CN115775679A

CN115775679A - Preparation method of high-performance sheet R-T-B rare earth permanent magnet

Info

Publication number: CN115775679A
Application number: CN202211556275.4A
Authority: CN
Inventors: 陈彪; 付松; 杨晓露; 马瑜琳; 章兆能; 王荣杰
Original assignee: Zhejiang Innuovo Magnetics Industry Co Ltd
Current assignee: Zhejiang Innuovo Magnetics Industry Co Ltd
Priority date: 2022-12-06
Filing date: 2022-12-06
Publication date: 2023-03-10

Abstract

The invention discloses a preparation method of a high-performance sheet R-T-B rare earth permanent magnet, which comprises the following steps: processing a magnet substrate into a magnetic sheet with the thickness of 0.5-10.0 mm, placing the magnetic sheet in a pickling solution for ultrasonic pickling for 10-180 s at room temperature, then ultrasonically cleaning the magnetic sheet for 1-5 min by using clear water, and drying the magnetic sheet, wherein the pickling solution is an aqueous solution containing 0.1-5 vol% of acidic surface passivation solution and 1.0-10 vol% of nitric acid; and depositing a heavy rare earth element diffusion layer on the surface of the magnetic sheet subjected to acid cleaning treatment, and then performing grain boundary diffusion treatment to obtain the high-performance sheet R-T-B rare earth permanent magnet. According to the invention, by controlling the pickling process, the corrosion depth of the R-rich phase of the crystal boundary on the surface layer of the magnet is controlled to be 0.1-0.4 times of the depth of the crystal boundary of the two main phases on the surface, so that the continuity of the R phase of the residual crystal boundary on the surface layer of the magnet and the crystal boundary diffusion source coating is ensured while the oxide layer on the surface of the magnet is fully removed, the crystal boundary diffusion effect of the magnet is improved, the bonding force between the crystal boundary diffusion source coating and the surface of the magnet is enhanced, and the diffusion source coating is prevented from falling off under stress. The use of low concentration acidic surface passivator can form a thin passivation layer on the surface of a magnetic sheet to prevent the magnetic sheet from sticking during diffusion aging.

Description

Preparation method of high-performance sheet R-T-B rare earth permanent magnet

Technical Field

The invention relates to a preparation method of a high-performance sheet R-T-B rare earth permanent magnet, belonging to the field of rare earth magnets.

Background

The grain boundary diffusion can obviously improve the coercive force of the R-T-B magnet under the condition of basically not reducing the remanence of the R-T-B magnet, so that the R-T-B rare earth permanent magnet with high remanence and high coercive force can be prepared by adopting the grain boundary diffusion technology. Before grain boundary diffusion, a magnet matrix needs to be processed by machining, and thus, the surface of the magnet has defects of oil stains, rusty spots, surface oxidation layers and the like. In order to ensure the bonding strength between the diffusion source coating and the matrix and the improvement effect of the magnetic property of the diffused magnet, the surface of the magnet needs to be treated, and oil stains, rusts and oxide layers on the surface of the magnet are usually removed in an acid washing mode in the actual production process.

The R-T-B rare earth permanent magnet mainly consists of a main phase R ₂ T ₁₄ B and a grain boundary R-rich phase, and the grain boundary R-rich phase has higher electronegativity and can be corroded preferentially during acid washing. After acid cleaning, the surface of the magnet has a corrosion appearance that the macroscopic corrosion depth is smaller than the corrosion depth of a grain boundary phase, so that gaps with certain width and depth exist among main phase grains. The pickling process is controlled to ensure that gaps with proper depth exist among main phase grains after pickling, so that the oxide layer on the surface of the magnet can be fully removed. In addition, the gaps with proper depth among the main phase grains can improve the bonding force between the grain boundary diffusion source coating and the magnet matrix, and prevent the grain boundary diffusion source coating from falling off from the surface of the magnet due to stress in the transfer process of the magnet. However, when the amount of corrosion is too large, the gaps between the main phase grains are deepened, which results in a weak bonding force between the main phase grains of the surface layer of the magnet and the magnet matrix, and a loose surface structure of the magnet. When the grain boundary diffusion source coating covers the loose layer, the bonding force between the magnet surface layer crystal grains and the matrix is weakened, so that the magnet surface layer crystal grains and the grain boundary diffusion source coating attached to the magnet surface layer crystal grains easily fall off together when the magnet is stressed in the transfer process. In addition, the crystal grain boundary is in melting state during diffusion processThe grain boundary R-rich phase of the state is a fast channel for atomic diffusion. When the corrosion amount of the crystal boundary R-rich phase is too large, a larger gap exists between the crystal boundary R-rich phase of the magnet and the crystal boundary diffusion source coating, so that the crystal boundary diffusion process of diffusion source atoms is hindered, and the crystal boundary diffusion efficiency is reduced. Thereby causing the diffusion source to accumulate on the surface of the magnet, inducing the generation of bulk diffusion, and finally reducing the magnetic performance of the diffusion magnet.

In the actual production process, after PVD, when carrying out high temperature grain boundary diffusion, the magnet is because surface activity is higher, and low melting point metal melts each other between the magnet and often produces the phenomenon of bonding, can cause the magnetic sheet to produce the unfilled corner owing to violent the collision in the process of part to make magnetic sheet apparent dimension variation, the product percent of pass reduces, increases the scrap rate, has increased the loss cost.

By regulating and controlling the magnet acid washing process, the R-rich phase on the surface crystal boundary of the magnet has proper corrosion depth, an oxide layer on the surface of the magnet can be fully removed, and the crystal boundary diffusion effect of the magnet is improved. In addition, the proper corrosion depth of the R-rich phase of the crystal boundary on the surface layer of the magnet can improve the bonding strength of the crystal boundary diffusion source coating and the magnet matrix, and reduce the falling-off condition of the diffusion source coating from the surface of the magnet in the transfer process of the magnet, thereby improving the consistency of the performance of final products and improving the qualification rate of products. The surface of the magnet is formed into a passive film by adding the low-concentration acidic surface passivator, so that the phenomenon of sticking when the magnet is subjected to diffusion aging treatment is prevented.

Disclosure of Invention

Aiming at the problems that when the R-T-B rare earth permanent magnet is not sufficiently pickled, the oxide layer on the surface of the magnet is not completely removed, and the bonding force between a grain boundary diffusion source coating and a matrix is weak; the invention provides a preparation method of a high-performance sheet R-T-B rare earth permanent magnet, which overcomes the defects that the consistency of product performance is low, the magnetic performance after diffusion is low and the product percent of pass is low because a diffusion source coating easily falls off along with main phase grains on the surface of a magnet although the bonding force between the diffusion source coating and the surface of the magnet is enhanced, the bonding force between the main phase grains on the surface layer of the magnet and a matrix is weakened.

