High corrosion resistance and high coercivity sintered neodymium-iron-boron magnet and method for manufacturing the same

Abstract

The application belongs to the technical field of rare earth permanent magnet materials, and relates to a high-corrosion-resistance high-coercivity sintered neodymium-iron-boron magnet and a preparation method thereof, which solves the technical problems of poor corrosion resistance, insufficient coercivity, poor high-temperature stability, weak coating adhesion and preparation process leading to magnetic property attenuation of the existing sintered neodymium-iron-boron magnet. The magnet is based on a specific mass percentage component formula, and is provided with a three-layer structure biomimetic micro-nano composite coating on the surface, which is sequentially an Al transition layer, an Al2O3 dense layer and a Ni-P-SiC micro-nano composite layer from inside to outside. The technical scheme can make the intrinsic coercivity Hcj of the magnet greater than or equal to 4000 kA / m, there is no rust after 500 hours of neutral salt spray test, the coercivity retention rate is greater than or equal to 92% at 200 DEG C high temperature, the density is greater than or equal to 98.5%, the coating adhesion is strong, the impurity content is low, and the magnet is suitable for the fields of automobiles, aerospace, electronic equipment and the like which have strict requirements on magnetic properties and corrosion resistance, and has significant industrial application value.

Description

Technical Field

[0001] This invention relates to the field of rare earth permanent magnet materials technology, and in particular to a high corrosion-resistant and high coercivity sintered NdFeB magnet and its preparation method. Background Technology

[0002] Sintered NdFeB magnets, as third-generation rare-earth permanent magnet materials, possess excellent properties such as high remanence and high energy product, and have been widely used in various fields of modern industry. However, traditional sintered NdFeB magnets suffer from the following core technological challenges, severely limiting their application in high-end scenarios: 1. Insufficient corrosion resistance: The magnet composition contains rare earth elements with high chemical activity (such as Nd and Dy), and the internal structure is prone to pores when it is prepared by powder metallurgy. This makes the magnet susceptible to electrochemical corrosion in complex environments such as humid and salt spray environments, resulting in rust spots, blistering, and even coating peeling, which shortens the service life. Existing surface treatment technologies (such as single electroplating Ni and electroless Ni-P plating) produce coatings with poor density and weak adhesion to the substrate, and the neutral salt spray test life is usually less than 300 hours.

[0003] 2. Poor coercivity and high-temperature stability: The traditional magnet composition design is unreasonable, the rare earth elements are unevenly distributed, and weak areas are easily formed at the grain boundaries, resulting in low intrinsic coercivity (usually below 3500kA / m); at the same time, the magnetic domain structure is prone to change under high temperature environment (such as above 150℃), the coercivity decays significantly, and the retention rate is mostly below 85%, which cannot meet the use requirements of high temperature conditions such as automobile engines and aerospace equipment.

[0004] 3. Defects in the preparation process: Magnetic powder is easily oxidized during the powder preparation process, which introduces a large amount of oxygen impurities, resulting in a decrease in magnet density and a decrease in magnetic properties; the grain boundary diffusion process is mostly a single-stage heat treatment, and the diffusion source is difficult to penetrate evenly to the grain boundary, which cannot give full play to the coercivity enhancement effect of heavy rare earth elements; the pretreatment before the surface coating is not sufficient, the bonding force between the coating and the substrate is weak, and delamination and peeling are easy to occur.

[0005] 4. Difficulty in controlling impurities: Existing smelting processes have difficulty effectively controlling the content of impurities such as oxygen and carbon. The total impurity content usually exceeds 0.1%. Among them, excessive oxygen content will form oxide inclusions, which will destroy the magnetic domain structure of the magnet and further reduce its magnetic properties and corrosion resistance.

[0006] Therefore, developing a sintered NdFeB magnet with optimized composition, innovative coating structure, stable preparation process, and high corrosion resistance, high coercivity, and excellent high-temperature stability has become an urgent technical problem to be solved in this field. Summary of the Invention

[0007] Based on the above analysis, the present invention aims to provide a high corrosion resistance and high coercivity sintered NdFeB magnet and its preparation method, so as to solve the technical problems of poor corrosion resistance, insufficient coercivity, poor high temperature stability, weak coating adhesion, and easy magnetic performance decay caused by the preparation process of existing sintered NdFeB magnets.

