JPH0451512B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a high-density composite sintered body containing aluminum nitride and boron nitride. Specifically, a molded body of a mixed powder consisting of aluminum nitride powder, hexagonal boron nitride powder, a group a metal of the periodic table, and at least one metal or metal compound selected from group a metals is heated at 2 to 100 atmospheres. Provided is a method for producing a composite sintered body, characterized by sintering at a temperature of 1750 to 2400°C in a nitrogen gas atmosphere. So-called non-oxide ceramics such as silicon nitride and silicon carbide are ceramics that have recently attracted particular attention as high-temperature, high-strength materials, but once these sintered bodies are sintered, it is difficult to give them an arbitrary shape by post-processing. This has become extremely difficult, and this problem is considered to be one of the major practical difficulties. For this reason, in the field of oxides, glass ceramics in which many fine mica crystals are precipitated in glass, and so-called mica ceramics obtained by hot-pressing natural or synthetic mica powder have recently been developed. These ceramics are machinable ceramics that can be cut and ground with ordinary tools, and are useful as ceramics with improved workability. Furthermore, among non-oxidizing ceramics, hexagonal boron nitride has a layered graphite structure, so its sintered body can be cut and ground with ordinary tools, and has been in practical use for a long time. However, although the above-mentioned various machinable ceramics have the advantage of being easy to process, they also have fundamental drawbacks such as being "soft" and "weak", so they cannot be widely used as ceramics. I'm not used to it. As a result of intensive research on non-oxide machinable ceramics, the present inventors have found that aluminum nitride, hexagonal boron nitride, and at least one metal compound selected from Group A metals or Group A metals of the periodic table. We have already developed and proposed a new composite sintered body. The composite sintered body is a sintered body in which crystal grains with polygonal mechanical fracture surfaces are closely packed, and thin layered crystal grains are interposed in part or all of the grain interface of the packed particles. It is a revolutionary ceramic that can be cut at high speed with ordinary tools, and has a bending strength of 35 kg/mm ² or more depending on the composition, which is comparable to high-strength ceramics. As a result of intensive research into the manufacturing method of the composite sintered body, the present inventors found that aluminum nitride powder,
obtained by sintering a mixed powder of hexagonal boron nitride powder and at least one metal compound selected from group a metals or group a metals of the periodic table under nitrogen gas pressure. The present inventors have discovered that the mechanical strength and thermal conductivity of the sintered body can be improved while maintaining the machinability with ordinary tools, and have completed the present invention. That is, the present invention uses aluminum nitride powder 50 to 97
parts by weight, 3 to 50 parts by weight of hexagonal boron nitride powder, and 0.1 to 10 parts by weight of at least one metal or metal compound selected from Group A metals and Group A metals of the periodic table.
This is a method for producing a composite sintered body, characterized by sintering a molded body of mixed powder consisting of parts by weight at a temperature of 1750 to 2400°C in a nitrogen gas atmosphere of 2 to 100 atmospheres. Any type of aluminum nitride powder can be used as the aluminum nitride powder of the present invention, but when obtaining a dense composite sintered body, the average particle diameter of the aluminum nitride powder (centrifugal particle size distribution analyzer, for example, CAPA500 manufactured by Horiba) is used. It is preferable that the average particle diameter of aggregated particles measured by methods such as 5 μm or less is 5 μm or less.
Powders with a diameter of 3 μm or less, most preferably 2 μm or less are used. Particularly suitable is a powder containing 70% by volume or more of particles of 3 μm or less. In addition, when obtaining a composite sintered body with high thermal conductivity, aluminum nitride powder with an AlN content (calculated from the nitrogen content of the AlN powder) of 90% by weight or more is preferably used, and even 94% by weight. % or more, and more preferably 97% by weight
The above powders are employed. The aluminum nitride powder preferably used in the present invention has an average particle diameter of 2 μm or less, contains 70% by volume or more of particles of 3 μm or less, has an oxygen content of 3.0% by weight or less, and has an aluminum nitride composition. When is AlN, the cationic impurity contained is
Aluminum nitride powder containing 0.5% by weight or less. When such aluminum nitride powder is used, it is preferably used in the present invention because it can improve the thermal conductivity of the resulting composite sintered body and suppress a decrease in mechanical strength at high temperatures. In particular, powders with an average particle diameter of 2 μm or less, and particles of 3 μm or less
Contains 70% by volume or more, oxygen content 1.5% by weight or less,
In addition, when using aluminum nitride powder containing 0.3% by weight or less of cationic impurities when the aluminum nitride composition is AlN, the resulting composite sintered body has improved thermal conductivity and mechanical strength at high temperatures. It is particularly preferably used in the present invention because it has a remarkable effect of suppressing the decrease in . The method for producing the aluminum nitride powder is not particularly limited, and any known method can be used. For example, a method in which metal aluminum is nitrided in nitrogen and then pulverized, a method in which alumina is reduced and nitrided, a method in which metal aluminum is arc discharged in nitrogen, and AlCl ₃ and
Method by gas phase reaction with NH ₃ , (NH ₄ ) ₃ AlF ₆ and
