A method for improving the bonding strength of NdFeB magnet coating and its application

Abstract

The present invention relates to a method and application for improving the bonding strength of NdFeB magnet coating, the method comprising: preparation of rare earth hydroxide suspension; double-sided spraying of NdFeB magnet; grain boundary thermal diffusion treatment. The method of the present invention pyrolyzes rare earth hydroxide into rare earth oxides and deposits them on the surface of sintered NdFeB magnet, which can effectively increase the diffusion depth of heavy rare earth elements in the magnet and accurately control the composition and thickness of the coating; saves the cost of preparing rare earth oxides from rare earth hydroxides, simplifies the preparation process of diffusion sources, and has high spraying efficiency; the obtained coating has high quality, low coating porosity, and high bonding strength between the diffusion source and the magnet interface; helps to promote the subsequent grain boundary diffusion, and provides a new idea for the industrial application of grain boundary diffusion technology.

Description

Technical Field

[0001] The invention relates to the technical field of NdFeB magnets, in particular to a method for improving the bonding force of NdFeB magnet coatings and its application. Background Art

[0002] NdFeB is an important permanent magnetic material. It is widely used in electronics, automobiles, aerospace and other fields due to its superior magnetic properties. The high remanence, high coercivity and good temperature stability of NdFeB magnets make it an important part of modern technology. However, with the advancement of science and technology, the market's requirements for the magnetic properties of NdFeB magnets are constantly increasing. The current research direction is mainly focused on improving the magnetic properties of NdFeB and saving rare earth raw materials. When preparing high-performance NdFeB by traditional methods, 1%-5% of heavy rare earth elements need to be added during the batching. Although the coercivity of the material can be improved, the addition of too much heavy rare earth will reduce the remanence of the material and also cause a waste of heavy rare earth resources.

[0003] The grain boundary diffusion method is to deposit a small amount of heavy rare earth diffusion source on the surface of the magnet. The heavy rare earth will penetrate into the interior of the magnet along the grain boundary phase during the subsequent heat treatment process, greatly improving the coercive force of the magnet while keeping the magnetic energy product and remanence unchanged or slightly reduced. At present, this is the most efficient and economical method to improve the coercive force of NdFeB magnets. The coating method, chemical bath deposition method and magnetron sputtering method are the mainstream methods for preparing heavy rare earth diffusion sources on the surface of NdFeB magnets, but the coating method and chemical bath deposition method are difficult to control the deposition amount of heavy rare earth diffusion sources, and the prepared diffusion sources have poor bonding strength with the magnet and are prone to fall off. The heavy rare earth diffusion source prepared by magnetron sputtering has a high density and a high bonding strength between the diffusion source and the magnet, but the efficiency of magnetron sputtering is low, the cost is high, and it is difficult to apply to industrial production. Summary of the invention

[0004] In view of the shortcomings of the prior art, the technical problem to be solved by the present invention is how to improve the bonding strength of the prepared diffusion source and the magnet (i.e. the bonding strength of the heavy rare earth coating and the NdFeB magnet substrate) while obtaining a NdFeB magnet coating with a small reduction in magnetic energy product and remanence and a high increase in coercive force. To this end, the present invention provides a method and application for improving the bonding strength of NdFeB magnet coatings.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0006] The present invention provides a method for improving the bonding force of a NdFeB magnet coating, comprising the following steps:

[0007] (1) Preparation of rare earth hydroxide suspension:

[0008] (1-1) adding ammonia water to a rare earth solution to generate rare earth hydroxide precipitate, filtering, washing, drying, grinding and setting aside;

[0009] (1-2) adding a suspension medium and a dispersant to the treated rare earth hydroxide precipitate in sequence, and then performing ball milling to obtain a rare earth hydroxide suspension; then adjusting the pH of the rare earth hydroxide suspension for later use;

[0010] (2) Double-sided spraying of NdFeB magnets:

[0011] (2-1) Pretreatment of NdFeB magnets: degreasing, pickling and sandblasting of NdFeB magnets are performed in sequence for standby use;

[0012] (2-2) using a suspension plasma spraying method to spray the NdFeB magnet on both sides to obtain a rare earth oxide coating; during the spraying, the rare earth hydroxide suspension obtained in step (1) is used to first atomize the rare earth hydroxide suspension, and then spray the front and back sides of the NdFeB magnet respectively;

[0013] (3) Grain boundary thermal diffusion treatment:

[0014] The double-sided sprayed NdFeB magnet is placed in a tempering furnace and vacuumed, and is first annealed and then tempered.

[0015] Preferably, in step (1), the rare earth solution is a heavy rare earth chloride solution and / or a light rare earth chloride solution; the light rare earth chloride solution is a PrCl3 solution; the heavy rare earth chloride solution is a chloride solution of any one or two or more of gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), and yttrium (Y);

[0016] The total concentration of rare earth ions in the rare earth solution is 0.6-2 mol / L;

[0017] Preferably, the rare earth solution is any one of DyCl3 solution, YCl3 solution, GdCl3 solution, TbCl3 solution, or a mixture of two or more thereof;

[0018] More preferably, the rare earth solution contains DyCl3 solution, and further contains any one of YCl3 solution, GdCl3 solution, TbCl3 solution, or a mixture of two or more thereof;

[0019] More preferably, the rare earth solution is a mixture of DyCl3 solution, YCl3 solution and TbCl3 solution; wherein the molar ratio of Dy:Y:Tb in the rare earth solution is (2-7):1:(2-7);

[0020] More preferably, the molar ratio of Dy:Y:Tb in the rare earth solution is 2:1:7;

[0021] Among them, the preferred rare earth solution of the present invention is a mixture of DyCl3 solution, YCl3 solution, and TbCl3 solution, and the molar ratio of Dy, Y, and Tb elements in the rare earth solution is optimized. By limiting the molar ratio of the three within a specific range, the prepared rare earth hydroxide suspension can make Y better play its role in improving the grain boundary structure under the spraying process conditions of the present invention, ensuring that more Y is enriched in the Nd-rich phase in a continuous band, so that the Nd-rich phase is more continuous and smooth, thereby significantly improving its coercivity;

[0022] Preferably, in step (1), the molar ratio of the total molar amount of the rare earth element in the rare earth solution to the hydroxyl ion in the ammonia water is 1:(4-8); the concentration of the ammonia water is 4-12 mol / L;

[0023] Preferably, in step (1), the suspending medium is deionized water and / or anhydrous ethanol; after adding the suspending medium, the mass of the rare earth hydroxide precipitate is 5-40% of the total mass of the rare earth hydroxide precipitate and the suspending medium;

[0024] More preferably, in step (1), the suspending medium is deionized water and anhydrous ethanol in a volume ratio of 1:1; after adding the suspending medium, the mass of the rare earth hydroxide precipitate is 8-25% of the total mass of the rare earth hydroxide precipitate and the suspending medium;

[0025] The suspension medium of the present invention is a mixture of deionized water and anhydrous ethanol. The addition of ethanol can increase the temperature of the plasma flame flow during subsequent plasma spraying to ensure that the temperature of the plasma flame flow can pyrolyze the rare earth hydroxide into rare earth oxide. If deionized water is used alone, too much heat of the flame flow will be consumed; and if anhydrous ethanol is used alone, the problem of too high calorific value of the flame flow will be caused.

