JP5326747B2 - Method for preventing degranulation of R-Fe-B sintered magnet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for efficiently preventing particle shedding caused by mechanical processing for an R-Fe-B-based sintered magnet. <P>SOLUTION: In the method for preventing the particle shedding of an R-Fe-B-based sintered magnet, grindstone processing is performed for the surface of the magnet followed by performing heat treatment at 200-600&deg;C under an atmosphere of 1&times;10<SP>2</SP>to 1&times;10<SP>5</SP>Pa of oxygen partial pressure and 0.1 to 1,000 Pa (excluding 1,000 Pa) of steam partial pressure to form a modified layer on the surface. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a method for preventing degranulation caused by mechanical processing on an R—Fe—B based sintered magnet.

  R-Fe-B-based sintered magnets represented by Nd-Fe-B-based sintered magnets are widely used today because they use resource-rich and inexpensive materials and have high magnetic properties. Although it is used in various fields, since it contains a highly reactive rare earth metal: R, it has the property of being easily oxidized and corroded in the atmosphere (Patent Document 1). Due to this characteristic, the R-Fe-B sintered magnet is generated by the force applied to the magnet by performing mechanical processing for the purpose of dimensional adjustment performed at the final stage in the manufacturing process. Moisture penetrates into the magnet from the work-deteriorated layer (a region where micro cracks and strains of several μm to several tens of μm deep from the surface), and corrodes the grain boundary phase (R-rich phase), thereby causing degranulation. As a result, the weight of the magnet is reduced. Although this phenomenon is well known among those skilled in the art, an effective prevention method has not yet been found.

Japanese Patent No. 3519069

  Then, an object of this invention is to provide the method of preventing effectively the graining produced by the mechanical processing with respect to a R-Fe-B type sintered magnet.

  As a result of intensive studies in view of the above points, the present inventor appropriately controlled oxygen partial pressure and water vapor partial pressure after performing grinding wheel processing exemplified by surface grinding on the magnet surface. It has been found that degranulation can be effectively prevented by modifying the magnet surface by performing a heat treatment in an oxidizing atmosphere.

The method for preventing degranulation of the R—Fe—B sintered magnet of the present invention completed on the basis of the above knowledge, as described in claim 1, after the grindstone is processed on the magnet surface, the oxygen partial pressure is By performing heat treatment at 200 ° C. to 600 ° C. in an atmosphere of 1 × 10 ² Pa to 1 × 10 ⁵ Pa and water vapor partial pressure of 0.1 Pa to 1000 Pa (excluding 1000 Pa), the inside of the magnet In order, the main layer having an R-concentrated layer containing R, Fe, B, and oxygen, having a laterally extending length of 0.5 μm to 30 μm and a thickness of 50 nm to 400 nm, at least R, Fe, and oxygen A modified layer having at least three outermost layers containing iron oxide mainly composed of hematite as a constituent component is formed (provided that a polyimide resin film is formed on the surface of the modified layer ) R-Fe-B firing Except for the magnet).
Further, the method for preventing degranulation of the R—Fe—B based sintered magnet according to claim 2 is the method for preventing degranulation of the R—Fe—B based sintered magnet according to claim 1, wherein the count is # 60 to # 400. Processing is performed using a grindstone having a particle size.
Further, the method for preventing degranulation of an R—Fe—B based sintered magnet according to claim 3 is the method for preventing degranulation of an R—Fe—B based sintered magnet according to claim 1 or 2, wherein the oxygen partial pressure and the water vapor content are reduced. The pressure ratio (oxygen partial pressure / water vapor partial pressure) is 1 to 400.
Further, the method for preventing the degranulation of the R—Fe—B based sintered magnet according to claim 4 is the method for preventing the degranulation of the R—Fe—B based sintered magnet according to any one of claims 1 to 3. The temperature rise to the temperature at which the heat treatment is performed is performed in an atmosphere having an oxygen partial pressure of 1 × 10 ² Pa to 1 × 10 ⁵ Pa and a water vapor partial pressure of 1 × 10 ⁻³ Pa to 100 Pa.
Furthermore, the method for preventing degranulation of an R—Fe—B based sintered magnet according to claim 5 is the modification of the method for preventing degranulation of an R—Fe—B based sintered magnet according to any one of claims 1 to 4. Compared with the composition of the unmodified magnet, the composition of the main layer in the layer is characterized in that the Fe content is decreased and the oxygen content is increased.

  ADVANTAGE OF THE INVENTION According to this invention, the method of preventing effectively the granulation which arises by the mechanical processing with respect to a R-Fe-B type sintered magnet can be provided.

