WO1989004735A1 - Process of preparing sintered shapes containing reinforcement - Google Patents

Process of preparing sintered shapes containing reinforcement Download PDF

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Publication number
WO1989004735A1
WO1989004735A1 PCT/US1988/004691 US8804691W WO8904735A1 WO 1989004735 A1 WO1989004735 A1 WO 1989004735A1 US 8804691 W US8804691 W US 8804691W WO 8904735 A1 WO8904735 A1 WO 8904735A1
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process
defined
article
ceramic
slurry
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PCT/US1988/004691
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French (fr)
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Wen-Cheng Wei
Bruce E. Novich
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Ceramics Process Systems Corporation
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Priority to US12564387A priority Critical
Priority to US125,643 priority
Priority to US18009288A priority
Priority to US180,092 priority
Priority to US270,248 priority
Priority to US27024888A priority
Application filed by Ceramics Process Systems Corporation filed Critical Ceramics Process Systems Corporation
Publication of WO1989004735A1 publication Critical patent/WO1989004735A1/en

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    • BPERFORMING OPERATIONS; TRANSPORTING
    • B22CASTING; POWDER METALLURGY
    • B22FWORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER
    • B22F3/00Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/22Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces for producing castings from a slip
    • B22F3/225Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces for producing castings from a slip by injection molding
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B22CASTING; POWDER METALLURGY
    • B22FWORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER
    • B22F3/00Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/22Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces for producing castings from a slip
    • B22F3/222Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces for producing castings from a slip by freeze-casting or in a supercritical fluid
    • CCHEMISTRY; METALLURGY
    • C04CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04BLIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B35/00Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/622Forming processes; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/626Preparing or treating the powders individually or as batches ; preparing or treating macroscopic reinforcing agents for ceramic products, e.g. fibres; mechanical aspects section B
    • C04B35/63Preparing or treating the powders individually or as batches ; preparing or treating macroscopic reinforcing agents for ceramic products, e.g. fibres; mechanical aspects section B using additives specially adapted for forming the products, e.g.. binder binders
    • C04B35/632Organic additives
    • CCHEMISTRY; METALLURGY
    • C04CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04BLIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B35/00Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/71Ceramic products containing macroscopic reinforcing agents
    • C04B35/78Ceramic products containing macroscopic reinforcing agents containing non-metallic materials
    • C04B35/80Fibres, filaments, whiskers, platelets, or the like
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C47/00Making alloys containing metallic or non-metallic fibres or filaments
    • C22C47/14Making alloys containing metallic or non-metallic fibres or filaments by powder metallurgy, i.e. by processing mixtures of metal powder and fibres or filaments
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B22CASTING; POWDER METALLURGY
    • B22FWORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER
    • B22F2998/00Supplementary information concerning processes or compositions relating to powder metallurgy

Abstract

Dense, flaw-free, complex shaped, three-dimensional, reinforced inorganic articles are formed by a process including the steps of providing a low viscosity suspension of solids, generally either ceramic or metallic, present at least at about 35 vol.%, reinforcement material having a high aspect ratio (e.g., fibers, whiskers, platelets), and dispersant, and shaping the suspension under low shear forces to produce a green article that can be pressureless sintered to net shape with minimal distortion. Shaping can be accomplished by casting or low pressure injection molding (less than about 100 psi) or by a freeze-forming and freeze-drying process. The low viscosity of the molding slurry and forming under low shear stress avoids reinforcement particle alignment. Accordingly, sintered articles exhibit a high tolerance to the originally molded shape and very little distortion during densification.

Description

"Process Of Preparing Sintered Shapes Containing Reinforcement"

BACKGROUND OF THE INVENTION

1. The field of the invention.

The present invention pertains to forming complex shaped sintered articles containing a ceramic or metallic matrix and high aspect ratio reinforcement particles (platelets, whiskers, and/or fibers) randomly dispersed therein. The forming operation can be performed under low pressure and the green article can be pressure-sintered, pressureless-sintered, or sinter-HIP'ed without distortion.

2. The state of the art.

Reinforced ceramic composites are of great importance as a class of engineered materials. Unreinforced ceramics typically display brittle mechanical behavior with catastrophic failure (low fracture toughness) whereas a brittle ceramic matrix reinforced with fibers, whiskers, or platelets results in an engineered material with a higher fracture toughness and a tendency toward a more predictable failure. (Whiskers are single crystal, high aspect ratio particles which typically have a tensile strength of about 7 GPa and an elastic modulus of about 700 GPa; fibers can be characterized as polycrystalline; both fibers and whiskers being high aspect ratio particles. Platelets are polycrystalline or single crystal platy high aspect ratio particles.) This increased fracture toughness generally results in increased chemical, wear, and erosion resistance, and as well as an improved structural integrity at high temperatures. Typical applications for such reinforced ceramics and metals include engine parts, cutting tools, bearings, and valves. Sintered silicon carbide whisker reinforced ceramic oxide and non-oxide composites are a relatively new class of engineering materials which have particular applications at high temperatures. The reinforcing whiskers are believed to be crack blunters and to absorb fracture energy, which imparts a higher fracture toughness to the brittle ceramic matrix. An analysis of toughening mechanisms, including the possible effect of crack deflection and whisker bridging/pullout, is discussed by J. Homeny et al., in "Processing and Mechanical Properties of SiC-Whisker-Al203- Matrix Composites," Amer. Cer. Soc. Bull.. vol. 66, no. 2, pp. 333-338 (1987) . Exemplary of these applications, Tiegs and Becher (J. Amer. Cer. Soc.. vol. 2, p. 339 (1986)) have shown that the presence of even low Sic whisker contents (that is, < 10% by volume) results in improvements to various properties, including thermal shock resistance, toughness, and high temperature strength. Thus, proper engineering of these materials should allow the structural ceramic component designer to achieve key property improvements.

