JP5880389B2 - Method for producing boron nitride polycrystal - Google Patents

Description

  The present invention relates to a boron nitride polycrystal, a method for producing the same, and a tool, and more particularly, to a boron nitride polycrystal having a main component of wurtzite boron nitride, a method for producing the same, and a tool.

  Conventionally, titanium nitride (TiN), titanium carbide (TiC), cobalt (Co), etc. have been used as sintering aids or binders in cubic boron nitride (cBN) sintered bodies used as cutting tools and anti-wear tools. It has been. These can be obtained by sintering cBN powder together with a sintering aid and a binder under a pressure of about 4 to 5 GPa. This sintered body contains about 10 to 40% of a binder, and this binder greatly affects the strength, heat resistance, and thermal diffusibility of the sintered body. In particular, when the iron-based material is used as a tool for cutting, the cutting edge is easily broken or cracked, and the tool life is shortened.

  As a technique for extending the tool life, a method of manufacturing a boron nitride sintered body without including a binder is known. In this method, hexagonal boron nitride (hBN) using a catalyst such as magnesium boronitride is used as a raw material, and this is subjected to reaction sintering. Since this method does not include a binder, the cBNs are strongly bonded to each other, and the thermal conductivity is as high as 6 to 7 W / cm ° C. Therefore, the boron nitride sintered body is used for a heat sink material, a TAB (Tape Automated Bonding) bonding tool, and the like.

  However, since some catalyst remains in this sintered body, when heat is applied, fine cracks due to a difference in thermal expansion between the catalyst and cBN are likely to occur. For this reason, the heat-resistant temperature is as low as about 700 ° C., which is a serious problem as a cutting tool. Further, since the particle size is as large as about 10 μm, the thermal conductivity is high, but the strength is weak and it cannot withstand heavy cutting.

  On the other hand, a boron nitride sintered body can also be obtained by subjecting atmospheric pressure type BN such as hBN to direct conversion sintering under ultra high pressure and high temperature. For example, JP-A-47-34099 (Patent Document 1) and JP-A-3-159964 (Patent Document 2) convert hBN into cBN under ultra-high pressure and high temperature to obtain a boron nitride sintered body. It is shown.

  In addition to cBN, wurtzite boron nitride (wBN) is known as the high-pressure phase of BN, and wBN is a material that is expected to have excellent mechanical strength like cBN. However, in boron nitride, cBN is a stable phase at high pressure, and wBN is a metastable phase. Therefore, wBN is extremely difficult to produce. Japanese Examined Patent Publication No. 58-23459 describes a method of producing a fine wBN powder by phase transition from normal pressure type hBN to wBN by impact compression such as explosion of explosives. Further, Japanese Patent Publication No. 58-23459 describes a boron nitride sintered body obtained by sintering this wBN powder together with a binder, and a boron nitride sintered body obtained by sintering wBN powder, cBN, and a binder.

JP 47-34099 A JP-A-3-159964 Japanese Patent Publication No. 63-394 Japanese Patent Publication No.58-23459

  However, since the time required to synthesize wBN by impact compression is an extremely short time in units of 1 / 1,000,000 second, a bulk polycrystalline body free from cracks and defects cannot be obtained, and only a powdery product can be produced. Further, the obtained wBN powder also contained many lattice defects in the crystal, and the hardness and strength were insufficient. Therefore, a boron nitride sintered body obtained by mixing and sintering such a wBN powder and a binder also has hardness, strength as a member for tools (particularly, cutting tools, wear-resistant tools, and grinding tools). Was insufficient.

  The present invention has been made to solve the above-described problems. A main object of the present invention is to provide a boron nitride polycrystal having high hardness and crack propagation resistance, a method for producing the same, and a tool.

  The polycrystalline boron nitride of the present invention comprises wurtzite boron nitride and unavoidable impurities, and has a maximum outer width of 0.3 mm or more. As described above, the polycrystalline boron nitride of the present invention is mainly composed of wurtzite boron nitride, and can have a bulk-like outer shape, not as a powder, and thus has high hardness and is resistant to cracking. It can have propagation properties.

  The polycrystalline boron nitride of the present invention comprises wurtzite boron nitride and inevitable impurities, and has a Knoop hardness of 40 GPa or more. Thus, since the boron nitride polycrystal of the present invention is mainly composed of wurtzite boron nitride, it can have high hardness.

The polycrystalline boron nitride of the present invention comprises wurtzite boron nitride and inevitable impurities, and has a fracture toughness value K _1c of 8.0 MPa · m ^1/2 or more. Thus, since the boron nitride polycrystal of the present invention is mainly composed of wurtzite boron nitride, it can have excellent toughness.