It is the schematic (side view) of an example of the continuous processing furnace suitable for implementation of the degreasing prevention method of the R-Fe-B type sintered magnet of this invention. It is a photograph which shows the result of the cross-sectional observation using the field emission type | mold scanning electron microscope of the surface-modified magnet body test piece in Example 1. FIG. It is a chart which shows the result of having analyzed the outermost layer which comprises the surface-modified part (surface modification layer) of the surface-modified magnet body test piece from the surface using the X-ray-diffraction apparatus.

In the method for preventing the degranulation of the R—Fe—B based sintered magnet of the present invention, after grinding the stone surface, the oxygen partial pressure is 1 × 10 ² Pa to 1 × 10 ⁵ Pa and the water vapor partial pressure is By performing heat treatment at 200 ° C. to 600 ° C. in an atmosphere of 0.1 Pa to 1000 Pa (excluding 1000 Pa), a modified layer is formed on the surface. Grinding wheel processing is a kind of mechanical processing, which causes the generation of a work-degraded layer, but the present inventor has found that the surface composition of the magnet is made uniform by performing grinding wheel processing. Combining an effective grinding wheel processing and a suitably controlled oxidation heat treatment has made it possible to achieve excellent anti-granulation prevention. That is, the surface of the R—Fe—B based sintered magnet is mainly composed of a main phase (R ₂ Fe ₁₄ B phase) and a grain boundary phase (R rich phase), and its composition is not uniform. In this state, a uniform oxidation heat treatment cannot be performed on the magnet. However, after grinding the magnet surface, the heat treatment is performed in an oxidizing atmosphere in which the oxygen partial pressure and the water vapor partial pressure of less than 10 hPa are appropriately controlled. By performing the step, it is possible to uniformly form a modified layer exhibiting excellent corrosion resistance with respect to the entire surface of the magnet having a uniform surface composition, thereby effectively preventing degranulation. Several methods for modifying the surface of a magnet by performing a heat treatment in an oxidizing atmosphere are already known. For example, JP 2006-156683 A, JP 2006-210864 A, and JP 2007-103523 A. Japanese Patent Application Laid-Open No. 2007-207936 describes a method of performing heat treatment using water vapor alone or by combining oxygen with water vapor to form an oxidizing atmosphere. However, when heat treatment is performed in an atmosphere with a high water vapor partial pressure (10 hPa (1000 Pa) or more) as proposed in these patent documents, a large amount of hydrogen is generated as a by-product due to an oxidation reaction that occurs on the surface of the magnet. However, there is a possibility that the magnetic properties may be deteriorated by occlusion of hydrogen generated by the magnet and embrittlement. On the other hand, if heat treatment is performed in an oxidizing atmosphere in which the oxygen partial pressure and the water vapor partial pressure of less than 10 hPa are appropriately controlled, surface modification that exhibits excellent corrosion resistance can be effectively performed on the magnet. In addition, it is possible to suppress a decrease in the magnetic properties of the magnet due to the mass production of hydrogen caused by the presence of excess water vapor.

  The grindstone processing performed on the magnet surface in the present invention means an operation of processing the magnet surface using a grindstone, and specific examples thereof include grinding and cutting. More specifically, for example, a surface grinding process using a surface grinding machine or a double-head grinding machine, which can be applied to a surface machining process such as a rectangular magnet or a plate magnet, or a curved surface such as an arcuate magnet is used. This can be applied to the surface processing of magnets having a shape grinding process performed using a general-purpose grinding wheel, etc., as well as a cutting process performed using a grinding wheel cutting machine. Any grindstone processing applied to magnets of various shapes may be used as long as it is to be processed. It is desirable that the grindstone used has a grain size of # 60 to # 400. If the count is less than # 60 (if the particle size is too coarse), the surface of the magnet may be processed more than necessary, which may adversely affect the dimensional accuracy of the magnet. On the other hand, if the count exceeds # 400 (If the particle size is too fine), the surface composition of the magnet may become insufficiently uniform. In addition, as for the rotation speed of a grindstone, 600 rpm-2000 rpm are desirable. Further, the feeding speed of the magnet to the processing apparatus is preferably 0.01 m / min to 5 m / min, and more preferably 0.1 m / min to 4 m / min. Grinding stone processing may be performed after another method such as cutting of the magnet or grinding for dimension adjustment. However, by grinding the grinding of the magnet or dimension adjustment by grinding wheel processing, Operation and uniform surface composition of the magnet can be achieved simultaneously.

