JP2019131410A - Bn sintered body excellent in corrosion resistance - Google Patents

Abstract

To provide a boron nitride sintered body having excellent corrosion resistance to molten metal containing rare earths.SOLUTION: In the boron nitride sintered body, the boron nitride content is 80.0 mass% or more. The boron nitride sintered body contains 0.3 mass% or more and 12.0 mass% or less of calcium in terms of calcium oxide, and 1.0 mass% or more and 18.0 mass% or less of yttrium in terms of yttrium oxide.SELECTED DRAWING: None

Description

  The present invention relates to a boron nitride sintered body suitable for an apparatus member, a part, which has excellent corrosion resistance against a molten metal containing a rare earth, and which produces a material for a rare earth magnet, and a method for producing the same.

  In office automation equipment, automobile parts, home appliances, etc., there is a strong demand for miniaturization, lightening, and high performance of motors for the purpose of miniaturization, lightening, energy saving, and high functionality. To make the motor smaller and lighter, a powerful magnet is required. A magnetic body made of an alloy containing a rare earth has strong magnetic properties, and is therefore widely used as a permanent magnet used in a motor.

  As a method for producing an alloy for a magnetic material containing a rare earth, there are a melt spinning method (single roll method), an atomizing method, a strip casting method and the like. For example, in the melt spinning method, a raw material of a predetermined alloy composition (for example, Nd + Fe + B alloy) containing rare earth is melted in a ceramic container, and the molten alloy is injected from a ceramic nozzle and rotated with respect to the nozzle. The alloy is rapidly cooled and solidified by being brought into contact with the outer peripheral surface and brought into contact with the outer peripheral surface to continuously form a ribbon-shaped ribbon alloy. In the atomizing method and the strip casting method, a molten alloy is sprayed or injected through a nozzle to rapidly cool the alloy, and the alloy is formed into a sheet shape, a thread shape, or a particle shape.

  Rare earth elements are very easy to oxidize, and when they are oxidized, the magnetic properties are degraded. For this reason, in the production of the alloy, most oxide-based materials are not suitable as materials for parts such as a melting crucible and a nozzle that come into contact with the alloy at a temperature at which the alloy melts. In particular, when an oxide-based material that oxidizes rare earth elements is used for the nozzle, it not only oxidizes the rare earth elements in the alloy, but also causes problems such as nozzle corrosion and wear and nozzle clogging due to the generated rare earth oxide. . For this reason, a boron nitride sintered body is generally used as the nozzle material.

Boron nitride is a chemically stable ceramic material with excellent heat resistance at high temperatures, but it is also a hardly sinterable material. Boron oxide (B ₂ O ₃ ) is used as a sintering aid in sintering according to the prior art. It is common. On the other hand, since B ₂ O ₃ has a melting point of about 450 ° C., it is not suitable for use at a high temperature. When a boron nitride sintered body is used in a high-temperature environment of 1000 ° C. or higher, a method in which the sintered body containing B ₂ O ₃ is subjected to high temperature treatment to volatilize and remove B ₂ O ₃ to obtain a high purity or B ₂ O _A method of performing high-pressure molding and firing such as HIP and CIP without adding ₃ has been developed (see Patent Document 1). However, the boron nitride produced by these methods has a lower density, strength, and the like than those using B ₂ O ₃ as a sintering aid.

Even in a magnetic alloy containing a rare earth, if the sintered body contains B ₂ O ₃ , the rare earth component reacts with B ₂ O ₃ to produce a rare earth oxide. Alternatively, a high-purity type boron nitride not containing B ₂ O ₃ is used for the nozzle to be sprayed. However, the high purity type boron nitride containing no B ₂ O ₃ has a low density and has many pores, so that there is a problem that the molten alloy penetrates into the pores and is easily melted. When boron nitride is melted, the nozzle life is shortened, which affects the productivity of the alloy. In particular, when the discharge port portion of the nozzle is melted, the injection amount of the molten alloy increases, and the cooling rate and the thickness of the alloy ribbon, the thickness of the alloy yarn, or the size of the alloy particles become nonuniform. If the cooling rate of the alloy is different, the state of crystallite size, crystallinity, etc. of the alloy will be different, resulting in quality variations as a magnetic material. Further, even when the thickness of the alloy ribbon, the thickness of the alloy yarn, or the size of the alloy particles are different, the magnetic characteristics are different, leading to a deterioration in quality as a magnet.

Japanese Patent No. 3942288

  An object of the present invention is to provide a boron nitride sintered material having excellent corrosion resistance and melt resistance against molten metal containing a rare earth in view of the above-described problems of the prior art.