  The boron nitride polycrystal of the present invention contains 95% by volume or more of wurtzite boron nitride, and the remainder is composed of other boron nitrides and inevitable impurities having a crystal structure different from that of wurtzite boron nitride. Thereby, since the boron nitride polycrystal composed mainly of wurtzite boron nitride has a bulk-like outer shape, not as a powder, it can have high hardness and crack propagation resistance.

  The other boron nitride may be cubic boron nitride. The other boron nitride may be compressed hexagonal boron nitride. Here, “compressed hexagonal boron nitride” refers to a crystal having a crystal structure similar to that of normal hBN and having a c-axis direction plane spacing smaller than that of normal hBN (0.333 nm).

In addition, the boron nitride polycrystal including 95% by volume or more of wurtzite-type boron nitride and the balance of the boron nitride and inevitable impurities having different crystal structures from the wurtzite-type boron nitride has an outer shape. The maximum width can be 0.3 mm or more. The boron nitride polycrystal may have a Knoop hardness of 40 GPa or more. The boron nitride polycrystal may have a fracture toughness value K _1c of 8.0 MPa · m ^1/2 or more.

  The tool which concerns on this invention is equipped with the said boron nitride polycrystal. In this way, a tool with high hardness and excellent crack propagation resistance can be realized.

  The method for producing a boron nitride polycrystal according to the present invention includes, as a starting material, a step of preparing atmospheric pressure boron nitride having a graphitization index of less than 3.5 in an X-ray diffraction method, and a step of heat-treating atmospheric pressure boron nitride. And the step of heat-treating includes a step of pressurizing and heating to a pressure of 8 GPa or higher and a temperature of 1200 ° C. or higher and lower than 2200 ° C., and a step of maintaining the conditions.

Thereby, the boron nitride polycrystal according to the present invention can be easily obtained.
In the heat treatment step, atmospheric pressure boron nitride can be directly converted into wurtzite boron nitride and simultaneously sintered.

  Since the boron nitride polycrystal according to the present invention is mainly composed of wurtzite boron nitride, it can have high hardness and crack propagation resistance.

It is a figure for demonstrating the tool which concerns on this Embodiment. It is a figure which shows the flow of the manufacturing method of the boron nitride polycrystal which concerns on this Embodiment.

  Embodiments of the present invention will be described below. The boron nitride (BN) sintered body according to the present embodiment contains 95% by volume or more of wurtzite boron nitride (wBN), and the balance is composed of cubic boron nitride (cBN) and inevitable impurities. Inevitable impurities are, for example, nitrogen, hydrogen, oxygen, etc., and the content is 0.1 volume% or less. At this time, the average particle size of wBN is about 500 nm or less. The sintered body does not substantially contain a binder, a sintering aid, a catalyst and the like. For this reason, the wBN particles, the wBN particles and the cBN particles, and the cBN particles are all firmly bonded, and the boron nitride polycrystal according to the present embodiment has a dense composite structure.

  The boron nitride polycrystal of the present embodiment has a maximum outer width of 0.3 mm or more. That is, the boron nitride polycrystal of the present embodiment is not a powder form but a bulk body (lump).

  Conventionally, wBN can only be produced as a powder, but cannot be produced as a bulk. Therefore, a conventional boron nitride polycrystal containing wBN is produced by sintering with wBN powder, cBN powder and a binder, and the hardness and crack propagation resistance are examined by the ratio of wBN or cBN to the binder. Or by adjusting the material of the binder. As a result, the boron nitride polycrystal containing wBN could not have sufficient hardness and crack propagation resistance as a tool member.

As described above, the boron nitride polycrystal of the present embodiment can be a bulk-like body configured without including a binder or the like. Therefore, the boron nitride polycrystal of the present embodiment can have a Knoop hardness of 40 GPa or more, and can further have a fracture toughness value K _1c of 8.0 MPa · m ^1/2 or more.

From the examples described later, the boron nitride polycrystal composed of wBN, cBN, and inevitable impurities had a Knoop hardness at room temperature of 48 GPa or more and a fracture toughness value K _1c of 8.6 MPa · m ^1/2 or more. Thus, it was confirmed that the boron nitride polycrystal of the present embodiment has high crack resistance.

