JP6798090B1 - Cubic boron nitride polycrystal and its manufacturing method - Google Patents

Abstract

A cubic boron nitride polycrystal containing 98.5% by volume or more of cubic boron nitride, the dislocation density of the cubic boron nitride is larger than 8 × 1015 / m2, and the cubic boron nitride polycrystal is a cubic boron nitride polycrystal. A plurality of crystal grains are included, and the median diameter d50 of the circle-equivalent diameter of the plurality of crystal grains is 0.1 μm or more and 0.5 μm or less.

Description

The present disclosure relates to a cubic boron nitride polycrystal and a method for producing the same. This application has priority based on Japanese Patent Application No. 2019-036261 filed on February 28, 2019, and PCT / JP2020 / 001436, an international application filed on January 17, 2020. Claim priority based on. All the contents of the Japanese patent application and the international application are incorporated herein by reference.

Cube boron nitride (hereinafter, also referred to as “cBN”) has hardness next to diamond, and is also excellent in thermal stability and chemical stability. For this reason, cubic boron nitride sintered bodies have been used as materials for tools.

As the cubic boron nitride sintered body, one containing about 10 to 40% by volume of a binder was used. However, the binder has been a cause of lowering the strength and thermal diffusivity of the sintered body.

In order to solve this problem, hexagonal boron nitride is directly converted to cubic boron nitride under ultra-high pressure and high temperature without using a binder, and at the same time, it is sintered to obtain a binder-free cubic boron nitride sintered. A method of gaining a body has been developed.

According to Japanese Patent Application Laid-Open No. 11-246271 (Patent Document 1), low crystalline hexagonal boron nitride is directly converted into a cubic boron nitride sintered body under ultra-high temperature and high pressure, and then sintered to obtain cubic nitride. A technique for obtaining a boron sintered body is disclosed.

Japanese Unexamined Patent Publication No. 11-246271

The cubic boron nitride polycrystals of the present disclosure are
A cubic boron nitride polycrystal containing 98.5% by volume or more of cubic boron nitride.
The dislocation density of the cubic boron nitride is greater than 8 × 10 ¹⁵ / m ² .
The cubic boron nitride polycrystal contains a plurality of crystal grains and contains a plurality of crystal grains.
The median diameter d50 of the plurality of crystal grains having a circle-equivalent diameter is 0.1 μm or more and 0.5 μm or less, which is a cubic boron nitride polycrystal.

The method for producing a cubic boron nitride polycrystal according to the present disclosure is as follows.
The method for producing a cubic boron nitride polycrystal described above.
The first step of preparing hexagonal boron nitride powder and
The hexagonal boron nitride powder is passed through the temperature and pressure within the stable region of the wurtzite-type boron nitride and heated and pressurized to a temperature of 1700 ° C. or higher and 2500 ° C. or lower and a pressure of 8 GPa or higher to obtain a boron nitride polycrystal. The second step to get the body and
The boron nitride polycrystal obtained in the second step is held at a temperature of 1700 ° C. or higher and 2500 ° C. or lower and a pressure condition of 8 GPa or higher for 3 minutes or longer and 60 minutes or lower to obtain a cubic boron nitride polycrystal. With a third step to obtain
The stable region of the wurtzite-type boron nitride is a region that simultaneously satisfies the following formula 1 and the following formula 2 when the temperature is T ° C. and the pressure is PGPa.
Equation 1: P ≧ -0.0037T + 11.301
Equation 2: P ≦ -0.085T + 117
In the heating and pressurizing path of the second step, the temperature at which the wurtzite-type boron nitride enters the stable region is 500 ° C. or lower.
The second step is the production of a cubic boron nitride polycrystal, which comprises a step of maintaining the temperature and pressure in the heating and pressurizing path at the temperature and pressure within the stable region of the wurtzite-type boron nitride for 10 minutes or more. The method.

FIG. 1 is a pressure-temperature phase diagram of boron nitride. FIG. 2 is a diagram for explaining an example of a method for producing a cubic boron nitride polycrystal of the present disclosure. FIG. 3 is a diagram for explaining another example of the method for producing a cubic boron nitride polycrystal of the present disclosure. FIG. 4 is a diagram for explaining another example of the method for producing a cubic boron nitride polycrystal of the present disclosure. FIG. 5 is a diagram for explaining a conventional example of a method for producing a cubic boron nitride polycrystal. FIG. 6 is a diagram for explaining the aspect ratio of the crystal grains.

[Issues to be resolved by this disclosure]
In recent years, in addition to iron-based materials, the processing of difficult-to-cut materials such as Ti alloys used as aircraft parts has been increasing. Generally, when high-load machining is performed on a difficult-to-cut material, the cutting edge tends to be chipped and the tool life tends to be shortened. Therefore, there is a demand for a tool that can exhibit an excellent tool life even in high-load machining of iron-based materials and difficult-to-cut materials.

Therefore, an object of the present invention is to provide a cubic boron nitride polycrystal that can have a long tool life when used as a tool, especially in high-load machining of iron-based materials and difficult-to-cut materials. And.
[Effect of this disclosure]
According to the present disclosure, the cubic boron nitride polycrystal can have a long tool life when used as a tool, particularly even in high-load machining of iron-based materials and difficult-to-cut materials.

[Explanation of Embodiments of the present disclosure]
First, embodiments of the present disclosure will be listed and described.

(1) The cubic boron nitride polycrystal of the present disclosure is a cubic boron nitride polycrystal containing 98.5% by volume or more of cubic boron nitride.
The dislocation density of the cubic boron nitride is greater than 8 × 10 ¹⁵ / m ² .
The cubic boron nitride polycrystal contains a plurality of crystal grains and contains a plurality of crystal grains.
The median diameter d50 of the plurality of crystal grains having a circle-equivalent diameter is 0.1 μm or more and 0.5 μm or less, which is a cubic boron nitride polycrystal.

According to the present disclosure, the cubic boron nitride polycrystal can have a long tool life when used as a tool, especially in high-load machining of iron-based materials and difficult-to-cut materials.

(2) The dislocation density is preferably 9 × 10 ¹⁵ / m ² or more. According to this, the fracture resistance of the tool is improved.

(3) The total content of the alkali metal element and the alkaline earth metal element of the cubic boron nitride polycrystal is preferably 10 ppm or less on a mass basis.

According to this, a tool using the cubic boron nitride polycrystal can have an excellent tool life even in ultra-high speed and high load machining of iron-based materials and difficult-to-cut materials.

(4) When the cross section of the cubic boron nitride polycrystal is observed at a magnification of 10000 times using a scanning electron microscope, the area ratio of the plate-like particles having an aspect ratio of 4 or more is 30 area% or less. It is preferable to have. A tool using the cubic boron nitride polycrystal can have an excellent tool life.

(5) The area ratio of the plate-like particles having an aspect ratio of 4 or more is preferably 5 area% or less. A tool using the cubic boron nitride polycrystal can have an even better tool life.

(6) The cubic boron nitride polycrystal preferably contains 0.01% by volume or more of compressed hexagonal boron nitride. A tool using the cubic boron nitride polycrystal can have an excellent tool life.

(7) The cubic boron nitride polycrystal preferably contains 0.1% by volume or more of wurtzite-type boron nitride. A tool using the cubic boron nitride polycrystal can have an excellent tool life.

(8) The dislocation density is preferably calculated using the modified Williamson-Hall method and the modified Warren-Averbach method. The dislocation density has a good correlation with the performance of the cubic boron nitride polycrystal.

(9) The dislocation density is preferably measured using synchrotron radiation as an X-ray source. The dislocation density has a good correlation with the performance of the cubic boron nitride polycrystal.

(10) The method for producing a cubic boron nitride polycrystal of the present disclosure is the above-mentioned method for producing a cubic boron nitride polycrystal.
The first step of preparing hexagonal boron nitride powder and
The hexagonal boron nitride powder is passed through the temperature and pressure within the stable region of the wurtzite-type boron nitride and heated and pressurized to a temperature of 1700 ° C. or higher and 2500 ° C. or lower and a pressure of 8 GPa or higher to obtain a boron nitride polycrystal. The second step to get the body and
The boron nitride polycrystal obtained in the second step is held at a temperature of 1700 ° C. or higher and 2500 ° C. or lower and a pressure condition of 8 GPa or higher for 3 minutes or longer and 60 minutes or lower to obtain a cubic boron nitride polycrystal. With a third step to obtain
The stable region of the wurtzite-type boron nitride is a region that simultaneously satisfies the following formula 1 and the following formula 2 when the temperature is T ° C. and the pressure is PGPa.
Equation 1: P ≧ -0.0037T + 11.301
Equation 2: P ≦ -0.085T + 117
In the heating and pressurizing path of the second step, the temperature at which the wurtzite-type boron nitride enters the stable region is 500 ° C. or lower.
The second step includes a step of maintaining the temperature and pressure in the heating and pressurizing path at the temperature and pressure within the stable region of the wurtzite-type boron nitride for 10 minutes or more.

The cubic boron nitride polycrystal obtained by this production method can have a long tool life when used as a tool, especially in high-load machining of iron-based materials and difficult-to-cut materials.

(11) The inrush temperature is preferably 300 ° C. or lower. According to this, the life of the tool using the obtained cubic boron nitride polycrystal is further improved.

(12) The second step preferably includes a step of maintaining the temperature and pressure in the heating and pressurizing path at the temperature and pressure within the stable region of the wurtzite-type boron nitride for 15 minutes or more. According to this, the life of the tool using the obtained cubic boron nitride polycrystal is further improved.

(13) In the second step, when the temperature and pressure in the heating and pressurizing path are T ° C. and the pressure is PGPa, the temperature in the region that simultaneously satisfies the following formula 1, the following formula 2 and the following formula 3. And it is preferable to include a step of holding at pressure for 10 minutes or more.
Equation 1: P ≧ -0.0037T + 11.301
Equation 2: P ≦ -0.085T + 117
Equation 3: P ≦ -0.0037T + 11.375

According to this, the life of the tool using the obtained cubic boron nitride polycrystal is further improved.

[Details of Embodiments of the present disclosure]
The cubic boron nitride polycrystal of the present disclosure and a method for producing the same will be described below with reference to the drawings.

[Embodiment 1: Cubic boron nitride polycrystal]
A cubic boron nitride polycrystal according to an embodiment of the present disclosure will be described.

<Cublic Boron Nitride Polycrystal>
The cubic boron nitride polycrystal of the present disclosure is a cubic boron nitride polycrystal containing 98.5% by volume or more of cubic boron nitride, and the dislocation density of the cubic boron nitride is 8 × 10 ¹⁵ / m. Larger than ² , the cubic boron nitride polycrystal contains a plurality of crystal grains, and the median diameter d50 of the circle-equivalent diameter of the plurality of crystal grains is 0.1 μm or more and 0.5 μm or less.

The cubic boron nitride polycrystal of the present disclosure is a sintered body, but since the sintered body is usually intended to contain a binder, the term "polycrystal" is used in the present disclosure. ..

When used as a tool, the cubic boron nitride polycrystal of the present disclosure can have a long tool life, especially in high-load machining of iron-based materials and difficult-to-cut materials. The reason for this is not clear, but it is presumed to be as shown in (i) to (iii) below.

(I) The cubic boron nitride polycrystal of the present disclosure contains 98.5% by volume or more of cubic boron nitride and substantially does not contain a binder, a sintering aid, a catalyst or the like. Therefore, the cubic boron nitrides are strongly bonded to each other, and the strength and thermal diffusivity of the cubic boron nitride polycrystal are improved. Therefore, a tool using the cubic boron nitride polycrystal can have a long tool life even in high-load machining of iron-based materials and difficult-to-cut materials.

(Ii) In the cubic boron nitride polycrystal of the present disclosure, the dislocation density of cubic boron nitride is larger than 8 × 10 ¹⁵ / m ² . The cubic boron nitride polycrystal has a relatively large number of lattice defects in the polycrystal and has a large strain, so that the strength is improved. Therefore, a tool using the cubic boron nitride polycrystal can have a long tool life even in high-load machining of iron-based materials and difficult-to-cut materials.

(Iii) The cubic boron nitride polycrystal of the present disclosure has a median diameter d50 (hereinafter, also referred to as “particle size”) having a diameter equivalent to a circle of a plurality of crystal grains contained therein, which is 0.1 μm or more and 0.5 μm. It is as follows. Conventionally, it has been considered that the cutting performance of a cubic boron nitride polycrystal is improved because the strength increases as the grain size of the crystal grains decreases. For this reason, the particle size of the crystal grains constituting the cubic boron nitride polycrystal was reduced (for example, the average particle size was less than 100 nm), but this tended to reduce the toughness. On the other hand, as described in (ii) above, the cubic boron nitride polycrystal of the present disclosure secures strength by increasing the dislocation density of cubic boron nitride, so that the particle size is larger than before. can do. Therefore, the cubic boron nitride polycrystal of the present disclosure has improved toughness and can have excellent crack propagation resistance. Therefore, a tool using the cubic boron nitride polycrystal can have a long tool life even in high-load machining of iron-based materials and difficult-to-cut materials.

In the above description, the cubic boron nitride polycrystal of the present disclosure has a long tool life in high-load machining of iron-based materials and difficult-to-cut materials, but the work material and processing method are limited to these. Not done. Examples of the work material include Ti alloys such as Ti6Al4V and cobalt-chromium alloys. Examples of the processing method include turning and milling.

