JP7033542B2 - Cubic boron nitride polycrystals and their manufacturing methods, as well as cutting tools and grinding tools - Google Patents

Description

The present invention relates to a cubic boron nitride polycrystal, which is a material that can be used for cutting tools, grinding tools, etc. that require high hardness, and a method for producing the same.

Boron nitride (BN) is mainly composed of crystals called hexagonal boron nitride (hexagonal BN: hBN), wurtzitic BN (wBN), and cubic boron nitride (cubic BN: cBN). There are three types with different structures. Among these BNs, cBN has the characteristics that it has the hardness next to diamond and is superior in thermal stability and chemical stability to diamond. In particular, diamond reacts easily with materials such as iron, nickel and titanium, whereas cBN does not react with those materials, so cBN is particularly a material for tools for machining workpieces containing iron, nickel or titanium. Are suitable.

hBN can be produced under normal pressure, whereas cBN and wBN are produced under high pressure. Patent Documents 1 and 2 and Non-Patent Document 1 contain cBN and wBN by heating the raw material hBN or pBN (described later) at a temperature of 1900 to 2300 ° C. while applying a pressure of 8 to 20 GPa. However, it is described that a cBN / wBN composite polycrystal, which is a polycrystal having an average particle size of cBN crystals of 500 nm or less, is produced. Here, pBN (pyrolytic boron nitride) is a normal pressure phase having a crystal structure oriented in the [001] direction like hBN, and is a vacuum thermal decomposition chemical vapor deposition (CVD) method. It is a boron nitride with a higher purity than hBN. In Comparative Example 2 of Patent Document 2 and Comparative Example 1 of Patent Document 3, polycrystals containing only cBN without containing wBN are described, but these polycrystals are average grains of crystals. It has a diameter exceeding 1000 nm and does not have high hardness.

Japanese Unexamined Patent Publication No. 2014-034487 JP-A-2015-205789

H. Sumiya et al., "Real indentation hardness of nano-polycrystalline cBN synthesized by direct conversion sintering under HPHT", Diamond and Related Materials, published by Elsevier, (Netherlands), Vol. 48, pp. 47-51, September 2014

The cBN / wBN composite polycrystals described in Patent Documents 1 and 2 and Non-Patent Document 1 have a Knoop hardness, which is one of the indexes representing hardness, at the maximum of 48 GPa. However, in order to efficiently and accurately cut and grind workpieces with high hardness such as high-speed steel and die steel, cutting tools and grinding tools made of materials with hardness of 50 GPa or more is necessary.

An object to be solved by the present invention is to provide a polycrystal of boron nitride having a hardness of 50 GPa or more.

The cubic boron nitride polycrystal (cBN polycrystal) according to the present invention, which has been made to solve the above problems, has a single-phase crystal structure of cubic boron nitride (cBN) having an average particle size of 200 nm or less. It is an unoriented polycrystal composed of, and is characterized by having a noup hardness of 50 GPa or more.

Here, "average particle size", "single phase" and "non-oriented" are defined as follows.
In the present invention, the "average particle size" refers to a value obtained by a cutting method using a transmission electron microscope (TEM) image based on JIS G 0551: 2013 (microscopic test method for steel-crystal particle size). In the cutting method, a circle is drawn on the TEM image, 6 straight lines (diameter) passing through the center of the circle are drawn at 30 ° intervals (12 in radius), and the number of crystal grains crossing each straight line is counted (however, a straight line). If the end of is inside the crystal grain, the number of the crystal grain is counted as 0.5), and the sum of all the straight lines of the number (the value of this sum is defined as the "number of crystal grains") is obtained. Then, the value obtained by dividing 6 times the diameter by the number of crystal grains is taken as the average particle size. The diameter of the circle drawn on the TEM image is not limited to a specific value, but it is desirable to set the number of crystal grains in one diameter to be about 10 to 40. In addition, it is desirable to acquire TEM images at a plurality of locations for one cBN polycrystal and obtain the average particle size from the average value of the plurality of locations.