A method using thermal decomposition of AlCl ₃ /NH ₃ etc. can be adopted. The boron nitride powder is not particularly limited, and any hexagonal boron nitride powder can be used. Typical examples that are generally suitably used are as follows. Hexagonal boron nitride powder that is generally preferably used has a boron nitride purity of 99.0% by weight or more and an average particle size of 5 μm or less. Furthermore, the method for producing the hexagonal boron nitride powder is not particularly limited, and any known method can be employed. For example, (1) converting H ₃ BO ₃ or Na ₂ B ₄ O ₇ to NH ₃ in the presence of urea.
A method of manufacturing by heating at 500 to 950°C in an atmosphere, (2) A method of manufacturing by reacting BCI ₃ and NH ₃ , (3) A method of manufacturing by heating Fe-B alloy at a temperature of 500 to 1400°C,
After that, a method of removing Fe by dissolving it with an acid, for example, can be adopted. Further, at least one metal or metal compound selected from the group a metals and group a metals of the periodic table (hereinafter sometimes simply referred to as a sintering aid).
is not particularly limited, and any known material can be used. Typical examples that are particularly preferably used are as follows. Generally, beryllium, calcium, strontium, barium, etc. are suitable as the metal of Group a of the periodic table. Yttrium or lanthanum group metals are preferably used as metals belonging to group a of the periodic table. More specifically, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium,
Preferred are dysprosium, holmium, erbium, thulium, ytterbium, lutetium, and especially yttrium, lanthanum, cerium, neodymium, and the like. These metal compounds of group a or group a of the periodic table are not particularly limited, and the metal compounds known as sintering aids for aluminum nitride powder and/or hexagonal boron nitride powder can be used. In general, compounds such as nitrates, carbonates, halides, and oxides are preferably used. Further, the composition ratio of each component constituting the composite sintered body of the present invention can be selected from a wide range depending on the properties required of the composite sintered body. Generally, for example, aluminum nitride is in the range of 50 to 97 parts by weight, preferably 65 to 95 parts by weight, boron nitride is in the range of 3 to 50 parts by weight, preferably 5 to 35 parts by weight, and 0.1 to 10 parts by weight of at least one metal or metal compound selected from group metals and group a metals;
It is preferable to select the content in the range of 0.2 to 5 parts by weight. The aluminum nitride powder, hexagonal boron nitride powder, and sintering aid described above are mixed, then shaped, and subjected to sintering under nitrogen gas pressure. Here, the sintering aid does not necessarily need to be added to the aluminum nitride powder or hexagonal boron nitride powder, and the sintering aid is added in advance to the aluminum nitride powder or hexagonal boron nitride powder during production. It may also be mixed into the raw material for producing the powder. The mixing of the aluminum nitride powder, boron nitride powder, and sintering aid in the present invention is not particularly limited, and may be dry mixing or wet mixing. A particularly preferred embodiment is wet mixing, ie mixing in the wet state using a liquid dispersion medium. The liquid dispersion medium is not particularly limited, and commonly used water, alcohols, hydrocarbons, or mixtures thereof are preferably used. In particular, methanol, ethanol,
Lower alcohols with 4 or less carbon atoms such as butanol. In addition, the wet mixing device used for the above mixing is not particularly limited and any known device may be used, but especially when used for mixing highly pure aluminum nitride powder and hexagonal boron nitride powder,
It is preferable to select a material that does not generate impurity components due to the material. For example, the material may be aluminum nitride itself, a plastic material such as polyethylene, polyurethane, nylon, or a material coated with these materials. Further, when obtaining the composite sintered body, addition and mixing of a binder, a deflocculant, a plasticizer, etc. in addition to the raw materials may be appropriately employed as necessary. Specific aspects of sintering in the present invention include:
The mixed powder containing the aluminum nitride powder, hexagonal boron nitride powder, and sintering aid is molded into the desired shape by a suitable molding method such as a dry press method, a rubber press, an extrusion method, an injection method, a doctor blade sheet molding method, etc. The most commonly used method is to mold the material into a shape, place it on a suitable crucible, pod material, etc., and sinter it at high temperature under nitrogen pressure. In another embodiment, a method is adopted in which the mixed powder consisting of the aluminum nitride powder, hexagonal boron nitride powder, and sintering aid is sintered under gas pressure while applying a mechanical pressure of 50 to 500 Kg/ ^cm2. be able to. The nitrogen gas pressure employed in the present invention ranges from 2 to 100 atmospheres. When sintering under 2 atmospheres, that is, around atmospheric pressure or under reduced pressure, it is often difficult to improve the thermal conductivity and mechanical strength of the composite sintered body due to thermal decomposition of AlN at high temperatures. Further, a gas pressure of 100 atmospheres or more is not preferable because it does not improve the effect much and is accompanied by equipment difficulties. The nitrogen gas pressure that can be suitably employed is in the range of 4 to 20 atmospheres. Further, in the present invention, the nitrogen gas atmosphere only needs to be composed mainly of nitrogen gas, and there is no problem even if it partially contains gases such as argon and helium. Sintering of the present invention is carried out at 1750-2400℃, preferably 1800-2400℃.
It is carried out in a temperature range of 2300℃. At temperatures below 1750°C, it may be difficult to improve the thermal conductivity and mechanical strength of composite sintered bodies, and at temperatures above 2400°C,
