JPH0127022B2 - - Google Patents

Description

[Detailed description of the invention]

There are two types of high-pressure phase boron nitride (BN), cubic crystal and wurtzite crystal, both of which have a hardness second only to diamond, and are used as materials for grinding and cutting.
It is considered extremely promising. It is already widely used for cutting purposes. Cubic type for cutting applications
Some sintered bodies made of BN bonded with metals such as Co are on the market on a trial basis. This metal-bonded BN sintered body is a type of cermet, and has a problem in terms of heat resistance. That is, it is unsuitable for applications where the temperature of the cutting edge increases, such as when cutting at high speed. The present invention has heat resistance as a bond,
It uses a metal compound with excellent wear resistance and is intended to provide high strength. The inventors first discovered cubic boron nitride (abbreviated as
Invented a highly hard sintered body for tools made by bonding CBN) with a metal compound with excellent heat resistance and high thermal conductivity, and filed a patent application (Japanese Patent Application No. 154570, No. 1983).
54666). The inventors applied the same idea to CBN to wurtzite BN, which is another high-pressure phase of boron nitride, and as a result of various studies, they arrived at the present invention. Wurtzite-type BN can be synthesized using a dynamic ultra-high pressure generation method using shock waves using hexagonal BN as a raw material. This method has the advantage that it can be produced at a lower cost than CBN, which is synthesized using a static ultra-high pressure device. In this synthesis method using the shock wave method, the duration of pressure and temperature during synthesis is short, so the time for crystal growth is limited, and the grain size of Wurtz-type BN crystals synthesized by this method is generally less than 10μ. many. Japanese Patent Publication No. 50-39444, filed by one of the inventors, discloses a sintered body obtained by using this wurtzite type BN as a raw material and sintering it under high temperature and high pressure. It has been shown that a sintered body made only of type BN has higher hardness than CBN single crystal and also has superior heat resistance. The inventors investigated the characteristics and performance of this wurtzite-type BN sintered body in detail, and found that there were many variations from one sintered body to another, and that there was room for improvement in both toughness and wear resistance in terms of performance as a tool. I discovered something. As mentioned above, wurtzite-type BN synthesized using shock waves is an extremely fine powder, and its particle shape is complex and its surface has many irregularities, so it has a large surface area. Therefore, gas is easily adsorbed on the particle surface, and oxides and hydroxides are formed on the particle surface. When sintering using such fine powder, it is not difficult to release this gas from the system if the material to be sintered is not sealed. However, as in the present invention,
When sintering under ultra-high pressure, the gas generated is
It is almost impossible to escape from the heating system. In such cases, it is common knowledge in the powder metallurgy industry to perform a degassing treatment in advance, but this poses a problem if the degassing temperature cannot be raised sufficiently. This is exactly the case in this case. That is, considering the transformation of wurtzite-type BN to a low-pressure phase, there is an upper limit to the heating temperature. The degassing process of fine powder involves the following stages depending on the temperature. First, at low temperatures, physically adsorbed substances and hygroscopic water are removed. Decomposition of chemisorbed substances and hydroxides then occurs. At the end, oxide remains. In the case of wurtzite-type BN, it is stable up to about 800°C, so it can be heated to this temperature in advance. Therefore, it can be considered that if the gas is degassed and heated in advance, the residual gas components remain in the form of oxides. Conversely, since it is desired that gas components remain in the sintered body as little as possible, it is preferable to remove all water and hydrogen as a preliminary treatment. However, this thermal degassing treatment cannot remove oxides on the particle surface; in the case of wurtzite-type BN particles, oxides remain, probably in the form of boron oxide (B ₂ O ₃ ). Boron oxide has a low melting point (450°C) and becomes glassy at high temperatures. It also easily absorbs moisture from the atmosphere and becomes hydroxide. Conventional wurtzite-type BN sintered bodies are sintered with this B ₂ O ₃ remaining on the surface of the crystal grains, resulting in variations in properties depending on the amount, and when used in cutting tools etc. This was one of the causes of reduced wear resistance and toughness. The present invention solves this problem and provides a high-performance, stable sintered body for tools. The inventors sintered a mixed powder of wurtzite-type BN and various heat-resistant compound powders and examined the characteristics and performance.
We have found that when carbides of group 4a and 5a metals are used, high-performance sintered bodies with stable properties can be obtained. The reason why such a good sintered body can be obtained when carbides of metals from groups 4a and 5a of the periodic table are used is considered to be as follows. That is, these compounds of metals of groups 4a and 5a of the periodic table are expressed in the form MC ₁ ± _x , where M is Ti, Zr, Hf of group 4a of the periodic table or V, Nb, Ta of group 5a (where x means the presence of atomic vacancies or relatively excess atoms of C) having a composition range with a certain width on the M-C phase diagram. Especially the group 4a metal Ti,