In the present invention, "single phase" means to irradiate a cBN polycrystal with CuKα rays (wavelength: 0.15418 nm, collimator: 0.1 mm) generated from an X-ray generator operating at a tube voltage of 40 KV and a tube current of 30 mA. In the X-ray diffraction pattern obtained thereby, a peak derived from a crystal other than the cBN crystal (particularly, a different crystal composed of wBN) is not detected.

The "non-oriented" in the cBN polycrystal according to the present invention means the intensity I ₍₂₂₀₎ of 220 peaks and the intensity I of 111 peaks of X-ray diffraction measured in bulk (without powdering) of the cBN polycrystal. It means that the ratio I ₍₂₂₀₎ / I _{(111) of (111) is} 0.15 or more. The measurement conditions for X-ray diffraction at that time are the same as the measurement conditions for confirming that the phase is single-phase.

In the present specification, among the cBN polycrystals of the present invention, those having an average particle size of 100 nm or less are referred to as “microstructure cBN nanopolycrystals”, and those having an average particle size of 50 nm or less are referred to as “ultrafine”. It is called "tissue cBN nanopolycrystal". In the configuration described later in which coarse crystal grains of cBN having an average particle size of 115 to 165 nm are dispersed in a fine structure having an average particle size of 70 nm or less, the coarse crystal grains also constitute the cBN polycrystal of the present invention. It is contained in the crystals. In other words, "coarse crystal grains" mean that the crystal grains are relatively large in comparison with the crystal grains having an average grain size of 70 nm or less, and a general cBN (not limited to that of the present invention). The crystal grains in the polycrystal are classified into fine crystal grains.

In the cBN polycrystal, when the average particle size of the crystals exceeds 200 nm, it is difficult to obtain a polycrystal in which the crystals are firmly bonded to each other. In addition, if the polycrystal contains crystals (different crystals) other than cBN (wBN, etc.), the different crystals become the starting points of slips and microcracks when a force is applied to the polycrystal from the outside, and the hardness decreases. Causes the problem. Furthermore, if the polycrystals are oriented, anisotropy occurs in the hardness, and the hardness varies depending on the direction. Therefore, in the present invention, (i) the average particle size of the crystal is as small as 200 nm or less, (ii) it is a single phase consisting only of cBN, and (iii) it is non-oriented, so that the conventional cBN many A cBN polycrystal having a high Knoop hardness of 50 GPa or more, which could not be realized with a crystal, can be obtained. The cBN polycrystal having such a high Knoop hardness can be suitably used as a material for a tool cutting edge of a cutting tool, an abrasive grain cutting edge of a grinding tool, and the like.

In the cBN polycrystal according to the present invention, it is desirable that the average particle size is 100 nm or less (microstructure cBN nanopolycrystal) and the Knoop hardness is 53 GPa or more. As a result, a material having a higher Knoop hardness and suitable for a tool cutting edge, an abrasive grain cutting edge, or the like can be obtained. In particular, in order to obtain a sharp tool cutting edge that can perform a mirror finish only by cutting and a sharp abrasive grain cutting edge for performing more accurate polishing processing, the average particle size is set. It is more desirable that the cBN polycrystal is 50 nm or less (that is, the cBN polycrystal is an ultrafine structure cBN nanopolycrystal).

The cBN polycrystal according to the present invention has a fine polycrystal structure in which coarse crystal grains of cBN having an average particle size of 115 to 165 nm are dispersed in a fine polycrystal structure of cBN having an average particle size of 70 nm or less. It is desirable that the polycrystal obtained by combining the coarse crystal grains and the coarse crystal grains has an average particle size of 100 nm or less (microstructure cBN nanopolycrystal) and a noup hardness of 53.5 GPa or more. Here, it is more desirable that the coarse crystal grains are 120 to 165 nm and the Knoop hardness is 54.5 GPa or more. Since the coarse crystal grains are dispersed in the fine polycrystalline structure as described above, the Knoop hardness can be made higher than that in the case of crystals having a nearly uniform particle size. Hereinafter, such a cBN polycrystal is referred to as a “coarse grain dispersed cBN nanopolycrystal”.

