A method for inducing directional diffusion of heavy rare earth elements along grain boundaries

Abstract

The embodiment of the application discloses a method for inducing heavy rare earth elements to diffuse along grain boundaries, comprising: pretreatment of a magnet; deposition of an inducing coating; deposition of an inducing coating composed of multi-principal element alloy metal material on the surface of a sintered neodymium-iron-boron magnet; deposition of a high-density buffer coating composed of multi-principal element alloy metal material and heavy rare earth material; deposition of a heavy rare earth coating composed of heavy rare earth material; and multi-stage heat diffusion treatment. The multi-principal element alloy metal contains no less than three types of metal elements; and the temperature of the multi-stage heat diffusion treatment process increases sequentially. The method combines multi-coating and corresponding heat treatment, greatly inhibits the phenomenon of heavy rare earth intracrystalline diffusion, induces heavy rare earth elements to diffuse along grain boundaries, and can not only further improve the utilization rate of heavy rare earth and the coercive force increment, but also reduce the decline amplitude of residual magnetism and magnetic energy product.

Description

Technical Field

[0001] This invention relates to the field of grain boundary diffusion technology for sintered NdFeB magnets, specifically to a method for inducing directional diffusion of heavy rare earth elements along grain boundaries. Background Technology

[0002] The demand for high-performance sintered neodymium iron boron (NdFeB) permanent magnets, a key material for drive motors in new energy vehicles, has increased significantly, with the market size reaching tens of thousands of tons. Heavy rare earth grain boundary diffusion technology is currently one of the most effective methods for manufacturing high coercivity and high remanence NdFeB magnets. Its basic principle is that at high temperatures, heavy rare earth elements enter the magnet along grain boundaries, replacing Nd atoms at the boundaries of the main phase grains to form a heavy rare earth-rich shell with a high magnetocrystalline anisotropy field. The precipitation of Nd atoms also repairs defects in the grain boundary phase, slowing down the nucleation of antimagnetic domains, thereby improving coercivity. Due to the selective diffusion of heavy rare earth elements, the consumption of heavy rare earth elements in grain boundary diffusion technology is much lower than that in traditional smelting, resulting in a greater increase in coercivity and a significantly reduced decrease in remanence and energy product.

[0003] Limited by the relatively low diffusion rate of heavy rare earth elements (HREEs), traditional grain boundary diffusion typically requires high temperatures and long durations to ensure sufficient diffusion rates and depths to form a large enough HREE shell to achieve the desired increase in coercivity. However, the high-temperature and long-duration diffusion process also accelerates the diffusion of HREEs into the main phase grains, especially in the shallow surface region of the magnet where intragranular diffusion is most pronounced. Intragranular diffusion alters the main phase structure, diluting the magnet's magnetism and significantly reducing remanence and energy product. Furthermore, it reduces the amount of HREEs participating in grain boundary diffusion, decreasing the diffusion depth and thus hindering the achievement of better coercivity gains, ultimately lowering the utilization rate of HREEs. Summary of the Invention

[0004] Therefore, embodiments of the present invention provide a method for inducing directional diffusion of heavy rare earth elements along grain boundaries. By combining multiple coatings and corresponding heat treatment methods, the intragranular diffusion phenomenon of heavy rare earth elements is suppressed to a large extent, and the directional diffusion of heavy rare earth elements along grain boundaries is induced. This not only further improves the utilization rate and coercivity increment of heavy rare earth elements, but also reduces the decrease in remanence and magnetic energy product.

[0005] To achieve the above objectives, the embodiments of the present invention provide the following technical solutions:

[0006] In one aspect of the present invention, a method for inducing directional diffusion of heavy rare earth elements along grain boundaries is provided, comprising:

[0007] S100. Pretreatment of magnets: The sintered NdFeB magnets are successively ground, polished and ultrasonically cleaned, and then dried.

[0008] S200, Deposition of induced coating: An induced coating composed of multi-principal element alloy metal material is deposited on the surface of sintered NdFeB magnets under a vacuum of 0.4 to 1.2 Pa and an inert gas atmosphere.

[0009] Deposition of S300 high-density buffer coating: Adjust the vacuum degree to 0.2-0.8 Pa and deposit a high-density buffer coating composed of multi-principal element alloy metal materials and heavy rare earth materials;

[0010] S400, deposition of heavy rare earth coating: Adjust the vacuum degree to 0.4~1.2 Pa, and deposit a heavy rare earth coating composed of heavy rare earth materials;

[0011] S500, multi-stage heat treatment: The sintered NdFeB magnets after deposition of heavy rare earth coatings undergo multiple thermal diffusion treatments followed by tempering to complete the directional diffusion of heavy rare earth elements in the sintered NdFeB magnets; among which...

[0012] Multi-principal element alloys contain no fewer than three types of metallic elements;

[0013] In step S500, the temperature of the multiple thermal diffusion processes increases sequentially.

[0014] In a preferred embodiment of the present invention, the metal elements in the multi-principal-element alloy metal are low-melting-point metal elements.

[0015] As a preferred embodiment of the present invention, the metal elements in the multi-principal alloy metal are selected from at least three of Al, Zn, Pr, Mg, and Cu.

[0016] As a preferred embodiment of the present invention, the difference in the molar number of each metal element in the multi-principal alloy metal does not exceed 15%.

[0017] As a preferred embodiment of the present invention, step S200 specifically includes:

[0018] S201. Evacuate to a vacuum level not exceeding 2×10⁻⁶. -2 After Pa, inert gas is introduced until the vacuum degree is 0.4 to 1.2 Pa;

[0019] S202. Under the premise of applying a negative bias voltage to the workpiece holder on which the sintered NdFeB magnet is placed, an inductive coating is deposited using vacuum deposition technology; wherein...

