THIN CERAMIC COMPOSITE BODIES AND METHODS FOR MAKING THE SAME

Abstract: Ceramic bodies may be prepared using a combination of thin structure formation techniques and metal oxidation techniques. Specifically, a thin form is provided comprising at least one parent metal, at least one organic or inorganic fiber, and optionally comprising at least one inorganic filler material. The thin form may then be infiltrated with a liquid or fusible composition such as a polyureasilazane to increase the flexibility and formability of the thin form. The thin form may then be shaped or configured, either individually or in stacks, and rigidized to form for example, a self-supporting structure. The thin form may then be heated to a temperature above the melting point of the parent metal to react the molten parent metal in situ with an oxidant to form an oxidation reaction product, which is at least partially three-dimensionally interconnected to form a thin ceramic body in a desired shape or configuration.

Description: DESCRIPTION THIN CERAMIC COMPOSITE BODIES AND METHODS FOR MAKING THE SAME Technical Field The present invention broadly relates to novel thin ceramic bodies and to methods of making the same, wherein the novel thin ceramic bodies exhibit superior properties as compared with conventional or known thin ceramic bodies. Specifically, this invention relates to a process for making strong, thin ceramic bodies possessing characteristics such as good dimensional stability and low coefficient of thermal expansion at high temperatures, and controlled porosity, prepared from thin forms exhibiting high green strength and enhanced formability. In particular, the invention relates to a technique for making thin ceramic bodies comprising providing at least one thin form containing at least one parent metal, at least one organic or inorganic fiber, and optionally at least one inorganic filler material, and contacting at least a poπion of the at least one thin form with at least one liquid or fusible composition to provide characteristics such as tackiness, formability and strength. The at least one thin form may be fired to densify the same by a metal oxidation process, thus creating a thin ceramic body of a desired shape. Further, the present invention is directed to the formation of complex shapes fabricated from such thin forms. Background An In recent years there has been increasing interest in the use of ceramics for structural applications traditionally served by metals. Many high temperature metals, including super-alloys and the refractory metals, such as molybdenum, have been used as turbine blades, fans, nozzles for jet engines, gas turbines, rockets, and the like, but such materials have not satisfactorily met the demands placed upon such structures in view of the increased high temperature requirements imposed. These materials tend to soften at high operating temperatures or require protective coatings against oxidation, and are in general difficult to machine into precise shapes. Other materials proposed for these uses, such as graphite, silicon carbide, and metal oxides, suffer from similar deficiencies, as well as other deficiencies such as brittleness and the like. The impetus for substituting ceramics in areas traditionally dominated by metals has been the superior properties of ceramics such as corrosion resistance, hardness. modulus of elasticity, and refractory capabilities when compared with the performance of metals in adverse environmental conditions. Examples of areas for such prospective substitution include engine components, heat exchangers, cutting tools, bearings and wear surfaces, pumps, and marine hardware. Ceramics also exhibit more favorable characteristics respecting dimensional stability and low coefficients of thermal expansion over wider temperature ranges than metals. Recently there has been a great interest in the formation of ceramic bodies which are capable of maintaining good dimensional stability and which possess low coefficients of thermal expansion at elevated temperatures for use in applications such as heat exchangers. In general, ceramic heat exchangers of the prior art have been fabricated by conventional methods including slip casting, isostatic pressing, cold uniaxial pressing or extrusion, or saturation of corrugated paper with ceramic slip. Structures formed by the above-mentioned processes may suffer from deficiencies, including high coefficients of thermal expansion, resulting in cracking during heat cycles, and poor green strength limiting the geometric complexity of the ceramic body. Shaped ceramic bodies have been formed by other methods including, for example, tape-casting and paper-making techniques. However, green bodies that are formed by these methods are often fragile and difficult to handle. Further, it is difficult to form dense ceramics by these methods which often incorporate a high amount of organic material to form stronger green bodies. Further, ceramic bodies traditionally have had limited usefulness in certain adverse environments which require materials possessing characteristics such as good structural integrity, resistance to thermal shock, and corrosion. Though fully densified ceramic materials may meet some of these requirements, manufacturing fully dense ceramics, particularly those having any complex shapes, is difficult when low-cost and high reproducibility are required. Limited success has been attained in efforts to manufacture dense ceramic components, whether as a shaped integral article or as a composite. For example, attempts to produce ceramic structures using metal precursors and air oxidation are disclosed in U.S. Patent No. 3,255,027, issued in the name of Talsma which teaches the formation of refractory bodies comprising a porosity of from about 15% to about 95 % . The products are formed by a process which mixes particulate aluminum or aluminum alloy with a metal oxide fluxing agent, and which may further comprise the addition of particulate filler refractory . The green body is fired at a temperature of at least 400°C in an oxygen containing atmosphere until at least 11 % by weight aluminum is oxidized to form a substantially continuous integral skeleton of crystalline refractory. A generally porous structure characterized as a structure having an interior void is developed having intrinsically low strength properties. U.S. Patent No. 3,493,938, issued in the name of Oberlin, proposes to overcome the limitations of Tals a and teaches an alumina containing thin-walled refractory strucmre with the formation of alumina bridges. Thin-walled refractory strucmres are made by coating a thin aluminum section with a composition containing a vanadium compound having a melting point above 600°C. a fluxing agent selected from the alkali metal and alkaline earth silicates and a refractory material or its precursor. The coated strucmre is fired in an oxygen atmosphere at a temperamre from 600 to 900 °C and then fired in an oxygen atmosphere at a temperamre from 900 °C to slightly below the melting point of the strucmre. U.S. Patent No. 3,299,002, issued in the name of Hare teaches the formation of rigid nonoxide refractory bodies having a porosity of at least 20% by a process comprising mixing particles of aluminum and aluminum alloys, a fluxing agent (metal oxides such as alkali metals and alkaline metals), and a filler refractory. The mixmre is shaped and fired in an oxidizing atmosphere to a temperamre of between 600 °C and the melting point of the filler refractory for a time sufficient to oxidize at least 90% of the aluminum. Both Talsma and Hare teach the use of fluxes and metal powders in admixture with other particulates to dissolve or break up the oxidation reaction products, facilitating oxidation of the surfaces and thereby yielding intrinsically porous, low strength ceramics. In U.S. Patent No. 3,262.763, Bechtold teaches the formation of refractory bodies having solid, continuous matrix structures containing nitride, and a metallic component. The essential constituents of these materials are aluminum, boron, nitrogen, and silicon. The refractory bodies are prepared by heating compacts comprising aluminum, silicon nitride, and boron in a nitrogen or oxygen containing atmosphere at a temperamre of between 700°C and 1500°C. The resulting refractory bodies may be electrically conductive or insulating, and contain some porosity and at least 5 % aluminum or silicon. U.S. Patent No. 3,421 ,863, issued in the names of Bawa et al. , teaches a method for making a cermet material which is electrically insulating at high temperatures. The cermet is formed by compression molding a mixture comprising aluminum powder and aluminum silicate powder and firing the molded article at a temperamre of about 1000°C to about 1400°C in an oxidizing environment. The above methods suffer from deficiencies which are inherent in current ceramic body formation techniques, such as the formation of green bodies with poor strength and flexibility, limited shape formation capabilities, and expensive and complex set-ups which are not amenable to continuous expedient or economical processing. Further, methods devoted to the fabrication of ceramic articles by in situ oxidation of precursor metals typically produce articles lacking strucmral integrity with poor contact wear and erosion resistance. Description of Commonly Owned U.S. Patents and Patent Applications The subject matter of this application is somewhat related to that of several commonly owned, copending applications and several commonly owned patents. Particularly, these patents and patent applications describe, among other things, ceramic matrix composites and methods of making the same (hereinafter sometimes referred to as Commonly Owned Ceramic Matrix Composite Patents and Patent Applications) and novel inorganic and inorganic/organic polymer compositions, and methods of making the same (hereinafter sometimes referred to as Commonly Owned Inorganic/Organic Polymer Patents and Patent Applications). The entire disclosures of all of the Commonly Owned Ceramic Matrix Composite and Inorganic/Organic Polymer Patents and Patent Applications are expressly incorporated herein by reference in their entirety . This is a continuation-in-part of Provisional U.S. Patent Application Serial No. 60/004913, filed on October 6, 1995, and entitled "Ceramic Bodies and Methods for Making the Same. " Commonly Owned U.S. Patent No. 4,851,375, issued in the names of Newkirk et al. , teaches a method of making self-supporting ceramic composite strucmres having embedded filler. The ceramic composites are obtained by oxidizing a parent metal to form a polycrystalline oxidation reaction product which infiltrates into a permeable mass of filler material. A parent metal, optionally provided with one or more dopants in or in contact with the metal, is placed adjacent to the permeable mass of filler and heated to provide a body of molten parent metal which is contacted with a suitable oxidant to result in the formation of the oxidation reaction product. Molten parent metal migrates through previously formed oxidation reaction product into contact with the oxidant, causing the oxidation reaction product to continue to grow, thereby embedding the filler material to provide a ceramic composite structure. Commonly Owned and Copending U.S. Patent Application Serial No. 08/451,581 , filed May 26, 1995, in the names of Marc S. Newkirk et al., is a descendant of U.S. Patent No. 4,851 ,375. Among other disclosures, this patent application expressly teaches that the parent metal may be provided in finely divided form, preferably admixed with the permeable mass comprising at least one filler material. Thus, the directed oxidation of molten parent metal may be conducted "in situ" . The bodies produced are typically porous, with pores being located at places where bodies of parent metal have oxidized, and the pores typically replicate at least a portion of the physical geometry of the parent metal body prior to oxidation. Thus, this patent application also teaches a technique for producing porous ceramic bodies whereby a measure of control over the size, shape and distribution of porosity is possible. Commonly Owned U.S. Patent No. 5,017,526, also issued in the names of Newkirk et al. , teaches a method for forming self-supporting ceramic composite bodies of desired shape by infiltrating permeable shaped preforms with polycrystalline matrix material comprising the oxidation reaction product obtained by the oxidation of a parent metal. The shaped composite bodies are formed by contacting a zone of the self- supporting preform with a body of molten parent metal, optionally with a dopant in or in contact with the metal at some time during the processing, and reacting the metal with a suitable oxidant to form an oxidation reaction product. Molten parent metal continues to migrate through the oxidation reaction product into contact with the oxidant and grows into the shaped preform, thereby providing composite strucmres of desired geometry. Commonly Owned U.S. Patent No. 4,929,704, in the name of Schwark, is directed to among other things, the preparation of polysilazane addition polymers which are prepared by the reaction of ammonia with one or more halogenated silicon compounds to produce cyclic silazane ammonolysis products, and then further reacting a metal-nitrogen