This is not preferable because the thermal decomposition of AlN increases. The sintering temperature may be appropriately determined depending on the particle size of the aluminum nitride powder and hexagonal boron nitride powder and the nitrogen gas pressure. The sintering time varies depending on the sintering temperature, but is usually selected from a range of 30 minutes to 10 hours. As is clear from the mechanical fracture surface of the composite sintered body obtained by the method of the present invention shown in FIG. 1 of the attached drawing, the mechanical fracture surface is filled with polygonal crystal grains. It is composed of thin layered crystal grains interposed in part or all of the grain interface of the filled particles. The crystal grains whose mechanically fractured surfaces are polygonal in FIG. 1 are sintered crystal grains of aluminum nitride. The aluminum nitride sintered grains are formed by a close packing of fine crystal grains whose mechanical fracture surfaces are distinguished from each other by clear contours, and the fracture surfaces of the fine crystal grains are The clear outline in is a polygonal shape, and the fine crystals have an average particle size in the fracture surface defined by the clear outline in the range of 0.3D to 1.8D, defined as D (μm). It is composed of at least 70% of crystal grains having a particle size of . Approximately 0.3D~1.8D
An aluminum nitride sintered body that has a fracture surface in which 70% or more of the particles are present in the range of , that is, an aluminum nitride sintered body that has polygonal particles of relatively uniform size on the fracture surface, has not been known so far. Another feature of the appearance of the mechanically fractured surface of aluminum nitride is that the crystal planes appearing on the fractured surface of individual particles form relatively smooth flat surfaces. This indicates that the crystal grains have almost no foreign phase (which usually appears as small circular depressions on the fracture surface) that are formed due to the incorporation of impurities or gas phase. ing. Furthermore, from FIG. 1, it is clear that the grains filled with polygonal crystal grains made of aluminum nitride have thin layered crystal grains interposed in part or all of the grain interface. These thin layered crystal grains are sintered crystal grains made of hexagonal boron nitride. As clearly shown in Figure 1, on the mechanically fractured surface of the sintered body, thin layered crystal grains made of hexagonal boron nitride existing at the grain interface of the aluminum nitride particles are machined into the composite sintered body of the present invention. It is estimated that this is an important factor that gives Brucellamic's properties. That is, the thin layer-like crystal grains made of hexagonal boron nitride existing at the grain interface of the aluminum nitride particles absorb the force applied from the outside during cutting and prevent the aluminum nitride sintered body from breaking.
We believe that this enables high-speed cutting of composite sintered bodies using ordinary tools. From a photograph of a mechanically fractured fracture surface of the composite sintered body obtained in the present invention, it is not possible to confirm at least one metal compound selected from Group A metals and Group A metals of the periodic table. The metal compound can generally be analyzed using an X-ray microanalyzer or a fluorescent X-ray analyzer.
In addition, the content may be confirmed to be reduced compared to the amount of the metal compound or the substance that can become the metal compound mixed with the aluminum nitride powder and hexagonal boron nitride powder before the sintering process. . That is, it is considered that a part of the metal compound or a substance that can become the metal compound may sublimate during sintering. However, although the amount present is small, the metal compound in the composite sintered body obtained by the present invention functions to assist in the sintering of aluminum nitride powder and hexagonal boron nitride powder, and It is thought that this, together with the crystalline boron nitride powder, is a factor that gives these composite sintered bodies the properties of machinable ceramics. The composite sintered body obtained by the present invention is a sintered body with high density and high purity, and generally has a purity of, for example, 90% or more, preferably 95% or more and 2.5g/cm ³ or more, preferably 2.7g/cm It has a density of ³ or more. Sintering under nitrogen gas pressure according to the present invention has a remarkable effect on improving the thermal conductivity and mechanical strength of the composite sintered body, as will be clear from the examples described below.
This is because pressurized nitrogen gas is used during high temperature sintering.
Sintering is accelerated to suppress thermal decomposition of AlN,
This is thought to be due to the fact that the pores in the sintered body are finally removed to a large extent. Like the normal pressure sintering method, the sintering method under pressurized nitrogen according to the present invention enables the production of products with complex shapes in large quantities, and therefore has extremely high industrial value. The present invention will be specifically illustrated below with reference to Examples, but the present invention is not limited to these Examples. Example 1 80 parts by weight of aluminum nitride powder with an average particle size of 1.31 μm and 3 μm or less and the composition shown in Table 1, and the ratio of particles with an average particle size of 2.5 μm and a particle size of 5 μm or less was 95% by volume. 20 parts by weight of hexagonal boron nitride powder with a purity of 99.5% and barium nitrate 3
Parts by weight were uniformly mixed in a ball mill using a nylon pot and a nylon-coated ball with ethanol as a dispersion medium. After drying the obtained mixed powder, about 12 g was rubber plated at a pressure of 2000 Kg/cm ² to form a disk-shaped compact with an outer diameter of about 40 mm and a thickness of about 6.3 mm. This compact was placed in a graphite crucible whose inner wall was coated with boron nitride powder.
It was fired at a temperature of 2200° C. for 3 hours under nitrogen gas pressure. The density of the obtained white sintered body is 2.93g/
It was warm at ^cm3 . A columnar test piece approximately 3 mm square was cut out from this sintered body, finished with 1500-grit sandpaper, and set at a crosshead speed of 0.5 mm/min.

