JP2004250278A - High purity ultrafine particle translucent cubic boron nitride sintered compact and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an ultrafine particle translucent cBN sintered compact which contains cBN in the amount of 100%, which has never been obtained by a conventional technology for synthesizing a high purity cBN sintered compact, and which has a fine structure such that the diameters of the constitutive particles are ≤0.1 μm and is translucent. <P>SOLUTION: The high purity ultrafine particle cBN sintered compact which contains cBN in the amount of 100%, has an average particle diameter in the sintered compact of ≤0.1 μm, and is translucent is manufactured by using, as a raw material, low pressure phase boron nitride, and sintering it at a pressure not lower than 9.5×10<SP>4</SP>atm and at a temperature of 1,700 to 1,900°C at which the cubic boron nitride (cBN) is thermodynamically stable without adding a sintering aid while accompanying a high pressure phase transition to the cubic crystal phase. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention has hardness next to diamond, is more stable than diamond for iron-based metals, and can be expected to bring technological innovation to the conventional machining field as a cutting tool for iron-based metals, abrasives, and the like. The present invention relates to a high-purity ultrafine particle translucent cubic boron nitride (cBN) sintered body and a method for producing the same.
[0002]
[Prior art]
Various forms of cBN sintered bodies have been used as cutting tools and abrasives for ferrous metals. These have excellent features in machining iron-based materials and occupy an important position in the machining field, which is a modern industrial base. The cBN sintered bodies used so far are composite sintered bodies sintered together with various sintering aids in the range of 40,000 to 50,000 atm, and the cBN content varies from about 40 to 90 wt%. Things are being developed. When used as a cutting tool, the content of cBN is controlled in accordance with the characteristics of the work material. In recent years, however, it has been found that a high-purity sintered body having a cBN content of 100% exhibits excellent cutting performance. (Non-Patent Document 1).
[0003]
As a method for synthesizing a high-purity cBN sintered body having a cBN content of 100%, a low-pressure phase boron nitride such as hexagonal material is used as a raw material and simultaneously sintering while promoting a phase transition to a high-pressure phase cBN under high pressure and high temperature. A reaction sintering method in which a reaction proceeds and a direct sintering method in which sintering is performed without using a sintering aid using cBN crystal particles prepared in advance as a raw material are known. Thus, synthesis of a dense translucent cBN sintered body having a constituent particle diameter of about 0.5 μm in the region of 77,000 atmospheres and 2000 ° C. has been reported to date (Non-Patent Documents 2, 3). ).
[0004]
On the other hand, it is known that the strength of a sintered body depends on the constituent particle diameter.However, in a structure in which fine particles are densely sintered, the strength of the sintered body is improved because the size of the internal microcracks becomes smaller. It is known to When the sintered body is used as a cutting tool, the surface roughness of the machined surface of the workpiece is affected by the size of the constituent particle diameter of the sintered body. Therefore, when the purpose is precision processing such as mirror finishing of a material, it is desired that the particle size of the sintered body be as small as possible.
[0005]
Further, increasing the strength of the translucent cBN sintered body has practical significance, such as application to not only cutting tools but also high-strength window materials as industrial applications. However, the light-transmitting high-purity cBN sintered body obtained to date has a particle size of about 0.5 μm even for a fine-grained high-purity cBN sintered body. Has not been reported.
[0006]
[Non-patent document 1]
H. Sumiya and S.M. Uesaka, J .; Mater. Res. , 35, 1181 (2000)
[Non-patent document 2]
M. Akaishi, et al. Mater. Sci. Let. , 12, 1883 (1993)
[Non-patent document 3]
T. Taniguchi et al. Mater. Res. , 14, 162 (1999)
[0007]
[Problems to be solved by the invention]
In order to improve the efficiency of the machining process represented by the automobile industry and realize environmental conservation, there is a demand for improving the characteristics of existing cBN sintered body tools. Fine control of the structure of the sintered body is indispensable for the improvement of the characteristics of the cBN sintered body tool. For this purpose, it is necessary to supply a high-purity cBN sintered body having a cBN content of 100% in which fine particles are densely sintered. It is essential.
[0008]
The synthesis of a high-purity cBN sintered body by an existing technique has been promoted in a 77,000 atmospheric pressure region. At this time, the constituent particle diameter of the translucent high-purity cBN sintered body that can be synthesized is a fine particle diameter. It is about 0.5 μm or more. A high-purity cBN sintered body having a fine structure with a particle size of 0.1 μm or less has not been obtained, and its application to a high-performance cutting tool for precision cutting, a high-strength window material, or the like has not been advanced.
[0009]
That is, the problem to be solved by the present invention is a fine structure with a constituent particle diameter of 0.1 μm or less, and a cBN content of at least 0.1 μm, which could not be achieved by the conventional high-purity cBN sintered body synthesis technology. The purpose is to obtain a 100% high-purity ultrafine particle translucent cBN sintered body.