The cBN polycrystal according to the present invention can be produced by the following method. That is, the method for producing a cubic boron nitride polycrystal according to the present invention is
The process of preparing a raw material that is composed of pyrolysis boron nitride (pBN), which is the starting material, and is oriented in the [001] direction.
It is characterized by having a step of directly converting the pyrolyzed boron nitride to cubic boron nitride (cBN) by holding the raw material at a pressure of 25 GPa or more at a predetermined temperature for a predetermined time.

Here, "aligned in the [001] direction" means the intensity I ₍₁₀₀₎ of 100 peaks and the intensity I of 002 peaks of X-ray diffraction measured in bulk of pyrolysis boron nitride (without powdering). It means that the ratio I ₍₁₀₀₎ / I _{(002) of (002) is} 0.15 or less. The measurement conditions for X-ray diffraction at that time are the same as the measurement conditions for confirming that the cBN polycrystal is single-phase.

In order to produce a cBN polycrystal having a single-phase crystal structure of cBN, it is necessary to use pyrolysis boron nitride oriented in the [001] direction as a raw material. Even if the raw materials are oriented in this way, the crystals are not oriented in the produced cBN polycrystal.

If the manufacturing pressure is lower than 25 GPa, different crystals consisting of wBN will be generated in the polycrystal in addition to the cBN crystals, so the lower limit of the pressure is set to 25 GPa. On the other hand, in BN, since cBN is obtained at the highest pressure, there is theoretically no upper limit to the pressure.

The predetermined temperature varies depending on the pressure, but can be determined by a preliminary experiment. For example, when the pressure is 25 GPa, if the temperature is lower than 1900 ° C, different crystals consisting of wBN will be generated in the polycrystal in addition to the cBN crystal, and if the temperature exceeds 2000 ° C, Since the average particle size of the cBN crystals in the polycrystal exceeds 200 nm, the predetermined temperature is set within the range of 1900 to 2000 ° C.

The predetermined time, that is, the time for holding at the pressure and temperature varies depending on the pressure and temperature, but can be determined by a preliminary experiment. If the holding time is too short, different crystals consisting of wBN will be mixed in, and if it is too long, the average grain size of the crystals will exceed 200 nm. In order to prepare hyperfine cBN nanopolycrystals, the retention time should be short within the range where different crystals composed of wBN are not mixed. In order to produce a coarse-grained dispersed cBN nanopolycrystal, the retention time is longer than in the case of producing an ultrafine structure cBN nanopolycrystal. For example, when the pressure is 25 GPa and the temperature is 1950 ° C., the cBN polycrystal according to the present invention can be produced by setting the holding time to 11 to 25 minutes, of which the holding time is 11 to 13 minutes. By setting the above, an ultrafine structure cBN nanopolycrystal can be produced, and by setting the holding time to 15 to 20 minutes, a coarse-grained dispersed cBN nanopolycrystal can be produced.

The raw material is GI based on the integrated intensity Area ₍₁₀₀₎ of 100 peaks of X-ray diffraction measured in the bulk of the pyrolyzed boron nitride, the integrated intensity Area ₍₁₀₁₎ of 101 peaks, and the integrated intensity Area ₍₁₀₂₎ of 102 peaks. It is desirable that the GI value represented by = (Area ₍₁₀₀₎ + Area ₍₁₀₁₎ ) / Area ₍₁₀₂₎ is 15 or more. The GI value means that the higher the value, the lower the crystallinity (here, the crystallinity of the raw material pBN). By using pBN having low crystallinity as a raw material in this way, it becomes difficult to generate different crystals composed of wBN.

According to the present invention, a polycrystal of boron nitride having a hardness of 50 GPa or more can be obtained.