polymer comprising a silazane ammonolysis product with 0.1 % to 30% by weight of an isocyanate, isothiocyanate, ketene. thioketene. carbodiimide, or carbon disulfide. Polysilazane addition polymers containing alkenyl or alkynyl groups can be cured by supplying energy to generate free radicals. This patent further describes a method for preparing polysilazane addition polymers in which the viscosity of the polymers can be controlled. The viscosity of such polymers may be tailored to be suitable for a specific intended end use. For example, high viscosities may be desirable for making fibers, and low viscosities may be desirable for polymers used to infiltrate porous ceramic bodies or produce thin films. U.S. Patent No. 5,001,090, also in the name of Schwark and which issued from a divisional application of U.S. Patent No. 4,929,704, relates to, among other things, silicon nitride ceramics which are produced by the pyrolysis of cured or uncured polysilazane addition polymers. Further, Commonly Owned U.S. Patent No. 5,021 ,533, issued in the name of Schwark, teaches poly(thio)ureasilazane compositions prepared by the reaction of an organic isocyanate with a polysilazane comprising Si-H bonds so as to effect a reaction of the isocyanate with the silicon-nitrogen bond of the polysilazane to create a composition in which the viscosity may be controlled, and which can be subsequently followed by a crosslinking reaction. Such crosslinking can be partially initiated by raising the temperature to not greater than 300 °C for a time sufficient to increase the viscosity to a value suitable for the desired end use of the polymer. Cured or uncured poly(thio)ureasilazanes can be pyrolyzed to yield a silicon-nitride containing ceramic material. Moreover, U.S. Patent No. 5,032,649, and U.S. Patent No. 5, 155, 181, both issued in the name of Schwark, disclose the preparation of organic amide-modified silazane polymers by the initial reaction of, for example, less than about 30 wt% of an organic amide with a polysilazane comprising Si-H bonds so as to effect reaction of the isocyanate with the silicon-nitrogen bond followed by a crosslinking reaction in which the by-product is hydrogen gas. Commonly Owned U.S. Patent Application Serial No. 08/223,294, filed April 4, 1994 (PCT Publication No. WO 95/12630), which was a continuation-in-part application of U.S. Patent Application Serial No. 08/148,044, filed November 5, 1993, both in the names of Becker et al. , is directed to, among other things, compositions which comprise uncrosslinked reaction mixtures comprising at least one organic monomer, oligomer, or polymer comprising a multiplicity of organic, electrophilic substituents, and at least one metal -containing polymer comprising a metal-nitrogen polymer. Further, this commonly owned application which is directed to compositions comprise reaction mixtures comprising at least one organic monomer, oligomer, or polymer comprising a multiplicity of organic, electrophilic substituents, and at least one of (a) silicon-nitrogen polymers, (b) aluminum-nitrogen polymers, (c) boron-nitrogen polymers and (d) polymer combinations thereof, which comprise a multiplicity of sequentially bonded repeat units, the compositions comprising the reaction products of the reaction mixtures, and the compositions obtained by crosslinking the reaction products of the reaction mixtures. Crosslinking may be effected through at least one of thermal-based, radiation-based, free radical-based or ionic-based crosslinking mechanisms. The compositions may be molded or shaped by various techniques into numerous useful articles. Furthermore, the compositions may be applied as coatings. adhesives, and the like by various techniques onto numerous articles to enhance the performance of such articles. Summary of the Invention The present invention is directed to thin, self-supporting ceramic bodies which may be prepared using a combination of shape formation techniques and metal oxidation techniques. Specifically, a thin form is provided comprising at least one parent metal, at least one organic or inorganic fiber, and optionally one or more inorganic filler materials. In one embodiment, thin forms may then be contacted with a liquid or fusible composition to impart desirable physical characteristics such as enhanced flexibility, formability, and strength, then shaped into a desired shape or configuration and rigidized by any method known in the art to render the thin form self-supporting. In a further embodiment, thin forms are heated to a temperamre above the melting point of the parent metal to render the matrix metal molten and at least partially undergo oxidation, thereby forming a desired oxidation reaction product. The oxidation reaction product may be at least partially three-dimensionally interconnected to produce strong, thin ceramic bodies. In addition, it may be desired to increase the porosity formed by the oxidation reaction of the parent metal by including fugitive materials into the materials utilized to produce the green body. The fugitive materials include, for example, polymer pellets, sawdust, corn starch, coconut charcoal, wood pulp, etc. These materials will volatilize or burn off when heated in accordance with the present invention. Further, complex shapes may be fabricated utilizing the thin forms of the present invention. The thin forms may be prepared by any method known in the art such as, for example, slush molding, blow molding, slip casting, tape casting and paper-making. In a preferred embodiment of the present invention, thin forms are prepared using a continuous processing technique in which the production of thin forms is expedient and economical, such as for example, a paper-making technique. Parent metals suitable for the present invention include for example, at least one metal or alloy selected from aluminum, titanium, silicon, zirconium, hafnium, tin, zinc, molybdenum, etc. , and may be present in at least one desirable form such as, for example, wires, fibers, particulates, spheres, powders, and the like. In another embodiment of the present invention, a dopant or dopants may be provided to facilitate oxidation of the parent metal to a ceramic. In a further preferred embodiment where at least one liquid or fusible composition is contacted with the thin form to impart, for example, formability, the composition also enhances the oxidation of the parent metal to a ceramic, creating thin ceramic bodies with superior strength properties without the need for providing separate dopant materials. In a preferred embodiment of the present invention, the at least one fusible or liquid composition comprises a liquid polymer which is capable of being pyrolyzed to a ceramic material, sometimes termed a "preceramic polymer" or "ceramer", which is contacted, deposited, coated, layered, infiltrated, etc. , onto or into the dried (but still green) thin form. In another embodiment, the at least one liquid or fusible composition may be admixed with the at least one parent metal, the at least one fiber, or the optional at least one inorganic filler material. The at least one thin form comprising at least one liquid or fusible composition has characteristics such as shapeability, adhesion, and strength, such that the at least one thin form may be shaped, for example, by cutting or molding, and subsequently rigidized to render the at least one thin form self-supporting in a desired shape or configuration. Where the liquid or fusible composition comprises, for example, a crosslinkable preceramic polymer, the thin form may remain self-supporting throughout the firing and oxidation process thereby eliminating any slumping and deformation of the structure. Further, where the at least one liquid or fusible composition comprises a preceramic polymer, the composition may decompose upon firing to form additional ceramic material creating a more dense thin ceramic body, thereby adding strength. In a further embodiment of the invention, the at least one thin form may be seamed or joined or bonded to the same or at least one other thin form, thereby creating complex shapes. Specifically, at least one thin form of the present invention may be contacted to the same or at least one other thin form along at least one overlapped edge or common edge. Upon firing to a temperature sufficient to initiate the metal oxidation process, the resulting thin ceramic bodies will be bonded to one another along the at least one overlapped edge or common seam. In another embodiment of the present invention, thin forms may be stacked as laminates. At least one liquid or fusible composition such as, for example, a preceramic polymer may be applied to the thin forms to provide characteristics such as tackiness, drapeability, and strength to thin forms which can be stacked and shaped into complexly shaped three-dimensional laminate bodies. Further, thin forms of the present invention may be stacked with materials comprising other compositions to form heterogeneous laminate strucmres comprising layers of varying compositions. Still further, certain mechanized paper-making techniques tend to impart a directionality to the properties of the thin bodies produced thereby by tending to align the fibers in rolling or drawing the paper. Additionally, thin shapable forms of the present invention may be alternated with, for example, fiber tows or pliable weave or mesh that may be provided to add characteristics such as strength, or alternatively may be subsequently substantially burned out at higher temperatures to create layered complex shaped three-dimensional strucmral compositions with internal chambers or channels. Definitions "Aluminum", as used herein, means and includes essentially pure metal (e.g. , relatively pure, commercially available unalloyed aluminum) or other grades of metal and metal alloys such as the commercially available metals having impurities and/or alloying constituents such as iron, silicon, copper, magnesium, manganese, chromium, zinc, etc., therein. An aluminum alloy for purposes of this definition is an alloy or intermetallic compound in which aluminum is the major constituent. "Balance Non-Oxidizing Gas", as used herein, means any gas present in addition to the primary or oxidizing gas (if utilized) comprising the vapor-phase oxidant that is either an inert gas or a reducing gas which is substantially non-reactive with the parent metal under the process conditions. Any oxidizing gas which may be present as an impurity in the gas(es) used should be insufficient to oxidize the parent metal to any substantial extent under the process conditions. "Barrier" or "barrier means", as used herein, means any material, compound, element, composition, or the like, which, under the process conditions, maintains some integrity, is not substantially volatile (i.e. , the barrier material does not volatilize to such an extent that it is rendered non-functional as a barrier) and is preferably permeable to a vapor-phase oxidant (if utilized) while being capable of locally inhibiting, poisoning, stopping, interfering with, preventing, or the like, continued growth of the oxidation reaction product. "Ceramic" , as used herein, should not be unduly construed as being limited to a ceramic body in the classical sense, that is, in the sense that it consists entirely of non- metallic and inorganic materials, but rather refers to a body which is predominantly ceramic with respect to either composition or dominant properties, although the body may contain minor or substantial amounts of one or more metallic constituents (isolated and/ or interconnected, depending on the processing conditions used to form the body) derived from the parent metal, or reduced from the oxidant or a dopant, most typically within a range of from about 1-40 percent by volume, but may include still more metal. "Ceramic Matrix Composite" or "CMC" or "Ceramic Composite Bodv". as used herein, means a material comprising a two- or three-dimensionally interconnected ceramic matrix which has embedded a preform or filler material, and may further include a parent metal phase embedded therein, possibly in a two-or three- dimensionally interconnected network. The ceramic matrix may include various dopant elements to provide a specifically desired microstructure, or specifically desired mechanical, physical, or chemical properties in the resulting ceramic matrix composite. "Dopants" , as used herein, means materials (parent metal constituents or constituents combined with and/or included in or on a filler, or combined with the oxidant) which, when used in combination with the parent metal, favorably influence or promote the oxidation reaction process and/or modify the growth process to alter the morphology, microstructure, and/or properties of the product. While not wishing to be bound by any particular theory or explanation of the function of dopants, it appears that some dopants are useful in promoting oxidation reaction product formation in cases where appropriate surface energy relationships between the parent metal and its oxidation reaction product do not intrinsically exist so as to promote such formation. Other dopants may improve the moφhology of the composite by enhancing the nucleation and uniformity of the growth of the oxidation reaction product, or improve the physical, chemical, and mechanical properties of the ceramic matrix composite