Colloidal isopressing

Details Colloidal isopressing Colloidal isopressing

2002-07-19

2004-09-07

Fiorilla, Christopher A. (Department: 1731)

Plastic and nonmetallic article shaping or treating: processes

Removal of liquid component or carrier through porous mold...

C264S621000, C264S651000, C264S667000

Reexamination Certificate

active

06787080

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the formation of products from powder slurries.
2. Description of Related Art
Ceramic components, ranging from silicon nitride turbocharger rotors, used in high-performance automobiles, to translucent aluminum oxide tubes, used in high efficiency yellow sodium lamps, are formed by molding a powder into the desired engineering shape. The powder compact is then densified to its final form by a high temperature heat treatment. Because advanced ceramic powders such as silicon nitride and alumina lack the plastic properties of traditional clay-based ceramics, conventional shape forming is carried out by either the pressure consolidation of a dry powder or by the pressure forming of a powder containing a large fraction of a polymer that imparts plasticity. As received, ceramic powders contain agglomerates, inorganic and organic inclusions, and other inhomogeneities that degrade both the electrical and mechanical properties of sintered, dense bodies. As a result, both of these commercial shaping methods suffer from inclusions, present in the powder and retained during shape forming and densification. These inclusions concentrate any applied stress to severely degrade the component's strength. Agglomeration via spray drying prior to dry pressing incorporates more contaminants from the drying air, and crack-like void spaces are generated when the spray-dried agglomerates do not fully deform. In addition, the large quantity of polymer required for plastic forming (on the order of 40% by volume) must be removed very slowly to avoid defect formation.
It is known that the reliability of ceramic components can be improved by processing the powder as a colloidal suspension [F. F. Lange, “Powder processing science and technology for increased reliability,”
J. Am. Ceram. Soc.
72, 3 (1989)]. When the ceramic particles are dispersed in a liquid, the slurry can be passed through a filter to remove all inclusions greater than the size defined by the filter. Reducing the inclusion size will increase the average strength and component reliability. Techniques currently employed to form engineering shapes from a slurry can be categorized as either consolidation or direct shaping methods.
Consolidation methods start with a slurry containing a low volume fraction of powder that is concentrated by either evaporation or pressure filtration. Examples include tape casting (evaporation) [R. E. Mistler, D. J. Shanefield, R. B. Runk, “Tape casting of ceramics,” in Ceramic Processing Before Firing, G. Y Onoda, L L Hench Eds., (Wiley-Interscience, New York, 1978) pp. 411-448], slip casting (for the production of ceramic green parts, in which hardening is achieved, as is well known, by water removal with capillary pressure via a porous mold) [J. S. Reed, in
Principles of Ceramic Processing
(Wiley-Interscience, New York, ed. 2, 1995) pp. 493-503], and pressure filtration (external overpressure) [F. F. Lange, K. T. Miller, “Pressure filtration: Consolidation kinetics and mechanics,”
Am. Ceram. Soc. Bull.
66, 1498 (1987)]. Because the initial volume fraction of powder is <0.40, these dispersed slurries can first be passed through a filter to remove strength degrading inclusions. Consolidation methods also have the capability to produce bodies with the highest relative density. However, because the liquid removed during consolidation must flow through the body as it consolidates, these methods generally require long periods within the mold. Additionally, tape casting and slip casting, are typically limited to thin (or thin walled) bodies.
Direct shaping methods start with a slurry containing a high volume fraction of powder (>0.50) that can still be either poured or injected into a mold. Unlike the consolidation methods, the volume fraction of powder does not change during molding. Highly repulsive interparticle potentials are needed to formulate flowable slurries containing a high volume fraction of powder. Within the mold, the slurry must be converted to an elastic body so the component can retain its shape upon removal from the mold. Direct shaping methods include injection molding, gel casting, direct coagulation casting and vibra-forming [J. A. Mangels, “Injection molding ceramics,”
Ceram. Eng. Sci.
3, 529 (1982); A. C. Young, O. O. Omatete, M. A. Janney, P. A. Menchhofer, “Gelcasting of alumina,”
J. Am. Ceram. Soc.
74, 612 (1991); T. J. Graule, F. H. Baader, L. J. Gauckler, “Shaping of ceramic green compacts direct from suspensions by enzyme catalyzed reactions,”
cfi/Ber. DKG
71, 317 (1994); and G. V. Franks, B. V. Velamakanni, F. F. Lange, “VibraForming and in-situ flocculation of consolidated, coagulated alumina slurries,”
J. Am. Ceram. Soc.
78, 1324 (1995)]. In the case of injection molding and gel casting, the slurry's liquid phase solidifies, respectively, by freezing or polymerization. For direct coagulation casting and vibra-forming, the particle network within the slurry is solidified by changing the pH of the slurry to the isoelectric point (the pH where the net surface charge on the particle is zero) via a temperature induced chemical reaction. Because slurries used for all direct shaping methods must contain the highest volume fraction of powder possible, they are too viscous to remove strength degrading inclusions by filtration. Although injection molding only requires very short periods within the mold, very long periods are needed to remove the polymer without causing cracking, blistering, etc. The periods required to convert the molded slurry into an elastic body is too long (10 minutes to several hours) to utilize either gel casting, direct coagulation casting or vibra-forming as rapid forming methods.
After a component is shaped by any one of these methods, the liquid within the powder compact must be removed before densification at high temperature. Shrinkage typically occurs during evaporative drying because the powder can further consolidate, driven by capillary (Laplace) pressure. Surface tensile stresses associated with shrinkage may arise if the exterior portion of the body, where evaporation initiates, is constrained by the interior. The magnitude of the tensile stress, which can induce cracking, depends on a number of factors including the initial relative density achieved during shape forming and the rate of drying [G. W. Scherer, “Theory of drying,”
J. Am. Ceram. Soc.
73, 3 (1990)]. Bodies produced by direct shaping methods are more prone to cracking because they exhibit greater shrinkage relative to the higher density, consolidated bodies.
Pujari et al., have shown that the average strength and reliability of tensile specimens can be greatly improved by filtering inclusions from slurries prior to compact formation by pressure filtration [V. K. Pujari et al., “Reliable ceramics for advanced heat engines,”
Am. Ceram. Soc. Bull.
74, 86 (April 1995)]. Unfortunately, most of the colloidal forming methods described above have only found niche industrial applications. For example, tape casting is one viable technique for fabrication of multilayer electronic packages. In general, the long forming periods, large fractions of polymer that must be removed prior to densification, and/or the inability to remove inclusions prior to shaping are the limiting factors for economical, industrial practice.
SUMMARY OF THE INVENTION
The present invention provides a new method to form ceramic components from colloidal suspensions of ceramic powders, in which isostatic, pressure is applied to a colloidal suspension, and which we call colloidal isopressing. The method starts with a slurry that can be filtered to remove strength degrading inclusions. After an initial, low pressure consolidation, which can use pressure filtration to create a fluid-like consolidated body, the shape forming method requires only a short isopressure period within a flexible mold. Following shaping, the saturated body can be rapidly dried without shri

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