DE3917734A1 - METHOD FOR PRODUCING CERAMIC MOLDED BODIES BY FREEZING AQUEOUS SLICKER - Google Patents

Abstract

A process for the production of high-density ceramic castings, in which an aqueous slip having a viscosity of 5 to 500 mPa.s is produced from a ceramic or metal powder, water and, if desired, dispersants, binders, a source of carbon and/or sintering aids, the slip is poured into a mould and frozen therein within not more than 90 seconds, and the frozen casting is removed from the mould, dried and sintered. Formation of dendritic ice crystals which impair the strength characteristics of the castings is prevented by a high rate of cooling without the need to add nuclei-forming additives.

Description

The invention relates to a method for producing high-density ceramic moldings, in which made of ceramic or metallic powder, auxiliaries and water Slurry is produced, the aqueous slip in a The mold is poured and frozen in it, the frozen one Casting is removed from the mold, dried and sintered.

A prerequisite for the further spread of ceramic Components in the field of functional and engineering ceramics is the economical production of large quantities. It is particularly important that the shape a close proximity to the final contour is achieved because on the one hand the Processing in the unsintered state usually because the insufficient strength of the green bodies is difficult, and on the other hand the processing in the Gesin tter condition due to the extremely high hardness of the ceramic Materials with diamond tools are very complex and ent speaking is costly.

as standardized shaping processes, in part already automated, axial pressing, cold iso static pressing, hot pressing or hot iso static presses applied. Leave with these procedures however, only geometrically simple molded articles are produced, and the procedures are relatively expensive.

More complex moldings, e.g. Turbocharger rotors, can be produced with the injection molding process. Disadvantageous However, the tools are very high in this process cost, especially with undercuts on the shape body in many parts and correspondingly complex shapes to lead. Another disadvantage is the high abrasive attack, due to the ceramic masses with high solids content on the injection molding machine parts, such as screw and cylinder, and occurs on the forms themselves and by appropriate ones  Hardening or protective layers must be minimized. Another problem is the high expenditure of time for that Dewax i.e. the removal of the ceramic Bulk plasticizers added. Depending on the component size This may take several weeks.

This has the disadvantages of classic ceramics known slip casting process is not due to the slip be produced on an aqueous basis. The wear of the Tools is because of the low viscosities and shear rates clearly lower. With the classic slip casting process the aqueous slip is poured into a plaster mold. Due to the capillary forces of the gypsum, the slip Removed water until a hard leather from the plaster mold Green body can be removed. With the hollow casting process can only produce larger moldings with a constant wall thickness getting produced. Smaller moldings are in full casting drive producible, but the body formation time increases with the square of the body thickness. The plaster molds also have a limited lifespan, each between 5 and 100 impressions, depending on the type of slip be dried in a controlled manner after each pouring process.

To eliminate these disadvantages and the casting without Removing damage from the mold is already done been proposing to freeze the slurry in the molds remove the frozen casting from the mold and then dry. This procedure exists however, there is a risk that larger ice crystals will form when freezing form, which lead to large pores when drying, which the Strength and uniformity of the sintered molded body deteriorate and lead to defects in the molded body. To Prevention of the formation of larger ice crystals are already various measures have been proposed.

DE-A-32 11 083 describes a freeze casting process  in which the casting compound hydrogen-binding substances, such as e.g. n-propyl sulfoxide, triethanolamine, urea, Dimethyl sulfoxide and others can be added. These Additives are said to freeze the water very much in the form enable small crystals.

EP-B-00 16 971 describes a method in which that of the slurry is a freeze-sensitive colloidal ceramic sol, especially silica sol, preferably with Lithium ion is stabilized, is added on slurry after placing in the mold heavily under cools and freezes. To hypothermia and that in to achieve substantial instant freezes the shape before applying the slurry with a treated hydrophobic liquid, so that the shape when Filling the slurry is still damp, and the filled one The mold is then placed in a cooling medium. Shape and cool medium should have a high thermal conductivity. With this Processes are slurries with very high viscosity used.

The addition of nucleating substances to those just described However, the process has disadvantages. The fabrics must then in a separate step before Can be removed or remain, as in the case of Kieselsols, as undesirable components in the ceramic Molded body. In particular for the production of moldings made of non-oxide ceramic materials Molded additives remaining out of the question.

