WO1994010101A1

WO1994010101A1 - Method of manufacturing mouldings from reaction-bonded, silicon-infiltrated silicon carbide, and a compression-moulding compound used as an intermediate in the method

Info

Publication number: WO1994010101A1
Application number: PCT/EP1993/003027
Authority: WO
Inventors: Albert Kerber; Mehmet Kara; Karl-Ludwig Eckert
Original assignee: Lonza-Werke G.M.B.H.
Priority date: 1992-11-02
Filing date: 1993-10-29
Publication date: 1994-05-11

Abstract

In the method proposed, mouldings are produced from reaction-bonded, silicon-infiltrated silicon carbide using graphite or carbon black as the source of carbon. A mixture of a water-soluble organic polymer, an aqueous wax emulsion and a hygroscopic organic compound is used as a temporary binder. Before the silicon-infiltration stage, the green compacts obtained by compression are subjected to heat treatment under controlled conditions. The method makes it possible to reach higher green densities and higher infiltrated densities. The mouldings produced are for instance suitable for use as slip rings and bearings for pumps, welding-torch nozzles and burner barrels.

Description

Process for the production of moldings from reaction-bound, silicon-infiltrated silicon carbide and molding compound as an intermediate product

The present invention relates to a process for the production of moldings from reaction-bonded, silicon-infiltrated silicon carbide. It further relates to a finished mass based on silicon carbide as an intermediate in the process according to the invention.

Silicon infiltrated silicon carbide (SiSiC) is a ceramic composite material that is becoming increasingly important due to its extreme hardness and wear resistance, its resistance to oxidation and corrosion (in the acidic, neutral and weakly basic range) and its good resistance to temperature changes and high temperatures as well as its gas tightness. Applications for silicon infiltrated silicon carbide are, for example, mechanical seals and bearings for pumps, combustion chambers, welding nozzles, flame tubes etc. (see, for example, M. Blecha et al., Conference room 123, 263-268 (1990)).

Shaped bodies made of reaction-bound, silicon-infiltrated silicon carbide are usually produced by molding a green body from silicon carbide grain which contains a certain proportion of elemental carbon and has an open porosity. The green body is then infiltrated with liquid silicon, which reacts with the free carbon and forms silicon carbide. Ideally, the finished molded body contains neither pores nor unreacted carbon, and the spaces between the particles are filled with metallic silicon.

H. Cohrt in Z. Werkstofftech gives an overview of the production, properties and application of reaction-bound silicon-infiltrated silicon carbide. 16, 277-285 (1985).

The shaping in the production of green bodies can in principle be carried out by all methods customary in ceramics, preferably by molding. For this it is necessary to add temporary binders and plasticizers in order to achieve sufficient strength of the green body and sufficient formability. To this end So far, mostly organic polymers in the form of thermoplastics or resins or thermosets (which harden during the pressing process or subsequently harden) have been used. The binders and plasticizers used up to now generally required a number of special heat treatments in controlled atmospheres (H. Cohrt, loc. Cit.). When using resins (M. Blecha et al., Loc. Cit.), It should also be noted that these can only be mixed in shortly before pressing in order to avoid uncontrolled hardening of the mass, or a special heat treatment for hardening ¬ need hardening. Finally, their use often requires the use of organic solvents, which requires additional effort to avoid environmental pollution. It is also known to use water soluble binders such as carboxymethyl cellulose. With this, however, only massive green densities of at most approx. 2.25 g / cm- ^> and densities of the infiltrated bodies of at most 3.03 g / cm ^ could be achieved.

It was an object of the present invention to provide a process for the preparation of reaction-bonded, silicon-infiltrated silicon carbide which requires only small amounts of binder, does not require organic solvents and provides green bodies of high density and sufficient strength, which end products without expensive heat treatments before silicon infiltration result in high density and strength. Furthermore, it was an object of the present invention to provide a storage-stable and readily processable, ready-to-use molding compound for carrying out the process according to the invention.

According to the invention the object is achieved by the method according to claim 1 and the molding compound according to claim 11.

It has been found that when graphite or carbon black is used as the carbon source and a combination of a water-soluble polymer with a wax and a hygroscopic organic compound as a temporary binder and pressing aid, very good processing properties, high green densities and excellent final densities are obtained even with relatively small amounts of organic additives the molded body can be reached. The process does not require any organic solvents and is easy to carry out. Because of the small amount of organic additives, the heat treatment before silicon infiltration can be carried out with a low level of pollutants, in addition relatively low temperatures are sufficient and no special atmosphere has to be created. A mixture of very fine and coarse grains or several grain fractions can be used as the silicon carbide grain mixture. Such mixtures are known per se, their structure is advantageously optimized depending on the intended use, size and shape of the molded body to be produced. This procedure is known to the person skilled in the art. When testing the method according to the invention, good results were obtained with mixtures of more than 15% by weight, preferably more than 20% by weight, of very fine grain with an average particle size of 1-10 μm and a "coarse grain" proportion with average particle sizes in the range 25- 150 μm achieved.