Oxidation heat treatment performed after grinding the magnet surface is performed with an oxygen partial pressure of 5 × 10 ³ in order to form a modified layer exhibiting excellent corrosion resistance on the magnet surface more effectively and at low cost. Pa~5 × ¹⁰ 4 Pa is ^{preferred, 1 × 10 4 Pa~4 × 10} 4 Pa is more preferable. The water vapor partial pressure is preferably 250 Pa to 900 Pa, and more preferably 400 Pa to 700 Pa. The ratio of oxygen partial pressure to water vapor partial pressure (oxygen partial pressure / water vapor partial pressure) is preferably 1 to 400, and more preferably 5 to 100. The oxidizing atmosphere in the processing chamber may be formed, for example, by individually introducing these oxidizing gases so as to have a predetermined partial pressure, or these oxidizing gases are included at a predetermined partial pressure. You may form by introduce | transducing the atmosphere which has a dew point. Further, an inert gas such as nitrogen or argon may coexist in the processing chamber.

  The reason why the heat treatment temperature is defined as 200 ° C. to 600 ° C. is that if the treatment is performed at a temperature lower than 200 ° C., it is difficult to perform a desired modification on the magnet surface. This is because if done, the magnetic properties of the magnet may be adversely affected, and the modified layer formed due to excessive modification of the magnet surface may fall off. The heat treatment temperature is preferably 250 ° C to 550 ° C, more preferably 300 ° C to 450 ° C. The processing time is preferably 1 minute to 3 hours.

The temperature rise from room temperature (for example, 10 ° C. to 30 ° C.) to the heat treatment temperature is performed in an atmosphere having an oxygen partial pressure of 1 × 10 ² Pa to 1 × 10 ⁵ Pa and a water vapor partial pressure of 1 × 10 ⁻³ Pa to 100 Pa. It is desirable to do. If the temperature raising step is performed in the air without controlling the atmosphere, for example, an oxidation reaction due to moisture contained in the air occurs at the time of temperature rising, and the magnetic characteristics of the magnet are reduced due to the large amount of hydrogen generated. There is a risk of inviting. In addition, since the amount of moisture contained in the atmosphere varies depending on the season, there is a risk that surface modification with stable quality throughout the year cannot be performed on the magnet. On the other hand, since the above atmosphere contains moderate oxygen and water vapor, the temperature raising process itself has a favorable effect on the surface modification of the magnet, and imparts excellent corrosion resistance to the magnet and suppresses deterioration of the magnetic properties. Contribute to. The rate of temperature increase from room temperature to the heat treatment temperature is preferably 100 ° C./hour to 1800 ° C./hour, and the temperature increase time is preferably 20 minutes to 2 hours. After the magnet is heated to the heat treatment temperature, it may be immediately transferred to the heat treatment step, or after the magnet is held for a while (for example, 1 to 60 minutes) in the atmosphere of the temperature increase step, the heat treatment step may be performed. Good.

The temperature lowering after the heat treatment is also desirably performed in an atmosphere having an oxygen partial pressure of 1 × 10 ² Pa to 1 × 10 ⁵ Pa and a water vapor partial pressure of 1 × 10 ⁻³ Pa to 100 Pa. By lowering the temperature in such an atmosphere, it is possible to prevent the surface of the magnet from condensing and causing corrosion during the process.

  The temperature raising process, the heat treatment process, and the temperature lowering process may be performed by sequentially changing the environment in the processing chamber in which the magnet is accommodated, or the processing chamber is divided into regions controlled by the respective environments, and the magnet is divided into each region. You may carry out by moving sequentially.

  FIG. 1 (a) shows a continuous processing furnace in which the temperature raising process, the heat treatment process, and the temperature lowering process can be performed by dividing the interior into regions controlled by the respective environments and moving the magnets sequentially to each region. It is a schematic diagram (side view) of an example. In the continuous processing furnace shown in FIG. 1 (a), each processing is performed while moving the magnet from the left to the right in the drawing by moving means such as a belt conveyor. Arrows indicate the flow of the atmospheric gas in each region formed by an unillustrated air supply means and exhaust means. The inlet of the temperature rising region and the outlet of the temperature falling region are partitioned by, for example, an air curtain, and the boundary between the temperature rising region and the heat treatment region and the boundary between the heat treatment region and the temperature lowering region are partitioned by, for example, the flow of the atmospheric gas indicated by the arrows (these This may be done mechanically with a shutter). FIG.1 (b) is a figure which shows the temperature change of the magnet which moves the inside of the continuous processing furnace shown to Fig.1 (a). If such a continuous processing furnace is used, surface modification with stable quality can be continuously performed for a large number of magnets.

