JPS6253473B2 - - 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 crystallization, 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, 386
~Disclosed on page 389. They are 1% by weight
It has been discovered that dense silicon carbide can be produced from powders containing 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 0.03 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. No. 3,852,099, 3,954,483 and 3,968,194.

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 in the range of 3.0 wt.
It was found that about 1.0% by weight is suitable for promoting high density of silicon carbide powder compacts. Beryllium can be used as a densification aid or in addition to other densification aids such as boron. Mixtures of beryllium and boron or other auxiliaries can therefore be used. Typically such adjuvants range from about 0.03 to about 3.0% by weight of the silicon powder.

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, beryllium or beryllium-containing additives should be added to the powder to approx.
Amounts should be utilized such that 0.03 to about 3.0% by weight is beryllium. 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 with a density of % is obtained according to the invention.

The beryllium additives of this invention can be used alone or mixed with other densification aids, most commonly boron. When using such additives,
The beryllium component can be completely replaced with boron or other densification aids, or can be partially replaced with boron or other densification aids. Generally, such mixtures, when ready to sinter, have a
Contains 3.0% by weight total densification aid.

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 20 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, with 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, in hot compaction, silicon carbide powder containing about 0.5 to about 5.0 weight percent excess carbon is mixed with beryllium or beryllium-containing additives to form a homogeneous mixture with a total of about 0.03 to about 3.0 weight percent beryllium present. form.
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 wt.% excess carbon is mixed with about 0.1 to about 1.0 wt.% beryllium added as _Be2C , and the mixture is placed in a graphite compression mold at about 2000 °C and about 5000 psi ( 352Kg/
It can be heated and compressed at a pressure of 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 beryllium-containing additives to form a
A homogeneous mixture is formed in which about 3.0% by weight of beryllium is present. 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 particularly, silicon carbide powder having a surface area of approximately 11 m ² /g and containing about 2.0% by weight excess carbon, suitably as Be ₂ C, or in elemental form, from about 0.03 to about 1.0 wt. Can be mixed with % beryllium. The resulting mixture can be compressed to a density of about 1.76 g/cm ² . Binders can be used to increase the flowability of the powder or to increase the green strength of the compacted product. The compacted powder compact is then heated, preferably in an inert atmosphere.
Sinter at approximately 2100°C for approximately 30 minutes. After cooling, the sintered powder typically has a density greater than 85% of the theoretical density.

The following examples illustrate the invention.

Example 1 Heat Compression 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
Chemical Company product) and 0.5 g of minus 325 mesh beryllium metal powder, the phenolic resin was decomposed by subsequent heating to hot compression temperatures, leaving a carbon residue. This mixture is compounded in a closed environment and then heated to approximately 2000 psi
(141 Kg/cm ² ) into a 3 inch (7.6 cm) slug. This slag is suitable for heat compaction by techniques well known in the art, namely by heating to a temperature of about 2000° C. at a pressure of about 4000 psi (281 kg/cm ² ). The time required to achieve final density was approximately 30 minutes. After densification,
Reduced pressure and lowered temperature. Products manufactured by the above technique can be expected to have a bulk density of 2.88 g/cm ³ (approximately 90% of the theoretical density) or higher.

Example 2 Pressureless Sintering 49.75 g of silicon carbide powder having the properties described in Example 1 and containing approximately 2% by weight of carbon added in the form of phenolic resin (Varcum Chemical Company No. 8121) were Mixed with 0.25 g of minus 325 mesh beryllium metal powder. 3% polyvinyl alcohol was added as a binder to increase the green strength of the powder mixture. 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 mixing well, this slurry was evaporated to dryness.
Pour the resulting powder mixture into a 1.125 inch (2.85 inch) diameter
cm), pellets weighing approximately 10 g, approximately 12,000 psi (844
Kg/cm ² ). The resulting powder compact was then placed in a graphite resistance heating element furnace and slowly heated to remove volatiles, then rapidly heated to about 2100°C and maintained at this temperature for 30 minutes.

A typical heating rate table is as follows: Room temperature to 150°C 30 minutes 150°C to 400°C 120 minutes 400°C to 800°C 60 minutes 800°C to 2100°C 180 minutes After sintering, turn off the power and The congealed product was left to cool in the oven. During the sintering process, an argon atmosphere was utilized. The powder compacts produced have a bulk density greater than 2.75 g/cm ³ (85% of the 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 2, except that no beryllium powder was added. The powder compact produced is approximately 2.25
It was found to have a bulk density of less than g/cm ³ (70% of the theoretical density).

Claims (1)

[Scope of Claims] 1. Silicon carbide powder containing 0.5 to 5% by weight of excess carbon, and approximately
A raw material powder mixture for producing a silicon carbide sintered body containing 0.03 to about 3.0% by weight of beryllium or a beryllium-containing compound. 2. The powder mixture of claim 1, wherein the powder contains about 0.5-5.0% by weight excess carbon. 3. A powder mixture according to claim 1, having an average particle size of less than 5 microns. 4. The powder mixture of claim 1, wherein the powder contains from about 0.03 to about 1.0% by weight beryllium. 5. A powder mixture according to claim 1, wherein the beryllium is elemental beryllium. 6. A powder mixture according to claim 1, wherein the beryllium is in the form of beryllium carbide. 7 (a) Forming a homogeneous powder mixture comprising silicon carbide powder containing from 0.5 to 5% by weight of excess carbon and from about 0.03 to about 3.0% by weight of beryllium, expressed as beryllium, of the powder; (b) The powder mixture is heated to a temperature of about 1900 to about 2200°C at about 1000 to about 10000 psi (70.3 to 703.1 Kg/
cm ² ), and (c) maintaining said temperature and said pressure for a time sufficient to obtain a silicon carbide product having a density greater than 85% of the theoretical density. A method of manufacturing silicon carbide products. 8 (a) forming a homogeneous powder mixture comprising silicon carbide powder containing from 0.5 to 5% by weight of excess carbon and from about 0.03 to about 3.0% by weight of beryllium, expressed as beryllium, of the powder; (b) forming the mixture into a green product; and (c) heating the green product in an inert atmosphere to about 1,950 to about 2,300
sintering at a temperature of 0.degree. Law. 9 About 0.03 to about 3.0% by weight of beryllium; about 0.5 to about
Approximately 5.0% excess carbon by weight: and 2.0 as impurities
85 with a theoretical density characterized by consisting of silicon carbide with the balance amount containing less than % by weight of other elements
Sintered ceramic products with a bulk density greater than %.