by modifying its microstructure. Dopants may be added to the filler material, they may be in the form of a gas, solid, or liquid under the process conditions, they may be included as constituents of the parent metal, or they may be added to any one of the constituents involved in the formation of the oxidation reaction product. Dopants may: (1) create favorable surface energy relationships which enhance or induce the wetting of the oxidation reaction product by the molten parent metal; and/or (2) form a "precursor layer" at the growth surface by reaction with alloy, oxidant, and/or filler, that (a) minimizes formation of a protective and coherent oxidation reaction product layer(s), (b) may enhance oxidant solubility (and thus permeability) in the molten parent metal, and/or (c) allow for transport of oxidant from the oxidizing atmosphere through any precursor oxide layer to combine subsequently with the molten parent metal to form another oxidation reaction product; and/or (3) cause microstructural modifications of the oxidation reaction product as it is formed or subsequently and/or alter the metallic constituent composition and properties of such oxidation reaction product; and/or (4) enhance growth nucleation and uniformity of growth of oxidation reaction product. "Filler" or "Filler Material", as used herein, means either single constituents or mixtures of constituents which are substantially non-reactive with and/or of limited solubility in the parent metal and/or oxidation reaction product and may be single or multi-phase. Fillers may be provided in a wide variety of forms, such as powders, flakes, platelets, microspheres, whiskers, bubbles, etc. , and may be either dense or porous. "Filler" may also include ceramic fillers, such as alumina or silicon carbide as fibers, chopped fibers, particulates, whiskers, bubbles, spheres, fiber mats, or the like, and coated fillers such as carbon fibers coated with alumina or silicon carbide to protect the carbon from attack, for example, by a molten aluminum parent metal. Fillers may also include metals. For example, refractory metals such as tungsten, tantalum and molybdenum could be used as fillers. "Green", as used herein in conjunction with filler materials and preforms, refers to a filler material or preform before any growth of oxidation reaction product into the filler material or preform has occurred. Thus a filler material or preform that has been fired at an elevated temperature (for example, to volatilize a binder) should be considered to be "green" so long as the filler material or preform has not been infiltrated by either the parent metal or the oxidation reaction product. "Growth Alloy", as used herein, means any alloy containing initially, or at some point during processing obtaining, a sufficient amount of requisite constituents to result in growth of oxidation reaction product therefrom. Growth alloy may differ from a parent metal in that the growth alloy may include constituents not present in the parent metal, but incorporated into the molten alloy during growth. "Liquid-Phase Oxidant" or "Liquid Oxidant". as used herein, means an oxidant in which the identified liquid is the sole, predominant or at least a significant oxidizer of the parent or precursor metal under the conditions of the process. Reference to a liquid oxidant means one which is a liquid under the oxidation reaction conditions. Accordingly, a liquid oxidant may have a solid precursor, such as a salt, which is molten at the oxidation reaction conditions. Alternatively, the liquid oxidant may have a liquid precursor (e.g. , a solution of a material) which is used to impregnate part or all of the filler and which is melted or decomposed at the oxidation reaction conditions to provide a suitable oxidant moiety. Examples of liquid oxidants as herein defined include low melting glasses. If a liquid oxidant is employed in conjunction with the parent metal and a filler, typically, the entire bed of filler, or that portion comprising the filler which is to be embedded within the desired ceramic matrix composite body, is impregnated with the oxidant (e.g. , by coating or immersion in the oxidant). "Nitrogen-Containing Gas Oxidant". as used herein, means a particular gas or vapor in which nitrogen is the sole, predominant or at least a significant oxidizer of the parent or precursor metal under the conditions existing in the oxidizing environment utilized. The nitrogen could be molecular nitrogen (i.e. , N2) or could be contained in a compound such as NH3. "Oxidant". as used herein, means one or more suitable electron acceptors or electron sharers and may be a solid, a liquid or a gas or some combination of these (e.g. , a solid and a gas) at the oxidation reaction conditions. Typical oxidants include, without limitation, oxygen, nitrogen, any halogen or a combination thereof, sulphur, phosphorus, arsenic, carbon, boron, selenium, tellurium, and or compounds and combinations thereof, for example, silica or silicates (as sources of oxygen), methane, ethane, propane, acetylene, ethylene, propylene (the hydrocarbon as a source of carbon), and mixtures such as air, H2/H2O and CO/CO2 (as sources of oxygen). The latter two (i.e. , H2/H2O and CO/CO2) being useful in reducing the oxygen activity of the environment. "Oxidation" , as used herein, means a chemical reaction in which an oxidant reacts with a parent metal, and that parent metal gives electrons to or shares electrons with the oxidant. " Oxidation Reaction Product", as used herein, means one or more metals in any oxidized state wherein the metal(s) has given up electrons to or shared electrons with another element, compound, or combination thereof. Accordingly, an "oxidation reaction product" under this definition includes the product of the reaction of one or more metals with one or more oxidants. "Oxygen-Containing Gas Oxidant". as used herein, means a particular gas or vapor in which oxygen is the sole, predominant or at least a significant oxidizer of the parent or precursor metal under the conditions existing in the oxidizing environment utilized. "Parent Metal" , as used herein, means that metal(s) (e.g. , aluminum, silicon, hafnium, molybdenum, titanium, tin. zinc and/or zirconium) which is the precursor of a polycrystalline oxidation reaction product and includes that metal(s) as an essentially pure metal, a commercially available metal having impurities and/or alloying constituents therein, or an alloy in which that metal precursor is the major constituent. When a specified metal is mentioned as the parent or precursor metal (e.g. , aluminum, etc.), the metal identified should be read with this definition in mind unless indicated otherwise by the context. "Preceramic Polvmer" or "Ceramer" . as used herein, means a polymeric material which upon pyrolysis at elevated temperature, chemically converts to a ceramic material, which may be crystalline, amorphous or some combination. "Preform" or "Permeable Preform", as used herein, means a porous mass of filler or filler material which is manufactured with at least one surface boundary which essentially defines a boundary for growing oxidation reaction product, such mass retaining sufficient shape integrity and green strength to provide dimensional fidelity prior to being infiltrated by the ceramic matrix. The mass should be sufficiently porous or permeable to: (1) allow the vapor-phase oxidant (if a vapor-phase oxidant is used) to permeate the preform and contact the parent metal; and (2) accommodate development or growth of oxidation reaction product. A preform typically comprises a bonded array or arrangement of filler, either homogeneous or heterogeneous, and may be comprised of any suitable material (e.g. , ceramic and/or metal paπiculates, powders, fibers, whiskers, etc.. and any combination thereof). A preform may exist either singularly or as an assemblage. "Reducible Substance", as used herein, means an element or compound which interacts with the molten parent metal and/or the oxidation reaction product (e.g., is reduced by the parent metal and/or oxidation reaction product and thus modifies the composition of the parent metal and/or provides an oxidant for formation of the oxidation reaction product). See also "Liquid Oxidant" and "Solid Oxidant. " " Solid-Phase Oxidant" or "Solid Oxidant". as used herein, means an oxidant in which the identified solid is the sole, predominant or at least a significant oxidizer of the parent or precursor metal under the conditions of the process. When a solid oxidant is employed in conjunction with the parent metal and a filler, it is usually dispersed throughout the entire bed of filler or that portion of the bed into which the oxidation reaction product will grow, the solid oxidant being, for example, particulates admixed with the filler or coatings on the filler particles. Any suitable solid oxidant may be thus employed including elements, such as boron or carbon, or reducible compounds, such as silicon dioxide or certain borides of lower thermodynamic stability than the boride reaction product of the parent metal. For example, when boron or a reducible boride is used as a solid oxidant for an aluminum parent metal, the resulting oxidation reaction product comprises aluminum boride. "Vapor-Phase Oxidant", as used herein, means an oxidant which contains or comprises a particular gas or vapor and further means an oxidant in which the identified gas or vapor is the sole, predominant or at least a significant oxidizer of the parent or precursor metal under the conditions existing in the oxidizing environment utilized. For example, although the major constituent of air is nitrogen, the oxygen content of air is the sole oxidizer for the parent metal because oxygen is a significantly stronger oxidant than nitrogen. Air therefore falls within the definition of an "Oxygen- Containing Gas Oxidant" but not within the definition of a "Nitrogen-Containing Gas Oxidant" (an example of a "nitrogen-containing gas" oxidant is forming gas, which typically contains about 96 volume percent nitrogen and about 4 volume percent hydrogen) as those terms are used herein and in the claims. "Copolvmer" . as used herein, means a polymer made from two or more monomers, oligomers or polymers corresponding to different repeat units, where the different repeat units are incorporated in the same polymeric molecule or chain. Copolymers include random copolymers. di-block copolymers, multiblock copolymers, alternating copolymers, graft copolymers, organic copolymers, inorganic copolymers, hybrid copolymers (e.g., both organic and inorganic backbone copolymers), organic graft copolymers, inorganic graft copolymers, hybrid graft copolymers (e.g. , both organic and inorganic grafts on the same copolymer), terpolymers, etc. "Hybrid Polvmer" or "Ceramer". as used herein, means an oligomer, polymer, copolymer or polymer alloy which is comprised of a plurality of metal-containing segments and a plurality of organic segments. The hybrid polymer or ceramer may be at least one of copolymeric or polymer alloy. Hybrid polymers or ceramers may include random copolymers, di-block copolymers, multiblock copolymers, alternating copolymers, graft copolymers, terpolymers, etc. " Liquid or Fusible Composition", as used herein, means a substance which is contacted with a green, thin ceramic body (e.g. , a preform) for the purpose of imparting or enhancing some desirable physical characteristic, such as, for example, adhesion, flexibility, formability and drapeability of the body. As used herein, such compositions include polymers and sols such as tetra ethyl ortho silicate. "Monomer" , as used herein, means a molecule or chemical compound comprising one repeat unit with an inherent capability of forming chemical bonds with the same and/or other monomers oligomers or polymers in such a manner that oligomeric and/or polymeric molecules or macromolecules are formed. Monomers include molecules or chemical compounds which are wholly organic, wholly inorganic or hybrid (i.e. , organic and inorganic). "Oligomer", as used herein, means a molecule or chemical compound which comprises several repeat units, generally from about 2 to about 10 repeat units. Oligomers have an inherent capability of forming chemical bonds with the same and/or other monomers and/or oligomers and/or polymers in such a manner that oligomeric and/or polymeric molecules or macromolecules are formed including molecules or chemical compounds which are wholly organic, wholly inorganic, or hybrid (i.e. , organic and inorganic). "Polymer" , as used herein, means a molecule or compound which comprises a large number of repeat units, generally greater than about 10 repeat units. Polymer includes thermosetting polymers, thermoplastic polymers, elastomers, amorphous polymers, crystalline polymers, semicrystalline polymers, homopolymers, heteropolymers, copolymers, polymer alloys, linear or unbranched polymers, branched polymers such as macromolecules comprising long branching, short branching or mixed long and short branching, cyclic polymers, crosslinkable polymers, crosslinked polymers, polymeric network polymers, interpenetrating polymeric networks, combinations thereof, etc. Additionally, polymers include wholly organic, wholly inorganic and hybrid (i.e. , organic and inorganic) chemical macromolecules. Brief Description of Figure 1 Figure 1 is a