JPH0457634B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a boron carbide compact, and particularly to a method for producing a high-density boron carbide compact by short-time pressure sintering. Boron carbide has a hardness second only to diamond, excellent wear resistance and corrosion resistance, and a high melting point.
It has characteristics such as high neutron absorption capacity. Based on this characteristic, it is used for abrasive materials, metal additives (hardening), nuclear reactor control materials, etc.
Furthermore, the molded products are used for nozzles such as sandblasting nozzles, liquid honing nozzles, and spray dryer nozzles, gauges, dressers, hard mortars and pestles, and hand wrappers. In particular, in applications that require wear resistance, boron carbide molded bodies preferably have a relative density of 90% or more from the viewpoint of durability, and practical application of boron carbide molded bodies with a relative density lower than 90%. The situation is almost non-existent. BACKGROUND ART Conventionally, when manufacturing a boron carbide molded body, it is known that fine powder of boron carbide is used and a pressureless sintering method or a pressure sintering method is used. However, the relative density of boron carbide compacts obtained by pressureless sintering is as low as approximately 80% (Japanese Patent Application Laid-Open No. 48-42009), and high-density compacts cannot be obtained by pressure sintering. However, it has the disadvantage of poor productivity because it takes a long time. The present inventors conducted various studies with the aim of compensating for these drawbacks of the conventional technology and obtaining a boron carbide molded body with a relative density of 90% or more in a short time by pressure sintering. It was found that the sintering properties, especially the sintering speed, were greatly affected. A crystallite here refers to the size of a region (three-dimensional) in which atoms are regularly arranged, and is the smallest unit of a perfect crystal when viewed from an x-ray perspective. From the results of numerous experiments, it was found that the smaller the size of the crystallites in the raw material powder, the more remarkable the growth of the crystallites and the faster the sintering speed, leading to the present invention. That is, the present invention provides 2MgO・B ₂ O ₃ and/or
Magnesium borate whose main component is 3MgO・B ₂ O ₃ ,
It is manufactured by reducing and carbonizing a mixed raw material of magnesium and carbon, and the size of the primary particles is
This is a method for producing a boron carbide molded body, which is characterized by pressurizing and sintering boron carbide powder having a crystallite size of 10 μm or less and a crystallite size of 800 Å or less. The present invention will be explained in more detail below. The boron carbide (hereinafter referred to as B ₄ C) powder used in the present invention can be produced by, for example, the method for producing boron carbide powder described in Japanese Patent Application No. 154410/1982 (Japanese Patent Application Laid-open No. 46909/1983). , a magnesium borate containing 2MgO・B ₂ O ₃ and/or 3MgO・B ₂ O ₃ as the main component is used, and a mixed raw material of magnesium and carbon is heated at a temperature of 1450 to 1650℃, especially 1600℃.
It is produced by reduction and carbonization at about ℃. The sinterability of B ₄ C powder is influenced by the size of the primary particles, resulting in a high-density B ₄ C compact (hereinafter referred to as a compact).
In order to obtain this, the size of the primary particles must be 10 μm or less, preferably 5 μm or less. The size of the primary particles is
If it exceeds 10 μm, sinterability deteriorates and it is difficult to obtain a high-density compact. The reason for this is that the specific surface area value, which is proportional to the surface activation energy value that directly affects sinterability, becomes smaller. Next, it is necessary that the crystallite size of the B ₄ C powder used in the present invention is 800 Å or less, preferably 600 Å or less. When the crystallite size exceeds 800 Å, the sintering speed becomes slow and it takes a long time for sintering. Furthermore, to explain the B ₄ C powder used in the present invention, the present inventors have discovered that the size of the primary particles is
It has been found that even B ₄ C powder with a particle size of 10 μm or less has good sinterability when the crystallite size is 800 Å or less, but when it exceeds 800 Å, the sinterability deteriorates. This is because in the sintering mechanism of B ₄ C, the growth of crystallites controls its sinterability, and the smaller the size of the crystallites of B ₄ C powder, the more remarkable the growth of crystallites. This is because sinterability is improved. A hot press furnace or a hot isostatic pressure furnace can be used as a pressure sintering device for pressure sintering the B ₄ C powder described above. For example, when using a hot press furnace, a carbon die serving as a molding frame is filled with B ₄ C powder and heated while being pressurized to obtain a molded body. The sintering conditions for pressure sintering the B ₄ C powder are normal, and the pressure sintering temperature is preferably 1800 to 2200°C, and the pressure sintering pressure is preferably 50 to 350 kg/cm ² . Pressure sintering temperature is less than 1800℃ or pressure sintering pressure is 50Kg/
If it is less than cm ² , it is not preferable because it takes a very long time to obtain a high-density molded product. Furthermore, when the pressure sintering temperature exceeds 2200℃, it is the melting point of B ₄ C.
This is not preferable because the temperature approaches 2450°C and a reaction occurs with the carbon die used as the molding frame, making it impossible to obtain a product of consistent quality. Furthermore, if the pressure sintering pressure exceeds 350 kg/cm ² , a high-density compact can be obtained in a short time, but the carbon die used as a reaction frame will be damaged extremely, which is not desirable in terms of production cost. do not have. As explained above, according to the method of the present invention, by using a predetermined B ₄ C powder and sintering it under pressure, it is possible to obtain a compact with a relative density of 90% or more in a short time. Yes, the effect of improving productivity is significant. Next, the present invention will be described in more detail with reference to examples. Example 1 B ₄ C powder with a primary particle size of 5 μm and a crystallite size of 600 Å was pressed into a die with a cold isostatic pressure of 50 kg/cm ² in advance, and then placed in a pressure sintering furnace. (Manufactured by Fuji Denpa Kogyo Co., Ltd., FHP-10)
was used for pressure sintering under the conditions shown in Table 1 Experiment Nos. 1 to 6 to obtain a cylindrical molded body with a diameter of 15 mm and a height of 20 mm. The results are shown in Table-1. The B ₄ C powder used was obtained from patent application No. 154410/1986.
Experiment No. 1 in the specification (Japanese Unexamined Patent Publication No. 60-46909)
It was obtained by reducing and carbonizing magnesium borate at 1600°C as described in . The physical properties of the B ₄ C powder and compact were measured by the following method. (1) Physical properties of B ₄ C powder (1) The size of the primary particles was determined using a scanning electron microscope (SEM Akashi Seisakusho MSM-4). (2) The size of crystallites is determined using an X-ray diffraction device (Rigaku
Geigerflex 2013 model) was used to calculate B ₄ C, which was corrected by the half-width of the 111 diffraction line of silicon, using powder X-ray diffraction method using silicon powder as an internal standard.
The average value of the crystallite size determined from the half width of the 012 and 104 diffraction lines was used. (2) Physical properties of the compact (1) The density was determined by dividing the weight of the compact by the volume calculated from the dimensions. (2) The relative density was determined by dividing the density of the compact by the theoretical density of B ₄ C, 2.51 g/cm ³ , and multiplying by 100. Furthermore, the required time is the time (minutes) from the start of heating and pressurizing until the end of heating and pressurizing. Comparative Example 1 For comparison, B ₄ C powder with a primary particle size of 15 μm and a crystallite size of 1000 Å or more is shown below.
1Except that it was used under the conditions shown in Experiment Nos. 7 to 12,
A molded article was obtained in the same manner as in Example 1. The results are shown in Table-1. In addition, B ₄ C powder was prepared by heating the B ₄ C powder of Example 1 at 2000°C.
It was obtained by heat treatment.

