GB2232115A

GB2232115A - Ceramic castings

Info

Publication number: GB2232115A
Application number: GB9011780A
Authority: GB
Inventors: Hartmond Kruener; Rainer Hamminger; Peter Selgert; Fritz Aldinger
Original assignee: Hoechst AG
Current assignee: Hoechst AG
Priority date: 1989-05-31
Filing date: 1990-05-25
Publication date: 1990-12-05
Anticipated expiration: 2010-05-25
Also published as: GB2232115B; GB9011780D0; JPH0319802A; DE3917734A1

Description

il k Process for the production of ceramic castings The invention relates

to a process for the production of ceramic castings, in which a slip is produced from a ceramic or metal powder and water, the aqueous slip is poured into a mould in which it is frozen and the frozen casting is removed from the mould, dried and sintered.

The wider utilization of ceramic structural units in the field of functional and engineering ceramics depends on the production by an economically viable method of a large number of units. It is particularly important that the shape imparted during casting comes very close to the final shape, since on the one hand fabrication in the unsintered state is normally very.difficult because of the low strength of the green body, while on the other fabrication in the sintered state is very timeconsuming and correspondingly expensive, since diamond tools have to be used because of the extreme hardness of the ceramic materials.

The standard casting techniques used, some of which are automated, are axial pressing, cold isostatic pressing, hot pressing and hot isostatic pressing. However, only geometrically simple castings can be produced by these techniques and the techniques are relatively expensive to carry out.

11 Castings of a more complex shape, for example turbo-supercharger rotors, may be produced by injection moulding. However, the very high cost of tools for producing multipart and correspondingly complicated shapes, especially if the castings need to be undercut, are a drawback of this process. A further drawback is abrasion of the injection moulding machinery parts, such as screws and cylinders, and of the moulds themselves by the ceramic compounds, which have a high solids content; this abrasion has to be minimized by the use of suitable reinforcements and protective coatings. In addition, the time required for dewaxing, i.e. removal of the plasticizer added to the ceramic compounds, can also be a problem; such dewaxing may take several weeks, depending on the size of the structural unit.

The slip-casting process, known from conventional ceramic technology, does not have these drawbacks, since the slips are water-based. Tool wear is significantly lower because of low viscosities and shear rates. In the classical slip-casting process the aqueous slip is poured into a plaster of Paris mould. Water is eliminated from the slip by the capillary forces of the plaster until a leather-hard green body can be withdrawn from the plaster inould. The hollow casting process can produce large castings only with uniform wall thickness. Smaller castings can be produced by the solid casting process, the time for forming the body increasing with the square i j of the body thickness. The plaster moulds have in addition only a limited life, viz. between 5 and 100 castings depending on the type of slip, and they must be dried under controlled conditions after each casting.

To overcome these drawbacks and to permit the casting to be removed from the mould undamaged. it has already been proposed to freeze the slip in the moulds. remove the frozen casting from the mould, and then dry it. However, this method involves the risk of large ice crystals forming in the freezing process; these cause large voids to form on drying which impair the strength and homogeneity of the sintered casting and lead to defects in the casting. Various measures have been proposed to prevent large ice crystals from forming.

DE-A-3,211.083 discloses a freeze-casting process in which hydrogenbinding substances, for example, n-propyl sulphoxide, triethanolamine, urea, dimethyl sulphoxide and others, are added to the casting material. These additives are said to allow the water to freeze in the form of very small crystals.

EP-B-0,016-971 discloses a process in which a colloidal ceramic sol, particularly a silica sol. which is susceptible to freezing and is stabilized, preferably with lithium ions, is added to the suspension, the suspension is strongly supercooled after being introduced into the mould, and frozen. In order to achieve the supercooling and the freezing, which is virtually t instantaneous, the mould is treated with a hydrophobic fluid prior to the introduction of the suspension, so that the mould is still moist after the suspension has been introduced, and the charged mould is then placed in a coolant. The mould and the coolant should possess very high thermal conductivity. This process uses suspensions of very high viscosity.

However, the addition of nuclei-forming substances in the processes just described has its drawbacks. The substances must be subsequently eliminated in a separate step prior to sintering, otherwise they remain, as in the case of silica sol, as undesirable components in the ceramic casting. It is undesirable for additives to remain in the casting, especially if the castings are to be produced from non-oxide ceramic materials.

EP-A-0,161,494 discloses a process in which a liquid binder, particularly water, is added to a ceramic powder, the mixture is introduced into a mould, cast under pressure, rapidly cooled, removed from the mould, dried and then sintered. Since in this process casting is effected by injection moulding, compression moulding, or other techniques in which pressure is exerted on the material in the mould, the process is expensive.