photograph of a thin ceramic body produced in accordance with Example 26. Detailed Description of the Invention and Preferredm Embodiment The present invention is directed to thin, self-supporting ceramic bodies which, among other things, have good dimensional stability and a low coefficient of thermal expansion at high temperatures. The thin ceramic bodies of the present invention may be prepared using any combination of thin structure formation techniques and metal - 15 - oxidation techniques. Specifically, at least one parent metal, at least one organic or inorganic fiber and, optionally, at least one inorganic filler are incoφorated into thin forms. In one preferred embodiment, thin forms further may be contacted with a liquid or fusible composition to enhance, for example, adhesion, flexibility, formability and drapeability of the thin forms. In another preferred embodiment of the present invention, thin forms may be heated to a temperamre above the melting point of the parent metal to render the matrix metal molten and at least partially undergo oxidation, thereby forming a desired oxidation reaction product to produce strong, thin ceramic bodies. In a further preferred embodiment, complex shapes may be fabricated by combining such thin forms into a virtually unlimited number of shapes and combinations. A thin form comprising at least one parent metal, at least one organic or inorganic fiber, and optionally at least one inorganic filler material may be formed by any formation technique known in the art. Thin forms may be produced by any method known in the art including, for example, slush molding, slip casting, tape casting, blow molding, paper making, and other similar techniques for making thin bodies. In a preferred embodiment, thin forms may be prepared using a continuous processing technique in which formation is expedient and economical, such as, for example, a paper-making technique. The thickness of the forms may be determined by such factors as the formation technique and the quantity and composition of fiber and filler materials. Forms made by any thin strucmre formation technique, including the above-mentioned techniques, may be shaped into simple or complex geometries from either individual or multiple forms. In a preferred embodiment of the present invention, at least a portion of at least one thin form may be contacted with at least one liquid or fusible composition to impart desirable characteristics including, for example, tackiness, adhesion, drapeability, flexibility and strength. Thin forms provided with at least one liquid or fusible composition may have, for example, enhanced drapeability or formability for cutting, bending, folding, shaping or molding into a desired geometry, and tackiness or adhesion for contacting with other forms in the creation of, for example, multiple component strucmres and laminates. At least one liquid or fusible composition may be applied to the thin form in any manner known in the art including, but not limited to, painting, spraying, dipping, soaking, brushing, and dusting (e.g. , in instances where the composition is applied as a fusible solid material). In another embodiment of the present invention, at least one liquid or fusible composition may be incoφorated into the thin form by admixing the composition with the at least one parent metal, the at least one fiber or with the optional at least one inorganic filler material. In a further alternative embodiment, the at least one inorganic or organic composition may further comprise a dispersion comprising particulate materials such as, for example, at least one fiber or filler material used in the preparation of the thin form. Thin forms which have been contacted with at least one liquid or fusible composition and which have been shaped or placed into a particular configuration may be subsequently rigidized into that configuration or shape by, for example, air drying or heating to dry the composition. In one embodiment, wherein the thin form is contacted with a crosslinkable liquid or fusible composition, the composition may be crosslinked by any method known in the art in an amount sufficient to rigidize the form and render the form self-supporting. For example, where the at least one liquid or fusible composition is a crosslinkable polymer, the composition may be crosslinked by. for example, radiation or chemical or thermal means. The use of a crosslinkable polymer allows thin forms to be rendered self-supporting, for example, by heating, thereby permitting the thin body to undergo subsequent firing steps without the occurrence of slumping or distortion during heating. ln a preferred embodiment, thin forms prepared by the process of the present invention comprising at least one parent metal provided in finely divided form (e.g. , provided as a plurality of small bodies), at least one organic or inorganic fiber, and, optionally, at least one inorganic filler, may be heated to a temperature above the melting point of the parent metal to initiate oxidation of the molten parent metal, and to form oxidation reaction product. In addition, it may be desired to increase the porosity formed by the oxidation reaction of the parent metal by including fugitive materials into the materials utilized to produce the green body. The fugitive materials include polymer pellets, sawdust, corn starch, coconut charcoal, wood pulp, etc. These materials will volatilize or burn off when heated in accordance with the present invention. Polymeric fugitive materials may comprise organic or inorganic polymeric filler materials including plastics or organic polymer fillers such as, for example, olefinics, vinylics, styrenics, acrylonitrilics, acrylics, polyesters, imides and cellulosic fugitive materials. By way of explanation of the oxidation process, but not wishing to be bound thereby, the molten metal is transported along channels at certain high energy grain intersections of the oxidation reaction product phase. It is well understood that any polycrystalline material exhibits a range of grain boundary energies (surface free energies) depending upon the degree of lattice misalignment at the interface between two adjacent crystals or grains of the same material. In general, grain boundaries of low angular misalignment exhibit low surface energies, while high angle boundaries have high surface energies, although the relationship may not be a simple, monotonically increasing function of the angle due to the occasional occurrence of more favorable atomic alignments at intermediate angles. Similarly, the lines along which three grains intersect also typically are high energy features in a polycrystalline microstructure. As further explained in the aforesaid Commonly Owned Ceramic Matrix Patents and Patent Application, but not wishing to be bound thereby, the parent metal and the oxidant apparently form a favorable polycrystalline oxidation reaction product having a surface free energy relationship with the molten parent metal such that within some portion of a temperamre region in which the parent metal is molten, at least some of the grain intersections (i.e. , grain boundaries or three-grain-intersections) of the polycrystalline oxidation reaction product are replaced by planar or linear channels of molten metal. For example, consider a grain boundary having a surface free energy greater than the alternative configuration of two substantially geometrically equivalent crystal/molten metal interface boundaries. In these circumstances, such a high energy grain boundary either will not form or will spontaneously decompose in favor of a planar channel of molten metal bounded by two crystal/metal interfaces. When the molten metal is maintained in the oxidizing environment and within the effective portion of the temperamre region, molten metal is drawn or transported along such channels in the direction of the oxidant. More specifically, this phenomenon occurs when (1) the liquid metal wets the crystalline oxidation reaction product phase (i.e. , Ysl c sg < where γ s j denotes the surface free energy of the crystal/molten metal interface and γ S g denotes the surface free energy of the crystal/vapor interface), and (2) the energy of some of the grain boundaries, γ D . is greater than twice the crystal/liquid metal interfacial energy, i.e. , γmax > Υgsl where Y max - s tne maximum grain boundary energy of the polycrystalline material. Molten metal channels of linear character can be formed in a similar way if metal replaces some or all of the three-grain-intersections in the material. Since the channels are at least partially interconnected, (i.e. , the grain boundaries of the polycrystalline material are interconnected), molten metal is transported through the polycrystalline oxidation reaction product to its surface and into contact with the oxidizing atmosphere, where the metal undergoes oxidation resulting in the continual growth of the oxidation reaction product. Furthermore, since the wicking of molten metal along channels is a much faster transport process than the ionic conduction mechanisms of most normal oxidation phenomena, the growth rate observed for the oxidation reaction product with this oxidation process is much faster than that typically observed in other oxidation phenomena. While the oxidation reaction product of the present invention is inteφenetrated by metal along high energy grain intersections, the polycrystalline oxidation reaction product phase is itself interconnected in one or more dimensions, preferably in three dimensions, along relatively low angle grain boundaries which do not meet the criterion γ D > 2γ s i- Thus, the ceramic matrix of the composites of this invention exhibits many of the desirable properties of the classical ceramic (i.e. , hardness, refractoriness, wear resistance, etc.) while deriving additional benefits from the presence of the distributed metal phase (notably higher toughness and resistance to fracture). In the embodiment of the present invention wherein a ceramic matrix body is formed, the ceramic matrix, which may be obtained by oxidation of a molten parent metal with a vapor-phase oxidant to form a polycrystalline oxidation reaction product, is characterized by an essentially single phase polycrystalline oxidation reaction product and distributed metal or voids or both, and by crystal lattice misalignments at oxidation reaction product crystallite grain boundaries which are less than the lattice misalignments between those neighboring oxidation reaction product crystallites having planar metal channels or planar voids, or both, disposed between said neighboring crystallites. In certain embodiments, substantially all of the grain boundaries in said oxidation reaction product phase have an angular mismatch between adjacent crystal lattices of less than about 5 degrees. One or more oxidants can be employed in the process of the present invention. Typically a vapor-phase oxidant is employed, the vapor-phase oxidant normally being gaseous, or at least gaseous under the process conditions. The vapor-phase oxidant provides an oxidizing atmosphere, such as atmospheric air. Typical vapor oxidants include, for example, elements, compounds or combinations of the following, including volatile or vaporizable elements, compounds, or constituents of compounds or mixtures: oxygen, nitrogen, a halogen, sulphur, phosphorus, arsenic, carbon, boron, selenium, tellurium, methane, ethane, propane, acetylene, ethylene, propylene (the hydrocarbons as a source of carbon), and mixtures such as air, H2/H2O and a CO/CO2, the latter two (i.e. , H2/H2O and CO/CO2) being useful in reducing the oxygen activity of the environment relative to desirable oxidizable constituents of the preform. Oxygen or gas mixtures containing oxygen (including air) are suitable vapor- phase oxidants, with air usually being preferred for obvious reasons of economy. When a vapor-phase oxidant is identified as containing or comprising a particular gas or vapor, this means a vapor-phase oxidant in which the identified gas or vapor is the sole, predominant or at least a significant oxidizer of the parent metal under the conditions obtained in the oxidizing environment utilized. For example, although the major constituent of air is nitrogen, the oxygen content of air is normally the sole oxidizer of the parent metal under the conditions obtained in the oxidizing environment utilized. Air therefore falls within the definition of an "oxygen-containing gas" oxidant but not within the definition of a "nitrogen-containing gas" oxidant. An example of a "nitrogen-containing gas" oxidant as used herein is "forming gas", which typically contains about 96 volume percent nitrogen and about 4 volume percent hydrogen. Nitrogen is a particularly preferred vapor-phase oxidant for producing the porous aluminum nitride matrix composite in which the parent metal comprises finely divided aluminum and which is admixed with the ceramic filler of the permeable mass. Argon gas has been found to be a useful inert gas for reducing the partial pressure and activity of the nitrogen vapor-phase oxidant. An oxidant which is liquid or solid at the process conditions may be employed in conjunction with, or independently from, the vapor-phase oxidant. Such additional oxidants may be particularly useful in enhancing oxidation of the parent metal preferentially within the permeable mass, rather