[0020] The operating parameters of the DC pulse bias power supply used to apply the negative bias voltage in step S202 are: frequency 40kHz~100kHz, duty cycle 50%~80%, voltage value -50V~-200V;

[0021] The vacuum coating technology in step S202 is DC pulse magnetron sputtering, and the operating parameters of the DC pulse magnetron sputtering process are: frequency 40kHz~100kHz, duty cycle 20%~80%, voltage 350V~600V;

[0022] The deposition time for step S200 is 2 to 10 minutes.

[0023] As a preferred embodiment of the present invention, the deposition process in step S300 specifically includes: under the premise of applying a negative bias voltage to the workpiece holder on which the sintered NdFeB magnet is placed, deposition is performed on the multi-principal element alloy metal target and the heavy rare earth target using a high ionization rate vacuum coating technology; wherein...

[0024] At least some of the operating parameters in the high ionization rate vacuum coating process increase or decrease linearly with the sputtering time of the multi-principal element alloy metal target and / or heavy rare earth target.

[0025] As a preferred embodiment of the present invention, the high ionization rate vacuum coating technology is high-power pulsed magnetron sputtering; and...

[0026] The operating parameters for the high-power pulsed magnetron sputtering process used to deposit multi-principal alloy metal targets are: initial voltage of 750V to 850V, pulse width of 100μs to 200μs, and frequency of 50Hz to 300Hz. During the deposition process, the pulse width and frequency remain constant, and the voltage decreases linearly to 400V to 500V with the increase of deposition time.

[0027] The operating parameters for the high-power pulsed magnetron sputtering process used for depositing heavy rare earth targets are: initial voltage of 400V to 500V, pulse width of 100μs to 200μs, and frequency of 50Hz to 00Hz. During the deposition process, the pulse width and frequency remain constant, and the voltage increases linearly with the deposition time to 750V to 850V.

[0028] The operating parameters of the DC pulse bias power supply used to apply the negative bias voltage in step S300 are: frequency 40kHz~100kHz, duty cycle 50%~80%, voltage value -100V~-150V;

[0029] The deposition time for step S300 is 10–20 min.

[0030] As a preferred embodiment of the present invention, the deposition process in step S400 specifically includes: depositing a heavy rare earth coating using vacuum deposition technology under the premise of applying a negative bias voltage to the workpiece holder on which the sintered NdFeB magnet is placed; wherein...

[0031] The operating parameters of the DC pulse bias power supply used to apply the negative bias voltage in step S400 are: frequency 40kHz~100kHz, duty cycle 50%~80%, voltage value -50V~-200V;

[0032] The vacuum coating technology in step S400 is DC pulse magnetron sputtering, and the operating parameters of the DC pulse magnetron sputtering process are: frequency 40kHz~100kHz, duty cycle 20%~80%, voltage 350V~600V;

[0033] The deposition time for step S400 is 20–60 min.

[0034] In a preferred embodiment of the present invention, the multiple thermal diffusion treatment includes at least a first thermal diffusion treatment and a second thermal diffusion treatment; and...

[0035] The two thermal diffusion processes were performed in a vacuum environment.

[0036] As a preferred embodiment of the present invention, the operating parameters for the first thermal diffusion treatment are: diffusion temperature 550℃~750℃, time 15min~60min, and vacuum degree less than 10. -2 Pa;

[0037] The operating parameters for the second thermal diffusion treatment are: diffusion temperature 800℃~900℃, time 5h~10h, and vacuum degree less than 10. -2 Pa.

[0038] As a preferred embodiment of the present invention, the working parameters of the tempering process are: tempering temperature of 450℃~490℃, tempering time of 3h~6h, and vacuum degree of less than 10. -2 Pa.

[0039] The embodiments of the present invention have the following advantages:

[0040] 1) This invention proposes to deposit a multilayer coating on the surface of sintered NdFeB magnets. The coating structure is specifically an inductive coating / high-density buffer coating / heavy rare earth coating. Combined with the corresponding heat treatment process, the goal of inducing the directional diffusion of heavy rare earth elements along the grain boundaries can be achieved in one step.

[0041] 2) This invention proposes depositing a multilayer coating on the surface of a sintered NdFeB magnet, followed by a first heat treatment. This induces the elements in the coating to enter the magnet and form a low-melting-point phase with the Nd-rich phase at the grain boundaries, improving the wettability of the grain boundary phase and optimizing its distribution characteristics. This results in a more uniform and continuous distribution of the Nd-rich phase. Since grain boundaries are the main channels for the diffusion of heavy rare earth elements, this treatment process serves two purposes: firstly, it repairs the grain boundary phase, providing more diffusion channels for heavy rare earth elements; secondly, the lower melting point of the grain boundary phase increases the diffusion rate of heavy rare earth elements along the grain boundaries. Both of these aspects can promote the directional diffusion of heavy rare earth elements along the grain boundaries.

[0042] 3) Due to the optimization of the grain boundary phase structure and distribution characteristics and the reduction of the melting point, the diffusion temperature of heavy rare earth can be appropriately reduced by optimizing the grain boundary diffusion process, so as to achieve low-temperature diffusion, thereby reducing the possibility of heavy rare earth diffusing into the main phase grain, and thus inducing more heavy rare earth to diffuse directionally along the grain boundary. Attached Figure Description

[0043] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0044] The structures, proportions, sizes, etc. illustrated in this specification are only for the purpose of assisting those skilled in the art in understanding and reading the content disclosed herein, and are not intended to limit the conditions under which the present invention can be implemented. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportions, or adjustments to the size, without affecting the effects and objectives that the present invention can produce, should still fall within the scope of the technical content disclosed in the present invention.