  That is, in the embodiment of the present invention, the boron nitride content is 80.0% by mass or more, and calcium is converted to calcium oxide in an amount of 0.3% by mass to 12.0% by mass, and yttrium is converted to yttrium oxide. A boron nitride sintered body comprising 1.0% by mass or more and 18.0% by mass or less as an amount can be provided.

  In a certain aspect of the above-described embodiment, the content of boron nitride in the sintered body is 80.0% by mass or more, and 0.6 mass% or more and 11.0 mass% or less when calcium is converted into calcium oxide. And 1.9 mass% or more and 17.0 mass% or less may be included as a yttrium oxide conversion amount. Further, the amount of boron oxide contained in the sintered body may be 0.3% by mass or less, or 0.1% by mass or less. In one embodiment, the relative density with respect to the true density of the sintered body may be 75% or more, the open porosity may be 7% or less, and the Shore hardness may be 13 or more.

In another embodiment of the present invention, 80.0% by mass or more of boron nitride, 0.3% by mass or more and 12.0% by mass or less of calcium oxide as a sintering aid, and a sintering aid as Mixing 1.0% by mass or more and 18.0% by mass or less of yttrium oxide to obtain a raw material mixture;
And sintering the raw material mixture at a temperature in the range of 1600 ° C. or higher and 2050 ° C. or lower.

  In an aspect of the above embodiment, the amount of calcium oxide in the raw material mixture is 0.6% by mass or more and 11.0% by mass or less, and the amount of yttrium oxide is 1.9% by mass or more and 17.0% by mass. It may be the following.

  By this invention, the boron nitride sintered compact material excellent in the corrosion resistance with respect to the molten alloy containing a rare earth, and melt-resistance can be obtained.

It is a SEM image of the section after use of the crucible created from the sintered compact of Example 3 concerning the present invention. It is an EDS image of the section after use of the crucible created from the sintered compact of Example 3 concerning the present invention. It is a SEM image of the section after use of the crucible created from the sintered compact of comparative example 3. It is an EDS image of the section after use of the crucible created from the sintered compact of comparative example 3.

The boron nitride sintered body of the present invention can be obtained by mixing calcium nitride powder and yttrium oxide powder as a sintering aid with boron nitride powder, molding and sintering. Boron nitride is a hardly sinterable material, and B ₂ O ₃ is generally used as a sintering aid for sintering in the prior art. However, B ₂ O ₃ is highly reactive with rare earths and is not preferred for use as a material for handling molten metals containing rare earths. Also, as a sintering aid for nitride ceramics such as silicon nitride and aluminum nitride, Al ₂ O ₃ , MgO, Y ₂ O _{3 and the} like are common, but Al ₂ O ₃ and MgO are also reactive with rare earths. Is expensive. Therefore, as a result of investigating and examining oxide ceramics that can be used as sintering aids for nitride ceramics and have low reactivity with rare earth metals, the present inventors have found that the combination of calcium oxide and yttrium oxide is boron nitride. It has been found that it exhibits an extremely excellent effect in sintering. In this specification, unless otherwise specified, “calcium oxide” refers to anhydrous calcium oxide.

  In order to sinter boron nitride, the sintering aid needs to be in a liquid phase. Calcium oxide and yttrium oxide are both high-melting point oxides, but if they are blended in appropriate amounts, they become complex oxides that have a lower melting point and can be made into a liquid phase even at temperatures of 2000 ° C. or lower. The blending amount is preferably 0.3 to 12.0% by mass of calcium oxide and 1.0 to 18.0% by mass of yttrium oxide, more preferably oxidized based on the total mass of the raw material mixture. Calcium is 0.6 to 11.0% by mass and yttrium oxide is 1.9 to 17.0% by mass. If the amount of calcium oxide or yttrium oxide is excessive, the melting point of the liquid phase becomes high, which may cause a problem that the sinterability is significantly lowered. If the amount of calcium oxide or yttrium oxide is too small, a problem may occur that the Shore hardness of the obtained sintered body is too low. The amount of calcium oxide and the amount of yttrium oxide in the sintered body are respectively equivalent amounts that can be calculated from the amounts of calcium element and yttrium element contained in the sintered body. Further, in this specification, the symbol tilde “˜” indicating a value range has a meaning of “... Or more,..., Or less” including a lower limit value and an upper limit value unless otherwise specified.