than beyond its surfaces. That is, the use of such liquid or solid oxidants may create an environment within the permeable mass more favorable to the oxidation kinetics of the parent metal than the environment outside the permeable mass. This enhanced environment is beneficial in promoting matrix development within the permeable mass to the boundary and minimizing overgrowth. When a solid oxidant is employed, it may be dispersed through the entire permeable mass or through a portion of the permeable mass, such as in particulate form and admixed with the permeable mass, or it may be utilized as coatings on the permeable mass particles. Any suitable solid oxidant may be employed depending upon its compatibility with the vapor-phase oxidant. Such solid oxidants may include suitable elements, such as boron or carbon, or suitable reducible compounds, such as silicon dioxide (as a source of oxygen) or silicon nitride (as a source of nitrogen) or certain borides of lower thermodynamic stability than the oxidation reaction product of the parent metal . In some instances, the oxidation reaction of the parent metal may proceed so rapidly with a solid oxidant that the oxidation reaction product tends to fuse due to the exothermic nature of the process. This occurrence can degrade the microstructural uniformity of the resulting ceramic matrix or ceramic matrix composite body. This rapid exothermic reaction can be ameliorated by mixing into the composition relatively inert fillers which absorb the excess heat. An example of such a suitable inert filler is one which is identical, or substantially identical, to the intended oxidation reaction product. If a liquid oxidant is employed, the liquid oxidant may be dispersed throughout the entire permeable mass or a portion thereof adjacent to the molten metal, provided such liquid oxidant does not prevent access of the vapor-phase oxidant (when utilized) to the molten parent metal. Reference to a liquid oxidant means one which is a liquid under the oxidation reaction conditions, and so a liquid oxidant may have a solid precursor, such as a salt, which is molten or liquid at the oxidation reaction conditions. Alternatively, the liquid oxidant may be a liquid precursor, e.g. a solution of a material, which is used to coat part or all of the porous surfaces of the permeable mass and which is melted or decomposed at the process conditions to provide a suitable oxidant moiety. Examples of liquid oxidants as herein defined include low melting glasses. In the embodiment in which the parent metal is distributed throughout the permeable mass as finely divided bodies, the permeable mass is charged to a furnace, supplied with an oxidant, and elevated to the appropriate temperature interval to effect the conversion from a moldable, pliable preform to a self-supporting strucmral component. Depending upon the reactivity of the filler relative to the parent metal, the heating cycle may vary. In the preferred embodiment of this invention, the permeable mass is charged to a furnace which has been preheated to the reaction temperamre. If dopants are utilized, they may be included in the permeable mass or alloyed into the parent metal, or both. The parent metal is melted, preferably without complete loss of the dimensional integrity of the permeable mass, but the temperamre is kept below the melting point of the oxidation reaction product and the filler. The molten parent metal reacts with the oxidant to form oxidation reaction product. The porosity of the permeable mass is sufficient to accommodate the oxidation reaction product without substantially disturbing or displacing the boundaries of the permeable mass. Continued exposure of the molten parent metal to the oxidizing environment induces transport of molten metal through the oxidation reaction product, progressively drawing molten metal into and through the oxidation reaction product towards the oxidant, causing progressive growth of the polycrystalline oxidation reaction product. The oxidation reaction product grows into interstitial spaces of the permeable mass. Concurrently, voids are created upon migration or transport of the molten metal, these voids tending to replicate the size and shape of the original finely divided parent metal bodies. If the volume percent of parent metal is too low, the resulting structure may be weak relative to a body formed utilizing a greater volume percent of metal. On the other hand, excessive parent metal may be undesirable, in that the final product may contain too much metal for the end-use application. The initial growth of oxidation reaction product from the molten metal fills at least a portion of the inter-particle pores of the permeable mass and creates voids, as noted above. Continuing the oxidation reaction process promotes the continual migration of residual molten metal outwardly through the oxidation reaction product. As in the case of bulk metal contacting an exterior surface of a permeable mass, oxidation continues until either the parent metal or oxidant is exhausted, the temperature is altered to be outside of the oxidation processing "window", or the developing reaction product contacts a barrier material. It should be understood that the resulting polycrystalline material may exhibit porosity which may be a partial or nearly complete replacement of the metal phase(s), but the volume percent of voids will depend largely on such conditions as temperature, time, type of parent metal, and dopant concentrations. Typically in these polycrystalline ceramic structures, the oxidation reaction product crystallites are interconnected in more than one dimension, preferably in three dimensions, and the metal may be at least partially interconnected. Because of the barrier means, the ceramic product has generally well-defined boundaries regardless of the metal volume content or porosity. The barrier means of this invention may be any suitable means which interferes, inhibits, or terminates growth or development of the oxidation reaction product. Suitable barrier means may be any material, compound, element, composition, or the like, which, under the process conditions of this invention, maintains some integrity, is not volatile and preferably is permeable to the vapor-phase oxidant while being capable of locally inhibiting, poisoning, stopping, interfering with, preventing, or the like, continued growth of the oxidation reaction product. It appears that one category of barrier means is that class of materials which is substantially non- wettable by the transported molten parent metal. A barrier of this type exhibits substantially no affinity for the molten metal, and growth is terminated or inhibited by the barrier means. Other barriers tend to react with the transported molten parent metal to inhibit further growth either by dissolving into and diluting the transported metal excessively or by forming solid reaction products, e.g. intermetallics, which obstruct the molten metal transport process. A barrier of this type may be a metal or metal alloy, including any suitable precursor thereto such as an oxide or a reducible metal compound, or a dense ceramic. Because of the nature of the growth inhibition or obstruction process with this type of barrier, growth may extend into or somewhat beyond the barrier before growth is terminated. Nevertheless, the barrier reduces any final machining or grinding that may be required of the product. As stated above, the barrier should preferably be permeable or porous, and therefore, when a solid, impermeable wall is used, the barrier should be opened in at least one zone or at one or both ends to permit the vapor-phase oxidant (when utilized) to contact the molten parent metal. Suitable barriers particularly useful in this invention, in the case of using aluminum parent metals, are, for example, calcium sulfate and calcium silicate, which are essentially non-wettable by the transported molten parent metal; and in the case of using silicon parent metals suitable barriers include, for example, boron nitride, titanium nitride, zirconium nitride and aluminum nitride, with boron nitride being the most preferred. Such barriers typically may be applied as a slurry or paste to the surfaces of a filler bed which preferably is preshaped as a preform. The barrier means also may include a suitable combustible or volatile material that is eliminated on heating, or a material which decomposes on heating, in order to increase the porosity and permeability of the barrier means. Still further, the barrier means may include a suitable refractory particulate to reduce any possible shrinkage or cracking which otherwise may occur during the process. Such a particulate having substantially the same coefficient of expansion as that of the filler bed is especially desirable. For example, if the preform comprises alumina and the resulting ceramic comprises alumina, the barrier may be admixed with alumina particulate, desirably having a mesh size of about 20-1000. The alumina particulate may be mixed with calcium sulfate, for example, in a ratio ranging from about 10: 1 to 1: 10, with the preferred ratio being about 1 : 1. In one preferred embodiment of the invention, the barrier means includes an admixture of calcium sulfate (i.e. Plaster of Paris) and portland cement. The portland cement may be mixed with the Plaster of Paris in a ratio of 10: 1 to 1 : 10, with the preferred ratio of portland cement to Plaster of Paris being about 1 :3. Where desired, portland cement may be used alone as the barrier material. Another preferred embodiment, when using aluminum parent metals, comprises Plaster of Paris admixed with silica in a stoichiometric amount, but there can be an excess of Plaster of Paris. During processing, the Plaster of Paris and silica react to form calcium silicate, which results in a particularly beneficial barrier in that it is substantially free of fissures. In still another embodiment, the Plaster of Paris is admixed with about 25-40 weight percent calcium carbonate. On heating, the calcium carbonate decomposes emitting carbon dioxide, thereby enhancing the porosity of the barrier means. The barrier means may be manufactured or produced in any suitable form, size, and shape, and preferably is permeable to the vapor-phase oxidant. The barrier means may be applied or utilized as a film, paste, slurry, pervious or impervious sheet or plate, or a reticulated or foraminous web such as a metal or ceramic screen or cloth, or a combination thereof. The barrier means also may comprise some filler and/or binder. In order to achieve minimal or no polycrystalline material overgrowth in the formation of ceramic composites, the barrier means may be placed on, or positioned in close proximity to, the defined surface boundary of any filler bed or preform. Disposal of the barrier means on the defined surface boundary of the bed or preform may be performed by any suitable means, such as by layering the defined surface boundary with the barrier means. Such layer of barrier means may be applied by painting, dipping, silk screening, evaporating, or otherwise applying the barrier means in liquid, slurry, or paste form, or by sputtering a vaporizable barrier means, or by simply depositing a layer of a solid particulate barrier means, or by applying a solid thin sheet or film of barrier means onto the defined surface boundary. With the barrier means in place, growth of the polycrystalline oxidation reaction product terminates upon reaching the defined surface boundary of the preform and contacting the barrier means. In a preferred embodiment of the present invention. a permeable shaped preform (described below in greater detail) is formed having at least one defined surface boundary with at least a portion of the defined surface boundary having, or superimposed with, the barrier means. It is understood that the term "preform" may include an assembly of separate preforms ultimately bonded into an integral composite. The permeable preform is part of the lay-up, and upon heating in a furnace, the parent metal and the preform may be exposed to or enveloped by the vapor phase oxidant, which may be used in combination with a solid or a liquid oxidant. The reaction process is continued until the oxidation reaction product has infiltrated the preform and comes in contact with the defined surface boundary having, or superimposed with, the barrier means. Most typically, the boundaries of the preform, and of the polycrystalline matrix, substantially coincide; but individual constiments at the surfaces of the preform may be exposed or may protrude from the matrix, and therefore infiltration and embedment may not be complete in terms of completely surrounding or encapsulating the preform by the matrix. The barrier means prevents, inhibits or terminates growth upon contact with the barrier means, and substantially no "overgrowth" of the polycrystalline material occurs. The resulting ceramic composite product includes a permeable mass infiltrated to its boundaries by a ceramic matrix comprising a polycrystalline material consisting essentially of the oxidation reaction product of the parent metal with an oxidant and, optionally, one or more metallic constituents such as non-oxidized constiments of the parent metal or reduced constiments of any reducible substances, or both. It further should be understood that voids are developed