Fabrication of ceramic-whisker composites has not been straightforward; elaborate schemes have been devised to attain full density of the composite matrix upon sintering. Whiskers, which are non-sinterable and inert, act as proppants which retard the densification of the matrix particles. Traditionally, hot pressing or sinter-HIP with sintering aids have been used to densify these composites. Hot Pressing (HP) involves uniaxially pressing a green ceramic compact under high temperature and pressure; this process is limited to simple shapes that can be uniaxially formed, such as disks and billets. The green ceramic compacts can have open porosity prior to sintering, and the compacts can have up to 50% total porosity. The Hot

Isostatic Pressing (HIP) process is where a green compact is formed, of either a simple or a complex shape, and the compact is subsequently sintered in an isostatic stressfield at high temperature and pressure (e.g., >20,000 psi). In this case, all of the porosity present in the green compact must be sealed off at the surface, i.e., all of the porosity must be "closed"; this can be accomplished either by first sintering the piece to closed porosity, typically < 10 vol.% porosity, or by encapsulating the compact in glass prior to HIP'ing. The technique generally preferred in the art is the sinter-HIP densification scheme, in which one must have the ability to pressureless sinter the ceramic to a density of approximately > 90-92% of theoretical density to effectively close the porosity. Further, because the second step (pressure sintering, HP, or HIP) can cause ceramic degradation at the expense of densification, and HP or HIP is a costly batch step requiring expensive process equipment and very careful sample preparation, the achievement of pressureless sintered densities approaching theoretical density for high whisker loadings would be very beneficial. Still further, pressure sintering typically results in an overfired ceramic matrix, often leading to degradation and formation of a glassy phase at the expense of full densification; accordingly, providing pressureless sintered samples of a high density would help to reduce the degradation caused in the pressure sintering step. Also, a typical hot pressed SiC whisker/alumina composite at full density will exhibit alumina grains in excess of 50 microns, an order of magnitude larger than the original alumina particles, thereby resulting in strength limiting defects in the stressed body. As a reinforcing component in the sintered ceramic matrix, Wei (U.S. Pat. No. 4,543,345) has shown that pressure sintered composites of silicon carbide whiskers and a fine ceramic powder (such as alumina, mullite, or boron carbide) provide a significant fracture toughening component to the brittle ceramic microstructure. Wei achieves the composites by sintering at 1600°C to 1950°C at pressures of 28 to 70 MPa.

Tiegs (U.S. Pat. No. 4,652,413) augmented Wei's composition by the addition of 0.5 to 5 wt.% of a sintering aid such as yttria to a body containing 5 to 10 vol.% silicon carbide whiskers, and the balance being 0.1 to 1.0 micron alumina. Tiegs begins with a granulated mixture of alumina, whiskers, and sintering aid, which is cold pressed into a pellet having a density of approximately 55% theoretical. The pellet is then two-step sintered to full density by first pressureless sintering at 1800°C to 95% of theoretical density and then pressure sintering at 20,000 psi to over 98% of theoretical density. Tiegs also states that pellets with 20 vol.% whiskers could not be sintered to over 75% theoretical density, although increased yttria content may be beneficial in achieving such a result. Moreover, Tiegs also states that pellets with whisker concentrations exceeding 10 vol.% may not be sinterable without pressure assistance.

Becher and Tiegs (in U.S. Pat. No. 4,657,877) also built upon Wei's work by the addition of transformation toughened tetragonal zirconia to the SiC whisker-mullite and SiC whisker-alumina matrix. The further addition of Zrθ2 to the whisker composite provided further increased toughness after sintering at 7 to 70 MPa in an inert atmosphere.

Thus, the art has continuously resorted to fabrication methods for reinforced composites which require a pressure sintering step to achieve full density and which are practically applicable only to simple shapes. The art has also suggested, in certain aspects, that pressure sintering and/or an increased amount of sintering aid is necessary for compositions having greater than a threshold volume fraction of whiskers. However, the use of a pressure sintering step inherently limits the complexity and geometry of the . composite articles fabricated.

It is also important to note that SiC whiskers commercially available vary dramatically in their properties. Tiegs and Becher (ORNL/TM-9947, 1985-86) reviewed SiC whisker reinforced ceramic composites, and thereby discovered significant differences among whiskers. Among various whiskers (i.e., those available from ARCO, Tokai Carbon, Tateho, Versar, and Los Alamos National

Laboratory) incorporated at 20 vol.% into an alumina matrix, sintered densities ranged from 3.72 to 3.83 g/cm3, strengths varied from 340 to 650 MPa, fracture toughnesses varied from 4.2 to 9.1 MPa-_/m, and weight losses during hot pressing varied from 0.8% to 5.17%. In some instances (with Tateho and Tokai Carbon whiskers) , while the whiskers appeared to be intact, SEM examination of a fracture surface revealed no SiC whiskers, believed to be due to adhesive interaction between the matrix and whisker phases, and thus no benefits of pullout or crack deflection. Tiegs, Becher, and Harris (ORNL/TM-9947, 1984-85) have also shown that for 20 vol.% SiC reinforced alumina including Tateho and ARCO whiskers, the respective fracture toughnesses were 5.1 and 7.8, and the respective flexural strengths were 535 and 700. The 1984-85 report also showed the good thermal shock resistance of 20 vol.% SiC-alumina composites up to about 900 °C. Thus, SiC whiskers can provide desired properties, but the properties vary widely among whiskers, which thereby limits the number of variables for which a certain ceramic can be engineered. See also T. Tiegs, "Business Outlook for Advanced Ceramics and Composites," Whisker-Reinforced Ceramic Composites: Present Status and Potential Trends. presentation at Oak Ridge National Laboratory on March 12, " 1987.

The fabrication of sintered and complex shaped reinforced ceramic and metallic parts has presented two principal challenges: (1) pressureless sintering of the whisker composite to full density or to closed porosity (i.e., suitable for subsequent HIP'ing) and (2) processing through sintering with uniform and predictable shrinkage. Pressure-assisted sintering of these composites can achieve essentially full density, but such a process is costly and can result in strength limiting defects, such as large grains and long grain boundaries; accordingly, pressureless sintering to full density would be a more desireable production avenue. Nevertheless, complex shape forming to net or near net shape by any method which includes a molding or casting forming step (i.e., fluid flow of the slip), is difficult to achieve due to alignment of high aspect ratio particles during the forming process; parts formed into complex shapes distort and shrink non-uniformly during the sintering process. In other words, despite the improved ability to densify whisker reinforced composites, particle alignment during forming has still prevented net shape fabrication. The present state of the art has observed that silicon nitride composites reinforced with silicon carbide whiskers, formed by injection molding and then densified by direct HIP'ing, show anisotropic shrinkage due to alignment of the whiskers; the investigators conclude that "It seems necessary to take this fact into account when the components of the composites are formed by injection molding." T. Kandori et al., "Directly HIP'd SiC-Whisker Reinforced Si3N4," ASM's Int. Conf. Whisker- and Fiber-Touσhened

