JPH0253388B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Silicon carbide, a crystalline compound of metallic silicon and non-metallic carbon, has long been known for its hardness, strength, and resistance to oxidation and corrosion. Silicon carbide has a low coefficient of expansion, good thermal conductivity, and retains high strength at high temperatures. Recently, techniques have been developed to produce dense silicon carbide objects from silicon carbide powder. Such techniques include reactive bonding, chemical vapor deposition, hot compression, and pressureless sintering (in which the article is first shaped and then sintered). Examples of these techniques are described in US Pat. The dense silicon carbide bodies produced in this way are excellent industrial materials for turbines, heat exchange units, pumps and other equipment exposed to severe wear and/or operation under high temperature conditions. It is used for manufacturing tools and components. The present invention relates to silicon carbide powder mixtures suitable for use in various methods of producing dense silicon carbide bodies by hot compaction or sintering, and ceramic articles made therefrom.

Various additives have been utilized to obtain high density and high strength silicon carbide ceramic materials.
For example, Alliegro, et al, J.
Ceram., Soc., Vol.39, No.11, Nov., 1965,
Disclosed on pages 386-389. They discovered that dense silicon carbide could be produced from powder containing 1% by weight aluminum. The products had a rupture modulus of 54,000 psi (3797 Kg/cm ² ) at room temperature and 70,000 psi (4922 Kg/cm ² ) at 1371°C. A recent advance is the use of boron as a densification agent, usually in the range of about 0.33 to about 3.0% by weight of the powder. The boron additive can be in the form of elemental boron or a boron-containing compound, such as boron carbide. Examples of boron-containing silicon carbide powders are described in US Pat.

Now, according to the present invention, it has been found that beryllium can be used as a densification aid in the production of sintered silicon carbide materials. Approximately 0.03 to approx. of powder
Beryllium and boron in the range of 3.0% by weight have been found to be primarily useful, and more particularly from about 0.1 to about 1.0% by weight of the powder to promote densification of silicon carbide powder compacts.

A starting silicon carbide powder containing about 0.5 to about 5.0 weight percent excess carbon is mixed with finely divided beryllium or beryllium-containing compounds. Preferably, the viscosity of both components is less than 5 microns, more preferably less than 2 microns. These ingredients are approximately
When smaller than 1.0 micron, very good distribution is obtained. To obtain densification, the beryllium or beryllium-containing additive and boron or boron-containing compound should be utilized in amounts such that about 0.03 to about 3.0% by weight of the powder is beryllium and boron. It has been found that the use of less than about 0.03% by weight does not substantially increase the density of the sintered product. Addition of beryllium in amounts greater than about 3.0% by weight provides little additional densification, leads to excessive grain growth and reduces the strength of the sintered product.

A bulk density of at least 75% of the theoretical density is required for most applications, and a bulk density of at least 85% of the theoretical density
A bulk density of is often required. Theoretical density of 85
A hot-pressed or sintered product having a density of 10% is obtained according to the invention.

Preferably, the silicon carbide source material is a submicron powder with a surface area greater than 8.0 m ² /g and containing about 0.5 to about 5.0 weight percent excess carbon. Powder compositions generally have a surface area of about 5 to about 50 m ² /g.
It has been found to be mainly useful. Excess carbon can be introduced, for example, during the manufacturing process by subsequent addition of carbon or carbonaceous material, or as a binder before sintering.

The starting materials for beryllium or beryllium-containing additives that have been found to be useful generally have a particle size of less than 50 microns, preferably less than 10 microns. Particle sizes smaller than 5 microns are primarily useful, and in order to easily distribute beryllium or beryllium-containing additives and silicon carbide powder to obtain a homogeneous mixture useful for sintering, approximately
Particle sizes smaller than 1.0 micron are very useful.

Other additives can be used but are unnecessary to promote densification during sintering. Preferably, sintering is carried out in an inert atmosphere, argon or helium being suitable as inert atmosphere. Reducing atmospheres can also be used.

The powder composition of the present invention can be used for heat compression or pressureless sintering. For example, silicon carbide powder containing about 0.5 to about 5.0 weight percent excess carbon may be mixed with beryllium or a beryllium-containing additive and boron or a boron-containing compound in a hot compaction to provide a total of about 0.03 to about 3.0 weight percent beryllium. and boron to form a homogeneous mixture. Put this mixture into a heated compression mold and heat it to 1000~10000psi.
(70.3 to 703.1Kg/cm ² ) under pressure of about 1900 to about 2200
Heating for sufficient time to a temperature of 75% of the theoretical density
Obtain silicon carbide products with greater density. More specifically, the surface area is approximately 11 m ² /g and approximately
Silicon carbide powder containing 2.0% by weight excess carbon added as _Be2C and boron carbide from about 0.1 to about
mixed with 1.0% by weight beryllium and boron;
Place this mixture in a graphite compression mold, approx.
It can be heat compressed at 2000° C. and a pressure of about 5000 psi (352 Kg/cm ² ). The silicon carbide products thus formed typically have densities greater than 85% of theoretical density and can be used as formed or machined into articles of complex shapes.

