Manufacturing method for composite diamond sintered bodies

Abstract

In particular, the objective is to provide a diamond sintered body that can be applied to the processing of all materials, including iron, does not accelerate the graphitization phase transition by binders, and can be manufactured under the same pressure and temperature conditions as currently common cobalt-based diamond sintered bodies (PCDs). [Solution] A method for producing a w-BN-based boron nitride bonded composite diamond sintered body, characterized by densely mixing diamond particles and wurtzite-type boron nitride (w-BN) powder, distributing the w-BN between the diamond particles to form a binary starting composition, and simultaneously subjecting the starting composition to a pressure of 4.5 GPa or more and less than 8 GPa and a temperature of 1200°C or more and less than 2000°C to bond adjacent diamond particles together via the inter-particle w-BN phase present between the particles, thereby integrating them.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a composite diamond sintered body in which diamond particles are consolidated and integrated through a mixed phase of wurtzite-type boron nitride (w-BN) or a mixed phase of wurtzite-type boron nitride and a relatively small amount of cubic boron nitride (c-BN).

Background Art

[0002] The present invention particularly relates to a diamond sintered body that can be applied to the processing of various material types including ferrous metal materials as a cutting tool element excellent in hardness and heat resistance, and can be used for cutting processes in a wide range of fields, and a method for manufacturing the same.

[0003] Sintered bodies obtained by binding powdered diamond, which is an abrasive with high hardness and excellent wear resistance, have been widely used for the production of chips and the like as suitable cutting tool materials. Such sintered bodies are also called polycrystalline diamond (PCD), and generally, a sintering aid metal in a molten state (for example, cobalt (Co)) is caused to flow between diamond powders under ultrahigh pressure and high temperature, and the diamond powders are integrated through the molten phase, and are widely used as tool materials.

[0004] However, cobalt as a binder acts as a catalyst for graphitizing diamond from around 700°C, and this effect becomes significant due to the heat generation during cutting, resulting in a problem of heat resistance that makes it difficult to use under high-temperature conditions.

[0005] Diamond sintered bodies using transition metals or ceramics such as carbides and nitrides of transition metals as binders instead of sintering aid metals have also been put into practical use. However, since the thermal expansion coefficients of metals and ceramics are significantly larger than that of diamond, the loss of diamond particles due to the strain generated at the interface between diamond and the binder at high temperatures has been a factor reducing the tool life.

[0006] On the other hand, the high reactivity of diamond itself with iron, the material being cut, at high temperatures is a significant drawback that cannot be ignored. Therefore, there is a need to develop a diamond-based mass that overcomes these problems inherent in diamond sintered bodies, can exhibit the extremely hard properties of diamond as a cutting tip material, and is also applicable to cutting iron-based materials.

[0007] Methods for preparing polycrystalline diamond (masses) without using cobalt are known. For example, methods using alkaline earth carbonates (Patent Document 1) or boron carbide (Patent Document 2) as binders instead of cobalt, methods using metal carbides (Patent Document 3), and methods for creating a single, integrated diamond without using a binder (Patent Document 4) are known.

[0008] In the method described in Patent Document 1, 0.5 to 15 wt% of boron (B) powder is added to diamond powder as a doping material to impart conductivity, and alkaline earth carbonate powders such as Mg and Ca are added as components to form the binder phase of the sintered body. In the first stage, conductivity is imparted to the diamond powder by the diffusion of B, and in the second stage, a conductive diamond sintered body is obtained by dissolving and filling the gaps between the diamond powder particles with the binder phase. These processes require ultra-high pressure and high temperature, and the second stage in particular is carried out at 6.0 to 9.0 GPa and 1600 to 2500°C.

[0009] The method described in Patent Document 2 requires the permeation of boron carbide, which has a melting point of 2450°C, into the spaces between diamond particles in a molten or semi-molten state. Even considering the decrease in sintering temperature due to fine powdering, heating to around 2000°C is required. Maintaining a thermodynamically stable state of diamond at this temperature requires maintaining an ultra-high pressure of 7 GPa or more, further increasing the burden on the sintering apparatus.

