JPH0347132B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing cubic boron nitride (hereinafter referred to as CBN) from hexagonal boron nitride (hereinafter referred to as HBN) using a novel catalyst. As is well known, CBN has a hardness close to that of diamond, and is superior to diamond in terms of chemical stability, so its demand as a grinding material (abrasive grain) is increasing. The industrial method for producing CBN as described above involves mixing HBN powder and catalyst powder, and then
A common method is to convert HBN into CBN by processing at high pressures of about 40 to 60 kbar and high temperatures of about 1,400 to 1,600 degrees Celsius. Catalysts used in such methods include nitrides of alkali metals or alkaline earth metals, or boron nitride-based ternary compounds consisting of alkali metals or alkaline earth metals, nitrogen, and boron, such as Ca ₃ B ₂ N ₄ and Li ₃ BN ₂ are known. In this method, hexagonal boron nitride is dissolved in the catalyst melt, and CBN is precipitated by taking advantage of the fact that CBN has a lower solubility in the eutectic melt than HBN under the synthesis conditions. . By the way, grinding materials (abrasive grains) need to have high mechanical strength, especially breaking strength, and in relation to strength, the particles must have good shape, that is, flat or acutely angled. It is required that the shape be as close to a sphere as possible without any irregularities, or that the surface have few irregularities.
However, as mentioned above, in the conventional cubic boron nitride manufacturing method using nitride (binary compound) or boron nitride-based ternary compound as a catalyst, it is not always possible to obtain CBN with sufficient mechanical strength and good shape. The reality is that this is not always possible.
In other words, in the conventional method using a catalyst, the actual situation is that strength and shape cannot be improved unless manufacturing conditions are controlled very precisely and complicated. Therefore, the present inventors conducted extensive experiments and research in order to establish a method for improving the strength and shape of CBN, and developed a new catalyst. Excellent CBN
was successfully manufactured. This novel catalyst has a molar ratio of X:BN (1~
1.4):2 and heated at 800 to 1300°C under an inert atmosphere such as _N2 or Ar. Here, X is Be, Mg, Ca, Sr, Ba
nitrides of Be ₃ N ₂ , Mg ₃ N ₂ , Ca ₃ N ₂ ,
It is a mixture of two or more selected from Sr ₃ N ₂ and Ba ₃ N ₂ . There is no particular restriction on the mixing ratio, but each component should be 10%.
It is preferable that it is above. The structure of the substance produced by this heat treatment is not clear. However, it is thought that it is not just a mixture. This is because the effects are clearly different when a mixture of these is used as a catalyst without heat treatment and when this product is used as a catalyst. Furthermore, it is important to use two or more types of nitrides such as Be and Mg. It is known that the above-mentioned Ca ₃ B ₂ N ₄ can be obtained by mixing Ca ₃ N ₂ and BN in a molar ratio of 1:2 and heating the mixture. However, according to the experiments of the present invention, when two or more kinds of alkaline earth nitrides are used,
It has been found that using a catalyst reacted with BN provides excellent CBN yield, particle properties, etc. In the above treatment, it is thought that the effect of heating is not apparent at temperatures below 800°C, and when the temperature exceeds 1300°C, evaporation is severe and the product is decomposed. A heating time of about 20 to 60 minutes is sufficient. The mixture melts exothermically in the above temperature range. From these points, it is presumed that some kind of compound was produced by heating the mixture. The reason for specifying the molar ratio above is that when mixed and heated to this ratio, the effect as a catalyst becomes greater. The molten material is cooled and solidified in an inert gas atmosphere, pulverized to about 150 mesh or less, and used as a catalyst. Next, a method for producing cubic boron nitride using the catalyst obtained as described above will be explained. First, to 100 parts by weight of a powder of hexagonal boron nitride, preferably of 150 mesh or less, 5 to 5 parts of a powder of preferably 150 mesh or less of the product as a catalyst is added.
50 parts by weight, preferably 10 to 30 parts by weight, are mixed uniformly and compacted. Alternatively, the hexagonal boron nitride powder and the above-mentioned catalyst powder are individually compacted into thin plate shapes, and these are alternately stacked in the above-mentioned mixing ratio. CBN is applied to the mixed powder compact or laminate thus obtained.
A high pressure of 40 to 60 kbar is applied at a high temperature in the thermodynamically stable region of , preferably 1300 to 1600°C, and maintained for 5 to 40 minutes. In this way, cubic boron nitride crystal grains are obtained. Note that these temperatures, pressures, and holding times are the same as conventional ones. As mentioned above, various means for applying high temperature and high pressure can be considered, but for example, the mixed powder compact or laminate is placed in a reaction vessel as shown in Fig. 1, and electricity is applied and pressure is applied using a press. Good. In FIG. 1, the outer wall 1 of the container is made of pyrofluorite as a pressure transmitting body in a cylindrical shape, and inside thereof a heater 2 made of a graphite cylinder and pyrofluorite 8 as a partition wall material are arranged. There is. Further, a current-carrying steel ring 3 and a current-carrying steel plate 4 are arranged at the upper and lower ends of the container, respectively, and inside thereof, a sintered alumina plate 5 and a pyrofilite 6 as a pressure transmitting body are arranged. A space surrounded by the pyrophyllite 6 and the pyrophyllite 8 serving as a partition wall material serves as a storage chamber 7 for accommodating the reaction raw materials. Examples and comparative examples in which cubic nitrogen boron was produced using the catalyst of the present invention are shown below. Examples 1 to 10 Compounds each pulverized to 150 mesh or less were mixed in the proportions shown in Table 1 and placed in a platinum container.
While flowing N ₂ gas at a flow rate of 8/min, the sample was heated and heated in an electric furnace and maintained under the conditions shown in the table.
The reaction product was cooled in an electric furnace in a _N2 gas stream, and then ground to 150 mesh or less in a _N2 gas atmosphere.