[0026] The present invention optimizes the content of rare earth hydroxide precipitates in the rare earth hydroxide suspension (i.e., the solid content of the rare earth hydroxide suspension). By limiting the solid content of the rare earth hydroxide suspension within a specific range, the prepared rare earth hydroxide suspension has good fluidity, and the distribution of the rare earth hydroxide in the suspension is more uniform, which is conducive to the subsequent spraying process. If the content of the rare earth hydroxide precipitates is too high, the prepared rare earth hydroxide suspension will have the problem of poor fluidity, which will cause the liquid delivery pipeline to be blocked in the subsequent spraying process, affecting the smooth progress of the spraying; if the content of the rare earth hydroxide precipitates is too low, the prepared rare earth hydroxide suspension will have the problem of poor stability, which will cause the sprayed coating to be uneven or even leaking in the subsequent spraying process.

[0027] Preferably, in step (1), the dispersant is polyacrylic acid, ammonium polyacrylate or sodium polyacrylate; the mass of the dispersant is 0.5-3.0% of the mass of the rare earth hydroxide precipitate;

[0028] Preferably, in step (1), the drying treatment is carried out at a temperature of 80-120°C and for a time of 8-12 h;

[0029] The ball milling treatment time is 0.5-8h; after ball milling, the median particle size of the particles in the rare earth hydroxide suspension is 0.5-4.0 μm;

[0030] More preferably, in the step (1), the ball milling treatment time is 0.5-3.5 hours; after ball milling, the median particle size of the particles in the rare earth hydroxide suspension is 0.5-2.5 μm;

[0031] Among them, the present invention optimizes the ball milling time and the median particle size of the particles in the rare earth hydroxide suspension after ball milling. By limiting the ball milling time and the median particle size within a specific range, the prepared rare earth hydroxide suspension has good dispersibility, reduces the surface energy of the rare earth hydroxide, enhances stability, and is conducive to the subsequent spraying process. If the median particle size of the particles in the rare earth hydroxide suspension is too high, the rare earth hydroxide suspension will have a sedimentation problem, resulting in poor stability of the suspension; if the median particle size of the particles in the rare earth hydroxide suspension is too low, there will be a viscosity problem, resulting in poor fluidity of the suspension. The stability and fluidity of the suspension are related to the subsequent spraying process, which indirectly affects the uniformity of the coating obtained by spraying and the deposition effect of the rare earth on the surface of the magnet.

[0032] Preferably, in step (1), the pH of the rare earth hydroxide suspension is adjusted to 7-10;

[0033] Preferably, in step (1), the absolute viscosity of the rare earth hydroxide suspension obtained is 1.0-5.0 mPa·s and the surface tension is 25-60 mN / m;

[0034] Among them, the present invention reasonably limits the absolute viscosity and surface tension of the rare earth hydroxide suspension. Within this specific range, the prepared rare earth hydroxide suspension has good transportability, which creates the prerequisite for the smooth progress of the subsequent spraying process, effectively avoids unevenness and missing coating of the sprayed coating, and also provides a guarantee for obtaining high coating-substrate interface bonding strength.

[0035] Preferably, in step (2), the NdFeB magnet is a sintered NdFeB magnet;

[0036] Among them, based on the subsequent grain boundary thermal diffusion treatment, the present invention selects sintered NdFeB magnets instead of bonded NdFeB magnets;

[0037] Preferably, in the step (2), the degreasing is performed by ultrasonic cleaning with an acetone solution, and the ultrasonic cleaning time is 10-30 min; the pickling is performed by pickling with a nitric acid solution having a mass fraction of 2.0-5.0 wt%, and the pickling time is 10-30 s; the material for sandblasting is 80-300 mesh brown corundum, the sandblasting angle is 30-60°, and the sandblasting time is 30-60 s;

[0038] Preferably, in step (2), the surface roughness of the NdFeB magnet after pretreatment is 5-20 μm;

[0039] Among them, under the preferred surface roughness of the NdFeB magnet of the present invention, the coating-substrate bonding strength is high.

[0040] Preferably, in the step (2), before spraying, the pretreated NdFeB magnet is preheated at a temperature of 100-280° C.; during the spraying process, compressed air is used to cool the front and back surfaces of the NdFeB magnet;

[0041] Preferably, in step (2), the process parameters involved in the suspension plasma spraying method are: plasma gas Ar flow rate is 30-100 L / min; plasma gas N2 flow rate is 30-100 L / min; plasma gas H2 flow rate is 10-50 L / min; atomizing gas flow rate of rare earth hydroxide suspension is 5-20 L / min; spraying distance is 40-100 mm; spraying power is 30-120 kW; the number of spraying times for each side of the NdFeB magnet is 5-10 times; after spraying, the single-side thickness of the obtained coating is 5-25 μm;

[0042] When the rare earth hydroxide suspension is atomized, the delivery rate of the rare earth hydroxide suspension is 20-60 mL / min;

[0043] Among them, in the spraying process of the present invention, rare earth hydroxide is pyrolyzed into rare earth oxide and deposited on the surface of the NdFeB magnet. The rare earth oxide or rare earth hydroxide (due to comprehensive reasons such as the heat flow of the spray gun, incomplete pyrolysis of the rare earth hydroxide may occur) can be in a molten state or a semi-molten state under the action of the high-temperature plasma flame flow to impact the surface of the NdFeB magnet, thereby obtaining a high coating-substrate bonding strength.

[0044] More preferably, in step (2), the process parameters involved in the suspension plasma spraying method are: plasma gas Ar flow rate is 60-90 L / min; plasma gas N2 flow rate is 60-90 L / min; plasma gas H2 flow rate is 30-50 L / min; atomizing gas flow rate of rare earth hydroxide suspension is 10-20 L / min; spraying distance is 60-100 mm; spraying power is 80-120 kW;

[0045] Before spraying, the preheating temperature of the NdFeB magnet is 100-150℃;

[0046] Among them, the present invention optimizes the process parameters involved in the suspension plasma spraying method. Under the spraying process set by the present invention, the rare earth hydroxide suspension particles can be successfully pyrolyzed into rare earth oxide particles and reach a molten or semi-molten state, and the molten or semi-molten particles can be successfully adhered to the surface of the magnet, which is conducive to the next step of grain boundary diffusion treatment, so that the magnet obtains high coercivity and effectively improves the bonding force between the magnet and the coating. If the spraying power (related to the gas flow rate) is too high, there is a problem of overburning the magnet, resulting in the magnet being unusable; if the spraying power is too low, there is a problem of incomplete heating of the rare earth hydroxide suspension particles, resulting in the rare earth hydroxide being unable to be successfully pyrolyzed into rare earth oxides, and even the suspended particles cannot reach a molten state and cannot adhere to the surface of the magnet.