From EP-A-01 61 494 a method is known in which a ceramic powder, a binder liquid, in particular Water is added to the mixture filled in a mold is molded under pressure, cooled quickly, from the mold is removed, dried and then sintered. Because the shape in this process by injection molding, compression molding  or some other technique when printing on that in the mold existing material is exercised, must be done Procedures costly.

The object of the present invention is to create a Process of the type mentioned, which is economically Lich can be carried out, no nucleating additives Prevent the formation of dendritic ice crystals dert, and the production of moldings with high equal shape and good strength properties as well complex shape.

This object is achieved by that specified in claim 1 Procedure solved.

Excessive ice crystal growth was found during the freezing step even without nucleating additives can be prevented if the casting with a high Cooling rate is frozen. Preferably the Casting within 60 s and particularly preferably within frozen from 20 s.

In order to achieve a high cooling rate prefers molds made of a material with high thermal conductivity ability that is preferably at least 200 W / (m · K) is. Preferred materials are e.g. Aluminum, copper and brass. If the molds are made of lower materials Thermal conductivity exist, become very thin-walled shapes used. Furthermore, to a high cooling rate to achieve the forms filled with the slip a very low temperature environment, e.g. in the cold room of a cryostat or in direct contact with the Cooling medium of a cryostat to be cooled. It is also possible the forms before filling the aqueous Pre-cool the slip to a low temperature. The  the cooling method used depends on the size and shape of the molded body.

For the production of moldings with a very small volume the aqueous slip can be in a mold at room temperature be poured and then put the mold in a cold room will. In the case of larger moldings, the mold is pre-cooled and after filling the slip into a cold room posed. If the shape has sufficient wall thickness points, and the casting is relatively small, the one bring in a cold room account for the heat capacity the shape is then sufficient to close the casting quickly enough freeze. However, this method is not for manufacturing of moldings with very different wall thicknesses suitable, because then there is a risk that the thin-walled Freeze parts early and thereby complete Prevent filling of the mold.

For the production of moldings with a complex shape, such as. Turbocharger rotors, it is most suitable one Use mold at room temperature, if necessary to deaerate and the aqueous slip under vacuum to fill. The mold is then preferably in the refrigerant a cryostat immersed to get through the improved Heat transfer to achieve a sufficiently rapid cooling. In this case, the mold must prevent the ingress of cold be protected.

In the process according to the invention, the aqueous slip does not contain any nucleating additives to prevent the formation of dendritic ice crystals. Rather, the formation of dendritic ice crystals is achieved solely through the high cooling rate. The moldings produced according to the invention thus have no large pores which reduce the strength. If necessary, dispersants, binders, carbon sources and / or sintering aids can be added to the aqueous slip. The substances normally used in technical ceramics are suitable as dispersants and binders. If silicon carbide moldings are produced, binders can be used which not only increase the strength of the dried green body, but also serve as a carbon source in order to reduce the silicon dioxide layers adhering to the silicon carbide primary powder particles during sintering. Suitable binders for this purpose are, for example, phenol-formaldehyde resols. To produce reaction-sintered silicon carbide moldings, carbon black and / or graphite is additionally added to the aqueous slip as a carbon source. In order to be able to sinter molded articles made of silicon carbide or silicon nitride without pressure, sintering aids, such as, for example, Y ₂ O ₃ , Al ₂ O ₃ , B, etc., are added to the aqueous slip.

As ceramic powders, for example Al ₂ O ₃ , 3A1 ₂ O ₃ × 2SiO ₂ , Al ₂ TiO ₄ , ZrO ₂ , SiC and Si ₃ N ₄ can be used in the process according to the invention. The method according to the invention is particularly suitable for the latter two ceramic materials. Silicon is used as the metallic powder for producing reaction-bonded silicon nitride moldings.

The ceramic or metallic powder can be used in the usual way be processed, i.e. in a solvent, if necessary Add sintering aid and / or carbon source ground, sieved and dried. Subsequently becomes the aqueous slurry from the conditioned powder if necessary with the addition of binder and / or dispersant posed. When processing ceramic or metallic Powder water used can be treatment and fro position of the aqueous slip in one step, where the additives used are added immediately.

