JPS6350310B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the use of liquid sintering aids in the process of sintering particulate silicon carbide materials to produce dense, hard products of industrial use.

Silicon carbide has long been known to be hard, strong, and has excellent resistance to oxidation and corrosion. Silicon carbide has a small coefficient of expansion, good heat transfer properties, and maintains high strength at high temperatures. Recently, methods have been developed for sintering silicon carbide powder to create high density silicon carbide materials. Dense silicon carbide finds use in the manufacture of turbines, heat exchangers, pumps, and other equipment or tools that undergo significant corrosion or wear, especially at high temperatures.

Ceramic bodies or densities are made from granular ceramic materials by various casting or molding methods. In the industry, hot or cold pressing methods, isostatic molding methods, slip casting methods, extrusion methods, injection or transfer molding methods, or tape and
A molding method known as a casting method can be used. When the shaped ceramic body is sintered at a temperature between approximately 1900°C and 2200°C, it becomes a dense and hard product.

Various additives have been used as sintering or densification aids to obtain high density, high strength silicon carbide ceramic materials. For example, a high-temperature pressing method for silicon carbide with a density of about 98% of the theoretical value by adding aluminum and iron as densifying agents is described in Alliegro et al., Journal of Ceramics Society (J.Ceram.Soc.). Volume 39
No. 11 (November 1956), pages 386-389. They found that dense silicon carbide could be made from powder containing 1% aluminum by weight. This product has a rupture coefficient of 54000psi at room temperature
(3797Kg/cm ² ), 70000psi (4922
kg/cm ² ). More recently, methods have been developed that use boron or beryllium as sintering or densification aids. Such adjuvants are typically added to the silicon carbide material powder in an amount of about 0.3 to about 5.0 weight percent boron or beryllium. Sintering aids can be added in the form of elemental boron or beryllium or in the form of compounds containing boron or beryllium. Boron is a preferred additive due to its good handling and use properties. Boron is usually used in the form of boron carbide. Silicon carbide powder containing boron is disclosed in US Pat. No. 3,852,099,
No. 3954483 and No. 3968194.

Sinterable silicon carbide powders also contain excess or unbonded carbon. Normal about 1.0 to about 4.0
An excess of carbon in the amount of % by weight is used. The excess carbon required for sintering can be in the form of carbon left in the product from a previous step, or can be added in the form of a carbon source material that provides the desired amount of excess carbon after sintering.
The excess carbon facilitates sintering and can reduce the amount of various impurities in the raw silicon carbide material that would otherwise remain in the finished product.

According to the invention, the sintering aid used to densify the silicon carbide material is used in liquid form. The boron source or sintering aid of the present invention is H ₃ BO ₃ ,
selected from solutions of B ₂ O ₃ or mixtures thereof;
The solvent can be any liquid in which H ₃ BO ₃ or B ₂ O ₃ is soluble, but water or alcohol are preferred due to their easy availability and low cost.

Suitably, the liquid sintering aid is added to the shaped porous silicon carbide body prior to sintering. In one embodiment of the invention, the sintering aid and carbon source are added together in liquid form. In such embodiments, the sintering aid can be dissolved in the carbon source, or the sintering aid and the carbon source can be dissolved in a common solvent, such as water or alcohol.

The silicon carbide source material is initially made in the form of a fine powder or powder. Preferably the particles have an average particle size of about 0.10 to
It is about 2.00μ, and the maximum is about 5.00μ. Although particle size is an important factor, surface area must also be carefully considered in determining the appropriate material. Therefore, it is preferred that the particles have a surface area of about 1 to 100 ^{m 2} /g. More preferably within this range, the particles have a surface area of about 5 to about 20 m ² /g.

Silicon carbide can be in alpha or beta phase or amorphous. Currently, the α (non-cubic) crystalline phase of silicon carbide is the most economically obtainable. The compositions of the present invention may contain substantially all, eg, 95% by weight or more, silicon carbide in the alpha phase, or may contain a mixture of various forms of silicon carbide. For example, mixtures in which the predominant amount (greater than 50%) is alpha phase are suitable for use. Silicon carbide materials can contain small amounts of impurities without adverse effects. Generally, a purity of at least about 95% by weight is required, and higher purity silicon carbide materials are desirable.

