WO1988007901A1

WO1988007901A1 - Molding and precision forming using highly loaded systems

Info

Publication number: WO1988007901A1
Application number: PCT/US1988/001257
Authority: WO
Inventors: Bruce E. Novich; Mark A. Occhionero
Original assignee: Ceramic Systems Corporation
Priority date: 1987-04-09
Filing date: 1988-04-11
Publication date: 1988-10-20
Also published as: AU1728688A

Abstract

A new process of molding uses a slip of narrow size range particles with a volume fraction of solids just below the rheological transition point so that the slip is pourable. Removal of a slight amount of pore fluid causes the slip to solidify, and which can be dried to a green piece with virtually no drying shrinkage.

Description

MOLDING AND PRECISION FORMING USING HIGHLY LOADED SYSTEMS Field of the Invention This invention relates to a novel process for forming ceramic particles, and in particular, to processes involving highly loaded dispersions.

Backcrround Art There are many advantages to using narrow size range and/or submicron particles as materials for high performance ceramics. The near uniform packing characteristic of these particles gives green pieces high strength and the fired pieces have good sintered microstructure characteristics. However, there are many problems associated with the processing of these powders.

It is typical to make suspensions of these powders with solids concentrations ranging between 30-50 volume percent. These type of dispersions are not suitable for many molding and casting procedures, including tape casting, because the low solids content results in a substantial (and sometimes uncontrollable) shrinkage upon drying. Linear shrinkages of 10% and greater are not uncommon, and this may result in warpage and/or cracking, rendering the piece unusable.

Further, conventional slip casting techniques are not well suited to these types of dispersions as the low permeability of both the cast particle layers and of the porous casting mold results in a slow processing time. Also, the maximum green densities obtained for conventionally slip cast suspensions are on the order of 50-55 volume fraction solids. It is usually necessary to add an organic binder in these systems to give the part adequate green strength to survive subsequent handling operations. Binders, in most cases, must be removed in a controlled manner prior to firing to prevent part distortion and cracking. Disclosure of Invention

We have found a method to mold and cast very highly loaded slurries, including those having narrow size range particles, which solves the aforementioned problems. The pieces do not exhibit a great deal of shrinkage during drying and are therefore less prone to warpage and cracking. Surprisingly, the high degree of green strength negates the need for a binder, and the problems associated with binder burnout during the firing process are avoided. Further, the process used is rapid and efficient. Brief Description of the Drawings Fig. 1 depicts viscosity as a function of volume percent solids for a narrow size range (0.5-0.8 micron) alumina powder used in accordance with this invention, and for a commercial alumina powder that has, in comparison to the foregoing pcwder, a relatively wide size range.

Fig. 2 depicts particle size distributions for the powders of Fig. 1.

Description of the Specific Embodiments The invention takes advantage of a sharp rheological transition between a workable and pourable liquid-like behavior, and shear thickening solidlike behavior observed in well dispersed suspensions prepared from powders, which in a preferred embodiment have narrow particle size distributions. These dispersions can be prepared with virtually any powder, ceramic or even metallic, by a variety of different methods, including those taught in U.S. patent application serial number 028,891, filed March 23, 1987, which is incorporated by reference herein.

The sharp rheological transition occurs at high solids concentrations (approximately 60 volume percent) where particles begin to interact in these well dispersed particle systems. All other things being equal, the sharpness of this transition is related to the breadth of the particle size ddstribution, with broad or wide distributions showing smoother transitions. This is illustrated in Fig. 1, which displays the transitions observed in a narrow-size (0.5-0.8 micrometers) and a commercially available relatively wide size-range distribution alumina powder (A-16 Superground available from Aluminum Company of America, Pittsburgh, PA) as a plot of viscosity versus concentration of solids. (Fig. 2 shows the particle size distribution of each of these powders.) Referring to Fig. 1, the curve of the narrower size distribution powder shows a sharp transition at approximately 60 volume percent solids. (It will be appreciated tαat powders having a broader size distribution than that of A-16 Superground will exhibit a less distinct and more gradual transition.) Above the transition, these suspensions exhibit a shear thickening behavior and become unworkable regardless of the mean particle size.