<Composition>
The cubic boron nitride polycrystals of the present disclosure contain 98.5% by volume or more of cubic boron nitride. As a result, the cubic boron nitride polycrystal has excellent hardness, and is also excellent in thermal stability and chemical stability.

The cubic boron nitride polycrystal has a total of 1.5% by volume of one or both of compression hexagonal boron nitride and wurtzite boron nitride in addition to cubic boron nitride to the extent that the effects of the present disclosure are exhibited. The following may be included. Here, the "compressed hexagonal boron nitride" has a crystal structure similar to that of ordinary hexagonal boron nitride, and the plane spacing in the c-axis direction is larger than that of normal hexagonal boron nitride (0.333 nm). Indicates a small one.

The cubic boron nitride polycrystal may contain unavoidable impurities as long as the effects of the present disclosure are exhibited. Inevitable impurities include, for example, hydrogen, oxygen, carbon, alkali metal elements (in the present specification, alkali metal elements include lithium (Li), sodium (Na), potassium (K)) and alkaline earth metals. Elements (in this specification, alkaline earth metal elements include calcium (Ca), magnesium (Mg), strontium (Sr), barium (Ba)), silicon (Si), aluminum (Al) and the like. be able to. When the cubic boron nitride polycrystal contains unavoidable impurities, the content of the unavoidable impurities is preferably 0.1% by mass or less. The content of unavoidable impurities can be measured by secondary ion mass spectrometry (SIMS).

Alkaline metal elements (lithium (Li), sodium (Na), potassium (K)) and alkaline earth metal elements (calcium (Ca), magnesium (Mg), strontium (Sr), barium) of cubic boron nitride polycrystals The total content of (Ba)) is preferably 10 ppm or less. Among the above unavoidable impurities, the alkali metal element and the alkaline earth metal element have a catalytic action on the phase conversion between hexagonal boron nitride and boron nitride. When the total content of the alkali metal element and the alkaline earth metal element of the cubic boron nitride polycrystal is 10 ppm or less, the tool using the cubic boron nitride polycrystal has a cutting edge and a work piece in a cutting environment. Even when the interface with the material is exposed to high temperature and high pressure, it is possible to satisfactorily suppress the progress of damage to the tool due to the conversion of part of the cubic boron nitride that constitutes the tool to hexagonal boron nitride. it can. The lower limit of the total content of the alkali metal element and the alkaline earth metal element in the hexagonal boron nitride polycrystal is preferably 0 ppm. That is, the total content of the alkali metal element and the alkaline earth metal element in the hexagonal boron nitride polycrystal is preferably 0 ppm or more and 10 ppm or less.

Conventional cubic boron nitride sintered bodies are produced using cBN abrasive grains as a starting material, for example, as described in JP-A-2006-201216. Here, the total content (content of the catalyst component in 1 mol of cBN) of the catalyst components (alkali metal element, alkaline earth metal element) remaining in the cBN abrasive grains is 2.4 × 10 ^{-4 to} 13 .5 × 10 ^-4 mol. Therefore, it is obvious to those skilled in the art that the total content of the catalyst components of the conventional cubic boron nitride polycrystal obtained by sintering the cBN abrasive grains is 0.01% by mass (100 ppm) or more. is there.

On the other hand, the cubic boron nitride polycrystal of the present disclosure uses hexagonal boron nitride as a starting material and heats and pressurizes the hexagonal boron nitride without using a catalyst to convert it into cubic boron nitride, as will be described later. You can get it. Therefore, the content of the catalyst component of the cubic boron nitride polycrystal can be 10 ppm or less on a mass basis.

The total content of silicon (Si) and aluminum (Al) in the cubic boron nitride polycrystal is preferably 50 ppm or less on a mass basis. According to this, the tool using the cubic boron nitride polycrystal body constitutes the tool even when the interface between the cutting edge and the work material is exposed to high temperature and high pressure in a cutting environment. It is possible to satisfactorily suppress the progress of damage to the tool due to the reaction of a part of boron with Si and Al.

It is preferable that the cubic boron nitride polycrystal is substantially free of binders, sintering aids, catalysts and the like. As a result, the strength and thermal diffusivity of the cubic boron nitride polycrystal are improved.

The content of cubic boron nitride in the cubic boron nitride polycrystal is preferably 98.5% by volume or more and 100% by volume or less, and more preferably 99% by volume or more and 100% by volume or less. The upper limit of the content of cubic boron nitride can be 100% by volume or less, 99.99% by volume or less, 99.9% by volume or less, and 99.89% by volume or less.

The total content of compressed hexagonal boron nitride and wurtzite boron nitride in the cubic boron nitride polycrystal is preferably 0% by volume or more and 1.5% by volume or less, and 0% by volume or more and 1% by volume or less. Preferably, 0% by volume is preferable. That is, it is preferable that the cubic boron nitride polycrystal does not contain either compressed hexagonal boron nitride or wurtzite boron nitride.

The content of compressed hexagonal boron nitride in the cubic boron nitride polycrystal is preferably 0% by volume or more and 1.5% by volume or less, preferably 0% by volume or more and 1% by volume or less, and preferably 0% by volume. That is, it is preferable that the cubic boron nitride polycrystal does not contain compressed hexagonal boron nitride.

The content of wurtzite-type boron nitride in the cubic boron nitride polycrystal is preferably 0% by volume or more and 1.5% by volume or less, preferably 0% by volume or more and 1% by volume or less, and preferably 0% by volume. That is, it is preferable that the cubic boron nitride polycrystal does not contain wurtzite-type boron nitride.

Hexagonal boron nitride, compression-type hexagonal boron nitride, and wurtzite-type boron nitride have low frictional resistance, which can reduce the adhesion of the work material during cutting and reduce the cutting resistance. Further, hexagonal boron nitride, compression-type hexagonal boron nitride and wurtzite-type boron nitride are softer than cubic boron nitride and have excellent crack propagation resistance. Therefore, depending on the processing application, it may be preferable that the cubic boron nitride polycrystal contains hexagonal boron nitride, compressed hexagonal boron nitride, and wurtzite boron nitride.

In the above case, the cubic boron nitride polycrystal preferably contains 0.01% by volume or more of compressed hexagonal boron nitride. The cubic boron nitride polycrystal preferably contains compression type hexagonal boron nitride in an amount of 0.01% by volume or more and 1.5% by volume or less, and preferably 0.01% by volume or more and 1% by volume or less. In this case, the cubic boron nitride polycrystal preferably contains 99.99% by volume or less of cubic boron nitride.

In the above case, the cubic boron nitride polycrystal preferably contains 0.1% by volume or more of wurtzite-type boron nitride. The cubic boron nitride polycrystal preferably contains 0.1% by volume or more and 1.5% by volume or less of wurtzite-type boron nitride, and preferably 0.1% by volume or more and 1% by volume or less. In this case, the cubic boron nitride polycrystal preferably contains 99.9% by volume or less of cubic boron nitride.

The content (% by volume) of cubic boron nitride, compression-type hexagonal boron nitride, and wurtzite-type boron nitride in the cubic boron nitride polycrystal can be measured by an X-ray diffraction method. The specific measurement method is as follows.

A cubic boron nitride polycrystal is cut with a diamond grindstone electrodeposition wire, and the cut surface is used as an observation surface.

An X-ray spectrum of a cut surface of a cubic boron nitride polycrystal is obtained using an X-ray diffractometer (“MiniFlex 600” (trade name) manufactured by Rigaku). The conditions of the X-ray diffractometer at this time are as follows.

Characteristic X-ray: Cu-Kα (wavelength 1.54 Å)
Tube voltage: 45kV
Tube current: 40mA
Filter: Multi-layer mirror Optical system: Concentration method X-ray diffraction method: θ-2θ method.

In the obtained X-ray spectrum, the following peak intensity A, peak intensity B and peak intensity C are measured.

Peak intensity A: The peak intensity of compressed hexagonal boron nitride excluding the background from the peak intensity near the diffraction angle 2θ = 28.5 °.

Peak intensity B: The peak intensity of wurtzite-type boron nitride excluding the background from the peak intensity near the diffraction angle 2θ = 40.8 °.

Peak intensity C: The peak intensity of cubic boron nitride excluding the background from the peak intensity near the diffraction angle 2θ = 43.5 °.

The content of compressed hexagonal boron nitride can be obtained by calculating the value of peak intensity A / (peak intensity A + peak intensity B + peak intensity C). The content of wurtzite-type boron nitride can be obtained by calculating the value of peak intensity B / (peak intensity A + peak intensity B + peak intensity C). The content of cubic boron nitride can be obtained by calculating the value of peak intensity C / (peak intensity A + peak intensity B + peak intensity C). Since compression-type hexagonal boron nitride, wurtzite-type boron nitride, and cubic boron nitride all have similar electronic weights, the above X-ray peak intensity ratio is the volume ratio in the cubic boron nitride polycrystal. Can be regarded as.

<Dislocation density>
In the cubic boron nitride polycrystals of the present disclosure, the dislocation density of cubic boron nitride is greater than 8 × 10 ¹⁵ / m ² . The cubic boron nitride polycrystal has a relatively large number of lattice defects in the polycrystal and has a large strain, so that the strength is improved. Therefore, a tool using the cubic boron nitride polycrystal can have a long tool life even in high-load machining of iron-based materials and difficult-to-cut materials. The dislocation density is preferably 9 × 10 ¹⁵ / m ² or more, and more preferably 1.0 × 10 ¹⁶ / m ² or more. The upper limit of the dislocation density is not particularly limited, but from the viewpoint of manufacturing, it can be 1.4 × 10 ¹⁶ / m ² . That is, the dislocation density is preferably 8 × ¹⁰ 15 / ^m greater than ² 1.4 × ¹⁰ 16 / ^{m 2} or less, more preferably 9 × ¹⁰ 15 / ^{m 2} or more 1.4 × ¹⁰ 16 / ^{m 2} or less, 1 More preferably, it is 0.0 × 10 ¹⁶ / m ² or more and 1.4 × 10 ¹⁶ / m ² or less.

In the present specification, the dislocation density is calculated by the following procedure.
A test piece composed of a cubic boron nitride polycrystal is prepared. The size of the test piece is 2.0 mm × 2.0 mm on the observation surface and 1.0 mm in thickness. Polish the observation surface of the test piece.

X-ray diffraction measurement was performed on the observation surface of the test piece under the following conditions, and the main orientations of cubic boron nitride (111), (200), (220), (311), (400), ( The line profile of the diffraction peak from each azimuth plane of 331) is obtained.

(X-ray diffraction measurement conditions)
X-ray source: Synchrotron radiation Device condition: Detector NaI (cut fluorescence by appropriate ROI)
Energy: 18 keV (wavelength: 0.6888 Å)
Spectral crystal: Si (111)
Incident slit: width 5 mm x height 0.5 mm
Light receiving slit: Double slit (width 3 mm x height 0.5 mm)
Mirror: Platinum coated mirror Incident angle: 2.5 mrad
Scanning method: 2θ-θscan
Measurement peaks: (111), (200), (220), (311), (400), (331) of cubic boron nitride. However, if it is difficult to obtain a profile due to texture and orientation, the peak of the surface index is excluded.

Measurement conditions: Make sure that the number of measurement points is 9 or more within the half width. The peak top intensity shall be 2000 counts or more. Since the tail of the peak is also used for analysis, the measurement range is about 10 times the half width.

The line profile obtained by the above-mentioned X-ray diffraction measurement has a shape that includes both true spread due to physical quantities such as non-uniform strain of the sample and spread due to the device. In order to determine the non-uniform strain and crystallite size, the device-derived components are removed from the measured line profile to obtain a true line profile. The true line profile is obtained by fitting the obtained line profile and the device-derived line profile by the pseudo Voigt function and subtracting the device-derived line profile. LaB ₆ was used as a standard sample for removing the diffraction line spread caused by the device. Further, when synchrotron radiation having high parallelism is used, the diffraction line spread due to the device can be regarded as zero.

The dislocation density is calculated by analyzing the obtained true line profile using the modified Williamson-Hall method and the modified Warren-Averbach method. The modified Williamson-Hall method and the modified Warren-Averbach method are known line profile analysis methods used to determine the dislocation density.

The formula of the modified Williamson-Hall method is represented by the following formula (I).

(In the above equation (I), ΔK is the half width of the line profile, D is the crystallite size, M is the arrangement parameter, b is the Burgers vector, ρ is the dislocation density, K is the scattering vector, and O (K ² C) is K. ² The higher term of C, C indicates the average value of the contrast factor.)
C in the above formula (I) is represented by the following formula (II).

C = C _h00 [1-q (h ² k ² + h ² l ² + k ² l ² ) / (h ² + k ² + l ² ) ² ] (II)
In above-mentioned formula (II), the coefficient q for each of the contrast factor C _h00 and contrast factor in screw dislocations and edge dislocations, using the calculation code ANIZC, slip system <110> {111}, the elastic stiffness C ₁₁ 8 It is calculated as .44 GPa, C ₁₂ is 1.9 GPa, and C ₄₄ is 4.83 GPa. The contrast factor C _h00 is 0.203 for spiral dislocations and 0.212 for blade dislocations. The coefficient q for the contrast factor is 1.65 for spiral dislocations and 0.58 for blade dislocations. The spiral dislocation ratio is fixed at 0.5, and the blade dislocation ratio is fixed at 0.5.

