JP2008010726A - Rare earth bond magnet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rare earth bond magnet equipped with precise rust proofing coat, which has sufficient mechanical strength and uniform film thickness and is excellent in oxygen shielding property. <P>SOLUTION: In the bond magnet consisting of rare earth iron based alloy magnetic powder and synthetic resin binder, the surface of the magnet is covered with an Ni based electroless deposition coating including phosphorus, and the thickness of coating is 2-15 μm, and a ratio V/S of magnet volume V to magnet superficial area S is more than 0.020(cc/cm<SP>2</SP>). The content quotient of phosphorus in the coating is 3-12%. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a rare earth bonded magnet, and more particularly to a rare earth bonded magnet having a rust preventive film.

  Rare earth bonded magnets exhibit excellent magnetic properties at each stage compared to ferrite-based bonded magnets, so they are widely used in various types of OA motors, automotive electrical motors, mobile phone vibration motors, etc., including HDD spindle motors for personal computers. Has been. However, since the NdFeB magnetic powder and SmFeN magnetic powder used as raw materials contain pure iron as an element, there is a problem that they are oxidized by oxygen in the atmosphere to cause rust and the magnetic performance is likely to deteriorate. For this reason, in the rare earth bonded magnet, a rust preventive film is formed on the magnet surface by epoxy-based cationic electrodeposition coating, spray coating, or the like in order to prevent oxidation and to exhibit excellent magnetic performance over a long period of time.

By the way, HDD spindle motors, etc., of course, have a magnetic characteristic that is not limited to the parts used in the product, and from the viewpoint of preventing magnetic powder contamination on the product, it has been required that the film is not easily damaged during handling. Resin-based coatings such as electrodeposition coating and spray coating did not exhibit sufficient corrosion resistance and mechanical strength during use. Therefore, a metal plating method has been put to practical use as a coating method that is superior in mechanical strength to resin coating and also has a high anticorrosion effect. For example, Patent Document 1 discloses a method of performing electrolytic Ni plating on an NdFeB-based bonded magnet. ing.
JP-A-4-276095

  However, if a relatively thin film is formed on a ring magnet or the like by the above conventional electrolytic Ni plating coating, the inner peripheral side plating film becomes thinner than the outer peripheral side, and the film thickness is not uniform. In the motor assembling process, film damage due to contact with the other metal part is a problem. In addition, the electrolytic Ni plating coating is also problematic in that the magnet is oxidized and the magnetic properties are lowered due to oxygen molecules entering from the plating pinhole during use at high temperatures.

  Therefore, the present invention solves such a problem, and provides a rare earth bonded magnet having a dense rust preventive film having sufficient mechanical strength, uniform film thickness and excellent oxygen barrier properties. Objective.

In order to achieve the above object, in the present invention, in a bonded magnet comprising a rare earth iron-based alloy magnetic powder and a synthetic resin binder, the surface of the magnet is coated with a film containing Ni-based electroless plating containing phosphorus, and the thickness of the film Is 2 μm to 15 μm, and the ratio V / S of the magnet volume V to the magnet surface area S is 0.020 (cc / cm ² ) or more. Here, it is preferable that the content ratio of phosphorus in the film is 3% to 12% and the micro Vickers hardness Hv is Hv = 500 to 1000. When exposed to an atmosphere of 120 ° C. for 1000 hours, the change over time in the weight increase due to oxidation per surface area of the bonded magnet is proportional to the 0.35th power of the elapsed time, and the proportionality coefficient k is 0.00030 or less. It is desirable to be. Furthermore, it is desirable that the entire or part of the base surface is coated with carbon powder.

  The rare earth bonded magnet of the present invention is provided with a dense rust preventive film having sufficient mechanical strength, uniform film thickness and excellent oxygen barrier properties.

  As the rare earth alloy magnetic powder for forming the bonded magnet, NdFeB-based or SmFeN-based powder can be used. The above magnetic powder may be used alone or in combination. Since magnetic powder contains pure iron as a constituent element, a coupling treatment is performed for the purpose of preventing oxidation during molding and at the stage of using the product. Coupling agents used include γ-aminopropyltriethoxysilane and γ-aminopropyltriethoxysilane for silane type, tetraisopropyl titanate, tetraisobutyl orthotitanate, etc. for titanate type, aluminum-based acetoalkoxyaluminum diisopropylate, etc. Zirconium-based zirconium tributoxy systemate can be used.

  The coupled magnetic powder is mixed with a synthetic resin binder and molded. Synthetic resins to be used include nylon 6, nylon 12, polyethylene, chlorinated polyethylene, polyvinyl chloride, polyphenylene sulfide, polypropylene, thermoplastic resins typified by ethylene vinyl acetate copolymer, and various epoxy resins of bisphenol type and novolac type. Thermosetting resins represented by phenol resin, melamine resin and urea resin can be used. When a thermoplastic resin is used as a binder, a ring-shaped bonded magnet can be formed by mixing with magnetic powder, followed by kneading granulation and then by injection molding or extrusion molding. In the case of a thermosetting resin, after mixing with magnetic powder, a green molded body having a desired shape is obtained by compression molding in a powder state, and then cured by heat treatment to complete the molding of a bonded magnet. The magnet shape to which the present invention is applied may be a ring shape, a cylindrical shape, a disk shape, a spherical shape, or the like, but is not limited thereto.