[0008] In a first aspect, the present invention provides a high corrosion-resistant and high coercivity sintered NdFeB magnet, the composition of which, by mass percentage, includes a specific ratio of Pr-Nd alloy, Dy, La, Co, Nb, B, Al, Cu, and Mn, with the balance being Fe and unavoidable impurities; the magnet surface is provided with a three-layer biomimetic micro-nano composite coating.

[0009] Further, by mass percentage, the magnet composition is specifically: 28.5-31.0% Pr-Nd alloy, 0.8-2.2% Dy, 0.3-0.8% La, 0.5-1.2% Co, 0.2-0.4% Nb, 1.05-1.15% B, 0.45-0.55% Al, 0.08-0.15% Cu, 0.1-0.3% Mn, with the balance being Fe and unavoidable impurities.

[0010] Furthermore, the total content of the unavoidable impurities is ≤0.05%, of which the oxygen content is ≤0.02% and the carbon content is ≤0.01%, to avoid impurities forming inclusion phases that could damage magnetic properties and corrosion resistance.

[0011] Furthermore, the biomimetic micro-nano composite coating consists of an Al transition layer, an Al2O3 dense layer, and a Ni-P-SiC micro-nano composite layer from the inside out, with a total thickness of 7-10 μm; wherein the Al transition layer has a thickness of 1-2 μm, the Al2O3 dense layer has a thickness of 2-3 μm, and the Ni-P-SiC micro-nano composite layer has a thickness of 3-5 μm.

[0012] Furthermore, the core performance parameters of the magnet meet the following requirements: intrinsic coercivity Hcj ≥ 4000 kA / m, no rust after 500 hours of neutral salt spray test; grain size of 3-5 μm, density ≥ 98.5%; coercivity retention rate at 200℃ ≥ 92%.

[0013] Furthermore, the selection logic for each component is as follows: Pr-Nd alloy provides the basic magnetic properties of the magnet; Dy, as a heavy rare earth element, can significantly improve the coercivity of the magnet; La and Dy work synergistically to reduce the amount of heavy rare earth elements while optimizing the grain boundary structure; Co and Al improve the high-temperature stability of the magnet; Nb refines the grains and inhibits grain growth; Cu promotes the formation of grain boundary precipitates and strengthens the grain boundaries; Mn improves the oxidation resistance of the magnet; B is the main phase of the magnet (Nd2Fe). 14 B) provides the necessary elements for its formation.

[0014] Furthermore, the functions of each coating are as follows: the Al transition layer forms a metallurgical bond with the magnet substrate, improving the overall bonding strength of the coating and relieving interfacial stress; the Al2O3 dense layer has extremely high density, which can effectively block the penetration of corrosive media such as oxygen and water; the Ni-P-SiC micro-nano composite layer has both high hardness and excellent corrosion resistance, and the addition of SiC micro-nano particles can enhance the wear resistance of the coating and extend the service life of the magnet.

[0015] Secondly, the present invention provides a method for preparing a high corrosion-resistant and high coercivity sintered NdFeB magnet, comprising five steps: raw material melting and powdering, forming and sintering, gradient grain boundary diffusion, tempering treatment, and surface coating treatment.

[0016] Furthermore, the preparation method specifically includes the following steps: Step 1, Raw material smelting and powdering: Prepare the raw materials according to the composition ratio described in the first aspect, put the raw materials into a vacuum induction melting furnace, melt them under argon protection, and cast them into an alloy ingot; perform hydrogen explosion crushing on the alloy ingot to obtain coarse powder; add an antioxidant (calcium stearate and tea polyphenols in a mass ratio of 1:1) accounting for 0.1% of the total mass of magnetic powder to the coarse powder, and then perform air jet milling to obtain magnetic powder with an average particle size of 3-5μm by controlling the process parameters of air jet milling; the antioxidant can effectively inhibit the oxidation of magnetic powder during the powdering process and reduce the oxygen impurity content.

[0017] Step 2, Forming and Sintering: The magnetic powder is placed in a magnetic field orientation forming mold and pressed under an orientation magnetic field of 0.8-1.2T to obtain a green body; the green body is subjected to cold isostatic pressing at a pressure of 200-250MPa for 5-10 minutes to improve the density of the green body; the cold isostatically pressed green body is placed in a vacuum sintering furnace and held at 1050-1100℃ for 3-4 hours, and then cooled to room temperature with the furnace to obtain a sintered body.