The modified layer formed on the surface of the R—Fe—B based sintered magnet by the above steps includes, in order from the inside of the magnet, a main layer containing R, Fe, B and oxygen, and at least R, Fe and oxygen. It has at least three layers of an outermost layer containing an amorphous layer and iron oxide mainly composed of hematite (α-Fe ₂ O ₃ ) as a constituent component. When the composition of the main layer in the surface-modified layer is compared with the composition of the magnet (material) that is not surface-modified, the Fe content is decreased and the oxygen content is increased. It is 2.5 mass%-15 mass%. The main layer in the surface modified layer may have an R-concentrated layer having a length extending in the lateral direction of 0.5 μm to 30 μm and a thickness of 50 nm to 400 nm. This R-concentrated layer is presumed to be formed by precipitation of R in the work strain part existing in the magnet, reinforcing the decrease in the strength of the magnet due to degranulation, etc. It is thought that it contributes to the improvement of the adhesive strength with the interposed parts. As for the outermost layer in the surface modified layer, it is desirable that 90% by mass or more of iron oxide contained as a constituent component is hematite. More preferably, it is 95 mass% or more, More preferably, it is 98 mass% or more. The fact that iron oxide contains hematite in a high ratio and does not contain magnetite (Fe ₃ O ₄ ) as much as possible contributes to imparting excellent corrosion resistance by performing surface modification of the magnet. By performing heat treatment in an oxidizing atmosphere in which the oxygen partial pressure and the water vapor partial pressure of less than 10 hPa are appropriately controlled, the outermost layer in the surface modified layer is composed of iron oxide containing hematite in a high ratio. Can be. Further, the surface of the outermost layer can be made uniform with a high surface coverage by hematite by performing the oxidation heat treatment after homogenizing the composition of the magnet surface by grinding. The surface coverage by hematite is desirably 90% or more, and more desirably 95% or more. In contrast, when heat treatment is performed in an atmosphere having a high water vapor partial pressure as described in Patent Documents 3 to 6, iron oxide constituting the outermost layer in the surface modified layer is magnetite. Contains at a high ratio. The ratio of hematite in iron oxide contained as a constituent component in the outermost layer can be obtained by analyzing from the magnet surface by, for example, Raman analysis. The amorphous layer located between the main layer and the outermost layer in the surface modified layer is a portion where stable crystals were not formed when R or Fe contained in the magnet was converted into an oxide by an oxidation reaction. It is thought that.

  The thickness of the surface modification layer formed on the surface of the R—Fe—B based sintered magnet is preferably 0.5 μm to 10 μm. If the thickness is too thin, sufficient corrosion resistance may not be exhibited. On the other hand, if the thickness is too thick, the magnetic properties of the magnet may be adversely affected. The thickness of the main layer in the surface modification layer is preferably 0.4 μm to 9.9 μm, and more preferably 1 μm to 7 μm. The thickness of the amorphous layer is preferably 100 nm or less, more preferably 70 nm or less (the lower limit is preferably 10 nm, for example). The thickness of the outermost layer is desirably 10 nm to 300 nm, and more desirably 50 nm to 200 nm.

Also, more than prior to oxidation heat treatment and / or after further under an atmosphere of an oxygen partial pressure of 1 × water vapor partial pressure of 1 × ¹⁰ -7 at ^{10 -2 Pa~50Pa Pa~1 × 10 -2 Pa} , 200 Heat treatment may be performed at a temperature of from 600C to 600C. By adding such heat treatment, the surface modification of the R—Fe—B based sintered magnet can be made more reliable. The treatment time is preferably 1 minute to 3 hours.

