JPS62108708A - Production of cubic boron nitride - Google Patents

Abstract

PURPOSE:To prolong the life of an apparatus and to obtain cubic boron nitride in a high yield by adding the boronitride of an alkali metal to pyrolytic boron nitride and holding them under conditions of high temp. and pressure within ranges in which cubic boron nitride is thermodynamically stable. CONSTITUTION:The boronitride of an alkali metal is added as a catalyst to pyrolytic boron nitride and they are held under conditions of high temp. and pressure within ranges in which cubic boron nitride is thermodynamically stable to convert the pyrolytic boron nitride into polycrystalline cubic boron nitride. The pyrolytic boron nitride used is highly oriented boron nitride synthesized by a special production method called a chemical vapor deposition method (CVD method). Lithium boronitride is preferably used as the boronitride of the alkali metal. It is necessary to add the catalyst by >=0.1mol% of the amount of the pyrolytic boron nitride as starting material.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for producing cubic boron nitride. Cubic boron nitride is a substance that has attracted attention because it has properties suitable for use as a tool material such as cutting tool inserts and grinding abrasive grains.

(Prior art) Cubic boron nitride has a hardness close to that of diamond, and is superior to diamond in terms of chemical stability, for example, it can withstand high temperatures in an oxidizing atmosphere and is particularly reactive with iron group elements. It is in increasing demand as a useful material for cutting and grinding high-speed steel, nickel, and cobalt-based heat-resistant, high-strength materials with mechanical properties far superior to those of diamond. It's coming.

The conventional manufacturing method of cubic boron nitride is (a)
A method of treating hexagonal boron nitride under high temperature and high pressure by adding an appropriate catalyst (temperature: approximately 1400°C or higher, pressure crab 45K)
More than bar; Kagaku Kogyo (1982) September issue.

55-58).

(b) A method of directly treating cubic boron nitride under high temperature and high pressure without adding a catalyst (temperature: about 1600°C or higher, preferably 2000-3000°C, pressure crab 55-85Kba)
r, preferably 65 to 75 Kbar; JP-A-55-16
Publication No. 7110).

etc. are known.

(Problems to be Solved by the Invention) However, these methods have the following problems.

In method (a), cubic boron nitride can be produced under milder high-temperature and high-pressure conditions than in method (b), but the cubic boron nitride obtained is usually single-crystal particles. When cubic boron nitride of such tunic crystal grains is used as grinding abrasive grains for difficult-to-cut materials, the grains tend to break due to the interstitial pillars peculiar to single crystal grains, and it has been observed that the wear resistance of the grinding wheel is improved. do not have.

Polycrystalline cubic boron nitride may be produced by adjusting the amount of catalyst added, but in this case, the catalyst components remain as impurities in the cubic boron nitride, resulting in a decrease in strength.

In the method (b), since it is a direct conversion without using a catalyst, the resulting cubic boron nitride is polycrystalline, has no interstitial pillars, and has significantly improved fracture toughness compared to single crystal.
When used as abrasive grains, it exhibits higher wear resistance than cubic boron nitride obtained by method (a). However, very severe high temperature and high pressure processing conditions such as 2000 to 2300°C and 65 to 75 Kbar are required, which tends to damage expensive high temperature and high pressure equipment, and therefore method (b) is unsuitable for industrial production. be.

Furthermore, in both methods (a) and (b), cubic boron nitride is used as a raw material.

Commercially available cubic boron nitride is boron oxide (B,
0. ) is manufactured by nitriding. For this reason, it is common to contain 5,000 to 10,000 ppm of oxygen as an impurity. Since the oxygen contained in such starting materials significantly inhibits the conversion of cubic boron nitride to cubic boron nitride, unreacted cubic boron nitride remains, which causes uneven cubic nitride. This causes the formation of boron and a decrease in the yield of cubic boron nitride. In order to prevent such a phenomenon, there is a drawback that a deoxidizing pretreatment step such as heating the raw material cubic boron nitride at 2000° C. for 2 hours or more in a nitrogen stream is required.