The aqueous slip has a viscosity of 5 to 500 mPa × s and preferably from 10 to 200 mPa × s. To  The aqueous slip generally has homogenization a solids content of 30 to 70 vol .-% and preferred from 45 to 65 vol%.

The aqueous slip is preferably used before filling degassed into the mold. In certain cases it is preferred to remove the mold before filling the aqueous slip ventilate and fill in the aqueous slip under vacuum. Depending on the shape of the molded body, it can be advantageous on the surfaces of the mold that with the come into contact with aqueous slip, a release agent, e.g. a commercial vacuum grease to apply to the To facilitate demolding.

After freezing, the frozen casting is removed from the mold. The subsequent drying takes place depending on the type and Shape of the molded body in a drying cabinet, climate cabinet, microwave oven or a freeze dryer investment. In any case, it must be ensured that the Sufficient moisture when thawing the casting is dissipated quickly. Freeze-drying is preferred applied.

If a binder is included, this can be done before Sintering step can be removed from the molded body. The Shaped body is then sintered using conventional methods. The term "sintering" is not only meant to be compression and Solidification of the molded body through temperature treatment with or without pressure but also reaction sintering include where the ceramic substance during sintering is formed or liquid silicon is infiltrated. For the production of reaction sintered silicon carbide Shaped bodies become a silicon firing in a known manner made with the liquid silicon in the molded body penetrates and with the carbon forming new Silicon carbide reacts. For the production of reactive bundles  which is shaped silicon nitride in a known manner burned in a nitrogen atmosphere, the nitrogen reacts with the silicon to form silicon nitride.

The freeze-molded articles show after sintering comparable strengths, densities and an identical Structure morphology such as isostatically pressed molded bodies.

In the following the invention is illustrated by the examples and Figures explained in more detail.

The drawing shows:

A valve produced according to Example 2, Fig. 1,

Fig. 2 is a ring prepared according to Example 3 and

Fig. 3 is a welding nozzle manufactured according to Example 4.

example 1

A mixture of 88% by weight of Si ₃ N ₄ and 10% by weight of Y ₂ O ₃ and 2% by weight of Al ₂ O ₃ as sintering aid was ground in isopropanol for 2 hours, sieved (20 μm) and evaporated in a rotary evaporator dried. The dry granules were sieved again (320 µm). An aqueous slip of 24.8% by weight of water, 0.2% by weight of ammonium salt of a polyacrylic acid (Dolapix CA®) as a dispersing agent, 3.0% by weight of ester wax (KST-Wachs®) as Binder and rest of the granulate produced. By using a transparent mixing vessel, controlled degassing was possible without risk of overflowing even with foaming slips.

An aluminum mold (thermal conductivity 240 W / (m × K) with a wall thickness of 1 mm was used for the production of Pipes with an outer diameter of 20 mm, one Inside diameter of 10 mm and a length of 300 mm with filled the slip and then one in the cold room Frozen cryostats (-95 ° C). After 60 s the casting became  removed from the mold and dried in a freeze dryer. The binder was then exposed to air in a chamber furnace removed at temperatures up to 500 ° C. The sintering of the Pipes were made in the temperature range between 1600 and 1800 ° C without pressure in a nitrogen atmosphere.

The tubes had an outer diameter after sintering of 14.8 mm, an inner diameter of 7.4 mm and one Length of 228 mm. Post-treatment of the pipes was limited short-term grinding and polishing. The The structure of the pipes was homogeneous and fine-grained (crystallite size <5µm). Bending samples made from the tubes resulted in a density of 98.5% of the theoretical Density a 4-point flexural strength of 750 MPa.

Example 2

The preparation of the ceramic mass and the preparation of the aqueous slip were carried out as in Example 1. A thick-walled casting mold made of copper (thermal conductivity 400 W / (m × K)) with a wall thickness of 10 mm was precooled to -70 ° C. in a cryostat. After filling in the slip, the frozen casting was removed from the mold after 30 s. The drying took place in a climatic cabinet at temperatures between -10 and + 80 ° C and relative air humidity between 80 and 20%. After dewaxing in air at 500 ° C, the valve was sintered and processed as in Example 1. The finished valve ( Fig. 1) had comparable material properties as in Example 1.

