JPWO2018088369A1 - Cubic boron nitride polycrystal, method for producing the same, cutting tool and grinding tool - Google Patents

Abstract

The cubic boron nitride (cBN) polycrystal according to the present invention is a non-oriented polycrystal having a single-phase crystal structure of cBN having an average grain size of 200 nm or less, and has a Knoop hardness of 50 GPa or more. . By setting the average particle size to 100 nm or less, the Knoop hardness can be set to 53 GPa or more, and by setting the average particle size to 50 nm or less, it can be suitably used for sharp tool cutting edges. A cBN coarse crystal grain having an average particle size of 120 to 165 nm is dispersed in a fine polycrystalline structure of cBN having an average grain size of 70 nm or less, and a polycrystal obtained by combining the fine polycrystalline structure and the coarse crystal grain A cBN polycrystal having an average particle size of 100 nm or less has a Knoop hardness of 54.5 GPa or more. The cBN polycrystal according to the present invention directly converts pBN to cBN by holding a raw material composed of pyrolytic boron nitride (pBN) and oriented in the [001] direction at a pressure of 25 GPa or higher and a predetermined temperature for a predetermined time. It is manufactured by doing.

Description

  The present invention relates to a cubic boron nitride polycrystal that is a material that can be used for a cutting tool, a grinding tool, or the like that requires high hardness, and a method for manufacturing the same.

  Boron nitride (BN) mainly includes crystals called hexagonal boron nitride (hexagonal BN: hBN), wurtzitic boron nitride (wBN), and cubic boron nitride (cubic BN: cBN). There are three types with different structures. Among these BNs, cBN has the second highest hardness after diamond, and has the characteristics that thermal stability and chemical stability are superior to diamond. In particular, diamond is easy to react with materials such as iron, nickel, and titanium, whereas cBN does not react with these materials, so cBN is particularly useful as a tool material for machining workpieces containing iron, nickel, or titanium. Is suitable.

  hBN can be produced at 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 at a temperature of 1900-2300 ° C while applying a pressure of 8-20 GPa to hBN or pBN (described later) as a raw material. In addition, 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 prepared. Here, pBN (pyrolytic boron nitride) is a normal pressure phase that has a crystal structure oriented in the [001] direction, similar to hBN, and is a low pressure pyrolysis chemical vapor deposition (CVD) method. Boron nitride with higher purity than hBN. In Comparative Example 2 of Patent Document 2 and Comparative Example 1 of Patent Document 3, there are described polycrystals which do not contain wBN and are composed only of cBN. The diameter exceeds 1000 nm and does not have high hardness.

JP 2014-034487 JP 2015-205789

H. Sumiya et al., “Real indentation hardness of nano-polycrystalline cBN synthesized by direct conversion under under HPHT” (Diamond and diamond and actual indentation hardness of cubic boron nitride produced by direct conversion firing under high pressure and high temperature), Diamond and Related Materials, published by Elsevier, (Netherlands), Volume 48, Pages 47-51, September 2014

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

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

  A 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 grain size of 200 nm or less. It is a non-oriented polycrystal made of the above, and has a Knoop hardness of 50 GPa or more.

Here, “average particle diameter”, “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 (steel-crystal grain size microscope test method). 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, straight lines) If the end of is in a crystal grain, the number of the crystal grains is counted as 0.5), and the sum of all the straight lines of this number (the value of this sum is referred to as “the number of crystal grains”) is obtained. A value obtained by dividing 6 times the diameter by the number of crystal grains is defined as an average particle diameter. The diameter of the circle drawn on the TEM image is not limited to a specific value, but it is desirable that the number of crystal grains in one diameter is about 10 to 40. Further, it is desirable to obtain TEM images at a plurality of locations for one cBN polycrystal and obtain an average particle diameter by an average value at the plurality of locations.

  In the present invention, “single phase” refers to irradiating 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 by this, a peak derived from other than cBN crystals (particularly, a different crystal composed of wBN) is not detected.

“Non-orientation” in the cBN polycrystal according to the present invention means 220 peak intensity I ₍₂₂₀₎ and 111 peak intensity I of X-ray diffraction measured in the bulk (without powder) of the cBN polycrystal. _This 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 the 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 “fine-structured 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 addition, in a configuration 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, which will be described later, the coarse crystal particles also constitute the cBN polycrystal of the present invention. To be included in the crystal. In other words, “coarse crystal grains” means that the crystal grains are relatively large in comparison with crystal grains having an average grain size of 70 nm or less, and are not limited to those of the present invention. The crystal grains in the polycrystal are classified into fine crystal grains.