Further, the relationship of the following equation (III) is established between the dislocation and the non-uniform strain by using the contrast factor C.

^{<Ε (L) 2> =} (ρCb 2 / 4π) ln (R e / L) (III)
(In the above formula (III), _{R e} represents an effective radius of the dislocation.)
From the relationship of the above equation (III) and the equation of Warren-Averbach, it can be expressed as the following equation (IV), and the dislocation density ρ and the crystallite size can be obtained as the modified Warren-Averbach method.

^{lnA (L) = lnA S (} L) - in ^{(πL 2 ρb 2/2)} ln (R e / L) (K 2 C) + O (K 2 C) 2 (IV) ( the formula (IV), A (L) is the Fourier series, a ^S (L) is the Fourier series regarding crystallite size, L is shows a Fourier length.)
For more information on the modified Williamson-Hall method and the modified Warren-Averbach method, see “T.Ungar and A.Borbely,“ The effect of dislocation contrast on x-ray line broadening: A new approach to line profile analysis ”Appl.Phys.Lett. ., vol.69, no.21, p.3173, 1996. ”and“ T.Ungar, S.Ott, P.Sanders, A.Borbely, J.Weertman, “Dislocations, grain size and planar faults in nanostructured copper Determined by high resolution X-ray diffraction and a new procedure of peak profile analysis "Acta Mater., Vol.46, no.10, pp.3693-3699, 1998."

<Crystal grains>
(Median diameter d50)
The median diameter d50 (hereinafter, also referred to as “median diameter d50”) of the equivalent circle diameter of the plurality of crystal grains contained in the cubic boron nitride polycrystal of the present disclosure is 0.1 μm or more and 0.5 μm or less. Conventionally, it has been considered that the smaller the grain size of cubic boron nitride polycrystals, the higher the strength and the better the cutting performance. For this reason, the particle size of the crystal grains constituting the cubic boron nitride polycrystal was reduced (for example, the average particle size was less than 100 nm), but this tended to reduce the toughness.

On the other hand, as described in (ii) above, the cubic boron nitride polycrystal of the present disclosure secures strength by increasing the dislocation density of cubic boron nitride, so that the particle size is larger than before. can do. Therefore, the cubic boron nitride polycrystal of the present disclosure has improved toughness and can have excellent crack propagation resistance. Therefore, a tool using the cubic boron nitride polycrystal can have a long tool life even in high-load machining of iron-based materials and difficult-to-cut materials.

(Area ratio of plate-shaped particles with aspect ratio of 4 or more)
When the cross section of a cubic boron nitride polycrystal is observed at a magnification of 10,000 times using a scanning electron microscope, the area ratio of plate-like particles having an aspect ratio of 4 or more is preferably 30 area% or less. .. In the conventional cubic boron nitride polycrystal, the decrease in toughness accompanying the reduction in particle size is compensated for by the presence of a plate-like structure in the cubic polycrystal. However, these plate-shaped particles suddenly fall off from the cutting edge during high-efficiency machining of a difficult-to-cut material, causing the cutting edge to be chipped, which causes variation and shortening of the tool life.

In the cubic boron nitride polycrystal according to the present embodiment, the content of plate-like particles having an aspect ratio of 4 or more is reduced. Therefore, the cubic boron nitride polycrystal is less likely to cause sudden chipping of the cutting edge due to plate-like particles, and has a long tool life even in high-load machining of iron-based materials and difficult-to-cut materials. it can.

The area ratio of the plate-like particles having an aspect ratio of 4 or more is preferably 30 area% or less. The area ratio of the plate-like particles having an aspect ratio of 4 or more is preferably 0 area% or more and 30 area% or less, and more preferably 0 area% or more and 5 area% or less.

(Measuring method of area ratio of plate-shaped particles with median diameter d50 and aspect ratio of 4 or more)
In the present specification, the median diameter d50 of the circle-equivalent diameter of a plurality of crystal grains contained in a cubic boron nitride polycrystal is the median diameter of a plurality of crystal grains at each of five arbitrarily selected measurement points. It means a value obtained by measuring each d50 and calculating an average value thereof.

In the present specification, the area ratio of the plate-like particles having an aspect ratio of 4 or more in the cubic boron nitride polycrystal is the plate-like particles having an aspect ratio of 4 or more at each of the five measurement points arbitrarily selected. It means the value obtained by measuring each of the area ratios of and calculating the average value of these.

As far as the applicant has measured, as long as the area ratio of the plate-like particles having a median diameter d50 and an aspect ratio of 4 or more is measured in the same sample, the selection point of the measurement field in the cubic boron nitride polycrystal is changed. It was confirmed that there was almost no variation in the measurement results even when calculated multiple times, and it was not arbitrary even if the measurement field was set arbitrarily.

When a cubic boron nitride polycrystal is used as a part of a tool, the portion of the cubic boron nitride polycrystal is cut out with a diamond grindstone electrodeposition wire or the like, and the cut out cross section is polished to perform the polishing. Five measurement points are arbitrarily set on the surface.

A method for measuring the area ratio of plate-shaped particles having a median diameter d50 having a circle-equivalent diameter of a plurality of crystal grains and an aspect ratio of 4 or more at each measurement location will be specifically described below.

The cubic boron nitride polycrystal is cut with a diamond grindstone electrodeposition wire or the like so that the measurement point is exposed, and the cut surface is polished. The measurement point on the polished surface is observed using an SEM (“JSM-7500F” (trade name) manufactured by JEOL Ltd.) to obtain an SEM image. The size of the measurement field of view is 12 μm × 15 μm, and the observation magnification is 10000 times.

For each of the five SEM images, with the grain boundaries of the crystal grains observed in the measurement field separated, using image processing software (Win Roof ver.7.4.5), the aspect ratio of each crystal grain and each Calculate the equivalent circle diameter of the crystal grains. Here, the aspect ratio means the value (major axis / minor axis) of the ratio of the major axis to the minor axis of the crystal grain on the cut surface. When the shape of the crystal grain is an indefinite shape as shown in FIG. 6, the aspect ratio is calculated according to the following procedures (a) to (c) using image processing software.

(A) The longest line segment (hereinafter, also referred to as "first line segment") that can be drawn inside the crystal grain (both ends are in contact with the crystal grain boundary) is specified, and the first line segment is specified. Measure the length L1.

(B) The longest line segment that is orthogonal to the above first line segment and can be drawn inside the crystal grain (both ends are in contact with the crystal grain boundary) (hereinafter, also referred to as "second line segment"). ) Is specified, and the length L2 of the second line segment is measured.

(C) The value (L1 / L2) of the ratio between the length L1 of the first line segment and the length L2 of the second line segment is calculated. The value of (L1 / L2) is used as the aspect ratio.

The median diameter d50 is calculated from the distribution of the equivalent circle diameters of all crystal grains in the measurement field of view. The area ratio of plate-shaped particles having an aspect ratio of 4 or more is calculated using the entire area of the measurement field of view as the denominator.

<Use>
The cubic boron nitride polycrystal of the present disclosure is preferably used for cutting tools, abrasion resistant tools, grinding tools and the like.

The cutting tool, the wear-resistant tool, and the grinding tool using the cubic boron nitride polycrystal of the present disclosure may be entirely composed of the cubic boron nitride polycrystal, or a part thereof (for example, a cutting tool). In the case of, only the cutting edge portion) may be composed of a cubic boron nitride polycrystal. Further, a coating film may be formed on the surface of each tool.

Cutting tools include drills, end mills, replaceable cutting tips for drills, replaceable cutting tips for end mills, replaceable cutting tips for milling, replaceable cutting tips for turning, metal saws, gear cutting tools, and reamers. , Taps, cutting tools, etc.

Examples of the wear-resistant tool include a die, a scriber, a scribing wheel, and a dresser. Examples of the grinding tool include a grinding wheel.

[Embodiment 2: Method for Producing Cubic Boron Nitride Polycrystal]
The method for producing a cubic boron nitride polycrystal of the present disclosure will be described with reference to FIGS. 1 to 5. FIG. 1 is a pressure-temperature phase diagram of boron nitride. 2 to 4 are views for explaining a method for producing a cubic boron nitride polycrystal of the present disclosure. FIG. 5 is a diagram for explaining a conventional example of a method for producing a cubic boron nitride polycrystal.

The method for producing a cubic boron nitride polycrystal of the present disclosure is the method for producing a cubic boron nitride polycrystal of the first embodiment. In the method for producing a cubic boron nitride polycrystal of the present disclosure, a first step of preparing hexagonal boron nitride powder (hereinafter, also referred to as “first step”) and the hexagonal boron nitride powder are subjected to wurtzite. The second step of obtaining a boron nitride polycrystal by passing through the temperature and pressure in the stable region of the boron nitride and heating and pressurizing to a temperature of 1700 ° C. or higher and 2500 ° C. or lower and a pressure of 8 GPa or higher (hereinafter, "" It is also referred to as "second step"), and the boron nitride polycrystal obtained in the second step is held for 3 minutes or more and 60 minutes or less under the conditions of a temperature of 1700 ° C. or higher and 2500 ° C. or lower and a pressure condition of 8 GPa or higher. It includes a third step (hereinafter, also referred to as “third step”) for obtaining a cubic boron nitride polycrystal. Here, the stable region of wurtzite-type boron nitride is a region that simultaneously satisfies the following equations 1 and 2 when the temperature is T (° C.) and the pressure is P (GPa).
Equation 1: P ≧ -0.0037T + 11.301
Equation 2: P ≦ -0.085T + 117
In the heating and pressurizing path of the second step, the inrush temperature of the wurtzite type boron nitride into the stable region is 500 ° C. or less, and in the second step, the temperature and pressure in the heating and pressurizing path are set to the wurtzite type boron nitride. Includes a step of holding the temperature and pressure within the stable region of the above for 10 minutes or more.

Before giving a detailed description of the method for producing a cubic boron nitride polycrystal of the present disclosure, a conventional example of a method for producing a cubic boron nitride polycrystal will be described in order to help understanding the method.

As shown in FIG. 1, boron nitride includes hexagonal boron nitride, which is a stable phase at normal temperature and pressure, cubic boron nitride, which is a stable phase at high temperature and high pressure, and from hexagonal boron nitride to cubic boron nitride. There are three phases of wurtzite-type boron nitride, which are semi-stable phases between the rearrangements of.

The boundary of each phase can be represented by a linear function. In the present specification, the temperature and pressure in the stable region of each phase can be indicated by using a linear function.

In the present specification, the temperature and pressure in the stable region of wurtzite-type boron nitride (referred to as “wBN stable region” in FIG. 1) are when the temperature is T (° C.) and the pressure is P (GPa). , Defined as the temperature and pressure that simultaneously satisfy the following formula 1 and the following formula 2.

Equation 1: P ≧ -0.0037T + 11.301
Equation 2: P ≦ -0.085T + 117

In the present specification, the temperature and pressure in the stable region of hexagonal boron nitride (referred to as “hBN stable region” in FIG. 1) are when the temperature is T (° C.) and the pressure is P (GPa). It is defined as the temperature and pressure that simultaneously satisfy the following formulas A and B, or the temperature and pressure that simultaneously satisfy the following formulas C and D.

Formula A: P ≦ -0.0037T + 11.301
Equation B: P ≦ -0.085T + 117
Formula C: P ≤ 0.0027T + 0.3333
Formula D: P ≧ -0.085T + 117

In the present specification, the temperature and pressure in the stable region of cubic boron nitride (referred to as “cBN stable region” in FIG. 1) are when the temperature is T (° C.) and the pressure is P (GPa). It is defined as a temperature and pressure that simultaneously satisfy the following formula D and the following formula E.

Formula D: P ≧ -0.085T + 117
Formula E: P ≧ 0.0027T + 0.3333

Conventionally, as a heating and pressurizing path for bringing hexagonal boron nitride to a temperature and pressure within a stable region of cubic boron nitride, the path shown in FIG. 5 (hereinafter, also referred to as “path of FIG. 5”). Was being considered.

In the path of FIG. 5, from the temperature and pressure (normal temperature and normal pressure) of the starting point S, the temperature (hereinafter, also referred to as “target temperature”) and pressure (hereinafter, “target pressure”) in the stable region of cubic boron nitride. When heating and pressurizing to (also referred to as), the pressure is first raised to the target pressure (about 8 GPa in FIG. 5) (arrow E1 in FIG. 5), and then the temperature is raised to the target temperature (about 1900 ° C. in FIG. 5). Raise (arrow E2 in FIG. 5). In the path of FIG. 5, since heating and pressurization are performed once each, the control of the heating and pressurizing operation is simple and has been conventionally adopted.

In the path of FIG. 5, the path passes through the stable region of wurtzite-type boron nitride on the way from the starting point S to the temperature and pressure in the stable region of cubic boron nitride. Conventionally, in order to shorten the cycle time of the manufacturing process, it has been considered that the time required for reaching the stable region of cubic boron nitride from the starting point S should be short. It is also found that the quality of the obtained cubic boron nitride polycrystal is improved by holding the wurtzite-type boron nitride polycrystal at a temperature and pressure in the stable region for a certain period of time in the heating and pressurizing path of the second step. It didn't exist. For this reason, the heating and pressurizing conditions were set so that the time required for passing through the stable region of the wurtzite-type boron nitride was shorter.