[0047] Preferably, in the step (2), the front side of the pretreated NdFeB magnet is first sprayed, and after spraying, when the temperature of the NdFeB magnet drops to 100-200°C, the back side of the NdFeB magnet is sprayed; the back side spraying process parameters are the same as the front side spraying process parameters;

[0048] Preferably, in step (3), the vacuum degree during the vacuum extraction is less than 1×10 -3 Pa;

[0049] Preferably, in the step (3), during the annealing treatment, the annealing temperature is 800-950°C and the holding time is 4-20 h; during the tempering treatment, the tempering temperature is 500-650°C and the holding time is 2-8 h.

[0050] More preferably, in the step (3), during the annealing treatment, the annealing temperature is 875-925°C and the holding time is 7-12 h; during the tempering treatment, the tempering temperature is 525-550°C and the holding time is 2-3 h.

[0051] The present invention optimizes the annealing and tempering process, and by setting reasonable annealing, tempering temperature and holding time, the rare earth elements sprayed on the surface of the magnet are fully diffused while there is no obvious growth of magnet grains, and the diffusion depth of the rare earth elements in the magnet is further increased, so that the coercive force of the magnet is effectively improved, and the bonding force between the substrate and the coating is further improved; if the annealing and tempering temperature is too high, not only the problem of magnet grain growth exists, but also the magnetic properties of the magnet are further deteriorated; if the annealing and tempering temperature is too low, the rare earth elements sprayed on the surface of the magnet are not sufficiently diffused, resulting in the coercive force of the magnet not being effectively improved, affecting the diffusion depth of the rare earth elements in the magnet and the bonding force between the substrate and the coating.

[0052] The invention provides an application of the method in preparing a high-bonding strength base material coating.

[0053] Preferably, the matrix material includes but is not limited to neodymium iron boron magnets.

[0054] The technical solution of the present invention achieves the following beneficial technical effects:

[0055] The present invention uses rare earth hydroxide as suspended particles to prepare a suspension, and uses rare earth oxides formed by pyrolysis of the rare earth hydroxide suspension as a diffusion source, combines the suspension plasma spraying technology and grain boundary thermal diffusion treatment, and reasonably optimizes the mixed types and molar ratios of rare earths in the suspension, the solid content of the suspension, the ball milling time, the median particle size of the diffusion source, the specific process parameters of the suspension plasma spraying, the temperature and time of the grain boundary thermal diffusion treatment, etc., so that the rare earth hydroxide is pyrolyzed into rare earth oxides and deposited on the surface of the sintered NdFeB magnet, and the diffusion depth of the heavy rare earth elements in the magnet is effectively improved, and the magnetic properties of the NdFeB magnet and the bonding force between the substrate and the coating (the maximum critical load is 122.2N) are further improved. The specific analysis is as follows.

[0056] The method of the present invention disperses rare earth hydroxide particles into a suspension medium to form a stable suspension, and injects the liquid into a plasma flame flow through a feeding system. The high temperature at the center of the plasma flame flow can thermally decompose the rare earth hydroxide into rare earth oxides during the spraying process, and then deposit them on the surface of the NdFeB magnet under the impact of the plasma flame flow to form a dense rare earth oxide coating.

[0057] The method of the present invention sprays the suspension evenly on the surface of the substrate through a plasma spray gun to prepare a high-quality coating with low porosity and high bonding strength; the material can be precisely controlled, and the composition and thickness of the coating can be precisely controlled during the spraying process; the spraying efficiency is high, and the production efficiency can be greatly improved; the diffusion depth of heavy rare earth elements in the magnet can be effectively increased, and the magnetic properties can be improved; the coating can be sprayed on different substrate materials, and the application range is very wide.

[0058] Compared with the method of directly using rare earth oxide as a diffusion source, since rare earth oxide is a downstream product of rare earth hydroxide, the present invention can save the cost of preparing rare earth oxide from rare earth hydroxide, not only simplifying the preparation process of the diffusion source, but also the diffusion source of the method of the present invention has high interface bonding strength with the magnet, solving the problem of poor coating-substrate bonding strength in coating method, chemical bath deposition method and the like, helping to promote subsequent grain boundary diffusion, helping to promote the development of the grain boundary diffusion industry, and providing new ideas for the industrial application of grain boundary diffusion technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 Scanning electron microscope image of the magnet coating obtained after the spraying step in Example 1 of the present invention;

[0060] Figure 2 Demagnetization curve of the sample corresponding to Example 3 of the present invention;

[0061] Figure 3 Scanning electron microscope image of the cross section of the magnet coating obtained after the spraying step in Example 3 of the present invention;

[0062] Figure 4 Demagnetization curve of the sample corresponding to Example 5 of the present invention.

[0063] Figure 5 Scanning electron microscope image of the cross section of the magnet coating obtained after the spraying step in Example 5 of the present invention; DETAILED DESCRIPTION

[0064] The raw materials used in the following embodiments of the present invention are:

[0065] Sintered NdFeB magnet: purchased from Jinmenghui Magnetic Materials Co., Ltd., item number Y240427, specification D24×6M; its remanence B r 11.58 kGs, coercivity H cj 15.71kOe, maximum magnetic energy product (BH) max It is 34.89MGOe.

[0066] Example 1 Preparation of heavy rare earth diffusion source by plasma spraying Dy(OH)3

[0067] This embodiment provides a method for improving the bonding strength of a NdFeB magnet coating, comprising the following steps:

[0068] (1) Preparation of rare earth hydroxide suspension:

[0069] (1-1) Ammonia water is selected as a mineralizer, and ammonia water is slowly added to a rare earth solution (DyCl3 solution) and stirred to generate a rare earth hydroxide precipitate; wherein the volume ratio of the rare earth solution (concentration of 1 mol / L) to ammonia water (concentration of 6 mol / L) is 1:1; the precipitate is filtered, washed, dried (temperature of 100°C, time of 8 h), and ground for later use;

[0070] (1-2) adding a suspension medium (deionized water and anhydrous ethanol in a volume ratio of 1:1) to the treated rare earth hydroxide precipitate, and adjusting the mass of the rare earth hydroxide precipitate to 8% of the total mass of the rare earth hydroxide precipitate and the suspension medium;

[0071] Then, a dispersant (sodium polyacrylate, the mass of which is 1.5% of the mass of the rare earth hydroxide precipitate) was added, and ball milling was performed for 0.5 h to obtain a rare earth hydroxide suspension (wherein the median particle size of the particles was 2.12 μm); then, the pH of the rare earth hydroxide suspension was adjusted to 7; a magnetic stirrer was used for stirring to obtain a stable and reliable rare earth hydroxide suspension for later use;

[0072] The absolute viscosity of the obtained rare earth hydroxide suspension is 2.7 mPa·s, and the surface tension is 39 mN / m. The suspension has good transportability.