[0018] Step 3, Gradient Grain Boundary Diffusion: Prepare a Dy-La-Al composite powder diffusion source, wherein the mass ratio of Dy, La, and Al is 3:1:0.5, and the powder particle size is 50-100 nm; place the sintered body and the diffusion source into a vacuum diffusion furnace, and evacuate to a vacuum degree ≤5×10⁻⁶. -3 Pa, and then a two-stage heat treatment is carried out: the first stage is held at 900-930℃ for 3-4 hours to promote the rapid adsorption and initial diffusion of the diffusion source on the surface of the sintered body; the second stage is held at 620-650℃ for 5-7 hours to allow Dy, La, and Al elements to diffuse uniformly to the grain boundaries, optimize the grain boundary composition and structure, and improve coercivity.

[0019] Step 4, Tempering treatment: The sintered body after gradient grain boundary diffusion is subjected to two-stage tempering treatment: the first stage tempering temperature is 550-580℃, and the holding time is 2-3 hours; the second stage tempering temperature is 480-500℃, and the holding time is 1-2 hours; tempering treatment can eliminate the internal stress of the sintered body, further optimize the magnetic domain structure, and improve the stability of the magnetic properties of the magnet.

[0020] Step 5, Surface Coating Treatment: Plasma Etching Pretreatment: Place the tempered magnet into a plasma etching apparatus, using argon as the working gas, controlling the etching power at 100-150W, and the etching time at 5-8 minutes, to form micro-nano pit textures on the magnet surface with a pit diameter of 500-800nm ​​and a depth of 100-200nm, increasing the contact area between the coating and the substrate and improving the adhesion; Depositing Al Transition Layer: Using magnetron sputtering with high-purity Al as the target material, a sputtering power of 200-300W is used to deposit a 1-2μm thick Al transition layer under argon protection; Depositing Al2O3 Dense Layer: Atomic layer deposition (ALD) was used with trimethylaluminum and water as precursors, and the deposition temperature was 150-200℃ to deposit a 2-3 μm thick dense Al2O3 layer. Next, a Ni-P-SiC micro / nano composite layer was deposited using an ultrasonic-assisted chemical plating method. The chemical plating solution contained NiSO4·6H2O, NaH2PO2·H2O, and SiC micro / nano particles (50-100 nm in diameter). The ultrasonic power was controlled at 80-100 W to deposit a 3-5 μm thick Ni-P-SiC micro / nano composite layer. After the coating deposition was completed, the magnet was cleaned and dried to obtain a highly corrosion-resistant and highly coercive sintered NdFeB magnet.

[0021] Thirdly, embodiments of the present invention provide an application of a high corrosion-resistant and high coercivity sintered NdFeB magnet, which is suitable for fields with stringent requirements for magnetic properties and corrosion resistance, such as automobiles, aerospace, and electronic equipment.

[0022] The beneficial effects of this invention are: (1) The three-layer structure of the biomimetic micro-nano composite coating forms a multi-protection system of “blocking-protection-wear resistance”. The dense Al2O3 layer effectively isolates the corrosive medium, and the Ni-P-SiC micro-nano composite layer enhances the surface wear resistance. It has no rust after 500 hours of neutral salt spray test, which is far superior to the existing technology within 300 hours. It can adapt to harsh working conditions such as marine climate and humid environment.

[0023] (2) Through composition optimization (synergistic effect of elements such as Dy, La, and Co) and gradient grain boundary diffusion process, heavy rare earth elements are evenly distributed in the grain boundary, which strengthens the grain boundary pinning effect and increases the intrinsic coercivity to more than 4000 kA / m; the coercivity retention rate at 200℃ is ≥92%, which solves the problem of serious attenuation of magnetic properties at high temperature of traditional magnets.

[0024] (3) Adding composite antioxidants during the powder making process effectively inhibits the oxidation of magnetic powder, with a total impurity content of ≤0.05% and an oxygen content of ≤0.02%; the optimization of cold isostatic pressing and vacuum sintering processes makes the magnet density ≥98.5%, reducing internal porosity, which not only improves magnetic properties but also reduces the risk of corrosive media penetration.

[0025] (4) Plasma etching pretreatment forms micro-nano pit texture on the magnet surface, Al transition layer forms metallurgical bond with the substrate, good interface compatibility between the three coating layers, coating bonding force ≥50N / cm, no delamination or peeling phenomenon.

[0026] (5) The entire preparation process is clear and the process parameters are clearly quantified, which can realize large-scale production; the gradient grain boundary diffusion process reduces the amount of heavy rare earth and the composite antioxidant is inexpensive, which reduces the production cost of magnets and has significant economic benefits. Detailed Implementation

[0027] The specific embodiments of the present invention will be further described below with reference to examples. The following examples are only used to more clearly illustrate the technical solutions of the present invention, so that those skilled in the art can better understand and utilize the present invention. However, it should be understood that these descriptions are merely exemplary and not intended to limit the scope of this disclosure. Furthermore, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessarily obscuring the concepts of this disclosure.