The differences in behavior between these two powders is believed to be related to the particle packing arrangements which occur. Well-dispersed particle suspensions with wider particle size distributions achieve higher packing efficiency by packing smaller particles in the interstices formed by larger particles. As a consequence, the transition between liquid-like and solid-like behavior occurs at higher solids contents than suspensions prepared from powders with a narrower particle size distribution. In addition, the transition between behaviors in wide particle size distribution powders is more gradual since particles are more mobile and can more easily be rearranged.

"Narrow size range" particles in accordance with the present invention have a standard deviation less than approximately 50% of the mean particle size, and preferably not more than approximately 25% of the mean particle size. Commercially available powders which meet this criteria include, but are not limited to: AKP-30 alimana, AKP-20 alumina (both available from Sumitomo Chemical Company, Ltd., Tokyo, Japan), SNE-10 silicon nitride (from Ube Industries, Ltd., Japan), and HSY-3 zirconia (from Zirconia Sales of America, American Vericulite Corp., Atlanta, GA). In the case of a narrow size range distribution, the ultimate packing efficiency is generally lower. Furthermore, particle mobility is hindered relative to the wider size distribution powders; particle-to-particle interactions are greater at any given concentration preventing stxuctural rearrangement. This is illustrated in Fig. 1 by the greater viscosity of the narrow distribution powder in comparison with the wide size distribution powder at any given particle concentration.

In accordance with this aspect of the invention, this rheological transition can be advantageously used for ceramic powder forτaing operations. To mold with a high solids, with narrow size range particle suspensions, a pourable suspension is prepared at just below the

"transition concentration" where the suspension is fluid. This suspension is suitable for a variety of forming operations, including, but not limited to: slip casting, drain casting, tape casting, and injection molding. A preferred typical solids concentration chosen is in the range of approximately 59 to 61 volume percent. One method of preparing these highly loaded suspensions can be found in the above-referenced patent application serial number 856,803.

One method used to obtain a solid green part is achieved by changing the solids concentration from the shear thinning (effectively liquid) behavior by slight desiccation, or removal of pore fluid. Removal of pore fluid may be by any known means including, but not limited to: evaporation, freeze-drying, vacuum-drying, removal by pressure, or by applying heat and driving the liquid off, or by molding in a mold that can absorb the pore fluid. The amount of pore fluid which is driven off to effect a change in state is minimal, typically less than about 10% pore fluid volume. As the suspension enters the thickening regime, particle interactions become sufficient to prevent particle rearrangement, and thus shrinkage of the formed part ceases. Effectively, suspensions are prepared slightly under the solids concentration of the green part (i.e. , just below the green density) . The small amount of drying required for solidification results in little volume shrinkage of the suspension to achieve the solids concentration of the green part. Using this method of solidification avoids many complications of drying and shrinkage that occur in conventional forming suspensions having lower solids contents.

An added benefit of the high solids, narrow sized, well dispersed suspensions is that the high packing fractions obtained yields parts with high green strength, making binder additions unnecessary. Packing fractions in these systems typically range between 61 and 68 volume percent solids, depending on the initial suspension concentration.

Although the effects and benefits of narrow size range particles have been illustrated with sutmicron powders, it will be appreciated now that the invention is also applicable to, and a sharp rheological transition exists with respect to, larger particle sizes that have a narrow size range. In such instances, the transition occurs at somewhat higher volume fractions of solids.

While molding of a narrow size range powders is a preferred use of this invention, it should be recognized and appreciated that this invention is not limited to this type of slurry. The invention operates with powders which do not necessarily meet the "narrow size range" definition set out herein, such as A-16 Superground. The invention also is applicable to slurries containing more than one narrow size range population, or to those slurries containing a narrow size range matrix material with a dopant of whiskers. Here the key is that the slurry is pourable yet has a sufficiently high particle density so that removal of a small amount of pore fluid, i.e., approximately 10%, is effective to cause the slurry to solidify. It should be appreciated that the above discussion is not limited to ceramic or refractory type materials, but is equally applicable to other types of materials as well, including metal powders. Ceramic powders of particular interest usable in this invention include, but are not limited to: alumina, silicon carbide, silicon nitride and its alloys, zirconias (including partially stabilized), cordierite, forsterite, mullite, titania, whiskers (e.g., silicon carbide, tungsten carbide), and any combination thereof.

Example 1 An aqueous suspension of alumina particles having a size range of approximately 0.5-0.8 microns as shown in Fig. 2, containing 59 volume percent solids and 2 wt % dispersant (Narlex LD-45 obtained from the National Starch Co., Bridgewater, NJ) was prepared. The suspension was pourable, exihibiting a viscosity of approximately 200 cP at 100 sec^-1 shear rate.