The technical scheme adopted by the invention is as follows:

a preparation method of a high-performance sheet R-T-B rare earth permanent magnet comprises the following steps:

(1) Processing a magnet substrate into a magnetic sheet with the thickness of 0.5-10.0 mm, placing the magnetic sheet in a pickling solution for ultrasonic pickling for 10-180 s at room temperature, then ultrasonically cleaning the magnetic sheet for 1-5 min by using clear water, and drying the magnetic sheet, wherein the pickling solution is an aqueous solution containing 0.1-5 vol% of acidic surface passivation solution and 1.0-10 vol% of nitric acid; obtaining the magnetic sheet after acid washing treatment;

(2) Depositing a heavy rare earth element diffusion layer on the surface of the magnetic sheet after the acid cleaning treatment;

(3) And carrying out grain boundary diffusion treatment on the magnetic sheet with the surface deposited with the diffusion source to prepare the high-performance sheet R-T-B rare earth permanent magnet.

In the step (1), the acidic surface passivation solution is a stainless steel pickling passivation solution. The stainless steel pickling passivation solution is prepared by compounding an environment-friendly inorganic acid oxidant serving as a main agent, a hydroxy acid compound high-efficiency corrosion inhibitor, an antifogging agent and the like, and is prepared by adopting a commercially available stainless steel pickling passivation solution. The invention preferably adopts pickling passivation solution ID4008 or DH365H of Guangdong Kai union passivation rust prevention technology Co.

The pickling process in the step (1) is to place the magnetic sheet in pickling solution with the concentration of acid surface passivation solution of 0.1-5 vol% and the concentration of nitric acid of 1-4 vol.% for ultrasonic pickling for 100-180 s at room temperature; or placing the magnetic sheet in an acid washing solution with the concentration of acid surface passivation solution of 0.1-5 vol% and the concentration of nitric acid of 4-8 vol.% for ultrasonic acid washing for 50-100 s at room temperature; or placing the magnetic sheet in an acid solution with the acid surface passivation solution concentration of 0.1-5 vol% and the nitric acid concentration of 8-10 vol.% for ultrasonic pickling for 10-50 s at room temperature.

Ultrasonic pickling is carried out for 180s when the concentration of the acid surface passivation solution in the pickling solution is 0.1-5 vol% and the concentration of the nitric acid is 1 vol%; when the concentration of the acid surface passivation solution in the pickling solution is 0.1-5 vol% and the concentration of nitric acid is 4vol.%, carrying out ultrasonic pickling for 100s; when the concentration of the acid surface passivation solution in the pickling solution is 0.1-5 vol% and the concentration of nitric acid is 8vol.%, carrying out ultrasonic pickling for 50s; the acid surface passivation solution in the acid washing solution has the concentration of 0.1-5 vol% and the nitric acid concentration of 10vol.% for 10s.

The Vickers hardness of the front and back surfaces of the magnetic sheet after acid pickling satisfies that H1/H2 is more than or equal to 1.05 and less than or equal to 1.25 ₁ The Vickers hardness of the surface of the magnetic sheet before pickling is H ₂ The Vickers hardness of the surface of the magnetic sheet after acid pickling is shown;

in the magnetic sheet after the acid cleaning treatment, the corrosion depth of the surface crystal boundary R-rich phase is 0.1-0.4 times of the depth of two main phase crystal boundaries on the surface of the magnet.

The magnet substrate in the step (1) is prepared by vacuum induction melting and melt spinning, hydrogen breaking, jet milling, orientation forming, isostatic pressing, vacuum sintering, high-temperature primary aging and low-temperature secondary aging. The magnet matrix is prepared by a preparation method known in the art.

The magnet substrate is generally machined into a magnetic sheet with a thickness of 0.5 to 10.0 mm.

In the step (2), a heavy rare earth element diffusion layer is preferably deposited on the surface of the magnetic sheet after the acid washing treatment by adopting a vapor deposition, magnetron sputtering or multi-arc ion plating mode.

In the step (2), the thickness of the heavy rare earth element diffusion layer is preferably 3 to 100 μm.

In the step (2), the heavy rare earth diffusion source is pure heavy rare earth element metal, heavy rare earth element hydride or an alloy of the heavy rare earth element and other metal elements, and the heavy rare earth element is at least one of Dy, tb or Ho.

In the step (2), the surface of the magnet perpendicular to the orientation direction is preferably covered with the heavy rare earth diffusion source, and the surface of the magnet not perpendicular to the orientation direction is preferably not covered with the heavy rare earth diffusion source.

In the step (3), the diffusion temperature of grain boundary diffusion is 800-1000 ℃, the heat preservation time is 5-25 h, after the heat preservation is finished, the temperature is increased to 400-650 ℃ after the temperature is cooled to be below 200 ℃, and the heat preservation time is 2-10 h, so that the high-performance slice R-T-B rare earth permanent magnet is prepared.

In the step (3), the absolute vacuum degree in the furnace is 10 after the grain boundary diffusion treatment reaches the diffusion temperature ^-2 ～10 ^-5 Pa。

The invention adopts vacuum induction melting and melt spinning, hydrogen breaking, jet milling, orientation forming, isostatic pressing, vacuum sintering, high-temperature first-stage aging and low-temperature second-stage aging to prepare the base magnet. Machining the base magnet into a magnetic sheet with the thickness of 0.5-10.0 mm, placing the magnetic sheet in an aqueous solution with the concentration of acidic surface passivation solution of 0.1-5 vol% and the concentration of nitric acid of 1.0-10.0 vol% for ultrasonic pickling for 10-180 s at room temperature, and then ultrasonically cleaning the magnetic sheet for 1-5 min by using clear water and drying the magnetic sheet. Depositing a heavy rare earth element diffusion layer with the thickness of 3-100 mu m on the surface of the magnetic sheet by adopting a mode of evaporation plating, magnetron sputtering or multi-arc ion plating, and then carrying out grain boundary diffusion treatment. The diffusion temperature is 800-1000 ℃, the heat preservation time is 5-25 h, after the heat preservation is finished, the temperature is increased to 400-650 ℃ after the temperature is cooled to be below 200 ℃, and the heat preservation time is 2-10 h, so that the high-performance sheet R-T-B rare earth permanent magnet is prepared.

After the magnet is machined, more oil stains, rusty spots and oxidation layers exist on the surface of the magnet, and the surface impurities can influence the binding force between the grain boundary diffusion source coating and the magnet and can also reduce the improvement effect of grain boundary diffusion on the coercive force. Therefore, the surface of the magnet is generally treated by pickling before the diffusion source coating is deposited on the surface of the magnet. It is important to control the amount of corrosion of the magnet during pickling. When the pickling time is too short and the corrosion amount of the surface layer of the magnet is too small, although the oil stain and the rust spot on the surface of the magnet can be sufficiently removed by a visual method, the oxide layer on the surface of the magnet is difficult to be completely removed. The surface oxide layer can obstruct the grain boundary diffusion process of heavy rare earth atoms, and finally influences the magnetic performance of the magnet. Meanwhile, when the corrosion amount of the surface of the magnet is small, the surface of the magnet is smooth, and the bonding force between the grain boundary diffusion source coating and the magnet substrate is weak. In the subsequent product transferring process, the grain boundary diffusion source coating is easy to fall off from the surface of the magnet when the magnet is stressed.