It is also within the scope of the present invention to use a single grain size instead of a mixture of different grain sizes. In this case, a relatively large amount of carbon (up to about 20% by weight) is expediently used.

Graphite is preferably used as the carbon source, soot can also be used. It is also within the scope of the invention to use another carbon source with a high carbon content, for example petroleum coke.

The carbon source is expediently used in an amount of 4-20% by weight, calculated as carbon and based on the amount of silicon carbide. Particularly good results are achieved with graphite in amounts of 6-8% by weight if the coarse grain fraction of the SiC offset is at an average grain size of approximately 50 μm.

For example, polyvinyl alcohol, carboxymethyl cellulose, polyethylene glycols, alginates or dextrins can be used as water-soluble polymers. Polyvinyl alcohol is particularly preferred.

If the green bodies still have to be subjected to mechanical finishing, it may be advantageous to use small amounts of resin as a temporary binder to further increase the strength. In general, however, the strength that can be achieved with the water-soluble polymers is sufficient. The wax used as a pressing aid according to the invention is expediently used as an aqueous emulsion. Such wax emulsions are commercially available, for example from Zschimmer & Schwarz, Lahnstein / Rhein.

According to the invention, a hygroscopic organic compound is used as a further pressing aid. A polyol, ie a dihydric or polyhydric alcohol, is preferably used for this. Glycols such as, for example, ethylene glycol, diethylene glycol, triethylene glycol or propylene glycol or compounds with three or more alcohol functions such as glycerol or the so-called sugar alcohols such as mannitol, sorbitol etc. are particularly suitable for this purpose.

The combination of temporary binder and pressing aids according to the invention is preferably used in a total amount (without the water used as solvent or for the preparation of the emulsion) of less than 10% by weight, based on the amount of silicon carbide. Amounts of less than 5% by weight are particularly preferred.

The mixture of silicon carbide grains and binder / pressing aid combination is produced in a manner known per se; it is advantageously concluded with a granulation and drying step.

The molding compound or granules obtained in this way is also the subject of the present invention. Under normal conditions, it can be kept practically indefinitely and can be pressed in without further pretreatment.

The pressing process is known per se. Pressures in the order of magnitude of 1000 bar (100 MPa) to 2500 bar (250 MPa) are advantageously used. To further increase the green density, an isostatic post-compression can advantageously be connected to the (uniaxial) compression molding process. This procedure is also known per se. By using the binder / pressing aid combination according to the invention, green bodies with densities of 2.45 to 2.57 g / cm- ^* - ^* can be obtained in this way. Comparative tests with water-soluble polymer alone gave only green densities up to 2.44 g / cm * - *.

The green bodies produced according to the invention can be subjected to silicon infiltration even without special heat treatment because of their low content of coking organic substance. However, it has proven advantageous to nevertheless provide a heat treatment step in order to achieve a controlled decomposition of the organic components. The heat treatment can be carried out in one step, temperatures of from 200 to 400 ° C. being expediently maintained. The temperature is preferably 220 to 300.degree. The heat treatment can take place in air, whereby the air can either freely enter or the air access is controlled by a powder bed. Silicon carbide powder is preferably used as the powder bed. This ensures an even temperature distribution and prevents contamination of the green bodies by foreign material. This heat treatment enables final densities of approx. 3J0 g / cm ^* - ^{5 to be} achieved in the subsequent silicon infiltration. The infiltration as such is carried out in a manner known per se, as is any post-processing that may be required.

example 1

Preparation of the binder solution

A binder performance was set up according to the following recipe and mixed thoroughly.

Polyvinyl alcohol solution 110 parts

(10% by weight, Mowiol ^R GE 04 - 86)

Wax emulsion 30 parts

(Zusoplast-R * WE8, Zschimmer and black) Glycerin 98% 20 parts

Ammonia solution l, 25% 10 parts

Desalinated water 40 parts

The solution thus obtained was only briefly stirred before use.

Example 2

Production of a molding compound

The raw materials were premixed dry in an Eirich mixer according to the following recipe:

Silicon carbide F230 (FEPA standard) 1600 g

F280 "800 g

F360 "600 g

UFB5 * 1000 g

Graphite (LONZA KS6, grain size <6μm) 300 g

* ultra fine, spec. Surface (BET) 5 ± 1 m-lg

Subsequently, 610-620 g of binder solution (see example 1) were added with a metering pump at a rate of approx. 1.5 ml / s while the mixer was running, so that granules formed. This was mixed for a further 3 minutes and then dried at 60 ° C. to a residual moisture of 2-3%. Example 3 Pellet Production

In each case 4 g of the molding compound from Example 2 were uniaxially pressed in a die of 14 mm diameter at 100 MPa and a holding time of 5 s.