However, in the cubic boron nitride polycrystal obtained by the route of FIG. 5, defects tend to occur during machining, and the tool life tends to be shortened. The present inventors analyzed and evaluated the cubic boron nitride polycrystal obtained by the route shown in FIG. 5 in order to investigate the reason, and found that the content of cubic boron nitride in the cubic boron nitride polycrystal. It was speculated that the rate would affect the tool life. Here, the content of cubic boron nitride in the cubic boron nitride polycrystal means that the cubic boron nitride polycrystal contains hexagonal boron nitride and / or wurtzite type boron nitride together with cubic boron nitride. In the case, it means the content of cubic boron nitride when the total content of cubic boron nitride, hexagonal boron nitride and wurtzite type boron nitride is used as the denominator.

Specifically, in the path of FIG. 5, since the retention time of wurtzite-type boron nitride in the stable region is short, the conversion rate from hexagonal boron nitride to wurtzite-type boron nitride decreases, and as a result, cubic crystals. It was speculated that the conversion rate to boron nitride also tended to decrease. Therefore, it is presumed that in the obtained cubic boron nitride polycrystal, the content of cubic boron nitride decreases, defects are likely to occur during machining, and the tool life tends to be shortened.

Furthermore, as a result of repeated studies by the present inventors, in the manufacturing process of the cubic boron nitride polycrystal, the inrush temperature of the wurtzite-type boron nitride into the stable region is related to the dislocation density of the cubic boron nitride. Therefore, it was speculated that it affects the strength of the obtained cubic boron nitride polycrystal.

The present inventors consider the above situation and the influence of the particle size of a plurality of crystal grains contained in the cubic boron nitride polycrystal on the toughness, and the pressure in the manufacturing process of the cubic boron nitride polycrystal. And the temperature path was investigated diligently. As a result, the present inventors have found heating and pressurizing conditions capable of obtaining a cubic boron nitride polycrystal capable of having a long tool life even in the processing of iron-based materials and difficult-to-cut materials.

In the conventional method for producing a cBN sintered body, cBN powder is used as a starting material, and the cBN powder is pressurized and then heated for sintering. It is inferred that the high-hardness cBN powders come into contact with each other by pressurization and dislocations are introduced into the cBN particles. However, as in the method for producing a cubic boron nitride polycrystal of the present disclosure, low hardness hBN powder is used as a starting material, and the hBN powder is heat-pressurized to be converted into a cubic boron nitride polycrystal. The dislocation density of the cubic boron nitride polycrystal was unpredictable. As a result of diligent studies, the present inventors have newly found the relationship between the heating and pressurizing conditions, the dislocation density of the cubic boron nitride polycrystal, and the tool performance.

Details of each step of the method for producing a cubic boron nitride polycrystal of the present disclosure will be described below with reference to FIGS. 2 to 4. In FIGS. 2 to 4, the arrows indicate the heating and pressurizing paths. If a circle is drawn at the tip of the arrow, it means that the arrow is held at that temperature and pressure for a certain period of time. Further, the routes shown in FIGS. 2 to 4 are examples, and the route is not limited thereto.

<First step>
Hexagonal boron nitride powder is prepared as a raw material for the cubic boron nitride polycrystal. The hexagonal boron nitride powder preferably has a purity (content of hexagonal boron nitride) of 98.5% or more, more preferably 99% or more, and most preferably 100%. The particle size of the hexagonal boron nitride powder is not particularly limited, but can be, for example, 0.1 μm or more and 10 μm or less.

<Second step>
Next, the hexagonal boron nitride powder passes through the temperature and pressure within the stable region of the wurtzite-type boron nitride, and has a temperature of 1700 ° C. or higher and 2500 ° C. or lower (hereinafter, also referred to as “reached temperature”) and. A boron nitride polycrystal is obtained by heating and pressurizing to a pressure of 8 GPa or more (hereinafter, also referred to as “reaching pressure”) (arrows A1, A2 and A3 in FIG. 2, arrows B1, B2, B3 and B4 in FIG. 3). In FIG. 4, arrows C1, C2, C3 and C4). In the heating and pressurizing path of the second step, the entry temperature of the wurtzite-type boron nitride into the stable region is 500 ° C. or lower. Further, the second step includes a step of maintaining the temperature and pressure in the heating and pressurizing path at the temperature and pressure in the stable region of the wurtzite-type boron nitride for 10 minutes or more (a1 in FIG. 2 and b1 in FIG. 3). , C1 and c2) in FIG.

The sintered body obtained in the second step has a lower content of cubic boron nitride than the cubic boron nitride polycrystal obtained after the third step. Therefore, in order to distinguish between the sintered body obtained by the second step and the cubic boron nitride polycrystal obtained after the third step, the sintered body obtained by the second step is used in the present specification. It is referred to as a boron nitride sintered body.

In the present specification, the temperature at which the wurtzite-type boron nitride enters the stable region means the temperature at the time when the wurtzite-type boron nitride reaches the stable region for the first time in the heating and pressurizing path of the second step. .. The inrush temperature is the temperature (about 250 ° C.) at the intersection of the arrow A2 and the line P = -0.0037T + 11.301 in FIG. 2, and in FIG. 3, the arrows B2 and P = -0.0037T + 11.301. It is the temperature at the intersection with the line of (about 250 ° C.), and in FIG. 4, it is the temperature at the intersection of the arrow C2 and the line of P = −0.0037T + 11.301 (about 250 ° C.).

In the heating and pressurizing path of the second step, the entry temperature of the wurtzite-type boron nitride into the stable region is 500 ° C. or lower. According to this, hexagonal boron nitride powder is converted to wurtzite-type boron nitride in an environment where atomic diffusion is unlikely to occur, and then converted to cubic boron nitride. Therefore, the finally obtained cubic boron nitride polycrystal has a relatively large number of lattice defects in the polycrystal, and the dislocation density of the cubic boron nitride is large and the strain is large, so that the strength is improved. doing. Therefore, a tool using the cubic boron nitride polycrystal can have a long tool life even in high-load machining of iron-based materials and difficult-to-cut materials.

The entry temperature of the wurtzite-type boron nitride into the stable region is preferably 300 ° C. or lower, more preferably 100 ° C. or lower. The lower the inrush temperature, the less likely it is that atomic diffusion will occur, and the more lattice defects tend to increase. The lower limit of the inrush temperature can be, for example, 10 ° C. The entry temperature of the wurtzite-type boron nitride into the stable region is preferably 10 ° C. or higher and 500 ° C. or lower, more preferably 10 ° C. or higher and 300 ° C. or lower, and further preferably 10 ° C. or higher and 100 ° C. or lower.

The second step includes a step of maintaining the temperature and pressure in the heating and pressurizing path at the temperature and pressure within the stable region of the wurtzite-type boron nitride for 10 minutes or more. According to this, hexagonal boron nitride powder, or powder containing hBN powder and a part thereof undergoing phase conversion, is retained for 10 minutes or more under temperature and pressure conditions within the stable region of wurtzite-type boron nitride. It will be. Since the retention time of wurtzite-type boron nitride in the stable region is long, the conversion rate from hexagonal boron nitride to wurtzite-type boron nitride is improved, and as a result, the conversion rate to cubic boron nitride is also improved. Therefore, in the finally obtained cubic boron nitride polycrystal, the content of cubic boron nitride increases, and defects are unlikely to occur during processing. Therefore, a tool using the cubic boron nitride polycrystal can have a long tool life even in high-load machining of iron-based materials and difficult-to-cut materials.

The holding time of the wurtzite-type boron nitride at temperature and pressure in the stable region is preferably 15 minutes or more, more preferably 30 minutes or more. The upper limit of the holding time is preferably 60 minutes from the viewpoint of manufacturing. The holding time is preferably 10 minutes or more and 60 minutes or less, more preferably 15 minutes or more and 60 minutes or less, and further preferably 30 minutes or more and 60 minutes or less.

In the second step, when the temperature and pressure in the heating and pressurizing path are T (° C.) and P (GPa), the following formula 1, the following formula 2 and the following formula 3 are simultaneously satisfied. It is preferable to include a step of holding the temperature and pressure for 10 minutes or more (hereinafter, also referred to as “second A step”).

Equation 1: P ≧ -0.0037T + 11.301
Equation 2: P ≦ -0.085T + 117
Equation 3: P ≦ -0.0037T + 11.375

The region that simultaneously satisfies the above formulas 1, 2 and 3 is a region near the boundary between the stable region of hexagonal boron nitride and the stable region of wurtzite-type boron nitride within the stable region of wurtzite-type boron nitride. is there. According to this, the hexagonal boron nitride powder or the powder containing hBN powder and a part thereof undergoing phase conversion is retained for 10 minutes or more under the temperature and pressure conditions that simultaneously satisfy the above formulas 1, 2 and 3. Will be done. By holding in this region for 10 minutes or more, lattice defects are more likely to occur. Therefore, it is considered that the finally obtained cubic boron nitride polycrystal has many lattice defects in the polycrystal and has a large strain, so that the strength is further improved. Therefore, a tool using the cubic boron nitride polycrystal can have a longer tool life even in high-load machining of iron-based materials and difficult-to-cut materials.

The holding time at temperature and pressure in the region satisfying the above formulas 1, 2 and 3 at the same time is more preferably 15 minutes or more, further preferably 20 minutes or more. The upper limit of the holding time is preferably 60 minutes from the viewpoint of manufacturing. The holding time is preferably 10 minutes or more and 60 minutes or less, more preferably 15 minutes or more and 60 minutes or less, and further preferably 20 minutes or more and 60 minutes or less.

When the second step includes a step of holding the temperature and pressure in the region satisfying the above formulas 1, 2 and 3 for 10 minutes or more, the second step is the temperature and pressure in the heating and pressurizing path, and then the temperature and pressure. Further, a step of holding the temperature and pressure in the region satisfying the following formula 2 and the following formula 4 for 1 minute or more (hereinafter, also referred to as “second B step”) can be included.

Equation 2: P ≦ -0.085T + 117
Equation 4: P> -0.0037T + 11.375

According to this, the conversion rate from hexagonal boron nitride to wurtzite-type boron nitride is further improved, and as a result, the conversion rate to cubic boron nitride is also improved. Therefore, in the finally obtained cubic boron nitride polycrystal, the content of cubic boron nitride increases, and defects are unlikely to occur during processing. Therefore, a tool using the cubic boron nitride polycrystal can have a long tool life even in high-load machining of iron-based materials and difficult-to-cut materials.

The holding time at temperature and pressure in the region satisfying the above formulas 2 and 4 is more preferably 10 minutes or more, further preferably 15 minutes or more. The upper limit of the holding time is preferably 60 minutes from the viewpoint of manufacturing. The holding time is preferably 1 minute or more and 60 minutes or less, more preferably 10 minutes or more and 60 minutes or less, and further preferably 15 minutes or more and 60 minutes or less.

In the second step, only the above-mentioned second A step may be performed, or the above-mentioned second B step may be performed after the above-mentioned second A step. Further, the second step may be a step of holding for 10 minutes or more in a region satisfying the above formulas 2 and 4.

The ultimate pressure in the second step is 8 GPa or more, preferably 10 GPa or more, and more preferably 13 GPa or more. The upper limit of the ultimate pressure is not particularly limited, but can be, for example, 15 GPa. The ultimate pressure in the second step is preferably 8 GPa or more and 15 GPa or less, more preferably 10 GPa or more and 15 GPa or less, and further preferably 13 GPa or more and 15 GPa or less.

In the second step, in the paths of FIGS. 2 to 4, pressurization is performed after heating, and further heating is performed, but the heating and pressurizing path is not limited to this. In the heating and pressurizing path, the temperature at which the wurtzite-type boron nitride enters the stable region is set to 500 ° C or less, the temperature and pressure within the stable region of the wurtzite-type boron nitride can be maintained for 10 minutes or more, and the ultimate temperature is 1700 ° C. Any path may be used as long as it can have a temperature of 2500 ° C. or lower and an ultimate pressure of 8 GPa or higher. For example, heating and pressurization may be performed at the same time.

As described above, a boron nitride polycrystal can be obtained by performing the second step on the hexagonal boron nitride powder.

<Third step>
After the above second step, the boron nitride polycrystal obtained by the second step has a temperature of 1700 ° C. or higher and 2500 ° C. or lower (hereinafter, also referred to as “final sintering temperature”) and a pressure of 8 GPa or higher. (Hereinafter, also referred to as "final sintering pressure".) The step of holding for 3 minutes or more and 60 minutes or less under the conditions is performed. As a result, the obtained cubic boron nitride polycrystal has a large content of cubic boron nitride, and a longer tool life can be achieved.

The final sintering temperature is preferably 1900 ° C. or higher and 2400 ° C. or lower. The final sintering pressure is preferably 8 GPa or more and 15 GPa or less, and more preferably 10 GPa or more and 15 GPa or less. The holding time in the third step is preferably 10 minutes or more and 20 minutes or less.