[0010]
[Means for Solving the Problems]
In the process of synthesizing a high-purity cBN sintered body by reaction sintering using low-pressure phase boron nitride such as hexagonal as a raw material, the minimum temperature condition for obtaining a good sintered body is from low-pressure phase boron nitride to cBN. Corresponds to the lower temperature limit at which the high pressure phase transition is completed. When the sintering is performed at a temperature lower than this, the low-pressure phase component remains in the structure of the sintered body, and a good sintered body cannot be obtained.
[0011]
On the other hand, the constituent particle diameter of the sintered body increases due to the grain growth with the increase of the sintering temperature, so it is necessary to keep the sintering conditions low in order to synthesize a sintered body of fine particles on the nanoscale order. Become. The lower limit of the synthesis temperature in the 77,000 atmosphere range is about 2000 ° C., and under this condition, a highly pure translucent cBN sintered body having a particle size of about 0.5 μm is synthesized. In order to synthesize a sintered body having a finer particle size than this, it is necessary to suppress grain growth, and for this purpose, it is necessary to reduce the sintering temperature.
[0012]
Although the phase transition behavior of the material from the low pressure phase to the high pressure phase depends on pressure and temperature, the phase transition behavior of boron nitride was examined in the range of 60,000 to 90,000 atmospheres. As a result, it was found that the temperature at which all the remaining low-pressure phase was converted to the cubic phase, which was the high-pressure phase, decreased with increasing pressure. That is, at a pressure higher than 77,000 atmospheres, the phase transition completion temperature decreases from the hexagonal phase to the cubic phase. As a result, the synthesis temperature can be reduced, and synthesis of an ultrafine particle sintered body in which grain growth is suppressed can be expected. Therefore, a synthesis experiment of a cBN sintered body was performed using a hexagonal boron nitride as a raw material at a pressure of 950,000 atm.
[0013]
As the hexagonal boron nitride raw material, a commercially available sintered body or hexagonal boron nitride powder can be used. However, usually, the hexagonal boron nitride as a raw material contains boron oxide as a main impurity, which causes a reduction in the strength of the obtained sintered body. Therefore, a heat treatment at 2000 ° C. for 2 hours was performed on the raw material hexagonal boron nitride in a nitrogen stream to reduce the oxygen impurity concentration in the raw material by about one digit from the initial 0.5%. Using the deoxygenated hexagonal boron nitride as a raw material, a high-pressure treatment was performed at 95,000 atmospheres at a temperature in the range of 1000 ° C. to 2000 ° C., and the characteristics of the obtained sintered body were evaluated. The obtained sintered body was a single phase of cBN at a synthesis temperature of 1700 ° C. or higher, and exhibited translucency. According to observation by a scanning electron microscope (SEM) and a transmission electron microscope (IEM), the constituent particle diameter was 0.1 μm or less, and the structure was uniform. No phase other than cBN was observed by X-ray diffraction, and the Vickers hardness was 55 GPa at a test load of 49 N, which was much higher than that of a cBN sintered body using a normal sintering aid.
[0014]
When the synthesis temperature was lower than 1700 ° C., the low-pressure phase BN component remained, and a decrease in the transmittance and a decrease in the hardness were observed. When the synthesis temperature exceeds 1900 ° C., the transmittance of the sintered body increases, but the hardness is reduced. According to the microstructure observation, grain growth up to a particle diameter of 1 μm or more was observed.
[0015]
From the above experiments, it was found that cubic boron nitride was thermodynamically stable at 95,000 atm and high-pressure phase to cubic phase under pressure and temperature conditions of 1700 ° C to 1900 ° C using hexagonal boron nitride as a raw material. It has been found that by sintering without addition of an auxiliary while accompanied by transition, a high-purity, high-hardness translucent cBN ultrafine particle sintered body having a dense structure with an average particle diameter of 0.1 μm or less can be synthesized. The present invention has been made based on this finding. The optimum temperature condition for synthesizing the fine-grained sintered body is between the lower limit temperature for completing the conversion from the low-pressure phase to the high-pressure phase and the upper limit temperature for suppressing the grain growth.
The optimum conditions depend on the pressure, and the higher the pressure, the wider the range of the optimum temperature can be. Therefore, if the pressure region is 950,000 atmospheres or more, the fine-grained sintered body can be synthesized. . Usually, a belt-type ultra-high pressure generator is suitable as this kind of high-pressure device, but such a device can generate a high pressure up to about 100,000 atmospheres. If another type of high-pressure generator (for example, a polyhedral high-pressure device) is used, higher pressure can be generated, but in this case, the sample volume becomes as small as about 10 mm ³ or less. For this reason, considering the economics of industrial production, the upper limit of the conditions for synthesizing the sintered body may be about 100,000 atmospheres using a belt-type high-pressure apparatus.
