JPS61132563A - Manufacture of boron nitride sintered body - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for producing a boron nitride sintered body, and more particularly, to a method for producing a dense sintered body by sintering under normal pressure.

[Conventional technology]

Boron nitride (hereinafter referred to as BN) has properties such as excellent thermal conductivity, coefficient of thermal expansion, good corrosion resistance, electrically insulating holes, good lubricity, machinability, and light weight. For this reason, they are used in industrial fields that require these properties, such as electrical insulating materials, high-temperature heat transfer materials, and corrosion-resistant materials, such as electric furnaces and processing of molten materials.

However, the high price of BN is one reason why the amount of BN used has not increased despite its wide range of applications.One reason for the high price is the manufacturing cost of the sintered body. The sintered body is manufactured by a hot press method, but in this case, the material costs of the hot press equipment and graphite dies are expensive, and the production efficiency is poor, which are the causes of increased costs.

One possible method for manufacturing BN sintered bodies at low cost is pressureless sintering, but there is a problem with the characteristic sintering behavior of BN. That is, sintering of BH progresses as crystal grains grow, and at this time, the volume of the sintered body expands. In a general sintering process, pores are removed and shrinkage occurs due to particle diffusion and grain growth, and therefore densification progresses, but the opposite phenomenon occurs in BN sintering. Therefore, in order to make a dense N sintered body, we have to rely on pressure sintering such as hot pressing, and since BN has poor self-sintering properties, it is difficult to sinter it alone. Pressure sintering also requires auxiliary agents such as B2O3 and CaO.

Conventionally, as an example of pressureless sintering and reaction sintering,
-38047 and Special Publication No. 48-43648, but both are BN and 5i02 or BN and A9. , Si, and the blending ratio of BN is in the range of 30 to 70%. Composites have limited applications because the original excellent properties of BN are diminished.

[Problem that the invention seeks to solve]

In the sintering method of BN, pressure sintering is expensive, and pressureless sintering does not make it dense.
The present invention proposes a method for manufacturing a dense S sintered body by suppressing expansion during sintering without pressure sintering.

[Means for solving problems]

The present invention involves making a compact as dense as possible in the molding stage, processing it using a lathe, etc., and then inserting the compact into a mold with high strength and low coefficient of thermal expansion without any gaps, and placing the entire mold in an inert atmosphere. It is sintered with Since a mold with a low coefficient of thermal expansion is used, the Ia! It is possible to suppress tension as much as possible, and to produce a sintered body having a density close to that of commercially available molded bodies.

The mold needs to have low thermal expansion and strength to overcome the expansion during the sintering process. In addition, the heat resistance temperature needs to be over 1700℃, so the materials include graphite, AmiN,
B4C, TiC1TiB2 are preferred.

Furthermore, the higher the density of the molded product, the better; the BN grains themselves have lubricity, so they have excellent moldability. If high pressure of 5 to 10t/cm' is applied during rubber press molding, the theoretical density will be 8.
Reach 5-90%.

The method of the present invention includes ■Molding of high-density compacts ■Machining (■ Insertion into low thermal expansion mold ■Baking It consists of four steps. The explanation will be given below.

Methods for forming the molded body include a rubber press method and a mold method.

The rubber press method is a method in which compression molding is performed by isotropically applying pressure to powder filled in a rubber mold, and it is possible to obtain a molded product with uniform density and low particle orientation. The shape of BN powder is a hexagonal plate, and since uniaxial molding such as a mold method causes particle orientation, a rubber press method of isostatic pressure molding is suitable.

Figure 1 shows the relationship between compact density and rubber press pressure. The curve 1 shows the case where BN powder is granulated with a spray dryer, and the 3rd curve shows the case without granulation, using granulated raw materials. and the density of the compact is improved. The BN powder in Figure 1 is 1
It is manufactured using ugliness as a raw material.

The reason why the rubber press pressure was selected to be 5 to 10 Ton/crn' is as follows. Rubber press pressure is 5Ton
/crn”, the density of the compact is significantly improved, but up to 5Ton
/Cm', the increase in density slows down to 10T/c
There is no improvement above m'. The higher the density of the BN sintered body, the better, but it can be put to practical use if it is 85% or more of the theoretical density.

Since the sintering method of the present invention is a method for producing a sintered body while maintaining the density of the BN compact, a sintered body with a theoretical density of 85% is a compact made using an Uhachi press of 5 Ton/cm'. Achieved by monkey.

Next, the molded body is inserted into the mold, and the narrower the gap between the two, the better.The molded body needs strength because it is finished by lathe processing, but the molded body of the 5Ton/crn' rubber press is strong enough to withstand the machining. has. On the other hand, the mold must be made of a material that has the properties of heat resistance, high temperature strength, and low expansion coefficient, such as Bed, Cr203, AjlN,
B4 C.

TiB2. Graphite can be used, and the coefficient of linear expansion of these materials is less than 1% at 1100°C.

After the BN molded body is inserted into the mold, both ends of the mold are closed with plugs that contact the BN molded body to suppress vertical expansion of the molded body. The plug is made of the same material as the mold. During the sintering process, the volatile matter in the BN molded body evaporates, so small holes are provided in the mold and plug. Further, if sintering is performed using a two-part mold, the sintered body can be easily taken out.

The reason why the sintering temperature is limited to 1700 to 2000°C is that below 1700°C, crystallization and grain growth during sintering are insufficient, and densification during sintering does not proceed.
At temperatures above 0''C, the sintering aid B2O3 evaporates and decreases, so sintering through the liquid phase does not proceed and sufficient grain growth does not occur, and the high temperature strength of the mold decreases. The limit is 2000°C.

〔Example〕

BN powder (-14m97%), B2O3 powder 3%, PV
24 in ethyl alcohol with addition of 1% A
Mixed time-wise. The mixture was granulated using a spray dryer (30 to 50 ALm).
After vibration-filling into a tube (mm inner diameter x 300 mm length), it was molded using a high-pressure rubber press at a pressure cuff of 000 kg/crrr.The molded body had a diameter of 18.3 mm and a length of 280 m.
It is m. The surface of the A-shaped body was processed to have a diameter of 18-0 mm, and the shape was inserted into a graphite mold. The graphite mold has a wall thickness of 10 mm, and the two vertical divisions are aligned, and the outer wall is 10 mm thick.
It has a structure that allows you to fit a ring. The upper and lower parts of the mold were sealed with threaded graphite cylinders. The mold was placed in a high-frequency heated graphite container and sintered at 1900° C. for 30 minutes in a N2 stream.

The sintered body taken out from the graphite mold is 18.5 mm.
The diameter and length are 28.2 mm, the bulk specific gravity is 1.95, and the porosity is 13.4%. Room temperature bending strength is a, 6kg/mm”
The bending strength after repeating 1500° C. and air cooling 10 times is 2.6 kg/mtn'.

〔Effect of the invention〕

It has become possible to sinter a BN sintered body by a method other than hot high-pressure sintering. This invention has the effect of reducing manufacturing costs and improving productivity.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between rubber press pressure and compact density.

Claims (1)

[Claims] 1 Hexagonal boron nitride is rubber press molded at a pressure of 5 to 10 t/cm^2, machined, then inserted into a low thermal expansion mold, and both ends are made of the same material as the mold and brought into contact with the BN molded body. A method for producing a BN sintered body, comprising: closing the BN sintered body, and then firing the BN sintered body at a temperature in the range of 1700 to 2000°C in a non-oxidizing atmosphere.