  In the cBN polycrystal, if the average crystal grain size 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 a crystal other than cBN (wBN, etc.) (heterocrystal), when the polycrystal is subjected to force from the outside, the heterocrystal becomes the starting point of slipping and microcracking, and the hardness decreases. Cause. Furthermore, when the polycrystal is oriented, anisotropy occurs in the hardness, and the hardness varies depending on the direction. Therefore, in the present invention, (i) the average grain size of the crystal is a small value of 200 nm or less, (ii) a single phase consisting only of cBN, and (iii) non-oriented, A cBN polycrystal having a high Knoop hardness of 50 GPa or more, which could not be realized with a crystal, can be obtained. Such a cBN polycrystal having a high Knoop hardness can be suitably used as a material for a cutting edge of a cutting tool, an abrasive cutting edge of a grinding tool, or the like.

  In the cBN polycrystal according to the present invention, it is preferable that the average particle diameter is 100 nm or less (fine-structured cBN nanopolycrystal) and the Knoop hardness is 53 GPa or more. Thereby, Knoop hardness is further high and a suitable material is obtained with a tool cutting edge, an abrasive grain cutting edge, etc. In particular, in order to obtain a sharp tool cutting edge that can be mirror-finished only by cutting, and a sharp abrasive cutting edge for performing higher-precision polishing, the average particle diameter is More desirably, it is 50 nm or less (that is, the cBN polycrystal is an ultrafine-structured cBN nanopolycrystal).

  In the cBN polycrystal according to the present invention, the cBN coarse crystal grains having an average particle size of 115 to 165 nm are dispersed in the fine polycrystal structure of cBN having an average particle size of 70 nm or less, and the fine polycrystal structure And a polycrystalline body (fine-structured cBN nanopolycrystal) having an average particle diameter of 100 nm or less, and a Knoop hardness of 53.5 GPa or more. Here, the coarse crystal grains are more preferably 120 to 165 nm and the Knoop hardness is 54.5 GPa or more. As described above, the coarse crystal grains are dispersed in the fine polycrystalline structure, so that the Knoop hardness can be made higher than that in the case where the grains are made of crystals having a nearly uniform grain 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 cubic boron nitride polycrystal manufacturing method according to the present invention is:
Preparing a raw material composed of pyrolytic boron nitride (pBN) as a starting material and oriented in the [001] direction;
A step of directly converting the pyrolytic boron nitride into cubic boron nitride (cBN) by holding the raw material at a pressure of 25 GPa or higher and a predetermined temperature for a predetermined time.

Here, “orientated in the [001] direction” means 100 peak intensity I ₍₁₀₀₎ and 002 peak intensity I of X-ray diffraction measured in a pyrolytic boron nitride bulk (without powder _{). (002)} the ratio I ₍₁₀₀₎ / I ₍₀₀₂₎ refers to that 0.15. The measurement conditions for X-ray diffraction at that time are the same as the measurement conditions for confirming that the above-mentioned cBN polycrystal is a single phase.

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

  Further, if the pressure during production is lower than 25 GPa, a different crystal composed of wBN is generated in the polycrystal in addition to cBN crystals, so the lower limit of the pressure is 25 GPa. On the other hand, in the case of BN, since cBN is obtained in the highest pressure state, there is no theoretical upper limit for the pressure.

  The predetermined temperature differs 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., a heterocrystal composed of wBN is generated in addition to cBN crystals in the polycrystal, and when the temperature exceeds 2000 ° C., Since the average particle size of cBN crystals in the polycrystal exceeds 200 nm, the predetermined temperature is set to a temperature in the range of 1900 to 2000 ° C.

  The predetermined time, that is, the time for holding at the pressure and temperature differs depending on the pressure and temperature, but can be determined by preliminary experiments. If the holding time is too short, a different crystal composed of wBN is mixed, and if it is too long, the average grain size of the crystal exceeds 200 nm. In order to fabricate an ultrafine textured cBN nanopolycrystal, the holding time is shortened within a range in which different crystals of wBN are not mixed. In order to produce a coarse-grain-dispersed cBN nanopolycrystal, the holding time is longer than that in the case of producing an ultrafine-structured 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, among which the holding time is 11 to 13 minutes. Thus, an ultrafine-structured cBN nanopolycrystal can be produced, and a coarse-grain-dispersed cBN nanopolycrystal can be produced by setting the retention time to 15 to 20 minutes.