Ceramics, Oak Ridge, TN (7-9 June 1988), page 29. In fact, the art appears so resigned to the presence of orientation

I or alignment during forming that methods have been devised to measure the degree of orientation and to correlate a corresponding relationship with composite toughness. See N.D. Corbin et al., "The Influence of Reinforcement Orientation on Si3N4 Matrix Composite Toughness," Ibid at page 31; see also T. Kandori et al., "SiC whisker reinforced Si3N4 composites," J. Mater. Sci. L.. vol. 6, p. 1356-1358 (1987), and S.T. Buijan et al. , "Dispersoid-Toughened

Silicon Nitride Composites," Final Report under contract DE- AC05-840R21400, ORNL/Sub/85-22011/1. The complex shape fooling of high aspect ratio- reinforced composites has been carried out fashion by the fiber-reinforced plastics industry, although sintering and densification of the composite is not a process step or consideration as it is in the ceramics industry. However, whisker orientation has been studied extensively because the whisker reinforced plastic composite properties are likewise affected by the orientation of the reinforcing phase in the plastic matrix. This work is accordingly of interest to the ceramics and metal reinforcement community as well, because the principal methods presently envisioned for fabricating molded components are plastic forming processes adapted for the ceramics industry, such as thermoplastic injection molding. . In the typical forming process, whether thermoplastic injection molding or slip casting, whiskers align with the flow field during the forming process, resulting in gross part distortion during densification; this distortion occurs as the sinterable particulate matrix shrinks around the rigid whiskers. If the whiskers are oriented in a particular direction, firing shrinkage will be small parallel to the orientation plane, while shrinkage will be extreme in the plane perpendicular to the orientation plane. For example, if a whisker-particulate composite is extruded uniaxially to form a tube and is then sintered, firing shrinkage in the axial direction will be a small percentage of the firing shrinkage in the radial direction. This is , due to the alignment of the whiskers parallel to the extrusion direction, which inhibits matrix densification between the whiskers, i.e. perpendicular to the whisker plane. Typically, shrinkage will be 4% axially and 20% radially, leaving a 5:1 shrinkage ratio with which it is essentially impossible to fabricate a part to desired dimensions with reasonable tolerances (e.g., 0.5%). Typically, three-dimensional, complex shaped ceramic parts are manufactured by a process analogous to thermoplastic injection molding, in which a ceramic or metallic powder is compounded with a mixture of thermoplastic resins at high torque and at high temperature. The resulting mixture has a dough-like consistency, which is why the compounding process is generally referred to a "kneading." Homogeneous particle dispersion is difficult and tedious to obtain in such a system, and traditionally has been a source of microstructural defects, such as holes and non-uniform particle packing.

The thermoplastic mixture is then fed into a high pressure injection molding machine, usually in the form of granules or pellets. The molding machine and the molds used are typically large and expensive because injection pressures can range from approximately 2500 psi to 30,000 psi, thus requiring mold clamping forces in the "tens of tons" range. As the pellets are fed into the inj ction molding machine, they are remelted and injected through a sprue into a mold cavity. The high viscosity and dough-like consistency of the molding composition can result in weld or knit lines, mold gate, sprue, and parting line textures, all of which can create property limiting defects. The high viscosity of the mix is primarily responsible for the particle alignment.

After the part is molded, the thermoplastic/ceramic composition is subjected to binder removal, which is a long (typically requiring days) , expensive, and deleterious process, particularly for a fine particle matrix typical of a high performance ceramic body. Initially, binder removal can result in bubble formation, delamination, and blistering of the part (as typically 40% by volume of the composite is a plastic material which is removed from the finely porous body) . Binder removal is commonly practiced by heating the polymer/ceramic composite beyond the polymer softening point; accordingly, dimensional tolerance is difficult to control because of fluids escaping from the softening composite matrix, and softening often coincides with the development of internal pressures due to gasification of the polymer (by vaporization or pyrolysis reactions) .

After binder removal, the porous particulate body is sintered at high temperatures so that the particles densify, resulting in a strong ceramic that is volumetrically smaller than the presintered (green) particulate part. Final machining is generally required due to poor dimensional tolerances, parting lines, and gate remnants remaining on the fired part. Moreover, the machining process commonly imparts defects to the fired part, thereby creating property limiting, especially strength limiting defects. An alternate approach to thermoplastic resin molding has been to substitute low temperature melting, low viscosity waxes in place of the thermoplastic resins. While this substitution allows for low pressure injection molding, the problems associated with dispersion, binder removal, machining, green strength, and dimensional tolerance have kept this particular system from wide commercial acceptance.

Historically, investigators have recognized the limitations that the binder has placed on the processing of complex shaped, three-dimensional parts. The art later began to understand and appreciate that the binder, which had allowed the ceramic and metal particles to be formed into a shape and later handled, was also the cause of many economic and performance problems. Rivers, U.S. Pat. No. 4,113,480, developed an aqueous-based injection molding process exclusively for metal powders using 1.5 to 3.5 wt.% (metal powder basis) of high viscosity methylcellulose additive to provide green strength. The resulting mixture of metal powder, water, and methylcellulose has a "plastic mass" consistency and could be injection molded at 8,000 psi. The molded mass is then thermally dried and the green part is conventionally sintered. Although binder burnout was eliminated by this particular process, defects still remain, as well as the costs associated with dispersion and molding of a high viscosity mix and the implementation of a necessary but difficult thermal drying step.

The use of a molding vehicle which could be frozen has been investigated as an alternate method for casting or molding without the use of thermoplastic carriers. Freeze forming has been described by Nesbit, U.S. Pat. No.

2,765,512, who describes casting a ceramic shape from a thick slip containing a hydrogen bonding medium (such as water) , a cryoprotectant, and ceramic particles which are then frozen into a shape while in the mold. The resulting frozen part was de olded, dried at room temperature and pressure, and subsequently fired. Downing et al., U.S. Pat. No. 3,885,005, has cast coarse grained refractory shapes from a slip containing 70% coarser than #200 mesh ceramic particles, water, and a silica sol binder. The resulting cast shape was subsequently frozen, causing the silica to gel and cementing the refractory particles together. The residual water was frozen and the part was demolded and heated to 200°F to thaw and dry the part. Tomilov, G.M. , and T.V. Smirnova, "Molding Quartz-Ceramic Articles Using an Aqueous-Slip Freezing Method," Glass & Ceramics, no. 10, pp. 655-6 (1977) also describe freeze molding a ceramic part that is later dried by the application of heat.

