A high life self-lubricating coating for sintered neodymium-iron-boron molds and a method for its production

Abstract

This invention discloses a high-life self-lubricating coating for sintered NdFeB molds and its preparation method, belonging to the field of mold surface treatment technology. The coating, from the inside out, comprises: a transition layer disposed on the surface of the mold substrate, the transition layer comprising Al and Mn; a support layer disposed on the surface of the transition layer, the support layer comprising an AlCoCrFeNi alloy; a self-lubricating functional layer disposed on the surface of the support layer, the self-lubricating functional layer comprising AlCoCrFeNi alloy, MoS2, and ZrN / GO composite nanoparticles; and a surface layer disposed on the surface of the self-lubricating functional layer, the surface layer comprising Al2O3. This invention achieves a functionally graded composite structure on the surface of the mold substrate, consisting of an AlMn alloy transition layer, an AlCoCrFeNi high-entropy alloy support layer, a self-lubricating functional layer containing MoS2 and ZrN / GO composite nanoparticles, and an Al2O3 surface layer, thereby significantly improving the bonding strength between the mold and the coating, reducing the surface friction coefficient, enhancing wear resistance, and extending the service life of the mold.

Description

Technical Field

[0001] This invention relates to the field of mold surface treatment technology, specifically to a long-life self-lubricating coating for sintered NdFeB molds and its preparation method. Background Technology

[0002] As the most powerful permanent magnet material currently available, the industrial production of sintered NdFeB magnets hinges on powder pressing, a crucial process determining the uniformity of the blank's density and its final magnetic properties. This typically requires pressing the relatively poor-flowability NdFeB powder into blanks with complex shapes under high pressures of hundreds of megapascals. In current production practices, Cr is commonly used as the mold material. 12 High-carbon, high-chromium cold work die steels such as MoV possess high base strength, hardenability, and a certain degree of wear resistance. However, during long-term continuous pressing, the die cavity surface is subjected to the combined effects of high pressure, fretting wear, and hard magnetic powder erosion, gradually revealing two major bottlenecks that restrict production efficiency and product quality. The first is the problem of mold sticking: under high pressure, neodymium iron boron powder easily adheres to the die cavity surface due to intermolecular forces and mechanical interlocking effects, leading to continuous deterioration of the die cavity surface finish. When the blank is demolded, the significantly increased surface frictional resistance causes defects such as scratches, marks, and even localized peeling. This not only directly deteriorates the product surface quality but also forces frequent production interruptions for mold cleaning, severely reducing production continuity and efficiency. Secondly, there is the issue of wear: sintered NdFeB powder itself has high hardness and fine particle size. During continuous pressing, the fine particles are easily embedded in the gap between the mold and act as abrasive. At the same time, there is frequent relative sliding between the blank and the mold cavity wall, which causes the mold cavity surface to gradually wear down and lose its original smoothness, resulting in secondary damage when the product is demolded. The mold needs to be disassembled and polished periodically. After multiple repairs, the cavity dimensions are out of tolerance and the precision is lost, resulting in scrap, which significantly increases the mold consumption cost and equipment downtime.

[0003] To address these challenges, researchers and engineers have explored various mold surface treatment technologies. However, existing single treatment methods all have performance limitations, making it difficult to achieve a balance between wear resistance and mold sticking prevention. Nitriding can form a high-hardness nitride diffusion layer on the mold surface, significantly improving surface hardness and abrasive wear resistance. However, its surface chemical activity is high, its self-lubricating properties are insufficient, and its friction coefficient is relatively large, limiting its effectiveness in alleviating the adhesion and accumulation of magnetic powder on the mold surface. Electroplating hard chrome is simple to operate and produces a high-hardness coating with good wear resistance. However, the electrodeposition process inevitably generates a microcrack network within the coating, which can easily become fatigue crack propagation sources under high-pressure cyclic loading. Furthermore, the bonding strength between the coating and the steel substrate is mainly mechanical, resulting in limited adhesion strength and a risk of peeling and flaking after long-term use. Physical vapor deposition (PVD) of hard ceramic coatings such as TiN and CrN has extremely high surface hardness and chemical stability. However, ceramic coatings are inherently brittle and have poor physical compatibility with steel substrates. They are prone to cracking, chipping, or even peeling when subjected to high pressure impact and cyclic stress. At the same time, their surface energy is high and their lubricity is still insufficient, which cannot effectively suppress magnetic powder adhesion.

[0004] In summary, existing surface treatment technologies cannot simultaneously meet the comprehensive performance requirements of strong and tough bonding, high wear resistance, and long-lasting self-lubrication. Therefore, developing a composite coating system that can take into account the toughness of the substrate, high wear resistance of the surface, and long-lasting self-lubricating function has become an urgent need to break through the technical bottleneck of sintered NdFeB magnet forming molds and improve the manufacturing level of the industry. Summary of the Invention

[0005] The purpose of this invention is to provide a long-life self-lubricating coating for sintered NdFeB molds and its preparation method. By sequentially setting an AlMn alloy transition layer, an AlCoCrFeNi high-entropy alloy support layer, a self-lubricating functional layer containing MoS2 and ZrN / GO composite nanoparticles, and an Al2O3 surface layer on the surface of the mold substrate, the invention solves the technical problem that existing mold surface treatment technologies cannot simultaneously achieve strong bonding between the coating and the substrate, high wear resistance, and long-lasting self-lubricating function, leading to wear, powder adhesion, and coating peeling of the mold under high pressure cyclic stress.

[0006] In a first aspect, embodiments of the present invention provide a long-life self-lubricating coating for sintering NdFeB molds, comprising, from the inside out: A transition layer is disposed on the surface of the mold substrate, the transition layer comprising Al and Mn; A support layer is disposed on the surface of the transition layer, the support layer comprising an AlCoCrFeNi alloy; A self-lubricating functional layer is disposed on the surface of the support layer, and the self-lubricating functional layer includes AlCoCrFeNi alloy, MoS2 and ZrN / GO composite nanoparticles; A surface layer is disposed on the surface of the self-lubricating functional layer, the surface layer comprising Al2O3.

[0007] As an optional implementation, the transition layer contains 85-92 wt% Al and 8-15 wt% Mn.

[0008] In this embodiment of the invention, the solid solubility of Mn in the Al matrix varies greatly with temperature. Through the HIPIMS process, Mn atoms can enter the Al lattice to form a supersaturated solid solution, which can effectively hinder dislocation movement and improve the shear strength of the transition layer itself. If the Al content is higher than 92%, the Mn content will be lower than 8%, resulting in a decrease in the hardness of the transition layer and an excess of toughness, leading to a significant decrease in bonding strength and easy overall peeling of the coating. When the Al content is lower than 85%, the Mn content will be higher than 15%, which will lead to the precipitation of brittle metal compounds such as Al6Mn. This will cause microcracks to form inside the transition layer when subjected to impact, which will propagate upwards and cause the coating to break apart.

[0009] As an optional implementation, in the support layer, the contents of Al, Co, Cr, Fe and Ni in the AlCoCrFeNi alloy are all 15~25 at.

[0010] In this embodiment of the invention, this ratio range maximizes configuration entropy, suppresses the precipitation of intermetallic compounds, and promotes the formation of a simple solid solution structure. Near-equal atomic ratio mixing leads to severe lattice distortion, resulting in a strong solid solution strengthening effect and a hardness greater than 700 HV. If Al is below 15%, the hardness and yield strength decrease; if Cr is below 15%, the chemical stability and oxidation resistance of the coating are significantly reduced; and if Co and Ni are below 15%, the alloy becomes brittle, making the support layer prone to hot cracking or brittle spalling under cyclic stress.

[0011] As an optional implementation, the contents of AlCoCrFeNi alloy, MoS2 and ZrN / GO composite nanoparticles in the self-lubricating functional layer are 70~85 vol%, 10~20 vol%, and 5~10 vol%, respectively.

[0012] As an optional implementation, the thickness of the transition layer is 2~5 μm; the thickness of the support layer is 20~40 μm; the thickness of the self-lubricating functional layer is 10~20 μm; and the thickness of the surface layer is 0.5~2 μm.

[0013] Secondly, embodiments of the present invention provide a method for preparing a long-life self-lubricating coating for sintering NdFeB molds, comprising the following steps: S1: Perform pretreatment on the mold substrate to make the surface of the mold substrate clean and the roughness Ra ≤ 0.1 μm; S2: An AlMn alloy layer is deposited on the surface of the mold substrate using high-energy pulsed magnetron sputtering technology. The AlMn alloy layer forms a metallurgical bond with the mold substrate. The thickness of the AlMn alloy layer is 2~5 μm. S3: An AlCoCrFeNi alloy layer is prepared on the surface of the AlMn alloy layer using laser cladding technology, and the thickness of the AlCoCrFeNi alloy layer is 20~40 μm; S4: A functional layer is co-deposited on the surface of the AlCoCrFeNi alloy layer using laser cladding technology. The functional layer uses AlCoCrFeNi alloy as the matrix phase and contains MoS2 lubricating phase and ZrN / GO composite nano-reinforcing phase. The thickness of the functional layer is 10~20 μm. S5: An Al2O3 layer is deposited on the surface of the functional layer using plasma-assisted chemical vapor deposition technology, and the thickness of the Al2O3 layer is 0.5~2 μm.

