Cubic boron nitride polycrystal and method for producing the same

Abstract

Cubic boron nitride polycrystal and method for producing the same. A cBN polycrystal (10) comprises a mixture of fine-grained crystals (11) formed of cBN and having a particle size within the range of 10 to 60 nm; coarse-grained crystals (12) formed of cBN and having a particle size within the range of 100 to 250 nm; and layered crystals (13) formed of a plurality of layered cBN plate-like crystals, wherein the maximum length in a direction perpendicular to the layering direction is less than 1000 nm, the aspect ratio obtained by dividing the maximum length by the maximum length in the layering direction is less than 3, and the average particle size of the fine-grained crystals, coarse-grained crystals, and layered crystals is less than 80 nm. The cBN polycrystal is produced by heating a raw material formed of hexagonal boron nitride to a first temperature within the range of 1300 to 1600°C under a pressure of 10 GPa or higher and maintaining the temperature for a first predetermined time, then heating the raw material to a second temperature within the range of 1700 to 2100°C and maintaining the temperature for a second predetermined time, and then cooling the raw material.

Description

Technical Field

[0001] The present invention relates to a cubic boron nitride polycrystal used in cutting tools, grinding tools, etc. requiring high hardness and high Young's modulus, and a method for producing the same. Background Art

[0002] Boron nitride (BN) primarily consists of three types with different crystal structures: hexagonal BN (hBN), wurtzite BN (wBN), and cubic BN (cBN). Of these BNs, cBN boasts a hardness second only to diamond and superior thermal and chemical stability. In particular, while diamond readily reacts with elements such as iron, nickel, and titanium, cBN is less reactive, making it suitable for tooling in a wide range of materials containing these elements, including steel, nickel alloys, and titanium alloys.

[0003] Patent Document 1 describes a method for producing a polycrystal containing cBN by sintering a raw material powder comprising a coarse hBN powder having an average particle size of 1 μm or greater, and a fine hBN powder having an average particle size of less than 100 nm, or a powder of BN other than hBN, such as amorphous BN, at a temperature of 2200°C or less and a pressure of 25 GPa or less, where the temperature and pressure satisfy a predetermined inequality. The cBN polycrystal contains fine-grained cBN with a maximum particle size of 100 nm or less and an average particle size of 70 nm or less, flake-like cBN with an average major diameter of 50 nm or more and 10,000 nm or less, and coarse-grained cBN with a minimum particle size exceeding 100 nm and an average particle size of 1,000 nm or less.

[0004] On the other hand, non-patent document 1 describes that pyrolyzed boron nitride (pBN), which is high-purity BN produced by CVD and has the same hexagonal crystal structure as hBN, is converted into cBN under pressure of 25 GPa and temperature of 1950°C, thereby producing a cBN polycrystal in which relatively coarse grains are mixed in a fine polycrystalline structure with an average grain size of approximately 85 nm.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2016-141609

[0008] Non-patent literature

[0009] Non-Patent Literature 1: Yoshio Ichida and five others, "Microstructure and Mechanical Properties of Nano-Polycrystalline cBN," Proceedings of the 60th High Pressure Symposium, published by the Japan Society of High Pressure Research, page 157, October 10, 2019 Summary of the Invention

[0010] Problems to be solved by the invention

[0011] If cutting tools and grinding tools are elastically deformed during processing, the processing accuracy is reduced. Therefore, the materials used in these tools are required not only to have high hardness, but also to have the property of not easily deforming elastically. To this end, it is necessary to improve the Young's modulus, which is an indicator of not easily deforming elastically. Patent Document 1 does not record the Young's modulus of the obtained cBN polycrystal. Non-Patent Document 1 records that the obtained cBN polycrystal has a Knoop hardness of 55.2 GPa and a Young's modulus of 1081.5 GPa, but a cBN polycrystal with a Knoop hardness equal to or higher than that and a higher Young's modulus is required.

[0012] An object of the present invention is to provide a cBN polycrystal having high Knoop hardness and high Young's modulus and a method for producing the same.

