JPS62108717A - 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 hydride of an alkali or alkaline earth 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 hydride of an alkali or alkaline earth 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). LiH, NaH, CaH2 or BaH2 may be used as the hydride of the alkali or alkaline earth 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 in which hexagonal boron nitride is treated under high temperature and pressure by adding an appropriate catalyst (temperature: about 1400℃ or more, pressure crab about 45K)
Over bar; Kagaku Kogyo (1982) September issue, 55-5
(page 8).

(b) A method of directly treating hexagonal boron nitride under high temperature and high pressure without adding a catalyst (temperature: 1600°C or higher, preferably 2000 to 3000°C, pressure crab 55 to 85kb)
ar.

Preferably 65 to 75 kbar; JP-A-167-1987
No. 110).

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 such single-crystal grain cubic boron nitride is used as abrasive grains for hard-to-cut materials, the grains tend to break due to the non-spacing characteristic of single-crystal grains, and the wear resistance of the grindstone is improved. I can't.

Polycrystalline cubic boron nitride may be produced by adjusting the amount of catalyst added, but in this case, the catalyst component remains as an impurity in the cubic boron nitride, leading to a decrease in strength.

In method (b), since the conversion is direct without using a catalyst, the resulting cubic boron nitride is polycrystalline, has no gaps, 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), hexagonal boron nitride is used as a raw material.

Usually commercially available hexagonal boron nitride is boron oxide (B2
03) 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 hexagonal boron nitride to cubic boron nitride, unreacted hexagonal 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 hexagonal 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 is capable of thermally decomposing boron nitride by adding an alkali metal or alkaline earth metal hydride as a catalyst to thermally decomposing 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 boron nitride into polycrystalline cubic boron nitride.

The pyrolytic boron nitride used in the present invention has a chemical vapor deposition strength (
It is a highly oriented boron nitride synthesized by a special m4 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 depositing nitrogen and boron 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 as.

Such pyrolytic boron nitride does not require any oxygen or oxides in its manufacturing process, and can be produced with extremely high purity.Moreover, pyrolytic boron nitride itself is extremely stable in air. Since the surface oxidation phenomenon observed in commercially available hexagonal boron nitride powders can be ignored, it can always be considered as boron nitride with extremely low oxygen content even without special treatment and handling.

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 alkali metal or alkaline earth metal hydride powder as a catalyst.

Examples of hydrides of alkali metals or alkaline earth metals include lithium hydride (LiH), sodium hydride (Nail), hydride (CaL), and barium hydride (BaH2).

The mixing ratio of the catalyst needs to be 0.1 mol % or more based on the pyrolytic boron nitride that is the raw material. If the amount is less than 0.1 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 a maximum of 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 normal hexagonal 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 hexagonal 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 and 65-75 Kbar. Therefore, the high temperature and high pressure equipment is less likely to be damaged, 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 process that is required when using ordinary hexagonal boron nitride as a starting material. be.

In the present invention, there is no need to perform deoxidation pretreatment of the starting material, and the fracture toughness (and resistance to It is possible to obtain polycrystalline cubic boron nitride with excellent abrasive properties and no residual catalyst in a high yield, and the polycrystalline cubic boron nitride that can be processed into F4 is suitable as an abrasive grain for grinding difficult-to-grind materials. It has certain characteristics.

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

Example 1 A commercially available pyrolytic boron nitride having a thickness of 1 mm was ground using a vibration mill to obtain a pyrolytic boron nitride powder having an average particle size of about 1 μm. The purity of this pyrolytic boron nitride powder was 99.9% by weight or more, which was extremely high purity.

This powder was mixed with lithium hydride (LiH) powder and formed into pellets, which were placed in a belt-type high-temperature, high-pressure device at a temperature of 1450°C and a pressure of 45 Kbar for 90 minutes.
The mixture was held for 0 minutes to obtain cubic boron nitride. Identification of the formed phase of the obtained cubic boron nitride by X-ray diffraction,
The residual catalyst components were determined by chemical analysis, the particle size was measured using 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 an 80/100 mesh to produce a resin bonded grindstone, subjected to wet surface grinding, and the grinding ratio was determined. .

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

Example 2 Calcium hydride (CaH2) powder was mixed with the same pyrolytic boron nitride powder as in Example 1 and formed into pellets, which was then treated at high temperature and pressure under the same conditions as in Example 1 to form a cube. Crystalline boron nitride was obtained. Regarding the obtained cubic boron nitride, the produced phase was identified, the particle size was measured, and the microstructure was observed in the same manner as in Example 1. Furthermore, a resin bonded grindstone was manufactured in the same manner as in Example 1, and a grinding test was conducted under the same conditions as in Example 1. These results are shown in Table 1.

Comparative example Commercially available high grade hexagonal boron nitride powder with average particle size of approximately 1 μm.
The product was exposed to a temperature of 1950° C. for 5 hours in a nitrogen gas atmosphere of 1 atm to obtain a hexagonal boron nitride powder having an oxygen content of 2% by weight or less. This powder was mixed with LiH powder, molded into pellets, and kept under the same temperature and pressure conditions in the same apparatus as in the example, and cubic boron nitride was used as a first-grade cubic boron nitride. Regarding the obtained cubic boron nitride, identification of the generated phase, quantitative determination of residual catalyst components, measurement of particle size, and observation of microstructure were performed in the same manner as in Examples. Furthermore, a grinding test was conducted under the same conditions as in the example. These results are shown in Table 1.

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 poor 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 the production method used by Tomari,
Since hot boron nitride is used as the starting material, there is no need for a process of deoxidizing the starting material.Moreover, by adding a cubic boron nitride conversion catalyst, it is possible to convert the conventional polycrystalline cubic boron nitride into Polycrystalline cubic boron nitride is manufactured under conditions of high temperature and pressure, which are much milder than the manufacturing conditions of This has the advantage of significantly improving productivity.

Claims (1)

[Claims] 1. Adding an alkali metal or alkaline earth metal hydride as a catalyst to pyrolyzed 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 pyrolytic 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.