A magnet and a method of making a magnet

Abstract

The present disclosure relates to a magnet comprising a first magnetic substrate having a surface layer with a thickness of 10-20 μm and a plating layer adhered to the surface layer, wherein the surface layer has cracks with a width of not more than 1.5 μm and an area crack density of 0.06-0.08 / μm in the surface layer; and wherein the length of cracks with a width of less than 0.5 μm accounts for not less than 80% of the total length of all cracks. The magnet provided by the present disclosure comprises a first magnetic substrate having a high-density surface layer, and thus has a strong anti-magnetic decay ability and a low high-temperature demagnetization rate.

Description

Technical Field

[0001] This disclosure relates to the field of permanent magnet technology, and more specifically, to a magnet and a method for manufacturing the magnet. Background Technology

[0002] Rare earth permanent magnets possess excellent magnetic properties and are widely used in electric motors, engines, voice coil motors, magnetic resonance imaging (MRI) scanners, communications, control instruments, and audio equipment. However, because rare earth permanent magnets contain easily oxidized Fe and rare earth elements, they are susceptible to oxidation and corrosion under natural conditions. Currently, the main method to address this problem is to deposit a protective coating on the surface of the rare earth permanent magnets using electroplating, electroless plating, or physical vapor deposition.

[0003] However, when using electroplating for corrosion protection, the antimagnetic attenuation ability of rare earth permanent magnets is significantly reduced after adding a coating, while the high-temperature demagnetization rate is significantly increased. Summary of the Invention

[0004] The purpose of this disclosure is to provide a magnet and a method for manufacturing the magnet, wherein the magnet has a low high-temperature demagnetization rate.

[0005] To achieve the above objectives, this disclosure provides a magnet comprising a first magnetic substrate and a coating. The first magnetic substrate has a surface layer with a thickness of 10–20 μm, and the coating is adhered to the surface layer. The surface layer contains cracks with a width not exceeding 1.5 μm, and the area crack density of the cracks in the surface layer is 0.06–0.08 / μm. For the cracks, the length of cracks with a width less than 0.5 μm accounts for not less than 80% of the total crack length.

[0006] Preferably, the area crack density of the crack in the surface layer is 0.06 to 0.07 / μm, and the length of cracks with a width of less than 0.5μm accounts for not less than 87% of the total crack length.

[0007] Optionally, the adhesion between the surface layer and the coating is 15–45 N / mm. 2 Preferably, the adhesion between the surface layer and the coating is 15–35 N / mm. 2 .

[0008] Optionally, the surface layer has a hardness of 580HV to 650HV, preferably 600HV to 630HV.

[0009] Optionally, the high-temperature demagnetization rate of the magnet is 1-4%, and the squareness is 85-98%; preferably, the high-temperature demagnetization rate of the magnet is 1.5-2%, and the squareness is 90-98%.

[0010] Optionally, the first magnetic matrix contains 28.5–32.5 wt% R, 0–5 wt% M, 0.9–1.1 wt% B, and the balance T and unavoidable impurity elements; preferably, the first magnetic matrix contains 29–32 wt% R, 1.5–4.5 wt% M, 0.95–1 wt% B, and the balance T and unavoidable impurity elements; wherein,

[0011] R includes at least one rare earth element, and preferably, R includes at least two lanthanide elements;

[0012] M includes at least one of Ti, V, Cu, Cr, Mn, Ni, Zr, Nb, Mo, Hf, Ta, W, Al, Ga, Si, Bi, and Sn;

[0013] T is a transition metal element including Fe, preferably T is Fe or a combination of Fe and Co, wherein the content of Co in the first magnetic matrix is ​​not greater than 5% by weight.

[0014] Optionally, the plating layer includes a first nickel plating layer, a copper plating layer adhered to the first nickel plating layer, and a second nickel plating layer adhered to the copper plating layer, wherein the thickness of the first nickel plating layer is 10-15 μm, the thickness of the copper plating layer is 10-15 μm, and the thickness of the second nickel plating layer is 10-15 μm; or,

[0015] The coating is a nickel coating with a thickness of 30–45 μm.

[0016] This disclosure also provides a method for preparing a magnet, the method comprising:

[0017] An intermediate magnet obtained by coating with a heat treatment medium;

[0018] The intermediate magnet coated with a heat treatment medium is heat-treated under vacuum conditions to obtain the magnet. The heat treatment conditions include: a vacuum degree not higher than 10 Pa, a heat treatment temperature of 450-550 °C, and a heat treatment time of 5-6 h; preferably, the vacuum degree is not higher than 1 Pa.

[0019] Optionally, when heat-treating an intermediate magnet coated with a heat treatment medium, the method further includes a programmed temperature rise step; wherein the programmed temperature rise includes:

[0020] In the first heating stage, the heat treatment temperature is increased to 90-100℃ at a heating rate of 3-10℃ / min, and then the second heating stage begins.