<Characteristics of cubic boron nitride polycrystal obtained by the pathways shown in FIGS. 2 to 4>
[Route in FIG. 2]
In the path of FIG. 2, the temperature is first raised from the starting point S to a predetermined temperature of 500 ° C. or lower (about 250 ° C. in FIG. 2) (arrow A1), and then the stable region of wurtzite-type boron nitride while maintaining the temperature. Pressurize to the inner pressure (about 13 GPa in FIG. 2) (arrow A2) and hold at the temperature (about 250 ° C.) and the pressure (about 13 GPa) for 10 minutes or more (a1 in FIG. 2). Then, while maintaining the pressure (about 13 GPa), the temperature is raised to 1700 ° C. or higher and 2500 ° C. or lower (about 2000 ° C. in FIG. 2) (arrow A3), and the temperature (about 2000 ° C.) and the pressure (about 2000 ° C.) are raised. At 13 GPa), hold for 3 minutes or more and 60 minutes or less (a2 in FIG. 2). In FIG. 2, the second step is indicated by arrows A1, A2 and A3, and a1, and the third step is indicated by a2.

In the route of FIG. 2, the inrush temperature of the wurtzite-type boron nitride into the stable region is 500 ° C. or lower (about 250 ° C.). According to this, hexagonal boron nitride powder is converted to wurtzite-type boron nitride in an environment where atomic diffusion is unlikely to occur, and then converted to cubic boron nitride. Therefore, the obtained cubic boron nitride polycrystal has a relatively large number of lattice defects in the polycrystal, and the dislocation density of the cubic boron nitride is large and the strain is large, so that the strength is improved. There is. Therefore, a tool using the cubic boron nitride polycrystal can have a long tool life even in high-load machining of iron-based materials and difficult-to-cut materials.

In the path of FIG. 2, the temperature and pressure in the stable region of the wurtzite-type boron nitride are maintained for 10 minutes or more. According to this, since the retention time of wurtzite-type boron nitride in the stable region is long, the conversion rate from hexagonal boron nitride to wurtzite-type boron nitride is improved, and as a result, the conversion rate to cubic boron nitride is also improved. improves. Therefore, in the obtained cubic boron nitride polycrystal, the content of cubic boron nitride increases, and defects are unlikely to occur during processing. Therefore, a tool using the cubic boron nitride polycrystal can have a long tool life even in high-load machining of iron-based materials and difficult-to-cut materials.

In the route of FIG. 2, the boron nitride polycrystal obtained in the second step is held at a temperature of 1700 ° C. or higher and 2500 ° C. or lower and a pressure condition of 8 GPa or higher for 3 minutes or longer and 60 minutes or shorter. As a result, the obtained cubic boron nitride polycrystal has a large content of cubic boron nitride and can have a longer tool life.

[Route in FIG. 3]
In the route of FIG. 3, the temperature is first raised from the starting point S to a predetermined temperature of 500 ° C. or less (about 250 ° C. in FIG. 3) (arrow B1), and then the following formula 1 and the following formula 2 are maintained while maintaining the temperature. And the pressure in the region that simultaneously satisfies the following formula 3 (about 10.4 GPa in FIG. 3) (arrow B2), and held at the temperature (about 250 ° C.) and the pressure (about 10.4 GPa) for 10 minutes or more. (B1 in FIG. 3).

Equation 1: P ≧ -0.0037T + 11.301
Equation 2: P ≦ -0.085T + 117
Equation 3: P ≦ -0.0037T + 11.375

Next, while maintaining the temperature (about 250 ° C.), the pressure is increased within the stable region of the wurtzite-type boron nitride (about 13 GPa in FIG. 3) (arrow B3). Subsequently, while maintaining the pressure (about 13 GPa), the temperature was raised to 1700 ° C. or higher and 2500 ° C. or lower (about 2000 ° C. in FIG. 3) (arrow B4), and the temperature (about 2000 ° C.) and the pressure (about 2000 ° C.) At about 13 GPa), hold for 3 minutes or more and 60 minutes or less (b2 in FIG. 3). In FIG. 3, the second step is indicated by arrows B1, B2, B3 and B4, and b1, and the third step is indicated by b2.

In the route of FIG. 3, the temperature at which the wurtzite-type boron nitride enters the stable region is 500 ° C. or lower (about 250 ° C.). According to this, hexagonal boron nitride powder is converted to wurtzite-type boron nitride in an environment where atomic diffusion is unlikely to occur, and then converted to cubic boron nitride. Therefore, the obtained cubic boron nitride polycrystal has a relatively large number of lattice defects in the polycrystal, and the dislocation density of the cubic boron nitride is large and the strain is large, so that the strength is improved. There is. Therefore, a tool using the cubic boron nitride polycrystal can have a long tool life even in high-load machining of iron-based materials and difficult-to-cut materials.

The path of FIG. 3 includes a step of holding for 10 minutes or more at a temperature and pressure in a region that simultaneously satisfies the above formula 1, the above formula 2, and the above formula 3. That is, the path of FIG. 3 has a temperature and pressure closer to the stable region of hexagonal boron nitride within the stable region of wurtzite-type boron nitride (that is, the stability of wurtzite-type boron nitride) as compared with the route of FIG. It is held for 10 minutes or more at the temperature and pressure near the boundary between the region and the stable region of hexagonal boron nitride. For this reason, lattice defects are more likely to occur than in the path of FIG. 2, and the cubic boron nitride polycrystal obtained in the path of FIG. 3 becomes the cubic boron nitride polycrystal obtained in the path of FIG. In comparison, the polycrystal has more lattice defects and the strain is large, so that the strength is considered to be further improved. Therefore, the tool using the cubic boron nitride polycrystal obtained by the route of FIG. 3 can have a longer tool life even in high-load machining of iron-based materials and difficult-to-cut materials.

In the route of FIG. 3, the boron nitride polycrystal obtained in the second step is held at a temperature of 1700 ° C. or higher and 2500 ° C. or lower and a pressure condition of 8 GPa or higher for 3 minutes or longer and 60 minutes or shorter. As a result, the obtained cubic boron nitride polycrystal has a large content of cubic boron nitride and can have a longer tool life.

[Route in FIG. 4]
In the route of FIG. 4, the temperature is first raised from the starting point S to a predetermined temperature of 500 ° C. or less (about 250 ° C. in FIG. 4) (arrow C1), and then the following formula 1 and the following formula 2 are maintained while maintaining the temperature. And the pressure in the region that simultaneously satisfies the following formula 3 (about 10.4 GPa in FIG. 4) (arrow C2), and held at the temperature (about 250 ° C.) and the pressure (about 10.4 GPa) for 10 minutes or more. (C1 in FIG. 4).

Equation 1: P ≧ -0.0037T + 11.301
Equation 2: P ≦ -0.085T + 117
Equation 3: P ≦ -0.0037T + 11.375

Next, while maintaining the temperature (about 250 ° C.), the pressure is increased within the stable region of the wurtzite-type boron nitride (about 13 GPa in FIG. 4) (arrow C3), and the temperature (about 250 ° C.) and the pressure (about 250 ° C.) are increased. Hold at about 13 GPa) (temperature and pressure shown in c2 of FIG. 4) for at least 1 minute. That is, in the path of FIG. 4, after the step of holding the formula 1, the formula 2 and the formula 3 at the same temperature and pressure for 10 minutes or more, the temperature and the temperature in the region satisfying the formula 2 and the formula 4 below are further satisfied. Includes a step of holding at pressure for 1 minute or longer.

Equation 2: P ≦ -0.085T + 117
Equation 4: P> -0.0037T + 11.375

Subsequently, while maintaining the above pressure (about 13 GPa), the temperature was raised to 1700 ° C. or higher and 2500 ° C. or lower (about 2000 ° C. in FIG. 4) (arrow C4), and the temperature (about 2000 ° C.) and the pressure. At (about 13 GPa), hold for 3 minutes or more and 60 minutes or less (c3 in FIG. 4). In FIG. 4, the second step is indicated by arrows C1, C2, C3 and C4, and c1 and c2, and the third step is indicated by c3.

In the path of FIG. 4, the temperature at which the wurtzite-type boron nitride enters the stable region is 500 ° C. or lower (about 250 ° C.). According to this, hexagonal boron nitride powder is converted to wurtzite-type boron nitride in an environment where atomic diffusion is unlikely to occur, and then converted to cubic boron nitride. Therefore, the obtained cubic boron nitride polycrystal has a relatively large number of lattice defects in the polycrystal, and the dislocation density of the cubic boron nitride is large and the strain is large, so that the strength is improved. There is. Therefore, a tool using the cubic boron nitride polycrystal can have a long tool life even in high-load machining of iron-based materials and difficult-to-cut materials.

The path of FIG. 4 includes a step of holding the temperature and pressure in a region that simultaneously satisfies the above formula 1, the above formula 2, and the above formula 3 for 10 minutes or more. That is, the path of FIG. 4 has a temperature and pressure closer to the stable region of hexagonal boron nitride within the stable region of wurtzite-type boron nitride (that is, the stability of wurtzite-type boron nitride) as compared with the route of FIG. It is held for 10 minutes or more at the temperature and pressure near the boundary between the region and the stable region of hexagonal boron nitride. Therefore, lattice defects are more likely to occur in the route of FIG. 4 than in the route of FIG. 2, and the cubic boron nitride polycrystal obtained in the route of FIG. 4 is a cubic crystal obtained in the route of FIG. Compared to the boron nitride polycrystal, the polycrystal has many lattice defects and the strain is large, so that the strength is considered to be further improved. Therefore, the tool using the cubic boron nitride polycrystal obtained by the route of FIG. 4 can have a longer tool life even in high-load machining of iron-based materials and difficult-to-cut materials.

In the path of FIG. 4, after the step of holding the temperature and pressure in the region satisfying the above formulas 1, 2 and 3 at the same time for 10 minutes or more, the pressure is further increased in the stable region of the wurtzite type boron nitride. (Approximately 13 GPa in FIG. 4) (arrow C3), the temperature (approximately 250 ° C.) and the pressure (approximately 13 GPa) are held for 1 minute or longer (c2 in FIG. 4). That is, in the path of FIG. 4, after the step of holding the formula 1, the formula 2 and the formula 3 at the same temperature and pressure for 10 minutes or more, the temperature and the temperature in the region satisfying the formula 2 and the formula 4 below are further satisfied. Includes a step of holding at pressure for 1 minute or longer.

Equation 2: P ≦ -0.085T + 117
Equation 4: P> -0.0037T + 11.375

Therefore, in the route of FIG. 4, the conversion rate from hexagonal boron nitride to wurtzite-type boron nitride is further improved as compared with the route in FIG. 3, and as a result, the conversion rate to cubic boron nitride is also further improved. Therefore, the cubic boron nitride polycrystal obtained by the route of FIG. 4 has an increased content of cubic boron nitride as compared with the cubic boron nitride polycrystal obtained by the route of FIG. Defects are unlikely to occur during processing. Therefore, a tool using the cubic boron nitride polycrystal can have a longer tool life even in high-load machining of iron-based materials and difficult-to-cut materials.

In the route of FIG. 4, the boron nitride polycrystal obtained in the second step is held at a temperature of 1700 ° C. or higher and 2500 ° C. or lower and a pressure condition of 8 GPa or higher for 3 minutes or longer and 60 minutes or shorter. As a result, the obtained cubic boron nitride polycrystal has a large content of cubic boron nitride, and a longer tool life can be achieved.

The present embodiment will be described in more detail with reference to Examples. However, these embodiments do not limit the present embodiment.

[Example 1]
In Example 1, the production conditions of the cubic boron nitride polycrystal, the composition of the obtained cubic boron nitride polycrystal (composition, median diameter of crystal grains, dislocation density), and the cubic boron nitride polycrystal. We investigated the relationship with the tool life when high-load machining of difficult-to-cut materials was performed with a tool using.

<Preparation of cubic boron nitride polycrystal>
The cubic boron nitride polycrystals of Samples 1-1 to 9 were prepared according to the following procedure.

(First step)
6 g of hexagonal boron nitride powder (“Dencaboron nitride” (trade name) manufactured by Denka Co., Ltd., particle size 5 μm) was prepared. The above hexagonal boron nitride powder was placed in a molybdenum capsule and installed in an ultra-high pressure and high temperature generator.

(2nd step and 3rd step)
[Sample 1-1, Sample 1-3, Sample 2-2, Sample 3-2, Sample 4 to Sample 9]
Using an ultra-high pressure and high temperature generator, the above hexagonal boron nitride powder was selected from the temperatures and pressures listed in the "Temperature" and "Pressure" columns of the "Starting point" in Table 1 to "First stage". The temperature and / or pressure were raised to the temperature and pressure described in the "Achieved temperature" and "Achieved pressure" columns, and maintained for the length described in the "Retention time" column.

Subsequently, while maintaining the temperature, the pressure was increased to the pressure described in the "reaching pressure" column of the "second stage" of Table 1, and the pressure was maintained for the length described in the "holding time" column.

Subsequently, while maintaining the pressure, the temperature is raised to the temperature described in the "reached temperature" column of the "third stage" in Table 1, and the temperature is maintained for the length described in the "holding time" column to cubic. A boron nitride polycrystal was obtained. In Sample 1-1, Sample 1-3, Sample 2-2, Sample 3-2, Sample 4 to Sample 9, the "reaching temperature", "reaching pressure" and "holding time" described in the "third stage" are described. The high temperature and high pressure treatment in "" corresponds to the third step.

[Sample 1-2]
Using an ultra-high pressure high temperature generator, the above hexagonal boron nitride powder was prepared from the temperatures and pressures listed in the "Temperature" and "Pressure" columns of the "Starting point" in Table 1 while maintaining the temperature. The pressure was increased to the pressure described in the "ultimated pressure" column of "first stage", and the pressure was maintained for the length described in the "holding time" column.