The R-rich phase in the grain boundary has higher electronegativity than the main phase, and when the acid concentration is increased or the acid washing time is prolonged, the R-rich phase in the grain boundary can be corroded preferentially. After acid cleaning, the surface of the magnet has a corrosion appearance that the macroscopic corrosion depth is smaller than the corrosion depth of the crystal boundary R-rich phase, so that gaps with certain width and depth exist among main phase crystal grains. The oxide layer on the surface of the magnet can be completely removed by increasing the corrosion amount on the surface of the magnet. In addition, gaps with larger depth exist among main phase grains, so that the bonding force between the grain boundary diffusion source coating and the magnet matrix can be improved, and the falling of the grain boundary diffusion source coating from the surface of the magnet in the transfer process of the magnet can be prevented. In the actual production process, the magnets which are diffused after PVD have high surface activity, and low-melting-point metals among the magnets are mutually fused to generate the phenomenon of sticking, so that the magnetic sheets are broken due to violent collision in the separation process, and the appearance sizes of the magnetic sheets are poor. When the corrosion amount of the R-rich phase in the magnet grain boundary is further increased, the bonding force between the main phase crystal grains on the surface layer of the magnet and the matrix is weakened, so that the main phase crystal grains on the surface layer of the magnet are easy to fall off. Therefore, although the gap with a certain depth between the main phase grains is beneficial to improving the bonding force between the diffusion source coating and the surface of the magnet, the bonding force between the main phase grains on the surface layer of the magnet and the matrix is weakened, and the grain boundary diffusion source coating is easy to fall off along with the main phase grains on the surface layer when the magnet is stressed. The magnetic performance of the magnet after the grain boundary diffusion is poor, and the qualification rate is low.

In addition, the fused grain boundary R-rich phase is a rapid channel for atomic diffusion in the grain boundary diffusion process, when a gap with too large depth exists between main phase grains, the distance between the grain boundary diffusion source coating and the grain boundary R-rich phase on the surface layer of the magnet is large, the grain boundary diffusion rate can be influenced, the grain boundary diffusion sources are greatly accumulated on the surface of the magnet, the generation of body diffusion is induced, and the magnetic performance of the magnet after diffusion is reduced. Experiments show that when the corrosion depth of the R-rich phase of the grain boundary on the surface of the magnet after acid cleaning is 0.1-0.4 times of the depth of the grain boundaries of two main phases on the surface of the magnet, an oxide layer on the surface of the magnet can be fully removed, and meanwhile, the gap in the depth range between the main phase grains can enhance the binding force between the grain boundary diffusion source coating and the surface of the magnet. In addition, the residual grain boundary R-rich phase can ensure the bonding force between the main phase grains on the surface of the magnet and the matrix, and ensure that the grain boundary diffusion source coating cannot fall off along with the main phase grains on the surface of the magnet due to stress. The purpose of improving the production efficiency and the crystal boundary diffusion effect of the magnet is achieved.

In actual production, if metallographic phase or SEM is adopted to observe the corrosion depth of the grain boundary R-rich phase, the production efficiency is seriously influenced due to more complicated operation. The invention discovers the corrosion depth and magnetism of the R-rich phase of the magnet grain boundary through experimentsThere is a significant link in the vickers hardness of the body surface. When the corrosion depth of the R-rich phase of the crystal boundary on the surface of the magnet is less than 0.1 time of the depth of the two main phase crystal boundaries on the surface of the magnet, the Vickers hardness of the surface of the magnet before and after acid cleaning meets the condition that H1/H2 is more than or equal to 1.0 and less than or equal to 1.05; when the corrosion depth of the R-rich phase of the crystal boundary on the surface of the magnet is 0.1 to 0.4 times of the depth of the crystal boundary of the two main phases on the surface, the Vickers hardness of the surface of the magnet after acid cleaning meets the condition that H1/H2 is more than or equal to 1.05 and less than or equal to 1.25; when the corrosion depth of the R-rich phase of the crystal boundary on the surface of the magnet is more than 0.4 times of the depth of the crystal boundary of the two main phases on the surface, the Vickers hardness of the surface of the magnet after acid cleaning meets the requirement of H1/H2>1.25 of, wherein H ₁ Vickers hardness of magnet surface before pickling, H ₂ The Vickers hardness of the magnet surface after acid cleaning. Therefore, in the actual production process, the corrosion degree of the grain boundary R-rich phase can be represented through the surface hardness change of the magnet, so that the production efficiency is improved while the corrosion depth of the grain boundary R-rich phase is reasonably controlled.

During magnet pickling, the concentration of acid and the pickling time influence the corrosion rate of the magnet, so that the concentration of acid and the pickling time need to be regulated and controlled so as to control the corrosion depth of the R-rich phase of the surface grain boundary. The magnetic sheet is ultrasonically pickled for 100 to 180 seconds in pickling solution with the concentration of acid surface passivation solution of 0.1 to 5vol percent and the concentration of nitric acid of 1 to 4vol percent at room temperature; or placing the magnetic sheet in an acid washing solution with the concentration of acid surface passivation solution of 0.1-5 vol% and the concentration of nitric acid of 4-8 vol.% for ultrasonic acid washing for 50-100 s at room temperature; or placing the magnetic sheet in an acid washing solution with the concentration of acid surface passivation solution of 0.1-5 vol% and the concentration of nitric acid of 8-10 vol.% for ultrasonic acid washing for 10-50 s at room temperature to ensure the proper corrosion depth of the R-rich phase of the crystal boundary of the surface layer of the magnet.

In order to ensure the bonding force between the grain boundary diffusion source coating and the matrix, the invention preferably adopts the modes of evaporation plating, magnetron sputtering or multi-arc ion plating to deposit a heavy rare earth element diffusion layer with the thickness of 3-100 mu m on the surface of the magnetic sheet, wherein the heavy rare earth diffusion source is pure heavy rare earth element metal, heavy rare earth element hydride or alloy of the heavy rare earth element and other metal elements, and the heavy rare earth element is at least one of Dy, tb or Ho. The R-T-B magnet grain boundary diffusion has anisotropy, namely the diffusion rate along the magnet orientation direction is high, and the diffusion rate along the grain boundary perpendicular to the magnet orientation direction is low. In the present invention, in order to ensure the coercive force increment of the magnet after grain boundary diffusion, the surface perpendicular to the orientation direction of the magnet must be coated with a grain boundary diffusion source. Although the coercive force of the magnet can be improved by coating other surfaces of the magnet with the grain boundary diffusion source, the grain boundary diffusion rate in the direction is slow, so that the generation of bulk diffusion is induced, and the reduction of the remanence of the magnet is increased. Therefore, it is preferable in the present invention that no grain boundary diffusion source layer is deposited on the other surface of the magnet.

The invention has the beneficial effects that: by controlling the pickling process, the corrosion depth of the R-rich phase of the crystal boundary on the surface layer of the magnet is controlled to be 0.1-0.4 times of the depth of the crystal boundary of the two main phases on the surface, so that oil stains, rusty spots and an oxide layer on the surface of the magnet can be completely removed after machining, and meanwhile, the continuity of the residual crystal boundary R phase on the surface layer of the magnet and a crystal boundary diffusion source coating is ensured, thereby improving the crystal boundary diffusion effect of the magnet. After acid cleaning, corrosion gaps with proper depth between main phase grains on the surface of the magnet can enhance the binding force between the grain boundary diffusion source coating and the magnet, ensure that the grain boundary diffusion source coating cannot fall off from the surface of the magnet due to stress in the transfer process of the magnet, and obtain a product with high performance, high consistency and high qualified rate. In addition, by establishing the relationship between the Vickers hardness of the surface of the magnet and the corrosion depth of the crystal boundary R-rich phase, the corrosion depth of the crystal boundary R-rich phase on the surface layer of the magnet can be represented by the change of the surface hardness of the magnet before and after pickling, the testing process is simplified, and the production efficiency is obviously improved. The surface of the magnet is formed into a passive film by adding the low-concentration acidic surface passivator, so that the phenomenon of sticking when the magnet is subjected to high-temperature grain boundary diffusion and aging treatment is prevented.