[0021] In the second heating stage, the heat treatment temperature is increased to 380-400℃ at a heating rate of 1-2℃ / min, and then the third heating stage begins.

[0022] In the third heating stage, the heat treatment temperature is increased to 450-550℃ at a heating rate of 0.5-1℃ / min.

[0023] Optionally, the heat treatment medium includes tin foil and / or aluminum foil, and the thickness of the heat treatment medium is 20-30 μm.

[0024] Optionally, the intermediate magnet includes a second magnetic substrate and a coating. The second magnetic substrate has a porous layer with a thickness of 10–20 μm. The coating is adhered to the porous layer. The porous layer contains cracks with a width not greater than 4.5 μm and an area crack density of 0.1–0.4 / μm. For the cracks, the length of cracks with a width greater than 0.5 μm accounts for not less than 70% of the total crack length.

[0025] The magnet provided by this disclosure includes a first magnetic substrate and a coating, wherein the first magnetic substrate has a high-density surface layer, and the magnet has strong antimagnetic attenuation capability and low high-temperature demagnetization rate.

[0026] Other features and advantages of this disclosure will be described in detail in the following detailed description section. Attached Figure Description

[0027] The accompanying drawings are provided to further illustrate the present disclosure and form part of the specification. They are used together with the following detailed description to explain the present disclosure, but do not constitute a limitation thereof. In the drawings:

[0028] Figure 1 This is a scanning electron microscope image of the magnet prepared according to the embodiments of this disclosure;

[0029] Figure 2 This is a scanning electron microscope image of the intermediate magnet prepared according to the embodiments of this disclosure. Detailed Implementation

[0030] The specific embodiments of this disclosure will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit this disclosure.

[0031] A first aspect of this disclosure provides a magnet comprising a first magnetic substrate and a coating. The first magnetic substrate has a surface layer with a thickness of 10–20 μm, and the coating is adhered to the surface layer. The surface layer contains cracks with a width not greater than 1.5 μm, and the area crack density of the cracks in the surface layer is 0.06–0.08 / μm. For the cracks, the length of cracks with a width less than 0.5 μm accounts for not less than 80% of the total crack length. Preferably, the area crack density of the cracks in the surface layer is 0.06–0.07 / μm, and the length of cracks with a width less than 0.5 μm accounts for not less than 87% of the total crack length.

[0032] In this embodiment of the disclosure, specifically, the area crack density refers to the sum of the cumulative crack length (μm) and the area (μm) of the cross-section. 2 The smaller the area crack density, the fewer cracks there are in the cross section.

[0033] Under normal circumstances, after machining and electroplating pretreatment (degreasing, pickling, etc.), a loose layer is formed on the surface of the treated magnetic material. The grain structure in this loose layer will be destroyed to form defects, resulting in a higher high-temperature demagnetization rate of the magnetic material after plating treatment.

[0034] The inventors of this disclosure have discovered that, in order to reduce the high-temperature demagnetization rate of magnetic materials after coating treatment, the main approach adopted in related technologies is to improve the coating. How to reduce the high-temperature demagnetization rate of magnetic materials after coating treatment more easily and efficiently without changing the coating is a problem that urgently needs to be solved in the field.

[0035] In this embodiment, the magnet includes a first magnetic substrate with a surface layer and a plating layer. The plating layer adheres to the surface of the first magnetic substrate. The surface layer of the first magnetic substrate has a relatively small number of cracks, and a large proportion of these cracks have a width of less than 0.5 μm, resulting in a high density. The increased surface density is due to the fact that during heat treatment of the intermediate magnet, grains damaged during machining or pre-plating treatment regrow and rearrange. Furthermore, rare-earth-rich phases from the magnetic substrate adjacent to the surface layer also enter the surface layer. This repairs defects caused by the destruction of the grain structure. Therefore, the magnet provided in this disclosure has a low high-temperature demagnetization rate.

[0036] According to this disclosure, the adhesion force between the surface layer and the coating can be 15–45 N / mm. 2 Preferably, the adhesion between the surface layer and the coating can be 15–35 N / mm. 2 The high adhesion between the surface layer and the coating gives the magnet a long service life.

[0037] According to this disclosure, the surface portion of the first magnetic substrate has a high density, and therefore the surface layer has a high hardness. Specifically, the hardness of the surface layer can be 580HV to 650HV, preferably 600HV to 630HV.

[0038] According to this disclosure, the magnet has excellent magnetic properties, including a low high-temperature demagnetization rate and a high squareness. Specifically, the high-temperature demagnetization rate of the magnet can be 1-4%, and the squareness can be 85-98%. Preferably, the high-temperature demagnetization rate of the magnet can be 1.5-2%, and the squareness can be 90-98%.