Subsequently, while maintaining the pressure, the temperature is raised to the temperature described in the "reached temperature" column of the "second stage" in Table 1, and the temperature is maintained for the length described in the "holding time" column to cubic. A boron nitride polycrystal was obtained. In Sample 1-2, the high-temperature and high-pressure treatment at the "reached temperature", "reaching pressure" and "holding time" described in the "second stage" corresponds to the third step.

[Sample 2-1, Sample 2-3, Sample 3-1 and Sample 3-3]
Using an ultra-high pressure and high temperature generator, the above hexagonal boron nitride powder was selected from the temperatures and pressures listed in the "Temperature" and "Pressure" columns of the "Starting point" in Table 1 to "First stage". The temperature and pressure were raised and increased to the temperatures and pressures described in the "Achieved temperature" and "Achieved pressure" columns, and the temperature was maintained for the length described in the "Retention time" column.

Subsequently, while maintaining the temperature, the pressure was increased to the pressure described in the "reaching pressure" column of the "second stage" of Table 1, and the pressure was maintained for the length described in the "holding time" column.

Subsequently, while maintaining the temperature, the pressure was increased to the pressure described in the "reaching pressure" column of the "third stage" of Table 1, and the pressure was maintained for the length described in the "holding time" column.

Subsequently, while maintaining the pressure, the temperature is raised to the temperature described in the "achieved temperature" column of the "fourth stage" in Table 1, and the temperature is maintained for the length described in the "holding time" column to cubic. A boron nitride polycrystal was obtained. In Sample 2-1 and Sample 2-3, Sample 3-1 and Sample 3-3, high-temperature and high-pressure treatment at the "reached temperature", "reaching pressure" and "holding time" described in the "fourth stage". Corresponds to the third step.

<Evaluation>
(Measurement of composition)
The content of cubic boron nitride in the above-mentioned cubic boron nitride polycrystal was measured by an X-ray diffraction method. Since the specific method of the X-ray diffraction method is as shown in the first embodiment, the description thereof will not be repeated. The results are shown in the "cBN content" column of Table 1.

In all the samples, no components other than cBN, wBN and compressed hBN were identified.

(Measurement of dislocation density)
Analyzing the dislocation density of cubic boron nitride in the cubic boron nitride polycrystal obtained above by using the modified Williamson-Hall method and the modified Warren-Averbach method for the line profile obtained by X-ray diffraction measurement. Calculated by Since the specific method for calculating the dislocation density is as shown in the first embodiment, the description thereof will not be repeated. The results are shown in the "Dislocation Density" column of Table 1.

(Measurement of median diameter d50 of crystal grains)
For the crystal grains contained in the cubic boron nitride polycrystal obtained above, the median diameter d50 having a diameter equivalent to a circle was measured. Since the specific method is as shown in the first embodiment, the description thereof will not be repeated. The results are shown in the "Median diameter (d50)" column of Table 1.

(Cutting test)
The cubic boron nitride polycrystal obtained above was cut by a laser and finished to produce a cutting tool having an insert model number NU-CNGA120408 (manufactured by Sumitomo Electric Hardmetal Corp.). Using the obtained cutting tool, intermittent cutting of a Ti6Al4V round bar (φ200 mm, with one V-shaped slit) was performed under the following cutting conditions, and the tool life was evaluated. The Ti6Al4V round bar, which is the work material, is a difficult-to-cut material.

(Cutting conditions)
Work material: Ti6Al4V round bar (φ200mm, with one V-shaped slit)
Tool shape:
Holder model number DCLNR2525 (manufactured by Sumitomo Electric Hardmetal Corp.)
Insert model number NU-CNGA120408 (manufactured by Sumitomo Electric Hardmetal Corp.)
Cutting speed: 180m / min
Feed amount: 0.1 mm / blade depth of cut: 0.15 mm
Coolant: WET
The above cutting conditions correspond to high-load machining of difficult-to-cut materials.

The tool life was measured by cutting under the above-mentioned cutting conditions and measuring the machining time until the missing amount of the tool observed from the flank surface became 100 μm or more. The longer the machining time, the better the fracture resistance and the longer the tool life. The results are shown in the "Tool Life" column of Table 1.

<Discussion>
[Sample 1-1 to Sample 1-3]
The methods for producing Samples 1-1 to 1-3 all correspond to Examples. The cubic boron nitride polycrystals of Samples 1-1 to 1-3 all contain 98.5% by volume or more of cubic boron nitride, and the dislocation density of cubic boron nitride is 8 × 10 ¹⁵ / m ² . It is large, and the median diameter d50 of the crystal grain is 0.1 μm or more and 0.5 μm or less, which corresponds to the example. It was confirmed that the tools using the cubic boron nitride polycrystals of Samples 1-1 to 1-3 have a long tool life even in high-load machining of difficult-to-cut materials.

Comparing the tool life of Samples 1-1 to 1-3, Samples 1-1 and 1-3 were longer than Samples 1-2. This is because the temperature and pressure reached in the first stage of Samples 1-1 and 1-3 are the temperatures and pressures that simultaneously satisfy the above formulas 1, 2 and 3, and at the temperature and the pressure. It is considered that because the step of holding the sample for 10 minutes or more is included, lattice defects are more likely to occur and the strength is higher than that of the sample 1-2 not including the step.

Furthermore, comparing the tool life of Sample 1-1 and Sample 1-3, Sample 1-1 was longer than Sample 1-3. This is because sample 1-1 has a longer retention time at the ultimate temperature and ultimate pressure (that is, within the stable region of wurtzite-type boron nitride) in the second stage than sample 1-3, and thus hexagonal crystals. It is considered that the conversion rate of boron nitride to wurtzite-type boron nitride is further improved, and as a result, the conversion rate to cubic boron nitride is higher than that of Sample 1-3, and the content of cubic boron nitride is large.

[Sample 2-1 to Sample 2-3]
The methods for producing Samples 2-1 to 2-3 all correspond to Examples. The cubic boron nitride polycrystals of Samples 2-1 to Sample 2-3 all contain 98.5% by volume or more of cubic boron nitride, and the dislocation density of cubic boron nitride is 8 × 10 ¹⁵ / m ² . It is large, and the median diameter d50 of the crystal grain is 0.1 μm or more and 0.5 μm or less, which corresponds to the example. It was confirmed that the tools using the cubic boron nitride polycrystals of Samples 2-1 to 2-3 have a long tool life even in high-load machining of difficult-to-cut materials.

Comparing the tool life of Samples 2-1 to 2-3, Samples 2-1 and 2-3 were longer than Samples 2-2. This is the temperature and pressure at which the temperature and pressure reached in the second stage of Samples 2-1 and 2-3 simultaneously satisfy the above formulas 1, 2 and 3, and at the temperature and pressure. It is considered that because the step of holding the sample for 10 minutes or more is included, lattice defects are more likely to occur and the strength is higher than that of the sample 2-2 not including the step.

Furthermore, comparing the tool life of Sample 2-1 and Sample 2-3, Sample 2-1 was longer than Sample 2-3. This is because sample 2-1 has a longer retention time at the ultimate temperature and pressure (that is, within the stable region of wurtzite-type boron nitride) in the third stage than sample 2-3, and thus hexagonal nitride. It is considered that the conversion rate from boron to wurtzite-type boron nitride is further improved, and as a result, the conversion rate to cubic boron nitride is higher than that of Sample 2-3, and the content of cubic boron nitride is large.

[Sample 3-1 to Sample 3-3]
All the methods for producing Samples 3-1 to 3-3 correspond to Examples. The cubic boron nitride polycrystals of Samples 3-1 to Sample 3-3 all contain 98.5% by volume or more of cubic boron nitride, and the dislocation density of cubic boron nitride is 8 × 10 ¹⁵ / m ² . It is large, and the median diameter d50 of the crystal grain is 0.1 μm or more and 0.5 μm or less, which corresponds to the example. It was confirmed that the tools using the cubic boron nitride polycrystals of Samples 3-1 to 3-3 have a long tool life even in high-load machining of difficult-to-cut materials.

Comparing the tool life of Samples 3-1 to 3-3, Samples 3-1 and 3-3 were longer than Samples 3-2. This is because the temperature and pressure reached in the second stage of Sample 3-1 and Sample 3-3 are the temperatures and pressures that simultaneously satisfy the above formulas 1, 2 and 3, and at the temperature and the pressure. It is considered that because the step of holding the sample for 10 minutes or more is included, lattice defects are more likely to occur and the strength is higher than that of the sample 3-2 not including the step.

Furthermore, comparing the tool life of Sample 3-1 and Sample 3-3, Sample 3-1 was longer than Sample 3-3. This is because sample 3-1 has a longer retention time at the ultimate temperature and ultimate pressure (that is, within the stable region of wurtzite-type boron nitride) in the third stage than sample 3-3. It is considered that the conversion rate of boron nitride to wurtzite-type boron nitride is further improved, and as a result, the conversion rate to cubic boron nitride is higher than that of Sample 3-3, and the content of cubic boron nitride is large.

[Sample 4, Sample 7]
Both the methods for producing Sample 4 and Sample 7 correspond to Examples. The cubic boron nitride polycrystals of Samples 4 and 7 both contain 98.5% by volume or more of cubic boron nitride, and the dislocation density of cubic boron nitride is larger than 8 × 10 ¹⁵ / m ² and crystal grains. The median diameter d50 of No. 1 is 0.1 μm or more and 0.5 μm or less, which corresponds to the example. It was confirmed that the tools using the cubic boron nitride polycrystals of Samples 4 and 7 have a long tool life even in high-load machining of difficult-to-cut materials.

Comparing the tool life of sample 4 and sample 7, sample 4 was longer. It is considered that this is because the sample 4 has a higher pressure in the third step than the sample 7 and has a large content of cubic boron nitride.

[Sample 5, Sample 6]
Both the sample 5 and the sample 6 have a temperature at which the wurtzite-type boron nitride rushes into the stable region exceeds 500 ° C., which corresponds to a comparative example. The cubic boron nitride polycrystals of Sample 5 and Sample 6 both have a dislocation density of 8 × 10 ¹⁵ / m ² or less of cubic boron nitride, and correspond to a comparative example. The tools using the cubic boron nitride polycrystals of Sample 5 and Sample 6 had a short tool life. This is because, in the production methods of Sample 5 and Sample 6, since the inrush temperature of wurtzite-type boron nitride into the stable region exceeds 500 ° C., lattice defects are unlikely to occur, and the obtained cubic boron nitride polycrystal is unlikely to occur. It is considered that the dislocation density became smaller in the body and the strength decreased.

[Sample 8]
The method for producing sample 8 does not include a step of holding the wurtzite-type boron nitride at a temperature and pressure in the stable region for 10 minutes or more, and corresponds to a comparative example. The cubic boron nitride polycrystal of Sample 8 has a content of 98.3% by volume of cubic boron nitride, which corresponds to a comparative example. The tool using the cubic boron nitride polycrystal of Sample 8 had a short tool life. This is because the method for producing sample 8 does not include a step of holding the wurtzite-type boron nitride at a temperature and pressure within the stable region for 10 minutes or more, so that the conversion rate from hexagonal boron nitride to wurtzite-type boron nitride is low. As a result, the conversion rate to cubic boron nitride is also low, and it is considered that the content of the obtained cubic boron nitride polycrystal is small. Further, in the method for producing sample 8, since the sintering pressure in the third step is lower than that of sample 4 and sample 7, the conversion rate to cubic boron nitride is low, and the obtained cubic boron nitride polycrystal is obtained. It is considered that the content of cubic boron nitride in the body was reduced, and as a result, the cubic boron nitride polycrystal of Sample 8 had a shorter tool life than Samples 4 and 7.

[Sample 9]
The method for producing sample 9 does not include a step of holding the wurtzite-type boron nitride at a temperature and pressure in the stable region for 10 minutes or more, and corresponds to a comparative example. The cubic boron nitride polycrystal of Sample 9 has a content of 98.2% by volume of cubic boron nitride, which corresponds to a comparative example. The tool using the cubic boron nitride polycrystal of Sample 9 had a short tool life. This is because the method for producing sample 9 does not include a step of holding the wurtzite-type boron nitride at a temperature and pressure within the stable region for 10 minutes or more, so that the conversion rate from hexagonal boron nitride to wurtzite-type boron nitride is low. As a result, the conversion rate to cubic boron nitride is also low, and it is considered that the content of cubic boron nitride is small in the obtained cubic boron nitride polycrystal. Further, in the method for producing sample 9, since the retention time of wurtzite-type boron nitride in the stable region is shorter than that of sample 4, the conversion rate to boron nitride is low, and the obtained cubic boron nitride is obtained. It is considered that the content of cubic boron nitride in the boron polycrystal was reduced, and as a result, the tool life of the cubic boron nitride polycrystal in Sample 9 was shorter than that in Sample 4.

[Example 2]
In Example 2, the production conditions of the cubic boron nitride polycrystal, the composition of the obtained cubic boron nitride polycrystal (composition, median diameter of crystal grains, dislocation density), and the cubic boron nitride polycrystal. We investigated the relationship with the tool life when high-load machining of iron-based materials was performed with a tool using.

<Preparation of cubic boron nitride polycrystal>
The cubic boron nitride polycrystals of Samples 10 to 16 were prepared according to the following procedure.