The present invention provides a process for the preparation of a ceramic casting in which an aqueous slip comprising water and a ceramic or metal powder and having a viscosity of 5 to 500 mPa.s is poured into a mould and, in not more than 90 seconds, is frozen therein, and the frozen casting is removed from the mould, dried, and sintered.

If desired, the slip also comprises one or more additional substances selected from dispersants, binders, sources of carbon, and sintering aids. Advantageously the slip consists of water and a ceramic powder and/or a metal powder and, optionally, one or more additional substances selected from dispersants, binders, sources of carbon, and sintering aids.

The invention makes it possible to provide a process for the production of high-density ceramic castings which can be performed economically, which does not require the introduction of nuclei-forming additives to prevent formation of dendritic ice crystals, and which allows the production of highly homogeneous castings of satisfactory strength and in complex shapes.

We have found that an excessive growth of ice crystals in the freezing stage can be prevented even without any nuclei-forming additives, if the casting is frozen with a high rate of cooling. The casting is preferably frozen within 60 seconds and particularly preferably within 20 seconds.

To attain a high rate of cooling, it is preferred to use moulds made from a material of high thermal conductivity, preferably not less than 200 W/(m.K). Preferred materials are. for example. aluminium, copper 4 and brass. If the moulds are composed of materials of lower thermal conductivity, then moulds having very thin walls are employed. Furthermore, to attain a high rate of cooling, the moulds filled with the slip may be cooled in a very low temperature environment, for example in the freezing chamber of a cryostat or in direct contact with the coolant of a cryostat. It is also possible to precool the moulds to a low temperature prior to introducing the aqueous slip. The cooling technique employed depends on the size and shape of the casting.

For the production of castings with a very small volume, the aqueous slip can be poured into a mould at room temperature and the mould can then be placed in a freezing chamber. For larger castings the mould is precooled, filled with the slip and placed in a freezing chamber: if the mould has sufficiently thick walls and the casting is relatively small, it is not necessary to place the precooled mould containing the casting in the freezing chamber, since the heat capacity of the mould is adequate to ensure a sufficiently rapid cooling of the casting. However, this process is unsuitable for the production of castings with highly variable wall thicknesses, since there is then the risk that the thinwalled parts will freeze early and thus prevent complete filling of the mould.

To produce complex-shaped castings, for example turbo-supercharger rotors, the most suitable method is to employ a mould at room temperature, evacuate it if necessary and introduce the aqueous slip under reduced pressure. The mould is then preferably immersed in the coolant of a cryostat in order to achieve a sufficiently rapid cooling rate by the improved heat transfer. In this case the mould must be protected against entry of the coolant.

In the process according to the invention the aqueous slip need not contain nuclei-forming additives for preventing the formation of dendritic ice crystals. It is more likely that the formation of dendritic ice crystals is prevented by the high cooling rate alone. The castings produced according to the invention are thus free from any large voids which would reduce their strength. one or more additional substances, for example, dispersants, binders, sources of carbon and/or sintering aids may be added to the aqueous slip, if desired. Examples of suitable dispersants and binders are substances customarily employed in ceramic technology. If silicon carbide castings are produced, the binders employed may be those which not only increase the strength of the dried green body, but at the same time act as sources of carbon in order to reduce during the sintering process silicon dioxide layers adhering to the silicon carbide primary powder particles. Suitable binders for this purpose are, for example, phenolformaldehyde resols. To produce reaction- sintered k silicon carbide castings, a source of carbon, for example, carbon black and/or graphite, is further added to the aqueous slip. In order to make possible the sintering of castings from silicon carbide or silicon nitride without the use of pressure, a sintering aid, for example Y203, A1203, B etc., may be added to the aqueous slip.

Examples of materials which may be used as ceramic powders in the process according to the invention, are A1203, 3A1203.2SiO2, A12Ti04. Zr02. SiC and Si3N4. The process according to the invention is particularly suitable for the two last-named ceramic materials. To produce reaction- bonded silicon nitride castings, silicon is used as metal powder. A example of a further metal powder which may be used in accordance with the invention is aluminium powder: if the slip is dried and heated in the presence of oxygen, an aluminium oxide ceramic material can be prepared.

The ceramic or metal powder can be compounded in any suitable manner, i.e. ground in a solvent, with or without the addition of a sintering aid and/or a source of carbon, sieved and dried. The aqueous slip is produced subsequently from the conditioned powder, with or without the addition of a binder and/or a dispersant. If water is used for the compounding of the ceramic or metal powder, compounding and production of the aqueous slip can be carried out in a single step, in which case the additives, if used, can be added at the same time.

The aqueous slip used in accordance with the invention has a viscosity of 5 to 500 mPa.s and preferably of 10 to 200 mPa.s. After homogenization, the aqueous slip advantageously has a solids content of 30 to 70% by volume and preferably 45 to 65% by volume.