The slurry was cast in an open Teflon ring mold having an inside diameter of approximately 2.75 inches (6.985 cm) and a thickness of approximately 0.25 inches (0.635 cm).

The piece was removed from the mold and fired, using the following schedule: room temperature to 350°C at 5°C/minute; held at 350ºC for one hour to burn out the dispersant and any residual water which may be present; then rapidly fired at 20°C/minute to 1500°C, where it was held for 4 hours.

Both the green piece and the fired piece were of excellent quality and showed very little shrinkage.

Example 2 The same general procedure of Example 1 was followed, except the slurry included AKP-20 alumina (available from Sumitomo Chemical Co., Ltd., Japan) a powder having a narrow size range of approximately 0.4-0.6 micrometers and polyvinyl alcohol at less than approximately 2 volume percent as a binder. The green pieces and fired pieces both were of excellent quality.

Example 3 The same general procedure of Example 1 was followed, except pieces were slip cast in a covered Plaster of Paris mold.

Example 4 The same general procedure of Example 1 was followed, except pieces were drain casted using a Plaster of Paris mold.

Claims

What is claimed is:

1. A process for forming an article comprising:

(a) preparing a pourable dispersion comprising particles, dispersant and at least one fluid, the dispersion having a high volume percent solids;

(b) forming the dispersion into a desired shape; and

(c) removing a small amount of fluid so that the dispersion solidifies.

2. A process according to claim 1, wherein the particles are ceramic.

3. A process according to claim 1, wherein the particles are metallic.

4. A process according to claim 2, wherein the volume fraction of solids is at least about 50 vol.%.

5. A process according to claim 2, wherein step (c) includes removing not more than 10% of the total amount of fluid.

6. A process according to claim 5, wherein the particles include at least one species of ceramic particle and at least one species of whisker.

7. A process according to claim 5, wherein step (a) includes preparing a dispersion containing more than one population of narrow size range particles.

8. A process according to claim 5, wherein step (b) includes introducing the dispersion into a slip mold.

9. A process according to claim 5, wherein step (b) includes introducing the dispersion into an injection mold.

10. A process according to claim 5, wherein step (b) includes introducing the dispersion into a drain casting mold.

11. A process according to claim 5, wherein step (c) includes removing fluid by desiccation.

12. A process according to claim 5, wherein step (c) includes removing fluid by applying heat.

13. A process according to claim 5, wherein step (c) includes removing fluid by freeze-drying.

14. A process according to claim 5, wherein step (c) includes removing fluid by evaporation.

15. A process according to claim 5, further including the step of drying the solidified dispersion.

16. A process according to claim 5, further including firing the green piece.

17. A process for forming an article comprising:

(a) preparing a pourable dispersion comprising narrow size range particles, at least one fluid, and dispersant, the dispersion having a high volume percent solids; (b) forming the dispersion into a desired share; and

(c) removing a small amount of fluid so that the dispersion solidifies.

18. A process according to claim 17, wherein step (a) includes preparing a pourable dispersion of narrow size range particles whose average size is less than approximately 1 micrometer.

19. A process according to claim 17, wherein step (a) includes preparing a pourable suspension having at least approximately 50 volume percent solids.

20. A process according to claim 17, wherein step (a) includes preparing a pourable suspension having at least approximately 55 volume percent solids.

21. A process according to claim 19, wherein step (b) includesintroducing the dispersion into a slip mold.

22. A process according to claim 19, wherein step (b) includes introducing the dispersion into an injection mold.

23. A process according to claim 19, wherein step (b) includes introducing the dispersion into a drain casting mold.

24. A process acxx>rding to claim 19, wherein step (c) includes removing fluid by desiccation.

25. A process according to claim 19, wherein step (c) includes removing fluid by applying heat.

2fi. A process according to claim 19, wherein step (c) includes removing fluid by freeze-drying.

27. A process according to claim 19, wherein step (c) includes removing fluid by evaporation.

28. A process according to claim 19, further including the step of drying the solidified dispersion.

29. A process according to claim 19, further including firing of the green piece.

30. A piece produced by the process of claim 1.

31. A ceramic piece produced by the process of claim 2.

32. A piece produced by the process of claim 3.