[0013] Solutions for solving problems

[0014] The cubic boron nitride polycrystal (cBN polycrystal) of the present invention for solving the above-mentioned problems is characterized in that the following crystals are mixed:

[0015] Fine-grained crystals formed of cubic boron nitride with a particle size ranging from 10 to 60 nm;

[0016] Coarse-grained crystals formed of cubic boron nitride, having a particle size within the range of 100 to 250 nm; and

[0017] A layered crystal composed of a plurality of stacked plate-like crystals of cubic boron nitride, wherein the maximum length in a direction perpendicular to the stacking direction is 1000 nm or less, and the aspect ratio obtained by dividing the maximum length in the stacking direction by the maximum length (thickness) is less than 3.

[0018] The average particle size of the fine-grained crystals, the coarse-grained crystals, and the lamellar crystals combined is 80 nm or less.

[0019] Here, the average particle size of the total of fine-grained crystals, coarse-grained crystals and layered crystals refers to the value obtained by the cutting method using a transmission electron microscope (TEM) image based on JIS G0551:2013 (steel-grain size test method). In the cutting method, a circle is drawn on the TEM image, and 6 straight lines (diameters) passing through the center of the circle are drawn at 30° intervals (12 for the radius), and the number of grains crossing each straight line (the total count of fine-grained crystals, coarse-grained crystals and layered crystals in the present invention) is counted (wherein, when the end of the straight line is in the grain, the number of grains counted is 0.5), and the sum of all the straight lines of the number is obtained (the value of the sum is set to "number of grains"). The value obtained by dividing 6 times the diameter by the number of grains is then set to the average particle size. It should be noted that the diameter of the circle depicted in the TEM image is not limited to a specific value, and is preferably specified in a manner such that the number of grains in one diameter is about 10 to 40. Furthermore, it is preferable to obtain TEM images of a plurality of locations for one cBN polycrystal and calculate the average particle size as the average value of these plurality of locations.

[0020] The particle size d of each fine-grained crystal and coarse-grained crystal can be calculated as follows: approximate the shape of each crystal in the TEM image with an ellipse, measure the major diameter a and minor diameter b of the ellipse on the TEM image, and then calculate the particle size d by approximating it by d = (a + b) / 2. Alternatively, the major diameter a and minor diameter b of the ellipse can be measured using the same method, and the area S of the ellipse can be calculated by S = π × a × b. The equivalent circle diameter is set as the particle size d, and the area S can be calculated by d = 2 × (S / π) 1 / 2 In addition, commercially available image processing software may automatically perform processing to determine the equivalent circle diameter after specifying the periphery of a crystal grain contained in an image. Therefore, the equivalent circle diameter determined from a TEM image using such image processing software may be specified as the grain size d.

[0021] The cBN polycrystal of the present invention can have a Knoop hardness of 60 GPa or more and a Young's modulus of 1082 GPa or more due to the mixed presence of fine-grained crystals, lamellar crystals, and coarse-grained crystals of cBN.

[0022] The total average particle size of the fine-grained crystals, the coarse-grained crystals, and the lamellar crystals is preferably 60 nm or less. This can further improve the Knoop hardness and Young's modulus.

[0023] The cBN polycrystal of the present invention can be produced by the following method. Specifically, the method for producing cubic boron nitride polycrystal (cBN polycrystal) of the present invention is characterized by heating a raw material composed of hexagonal boron nitride to a first temperature within a range of 1300-1600°C under a pressure of 10 GPa or higher and maintaining the temperature for a first predetermined time, then heating the raw material to a second temperature within a range of 1700-2100°C and maintaining the temperature for a second predetermined time, and then cooling the raw material.

[0024] Here, the raw material “hexagonal boron nitride” includes the above-mentioned hBN and pBN.

