JPH0314495B2 - - 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 crushing 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.By using this, CBN with excellent strength, shape, etc. It was successfully manufactured. This new catalyst contains _{Li 3} N: It is obtained by heating. In the above, X is Mg ₃ N, Sr ₃ N ₂ or Be ₃ N ₂ . The structure of the substance produced by this heat treatment is not clear. Moreover, it is thought that it is not just a mixture. This is because the effects are different when CBN is produced using a mixture of these as a catalyst and when this product is used as a catalyst. In the above treatment, the effect of heating does not appear below 800°C. Moreover, when the temperature exceeds 1300°C, it is thought that decomposition of the product occurs. Heating time is 20-60
A minute or so is sufficient. The mixture melts exothermically in the above temperature range. From these points, it is presumed that some kind of compound was produced from 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, 5 to 50 parts by weight, preferably 10 to 30 parts by weight of powder of the above product, preferably 150 mesh or less, is blended as a catalyst to 100 parts by weight of hexagonal boron nitride powder, preferably 150 mesh or less, and homogeneously mixed. The mixture is mixed 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. The thermodynamically stable distribution range of CBN for the mixed powder compact or laminate obtained in this way, preferably 1300~
Applying high pressure of 40 to 60 kbar at a high temperature of 1600℃,
Hold for ~40 minutes. In this way, cubic boron nitride crystal grains are obtained. Note that these temperatures, pressures,
The retention time is the same as before. 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, a container outer wall 1 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 pyrophyllite 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 in which cubic boron nitride was produced using the catalyst of the present invention and comparative examples in which cubic boron nitride was produced using a known substance as a catalyst are shown below. Examples 1 to 6 Compounds each pulverized to 150 mesh or less were mixed in the proportions shown in Table 1, placed in a platinum container, and heated and heated in an electric furnace while flowing N ₂ gas at a flow rate of 8/min. and maintained under the conditions shown in the same 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, Li ₃ BN ₂ powder (Comparative Example 1)
and a mixture of Li ₃ N powder, Mg ₃ N ₂ powder, and BN powder at a molar ratio of 1.1:1.2:3 (Comparative Example 2) and
Li ₃ N powder, Sr ₃ N ₂ powder, and BN powder in a molar ratio of 1:
CBN was produced in the same manner as in the example using a 1:3 mixture (comparative example 3) as a catalyst. Table 2 shows the conditions and results of these Examples and Comparative Examples.

[Table] The crushing test in Table 2 was conducted as follows. i.e. 10mm diameter made of WC-Co
Diameter 100~ on the bottom cylinder of the upper and lower cylinders
One sample particle 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 σt of the grains was determined from the total load W at which the grains fractured using the following formula (1), as is well known: σt=W/(0.32A) (1). 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). Formula (1) is clarified by Hiroyuki Yoshikawa, for example, in "RIKEN Report Vol. 39, No. 6" (published in 1963), page 310. In addition, in the table, the yield is the blended HBN (excluding catalyst)
It 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 4, and FIG. 3 shows Comparative Example 3. 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. 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 high-temperature, high-pressure treatment time for CBN production can be shortened, and the time that the mold is exposed to high temperature and high pressure is shortened accordingly, extending the life of the mold. Since Li ₃ N, 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 is This enables smooth dissolution and precipitation of HBN, resulting in high-quality
Generated by 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 1A1} 180 ^{D ×} 10 T × 3 20μ/pass grinding fluid Soluble type work material SKH-57 (H _RC = 62) The results are as follows. Example 4 Comparative Example 3 Grinding ratio 580 410 (Grinding ratio = grinding amount of work material (volume) / grinding wheel wear amount (volume))

[Brief explanation of the drawing]

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 4 of the present invention. FIG. 3 is an enlarged micrograph (100 times magnification) of CBN grains obtained in Comparative Example 3. 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, Li ₃ is used as the catalyst. It is characterized by using a mixture of N:X:BN in a molar ratio of (1 to 1.4): (1 to 1.4): 3 and calcined in an inert atmosphere at 800°C to 1300°C in advance. Manufacturing method of cubic boron nitride (in the above, X is Mg ₃ N ₂ or Sr ₃ N ₂ or
_{Be3N2 )} .