JP2006169080A - Method of manufacturing cubic boron nitride polycrystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cubic boron nitride (cBN) sintered compact (polycrystal) in which sintering of cBN particles is accelerated even under mild high-pressure and high-temperature conditions and which has high purity and is sufficiently securely sintered. <P>SOLUTION: In the method for manufacturing the cBN polycrystal, the surface of the cBN powder of the raw material is made into the higher purity and is stabilized by a plasma treatment in hydrogen. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a cubic boron nitride sintered body having a dense and fine polycrystalline structure, and in particular, a binder having high hardness, high strength and excellent thermal characteristics, which can be used for cutting of iron-based materials. The present invention relates to a cubic boron nitride polycrystal which does not contain bismuth and a method for producing the same.

  Cubic boron nitride (hereinafter abbreviated as cBN) has a hardness second to diamond and has high thermal and chemical stability, and is widely used as a cutting tool for iron-based materials as is well known in the past. It has been.

cBN can synthesize | combine (direct conversion) non-catalyst normal pressure type | mold BN, such as hBN, under ultra-high pressure and high temperature. It is known that a cBN sintered body containing no binder can be produced by sintering simultaneously with this hBN → cBN conversion.
For example, Patent Literature 1 and Patent Literature 2 disclose a method of obtaining a cBN sintered body by converting hBN to cBN at an ultrahigh pressure and high temperature. However, in order to obtain a sufficiently strong sintered body, severe pressure temperature conditions of 7 GPa and 2100 ° C. or more are required, and there are problems in cost and productivity for industrial production. Patent Document 3 and Patent Document 4 show a method for producing a cBN sintered body using a pyrolytic boron nitride (pBN) raw material. In this case as well, in order to obtain a strong cBN sintered body, a temperature of 2000 ° C. or higher is required, and in addition to the same problems as described above, the raw material pBN is extremely expensive, and compressed hBN tends to remain. There are problems such as a strong orientation (anisotropic) property and a tendency to cause lamellar cracks and peeling.

As a method for obtaining cBN by direct conversion under milder conditions, for example, Patent Document 5 discloses a method using hexagonal boron nitride having an average primary particle size of 3 μm or less as a raw material. As a result, cBN can be obtained under conditions of 6 GPa and 1100 ° C. However, since hexagonal boron nitride is a fine powder, it contains several percent of boron oxide impurities and adsorbed gas, so that sintering does not proceed sufficiently, and oxidation Since many objects are contained in the sintered body, a sintered body having high hardness, high strength and excellent heat resistance cannot be obtained, and cannot be used for a cutting tool.
JP 47-34099 A JP-A-3-159964 Japanese Patent Publication No. 63-394 JP-A-3-159964 Japanese Patent Publication No. 49-27518 T. Taniguchi, M. Akaishi, S. Yamaoka, "The 3rd NIRIM International Symposium on Advanced Materials"(ISAM'96), pp.275-280

  Since it is very difficult to directly sinter cBN particles (powder) alone, cBN sintered bodies generally used as cutting tools include metals such as Al and Co, and ceramics such as TiC and TiN. Is used as a binder by sintering cBN powder under ultra high pressure. For this reason, generally the commercially available cBN sintered compact contains about 10-40 volume% of the said binder. This binder greatly affects the strength, heat resistance, and heat dissipation of the sintered body.Especially when cutting iron-based materials at high speeds, the cutting edge is prone to chipping and cracking, resulting in a long tool life. It becomes very short. Such a problem is inevitable as long as the sintered body contains a binder.