[0014] As an optional implementation, the pretreatment includes sequentially chamfering, polishing, and ultrasonic cleaning of the mold substrate with acetone and anhydrous ethanol.

[0015] As an optional implementation, the process parameters of the high-energy pulsed magnetron sputtering technology described in S2 include: a background vacuum of <5×10⁻⁶. -3 Pa, deposition temperature 350~450 ℃, pulse frequency 50~150 Hz, duty cycle 5%~10%.

[0016] As an optional implementation, the process parameters for preparing the AlCoCrFeNi alloy layer using the laser cladding technology described in S3 include: laser power of 1.8~2.5 kW (slightly lower than the support layer to prevent excessive burning of MoS2), scanning speed of 8~12 mm / s, spot diameter of 2.5~3.0 mm, powder feeding rate of 15~25 g / min, and overlap rate of 40~50%.

[0017] As an optional implementation, the process parameters for co-depositing the functional layer using laser cladding technology in S4 include: laser power 1.5~2.0 kW, scanning speed 8~12 mm / s, powder feeding rate 10~20 g / min, and overlap rate 30~50%; after co-deposition, stress relief is achieved by holding the layer at 450~500 ℃ for 1.5~2.5 h.

[0018] Compared with the prior art, the embodiments of the present invention have the following advantages and beneficial effects: 1. In this embodiment of the invention, a functionally graded composite structure is formed on the surface of the mold substrate by sequentially setting an AlMn alloy transition layer, an AlCoCrFeNi high-entropy alloy support layer, a self-lubricating functional layer containing MoS2 and ZrN / GO composite nanoparticles, and an Al2O3 surface layer from the inside out. This structure enables modulus matching and chemical synergy among the layers. The AlMn transition layer utilizes the high ionization rate generated by HIPIMS technology to achieve metallurgical bonding with the steel substrate and alleviate thermal stress mismatch. The AlCoCrFeNi high-entropy alloy support layer forms a stable single phase through the high-entropy effect and hysteresis diffusion effect. The solid solution bears high pressure and prevents collapse. In the self-lubricating functional layer, MoS2 forms a continuous lubricating film under frictional shearing to achieve continuous friction reduction. ZrN / GO composite nanoparticles achieve synergistic toughening and friction reduction through the hard reinforcement of ZrN and the crack bridging of GO. The Al2O3 surface layer acts as a dense physical barrier to block external oxidation and erosion. Thus, the coating has strong toughness, high wear resistance and long-lasting self-lubricating function at the same time. It significantly reduces the friction coefficient of the mold surface, greatly improves wear resistance and fatigue resistance, effectively prevents the coating from peeling off under long-term cyclic stress, and extends the service life of the mold under harsh working conditions.

[0019] 2. This invention employs an organic combination of three preparation technologies: high-energy pulsed magnetron sputtering, laser cladding, and plasma-assisted chemical vapor deposition (PCVD) to construct a multi-layer gradient composite coating system on the surface of a mold substrate. HIPIMS technology utilizes the high ionization rate generated by high peak power density to deposit AlMn alloy particles onto the substrate surface at high energy, forming an extremely dense thin film structure without columnar crystals. This achieves a metallurgical-grade bond between the transition layer and the steel substrate, fundamentally solving the problem of insufficient adhesion in traditional coatings. Laser cladding technology uses a high-energy laser beam to melt and rapidly solidify AlCoCrFeNi high-entropy alloy powder and functional mixed powder containing MoS2 and ZrN / GO layer by layer. Utilizing the multi-principal high-entropy effect and hysteretic diffusion effect of the high-entropy alloy, a stable single-phase solid solution structure is formed, endowing the support layer with excellent strength, toughness, and load-bearing capacity. Simultaneously, the MoS2 lubricating phase and ZrN / GO reinforcing phase in the self-lubricating functional layer are uniformly embedded in the high-entropy alloy substrate. During friction, MoS2 undergoes interlayer deformation under shear force. The ZrN / GO composite nanoparticles, through the slippage and formation of a continuous lubricating film on the surface, achieve toughening and friction reduction through the hardening reinforcement of ZrN to resist abrasive wear and the lamellar slippage and crack bridging of GO. The three work synergistically to achieve the effect of "toughening and reducing friction". PACVD technology uses plasma to activate chemical reactions to achieve dense deposition of Al2O3 surface layer at a lower temperature, avoiding the damage of high temperature to the structure and properties of the underlying functional layer. This surface layer acts as a physical barrier to effectively block the oxidation and erosion of the internal lubricating phase by external water vapor and oxygen, while providing low friction characteristics in the initial contact stage. Through the synergy of the above processes and the modulus gradient design of the multilayer structure, the coating system achieves a smooth transition of hardness from the substrate to the surface, avoiding stress accumulation at a single interface. This allows the mold to simultaneously possess extremely high coating bonding strength, excellent wear resistance and fatigue resistance, long-lasting self-lubricating function, and good chemical stability. It significantly reduces product damage caused by sintered NdFeB powder sticking to the mold, ensures product dimensional stability, greatly extends the mold service life, and reduces production costs. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of the present invention and should not be considered as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort. In the drawings: Figure 1 This is a schematic diagram of the coating structure provided in an embodiment of the present invention; Figure 2 This is a surface structure diagram of the coating prepared in Example 1 of the present invention after 200,000 adaptation cycles.

[0021] Figure 3This is a surface structure diagram of the nitrided mold in Comparative Example 1 after 40,000 cycles of adaptation. Figure 4 The image shows the surface structure of the mold in Comparative Example 2 after 40,000 cycles of adaptation following ordinary polishing. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0023] Therefore, the detailed description of the embodiments of the present invention provided below is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0024] This invention provides a method for preparing a long-life self-lubricating coating for sintering NdFeB molds, comprising the following: Step 1: Matrix Pretreatment For Cr 12 The MoV mold steel substrate undergoes precision machining, with chamfering applied to the edges of the mold cavity at angles of 30°–45° and widths of 0.5–1.0 mm. The substrate surface is then mechanically polished to achieve a surface roughness Ra ≤ 0.1 μm. After polishing, the substrate is ultrasonically cleaned sequentially with acetone and anhydrous ethanol at a frequency of 40 kHz. The acetone cleaning time is 10–15 min, and the anhydrous ethanol cleaning time is 10–15 min, respectively, to remove surface oil and impurities, ensuring a clean surface free of stress concentration points. This substrate pretreatment aims to eliminate microscopic defects and stress concentration sources on the mold substrate surface, providing a clean and smooth physical basis for the subsequent coating adhesion and preventing poor coating bonding due to surface contamination or excessive roughness.

[0025] Step 2: Preheating The pretreated mold substrate is placed in a heating furnace for preheating at a temperature of 300–500 °C for 20–60 min. This reduces thermal stress between the coating and the substrate during subsequent deposition, improving coating adhesion. Preheating alleviates residual interfacial stress caused by thermal expansion mismatch by reducing the temperature gradient between the mold substrate and the deposition material during coating preparation, preventing cracking or peeling of the coating during cooling.

[0026] Step 3: Deposition of the transition layer An AlMn alloy transition layer was deposited on the preheated mold substrate surface using high-energy pulsed magnetron sputtering (HIPIMS) technology. Specific process parameters are as follows: base vacuum degree <5×10⁻⁶. -3 The deposition temperature was 350–450 °C, the pulse frequency was 50–150 Hz, the duty cycle was 5%–10%, the sputtering target was an AlMn alloy target with an Al content of 85–92 wt% and a Mn content of 8–15 wt%, the target-substrate distance was 60–100 mm, the working gas was high-purity Ar gas with a flow rate of 30–50 sccm, the deposition bias voltage was -100 to -300 V, and the deposition time was 30–90 min. The prepared AlMn transition layer had a thickness of 2–5 μm, which formed a metallurgical bond with the mold substrate with a bond strength >60 MPa, while effectively preventing external corrosive media from penetrating the substrate. HIPIMS technology utilizes high peak power density to generate high ionization rate, enabling sputtered particles to reach the substrate surface with high energy, forming an extremely dense, columnar crystal-free thin film structure; Mn dissolves into the Al matrix to form a supersaturated solid solution, producing a lattice distortion strengthening effect. This transition layer acts as a "chemical bridge," forming a metallurgical bond with the steel substrate, mitigating the thermal stress mismatch between the coating system and the substrate through modulus gradient design, and providing a solid adhesion foundation for the upper support layer.

