GB2107635A

GB2107635A - Process of forming a refractory shape using slip casting

Info

Publication number: GB2107635A
Application number: GB08218045A
Authority: GB
Inventors: Gerald Quay Weaver; John Charles Logan
Original assignee: Norton Co
Current assignee: Saint Gobain Abrasives Inc
Priority date: 1978-09-18
Filing date: 1982-06-22
Publication date: 1983-05-05
Also published as: GB2107635B

Abstract

A process of forming a refractory shape, comprising: (a) providing a castable slip; (b) casting said slip into a plaster of paris casting mold so as to provide a green casting, wherein said mold has been treated by incorporating into the plaster of paris mold mix 0.1 to 4.0% by weight of an organic polymer in the form of an aqueous solution thereof; (c) decomposing said mold to facilitate removal therefrom of said green casting, said decomposing being such that said mold containing a green casting is sufficiently heated in an oxygen containing atmosphere so as sufficiently to oxidise said polymer and thereby weaken said mold to permit ready removal of said green casting from said mold; and (d) removing said green casting from said decomposed mold. a

Description

SPECIFICATION Process of forming a refractory shape using slip casting In the formation of intricate cast shapes of silicon carbide having delicate, thin, complex, curved surfaces, it is desirable to utilize a mold such as plaster of paris which can be conveniently formed by known processes. In employing such a mold it is desirable to use a slip cast technique as described in Fredriksson United States Patent No. 2,964,823. However, the green casting formed by slip casting silicon carbide is relatively fragile, and in a complex mold system, it is extremely difficult to remove the mold without harming the green casting or it can be very costly to make a multi-piece mold.

Recrystallization of silican carbide powder to form high temperature elements goes back at least 75 years to United States Patent No. 650,234 issued to Fitzgerald. A number of patents issued to Tone (such as United States Patents Nos. 913,324 and 1,013,701) show the siliconization of silicon carbide. Similarly, Hutchins United States Patent No. 1,266,478 shows the in situ formation of silicon carbide in a body by reaction between silicon and carbon incorporated in the body. Fredriksson United States Patent No.

2,964,823 shows the slip-casting of silicon carbide bodies which can then be fired to recrystallization temperature. Johnson United States Patent No.

3,079,273 shows the formation of objects from a mixture of silicon carbide, graphite and polymeric plastic. The objects are molded from the plastic and joined by part of the same mix and then fired at an elevated temperature to carbonize the plastic and siliconize the carbon contained in the mixture.

Alliegro United States Patent No. 3,482,791 describes the slip casting of a silicon carbide refiner piate which is then siliconized to increase its impermeability.

In Schleicher United States Patent No. 2,303,303 there is described a decomposable slip casting mold containing wood flourwhich can be burned out to weaken the mold.

LeBlanc et al United States Patent No. 3,549,473 describes the addition of organic resin to a ceramic.

There is a similar disclosure in the old United States Patent No.1,450,140 issued to Danahower.These latter two patents do not relate to slip casting, however.

In the parent U.K. patent application from which the present application is divided, there is described and claimed a process of forming a refractory shape, comprising: (a) providing a castable slip having as additive 1 to 10% of water miscible resin curable at a temperature below 200or; (b) casting said slip into a casting mold; (c) curing said resin so as to enable said cast slip to provide in said mold a green casting having substantial strength; (d) decomposing said mold to facilitate removal therefrom of said green casting; and (e) removing said green casting from said decomposed mold.

According to the present invention, there is provided a process of forming a refractory shape, comprising: (a) providing a castable slip; (b) casting said slip into a plaster of paris casting mold, so as to provide a green casting, wherein said mold has been treated by incorporating into the plaster of paris mold mix 0.1 to 4.0% by weight of an organic polymer in the form of an aqueous solution thereof; (c) decomposing said mold to facilitate removal therefrom of said green casting, said decomposing being such that said mold containing a green casting is sufficiently heated in an oxygen containing atmosphere so as sufficiently to oxidise said polymer and thereby weaken said mold to permit ready removal of said green casting from said mold; and (d) removing said green casting from said decomposed mold.

In carrying out the present invention, said slip can comprise particles of silicon carbide. Such a slip can comprise a bimodal mixture (as hereinafter defined) of silicon carbide particles, wherein a portion of said particles has an average size of the order of 3 microns, and another portion of said particles has an average size of less than substantially 74 microns.

Surface of said plaster of paris mold can have been treated so that porosity of that surface is less than that of plaster of paris. Internal surface of such a plaster of paris mold can have been lacquered to reduce permeability of that surface to water. Said mold containing said cast slip can be placed under hydraulic pressure so as to consolidate said cast slip, e.g. said hydraulic pressure can be of the order of 1.4 Kg/cm2.