The schematic block diagram of the pressure medium used in one Embodiment of the manufacturing method of the cBN polycrystal according to this invention. The schematic block diagram of the static ultrahigh pressure application apparatus used in the manufacturing method of the cBN polycrystal of this embodiment. The X-ray diffraction pattern in the embodiment ((c), (d)) and the comparative example ((a), (b), (e)) of the cBN polycrystal according to the present invention. TEM images of cross sections in the present embodiment (a) and comparative example (b). The figure which shows the method of obtaining the average particle diameter from a TEM image. The graph which shows the relationship between the heating temperature and the average particle diameter in this Embodiment and a comparative example. TEM images of cross sections in the present embodiments (a) to (c) and comparative example (d). Schematic diagram of coarse-grained dispersed cBN nanopolycrystal. The graph which shows the relationship between the holding time and the average particle diameter in this Embodiment and a comparative example. The graph which shows the relationship between the average particle diameter and Knoop hardness in this Embodiment and a comparative example.

An embodiment of the cBN polycrystal according to the present invention and a method for producing the same will be described with reference to FIGS. 1 to 10.

(1) Embodiment of the method for producing a cBN polycrystal according to the present invention An embodiment of the method for producing a cBN polycrystal according to the present invention will be described with reference to FIGS. 1 and 2. In the present embodiment, a commercially available plate-shaped pBN oriented in the [001] direction is used as the raw material 11. The GI value of the raw material 11 used in this embodiment is 17.6. First, the raw material 11 is produced by cutting pBN into a disk shape having a diameter of 3 mm using a laser processing device. As shown in FIG. 1, the raw material 11 is housed in a pressure medium 17 including a pressure medium body 17a, an upper pressure medium 17b, and a lower pressure medium 17c. At that time, first, the raw material 11 is charged into a metal foil capsule 12, and the capsule 12 is placed in a cylindrical sleeve 13. Further, the heat insulating material 14 is packed above and below the raw material body 11. The outside of the sleeve 13 is surrounded by a heater 15 made of metal leaf, a heat insulating material 16 is arranged around the heater 15, and the sleeve 13 is placed in a cylindrical pressurization chamber opened in the center of the pressure medium body 17a. The upper and lower parts of the main body 17a are sandwiched between a quadrangular pyramid-shaped upper pressure medium 17b and a lower pressure medium 17c. An upper metal leaf electrode 18a and a lower metal leaf electrode 18b for supplying power to the heater 15 are interposed between the pressure medium body 17a and the upper pressure medium 17b, and between the pressure medium body 17a and the lower pressure medium 17c, respectively. .. Note that FIG. 1 is a schematic diagram, and the dimensions of each part do not represent an accurate ratio.

The raw material 11 housed in the pressure medium 17 in this way is subjected to the high pressure and high temperature described later by the static ultrahigh pressure applying device 20 shown in FIG. The static ultra-high pressure applying device 20 used here is called a "Kawai-type multi-anvil type high pressure generator" and is composed of a hydraulically driven piston 21, a guide block 22, and a press frame 23 for fixing them. A first-stage anvil 24, which is composed of six blocks made of special steel and has a cubic space formed therein, is fixed to the guide block 22. In this cubic space, the second stage anvil 25, which is composed of eight blocks made of tungsten carbide and has a regular octahedral space formed inside, is housed. The pressure medium 17 is housed in this octahedral space. By applying pressure to the first stage anvil 24 by the piston 21, the second stage anvil 25 is compressed, thereby compressing the pressure medium 17, and finally the following predetermined pressure is applied to the sample. Further, the heater 15 is energized from the external power supply 29 via the guide block 22, the first-stage anvil 24, the second-stage anvil 25, the upper metal leaf electrode 18a, and the lower metal foil electrode 18b, and the sample is supplied to the predetermined predetermined state below. By heating to a temperature, the cBN polycrystal of the present embodiment can be obtained.