Sublimative drying by freeze drying (lyophilization) has been shown to be less destructive to the particle fabric in the green part during drying than thermal drying. See A. Kwiatkowski et al. , "Preparation of Corundum and Steatite Ceramics by the Freeze Drying Method," Ceramurcria

Internationalr vol. 6, no. 2, pp. 79-82 (1980). Dennery et . al., U.S. Pat. No. 3,567,520, in making metal parts from powdered metals, formed an aqueous-based paste sheet into a part, the part was frozen at -60°F, and then freeze dried to overcome thermal drying stresses which could be destructiv _e to the part. Maxwell et al., U.S. Pat. No. 2,893,102, cast and molded thicker parts from an aqueous ceramic slip in which the slip and mold were frozen in a C02 bath followed by freeze drying and sintering. As a slight departure from the art thus described. Weaver et al., U.S. Pat. No.

4,341,725, describes the use of a cryoprotectant as an additive in an aqueous suspension to inhibit ice crystal growth, which, after drying, can cause severe strength limiting defects. Weaver et al. claim that the previous prior art would result in "low performance" articles riddled with scars resulting from ice crystal formation. By using hydrogen bonding additives in a hydrogen bonding medium, Weaver et al. claimed to limit the size of ice crystals formed to those on the order of 20-50 microns. Takahashi, in European Patent Applications Nos. 160,855 and 161,494, describes a method for freeze-pressure molding inorganic powders. That method includes providing a flocculated feedstock, shaping the feedstock under high pressure (at least about 2800 psi (200 kgf/cm2) ) , consolidating the shaped feedstock (including removing a portion of the fluid medium), and freezing under pressure.to form a frozen shape; the resulting shape is dried, such as by freeze drying, and then conventionally sintered. Besides the necessity for high pressure forming and consolidation, the Takahashi process has another disadvantage of being limited to particles having a size not greater than about 1 (one) micron. In general, a perusal of the Takahashi patent applications shows that that process is designed to overcome or avoid volume expansion on freezing by mechanical means. Still further, Takahashi achieves what is termed a "high density" article, although for alumina the sintered density is only about 85% of theoretical.

SUMMARY OF THE INVENTION Accordingly, it would be desireable to provide a reinforced ceramic article having a high density and good physical properties, having a complex shape, and formed by a process having only one sintering step. More specifically, it would be beneficial to provide an article having 10 vol.% whiskers, a density of at least 99% of theoretical, in a complex shape (such as a turbocharger rotor) , by pressureless sintering, to a tolerance of < 0.001 inch/inch for an alumina body. Still further, it would be advantageous to provide such an article which has the reinforcing particles randomly oriented, which would facilitate near net shape fabrication.

The present invention pertains to forming complex shaped articles containing a ceramic or metallic matrix having, as reinforcement, high aspect ratio particles (platelets, whiskers, and fibers) , by a process including providing a low viscosity aqueous or non-aqueous slurry including both matrix and reinforcement solids, freeze- forming the slurry into a desired shape, drying the frozen shape by non-destructive sublimation to provide a green article, and pressureless sintering the green article without deformation. The forming operation can be performed under low pressure, and an alumina sintered article can be fabricated having a density of >99% T.D. for 10 vol.% SiC whiskers and >93% T.D. for 20 vol.% whiskers. We have invented a process for producing complex shaped, three-dimensional fiber, whisker, and platelet reinforced ceramic and/or metallic matrix composite parts having nearly ideal and tailored microstructures, which process uniformly and with reproducible yields. This process by which uniform ceramic and metallic microstructures can be produced preferably includes the steps of mixing solids, dispersant, and fluid vehicle to provide a nearly ideally-dispersed suspension, filtering (preferably after deairing) , low pressure injection molding or casting, freezing without the formation of textures or property limiting defects, and gentle but rapid drying essentially by sublimation to form a green ceramic; an alternate route includes cooling the mold to below the freezing temperature of the fluid vehicle and then injecting the slurry into the chilled mold to simultaneously shape and freeze the slurry. The low viscosity, highly dispersed system can incorporate ultrahigh surface area particles (e.g., greater than 100 ι~~/q) , which leads to rapid, lower temperature sintering due to the increased densification driving force, and results in a uniform green article.

Further, the present process eliminates the conventional binder removal step and, because the green article can be formed to a high dimensional tolerance, reduces or eliminates the machining steps; both of these steps are traditional sources for defects which lead to non-uniformity of the final part. Still further, larger cross-sections can be made using the present invention while also attaining a high surface detail replication and resolution. Additionally, the present net shape forming process can avoid the need to machine a whisker-containing composite, thereby avoiding a health risk as mineral whiskers are a known asbestosis hazard. The invention is described below in more detail with respect to various specific embodiments.

Other features of the present invention, especially relating to the use of a non-aqueous vehicle, include facile mold release, less volume expansion upon freezing, the capability of a broader range of molding and freezing temperatures (both higher and lower) , faster drying times, and the ability to process materials incompatible with water. Although not all non-aqueous vehicles will exhibit all of these features, even singly these features are important and beneficial considerations for providing an economically feasible processing system.

The present invention also provides a composite ceramic or metallic article having a uniform, homogeneous composition including randomly oriented high aspect ratio reinforcment particles, a smooth surface finish, a dense or uniformly porous defect-free microstructure, a texture-free surface, and a high dimensional tolerance. In essence, this invention provides the ability to control defects on a particle size scale, in contrast to the 20-50 micron defects with which the art such as Weaver has typically been concerned. Whiskers remain randomly oriented during the injection process due to the low shear transfer from the mold walls to the particles due to the low viscosity suspension rheology. In summary, this invention provides a forming process that overcomes reinforcement particle alignment due to either or both of shear transfer and viscosity. Highly loaded slurries can be molded by a variety of techniques, including that provided by the present invention which is also suitable for low volume slurries.