[0045] Figure 1 This is a schematic diagram of the structure of the multilayer coating prepared by the method of the present invention;

[0046] Figure 2 This is a schematic diagram of the vacuum coating apparatus used in the method of the present invention;

[0047] Figure 3 This is a diagram showing the Dy and Nd elemental composition distribution of the deposited NdFeB magnet at a depth of 20 μm in Example 1 of the present invention;

[0048] Figure 4 This is a diagram showing the Dy and Nd elemental composition distribution of the deposited NdFeB magnet at a depth of 20 μm in Comparative Example 1 of this invention;

[0049] Figure 5 This is a shallow backscattered image of the NdFeB magnet after deposition in Example 2 of the present invention;

[0050] Figure 6 This is a shallow backscattered image of the NdFeB magnet after deposition in Comparative Example 2 of this invention;

[0051] Figure 7 This is a backscattered image of the deposited NdFeB magnet at a depth of 50 μm in Example 3 of the present invention;

[0052] Figure 8 This is a backscattered image of the deposited NdFeB magnet in Comparative Example 3 of the present invention at a depth of 50 μm;

[0053] Figure 9 A flowchart of the method provided by the present invention.

[0054] In the picture:

[0055] 1-Cavity; 2-NdFeB magnet; 3-Workpiece holder; 4-DC pulse bias power supply; 5-First DC pulse magnetron sputtering power supply; 6-First high-power pulse magnetron sputtering power supply; 7-Inductive metal target; 8-Heavy rare earth target; 9-Second DC pulse magnetron sputtering power supply; 10-Second high-power pulse magnetron sputtering power supply; 11-Gas introduction system; 12-Vacuum acquisition system. Detailed Implementation

[0056] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0057] The technical solution of this application will be described in detail below with reference to the accompanying drawings.

[0058] The method proposed in this invention for inducing directional diffusion of heavy rare earth elements along grain boundaries is used to prepare a multilayer coating structure on the surface of sintered NdFeB magnets, as shown in the figure. Figure 1 As shown, it includes a sintered NdFeB magnet, and an inductive coating, a high-density buffer coating, and a heavy rare earth coating sequentially disposed on the sintered NdFeB magnet.

[0059] At the same time, such as Figure 2 As shown, a vacuum deposition apparatus is provided for the method of inducing directional diffusion of heavy rare earth elements along grain boundaries according to the present invention. The method of inducing directional diffusion of heavy rare earth elements along grain boundaries proposed in this invention specifically includes:

[0060] Step 1: Pretreatment before vacuum coating. The sintered NdFeB magnet 2 is subjected to grinding and ultrasonic cleaning in sequence to remove rust and oil stains from the magnet surface, followed by drying.

[0061] Step 2: First coating layer – induced coating deposition, which can be prepared using vacuum deposition techniques such as magnetron sputtering. Specifically, this invention employs... Figure 2 The vacuum coating apparatus shown is used for coating preparation. First, the prepared neodymium iron boron magnet 2 is placed on the workpiece holder 3, and the vacuum level in the cavity 1 is reduced to 2 × 10⁻⁶ using the vacuum acquisition system 12. -2 Below Pa; an inert gas (e.g., Ar gas can be specifically selected considering cost and other factors) is introduced through the gas introduction system 11 to make the vacuum degree in the cavity 1 0.4Pa-1.2Pa. The first DC pulse magnetron sputtering power supply 5 is turned on, and the sputtering inductive metal target 7 (such as AlZnPr, MgAlZnCu, etc., a multi-principal element alloy target composed of three or more metals in equimolar or near equimolar proportions, with low melting point elements as the main component; the selection of low melting point here is mainly based on the type of heavy rare earth coating, which only needs to be lower than the melting point of the heavy rare earth used later, and can generally be considered as a low melting point metal type) is selected. The specific parameter range of the first DC pulse magnetron sputtering power supply 5 during the magnetron sputtering process is: frequency 40kHz-100kHz, duty cycle 20%-80%, voltage 350V-600V. At the same time, a negative bias voltage is applied to the workpiece holder 3 through the DC pulse bias power supply 4. The specific parameter range of the DC pulse bias power supply 4 during the negative bias process is as follows: frequency 40kHz-100kHz, duty cycle 50%-80%, voltage value -50V to -200V, coating deposition time 2min-10min.

[0062] Step 3: The second coating layer—a high-density buffer coating—can be prepared using high-ionization vacuum deposition technology. Turn off the first DC pulsed magnetron sputtering power supply 5. Adjust the Ar gas flow rate to achieve a gas pressure of 0.2 Pa–0.8 Pa in chamber 1. Turn on the first high-power pulsed magnetron sputtering power supply 6 and the second high-power pulsed magnetron sputtering power supply 10 to sputter the inductively coupled metal target 7 and the heavy rare earth target 8 (wherein the heavy rare earth target 8 is composed of Dy and / or Tb). The initial voltage of the first high-power pulsed magnetron sputtering power supply 6 is 750 V–850 V, the pulse width is 100 μs–200 μs, and the frequency is 50 Hz–300 Hz. During deposition, the pulse width and frequency remain constant, and the voltage decreases linearly with deposition time to 400 V–500 V (at the end). The initial voltage of the second high-power pulsed magnetron sputtering power supply 10 is 400V-500V, the pulse width is 100μs-200μs, and the frequency is 50Hz-300Hz. During deposition, the pulse width and frequency remain constant, and the voltage increases linearly with deposition time to 750V-850V (at the end). During deposition, a negative bias is applied to the workpiece holder 3 via a DC pulsed bias power supply 4. The specific parameter range of the DC pulsed bias power supply 4 during the negative bias application process is: frequency 40kHz-100kHz, duty cycle 50%-80%, voltage -100V to -150V; high-density buffer coating deposition time 10min-20min. Through the above specific embodiment, since the voltage change is positively correlated with the sputtering coefficient of the corresponding target material, further dynamic adjustment of the voltage during the high-power pulsed magnetron sputtering process allows the concentration of each element in the coating to exhibit a gradient change along the deposition direction during this dynamic process of coating deposition. This highly dense buffer coating comprises all elements found in the first and third coating layers. Furthermore, based on the aforementioned conditions, the concentration of elements in this coating exhibits a gradient change along the deposition direction during the gradual deposition process. It should be further noted that to ensure the high density of this coating, a high ionization rate vacuum deposition technique, such as high-power pulsed magnetron sputtering, is required. However, this is not a limitation; any vacuum deposition technique capable of generating high-density plasma, such as ion source-assisted vacuum deposition, can be used. In summary, based on the objectives of this invention, the use of a high ionization rate vacuum deposition technique, with its high-density plasma characteristics, can improve the migration rate of deposited particles on the coating surface and enhance the bombardment effect of high-energy particles, suppressing the "shadowing" effect of particles during deposition, reducing microscopic defects in the coating, and thus improving its density.