In the present embodiment, the pressure applied to the raw material 11 is 23 to 30 GPa, and the heating temperature of the raw material 11 is 1900 to 2000 ° C. In the present invention, there is no particular upper limit of the pressure applied to the raw material 11, but in the present embodiment, the maximum value of the pressure is set to 30 GPa due to the specifications of the static ultrahigh pressure applying device 20. For comparison, similar experiments were performed at a pressure of 25 GPa over a temperature range of 1600 ° C or higher and lower than 1900 ° C, and above 2000 ° C and up to 2600 ° C. Furthermore, for comparison, a similar experiment was performed when the temperature was 1950 ° C and the pressure was 23 GPa. In addition, when the pressure was 25 GPa, experiments were conducted on a plurality of examples in which the holding time (holding time) at a predetermined pressure and temperature was different.

(2) Embodiment and Comparative Example of Cubic Boron Nitride Polycrystal According to the Present Invention First, the pressure at the time of fabrication is 25 GPa, the holding time is 20 minutes, and the heating temperature is within the range of 1600 ° C to 2600 ° C. Multiple different samples were made. Among the samples prepared in FIG. 3, the heating temperatures are (a) 1600 ° C (comparative example), (b) 1800 ° C (comparative example), (c) 1900 ° C (this embodiment), and (d) 1950 ° C (this). The results of X-ray diffraction measurement are shown for the cases of (Embodiment) and (e) 2150 ° C (Comparative Example). For this X-ray diffraction measurement, CuKα rays (wavelength: 0.15418 nm, collimator: 0.1 mm) generated from an X-ray generator operating at a tube voltage of 40 KV and a tube current of 30 mA were used. In all the samples, a peak derived from the (111) plane of cBN can be seen near 2θ = 43.3 °. On the other hand, in (a) and (b), 100 peaks of wBN (different crystal) (with arrows in the figure) can be seen at the position where 2θ is slightly smaller than the 111 peak of cBN (around 2θ = 40.8 °). On the other hand, 100 peaks of wBN are not seen in (c), (d) and (e). In addition, no peaks derived from the second phase such as wBN and hBN are observed in any of the samples. From these results, the comparative examples (a) and (b) are not single-phase cBNs, whereas the comparative examples (c) and (d) and the comparative example (e) have. It was confirmed that a single-phase cBN was obtained.

Next, among the samples prepared under the above conditions of "heating temperature: 1600 ° C to 2600 ° C, pressure: 25 GPa, holding time: 20 minutes", the heating temperature is (a) 1950 ° C (corresponding to FIG. 3 (d)). FIG. 4 shows a transmission electron microscope (TEM) image of the cross section of each of (b) at 2150 ° C (same as (e)). The average particle size is 97.2 nm in (a) of the present embodiment, whereas it is 318 nm in (b) of the comparative example. Here, as shown in FIG. 5, for the average particle size, after drawing a circle 31 in the TEM image, six straight lines 32 passing through the center of the circle 31 are drawn at intervals of 30 °, and crystal grains crossing each straight line 32. Was counted and calculated by dividing the sum of all the straight lines by 6 times the diameter of the straight line 32. The circle 31 is set so that the number of crystal grains in one diameter is about 10 to 40, and three sets of such combinations of the circle 31 and the straight line 32 are drawn for one sample to obtain the average particle size for each set. Furthermore, the final average particle size value was determined by obtaining the average value thereof.

FIG. 6 shows the results of determining the average particle size of all the samples prepared under the conditions of "heating temperature: 1600 ° C to 2600 ° C, pressure: 25 GPa, holding time: 20 minutes". A single-phase cBN with an average particle size of 200 nm or less is obtained in a heating temperature range of 1900 to 2000 ° C.

Next, the heating temperature was 1950 ° C., the pressure was 25 GPa, and multiple samples having different holding times within the range of 1 to 60 minutes were prepared. From the results of X-ray diffraction measurements, single-phase cBN was obtained in all of these samples with a retention time of 11 to 60 minutes, whereas wBN was mixed in the samples with a retention time of 1 to 10 minutes. It was confirmed that

FIG. 7 shows a TEM image of a cross section of a sample having a retention time of (a) 11 minutes, (b) 20 minutes, (c) 25 minutes, and (d) 30 minutes. The TEM image in FIG. 7 (b) is the same as that in FIG. 4 (a). The average particle size is 40.8 nm in (a), 97.2 nm in (b), and 156 nm in (c) of the present embodiment, which are all within 200 nm, whereas they are comparative examples (d). ) Has a large value of 322 nm.