BRIEF DESCRIPTION OF THE FIGURES Fig. 1 is a photograph showing two alumina hollow hub rotors, the lighter colored one without whisker reinforcement and the darker one including whisker reinforcement. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS The present invention represents an improved departure from the invention described in U.S. patent application serial no. 07/125,643, incorporated herein by reference. That co-pending application generally describes making a ceramic body by: forming a slurry including solid particles, a dispersant, at least one cryoprotectant, and the remainder aqueous suspension vehicle; shaping the slurry in a mold and freezing the mold to form a frozen shape; freeze drying the frozen shape before thawing; and sintering the dried shape. The reader is referred to that application for more details regarding aqueous systems. The departure of the present invention is to the extent of the addition of high aspect ratio particles to the formed slurry. As a more distinct departure from that co-pending application, the invention of the U.S. patent application serial no. 07/180,092 (also incorporated herein by reference) is particularly directed to forming a ceramic using a non-aqueous system; as stated above, there are numerous features and advantages relating to non-aqueous systems. For example, aluminum nitride cannot be processed in an aqueous system, or densified in an air atmosphere, due to its susceptibility to oxidation (e.g., reacting with water) . Similarly, certain metals, such as those which are iron-ba'sed, are susceptible to oxidation reactions, and therefore can be processed more purely in a non-aqueous system; that is, only if the reaction with water is sufficiently slow and only if processing is carried out sufficiently quickly can the amount of reaction product (e.g., rust when iron or steel is processed in water) resulting from processing in an aqueous system can be maintained at an insignificant level.

Although this invention is illustrated below with aluminum oxide and non-oxide aluminum nitride, this invention is applicable to other non-oxide, as well as oxide, ceramics. Such ceramic compositions include, but are not limited to, aluminum nitride, aluminum oxide, aluminum silicates, barium carbides, barium carbonates, barium titanates, boron carbonates, boron nitrates, beryllium oxides, calcium alumino silicates, lanthanum borates, lead titanates, lead zirconates, silicon carbides, silicon nitride, tungsten carbides, titanates, and mixtures thereof. Also as stated, the present invention is applicable to powdered metals, such as, but not limited to, aluminum, nickel, iron, titanium, copper, tungsten, beryllium, and mixtures and alloys thereof, such as steels (e.g., stainless, low alloy, copper, silicon, and the like) , brasses, bronze, and so forth. For the sake of simplicity, the term "solids" will be used to illustrate the invention, and it is to be understood that that term connotes both ceramic and metallic sinterable matrix solids as well as mixtures thereof; thus, the present invention also contemplates the production of composite metal-ceramic structures and the subsequent sintering thereof.

I

In general, the starting powders can have an average particle size range of from about 0.1 micron to about 150 microns (about 100 mesh) . The surface area of the particles can be in the range of from about 1 to about 100 m2/g (B.E.T.), and a surface area of less than about 25 m2/g s generally preferred. However, it is important to note that surface area is a principle driving force for densification, and that for rather spherical, essentially non-porous particles the surface area (because it is then directly proportional to the volume fraction of solids) is the •principle effect on the slurry viscosity. Moreover, smaller particles (e.g., "fines") also contribute to increasing slurry viscosity. By the methods taught in the U.S. patent applications serial numbers 07/036,377, 07/045,684, and 06/856,803, mentioned below and incorporated by reference herein, a low viscosity slurry of high surface area particles can be provided even at high solids loading (e.g., > 55 vol.%) .

Various reinforcement materials are suitable to the extent they are compatible with the matrix solids and the slurry components, both in suspension and during densification. Exemplary whiskers include those composed of silicon nitride, silicon oxynitride, silicon carbide, boron carbide, titanium diboride, aluminum oxide, magnesium oxide, and silica. Exemplary fibers include those composed of silicon carbide, tungsten carbide, carbon, silicon nitride, silicon oxynitride, zirconia, alumina-silica, and silica. Exemplary platelets include those composed of silicon carbide, boron carbide, boron nitride, titanium diboride, and carbon. Of course, mixtures of various components within and among each of these groups may be fabricated.

The starting powder is then processed into the form of a pourable slurry having at least about 35 vol.% solids. Such slurries and methods for producing the same are described in co-pending applications serial numbers

06/856,803, filed 25 April 1986, 07/036,377, filed 09 April 1987, 07/045,684, filed 01 May 1987, and 07/242,726, filed 09 Spetember 1988, all o 'which are incorporated by reference herein (hereinafter "the liquefaction applications") . In general, such slurries are formed by dispersing the starting powder under high energy in a liquid vehicle to provide a well-dispersed suspension; the powders are "well-dispersed" when they are present in the slurry as primary particles as opposed to particle agglomerates and floes. Mixing under high energy can be accomplished by the use of a dispersing device, preferably a high energy vibratory mill such that available from Sweco, Inc. , Los Angeles, CA, or a Red Devil brand paint shaker, or a GYROMIXER brand mixer available from Miller Paint Co. , Toledo, OH can be used. As described more fully in the

07/036,377 application, all solids mixing is preferably in a staged order of addition, where first a slurry of the matrix material is prepared and then the reinforcement dopant is added by stages in aliquots to make the final slurry. In a preferred embodiment, the ceramic matrix material is composed of an ultrafine ceramic particulate matrix, which we have found promotes the pressureless sintering process for reinforced ceramic and metal composites. The resulting microstructures are dense, uniform, and fine grained at various reinforcement loadings. As described in the co-pending U.S. patent application serial number 269,671 , filed 09 November 1988 [attorney docket 124], incorporated herein by reference, the use of a sintering aid is preferable for pressureless sintering. As disclosed therein, the highest densities are achieved for an ultrafine matrix material (i.e., average particle size less than about one-half micron) and about 2-10 vol.% of a sintering aid

(e.g., yttria for alumina). In another embodiment, one may desire to utilize the method taught by Tiegs in U.S. Pat. No. 4,652,413, discussed above, in which case the article can be made with, the 0.1 to 1.0 micron powder described by Tiegs but formed according to the present invention. In general and as stated above, the matrix material average particle size is preferebly between about 0.1 and 5.0 microns.