The raw material, integrated intensity Area of 100 peaks of X-ray diffraction was measured in bulk The pyrolytic boron nitride _(100), integrated intensity Area ₍₁₀₁₎ of the 101 peak and 102 peak GI by integral intensity Area ₍₁₀₂₎ of 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 pBN as a raw material). Thus, by using pBN having low crystallinity as a raw material, it becomes difficult to produce a different crystal composed of wBN.

  According to the present invention, a boron nitride polycrystal 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 which concerns on this invention. The schematic block diagram of the static ultrahigh pressure provision apparatus used in the manufacturing method of the cBN polycrystal of this embodiment. The X-ray-diffraction pattern in embodiment ((c), (d)) and comparative example ((a), (b), (e)) of cBN polycrystal which concerns on this invention. The TEM image of the cross section in this embodiment (a) and a comparative example (b). The figure which shows the method of calculating | requiring an average particle diameter from a TEM image. The graph which shows the relationship between the heating temperature and average particle diameter in this embodiment and a comparative example. The TEM image of the cross section in this embodiment (a)-(c) and the comparative example (d). Schematic diagram of coarse-grain dispersed cBN nanopolycrystal. The graph which shows the relationship between the retention time and 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 a cBN polycrystal and a method for producing the same according to the present invention will be described with reference to FIGS.

(1) One Embodiment of Method for Producing cBN Polycrystal According to the Present Invention An embodiment of a method for producing a cBN polycrystal according to the present invention will be described with reference to FIGS. In the present embodiment, a commercially available plate-like pBN oriented in the [001] direction is used as the raw material body 11. The GI value of the raw material body 11 used in this embodiment is 17.6. First, the raw material body 11 is produced by cutting out pBN into a disk shape having a diameter of 3 mm using a laser processing apparatus. As shown in FIG. 1, the raw material body 11 is accommodated in a pressure medium 17 including a pressure medium body 17a, an upper pressure medium 17b, and a lower pressure medium 17c. In that case, first, the raw material body 11 is inserted into the capsule 12 made of metal foil, and this capsule 12 is put into the cylindrical sleeve 13. In addition, a 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 a metal foil, and a heat insulating material 16 is disposed around the sleeve 15 and placed in a cylindrical pressurizing chamber opened at the center of the pressure medium body 17a. The upper and lower sides of the main body 17a are sandwiched between a quadrangular pyramid upper pressure medium 17b and a lower pressure medium 17c. 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, an upper metal foil electrode 18a and a lower metal foil electrode 18b for supplying electric power to the heater 15 are interposed, respectively. . FIG. 1 is a schematic diagram, and the dimensions of each part do not represent an accurate ratio.

  Thus, the below-mentioned high pressure and high temperature are provided to the raw material body 11 accommodated in the pressure medium 17 by the static ultrahigh pressure applying device 20 shown in FIG. The static super-high pressure applying device 20 used here is called a “Kawai-type multi-anvil high-pressure generator”, and includes a hydraulically driven piston 21, a guide block 22, and a press frame 23 for fixing them. Fixed to the guide block 22 is a first-stage anvil 24 made of six blocks made of special steel and having a cubic space formed therein. In this cubic space, the second-stage anvil 25, which is composed of eight blocks made of tungsten carbide and in which a regular octahedral space is formed, is accommodated. The pressure medium 17 is accommodated in this regular 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 applying the following predetermined pressure to the sample. In addition, the heater 15 is energized from the external power source 29 through the guide block 22, the first stage anvil 24, the second stage anvil 25, the upper metal foil electrode 18a, and the lower metal foil electrode 18b, and the sample is subjected to the following predetermined By heating to a temperature, the cBN polycrystal of this embodiment can be obtained.

  In the present embodiment, the pressure applied to the raw material body 11 is 23 to 30 GPa, and the heating temperature of the raw material body 11 is 1900 to 2000 ° C. In the present invention, the upper limit of the pressure applied to the raw material body 11 is not particularly limited. However, in the present embodiment, the maximum pressure value is set to 30 GPa due to the specifications of the static ultrahigh pressure applying device 20. For comparison, a similar experiment was also performed in the temperature range of 1600 ° C. to less than 1900 ° C. and over 2000 ° C. up to 2600 ° C. when the pressure was 25 GPa. Further, for comparison, the same experiment was performed when the temperature was 1950 ° C. and the pressure was 23 GPa. In the case where the pressure was 25 GPa, experiments were conducted on a plurality of examples having different holding times (holding times) at a predetermined pressure and temperature.