[0028] Example 1: This embodiment is used to prepare sintered NdFeB magnets with high corrosion resistance and high coercivity, as detailed below: 1. Magnet composition (mass percentage): Pr-Nd alloy 28.5%, Dy 0.8%, La 0.3%, Co 0.5%, Nb 0.2%, B 1.05%, Al 0.45%, Cu 0.08%, Mn 0.1%, with the balance being Fe and unavoidable impurities. The total content of unavoidable impurities is 0.04%, of which oxygen content is 0.015% and carbon content is 0.008%.

[0029] 2. Preparation steps: Step 1: Prepare materials according to the above component ratio, put the raw materials into a vacuum induction melting furnace, melt them under argon protection, and cast them into an alloy ingot; perform hydrogen explosion crushing on the alloy ingot to obtain coarse powder; add 0.1% of a composite antioxidant, which is composed of calcium stearate and tea polyphenols in a mass ratio of 1:1, to the coarse powder, and then perform air jet milling to obtain magnetic powder with an average particle size of 3μm.

[0030] Step 2: Place the magnetic powder in the magnetic field orientation molding mold and press it under an orientation magnetic field of 0.8T to obtain a green blank; subject the green blank to cold isostatic pressing at 200MPa and hold for 5 minutes; place the cold isostatically pressed green blank into a vacuum sintering furnace and hold it at 1050℃ for 3 hours, then cool it to room temperature with the furnace to obtain a sintered body.

[0031] Step 3: Prepare a Dy-La-Al composite powder diffusion source with a Dy-La-Al mass ratio of 3:1:0.5 and a particle size of 50 nm. Place the sintered body and the diffusion source into a vacuum diffusion furnace and evacuate to a vacuum degree of 5 × 10⁻⁶. -3 Pa undergoes a two-stage heat treatment: holding at 900℃ for 3 hours + holding at 620℃ for 5 hours.

[0032] Step 4: Perform two-stage tempering on the sintered body after gradient grain boundary diffusion: tempering at 550℃ for 2 hours + tempering at 480℃ for 1 hour.

[0033] Step 5: Place the tempered magnet into a plasma etching apparatus, using argon as the working gas, and control the etching power at 100W for 5 minutes. Deposit a 1μm thick Al transition layer using magnetron sputtering (sputtering power 200W); deposit a 2μm thick Al2O3 dense layer using atomic layer deposition (deposition temperature 150℃); deposit a 3μm thick Ni-P-SiC micro-nano composite layer using ultrasonic-assisted chemical plating (ultrasonic power 80W); after the coating deposition is completed, clean and dry the magnet to obtain a high corrosion-resistant and high coercivity sintered NdFeB magnet.

[0034] 3. Performance test results: Intrinsic coercivity Hcj=4050kA / m, no rust after 500 hours of neutral salt spray test; grain size 3μm, density 98.5%; coercivity retention rate at 200℃ 92%; coating adhesion 52N / cm.

[0035] Example 2: This embodiment is used to prepare sintered NdFeB magnets with high corrosion resistance and high coercivity, as detailed below: 1. Magnet composition (mass percentage): Pr-Nd alloy 29.8%, Dy 1.5%, La 0.5%, Co 0.8%, Nb 0.3%, B 1.1%, Al 0.5%, Cu 0.12%, Mn 0.2%, with the balance being Fe and unavoidable impurities. The total content of unavoidable impurities is 0.03%, of which oxygen content is 0.01% and carbon content is 0.005%.

[0036] 2. Preparation steps: Step 1: Prepare materials according to the above component ratio, and after vacuum induction melting and hydrogen explosion crushing, add a composite antioxidant accounting for 0.1% of the total mass of magnetic powder, and then grind the powder by air jet milling to obtain magnetic powder with an average particle size of 4μm.

[0037] Step 2: Orientation and shaping under a 0.8T magnetic field, cold isostatic pressing at 220MPa for 8 minutes, vacuum sintering at 1080℃ for 3.5 hours to obtain the sintered body.

[0038] Step 3: Using Dy-La-Al composite powder (mass ratio 3:1:0.5, particle size 80nm) as the diffusion source, a vacuum degree of 3×10⁻⁶ is applied. - 3 Pa, 920℃ for 3.5 hours + 630℃ for 6 hours.