[0027] Step 4: Prepare the support layer A high-entropy AlCoCrFeNi alloy support layer was prepared on the surface of the transition layer using laser cladding technology. Specific process parameters are as follows: Equivalent or near-equivalent atomic ratio AlCoCrFeNi high-entropy alloy spherical powders prepared by gas atomization were used as the cladding material. The atomic percentage of each element was 15–25 at%, the powder particle size distribution was 15–53 μm (D50 approximately 35 μm), and the purity was ≥99.5%. Before cladding, the powder was placed in a vacuum drying oven and dried at 120 ± 5 ℃ for 4 h to remove physically adsorbed water. The cladding equipment uses a continuous fiber laser (wavelength 1064 nm) with a spot diameter of 2.5–3.0 mm, a laser power of 1.8–2.5 kW, and a scanning speed of 8–12 mm / s. A synchronous powder feeding method is employed, with a powder feeding rate of 15–25 g / min. High-purity Ar gas is used as the carrier gas at a flow rate of 8–10 L / min, and high-purity Ar gas is used as the protective gas at a flow rate of 15–20 L / min. A serpentine scanning path is employed, with an overlap rate of 40%–50% between adjacent cladding channels. After cladding, the mold is slowly cooled to room temperature either in the furnace or in air to release residual thermal stress. The prepared high-entropy alloy support layer has a thickness of 20–40 μm and a hardness ≥ 700 HV. High-entropy alloys utilize the multi-principal-element high-entropy effect, lattice distortion effect, and hysteresis diffusion effect to form a stable single-phase solid solution structure during the rapid solidification process of laser cladding, endowing the layer with extremely high hardness, excellent toughness, and high-temperature stability. As the "mechanical foundation" of the self-lubricating functional layer above, this layer bears the main load during high-pressure pressing, preventing the functional layer from undergoing plastic deformation or crushing due to direct high pressure, and avoiding coating collapse failure caused by the "eggshell effect".

[0028] Step 5: Prepare the self-lubricating functional layer A self-lubricating functional layer was co-deposited on the surface of a support layer using laser cladding technology. This functional layer consists of a matrix phase, a lubricating phase, and a reinforcing phase. The matrix phase is AlCoCrFeNi high-entropy alloy powder, with a volume fraction of 70–85%; the lubricating phase is MoS2 powder, with a volume fraction of 10–20%; and the reinforcing phase is ZrN / GO composite nanoparticles, with a particle size of 10–100 nm and a volume fraction of 5–10%. The high-entropy alloy matrix phase provides structural strength as a framework. During friction, MoS2 undergoes interlaminar slip under shear force, forming a continuous lubricating film on the coating surface, achieving sustained friction reduction. In the ZrN / GO composite nanoparticles, ZrN provides hard reinforcement to resist abrasive wear, while GO utilizes its large specific surface area to anchor the ZrN nanoparticles and achieves toughening and friction reduction through lamellar slip and crack bridging mechanisms. The three phases work synergistically to achieve a comprehensive effect of both toughening and friction reduction. Specifically: (1) Powder premix AlCoCrFeNi high-entropy alloy powder, MoS2 powder, and pre-prepared ZrN / GO composite nanoparticles were precisely weighed according to the above volume ratio and then placed in a planetary high-energy ball mill for ball milling and mixing. The ball mill jar and grinding balls were made of cemented carbide or zirconium oxide, with a ball-to-powder ratio of 5:1, a rotation speed of 200-300 rpm, and a milling time of 2-4 h. During the ball milling process, 1-2% of the total powder mass was added as a grinding aid to prevent powder agglomeration. The ball milling was carried out under Ar gas protection using an intermittent method, with a 10-minute pause after every 30 minutes of milling to prevent overheating and oxidation of MoS2. After mixing, the mixed powder was placed in a vacuum drying oven and dried at 80 °C for 2 h. High-energy ball milling enables the three powders to be uniformly dispersed and initially alloyed through mechanochemical action. The addition of stearic acid reduces the surface energy of the powder to prevent agglomeration. Ar gas protection and intermittent ball milling prevent MoS2 from oxidizing and failing due to frictional heat generation, ensuring that each functional phase is uniformly distributed in the matrix during the subsequent cladding process.

[0029] (2) Laser co-melting cladding The mold substrate with the support layer is preheated to 200-300 °C to reduce thermal stress. Premixed powder is co-deposited onto the support layer surface using laser cladding technology. Specific process parameters are as follows: laser power 1.5-2.0 kW, scanning speed 8-12 mm / s, spot diameter 2.5-3.0 mm, using either synchronous powder feeding or a pre-coating method. If using the pre-coating method, 5-10 wt% polyvinyl alcohol (PVA) is added as a binder to form a paste, with a coating thickness of approximately 0.5 mm. The paste is then allowed to dry at 100 °C for 1 h before laser scanning. The powder feeding rate is 10-20 g / min, the carrier gas is high-purity Ar with a flow rate of 8-10 L / min, the protective gas is high-purity Ar with a flow rate of 15-20 L / min, and a serpentine scanning path is used with an overlap rate of 30-50%. After cladding, the mold is immediately placed in a heat treatment furnace and held at 450-500 °C for 1.5-2.5 h to relieve stress, followed by slow cooling to room temperature in the furnace. This stress relief step is crucial to prevent microcracks in the self-lubricating functional layer. The prepared self-lubricating functional layer has a thickness of 10-20 μm. The laser power is reduced compared to the support layer preparation to avoid excessive energy causing decomposition and burn-off of the heat-sensitive MoS2 lubricating phase. The stress relief heat treatment after cladding promotes dislocation rearrangement and residual stress release, preventing microcracks in the functional layer due to cyclic stress during subsequent service, and ensuring the stable existence and synergistic effect of the lubricating and reinforcing phases in the matrix.

[0030] Step Six: Deposition Surface Layer A dense Al₂O₃ surface layer was deposited on the surface of the self-lubricating functional layer using plasma-assisted chemical vapor deposition (PACVD). Specific process parameters were as follows: deposition temperature 150–300 °C, deposition pressure 50–200 Pa, bias voltage -50 to -200 V, RF power 300–800 W, reactant gas a mixture of trimethylaluminum (TMA) and O₂, TMA carrier gas high-purity Ar, TMA flow rate 5–15 sccm, O₂ flow rate 20–50 sccm, Ar flow rate 50–100 sccm, and deposition time 30–120 min. The prepared Al₂O₃ surface layer had a thickness of 0.5–2 μm. This extremely dense layer effectively protected the internal self-lubricating functional layer from corrosion by moisture and oxygen in the environment. PACVD technology utilizes plasma to activate chemical reactions, achieving dense deposition of Al2O3 at lower temperatures, thus avoiding damage to the structure and properties of the underlying functional layer due to high temperatures. The Al2O3 surface layer acts as a physical barrier, and its chemical inertness and density effectively prevent external moisture and oxygen from oxidizing and damaging the internal MoS2 lubricating phase. At the same time, it provides low friction characteristics by utilizing its inherent low surface energy during the initial contact stage, forming a temporal synergy of "early protection - mid-term continuous lubrication" with the underlying functional layer.

[0031] It should be noted that the preparation of ZrN / GO composite nanoparticles includes the following: ZrN / GO composite nanoparticles were prepared using an electrostatic self-assembly method, with the following steps: First, graphene oxide (GO) was dispersed in anhydrous ethanol at a concentration of 0.5–2.0 mg / mL, and sonicated for 1 h to form a uniform suspension. Then, ZrN nanoparticles modified with a silane coupling agent were added to the suspension at a ZrN to GO mass ratio of (2–5):1. Through charge attraction, the ZrN nanoparticles were uniformly adsorbed onto the GO sheet surface. Subsequently, the mixed suspension was spray-dried at an inlet air temperature of 150–200 °C and an outlet air temperature of 80–100 °C. Finally, the dried powder was vacuum annealed at 400 °C with a vacuum degree <10. -2 The annealing time is 1-2 h to form stable ZrN / GO composite nanoparticles. Silane coupling agent modification imparts surface charge to the ZrN nanoparticles, enabling them to self-assemble with the oppositely charged GO sheets through electrostatic attraction. The large specific surface area and abundant oxygen-containing functional groups of GO provide numerous anchoring sites for ZrN, ensuring uniform dispersion of the nanoparticles without agglomeration. Spray drying rapidly solidifies the composite structure, while vacuum annealing removes residual organic groups and enhances the interfacial bonding between ZrN and GO, ultimately forming stable composite nanoparticles with both hardening and lubrication / toughening functions.