[0039] Specifically, the high-temperature demagnetization rate mentioned in this embodiment refers to the rate by which the magnetic flux value of a magnet decreases after being placed at 100°C for 2.5 hours and then cooled to room temperature.

[0040] According to this disclosure, the composition of the first magnetic matrix can vary within a certain range. For example, the first magnetic matrix may contain 28.5–32.5% by weight of R, 0–5% by weight of M, 0.9–1.1% by weight of B, and the balance of T and unavoidable impurity elements; preferably, the first magnetic matrix may contain 29–32% by weight of R, 1.5–4.5% by weight of M, 0.95–1% by weight of B, and the balance of T and unavoidable impurity elements; wherein R may include at least one rare earth element, preferably, R may include at least two lanthanide elements, more preferably, R may include Y, Nd, and Pr; M may include at least one of Ti, V, Cu, Cr, Mn, Ni, Zr, Nb, Mo, Hf, Ta, W, Al, Ga, Si, Bi, and Sn; T may be a transition metal element including Fe, preferably, T may be Fe or a combination of Fe and Co, wherein the content of Co in the first magnetic matrix is ​​not greater than 5% by weight.

[0041] According to this disclosure, the composition and thickness of the plating layer can vary within a certain range. For example, the plating layer may include a first nickel plating layer, a copper plating layer adhered to the first nickel plating layer, and a second nickel plating layer adhered to the copper plating layer, wherein the thickness of the first nickel plating layer may be 10-15 μm, the thickness of the copper plating layer may be 10-15 μm, and the thickness of the second nickel plating layer may be 10-15 μm; or, the plating layer may be a nickel plating layer, and the thickness of the nickel plating layer may be 30-45 μm.

[0042] A second aspect of this disclosure provides a method for preparing a magnet, the method comprising: coating an intermediate magnet obtained by plating with a heat treatment medium; subjecting the intermediate magnet coated with the heat treatment medium to heat treatment under vacuum conditions to obtain the magnet, wherein the heat treatment conditions include: a vacuum degree not exceeding 10 Pa, a heat treatment temperature of 450–550 °C, and a heat treatment time of 5–6 h; preferably, the vacuum degree not exceeding 1 Pa.

[0043] The inventors of this disclosure have discovered that the heat treatment temperature plays a crucial role in the performance of the magnet after heat treatment. Specifically, when the heat treatment temperature is below 450°C, the grain regrowth and rearrangement are slow, making it difficult to repair defects. When the heat treatment temperature is above 550°C, the rare earth-rich phase in the adjacent magnetic matrix becomes a liquid phase and precipitates out, accumulating in large quantities on the surface of the first magnetic matrix. This results in a significant reduction in the adhesion between the coating and the first magnetic matrix.

[0044] According to this disclosure, in order to further improve the magnetic properties of the sintered magnet, the method may further include a programmed temperature rise step during the heat treatment of the intermediate magnet coated with the heat treatment medium; wherein the programmed temperature rise may include: in a first heating stage, raising the heat treatment temperature to 90-100°C at a heating rate of 3-10°C / min, and then entering a second heating stage; in the second heating stage, raising the heat treatment temperature to 380-400°C at a heating rate of 1-2°C / min, and then entering a third heating stage; in the third heating stage, raising the heat treatment temperature to 450-550°C at a heating rate of 0.5-1°C / min.

[0045] In this embodiment of the present disclosure, specifically, before heat treatment of the intermediate magnet, the intermediate magnet is coated with a heat treatment medium. During the heat treatment heating process, a low and gradually decreasing heating rate is adopted. These operations can improve the atmosphere of the intermediate magnet during the heat treatment heating process, reduce the fluctuation of the gas inside the intermediate magnet during the heating process, avoid gas fluctuation peaks in the sintering furnace, and keep the vacuum degree in the sintering furnace within a stable range, which is beneficial to improving the consistency of the magnet.

[0046] Simultaneously, controlling the heating rate during heat treatment helps control the rate of grain regrowth and rearrangement, ensuring uniform grain growth at defect locations and ultimately improving the coercivity and squareness of the magnet. Furthermore, the holding stage after heat treatment allows residual hydrogen gas from the coating process to escape further, effectively reducing the destructive effect of residual hydrogen gas on the interface between the coating and the first magnetic substrate. This further improves the corrosion resistance of the coating and enhances the adhesion between the coating and the first magnetic substrate.

[0047] According to this disclosure, the heat treatment medium can be selected within a certain range. For example, the heat treatment medium may include tin foil and / or aluminum foil, and the thickness of the heat treatment medium may be 20 to 30 μm.