[0063] Step 4: The third coating layer—a heavy rare earth coating—can be prepared using vacuum deposition techniques such as magnetron sputtering. Turn off the first high-power pulsed magnetron sputtering power supply 6 and the second high-power pulsed magnetron sputtering power supply 10. Adjust the Ar gas flow rate to achieve a gas pressure of 0.4 Pa–1.2 Pa in chamber 1. Turn on the second DC pulsed magnetron sputtering power supply 9 to sputter the heavy rare earth target 8 (composed of Dy and / or Tb). Specific parameters for the magnetron sputtering process are: frequency 40 kHz–100 kHz, duty cycle 20%–80%, voltage 350 V–600 V. Simultaneously, apply a negative bias to the workpiece holder 3 via a DC pulsed bias power supply 4. Specific parameters for the DC pulsed bias power supply 4 during the negative bias application process are: frequency 40 kHz–100 kHz, duty cycle 50%–80%, voltage -50 V to -200 V, and heavy rare earth coating deposition time of 20–60 min.

[0064] Step 5: First heat treatment. The NdFeB magnet 2 is placed in a vacuum heat treatment furnace to complete the first vacuum heat treatment, promoting the entry of elements contained in the inductive coating into the magnet's grain boundary phase and optimizing the grain boundary phase. The heat treatment parameters are: diffusion temperature 550℃-750℃, time 15min-60min, vacuum degree less than 10... -2 Pa. Based on the setting of this diffusion temperature and diffusion time, it is possible to effectively reduce, or even greatly avoid, the induced coating entering the interior of the main phase and affecting the remanence and magnetic energy product of the magnet, and further improve the optimization effect of the induced coating on the grain boundary phase of the magnet.

[0065] Step 6: Second heat treatment. Maintain the existing vacuum heat treatment state, increase the heat treatment temperature to complete the grain boundary diffusion of Dy or Tb heavy rare earth elements, followed by tempering. The heat treatment parameters are as follows: diffusion temperature 800℃-900℃, time 5h-10h; tempering temperature 450℃-490℃, time 3h-6h; vacuum degree less than 10 -2 Pa.

[0066] Step 7: After the sample cools to room temperature, remove the sample to complete the processing of the sintered NdFeB magnet 2.

[0067] The above technical solution adopts a composite method of induced coating, high-density buffer coating, and heavy rare earth coating, combined with corresponding multi-stage heat treatment operations, to achieve the effect of induced directional diffusion of heavy rare earth along the grain boundaries.

[0068] Furthermore, this invention proposes depositing a multilayer coating on the surface of a sintered NdFeB magnet, with the second layer being a highly dense buffer coating. First, this coating is prepared using high-power pulsed magnetron sputtering, resulting in fewer defects, higher density, and stronger resistance to element diffusion. Second, this coating contains all elements found in the inductive coating and the heavy rare earth coating, with element concentrations exhibiting a gradient trend. The coating's melting point is higher than that of the first inductive coating and the Nd-rich phase, thus preventing significant diffusion of this coating itself into the first and third layers during the first heat treatment. Third, this layer is prepared by co-sputtering a multi-principal alloy target and a heavy rare earth target, containing at least four or more elements. The increased elemental content intensifies lattice distortion within the coating, significantly enhancing its ability to impede element diffusion. Therefore, this coating effectively hinders element diffusion. Without this buffer layer, the inductive coating and the heavy rare earth coating would interdiffuse during the first heat treatment, causing heavy rare earth elements to prematurely enter the grain boundary phase during the first heat treatment. Because of the high melting point of heavy rare earth elements, the optimization of grain boundary phase by the induced coating will be affected, thereby reducing the effectiveness of this method in inducing the directional diffusion of heavy rare earth elements along the grain boundary.

[0069] The technical solution of the present invention will be further described below through specific embodiments.

[0070] Example 1

[0071] In this embodiment, a sintered NdFeB magnet of grade N52, designated A, with dimensions of 25mm × 25mm × 3mm, was used. AlZnPr was selected as the inductive metal target, and Dy as the heavy rare earth target. AlZnPr and (AlZnPr) were sequentially deposited on the 25mm × 25mm surface. x Dy y A Dy coating is applied, followed by vacuum heat treatment. The specific process is as follows:

[0072] Step 1: Pretreatment before vacuum coating. The sintered NdFeB magnets are sequentially ground and ultrasonically cleaned to remove rust and oil from the magnet surface, followed by drying.