Further, in the present embodiments, in (a) and (c), the particle sizes of the particles appear to be relatively close to each other, whereas in (b), the particles are contained in a structure composed of particles having a relatively small particle size. Relatively large particles appear to be dispersed. When the average particle size of each of the relatively small particles (particles having a crystalline structure) and the relatively large particles (coarse crystals) was visually determined, it was 68 nm for the particles having a crystalline structure and 165 nm for the coarse crystals. Such a structure corresponds to the above-mentioned coarse-grained dispersed cBN nanopolycrystal. FIG. 8 schematically shows a coarse-grained dispersed cBN nanopolycrystal 30. The coarse-grained dispersed cBN nanopolycrystal 30 has a cBN having an average particle size of 115 to 165 nm, preferably 120 to 165 nm, in a fine polycrystal structure composed of fine polycrystals 31 of cBN having an average particle size of 70 nm or less. It has a structure in which the coarse crystal grains 32 of the above are dispersed. The preparation conditions for the coarse-grained dispersed cBN nanopolycrystal prepared in this embodiment are shown in Table 1 together with the average particle size and Knoop hardness (Knoop hardness will be described later). In addition, "whole" in Table 1 refers to the average particle size of a polycrystalline body in which a fine polycrystalline structure and coarse crystal grains are combined.

FIG. 9 shows the results of determining the average particle size of the sample prepared under the conditions of "heating temperature: 1950 ° C., pressure: 25 GPa, holding time: 1 to 60 minutes". Single-phase cBN with an average particle size of 200 nm or less can be obtained with a retention time in the range of 11 to 25 minutes.

FIG. 10 shows the preparation under the conditions of "heating temperature: 1600 ° C to 2600 ° C, pressure: 25 GPa, holding time: 20 minutes" and "heating temperature: 1950 ° C, pressure: 25 GPa, holding time: 1 to 60 minutes". The results of measuring the Knoop hardness for all the samples are shown. Here, the measurement results are shown in a graph in which the vertical axis is Knoop hardness and the horizontal axis is the reciprocal d- ^{1 / 2} of the square root of the average particle size d. From this measurement result, in the single-phase cBN polycrystal, which is a sample without ^wBN , the linear function of d-1 / 2 (straight line in the figure) except for the coarse-grained dispersed cBN nanopolycrystal. It has a tendency to be approximated, and when the average particle size d is 200 nm or less (d-1 / 2 is 0.071 nm- ^{1 / 2} or more), the ^noup hardness is 50 GPa or more, which is higher than that of the conventional cBN polycrystal. Have. When the average particle size d is 100 nm or less (d- ^{1 / 2} is 0.1 nm- ^{1 / 2} or more), the Knoop hardness is 53 GPa or more. On the other hand, in the example in which wBN is mixed instead of single-phase cBN, the Knoop hardness value is lower than the value corresponding to the above-mentioned linear function, and the phase transformation is insufficient, so that the second phase remains. It was confirmed that the hardness decreased.

Since these cBN polycrystals of the present embodiment have a high Knoop hardness of 50 GPa or more, they can be suitably used for tool cutting edges of cutting tools and abrasive grain cutting edges of grinding tools.

In addition, the coarse-grained dispersed cBN nanopolycrystal has a Knoop hardness higher than the value corresponding to the above-mentioned linear function (see Table 1), and the Knoop hardness value exceeds 53.5 GPa in this embodiment. ing. In particular, in the sample in which the average grain size d of the whole is 97.2 nm (d- ^{1 / 2} is 0.101) and the average grain size of the coarse crystal grains is 165 nm, the Knoop hardness value exceeds 54.5 GPa (Table). In the sample with 84.6 nm (d- ^{1 / 2} is 0.109) and the average grain size of coarse crystal grains is 130.2 nm, the Knoop hardness value exceeds 55 GPa (Table 1). The bottom row of. Data with arrows in FIG. 10). As described above, it was confirmed that the hardness was further increased by dispersing the coarse particles in the fine polycrystalline structure.