Examples of the R—Fe—B based sintered magnet to which the present invention is applied include those manufactured by the following manufacturing method.
An alloy containing 25% by mass or more and 40% by mass or less of rare earth element R, 0.6% by mass to 1.6% by mass of B (boron), the balance Fe and inevitable impurities is prepared. Here, a part of R may be substituted with a heavy rare earth element RH. Further, a part of B may be substituted by C (carbon), and a part of Fe (50% by mass or less) is substituted by another transition metal element (for example, Co or Ni). Also good. This alloy is suitable for a variety of purposes, including Al, Si, Ti, V, Cr, Mn, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Ag, In, Sn, Hf, Ta, W, Pb, and About 0.01 to 1.0% by mass of at least one additive element M selected from the group consisting of Bi may be contained.
The above-mentioned alloy can be suitably produced by rapidly cooling a molten raw material alloy by, for example, a strip casting method. Hereinafter, preparation of a rapidly solidified alloy by a strip casting method will be described.
First, a raw material alloy having the above composition is melted by high-frequency melting in an argon atmosphere to form a molten raw material alloy. Next, after holding this molten metal at about 1350 ° C., it is rapidly cooled by a single roll method to obtain, for example, a flake-shaped alloy ingot having a thickness of about 0.3 mm. The alloy slab thus produced is pulverized into, for example, 1 to 10 mm flakes before the next hydrogen pulverization treatment. In addition, the manufacturing method of the raw material alloy by a strip cast method is disclosed by US Patent 5,383,978 specification, for example.
[Coarse grinding process]
The alloy slab coarsely crushed into flakes is accommodated in the hydrogen furnace. Next, a hydrogen embrittlement treatment process (hereinafter sometimes referred to as “hydrogen pulverization treatment” or simply “hydrogen treatment”) is performed inside the hydrogen furnace. When the coarsely pulverized powder alloy powder after the hydrogen pulverization treatment is taken out from the hydrogen furnace, the takeout operation is preferably performed in an inert atmosphere so that the coarsely pulverized powder does not come into contact with the atmosphere. By doing so, it is possible to prevent the coarsely pulverized powder from oxidizing and generating heat, and to suppress the deterioration of the magnetic properties of the magnet.
By the hydrogen pulverization treatment, the rare earth alloy is pulverized to an average particle size of 500 μm or less. After the hydrogen pulverization treatment, the embrittled raw material alloy is preferably crushed more finely and cooled. When the raw material is taken out in a relatively high temperature state, the cooling process time may be relatively long.
[Fine grinding process]
Next, the coarsely pulverized powder is finely pulverized using a jet mill pulverizer. A cyclone classifier is connected to the jet mill crusher used in the present embodiment. The jet mill pulverizer is supplied with the rare earth alloy (coarse pulverized powder) coarsely pulverized in the coarse pulverization step, and pulverizes in the pulverizer. The powder pulverized in the pulverizer is collected in a collection tank through a cyclone classifier. In this way, a fine powder of about 0.1 to 20 μm (typically an average particle size of 3 to 5 μm) can be obtained. The pulverizer used for such fine pulverization is not limited to a jet mill, and may be an attritor or a ball mill. In grinding, a lubricant such as zinc stearate may be used as a grinding aid.
[Press molding]
In this embodiment, for example, 0.3 wt% of a lubricant is added to and mixed with the magnetic powder produced by the above method in a rocking mixer, and the surface of the alloy powder particles is coated with the lubricant. Next, the magnetic powder produced by the above-described method is molded in an orientation magnetic field using a known press machine. The intensity of the applied magnetic field is, for example, 1.5 to 1.7 Tesla (T). The molding pressure is set so that the green density of the molded body is, for example, about 4 to 4.5 g / cm ³ .
[Sintering process]
With respect to said powder molded body, the step of holding at a temperature in the range of 650 to 1000 ° C. for 10 to 240 minutes, and then sintering at a temperature higher than the above holding temperature (for example, 1000 to 1200 ° C.). It is preferable to sequentially perform the further steps. During sintering, particularly when a liquid phase is generated (when the temperature is in the range of 650 to 1000 ° C.), the R-rich phase in the grain boundary phase begins to melt and a liquid phase is formed. Then, sintering progresses and a sintered magnet body is formed. After the sintering step, aging treatment (400 ° C. to 700 ° C.) and grinding for dimension adjustment may be performed.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is limited to this and is not interpreted.