[0073] Step 2: First coating layer—Induced coating deposition. This invention employs, as follows: Figure 2 The vacuum coating apparatus shown is used for coating preparation. First, the treated NdFeB magnet is placed on the workpiece holder, and the vacuum level in the cavity is reduced to 5 × 10⁻⁶ using a vacuum system. -3Pa; Ar gas is introduced through the gas introduction system to achieve a vacuum of 0.4 Pa in the cavity. The first DC pulse magnetron sputtering power supply is turned on to sputter the inductively coupled metal AlZnPr target. Specific parameters: frequency 40kHz, duty cycle 80%, voltage 400V. Simultaneously, a negative bias voltage is applied to the workpiece holder through a DC pulse bias power supply. Specific parameters: frequency 40kHz, duty cycle 80%, voltage -75V, coating deposition time 5min;

[0074] Step 3: Second Coating Layer—High-Density Buffer Coating. Turn off the first DC pulsed magnetron sputtering power supply. Adjust the Ar gas flow rate to achieve a vacuum chamber pressure of 0.3 Pa. Turn on the first and second high-power pulsed magnetron sputtering power supplies to sputter the inductively coupled metal AlZnPr target and the heavy rare earth Dy target, respectively. The first high-power pulsed magnetron sputtering power supply has an initial voltage of 800 V, a pulse width of 100 μs, and a frequency of 100 Hz. During deposition, the pulse width and frequency remain constant, and the voltage linearly decreases to 400 V (at the end) with deposition time. The second high-power pulsed magnetron sputtering power supply has an initial voltage of 400 V, a pulse width of 100 μs, and a frequency of 100 Hz. During deposition, the pulse width and frequency remain constant, and the voltage linearly increases to 800 V (at the end) with deposition time. During coating deposition, a negative bias voltage is applied to the workpiece holder using a DC pulsed bias power supply. Specific parameters: frequency 40kHz, duty cycle 80%, voltage -100V; high-density buffer coating deposition time 10min.

[0075] Step 4: Third Coating Layer – Heavy Rare Earth Coating. Turn off the first and second high-power pulsed magnetron sputtering power supplies. Adjust the Ar gas flow rate to achieve a vacuum chamber pressure of 0.4 Pa. Turn on the second DC pulsed magnetron sputtering power supply to sputter the heavy rare earth Dy target. Specific parameters: frequency 40 kHz, duty cycle 80%, voltage 500 V. Simultaneously, apply a negative bias to the workpiece holder via a DC pulsed bias power supply. Specific parameters: frequency 40 kHz, duty cycle 80%, voltage -100 V, coating deposition time 40 min.

[0076] Step 5: First heat treatment. The NdFeB magnet is placed in a vacuum heat treatment furnace to complete the first vacuum heat treatment, which promotes the entry of AlZnPr elements contained in the inductive coating into the grain boundary phase of the magnet, thus optimizing the grain boundary phase. The heat treatment parameters are: diffusion temperature 630℃, time 20 min, vacuum degree 8×10⁻⁶. -3 Pa.

[0077] Step 6: Second heat treatment. Maintaining the existing vacuum heat treatment state, increase the heat treatment temperature to complete the grain boundary diffusion of Dy element, followed by tempering. The heat treatment parameters are: diffusion temperature 850℃, time 6h; tempering temperature 450℃, time 4.5h; vacuum degree 8×10⁻⁶. -3 Pa.

[0078] Step 7: After the sample cools to room temperature, remove the sample to complete the processing of the sintered NdFeB magnet.

[0079] Example 2

[0080] In this embodiment, a sintered NdFeB magnet of grade N52, designated as B, with dimensions of 25mm × 25mm × 3mm, was used. MgAlZnCu was selected as the inductive metal target, and Dy was selected as the heavy rare earth target. MgAlZnCu and (MgAlZnCu) were sequentially deposited on the 25mm × 25mm surface. x Dy y A Dy coating is applied, followed by vacuum heat treatment. The specific process is as follows:

[0081] Step 1: Pretreatment before vacuum coating. The sintered NdFeB magnets are sequentially ground and ultrasonically cleaned to remove rust and oil from the magnet surface, followed by drying.

[0082] Step 2: First coating layer—Induced coating deposition. This invention employs, as follows: Figure 2 The vacuum coating apparatus shown is used for coating preparation. First, the treated NdFeB magnet is placed on the workpiece holder, and the vacuum level in the cavity is reduced to 4 × 10⁻⁶ using a vacuum acquisition system. -3 Pa; Ar gas is introduced through the gas introduction system to achieve a vacuum of 0.5 Pa in the cavity. The first DC pulse magnetron sputtering power supply is turned on to sputter the inductively coupled metal MgAlZnCu target. Specific parameters: frequency 50kHz, duty cycle 60%, voltage 500V. Simultaneously, a negative bias voltage is applied to the workpiece holder through a DC pulse bias power supply. Specific parameters: frequency 40kHz, duty cycle 60%, voltage -100V, coating deposition time 10min;

[0083] Step 3: Second Coating Layer—High-Density Buffer Coating. Turn off the first DC pulsed magnetron sputtering power supply. Adjust the Ar gas flow rate to achieve a vacuum chamber pressure of 0.5 Pa. Turn on the first and second high-power pulsed magnetron sputtering power supplies to sputter the inductively coupled metal MgAlZnCu target and the heavy rare earth Dy target, respectively. The first high-power pulsed magnetron sputtering power supply has an initial voltage of 850 V, a pulse width of 200 μs, and a frequency of 100 Hz. The pulse width and frequency remain constant during deposition, and the voltage linearly decreases to 450 V (at the end) with deposition time. The second high-power pulsed magnetron sputtering power supply has an initial voltage of 500 V, a pulse width of 100 μs, and a frequency of 200 Hz. The pulse width and frequency remain constant during deposition, and the voltage linearly increases to 850 V (at the end) with deposition time. During coating deposition, a negative bias voltage is applied to the workpiece holder using a DC pulsed bias power supply. Specific parameters: frequency 80kHz, duty cycle 60%, voltage -120V; high-density buffer coating deposition time 15min.

[0084] Step 4: Third Coating Layer – Heavy Rare Earth Coating. Turn off the first and second high-power pulsed magnetron sputtering power supplies. Adjust the Ar gas flow rate to achieve a vacuum chamber pressure of 0.7 Pa. Turn on the second DC pulsed magnetron sputtering power supply to sputter the heavy rare earth Dy target. Specific parameters: frequency 100 kHz, duty cycle 50%, voltage 550 V. Simultaneously, apply a negative bias to the workpiece holder using a DC pulsed bias power supply. Specific parameters: frequency 80 kHz, duty cycle 80%, voltage -150 V, coating deposition time 30 min.