[0010] In the method described in Patent Document 3, a reaction is disclosed in which a starting material containing diamond particles and transition metal powder is subjected to a reaction temperature of 2000°C or higher, a ceramic phase containing transition metal carbides is formed by the reaction between the components of the starting material, and the diamond particles are bonded to each other via the ceramic phase to form a diamond-based sintered composite material. In this reaction, a combustion synthesis reaction technique is used in the ceramic formation reaction to at least partially melt and densely integrate the entire material.

[0011] The method described in Patent Document 4 involves the simultaneous direct conversion of graphite to diamond and sintering, resulting in a tough sintered body composed solely of diamond. However, in order to ensure the thermodynamic stability of diamond during the high-temperature reaction, it is necessary to maintain an even higher pressure of 8 GPa or more. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] Japanese Patent Publication No. 2008-133173 [Patent Document 2] U.S. Patent No. 3,136,615 [Patent Document 3] Japanese Patent Publication No. 2021-70091 [Patent Document 4] Japanese Patent Publication No. 2012-106925 [Non-patent literature]

[0013] [Non-Patent Document 1] RC DeVries Report No. 72 CRD178 June 1972, TECHNICAL INFORMATION SERIES, General Electric. [Non-Patent Document 2] Z. Pan et al., Physical Review Letters, 102, 055503 (2009). The American Physical Society. [Non-Patent Document 3] Akashi, Tamotsu et al., Journal of the Chemical Society of Japan, 1981 (9), pp. 1416-1421. [Overview of the project] [Problems that the invention aims to solve]

[0014] The present invention aims to provide a diamond sintered body that can be applied to the processing of all materials including iron, does not accelerate the graphitization phase transition by the binder, and can be manufactured under the above-mentioned pressure and temperature (ultra-high pressure and high temperature), which are currently common conditions for manufacturing cobalt-based diamond sintered bodies (PCDs). [Means for solving the problem]

[0015] The inventors have discovered that wurtzite-type boron nitride (w-BN), which had previously been considered only a secondary material in the production of sintered diamond particle bodies, can actually serve as an effective binder. Based on this discovery, they have established a method for producing sintered bodies with excellent hardness and heat resistance by constructing a binary starting composition by blending both materials in an effective composition ratio and processing it under a suitable high-pressure and temperature range.

[0016] The gist of the present invention is to densely mix diamond particles and wurtzite-type boron nitride (w-BN) powder, distribute the w-BN between the diamond particles to form a binary starting composition, and simultaneously subject the starting material to a pressure of 4.5 GPa or more and less than 8 GPa and a temperature of 1200°C or more and less than 2000°C to bond and integrate adjacent diamond particles via the w-BN phase present between the particles. [Effects of the Invention]

[0017] The sintered body of the present invention is a binary system composed of diamond and wurtzite-type boron nitride (w-BN) as a sintering material. In addition to w-BN approximating diamond in physical properties such as the coefficient of thermal expansion, it is a high thermal conductivity material second only to diamond. Since it does not contain other sintering aids, especially components such as cobalt (Co) that promote the graphitization of diamond, even in tools where the cutting edge is used in a red-hot state, graphitization does not occur, and it has the feature of maintaining stable machining performance due to an excellent heat dissipation effect.

Embodiments for Carrying Out the Invention

[0018] The production of the sintered body according to the present invention is carried out by treating a starting composition composed of a binary system of diamond and w-BN under ultrahigh pressure and high temperature. Diamond accounts for 60% or more and 90% or less of the total mass, and w-BN containing a small amount of inevitable impurities accounts for the remaining amount (10% or more and 40% or less). As will be described later, a part of the w-BN phase in the starting composition can be transformed into cubic boron nitride (c-BN) under the processing pressure and temperature conditions of the present invention, but this is not a problem in the present invention.