[0048] According to this disclosure, the intermediate magnet can be obtained by coating a second magnetic substrate. Specifically, the intermediate magnet may include a second magnetic substrate and a coating. The second magnetic substrate has a porous layer with a thickness of 10-20 μm. The coating is adhered to the porous layer. The porous layer contains cracks with a width not greater than 4.5 μm and an area crack density of 0.1-0.4 / μm. For the cracks, the length of cracks with a width of 0.5 μm or more accounts for not less than 70% of the total crack length.

[0049] In this embodiment of the present disclosure, specifically, after pre-plating treatment and plating treatment, the second magnetic substrate has a loose layer with a rough surface and porous texture. The grain structure in this loose layer is destroyed during the pre-plating treatment and plating treatment, and it contains many large cracks. This loose layer is transformed into the surface layer of the first magnetic substrate after the heat treatment operation in this embodiment of the present disclosure.

[0050] Furthermore, the intermediate magnet can be obtained from alloy raw materials through melting, granulation, orientation molding, sintering, grinding, pre-plating treatment and plating treatment, wherein the pre-plating treatment includes alkaline washing to remove oil and acid washing to remove rust, and the plating treatment is electroplating.

[0051] Specifically, the intermediate magnet can be prepared by the following method:

[0052] (1) The alloy raw material is loaded into a vacuum induction melting furnace, and an alloy quick-solidification sheet with a thickness of 0.3-0.5 mm is prepared by quick-solidification process; then the alloy quick-solidification sheet is hydrogen-crushed and ground into powder with an average particle size of 3-5 μm by air jet mill. During the grinding process, a certain amount of air, carbon-containing solvent, lubricant and antioxidant can be filled into the grinding chamber of the air jet mill; then the powder is oriented and shaped in a magnetic field with a magnetic field strength of 2T and isostatically pressed, and then sintered at a sintering temperature of 950-1000℃ for 8-10h. The sintered product is aged for 3-4h at 900-1000℃ and then aged for 4-5h at 500-550℃ to obtain a magnetic blank;

[0053] (2) The magnetic blank is cut and processed into a magnetic substrate of a predetermined size, and then the magnetic substrate is ground by mechanical vibration grinding and / or rolling chamfering. The mechanical vibration grinding and rolling chamfering are performed by vibrating or rolling the abrasive and the workpiece in a vibratory grinder for 6 to 12 hours. The shape of the abrasive can be spherical, elliptical, triangular prism, cylindrical, conical, triangular pyramidal or various irregular shapes. The material of the abrasive can be alloy steel or ceramic materials such as Al2O3, SiO2, SiC. The abrasive can be used alone or in combination.

[0054] (3) Use an alkaline degreasing agent to perform alkaline washing and degreasing on the polished magnetic substrate. The temperature of alkaline washing and degreasing can be 45-75℃ and the time can be 5-30min. The alkaline degreasing agent may include 20g / L sodium phosphate, 10g / L sodium carbonate and 10g / L sodium hydroxide.

[0055] (4) Use an acidic solution to pickle and remove rust from the degreased magnetic substrate. The acidic solution can be at least one of dilute sulfuric acid, dilute hydrochloric acid or dilute nitric acid with a pH value of 1 to 5 and a concentration of 1 to 5%. The pickling and rust removal time can be 1 to 120 seconds.

[0056] (5) The rust-removed magnetic substrate is placed in a first electroplating solution for a first electroplating treatment. The first electroplating solution may include 200-350 g / L nickel sulfate, 30-60 g / L nickel chloride, 30-60 g / L boric acid, 0.08 g / L sodium dodecyl sulfate, and the remainder water. The pH value may be 3.8-4.8, the temperature of the first electroplating treatment may be 40-60°C, and the time may be 30-90 minutes. Then, the magnetic substrate treated with the first electroplating treatment is placed in a second electroplating solution. A second electroplating treatment is performed in the plating solution, which may include 250-350 g / L potassium pyrophosphate, 30-70 g / L copper pyrophosphate, and the remainder water. The pH value of the second electroplating treatment may be 7.5-9.0, the temperature of the second electroplating treatment may be 30-50℃, and the time may be 30-90 minutes. Then, the magnetic substrate that has undergone the second electroplating treatment is placed in the first electroplating solution for a third electroplating treatment. The temperature of the third electroplating treatment may be 45-55℃, and the time may be 30-90 minutes.

[0057] (6) The magnetic substrate that has undergone the third electroplating treatment is air-dried to obtain the intermediate magnet.

[0058] The above-described method for preparing intermediate magnets is only for illustrative purposes and is not intended to limit the scope of this disclosure. The magnetic matrix of this disclosure can also be prepared using other methods in the art.

[0059] The present disclosure will be further illustrated by the following examples, but the present disclosure is not limited thereto.

[0060] Unless otherwise specified, the raw materials, reagents, instruments and equipment involved in the embodiments of this disclosure can all be obtained by purchase.