[0032] For example, the specific preparation process of ZrN nanopowder modified with silane coupling agent is as follows: Select a silane coupling agent, dissolve it in a 95% ethanol solution to prepare a hydrolysis solution with a concentration of 1~3wt%, adjust the pH value to 4~5, and stir for 30 min for hydrolysis. Then, ZrN nanoparticles were added to the hydrolysate, with the mass ratio of powder to coupling agent being 100:(1~3). The reaction was carried out under constant temperature magnetic stirring at 60~80℃ for 2~4 hours to fully coat the ZrN particles with silane molecules. After the reaction was completed, the powder was washed three times with anhydrous ethanol by centrifugation to remove excess coupling agent, and then dried in a vacuum drying oven at 60°C for 12 h to obtain ZrN nanoparticles modified with silane coupling agent.

[0033] The modified ZrN surface was successfully grafted with amino groups, making the powder positively charged in ethanol, thus allowing it to tightly bond with the negatively charged GO sheets through electrostatic self-assembly.

[0034] In this embodiment of the invention, preferably, the degree of modification is based on the formation of a monomolecular or multimolecular coating layer on the ZrN surface by the silane coupling agent, and the mass loss of surface organic functional groups is controlled between 2 and 5 wt% by thermogravimetric analysis.

[0035] By following the above steps, refer to Figure 1 As shown, in Cr 12 A transition layer, a support layer, a self-lubricating functional layer, and a surface layer are sequentially prepared on the surface of the MoV mold steel substrate to form a multifunctional gradient composite coating. This coating achieves a smooth transition in hardness from the substrate to the surface through modulus matching and chemical synergy between the layers, avoiding stress accumulation at a single interface. This simultaneously solves the problems of wear, powder adhesion, and coating peeling under high-pressure cyclic stress, significantly improving the service life of the mold.

[0036] To better demonstrate the effects of the present invention, the effects of specific embodiments will be tested and verified below.

[0037] Example 1: This embodiment of the invention provides a method for preparing a long-life self-lubricating coating for sintering NdFeB molds, comprising the following: Step 1: Matrix Pretreatment For Cr 12 The MoV mold steel substrate is precision machined, and the edges of the mold cavity are chamfered at a 40° angle with a width of 0.8 mm. The substrate surface is then mechanically polished to achieve a surface roughness Ra ≤ 0.1 μm. After polishing, the substrate is ultrasonically cleaned sequentially with acetone and anhydrous ethanol at a frequency of 40 kHz. The acetone cleaning time is 12 min, and the anhydrous ethanol cleaning time is 12 min, to remove surface oil and impurities, ensuring a clean substrate surface free of stress concentration points.

[0038] Step 2: Preheating The pretreated mold substrate is placed in a heating furnace for preheating at 400°C for 40 minutes to reduce thermal stress between the coating and the substrate during subsequent deposition and improve coating adhesion.

[0039] Step 3: Deposition of the transition layer An AlMn alloy transition layer was deposited on the preheated mold substrate surface using high-energy pulsed magnetron sputtering (HIPIMS) technology. Specific process parameters are as follows: base vacuum degree <5×10⁻⁶. -3 The deposition temperature was 400℃, the pulse frequency was 100Hz, the duty cycle was 8%, the sputtering target was an AlMn alloy target with an Al content of 90 wt% and a Mn content of 10 wt%, the target-substrate distance was 80 mm, the working gas was high-purity Ar gas with a flow rate of 40 sccm, the deposition bias voltage was -100 to -300 V, and the deposition time was 60 min. The prepared AlMn transition layer had a thickness of 3 μm. This layer formed a metallurgical bond with the mold substrate, with a bond strength >60 MPa, and effectively prevented external corrosive media from penetrating the substrate.

[0040] Step 4: Prepare the support layer A high-entropy AlCoCrFeNi alloy support layer was prepared on the transition layer surface using laser cladding technology. Specific process parameters are as follows: Equivalent or near-equiatomic AlCoCrFeNi high-entropy alloy spherical powder was prepared by gas atomization as the cladding material, with each element having an atomic percentage of 20 at%, a particle size distribution of 15–53 μm (D50 approximately 35 μm), and a purity ≥99.5%. Before cladding, the powder was placed in a vacuum drying oven and dried at 120 °C for 4 h to remove physically adsorbed water. The cladding equipment used a continuous fiber laser (wavelength 1064 nm) with a spot diameter of 2.8 mm, a laser power of 2 kW, a scanning speed of 10 mm / s, synchronous powder feeding at a rate of 20 g / min, high-purity Ar as the carrier gas at a flow rate of 9 L / min, and high-purity Ar as the protective gas at a flow rate of 18 L / min. A serpentine scanning path was used, with an overlap rate of 45% between adjacent cladding channels. After cladding, the mold is slowly cooled to room temperature in the furnace or in air to release residual thermal stress. The prepared high-entropy alloy support layer has a thickness of 30 μm and a hardness ≥ 700 HV.

[0041] Step 5: Prepare the self-lubricating functional layer A self-lubricating functional layer was co-deposited on the surface of the support layer using laser cladding technology. This functional layer consists of a matrix phase, a lubricating phase, and a reinforcing phase. The matrix phase is AlCoCrFeNi high-entropy alloy powder, with a volume fraction of 80%; the lubricating phase is MoS2 powder, with a volume fraction of 12%; and the reinforcing phase is ZrN / GO composite nanoparticles with a particle size of 10–100 nm and a volume fraction of 8%. Specifically: (1) Powder premix AlCoCrFeNi high-entropy alloy powder, MoS2 powder, and pre-prepared ZrN / GO composite nanoparticles were precisely weighed according to the above volume ratio and then placed in a planetary high-energy ball mill for ball milling and mixing. The ball mill jar and grinding balls were made of cemented carbide or zirconium oxide, with a ball-to-powder ratio of 5:1, a rotation speed of 250 rpm, and a milling time of 3 h. During the ball milling process, stearic acid, accounting for 1.5% of the total powder mass, was added as a grinding aid to prevent powder agglomeration. The ball milling was carried out under Ar gas protection using an intermittent method, with a 10-minute pause after every 30 minutes of milling to prevent overheating and oxidation of MoS2. After mixing, the mixed powder was placed in a vacuum drying oven and dried at 80 °C for 2 h.

[0042] (2) Laser co-melting cladding The mold substrate with the support layer was preheated to 250℃ to reduce thermal stress. Premixed powder was co-deposited onto the support layer surface using laser cladding technology. Specific process parameters were as follows: laser power 1.8kW, scanning speed 10 mm / s, spot diameter 2.8 mm, using either synchronous powder feeding or a pre-coating method. If the pre-coating method was used, 18wt% polyvinyl alcohol (PVA) was added as a binder to mix the powder into a paste, with a coating thickness of approximately 0.5 mm. The paste was then allowed to dry at 100℃ for 1 hour before laser scanning. The powder feeding rate was 15 g / min, the carrier gas was high-purity Ar with a flow rate of 9 L / min, the protective gas was high-purity Ar with a flow rate of 18 L / min, and a serpentine scanning path was used with an overlap rate of 40%. After cladding, the mold was immediately placed in a heat treatment furnace and held at 480℃ for 2 hours to relieve stress, then slowly cooled to room temperature with the furnace. This stress relief step is crucial for preventing microcracks from forming in the self-lubricating functional layer. The prepared self-lubricating functional layer has a thickness of 15 μm.

[0043] The ZrN / GO composite nanoparticles were prepared using an electrostatic self-assembly method, with the following steps: First, graphene oxide (GO) was dispersed in anhydrous ethanol at a concentration of 1 mg / mL, and ultrasonically treated for 1 h to form a uniform suspension. Then, ZrN nanoparticles modified with a silane coupling agent were added to the suspension at a ZrN to GO mass ratio of 3:1. Through charge attraction, the ZrN nanoparticles were uniformly adsorbed onto the GO sheet surface. Subsequently, the mixed suspension was spray-dried at an inlet air temperature of 180 ℃ and an outlet air temperature of 90 ℃. Finally, the dried powder was vacuum annealed at 400 ℃ with a vacuum degree <10. -2 Pa, annealing time of 1.5 h, to form stable ZrN / GO composite nanoparticles.

[0044] Step Six: Deposition Surface Layer A dense Al₂O₃ surface layer was deposited on the surface of the self-lubricating functional layer using plasma-assisted chemical vapor deposition (PACVD). Specific process parameters were as follows: deposition temperature 200 °C, deposition pressure 100 Pa, bias voltage -50 to -200 V, RF power 500 W, reactant gas mixture of trimethylaluminum (TMA) and O₂, TMA carrier gas high-purity Ar, TMA flow rate 10 sccm, O₂ flow rate 25 sccm, Ar flow rate 80 sccm, and deposition time 80 min. The prepared Al₂O₃ surface layer had a thickness of 1 μm. This extremely dense layer effectively protected the internal self-lubricating functional layer from corrosion by moisture and oxygen in the environment.