  The boron nitride polycrystal according to the present embodiment is suitable for a tool member because it has high hardness and excellent crack propagation resistance and has a maximum outer width of 0.3 mm or more. For example, the boron nitride polycrystal according to the present embodiment can be used for cutting tools, wear-resistant tools, grinding tools, and the like, and more specifically, precision tools such as cutting tools, dies, and micro tools, etc. Can be used. With reference to FIG. 1, the schematic of the cutting tool provided with the boron nitride polycrystal which concerns on this Embodiment is shown. The boron nitride polycrystalline body 1 is fixed to a predetermined region of the base metal 2 via the brazing layer 3 and the metallized layer 4. The tool according to the present embodiment manufactured in this way is more than a tool including a boron nitride polycrystal obtained by mixing and sintering a wBN powder and a binder prepared by conventional impact compression. Long life.

  Next, with reference to FIG. 2, the manufacturing method of the boron nitride polycrystal which concerns on this Embodiment is demonstrated. The method for producing a polycrystalline boron nitride according to the present embodiment includes a step of preparing atmospheric pressure boron nitride (BN) having a graphitization index of less than 3.5 in an X-ray diffraction method as a starting material (S10); And a step (S20) of heat-treating the normal pressure type BN.

  First, in step (S10), hexagonal boron nitride (hBN) is prepared as a starting material. hBN has a graphitization index (GI value) of less than 3.5 in the X-ray diffraction method and is highly crystalline. The GI value is a value derived by introducing the areas of the three peaks of x-ray diffraction of hBN, that is, the peaks of (100), (101), and (102) into Equation 1.

When the crystallinity of hBN is improved, the GI value is decreased. Here, I _(XXX) is the area of the diffraction peak on the (XXX) plane of the hBN crystal.

  Next, in step (S20), atmospheric pressure type BN is heat-treated to cause phase transition to wBN, and further sintered to produce a boron nitride polycrystal. Specifically, the step (S20) includes a step (S21) of pressurizing and heating the normal pressure BN to a pressure of 8 GPa or more and a temperature of 1300 ° C. or more and less than 2200 ° C., and a step of maintaining the condition (S22). Including. At this time, the upper limit value of the synthesis pressure may be any value as long as wBN is thermodynamically stable, and the upper limit value of the pressure is actually determined by the ultrahigh pressure and high temperature generator to be used (for example, about 25 GPa). In step (S22), the time for maintaining the pressure temperature condition in step (S21) is 120 seconds or more.

  In the method for producing a polycrystalline boron nitride according to the present embodiment, what is particularly important is the graphitization index (GI value) of the starting material (hBN) prepared in the step (S01), and the step (S02). ) Temperature and pressure conditions.

  When the GI value of the starting material is 3.5 or more and the pressure is about 20 GPa, the content of wBN produced in the step (S02) decreases.

  When the heating temperature in the step (S02) is 2200 ° C. or higher, wBN that is a metastable phase undergoes phase transition to cBN that is a stable phase. When the heating temperature is less than 1300 ° C., the unconverted starting material remains in the obtained boron nitride polycrystal.

  An appropriate heating temperature for synthesizing the boron nitride polycrystal of the present embodiment varies depending on the synthesis pressure and the GI value of the starting material. For example, when the GI value of the starting material is low, a boron nitride polycrystal containing 95% by volume or more of wBN can be obtained even under pressure temperature conditions where the synthesis pressure is low and the heating temperature is high. Conversely, when the starting material has a high GI value, the boron nitride polycrystal according to the present embodiment can be obtained by using a pressure temperature condition in which the synthesis pressure is high and the heating temperature is low.

  By doing in this way, the manufacturing method of the boron nitride polycrystal according to the present embodiment is not based on the impact compression method, and without adding a sintering aid, a catalyst, or the like, wBN, cBN, and inevitable impurities. The polycrystalline boron nitride made of can be obtained as a bulk-like body having a maximum outer width of 0.3 mm or more.

From examples described later, using a highly crystalline hBN having a graphitization index of less than 3.5 as a starting material, in the step (S02), the pressure is about 16 to 20 GPa and the temperature is about 1300 to 1800 ° C. The boron nitride polycrystal obtained by sintering had a Knoop hardness at room temperature of 45 GPa or more and a fracture toughness value K _1c of 8.2 MPa · m ^1/2 or more under the condition of a test load of 4.9 N. It was. Thus, it was confirmed that the boron nitride polycrystal of the present embodiment has high crack propagation resistance. However, it is considered that a boron nitride polycrystal having similar characteristics can be obtained even under sintering conditions of a pressure of about 8 GPa or more and less than about 2200 ° C.

  As described above, the boron nitride polycrystal according to the present embodiment is composed of wBN, cBN and unavoidable impurities, and has a maximum outer width of 0.3 mm or more, so it has high hardness and high fracture toughness value. Excellent crack propagation resistance. Therefore, the boron nitride polycrystal according to the present embodiment can be used for cutting tools, wear-resistant tools, grinding tools, and the like. In addition, according to the method for manufacturing a boron nitride polycrystal according to the present embodiment, highly crystalline hBN is prepared as a starting material in step (S01), and the starting material is supplied at a predetermined pressure and temperature in step (S02). By applying pressure and heating under conditions, and further maintaining the conditions, a boron nitride polycrystal as described above can be produced.