[0061] Example 1

[0062] This embodiment is used to prepare an intermediate magnet, and includes the following operations:

[0063] (1) Alloy raw materials (7% Pr, 21% Nd, 2% Tb, 1.5% Co, 0.15% Cu, 0.15% Zr, 0.25% Ga, 1.0% B and balance Fe) were loaded into a vacuum induction melting furnace and a rapidly solidified alloy sheet with a thickness of 0.3 mm was prepared by a rapid solidification process. The rapidly solidified alloy sheet was then hydrogen-broken and ground into powder with an average particle size of 4.0 μm using an air jet mill. During the grinding process, a certain amount of air, carbon-containing solvent, lubricant and antioxidant were introduced into the grinding chamber of the air jet mill. The powder was then oriented and shaped in a magnetic field with a magnetic field strength of 2T and subjected to isostatic pressing. After that, it was sintered at a temperature of 1000℃ for 8 hours. The sintered product was aged for 3 hours at 900℃ and then aged for 5 hours at 500℃ to obtain a magnetic blank.

[0064] (2) The magnetic blank is cut and processed into a magnetic substrate with a size of 8.3*8.5*0.8mm. Then, the spherical SiO2 abrasive and the magnetic substrate are conventionally polished in a vibratory mill for 12 hours to obtain the polished magnetic substrate.

[0065] (3) The polished magnetic substrate was degreased by alkaline degreasing agent (20 g / L sodium phosphate, 10 g / L sodium carbonate and 10 g / L sodium hydroxide) at 45°C for 30 min to obtain the degreased magnetic substrate.

[0066] (4) The degreased magnetic substrate was pickled and derusted for 60 seconds using an acidic solution (dilute nitric acid with a pH of 3.0) to obtain the derusted magnetic substrate;

[0067] (5) The rust-removed magnetic substrate is placed in a first electroplating solution with a pH of 4.0 (200 g / L nickel sulfate, 30 g / L nickel chloride, 30 g / L boric acid, 0.08 g / L sodium dodecyl sulfate and the remainder water) for a first electroplating treatment, wherein the temperature of the first electroplating treatment is 40°C and the time is 60 minutes; then the magnetic substrate after the first electroplating treatment is placed in a second electroplating solution with a pH of 7.5 (250 g / L potassium pyrophosphate, 30 g / L copper pyrophosphate and the remainder water) for a second electroplating treatment, wherein the temperature of the second electroplating treatment is 30°C and the time is 40 minutes; then the magnetic substrate after the second electroplating treatment is placed in the first electroplating solution for a third electroplating treatment, wherein the temperature of the third electroplating treatment is 45°C and the time is 60 minutes;

[0068] (6) The magnetic substrate that has undergone the third electroplating treatment is air-dried to obtain an intermediate magnet.

[0069] The cross-section of the intermediate magnet prepared in this embodiment was scanned by electron microscopy, and the scanning results are as follows: Figure 2 As shown, by Figure 2 It can be seen that the intermediate magnet consists of a second magnetic substrate with a 20μm thick porous layer and a coating. The coating adheres to the surface of the porous layer of the second magnetic substrate. This porous layer contains cracks, and the area crack density of these cracks in the porous layer is 0.125 / μm. The maximum width of the cracks is 4.5μm, and the length of cracks with a width greater than 0.5μm accounts for 81.9% of the total crack length. After removing the coating, the hardness of the porous layer of the second magnetic substrate was tested, and the results showed that the hardness of the porous layer of the second magnetic substrate was 500HV.

[0070] Example 2

[0071] This embodiment illustrates the magnet preparation method of this disclosure, including the following steps:

[0072] (1) The intermediate magnet prepared in Example 1 was tightly wrapped with a heat treatment medium (25μm thick tin foil), then placed in a tin box, and then placed in a graphite box;

[0073] (2) The graphite box in step (1) is placed in a sintering furnace with a vacuum degree of 10 Pa for programmed temperature rise heat treatment to obtain the magnet of this embodiment. The programmed temperature rise heat treatment conditions include: in the first heating stage, the heat treatment temperature is raised to 100°C at a heating rate of 5°C / min, and then the second heating stage is entered; in the second heating stage, the heat treatment temperature is raised to 390°C at a heating rate of 2°C / min, and then the third heating stage is entered; in the third heating stage, the heat treatment temperature is raised to 450°C at a heating rate of 0.5°C / min, and then the temperature is held at 450°C for 6 hours; after the holding period, the magnet is cooled to room temperature in a nitrogen atmosphere at a cooling rate of 7°C / min.