The compacts were then isostatically compacted at 250 MPa. Green densities of 2.50 - 2.58 g / cm * ^ were achieved.

For comparison, compacts were produced under the same conditions from a molding compound in which the wax emulsion in the binder solution was omitted. Only green densities of 2.40 - 2.44 g / cm ** - * were achieved.

Example 4 Infiltration

The compacts obtained according to Example 3 were heated to 300 ° C. in air in a powder bed made of silicon carbide (grain size 0.2-0.5 mm). Alternatively, some of the compacts were heated free-standing to 300 ° C or 260 - 280 ° C.

The pre-treated compacts were placed in a graphite crucible, the bottom of which had previously been coated with an approx. 10 mm thick layer of SiC (grain size 0.5 - 1 mm) and a 5 - 10 mm thick layer of a mixture of 75% by weight SiC (grain size 0 - 0.2 mm) and 25% by weight of silicon powder.

The compacts were then showered with a further SiC / Si mixture so that they were completely embedded in it. Under an argon atmosphere (pressure 4 mbar) the whole arrangement was then heated to 1480 ° C according to the following firing curve:

20 - 400 ° C 5 K / min 400 - 1100 ° C 28 K min 1100 - 1300 ° C 20 K / min 1300 - 1480 ° C 10 K / min

The temperature of 1480 ° C. was maintained for a further 10 hours, then the mixture was cooled to room temperature.

The samples infiltrated in this way had the following densities:

pretreated in a powder bed at 300 ° C: 3.07 - 3J 0 g / cm ³ free-standing """: 3.04 - 3.05 g / cm ³ at 260 - 280 ° C": 3.07 - 3.09 g / cm ³

Claims

Claims:

1. Process for the production of moldings from reaction-bound silicon-infiltrated silicon carbide by producing a mixture of silicon carbide grains, a carbon source and a temporary binder, optionally granulating the mixture, pressing to a green body, optionally heat treatment of the green body, and infiltration of the porous Green body with liquid silicon, characterized in that graphite and / or soot is used as the carbon source and a mixture of a water-soluble organic polymer, an aqueous wax emulsion and a hygroscopic organic compound is used as the temporary binder and pressing aid.

2. The method according to claim 1, characterized in that polyvinyl alcohol is used as the water-soluble polymer.

3. The method according to claim 1 or 2, characterized in that a polyol is used as the hygroscopic organic compound.

4. The method according to claim 3, characterized in that a compound from the group consisting of ethylene glycol, diethylene glycol, triethylene glycol, glycerol, propylene glycol is used as the polyol.

5. The method according to claim 4, characterized in that glycerol is used as the polyol.

6. The method according to one or more of claims 1 to 5, characterized in that the water-soluble polymer, the wax and the hygroscopic organic compound are used in an amount of less than 10% by weight, based on the amount of silicon carbide.

7. The method according to claim 6, characterized in that the water-soluble polymer, the wax and the hygroscopic organic compound in an amount of less than 5 wt.%, Based on the amount of silicon carbide, are used.

8. The method according to one or more of claims 1 to 7, characterized in that a heat treatment is carried out at 200 to 400 ° C in an air atmosphere before the silicon infiltration.

9. The method according to claim 8, characterized in that the heat treatment is carried out in a powder bed.

10. The method according to claim 8, characterized in that the heat treatment is carried out with free air access at 220 to 300 ° C.

11. molding compound for the production of moldings from reaction-bound, silicon-infiltrated silicon carbide, consisting of silicon carbide grains, a carbon source and a temporary binder, characterized in that they are graphite and / or carbon black as a carbon source and a mixture as a temporary binder and pressing aid contains a water-soluble polymer, a wax and a hygroscopic organic compound.

12. molding compound according to claim 11, characterized in that it contains polyvinyl alcohol as the water-soluble polymer.

13. molding compound according to claim 11 or 12, characterized in that it contains a polyol as a hygroscopic organic compound.

14. molding compound according to claim 13, characterized in that it contains as a polyol a compound from the group ethylene glycol, diethylene glycol, triethylene glycol, glycerol, propylene glycol.

15. molding compound according to claim 14, characterized in that it contains glycerol as the polyol.

16. molding compound according to one or more of claims 11 to 15, characterized gekenn¬ characterized in that it contains the water-soluble polymer, the wax and the hygroscopic organic compound in an amount of less than 10 wt .-%, based on the amount of silicon carbide.

17. molding compound according to claim 16, characterized in that it contains the water-soluble polymer, the wax and the hygroscopic organic compound in an amount of less than 5% by weight, based on the amount of silicon carbide.