[0016]
That is, the present invention provides a high-purity ultrafine-particle translucent sintered body that could not be obtained by the prior art and the prior art by treating the low-pressure phase boron nitride under a higher pressure condition than the prior art. Things.
[0017]
【Example】
Hereinafter, the present invention will be described with reference to examples and drawings.
Example 1
A hexagonal boron nitride sintered body (particle diameter: about 0.5 μm) that has been subjected to a deoxidation treatment by heat treatment at 1500 ° C. in a vacuum and 2000 ° C. in a nitrogen stream is filled in a tantalum capsule in a high-pressure container, and a belt type super Sintering was performed for 30 minutes at a pressure and temperature of 95,000 atmospheres and 1700 ° C. using a high pressure generator. At this time, no cBN sintering aid was added. The heating rate was about 5 ° C./min. After cooling at about 500 ° C./min, the pressure was released and the sample was collected together with the tantalum capsules in the pressure vessel.
[0018]
The tantalum capsule was removed by mechanical or chemical treatment (a mixture of hydrofluoric acid and nitric acid), and the sample was recovered. The sample was evaluated by hardness measurement, SEM observation of the fractured surface, TEM observation, and phase identification by X-ray diffraction after polishing with diamond abrasive grains. From the X-ray diffraction pattern of the sintered body shown in FIG. 1, the sintered body is a single phase of cBN, and as shown in the SEM and TEM observation photographs of FIGS. 2 and 3, the sintered body has a uniform average particle diameter of 0.1 μm or less. It had a texture and no coarse particles due to abnormal grain growth or the like were observed.
[0019]
Further, a hardness test showed that the sintered body was a high hardness and high purity cBN fine particle sintered body having a Vickers hardness of about 55 GPa (a load of 49 N). The sintered body showed the wavelength dependence of the light transmittance as shown in FIG. When the thickness of the sintered body was 0.6 mm, the light transmittance in a visible light region (wavelength 500 to 800 nm) to an infrared region (wavelength 3000 nm) was about 10%. The sintered body exhibits translucency as seen in an optical micrograph taken together with FIG.
[0020]
Comparative Example 1
Under the sintering conditions of 950,000 atmospheres and 1600 ° C. or lower in the process described in Example 1, the components of the low-pressure phase boron nitride remained in the sintered body and did not become a single-phase cBN sintered body. Further, when the sintering temperature exceeds 1900 ° C., the obtained sintered body is a single phase of cBN and has high purity and high translucency, but grain growth is observed in the structure, and the hardness of the sintered body decreases. It was observed.
[0021]
In the method for producing a high-purity fine-grained cBN sintered body of the present invention, it was clarified from the comparative example that the sintering temperature was important for obtaining a good sintered body structure. Examples and Comparative Examples show that it is important to optimize the sintering temperature under higher pressure conditions than in the prior art when producing a high-purity ultrafine translucent cBN sintered body in the present invention. I have.
[0022]
【The invention's effect】
In the present invention, the reaction sintering method using low-pressure phase boron nitride as a raw material is performed in a 95,000-atmosphere region, so that the light-transmitting high hardness having an average particle diameter of 0.1 μm or less, which cannot be obtained by the conventional technology. It has become possible to supply a cBN ultrafine particle sintered body.
[Brief description of the drawings]
FIG. 1 is an X-ray diffraction pattern of a high-purity translucent ultrafine particle cBN sintered body synthesized in Example 1.
FIG. 2 is a scanning electron micrograph of a fractured surface of the high-purity translucent ultrafine particle cBN sintered body synthesized in Example 1 instead of a drawing.
FIG. 3 is a transmission electron micrograph instead of a drawing of a fracture surface of the high-purity translucent ultrafine particle cBN sintered body synthesized in Example 1.
FIG. 4 is a drawing-substitute optical microscope photograph showing the wavelength dependence of the light transmittance of the high-purity translucent ultrafine particle cBN sintered body synthesized in Example 1 and the translucency of the sintered body.

Claims (2)

Using low-pressure phase boron nitride as a raw material, cubic boron nitride (hereinafter referred to as cBN) is thermodynamically stable at a pressure of 950,000 atmospheres or more and 1700 ° C or more and 1900 ° C or less. High-purity ultrafine cBN sintered body having a cBN content of 100%, a sintered body having an average particle diameter of 0.1 μm or less, and exhibiting light transmissivity, which is sintered with no aid added while accompanied by a phase transition. Using low-pressure phase boron nitride as a raw material, cubic boron nitride (hereinafter referred to as cBN) is thermodynamically stable at a pressure of 950,000 atmospheres or more and 1700 ° C or more and 1900 ° C or less. High-purity ultrafine cBN particles having a cBN content of 100%, a sintered body having an average particle diameter of 0.1 μm or less, and exhibiting light transmissivity, characterized by sintering without the addition of an auxiliary while accompanied by a phase transition. Manufacturing method of sintered body.

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