Example 1
Nd: 16.4, Pr: 4.7, Dy: 9.4, B: 1.00, Co: 2.0, Al: 0.15, Ga: 0.07, Cu: 0.1, balance: An alloy flake having a composition of Fe (unit: mass%) and having a thickness of 0.2 to 0.3 mm was produced by strip casting.
Next, this alloy flake was filled in a container and accommodated in a hydrogen treatment apparatus. Then, the hydrogen treatment apparatus was filled with hydrogen gas at a pressure of 500 kPa, so that hydrogen was occluded in the alloy flakes at room temperature and then released. By performing such a hydrogen treatment, the alloy flakes were embrittled to produce an amorphous powder having a size of about 0.15 to 0.2 mm.
After adding 0.04 wt% zinc stearate as a grinding aid to the coarsely pulverized powder produced by the above hydrogen treatment and mixing, an average powder particle size of about 3 μm is obtained by performing a pulverization step with a jet mill device. A fine powder was made and recovered in mineral oil to prevent oxidation.
The fine powder thus produced was molded by a press apparatus to produce a powder compact. Specifically, the powder particles were compressed in a magnetic field-oriented state in an applied magnetic field and pressed. Thereafter, the molded body was extracted from the press apparatus, and a degreasing process at 200 ° C. for 2 hours and a sintering process at 1050 ° C. for 4 hours were performed in a vacuum furnace to obtain a sintered body block.
The surface of the obtained sintered body block was subjected to an aging treatment at 480 ° C. for 8 hours in a vacuum, and then surface grinding using a surface grinder (manufactured by Daisho Seiki Co., Ltd.) : # 100, rotational speed of the grindstone: 1500 rpm, magnet feeding speed to grinding machine: 0.6 m / min), sintered magnet whose thickness is adjusted to 6 mm × length 7 mm × width 7 mm (hereinafter referred to as “magnet test”) Referred to as "piece").
Next, this magnet body test piece was washed with alcohol, and then at 2 ° C. at 410 ° C. in an atmosphere of dew point 0 ° C. in the atmosphere (oxygen partial pressure 20000 Pa, water vapor partial pressure 600 Pa, oxygen partial pressure / water vapor partial pressure = 33.3). By performing heat treatment for a time, a surface-modified magnet body test piece was obtained. The temperature of the magnet specimen from room temperature to the heat treatment temperature is about 900 ° C./hour in an atmosphere with a dew point of −40 ° C. (oxygen partial pressure 20000 Pa, water vapor partial pressure 12.9 Pa). (The temperature rising time was 25 minutes). Further, the temperature drop after the heat treatment was performed in the same atmosphere. After this magnet body test piece was resin-filled and polished, a sample was prepared using an ion beam cross-section processing apparatus (SM09010: manufactured by JEOL Ltd.), and a field emission scanning electron microscope (S-4300: manufactured by Hitachi High-Technologies Corporation) was used. The results of cross-sectional observation are shown in FIG. As is clear from FIG. 2, at this observation point, the thickness of the modified layer formed on the surface of the magnet test piece is about 5.2 μm, and this modified layer is composed of a plurality of layers, at least the main layer. It was found that there was a layer and an outermost layer having a thickness of about 120 nm. Further, in the modified layer, a layered structure composed of R having a thickness of about 100 nm and a length of about 5 μm (an R-concentrated layer having an R composition of 85% by mass or more) extends in the horizontal direction (approximately the same as the surface of the magnet body). It was confirmed that the film was formed in a parallel direction. Table 1 shows the results of analyzing the composition of the main layer in the modified layer and the composition of the material (magnet body test piece) using an energy dispersive X-ray analyzer (Genesis 2000: manufactured by EDAX). As is clear from Table 1, it was found that the main layer in the modified layer had a very high oxygen content while the Fe content was lower than that of the raw material. Furthermore, as a result of performing cross-sectional observation near the surface of the surface-modified magnetic body test piece using a transmission electron microscope (HF2100: manufactured by Hitachi High-Technology Corporation), the main layer and thickness are selected at the selected observation point. It was found that there was a layer having a thickness of about 60 nm between the outermost layers of about 150 nm. This layer was found to be amorphous (by electron diffraction analysis). As a result of analyzing the composition of the amorphous layer and the outermost layer in the modified layer using an energy dispersive X-ray analyzer (EDX: manufactured by NORAN), there is almost no R in the outermost layer in the modified layer. It was found that it was composed of iron oxide, and the amorphous layer was composed of a composite oxide of R and Fe. Moreover, the result of having analyzed the outermost layer in the modified layer of the surface-modified magnetic body test piece from the surface using an X-ray diffractometer (RINT2400: manufactured by Rigaku) is shown in FIG. As is apparent from FIG. 3, it was found that the outermost layer in the modified layer was a layer mainly composed of hematite (♦ in the figure: peak of hematite). The outermost layer mainly composed of hematite is formed by oxidizing part of the main phase (R ₂ Fe ₁₄ B) of the raw material by heat treatment, so that Fe flows out of the main phase and is oxidized. Was guessed. Furthermore, as a result of analyzing the outermost layer in the modified layer of the surface-modified magnetic body test piece from the surface using a Raman spectroscopic analyzer (manufactured by Holo Lab 5000R: KAISER OPTICAL SYSTEM), a constituent component is formed on the outermost layer. It was found that all of the iron oxide contained as (100% by mass) is hematite and the surface coverage by hematite is 96.1%. In addition, a pressure cooker test (125 ° C., 85% RH, 0.2 MPa, the same applies hereinafter) is performed on the surface-modified magnet body test piece for 200 hours, and the weight reduction amount of the magnet due to degranulation (from before the test) (By comparison) was 0.07 g / m ² . From the above results, it was found that according to the above method, degranulation can be effectively prevented.