Before said decomposition step (c) in the present invention, said mold containing said green casting can be heated to increase the green strength of said green casting. Before said decomposition step (c) in the present invention, said mold containing said green casting can be heated to in excess of substantially 600"C for sufficient time to oxidise silicon carbide so as to increase green strength of a green casting.

Said decomposition step (c) is such that said plaster of paris mold containing a green casting can be sufficiently heated in an oxygen containing atmosphere so as sufficiently to oxidise said polymer and thereby weaken said mold, to permit ready removal of said green casting from said mold.

The present invention also includes a said refractory shape obtained by a said process of the present invention. Some shapes are in the form of an airfoil of a turbine blade; in the form of a turbine rotor; or in the form of a turbine wheel. In providing a said refractory shape, said removed green casting can have been heated to recrystallisation temperature.

Said recrystallised casting can have been provided with additional carbon by utilising furfuryl alcohol.

Said recrystallised casting can have been heated in a silicon-containing atmosphere at a temperature of the order of 2000"C.

Referring now to the present invention, in general a castable slip of a biomodal distribution (as hereinafter defined) of silicon carbide particles is prepared. The slip is then cast into a plaster of paris mold which can be ultimately destroyed to permit removal of the mold from the green casting. Prior to the removal of the mold from the casting, the green casting is partially strengthened in one of several ways. In one method, a curable resin is added to the slip and this curable resin is cured prior to removal from the mold. In another method of strengthening the green casting, it is heated, while still in the mold, to temperatures sufficiently high to slightly oxidize the silicon carbide and thereby form a glassy bond which strengthens the green casting.

The plaster of paris mold is preferably treated, at least on its inner surface which contacts the slip, so as to appreciably slow down the absorption of water from the slip to permit complete filling of the mold, this being particularly important when very thin sections, such as airfoils of turbine blades and the like, are to be formed. The curable or otherwise settable resin preferably is one which is cured or set at a relatively low temperature of the order of 200"C or less.

The green casting removed from the mold is preferably fired to a sufficiently elevated temperature to recrystallize the silicon carbide and form a relatively rugged structure. This product will have some residual carbon remaining after the firing operation due to the decomposition of the resin binder. Where a fairly dense product is desired, additional carbon can be provided in the interstices ofthe relatively porous recrystallized silicon carbide structure by utilizing furfuryl alcohol, as described in Hevroth United States Patent No. 2,431,326 or Taylor United States Patent No. 3,205,043.As final step in the preparation of the product, the carbon-containing porous silicon carbide is fired in a silicon-containing atmosphere at a temperature of the order of 2000"C to convert all of the contained carbon to silicon carbide and, if desired, to completely impregnate the body with free silicon.

A lacquer, such as L-1 8 Clear Lacquer sold by Raffi and Swanson Inc., can be used to coat the interior of the plaster of paris mold, and reagents otherthan hydrochloric can be used to decompose the plaster of paris mold.

It is often desirable that the material of the plaster of paris mold be relatively soft to prevent the mold from exerting stresses on the green casting during the heat treatment of the green casting to give it additional green strength. In order to obtain such a relatively soft mold, the present invention provides adding an aqueous solution of an organic polymer to the plaster of paris slip just before the plaster of paris mold is cast. Preferably this organic polymer is methylcellulose, sold under the trademark Methocel.

The introduction of methylcellulose causes formation of very fine bubbles in the plaster of paris mix.

The bubbles subsequently cause porosity in the final plaster of paris mold. Because these bubbles are extremely small and well distributed they form a smooth surfaced, but porous, soft plaster which can be easily decomposed. The methylcellulose serves a second function and that is the suspension of undissolved plaster of paris particles. If one were to use the same plaster of paris to water ratio, without methylcellulose, as can be used with methylcellulose, the plaster of paris would tend to settle a great deal. In the course of working with methylcellulose, it was found that the amount of methylcellulose could be changed considerably. When the high concentrations of methylcellulose are used, it can serve as a combustible component in the plaster of paris mix, and, when burned out, drastically weaken the plaster of paris mold.

The present invention is illustrated by the following example utilising methylcellulose.

EXAMPLE 200 grams of plaster of paris are added to 2 grams of water. This mixture is mixed for 3 minutes and then a pinch of already set plaster of paris which has been ground up is added to cause more rapid setting. Mixing is continued for 9 minutes. Thereafter a methylcellulose solution is added for an additional mixing time of 2 minutes. Thereafter the mix is poured to form a plaster of paris mold. The amount of methylcellulose may be in the range from 0.1 to 4.0%, by weight, this amount being added in the form of a methylcellulose solution whose concentration is in the range from 1 to 8% methylcellulose. The upper concentration of methylcellulose is reached when the mold shrinks excessively during drying. In general, to 15% of a 2% methylcellulose solution is quite satisfactory.

When the plaster of paris mold is to be decomposed by burning it is preferred that a somewhat higher concentration of the methylcellulose be utilized.

