GB2104103A

GB2104103A - Forming shaped silicon carbide bodies

Info

Publication number: GB2104103A
Application number: GB08223926A
Authority: GB
Inventors: Fukuzi Matsumoto; Yasushi Matsudaira
Original assignee: Shin Etsu Chemical Co Ltd
Current assignee: Shin Etsu Chemical Co Ltd
Priority date: 1981-08-21
Filing date: 1982-08-19
Publication date: 1983-03-02
Also published as: JPS5832070A; GB2104103B; FR2511665B1; DE3231100A1; FR2511665A1; DE3231100C2; JPS6327305B2

Abstract

A method for forming a shaped body of silicon carbide comprises calcining a shaped body formed from a blend of silicon carbide and carbon powders with an organic binder; impregnating at least the surface layer of the calcined body with an organic solution containing a material capable of isolating free carbon or silicon carbide by thermal decomposition; drying the impregnated body; and contacting the dried body with molten silicon which permeates into the body and reacts with carbon to form silicon carbide in situ.

Description

SPECIFICATION Forming shaped silicon carbide bodies This invention relates to a method for forming shaped bodies of silicon carbide.

For use in severe conditions, e.g. at very high temperatures or in corrosive atmospheres, silicon carbide is a desirable structure material, owing to its high melting point, inert nature and high hardness. These very properties make it difficult to form shaped bodies of silicon carbide, which are accurately dimensioned. For example, conventional powder metallurgical technology, in which a compact powder mixture is sintered, is well-nigh inapplicable to silicon carbide owing to the high sintering temperature which is necessary. Metallurgical shaping of silicon carbide powder compacts can only be achieved by pulverising the silicon carbide to an extremely fine powder having a particle diameter of 1 sst4m or smaller and the powder compact must be heated at a temperature of 1 900 to 23000C, preferably, by use of a hot press.In addition, shaping by mechanical working can be performed only with very large costs due to the extremely high hardness of silicon carbide.

Therefore, the method currently practised for obtaining shaped bodies of silicon carbide is the so-called reactive infiltration method in which silicon carbide and carbon each in the form of a fine powder are blended together, preferably with addition of an organic binder, and then this powder blend is shaped or molded into a formed body having dimensions approximating those of the desired finished shaped body. The thus shaped body is then calcined at a suitable temperature and, after machining if necessary, the calcined body is brought into contact with molten silicon so that the molten silicon infiltrates into the calcined body composed of the silicon carbide and carbon to react with the carbon and convert it into silicon carbide in situ, whereby a solid shaped body of silicon carbide is obtained.

Such a method is described, for example, in the specifications of U.S. Patents 2,938,807 and 3,495,939.

The above-described method of reactive infiltration is very advantageous over the conventional powder metallurgical method by simple sintering because the temperature required for processing may be in the range from the melting temperature of silicon, i.e. 14140C, up to 17000C at the highest and the difference between the dimensions of the calcined body and the finished body is only 1 to 2% or small, to ensure high accuracy in dimensions of the products even without machining along with the uniformity of the properties of the shaped bodies.

Therefore, this method of reactive infiltration is almost exclusively used for the preparation of a shaped body of silicon carbide of a large size or complicated form.

In contrast to the above-described advantages, the method of the reactive infiltration is not free from several problems. For example, it is rather difficult to obtain a shaped body of silicon carbide having a sufficiently high density as close as possible to the true density (3.21 g/cm3) of silicon carbide. In addition, the molten silicon with which the calcined body is brought into contact in the step of infiltration unavoidably adheres the clogs more or less to the surface of the calcined body and solidifies there, so that the solidified silicon clogging to the surface after infiltration and cooling of the shaped body must be removed by some means.

That is, whereas it is desirable to decrease the unreacted carbon and silicon remaining in the shaped body after infiltration to as small as possible amounts, by reacting the carbon in the calcined body with a theoretical amount of metallic silicon, sometimes the molten silicon infiltrating into the calcined body reacts with the carbon in the outer layer of the calcined body to be converted into silicon carbide, which forms a barrier for the further infiltration of the molten silicon into the depth or core portion of the calcined body, so that free carbon and metallic silicon may remain in considerable amounts, unreacted, in the core portion and outer layer, respectively, of the shaped body resulting, as a consequence, in an insufficiently low density of the finished shaped body of silicon carbide.