(2) Embodiment and comparative example of cubic boron nitride polycrystal according to the present invention First, the pressure during production is 25 GPa, the holding time is 20 minutes, and the heating temperature is within the range of 1600 ° C to 2600 ° C. 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), (d) 1950 ° C. (main) Embodiment), (e) The results of X-ray diffraction measurement for 2150 ° C. (comparative example) are shown. 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 any sample, a peak derived from the (111) plane of cBN is observed around 2θ = 43.3 °. On the other hand, in (a) and (b), 100 peaks of wBN (different crystals) (shown with arrows in the figure) are found at positions where 2θ is slightly smaller than cBN's 111 peak (around 2θ = 40.8 °). On the other hand, 100 peaks of wBN are not observed in (c), (d) and (e). In addition, no peak derived from the second phase such as wBN or hBN is observed in any sample. From these results, the comparative examples (a) and (b) are not single-phase cBN, whereas the present embodiment (c) and (d) and the comparative example (e) It was confirmed that single-phase cBN was obtained.

  Next, among the samples prepared under the 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)) And (b) FIG. 4 shows a transmission electron microscope (TEM) image of the cross section for each of those at 2150 ° C. ((e)). The average particle diameter is 97.2 nm in the present embodiment (a), whereas it is 318 nm in the comparative example (b). Here, as shown in FIG. 5, the average grain size is a crystal grain that draws a circle 31 in the TEM image, draws six straight lines 32 passing through the center of the circle 31 at 30 ° intervals, and crosses each straight line 32. Was obtained by dividing 6 times the diameter of the straight line 32 by the sum of all the straight lines. The circle 31 is determined so that the number of crystal grains in one diameter is about 10 to 40, and three combinations of the circle 31 and the straight line 32 are drawn for one sample to obtain an average grain size for each group. Further, the final average particle size was determined by obtaining an average value of them.

  FIG. 6 shows the results of determining the average particle diameter for 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 having an average particle size of 200 nm or less and a heating temperature in the range of 1900 to 2000 ° C. is obtained.

  Next, a plurality of different samples were manufactured at a heating temperature of 1950 ° C., a pressure of 25 GPa, and a holding time in the range of 1 to 60 minutes. From the results of X-ray diffraction measurement, all of these samples with a retention time of 11 to 60 minutes have single-phase cBN, whereas samples with a retention time of 1 to 10 minutes contain wBN. It was confirmed that

  FIG. 7 shows a cross-sectional TEM image of a sample whose holding time is (a) 11 minutes, (b) 20 minutes, (c) 25 minutes, and (d) 30 minutes. Note that the TEM image in FIG. 7B is the same as that in FIG. The average particle size is 40.8 nm in the present embodiment (a), 97.2 nm in (b), and 156 nm in (c), both of which are within 200 nm or less, compared to the comparative example (d ) Is a large value of 322nm.

Further, in this embodiment, in (a) and (c), the particle sizes appear to be relatively close to each other, whereas in (b), in the tissue composed of particles having a relatively small particle size, It appears that relatively large particles are dispersed. When the average particle diameters were determined by dividing the particle size into relatively small particles (grains of crystal structure) and relatively large particles (coarse crystals), it was 68 nm for grains of crystal structure and 165 nm for coarse crystals. Such a structure corresponds to the aforementioned coarse-grain-dispersed cBN nanopolycrystal. FIG. 8 schematically shows a coarse-grain-dispersed cBN nanopolycrystal 30. The coarse-grain-dispersed cBN nanopolycrystalline material 30 is a cBN having an average particle size of 115 to 165 nm, preferably 120 to 165 nm in a fine polycrystalline structure composed of cBN fine polycrystal 31 having an average particle size of 70 nm or less. The coarse crystal grains 32 are dispersed. The production conditions of the coarse-grain-dispersed cBN nanopolycrystal produced in this embodiment are shown in Table 1 together with the average particle diameter and Knoop hardness (Knoop hardness will be described later). In addition, “whole” in Table 1 indicates an average particle diameter of a polycrystalline body obtained by combining a fine polycrystalline structure and coarse crystal grains.

  FIG. 9 shows the results of determining the average particle diameter of the sample prepared under the conditions of “heating temperature: 1950 ° C., pressure: 25 GPa, holding time: 1 to 60 minutes”. A single-phase cBN having an average particle size of 200 nm or less is obtained within a holding time of 11 to 25 minutes.