Example 3

Commercial α- SiC powder with a specific surface area <10 m ² / g and an average grain size <1 µm was used as the starting material. The powder was homogenized with 0.4% by weight of B as a sintering aid and 4.0% by weight of an aqueous phenol-formaldehyde resole in a laboratory planetary ball mill. The slip was sieved and degassed and had a solids content of 71% by weight before casting.

The slip was cast into a ring shape (25 ° C) made of brass (thermal conductivity 200 W / (m × K)) with a wall thickness of 0.5 mm, brought into direct contact with the cooling medium of a cryostat (-65 ° C) and demolded after 60 s. To improve the mold release, the brass surface had previously been coated with a silicone grease. Drying was carried out in the freeze dryer (24 h). The casting skin on the top of the ring was removed in the green state and the ring was then sintered under Ar at 2100 ° C. without pressure. The shrinkage was 23%. After sintering, there was a brief grinding and polishing. The finished ring is shown in Fig. 2. The 4-point bending strength was 390 MPa with a density of 97.2% of the theoretical density. The structure was fine-grained with a crystallite size <5 µm.

Example 4

The aqueous slip was prepared and prepared as in Example 3. The conditioned slip was poured into a mold made of polytetrafluoroethylene with a wall thickness of 0.2 mm, brought into direct contact with the cooling medium of a cryostat (-65 ° C) and after 90 s demolded. The drying in the climatic cabinet was carried out as in Example 2, sintering and finishing according to Example 3. The welding nozzle produced then had the shape shown in FIG. 3 and had comparable material properties to the ring in Example 3.

Claims (16)

1. A process for the production of high-density ceramic moldings, in which a slip is produced from ceramic powder, auxiliary substances and water, the aqueous slip is poured into a mold and frozen therein, the frozen casting is removed from the mold, dried and sintered, thereby characterized in that the aqueous slip, consisting of ceramic or metallic powder, water and optionally dispersing agent, binder, carbon source, and / or sintering aid, has a viscosity of 5 to 500 mPa × s and is frozen within no more than 90 s . 2. The method according to claim 1, characterized in that the aqueous slip within no more than 60 s is frozen. 3. The method according to claim 2, characterized in that the aqueous slip within no more than 20 s is frozen. 4. The method according to any one of claims 1 to 3, characterized characterized in that the aqueous slip into a mold made of a material with a thermal conductivity of at least 200 W / (m × K) is poured. 5. The method according to claim 4, characterized in that the aqueous slip in a form made of aluminum, brass or copper is cast. 6. The method according to any one of claims 1 to 5, characterized characterized in that the aqueous slip into a very thin-walled mold is poured.   7. The method according to any one of claims 1 to 6, characterized characterized in that the slip poured into the mold is frozen in the cold room of a cryostat. 8. The method according to any one of claims 1 to 6, characterized characterized in that the slip poured into the mold is frozen in the coolant of a cryostat. 9. The method according to any one of claims 1 to 8, characterized characterized in that the ceramic powder is silicon carbide or is silicon nitride. 10. The method according to any one of claims 1 to 8, characterized characterized in that the metallic powder is silicon. 11. The method according to any one of claims 1 to 10, characterized characterized in that the aqueous slip before Filling into the mold is degassed. 12. The method according to any one of claims 1 to 11, characterized characterized in that the form before filling the vented aqueous slip and the aqueous slip is filled in under vacuum. 13. The method according to any one of claims 1 to 12, characterized characterized in that the ceramic or metallic Powder if necessary with the addition of sintering aids and / or Carbon source in a non-aqueous solvent prepared, dried and with water if necessary with the addition from binder and dispersant to aqueous slip is processed. 14. The method according to any one of claims 1 to 12, characterized characterized that the processing of the ceramic or metallic powder, if necessary with binder, sinter auxiliaries, carbon source and / or dispersant  medium and the preparation of the aqueous slip be carried out in one step. 15. The method according to any one of claims 1 to 14, characterized characterized in that when adding a binder before A dewaxing step is provided. 16. The method according to any one of claims 1 to 15, characterized characterized in that the frozen casting after the Remove from the mold in a freeze dryer layer is dried.