  Since a fine hole exists on the surface of the ring-shaped bonded magnet, it is preferable to coat the surface of the bonded magnet with an inorganic fine powder. Carbon, mica, talc, calcium carbonate, etc. can be used as this inorganic fine powder. As a coating method, there can be used a method in which a bond magnet, a vibration medium and an inorganic fine powder are filled in a vibration barrel, a method in which these inorganic fine powders are dispersed in a solvent, and a method of immersing in the liquid. It is preferable to coat the inorganic fine powder and then immerse it in a palladium chloride solution to perform a catalytic treatment for promoting metal deposition in the following electroless plating.

  The rare earth bonded magnet thus formed is subjected to Ni-P alloy electroless plating. By this alloy electroless plating, the micro Vickers hardness of the coating containing P was about Hv = 500 to 900, which was several times that of electrolytic Ni plating. In addition, the electrolytic Ni plating film is a crystalline magnetic material, but the Ni-P electroless plating film is amorphous and non-magnetic. Therefore, the magnetic field lines of the magnet are adjacent to the adjacent poles through the film like the electrolytic Ni plating film. The problem of leakage does not occur. In addition, since electroless Ni—P plating has a better sealing effect (covering property) against fine holes in the magnet base than conventional electrolytic Ni plating, oxygen atoms do not easily enter the magnet. Here, since the micro Vickers hardness Hv of the film in the electroless plating is substantially proportional to the amount of P contained in the film, in order to obtain the micro Vickers hardness Hv = 500 to 1000, the content ratio of P in the film Is preferably 3% to 12%. If it is 3% or less, the film hardness is insufficient, and if it exceeds 12%, the film becomes magnetized, which is not preferable.

  In plating, a commercially available plating bath can be used, but a uniquely prepared bath can also be used. This electroless plating bath contains metal salts such as chlorides and sulfates, reducing agents such as hydrazine and sodium hydrogen phosphate, sacrificial agents such as ammonia and citric acid, pH adjusting agents, buffering agents, etc. Is done. Unlike electroplating, electroless plating has no unevenness in the metal deposition thickness of the ring-shaped inner periphery, outer periphery, and corner, and can achieve a uniform finish. In addition, prior to performing the Ni-P-based electroless plating, electroless Cu plating may be performed. By forming the Ni-P film on the Cu film, the adhesion is improved.

The inventors have found that the larger the ratio of the magnet surface area to the magnet volume, the more the influence of weight increase due to oxidation is suppressed. When the magnet is exposed in a high temperature atmosphere, oxygen atoms that pass through or penetrate the diffusion of the film cause an oxidation reaction to proceed from the magnetic powder directly under the film to the back of the magnet. In order to suppress the influence of the decrease, it has been found effective to set the ratio of the magnet volume V and the magnet surface area S to a certain value or more. Specifically, the value V / S obtained by dividing the magnet volume V by the magnet surface area S is preferably 0.020 (cc / cm ² ) or more.

  The inventors have further found that the value of the increase in oxidation per surface area of the bonded magnet exposed under high temperature is approximately proportional to the 0.35th power of the exposure time. And the anticorrosive film of the bonded magnet of the present invention has found that the proportionality factor of the oxidation increase value to the 0.35th power of the elapsed time is small and the progress of the oxidation is slow compared with the conventional resin coating or electrolytic plating film. It was. FIG. 1 shows a change line x with time of oxidation increase, and the proportional coefficient k is preferably 0.00030 or less.

  The film thickness of the electroless Ni—P plating is preferably 2 μm to 15 μm. If the thickness is 2 μm or less, the film is difficult to be uniform and the film strength is not sufficient. In addition, fine pinholes are generated, and oxygen may enter the magnet through the pinholes during use at high temperatures, and oxidation may proceed. On the other hand, when the film thickness is set to 15 μm or more, the distance between the magnet surface and the motor core or the like is increased, which causes a problem that sufficient magnetic force cannot be used. Hereinafter, examples and comparative examples will be described with reference to Table 1.

(Example 1)
The NdFeB alloy magnetic powder produced by the ultra-quenching method is treated with 0.1 wt% aminopropyltriethoxysilane, then mixed with a novolac type epoxy resin and dried to obtain a mixture having a magnetic powder weight ratio of 93 wt%. It was. A ring-shaped molded body having an outer diameter of 9 mm, an inner diameter of 7 mm, and a length of 1.4 mm was produced using a compression molding machine with a pressing force of 10 ton / cm ² , and heat-cured at 180 ° C. The ratio V / S of the volume V per surface area S of this molded body is 0.029 cc / cm ² . Thereafter, a Ni—P film (P content 8% in the film) was formed in an electroless plating bath having the conditions and composition shown in Table 2. In this case, the film thickness is 11 μm to 12 μm, which is almost uniform on the inner and outer circumferences, the micro Vickers hardness Hv of the film is sufficiently large as 700, and the proportional coefficient k of the change with time in oxidation increase under 120 ° C. exposure is 0.00017 (g・ Cm ⁻² · Hr ^−0.35 ) and sufficiently small.