Boron nitride as a raw material contains oxygen as an impurity and often exists as B ₂ O ₃ . Since B ₂ O ₃ reacts with rare earths, it is preferable to have as little as possible. When the amount of B ₂ O ₃ is small, a complex oxide is formed and stabilized with calcium oxide and yttrium oxide, so that the reaction with the rare earth in the molten alloy is suppressed. However, if B ₂ O ₃ is present in excess, the reactivity with the rare earth becomes strong. For this reason, the amount of B ₂ O ₃ in the sintered body is preferably 0.3% by mass or less, more preferably 0.2% by mass or less, still more preferably 0.1% by mass or less, and still more preferably substantially present. do not do. Here, “substantially non-existent” means that the amount is below the lower limit of detection, excluding the amount inevitably mixed in the production process (this is “0.0 mass%”, etc.) May be written).

The boron nitride sintered body of the present invention has a hexagonal crystal form, and the shape of the particles is scaly. Boron nitride is difficult to sinter and is porous due to the flake-like particle shape. The relative density of conventional high purity type boron nitride without adding sintering aid or removing B ₂ O ₃ after sintering is usually about 60 to less than 85%, and the open porosity is usually 10% or more. It is. High purity boron nitride has low reactivity with rare earth elements, but the penetration of molten metal into the pores promotes peeling and dropping of boron nitride particles from the surface of the sintered body, which is one of the causes of erosion. . On the other hand, in the embodiment of the present invention, by using a sintering aid containing a combination of yttrium oxide and calcium oxide, the relative density (that is, relative density to the true density) of the obtained sintered body is 75% or more. It is possible to achieve 80% or more, and it is possible to realize an open porosity of 7% or less, preferably 5% or less, and more preferably 3% or less. For example, in some embodiments, the relative density can be in the range of 75% to 95% and the open porosity can be in the range of 5% or less. That is, according to the embodiment of the present invention, the penetration of the molten metal into the sintered body can be reduced, and the melt resistance can be improved.

  Further, the boron nitride sintered body according to the embodiment of the present invention is preferably sintered by using a sintering aid containing a combination of yttrium oxide and calcium oxide, so that the Shore hardness is preferably 13 or more, more preferably 14 The above can be realized. On the other hand, the Shore hardness of the high-purity type boron nitride according to the prior art is about 11 to 12, and the sintered body according to the embodiment of the present invention has excellent wear resistance as a material (material). Understood. This is because the wear resistance of a material generally correlates with hardness, and the higher the hardness, the better the wear resistance. Although there is no restriction | limiting in particular in the upper limit of Shore hardness, For example, 24 or less may be sufficient.

  The boron nitride sintered body according to the embodiment of the present invention is less susceptible to erosion / penetration by a rare earth even when touched with a molten metal containing a rare earth, and has excellent durability. For this reason, the boron nitride sintered compact which concerns on embodiment of this invention can be conveniently used as an apparatus member used for manufacture of the alloy containing a rare earth, for example. Preferably, it can be suitably used as an apparatus member (for example, a nozzle, a nozzle support member, a crucible, etc.) for producing an alloy that can be used for magnetic materials such as neodymium and samarium. In a preferred embodiment, the boron nitride sintered body can also be used as a nozzle material when an alloy powder containing a rare earth (for example, an alloy powder containing neodymium or samarium) is produced by a melt spinning method or the like. is there. The shape of the discharge port of such a nozzle includes a hole shape and a slit shape, and a discharge hole diameter of about 0.5 to 2 mm or a discharge slit width of about 0.2 to 1 mm is common. Since the molten metal continuously passes through the inside, there is a problem that the discharge port or the slit of the nozzle is worn by friction with the molten metal and gradually spreads. By using the boron nitride sintered body according to the embodiment of the present invention for such a nozzle or slit, it is possible to exhibit high wear resistance, extend the service life, and improve the productivity.

  Further, as described above, the boron nitride sintered body according to the embodiment of the present invention has low reactivity to molten rare earth and low meltability (that is, high corrosion resistance), and therefore, a molten metal containing rare earth. It can also be suitably used as a crucible for preparing the.

In the boron nitride sintered body of the present invention, the boron nitride powder is oxidized to 80.0% by mass or more, preferably 80.0 to 98.7% by mass, more preferably 80.0 to 97.5% by mass. Calcium powder is 0.3 to 12.0% by mass, preferably 0.6 to 11.0% by mass, and yttrium oxide powder is 1.0 to 18.0% by mass, preferably 1.9 to 17. What added 0 mass% can be mixed and it can be set as raw material powder (raw material mixture). The raw material powder is molded and sintered at a temperature in the range of 1600 to 2050 ° C. to obtain the boron nitride sintered body. In a preferred embodiment, no B ₂ O ₃ is added to the raw material mixture. This means that B ₂ O ₃ is not intentionally added to the raw material mixture except for B ₂ O ₃ contained as an inevitable impurity. In addition, the sintered body according to the embodiment of the present invention can be produced in an inert atmosphere such as a nitrogen atmosphere.