  Moreover, although the boron nitride polycrystal according to the present embodiment includes cBN as boron nitride having a crystal structure different from that of wBN, the present invention is not limited to this. For example, the other boron nitride having a crystal structure different from wBN may be compressed hexagonal boron nitride. Moreover, cBN and compressed boron nitride may be mixed. Even in this case, the boron nitride polycrystal of the present invention has a dense composite structure, and can have sufficient hardness and crack propagation resistance as a tool member.

  As described above, the boron nitride polycrystal according to the present embodiment includes 95% by volume or more of wBN, and the balance is composed of cBN as another boron nitride having a crystal structure different from that of wBN, and unavoidable impurities. ing. However, the polycrystalline boron nitride of the present invention can be composed of wBN and inevitable impurities. At this time, the inevitable impurities are 0.1% by volume or less. That is, the polycrystalline boron nitride of the present invention can contain only wBN as boron nitride without containing other boron nitride having a crystal structure different from that of wBN while keeping the impurity concentration extremely low. Even in this case, the boron nitride polycrystal of the present invention has a dense composite structure, and can have sufficient hardness and crack propagation resistance as a tool member.

  The method for producing a boron nitride polycrystal comprising only wBN and inevitable impurities has basically the same structure as the method for producing a boron nitride polycrystal according to the present embodiment, but the GI value and pressure of the starting material. The range of temperature conditions is more limited. For example, when the GI value is 3.0 or more and less than 3.4, the combined pressure may be about 20 GPa and the heating temperature may be about 1300 ° C. Further, when the GI value is about 2.0 or more and less than 3.0, the combined pressure may be about 19 GPa and the heating temperature may be about 1400 ° C. or more and 1500 ° C. or less. When the GI value is about 1.0 or more and less than 2.0, the combined pressure may be about 18 GPa and the heating temperature may be about 1400 ° C. or more and 1800 ° C. or less.

From the examples described later, the boron nitride polycrystal containing only wBN as boron nitride has a Knoop hardness at room temperature of 45 GPa or more under a test load of 4.9 N, and a fracture toughness value K _1c of 8.2 MPa · m. It was more than ^1/2 .

  Examples of the present invention will be described below.

  Boron nitride polycrystals according to Examples 1 and 2 were produced by the following method. First, a highly crystalline hBN having a GI value of 3.1 in the form of a commercially available pellet was used as a starting material. The starting material was put into a capsule made of a refractory metal and held for 20 minutes under the pressure and temperature conditions shown in Table 1 using an ultrahigh pressure and high temperature generator to produce a boron nitride polycrystal.

  Boron nitride polycrystals according to Examples 3 and 4 were produced by the following method. First, a highly crystalline hBN having a GI value of 1.1 having a commercially available pellet form was used as a starting material. The starting material was put into a capsule made of a refractory metal and held for 20 minutes under the pressure and temperature conditions shown in Table 1 using an ultrahigh pressure and high temperature generator to produce a boron nitride polycrystal.

  A boron nitride polycrystal according to Comparative Example 1 was produced by the following method. First, as a starting material, commercially available pellet-shaped hBN having a GI value of 3.6 was used. The starting material was put in a capsule made of a refractory metal, a pressure of 7.7 GPa was generated with an ultrahigh pressure generator, and held at a temperature of 2300 ° C. for 15 minutes to prepare a boron nitride polycrystal.

  Boron nitride polycrystals according to Comparative Examples 2 and 3 were produced by the following method. First, a highly crystalline hBN having a GI value of 1.1 having a commercially available pellet form was used as a starting material. The starting material was put into a capsule made of a refractory metal and held for 20 minutes under the pressure and temperature conditions shown in Table 1 using an ultrahigh pressure and high temperature generator to produce a boron nitride polycrystal.

  The composition, particle size, hardness, and fracture toughness value of the boron nitride polycrystals of Examples 1-4 and Comparative Examples 1-3 obtained as described above were measured by the following methods.

  The composition of each phase was obtained by identifying each phase with an X-ray diffractometer. The X-ray source of this apparatus is Cu, which is a Kα ray with a wavelength of 1.54 mm.