[0025] In the method for producing cBN polycrystals of the present invention, the raw material is heated at a first temperature under a pressure of 10 GPa or higher. The majority of the raw material is converted to granular cBN crystals through a diffusion-type phase transformation of hBN, while the remaining raw material is converted to lamellar wBN crystals primarily through a diffusionless phase transformation (martensitic phase transformation). This results in a crystalline structure in which these granular cBN crystals and lamellar wBN crystals coexist. Furthermore, during a first predetermined time, a portion of the lamellar wBN crystals transforms into lamellar cBN crystals, while the majority of the remaining crystals maintain the morphology of lamellar wBN crystals. The lamellar wBN crystals are crushed and miniaturized during this first predetermined time due to their mixing with the harder granular cBN crystals and lamellar cBN crystals under ultrahigh pressure.

[0026] When this crystalline structure, containing a mixture of cBN granular crystals, cBN lamellar crystals, and refined wBN lamellar crystals, is heated to a second temperature, the majority of the refined wBN lamellar crystals transform into cBN lamellar crystals through a martensitic transformation. Immediately after reaching the second temperature, some wBN lamellar crystals remain, but within the second predetermined time, they are completely transformed into cBN lamellar crystals. As a result, a single-phase cBN polycrystal is formed overall. Numerous lattice defects, such as twins and stacking faults, exist on the surface and within these cBN lamellar crystals, contributing to the increased hardness and Young's modulus of the polycrystal.

[0027] In addition, as the heating time at the second temperature passes, new cBN nuclei are formed around dangling bonds near such lattice defects, and recrystallization toward new grain growth occurs under ultrahigh pressure, thereby forming fine-grained cBN crystals. This is dynamic recrystallization, and many deformations and defects are also introduced into the fine-grained cBN crystals, which is the main reason for improving the hardness and Young's modulus of the polycrystal. A portion of the layered crystals of cBN does not undergo this dynamic recrystallization, and remains in a state that maintains its morphology and improves the hardness and Young's modulus of the polycrystal. On the other hand, the granular cBN crystals generated by heating at the first temperature gradually undergo grain growth as the temperature is subsequently increased to the second temperature and the holding time passes, transforming into coarse-grained cBN crystals. Since this is also grain growth under ultrahigh pressure, many lattice deformations and defects are also present in the coarse-grained cBN crystals, which is the main reason for improving the hardness and Young's modulus of the polycrystal. Through the above process, the cBN polycrystal of the present invention in which three types of crystals, namely, fine-grained cBN crystals, lamellar cBN crystals, and coarse-grained cBN crystals, are mixed is obtained.

[0028] However, if the heating time at the second temperature is too short, part of the layered crystals of wBN will not be transformed into layered crystals of cBN and will remain. On the other hand, if the heating time at the second temperature is too long, all the layered crystals will be recrystallized and disappear, and the fine-grained crystals generated will become coarse-grained. The previously coarse-grained crystals will also be further coarsened, and the overall structure of the cBN polycrystal will become coarser. Therefore, the second prescribed time is prescribed by conducting preliminary experiments in such a way that the layered crystals of wBN will not remain, and the fine-grained crystals, coarse-grained crystals and layered crystals of cBN will all remain. The existence ratio of fine-grained crystals, coarse-grained crystals and layered crystals is preferably in the range of (13 to 20): (60 to 75): (10 to 30) in terms of area ratio. According to experiments conducted by the inventors of the present application, if, for example, the pressure is within the range of 10 to 25 GPa, the second temperature is within the range of 1700 to 1950°C, and the second prescribed time is within the range of 4 to 7 minutes, no layered crystals of wBN will remain, and fine-grained crystals, coarse-grained crystals, and layered crystals may all remain, and the Knoop hardness may be set to above 60 GPa, and the Young's modulus may be set to above 1082 GPa.

[0029] Furthermore, if the first predetermined time is too long, the rate of phase transformation from wBN layered crystals to cBN layered crystals increases during heating at the first temperature, making it difficult for the wBN layered crystals to remain, making it difficult to achieve miniaturization through the fragmentation of the wBN layered crystals. Consequently, after subsequent heating at the second temperature, coarse cBN layered crystals with an aspect ratio of 3 or greater are produced. This causes the overall structure of the cBN polycrystal to become coarser, making it impossible to achieve a sufficiently high Knoop hardness. Therefore, the first predetermined time is also set, through preliminary experiments, to achieve a Knoop hardness exceeding 60 GPa.