[0085] Step 5: First heat treatment. The NdFeB magnet is placed in a vacuum heat treatment furnace to complete the first vacuum heat treatment, which promotes the induction of Mg, Al, Zn, and Cu elements contained in the coating into the magnet's grain boundary phase, thus optimizing the grain boundary phase. The heat treatment parameters are: diffusion temperature 600℃, time 35 min, vacuum degree 9 × 10⁻⁶. -3 Pa.

[0086] Step 6: Second heat treatment. Maintaining the existing vacuum heat treatment state, increase the heat treatment temperature to complete the grain boundary diffusion of Dy element, followed by tempering. The heat treatment parameters are as follows: diffusion temperature 870℃, time 5h; tempering temperature 460℃, time 5h; vacuum degree 9×10⁻⁶. -3 Pa.

[0087] Step 7: After the sample cools to room temperature, remove the sample to complete the processing of the sintered NdFeB magnet.

[0088] Example 3

[0089] In this embodiment, a sintered NdFeB magnet of grade 45SH, designated C, with dimensions of 25mm × 25mm × 3mm, was used. CuZnPr was selected as the inductive metal target, and Tb as the heavy rare earth element target. CuZnPr and (CuZnPr) were sequentially deposited on the 25mm × 25mm surface. x Tb y A Tb coating is applied, followed by vacuum heat treatment. The specific process is as follows:

[0090] Step 1: Pretreatment before vacuum coating. The sintered NdFeB magnets are sequentially ground and ultrasonically cleaned to remove rust and oil from the magnet surface, followed by drying.

[0091] Step 2: First coating layer—Induced coating deposition. This invention employs, as follows: Figure 2 The vacuum coating apparatus shown is used for coating preparation. First, the treated NdFeB magnet is placed on the workpiece holder, and the vacuum level in the cavity is reduced to 3 × 10⁻⁶ using a vacuum acquisition system. -3 Pa; Ar gas is introduced through the gas introduction system to achieve a vacuum of 0.7 Pa in the cavity. The first DC pulse magnetron sputtering power supply is turned on to sputter the inductively coupled metal CuZnPr target. Specific parameters: frequency 100kHz, duty cycle 70%, voltage 550V. Simultaneously, a negative bias voltage is applied to the workpiece holder through a DC pulse bias power supply. Specific parameters: frequency 90kHz, duty cycle 70%, voltage -150V, coating deposition time 3min;

[0092] Step 3: Second Coating Layer—High-Density Buffer Coating. Turn off the first DC pulsed magnetron sputtering power supply. Adjust the Ar gas flow rate to achieve a vacuum chamber pressure of 0.7 Pa. Turn on the first and second high-power pulsed magnetron sputtering power supplies to sputter the inductively coupled metal CuZnPr target and the heavy rare earth Tb target, respectively. The first high-power pulsed magnetron sputtering power supply has an initial voltage of 750 V, a pulse width of 150 μs, and a frequency of 300 Hz. The pulse width and frequency remain constant during deposition, and the voltage linearly decreases to 500 V (at the end) with deposition time. The second high-power pulsed magnetron sputtering power supply has an initial voltage of 450 V, a pulse width of 200 μs, and a frequency of 150 Hz. The pulse width and frequency remain constant during deposition, and the voltage linearly increases to 800 V (at the end) with deposition time. During coating deposition, a negative bias voltage is applied to the workpiece holder using a DC pulsed bias power supply. Specific parameters: frequency 80kHz, duty cycle 60%, voltage -100V; high-density buffer coating deposition time 20min.

[0093] Step 3: Third Coating Layer—Heavy Rare Earth Coating. Turn off the first and second high-power pulsed magnetron sputtering power supplies. Adjust the Ar gas flow rate to achieve a vacuum chamber pressure of 1 Pa. Turn on the second DC pulsed magnetron sputtering power supply to sputter the heavy rare earth Tb target. Specific parameters: frequency 100kHz, duty cycle 70%, voltage 500V. Simultaneously, apply a negative bias to the workpiece holder via a DC pulsed bias power supply. Specific parameters: frequency 60kHz, duty cycle 80%, voltage -100V, coating deposition time 55min.

[0094] Step 4: First heat treatment. The NdFeB magnet is placed in a vacuum heat treatment furnace to complete the first vacuum heat treatment, which promotes the induction of CuZnPr elements contained in the coating into the magnet's grain boundary phase and optimizes the grain boundary phase. The heat treatment parameters are: diffusion temperature 700℃, time 50 min, vacuum degree 9×10⁻⁶. -3 Pa.

[0095] Step 6: Second heat treatment. Maintaining the existing vacuum heat treatment state, increase the heat treatment temperature to complete Tb grain boundary diffusion, followed by tempering. The heat treatment parameters are: diffusion temperature 890℃, time 8h; tempering temperature 460℃, time 6h; vacuum degree 9×10⁻⁶. -3 Pa.

[0096] Step 7: After the sample cools to room temperature, remove the sample to complete the processing of the sintered NdFeB magnet.

[0097] Comparative Example 1

[0098] The grade and size of the NdFeB magnet A used are exactly the same as those in Example 1. The difference in operation method is that the second and third steps are skipped, and the fourth step is directly entered to deposit a Dy coating with the same content as in Example 1, followed by the same heat treatment process.

[0099] Comparative Example 2

[0100] The grade and size of the NdFeB magnet B used are exactly the same as those in Example 2. The difference from the operation method in Example 1 is that the second and third steps are skipped, and the fourth step is directly entered to deposit a Dy coating with the same content as in Example 2, and the same heat treatment process is used.