(Means for Solving the Problems) The present inventors have conducted extensive research with the aim of solving the above-mentioned drawbacks of the conventional manufacturing method of cubic boron nitride, and have achieved We have discovered a method for producing polycrystalline cubic boron nitride in high yield, which has high fracture toughness, excellent wear resistance, and does not contain any residual catalyst under mild conditions of high temperature and high pressure.

The present invention produces pyrolytic boron nitride by adding an alkali metal boron nitride as a catalyst to pyrolytic boron nitride and maintaining it under high temperature and high pressure conditions within the thermodynamic stability range of cubic boron nitride. This is a method for producing cubic boron nitride, which is characterized by converting it into polycrystalline cubic boron nitride.

The pyrolytic boron nitride used in the present invention has a chemical vapor deposition strength (
It is highly oriented boron nitride synthesized by a special manufacturing method called CVD method. Synthesis of pyrolytic boron nitride by the CVD method uses a boron halide such as boron trichloride and ammonia as gaseous raw materials, as disclosed in U.S. Pat. No. 3,152,006, for example. temperature 14
This is achieved by precipitating nitrogen and boron from the gas phase on the surface of a suitable substrate under conditions of 50°C to 2300°C and a pressure of 50 Torr or less. It is commercially available.

This type of pyrolytic boron nitride does not require any oxygen or oxides in the manufacturing process, and can be manufactured with extremely high purity. Moreover, pyrolytic boron nitride itself is extremely stable in air. Since the surface oxidation phenomenon observed in commercially available cubic boron nitride powders is negligible, no special treatment and handling is required (although it can always be considered as boron nitride with extremely low oxygen content).

Commercially available pyrolytic boron nitride is a plate-shaped molded product, but it is highly flexible and can be easily crushed. The pyrolytic boron nitride powder is then mixed with an alkali metal boron nitride powder as a catalyst. Lithium boronitride (LiJ) is suitable as the alkali metal boronitride.

The mixing ratio of the catalyst needs to be 0.1 mol % or more with respect to the pyrolytic boron nitride that is the raw material. 0.1
If the amount is less than mol %, the conversion to cubic boron nitride will not be complete. Even if too much catalyst is added, no increase in effect is observed, and at most about 10 mol % is sufficient.

Using the catalyst-containing pyrolytic boron nitride powder obtained in this way as a starting material, pyrolytic boron nitride is converted to cubic boron nitride, which is not found in conventional cubic boron nitride synthesis methods. Various advantages can be obtained. First, unlike when ordinary cubic boron nitride is used as a raw material, polycrystalline cubic boron nitride is produced even in the presence of a catalyst. In this case, even if the amount of catalyst added is not strictly controlled, the cubic boron nitride produced is polycrystalline, and there is hardly any residual catalyst that could cause particle destruction. The reason for this is not clear, but the formation mechanism is different between ordinary hexagonal boron nitride and pyrolytic boron nitride, and the difference in microstructure caused by this is due to
This is thought to be reflected in the microstructure of cubic boron nitride after conversion. In addition, since the starting material contains a catalyst for converting cubic boron nitride, the high temperature and high pressure conditions required for conversion to cubic boron nitride can be changed to mild conditions in the thermodynamic stability range of cubic boron nitride, such as 1450 ~1500℃, 45
~50Kbar is sufficient. Therefore, in the method of the present invention, the conditions required when conventionally known cubic boron nitride is directly treated under high temperature and high pressure conditions without adding a catalyst, such as 2
Polycrystalline cubic boron nitride is produced under much milder conditions than 000-2300°C + 65-75 Kbar. Therefore, damage to the high-temperature and high-pressure equipment is less likely to occur, the life of the equipment can be extended, and industrial productivity can be significantly improved. Furthermore, pyrolytic boron nitride, which is the starting material in the present invention, does not contain any oxygen or oxides during its manufacturing process, and pyrolytic boron nitride itself has excellent stability in air and oxidation resistance. Therefore, the oxygen content is extremely low, and the advantage is that cubic boron nitride can be obtained in high yield without the need for the deoxidizing pretreatment step that is required when using ordinary cubic boron nitride as a starting material. be.