[0030] Effects of the Invention

[0031] According to the present invention, it is possible to obtain a cubic boron nitride polycrystal (cBN polycrystal) having a high Knoop hardness of 60 GPa and a high Young's modulus of 1082 GPa or more. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 A flow chart showing one embodiment of a method for producing a cBN polycrystal according to the present invention.

[0033] Figure 2 A diagram showing the relationship between heating temperature and time in the method for producing a cBN polycrystal according to the present embodiment.

[0034] Figure 3 A diagram schematically showing a cBN polycrystal according to this embodiment.

[0035] Figure 4 This is a transmission electron microscope (TEM) photograph of an example of the cBN polycrystal according to the present embodiment.

[0036] Figure 5A A diagram showing the results of X-ray diffraction measurement of a sample obtained by rapid cooling after heat treatment at a first temperature for a first time (without heat treatment at a second temperature).

[0037] Figure 5B Indicates that Figure 4 The graph shows the results of X-ray diffraction measurement of cBN polycrystal.

[0038] Figure 6A This is a high-resolution transmission electron microscope (HR-TEM) photograph of a layered crystal, obtained by magnifying and photographing an example of the cBN polycrystal according to the present embodiment.

[0039] Figure 6B This is an HR-TEM photograph of a layered crystal, which is an example of the cBN polycrystal according to the present embodiment, and is obtained by further magnifying it.

[0040] Figure 7A schematic diagram showing a process for obtaining the cBN polycrystal according to this embodiment.

[0041] Figure 8 A graph showing the relationship between the second time t2 during production and the measured value of the Knoop hardness of the produced cBN polycrystal.

[0042] Figure 9 A graph showing the relationship between the second time t2 during production and the measured value of the Young's modulus of the produced cBN polycrystal.

[0043] Description of Reference Numerals

[0044] 10…Cubic boron nitride (cBN) polycrystal

[0045] 11…fine-grained crystals (cBN)

[0046] 12…(cBN) coarse-grained crystals

[0047] 13…Layered crystals (cBN)

[0048] 92…cBN granular crystals

[0049] 93…wBN layered crystals

[0050] SF…Stacking Fault

[0051] T…Twin

[0052] TB…Twin Boundary

[0053] TT…Distance DETAILED DESCRIPTION

[0054] use Figures 1 to 9 Embodiments of the cBN polycrystal and the method for producing the same according to the present invention will be described.

[0055] First, the edge reference Figure 1 Flowchart and Figure 2 The method for producing the cBN polycrystal according to this embodiment will be described with reference to the drawings.

[0056] First, hexagonal boron nitride (BN) is prepared as a raw material (step 1). Hexagonal BN can use hBN or pBN. In the examples and comparative examples described below, pBN is obtained by cutting a commercially available plate-like pBN (plate thickness 3 mm) into a disk shape of 3 mm in diameter and 3 mm in height using a laser processing device, and hBN is obtained by filling powdered hBN into a superhard mold for powder molding with an inner diameter of 3 mm and compacting it with a powder molding machine at a pressure of 200 MPa to obtain a granular raw material of 3 mm in diameter and 3 mm in height.

[0057] A pressure P of 10 GPa or more is applied to the raw material (step 2). There is no upper limit to the pressure P. While maintaining the pressure P, the temperature is raised from room temperature to a first temperature (first temperature) T1 in the range of 1300 to 1600°C, preferably in the range of 1350 to 1500°C (step 3). The heating rate is preferably in the range of 50 to 120°C / minute. Then, the first temperature T1 is maintained for a first predetermined time (first time) t1 while maintaining the pressure P (step 4). The first time t1 is appropriately determined together with the second predetermined time (second time) t2 described later through preliminary experiments.