[0073] (2) Double-sided spraying of sintered NdFeB magnets:

[0074] (2-1) Pretreatment of sintered NdFeB magnets: Degreasing, pickling and sandblasting are performed on the sintered NdFeB magnets in sequence for standby use;

[0075] Among them, acetone ultrasonic cleaning was used for degreasing, and the ultrasonic cleaning time was 30 min; the pickling was carried out by nitric acid solution with a mass fraction of 4.0 wt%, and the pickling time was 10 s; the sandblasting material was 100 mesh brown corundum, the sandblasting angle was 45°, and the sandblasting time was 30 s; the surface roughness of the NdFeB magnet after pretreatment was 9.2μm;

[0076] (2-2) using a suspension plasma spraying method, first spraying the front side of the NdFeB magnet, and then spraying the back side of the NdFeB magnet after the temperature of the NdFeB magnet drops to 120°C; the back side spraying process parameters are the same as the front side spraying process parameters, and a rare earth oxide coating with a nanostructure is obtained, and the single-side thickness of the coating is 6.6 μm;

[0077] Before spraying, the NdFeB magnet is preheated at a temperature of 100°C; after preheating, the NdFeB magnet is sprayed, and during the spraying process, compressed air is used to cool the front and back sides of the NdFeB magnet; during spraying, the rare earth hydroxide suspension obtained in step (1) is used to atomize the rare earth hydroxide suspension, and during atomization, the liquid delivery rate of the rare earth hydroxide suspension is 30 mL / min;

[0078] The process parameters involved in the suspension plasma spraying method are: plasma gas Ar flow rate is 72L / min; plasma gas N2 flow rate is 72L / min; plasma gas H2 flow rate is 36 L / min; atomizing gas flow rate of rare earth hydroxide suspension is 15 L / min; spraying distance is 100 mm; spraying power is 103kW; the number of spraying passes for both the front and back NdFeB magnets is 5;

[0079] (3) Grain boundary thermal diffusion treatment:

[0080] The sprayed magnet is placed in a tempering furnace and evacuated to a vacuum degree of 6×10 -4 Pa, first annealing treatment and then tempering treatment. Annealing temperature is 925℃, holding time is 8 h; tempering temperature is 525℃, holding time is 2 h.

[0081] Example 2 Preparation of composite diffusion source by plasma spraying of Dy(OH)3 doped with Y(OH)3

[0082] This embodiment provides a method for improving the bonding strength of a NdFeB magnet coating, comprising the following steps:

[0083] (1) Preparation of rare earth hydroxide suspension:

[0084] (1-1) Ammonia water is selected as a mineralizer, and ammonia water is slowly added to a rare earth solution (a mixed solution of DyCl3 and YCl3, with a molar ratio of Dy:Y of 7:3) and stirred to generate a rare earth hydroxide precipitate; wherein the volume ratio of the rare earth solution (total rare earth ion concentration is 1.2 mol / L) to ammonia water (concentration is 7 mol / L) is 1:1; the precipitate is filtered, washed, dried (temperature is 80°C, time is 10 h), and ground for later use;

[0085] (1-2) adding a suspension medium (deionized water and anhydrous ethanol in a volume ratio of 1:1) to the treated rare earth hydroxide precipitate, and adjusting the mass of the rare earth hydroxide precipitate to 10% of the total mass of the rare earth hydroxide precipitate and the suspension medium;

[0086] Then, a dispersant (polyacrylic acid, the mass of which is 0.5% of the mass of the rare earth hydroxide precipitate) is added, and ball milling is performed for 1.0 h to obtain a rare earth hydroxide suspension (wherein the median particle size of the particles is 1.98 μm); then the pH of the rare earth hydroxide suspension is adjusted to 7; a magnetic stirrer is used for stirring to obtain a stable and reliable rare earth hydroxide suspension for use;

[0087] The absolute viscosity of the obtained rare earth hydroxide suspension is 3.1 mPa·s, and the surface tension is 44 mN / m. The suspension has good transportability.

[0088] (2) Double-sided spraying of sintered NdFeB magnets:

[0089] (2-1) Pretreatment of sintered NdFeB magnets: Degreasing, pickling and sandblasting are performed on the sintered NdFeB magnets in sequence for standby use;

[0090] Among them, acetone ultrasonic cleaning was used for degreasing, and the ultrasonic cleaning time was 30 min; the pickling was carried out by nitric acid solution with a mass fraction of 5.0 wt%, and the pickling time was 10 s; the sandblasting material was 100 mesh brown corundum, the sandblasting angle was 45°, and the sandblasting time was 35 s; the surface roughness of the NdFeB magnet after pretreatment was 10.3μm;

[0091] (2-2) using a suspension plasma spraying method, first spraying the front side of the NdFeB magnet, and then spraying the back side of the NdFeB magnet after the temperature of the NdFeB magnet drops to 120°C; the back side spraying process parameters are the same as the front side spraying process parameters, and a rare earth oxide coating with a nanostructure is obtained, and the single-side thickness of the coating is 10.1 μm;

[0092] Before spraying, the NdFeB magnet is preheated at a temperature of 100°C; after preheating, the NdFeB magnet is sprayed, and during the spraying process, compressed air is used to cool the front and back sides of the NdFeB magnet; during spraying, the rare earth hydroxide suspension obtained in step (1) is used to atomize the rare earth hydroxide suspension, and during atomization, the liquid delivery rate of the rare earth hydroxide suspension is 30 mL / min;

[0093] The process parameters involved in the suspension plasma spraying method are: plasma gas Ar flow rate is 80L / min; plasma gas N2 flow rate is 80 L / min; plasma gas H2 flow rate is 40 L / min; atomizing gas flow rate of rare earth hydroxide suspension is 15 L / min; spraying distance is 100 mm; spraying power is 108kW; the number of spraying passes for both the front and back NdFeB magnets is 10 times;

[0094] (3) Grain boundary thermal diffusion treatment:

[0095] The sprayed magnet is placed in a tempering furnace and evacuated to a vacuum degree of 7×10 -4 Pa, annealing treatment is first performed and then tempering treatment. The annealing temperature is 900℃, and the holding time is 10 h; the tempering temperature is 525℃, and the holding time is 2 h.