[Table] The powder of 150 mesh or less obtained in each of the above examples and the HBN powder of 150 mesh or less were uniformly mixed in a nitrogen atmosphere, and the surface pressure was 700 Kg/cm ²
It was formed into a cylindrical shape with an outer diameter of 20 mm and a length of 20 mm, placed in a container shown in Figure 1, and processed with a high-pressure press.
CBN was generated. For comparison, each powder used in Example 1 (Comparative Example 1) and the powder used in Example 7 (Comparative Example 7)
were not calcined in advance, but were simply mixed in the same molar ratio as in each example, and the catalyst was used in the same manner as in the examples.
Manufactured CBN. Comparative Examples 2 to 6 Only one type of nitride such as Be and Mg and BN
Beryllium nitride and the like were produced under the mixing conditions shown in Table 2. Other conditions such as the particle size of the raw material at this time were the same as in Examples 1 to 10. Powders of 150 mesh or less of each of the produced compounds and HBN powder of 150 mesh or less were uniformly mixed in a nitrogen atmosphere, molded under the same conditions as Examples 1 to 10, and processed with a high-pressure press. ,
CBN was generated.

[Table] Table 3 shows the conditions and results of these Examples and Comparative Examples.

【table】

[Table] In Table 3, the destructive tests were conducted as follows. i.e. diameter 10 made of WC-Co
Diameter 100 on the bottom cylinder of the upper and lower cylinders in mm
One sample grain of ~150 μm was placed, and the upper cylinder was lowered by driving a DC motor. Then, the position where the upper cylinder contacts the sample grain on the lower cylinder was electrically detected, and the corresponding distance D between the surfaces of the upper and lower cylinders was determined, and this was taken as the diameter of the grain. As the load was further increased, the fracture strength σ t of the grain was determined from the total load W at which the grain fractured using the following formula (1), σ _t =W/(0.32A)...(1), as is _well known. . However, in reality, the above-mentioned test was conducted on 50 samples each, the average value of D and the average value of W were determined, and the average breaking strength was calculated from equation (1). Note that formula (1) is clarified by Hiroyuki Yoshikawa, for example, in "RIKEN Report Vol. 39, No. 6" (published in 1960), page 310. In addition, in the table, the yield is the blended HBN (excluding catalyst)
This is the ratio of CBN produced to Electron micrographs are shown of representative examples of CBN grains obtained in the above Examples and Comparative Examples. The magnification is
It is 100 times more. FIG. 2 shows Example 1, and FIG. 3 shows Comparative Example 1. The same was true for other Examples and Comparative Examples. As can be seen from this photo, the CBN according to the present invention has a nearly spherical shape as a whole;
Moreover, it can be seen that the surface has a smooth shape with few minute irregularities. As shown in Table 3, nitrides such as Be and Mg are
Compared to a composite catalyst in which only seeds are reacted with BN, using a composite catalyst in which two or more types of nitrides and BN are reacted as in the present invention improves CBN yield, grain strength, and other characteristics. You can see that it will be excellent. Furthermore, according to the present invention, in addition to being able to increase the yield of CBN, there are the following effects. Since the catalyst composition is pre-calcined, the time required for high-temperature, high-pressure treatment to generate CBN can be shortened, and the time that the mold is exposed to high temperature and high pressure is correspondingly shortened, extending the life of the mold. Since Be ₃ N ₂ , Mg ₃ N ₂ , etc. are mixed with BN and treated in advance, it is thought that a reaction occurs during this time, and this reaction does not occur during CBN production, and the catalyst This enables smooth dissolution and precipitation of HBN during the process, producing high-grade CBN. The pre-calcined catalyst is thought to have a stable structure, and although it could previously only be handled in a nitrogen atmosphere box, it is sufficiently stable in the atmosphere, making it extremely easy to store and handle. Improves reproducibility in manufacturing. Reference Example Grinding tests for representative examples of abrasive grains obtained in the above Examples and Comparative Examples are shown below. The grain size was #120/140 according to the JIS standard, and an electrodeposited grindstone was manufactured according to a conventional method. The grindstone specifications and grinding conditions are as follows. Grinding method Wet surface grinding (traverse cut) Grinding wheel specifications IAI ^{180 D} × 10 ^{T ×} 3 /Pass grinding fluid Soluble type work material SKH-57 (H _RC = 62) The results are as follows. Example 1 Comparative Example 1 Grinding ratio 610 420 (Grinding ratio = grinding amount of work material (volume) / grinding wheel wear amount (volume))

[Brief explanation of drawings]

FIG. 1 is a vertical cross-sectional view showing an example of a reaction vessel used in producing CBN, and FIG. 2 is an enlarged micrograph (100x) of CBN grains obtained in Example 1 of the present invention. FIG. 3 is an enlarged micrograph (100 times magnification) of CBN grains obtained in Comparative Example 1. DESCRIPTION OF SYMBOLS 1... Container outer wall, 2... Heater, 3... Steel plate ring for energizing, 4... Steel plate for energizing, 5... Alumina plate, 7... Raw material storage chamber.

Claims (1)

[Claims] 1. When synthesizing cubic boron nitride by holding hexagonal boron nitride and a catalyst in a high temperature and high pressure region where cubic boron nitride is thermodynamically stable, X:BN is used as the catalyst. A method for producing cubic boron nitride, characterized in that the molar ratio of (1 to 1.4):2 is used, and the product is fired in an inert atmosphere at 800°C to 1300°C (the above X is
A mixture of two or more nitrides selected from Be, Mg, Ca, Sr, and Ba nitrides).