According to the invention, a sintering aid, or a combination of a sintering aid and a carbon source material, is added to the formed porous silicon carbide body after torrefaction and before sintering. Such porous bodies are produced by a molding or casting method in which a green compact is produced, roasted, and then sintered. sintering aid,
Alternatively, it is particularly advantageous for injection molding and slip casting processes to add the combination of sintering aid and carbon source material after torrefaction, just before sintering. In practicing the invention, the silicon carbide material is first mixed with various additives, such as sintering aids, carbon source materials, resin binders, mold release agents, and additives that reduce the viscosity of the mixture. Sintering aids and/or carbon source materials often have an adverse effect on the bonding, release, or flow properties of the mixture. Removal of the sintering aid, or combination of sintering aid and carbon source material, facilitates molding operations and significantly increases the number of useful bonding resins and processing additives that can be used in such mixtures.

In the sintering aid of the present invention, an amount of sintering aid is added to the torrefied green compact to provide from about 0.3% to about 5.0% boron by weight of the silicon carbide material, but within this range. It is suitable to use about 0.5% to about 4.0%. When using less than about 0.3 weight of boron,
Sintering is generally not carried out effectively. About boron
If it is contained in an amount of 5.0% by weight or more, densification will not be significantly improved, and excessive boron may have an adverse effect. The concentration of this solution is adjusted so that the boron is substantially uniformly dispersed within the particulate material and provides boron within the aforementioned range during subsequent sintering. The sintering aid solution can be relatively concentrated and, after saturating or impregnating the pore area, the excess can be allowed to drip off. The amount of boron added can be determined by simply weighing the granular silicon carbide material before and after treatment with the liquid sintering agent, and calculating the amount of boron from the increase in weight.

The present invention also includes the use of a combination of sintering aid and excess carbon source in liquid form. Dissolving the sintering aid into excess carbon source material or
Alternatively, it is appropriate to dissolve the sintering aid and excess carbon source material in a common solvent. Excess or bondable carbon can be used in amounts of about 0.05 to 5.0% by weight of the silicon carbide material to facilitate sintering, with about 1.0 to about 4.0% by weight within this range being particularly useful. The carbon source can be any carbonaceous material with which the sintering aid can be mixed without adverse effects or in a common solvent with the sintering aid. Sugars such as sucrose and dextrose, corn syrup, furfural, furfuryl alcohol, tetrahydrofurfuryl alcohol, phenolic resins, polyphenylene resins, and furan resins are typical useful carbon source materials. Typically, the carbonization value of the carbon source material is from about 15 to about 80% by weight. The appropriate amounts of sintering aid and carbon source material can therefore be easily calculated.
It is assumed that the sintering aid of the present invention is reduced during sintering. The amount of carbon source can be adjusted to provide the amount of carbon required for such reduction and the amount of excess carbon required for sintering.

The liquid auxiliaries of the present invention can be prepared by simply dissolving H ₃ BO ₃ , B ₂ O ₂ or mixtures thereof in a suitable solvent. The chemical formulas H ₃ BO ₃ and B ₂ O ₃ , i.e. boric acid and boron oxide, are to be read as also including the hydrates of H ₃ BO ₃ and B ₂ O ₃ . The solvent is
Any liquid in which H ₃ BO ₃ and B ₂ O ₃ are soluble, water or alcohol are suitable. In addition to the porous body, the sintering aid of the present invention is about 0.3 to about 5.0, preferably about 0.05 to about
Make sure to give 4.0% boron by weight. The concentration of the sintering agent solution is adjusted to facilitate substantially uniform dispersion of the boron source within the compact.

The sintering aid of the present invention can be dissolved in a carbon source to provide a liquid containing a combination of the sintering aid and the carbon source material. Suitable carbon sources that can be used in this embodiment are those in which the sintering aid can be dissolved. Particularly suitable are alcohols such as furfuryl alcohol and tetrahydrofurfuryl alcohol. Excess or bondable carbon accounts for about 0.05 to about
5.0, preferably from about 1.0 to about 4.0% by weight to facilitate sintering. Generally, the carbonaceous material decomposes upon heating to yield from about 15 to about 80% by weight of free or bound carbon. In this embodiment of the invention, a suitable amount of sintering aid is dissolved into a suitable amount of carbon source material, suitably with mixing, to provide an excess of carbon within the desired range.