The liquid vehicle portion of the suspension can be aqueous or non-aqueous, the design criteria being dependent upon the susceptibility of the matrix material and reinforcement material to react with the vehicle, and the ability of the dispersant (discussed below) to function with each. The "vehicle" can be composed of a polymerizable binder, as described in co-pending U.S. patent application serial numbers 07/054,628, filed 27 May 1987, and 07,249,210, filed 23 September 1988, both incorporated herein by reference, an already polymerized binder (e.g., dissolved in a solvent) , or any combination thereof. With reference to the preferred molding method described below, the "liquid" vehicle can have any freezing point, preferably less than about ambient room temperature, and in solid form preferably has a relatively high vapor pressure and low latent heat of sublimation. That is, the vehicle can be a non-aqueous material that is, or can be made to be, essentially liquid and is later solidified or frozen; for example, the "liquid" vehicle may be a solid at room temperature and, accordingly, a slurry including the vehicle would be provided by maintaining the composition at an elevated temperature; the slurry would then be "frozen" by reducing the temperature such that the vehicle becomes a solid. Therefore, terms used herein such as "freezing" and "frozen" refer to solidifying the vehicle or to the vehicle in a solidified state. In a broader sense, the vehicle can be composed of a polymeric or polymerizable binder, the latter being more fully described in the above-mentioned applications.

While this invention is later illustrated primarily by the use of cyclohexane as the non-aqueous liquid vehicle, other non-aqueous vehicles include: aromatic hydrocarbons, such as benzene and toluene; aldehydes, such as propanal; ketones, such as methyl ethyl ketone and cyclohexanone; alcohols, such as methanol, ethanol, propanol, glycerol, and neopentyl alcohol; amides, such as formamide; amines, such as pyridine; ethers, such as dioxane; alkanes such as methane, ethane, butane, isobutane, hexane, cyclohexane, and dodecane; acetonitrile; and the like and compatible mixtures thereof. Of course, when forming is by methods such as described in the 07/125,643 application or for any other casting method, the vehicle can be aqueous.

The slurry is formed by mixing the solids and the vehicle (in liquid form) under high energy in the presence of a dispersant. Exemplary dispersants include quaternary ammonium salts (e.g., EMCOL CC-55, a cationic polypropoxy quaternary ammonium acetate, available from Witco Chem. Corp., Perth Amboy, NJ) , amine oxides, alkyl betaines, and similar compounds which are inert with respect to the ceramic and/or metallic composition. Other exemplary dispersants include coupling agents, such as those based on silicon (e.g., gamma-aminopropylmethoxy silane) . Various classes of preferred dispersants are described in the co- pending liquefaction applications described above. The dispersant not only aids in providing a well-dispersed suspension, but, to some extent, is believed to act as a binder for the green article (although not analogous to polymeric binders conventionally used in the ceramics art because the green article formed herein is porous, whereas conventional ceramic/polymeric binder composites are essentially non-porous) . Accordingly, the dispersant is present in amounts effective to provide a well-dispersed solids suspension; generally, this includes amounts of from about 0.5 to about 5 wt.% based on the solids weight, more preferably in the approximate range of 1.5 to 2.7 wt.%. The slurry may also include an internal lubricant to aid in de-molding. Such mold release agents include compounds such as oleic acid, stearic acid, waxes such as polyethylene glycol, and the like and mixtures thereof, and are generally present in the slurry in amounts of about 1-3 vol.% based on the pore fluid (liquid vehicle) portion. All of the components are formed into a pourable slurry, having a solids content of at least about 35 vol.% and a viscosity of less than about 5,000 cPs at 100 sec"1 shear rate, preferably less than about 1000 cPs at 100 sec"1. The slurry is preferably pseudoplastic and while it may also exhibit dilatant characteristics it is sufficient if the slurry can flow in a continuous stream under gravity head.

The composite slurry most preferably has virtually no volume change upon freezing for the preferred method described below; however, a composite volume change on freezing of less than about ± 4 vol.% is within the general tolerance limits of the present invention. As a departure from the co-pending application serial number 07/125,643, we have discovered that a non-aqueous slurry does not specifically require a cryoprotectant because the composite (i.e., the composite of solids, vehicle, and dispersant) volume change on freezing is negligible; accordingly, the major causes of the defects discussed in the background section are not encountered with non-aqueous slurries. Moreover, we have unexpectedly discovered that volume changes of less than about ± 4 vol.% on freezing are sufficiently negligible that a cryoprotectant is still not necessary even though there is a small volume change on freezing. While not desirous of being constrained to a particular theory, it is believed that if the rate of nucleation is significantly greater than the rate of crystal growth, then volume expansion on freezing is virtually avoided, even if the vehicle exhibits some hydrogen bonding.

The composite slurry, including the well-dispersed solid particles, reinforcement particles, liquid vehicle, and dispersant, is de-aired and then shaped by means of a mold using such processes as casting (e.g., tape casting, open mold casting) or extruding (e.g., injection molding). In particular, low pressure injection molding is a preferred method for shaping the slurry. The term "low pressure" is meant to indicate injection pressures generally less than about 400 psi,.and especially less than about 100 psi; in fact, the present slurries are suitable for casting under gravity head (under which they flow in a continuous stream) , and for dilatant slurries, casting under gravity head may be the only available forming method. Another aspect of this invention is that the slurries can be shaped and subsequently frozen in a closed system having a fixed volume. The mold material can be soft or hard, such as silicone rubber, wax, alumina, or tool steel. A frozen part can be formed by shaping the slurry in a mold which is already cooled to below the freezing temperature of the liquid vehicle or by subsequently freezing the mold containing the previously introduced and shaped slurry. The cooling rate, especially in the situation where a filled mold is subsequently cooled, can be either fast or slow, although fast cooling rates are preferred. Another aspect of the present invention is that the frozen part can be formed without the requirement for a substantial clamping pressure on the mold; rather, the present invention provides for freeze-forming at pressures not significantly different than atmospheric. The frozen part is "substantially frozen"; that is, the vehicle need not be completely frozen, but to the extent that some of the vehicle remains as a liquid phase, that phase is believed to be homogeneously dispersed throughout the frozen vehicle matrix. This is in contrast to the Takahashi patent applications, where the fluid is frozen only to provide a shell of sufficient thickness and strength to withstand release from the mold. Although not desirous of being constrained to a particular theory, it appears that at least about 95 vol.% of the vehicle is frozen when practicing the present invention.