The above-mentioned problem of clogging molten silicon on the surface of the shaped body is, therefore, caused not only by the simple wetting of the surface in the bath of molten silicon but also by the exudation of the unreacted silicon in the outer layer of the shaped body due to the volume increase in the solidification.

Removal of such metallic silicon clogging to the surface of the shaped body can of course be performed by a mechanical means such as sand blasting, even though such a mechanical method is disadvantageous due to the high costs therefor as well as the decreased dimensional accuracy of the finished shaped body. Accordingly, there have been made several attempts to remove the clogging silicon by a chemical means using a chemical solution which is inert to silicon carbide but capable of dissolving metallic silicon, such as a mixture of nitric and hydrofluoric acids or an aqueous solution of sodium hydroxide at 500C or higher.Unfortunately, this chemical method for the removal of clogging silicon has great disadvantages; discoloration to greenish-yellow takes place on the surface of the shaped body after the chemical treatment and the thus discolored surface layer has a greatly decreased hardness. Therefore, such a discolored surface layer with decreased hardness must also be removed by a mechanical means such as sand blasting or barreling, again leading to increased costs and decreased dimensional accuracy of the finished shaped body.

Thus, there has been an eager demand for developing a simple and efficient method for the preparation of a shaped body of silicon carbide provided with neatly finished surface and high dimensional accuracy as well as high surface hardness without elaborate and costly finishing treatments.

It is therefore an object of the present invention to provide a novel and improved method for the preparation of a shaped body of silicon carbide by reactive infiltration of molten silicon into a calcined body of a powder compact of silicon carbide and carbon, according to which the finished shaped body of silicon carbide is provided with a neat surface and imparted with high dimensional accuracy and high surface hardness without costly finishing treatments.

The method of the present invention for the preparation of a shaped body of silicon carbide by the reactive infiltration of molten silicon comprises the steps of (a) blending silicon carbide and carbon each in a powdery form together with an organic binder to give a powdery blend, (b) shaping the powdery blend into a formed body, (c) calcining the formed body, (d) impregnating at least the surface layer of the calcined body with an organic solution containing a material capable of isolating free carbon or silicon carbide by thermal decomposition followed by drying, and (e) bringing the thus dried body into contact with molten silicon to cause infiltration thereof into the body and reaction thereof with carbon to form silicon carbide in situ.

The above outlined method of the present invention has been established as a result of the extensive investigations undertaken by the inventors leading to a discovery that, when the silicon clogging to the surface of the shaped body after infiltration and cooling as prepared by the above described inventive method is removed by a chemical means using a mixture of nitric and hydrofluoric acids or a hot aqueous solution of sodium hydroxide, the surface of the shaped body is remarkably and unexpectedly resistive against the attack of the chemicals without discoloration or decrease of hardness so that a shaped body of silicon carbide having satisfactory surface conditions and high dimensional accuracy can be obtained even without costly machining for finishing.

In the above outlined inventive method, details of the steps (a) to (c) are much the same as in the conventional method for the preparation of a shaped body of silicon carbide by the reactive infiltration and hence need not be described in great detail.

Thus, powders of silicon carbide and carbon are first blended uniformly together with an organic binder to give a powdery blend. The powders of silicon carbide and carbon should have a particle size as fine possible in order to obtain an intimate blend of the powders. Usually, it is preferable that the powder of silicon carbide has a particle size not exceeding 20 4m and the powder of carbon, which may be graphitic or amorphous, has a particle size not exceeding 2 ,um. Any suitable organic binder may be used; it is usually selected from methyl celluloses, phenolic resins, silicone resins and the like.

Blending ratio of the silicon carbide and carbon is of great importance from the standpoint of obtaining a shaped body of silicon carbide having a density as high as possible or as close as possible to the true density of silicon carbide. That is, although it is an ideal and desirable condition that the free carbon contained in the calcined body of the powder compact has been completely converted to silicon carbide by the reaction with the molten silicon infiltrating thereinto without leaving unreacted silicon, it is usually necessary to have an excess amount of molten silicon infiltrating into the calcined body in order that no free carbon remains unreacted in the body after the infiltration treatment, so that unreacted metallic silicon unavoidably remains more or less resulting in the decreased density of the finished shaped body of silicon carbide.Therefore, a conventional measure undertaken in this regard is the use of a higher molding pressure in the step of shaping the powdery blend of silicon carbide and carbon into a formed body or the use of an increased amount of carbon relative to the powder of silicon carbide although these methods are necessarily accompanied by some disadvantages when practiced industrially. In the inventive methods, the amount of carbon in the powdery blend with the powder of silicon carbide is preferably in the range from 25 to 50 parts by weight per 100 parts by weight of the powder of silicon carbide. The amount of the organic binder is usually in the range from 5 to 50 parts by weight per 100 parts by weight of the total amount of silicon carbide and carbon. Blending of these components may be performed in a suitable blending machine such as ball mills.If necessary, blending may be performed with addition of a suitable amount of an organic solvent capable of dissolving the organic binder.