[0074] The cross-section of the magnet in this embodiment was scanned by electron microscopy, and the scanning results are as follows: Figure 1 As shown, by Figure 1 It can be seen that the magnet consists of a first magnetic substrate and a coating. The first magnetic substrate has a surface layer with a thickness of 20 μm, and the coating is adhered to this surface layer. The surface layer contains cracks, and the area crack density of these cracks is 0.06 / μm. The maximum width of the cracks is 1.5 μm, and the length of cracks with a width less than 0.5 μm accounts for 89% of the total crack length. After removing the coating, the hardness of the surface layer of the first magnetic substrate was tested, and the results showed that the hardness of the surface layer of the first magnetic substrate was 600 HV.

[0075] Example 3

[0076] The magnet of this embodiment was prepared according to the method of Example 2, except that the programmed temperature rise heat treatment conditions of this embodiment include: in the first heating stage, the heat treatment temperature is raised to 90°C at a heating rate of 3°C / min, and then the second heating stage is entered; in the second heating stage, the heat treatment temperature is raised to 380°C at a heating rate of 1°C / min, and then the third heating stage is entered; in the third heating stage, the heat treatment temperature is raised to 500°C at a heating rate of 0.5°C / min, and then held at 500°C for 5.5 hours.

[0077] The magnet of this embodiment was tested according to the testing method of Example 2. The test results showed that the magnet is composed of a first magnetic substrate and a coating. The first magnetic substrate has a surface layer with a thickness of 10 μm, and the coating is adhered to the surface layer. The surface layer contains cracks, and the area crack density of the cracks in the surface layer is 0.08 / μm. The maximum width of the crack is 1.4 μm, and the length of cracks with a width of less than 0.5 μm accounts for 88.2% of the total crack length. The hardness of the surface layer is 580 HV.

[0078] Example 4

[0079] The magnet of this embodiment was prepared according to the method of Example 2, except that the programmed temperature rise heat treatment conditions of this embodiment include: in the first heating stage, the heat treatment temperature is raised to 95°C at a heating rate of 10°C / min, and then the second heating stage is entered; in the second heating stage, the heat treatment temperature is raised to 400°C at a heating rate of 1.5°C / min, and then the third heating stage is entered; in the third heating stage, the heat treatment temperature is raised to 550°C at a heating rate of 1°C / min, and then the temperature is held at 550°C for 5 hours.

[0080] The magnet of this embodiment was tested according to the testing method of Example 2. The test results showed that the magnet is composed of a first magnetic substrate and a coating. The first magnetic substrate has a surface layer with a thickness of 15 μm, and the coating is adhered to the surface layer. The surface layer contains cracks, and the area crack density of the cracks in the surface layer is 0.08 / μm. The maximum width of the crack is 1.5 μm, and the length of cracks with a width of less than 0.5 μm accounts for 86% of the total crack length. The hardness of the surface layer is 620 HV.

[0081] Example 5

[0082] This embodiment illustrates another method for preparing a magnet according to the present disclosure, including the following steps:

[0083] (1) The intermediate magnet prepared in Example 1 was tightly wrapped with a heat treatment medium (tin foil after 25 μm), then placed in a tin box, and then placed in a graphite box;

[0084] (2) The graphite box in step (1) is placed in a sintering furnace with a vacuum of 5 Pa for heat treatment to obtain the magnet of this embodiment. The heat treatment conditions include: heat treatment temperature of 450°C and time of 6h. After the heat treatment is completed, the magnet is cooled to room temperature in a nitrogen atmosphere at a cooling rate of 8°C / min.

[0085] The magnet of this embodiment was tested according to the testing method of Example 2. The test results showed that the magnet is composed of a first magnetic substrate and a coating. The first magnetic substrate has a surface layer with a thickness of 18 μm, and the coating is adhered to the surface layer. The surface layer contains cracks, and the area crack density of the cracks in the surface layer is 0.07 / μm. The maximum width of the crack is 1.5 μm, and the length of cracks with a width of less than 0.5 μm accounts for 86% of the total crack length. The hardness of the surface layer is 650 HV.

[0086] Example 6

[0087] The magnet of this embodiment was prepared according to the method of Example 5, except that the vacuum degree of this embodiment is 2 Pa, the heat treatment temperature is 500 °C, and the heat treatment time is 5.5 h.

[0088] The magnet of this embodiment was tested according to the testing method of Example 2. The test results showed that the magnet is composed of a first magnetic substrate and a coating. The first magnetic substrate has a surface layer with a thickness of 13 μm, and the coating is adhered to the surface layer. The surface layer contains cracks, and the area crack density of the cracks in the surface layer is 0.08 / μm. The maximum width of the crack is 1.5 μm, and the length of cracks with a width of less than 0.5 μm accounts for 84% of the total crack length. The hardness of the surface layer is 630 HV.

[0089] Example 7

[0090] The magnet of this embodiment was prepared according to the method of Example 5, except that the vacuum degree of this embodiment is 1 Pa, the heat treatment temperature is 550 °C, and the heat treatment time is 5 h.