After a frozen part is provided by freeze-forming, a green article is formed by de-molding the frozen part and drying to remove the solidified vehicle. As used herein, the "drying" process involves primarily non-destructive sublimation of the frozen pore vehicle. Processing may be accomplished in discrete stages wherein the article is slowly heated during the non-destructive sublimation. Accordingly, some evaporation may occur during the predominantly sublimative drying process. Nevertheless, such an occurrence is included within the scope of the present invention. An advantage of "freeze drying" is that there is very little shrinkage, and any shrinkage that does occur is very controlled. The resulting green part, having a density of about 40% of theoretical, is easily handled and is then put into a sintering furnace. Accordingly, the present invention obviates the need for a discrete binder burnout step; moreover, this invention provides an essentially binderless forming process. In view of the present definition of a vehicle as a component that is solidified and removed essentially by sublimative drying, it should be noted that "freeze drying" and "lyophilization" as used herein refer to non-destructive sublimative drying by which the solidified vehicle is removed as a gas; although some evaporative drying may be occurring along with the sublimative drying, the principal mechanism of vehicle removal is sublimation. Thus, the vehicle is removed under temperature and pressure conditions such that the primary operation is a physical change from the solid phase to the vapor phase; accordingly, as just noted, both the freezing and drying operations are physical processes.

For conventional molding processes, drying times and conditions are easily determinable by the skilled artisan.

The resulting dried green article can be sintered by conventional techniques. Such techniques, including sintering times, temperatures, and atmospheres, are well- known to those of ordinary skill in this art, and such variables are easily determinable for various ceramic or metallic compositions.

The invention will be further described by the following examples, which are meant to be illustrative and not limiting.

EXAMPLE I

This example describes the production of three dimensional, complex shape: a four-vane rotor about 2" in dia. and about 2" high. The rotor geometry includes a hollow conical hub to which four helically shaped vanes are attached, having a 90° angle between their leading and tailing edges; the vanes have a cross sectional thickness of about 0.25" tapering to about 0.125" at their outer edges.

A molding suspension was prepared in accordance with the above-referenced application serial number 07/036,377 with approximate amounts of the following:

932 g. alumina (Sumitomo AKP-30 brand, 0.3-0.5 μm,

9 m2/g; available from Sumitomo Chemical Co., Tokyo, Japan);

89 g. yttria (identified as FINEST; available from

Hermann C. Stark Co., Berlin, Germany) ; 170 g. SiC whiskers

(identified as SCW#2, used as- received, available from Tateho Chem. Ind. Co. Ltd., distributed by ICP Group, New York, NY) ;

337 g. distilled water;

(although preferred, distilled water is not necessary and tap water can be used)

15 g. dispersant

(identified as NARLEX LD-45, an ammonium acrylate-based copolymer, available from National Starch & Chem.

Corp., Bridgewater, NJ) ; and

15 g. dimethyl sulfoxide (DMSO) . The components were batched and milled in accordance with the above-referenced application 07/125,643 to achieve a pourable, injectable, stable suspension having a viscosity of about 500 cPs, and a total solids loading of about 46 vol.%, composed of a whisker solids loading of about 17.4 vol.% and a matrix solids loading of about 2S.6 vol.%. The suspension was deaired and injected at about 50 psi into a mold previously frozen at about -78°C. Within about one minute, the part was frozen and was thereafter demolded. The demolded part was subsequently dried in accordance with the procedure in the 07/125,643 application. The dried part was fired at about 1770°C for two hours in argon and densified to >95% T.D. ; sintering can also be done in helium.

The rotor geometry was maintained during densification as the vanes were concentric to the shaft with the with the 90° edge maintained between the leading and tailing edges. The open hub conical section was undistorted and maintained the constant taper over the approximately 2" cone height. This can be seen in Fig. 1, which shows a photograph of an alumina rotor on the left (lighter colored) and an SiC whisker reinforced alumina rotor on the right (darker colored) ; both rotors are shown as-sintered without any machining. EXAMPLE II

The same mold and procedures used to make the rotor of

Example I were used to mold a whisker composite formulation containing the approximate amounts of the following:

935 g. alumina (Sumitomo AKP-30 brand) ; yttria (Hermann C. Stark Co. FINEST brand) ; partially stabilized zirconia

(available as grade HSY-3, Zirconia Sales of America, Inc., Atlanta, GA) ; . SiC whiskers (available from Tateho) ; water; dispersant (NARLEX LD-45) ; and

Figure imgf000026_0001
DMSO.

The materials were batched to provide a composite slurry having a viscosity of about 500 cPs, about 48 vol.% total solids, and about 15 vol.% whiskers. The slurry was molded and sintered as in the above example that retained the geometric features and tolerances of the rotor fabricated as described in Example I.

EXAMPLE III

The same rotor mold and procedures used in the above examples were used to fabricate a composite from the approximate amounts of the following: 492 g. silicon nitride

(identified as HIGH GRADE, available from Elkem Corp., Pittsburgh, PA);

133 g. cerium oxide (identified as brand 5350, available from Unocal-Molycorp, York, PA) ;

100 g. SiC whiskers (Tateho) ; 187 g. cyclohexane; and

24 g. dispersant

(identified as GAFAC RE-610, an anionic polyoxyethylenenonylphenylether phosphate, available from GAF Corp., Wayne, NJ) .