The thus prepared powdery blend of silicon carbide and carbon with addition of an organic binder, either with the organic solvent or after drying, is shaped into a body of the desired form.

The method for shaping the powdery blend into a formed body is not critical and any method known in the technology of ceramics and powder metallurgy may be applicable including compression molding, for example, in a rubber press, extrusion molding with an extruder machine, slip casting of the slurry of the powdery blend and the like. When compression molding is undertaken, the pressure should preferably be at least 400 kg/cm2. It should be noted here that the formed body after the following step of calcination would have a density in the range from 70 to 90% of the theoretical density calculated from the true densities of silicon carbide and carbon and the blending ratio thereof.

This is because, when the density of the calcined body is below 70% of the theoretical value, the resultant shaped body of silicon carbide after the treatment of reactive infiltration cannot have a sufficiently high density without repeating the treatment in the step (d) followed by thermal decomposition of the thermally decomposable material, while an excessively high density of the calcined body above 90% of the theoretical value may cause difficulties in the infiltration of molten silicon into the core portion of the calcined body, consequently leaving a considerable amount of unreacted metallic silicon.When the treatment in the step (d) is to be performed with an organic solution containing a material capable of isolating silicon carbide by thermal decomposition, the density of the calcined body should preferably be in the range from 85 to 90% of the theoretically calculated value because no carbon content is added to the initially formulated amount of carbon by the thermal decomposition to react with the infiltrating molten silicon.

The next step is the calcination of the formed body of the powdery blend which is carried out at a temperature in the range from 500 to 1 0000C for a length of time, for example, from 1 to 20 hours in an inert atmosphere of nitrogen, argon and the like. In this treatment of calcination, the organic binder in the formed body is almost completely decomposed and the body is imparted with sufficient mechanical strength to withstand handling thereafter. If necessary, the calcined body is mechanically worked to modify the dimensions and the form in compliance with the desired finished shaped body.

The next step is the impregnation of at least the surface layer of the thus calcined body of the powder compact of silicon carbide and carbon with an organic solution containing a material capable of isolating free carbon or silicon carbide by thermal decomposition. Suitable materials capable of isolating free carbon by thermal decomposition are exemplified by certain synthetic resins such as phenolic resins, polyester resins, polystyrenes, epoxy resins and the like, various kinds of non-volatile fats and oils, paraffin waxes and the like. The material capable of isolating silicon carbide by thermal decomposition is typically a polycarbosilane composed of the recurring monomeric units represented by the formula -SiR2-CH2-, in which R is a hydrogen atom or a monovalent hydrocarbon group such as methyl or phenyl.These thermally decomposable materials are used dissolved in an organic solvent such as benzene, toluene, xylene, acetone, nhexane and the like, according to the solubility of the respective material. The concentration of these thermally decomposable materials in the organic solution is preferably in the range from 10 to 50% by weight in order to ensure sufficient impregnation of the calcined body with the material after subsequent drying. If desired, the organic solution may further contain a finelydivided carbonaceous material such as carbon black, soot and the like with an object to control the carbon content in the calcined body.

The calcined body thus impregnated with the organic solution is then dried and, though not always necessarily, heated to thermally decompose the decomposable material to isolate free carbon or silicon carbide. The temperature of this thermal decomposition treatment is determined naturally depending on the type of the thermally decomposable material. When it is desired to have a relatively large amount of the isolated free carbon or silicon carbide by the thermal decomposition, the cycle of impregnation with the organic solution, drying and thermal decomposition may be repeated several times.