[0058] After the first time t1 has elapsed, the temperature is raised from the first temperature T1 to a second temperature (second temperature) T2 within the range of 1700 to 2100°C, preferably within the range of 1700 to 1950°C, while maintaining the pressure P (step 5). At this time, the heating rate is preferably within the range of 50 to 120°C / minute. Next, the second temperature T2 is maintained for a second predetermined time (second time) t2 while maintaining the pressure P (step 6). After the second time t2 has elapsed, the temperature is lowered to room temperature, and then the pressure P is reduced to normal pressure (step 7), thereby obtaining the cBN polycrystal of this embodiment.

[0059] Figure 3 The cBN polycrystal 10 of the present embodiment is schematically shown. In the cBN polycrystal 10, fine-grained crystals 11, coarse-grained crystals 12, and layered crystals 13 formed of cBN are mixed. The particle size of the fine-grained crystals 11 is in the range of 10 to 60 nm, and the particle size of the coarse-grained crystals 12 is in the range of 100 to 250 nm. The layered crystals 13 are formed by stacking a plurality of sheet crystals. Regarding the size of the layered crystals 13, the maximum length in the direction perpendicular to the stacking direction of the sheet crystals is less than 1000 nm, and the value obtained by dividing the maximum length in this direction by the thickness as the maximum length in the stacking direction (aspect ratio) is less than 3. Since the thickness of each sheet crystal is about several nm to 10 nm, having an aspect ratio of less than 3 means that one layered crystal 13 is formed by stacking a plurality of sheet crystals.

[0060] Figure 4 The following is a transmission electron microscope (TEM) photograph of an example (Example 1) of the cBN polycrystal 10 of the present embodiment produced by the above method. In Example 1, the above-mentioned pBN was used as a raw material, and the cBN polycrystal 10 was produced under the following conditions: pressure P = 25 GPa, first temperature T1 = 1450°C, first time t1 = 10 minutes, second temperature T2 = 1950°C, and second time t2 = 5 minutes. Figure 4As shown, the resulting cBN polycrystal 10 contains a mixture of fine-grained crystals 11, coarse-grained crystals 12, and lamellar crystals 13. In this sample, the abundance ratio of fine-grained crystals 11, coarse-grained crystals 12, and lamellar crystals 13 is 64:15:21 by area ratio. Furthermore, the average particle size of the fine-grained crystals 11, coarse-grained crystals 12, and lamellar crystals 13 combined was determined to be 46.3 nm.

[0061] The X-ray diffraction measurement results of the cBN polycrystal 10 of Example 1 using CuKα rays (wavelength 15.4 nm) are as follows: Figure 5B At the same time, for comparison, the results of X-ray diffraction measurement of a sample obtained by rapid cooling after heat treatment at the first temperature T1 for the first time t1 (without heat treatment at the second temperature) at an intermediate stage during the production of the cBN polycrystal corresponding to Example 1 are shown as follows: Figure 5A shown. Figure 5A In addition to the peaks derived from cBN, peaks derived from wBN were also found. Figure 5B No peak derived from wBN was found in the graph, and the cBN polycrystal 10 of Example 1 was confirmed to be a single-phase cBN polycrystal.

[0062] In order to investigate the structure of the layered crystal 13 in further detail, Figure 4 The cBN polycrystal 10 and a part of the layered crystal 13 are magnified and photographed in HR-TEM (high resolution transmission electron microscope). Figure 6A As shown, the HR-TEM photo obtained by further enlarging it is as shown Figure 6B As shown. From these photos, we can see the appearance of multiple plate-like crystals stacked together. In particular, Figure 6B It is shown that within the layered crystals 13, the plate-like crystals are stacked with crystal planes {111}, and these crystal planes further form numerous twins T as twin boundaries TB. The distance TT between twin boundaries TB is less than 10 nm. Furthermore, numerous stacking faults SF are formed within the layered crystals 13. The presence of these twins T and stacking faults SF in the layered crystals 13 is believed to be one of the main reasons for the increased Knoop hardness and Young's modulus of the cBN polycrystal 10. Furthermore, these twins T and stacking faults SF also have the effect of inhibiting the progression of cracks in the cBN polycrystal 10.