(Example 2)
The sintered body block obtained by the same method as in Example 1 was subjected to an aging treatment under the same conditions as in Example 1, and then surface grinding was performed under the same conditions as in Example 1 to obtain a thickness of 6 mm × length of 7 mm × A sintered magnet whose dimensions were adjusted to 7 mm in width (hereinafter referred to as “magnet body test piece”) was obtained. After the magnet body test piece was washed with alcohol, heat treatment was performed under the same conditions as in Example 1 except that the heat treatment time was 30 minutes, thereby obtaining a surface-modified magnet body test piece. When this magnet body test piece was evaluated in the same manner as in Example 1, the modified layer formed on the surface of the magnet body test piece had a thickness of about 1.9 μm, and the configuration was obtained in Example 1. It was found that this was the same as the modified layer in the surface-modified magnetic body test piece (the thickness of the outermost layer: about 60 nm). A pressure cooker test was performed on the surface-modified magnet body test piece for 200 hours, and the weight loss of the magnet due to degranulation (by comparison with before the test) was measured to be 0.21 g / m ² . . From the above results, it was found that according to the above method, degranulation can be effectively prevented.

(Example 3)
The sintered body block obtained by the same method as in Example 1 was subjected to surface grinding under the same conditions as in Example 1, and the size was adjusted to 6 mm in thickness, 7 mm in length, and 7 mm in width. This was referred to as “body specimen”. After this magnet body test piece was washed with alcohol, an aging treatment was performed under the same conditions as in Example 1. Next, the magnet body test piece subjected to aging treatment was subjected to heat treatment under the same conditions as in Example 1 except that the heat treatment time was set to 30 minutes, thereby obtaining a surface-modified magnet body test piece. . When this magnet body test piece was evaluated in the same manner as in Example 1, the modified layer formed on the surface of the magnet body test piece had a thickness of about 2.2 μm, and the configuration was obtained in Example 1. It was found that this was the same as the modified layer in the surface-modified magnetic body specimen (thickness of the outermost layer: about 75 nm). A pressure cooker test was performed on the surface-modified magnet body test piece for 200 hours, and the weight loss of the magnet due to degranulation (by comparison with before the test) was measured to be 0.09 g / m ² . . From the above results, it was found that according to the above method, degranulation can be effectively prevented.

(Comparative Example 1)
A pressure cooker test was performed for 200 hours on the magnet test piece obtained in the same manner as in Example 2, and the weight loss of the magnet due to degranulation (by comparison with that before the test) was measured to find 1.55 g / m ^2. The occurrence of degranulation was observed.

(Comparative Example 2)
A pressure cooker test was performed for 200 hours on the magnet body test piece obtained by the same method as in Example 3, and the weight loss of the magnet due to degranulation (by comparison with before the test) was measured. It was 0.6 g / m ² , and although the degree was weaker than that of Comparative Example 1, occurrence of degranulation was observed.