by a partial or essentially complete displacement of the finely divided parent metal, but the volume percent of voids will depend largely on such conditions as temperamre, time, type of parent metal, volume fraction of parent metal, and dopant concentrations. Typically, some voids are completely isolated (closed), while others may be part of a network which opens to the exterior of the body. Typically in these polycrystalline ceramic structures, the oxidation reaction product crystallites are interconnected in more than one dimension, preferably in three dimensions, and the metallic constituents resulting from the transport of molten parent metal may be at least partially interconnected. The ceramic composite product of this invention has generally well-defined boundaries and possesses the approximate dimensions and geometric configuration of the original permeable mass. The polycrystalline ceramic composite may comprise metallic constituents such as non-oxidized parent metal, the amount depending largely on such factors as process conditions, alloying constiments in the parent metal, and dopants, although in certain cases it may contain substantially no metal. The volume percent metal may be tailored to meet the desired end-use properties for the product, and for several applications, such as engine components, it may be preferred to have a metal content in the finished component of about 5-10 percent or less. In accordance with this preferred embodiment, it will be observed that the filler is essentially nonreactive with the parent metal under the process conditions. Further, although the invention is described herein with particular emphasis to aluminum and specific embodiments of aluminum parent metal, this reference is for illustration puφoses only, and it is to be understood that other metals can be doped to meet, the criteria of this invention. Parent metals suitable for incoφoration into the thin forms of the present invention may comprise at least one of relatively pure metals, commercially available metals with impurities and/or alloying constituents, alloys in which the metal precursor is the major constituent, and intermetallic compounds of the metals. Parent metals suitable for the practice of the present invention may comprise metals and alloys of metals such as, for example, aluminum, titanium, silicon, zirconium, hafnium, tin, molybdenum, etc. For the puφoses of the present invention, the term "metal" as used herein is intended to include, for example, metalloids or semimetallic substances such as silicon and the like. In one embodiment of the present invention, for example, where the parent metal oxidizes to form an oxidation reaction product, parent metals may be of a suitable size and geometry so as to form voids upon oxidation which inversely replicate the parent metal shape, thereby contributing to, for example, the desired thermal or structural properties. Parent metals may comprise at least one form including, for example, metal sheets, wires, whiskers, particulate, spheres, finely divided metal powders, and the like. In a preferred embodiment of the present invention, the parent metal comprises a finely divided parent metal capable of forming an oxidation reaction product, such as, for example, aluminum powder. The finely divided parent metal bodies incoφorated into at least a portion of the permeable mass should be of a suitable size so as to form voids by inverse replication upon transport of the metal. Ideally, the voids are sufficiently numerous so as to enhance the ceramic properties, but yet not be so large or numerous as to detrimentally impact the structural integrity of the product. Further, metal particulate which is too finely divided poses a high safety risk in that the large surface area-to-volume ratio of such particulate poses an explosion hazard should they become airborne in sufficient concentrations. Thus, a particle size for the parent metal of about 50 to 500 grit (500 to 171 microns), preferably about 100 to 280 grit (173 to 40 microns), is useful. The terms "particulate" or "particle" with respect to the filler is used broadly to include powders, fibers, whiskers, spheres, platelets, agglomerates, and the like. Certain parent metals under specific conditions of temperamre and oxidizing atmosphere meet the criteria necessary for the oxidation phenomenon of the present invention with no special additions or modifications. However, as described in the aforesaid Commonly Owned Ceramic Matrix Patents and Patent Application, dopant materials used in combination with the parent metal can favorably influence or promote the oxidation reaction process. While not wishing to be bound by any particular theory or explanation of the function of the dopants, it appears that some of them are useful in those cases where appropriate surface energy relationships between the parent metal and its oxidation reaction product do not intrinsically exist. Thus, certain dopants or combinations of dopants, which reduce the solid-liquid interfacial energy, will tend to promote or accelerate the development of the polycrystalline strucmre formed upon oxidation of the metal into one containing channels for molten metal transport, as required for the process of the present invention. Another function of the dopant materials may be to initiate the ceramic growth phenomenon, apparently either by serving as a nucleating agent for the formation of stable oxidation product crystallites. or by disrupting an initially passive oxidation product layer in some fashion, or both. This latter class of dopants may not be necessary to create the ceramic growth phenomenon of the present invention, but such dopants may be important in reducing any incubation period for the initiation of such growth to within commercially practical limits for certain parent metal systems. Still another function of dopants may be to control the rate of formation of oxidation reaction product. For example, in addition to the above-described dopants, certain dopants may be utilized to accelerate or decelerate the rate of the oxidation reaction, thus improving, for example, moφhology and/or uniformity of the product. Those dopants may assist in obtaining net or near net shapes. The function or functions of the dopant material can depend upon a number of factors other than the dopant material itself. These factors include, for example, the particular parent metal utilized, the end product desired, the particular combination of dopants when two or more dopants are used, the use of an externally applied dopant in combination with an alloyed dopant, the concentration of the dopant, the oxidizing environment utilized, and the process conditions. The dopant or dopants (1) may be provided as alloying constiments of the parent metal, (2) may be applied to at least a portion of the surface of the parent metal, or (3) may be applied to or supplied by the filler or a part of the filler bed, or any combination of two or more of techniques (1), (2) and (3) may be employed. For example, an alloyed dopant may be used in combination with an externally applied dopant. In the case of technique (3), where a dopant or dopants are applied to the filler, the application may be accomplished in any suitable manner, such as by dispersing the dopants throughout part or the entire mass of filler in fine-droplet or particulate form, preferably in a portion of the bed of filler adjacent the parent metal. Application of any of the dopants to the filler may also be accomplished by applying a layer of one or more dopant materials to and within the bed, including any of its internal openings, interstices, passageways, intervening spaces, or the like, that render it permeable. A source of the dopant may also be provided by placing a rigid body containing the dopant in contact with and between at least a portion of the parent metal surface and the filler bed. For example, if a silicon dopant is required, a thin sheet of silicon-containing glass or other material can be placed upon a surface of the parent metal. When the parent metal overlaid with the silicon-containing material is melted in an oxidizing environment (e.g. , in the case of aluminum in air, between about 850°C to about 1450°C, preferably about 900°C to about 1350°C), growth of the polycrystalline ceramic material into the permeable filler occurs. In the case where the dopant is externally applied to at least a portion of the surface of the parent metal, the polycrystalline oxide structure generally grows into the permeable filler substantially beyond the dopant layer (i.e., to beyond the depth of the applied dopant layer). In any case, one or more of the dopants may be externally applied to the parent metal surface and/or to the permeable bed of filler. Additionally, dopants alloyed within the parent metal and/or externally applied to the parent metal may be supplemented by dopant(s) applied to the filler bed. Thus, any concentration deficiencies of the dopants alloyed within the parent metal and/or externally applied to the parent metal may be supplemented by additional quantities of the respective dopant(s) applied to the filler bed, and vice versa. Other dopants which are effective in promoting polycrystalline oxidation reaction product growth, for aluminum-based parent metal systems are, for example, silicon, germanium, tin and lead, especially when used in combination with magnesium or zinc. One or more of these other dopants, or a suitable source of them, may be alloyed into the aluminum parent metal system at concentrations for each of from about 0.5 to about 15% by weight of the total alloy; however, more desirable growth kinetics and growth moφhology are obtained with dopant concentrations in the range of from about 1-10% by weight of the total parent metal alloy. Lead as a dopant is generally alloyed into the aluminum-based parent metal at a temperature of at least 1000°C so as to make allowances for its low solubility in aluminum; however, the addition of other alloying components, such as tin. will generally increase the solubility of lead and allow the alloying materials to be added at a lower temperamre. One or more dopants may be used depending upon the circumstances, as explained above. For example, in a system wherein the parent metal comprises aluminum and the oxidant comprises air, particularly useful combinations of dopants include (a) magnesium and silicon or (b) magnesium, zinc and silicon. In such examples, a preferred magnesium concentration falls within the range of from about 0.1 to about 3 % by weight of the aluminum parent metal, for zinc in the range of from about 1 to about 6% by weight of the aluminum parent metal, and for silicon in the range of from about 1 to about 10% by weight of the aluminum parent metal. The concentration range for any one dopant will depend on such factors as the combination of dopants and the process temperamre. Concentrations within the above-mentioned range appear to initiate oxidation reaction product growth, enhance metal transport and favorably influence the growth moφhology of the resulting oxidation reaction product. Additional examples of dopant materials, useful with an aluminum parent metal, include sodium, lithium, calcium, boron, phosphorus, yttrium, barium, strontium, zirconium, gallium, lanthanum, titanium, chromium, cerium and nickel, which may be used individually or in combination with one or more other dopants depending on the oxidant and process conditions. Sodium and lithium may be used in very small amounts in the parts per million range, typically about 100-200 parts per million, and each may be used alone or together, or in combination with other dopant(s). Rare earth elements such as cerium, lanthanum, praseodymium, neodymium and samarium are also useful dopants, and herein again especially when used in combination with other dopants. The precise function of the dopant(s) may vary depending upon which process conditions are used, which parent metal is used, which oxidant is used, other dopants that may be present, etc. For example, it is possible that under one set of conditions a certain dopant may tend to initiate growth, but under a different set of conditions the same dopant may tend to control the rate of formation of oxidation reaction product. Thus, it may be difficult to categorize completely the function of any one particular dopant. Liquid or fusible compositions suitable for the practice of the present invention include at least one inorganic or organic monomeric, oligomeric or polymeric composition. In a preferred embodiment, the at least one liquid or fusible composition comprises at least one silicon-containing compound, such as, for example, tetra ethyl ortho silicate. In a further preferred embodiment, the at least one liquid or fusible composition may comprise at least one polymer, and in a particularly preferred embodiment the at least one polymer comprises a preceramic polymer composition, such as, for example, polysiloxanes, polycarbosilanes, polysilazanes. and polyureasilazanes. Where the composition comprises a solid fusible composition, the composition may be meltable, or the composition may be dissolved or dispersed in, for example, an aqueous or organic solution before contacting the thin form. It was suφrisingly discovered that where the at least one liquid or fusible composition comprises a preceramic composition, favorable