  The average particle size was measured with a scanning electron microscope. A cutting method was used as a method for obtaining the average particle diameter. In this method, first, a circle is written on an image of a scanning electron microscope (SEM), eight straight lines are drawn radially from the center of the circle to the outer periphery of the circle, and the number of lines that cross the grain boundary in the circle is counted. Then, the average intercept length is obtained by dividing the length of the straight line by the number of crossing, and the average crystal grain size is obtained by multiplying the average intercept length by 1.128. In addition, since the particle size of the boron nitride polycrystal of the comparative example 2 was 1000 nm or more even if it was small, the average particle size was not able to be calculated using the cutting method.

  The magnification of the SEM image used to use the cutting method is 30000 times. The reason for this is that at a magnification less than this, the number of grains in the circle increases, making it difficult to see the grain boundaries, causing miscounting, and increasing the possibility of including a plate-like structure when drawing a line. is there. Further, when the magnification is higher than this, the number of grains in the circle is too small, and an accurate average particle diameter cannot be calculated.

  In this experiment, three SEM images obtained by photographing different locations were used for one sample. The average value was made into the average particle diameter using the cutting method with respect to each SEM image.

  Knoop hardness was measured as a hardness measurement. A micro Knoop indenter was used to measure Knoop hardness. The load at this time was set to 4.9N. VLPAK2000 manufactured by Mitutoyo Corporation was used as a measuring instrument. The measurement was performed five times, and the average value of the three values excluding the smallest value and the largest value was taken as the hardness of the sample.

Fracture toughness value _K1C was measured from the length of a crack generated when measuring Vickers hardness according to JIS-R-1607. The load when measuring the Vickers hardness was 98N. The measurement was performed five times, and the average value of the three values excluding the smallest value and the largest value was taken as the fracture toughness value of the sample.

  Table 1 shows the results of composition, particle size, and hardness of the boron nitride polycrystals of Examples 1-4 and Comparative Examples 1-3. In Table 1, the particle size of Comparative Example 2 is not the average particle size but the minimum value of the particle size.

As shown in Table 1, the boron nitride polycrystals of Examples 1 to 4 were found to contain 95% by volume or more of wBN. Specifically, the boron nitride polycrystals of Examples 1 and 3 had a wBN composition ratio of 100% by volume. The boron nitride polycrystal of Example 2 had a composition ratio of wBN of about 97% by volume, and contained hBN and cBN as other boron nitrides having a different crystal structure from wBN. The boron nitride polycrystal of Example 4 had a wBN composition ratio of about 99.9% by volume, and contained hBN as another boron nitride having a crystal structure different from that of wBN. Moreover, in the boron nitride polycrystal of Examples 1-4, the average particle diameter of the granular crystal was 32 to 101 nm. Moreover, the boron nitride polycrystals of Examples 1 to 4 had a hardness of 40 GPa or more and a fracture toughness value of 8.0 MPa · m ^1/2 or more.

On the other hand, the boron nitride polycrystals of Comparative Examples 1 and 2 did not have wBN, and the boron nitride polycrystal of Comparative Example 3 had a small composition ratio of wBN of about 30% by volume. Moreover, the hardness of the boron nitride polycrystals of Comparative Examples 1 and 3 was comparable to those of the boron nitride polycrystals of Examples 1 to 4, but the fracture toughness value was 7.9 MPa · m ^1/2 or less. there were. The boron nitride polycrystal of Comparative Example 2 was lower in both hardness and fracture toughness than the boron nitride polycrystal of Examples 1 to 4. In addition, since the boron nitride polycrystals of Examples 1 to 4 and Comparative Examples 1 to 3 were produced by a direct conversion method, inevitable impurities were all below the detection limit in the X-ray diffraction method.

  From Table 1, it was confirmed that the boron nitride polycrystals of Examples 1 to 4 had improved crack propagation resistance compared to the boron nitride polycrystals of Comparative Examples 1 to 3.

  Although the embodiments and examples of the present invention have been described above, various modifications can be made to the above-described embodiments and examples. Further, the scope of the present invention is not limited to the above-described embodiments and examples. The scope of the present invention is defined by the terms of the claims, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  1 Boron nitride polycrystal, 2 base metal, 3 brazing layer, 4 metallized layer.

Claims (2)

Preparing a normal pressure boron nitride having a graphitization index of less than 3.5 in the X-ray diffraction method as a starting material;
Heat treating the atmospheric pressure boron nitride,
The step of performing the heat treatment includes a step of pressurizing and heating to a pressure of 8 GPa or more and a temperature of 1200 ° C. or more and less than 2200 ° C., and a step of maintaining the condition. 2. The method for producing a boron nitride polycrystal according to claim 1 , wherein in the heat treatment, the atmospheric pressure boron nitride is directly converted into wurtzite boron nitride and simultaneously sintered.

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