The components were batched and deaired to provide a slurry having a viscosity of about 500 cPs, a total solids loading of about 44 vol.% and a whisker loading of about 15 vol.%. Further processing was similar to that of the foregoing examples, but modified as described in the afore-mentioned application 07/180,092 regarding non-aqueous systems. The green rotors were sintered at 1780 -C for two hours in nitrogen. The pressureless sintered rotors exhibited a density of >85% T.D. and retained the geometric features and tolerances of the mold. It is critical to note that while the preferred molding process is shown in all of the examples, the present invention is more broadly directed to any molding process that can include low pressure forming, and if the slip is non-dilatent then high pressure forming processes can also be used. For example, various processes have been recently developed which include injection molding a ceramics composition that is gelled, dried, and fired. See, e.g., J. Cesarano III et al. , "Thermal Gelation of Aqueous Alumina Suspensions," and Fanelli et- al., "New Aqueous Process for the Injection Molding of Ceramic Powders," Amer. Cer. Soc.. 90th Ann. Mtg. , May 1-5, 1988, pp. 287-288; see also Fanelli et al., U.S. Pat. No. 4,734,237; a gel casting process was also developed by Mark Janney at Oak Ridge National Laboratory (personal communcation dated June 23, 1988) . A similar method was disclosed by A.F. Henriksen, "Injection- molded and Sintered Metal, Ceramic and Cermet Parts," Gorham Int'l. Inc., Feb. 1-3, 1987, which exploits a proprietary process including providing a wax-based slurry, forming such as by low pressure injection molding, dewaxing by a propri¬ etary process, and firing. Although none of the disclosures just described mentions the possible presence of high aspect ratio particles in their respective forming slips or slurries, we have discovered that there are two important aspects to forming a sinterable article having randomly oriented reinforcement. One aspect is low viscosity for the slurry, most preferably less than 1000 cP at 100 sec"1, and in the best cases the slurry behaves as a Newtonian fluid. The other aspect is a forming process where the transfer of shear forces to the reinforcement particles during fluid flow is avoided. While injection pressures of less than 100 psi are preferred, most preferably less than 25 psi, there are a number of parameters which influence the transfer of shear forces to the high aspect ratio reinforcement particles during flow, including the viscosity of the slurry, the size of the flow channel, the size of the gate, the mold cavity size and geometry, and the like. Nevertheless, the use of a low viscosity slurry is essential from a practical point of view because a high viscosity slurry would require molding by plug flow, which is unacceptably slow and expensive.

The present invention is also applicable to tape casting formulations, where it is generally desired to orient the whiskers in the slip. See, e.g., S.R. Gurkovich et al., "Orientation Effects in SiC Whiskers Reinforced A1203 Prepared by Tape Casting," Amer. Cer. Soc.. op. cit.. p. 165. That is, by the present method, a tape can be cast with randomly oriented whiskers. The foregoing descriptions and examples are meant to il¬ lustrate the present invention and not to limit the invention. Various modifications ma become apparent to the skilled artisan upon reviewing this specification, which are intended to be within the scope and spirit of the invention as defined by the claims.

Claims

That which is claimed is:
1. A process for producing a sinterable green article, comprising:
(a) providing a low viscosity, pourable, binderless slurry composed of (i) a vehicle, (ii) a dispersant,
(iii) well-dispersed sinterable matrix solids present in an amount of at least about 35 vol.%, and (iv) well-dispersed reinforcement material having a high aspect ratio and present in an amount of at least about 5 vol.%; and
(b) shaping the slurry under low shear into a green article of a desired shape having the reinforcement material essentially randomly oriented and homogeneously dispersed therein.
2. The process as defined by claim 1, further comprising the step of pressureless sintering the green article to produce a densified ceramic article.
3. The process as defined by claim 1, wherein the matrix solids are ceramics selected from the group consisting of aluminum oxide, aluminum nitride, aluminum oxide, aluminum silicates, barium carbides, barium carbonates, barium titanates, boron carbonates, boron nitrates, beryllium oxides, calcium alumino silicates, lanthanum borates, lead titanates, lead zirconates, silicon carbides, silicon nitride, tungsten carbides, and mixtures thereof.
4. The process as defined by claim 1, wherein the vehicle is is benzene, toluene, methyl ethyl ketone, cyclohexanone, methanol, ethanol, propanol, glycerol, neopentyl alcohol, pentaerythritol, π-dodecanol, formamide, pyridine, dioxane, methane, ethane, butane, hexane, cyclohexane, acetonitrile, or mixtures thereof.
5. The process as defined by claim 1, wherein the matrix solids are metallic, and further comprising the step of pressureless sintering the green article to produce a densified metallic article.
6. The process as defined by claim 5, wherein the matrix solids are selected from the group consisting of aluminum, nickel, iron, titanium, copper, tungsten, beryllium, and mixtures and alloys thereof.
7. The process as defined by claim 1, wherein the step of shaping is by casting.
8. The process as defined by claim 1, wherein the step of shaping is by low pressure injection molding.
9. The process as defined by claim 1, wherein the slurry further includes less than about 2 vol.% of an internal lubricant based on the liquid vehicle volume.
10. The process as defined by claim 1, wherein the vehicle is aqueous.
11. The process as defined by claim 1, wherein the reinforcement material is a non-sinterable material selected from the group consisting of fibers, whiskers, platelets, and mixtures thereof.
12. The process as defined by claim 11, wherein the reinforcement material is present in an amount of about 5 vol.% to about 30 vol.%.
13. The process as defined by claim 1, wherein the reinforcement material is selected from high aspect ratio particles composed of silicon carbide, tungsten carbide, boron carbide, boron nitride, titanium diboride, silicon oxynitride, silicon nitride, carbon, alumina, alumina- silica, silica, zirconia, magnesium oxide, and mixtures thereof.
14. The process as defined by claim 1, wherein the subprocess of shaping further comprises the steps of:
(i) providing a mold of a desired shape at a temperature below the freezing point of the vehicle;
(ii) injecting the slurry into the mold under low pressure to form a frozen article; and
(iii) drying the frozen article by predominantly sublimative drying to form a sinterable green article having the reinforcement material essentially randomly oriented and homogeneously dispersed therein.
15. The process as define by claim 1, wherein the subprocess of shaping further comprises the steps of:
(i) providing a mold of a desired shape; (ii) injecting the slurry into the mold under low pressure such that the reinforcing material is essentially randomly oriented and homogeneously dispersed therein;
(iii) chilling the filled mold to below the freezing point of the vehicle to form a frozen article; and
(iv) drying the frozen article to form a sinterable green article having the reinforcement material randomly oriented therein.
16. The process as defined by claim 1, further comprising the step of densifying the article by hot isostatic pressing.
17. A process for forming a whisker-reinforced ceramic composite including the steps of providing and shaping a slip composed of ceramic matrix particles and reinforcing whiskers into a green article and sintering the green article to make a ceramic, wherein the improvement comprises making a net shape ceramic by providing a low viscosity slip and shaping under low shear.
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EP0419151A2 (en) * 1989-09-18 1991-03-27 Ngk Insulators, Ltd. Sintered ceramic composite body and method of manufacturing same
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US20120157358A1 (en) * 2010-01-29 2012-06-21 Oxane Materials, Inc. Self-Toughened High-Strength Proppant and Methods Of Making Same
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US20130244914A1 (en) * 2010-09-21 2013-09-19 Oxane Materials, Inc. Light Weight Proppant With Improved Strength And Methods Of Making Same

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