[Table] As a result of measuring the three-point bending strength with a span of 20 mm, an average value of 42 Kg/mm ² was obtained. Also from the same sample, approximately 10 mm in diameter
A disk-shaped sample with a diameter of 25 mm and a thickness of 25 mm was cut out, and its thermal conductivity at room temperature was determined by the laser flash method to be 93 W/mK. Furthermore, when the workability of the composite sintered body obtained in this example was investigated, it was found that it was easy to perform both drilling with a carbide drill and high-speed cutting with a carbide cutting tool, and was free-cutting. Example 2 Table 2 shows the results of sintering in the same manner as in Example 1 with different composition ratios of aluminum nitride, hexagonal boron nitride, and group a or group a metal compounds of the periodic table. Example 3 Approximately 12 g of the same mixed powder of aluminum nitride powder, hexagonal boron nitride powder, and barium nitrate as used in Example 1 was filled into a graphite mold with an inner diameter of 40 mm whose inner wall was coated with boron nitride powder, and 200 kg/ While applying uniaxial mechanical pressure of cm ²
Sintering was performed at 2100°C for 3 hours under nitrogen gas pressure of 9.8 atm. The obtained sintered body had a density of 2.95 g/cm ³ and could be easily cut. The bending strength and thermal conductivity of this sintered body were measured under the same conditions as in Example 1 and were found to be 44 Kg/mm ² and 89 W/mK, respectively. Example 4 As aluminum nitride, the average particle size is 1.8 μm.
3 μm or less accounts for 75% by volume, and is the main impurity.
A mixed powder was prepared in the same manner as in Example 1, except that aluminum nitride powder containing 0.07% Fe, 0.03% Si, and 0.08% Ti and having an oxygen content of 2.1 wt% was used. This mixed powder was molded into a disk shape under the same conditions as in Example 1, and this molded body was sintered at a temperature of 2150° C. for 3 hours under nitrogen gas pressure of 9.8 atmospheres. The obtained sintered body had a density of 2.89 g/cm ³ and was a machinable ceramic that could be easily drilled and ground with a carbide drill or cutting tool. The bending strength and thermal conductivity of this sintered body were 31 Kg/mm ² and 47 W/mK, respectively.

[Table] Comparative Examples 1 to 2 The same procedure as in Example 1 was carried out except that the mixing ratio of AlN, BN and metal compounds and firing conditions in Example 1 were changed as shown in Table 3. The physical properties of the obtained sintered body were as shown in Table 3.

【table】

【table】 [Brief explanation of drawings]

FIG. 1 is a scanning electron micrograph showing the sintered body particle structure of the mechanically fractured surface of the composite sintered body obtained in Example 1.

Claims (1)

[Claims] 1 50 to 97 parts by weight of aluminum nitride powder, 3 to 50 parts by weight of hexagonal boron nitride powder, and at least 1 selected from group a metals and group a metals of the periodic table.
Manufacture of a composite sintered body characterized by sintering a compact of mixed powder consisting of 0.1 to 10 parts by weight of a metal or metal compound at a temperature of 1750 to 2400°C in a nitrogen gas atmosphere of 2 to 100 atm. Method.