The sintering aid and carbon source material of the present invention can be dissolved in a common solvent for both to create a combination liquid of the sintering aid and carbon source material. In this embodiment, water is suitable for use as both solvents. Examples of water-soluble carbon source materials that can be used are sugars such as sucrose and dextrose, corn syrup, and alcohols such as furfuryl alcohol. Alcohol can also be used as both solvents. Ethyl, methyl, propyl and isopropyl alcohols are particularly useful. Phenol formaldehyde resins are particularly suitable for use as carbon source materials when short chain alcohols are used as common solvents. The sintering aid and carbon source material can be dissolved in a common solvent, suitably by mixing, to create a combined sintering aid and carbon source material solution.

When carrying out the method of the invention, a liquid sintering agent or a combination of a liquid sintering aid and a carbon source is dispersed in a green torrefied compact, which is then sintered to form a dense compact. Manufacture silicon carbide products.

The method used to shape the green body is not critical to the process of the invention. Generally, in such methods, a particulate silicon carbide feedstock is first mixed with a resin binder and other suitable additives, such as a mold release agent, a lubricant, a viscosity aid, and an excess carbon source. This mixture is then shaped by known methods such as cold pressing, hydraulic forming, slip casting, tape casting, injection or transfer molding. Usually about 500 to about 1000 raw molded bodies
Motsu is usually roasted at about 700 to 900 degrees Celsius.
This results in a product with an open pore structure that is particularly suitable for processing with the liquid sintering agent of the invention or the combination of liquid sintering agent and carbon source material. The sintering aid or the combination of the sintering aid and the carbon source material is impregnated into the compact, either by applying pressure or by utilizing the capillary action of the pores, and substantially uniformly distributed throughout the torrefied compact. Can be dispersed. However, the preferred method is vacuum filling. In this method, a compact is evacuated, and a liquid sintering aid or a combination of a liquid sintering aid and a carbon source material is supplied to the compact. After dispersing the sintering aid or the combination of sintering aid and carbon source material, the impregnated compact is
It is sintered at a temperature of 2200℃ to form a dense and hard molded product.

The invention is not limited to the specific embodiments of the sintered powder and sintering method described above, and those skilled in the art will appreciate that various modifications may be made without departing from the spirit and scope of the invention. can.

Claims (1)

[Claims] 1. (a) Producing a green molded body consisting of a granular silicon carbide material and a resin binder, (b) Roasting the green molded body at a temperature of about 500°C to about 1000°C. (c) dispersing in the porous body a liquid sintering aid selected from the group consisting of solutions of H ₃ BO ₃ , B ₂ O ₃ and mixtures thereof; (d) dispersing the porous body; A method for producing a sintered silicon carbide product, comprising the step of sintering a treated porous body at about 1900° to 2200°C to produce a sintered silicon carbide product. 2. The method according to claim 1, wherein the liquid sintering aid is a solution of H ₃ BO ₃ . 3. The method according to claim 2, wherein the liquid sintering aid is an aqueous solution of H ₃ BO ₃ . 4. The method according to claim 1, wherein the liquid sintering aid is an aqueous solution. 5. The method according to claim 1, wherein the liquid sintering aid is an alcohol solution. 6. The method of claim 1, wherein a sintering aid is dispersed into the torrefied green compact in an amount sufficient to add about 0.3 to about 5.0 weight percent boron. 7. The method according to claim 1, wherein the liquid sintering aid contains a carbon source. 8. The method of claim 1, wherein the liquid sintering aid is present in solution together with the carbon source. 9. The method according to claim 8, wherein the liquid sintering aid contains a carbon source as a solvent. 10. The method according to claim 8, wherein the solution is an aqueous solution. 11. The method according to claim 8, wherein the solution is an alcoholic solution. 12. The method according to claim 1, wherein a liquid sintering aid is dispersed inside the green compact by a vacuum filling method. 13. The method of claim 1, wherein the liquid sintering aid is dispersed within the green compact by capillary action.