(Example 2)
A disc-shaped magnet having an outer diameter of 9 mm and a length (thickness) of 1.4 mm was prepared with the same composition as in Example 1, and the same electroless plating was performed. The ratio V / S of the volume V per surface area S of this molded body is 0.053 cc / cm ² . The thickness of the Ni—P film (P content 8% in the film) is about 13 μm. The micro Vickers hardness Hv of the film in this case was sufficiently large as 630, and the proportionality coefficient k of the change over time in oxidation increase was sufficiently small as 0.00015 (g · cm ⁻² · Hr ^−0.35 ).

(Example 3)
A ring-shaped magnet having the same composition as in Example 1 was prepared, and the surface of the molded body was covered with carbon powder, followed by the same electroless plating. The ratio V / S of the volume V per surface area S of the molded body is 0.029 cc / cm ² . In this case, the thickness of the Ni-P film (P content 4% in the film) is about 12 μm to 13 μm and is almost uniform on the inner and outer circumferences, and the micro Vickers hardness Hv of the film is sufficiently large as 550, which is proportional to the change over time in oxidation increase The coefficient k was 0.00012 (g · cm ⁻² · Hr ^−0.35 ) and sufficiently small.

Example 4
A ring magnet having an outer diameter of 20 mm, an inner diameter of 18 mm, and a length of 10 mm having the same composition as in Example 1 was prepared and subjected to the same electroless plating. The ratio V / S of the volume V per surface area S of the molded body is 0.045 cc / cm ² . In this case, the thickness of the Ni-P film (P content 10% in the film) is about 11 μm, uniform on the inner and outer circumferences, the micro Vickers hardness Hv of the film is sufficiently large as 900, and the proportionality coefficient k of the change over time in oxidation increase is 0.00017 sufficiently as small as ^{(g · cm -2 · Hr -0.35} ).

(Comparative Example 1)
Electrolytic Ni plating was carried out in a rotating barrel with the composition shown in Table 3 on a ring magnet manufactured with the same composition and the same shape as in Example 1. The volume ratio V / S per surface area of the molded body is 0.029 cc / cm ² . In this case, the film thickness is not uniform, with the outer circumference being about 20 μm and the inner circumference being about 5 μm, and the micro Vickers hardness Hv of the film is as small as 150. Further, the proportional coefficient k of the oxidation increase aging under 120 ° C. exposure increases the ^{0.00039 (g · cm -2 · Hr} -0.35).

(Comparative Example 2)
A ring-shaped molded body having an outer diameter of 9 mm, an inner diameter of 8 mm, and a length of 1.4 mm was produced in the same manner and in the same manner as in Example 1, and the same electroless plating as in Example 1 (P content in the coating was 8). %). The ratio V / S of the volume V per surface area S of this molded body is 0.018 cc / cm ² . In this case, the proportional coefficient k of the oxidation increment aging increases with ^{0.00033 (g · cm -2 · Hr} -0.35).

(Comparative Example 3)
A ring-shaped molded body having an outer diameter of 20 mm, an inner diameter of 18 mm, and a length of 10 mm was manufactured in the same manner and in the same manner as in Example 1, and electrolytic Ni plating was performed in a rotating barrel under the conditions and compositions shown in Table 3. . The ratio V / S of the volume V per surface area S of this molded body is 0.045 cc / cm ² . In this case, the film thickness is non-uniform such that the outer circumference is about 12 μm and the inner circumference is about 4 μm, and the micro Vickers hardness Hv of the film is as small as 130. Proportionality factor k of oxidation weight gain aging increases the ^{0.00051 (g · cm -2 · Hr} -0.35).

It is a graph which shows the time-dependent change of oxidation increase.

Claims (4)

In a bonded magnet comprising a rare earth iron-based alloy magnetic powder and a synthetic resin binder, the surface of the magnet is coated with a Ni-based electroless plating film containing phosphorus, and the thickness of the film is 2 μm to 15 μm. A rare earth bonded magnet having a volume V ratio V / S of 0.020 (cc / cm ² ) or more. 2. The rare earth bonded magnet according to claim 1, wherein the content ratio of phosphorus in the film is 3% to 12%, and the micro Vickers hardness Hv is Hv = 500 to 1000. 3. When exposed to an atmosphere at 120 ° C. for 1000 hours, the change over time in the weight increase due to oxidation per surface area of the bonded magnet is proportional to the 0.35th power of the elapsed time, and the proportionality constant is 0.00030 or less. 3. A rare earth bonded magnet according to 1 or 2. The rare earth bonded magnet according to any one of claims 1 to 3, wherein the entire or part of the base surface is coated with carbon powder.

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