Drawings

FIG. 1 shows SEM secondary electron images of the surfaces of different magnets after acid pickling in example I, wherein (a), (b), and (c) are SEM secondary electron images of the surfaces of magnet bodies after acid pickling in experiment Nos. 1, 2, and 4, respectively.

FIG. 2 is a schematic diagram of the bonding condition of the grain boundary diffusion source coating and the matrix when the grain boundary R-rich phase on the surface of the magnet has different corrosion depths, wherein (a), (b) and (c) are respectively schematic diagrams of the bonding condition of the grain boundary diffusion source coating and the matrix when the corrosion depth of the grain boundary R-rich phase on the surface of the magnet is lower than the recommended range of the invention, in the recommended range of the invention and higher than the recommended range of the invention.

FIG. 3 shows SEM secondary electron images of the surfaces of different magnets after acid pickling in example II, wherein (a), (b), and (c) are SEM secondary electron images of the surfaces of magnet bodies after acid pickling in Experimental Nos. 10 to 12, respectively.

FIG. 4 shows SEM secondary electron images of the pickled surfaces of different magnets in the fourth example, wherein (a), (b) and (c) are SEM secondary electron images of the pickled surfaces of the magnets in Experimental Nos. 16 to 18, respectively.

Detailed Description

The technical solution of the present invention is further described with reference to the following specific examples, but the scope of the present invention is not limited thereto.

The base magnet is prepared by adopting vacuum induction melting and melt spinning, hydrogen breaking, jet milling, orientation forming, isostatic pressing, vacuum sintering, high-temperature first-stage aging and low-temperature second-stage aging.

Machining a magnet substrate into a magnetic sheet with the thickness of 0.5-10.0 mm, placing the magnetic sheet in a pickling solution for ultrasonic pickling for 10-180 s at room temperature, then ultrasonically cleaning the magnetic sheet for 1-5 min by using clear water, and drying the magnetic sheet, wherein the pickling solution is an aqueous solution added with 0.1-5 vol% of acidic surface passivation solution and 1.0-10 vol% of nitric acid.

Depositing a heavy rare earth element diffusion layer with the thickness of 3-100 mu m on the surface of the magnetic sheet by adopting a vapor deposition, magnetron sputtering or multi-arc ion plating mode. The heavy rare earth diffusion source is pure heavy rare earth metal, heavy rare earth hydride or alloy of the heavy rare earth and other metal elements, and the heavy rare earth element is at least one of Dy, tb or Ho. The surface of the magnet perpendicular to the orientation direction must deposit the heavy rare earth element grain boundary diffusion source, and the other surface of the magnet is preferably not deposited with the heavy rare earth element grain boundary diffusion source.

Heating the magnet deposited with the diffusion source coating to 800-1000 ℃ to carry out grain boundary diffusion treatment. The vacuum degree in the furnace is higher than 10 after the highest temperature is reached ^-2 Pa, and keeping the temperature for 5-25 h. And cooling to below 200 ℃ after heat preservation, then heating to 400-650 ℃, and preserving heat for 2-10 h to obtain the high-performance sheet R-T-B rare earth permanent magnet.

And (3) carrying out surface treatment on the final magnet by adopting a sand blasting process, removing the residual diffusion source coating on the surface, measuring the magnetic property of the magnet by adopting NIM magnetic property testing equipment, and measuring the components of the magnet by adopting ICP. And measuring the Vickers hardness of the surface of the magnet before and after pickling by using a Vickers hardness meter, observing the surface corrosion morphology of the magnet by using an SEM (scanning electron microscope), and measuring the corrosion depth of the crystal boundary R-rich phase on the surface of the magnet by using a laser confocal microscope. And testing the bonding force of the crystal boundary diffusion source coating and the magnet matrix by adopting a Baige experiment, testing the bonding force of 20 products in each experimental group, and calculating the qualification rate by taking the falling area of the diffusion source coating less than 5% as a qualification standard.

The first embodiment is as follows:

component Nd ₂₃ Pr ₇ B _0.95 Ga _0.15 Cu _0.15 Zr _0.2 Fe _68.55 After proportioning, preparing the base magnet by adopting vacuum induction melting and melt spinning, hydrogen breaking, jet milling, orientation forming, isostatic pressing, vacuum sintering, high-temperature first-stage aging and low-temperature second-stage aging.

The magnet substrate was machined into magnet pieces of length × width × height 10mm × 10mm × 3mm, with the height direction being the magnet orientation direction. Placing the magnetic sheet in pickling solution added with different proportions of acidic surface passivation solution and nitric acid at room temperature for ultrasonic pickling for 80s, then ultrasonically cleaning the magnetic sheet for 1min by using clear water, and drying the magnetic sheet. The adopted acidic surface passivation solution is pickling passivation solution ID4008 produced by Guangdong Kai union passivation and rust prevention technology Co.

And a pure Tb diffusion source layer with the thickness of 10 mu m is deposited on the surface of the magnetic sheet vertical to the orientation direction by adopting a multi-arc ion plating mode, and heavy rare earth element diffusion sources are not deposited on other surfaces.

And heating the magnet with the diffusion source coating deposited to 910 ℃ to perform high-temperature grain boundary diffusion. The absolute vacuum degree in the furnace after the highest temperature is reached is 10 ^-2 Pa～10 ^-5 Pa, and keeping the temperature for 10h. Cooling at a cooling speed of not less than 80 ℃/min after the heat preservation is finished until the temperature of the magnet is lower than 200 ℃. The magnet was then heated to 520 ℃ and incubated for 3h. Cooling to below 200 ℃ at a cooling speed of not less than 80 ℃/min after finishing the heat preservation.

And (3) carrying out surface treatment on the final magnet by adopting a sand blasting process, removing the residual diffusion source coating on the surface, measuring the magnetic property of the magnet by adopting NIM magnetic property testing equipment, and measuring the components of the magnet by adopting ICP. And measuring the Vickers hardness of the surface of the magnet before and after pickling by using a Vickers hardness meter, observing the surface corrosion morphology of the magnet by using an SEM (scanning electron microscope), and measuring the corrosion depth of the crystal boundary R-rich phase on the surface of the magnet by using a laser confocal microscope. And measuring the roughness of the micro-area on the surface of the material by using a laser confocal microscope, and obtaining the depth of the two main phase grain boundaries from the roughness detection. And testing the bonding force of the crystal boundary diffusion source coating and the magnet matrix by adopting a Baige experiment, testing the bonding force of 20 products in each experimental group, and calculating the qualification rate by taking the falling area of the diffusion source coating less than 5% as a qualification standard.