In a preferred embodiment of the present invention, the boron nitride sintered body preferably has one or more of the following characteristics, more preferably a combination of two or more, and all combinations It is even more preferred to have
(1) The B ₂ O ₃ content in the sintered body is 0.3% by mass or less, more preferably 0.1% by mass or less, and still more preferably substantially absent.
(2) The relative density with respect to the true density of the sintered body is 75% or more, more preferably 80% or more.
(3) The open porosity of the sintered body is 7% or less, more preferably 5% or less, and still more preferably 3% or less.
(4) The Shore hardness of the sintered body is 13 or more, more preferably 14 or more.

(Examples 1-7)
Boron nitride powder (Denka Co., Ltd., oxygen content 1.0 wt%, average particle size 20 μm), anhydrous calcium oxide powder (Kishida Chemical Co., average particle size 30 μm), yttrium oxide powder (manufactured by Anan Kasei Co., Ltd., The average particle size is 4.5 μm) in the proportions shown in Table 1, filled in a cylindrical resin pot with an internal volume of 10 L together with φ10 mm silicon nitride media, mixed for 2 hours in a ball mill, and blended as shown in Table 1. A mixed powder of boron nitride was obtained. This mixed powder was filled in a graphite die having an inner diameter of 140 mm and subjected to hot press sintering at a temperature of 2000 ° C. and a pressure of 20 MPa to obtain a boron nitride sintered body.

  The oxygen content of the boron nitride powder was measured using an O / N simultaneous analyzer (EMGA-620W / C) manufactured by Horiba.

  The average particle size of boron nitride powder, calcium oxide powder, and yttrium oxide powder was measured by adding 60 mg of a measurement sample to 200 cc of pure water mixed with 2 ml of a 20 wt% aqueous solution of sodium hexametaphosphate, and ultrasonic homogenizer (manufactured by Nippon Seiki Seisakusho). , Trade name "US-300") for 3 minutes, and then measured with Microtrac (trade name "MT3300EXII", manufactured by Nikkiso Co., Ltd.) Pure water was used as a solvent for the microtrac circulator, and a measurement sample Was adjusted to an appropriate concentration.

  The relative density and open porosity of the boron nitride sintered body are determined by Archimedes method in accordance with JIS R 1634: 1998 after grinding the outer periphery of about 3 mm to remove graphite adhering to the surface and expose a clean surface. Measured and calculated. The results are shown in Table 1.

  The Shore hardness of the boron nitride sintered body was measured in accordance with JIS Z 2246: 2000 using a D-type manufactured by Shimadzu Corporation by cutting out a test piece of 40 mm length × 30 mm width × 10 mm thickness from the sintered body. The measurement results are shown in Table 1.

A test piece having a length of 50 mm, a width of 50 mm, and a thickness of about 3 mm was cut out from the boron nitride sintered body and finely pulverized with a silicon nitride mortar to obtain a powder. A certain amount of this powder was put into a dry weight (W0) container and dried at 150 ° C. for 18 hours or more. After drying, the container and powder were cooled to room temperature in a desiccator whose humidity was controlled, and the weight (W1) was measured immediately. Thereafter, methyl alcohol was added to the powder in the container, and B ₂ O ₃ was eluted in methyl alcohol. Then, B ₂ O ₃ eluted by warm air drying was volatilized and removed together with methyl alcohol. The container containing the powder was again transferred into a desiccator, cooled to room temperature, and the weight (W2) was measured. Based on each measured weight (W0, W1, W2), the amount of B ₂ O ₃ was calculated by the following formula. The calculation results of the B ₂ O ₃ amount are shown in Table 1.

  Further, the remaining part of the boron nitride sintered body obtained by hot press sintering was pulverized with a silicon nitride mortar, and the contained components were measured by XRF. As a result, it was confirmed that the boron nitride sintered body obtained by sintering under the conditions described in Table 1 had amounts of B, Ca, and Y metal elements substantially equal to the amounts shown in Table 1. Other metal elements were not confirmed in the sintered body. The results are shown in Table 1.

  The sintered body was processed to obtain a crucible having an outer diameter of 50 mm, an inner diameter of 30 mm, a height of 50 mm, and a depth of 40 mm. In this crucible, 60 g of Nd-based alloy pieces having a composition ratio of 21.5 wt% Nd-76.5 wt% Fe-1.0 wt% B-1.0 wt% Dy were placed and heated to 1350 ° C. in a nitrogen atmosphere in a vacuum atmosphere furnace. The Nd alloy was melted and held for 1 hour and then cooled. After cooling, it was cut to observe the interface between the hardened alloy and the crucible. The distribution state of Nd and Fe elements on the alloy side, the ceramic side, and the interface was observed with a scanning electron microscope SEM and EDS.