[0096] Example 3 Preparation of composite diffusion source by plasma spraying of Gd and Y co-doped Dy(OH)3

[0097] This embodiment provides a method for improving the bonding strength of a NdFeB magnet coating, comprising the following steps:

[0098] (1) Preparation of rare earth hydroxide suspension:

[0099] (1-1) Ammonia water is selected as a mineralizer, and ammonia water is slowly added to a rare earth solution (a mixed solution of DyCl3, GdCl3 solution and YCl3, with a molar ratio of Dy:Gd:Y of 7:1:2) and stirred to generate a rare earth hydroxide precipitate; wherein the volume ratio of the rare earth solution (total rare earth ion concentration is 0.95 mol / L) to ammonia water (concentration is 6 mol / L) is 1:1; the precipitate is filtered, washed, dried (temperature is 90°C, time is 10 h), and ground for later use;

[0100] (1-2) adding a suspension medium (deionized water and anhydrous ethanol in a volume ratio of 1:1) to the treated rare earth hydroxide precipitate, and adjusting the mass of the rare earth hydroxide precipitate to 15% of the total mass of the rare earth hydroxide precipitate and the suspension medium;

[0101] Then, a dispersant (ammonium polyacrylate, the mass of which is 3.0% of the mass of the rare earth hydroxide precipitate) was added, and ball milling was performed for 2.0 hours to obtain a rare earth hydroxide suspension (wherein the median particle size of the particles was 1.56 μm); then, the pH of the rare earth hydroxide suspension was adjusted to 8; a magnetic stirrer was used for stirring to obtain a stable and reliable rare earth hydroxide suspension for later use;

[0102] The absolute viscosity of the obtained rare earth hydroxide suspension is 3.8 mPa·s and the surface tension is 45 mN / m. The suspension has good transportability.

[0103] (2) Double-sided spraying of sintered NdFeB magnets:

[0104] (2-1) Pretreatment of sintered NdFeB magnets: Degreasing, pickling and sandblasting are performed on the sintered NdFeB magnets in sequence for standby use;

[0105] Among them, acetone ultrasonic cleaning was used for degreasing, and the ultrasonic cleaning time was 30 min; the pickling was carried out by nitric acid solution with a mass fraction of 2.5 wt%, and the pickling time was 30 s; the sandblasting material was 80 mesh brown corundum, the sandblasting angle was 45°, and the sandblasting time was 60 s; the surface roughness of the NdFeB magnet after pretreatment was 18.6μm;

[0106] (2-2) using a suspension plasma spraying method, first spraying the front side of the NdFeB magnet, and then spraying the back side of the NdFeB magnet after the temperature of the NdFeB magnet drops to 120°C; the back side spraying process parameters are the same as the front side spraying process parameters, and a rare earth oxide coating with a nanostructure is obtained, and the single-side thickness of the coating is 20.1 μm;

[0107] Before spraying, the NdFeB magnet is preheated at a temperature of 100°C; after preheating, the NdFeB magnet is sprayed, and during the spraying process, compressed air is used to cool the front and back sides of the NdFeB magnet; during spraying, the rare earth hydroxide suspension obtained in step (1) is used to atomize the rare earth hydroxide suspension, and during atomization, the liquid delivery rate of the rare earth hydroxide suspension is 20 mL / min;

[0108] The process parameters involved in the suspension plasma spraying method are: plasma gas Ar flow rate is 70L / min; plasma gas N2 flow rate is 70 L / min; plasma gas H2 flow rate is 35 L / min; atomizing gas flow rate of rare earth hydroxide suspension is 12L / min; spraying distance is 90 mm; spraying power is 97 kW; the number of spraying passes for both the front and back NdFeB magnets is 8 times;

[0109] (3) Grain boundary thermal diffusion treatment:

[0110] The sprayed magnet is placed in a tempering furnace and evacuated to a vacuum degree of 5×10 -4 Pa, first annealing treatment and then tempering treatment. Annealing temperature is 900℃, holding time is 10 h; tempering temperature is 550℃, holding time is 2 h.

[0111] Example 4 Preparation of composite diffusion source by plasma spraying of Tb(OH)3 doped with Dy(OH)3

[0112] This embodiment provides a method for improving the bonding strength of a NdFeB magnet coating, comprising the following steps:

[0113] (1) Preparation of rare earth hydroxide suspension:

[0114] (1-1) Ammonia water is selected as a mineralizer, and ammonia water is slowly added to a rare earth solution (a mixed solution of TbCl3 and DyCl3, with a molar ratio of Tb:Dy of 8:2) and stirred to generate a rare earth hydroxide precipitate; wherein the volume ratio of the rare earth solution (total rare earth ion concentration is 0.8 mol / L) to ammonia water (concentration is 5 mol / L) is 1:1; the precipitate is filtered, washed, dried (temperature is 80°C, time is 12 h), and ground for later use;

[0115] (1-2) adding a suspension medium (deionized water and anhydrous ethanol in a volume ratio of 1:1) to the treated rare earth hydroxide precipitate, and adjusting the mass of the rare earth hydroxide precipitate to 20% of the total mass of the rare earth hydroxide precipitate and the suspension medium;

[0116] Then, a dispersant (ammonium polyacrylate, the mass of which is 0.5% of the mass of the rare earth hydroxide precipitate) was added, and ball milling was performed for 3.0 hours to obtain a rare earth hydroxide suspension (wherein the median particle size of the particles was 1.26 μm); then, the pH of the rare earth hydroxide suspension was adjusted to 8; a magnetic stirrer was used for stirring to obtain a stable and reliable rare earth hydroxide suspension for later use;

[0117] The absolute viscosity of the obtained rare earth hydroxide suspension is 4.3 mPa·s, and the surface tension is 49 mN / m. The suspension has good transportability.