To burn out the polymer, the mold with the silicon carbide slip casting is heated from room temperature to about 650 to 700"C over a time schedule of 15"C per hour. After cooling to room temperature, the plaster mold is extremely soft and may be easily brushed from the piece. During this heating cycle, the silicon carbide green casting is slightly oxidized to form a glassy phase which substantially increases the strength of the green casting.

Although methylcellulose has been described above, numerous other water soluble organic polymers may be employed which can be burned out ot weaken the mold. Examples of such organic polymers are: carboxylvinyl polymers of the type sold by B. F. Goodrich underthe trademark Carbopol 934, xanthan gums as sold by Kelco Company under the trademark Keizon or an ammonium alginate as sold by Kelco Company under the trademark Superloid.

It should be noted, that although the foregoing description of the invention is presented within the context of forming thin walled, delicate, and com plex shapes, this is not to be construed as a limitation. The invention process is also applicable to forming large and small, thin and thick, complex and simple shapes.

Likewise, the examples set out above utilize silicon carbide, but the process can be used with silicon metal powder, for example, which maythen be nitrided as taught by E. R. W. May in United States Patent No.3,819,786 or by N. L. Parr et al in United States Patent No. 3,222,438. The methods described herein are also amenable to forming intricate shapes of other powdered refractory materials such as silicon nitride, aluminium oxide, composites of silicon carbide and silicon nitride, and the like when a curable resin is used to give green strength to the casting. Any refractory compound powder that oxidizes slightly to form a stable glass (e.g. B203, SiO2 or TiO2) to strengthen the green casting can be used when the green casting is strengthened by heating in air.

For the purpose of the present specification and the appended claims, the expressions "bimodal mixture" and "biomodal distribution" refer to a mixture of two populations of particle sizes. A population has a distribution defined by the normal distribution curve. When the two populations are mixed, there is a double peak size plotted against frequency.

All of the contents of the parent U.K. patent application No. 37167/78 should be referred to, and the whole of those contents are hereby imported into the description of the present divisional application.

Claims

1. A process of forming a refractory shape, comprising: (a) providing a castable slip; (b) casting said slip into a plaster of paris casting mold so asto provide a green casting, wherein said mold has been treated by incorporating into the plaster of paris mold mix 0.1 to 4.0% by weight of an organic polymer in the form of an aqueous solution thereof; (c) decomposing said mold to facilitate removal therefrom of said green casting, said decomposing being such that said mold containing a green casting is sufficiently heated in an oxygen containing atmosphere so as sufficiently to oxidise said polymer and thereby weaken said mold to permit ready removal of said green casting from said mold; and (d) removing said green casting from said decomposed mold.

2. A process as claimed in claim 1, wherein said slip comprises particles of silicon carbide.

3. A process as claimed in claim 2, wherein said castable slip comprises a bimodal mixture of silicon carbide particles, wherein a portion of said particles has an average size of the order of 3 microns, and another portion of said particles has an average size of less than substantially 74 microns.

4. A process as claimed in any one of claims 1 to 3, wherein said polymer is methylcellulose.

5. A process as claimed in claim 4, wherein said aqueous solution of methylcellulose has a concentration in the range 1 to 8% methylcellulose.

6. A process as claimed in any one of claims 1 to 5, wherein surface of said mold has been treated so that porosity of that surface is less than that of plaster of paris.

7. A process as claimed in claim 6, wherein internal surface of said plaster of paris mold has been lacquered to reduce permeability of that surface to water.

8. A process as claimed in any one of claims 1 to 7, wherein said mold containing said cast slip is placed under hydraulic pressure so as to consolidate said cast slip.

9. A process as claimed in claim 8, wherein said hydraulic pressure is of the order of 1.4 Kg/cm2.

10. A process as claimed in any one of claims 1 to 9, wherein before said decomposition step (c), said mold containing said green casting is heated to increase the green strength of said green casting.

11. A process as claimed in claim 10, wherein before said decomposition step (c), said mold containing said green casting is heated to in excess of substantially 600"C for sufficient time to oxidise silicon carbide so as to increase green strength of a green casting.

12. A process as claimed in claim 1, substantially as described in the Example.

13. A said refractory shape obtained by a process as claimed in any one of claims 1 to 12.

14. A refractory shape as claimed in claim 13, when in the form of an airfoil of a turbine blade.

15. A refractory shape as claimed in claim 13, when in the form of a turbine rotor.

16. A refractory shape as claimed in claim 13, when in the form of a turbine wheel.

17. A refractory shape as claimed in any one of claims 13to 16, when said removed green casting has been heated to recrystallisation temperature.

18. A refractory shape as claimed in claim 17, when said recrystallised casting has been provided with additional carbon by utilizing furfuryl alcohol.

19. A refractory shape as claimed in claim 17 or 18, when said recrystallised casting has been heated in a silicon-containing atmosphere at a temperature of the order of 2000 C.