[0101] Comparative Example 3

[0102] The grade and size of the NdFeB magnet C used are exactly the same as those in Example 3. The difference in operation method is that the second and third steps are skipped, and the fourth step is directly entered to deposit a Tb coating with the same content as in Example 3, and the same heat treatment process is used.

[0103] Detection Example 1

[0104] The tests for Example 1 and Comparative Example 1 were performed using the following specific methods:

[0105] The elemental composition distribution of Dy and Nd at a depth of 20 μm in the magnet was characterized using an electronic probe microanalyzer (EPMA). Figure 4 As shown, in Comparative Example 1, Dy element exhibits significant intracrystalline diffusion, with a large amount of Dy element entering the main phase; as... Figure 3 As shown, in Example 1, the Dy element is more located at the main phase interface, and its distribution is more uniform and continuous, effectively suppressing grain boundary diffusion. Furthermore, compared to Comparative Example 1, the Nd-rich phase in Example 1, existing as corner phases and isolated phases, is improved, exhibiting a more uniform and fine distribution, which is beneficial for clearing grain boundary diffusion channels for heavy rare earth elements. These results fully demonstrate that the method proposed in this invention can induce directional diffusion of the heavy rare earth element Dy along grain boundaries.

[0106] The magnetic properties of Example 1 and Comparative Example 1 were measured using a pulsed magnetic field strength meter, and the results are shown in Table 1. Compared with Comparative Example 1, the decrease in remanence of Example 1 was reduced by 0.08 kGs, while the increase in coercivity was increased by 0.59 kOe. This result is consistent with the distribution characteristics of Dy and Nd elements in the magnet, verifying that the method proposed in this invention can induce directional diffusion of heavy rare earth elements along the grain boundaries to suppress intragranular diffusion of heavy rare earth elements. This not only achieves a higher increase in coercivity and reduces the decrease in remanence and magnetic energy product, but also further improves the utilization rate of heavy rare earth elements.

[0107] Table 1

[0108] serial number Remanence Br (kG) Coercivity Hcj(kOe) Example 1 14.36 15.87 Comparative Example 1 14.28 15.28

[0109] Detection Example 2

[0110] The tests for Example 2 and Comparative Example 2 were performed using the following methods:

[0111] The cross-sectional morphology of the magnet was analyzed using scanning electron microscopy (SEM), and its backscattered image was obtained. For example... Figure 5 and Figure 6 As shown, the white area represents the Nd-rich phase, and the light gray area represents (Nd, Dy)₂Fe. 14 B-shell phase, dark gray region is Nd2Fe 14 B is the main phase. For example... Figure 6 As shown, it is evident that in Comparative Example 2, a large area of ​​light gray exists in the shallow surface region (within approximately 20 μm of the magnet surface), representing a relatively severe intracrystalline diffusion of Dy element; while... Figure 5As shown in Example 2, the intracrystalline diffusion region of Dy element is significantly reduced. The above results fully demonstrate that the method proposed in this invention can induce the directional diffusion of heavy rare earth element Dy along the grain boundaries.

[0112] The magnetic properties of Example 2 and Comparative Example 2 were measured using a pulsed magnetic field strength meter, and the results are shown in Table 2. Compared with Comparative Example 2, the decrease in remanence of Example 2 was reduced by 0.16 kGs, while the increase in coercivity was increased by 1.12 kOe. This result is consistent with the results of the magnet backscattering image, verifying that the method proposed in this invention can induce directional diffusion of heavy rare earth elements along the grain boundaries to suppress intragranular diffusion of heavy rare earth elements. This not only achieves a higher increase in coercivity and reduces the decrease in remanence and magnetic energy product, but also further improves the utilization rate of heavy rare earth elements.

[0113] Table 2

[0114] serial number Remanence Br (kG) Coercivity Hcj(kOe) Example 2 14.08 16.41 Comparative Example 2 13.92 15.29

[0115] Detection Example 3

[0116] The tests for Example 3 and Comparative Example 3 were performed using the following specific methods:

[0117] The cross-sectional morphology of the magnet was analyzed using scanning electron microscopy (SEM), and backscattered images at a depth of 50 μm were acquired. Figure 7 and Figure 8 As shown, the white area represents the Nd-rich phase, and the light gray area represents (Nd, Tb)₂Fe. 14 B-shell phase, dark gray region is Nd2Fe 14 Phase B. It is clearly visible that, as... Figure 8 As shown, in Comparative Example 3, the light gray area on the surface of the main phase grains is relatively large (especially noticeable in the upper region of the backscattered image), representing (Nd,Tb)₂Fe. 14 The thicker B shell indicates more severe intragranular diffusion of Tb. In comparison, such as... Figure 7 As shown, in Example 3, (Nd,Tb)2Fe 14 The B-shell is relatively thin. These results clearly demonstrate that the method proposed in this invention can induce directional diffusion of the heavy rare earth element Tb along grain boundaries.

[0118] The magnetic properties of Example 3 and Comparative Example 3 were measured using a pulsed magnetic field strength meter, and the results are shown in Table 3. Compared with Comparative Example 3, the decrease in remanence of Example 3 was reduced by 0.04 kGs, while the increase in coercivity was increased by 0.37 kOe. This result is consistent with the results of the magnet backscattering image, verifying that the method proposed in this invention can induce directional diffusion of heavy rare earth elements along the grain boundaries to suppress intragranular diffusion of heavy rare earth elements. This not only achieves a higher increase in coercivity and reduces the decrease in remanence and magnetic energy product, but also further improves the utilization rate of heavy rare earth elements.