(First step)
6 g of a commercially available hexagonal boron nitride powder (particle size 5 μm) was prepared. The above hexagonal boron nitride powder was placed in a molybdenum capsule and installed in an ultra-high pressure and high temperature generator.

(2nd step and 3rd step)
[Sample 1 to Sample 16]
Using an ultra-high pressure and high temperature generator, the above hexagonal boron nitride powder was prepared from the temperatures and pressures listed in the "Temperature" and "Pressure" columns of the "Starting point" in Table 1 while maintaining the pressure. The temperature was raised to the temperature described in the "reached temperature" column of "first stage", and the temperature was maintained for the length described in the "holding time" column.

Subsequently, while maintaining the temperature, the pressure was increased to the pressure described in the "reaching pressure" column of the "second stage" of Table 1, and the pressure was maintained for the length described in the "holding time" column.

Subsequently, while maintaining the pressure, the temperature is raised to the temperature described in the "reached temperature" column of the "third stage" in Table 1, and the temperature is maintained for the length described in the "holding time" column to cubic. A boron nitride polycrystal was obtained. The high-temperature and high-pressure treatment at the "reached temperature", "reaching pressure" and "holding time" described in the "third stage" corresponds to the third step.

<Evaluation>
(Measurement of composition)
The content of cubic boron nitride in the above-mentioned cubic boron nitride polycrystal was measured by an X-ray diffraction method. Since the specific method of the X-ray diffraction method is as shown in the first embodiment, the description thereof will not be repeated. The results are shown in the "cBN content" column of Table 2.

(Measurement of dislocation density)
Analyzing the dislocation density of cubic boron nitride in the cubic boron nitride polycrystal obtained above by using the modified Williamson-Hall method and the modified Warren-Averbach method for the line profile obtained by X-ray diffraction measurement. Calculated by Since the specific method for calculating the dislocation density is as shown in the first embodiment, the description thereof will not be repeated. The results are shown in the "Dislocation Density" column of Table 2.

(Measurement of median diameter d50 of crystal grains)
For the crystal grains contained in the cubic boron nitride polycrystal obtained above, the median diameter d50 having a diameter equivalent to a circle was measured. Since the specific method is as shown in the first embodiment, the description thereof will not be repeated. The results are shown in the "Median diameter (d50)" column of Table 2.

(Cutting test)
The cubic boron nitride polycrystal obtained above was cut by a laser and finished to produce a cutting tool of insert model number SNEW1203ADTR (manufactured by Sumitomo Electric Hardmetal Corp.). Using the obtained cutting tool, face milling of a gray cast iron FC300 block material (80 mm × 300 mm × 150 mm) was performed under the following cutting conditions, and the tool life was evaluated.

(Cutting conditions)
Work material: Gray cast iron FC300 block material (80 mm x 300 mm x 150 mm)
Tool shape:
Cutter model number FMU4100R (manufactured by Sumitomo Electric Hardmetal Corp.)
Insert model number SNEW1203ADTR (manufactured by Sumitomo Electric Hardmetal Corp.)
Cutting speed: 2400m / min
Feed amount: 0.15 mm / blade depth of cut: 0.4 mm
Coolant: DRY
The above cutting conditions correspond to high-load machining of iron-based materials.

The tool life was measured by cutting under the above-mentioned cutting conditions and measuring the machining time until the missing amount of the tool observed from the flank surface became 250 μm or more. The longer the machining time, the better the fracture resistance and the longer the tool life. The results are shown in the "Tool Life" column of Table 2.

<Discussion>
[Sample 10]
The method for producing the sample 10 has a holding time of less than 3 minutes (2 minutes) in the third step (third step), and corresponds to a comparative example. The cubic boron nitride polycrystal of Sample 10 has a grain size d50 of less than 0.1 μm (0.08 μm), which corresponds to a comparative example. The tool using the cubic boron nitride polycrystal of Sample 10 had a short tool life. It is considered that this is because, in the sample 10, the holding time in the third step was short, so that the grain growth was insufficient and the median diameter d50 of the crystal grains became small.

[Sample 11 and Sample 12]
Both the methods for producing Sample 11 and Sample 12 correspond to Examples. The cubic boron nitride polycrystals of Samples 11 and 12 both contain 98.5% by volume or more of cubic boron nitride, and the dislocation density of cubic boron nitride is larger than 8 × 10 ¹⁵ / m ² and crystal grains. The median diameter d50 of No. 1 is 0.1 μm or more and 0.5 μm or less, which corresponds to the example. It was confirmed that the tools using the cubic boron nitride polycrystals of Samples 11 and 12 have a long tool life even in high-load machining of iron-based materials.

[Sample 13]
The method for producing the sample 13 has a holding time of more than 60 minutes (70 minutes) in the third step (third step), which corresponds to a comparative example. The cubic boron nitride polycrystal of Sample 13 has a median diameter d50 of crystal grains of more than 0.5 μm (0.59 μm), which corresponds to a comparative example. The tool using the cubic boron nitride polycrystal of Sample 13 had a short tool life. It is considered that this is because, in the sample 13, since the holding time in the third step was long, the grain growth proceeded excessively and the median diameter d50 of the crystal grains became large.

[Sample 14]
In the method for producing the sample 14, the temperature in the third step (third step) is less than 1700 ° C. (1650 ° C.), which corresponds to a comparative example. The cubic boron nitride polycrystal of Sample 14 has a grain size d50 of less than 0.1 μm (0.09 μm), which corresponds to a comparative example. The tool using the cubic boron nitride polycrystal of Sample 14 had a short tool life. It is considered that this is because, in the sample 14, the temperature in the third step was low, so that the grain growth was insufficient and the median diameter d50 of the crystal grains became small.

[Sample 15]
The method for producing the sample 15 has a temperature of more than 2500 ° C. (2550 ° C.) in the third step (third step), which corresponds to a comparative example. The cubic boron nitride polycrystal of Sample 15 has a median diameter d50 of crystal grains of more than 0.5 μm (0.60 μm), which corresponds to a comparative example. The tool using the cubic boron nitride polycrystal of Sample 15 had a short tool life. It is considered that this is because, in the sample 15, since the temperature in the third step was high, the grain growth proceeded excessively and the median diameter d50 of the crystal grains became large.

[Sample 16]
In the method for producing the sample 16, the temperature at which the wurtzite-type boron nitride enters the stable region exceeds 500 ° C. (1000 ° C.), which corresponds to a comparative example. The cubic boron nitride polycrystal of Sample 16 has a dislocation density of 8 × 10 ¹⁵ / m ² or less (6.6 × 10 ¹⁵ / m ² ), which corresponds to a comparative example. The tool using the cubic boron nitride polycrystal of Sample 16 had a short tool life. It is considered that this is because in the sample 16, since the inrush temperature of the wurtzite-type boron nitride into the stable region exceeds 500 ° C., lattice defects are unlikely to occur, the dislocation density becomes small, and the strength decreases.

[Example 3]
In Example 3, the content of alkali metal elements and alkaline earth metals in the cubic boron nitride polycrystal and the ultra-high speed and high load machining of difficult-to-cut materials with a tool using the cubic boron nitride polycrystal. The relationship with the tool life when this was done was investigated.

<Preparation of cubic boron nitride polycrystal>
The cubic boron nitride polycrystals of Samples 17 to 19 were prepared according to the following procedure. The sample 17 was produced by the same raw materials and manufacturing process as the above sample 2-2, and the obtained cubic boron nitride polycrystal is also the same as the sample 2-2.

(First step)
[Sample 17, Sample 18]
6 g of a commercially available hexagonal boron nitride powder (particle size 5 μm) was prepared. The above hexagonal boron nitride powder was placed in a molybdenum capsule and installed in an ultra-high pressure and high temperature generator.

[Sample 19]
A commercially available cubic boron nitride powder (average particle size 4 μm) was heat-treated at 1900 ° C. for 1 hour under an argon atmosphere. As a result, cubic boron nitride was converted to hexagonal boron nitride, and hexagonal boron nitride powder was obtained. The obtained hexagonal boron nitride powder was placed in a molybdenum capsule and installed in an ultra-high pressure and high temperature generator.

(2nd step and 3rd step)
[Sample 17 to 19]
Using an ultra-high pressure high temperature generator, the above hexagonal boron nitride powder was prepared from the temperatures and pressures listed in the "Temperature" and "Pressure" columns of the "Starting point" in Table 3 while maintaining the pressure. The temperature was raised to the temperature described in the "reached temperature" column of "1 step", and the temperature was maintained for the length described in the "holding time" column.

Subsequently, while maintaining the temperature, the pressure was increased to the pressure described in the "reaching pressure" column of the "second stage" of Table 3, and the pressure was maintained for the length described in the "holding time" column.

Subsequently, while maintaining the pressure, the temperature is raised to the temperature described in the "reached temperature" column of the "third stage" in Table 3, and the temperature is maintained for the length described in the "holding time" column to cubic. A boron nitride polycrystal was obtained. The high-temperature and high-pressure treatment at the "reached temperature", "reaching pressure" and "holding time" described in the "third stage" corresponds to the third step.

<Evaluation>
(Measurement of content of cubic boron nitride, measurement of dislocation density, measurement of median diameter d50 of crystal grains)
With respect to the cubic boron nitride polycrystal obtained above, the content of cubic boron nitride, the dislocation density of cubic boron nitride, and the median diameter d50 of the crystal grains were measured. Since the specific measurement method is as shown in the first embodiment, the description thereof will not be repeated. The results are shown in the columns of "cBN content", "cBN dislocation density", and "median diameter (d50)" in Table 3.

In all the samples, no components other than cBN, wBN and compressed hBN were identified.

(Measurement of total content of alkali metal elements and alkaline earth metal elements)
The total content of alkali metal elements and alkaline earth metal elements in the cubic boron nitride polycrystals obtained above was measured by SIMS. Since the specific measurement method is as shown in the first embodiment, the description thereof will not be repeated. The total content of alkali metal elements and alkaline earth metal elements is shown in the "Alkaline metal / alkaline earth metal content" column of Table 3.

(Cutting test)
The cubic boron nitride polycrystal obtained above was cut by a laser and finished to produce a cutting tool having an insert model number NU-CNGA120408 (manufactured by Sumitomo Electric Hardmetal Corp.). Using the obtained cutting tool, intermittent cutting of a Ti6Al4V round bar (φ200 mm, with one V-shaped slit) was performed under the following cutting conditions, and the tool life was evaluated. The Ti6Al4V round bar, which is the work material, is a difficult-to-cut material.

(Cutting conditions)
Work material: Ti6Al4V round bar (φ200mm, with one V-shaped slit)
Tool shape:
Holder model number DCLNR2525 (manufactured by Sumitomo Electric Hardmetal Corp.)
Insert model number NU-CNGA120408 (manufactured by Sumitomo Electric Hardmetal Corp.)
Cutting speed: 250 m / min
Feed amount: 0.1 mm / blade depth of cut: 0.15 mm
Coolant: WET
The above cutting conditions correspond to ultra-high speed and high load machining of difficult-to-cut materials.

The tool life was measured by cutting under the above-mentioned cutting conditions and measuring the machining time until the missing amount of the tool observed from the flank surface became 100 μm or more. The longer the machining time, the better the fracture resistance and the longer the tool life. The results are shown in the "Tool Life" column of Table 3.

<Discussion>
[Sample 17 to 19]
The methods for producing Samples 17 to 19 all correspond to Examples. The cubic boron nitride polycrystals of Samples 17 to 19 all contain 98.5% by volume or more of cubic boron nitride, and the dislocation density of cubic boron nitride is larger than 8 × 10 ¹⁵ / m ² and crystal grains. The median diameter d50 of No. 1 is 0.1 μm or more and 0.5 μm or less, which corresponds to the example. The tools using the cubic boron nitride polycrystals of Samples 17 to 19 were able to achieve a tool life of 15 minutes or more even in ultra-high speed and high load machining of difficult-to-cut materials.

Samples 17 and 18 had a longer tool life than sample 19. This is because the cubic boron nitride polycrystals of Samples 17 and 18 have a total content of alkali metal elements and alkaline earth metal elements of 10 ppm or less, and the interface between the tool cutting edge and the work material in a cutting environment. It is considered that the conversion from cubic boron nitride to hexagonal boron nitride is unlikely to occur even when the temperature and pressure are high.

The composition, the total content of the alkali metal element and the alkaline earth metal element, the median diameter d50 of the crystal grains, and the dislocation density of cubic boron nitride were different between Sample 17 and Sample 18. It is considered that this is due to variations in the amount of impurities and the particle size of the raw material hexagonal boron nitride.

[Example 4]
In Example 4, the content of compressed hexagonal boron nitride and wurtzite boron nitride in the cubic boron nitride polycrystal and the high load of the difficult-to-cut material with a tool using the cubic boron nitride polycrystal. The relationship with the tool life when machining was performed was investigated.

<Preparation of cubic boron nitride polycrystal>
The cubic boron nitride polycrystals of Samples 17 to 19 were prepared according to the following procedure.

(First step)
6 g of a commercially available hexagonal boron nitride powder (particle size 5 μm) was prepared. The above hexagonal boron nitride powder was placed in a molybdenum capsule and installed in an ultra-high pressure and high temperature generator.