In FIG. 10, it was produced 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 result of having measured Knoop hardness about all the samples is shown. Here, the measurement results are shown in a graph in which the vertical axis represents Knoop hardness and the horizontal axis represents the reciprocal d ^-1/2 of the square root of the average particle diameter d. From this measurement result, in the single-phase cBN polycrystal that is a sample not mixed with wBN, it is a linear function of d ^−1/2 (straight line in the figure) except for the coarse-grain dispersed cBN nanopolycrystal. Higher than conventional cBN polycrystals with Knoop hardness of 50 GPa or more when the average particle diameter d is 200 nm or less (d ^-1/2 is 0.071 nm ^-1/2 or more). Have Further, 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. It was confirmed that the hardness decreased.

  These cBN polycrystals of this embodiment have a high Knoop hardness of 50 GPa or more, and therefore can be suitably used for tool cutting edges of cutting tools and abrasive grain cutting edges of grinding tools.

Further, the coarse-grain dispersed cBN nanopolycrystal has a Knoop hardness higher than the value corresponding to the above-mentioned linear function (see Table 1), and in this embodiment, the Knoop hardness value exceeds 53.5 GPa. ing. In particular, the sample having an overall average particle diameter d of 97.2 nm (d ^-1/2 is 0.101) and an average particle diameter of coarse crystal grains of 165 nm has a Knoop hardness value exceeding 54.5 GPa (Table 1), d is 84.6 nm (d ^−1/2 is 0.109), and the average grain size of the coarse grains is 130.2 nm, the Knoop hardness value exceeds 55 GPa (Table 1). (The bottom row of Fig. 10. Data with an arrow in Fig. 10). Thus, it was confirmed that the hardness is further increased by the dispersion of the coarse grains in the fine polycrystalline structure.

  Furthermore, in this embodiment, as shown in FIG. 10, an ultrafine-structured cBN nanopolycrystal having an average particle diameter d of 50 nm or less and a Knoop hardness of 53 GPa or more is also obtained. The ultra-fine textured cBN nanopolycrystal has a sharp tool cutting edge that can be mirror-finished only by cutting, and a sharp abrasive cutting edge that enables high-precision polishing. In order to obtain, it can use suitably.

  So far, the case where the pressure at the time of production is 25 GPa has been described, but cBN polycrystals were also produced by the same method as described above in the case of 23 GPa and 30 GPa.

  When the pressure was 23 GPa, the production temperature 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). Was approximately 0%, and single-phase cBN was obtained, but the average crystal grain size was 276 nm, which was larger than the above-mentioned 200 nm, and the cBN polycrystal according to the present invention was not obtained. The Knoop hardness of this sample was measured and found to be 49.5 GPa, which was smaller than the lower limit value of 50 GPa in the cBN polycrystal according to the present invention.

  When the pressure was 30 GPa, the production temperature was 1800 ° C. The obtained sample was a single-phase cBN with a wBN content of 0%, and the average crystal grain size 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.

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

Claims (8)

  A cubic boron nitride polycrystal comprising a non-oriented polycrystal having a single-phase crystal structure of cubic boron nitride having an average particle diameter of 200 nm or less and having a Knoop hardness of 50 GPa or more.   The cubic boron nitride polycrystal according to claim 1, wherein the average grain size is 100 nm or less and the Knoop hardness is 53 GPa or more.   In the fine polycrystalline structure of cubic boron nitride having an average particle diameter of 70 nm or less, coarse crystalline grains of cubic boron nitride having an average particle diameter of 115 to 165 nm are dispersed, and the fine polycrystalline structure and the coarse crystal are dispersed. 3. The cubic boron nitride polycrystal according to claim 1, wherein a polycrystal having an average grain size of 100 nm or less and a Knoop hardness of 53.5 GPa or more. body.   4. The cubic boron nitride polycrystal according to claim 3, wherein the coarse crystal grains have an average grain size of 120 to 165 nm and the Knoop hardness is 54.5 GPa or more.   The cubic boron nitride polycrystal according to claim 2, wherein the average particle size is 50 nm or less.   A cutting tool comprising a tool cutting edge made of the cubic boron nitride polycrystal according to any one of claims 1 to 5.   A grinding tool comprising an abrasive cutting edge made of the cubic boron nitride polycrystal according to any one of claims 1 to 5. Preparing a raw material composed of pyrolytic boron nitride as a starting material and oriented in the [001] direction;
A method of directly converting the pyrolytic boron nitride into cubic boron nitride by holding the raw material at a pressure of 25 GPa or higher and a predetermined temperature for a predetermined time, to produce a cubic boron nitride polycrystal.