[0045] Through the above steps, in Cr 12 A transition layer, a support layer, a self-lubricating functional layer, and a surface layer are sequentially prepared on the surface of the MoV mold steel substrate to form a multifunctional gradient composite coating.

[0046] Example 2: This embodiment of the invention provides a method for preparing a long-life self-lubricating coating for sintering NdFeB molds, comprising the following: Step 1: Matrix Pretreatment For Cr 12 The MoV mold steel substrate is precision machined, and the edges of the mold cavity are chamfered at a 30° angle with a width of 0.5 mm. The substrate surface is then mechanically polished to achieve a surface roughness Ra ≤ 0.1 μm. After polishing, the substrate is ultrasonically cleaned sequentially with acetone and anhydrous ethanol at a frequency of 40 kHz. The acetone cleaning time is 10 min, and the anhydrous ethanol cleaning time is 10 min, to remove surface oil and impurities, ensuring a clean substrate surface free of stress concentration points.

[0047] Step 2: Preheating The pretreated mold substrate was placed in a heating furnace for preheating at 300 ℃ for 60 min to reduce thermal stress between the coating and the substrate during subsequent deposition and improve coating adhesion.

[0048] Step 3: Deposition of the transition layer An AlMn alloy transition layer was deposited on the preheated mold substrate surface using high-energy pulsed magnetron sputtering (HIPIMS) technology. Specific process parameters are as follows: base vacuum degree <5×10⁻⁶. -3 The deposition temperature was 350℃, the pulse frequency was 50Hz, the duty cycle was 5%, the sputtering target was an AlMn alloy target with an Al content of 85wt% and a Mn content of 15wt%, the target-substrate distance was 60mm, the working gas was high-purity Ar gas with a flow rate of 30 sccm, the deposition bias voltage was -100 to -300 V, and the deposition time was 30min. The prepared AlMn transition layer had a thickness of 2 μm, which formed a metallurgical bond with the mold substrate with a bond strength >60 MPa, while effectively preventing external corrosive media from penetrating the substrate.

[0049] Step 4: Prepare the support layer A high-entropy AlCoCrFeNi alloy support layer was prepared on the transition layer surface using laser cladding technology. Specific process parameters are as follows: Equivalent or near-equivalent atomic ratio AlCoCrFeNi high-entropy alloy spherical powder was prepared by gas atomization as the cladding material. The atomic percentages of each element were 15 at%, 15 at%, 25 at%, 25 at%, and 20 at%, respectively. The powder particle size distribution was 15–53 μm (D50 approximately 35 μm), and the purity was ≥ 99.5%. Before cladding, the powder was placed in a vacuum drying oven and dried at 115 °C for 4 h to remove physically adsorbed water. The cladding equipment employed a continuous fiber laser (wavelength 1064 nm) with a spot diameter of 2.5 mm, a laser power of 1.8 kW, and a scanning speed of 8 mm / s. A synchronous powder feeding method was used at a powder feeding rate of 15 g / min. High-purity Ar gas was used as the carrier gas at a flow rate of 8 L / min, and high-purity Ar gas was used as the protective gas at a flow rate of 15 L / min. A serpentine scanning path was employed, with an overlap rate of 40% between adjacent cladding channels. After cladding, the mold was slowly cooled to room temperature either in the furnace or in air to release residual thermal stress. The prepared high-entropy alloy support layer had a thickness of 20 μm and a hardness ≥ 700 HV.

[0050] Step 5: Prepare the self-lubricating functional layer A self-lubricating functional layer was co-deposited on the surface of the support layer using laser cladding technology. This functional layer consists of a matrix phase, a lubricating phase, and a reinforcing phase. The matrix phase is AlCoCrFeNi high-entropy alloy powder, with a volume fraction of 70%; the lubricating phase is MoS2 powder, with a volume fraction of 20%; and the reinforcing phase is ZrN / GO composite nanoparticles with a particle size of 10–100 nm, with a volume fraction of 10%. Specifically: (1) Powder premix AlCoCrFeNi high-entropy alloy powder, MoS2 powder, and pre-prepared ZrN / GO composite nanoparticles were precisely weighed according to the above volume ratio and then placed in a planetary high-energy ball mill for ball milling and mixing. The ball mill jar and grinding balls were made of cemented carbide or zirconium oxide, with a ball-to-powder ratio of 5:1, a rotation speed of 250 rpm, and a milling time of 3 h. During the ball milling process, 1% stearic acid by weight of the total powder was added as a grinding aid to prevent powder agglomeration. The ball milling was carried out under Ar gas protection using an intermittent method, with a 10-minute pause after every 30 minutes of milling to prevent overheating and oxidation of MoS2. After mixing, the mixed powder was placed in a vacuum drying oven and dried at 80 °C for 2 h. (2) Laser co-melting cladding The mold substrate with the support layer was preheated to 200℃ to reduce thermal stress. Premixed powder was co-deposited onto the support layer surface using laser cladding technology. Specific process parameters were as follows: laser power 1.5 kW, scanning speed 8 mm / s, spot diameter 2.5 mm, using either synchronous powder feeding or a pre-coating method. If the pre-coating method was used, 5 wt% polyvinyl alcohol (PVA) was added as a binder to mix the powder into a paste, with a coating thickness of approximately 0.5 mm. The paste was then allowed to dry at 100℃ for 1 h before laser scanning. The powder feeding rate was 10 g / min, the carrier gas was high-purity Ar with a flow rate of 8 L / min, the protective gas was high-purity Ar with a flow rate of 15 L / min, and a serpentine scanning path was used with an overlap rate of 30%. After cladding, the mold was immediately placed in a heat treatment furnace and held at 450℃ for 2 h to relieve stress, then slowly cooled to room temperature with the furnace. This stress relief step is crucial for preventing microcracks from forming in the self-lubricating functional layer. The prepared self-lubricating functional layer has a thickness of 10 μm.

[0051] It should be noted that the ZrN / GO composite nanoparticles were prepared using an electrostatic self-assembly method. The specific steps are as follows: First, graphene oxide (GO) was dispersed in anhydrous ethanol at a concentration of 0.5 mg / mL, and ultrasonically treated for 1 h to form a uniform suspension. Then, ZrN nanoparticles modified with a silane coupling agent were added to the suspension at a ZrN to GO mass ratio of 2:1. Through charge attraction, the ZrN nanoparticles were uniformly adsorbed onto the GO sheet surface. Subsequently, the mixed suspension was spray-dried at an inlet air temperature of 150 °C and an outlet air temperature of 80 °C. Finally, the dried powder was vacuum annealed at 400 °C with a vacuum degree <10. -2 Pa, annealing time of 1 h, to form stable ZrN / GO composite nanoparticles.

[0052] Step Six: Deposition Surface Layer A dense Al₂O₃ surface layer was deposited on the surface of the self-lubricating functional layer using plasma-assisted chemical vapor deposition (PACVD). Specific process parameters were as follows: deposition temperature 200℃, deposition pressure 150 Pa, bias voltage -50 to -200 V, RF power 300 W, reactant gas mixture of trimethylaluminum (TMA) and O₂, TMA carrier gas high-purity Ar, TMA flow rate 5 sccm, O₂ flow rate 20 sccm, Ar flow rate 50 sccm, and deposition time 30 min. The prepared Al₂O₃ surface layer had a thickness of 0.5 μm. This extremely dense layer effectively protected the internal self-lubricating functional layer from corrosion by moisture and oxygen in the environment.

[0053] Through the above steps, in Cr 12 A transition layer, a support layer, a self-lubricating functional layer, and a surface layer are sequentially prepared on the surface of the MoV mold steel substrate to form a multifunctional gradient composite coating.

[0054] Example 3: This embodiment of the invention provides a method for preparing a long-life self-lubricating coating for sintering NdFeB molds, comprising the following: Step 1: Matrix Pretreatment For Cr 12 The MoV mold steel substrate is precision machined, and the edges of the mold cavity are chamfered at a 45° angle with a width of 1.0 mm. The substrate surface is then mechanically polished to achieve a surface roughness Ra ≤ 0.1 μm. After polishing, the substrate is ultrasonically cleaned sequentially with acetone and anhydrous ethanol at a frequency of 40 kHz. The acetone cleaning time is 15 min, and the anhydrous ethanol cleaning time is 15 min, to remove surface oil and impurities, ensuring a clean substrate surface free of stress concentration points.

[0055] Step 2: Preheating The pretreated mold substrate is placed in a heating furnace for preheating at 500 ℃ for 20 min to reduce thermal stress between the coating and the substrate during subsequent deposition and improve coating adhesion.