When impregnation of the calcined body with the organic solution is desired only at the surface layer thereof, it is sometimes sufficient to merely apply the organic solution to the surface of the calcined body by brushing, spraying or like coating method. It is, however, usually preferable that the impregnation of the calcined body with the organic solution is not limited to the surface layer but should reach the core portion of the calcined body by dipping the calcined body in the organic solution for at least 30 minutes. In order to obtain complete impregnation of the calcined body with the organic solution to the core portion, impregnation under vacuum or pressurization is sometimes recommended, in particular, when the calcined body is large.

As is mentioned above, the treatment of thermal decomposition following the impregnation of the calcined body with the organic solution is not always indispensable because the calcined body impregnated with the organic solution and dried is, irrespective of whether the step of the thermal decomposition is undertaken or omitted, necessarily subjected to the subsequent reactive infiltration of molten silicon by contacting therewith at a temperature much higher than the decomposition temperature of the impregnating thermally decomposable material.

The final step in the inventive method is the reactive infiltration of molten metallic silicon into the calcined body after impregnation with the thermally decomposable material followed by drying and, optionally, heat treatment to thermally decompose the material. The method of this reactive infiltration of molten silicon is also well known in the art and need not be described in great detail. Thus, the calcined body is heated up to a temperature higher than the melting temperature of silicon, i.e. 141 40C, in vacuo or in an inert atmosphere and brought into contact with molten silicon at least at an end portion thereof so that the molten silicon infiltrates into the calcined body and reacts with the carbon content therein to form silicon carbide in situ whereby the calcined body is converted to a highdensity shaped body of silicon carbide.

In this case, the amount of the molten silicon infiltrating into the calcined body is usually more than sufficient to react with the free carbon content in the calcined body so that the shaped body thus obtained is a composite body of silicon carbide with unreacted metallic silicon more or less. In addition, as is mentioned above, the unreacted silicon adheres and clogs to the surface of the shaped body not only from outside but also by the exudation from inside of the body by the volume increase in solidification. Therefore, it is usually necessary to remove such clogging silicon from the surface of the shaped body by some means.In the shaped body of silicon carbide prepared in accordance with the inventive method, chemical means is suitable for the removal of metallic silicon and a mixture of nitric and hydrofluoric acids or a hot aqueous solution of sodium hydroxide can be used suitably.

Unexpectedly and differently from the shaped bodies of silicon carbide prepared by the conventional method, the surface of the shaped body according to the invention is highly resistive against the attack of such chemical solutions, remaining black, without discoloration. In addition, the hardness of the surface of the thus cleaned shaped body is not decreased at all by the chemical treatment so that the shaped body of silicon carbide with the chemically cleaned surface can be used as such without mechanical finishing such as sand blasting with the surface as the so-called as-cast surface contributing to a great reduction of the manufacturing costs of such a shaped body of silicon carbide.

in the following, the method of the present invention is described in further detail by way of examples.

Example 1 A powdery blend prepared by uniformly blending in a ball mill 2.75 kg of a commercially available green silicon carbide in a powdery form having an average particle diameter of 5.7 Mm, 1.5 kg of carbon powder having an average particle diameter of 1.0 ,um and 0.75 kg of a phenolic resin with addition of 2 liters of acetone, was dried and shaped by compression molding in a metal mold under a pressure of 1 ton/cm2 into a ring-wise form having an outer diameter of 30 mm, an inner diameter of 20 mm and a height of 5 mm.

The thus formed ring-wise body was calcined in an atmosphere of argon at a temperature of 6000C for 10 hours and then dipped in a 30% acetone solution of a phenolic resin at room temperature for 1 hour followed by drying. One hour of dipping was sufficient for the organic solution to reach the core portion of the ring-wise calcined body.

The resin-impregnated calcined body was broght into contact with a bath of molten metallic silicon kept at 1 6000C in a vacuum furnace so that the molten silicon infiltrated into the calcined body to react in situ with the free carbon therein and convert the body into a composite shaped body of silicon carbide and silicon with a relatively small amount of solidified silicon clogging on to the surface of the shaped body, which was dissolved away by dipping the body in a 30% aqueous solution of sodium hydroxide at 700C for 4 hours. The thus treated shaped body had a blackish appearance and the condition of the surface was satisfactorily clean without finishing by mechanical working. The densities of 5 pieces of the ring-wise shaped bodies prepared in the above described manner ranged from 3.079 to 3.088 g/cm3 with an average of 3.084 g/cm3.