  In the observation on the ceramic side, the presence or absence of erosion, penetration, and diffusion of the metal component from the interface with the Nd alloy into the ceramic was confirmed.

  In the observation on the alloy side, it is confirmed from the image of the element distribution of EDS whether there is a place where the distribution state of the Nd element and the Fe element is locally gathered or a place where the distribution state does not exist locally. Or the thing which does not have the coarse dense spot of Fe element was made uniform, and the thing which existed was made nonuniform. The results are shown in Table 1.

(Comparative Examples 1-3)
In Comparative Examples 1 to 3, hot press sintering was performed at the blending ratios of Comparative Examples 1 to 3 described in Table 1 as in Examples 1 to 7. Here, Comparative Example 3 is obtained by adding B ₂ O ₃ powder to a sintering aid. The open porosity, Shore hardness, and the amount of B ₂ O ₃ were measured in the same manner as in the above examples, and a corrosion resistance test with an Nd alloy was performed. The results are shown in Table 1.

Comparative Example 1 described in Table 1 had a smaller amount of auxiliary agent than Examples 1 to 7, while Comparative Example 2 had a larger amount of auxiliary agent than Examples 1 to 7, and both had increased open porosity. In the cross-sectional observation, in the comparative example having a large open porosity, the alloy penetrated into the pores, and in comparative example 2, a portion where the crucible interface was further broken was also observed. In Comparative Example 3, the amount of B ₂ O ₃ in the sintered body was large, an Nd component layer was formed at the interface between the crucible and the alloy layer, and the alloy structure became non-uniform.

  The SEM image of the cross section of the sintered body of Example 3 is shown in FIG. 1, and the EDS image is shown in FIG. It can be seen that the alloy structure is uniform and there is no alloy penetration into the ceramic side. In addition, the light-colored portion (the gray portion in the color SEM image and the green portion in the color EDS image) at the top of FIGS. 1 and 2 is an alloy layer. The lower dark color portion (the black portion in the color SEM image / EDS image) is the ceramic layer.

  Moreover, the SEM image of the sintered compact cross section of the comparative example 3 was shown in FIG. 3, and the EDS image was shown in FIG. It can also be seen that the alloy structure is non-uniform and the alloy permeates into the ceramic side. Further, it is understood that a layer of Nd component is formed at the interface. 3 and 4 is an alloy layer (the light-colored portion in the color SEM image and the blue-green portion in the color EDS image). The lower dark color portion (the black portion in the color SEM image / EDS image) is the ceramic layer.

Claims (9)

  The content of boron nitride is 80.0% by mass or more, calcium is 0.3 to 12.0% by mass in terms of calcium oxide, and 1.0% by mass or more in terms of yttrium oxide. A boron nitride sintered body containing 18.0% by mass or less.   The content of boron nitride is 80.0% by mass or more, calcium is 0.6 to 11.0% by mass in terms of calcium oxide, and 1.9% by mass in terms of yttrium oxide. The boron nitride sintered body according to claim 1, comprising 17.0% by mass or less.   The boron nitride sintered body according to claim 1 or 2, wherein an amount of boron oxide contained in the sintered body is 0.3 mass% or less.   The boron nitride sintered body according to claim 3, wherein the amount of boron oxide contained in the sintered body is 0.1 mass% or less.   The boron nitride sintered body according to any one of claims 1 to 4, wherein a relative density with respect to a true density of the sintered body is 75% or more.   The boron nitride sintered body according to any one of claims 1 to 5, wherein an open porosity of the sintered body is 7% or less.   7. The boron nitride sintered body according to claim 1, wherein the sintered body has a Shore hardness of 13 or more. 80.0% by mass or more of boron nitride, 0.3% by mass to 12.0% by mass of anhydrous calcium oxide as a sintering aid, and 1.0% by mass or more of 18.0% by mass as a sintering aid. Mixing a yttrium oxide having a mass% or less to obtain a raw material mixture;
Sintering the raw material mixture at a temperature in the range of 1600 ° C. or higher and 2050 ° C. or lower.   The amount of anhydrous calcium oxide in the raw material mixture is 0.6 mass% or more and 11.0 mass% or less, and the amount of yttrium oxide is 1.9 mass% or more and 17.0 mass% or less. The manufacturing method as described.