[0056] Step 3: Deposition of the transition layer An AlMn alloy transition layer was deposited on the preheated mold substrate surface using high-energy pulsed magnetron sputtering (HIPIMS) technology. Specific process parameters are as follows: base vacuum degree <5×10⁻⁶. -3 The deposition temperature was 450 ℃, the pulse frequency was 150 Hz, the duty cycle was 10%, the sputtering target was an AlMn alloy target with an Al content of 92 wt% and a Mn content of 8 wt%, the target-substrate distance was 100 mm, the working gas was high-purity Ar gas with a flow rate of 50 sccm, the deposition bias voltage was -100 to -300 V, and the deposition time was 90 min. The prepared AlMn transition layer had a thickness of 5 μm. This layer formed a metallurgical bond with the mold substrate, with a bond strength >60 MPa, and effectively prevented external corrosive media from penetrating the substrate.

[0057] Step 4: Prepare the support layer A high-entropy AlCoCrFeNi alloy support layer was prepared on the transition layer surface using laser cladding technology. Specific process parameters are as follows: Equivalent or near-equivalent atomic ratio AlCoCrFeNi high-entropy alloy spherical powders prepared by gas atomization were used as the cladding material. The atomic percentages of each element were 25 at%, 20 at%, 25 at%, 15 at%, and 15 at, respectively. The powder particle size distribution was 15–53 μm (D50 approximately 35 μm), and the purity was ≥ 99.5%. Before cladding, the powder was placed in a vacuum drying oven and dried at 125°C for 4 hours to remove physically adsorbed water. The cladding equipment employed a continuous fiber laser (wavelength 1064 nm) with a spot diameter of 3.0 mm, a laser power of 2.5 kW, and a scanning speed of 12 mm / s. A synchronous powder feeding method was used at a powder feeding rate of 25 g / min. High-purity Ar gas was used as the carrier gas at a flow rate of 10 L / min, and high-purity Ar gas was used as the protective gas at a flow rate of 20 L / min. A serpentine scanning path was employed, with a 50% overlap between adjacent cladding channels. After cladding, the mold was slowly cooled to room temperature either in the furnace or in air to release residual thermal stress. The prepared high-entropy alloy support layer had a thickness of 40 μm and a hardness ≥ 700 HV.

[0058] Step 5: Prepare the self-lubricating functional layer A self-lubricating functional layer was co-deposited on the surface of the support layer using laser cladding technology. This functional layer consists of a matrix phase, a lubricating phase, and a reinforcing phase. The matrix phase is AlCoCrFeNi high-entropy alloy powder, with a volume fraction of 85%; the lubricating phase is MoS2 powder, with a volume fraction of 10%; and the reinforcing phase is ZrN / GO composite nanoparticles with a particle size of 10–100 nm, with a volume fraction of 5%. Specifically: (1) Powder premix AlCoCrFeNi high-entropy alloy powder, MoS2 powder, and pre-prepared ZrN / GO composite nanoparticles were precisely weighed according to the above volume ratio and then placed in a planetary high-energy ball mill for ball milling and mixing. The ball mill jar and grinding balls were made of cemented carbide or zirconium oxide, with a ball-to-powder ratio of 5:1, a rotation speed of 300 rpm, and a milling time of 4 h. During the ball milling process, 2% stearic acid (by weight of the total powder) was added as a grinding aid to prevent powder agglomeration. The ball milling was carried out under Ar gas protection using an intermittent method, with a 10-minute pause after every 30 minutes of milling to prevent overheating and oxidation of MoS2. After mixing, the mixed powder was placed in a vacuum drying oven and dried at 80 °C for 2 h.

[0059] (2) Laser co-melting cladding The mold substrate with the support layer was preheated to 300 °C to reduce thermal stress. Premixed powder was co-deposited onto the support layer surface using laser cladding technology. Specific process parameters were as follows: laser power 2.0 kW, scanning speed 12 mm / s, spot diameter 3.0 mm, using either synchronous powder feeding or a pre-coating method. If the pre-coating method was used, 10 wt% polyvinyl alcohol (PVA) was added as a binder to mix the powder into a paste, with a coating thickness of approximately 0.5 mm. The paste was then allowed to dry at 100 °C for 1 h before laser scanning. The powder feeding rate was 20 g / min, the carrier gas was high-purity Ar with a flow rate of 10 L / min, the protective gas was high-purity Ar with a flow rate of 20 L / min, and a serpentine scanning path was used with an overlap rate of 50%. After cladding, the mold was immediately placed in a heat treatment furnace and held at 500 °C for 2 h to relieve stress, followed by slow cooling to room temperature with the furnace. This stress relief step is crucial for preventing microcracks from forming in the self-lubricating functional layer. The prepared self-lubricating functional layer has a thickness of 20 μm.

[0060] It should be noted that the ZrN / GO composite nanoparticles were prepared using an electrostatic self-assembly method, with the following specific steps: First, graphene oxide (GO) was dispersed in anhydrous ethanol at a concentration of 2.0 mg / mL, and sonicated for 1 h to form a uniform suspension. Then, ZrN nanoparticles modified with a silane coupling agent were added to the suspension at a ZrN to GO mass ratio of 5:1. Through charge attraction, the ZrN nanoparticles were uniformly adsorbed onto the GO sheet surface. Subsequently, the mixed suspension was spray-dried at an inlet air temperature of 200 °C and an outlet air temperature of 100 °C. Finally, the dried powder was vacuum annealed at 400 °C with a vacuum degree <10. -2 Pa, annealing time of 2 h, to form stable ZrN / GO composite nanoparticles.

[0061] Step Six: Deposition Surface Layer A dense Al₂O₃ surface layer was deposited on the surface of the self-lubricating functional layer using plasma-assisted chemical vapor deposition (PACVD). Specific process parameters were as follows: deposition temperature 300 °C, deposition pressure 200 Pa, bias voltage -50 to -200 V, RF power 800 W, reactant gas mixture of trimethylaluminum (TMA) and O₂, TMA carrier gas high-purity Ar, TMA flow rate 15 sccm, O₂ flow rate 50 sccm, Ar flow rate 100 sccm, and deposition time 120 min. The prepared Al₂O₃ surface layer was 2 μm thick and extremely dense, effectively protecting the internal self-lubricating functional layer from corrosion by moisture and oxygen in the environment.

[0062] Through the above steps, in Cr 12 A transition layer, a support layer, a self-lubricating functional layer, and a surface layer are sequentially prepared on the surface of the MoV mold steel substrate to form a multifunctional gradient composite coating.

[0063] Example 4: This embodiment of the invention provides a method for preparing a long-life self-lubricating coating for sintering NdFeB molds. The difference from Example 1 is that the thickness of the transition layer is 1 μm, while the remaining steps remain unchanged. Example 5: This embodiment of the invention provides a method for preparing a long-life self-lubricating coating for sintering NdFeB molds. The difference from Example 1 is that the thickness of the support layer is 10 μm, while the other steps remain unchanged.

[0064] Example 6: This embodiment of the invention provides a method for preparing a long-life self-lubricating coating for sintering NdFeB molds. The difference from Example 1 is that the thickness of the self-lubricating functional layer is 5 μm, while the other steps remain unchanged.

[0065] Example 7: This embodiment of the invention provides a method for preparing a long-life self-lubricating coating for sintering NdFeB molds. The difference from Example 1 is that the thickness of the surface layer is 0.3 μm, while the other steps remain unchanged.

[0066] Example 8: This embodiment of the invention provides a method for preparing a long-life self-lubricating coating for sintering NdFeB molds. The difference from Example 1 is that the thickness of the transition layer is 2 μm, while the other steps remain unchanged.

[0067] Example 9: This embodiment of the invention provides a method for preparing a long-life self-lubricating coating for sintering NdFeB molds. The difference from Example 1 is that the thickness of the transition layer is 5 μm, while the other steps remain unchanged.

[0068] Example 10: This embodiment of the invention provides a method for preparing a long-life self-lubricating coating for sintering NdFeB molds. The difference from Example 1 is that the thickness of the support layer is 20 μm, while the other steps remain unchanged.

[0069] Example 11: This embodiment of the invention provides a method for preparing a long-life self-lubricating coating for sintering NdFeB molds. The difference from Example 1 is that the thickness of the support layer is 50 μm, while the other steps remain unchanged.

[0070] Example 12: This embodiment of the invention provides a method for preparing a long-life self-lubricating coating for sintering NdFeB molds. The difference from Example 1 is that the thickness of the self-lubricating functional layer is 10 μm, while the other steps remain unchanged.

[0071] Example 13: This embodiment of the invention provides a method for preparing a long-life self-lubricating coating for sintering NdFeB molds. The difference from Example 1 is that the thickness of the self-lubricating functional layer is 20 μm, while the other steps remain unchanged.

[0072] Example 14: This embodiment of the invention provides a method for preparing a long-life self-lubricating coating for sintering NdFeB molds. The difference from Example 1 is that the thickness of the surface layer is 0.5 μm, while the other steps remain unchanged.

[0073] Example 15: This embodiment of the invention provides a method for preparing a long-life self-lubricating coating for sintering NdFeB molds. The difference from Example 1 is that the thickness of the surface layer is 2 μm, while the other steps remain unchanged.