Further, in the present embodiment, as shown in FIG. 10, an ultrafine structure cBN nanopolycrystal having an average particle size d of 50 nm or less and a Knoop hardness of 53 GPa or more is also obtained. The ultra-fine structure cBN nanopolycrystal has a sharp tool cutting edge that can be mirror-finished just by cutting, and a sharp abrasive grain cutting edge for more accurate polishing. It can be suitably used for obtaining.

Up to this point, the case where the pressure at the time of preparation is 25 GPa has been described, but the cBN polycrystal was prepared by the same method as above for the cases of 23 GPa and 30 GPa.

When the pressure was 23 GPa, the temperature at the time of preparation was 1950 ° C. (the median value within the temperature range in which the cBN polycrystal according to the present invention was obtained when the pressure was 25 GPa), and the wBN content was obtained. Although a single-phase cBN was obtained at almost 0%, the average grain size of the crystals was 276 nm, which was larger than the above-mentioned 200 nm, and the cBN polycrystal according to the present invention could not be obtained. The Knoop hardness of this sample was measured to be 49.5 GPa, which was smaller than the lower limit of 50 GPa in the cBN polycrystal according to the present invention.

When the pressure was 30 GPa, the temperature at the time of fabrication was 1800 ° C. The obtained sample was a single-phase cBN having a wBN content of 0%, and the average grain size of the crystals was 47 nm. The Knoop hardness was 55.6 GPa. Therefore, it can be said that the cBN polycrystal according to the present invention was obtained under the pressure and temperature conditions.

11 ... Raw material 12 ... Capsule 13 ... Sleeve 14 ... Heat insulating material 15 ... Heater 16 ... Heat insulating material 17 ... Pressure medium 17a ... Pressure medium body 17b ... Upper pressure medium 17c ... Lower pressure medium 18a ... Upper metal leaf electrode 18b ... Lower Side metal leaf electrode 20 ... Static ultra-high pressure applying device 21 ... Piston 22 ... Guide block 23 ... Press frame 24 ... First stage anvil 25 ... Second stage anvil 29 ... Power supply

Claims (6)

An unoriented polycrystal consisting of a single-phase crystal structure of cubic boron nitride,
In the fine polycrystalline structure of cubic boron nitride having an average particle size of 70 nm or less and a predetermined maximum particle size , the average particle size is 115 to 165 nm and the particle size is larger than the maximum particle size. A plurality of coarse crystal grains of cubic boron nitride are individually isolated and dispersed.
The average particle size of the polycrystalline body obtained by combining the fine polycrystalline structure and the coarse crystal grains is 100 nm or less.
A cubic boron nitride polycrystal characterized by a Knoop hardness of 53.5 GPa or higher. The cubic boron nitride polycrystal according to claim 1, wherein the coarse crystal grains have an average particle size of 120 to 165 nm and a Knoop hardness of 54.5 GPa or more. The cubic boron nitride polycrystal according to claim 1 or 2, wherein the coarse crystal grains have an average particle size of 135 to 165 nm. A cutting tool comprising a tool cutting edge made of the cubic boron nitride polycrystal according to any one of claims 1 to 3 . A grinding tool comprising an abrasive grain cutting edge made of the cubic boron nitride polycrystal according to any one of claims 1 to 3 . The method for producing a cubic boron nitride polycrystal according to any one of claims 1 to 3.
The process of preparing a raw material that is oriented in the [001] direction and consists only of pyrolysis boron nitride, which is the starting material.
A method for producing a cubic boron nitride polycrystal, which comprises a step of directly converting the pyrolyzed boron nitride into cubic boron nitride by holding the raw material at a pressure of 25 GPa or more at a predetermined temperature for a predetermined time.

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