The nitric acid concentration, the volume concentration of the acid-washed surface passivating agent, the final magnet Tb content, and the magnet room temperature (20 ℃) magnetic properties of the acid-washed samples of experiment Nos. 1 to 9 are shown in Table 1:

TABLE 1

From the data in table 1, it can be seen that when the concentration of nitric acid and the volume concentration of the acid-washing surface passivating agent are within the recommended range of the present invention (0.1-5 vol% of the acidic surface passivating solution and 1.0-10 vol.% of nitric acid), the Tb content of the magnet after grain boundary diffusion is high, and the remanence and the coercive force are both at high levels. When the concentration of nitric acid and the volume concentration of the acid-washing surface passivator are outside the recommended range, the magnetic performance of the magnet is poor due to the influence on the grain boundary diffusion effect caused by incomplete removal of the oxide layer on the surface of the magnet, too large corrosion depth of the grain boundary R-rich phase or thicker thickness of the passivated film on the surface of the magnetic sheet.

As can be seen from surface SEM secondary electron images of the magnet after acid washing of the experiment Nos. 1, 2 and 4, when the acid washing time of the magnet is fixed to 80s and the acid concentration is lower than the recommended range (4-8 vol% is recommended for the nitric acid concentration) of the invention (experiment No. 1), the corrosion amount of the surface of the magnet is small; when the acid concentration is higher than the recommended range of the invention, the corrosion of the magnet surface is more serious, not only the crystal boundary R-rich phase has larger corrosion depth, but also the main phase crystal grains on the magnet surface are also seriously corroded; when the acid concentration is in the recommended range of the invention, the R-rich phase of the magnet grain boundary has a relatively proper corrosion depth, and the main phase grains on the surface layer of the magnet are not obviously corroded.

In addition, it can be seen from experiments No.2, no.6, no.7, no.8 and No.9 that when the content of the acidic surface-passivating solution is within the range recommended in the present invention (0.1 to 5vol% of the acidic surface-passivating solution), although the diffusion effect of the magnetic sheet is slightly lowered with respect to the case where the acidic surface-passivating solution is not added, the influence on the magnetic properties of the magnetic sheet is small; however, compared with experiment No.6 without the acidic surface passivation solution, the magnetic sheets in other experiment groups do not generate the sticking phenomenon after the diffusion aging, while the magnetic sheets in experiment No.6 group generate the serious sticking phenomenon after the diffusion aging, which is not beneficial to the separation of the magnetic sheets and generates a large amount of waste products. However, when the content of the acidic surface passivation solution exceeds the recommended range of the invention, as shown in experiment No.9, the concentration of the surface passivation solution is 10%, and the range is exceeded, the experiment result shows that the surface grain passivation film of the magnetic sheet is thicker, the diffusion effect of the magnet is obviously reduced, and the performance of the magnet is greatly reduced.

The corrosion depth of the crystal boundary R-rich phase on the surface of the magnet is tested by adopting a laser confocal microscope, the hardness of the magnet is tested by adopting a Vickers hardness tester, the Vickers hardness change of the surface of the magnet before and after the acid washing of the experiment No. 1-experiment No.9 and the corrosion depth of the crystal boundary R-rich phase on the surface of the magnet after the acid washing are shown in the table 2:

TABLE 2

The method measures the average size of the main phase grains on the surface layer of the magnet, and takes the average size of the main phase grains on the surface layer of the magnet as the depth of the grain boundary of two main phases on the surface. As can be seen from the data in Table 2, when the depth of corrosion of the R-rich phase on the surface grain boundary of the magnet is 0.1 to 0.4 times the depth of the two main phase grain boundaries on the surface, the R-rich phase on the grain boundary between the main phase grains is corroded to a certain extent, the bonding force between the main phase grains is weakened, so the Vickers hardness of the surface of the magnet is reduced to a certain extent, and the Vickers hardness ratio H1/H2 of the surface of the magnet before and after pickling is 1.05 to 1.25. When the corrosion depth of the R-rich phase of the crystal boundary on the surface of the magnet is less than 0.1 time of the depth of the crystal boundary of the two main phases on the surface, the Vickers hardness of the surface of the magnet does not change greatly because the corrosion amount of the R-rich phase of the crystal boundary is less, and the Vickers hardness ratio of the surface of the magnet before and after acid cleaning is H1/H2<1.05. When the corrosion amount of the R-rich phase of the crystal boundary on the surface of the magnet is further deepened, the bonding force among main phase crystal grains on the surface layer of the magnet is further weakened, the magnet is more easily cracked when stressed, the Vickers hardness of the surface is further reduced, and the Vickers hardness H1/H2 of the surface of the magnet before and after acid cleaning is more than 1.25. In addition, when the content of the pickling passivation solution exceeds the recommended range of the present invention (experiment No. 9), although the corrosion depth of the surface of the magnetic sheet is reduced, the thickness of the grain passivation film on the surface of the magnetic sheet is thick, and the pickling causes the loss of the R-rich phase among grains, so that the bonding force among the surface grains is reduced, thereby reducing the hardness value of the magnet.

It can be seen that the corrosion depth of the grain boundary R-rich phase on the surface of the magnet has a large correlation with the Vickers hardness of the surface of the magnet, so that the acid washing degree of the magnet can be represented by the change of the Vickers hardness of the surface of the magnet before and after acid washing.

The results of the percent pass experiments of the magnets of experiment No.1, experiment No.2, experiment No.4 and experiment No.5 after acid washing and deposition of the grain boundary diffusion source coating are shown in Table 3:

TABLE 3

Experiment No.	Percent of pass
		1	92.3％
2	99.5％
		4	95.8％
5	89.2％

As can be seen from the data in Table 3, when the corrosion depth of the R-rich phase of the grain boundary on the surface of the magnet exceeds the recommended range of the invention, the bonding force between the grain boundary diffusion source coating on the surface of the magnet and the matrix is reduced, which is reflected in that the qualification rate of defective products in the Baige experiment is reduced.

The schematic diagram of the bonding condition of the grain boundary diffusion source and the matrix when the corrosion depth of the grain boundary R-rich phase on the surface of the magnet is lower than the recommended range of the invention, is in the recommended range of the invention and is higher than the recommended range of the invention is shown in fig. 2, and fig. 2 shows that when the corrosion depth of the grain boundary R-rich phase on the surface of the magnet is smaller (< 0.1 time of the depth of the grain boundary of the two main phases on the surface), the surface of the magnet is smoother, and at the moment, the grain boundary diffusion source coating can only be attached to the surface of the magnet and has smaller bonding force with the matrix, so the grain boundary diffusion source coating is easy to fall off from the surface of the magnet when a hundred-grid experiment or the magnet is stressed. When the R-rich phase of the grain boundary on the surface of the magnet has proper corrosion depth (0.1-0.4 times of the depth of the grain boundary of the two main phases on the surface), part of the grain boundary diffusion source coating can enter the gaps of the main phase grains, and the bonding force between the grain boundary diffusion source and the surface of the magnet is enhanced, so that the diffusion source coating is not easy to fall off. And when the corrosion depth of the R-rich phase of the crystal boundary of the surface layer of the magnet is too large (> 0.4 times of the depth of the crystal boundary of the two main phases of the surface), although the bonding force between the crystal grain boundary diffusion source coating and the surface of the magnet is enhanced, the bonding force between the crystal grain of the main phase of the surface layer of the magnet and the matrix of the magnet is weakened, so that the crystal grain of the main phase of the surface layer of the magnet is easy to fall off from the matrix of the magnet, and the crystal grain boundary diffusion source coating attached to the surface layer of the magnet also falls off.