[0063] The Knoop hardness and Young's modulus of the cBN polycrystal 10 of Example 1 were measured. The results showed that the Knoop hardness was 65.6 GPa and the Young's modulus was 1085.7 GPa. Thus, the cBN polycrystal 10 of Example 1 had a high Knoop hardness of 60 GPa and a Young's modulus of 1082 GPa or greater, which is higher than that of conventional cBN polycrystals.

[0064] The process of forming the cBN polycrystalline 10 uses Figure 7 First, when a raw material composed of hexagonal BN is heated to a first temperature T1 under a pressure of 10 GPa or more, granular crystals 92 of cBN and layered crystals 93 of wBN are formed ( Figure 7 If the first temperature T1 is maintained for the first time t1, a portion of the wBN layered crystals 93 is transformed into cBN layered crystals 13 by martensitic transformation ( Figure 7 (upper right figure), and most of it remains in the form of wBN layered crystals 93. Then, the remaining wBN layered crystals 93 are crushed and refined by the hard cBN layered crystals 13 and granular crystals 92 under high pressure. If the temperature is further increased from the first temperature T1 to the second temperature T2, the phase transformation from the refined wBN layered crystals 93 to the cBN layered crystals 13 occurs ( Figure 7 ), during the second time t2 at the second temperature T2, the layered crystals 93 of wBN are all transformed into the layered crystals 13 of cBN ( Figure 7 During the second time t2 at the second temperature T2, new cBN nuclei are formed in a portion of the cBN layered crystals 13, starting from dangling bonds near the lattice defects, and dynamic recrystallization occurs, which leads to the growth of new fine cBN crystals. As a result, a portion of the cBN layered crystals 13 is transformed into cBN fine crystals 11 ( Figure 7 (lower middle figure). Thus, a cBN polycrystal 10 containing a mixture of fine cBN crystals 11, coarse cBN crystals 12, and layered cBN crystals 13 is obtained. If the second time t2 is too short, some of the layered wBN crystals 93 will remain. Therefore, the second time t2 is set so that all of the layered wBN crystals 93 are converted into layered cBN crystals 13.

[0065] If the heating at the second temperature T2 is continued after the second time t2, all the cBN layered crystals 13 are transformed into cBN fine crystals 13 by dynamic recrystallization and disappear, and the cBN fine crystals 13 formed before and after the second time t2 undergo grain growth and coarsen ( Figure 7 (lower right figure). If this is done, the cBN polycrystal 10 of this embodiment, which has high Knoop hardness and Young's modulus, cannot be obtained. Therefore, the second time t2 needs to be set so that the cBN fine crystals 11 and the cBN lamellar crystals 13 remain.

[0066] In preliminary experiments to determine the conditions for the second time t2 described herein, it is sufficient to confirm the formed crystals using TEM images or to confirm the presence of residual wBN using X-ray diffraction measurements for multiple samples produced at different second times t2. Alternatively, the Knoop hardness and Young's modulus of multiple similarly produced samples can be measured. If these values are 60 GPa or higher and 1082 GPa or higher, it is assumed that the conditions for each crystal are met.

[0067] Next, cBN polycrystals 10 were produced under various conditions, varying in first temperature and first time, second temperature and second time, and pressure, and the average grain size, the abundance ratio (area ratio) of fine-grained crystals 11, coarse-grained crystals 12, and lamellar crystals 13, Knoop hardness, and Young's modulus were measured (Examples 2 to 8). Similar measurements were also performed on Comparative Examples 1 and 2, where the second time was longer than that of Examples 1 to 7. Furthermore, similar measurements were performed on Comparative Examples 3 to 7, which were produced by heating from room temperature to a predetermined temperature and maintaining that temperature for a predetermined time, without performing two-stage heating at the first and second temperatures. The production conditions and measurement results for the samples of Examples 1 to 8 and Comparative Examples 1 to 7, including Example 1, are shown in Table 1. It should be noted that in Comparative Examples 3 to 7, for convenience, the heating temperature and the time maintained at that temperature are listed in the columns for the first temperature and first time, but these do not correspond to the first temperature and first time in the present invention.