[0074] Example 16: This embodiment of the invention provides a method for preparing a high-life self-lubricating coating for sintered NdFeB molds. The difference from Example 1 is that the matrix phase in the self-lubricating functional layer is AlCoCrFeNi high-entropy alloy powder with a volume fraction of 92.5%; the lubricating phase is MoS2 powder with a volume fraction of 5%; and the reinforcing phase is ZrN / GO composite nanoparticles with a particle size of 10~100 nm and a volume fraction of 2.5%. The remaining steps remain unchanged.

[0075] Example 17: This embodiment of the invention provides a method for preparing a high-life self-lubricating coating for sintered NdFeB molds. The difference from Example 1 is that the matrix phase in the self-lubricating functional layer is AlCoCrFeNi high-entropy alloy powder with a volume fraction of 75%; the lubricating phase is MoS2 powder with a volume fraction of 15%; and the reinforcing phase is ZrN / GO composite nanoparticles with a particle size of 10~100 nm and a volume fraction of 10%. The remaining steps remain unchanged.

[0076] Comparative Example 1: For Cr 12 The MoV mold steel matrix undergoes nitriding treatment, specifically including the following: (1) Cr 12 Advanced tempering treatment of MoV mold steel matrix: The matrix needs to be quenched and reflowed at high temperature to obtain a uniform sorbite structure. The tempering temperature should be higher than the nitriding temperature to ensure that the matrix has sufficient supporting strength.

[0077] (2) Surface cleaning: The mold surface is finely polished and ultrasonically cleaned with anhydrous ethanol to remove oil and oxide film, so as to ensure uniform penetration of nitrogen atoms; The gas nitriding process parameters are as follows: Nitriding temperature: controlled at 520℃. This temperature ensures a high diffusion rate of nitrogen atoms while avoiding Cr... 12 MOV steel softens; Insulation time: 30 hours. This ensures the formation of a nitrided layer with a thickness of approximately 0.2 mm, meeting basic wear resistance requirements; Nitrogen decomposition rate: Stage 1: 22%, Stage 2: 35%; Cooling method: After nitriding, the mold should be cooled to below 150°C in the furnace before being removed from the furnace and air-cooled to reduce deformation caused by thermal stress.

[0078] Comparative Example 2: For Cr 12 The MoV mold steel substrate undergoes a standard polishing process, specifically including the following: Polish with an oilstone, using 400#, 800#, and 1200# in sequence from coarse to fine. Then, use abrasive paste on a wool felt for fine polishing (set the polisher speed to 600 rpm and the polishing pressure to a moderate level to avoid burning due to local overheating). After finishing, clean with ultrasonic anhydrous ethanol.

[0079] Comparative Example 3: A method for preparing a coating for sintering NdFeB molds is provided. The difference from Example 1 is that the transition layer uses a Ni-P amorphous alloy, while the remaining steps remain unchanged. Specifically: The transition layer is prepared using a Ni-P amorphous alloy via chemical plating. The plating solution formulation is as follows: Main salt: nickel sulfate, concentration 28 g / L; reducing agent: sodium hypophosphite, concentration 22 g / L; complexing agent: sodium citrate, total concentration 20 g / L; buffer: sodium acetate, concentration 12 g / L; stabilizer: potassium iodate, concentration 2 mg / L. By adjusting the pH value and the ratio of reducing agent, the mass fraction of scale in the coating is ensured to be 10~13wt% to obtain a completely amorphous structure; Process steps and key parameters: pH value of plating solution: 4.8 (acidic solution), adjusted in real time using ammonia or dilute sulfuric acid; Plating temperature: kept constant at 88℃; Deposition rate: controlled at 12 μm / H, duration approximately 18 min to obtain the transition layer; Hydrogen removal treatment: After plating, the plated material is left to stand and bake at 200°C for 2 hours to eliminate the risk of hydrogen breakage.

[0080] The coated Cr materials prepared in Examples 1-17 and Comparative Example 3 were used. 12 MoV mold steel matrix, Cr treated in Comparative Example 1 and Comparative Example 2 12 The MoV mold steel matrix underwent performance testing, including the following tests: (1) Coating bonding strength: tensile test method, GB / T 8642-2002, requirement >60MPa; (2) Microhardness: Micro Vickers hardness tester, GB / T 4340.1-2009, load 200g; (3) Coefficient of friction: The ball-and-disc friction and wear tester was used, referring to ASTM G99, and the pairing was sintered NdFeB balls; (4) Wear rate: The calculation formula is W=V / (F⋅L), where W is the wear rate (mm). 3 / N·m), V is the wear volume (mm). 3 F is the normal load (N), and L is the sliding distance (m). (5) Mold life: Counted during continuous production in the industrial field until the product shows obvious scratches or powder sticking.

[0081] The test results are shown in Table 1 below:

[0082] Combined with Table 1, Figure 2 , Figure 3 and Figure 4 As can be seen, Embodiment 1 of the present invention has excellent overall performance, and the actual service life of the mold exceeds 200,000 cycles.

[0083] Compared with Example 1, Example 4 reduced the thickness of the transition layer to 1 μm, while the rest was the same as Example 1. Due to the excessively thin transition layer, the dense, columnar crystal structure formed by HIPIMS deposition was insufficient, the lattice distortion strengthening effect was weakened, the metallurgical bonding area with the substrate was reduced, resulting in a significant reduction in bonding force and a marked decrease in coating bonding strength. During use, the coating was prone to peeling, and the mold service life was reduced to more than 80,000 cycles.

[0084] Compared with Example 1, Example 5 reduced the thickness of the support layer to 10 μm. Due to the thinning of the high-entropy alloy bearing layer, the single-phase solid solution structure formed by the high-entropy effect and the hysteresis diffusion effect provides insufficient mechanical support for the functional layer above, resulting in a significant reduction in overall hardness. It is prone to plastic deformation during high-pressure pressing, and its ability to resist the indentation and cutting of high-hardness NdFeB powder decreases. The mold is damaged after more than 120,000 uses.

[0085] Compared with Example 1, Example 6 reduced the thickness of the self-lubricating functional layer to 5 μm. Due to the reduction in the absolute content of MoS2 lubricating phase and ZrN / GO reinforcing phase, the thickness of the continuous lubricating film formed by MoS2 under shear force during friction was insufficient, resulting in reduced lubrication function and a significant increase in the coefficient of friction. At the same time, the resistance of ZrN / GO composite nanoparticles to abrasive wear was weakened, leading to a multiple increase in wear rate and a mold service life of only over 60,000 cycles.

[0086] Compared with Example 1, Example 7 has a surface layer thickness of 0.3 μm. Due to the excessively thin Al2O3 physical barrier, its density decreases, weakening its ability to prevent external water vapor and oxygen from eroding the internal lubricating phase. MoS2 is prone to oxidation failure. At the same time, the low friction characteristics in the initial contact stage are insufficient, the friction coefficient increases, the wear rate increases, and the mold service life is reduced to more than 80,000 cycles.

[0087] Compared with Example 1, Examples 8 and 9 have transition layer thicknesses of 2 μm and 5 μm, respectively. Experiments have verified that 3 μm is the optimal thickness. If the thickness is too thin, the metallurgical bonding strength is insufficient; if the thickness is too thick, the toughness of the coating system decreases due to the increased brittleness of the AlMn alloy. In both cases, the service life is slightly reduced.

[0088] Compared with Example 1, Examples 10 and 11 have support layer thicknesses of 20 μm and 40 μm, respectively. Experiments have verified that 30 μm is the optimal thickness. This thickness can ensure sufficient mechanical support of the functional layer by the high-entropy alloy, while avoiding the accumulation of thermal stress and increased processing costs caused by excessive thickness. The effects of other thicknesses did not reach the optimal level of Example 1.

[0089] Compared with Example 1, Examples 12 and 13 have self-lubricating functional layer thicknesses of 10 μm and 20 μm, respectively. Experiments have verified that 15 μm is the optimal thickness. This thickness achieves the best balance between the MoS2 lubricating phase content and the ZrN / GO reinforcing phase content, ensuring both continuous friction reduction and sufficient wear resistance. Other thicknesses failed to achieve the optimal synergistic effect.

[0090] Examples 14 and 15 were compared with Example 1, with surface layer thicknesses of 0.5 μm and 2 μm, respectively. Experiments verified that 1 μm was the optimal thickness. If it was too thin, the protective ability would be insufficient, and if it was too thick, the brittleness of the Al2O3 ceramic layer would increase, making it prone to cracking under high pressure cyclic stress. In both cases, the optimal effect could not be achieved.