  On the other hand, as a cBN sintered body containing no binder, a sintered body obtained by reactive sintering using hexagonal boron nitride (hBN) as a raw material using a catalyst such as magnesium boronitride is known. Since this sintered body has no binder and cBN particles are strongly bonded, the thermal conductivity is as high as 6 to 7 W / cm ° C., and is used for heat sink materials, TAB bonding tools, and the like. However, since some catalyst remains in the sintered body, when heat is applied, fine cracks due to a difference in thermal expansion between the catalyst and cBN are likely to occur. For this reason, the heat-resistant temperature is as low as about 700 ° C., which is a serious problem as a cutting tool. Further, since the particle size is as large as about 10 μm, the thermal conductivity is high, but the strength is not sufficient and it is not possible to cope with intermittent cutting with a large load.

  Therefore, the object of the present invention is to promote cBN particle sintering even under fairly mild high pressure and high temperature conditions such as 6 to 6.5 GPa and 1800 to 2000 ° C. The object is to provide a knot (polycrystal).

  As shown in Non-Patent Document 1, in order to directly sinter cBN powder at high temperature and high pressure, a very high pressure temperature condition of 7.7 GPa or more and 2300 ° C. or more is required. There are problems and the main reasons why such a high pressure temperature is necessary are as follows: 1) Since cBN is very hard and has a high melting point, at a pressure temperature lower than this, atomic diffusion becomes insufficient. A sufficiently strong bond between particles cannot be obtained, and 2) a boron oxide film and a gas containing oxygen are adsorbed on the surface of the raw material cBN particles. The second object of the present invention is to eliminate such a cause because it is considered that there is a considerable obstruction. Here, in order to remove boron oxide and adsorbed gas formed on the surface of the cBN powder, for example, in the non-patent document, high-temperature heat treatment at 1000 ° C. is performed in vacuum. Although the cBN powder surface is cleaned by such treatment, it becomes a very active surface state, and when it is taken out into the air after the heat treatment, it rapidly oxidizes and adsorbs, which becomes a factor inhibiting sintering. .

  As a result of diligent efforts to achieve the above object, the present inventors have effectively removed oxygen by treating the cBN powder as a raw material at a high temperature in a hydrogen stream in a plasma state, and at the same time, the surface is terminated with hydrogen atoms. Thus, by obtaining the knowledge that a stable surface state can be obtained and using this as a raw material, cBN particles can be sintered even under fairly mild high pressure and high temperature conditions of 6 to 6.5 GPa and 1800 to 2000 ° C. It was found that a cBN sintered body (polycrystalline body) that was promoted and sintered with high purity and sufficiently strong was obtained. Oxidation and adsorbed gas that inhibits sintering are completely removed from the surface of the cBN powder by plasma treatment in hydrogen, preferably in a hydrogen stream, and the hydrogen formed on the surface serves as a catalyst for the mild reaction as described above. It seems that the process of atom transfer → sintering between cBN particles was promoted even under high pressure and high temperature conditions. Here, the surface of the cBN powder after the plasma treatment in hydrogen is in a stable state because hydrogen atoms are terminated, and there is no problem of oxidation or gas adsorption when taking out from the furnace.

  The cBN polycrystal obtained in this way has high hardness and does not contain conventional binders such as metals and ceramics, and therefore has excellent thermal stability and thermal crack resistance. For this reason, it is thought that it has the very outstanding performance as a cutting tool, and the cutting test was done. As a result, it was confirmed that the wear resistance was more than twice as good as that of a cBN sintered body containing a conventional binder, and the present invention was achieved.

  That is, the present invention relates to a method for producing a cubic boron nitride polycrystal, wherein the surface of a raw material cubic boron nitride powder is purified and stabilized by plasma treatment in hydrogen. This is a method for producing a boron polycrystal. The plasma treatment in the present invention is performed by generating plasma by microwave discharge (for example, 2.45 GHz) or high-frequency discharge (for example, radio wave discharge, for example, 13.56 MHz) according to a conventional method.

  Needless to say, the method of the present invention is suitably applied to a cBN sintered body containing no binder, but can also be applied to a cBN sintered body containing a binder. Since the surface of the cBN powder is cleaned by the hydrogen plasma treatment, it can effectively act even in sintering in a system containing a binder to improve the sinterability.