For comparison, the same preparation procedure as above was repeated excepting the omission of the impregnation of the calcined body with the phenolic resin. The finished ring-wise shaped bodies, after the chemical treatment with the aqueous sodium hydroxide solution, were green in color and the surface was rugged and not in a condition suitable for use without finishing by mechanical working. The densities of 5 pieces of these comparative ring-wise shaped bodies ranged from 3.026 to 3.038 g/cm3 with an average of 3.030 g/cm3.

Example 2 A powdery blend was prepared by uniformly blending in a ball mill 2.5 kg of a commercially available green silicon carbide in a powdery form having an average particle diameter of 9.5 ym, 1.5 kg of an artifical graphite in a powdery form having an average particle diameter of 1.0 ym and 2.0 kg of a silicone resin (KR-260, a product of Shin-Etsu Chemical Co., Ltd.) with addition of 2 liters of toluene followed by drying. The powdery blend was shaped into a ring-wise body in the same manner as in Example 1, which was calcined for 10 hours at 800or in an atmosphere of argon.

The thus caicined bodies were dipped for 1 hour at room temperature in an acetone solution of a phenolic resin in a concentration of either 30%, 40% or 50% by weight followed by drying.

The resin-impregnated calcined bodies were then subjected to the reactive infiltration of molten silicon and removal of the clogging silicon on the surface in the same manner as in Example 1 to give composite shaped bodies of silicon carbide and silicon. The surface of each of the thus cleaned shaped bodies was blackish in color and in good condition, suitable for use without finishing by mechanical working. The densities of the shaped bodies, prepared in twos for each of the resin solutions of 30%, 40% and 50% concentrations, were 3.098 and 3.104 g/cm3 for the 30% solution, 3.104 and 3.112 g/cm3 for the 40% solution and 3.123 and 3.122 g/cm3 for the 50% solution.

Example 3 The ring-wise bodies formed and calcined in the same manner as in Example 1 were dipped for 1 hour at room temperature in a toluene solution containing either 30%, 40% or 50% by weight of a polydimethylcarbosilane, followed by drying.

These calcined bodies, after impregnation with the polycarbosilane, were subjected to the reactive infiltration of molten silicon and removal of the clogging silicon on the surface in the same manner as in Example 1 to give composite shaped bodies of silicon carbide and silicon. The surface of each of these shaped bodies was blackish in color and in good condition suitable for use without finishing by mechanical working. The densities of the shaped bodies, prepared in twos for each of the polycarbosilane solutions of 30%, 40% and 50% concentrations, were 3.065 and 3.061 g/cm3 for the 30% solution, 3.069 and 3.074 g/cm3 for the 40% solution and 3.085 and 3.084 g/cm3 for the 50% solution.

Claims

1. A method for forming a shaped body of silicon carbide, which comprises calcining a shaped body formed from a blend of silicon carbide and carbon powders with an organic binder; impregnating at least the surface layer of the calcined body with an organic solution containing a material capable of isolating free carbon or silicon carbide by thermal decomposition; drying the impregnated body; and contacting the dried body with molten silicon which permeates into the body and reacts with carbon to form silicon carbide in situ.

2. A method according to claim 1, in which from 20 to 50 parts by weight of carbon powder are blended with 100 parts by weight of silicon carbide powder.

3. A method according to claim 1 or claim 2, in which the organic binder is a methylcellulose, a phenolic resin or a silicone resin.

4. A method according to any preceding claim, in which the shaped body is calcined at from 500 to 10000C.

5. A method according to any preceding claim, in which the bulk density of the calcined body is from 70 to 90% of the corresponding mixture of silicon carbide and carbon.

6. A method according to any preceding claim, in which the organic solution contains from 10 to 50% by weight of the said material.

7. A method according to any preceding claim, in which the said material is capable of isolating free carbon by thermal decomposition and is selected from phenolic resins, polyester resins, polystyrenes, epoxy resins, non-volatile fats and oils and paraffin waxes.

8. A method according to any of claims 1 to 6, in which the said material is capable of isolating silicon carbide by thermal decomposition and is a polycarbosilane.

9. A method according to any preceding claim, in which, before contacting the body with molten silicon, the said material is thermally decomposed.

10. A method according to claim 9, in which the thermal decomposition is conducted before the drying.

11. A method according to claim 9, in which the impregnation and thermal decomposition steps are conducted at least twice.

1 2. A method according to claim 10, in which the impregnation, drying and thermal decomposition steps are repeated at least twice.

13. A method according to claim 1, substantially as described in any of the Examples.