(Summary)
Table 2 summarizes the results of the pressure cooker test performed in each of Examples 1 to 3, Comparative Example 1, and Comparative Example 2. As is clear from Comparative Example 1, even when the mechanical processing was performed by surface grinding, which is grinding wheel processing, occurrence of degranulation was observed. Further, as apparent from Comparative Example 2, degranulation was reduced by performing the aging treatment after the grinding wheel was processed. It was considered that this was due to the fact that the grain boundary components that had become a liquid phase by aging treatment were supplied from the inside of the magnet along the fine cracks to the work-degraded layer, and the work-degraded layer was repaired.
On the other hand, as is apparent from Examples 1 to 3, by performing a grindstone process on the magnet surface, by performing a heat treatment in an oxidizing atmosphere in which the oxygen partial pressure and water vapor partial pressure are appropriately controlled. By modifying the magnet surface, degranulation could be effectively prevented. This is because the surface composition of the magnet is made uniform by performing the grinding wheel processing on the magnet surface before performing the oxidation heat treatment, which makes it possible to perform a uniform oxidation heat treatment on the entire surface of the magnet. The main component is a main layer having at least oxygen content higher than that of the material, an amorphous layer composed of a composite oxide of R and Fe, and iron oxide mainly composed of stable hematite. It was considered that the modified layer having the structure having the outermost layer was formed over the entire surface of the magnet and effectively prevented moisture from entering into the magnet. The surface composition of the magnet is made uniform by the grinding wheel processing because the grinding powder generated by the grinding wheel processing (main component is Fe constituting the main phase) is reattached to the magnet surface and stretched on the surface. It was considered that the re-adhesion and stretching of the grinding powder on the surface were repeated, and that the irregularities on the magnet surface were stretched by grinding wheel processing. In addition, the layered structure composed of R confirmed in the modified layer in Example 1 was converted into a liquid phase by R flowing out of the main phase because a part of the main phase of the material was decomposed by the heat treatment or by heat treatment. It is speculated that the grain boundary component is formed by being supplied to a crack portion slightly generated in the modified layer due to the difference in thermal expansion coefficient between the raw material and the modified layer. It was thought that it contributed to the repair of the processing deteriorated layer. Further, in Example 2 and Example 3, the layered structure composed of R confirmed in the modified layer in Example 1 could not be confirmed under the present observation conditions, but Example 2 was a comparative example. Since the effect superior to the degranulation preventing effect due to the aging treatment in No. 2 was obtained, and in Example 3, an effect superior to the effect obtained in Example 2 was obtained, the grinding wheel processing obtained by the present invention and It was found that the anti-granulation effect by the combination of oxidative heat treatment is superior to the anti-granulation effect by the aging treatment.
These results are synergistic between the homogenization of the surface composition by grinding wheel processing and the formation of the surface modification layer by heat treatment in an oxidizing atmosphere in which the oxygen partial pressure and the water vapor partial pressure of less than 10 hPa are appropriately controlled. The modified layer formed on the surface of the magnet effectively prevents moisture from entering the inside of the magnet, and against fine cracks existing in the work-degraded layer and work distortions inherent in the layer. It is considered that R flowing out from the main phase and liquid phase grain boundary components are obtained by effectively repairing these. The grain boundary component that has become a liquid phase by the aging treatment is supplied to the work-deteriorated layer and repaired, but this action is effective against cracks, but it is effective against work strains inherent in the layer. It seems not necessarily so. For this reason, the processing strain inherent in the processing deteriorated layer is manifested by bonding the magnet to another member or exposing the magnet to a harsh environment, thereby causing grain loss. In the present invention, a modified layer is formed on the surface of the magnet whose composition has been made uniform by grinding, thereby further utilizing the difference in coefficient of thermal expansion between the material and the surface modified layer to further increase the magnet surface. By imparting strain, the processing strain inherent in the work-deteriorated layer is positively manifested, and the manifested processing strain is repaired by R flowing out of the main phase by heat treatment or a liquid phase grain boundary component. It is inferred that the layered structure composed of R confirmed in the modified layer in Example 1 is a trace of the working strain thus repaired.

INDUSTRIAL APPLICABILITY The present invention has industrial applicability in that it can provide a method for effectively preventing degranulation caused by mechanical processing on an R—Fe—B based sintered magnet.

Claims (5)

After grindstone processing on the magnet surface, in an atmosphere with an oxygen partial pressure of 1 × 10 ² Pa to 1 × 10 ⁵ Pa and a water vapor partial pressure of 0.1 Pa to 1000 Pa (excluding 1000 Pa), 200 ° C. to By performing heat treatment at 600 ° C., the surface contains R, Fe, B and oxygen in order from the inside of the magnet, the length extending in the lateral direction is 0.5 μm to 30 μm, and the thickness is 50 nm to 400 nm. Forming a modified layer having at least three layers of a main layer having a chemical layer, an amorphous layer containing at least R, Fe and oxygen, and an outermost layer containing iron oxide mainly composed of hematite as a constituent component R-Fe-B sintered magnets (excluding R-Fe-B sintered magnet to form a polyimide resin film on the surface of the modified layer) shedding prevention how the to.   2. The method for preventing the degranulation of an R—Fe—B based sintered magnet according to claim 1, wherein the grinding is performed using a grindstone having a grain size of # 60 to # 400.   The ratio of oxygen partial pressure and water vapor partial pressure (oxygen partial pressure / water vapor partial pressure) is set to 1 to 400, and the method for preventing degranulation of an R-Fe-B sintered magnet according to claim 1 or 2. The temperature rise from room temperature to the temperature at which heat treatment is performed is performed in an atmosphere having an oxygen partial pressure of 1 × 10 ² Pa to 1 × 10 ⁵ Pa and a water vapor partial pressure of 1 × 10 ⁻³ Pa to 100 Pa. The method for preventing the degranulation of the R—Fe—B based sintered magnet according to claim 1.   The composition of the main layer in the modified layer is such that the Fe content is decreased and the oxygen content is increased compared to the composition of the unmodified magnet. A method for preventing degranulation of an R—Fe—B sintered magnet according to claim 1.