[0091] Comparing Comparative Example 3 with Example 1, the transition layer uses Ni-P amorphous alloy instead of AlMn alloy. Since the Ni-P and the steel substrate are chemically deposited, the bonding method is mainly mechanical interlocking, which fails to form the metallurgical bonding as when preparing AlMn layer with HIPIMS. Moreover, the lattice distortion strengthening effect and stress buffering capacity of Ni-P amorphous alloy are far inferior to AlMn supersaturated solid solution, resulting in a serious reduction in coating bonding strength and a significant shortening of mold life.

[0092] Comparing Examples 16 and 17 with Example 1, by changing the proportions of the self-lubricating functional layer, it was found that the optimal proportions were 70-85 vol% matrix phase, 10-20 vol% MoS2 lubricating phase, and 5-10 vol% ZrN / GO reinforcing phase. When the MoS2 content was too high, although the lubricating phase increased, the high-entropy alloy matrix phase decreased relatively, resulting in a decrease in the overall hardness of the coating and insufficient load-bearing capacity, which in turn led to an increase in the wear rate. The effects of the other proportions did not reach the level of Example 1.

[0093] Comparative Example 1 is a nitriding treatment. Although the nitrided layer has high hardness and certain wear resistance, its surface chemical activity is high, its self-lubricating performance is insufficient, the friction coefficient is as high as 0.65, magnetic powder is easy to adhere and accumulate, and it lacks the synergistic protection of the gradient composite structure. After only 40,000 uses, it has problems such as powder adhesion and wear, requiring maintenance.

[0094] Comparative Example 2 is a normal polishing treatment. The mold surface has low hardness and no lubricating protective layer, resulting in a higher coefficient of friction. The high-hardness NdFeB powder directly cuts the mold surface, causing extremely severe wear. The service life is only about 4,000 cycles before it needs to be repaired due to powder adhesion and wear.

[0095] Therefore, it can be seen that in the preferred embodiment 1 of the present invention, the thickness and composition ratio of each layer have been systematically verified. The thickness configuration of the transition layer (3μm), support layer (30μm), self-lubricating functional layer (15μm), and surface layer (1μm), along with the material combination of AlMn alloy, AlCoCrFeNi high-entropy alloy, MoS2 / ZrN-GO composite system, and Al2O3, can achieve optimal modulus matching and chemical synergy between layers, thereby achieving the superior effect of the present invention. Compared with nitriding treatment, the mold service life of the preferred embodiment of the present invention is increased from 40,000 cycles to 200,000 cycles, the friction coefficient is reduced from 0.65 to 0.38, and the wear rate is reduced by more than 60%, resulting in significant economic benefits.

[0096] Compared with the prior art, the embodiments of the present invention have the following outstanding advantages: This invention employs a gradient composite structure design of "transition layer-support layer-functional layer-surface layer" to achieve a smooth transition in hardness from the mold substrate to the coating surface, avoiding coating cracking and peeling caused by stress accumulation at a single interface. The AlMn alloy transition layer utilizes the high ionization rate of HIPIMS technology to form a metallurgical bond with the steel substrate, fundamentally solving the problem of insufficient coating adhesion. The AlCoCrFeNi high-entropy alloy support layer forms a stable single-phase solid solution through high-entropy effect, lattice distortion effect, and hysteresis diffusion effect, providing a high-strength and tough "backbone" support for the entire coating system. The self-lubricating functional layer containing MoS2 and ZrN / GO composite nanoparticles achieves a balance between excellent lubrication and wear resistance through the continuous friction reduction caused by interlayer slippage of MoS2 and the synergistic toughening effect of hardening and crack bridging by ZrN / GO. The dense Al2O3 surface layer, through PACVD low-temperature deposition technology, acts as a "skin" barrier to effectively block external oxidation and provide initial low-friction characteristics.

[0097] In terms of material selection, the embodiments of the present invention use a high-entropy alloy as both the support layer and the functional layer matrix. By utilizing its unique high-entropy effect, it achieves high strength, toughness, wear resistance and thermal stability that are difficult to achieve with traditional alloys. The introduced ZrN / GO composite nano-reinforcing phase anchors ZrN nanoparticles through the large specific surface area of ​​GO, realizing the stable distribution of the lubricating phase and the synergistic toughening of the reinforcing phase.

[0098] In terms of process combination, three advanced technologies, HIPIMS, laser cladding and PACVD, are organically combined to give full play to their respective advantages in dense bonding, thick metallurgical cladding and low-temperature high-quality deposition, ensuring the high quality and high performance of the overall coating. Laboratory tests and industrial verification show that the coating of this invention reduces the coefficient of friction of the mold to below 0.4, reduces the wear rate by more than 60% compared with the traditional nitriding mold, and increases the service life by 4-5 times, from 40,000 cycles to 200,000 cycles, which greatly reduces downtime and mold replacement costs.

[0099] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A long-life self-lubricating coating for sintering NdFeB dies, characterized in that, From the inside out, the following are included: A transition layer is disposed on the surface of the mold substrate, the transition layer comprising Al and Mn; A support layer is disposed on the surface of the transition layer, the support layer comprising an AlCoCrFeNi alloy; A self-lubricating functional layer is disposed on the surface of the support layer, and the self-lubricating functional layer includes AlCoCrFeNi alloy, MoS2 and ZrN / GO composite nanoparticles; A surface layer is disposed on the surface of the self-lubricating functional layer, the surface layer comprising Al2O3.

2. The long-life self-lubricating coating for sintering NdFeB dies according to claim 1, characterized in that, In the transition layer, the content of Al is 85~92wt% and the content of Mn is 8~15wt%.

3. The long-life self-lubricating coating for sintering NdFeB dies according to claim 2, characterized in that, In the support layer, the contents of Al, Co, Cr, Fe and Ni in the AlCoCrFeNi alloy are all 15~25 at.

4. The long-life self-lubricating coating for sintering NdFeB dies according to claim 3, characterized in that, The contents of AlCoCrFeNi alloy, MoS2 and ZrN / GO composite nanoparticles in the self-lubricating functional layer are 70~85 vol%, 10~20 vol%, and 5~10 vol%, respectively.

5. A high-life self-lubricating coating for sintering NdFeB dies according to any one of claims 1 to 4, characterized in that, The thickness of the transition layer is 2~5 μm; the thickness of the support layer is 20~40 μm; the thickness of the self-lubricating functional layer is 10~20 μm; and the thickness of the surface layer is 0.5~2 μm.

6. A method for preparing a long-life self-lubricating coating for sintering NdFeB molds as described in claim 5, characterized in that, Includes the following steps: S1: Perform pretreatment on the mold substrate to make the surface of the mold substrate clean and the roughness Ra ≤ 0.1 μm; S2: An AlMn alloy layer is deposited on the surface of the mold substrate using high-energy pulsed magnetron sputtering technology. The AlMn alloy layer forms a metallurgical bond with the mold substrate. The thickness of the AlMn alloy layer is 2~5 μm. S3: An AlCoCrFeNi alloy layer is prepared on the surface of the AlMn alloy layer using laser cladding technology, and the thickness of the AlCoCrFeNi alloy layer is 20~40 μm; S4: A functional layer is co-deposited on the surface of the AlCoCrFeNi alloy layer using laser cladding technology. The functional layer uses AlCoCrFeNi alloy as the matrix phase and contains MoS2 lubricating phase and ZrN / GO composite nano-reinforcing phase. The thickness of the functional layer is 10~20 μm. S5: An Al2O3 layer is deposited on the surface of the functional layer using plasma-assisted chemical vapor deposition technology, and the thickness of the Al2O3 layer is 0.5~2 μm.

7. The method for preparing a long-life self-lubricating coating for sintering NdFeB molds according to claim 6, characterized in that, The pretreatment includes sequentially chamfering, polishing, and ultrasonic cleaning of the mold substrate with acetone and anhydrous ethanol.

8. The method for preparing a long-life self-lubricating coating for sintering NdFeB molds according to claim 6, characterized in that, The process parameters for the high-energy pulsed magnetron sputtering technology described in S2 include: a base vacuum of < 5 × 10⁻⁶. -3 Pa, deposition temperature 350~450 ℃, pulse frequency 50~150 Hz, duty cycle 5%~10%.

9. A method for preparing a long-life self-lubricating coating for sintering NdFeB molds according to claim 6, characterized in that, The process parameters for preparing AlCoCrFeNi alloy layers using laser cladding technology described in S3 include: laser power 1.8~2.5 kW, scanning speed 8~12 mm / s, spot diameter 2.5~3.0 mm, powder feeding rate 15~25 g / min, and overlap rate 40~50%.

10. The method for preparing a long-life self-lubricating coating for sintering NdFeB molds according to claim 6, characterized in that, The process parameters for co-depositing functional layers using laser cladding technology described in S4 include: laser power 1.5~2.0 kW, scanning speed 8~12 mm / s, powder feeding rate 10~20 g / min, and overlap rate 30~50%; after co-deposition, stress relief is achieved by holding the layer at 450~500 ℃ for 1.5~2.5 h.

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