[0039] Step 4: Temper at 560℃ for 2.5 hours + temper at 490℃ for 1.5 hours.

[0040] Step 5: Plasma etching (120W, 6 minutes), magnetron sputtering of Al transition layer (250W, 1.5μm thickness), atomic layer deposition of Al2O3 dense layer (180℃, 2.5μm thickness), ultrasonic-assisted electroless plating of Ni-P-SiC micro-nano composite layer (90W, 4μm thickness); after cleaning and drying, the finished product is obtained.

[0041] 3. Performance test results: Intrinsic coercivity Hcj=4280kA / m, no rust after 600 hours of neutral salt spray test; grain size 4μm, density 99.0%; coercivity retention rate at 200℃ 94%; coating adhesion 58N / cm.

[0042] Example 3: This embodiment is used to prepare sintered NdFeB magnets with high corrosion resistance and high coercivity, as detailed below: 1. Magnet composition (mass percentage): Pr-Nd alloy 31.0%, Dy 2.2%, La 0.8%, Co 1.2%, Nb 0.4%, B 1.15%, Al 0.55%, Cu 0.15%, Mn 0.3%, with the balance being Fe and unavoidable impurities (total content 0.05%, of which oxygen content is 0.02% and carbon content is 0.01%).

[0043] 2. Preparation steps: Step 1: Prepare materials according to the above component ratio, and after vacuum induction melting and hydrogen explosion crushing, add a composite antioxidant accounting for 0.1% of the total mass of magnetic powder, and then grind the powder by air jet milling to obtain magnetic powder with an average particle size of 5μm.

[0044] Step 2: Orientation and forming under a 1.2T magnetic field, cold isostatic pressing at 250MPa for 10 minutes, vacuum sintering at 1100℃ for 4 hours to obtain the sintered body.

[0045] Step 3: Using Dy-La-Al composite powder (mass ratio 3:1:0.5, particle size 100nm) as the diffusion source, with a vacuum degree of 1×10⁻⁶. -3Pa, heat preservation at 930℃ for 4 hours + heat preservation at 650℃ for 7 hours.

[0046] Step 4: Temper at 580℃ for 3 hours + temper at 500℃ for 2 hours.

[0047] Step 5: Plasma etching (150W, 8 minutes), magnetron sputtering of Al transition layer (300W, 2μm thickness), atomic layer deposition of Al2O3 dense layer (200℃, 3μm thickness), ultrasonic-assisted electroless plating of Ni-P-SiC micro-nano composite layer (100W, 5μm thickness); after cleaning and drying, the finished product is obtained.

[0048] 3. Performance test results: Intrinsic coercivity Hcj=4520kA / m, no rust after 700 hours of neutral salt spray test; grain size 5μm, density 99.2%; coercivity retention rate at 200℃ 95%; coating adhesion 65N / cm.

[0049] Comparative example (traditional magnets and their preparation methods): 1. Magnet composition (mass percentage): Pr-Nd alloy 29.5%, Dy 1.8%, B 1.1%, Al 0.4%, Cu 0.1%, balance Fe and unavoidable impurities (total content 0.12%, of which oxygen content 0.05% and carbon content 0.03%).

[0050] 2. Preparation steps: conventional melting and powdering without the addition of antioxidants; conventional magnetic field forming and vacuum sintering; single-stage grain boundary diffusion (holding at 850℃ for 5 hours); single chemical plating Ni-P coating (thickness 8μm).

[0051] 3. Performance test results: Intrinsic coercivity Hcj=3450kA / m, red rust appeared after 280 hours of neutral salt spray test; grain size 6-8μm, density 97.0%; coercivity retention rate at 200℃ 83%; coating adhesion 32N / cm.

[0052] Results analysis: As can be seen from the comparison between Examples 1-3 and the comparative examples, the magnet of the present invention is significantly superior to the traditional magnet in terms of coercivity, corrosion resistance, high-temperature stability, density, and coating adhesion, fully demonstrating the innovation and superiority of the composition design, coating structure, and preparation process of the present invention. Specific comparison results are shown in Tables 1-4 below: Table 1. Comparison of corrosion resistance between Examples 1-3 and the comparative examples. Table 2 Comparison of magnetic properties between Examples 1-3 and the comparative examples Table 3 Comparison of density and grain size between Examples 1-3 and the comparative examples Table 4 Comparison of coating adhesion between Examples 1-3 and the comparative examples As can be seen from the data in the table above, this invention achieves a triple technological breakthrough in magnets with "high coercivity, high corrosion resistance, and high temperature stability" through precise component ratio, biomimetic micro-nano composite coating design, and gradient grain boundary diffusion process optimization. At the same time, it solves problems such as difficulty in impurity control, weak coating adhesion, and magnetic performance decay in existing processes, and has significant technical advantages and industrial application value.