In pressureless sintering, silicon carbide powder containing about 0.5 to about 5.0 weight percent excess carbon is mixed with beryllium or a beryllium-containing additive and boron or a boron-containing compound to produce about 0.03 to about 3.0 weight percent beryllium. and boron form a homogeneous mixture. This homogeneous mixture is then shaped into a green product. Suitable additives that increase particle flow and binding can be incorporated into the starting mixture. The green product is subsequently sintered in an inert or reducing atmosphere at about 1950 DEG to about 2300 DEG C. for a sufficient period of time to obtain a silicon carbide product having a density greater than 75% of theoretical density. More specifically, silicon carbide powder having a surface area of approximately 11 m ² /g and containing approximately 2.0% by weight excess carbon, suitably as Be ₂ C or as elemental or as boron carbide or as elemental Can be mixed with about 0.03 to about 1.0% by weight beryllium and boron. The resulting mixture can be compressed to a density of about 1.766 g/cm ² .
Binders can be used to increase the flowability of the powder or to increase the green strength of the compacted product. Then,
The compacted powder compact is sintered at about 2100° C. for about 30 minutes, preferably in an inert atmosphere. After cooling, the sintered powder typically has a density greater than 85% of the theoretical density.

The following examples illustrate the invention.

Example 1 Additive Mixture Silicon carbide powder with the following specifications was used as starting material. The silicon carbide powder had a surface area greater than 8.0 m ² /g and the following analytical values expressed in weight percent: Oxygen 0.8 Small Iron 0.2 Aluminum 0.4 Nickel 0.1 Titanium 0.1 Tungsten 0.5 Free Silicon 0.4 Carbide Silicon Greater than 97.5 97.5 g of the above powder was mixed with 4.8 g of phenolic resin (known as Resin No. 8121, Varcum
A typical silicon carbide powder containing approximately 2% by weight excess carbon added as a phenolic resin is mixed with enough boron carbide to provide 0.1% by weight boron in a 30 g powder batch. and mixed with enough beryllium carbide to provide 0.1% beryllium by weight. The boron carbide and beryllium carbide powders had particle sizes less than 10 microns. 5% of a binder containing polyvinyl alcohol and a plasticizer was added to the mixture to increase the green strength of the compacted powder compact. The preparation of this mixture was carried out in a closed environment. Wet mixing techniques were used to ensure good dispersion of the additives in the powder. This mixture was slurried in a mixture of 80% methyl alcohol and 20% water by volume. After thorough mixing, the slurry was evaporated to dryness to obtain a powder mixture.

The powder is compressed at a pressure of approximately 16,000 psi (1125 Kg/cm ² ) to a diameter of 0.5 inches (1.27 cm) and a weight of approximately 0.75 g.
It was made into pellets. These pellets were placed in a small crucible made from the same powder mixture;
The crucible was closed and the pellets and crucible were calcined in an inert atmosphere at a temperature of approximately 2150° C. for approximately 30 minutes. The sintered pellets, which initially had a density of 1.71 g/cm ³ (53% of theoretical density) before sintering, were found to have a density of 2.9 g/cm ³ (93% of theoretical density).

Example 2 Additive Mixture Using the procedure of Example 1, sufficient beryllium carbide was added to the powder mixture to provide 0.33% by weight Be, while all other ingredients were maintained as described above. Diameter 0.5 inch (1.27 cm), weight approximately 0.75
g of pellets were pressed from this mixture, placed in a crucible made from powder having the same composition as the pellets, and capped. The pellets and crucible were sintered at a concentration of approximately 2150°C for approximately 30 minutes in an inert atmosphere. Before sintering, the average density of the compressed powder compact was 1.70 g/cm ³ (53% of the theoretical density), but the average density of the powder compact after sintering at 2150°C was 3.02 g/cm ³ (theoretical density). (94.1%).

Example 3 Additive Mixture Using the procedure of Example 1, sufficient boron carbide was added to a silicon carbide powder containing approximately 2% by weight excess carbon to provide 0.2% by weight boron in the powder mixture. During the addition, enough beryllium carbide was added to give 0.03% beryllium in the powder mixture by weight. This mixture consisted of 0.08 g of minus 325 mesh boron carbide, 0.02 g of minus 325 mesh beryllium carbide, and 29.90 g of Example 1
consisting of silicon carbide having the properties described, containing 2% by weight of excess carbon added in the form of phenolic resin, and combining this mixture with 5% by weight of a binder system containing polyvinyl alcohol and a plasticizer. Wet mixed thoroughly. After drying, the powder is heated to 16000psi
(1125 Kg/cm ² ) into pellets approximately 0.5 inches (1.27 cm) in diameter and weighing approximately 0.75 g.
The sample thus prepared was placed in a crucible of the same composition as the pellets, the crucible was covered, and the crucible and sample were sintered at about 2150° C. in an inert atmosphere for about 30 minutes. The sample was initially approximately 1.72 g/cm ³
(54% of the theoretical density), and after sintering almost
It had a bulk density of 3.15 g/cm ² (98% of theoretical density).

Comparative Example 1 Control A phenolic resin (Varcum Chemical Company No.
50 g of silicon carbide powder containing approximately 2% by weight excess carbon added in the form of 8121) was prepared and sintered according to the procedure of Example 1, except that boron carbide and beryllium carbide were not added. The powder compacts produced were found to have a bulk density of less than about 2.25 g/cm ³ (70% of theoretical density).

Claims (1)

[Claims] 1. Silicon carbide, containing 0.5-5.0% by weight of excess carbon and 0.03-3.0% by weight of a mixture of beryllium and boron, wherein the beryllium and boron are elemental beryllium, beryllium-containing compounds, A raw material powder mixture for producing a silicon carbide sintered body, characterized in that it is selected from the group of elemental boron, boron-containing compounds, or mixtures thereof. 2. A theory characterized in that it consists of a mixture of 0.03-3.0% by weight of beryllium and boron; 0.5-5.0% by weight of excess carbon; and a balance amount of silicon carbide containing less than 2.0% by weight of other elements as impurities. Sintered ceramic products with a bulk density greater than 85% of the density.