The aqueous slip is preferably degassed prior to being introduced in the mould. In some cases it is preferred to evacuate the mould prior to introducing the aqueous slip and to introduce the aqueous slip under reduced pressure. Depending on the shape of the casting, it may be advantageous to apply to the surfaces of the mould which come into contact with the aqueous slip a release agent, for example, a commercial vacuum grease, to facilitate demoulding.

The frozen casting is demoulded after freezing. Subsequent drying may be carried out, depending on the type and shape of the casting, in, for example, a drying cabinet, a humidity chamber, a microwave instrument or in freeze-drying equipment. Whatever drying method is used, it is important to ensure that, when the casting thaws, moisture is removed sufficiently quickly. Freeze-drying is preferably used.

If a binder is present, it may be removed from the casting prior to the sintering stage. The casting is then sintered in any suitable manner. The term Isintering' encompasses not only consolidation and compaction of the casting by heat treatment with or without pressure, but also reaction sintering,.in which the ceramic substance is formed during the sintering or becomes penetrated by liquid silicon. To produce reaction-sintered silicon carbide castings, siliconforming firing in which liquid silicon penetrates the casting and reacts with carbon forming new silicon carbide, may be carried out. To produce reaction- bonded silicon nitride castings, firing may be carried out in for example, a nitrogen atmosphere, the nitrogen reacting with the silicon with the formation of silicon nitride.

Sintered freeze-cast castings produced in accordance with the invention may possess strengths and densities comparable to, and a structural morphology identical with, isostatically moulded castings.

The following Examples and drawings illustrate the invention. In the drawings, in which dimensions are given in mm: Figure I represents a valve produced according to Example 2; Figure 2 represents a ring produced according to Example 3; and Figure 3 represents a welding nozzle produced according to Example 4. The viscosities of the slips used in the Examples are in the range of from 10 to 50 mPa.s.

i Z - 1 1 - Exsimple 1 A mixture of 88% by weight Of Si3N4P 10% by weight Of Y203 and 2% by weight of A1203 as sintering aid was ground for 2 hours in isopropanol, sieved (20 gn) and dried in a rotary evaporator. The dry granules were sieved for a second time (320 gm). An aqueous slip was produced in a stirred vessel from 24.8% by weight of water. 0.2% by weight of ammonium polyacrylate (Dolapix CA:Dolapix is a trade mark) as dispersant, 3.0% by weight of ester wax (KST Wax: trade mark) as binder and the granules as the remaining percentage. By using a translucent stirred vessel it was possible to carry out controlled degassing without danger of any strongly foaming slip running over.

For the production of tubes having an external diameter of 20 mm, an internal diameter of 10 mm and a length of 300 mm, the slip was introduced into an aluminium mould (thermal conductivity 240 W/(m.K)) with a 1 am wall thickness and then frozen in the freezing chamber of a cryostat (-95C). After 60 seconds the casting was demoulded and dried in freeze-drying equipment. The binder was subsequently eliminated in a chamber furnace open to air at temperatures of up to 500C. The tube was sintered in a nitrogen atmosphere in a temperature range of from 1600 to 1800C without any pressure being applied.

After sintering, the tubes had an external diameter of 14.8 mm, an internal diameter of 7.4 mm, and a length of 228 mm. Subsequent treatment of the tubes was limited to brief grinding and polishing. The structure of the tubes was uniform and the particle size was small (crystal size < 5 pm). Bending tests performed on the tubes indicated a 4-point flexural breaking strength of 750 MPa at a density of 98.5% of the theoretical density.

Example 2

The compounding of the ceramic compound and the production of the aqueous slip was carried out as in Example 1. A thick-walled mould made of copper (thermal conductivity 400 W/(m.K)) with a 10 mm wall thickness was precooled in a cryostat at -70C. The slip was introduced into the mould, and the frozen casting was demoulded after 30 seconds. Drying was effected in a humidity chamber at temperatures of from -10 to +80C and relative humidities of from 80 to 20%. After dewaxing in air at 500C the valve was sintered and aftertreated as in Example 1. The finished valve (Figure 1) had material characteristics similar to those of Example 1.

Example 3 commercial a-siC powder of a specific surface area > 10 m2/g and an average particle size < 1 gm was used as the starting material The powder was homogenized in water in a standard laboratory planetary ball mill with 0.4% by weight of boron as sintering aid and 4.0% by weight of an aqueous phenol- formaldehyde resol. The slip was sieved and degassed and had a solids content of 71% by weight prior to casting.