In the present invention, there is no need to perform deoxygenation pretreatment of the starting material, and the material has high fracture toughness and resistance under mild temperature and high pressure conditions that are advantageous for industrial production within the thermodynamic stability range of cubic boron nitride. Polycrystalline cubic boron nitride with excellent abrasive properties and containing no residual catalyst can be obtained at a high yield, and the obtained polycrystalline cubic boron nitride is suitable as abrasive grains for hard-to-grind grinding. It has certain characteristics.

(Example) The present invention will be described below with reference to Examples and Comparative Examples.

EXAMPLE Commercially available pyrolytic boron nitride having an IN thickness was pulverized using a vibration mill to obtain pyrolytic boron nitride powder having an average particle size of about 1 μm. The purity of this pyrolytic boron nitride powder is 99.9% by weight.
Thus, the purity was extremely high.

This powder was mixed with lithium boronitride (Li:+N) powder, molded into a pellet, and held in a belt-type high-temperature, high-pressure device at a temperature of 1450°C and a pressure of 50 Khar for 90 minutes. I got the basics. Regarding the obtained cubic boron nitride, the produced phase was identified by X-ray diffraction, the residual catalyst component was quantified by chemical analysis, the particle size was measured by an electron microscope, and the microstructure was observed. These results are shown in Table 1.

Furthermore, the cubic boron nitride particles were plated with approximately 50% nickel by weight, and then sieved to 80/100 maisone to manufacture a resin bonded grindstone, which was subjected to wet surface grinding to determine the grinding ratio. .

The results are shown in Table 1. In addition, Table 2 shows the manufacturing conditions and grinding conditions for the grindstone.

Comparative example Commercially available high-purity cubic boron nitride powder with an average particle size of approximately 1 μm
The powder was exposed to a temperature of 1950° C. in a nitrogen gas atmosphere of 1 atm for 5 hours to obtain a cubic boron nitride powder having an oxygen content of 2% or less. This powder was mixed with Li and BNz powders, molded into pellets, and kept under the same temperature and pressure conditions in the same apparatus as in the example to obtain cubic boron nitride. Regarding the obtained cubic boron nitride, identification of the generated phase, determination of residual catalyst components,
The particle size was measured and the microstructure was observed. Furthermore, a grinding test was conducted under the same conditions as in the example. These results are shown in Table 1.
Shown below.

In addition, in Table 1, the absence of crystal habit in the microstructure means high fracture toughness, and the high grinding ratio means excellent wear resistance, which makes it suitable for application to tool materials such as abrasive grains for grinding wheels. means it is suitable for

(Effects of the Invention) According to the present invention, polycrystalline cubic boron nitride with high fracture toughness and excellent wear resistance can be obtained, and therefore the cubic boron nitride produced can be used as abrasive grains for grinding wheels, etc. Suitable for application to tool materials. In particular, in the present invention, unlike conventional manufacturing methods,
Since pyrolytic boron nitride is used as the starting material, there is no need for a pretreatment step for deoxidizing the starting material, and by adding a cubic boron nitride conversion catalyst, it is possible to produce polycrystalline cubic boron nitride using conventional methods. Polycrystalline cubic boron nitride can be produced under conditions of high temperature and high pressure, which are much milder than the conventional conditions, so damage to high temperature and high pressure equipment is less likely to occur, making it possible to extend the life of the equipment and significantly improve industrial productivity. There is an advantage that

Claims (1)

[Claims] 1. Pyrolytic decomposition is carried out by adding an alkali metal boronitride as a catalyst to pyrolytic boron nitride and maintaining it under high temperature and high pressure conditions within the thermodynamic stability range of cubic boron nitride. A method for producing cubic boron nitride, comprising converting boron nitride into polycrystalline cubic boron nitride. 2. The method according to claim 1, wherein the pyrolytic boron nitride has a purity of 99.9% by weight or more.