite coating system with tungsten carbide as the framework, nickel-chromium-molybdenum alloy as the strong and corrosion-resistant matrix, and carbon nanotubes and lanthanum oxide as functional modifiers, and combining it with a fully optimized supersonic flame spraying process, the technical challenge of balancing conductivity and corrosion resistance in traditional cemented carbide coatings was effectively solved. Specifically, the nickel-chromium-molybdenum ternary alloy binder phase utilizes the synergistic effect of its components to significantly improve the coating's resistance to pitting corrosion and passivation while ensuring high bonding strength. The introduction of trace amounts of carbon nanotubes constructs a highly efficient microscopic conductive network without disrupting the coating's mechanical structure, significantly reducing volume resistivity. Furthermore, the interface purification and densification effects of lanthanum oxide further eliminate porosity defects and block the penetration channels of corrosive media. The final prepared coating exhibits a bonding strength >89 MPa, a volume resistivity as low as 4.86 mΩ⋅cm, and a neutral salt spray resistance exceeding 1500 hours, maintaining stable electrical transport and physical protection functions even under extreme corrosive environments.

[0072] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A process for preparing a conductive and corrosion-resistant hard alloy coating, characterized in that, Includes the following steps: S1. Weigh out 15.0wt%~25.0wt% of nickel-chromium-molybdenum binder powder, 1.0wt%~3.0wt% of functional modifier, and the remaining tungsten carbide hard phase powder by mass percentage. S2. Add the weighed raw material powders of each component into a ball mill jar, add grinding media and process control agent and ball mill to mix, obtain a mixed slurry, spray granulate the mixed slurry, dry it and then sinter it to obtain spherical composite precursor powder. S3. Clean, degrease and roughen the surface of the workpiece substrate by sandblasting; S4. The spherical composite precursor powder is loaded into a powder feeder, and a supersonic flame spraying process is used to deposit the semi-molten particles onto the pretreated substrate surface to form a conductive and corrosion-resistant coating.

2. The preparation process of the conductive and corrosion-resistant hard alloy coating according to claim 1, characterized in that, The mass ratio of each metal component inside the binder phase powder is nickel:chromium:molybdenum = 30~40:25~35:30~40.

3. The preparation process of the conductive and corrosion-resistant hard alloy coating according to claim 2, characterized in that, The binder powder accounts for 19.2 wt% of the total mass of the raw materials.

4. The preparation process of the conductive and corrosion-resistant hard alloy coating according to claim 3, characterized in that, The functional modifier is prepared by mixing carbon nanotubes and lanthanum oxide, wherein the amount of carbon nanotubes added is 0.8wt%~1.2wt% and the amount of lanthanum oxide added is 0.4wt%~0.6wt%.

5. The preparation process of the conductive and corrosion-resistant hard alloy coating according to claim 4, characterized in that, The mass ratio of each metal component in the binder phase powder is nickel:chromium:molybdenum = 1:1:1; the amount of carbon nanotubes added is 1.0 wt%; and the amount of lanthanum oxide added is 0.5 wt%.

6. The preparation process of the conductive and corrosion-resistant hard alloy coating according to claim 1, characterized in that, The ball milling process uses anhydrous ethanol as the process control agent, the ball-to-material ratio is (5~10):1, the ball milling speed is 250~350 r / min, and the ball milling time is 20~30 hours. The sintering process is carried out under a hydrogen protective atmosphere, with the temperature increased to 900℃~1000℃ at a heating rate of 3~5℃ / min, and held for 45~90 minutes. Hydrogen is used to reduce the oxides on the powder surface and achieve micro-area metallurgical bonding.

7. The preparation process of the conductive and corrosion-resistant hard alloy coating according to claim 1, characterized in that, The sandblasting roughening treatment uses 46-60 mesh white corundum sand, with a sandblasting pressure of 0.4-0.6 MPa, and controls the surface roughness of the substrate to be between 3.0 μm and 5.0 μm.

8. The preparation process of the conductive and corrosion-resistant hard alloy coating according to claim 1, characterized in that, The oxygen-fuel ratio of the supersonic flame spraying is 0.95~1.02, and the oxygen flow rate is 17800~19500 m³ / h.

9. The preparation process of the conductive and corrosion-resistant hard alloy coating according to claim 8, characterized in that, The supersonic flame spraying has a spraying distance of 300~380mm, a powder feeding rate of 30~60g / min, a carrier gas flow rate of 170~255m³ / h, and a spray gun moving speed controlled at 300~500mm / s.

10. A conductive and corrosion-resistant hard alloy coating, characterized in that, Prepared by the method described in any one of claims 1 to 9, the conductive corrosion-resistant coating has a bonding strength ≥75MPa, a volume resistivity ≤6.0mΩ⋅cm, and a self-corrosion potential ≥-0.15V.

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