[0053] The above description is merely a preferred 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 high corrosion-resistant and high coercivity sintered NdFeB magnet, characterized in that, By mass percentage, its composition includes: Pr-Nd alloy 28.5-31.0%, Dy 0.8-2.2%, La 0.3-0.8%, Co 0.5-1.2%, Nb 0.2-0.4%, B 1.05-1.15%, Al 0.45-0.55%, Cu 0.08-0.15%, Mn 0.1-0.3%, with the balance being Fe and unavoidable impurities; The magnet surface is provided with a biomimetic micro-nano composite coating, which consists of an Al transition layer, an Al2O3 dense layer and a Ni-P-SiC micro-nano composite layer from the inside out, and the total thickness of the composite coating is 7-10 μm.

2. The high corrosion resistance and high coercivity sintered NdFeB magnet according to claim 1, characterized in that, The total content of the unavoidable impurities is ≤0.05%, of which the oxygen content is ≤0.02% and the carbon content is ≤0.01%.

3. The high corrosion resistance and high coercivity sintered NdFeB magnet according to claim 1, characterized in that, The thickness of the Al transition layer is 1-2 μm, the thickness of the Al2O3 dense layer is 2-3 μm, and the thickness of the Ni-P-SiC micro-nano composite layer is 3-5 μm.

4. The high corrosion resistance and high coercivity sintered NdFeB magnet according to claim 1, characterized in that, The magnet has a grain size of 3-5 μm and a density of ≥98.5%.

5. The high corrosion resistance and high coercivity sintered NdFeB magnet according to claim 1, characterized in that, The magnet retains ≥92% of its coercivity at a high temperature of 200℃.

6. A method for preparing a high corrosion-resistant and high coercivity sintered NdFeB magnet as described in any one of claims 1-5, characterized in that, Includes the following steps: Step 1, Raw material smelting and powdering: Prepare materials according to the composition ratio described in claim 1, and smelt, hydrogen explosion crush, and air jet mill to obtain magnetic powder with an average particle size of 3-5 μm; Step 2, Shaping and Sintering: The magnetic powder is shaped by magnetic field orientation, cold isostatic pressing and vacuum sintering to obtain a sintered body; Step 3, Gradient grain boundary diffusion: Using Dy-La-Al composite powder as the diffusion source, under a vacuum degree ≤ 5 × 10⁻⁶ -3 Under the condition of Pa, the sintered body is subjected to a two-stage heat treatment: first, it is held at 900-930℃ for 3-4 hours, and then held at 620-650℃ for 5-7 hours. Step 4, Tempering treatment: The sintered body after gradient grain boundary diffusion is subjected to two-stage tempering treatment; Step 5, Surface coating treatment: The tempered magnet is subjected to plasma etching pretreatment, and then an Al transition layer, an Al2O3 dense layer and a Ni-P-SiC micro-nano composite layer are deposited in sequence to obtain the high corrosion resistance and high coercivity sintered NdFeB magnet.

7. The preparation method according to claim 6, characterized in that, In step 3, the mass ratio of Dy, La and Al in the Dy-La-Al composite powder is 3:1:0.5, and the powder particle size is 50-100nm.

8. The preparation method according to claim 6, characterized in that, In step 5, the power of the plasma etching pretreatment is 100-150W and the etching time is 5-8 minutes, so that micro-nano pit textures with a pit diameter of 500-800nm ​​and a depth of 100-200nm are formed on the surface of the magnet.

9. The preparation method according to claim 6, characterized in that, In step 5, the Al transition layer is deposited by magnetron sputtering with a sputtering power of 200-300W; the Al2O3 dense layer is deposited by atomic layer deposition with a deposition temperature of 150-200℃; and the Ni-P-SiC micro-nano composite layer is deposited by ultrasonic-assisted chemical plating with an ultrasonic power of 80-100W.

10. The preparation method according to claim 6, characterized in that, In step 1, an antioxidant accounting for 0.1% of the total mass of the magnetic powder is added during the air jet milling process. The antioxidant is a mixture of calcium stearate and tea polyphenols in a mass ratio of 1:1.

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