properties are imparted whereby the liquid or fusible composition facilitates the formation of the oxidation reaction product, thus obviating the need for additional compositions such as conventional dopants. Particularly preferred are liquid or fusible compositions which contribute, for example, both to shape formation and to the oxidation of the parent metal, and include, for example, silicon-containing preceramic polymer compositions. It was further suφrisingly discovered that the use of a dopant in the thin form in combination with the liquid or fusible composition such as a silicon-containing preceramic polymer liquid or fusible compositions, favorable properties are imparted which enhance the oxidation reaction, thereby creating a stronger, more coherent body than was obtained when using only a dopant without the presence of the liquid or fusible composition. Although not wishing to be bound by theory, it is believed that where no conventional dopants are provided, and a silicon-containing liquid or fusible composition is provided, the liquid or fusible composition exhibits at least some doping characteristics, thereby enhancing the formation of the oxidation reaction product. It is further believed that the liquid or fusible composition may enhance the oxidation reaction product formation by enhancing the wetting character of the surface of the at least one fiber and inorganic filler material, and facilitating formation of the oxidation reaction product on or near the surface of the at least one fiber or filler material. Fibers suitable for the practice of the present invention comprise at least one fiber selected from the group consisting of organic and inorganic fibers. Organic fibers may include, for example, pulp such as hardwood or softwood pulp, polymeric pulp such as polyolefin pulp, cotton fibers, carbon fibers, and the like. Inorganic fibers may comprise, for example, ceramic, glass, and metal fibers. Fibers may be present as, for example, individual fibers, fiber mats, weaves, mesh, uniaxially arranged fiber tows. chopped fibers, and the like. In a particularly preferred embodiment, the inorganic fibers may be coated with at least one debond layer so as to provide for one or more ceramic toughening mechanisms such as crack deflection and fiber debonding/pullout during mechanical stressing of the ceramic material. In another embodiment for example, where at least some of the fibers comprise organic fibers, fibers may be added with specific dimensions and in specific quantity to achieve a desired level of porosity by burning out at least a portion of said fibers when firing the thin form, for example, during oxidation of the parent metal. Some inorganic fibers which are desirable for use in the present thin ceramic bodies are inherently reactive with one or more species under the local processing conditions, but it is the parent metal, especially an aluminum parent metal, which typically poses the greatest problem with respect to chemical attack of such fibers. The use of such fiber in the presence of molten aluminum typically necessitates a coating resistant to the corrosive effects of aluminum if survival of the fiber is desired. Experience with directing the oxidation of a molten parent metal such as aluminum into an adjacent permeable mass has shown that while CVD SiC coatings, for example, can be adequately protective of a substrate fiber, other coating processes, such as treating the fiber in a bath of colloidal alumina, are less efficacious. It has been discovered, however, that when directed metal oxidation is conducted "in-situ"; that is, in the form wherein the parent metal is in the form of a plurality of small bodies or a reticulated strucmre distributed throughout the permeable mass, the propensity of the parent metal to corrode such "reactive" fillers is much less. Thus, the high volume, low cost coating techniques, such as those taught by U.S. Patent No. 5, 165,996, to Jacobson, the entire disclosure of which is herein incoφorated by reference, are sufficient under the "in-situ directed metal oxidation" conditions to protect fillers which would otherwise react with molten parent metal. A room temperamre strength determination of the ceramic material produced in accordance with Example 22, below, exhibited a degree of toughening behavior, e.g. , fiber debonding and pull-out from the matrix. This result suggests that through intentional application of debond coatings, the amount and onset of toughening through the debond/pull-out mechanism might be tailorable. Further, it should be possible to achieve both debonding and protection of otherwise reactive or unstable inorganic fibers in the content of the present invention. For example, U.S. Patent 5,389,450, to Kennedy et al., teaches a duplex coating system for reinforcement materials used in composite materials. According to this Commonly Owned U.S. Patent, whose disclosure is incoφorated herein by reference, one or more outer coatings provide protection from chemical degradation of the underlying substrate, and one or more debond coatings are supplied between the substrate and the protective coating(s) to permit pull-out of the reinforcement to occur under mechanical load. Filler materials suitable for the process of the present invention include, for example, at least one of glass, ceramic, metal and carbon filler materials. Ceramic filler materials may comprise, for example, at least one of borides, oxides, carbides, and nitrides. Examples of ceramic fillers suitable for this invention include, for example, clays, oxide ceramics and non-oxide ceramics, such as alumina, silicon carbide, silicon nitride, boron carbide, zirconium oxide, zirconium boride, zirconium nitride, titanium oxide, and the like. Metal filler materials may include metals and metalloids of IUPAC Groups 1 through 12, the lanthanide series metals, metal and metalloids of IUPAC groups 13 and 14, including boron and silicon. Filler materials may comprise at least one geometry such as, for example, powders, fibers, fiber mats, wires, whiskers, flakes, platelets, bubbles, spheres, and particulates. The at least one filler material may further comprise either reactive or unreactive filler materials. In one embodiment, for example, where a strong, thin ceramic body is desired, wire, such as tungsten wire, may be added to the thin form to provide additional structural support. In an embodiment where, for example, paper-making techniques are used in the preparation of thin forms, ceramics in the form of, for example, ceramic powders are particularly preferred filler materials. Further, where thin forms are provided by paper-making techniques, additional filler materials such as cationic wet strength resin (for strength in sheet formation) and anionic polymers (for floe formation) may be added. In a further embodiment, where a dense ceramic product is desired, a filler material comprising a sintering aid may be added to the mixmre. The sintering aid used will depend upon which ceramic filler material is used. For example, alumina and yttria may comprise preferable sintering aids for silicon nitride powder-containing forms, and CaO, MgO, and Siθ2 are often used with alumina powder. Thin forms provided by the process of the present invention which have been contacted with at least one liquid or fusible composition are typically stronger and more flexible than green bodies formed by prior art methods, particularly when wet. Thin forms which have been, for example, coated or impregnated with a preceramic polymer can be stored in an unfired state. Stored thin forms may be subsequently formed into single layer complex shapes, for example, by cutting or molding, or the thin forms may be stacked or wound to build shapes such as laminates, tubes or honeycombs utilizing either a continuous individual thin form or a plurality of thin forms. Where a liquid or fusible composition such as a polymer is applied to a thin form to create a tacky surface, multilayer strucmres may be prepared from stacking thin forms without the requirement of a separate step for applying an adhesive, which is often required by prior art methods. In a further embodiment, thin forms of the present invention may be stacked with other layers of the same composition to form homogeneous laminate structures or with different compositions which may be reactive, non-reactive or partially reactive with the thin bodies, to form heterogeneous laminate strucmres comprising layers of varying compositions. For example, thin forms of the present invention may be alternated with, for example, other thin form compositions prepared by the process of the present invention, or fiber tows or pliable weave or mesh that may, for example, subsequently at least partially burn out at higher temperamres to create multiple layered complex-shaped, three-dimensional structural compositions with layers of varying compositions or porosities, or internal chambers or channels where all or part of certain compositions have burned out. Still further, certain mechanized paper-making techniques, such as for example, those whereby the pulp is poured onto a moving belt and de-watered, tend to impart a directionality to one or more properties of the paper product. This directionality can survive the timing operation. One manifestation of this directionality is the at-least partial alignment of the fiber component of the thin form in the direction of travel of the paper pulp or wet paper. In another example, thin forms may be alternated with metal tows which may be oxidized in situ and may infiltrate into the thin form layers creating increased strength. Stacked thin forms may be heated to a temperature sufficient to initiate metal oxidation, thereby bonding contiguous layers and providing a strong bond between layers. Further, barrier materials may be selectively placed between the layers to inhibit oxidation growth at specific sites. In another embodiment, a spacer or stand-off may be provided between layers to create supports or channels for the transport of air or liquid between the layers of the thin ceramic bodies. The material making up the spacer or stand-off may include any material which provides sufficient spacing between layers and which is compatible with the formation process, including for example, ceramics, metals, polymers, or glass, and may be incoφorated between layers by, for example, contacting with a tacky liquid or fusible composition. Alternatively, the spacer may be bonded by the formation of oxidation reaction product between the thin form layers and the spacing materials. In another embodiment of the present invention, thin ceramic bodies may be formed by seaming or joining or bonding one or more thin forms to provide complex shapes. Specifically, the thin forms of the present invention may be contacted to one another along overlapped or a common edges. In one embodiment, the overlapping edges of at least one thin form may be held together by the formation of a tacky surface upon contacting with at least one liquid or fusible compositions. Upon firing to a temperature sufficient to initiate oxidation of the parent metal, the resulting thin ceramic bodies may be bonded to one another along the common seam. Moreover, the developing oxidation reaction product is capable of bridging the interface between the two bodies, thereby adding to the strength of the bond between the bodies above and beyond that which is due to the bonding action of the liquid or fusible composition. The following examples are intended to be nonlimiting illustrations of some of the embodiments of the present invention. Example 1 The following example demonstrates the formation of a thin ceramic body comprising alumina and silicon carbide, from a mixmre comprising an aluminum alloy powder (380M alloy comprising approximately 8.5-7.5 wt% Si, 4.0-3.0 wt% Cu, 3.5- 2.7 wt% Zn, 0.30-0.20 wt% Mg, 1.0-0.7 wt% Fe, balance aluminum (Alcan, Montreal, Canada), silicon carbide (Norton Company, Worcester, MA), and hardwood pulp (James River, Richmond, VA). An aqueous mixture was prepared comprising aluminum alloy powder (about 220 grams of 380M alloy), silicon carbide (about 99 grams of 220 grit, about 55 grams of 500 grit, about 33 grams of 800 grit, and about 33 grams of 1200 grit), and hardwood pulp (about 2800 grams of 10% by weight wood pulp in water) mixed with water to a volume of about 16 liters. One liter of the resulting slurry was diluted with approximately 1 liter of water and agitated. Approximately 30 mis of a 0.1 % solution of Reten® 235 anionic synthetic resin (Hercules, Inc. , Wilmington, DE) and about 3 mis of Kymene® 557H cationic synthetic resin (Hercules, Inc., Wilmington, DE) was added to the slurry to cause floe formation. The slurry was flocced and poured into a handsheet-former. Water was drained and a vacuum was applied to further remove water, leaving a thin structure. The thin strucmre was placed between two pieces of felt, placed through a wringer to squeeze excess water from the structure, and removed from the screen. The thin structure was dried at about 250 °F for about 3 to 4 minutes and heat pressed to dry to completion. Dried thin strucmres were contacted with the liquid composition comprising CERASET™ SN inorganic polymer (a silazane-based preceramic polymer from Lanxide Performance Materials, Inc. , Newark, DE) and further comprising about 