The corrosion depth of the R-rich phase of the crystal boundary on the surface of the magnet also has obvious influence on the diffusion effect of the crystal boundary, and when the corrosion depth of the R-rich phase of the crystal boundary on the surface layer of the magnet is less than 0.1 time of the depth of the crystal boundary of the two main phases on the surface, because the corrosion amount of the surface of the magnet is too small, a surface oxidation layer cannot be removed completely. The oxide layer on the surface of the magnet, especially the oxide which exists in the R-rich phase of the magnet grain boundary, can play a strong role in hindering the grain boundary diffusion, and reduces the grain boundary diffusion efficiency. The bulk accumulation of heavy rare earth elements on the surface of the magnet induces bulk diffusion, so that the coercivity increment of the final magnet is reduced, and the remanence decrement is increased. When the corrosion depth of the R-rich phase of the surface grain boundary of the magnet is more than 0.4 times of the depth of the two main phase grain boundaries on the surface, deeper gaps exist between the main phase grains. At this time, the distance between the grain boundary diffusion source coating and the residual grain boundary R-rich phase of the magnet is large, as shown in FIG. 2 (c). The discontinuity of the grain boundary diffusion source coating and the R-rich phase of the magnet grain boundary can also obstruct the atomic diffusion, reduce the grain boundary diffusion effect, induce the generation of body diffusion and further reduce the magnetic performance of the magnet. When the corrosion depth of the R-rich phase of the crystal boundary on the surface layer of the magnet is 0.1-0.4 times of the depth of the crystal boundary of the two main phases on the surface, not only can an oxide layer on the surface of the magnet be fully removed, but also the continuity of the R-rich phase of the crystal boundary and a crystal boundary diffusion source coating can be ensured, so that the magnetic performance of the magnet after the crystal boundary diffusion is ensured.

According to the invention, through regulating and controlling the pickling process, the corrosion depth of the R-rich phase of the crystal boundary on the surface layer of the magnet is controlled to be 0.1-0.4 times of the depth of the crystal boundary of the two main phases on the surface, so that the oxide layer on the surface of the magnet can be fully removed, and the continuity of the residual crystal boundary R phase and the crystal boundary diffusion source coating on the surface layer of the magnet can be ensured, thereby improving the crystal boundary diffusion effect of the magnet. In addition, the proper corrosion depth of the R-rich phase of the crystal boundary of the surface layer of the magnet can enhance the bonding force between the crystal boundary diffusion source coating and the surface of the magnet, and the phenomenon that the consistency of product performance is poor and the qualified rate is reduced because the crystal boundary diffusion source coating falls off from the surface of the magnet when the magnet is stressed is prevented. Meanwhile, the invention discovers that the correlation between the corrosion depth of the R-rich phase of the crystal boundary of the surface of the magnet and the Vickers hardness of the surface of the magnet is larger during acid cleaning. Therefore, the corrosion depth of the grain boundary R-rich phase can be represented by testing the Vickers hardness change of the surface of the magnet before and after pickling, thereby simplifying the testing process and improving the production efficiency. In addition, as the acid concentration increases, the corrosion rate of the magnet increases, and the difficulty in controlling the corrosion depth of the grain boundary R-rich phase increases, so that the acid pickling solution in the present invention is suitable in a nitric acid concentration of 1.0 to 10.0vol.%.

Example two:

The magnet substrate was machined into magnet pieces of length × width × height 10mm × 10mm × 3mm, with the height direction being the magnet orientation direction. The magnetic sheet is placed in an aqueous solution added with 1.0vol% of acid-washing surface passivator and 5.0vol.% of nitric acid for ultrasonic acid washing for different times at room temperature, and then is ultrasonically washed by clean water for 1min and then is dried. The adopted acidic surface passivation solution is pickling passivation solution ID4008 produced by Guangdong Kai union passivation and rust prevention technology Co.

And (3) carrying out surface treatment on the final magnet by adopting a sand blasting process, removing the residual diffusion source coating on the surface, then measuring the magnetic property of the magnet by adopting NIM magnetic property testing equipment, and measuring the components of the magnet by adopting ICP. The Vickers hardness of the surface of the magnet before and after pickling is measured by adopting a Vickers hardness meter, the corrosion morphology of the surface of the magnet is observed by adopting an SEM, and the corrosion depth of the crystal boundary R-rich phase on the surface of the magnet is measured by adopting a laser confocal microscope. And testing the bonding force of the crystal boundary diffusion source coating and the magnet matrix by adopting a Baige experiment, testing the bonding force of 20 products in each experimental group, and calculating the qualification rate by taking the falling area of the diffusion source coating less than 5% as a qualification standard.

The acid washing time, the final Tb content of the magnet, and the magnetic properties at room temperature (20 ℃ C.) of the magnet of experiment Nos. 10 to 12 are shown in Table 4:

TABLE 4

Experiment No.	Acid washing time(s)	Tb content (wt.%)	Br(kGs)	Hcj(kOe)
					10	20	0.22	14.25	20.8
11	80	0.25	14.33	22.1
					12	200	0.21	14.19	20.7

As can be seen from the data in Table 4, for experiment No.11, when the pickling process was within the recommended range (50-100 s) of the present invention, the Tb content of the magnet after grain boundary diffusion was high, and both the remanence and the coercive force were at high levels. When the pickling process exceeds the recommended range of the invention, the magnet has lower magnetic performance due to incomplete removal of the oxide layer on the surface of the magnet (experiment No. 10) or too great corrosion depth of the R-rich phase in the grain boundary (experiment No. 12), which affects the diffusion effect of the grain boundary.

The SEM secondary electron images of the surfaces of the magnet bodies of experiment Nos. 10 to 12 after acid washing are shown in FIGS. 3 (a) to (c). When the concentration of nitric acid is 5vol% and the corresponding pickling time is lower than the recommended range of the invention, the corrosion amount of the surface layer of the magnet is less; when the pickling time is longer than the recommended range of the invention, the surface of the magnet is seriously corroded, not only the grain boundary R-rich phase has larger corrosion depth, but also the main phase crystal grains on the surface of the magnet are seriously corroded; when the pickling time is within the recommended range of the invention, the R-rich phase of the magnet surface grain boundary has a relatively proper corrosion depth, and the main phase grains of the magnet surface are not obviously corroded.

The vickers hardness changes of the surfaces before and after the magnet is pickled, the corrosion depth of the grain boundary R-rich phase on the surface of the magnet after pickling, and the pass rates of the baige experiment are shown in table 5.

TABLE 5

When the pickling time is within the recommended range of the invention, the corrosion depth of the R-rich phase of the crystal boundary on the surface of the magnet is 0.1-0.4 times of the depth of the crystal boundary of the two main phases on the surface, and the Vickers hardness ratio H1/H2 of the surface of the magnet before and after pickling is 1.05-1.25. At the moment, the bonding force between the grain boundary diffusion source coating and the surface of the magnet can be enhanced while the oxide layer on the surface of the magnet can be fully removed, and the defect of low product percent of pass caused by the fact that the grain boundary diffusion source coating falls off from the surface of the magnet when the magnet is stressed is avoided.