[0019] In the present invention, the w-BN mixed with diamond and used as part of the starting material is a high-pressure metastable phase of boron nitride, and industrially it is produced by applying an impact load by the explosion of gunpowder to hexagonal boron nitride (hereinafter h-BN). Since the duration of the synthesis reaction under the impact load is an extremely short time in the order of microseconds, crystal growth does not progress, and the reaction product is obtained in the form of an aggregate (powder) of polycrystalline particles of about several tens of μm in which primary particles of several tens of nm are mosaically aggregated.

[0020] Furthermore, a large amount of lattice defects are contained in the primary particles. In addition to the defect repair function at high temperatures, the movement of atoms accompanying the transition from the metastable phase to the stable phase cubic boron nitride (c-BN) is also recognized as the driving force for the sintering reaction.

[0021] Diamond and the high-pressure phase of boron nitride, particularly c-BN and w-BN, have similar physical properties, and it can be said that w-BN and c-BN are especially similar. For example, the lattice constant of diamond is 3.57 Å, while that of c-BN is 3.62 Å, and the coefficient of thermal expansion of diamond is 4.5 × 10⁻⁶. -4 (750℃), c-BN is 4.3 × 10 -4 (700℃) (Non-Patent Literature 1). Although data on the thermal expansion coefficient of w-BN is not available, it is estimated to be approximately equal to that of c-BN based on the similarity of its atomic composition and lattice constant. On the other hand, while the Hv hardness of diamond is said to be 90 GPa and that of c-BN is 45 GPa, it is also said that w-BN has a hardness of 114 GPa (Non-Patent Literature 2), which exceeds that of diamond.

[0022] w-BN is a high-pressure stable phase of the BN system, but it is known to gradually transition to c-BN at temperatures above 1200°C under high-pressure conditions. Theoretically, the transition rate increases with increasing temperature, but in actual operation, it is also affected by processing conditions and methods, such as the particle size and residual impurities of the w-BN used, the method of filling the apparatus, and the heating method.

[0023] Since both the sintering reaction via w-BN and the phase transition to c-BN that may occur during the sintering reaction in this invention are solid-phase reactions, it is essential that the amount of w-BN added be at least 5% by mass, preferably 10% or more, and that it be uniformly distributed throughout the entire area surrounding the diamond particles and throughout the entire diamond particle layer.

[0024] The size of the diamond particles can be selected depending on the application of the resulting sintered body, but generally, micron-sized abrasive grains with an average particle size of 1 to 20 μm or less are preferred from the standpoint of compatibility with conventionally used diamond sintered bodies. Furthermore, when hardness is a priority in the composite sintered body, it is preferable to have a diamond particle ratio (mass) of 2 / 3 or more in the starting mixture.

[0025] In the method of the present invention, since the sintering process is by solid-phase reaction, direct bonding between diamond particles cannot be expected, and general particle rearrangement, dissolution, and precipitation that are common in liquid-phase sintering cannot be expected. Therefore, it is necessary to reduce the gaps between diamond particles in advance. As a method for this, it is effective to compose the diamond portion of the starting material with multiple granular products with different average particle diameters, that is, main diamond particles with a larger average particle diameter and diamond particles with a smaller average particle diameter for gap filling. In this case, it is particularly preferable to use diamond particles with an average particle diameter of 1 / 4 or less of that of the main diamond particles.

[0026] Several methods can be used to prepare the starting material. For example, one method involves first blending (mixing) several whole-grain diamond products with different particle sizes, and then mixing them with w-BN. Alternatively, to distribute the powdered w-BN as a binder throughout, w-BN and gap-filling diamond particles are mixed first, and then this mixture is mixed with the main diamond particles to form the starting material. A standard ball mill can be used to mix the components of the starting material.

[0027] The sintering reaction of diamond via w-BN according to the present invention can be clearly confirmed at temperatures of 1400°C or higher. Therefore, in the present invention, it is preferable to set the sintering reaction temperature to 1400°C or higher, and the reaction duration to 5 minutes or more, taking into account that the sintering process is a solid-phase reaction with a slow diffusion rate.