[0119] Table 3

[0120] serial number Remanence Br (kG) Coercivity Hcj(kOe) Example 3 13.43 29.16 Comparative Example 3 13.39 28.79

[0121] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for inducing directional diffusion of heavy rare earth elements along grain boundaries, characterized in that, include: S100. Pretreatment of magnets: The sintered NdFeB magnets are successively ground, polished and ultrasonically cleaned, and then dried. S200, Deposition of induced coating: An induced coating composed of multi-principal element alloy metal material is deposited on the surface of sintered NdFeB magnets under a vacuum of 0.4 to 1.2 Pa and an inert gas atmosphere. Deposition of S300 high-density buffer coating: Adjust the vacuum degree to 0.2-0.8 Pa and deposit a high-density buffer coating composed of multi-principal element alloy metal materials and heavy rare earth materials; S400, deposition of heavy rare earth coating: Adjust the vacuum degree to 0.4~1.2 Pa, and deposit a heavy rare earth coating composed of heavy rare earth materials; S500, multi-stage heat treatment: The sintered NdFeB magnets after deposition of heavy rare earth coatings undergo multiple thermal diffusion treatments followed by tempering to complete the directional diffusion of heavy rare earth elements in the sintered NdFeB magnets; among which... The multi-principal element alloy metal contains no less than three types of metallic elements; and the metallic elements in the multi-principal element alloy metal are low-melting-point metallic elements. The deposition process in step S300 specifically includes: under the premise of applying a negative bias voltage to the workpiece holder on which the sintered NdFeB magnet is placed, high-power pulsed magnetron sputtering is used to deposit multi-principal element alloy metal targets and heavy rare earth targets respectively; wherein, At least some of the operating parameters in the high-power pulsed magnetron sputtering process increase or decrease linearly with the sputtering time of multi-principal alloy metal targets and / or heavy rare earth targets. In step S500, the temperature of the multiple thermal diffusion processes increases sequentially.

2. The method for inducing directional diffusion of heavy rare earth elements along grain boundaries according to claim 1, characterized in that, The metallic elements in the multi-principal alloy metal are selected from at least three of Al, Zn, Pr, Mg, and Cu.

3. The method for inducing directional diffusion of heavy rare earth elements along grain boundaries according to claim 2, characterized in that, The difference in the molar number of each metal element in the multi-principal-element alloy metal does not exceed 15%.

4. A method for inducing directional diffusion of heavy rare earth elements along grain boundaries according to any one of claims 1-3, characterized in that, Step S200 specifically includes: S201. Evacuate to a vacuum level not exceeding 2×10⁻⁶. -2 After Pa, inert gas is introduced until the vacuum degree is 0.4 to 1.2 Pa; S202. Under the premise of applying a negative bias voltage to the workpiece holder on which the sintered NdFeB magnet is placed, an inductive coating is deposited using vacuum deposition technology; wherein... The operating parameters of the DC pulse bias power supply used to apply the negative bias voltage in step S202 are: frequency 40kHz~100kHz, duty cycle 50%~80%, voltage value -50V~-200V; The vacuum coating technology in step S202 is DC pulse magnetron sputtering, and the operating parameters of the DC pulse magnetron sputtering process are: frequency 40kHz~100kHz, duty cycle 20%~80%, voltage 350V~600V; The deposition time for step S200 is 2 to 10 minutes.

5. A method for inducing directional diffusion of heavy rare earth elements along grain boundaries according to any one of claims 1-3, characterized in that, The operating parameters for the high-power pulsed magnetron sputtering process used to deposit multi-principal alloy metal targets are: initial voltage of 750V to 850V, pulse width of 100μs to 200μs, and frequency of 50Hz to 300Hz. During the deposition process, the pulse width and frequency remain constant, and the voltage decreases linearly to 400V to 500V with the increase of deposition time. The operating parameters for the high-power pulsed magnetron sputtering process used for depositing heavy rare earth targets are: initial voltage of 400V to 500V, pulse width of 100μs to 200μs, and frequency of 50Hz to 00Hz. During the deposition process, the pulse width and frequency remain constant, and the voltage increases linearly with the deposition time to 750V to 850V. The operating parameters of the DC pulse bias power supply used to apply the negative bias voltage in step S300 are: frequency 40kHz~100kHz, duty cycle 50%~80%, voltage value -100V~-150V. The deposition time for step S300 is 10–20 min.

6. A method for inducing directional diffusion of heavy rare earth elements along grain boundaries according to any one of claims 1-3, characterized in that, The deposition process in step S400 specifically includes: depositing a heavy rare earth coating using vacuum deposition technology, under the premise of applying a negative bias voltage to the workpiece holder on which the sintered NdFeB magnet is placed; wherein, The operating parameters of the DC pulse bias power supply used to apply the negative bias voltage in step S400 are: frequency 40kHz~100kHz, duty cycle 50%~80%, voltage value -50V~-200V. The vacuum coating technology in step S400 is DC pulse magnetron sputtering, and the operating parameters of the DC pulse magnetron sputtering process are: frequency 40kHz~100kHz, duty cycle 20%~80%, voltage 350V~600V; The deposition time for step S400 is 20–60 min.

7. A method for inducing directional diffusion of heavy rare earth elements along grain boundaries according to any one of claims 1-3, characterized in that, Multiple thermal diffusion treatments include at least a first thermal diffusion treatment and a second thermal diffusion treatment; and, The two thermal diffusion processes were performed in a vacuum environment.

8. The method for inducing directional diffusion of heavy rare earth elements along grain boundaries according to claim 7, characterized in that, The operating parameters for the first thermal diffusion treatment are: diffusion temperature 550℃~750℃, time 15min~60min, and vacuum degree less than 10. -2 Pa; The operating parameters for the second thermal diffusion treatment are: diffusion temperature 800℃~900℃, time 5h~10h, and vacuum degree less than 10. -2 Pa.

9. A method for inducing directional diffusion of heavy rare earth elements along grain boundaries according to any one of claims 1-3, characterized in that, The working parameters for the tempering process are: tempering temperature 450℃~490℃, tempering time 3h~6h, and vacuum degree less than 10. -2 Pa.

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