In this embodiment, when the etching time is short and the etching depth of the R-rich phase in the surface layer grain boundary of the magnet is less than 0.1 times the depth of the two main phase grain boundaries on the surface, the surface oxide layer is not removed completely because the etching amount of the surface layer of the magnet is too small. The surface oxide layer can play a strong role in hindering the grain boundary diffusion, and the grain boundary diffusion efficiency of the magnet is reduced. The heavy rare earth elements are accumulated on the surface of the magnet in a large amount to induce the diffusion of the magnet, so that the coercive force increment of the final magnet is reduced, and the remanence decline is increased. When the corrosion depth of the R-rich phase of the surface grain boundary of the magnet is more than 0.4 times of the depth of the two main phase grain boundaries on the surface, deeper gaps exist between the main phase grains. At this time, a larger gap exists between the grain boundary diffusion source coating and the residual grain boundary R-rich phase of the magnet. The discontinuity of the grain boundary diffusion source coating and the R-rich interphase of the magnet grain boundary can also hinder the diffusion of atoms, reduce the diffusion effect of the grain boundary, induce the generation of body diffusion and further reduce the magnetic performance of the magnet. When the corrosion depth of the R-rich phase of the crystal boundary on the surface layer of the magnet is 0.1-0.4 times of the depth of the crystal boundary of the two main phases on the surface, not only can an oxide layer on the surface of the magnet be fully removed and the bonding force between the diffusion source coating and the surface of the magnet be ensured, but also the continuity between the residual crystal boundary R-rich phase of the surface layer of the magnet and the crystal boundary diffusion source coating can be ensured, thereby ensuring the magnetic performance of the magnet after crystal boundary diffusion.

In the invention, in order to more accurately control the corrosion depth of the grain boundary R-rich phase, the concentration of the nitric acid is 1.0-10.0 vol.%. And the corrosion amount of the grain boundary R-rich phase is correspondingly increased along with the prolonging of the corrosion time. Therefore, the corrosion time needs to be reasonably regulated and controlled according to different acid concentrations. In the invention, when the acid concentration is 1-4 vol.%, the magnet acid washing time is 100-180 s at room temperature; when the acid concentration is 4-8 vol.%, the magnet acid washing time is 50-100 s at room temperature; when the acid concentration is 8-10 vol.%, the acid washing time of the magnet is 10-50 s at room temperature, so that the proper corrosion depth of the R-rich phase of the surface grain boundary of the magnet is ensured.

Example three:

component Nd _22.5 Pr _7.5 Dy _1.0 B _0.95 Al _0.5 Cu _0.15 Ga _0.15 Zr _0.1 Fe _67.15 After proportioning, adopting vacuum induction melting melt spinning, hydrogen breaking, jet milling, orientation forming, isostatic pressing, vacuum sintering, high-temperature first-stage aging and low-temperature second-stage aging to prepareA base magnet.

The magnet substrate was machined into magnet pieces of length × width × height 10mm × 10mm × 3mm, with the height direction being the magnet orientation direction. And (3) placing the magnetic sheet in an aqueous solution added with 1.0vol% of acid-washing surface passivator and 5.0vol.% of nitric acid for ultrasonic acid washing for 80s at room temperature, then ultrasonically washing the magnetic sheet for 1min by using clear water, and then drying the magnetic sheet. The adopted acidic surface passivation solution is an acid pickling passivation solution DH365H produced by Guangdong Kai union passivation and rust prevention technology Co.

Experiment No.13 adopts the mode of multi-arc ion plating to deposit pure Tb diffusion source layers with the thickness of 10 mu m on two surfaces of the magnet vertical to the orientation direction, and no heavy rare earth element diffusion source is deposited on other surfaces. Experiment No.14 the magnet was coated with multi-arc ion to deposit a pure Tb diffusion source layer with a thickness of 10 μm on all surfaces of the magnet.

And heating the magnet with the diffusion source coating deposited to 890 ℃ to perform high-temperature grain boundary diffusion. The absolute vacuum degree in the furnace after the highest temperature is reached is 10 ^-2 Pa～10 ^-5 Pa, and keeping the temperature for 10h. Cooling at a cooling speed of not less than 80 ℃/min after the heat preservation is finished until the temperature of the magnet is lower than 200 ℃. The magnet was then heated to 510 ℃ and incubated for 3h. Cooling to below 200 ℃ at a cooling speed of not less than 80 ℃/min after finishing the heat preservation.

And (3) carrying out surface treatment on the final magnet by adopting a sand blasting process, removing the residual diffusion source coating on the surface, measuring the magnetic property of the magnet by adopting NIM magnetic property testing equipment, and measuring the components of the magnet by adopting ICP.

The final magnet compositions and magnetic properties of experiment No.13 and experiment No.14 are shown in Table 6.

TABLE 6

Experiment No.	Tb content (wt.%)	Br(kGs)	Hcj(kOe)
				13	0.23	14.2	21.7
14	0.31	13.4	22.3

The R-T-B magnet has strong anisotropy in grain boundary diffusion, and the grain boundary diffusion rate along the orientation direction is greater than the grain boundary diffusion rate perpendicular to the orientation direction. In the invention, the experimental No.13 magnet deposits the grain boundary diffusion source on two surfaces vertical to the orientation direction, and the removal of the oxide layer on the surface of the magnet by combining the pickling process can ensure that the magnet has higher coercive force increment after the grain boundary diffusion, and meanwhile, the decline amount of remanence after the diffusion is lower. When the surface of the magnet parallel to the orientation direction is also deposited with a grain boundary diffusion source, because the grain boundary diffusion rate along the direction vertical to the orientation direction of the magnet is low, although the oxide layer on the surface of the magnet is sufficiently removed after acid cleaning, the low grain boundary diffusion rate causes heavy rare earth elements to be accumulated on the surface of the magnet, and the body diffusion is promoted. Therefore, the increase in coercive force of the magnet after diffusion in the magnet of experiment No.14 is higher than that in experiment No.13, but the decrease in remanence is significantly increased. Therefore, in the present invention, it is preferable that both surfaces perpendicular to the magnet orientation direction need to be coated with the grain boundary diffusion source coating, while the surfaces parallel to the magnet orientation direction do not deposit the grain boundary diffusion source.

Example four:

component Nd _22.5 Pr _7.5 Dy _1.0 B _0.95 Al _0.5 Cu _0.15 Ga _0.15 Zr _0.1 Fe _67.15 (mass ratio) mixingAnd preparing a base magnet by adopting vacuum induction melting and melt spinning, hydrogen breaking, jet milling, orientation forming, isostatic pressing, vacuum sintering, high-temperature first-stage aging and low-temperature second-stage aging after the materials are fed.

The magnet substrate was machined into magnet pieces of length × width × height 10mm × 10mm × 3mm, with the height direction being the magnet orientation direction. The magnetic sheet is placed in an aqueous solution added with 1.0vol% of acid-washing surface passivator and 5.0 vol% of different acids for ultrasonic acid washing for 80s at room temperature, and then cleaned with clear water for 1min and dried. The adopted acidic surface passivation solution is pickling passivation solution ID4008 produced by Guangdong Kai union passivation and rust prevention technology Co.