(2nd step and 3rd step)
Using an ultra-high pressure high temperature generator, the above hexagonal boron nitride powder was prepared from the temperatures and pressures listed in the "Temperature" and "Pressure" columns of the "Starting point" in Table 4 while maintaining the pressure. The temperature was raised to the temperature described in the "reached temperature" column of "1 step", and the temperature was maintained for the length described in the "holding time" column.

Subsequently, while maintaining the temperature, the pressure was increased to the pressure described in the "reaching pressure" column of the "second stage" in Table 4, and the pressure was maintained for the length described in the "holding time" column.

Subsequently, while maintaining the pressure, the temperature is raised to the temperature described in the "reached temperature" column of the "third stage" of Table 4, and the temperature is maintained for the length described in the "holding time" column to cubic. A boron nitride polycrystal was obtained. The high-temperature and high-pressure treatment at the "reached temperature", "reaching pressure" and "holding time" described in the "third stage" corresponds to the third step.

<Evaluation>
(Measurement of composition, measurement of dislocation density, measurement of crystal grain median diameter d50)
Regarding the cubic boron nitride polycrystal obtained above, the composition (content of cubic boron nitride, content of compressed hexagonal boron nitride (hereinafter, also referred to as “comp.hBN”), wurtzite type nitride). The content of boron), the rearrangement density of cubic boron nitride, and the median diameter d50 of the crystal grains were measured. Since the specific measurement method is as shown in the first embodiment, the description thereof will not be repeated. The results are shown in the columns of "cBN content", "comp.hBN content", "wBN content", "cBN dislocation density", and "median diameter (d50)" in Table 4.

In all the samples, no components other than cBN, wBN and compressed hBN were identified.

(Cutting test)
The cubic boron nitride polycrystal obtained above was cut by a laser and finished to produce a cutting tool having an insert model number NU-CNGA120408 (manufactured by Sumitomo Electric Hardmetal Corp.). Using the obtained cutting tool, intermittent cutting of a Ti6Al4V round bar (φ200 mm, with one V-shaped slit) was performed under the following cutting conditions, and the tool life was evaluated. The Ti6Al4V round bar, which is the work material, is a difficult-to-cut material.

(Cutting conditions)
Work material: Ti6Al4V round bar (φ200mm, with one V-shaped slit)
Tool shape:
Holder model number DCLNR2525 (manufactured by Sumitomo Electric Hardmetal Corp.)
Insert model number NU-CNGA120408 (manufactured by Sumitomo Electric Hardmetal Corp.)
Cutting speed: 120m / min
Feed amount: 0.1 mm / blade depth of cut: 0.15 mm
Coolant: WET
The above cutting conditions correspond to high-load machining of difficult-to-cut materials.

The tool life was measured by cutting under the above-mentioned cutting conditions and measuring the machining time until the missing amount of the tool observed from the flank surface became 100 μm or more. The longer the machining time, the better the fracture resistance and the longer the tool life. The results are shown in the "Tool Life" column of Table 4.

<Discussion>
[Sample 20 to 22]
The methods for producing Samples 20 to 22 all correspond to Examples. The cubic boron nitride polycrystals of Samples 20 to 22 all contain 98.5% by volume or more of cubic boron nitride, and the dislocation density of cubic boron nitride is larger than 8 × 10 ¹⁵ / m ² and crystal grains. The median diameter d50 of No. 1 is 0.1 μm or more and 0.5 μm or less, which corresponds to the example. The tools using the cubic boron nitride polycrystals of Samples 20 to 22 were able to achieve a long tool life even in high-load machining of difficult-to-cut materials.

Sample 20 and Sample 21 had a longer tool life than Sample 22. It is considered that this is because the cubic boron nitride polycrystals of Sample 20 and Sample 21 contain compression hexagonal boron nitride and / or wurtzite boron nitride, and the adhesion resistance and slidability are improved.

[Sample 23]
The method for producing sample 23 does not include a step of holding the wurtzite-type boron nitride at a temperature and pressure in the stable region for 10 minutes or more, and corresponds to a comparative example. The cubic boron nitride polycrystal of Sample 23 has a content of 98.2% by volume of cubic boron nitride, which corresponds to a comparative example. The tool using the cubic boron nitride polycrystal of Sample 23 had a short tool life. This is because the method for producing sample 23 does not include a step of holding the wurtzite-type boron nitride at a temperature and pressure within the stable region for 10 minutes or more, so that the conversion rate from hexagonal boron nitride to wurtzite-type boron nitride is low. As a result, the conversion rate to cubic boron nitride is also low, and it is considered that the content of the obtained cubic boron nitride polycrystal is small.

[Example 5]
In Example 5, the area ratio of the plate-like particles in the cubic boron nitride polycrystal and the tool life when high-load machining of an iron-based material is performed with a tool using the cubic boron nitride polycrystal. I investigated the relationship between.

<Preparation of cubic boron nitride polycrystal>
The cubic boron nitride polycrystals of Samples 24 to 28 were prepared according to the following procedure.

(First step)
6 g of a commercially available hexagonal boron nitride powder (particle size 5 μm) was prepared. The above hexagonal boron nitride powder was placed in a molybdenum capsule and installed in an ultra-high pressure and high temperature generator.

(2nd step and 3rd step)
Using an ultra-high pressure and high temperature generator, the above hexagonal boron nitride powder was prepared from the temperatures and pressures listed in the "Temperature" and "Pressure" columns of the "Starting point" in Table 5 while maintaining the pressure. The temperature was raised to the temperature described in the "reached temperature" column of "1 step", and the temperature was maintained for the length described in the "holding time" column.

Subsequently, while maintaining the temperature, the pressure was increased to the pressure described in the "reaching pressure" column of the "second stage" in Table 5, and the pressure was maintained for the length described in the "holding time" column.

Subsequently, while maintaining the pressure, the temperature is raised to the temperature described in the "reached temperature" column of the "third stage" in Table 5, and the temperature is maintained for the length described in the "holding time" column to cubic. A boron nitride polycrystal was obtained. The high-temperature and high-pressure treatment at the "reached temperature", "reaching pressure" and "holding time" described in the "third stage" corresponds to the third step.

<Evaluation>
(Measurement of content of cubic boron nitride, measurement of dislocation density, measurement of median diameter d50 of crystal grains)
With respect to the cubic boron nitride polycrystal obtained above, the content of cubic boron nitride, the dislocation density of cubic boron nitride, and the median diameter d50 of the crystal grains were measured. Since the specific measurement method is as shown in the first embodiment, the description thereof will not be repeated. The results are shown in the columns of "cBN content", "cBN dislocation density", and "median diameter (d50)" in Table 5.

In all the samples, no components other than cBN, wBN and compressed hBN were identified.

(Measurement of area ratio of plate-shaped particles)
The area ratio of the plate-like particles was measured for the cubic boron nitride polycrystals obtained above. Since the specific measurement method is as shown in the first embodiment, the description thereof will not be repeated. The results are shown in the "Area ratio of plate-like particles" column of Table 5.

(Cutting test)
The cubic boron nitride polycrystal obtained above was cut by a laser and finished to produce a cutting tool of insert model number SNEW1203ADTR (manufactured by Sumitomo Electric Hardmetal Corp.). Using the obtained cutting tool, face milling of a gray cast iron FC300 block material (80 mm × 300 mm × 150 mm) was performed under the following cutting conditions, and the tool life was evaluated.

(Cutting conditions)
Work material: Gray cast iron FC300 block material (80 mm x 300 mm x 150 mm)
Tool shape:
Cutter model number FMU4100R (manufactured by Sumitomo Electric Hardmetal Corp.)
Insert model number SNEW1203ADTR (manufactured by Sumitomo Electric Hardmetal Corp.)
Cutting speed: 2250 m / min
Feed amount: 0.13 mm / blade depth of cut: 0.45 mm
Coolant: DRY
The above cutting conditions correspond to high-load machining of iron-based materials.

The tool life was measured by cutting under the above-mentioned cutting conditions and measuring the machining time until the missing amount of the tool observed from the flank surface became 250 μm or more. The longer the machining time, the better the fracture resistance and the longer the tool life. The results are shown in the "Tool Life" column of Table 5.

[Sample 24-Sample 28]
The methods for producing Samples 24 to 28 all correspond to Examples. The cubic boron nitride polycrystals of Samples 24 to 28 all contain 98.5% by volume or more of cubic boron nitride, and the dislocation density of cubic boron nitride is larger than 8 × 10 ¹⁵ / m ² and crystal grains. The median diameter d50 of No. 1 is 0.1 μm or more and 0.5 μm or less, which corresponds to the example. It was confirmed that the tools using the cubic boron nitride polycrystals of Samples 24 to 28 have a long tool life even in high-load machining of iron-based materials.

Comparing Samples 24 to 28, it was confirmed that the smaller the area ratio of the plate-like particles, the longer the tool life. It is considered that this is because the smaller the area ratio of the plate-shaped particles, the less likely it is that the cutting edge is suddenly chipped due to the plate-shaped particles.

Although the embodiments and examples of the present disclosure have been described as described above, it is planned from the beginning that the configurations of the above-described embodiments and examples may be appropriately combined or modified in various ways.

The embodiments and examples disclosed this time should be considered as exemplary in all respects and not restrictive. The scope of the present disclosure is shown by the scope of claims rather than the embodiments and examples described above, and is intended to include meaning equivalent to the scope of claims and all modifications within the scope.

Claims (13)

A cubic boron nitride polycrystal containing 98.5% by volume or more of cubic boron nitride.
The dislocation density of the cubic boron nitride is greater than 8 × 10 ¹⁵ / m ² .
The cubic boron nitride polycrystal contains a plurality of crystal grains and contains a plurality of crystal grains.
A cubic boron nitride polycrystal having a median diameter d50 having a circle-equivalent diameter of the plurality of crystal grains of 0.1 μm or more and 0.5 μm or less. The cubic boron nitride polycrystal according to claim 1, wherein the dislocation density is 9 × 10 ¹⁵ / m ² or more. The cubic boron nitride polycrystal according to claim 1 or 2, wherein the total content of the alkali metal element and the alkaline earth metal element of the cubic boron nitride polycrystal is 10 ppm or less on a mass basis. When the cross section of the cubic boron nitride polycrystal is observed at a magnification of 10000 times using a scanning electron microscope, the area ratio of the plate-like particles having an aspect ratio of 4 or more is 30 area% or less. The cubic boron nitride polycrystal according to any one of claims 1 to 3. The cubic boron nitride polycrystal according to claim 4, wherein the area ratio of the plate-like particles having an aspect ratio of 4 or more is 5 area% or less. The cubic boron nitride polycrystal according to any one of claims 1 to 5, wherein the cubic boron nitride polycrystal contains 0.01% by volume or more of compressed hexagonal boron nitride. The cubic boron nitride polycrystal according to any one of claims 1 to 6, wherein the cubic boron nitride polycrystal contains 0.1% by volume or more of wurtzite-type boron nitride. The cubic boron nitride polycrystal according to any one of claims 1 to 7, wherein the dislocation density is calculated by using the modified Williamson-Hall method and the modified Warren-Averbach method. The cubic boron nitride polycrystal according to any one of claims 1 to 8, wherein the dislocation density is measured using synchrotron radiation as an X-ray source. The method for producing a cubic boron nitride polycrystal according to any one of claims 1 to 9.
The first step of preparing hexagonal boron nitride powder and
The hexagonal boron nitride powder is passed through the temperature and pressure within the stable region of the wurtzite-type boron nitride and heated and pressurized to a temperature of 1700 ° C. or higher and 2500 ° C. or lower and a pressure of 8 GPa or higher to obtain a boron nitride polycrystal. The second step to get the body and
The boron nitride polycrystal obtained in the second step is held at a temperature of 1700 ° C. or higher and 2500 ° C. or lower and a pressure condition of 8 GPa or higher for 3 minutes or longer and 60 minutes or lower to obtain a cubic boron nitride polycrystal. With a third step to obtain
The stable region of the wurtzite-type boron nitride is a region that simultaneously satisfies the following formula 1 and the following formula 2 when the temperature is T ° C. and the pressure is PGPa.
Equation 1: P ≧ -0.0037T + 11.301
Equation 2: P ≦ -0.085T + 117
In the heating and pressurizing path of the second step, the temperature at which the wurtzite-type boron nitride enters the stable region is 500 ° C. or lower.
The second step is the production of a cubic boron nitride polycrystal, which comprises a step of maintaining the temperature and pressure in the heating and pressurizing path at the temperature and pressure within the stable region of the wurtzite-type boron nitride for 10 minutes or more. Method. The method for producing a cubic boron nitride polycrystal according to claim 10, wherein the inrush temperature is 300 ° C. or lower. The second step according to claim 10 or 11, further comprising a step of maintaining the temperature and pressure in the heating and pressurizing path at the temperature and pressure within the stable region of the wurtzite-type boron nitride for 15 minutes or more. Method for producing a cubic boron nitride polycrystal. In the second step, the temperature and pressure in the heating and pressurizing path are the temperatures and pressures in the region that simultaneously satisfy the following formula 1, the following formula 2 and the following formula 3 when the temperature is T ° C. and the pressure is PGPa. Including the step of holding for 10 minutes or more,
Equation 1: P ≧ -0.0037T + 11.301
Equation 2: P ≦ -0.085T + 117
Equation 3: P ≦ -0.0037T + 11.375
The method for producing a cubic boron nitride polycrystal according to any one of claims 10 to 12.

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