Zr and Hf have a wide range of existence. For example, taking the carbide expressed in the form of MC ₁ ± _x as an example, the reason why a good sintered body is obtained when MC ₁ ± _x is added is considered to be as follows. That is, on the surface of the wurtzite-type BN powder, there is an oxide, probably in the form of B ₂ O ₃ , that cannot be removed by heating and degassing in a vacuum, for example.
When this B ₂ O ₃ and M corresponding to (free M) of MC ₁ ± _x react, B ₂ O ₃ +4M→MB ₂ +3MO (1) and no gas is generated. MO and MC form a mutual solid solution. This is thought to be the reason why a good sintered body can be obtained when carbides of metals from groups 4a and 5a of the periodic table, represented by MC ₁ ± _x , are added. Further, these compounds of metals of groups 4a and 5a of the periodic table can form solid solutions with carbides and nitrides of Cr, Mo, and W, which are metals of group 6a of the periodic table. Such composite carbides, nitrides, and carbonitrides can also be applied to the sintered body of the present invention as long as they have an existence range expressed in the form (M, M') C ₁ ± _x . The inventors used wurtzite-type BN and these compound powders ( ₁
As a result of prototyping sintered bodies by mixing various composition ranges of (1 ± _x ) and investigating the properties and performance, the value of ( _{1 ±} It was confirmed that a strong and good quality sintered body can be stably obtained when a compound powder with holes is used. TiC ₁ ±
The case where a sintered body is created by mixing _x will be explained in more detail. Considering the case of TiC addition of 60% and 10%, the amount of oxygen that wurtzite-type BN retains even after preheating is probably 1% at most, so in these extreme cases, TiC _1-x Calculating the minimum required amount of x for TiC Wurtzite BN 60% 40% x = 0.056 10% 90% x = 0.757. That is, when the amount of TiC is large, it is sufficient to use TiC _1-x with a small x value, and when it is small, it is necessary to use a TiC with a large x value. As shown in Figure 1, TiC exists in a wide range of areas. The lower limit is approximately
TiC is _0.5 . The upper limit is TiC _0.98 . Therefore 90% BN−
In the case of 10% TiC, if the amount of oxygen is large, TiC and Ti
It is necessary to add a mixture of However, this is not preferable because the amount of TiC used as a binder decreases, and rather the amount of oxygen in the BN used should be reduced. The preferred value of (1-X) varies depending on the metal element used, but in the case of TiC _1-x , B ₂ O ₃ is decomposed during sintering and exists as a Ti (C, O) phase in the sintered body. The range is approximately (1-X)0.6
It is. The Ti--C--O solid solution phase remaining in _the sintered body is a stable compound that has much higher heat resistance and hardness than _B2O3 . Also, when B ₂ O ₃ decomposes as shown in equation (1) above,
Although TiB ₂ is produced, it does not deteriorate the performance of the sintered body because it is also a highly hard and heat-resistant compound. Figure 2 is a phase diagram of the Ti-C-B system.
It has been shown that some B is dissolved as a solid solution in TiC _1-x , and the compounds produced by the reaction between B ₂ O ₃ and TiC _1-x include TiB ₂ , TiB, and Ti-C-O solid solution. niTi
It is believed that a -C-O-B solid solution may also be formed. Now, the amount of the compound powder added to the wurtzite type BN in the sintered body of the present invention needs to be selected depending on the oxygen content in the raw BN powder and the composition of the compound powder to be added. Limited in terms of performance when used as The content of wurtzite-type BN is expressed as volume % in the sintered body.
If it is less than 10%, the hardness of the sintered body is low, and there is no noticeable difference in performance from a sintered body that does not contain BN. Further, if the amount of the added compound in the sintered body is less than 10% by volume, most of the oxides contained in the wurtzite-type BN powder are decomposed, which is insufficient to fully exhibit the effects of the present invention. A particularly suitable composition range is wurtzite type.
The content of BN is in the range of 30 to 70% by volume in the sintered body. In this limited composition range, by selecting the particle sizes of the raw BN powder and the additive compound powder as described later, the additive compound according to the present invention forms a continuous binder phase in the sintered body, and the surface of the BN particles is Since the BN powder is in an enclosed state, the oxides on the surface of the raw BN powder are completely decomposed and dissolved into the binder phase compound during sintering, making it possible to obtain a tough sintered body. In producing the sintered body of the present invention, the particle size of the wurtzite-type BN powder used is not particularly limited, but the particle size of BN obtained by the shock wave method is generally 10μ.
It is the following fine powder. This BN powder and periodic table number
Mix compound powder mainly consisting of carbides of group 4a and 5a metals. The particle size of this compound powder must be finely dispersed between the BN powder particles in order to exhibit the effects of the present invention. Therefore, it is better to use particles with a particle size finer than that of BN powder, and in the present invention, all powders with a particle size of 10 μm or less, preferably 1 μm or less are used. This mixed powder or stamped body is heated and degassed to a temperature of 800°C or less under a vacuum of 10 ^-4 mmHg or higher, and then sintered at high pressure and high temperature using an ultra-high pressure device. The ultra-high pressure equipment used is a girdle-type, belt-type, etc. equipment used for diamond synthesis. A graphite cylinder is used as the heating element, and talc and NaCl are placed inside it.