The slip was poured into a ring mould (25C) made of brass (thermal conductivity 200 W/(m.K)) with a 0.5 mm wall thickness, brought into direct contact with the coolant of a cryostat (-65C), and demoulded after 60 seconds. To improve the release properties, the surface of the brass had been precoated with a silicone grease. Drying was carried out in a freeze-drier (24 h). The casting skin on the upper side of the ring was removed in the green state and the ring was then sintered under Ar at 2100C without any pressure being applied. The shrinkage was 23%. After the sintering, the casting was briefly ground and polished. The finished ring is represented in Figure 2. The 4-point flexural breaking strength was 390 MPs at a finished density of 97.2% of theoretical density. The structure consisted of fine particles and had a crystal size < 5 gm.

Example 4

The compounding and production of the aqueous slip was carried out as in Example 3. The conditioned slip was poured into a polytetrafluoroethylene mould with a 0.2 mm wall thickness, brought into direct contact with 1 1.

- 14 the coolant of a cryostat (-650C), and demoulded after 90 seconds. Drying was effected in a humidity chamber as in Example 2, while sintering and aftertreatment corresponded to Example 3. The welding nozzle produced had the shape and dimensions shown in Figure 3 and its material characteristics were comparable to those of the ring in Example 3.

7 W - is -

Claims

CLAIMS:

1. A process for the preparation of a ceramic casting in which an aqueous slip comprising water and a ceramic or metal powder and having a viscosity of 5 to 500 mPa.5 is poured into a mould and, in not more than 90 seconds, is frozen therein, and the frozen casting is removed from the mould, dried, and sintered.

2. A process as claimed in claim 1, wherein the slip also comprises one or more additional substances selected from dispersants, binders, sources of carbon, and sintering aids.

3. A process as claimed in claim 1, wherein the slip consists of water and a ceramic powder and/or a metal powder and, optionally, one or more additional substances selected from dispersants, binders, sources of carbon, and sintering aids.

4 A process as claimed in any one of claims 1 to 3, in which the slip is frozen within not more than 60 seconds. 5. A process as claimed in claim 4, in which the slip is frozen within not mote than 20 seconds. 6. A process as claimed in any one of claims 1 to 5, in which the mould is made of a material having a thermal conductivity of not less than 200 w/(m.K). 7. A process as claimed in any one of claims 1 to 6, in which the mould is made of aluminium. brass or copper. 8. A process as claimed in any one of claims 1 to 7, in which the mould has very thin walls. 9. A process as claimed in any one of claims 1 to 8, in which the slip contained in the mould is frozen in the freezing chamber of a cryostat. 10. A process as claimolad in any one of claims 1 to 8, which the slip contained in the mould is frozen in the coolant of a cryostat. 11. A process as claimed in any one of claims 1 to 10, in which the ceramic powder comprises silicon carbide or silicon nitride. 12. A process as claimed in any one of claims 1 to 11, in which the metal powder comprises silicon. 13. A process as claimed in any one of claims 1 to 12, in which the slip is degassed before being poured into the mould. 14. A process as claimed in any one of claims 1 to 13, in which the mould is evacuated before the slip is introduced into it and the slip is introduced into the mould under reduced pressure. 15. A process as claimed in any one of claims 1 to 14, in which the ceramic or metal powder, with or without the addition of a sintering aid and/or a source of carbon, is compounded in a non-aqueous solvent, and dried and processed with water to the aqueous slip with or without the addition of a binder and/or dispersant. 16. A process as claimed in any one of claims 1 to 14, in which the compounding of the ceramic or metal powder, 1 A k with or without the addition of one or nore substances selected from binders, sintering aids, sources of carbon and dispersants, and the production of the aqueous slip are performed in a single step. 17. A process as claimed in any one of claims 1 to 16, in which, if the slip contains a binder, a dewaxing stage is introduced prior to the sintering stage. 18. A process as claimed in any one of claims 1 to 17, in which the frozen casting, having been removed from the mould, is dried in freeze-drying equipment. 19. A process for the production of high-density ceramic castings, in which a slip is produced from a ceramic or metal powder, auxiliaries and water, the aqueous slip is poured into a mould in which it is frozen, the frozen casting is removed from the mould, dried and sintered, the aqueous slip, which consists of a ceramic or metal powder, water and, if desired, dispersants. binders, a source of carbon and/or sintering aids, having a viscosity of 50 to 500 mPa.s and being frozen within not more than 90 seconds. 20. A process for the preparation of a ceramic-casting carried out substantially as described in any of the Examples herein. 21. A casting prepared by a process as claimed in any one of claims 1 to 20. 22. A casting substantially as described herein with reference to, and as illustrated by. any of the 1 accompanying drawings. 23. Any novel feature described herein, or any novel combination of hereindescribed features Published 1990 at The Patent Office. State House. 66.71 High Holborn. London IITC 1 R 4TP Further copies mky be obtained from The Patent OfficeSales Branch, St Mary Cray. Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray. Kent, Con. 1/87