And (3) carrying out surface treatment on the final magnet by adopting a sand blasting process, removing the residual diffusion source coating on the surface, then measuring the magnetic property of the magnet by adopting NIM magnetic property testing equipment, measuring the components of the magnet by adopting ICP (inductively coupled plasma), and observing the surface appearance of the magnet by adopting SEM (scanning electron microscope).

The acid types, final magnet Tb contents, and magnet room temperature (20 ℃ C.) magnetic properties of experiment Nos. 15 to 18 are shown in Table 7:

TABLE 7

Experiment No.	Acid type	Tb content (wt.%)	Br(kGs)	Hcj(kOe)
					15	Nitric acid	0.24	14.3	21.9
16	Hydrochloric acid	0.23	14.1	19.4
					17	Sulfuric acid	0.23	14.1	19.8
18	Oxalic acid	0.19	14.2	20.0

As can be seen from the data in Table 7, there is a large difference in the magnetic properties of the final magnet after the magnet surface was acid-washed with different kinds of acids. When nitric acid is used as the pickling solution, the coercive force increment of the magnet after grain boundary diffusion is large, and the remanence is also at a higher level. When hydrochloric acid, sulfuric acid and oxalic acid are used as pickling solution, the coercive force increment of the magnet after grain boundary diffusion is low, and the remanence is also obviously reduced.

The weight loss of the magnet after 5 minutes of acid washing in different acid solutions was tested and the results are shown in table 8.

TABLE 8

Experiment No.	Weight loss mg/cm ²
		15	0.105
16	0.317
		17	0.325
18	0.051

As is clear from the data in Table 8, when sulfuric acid or hydrochloric acid was used as the pickling solution, the magnet etching rate was about 3 times that when nitric acid was used. This results in the magnet having grain boundary R-rich phase corroding too fast and the process is difficult to control when sulfuric acid or hydrochloric acid is used as the pickling solution. The amount of residual grain boundary R-rich phase on the surface of the magnet after corrosion is less, and the distance between the grain boundary diffusion source coating and the residual grain boundary R-rich phase on the surface of the magnet is larger. The grain boundary diffusion effect of heavy rare earth atoms is hindered, and the body diffusion is promoted, so that the coercive force increment of the final magnet is reduced, and the remanence decline is increased.

It can also be seen from the data in Table 7 that the increase in coercivity of the magnet is significantly lower when hydrochloric acid is used as the acid wash than other types of acids. The reason is that the chloride ion radius in the hydrochloric acid is small, the permeability is strong, rust spots can be generated in the grain boundary R-rich phase permeating into the magnet, and the diffusion rate of the heavy rare earth elements along the grain boundary is seriously hindered, so that the magnetic performance of the magnet is obviously reduced when the hydrochloric acid is used as the acid washing solution.

The SEM secondary electron images of the surfaces of the magnets after acid washing of the magnets of experiment nos. 16 to 18 are shown in fig. 4, and it can be seen from the SEM secondary electron images of the surfaces of the magnets after acid washing with different acids that the corrosion depth of the surfaces of the magnets after corrosion with hydrochloric acid and sulfuric acid is very large, the R-rich phase of the grain boundaries between the main phase grains of the surface layer of the magnets is almost completely corroded, and the main phase grains of the surface layer of the magnets are also significantly corroded. When oxalic acid is used as the pickling solution, the corrosion effect of the surface of the magnet is weak, and the oxalic acid has a similar passivation effect on the surface of the magnet. This has a weak cleaning effect on the oxide layer, which is not favorable for removing the oxide layer. Under the passivation action of an oxide layer on the surface of the magnet and oxalic acid, the crystal boundary diffusion efficiency is obviously reduced, and the magnetic performance of the magnet after diffusion is also reduced. Therefore, nitric acid is preferably used as the acid washing solution in the present invention.

Claims

1. A preparation method of a high-performance sheet R-T-B rare earth permanent magnet is characterized by comprising the following steps:

2. The method for preparing high-performance R-T-B rare earth permanent magnet flakes according to claim 1, wherein in step (1), the acidic surface passivating solution is a stainless steel pickling passivating solution.

3. The method for preparing a high-performance R-T-B rare earth permanent magnet sheet according to claim 1, wherein in the step (1), the pickling process comprises placing the magnetic sheet in a pickling solution with an acidic surface passivation solution concentration of 0.1-5 vol% and a nitric acid concentration of 1-4 vol.% at room temperature for ultrasonic pickling for 100-180 s; or placing the magnetic sheet in an acid washing solution with the concentration of acid surface passivation solution of 0.1-5 vol% and the concentration of nitric acid of 4-8 vol.% for ultrasonic acid washing for 50-100 s at room temperature; or placing the magnetic sheet in an acid washing solution with the concentration of acid surface passivation solution of 0.1-5 vol% and the concentration of nitric acid of 8-10 vol.% for ultrasonic acid washing for 10-50 s at room temperature.

4. The method for producing a high-performance R-T-B rare-earth permanent magnet flake as claimed in claim 1, wherein in said step (1), vickers hardness of surface of said magnetic sheet before and after pickling satisfies 1.05. Ltoreq.H 1/H2. Ltoreq.1.25, H ₁ The surface Vickers hardness, H, of the magnetic sheet before pickling ₂ The Vickers hardness of the surface of the magnetic sheet after pickling was used.

5. The method for preparing R-T-B rare earth permanent magnet flakes according to claim 1, wherein in step (1), the R-rich phase corrosion depth of the surface grain boundary of the magnetic flakes after acid washing treatment is 0.1-0.4 times the depth of the two main phase grain boundaries on the surface of the magnet.

6. The method for preparing R-T-B rare earth permanent magnet of high performance sheet according to claim 1, wherein in the step (2), a heavy rare earth element diffusion layer with a thickness of 3-100 μm is deposited on the surface of the magnetic sheet after acid pickling by means of evaporation, magnetron sputtering or multi-arc ion plating.

7. The method for preparing high-performance R-T-B rare earth permanent magnet as claimed in claim 1, wherein in the step (2), the heavy rare earth diffusion source is pure heavy rare earth metal, heavy rare earth hydride or alloy of heavy rare earth element and other metal elements, and the heavy rare earth element is at least one of Dy, tb or Ho.

8. The method for preparing high-performance R-T-B rare earth permanent magnet flakes according to claim 1, wherein in step (2), the surface of the magnet perpendicular to the orientation direction is covered with the heavy rare earth diffusion source, and the surface of the magnet not perpendicular to the orientation direction is not covered with the heavy rare earth diffusion source.

9. The method for preparing the high-performance R-T-B rare earth permanent magnet slice as claimed in claim 1, wherein in the step (3), the diffusion temperature of grain boundary diffusion is 800-1000 ℃, the heat preservation time is 5-25 h, after the heat preservation is finished, the temperature is raised to 400-650 ℃ after the temperature is cooled to be below 200 ℃, and the heat preservation time is 2-10 h, so that the high-performance R-T-B rare earth permanent magnet slice is prepared.

10. The method for preparing R-T-B rare earth permanent magnet flakes according to claim 9, wherein in step (3), the degree of vacuum in the furnace is 10 after the grain boundary diffusion treatment reaches the diffusion temperature ^-2 ～10 ^-5 Pa。