[0118] (2) Double-sided spraying of sintered NdFeB magnets:

[0119] (2-1) Pretreatment of sintered NdFeB magnets: Degreasing, pickling and sandblasting are performed on the sintered NdFeB magnets in sequence for standby use;

[0120] Among them, acetone ultrasonic cleaning was used for degreasing, and the ultrasonic cleaning time was 30 min; the pickling was carried out by nitric acid solution with a mass fraction of 2.5 wt%, and the pickling time was 20 s; the sandblasting material was 150 mesh brown corundum, the sandblasting angle was 60°, and the sandblasting time was 50 s; the surface roughness of the NdFeB magnet after pretreatment was 15.3μm;

[0121] (2-2) using a suspension plasma spraying method, first spraying the front side of the NdFeB magnet, and then spraying the back side of the NdFeB magnet after the temperature of the NdFeB magnet drops to 120°C; the back side spraying process parameters are the same as the front side spraying process parameters, and a rare earth oxide coating with a nanostructure is obtained, and the single-side thickness of the coating is 11.3 μm;

[0122] Before spraying, the NdFeB magnet is preheated at a temperature of 100°C; after preheating, the NdFeB magnet is sprayed, and during the spraying process, compressed air is used to cool the front and back sides of the NdFeB magnet; during spraying, the rare earth hydroxide suspension obtained in step (1) is used to atomize the rare earth hydroxide suspension, and during atomization, the liquid delivery rate of the rare earth hydroxide suspension is 26 mL / min;

[0123] The process parameters involved in the suspension plasma spraying method are: plasma gas Ar flow rate is 60L / min; plasma gas N2 flow rate is 60 L / min; plasma gas H2 flow rate is 32 L / min; atomizing gas flow rate of rare earth hydroxide suspension is 16 L / min; spraying distance is 65 mm; spraying power is 82 kW; the number of spraying passes for both the front and back NdFeB magnets is 5;

[0124] (3) Grain boundary thermal diffusion treatment:

[0125] The sprayed magnet is placed in a tempering furnace and evacuated to a vacuum degree of 6×10 -4 Pa, first annealing treatment and then tempering treatment. Annealing temperature is 875℃, holding time is 12 h; tempering temperature is 550℃, holding time is 2 h.

[0126] Example 5 Preparation of composite diffusion source by suspension plasma spraying of Dy and Y co-doped Tb(OH)3

[0127] This embodiment provides a method for improving the bonding strength of a NdFeB magnet coating, comprising the following steps:

[0128] (1) Preparation of rare earth hydroxide suspension:

[0129] (1-1) Ammonia water is selected as a mineralizer, and ammonia water is slowly added to a rare earth solution (a mixed solution of DyCl3, YCl3 and TbCl3 solutions, with a molar ratio of Dy:Y:Tb of 2:1:7) and stirred to generate a rare earth hydroxide precipitate; wherein the volume ratio of the rare earth solution (total rare earth ion concentration is 1.5 mol / L) to ammonia water (concentration is 8 mol / L) is 1:1; the precipitate is filtered, washed, dried (temperature is 100°C, time is 8 h), and ground for later use;

[0130] (1-2) adding a suspension medium (deionized water and anhydrous ethanol in a volume ratio of 1:1) to the treated rare earth hydroxide precipitate, and adjusting the mass of the rare earth hydroxide precipitate to 25% of the total mass of the rare earth hydroxide precipitate and the suspension medium;

[0131] Then, a dispersant (ammonium polyacrylate, the mass of which is 1.0% of the mass of the rare earth hydroxide precipitate) is added, and ball milling is performed for 3.5 hours to obtain a rare earth hydroxide suspension (wherein the median particle size of the particles is 1.01 μm); then the pH of the rare earth hydroxide suspension is adjusted to 7; a magnetic stirrer is used for stirring to obtain a stable and reliable rare earth hydroxide suspension for use;

[0132] The absolute viscosity of the obtained rare earth hydroxide suspension is 4.6 mPa∙s and the surface tension is 55 mN / m. The suspension has good transportability.

[0133] (2) Double-sided spraying of sintered NdFeB magnets:

[0134] (2-1) Pretreatment of sintered NdFeB magnets: Degreasing, pickling and sandblasting are performed on the sintered NdFeB magnets in sequence for standby use;

[0135] Among them, acetone ultrasonic cleaning was used for degreasing, and the ultrasonic cleaning time was 15 minutes; the pickling was carried out by nitric acid solution with a mass fraction of 5.0 wt%, and the pickling time was 15 seconds; the sandblasting material was 120 mesh brown corundum, the sandblasting angle was 30 degrees, and the sandblasting time was 30 seconds; the surface roughness of the NdFeB magnet after pretreatment was 8.2μm;

[0136] (2-2) using a suspension plasma spraying method, first spraying the front side of the NdFeB magnet, and then spraying the back side of the NdFeB magnet after the temperature of the NdFeB magnet drops to 120°C; the back side spraying process parameters are the same as the front side spraying process parameters, and a rare earth oxide coating with a nanostructure is obtained, and the single-side thickness of the coating is 19.1 μm;

[0137] Before spraying, the NdFeB magnet is preheated at a temperature of 100°C; after preheating, the NdFeB magnet is sprayed, and during the spraying process, compressed air is used to cool the front and back sides of the NdFeB magnet; during spraying, the rare earth hydroxide suspension obtained in step (1) is used to atomize the rare earth hydroxide suspension, and during atomization, the liquid delivery rate of the rare earth hydroxide suspension is 42 mL / min;

[0138] The process parameters involved in the suspension plasma spraying method are: plasma gas Ar flow rate is 90L / min; plasma gas N2 flow rate is 90 L / min; plasma gas H2 flow rate is 46 L / min; atomizing gas flow rate of rare earth hydroxide suspension is 15 L / min; spraying distance is 75 mm; spraying power is 120kW; the number of spraying passes for both the front and back NdFeB magnets is 5;

[0139] (3) Grain boundary thermal diffusion treatment:

[0140] The sprayed magnet is placed in a tempering furnace and evacuated to a vacuum degree of 2×10 -4 Pa, annealing treatment is first performed and then tempering treatment is performed. The annealing temperature is 925℃ and the holding time is 7 h; the tempering temperature is 525℃ and the holding time is 2 h.

[0141] Comparative Example 1

[0142] This comparative example provides a method for preparing a NdFeB magnet coating, which is different from Example 5 in that the molar ratio of Dy:Y:Tb in the rare earth mixed solution of this comparative example is different.

[0143] In step (1-1) of this comparative example, DyCl3, YCl3 and TbCl3 solutions are mixed, wherein the molar ratio of Dy:Y:Tb in the mixed solution is 1:8:1;

[0144] The other steps and parameters are the same as those in Example 5.

[0145] Comparative Example 2

[0146] This comparative example provides a method for preparing a NdFeB magnet coating. The difference from Example 5 is that in this comparative example, different masses of the suspending medium are added to the treated rare earth hydroxide precipitate.

[0147] Step (1-2) of this comparative example: adding a suspension medium (deionized water and anhydrous ethanol in a volume ratio of 1:1) to the treated rare earth hydroxide precipitate, and adjusting the mass of the rare earth hydroxide precipitate to 3% of the total mass of the rare earth hydroxide precipitate and the suspension medium;

[0148] The other steps and parameters are the same as those in Example 5.

[0149] Comparative Example 3

[0150] This comparative example provides a method for preparing a NdFeB magnet coating, which differs from Example 5 in that the ball milling time and the median particle size of the particles in the obtained rare earth hydroxide suspension are different in this comparative example.

[0151] Step (1-2) of this comparative example: adding a dispersant and subjecting to ball milling for 0.2 h to obtain a rare earth hydroxide suspension (wherein the median particle size of the particles is 5.3 μm);

[0152] The other steps and parameters are the same as those in Example 5.