[0091] The magnet of this embodiment was tested according to the testing method of Example 2. The test results showed that the magnet is composed of a first magnetic substrate and a coating. The first magnetic substrate has a surface layer with a thickness of 20 μm, and the coating is adhered to the surface layer. The surface layer contains cracks, and the area crack density of the cracks in the surface layer is 0.06 / μm. The maximum width of the crack is 1.5 μm, and the length of cracks with a width of less than 0.5 μm accounts for 85% of the total crack length. The hardness of the surface layer is 610 HV.

[0092] Comparative Example 1

[0093] The magnets of this comparative example were prepared according to the method of Example 5, except that the heat treatment temperature in this comparative example was 400°C.

[0094] The magnet of this comparative example was tested according to the testing method of Example 2. The test results showed that the magnet consisted of a magnetic substrate with a 30 μm thick porous layer and a coating. The coating adhered to the surface of the porous layer of the magnetic substrate. The porous layer contained cracks, and the area crack density of the cracks in the porous layer was 0.2 / μm. The maximum width of the cracks was 4.3 μm, and the length of cracks with a width of 0.5 μm or more accounted for 79.8% of the total crack length. The hardness of the porous layer was 400 HV.

[0095] Comparative Example 2

[0096] The magnets of this comparative example were prepared according to the method of Example 5, except that the heat treatment temperature in this comparative example was 600°C.

[0097] The magnet of this comparative example was tested according to the testing method of Example 2. The test results showed that the magnet consisted of a magnetic substrate with a 25 μm thick porous layer and a coating. The coating adhered to the surface of the porous layer of the magnetic substrate. The porous layer contained cracks, and the area crack density of the cracks in the porous layer was 0.3 / μm. The maximum width of the cracks was 4.2 μm, and the length of cracks with a width of 0.5 μm or more accounted for 74.6% of the total crack length. The hardness of the porous layer was 500 HV.

[0098] Comparative Example 3

[0099] The magnet of this comparative example was prepared according to the method of Example 5, except that: in this comparative example, the intermediate magnet was not wrapped with a heat treatment medium, and the intermediate magnet was directly used for the sintering operation in step (2).

[0100] The magnet of this comparative example was tested according to the testing method of Example 2. The test results showed that the magnet consisted of a magnetic substrate with a 23 μm thick porous layer and a coating. The coating adhered to the surface of the porous layer of the magnetic substrate. The porous layer contained cracks, and the area crack density of the cracks in the porous layer was 0.2 / μm. The maximum width of the cracks was 4 μm, and the length of cracks with a width of 0.5 μm or more accounted for 68% of the total crack length. The hardness of the porous layer was 450 HV.

[0101] Test case

[0102] Ten pieces each of the intermediate magnet prepared in Example 1, the magnets prepared in Examples 2-7, and the magnets prepared in Comparative Examples 1-3 were taken and tested as follows.

[0103] (1) Measure and record the first magnetic flux value of each intermediate magnet or magnet, then place each intermediate magnet or magnet in an oven at 100°C for 2.5 hours, then remove it and allow it to cool naturally to room temperature. Measure and record the second magnetic flux value of each intermediate magnet or magnet again, and then calculate the high-temperature demagnetization rate of each intermediate magnet or magnet according to the following formula:

[0104] High-temperature demagnetization rate = (First magnetic flux value - Second magnetic flux value) / First magnetic flux value × 100%

[0105] The high-temperature demagnetization rates of 10 intermediate magnets or magnets were calculated and averaged to be used as the high-temperature demagnetization rates corresponding to Examples 1, 2-7 and Comparative Examples 1-3. The results are shown in Table 1.

[0106] (2) Measure and record the intrinsic coercivity, remanence and squareness of each intermediate magnet or magnet, and calculate the intrinsic coercivity, remanence and squareness of 10 intermediate magnets or magnets and take the average value. These values ​​are used as the intrinsic coercivity, remanence and squareness corresponding to Examples 1, 2 to 7 and Comparative Examples 1 to 3. The results are shown in Table 1.

[0107] (3) For the coating, the adhesion force between the coating and the magnetic substrate is tested in each intermediate magnet or magnet. Each intermediate magnet or magnet is fixed on the mold, and a small metal block of a fixed area is glued to its surface. Then, the small metal block is pulled up by a tensile tester until the coating separates from the magnetic substrate. The tensile force value at this time is the adhesion force. The test results are shown in Table 1.

[0108] Table 1

[0109]

[0110] As can be seen from Table 1, the magnet provided in this disclosure has excellent magnetic properties with a low high-temperature demagnetization rate, and the magnetic substrate and the coating in the magnet have a high adhesion force.

[0111] The preferred embodiments of the present disclosure have been described in detail above with reference to the accompanying drawings. However, the present disclosure is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present disclosure, various simple modifications can be made to the technical solutions of the present disclosure, and these simple modifications all fall within the protection scope of the present disclosure.