The CBN mixed powder molded body is wrapped with an insulating material such as. A pressure medium such as pyroferrite is placed around the graphite heating element. Sintering pressure and temperature conditions are the third
Although it is desirable to carry out the process within the stable region of the high-pressure phase type boron nitride shown in the figure (i.e., the temperature and pressure conditions above the line A-A' in Figure 3), this equilibrium line is not necessarily determined accurately. This is just one guideline. The B-B' line in Figure 3 shows the metastable region of wurtzite-type BN, and under the pressure and temperature conditions in the region surrounded by the B-B' line and the A-A' line, It is known that the BN type converts from the cubic type to the cubic type BN. In producing the sintered body of the present invention, part or all of the wurtzite type BN may be converted into cubic boron nitride by sintering within this region. When the sintered body of the present invention is used, for example, as a cutting tool, it is sufficient that the hard sintered body layer containing high-pressure phase boron nitride forms the cutting edge portion of the tool,
The supporting portion may be made of another high-strength material that can withstand stress and heat applied to the tool during cutting. A suitable material for use as such a support is a high-strength cemented carbide, which has the same rigidity as the hard sintered body layer. In particular, WC-based cemented carbide has good thermal conductivity and is ideal as a support for a hard sintered body layer. This support and the sintered body of the present invention containing high-pressure phase type boron nitride are bonded together during sintering under high pressure and high temperature, in contact with the stamped body of the mixed powder that forms this hard sintered body layer. Cemented carbide can be placed and sintered and joined at the same time. If this hard sintered body forming part is composed only of wurtzite boron nitride powder, it is difficult to obtain a sintered body that is firmly bonded to the cemented carbide support even under high pressure and high temperature.
In the sintered body of the present invention, a compound phase mainly composed of carbides of Group 4a and 5a metals of the periodic table is finely dispersed between the boron nitride particles, and these compounds and the cemented carbide are also present at the bonding interface. Stronger bonding can be obtained because it has greater chemical affinity than boron nitride. The sintered body of the present invention has excellent wear resistance, high hardness and toughness, and is used for tool materials and wire drawing dies for cutting high hardness steel such as hardened steel and high hardness cast iron. . Examples will be described below. Example 1 Wurtzite type BN made by the shock wave method with a particle size of 2 microns or less has a carbon content of 14.9% with a volume of 40%.
Add TiC powder with a particle size of 1 micron (TiC _0.7 ),
Wet ball mill mixing was performed for 50 hours using acetone as a solvent. As a result of analysis, the oxygen content of the wurtzite-type BN powder as a raw material was 0.7%. This mixed powder was pressed to have an outer diameter of 10 mm and a thickness of 1.5 mm. This embossed body was placed in a hollow cylindrical container with a bottom made of iron. Place this in a vacuum furnace at 700℃ x 20
Degassing was performed under the condition of 10 ^-5 mmHg for 10 min. This degassed product was placed in a belt device, which is one of the ultra-high pressure devices mainly used for diamond synthesis. Pyroferrite was used as the pressure medium and graphite as the heater. Increase the pressure to 55Kb and then increase the temperature to 1250℃.
Hold for 15 minutes. After lowering the temperature, the pressure was reduced and the sintered body was taken out. After grinding the sintered body using a diamond grindstone, it was wrapped with diamond paste. The Vickers hardness was measured on the wrapped surface and was approximately
It was 3800Kg/ ^mm2 . In addition, when the same surface was examined by X-ray diffraction, it was found that wurtzite-type BN and Ti(C,O)
In addition to the diffraction lines corresponding to the solid solution, a weak diffraction line of TiB ₂ was observed. Example 2 Wurtzite-type BN powder and TiC _0.8 similar to Example 1
An embossed body was made from the mixed powder. WC- pre-sintered in a steel bottomed container similar to Example 1.
A disk made of cemented carbide having a composition of 6% Co and having an outer diameter of 10 mm and a thickness of 3 mm was placed, and the embossed body was placed in contact with the disk. Thereafter, a sintered body was obtained in the same manner as in Example 1.
The 1 mm thick layer of hard sintered body containing wurtzite-type BN was firmly bonded to the cemented carbide disk. A notch was made in the sintered body using a diamond cutting blade, and the cut was broken to observe the cross section of the sintered body. Wurtzite type using an X-ray microanalyzer
When the interface between the hard sintered body layer containing BN and the cemented carbide was observed, no diffusion of Co in the cemented carbide into the hard sintered body layer was observed. The sintered body joined to this cemented carbide was brazed to a steel support to create a cutting tool. This cutting tool was used to cut a heat-treated material of SKD11 with a Rockwell C scale hardness of 60. Even after cutting for 20 minutes at a cutting speed of 100 m/min, depth of cut of 0.2 mm, and feed rate of 0.1 mm per revolution using water-soluble cutting oil, the flank wear width was 0.18 mm, making it possible to cut for even longer periods of time. . On the other hand, JIS classification which is the hardest cemented carbide
When cutting with KO ₁ , flank wear of more than 0.20 mm was observed in just 30 seconds, and further cutting was impossible. Example 3 Various compounds shown in Table 1 were blended with the wurtzite boron nitride used in Example 2.