[0028] According to the present invention, a phase transition from w-BN to c-BN may occur during sintering under high temperature and high pressure. In such a phase transition, Non-Patent Literature 3 states that the Hv hardness of the sintered product is 1 × 10⁻¹⁰ when the phase transition rate from w-BN to c-BN is 90%. 4 kg / mm 2 Since it is noted that it reaches its highest value, it is considered advantageous from the perspective of manufacturing hard materials. [Examples]

[0029] As sintering raw materials, diamond particles of Tomei Diamond's IMM8-16 (average particle size approximately 10 μm) and w-BN powder (particle size 1-3 μm or less) obtained from Nippon Engis Co., Ltd. were weighed in a mass ratio of 9:1 and mixed into a powder using a Turbra (product name) shaker mixer. This mixed powder was filled into a niobium (Nb) capsule with an inner diameter of 63 mm and a depth of 6 mm, subjected to vacuum degassing treatment at 700°C, and then sintered by simultaneously applying ultra-high pressure and high temperature of an estimated pressure of 5.0 GPa and an estimated temperature of 1400°C and holding for 20 minutes.

[0030] The sintered product is polished on both sides and has a density of 3.48 g / cm³. 3 A black disk with a Vickers hardness of 6.8 GPa and a flexural strength of 1.01 GPa was obtained. w-BN was observed around 2θ=41° in the X-ray diffraction pattern. <100> The peak height is the diamond <111> The peak was only 1 / 50th of the main peak, and no peak corresponding to c-BN was observed. [Examples]

[0031] As sintering raw materials, 75% by mass of a 3:1 mixture of Tomei Diamond's IMM8-16 (average particle size approximately 10 μm) and IMM1 / 2-3 (average particle size approximately 1.6 μm) diamond particles, along with 25% by mass of the same w-BN powder, were used. These mixed powders were sintered using the same method as above, at an estimated pressure of 6.5 GPa and an estimated temperature of 1550 °C, with a holding time of 15 minutes. The sintered product had a density of 3.46 g / cm³. 3 It has a Vickers hardness of 7.0 GPa and a flexural strength of 1.08 GPa, and the height of the w-BN peak in the X-ray diffraction pattern is that of diamond. <111> The c-BN peak is about 1 / 16th of the main peak, and together it accounts for about 1 / 30th of the total c-BN. <111> The existence of a peak that is presumed to be present was observed.

Claims

1. A method for producing a w-BN-based boron nitride bonded composite diamond sintered body, characterized by densely mixing diamond particles and wurtzite-type boron nitride (w-BN) powder, distributing the w-BN between the diamond particles to form a binary starting composition, and simultaneously subjecting the starting composition to a pressure of 4.5 GPa or more and less than 8 GPa and a temperature of 1200°C or more and less than 2000°C to bond adjacent diamond particles together via the w-BN phase present between the diamond particles, thereby integrating them.

2. The method according to claim 1, wherein the binary starting composition contains, by mass ratio of 60 to 90% diamond particles and 40 to 10% wurtzite-type boron nitride (w-BN) with respect to the total amount of the mixture.

3. The method according to claim 1, wherein the w-BN phase present between the diamond particles contains cubic boron nitride (c-BN) generated by a transition of a portion of the w-BN phase.

4. The method according to claim 1, wherein the w-BN phase present between the diamond particles is composed only of w-BN and unavoidable impurities.

5. The method according to claim 1, wherein the average particle size of the diamond particles is 1 to 20 μm.

6. The method according to claim 1 or 5, wherein the diamond particles constituting the starting composition are a mixture of main diamond particles having a large average particle diameter and gap-filling diamond particles having a smaller particle diameter.

7. The method according to claim 6, wherein the main diamond particles and gap-filling diamond particles are each composed of a plurality of sized particles with different average particle sizes.

8. The method according to claim 6, wherein the main diamond particles and gap-filling diamond particles are each composed of a plurality of sized particles with different average particle sizes, and the average particle size of the gap-filling diamond particles is 1 / 4 or less compared to the average particle size of the main diamond particles.

9. (1) First, the gap-filling diamond particles and w-BN are mixed to make a preliminary mixture, and then (2) Mix the preliminary mixture with the main diamond particles. The method according to claim 1, wherein a starting composition is formed by the means described herein.