[0112] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, this disclosure will not describe the various possible combinations separately.

[0113] Furthermore, various different embodiments of this disclosure can be combined in any way, as long as they do not violate the spirit of this disclosure, they should also be regarded as the content disclosed in this disclosure.

Claims

1. A magnet, characterized in that, The magnet includes a first magnetic substrate and a coating. The first magnetic substrate has a surface layer with a thickness of 10–20 μm. The coating is adhered to the surface layer. The surface layer contains cracks with a width not exceeding 1.5 μm and an area crack density of 0.06–0.08 / μm. For the cracks, the length of cracks with a width less than 0.5 μm accounts for not less than 80% of the total crack length. The first magnetic matrix contains 28.5–32.5 wt% R, 0–5 wt% M, 0.9–1.1 wt% B, and the balance T and unavoidable impurity elements; wherein, R includes at least one rare earth element; M includes at least one of Ti, V, Cu, Cr, Mn, Ni, Zr, Nb, Mo, Hf, Ta, W, Al, Ga, Si, Bi, and Sn; T represents transition metal elements, including Fe.

2. The magnet according to claim 1, wherein, The area crack density of the crack in the surface layer is 0.06 to 0.07 / μm, and the length of cracks with a width of less than 0.5μm accounts for not less than 87% of the total crack length.

3. The magnet according to claim 1 or 2, wherein, The adhesion between the surface layer and the coating is 15–45 N / mm. 2 .

4. The magnet according to claim 3, wherein, The adhesion between the surface layer and the coating is 15–35 N / mm. 2 .

5. The magnet according to claim 1 or 2, wherein, The surface layer has a hardness of 580HV to 650HV.

6. The magnet according to claim 5, wherein, The surface layer has a hardness of 600HV to 630HV.

7. The magnet according to claim 1 or 2, wherein, The magnet has a high-temperature demagnetization rate of 1-4% and a squareness of 85-98%.

8. The magnet according to claim 7, wherein, The magnet has a high-temperature demagnetization rate of 1.5-2% and a squareness of 90-98%.

9. The magnet according to claim 1, wherein, The first magnetic matrix contains 29-32% by weight of R, 1.5-4.5% by weight of M, 0.95-1% by weight of B, and the balance of T and unavoidable impurity elements.

10. The magnet according to claim 1 or 9, wherein, R includes at least two lanthanide elements; T is Fe or a combination of Fe and Co, wherein the content of Co in the first magnetic matrix is ​​not greater than 5% by weight.

11. The magnet according to claim 10, wherein, The R includes Y, Nd, and Pr.

12. The magnet according to claim 1 or 2, wherein, The plating layer includes a first nickel plating layer, a copper plating layer adhered to the first nickel plating layer, and a second nickel plating layer adhered to the copper plating layer, wherein the thickness of the first nickel plating layer is 10-15 μm, the thickness of the copper plating layer is 10-15 μm, and the thickness of the second nickel plating layer is 10-15 μm; or... The coating is a nickel coating with a thickness of 30–45 μm.

13. A method for preparing the magnet according to any one of claims 1-12, characterized in that, The method includes: An intermediate magnet obtained by coating with a heat treatment medium; The intermediate magnet coated with a heat treatment medium is heat-treated under vacuum conditions to obtain the magnet. The heat treatment conditions include: a vacuum degree not exceeding 10 Pa, a heat treatment temperature of 450–550 °C, and a heat treatment time of 5–6 h.

14. The method according to claim 13, wherein, The vacuum level is not higher than 1 Pa.

15. The method according to claim 14, wherein, When heat-treating an intermediate magnet coated with a heat treatment medium, the method further includes a programmed temperature rise step; wherein the programmed temperature rise includes: In the first heating stage, the heat treatment temperature is increased to 90-100℃ at a heating rate of 3-10℃ / min, and then the second heating stage begins. In the second heating stage, the heat treatment temperature is increased to 380-400℃ at a heating rate of 1-2℃ / min, and then the third heating stage begins. In the third heating stage, the heat treatment temperature is increased to 450-550℃ at a heating rate of 0.5-1℃ / min.

16. The method according to any one of claims 13-15, wherein, The heat treatment medium includes tin foil and / or aluminum foil, and the thickness of the heat treatment medium is 20-30 μm.

17. The method according to any one of claims 13-15, wherein, The intermediate magnet includes a second magnetic substrate and a coating. The second magnetic substrate has a porous layer with a thickness of 10–20 μm. The coating is adhered to the porous layer. The porous layer contains cracks with a width not greater than 4.5 μm and an area crack density of 0.1–0.4 / μm. For the cracks, the length of cracks with a width greater than 0.5 μm accounts for not less than 70% of the total crack length.

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