[0153] Comparative Example 4

[0154] This comparative example provides a method for preparing a NdFeB magnet coating, which is different from Example 5 in that the process parameters involved in the suspension plasma spraying method of this comparative example are different.

[0155] In step (2-2) of this comparative example, before spraying, the preheating temperature of the NdFeB magnet is 65°C; during atomization, the feeding rate of the rare earth hydroxide suspension is 15 mL / min;

[0156] The plasma gas Ar flow rate is 20L / min; the plasma gas N2 flow rate is 15L / min; the plasma gas H2 flow rate is 5L / min; the atomizing gas flow rate of the rare earth hydroxide suspension is 3L / min; the spraying distance is 30mm; the spraying power is 25kW; the number of spraying passes for the front and back NdFeB magnets is 10 times; the single-sided thickness of the rare earth oxide coating obtained in this comparative example is 4.2μm;

[0157] The other steps and parameters are the same as those in Example 5.

[0158] Comparative Example 5

[0159] This comparative example provides a method for preparing a NdFeB magnet coating. The difference from Example 5 is that the parameters of the grain boundary thermal diffusion treatment in this comparative example are different.

[0160] In step (3) of this comparative example, the annealing temperature is 1000°C and the holding time is 8 h; the tempering temperature is 450°C and the holding time is 2 h;

[0161] The other steps and parameters are the same as those in Example 5.

[0162] Performance testing and analysis

[0163] (1) The microstructure morphology of the magnet coating obtained after spraying in step (2) was observed using a scanning electron microscope (SEM). The results are as follows: Figure 1 , 3 shown.

[0164] in, Figure 1 This is a scanning electron microscope image of the magnet coating obtained after spraying in step (2) of Example 1. Figure 1 It can be seen that the coating is formed by a large number of flat deposits stacked layer by layer. This is because during the spraying process, the round particles in the molten or semi-molten state are deformed after impacting the magnet surface under the action of the plasma flame flow, and finally adhere to the magnet surface in a flat shape.

[0165] Figure 3 This is a scanning electron microscope image of the cross section of the magnet coating obtained after spraying in step (2) of Example 3. Figure 3 It can be seen that the coating is uniform and dense without transverse cracks, and the coating-magnet interface is well bonded and has a certain bonding strength.

[0166] Figure 5 This is a scanning electron microscope image of the cross section of the magnet coating as prepared in Example 5. Figure 5It can be seen that the coating thickness is evenly spread without pores or defects, and the coating-magnet interface is tightly bonded, which can effectively withstand external forces. At the same time, the tight interface bonding is conducive to the subsequent grain boundary diffusion.

[0167] (2) The bonding strength of the coating was tested by an automatic scratch tester for coating adhesion. The specific test conditions were as follows: loading speed 100 N / min, end load 100 N, scratch length 10 mm, and the test method was acoustic emission. The loading indenter was a diamond with a cone angle of 120° and a tip radius of R = 0.2 mm.

[0168] Among them, the critical load of the specimens corresponding to each embodiment is between 90-130 N, the coating-magnet interface bonding strength is high, and the high-strength interface bonding is conducive to subsequent grain boundary diffusion. The specific results are shown in Table 1.

[0169] (3) The magnetic properties of the samples were tested using a NIM-2000H automatic magnetic properties tester. The results are shown in Table 1. The demagnetization curves of the obtained samples (NdFeB magnets / Diffusion magnets treated according to the method of the embodiment and original magnets without any treatment) are shown in Table 1. Figure 2 , Figure 4 shown.

[0170] Table 1 Performance data of samples obtained from different embodiments and comparative examples

[0171]

[0172] In the table, “original magnet” corresponds to: sintered NdFeB magnet without any treatment.

[0173] According to Table 1, when the magnetic energy product and remanence are slightly reduced, the coercive force of the specimen corresponding to Example 5 is the highest (4.02 kOe) and the critical load is the largest (122.2 N). The specific analysis is as follows:

[0174] ① Example 5 uses a mixed rare earth solution of DyCl3, YCl3 and TbCl3, and specifically defines the molar ratio of Dy:Y:Tb, wherein Dy and Tb have a higher magnetocrystalline anisotropy field, diffuse into the interior of the magnet to form a Dy-rich, Tb-rich core-shell structure, which plays a key role in inhibiting the nucleation of reverse magnetization domains. The incorporation of Y can replace part of Dy and Tb to save costs, and the incorporation of Y can improve the grain boundary structure. Y is distributed in the Nd-rich phase in a continuous band, which makes the Nd-rich phase continuous and smooth, which is beneficial to the diffusion of Dy and Tb into the interior of the magnet and is beneficial to the improvement of coercive force.

[0175] ② In Example 5, the ball milling time is increased to 3.5 h, and the median diameter of the suspension is reduced to 1.01 μm, obtaining small-sized suspended particles. The small-sized particles have a larger heating area in the plasma flame flow, are easier to reach a molten state, and produce a higher interface bonding strength after colliding with the magnet, which makes it easier for the heavy rare earth to migrate into the interior of the magnet during grain boundary diffusion, which is beneficial to the improvement of coercive force.

[0176] ③ Example 5 adjusts the spraying process parameters to increase the coating deposition thickness, obtain a larger chemical potential gradient, further increase the diffusion driving force, promote the migration of heavy rare earth elements into the interior of the magnet, and thus obtain high coercivity.

[0177] ④ Example 5 further improves the spray gun power by adjusting the spray process parameters, increases the H2 flow rate so that the suspended particles obtain higher heat in the plasma flame, and increases the Ar and N2 flow rates so that the plasma flame has a higher impact force. The molten or semi-molten suspended particles obtain a higher coating-substrate interface bonding strength under the action of the high impact force of the plasma flame.

[0178] ⑤ Example 5 By adjusting the temperature and time of annealing and tempering in the grain boundary thermal diffusion treatment, it is ensured that elements such as Dy and Tb diffuse smoothly into the interior of the magnet, and the growth of the main phase grains of NdFeB is prevented, which causes the deterioration of the magnet performance. Too high heat treatment temperature and long heat treatment time will promote the growth of the main phase grains of NdFeB, which will deteriorate the magnet performance; too low heat treatment temperature and short heat treatment time make it difficult for elements such as Dy and Tb to diffuse into the interior of the magnet or insufficient diffusion occurs, so that the coercive force of the magnet cannot be improved.