[Table] As can be seen from Table 1, the time required for sintering becomes shorter as the particle size and crystallite size become smaller. Example 2 2200 using B ₄ C powder of Table 2 Experiment Nos. 13 to 15
The same procedure as in Example 1 was carried out except that a molded body was obtained under pressure sintering conditions of -350 Kg/ ^cm2 . The results are shown in Table-2. Comparative Example 2 The same procedure as in Example 2 was carried out except that the B ₄ C powder of Experiment Nos. 16 to 18 was used to obtain a molded body for comparison. The results are shown in Table-2. As can be seen from Table 2, the smaller the crystallite size and particle size, the shorter the time required and the better the productivity. In addition, the B ₄ C powder in Experiment Nos. 13 to 15 was obtained from a patent application filed in 1982.
154410 (Japanese Unexamined Patent Publication No. 60-46909) was used, and
B ₄ C powders No. 16 to 18 were commercially available products (Eagle Pitcher Co., Ltd.).

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Claims (1)

[Claims] 1 Manufactured by reducing and carbonizing a mixed raw material of magnesium borate, magnesium, and carbon whose main components are 2MgO・B ₂ O ₃ and/or 3MgO・B ₂ O ₃ , and the size of the primary particles is 10 μm. 1. A method for producing a boron carbide molded body, which comprises pressurizing and sintering boron carbide powder having a crystallite size of 800 Å or less.