  According to the present invention, oxygen is effectively removed by treating the cBN particles at a high temperature in a hydrogen stream in a plasma state, and at the same time, the surface is terminated with hydrogen atoms to obtain a stable surface state. The cBN polycrystal obtained in this way has high hardness, excellent thermal stability and thermal crack resistance, and can exhibit excellent performance when used as a cutting tool.

A cBN powder having a particle size of 0.1 to 10 μm, preferably 1 to 3 μm, is subjected to 500 to 1500 ° C., preferably 900 to 1300 in a hydrogen stream at 1 to 200 torr reduced pressure, preferably 50 to 150 torr reduced pressure in a microwave reactor. Hydrogen plasma treatment with microwave plasma (for example, 2.45 GHz) is performed at a temperature of 1 ° C. for 1 to 60 minutes. The stirring of the powder after the treatment and the treatment under the same conditions is repeated a plurality of times, preferably 2 to 10 times. This is put in a Mo capsule and treated at a temperature of 6 to 6.5 GPa and 1800 to 2000 ° C. for 5 to 60 minutes using a belt type ultra high pressure generator. By doing so, a polycrystalline body in which cBN particles are firmly bonded is obtained. In the cutting test conducted using this polycrystal, the tool life is more than twice as long as a cBN sintered body containing a commercially available binder.
EXAMPLES Hereinafter, although an Example is given and the cBN sintered compact of this invention is demonstrated concretely, this invention is not limited to this.

Example 1
The 1-3 μm cBN powder was subjected to hydrogen plasma treatment with microwave plasma at 1000 ° C. for 30 minutes in a 100 torr reduced-pressure hydrogen stream using a microwave reactor. Stirring the treated powder and treating it under the same conditions was repeated three times. This was placed in a Mo capsule and treated for 15 minutes at a temperature of 6.5 GPa and 2000 ° C. using a belt-type ultrahigh pressure generator. A polycrystalline body in which cBN particles were strongly sintered was obtained. When a cutting test was performed using this polycrystal, it was found that the tool life was twice or more that of a cBN sintered body containing a commercially available binder.

(Comparative Example 1)
The 1-3 μm cBN powder was treated for 1 hour at 1000 ° C. in a vacuum of 10 ⁻³ torr. This was subjected to a high-pressure and high-temperature treatment under the same conditions as in Example 1. The obtained polycrystal was insufficiently sintered and could not withstand the cutting test.

(Example 2)
1 to 3 μm of cBN powder was subjected to hydrogen plasma treatment with microwave plasma at 1000 ° C. for 30 minutes in a 100 torr reduced-pressure hydrogen stream using a microwave reactor. Stirring the treated powder and treating under the same conditions was repeated 6 times. This was put in a Mo capsule and treated for 15 minutes at a temperature of 6.5 GPa and 1900 ° C. using a belt-type ultrahigh pressure generator. A polycrystalline body in which cBN particles were strongly sintered was obtained. When a cutting test was performed using this polycrystal, it was found that the tool life was twice or more that of a cBN sintered body containing a commercially available binder.

(Example 3)
1 to 3 μm of cBN powder was subjected to hydrogen plasma treatment with microwave plasma at 1300 ° C. for 30 minutes in a 100 torr vacuum hydrogen stream using a microwave reactor. Stirring the treated powder and treating it under the same conditions was repeated three times. This was put in a Mo capsule and treated for 15 minutes at a temperature of 6.5 GPa and 1800 ° C. using a belt-type ultrahigh pressure generator. A polycrystalline body in which cBN particles were strongly sintered was obtained. When a cutting test was performed using this polycrystal, it was found that the tool life was twice or more that of a cBN sintered body containing a commercially available binder.

Claims (1)

In the method for producing cubic boron nitride polycrystal, the surface of the raw material cubic boron nitride powder is made highly purified and stabilized by plasma treatment in hydrogen. Method.