[0068] [Table 1]

[0069]

[0070] The samples of Examples 1 to 8 all had an average particle size of 80 nm or less, all contained fine-grained crystals 11, coarse-grained crystals 12, and layered crystals 13, and none contained medium-grained crystals. The samples of Examples 1 to 8 all had high values of Knoop hardness, 60 GPa or greater, and high values of Young's modulus, 1082 GPa or greater. In contrast, in Comparative Examples 1 to 7, layered crystals 13 were absent or present at a negligible rate of 5% or less by area. In particular, in Comparative Examples 1 and 2, although two-stage heating at the first and second temperatures was performed similarly to Examples 1 to 8, the second heating time was longer than in Examples 1 to 8 (4 to 6 minutes in Examples 1 to 8, 10 to 15 minutes in Comparative Examples 1 and 2). As a result, layered crystals 13 were absent or reduced to a negligible level, resulting in lower Knoop hardness and Young's modulus compared to Examples 1 to 8.

[0071] Next, the Knoop hardness ( Figure 8 ) and Young's modulus ( Figure 9 )’s measurement results. Figure 8 and Figure 9 Also shown are samples that were maintained at the first temperature (1450°C) for a specified period of time and then cooled to room temperature without increasing the temperature; samples that were immediately cooled after reaching the second temperature (the second time was 0); and a comparative example that was heated from room temperature to 1950°C and maintained at this temperature for a specified period of time without performing two-stage heating at the first and second temperatures. Figure 8 and Figure 9 It can be seen that when the second time is between 4 and 7 minutes, the Knoop hardness can be set to 60 GPa or more, and the Young's modulus can be set to 1082 GPa or more.

[0072] The cBN polycrystal 10 obtained in this embodiment has the following characteristics: high Knoop hardness and Young's modulus, and therefore can be suitably used as a material for cutting edges of cutting tools and abrasive grains of grinding tools. In addition, the cBN polycrystal 10 can also be suitably used as an indenter for hardness measurement.

Claims

1. A cubic boron nitride polycrystal, characterized in that: The following crystals are mixed: Fine-grained crystals formed of cubic boron nitride with a particle size ranging from 10 to 60 nm; Coarse-grained crystals, which are formed of cubic boron nitride and have a particle size in the range of 100 to 250 nm; and A layered crystal composed of a plurality of stacked plate-like crystals of cubic boron nitride, wherein the maximum length in a direction perpendicular to the stacking direction is less than 1000 nm, and the aspect ratio obtained by dividing the maximum length by the maximum length in the stacking direction is less than 3. The abundance ratio of the fine-grained crystals, the coarse-grained crystals, and the lamellar crystals is within the range of (13-20):(60-75):(10-30) in terms of area ratio, The average particle size of the fine-grained crystals, the coarse-grained crystals, and the lamellar crystals combined is 80 nm or less.

2. The cubic boron nitride polycrystal according to claim 1, characterized in that The average particle size of the fine-grained crystals, the coarse-grained crystals, and the lamellar crystals combined is 60 nm or less.

3. A method for producing cubic boron nitride polycrystals, characterized in that: The method is a method for producing the cubic boron nitride polycrystal according to claim 1 or 2, A raw material formed of hexagonal boron nitride is heated to a first temperature in the range of 1300-1600°C under a pressure of 10 GPa or more and maintained for a first predetermined time, then heated to a second temperature in the range of 1700-2100°C and maintained for a second predetermined time, and then cooled.

4. A cutting tool, characterized in that: It has a cutting edge formed of the cubic boron nitride polycrystal according to claim 1 or 2.

5. A grinding tool, characterized in that: The invention comprises abrasive grains formed of the cubic boron nitride polycrystal according to claim 1 or 2.

6. A hardness measuring indenter, characterized in that: It is formed of the cubic boron nitride polycrystal according to claim 1 or 2.

Images