[Table] A sintered body was produced in the same manner as in Example 1.
The pressure and temperature conditions during sintering were as shown in Table 1, and all sintering was carried out under these conditions for 15 minutes. All of them were good sintered bodies, and the hardness measurement results were as shown in Table 1.

[Brief explanation of drawings]

FIG. 1 is for explaining the range of TiC used in the sintered body of the present invention on the Ti-C phase diagram. FIG. 2 is a ternary system phase diagram of Ti-C-B, which is used to explain the state in which the additive compound of the sintered body of the present invention exists in the sintered body. FIG. 3 is for explaining the manufacturing conditions of the sintered body of the present invention, and shows the region where high-pressure phase type boron nitride exists on the P-T phase diagram.

Claims (1)

[Claims] 1. Wurtzite boron nitride powder and periodic table 4a,
When the Group 5a metal is M, the value of MC ₁ ± _x ( ₁ ± _x ) is 95% or less of the upper limit of the stable state of the metal carbide.
In a sintered body obtained by hot-pressing a mixed powder of a compound powder mainly composed of 60% or more carbide under high pressure and high temperature, wurtzite-type boron nitride or a part of wurtzite-type boron nitride during sintering. The total amount of high-pressure phase boron nitride, which is all converted to cubic boron nitride, is 10% or more and 90% in the volume of the sintered body.
M contains the following, and the remainder is formed by solid solution of the metal carbide and almost the entire amount of oxygen in the wurtzite boron nitride.
A highly hard sintered body for a tool, characterized in that it mainly consists of a solid solution compound phase in the form of (C, O). 2 In the sintered body according to claim 1, M(C,
A sintered body for a tool with high hardness, characterized in that the solid solution phase of O) forms a continuous binder phase in the sintered body.