[0179] In addition, by comparing the examples and comparative examples in Table 1, it can be seen that: ① Changing the doping ratio of elements will have a greater impact on the coercive force of the magnet. The magnetocrystalline anisotropy field of Y is low, and more Y cannot significantly improve the coercive force of the magnet after entering the interior of the magnet. Y mainly plays the role of optimizing the grain boundary phase and assisting the diffusion of Dy and Tb. ② Adjusting the mass ratio of rare earth hydroxide precipitation will change the deposition efficiency of a single spraying. If the deposition efficiency is too low and the number of spraying is small, the coating will be thinner, resulting in a small diffusion driving force and unable to diffuse deep into the magnet. ③ Changing the median particle size of the suspended particles will affect the melting effect of the suspended particles in the plasma flame flow. First, if the median particle size is large, the suspension is unstable and prone to sedimentation; secondly, larger suspended particles are difficult to melt in the plasma flame flow, which leads to low deposition efficiency and difficulty in obtaining high coating-magnet interface bonding force. ④ The spray gun power is too low, which makes it difficult for the suspended particles to reach a molten or semi-molten state. The essential reason is the same as ③. The coating-magnet interface bonding force is poor, and it is even difficult to form a coating on the magnet surface. ⑤ The heat treatment process is a key link in grain boundary diffusion. Excessive annealing temperature or long heat treatment time will cause abnormal grain growth, which makes it impossible for the grain boundary phase to effectively isolate the main phase grains, resulting in deterioration of magnetic properties.

[0180] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the claims of this patent application.

Claims

1. A method for improving the bonding strength of NdFeB magnet coating, characterized in that: The following steps are involved: (1) Preparation of rare earth hydroxide suspension: (1-1) adding ammonia water to a rare earth solution to generate rare earth hydroxide precipitate, filtering, washing, drying, grinding and setting aside; (1-2) adding a suspension medium and a dispersant to the treated rare earth hydroxide precipitate in sequence, and then performing ball milling to obtain a rare earth hydroxide suspension; then adjusting the pH of the rare earth hydroxide suspension for later use; The rare earth solution is a mixture of DyCl3 solution, YCl3 solution and TbCl3 solution; The total concentration of rare earth ions in the rare earth solution is 0.6-2 mol / L; wherein the molar ratio of Dy:Y:Tb in the rare earth solution is (2-7):1:(2-7); The ball milling treatment time is 0.5-8h; after ball milling, the median particle size of the particles in the rare earth hydroxide suspension is 0.5-4.0 μm; (2) Double-sided spraying of NdFeB magnets: (2-1) Pretreatment of NdFeB magnets: degreasing, pickling and sandblasting of NdFeB magnets are performed in sequence for standby use; (2-2) Using a suspension plasma spraying method, double-sided spraying of a NdFeB magnet is performed to obtain a rare earth oxide coating; During spraying, the rare earth hydroxide suspension obtained in step (1) is first atomized and then sprayed on the front and back surfaces of the NdFeB magnet respectively; The process parameters involved in the suspension plasma spraying method are: plasma gas Ar flow rate is 30-100 L / min; plasma gas N2 flow rate is 30-100 L / min; plasma gas H2 flow rate is 10-50 L / min; atomization gas flow rate of rare earth hydroxide suspension is 5-20 L / min; spraying distance is 40-100 mm; spraying power is 30-120 kW; when the rare earth hydroxide suspension is atomized, the liquid delivery rate of the rare earth hydroxide suspension is 20-60 mL / min; (3) Grain boundary thermal diffusion treatment: The double-sided sprayed NdFeB magnet is placed in a tempering furnace and vacuumed, and annealed first and then tempered; The vacuum degree during vacuum extraction is less than 1×10 -3 Pa; during the annealing treatment, the annealing temperature is 800-950 ° C, and the holding time is 4-20 h; during the tempering treatment, the tempering temperature is 500-650 ° C, and the holding time is 2-8 h.

2. The method according to claim 1, characterized in that The concentration of the ammonia water is 4-12 mol / L; The suspension medium is deionized water and / or anhydrous ethanol; after adding the suspension medium, the mass of the rare earth hydroxide precipitate is 5-40% of the total mass of the rare earth hydroxide precipitate and the suspension medium; The dispersant is polyacrylic acid, ammonium polyacrylate or sodium polyacrylate; the mass of the dispersant is 0.5-3.0% of the mass of the rare earth hydroxide precipitate; The drying process is carried out at a temperature of 80-120° C. and for a time of 8-12 h.

3. The method according to claim 1, characterized in that In the step (1), the suspension medium is deionized water and anhydrous ethanol in a volume ratio of 1:1; after the suspension medium is added, the mass of the rare earth hydroxide precipitate is 8-25% of the total mass of the rare earth hydroxide precipitate and the suspension medium; The ball milling treatment time is 0.5-3.5h; after ball milling, the median particle size of the particles in the rare earth hydroxide suspension is 0.5-2.5 μm; The pH value of the rare earth hydroxide suspension is adjusted to 7-10; the absolute viscosity of the obtained rare earth hydroxide suspension is 1.0-5.0 mPa·s and the surface tension is 25-60 mN / m.

4. The method according to claim 1, characterized in that In the step (2), the NdFeB magnet is a sintered NdFeB magnet; The oil removal adopts acetone ultrasound, and the ultrasonic cleaning time is 10-30 min; the pickling adopts nitric acid solution with a mass fraction of 2.0-5.0 wt% for pickling, and the pickling time is 10-30 s; the material for sandblasting is 80-300 mesh brown corundum, the sandblasting angle is 30-60°, and the sandblasting time is 30-60 s; The surface roughness of the NdFeB magnet after pretreatment is 5-20μm.

5. The method according to claim 1, characterized in that In the step (2), before spraying, the pretreated NdFeB magnet is preheated at a temperature of 100-280°C; During the spraying process, compressed air is used to cool the front and back of the NdFeB magnets; During spraying, the front side of the NdFeB magnet is sprayed first, and after spraying, when the temperature of the NdFeB magnet drops to 100-200°C, the back side of the NdFeB magnet is sprayed; the back side spraying process parameters are the same as the front side spraying process parameters.

6. The method according to claim 1, characterized in that In the step (2), the process parameters involved in the suspension plasma spraying method are: plasma gas Ar flow rate is 60-90 L / min; plasma gas N2 flow rate is 60-90 L / min; plasma gas H2 flow rate is 30-50 L / min; atomizing gas flow rate of rare earth hydroxide suspension is 10-20 L / min; spraying distance is 60-100 mm; spraying power is 80-120 kW; the number of spraying times for each side of the NdFeB magnet is 5-10 times; after spraying, the single-side thickness of the obtained coating is 5-25 μm.

7. The method according to claim 1, characterized in that In the step (3), during the annealing treatment, the annealing temperature is 875-925°C and the holding time is 7-12 h; during the tempering treatment, the tempering temperature is 525-550°C and the holding time is 2-3 h.

8. Use of the method according to any one of claims 1 to 7 in preparing a substrate material coating with high bonding strength.

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