1 % of dicumyl peroxide (Hercules, Inc. , Wilmington, DE) as a curing agent, by brushing or dipping the structures. Excess liquid polymer mixture was blotted from the surface of the thin strucmre. The coated structure was placed in a heated mold (in air and at a temperature of about 150°C) to obtain the desire contour and to achieve crosslinking of the liquid polymer mixture to form a rigid structure. The structure was further heated by firing in air to about 950°C for about 2 to 5 hours to burn out the wood pulp and to convert the thin structure to a ceramic. Examples 2-12 The following Examples 2-12 demonstrate the formation of thin ceramic bodies comprising alumina and silicon carbide made substantially according to the method of Example 1. In Examples 2-12, the compositions of materials such as metal/alloy, fiber and filler were varied as noted in Table 1 to demonstrate the ability to modify the compositions of the thin forms made by the processes of the present invention. The following materials were used in the formation of the thin ceramic bodies of Examples 2-12: Aluminum alloy powder (380M alloy comprising approximately 8.5-7.5 wt% Si, 4.0-3.0 wt% Cu, 3.5-2.7 wt% Zn, 0.30-0.20 wt% Mg, 1.0-0.7 wt% Fe, balance aluminum, Alcan, Montreal, Canada), aluminum alloy powder (comprising 3 % Sr, 1 % Si, 4% Ni, balance aluminum; Valimet, Inc. , Stockton, CA), aluminum powder (99.97% pure aluminum, grade 120L, Alcoa Specialty Metals, Rockdale, TX), silicon carbide (all grit sizes, Norton Company, Worcester, MA), hardwood and softwood pulp (James River, Richmond, VA), KAOWOOL® fibers (chopped fiber blanket, Thermal Ceramics, Augusta, GA), FIBERFRAX® KMTX (chopped fiber, Carborundum, Niagara Falls, NY), Reten® 235 anionic synthetic resin (Hercules, Inc. , Wilmington, DE), Kymene® 557H cationic synthetic resin (Hercules, Inc. , Wilmington, DE). VICTAWET® 12 (Dentsply Int'l Inc. , Ransom and Randolph, Maumee, OH), and CERASET™ SN inorganic polymer (sold by Lanxide Performance Materials, Inc. , Newark, DE). TABLE 1 Example Metal/ Allov Fiber Filler 2 660g 380M 2670g hardwood pulp 2 300g 220 grit SiC aluminum alloy 1 360g 500 grit SiC 3 660g 380M 2670g hardwood pulp 660g 1000 grit SiC aluminum alloy 4 660g 380M 2670g hardwood pulp 660g 500 grit SiC aluminum alloy 5 660g 2670g hardwood pulp 660g 1000 grit SiC aluminum alloy 3 6 660g 380M 1335g hardwood pulp 660g 500 grit SiC aluminum alloy 1139g KAOWOOL® fibers 7 660g 380M l lOOg 1 : 1 hardwood 660g 500 grit SiC aluminum alloy pulp: softwood pulp 2 227g FIBERFRAX® KMT 8 660g 380M 1500g 1 : 1 hardwood 660g 500 grit SiC aluminum alloy pulp: softwood pulp 9 75g 380M 56g 1 : 1 hardwood aluminum alloy pulp: softwood pulp 10 65g 380M 56g 1 : 1 hardwood lOg 500 grit SiC aluminum alloy pulp: softwood pulp 11 55g 380M 56g 1 : 1 hardwood 20g 500 grit SiC aluminum alloy pulp: softwood pulp 12 660g aluminum 2000g softwood pulp 2 660g 500 grit SiC metal powder 4 10 mis VICTAWET® 12* 380M Aluminum alloy (Alcan, Montreal, Canada) Hardwood pulp, hardwood pulp:softwood pulp mixtures, and softwood pulp are of a 10% by weight wood pulp in water. Aluminum alloy (Valimet, Inc., Stockton, CA) Aluminum metal powder (Alcoa Specialty Metals, Rockdale, TX) * Example 12: 51 mis of Kymene® 557H was added to the slurry after the addition of aluminum metal powder, prior to diluting 1 liter of slurry with 1 liter of water. Examples 13-14 The following Examples 13-14 demonstrate the formation of thin ceramic bodies comprising alumina and silicon carbide made substantially according to the method of Example 12. In Examples 12, 13 and 14, the volume of slurry which was diluted with 1 liter of water was varied as indicated in Table 2 to demonstrate the ability to effect changes in thin ceramic body thickness. Additionally, the amount of Reten® 235 anionic synthetic resin (Hercules, Inc. , Wilmington, DE) which was added to the slurry for flocculating was varied as indicated in Table 2. The following materials were used in the formation of the thin ceramic bodies of Examples 13-14: Aluminum metal (99.97% pure, grade 120L, Alcoa Specialty Metals. Rockdale, TX), softwood pulp (James River, Richmond, VA), 500 grit silicon carbide (Norton Company, Worcester, MA), Reten® 235 anionic synthetic resin (Hercules. Inc. , Wilmington, DE), Kymene® 557H cationic synthetic resin (Hercules, Inc. , Wilmington, DE), VICTAWET® 12 (Dentsply Int Inc. , Ransom and Randolph, Maumee, OH), and CERASET™ SN inorganic polymer (sold by Lanxide Performance Materials, Inc. , Newark, DE). TABLE 2 Volume of Slurry Thickness Diluted with Amount of of Thin Example 1 Liter of Water Reten-^ 235 Ceramic Bodv 12 1 liter 60 mis 0.058" 13 900 mis 40 mis 0.050" 14 500 mis 30 mis 0.032" Examples 15-18 The following Examples 15-18 demonstrate among other features (1) that the absence of a dopant or initiator material to initiate the directed metal oxidation process fails to result in a three-dimensionally interconnected alumina matrix in the final fired body; and (2) that CERASET™ SN inorganic polymer functions as such a dopant under the local process conditions. Four thin ceramic bodies were prepared substantially in accordance with the procedure outlined in Example 1. Further, the ratios of the raw materials were substantially as described in Example 1; however, where the specific raw materials 1 1 ' 9^ deviated from those used in Example 1 , the specific raw material actually used are specified in Table 3. The ceramic bodies of Examples 15 and 17 were not soaked in the CERASET™ SN inorganic polymer prior to firing at a temperature of about 900 °C which was maintained for about 4 hours. Table 3 also reports the initial and final thicknesses of the ceramic bodies as well as a generalized description of the appearance of the bodies upon firing. Monolithic (e.g. , self- supporting) bodies were produced in each instance with the exception of Example 17, which yielded loosely bound aluminum oxide powder. Thus, this series of examples demonstrates that self-supporting ceramic bodies may be produced where the starting compositions contain a dopant such as magnesium or zinc alloyed into the parent metal or where the bodies contain CERASET™ SN inorganic polymer. Thus, under the local process conditions, CERASET™ SN acts substantially as a dopant in initiating directed metal oxidation. This result is significant because it means that the atomized parent metal powder does not have to be custom-made to incoφorate dopant elements: instead, commercially available (e.g. , less expensive) atomized commercially pure aluminum may be used as a parent metal with good results. TABLE 3 CERASET™ SN Parent Polymer Initial Final Example Metal Present? Thickness Thickness Appearance 15 380M No 0.8 mm 2.4 mm Weak mono¬ lithic thick plate 16 380M Yes 0.8 mm 0.6 mm Monolithic thin plate 17 120L No 1.4 mm 8.9 mm Thick powder bed 18 120L Yes 1.4 mm 1.4 mm Monolithic thin plate Paper-making raw materials present in same ratios as in Example 1 2 Filler is 500 grit SiC (Norton Co.) Example 19 The following example demonstrates the formation of a thin ceramic body comprising alumina and silicon carbide, from a mixture comprising an aluminum powder (99.97% pure aluminum, grade 120L, Alcoa Specialty Metals, Rockdale. TX), silicon carbide (Norton Company, Worcester, MA) and softwood and hardwood pulp (James River, Richmond, VA). A mixmre was prepared comprising silicon carbide (about 660 grams of 500 grit), softwood pulp (about 2000 grams of 10% by weight in water) and VICTAWET® 12 (10 mis, Dentsply, Ranson and Randolph, Maumee, OH) blending until well dispersed, then further adding about 51 mis of Kymene® 557H cationic synthetic resin (Hercules, Inc. , Wilmington, DE) mixing until well dispersed, and then blending in aluminum powder (about 660 grams). The mixmre was combined with water to a volume of about 16 liters. Approximately 900 mis of the resulting slurry was diluted with about 1 liter of water and agitated. Approximately 30 mis of a 0.1 % solution of Reten® 235 anionic synthetic resin (Hercules Inc. , Wilmington, DE) was added to the slurry to cause floe formation. The slurry was flocced and poured into a handsheet- former. Water was drained and a vacuum was applied to further remove water leaving a thin strucmre. The thin strucmre was placed between two pieces of felt, placed through a wringer to squeeze excess water from the strucmre. and removed from the screen. The thin strucmre may be air dried or dried at about 250°F for about 3 to 4 minutes and heat pressed to dry to completion. Dried thin structures measuring about 1 inch x 4 inch x 0.055 inches (about 2.54 cm x 10.16 cm x 0.14 cm) were repeatedly contacted with the liquid polymer mixmre comprising about 2 grams of polycarbosilane (Nicalon Polycarbosilane, #X9- 6349, Dow Corning) dissolved in about 10 grams of toluene (Aldrich, Milwaukee, WI), by dipping the strucmres in the solution. The thin structures coated with the solution were shaped and air dried to rigidize the structures. Approximately 1.5 grams of polycarbosilane was absorbed into the structure. The strucmre was further heated by firing in air at about 100°C/hr to about 950°C. holding for about 4 hours to oxidize the aluminum metal and to burn out the wood pulp. Thus, this Example demonstrates, among other things, that silazane-based polymers other than CERASET™ SN inorganic polymer also possess directed metal oxidation dopant qualities under the local processing conditions. Example 20 The present example demonstrates, among other things, the formation of a complex, arch-shaped, rectangular ceramic body comprising two thin rectangular forms sealed along the longitudinal edges and further comprising a cavity between the two rectangular forms, whereby the forms are separated from each other by a spacing material to form the cavity. Two rectangular forms, one having slightly smaller lateral dimension than the other, were cut from a roll of thin form measuring about 0.045 inch thick (1.1 mm) by about 24 inches (61 cm) wide by hundreds of feet (tens of meters) long, then thoroughly dried. This roll of ceramic paper has been produced on a Fourdrinier papermaking machine operated at a belt speed of about 10 feet per minute (3 meters per minute). The proportions of parent metal, wood pulp and ceramic filler were substantially the same as in Example 12. The compositions of these materials were also as the Example 12 materials except that Grade A- 10 alumina (Alcoa Industrial Chemicals Div. , Bauxite, AR, 2-3 microns ave. particle size) was substituted weight for unit weight for the silicon carbide particulate. The Grade 101 L atomized aluminum which was used in the present Example (Alcoa Specialty Metals) had the same chemical composition as the Grade 120L aluminum used in Example 12 but a slightly finer particle size. The size of the batch was based upon an amount of atomized aluminum metal of about 170 kg. Sufficient water was added to this paper-making slurry to adjust the fraction of solids at about 9 percent by weight. The two dried rectangular forms were impregnated with a liquid composition comprising CERASET™ SN inorganic polymer (sold by Lanxide Performance Materials, Inc. , Newark, DE) and 1 % dicumyl peroxide (Hercules, Inc. , Wilmington, DE). Spacing material was prepared comprising approximately 30 wt% metal alloy, approximately 30 wt% of the filler material, and approximately 40 wt% of the liquid composition of Example 4 to produce a paste consistency . The impregnated forms were placed into separate forming molds. An appropriate quantity of the spacing material was placed into regularly spaced depressions in one of the forms, so that upon placing the rectangular forms in the molds the spacing material contacted the concave side of one form and the spacing material contacted the convex side of the form having the smaller lateral dimension. The molds were heated to about 150°C for a time sufficient raise the temperature of the mold to about 150°C, and then held at about 150°C for about 10 minutes to cure the liquid composition, thereby shaping and rigidizing the rectangular forms and adhering the spacing materials thereto. The arch- shaped rectangular form having the smaller lateral dimension was molded in an arch with slightly smaller radius than the other form. The rigid rectangular forms were removed from the mold and stacked with the spacing material positioned between the two sections, forming a cavity. The liquid composition was contacted along the two longitudinal edges of the two rectangular forms to form a seal by heating to about 150°C for a time sufficient to cure the liquid composition. The cured assembly was fired to about 950°C for about 4 hours to oxidize the parent metal and to create a complex, shaped ceramic body. Thus, the present Example demonstrates a number of significant results, including the substitution of alumina for silicon carbide and scaleup of the sheet- forming process from grams to kilograms and from single handmade sheets to semiautomated processing of rolls of the ceramic material many square meters in area. Examples 21-23 The following Examples demonstrate, among other things, the production of thin ceramic forms according to the present invention which incoφorates an inorganic fibrous material, specifically silica. Thin ceramic forms were produced using the ingredients listed in Table 4 with processing substantially in accordance with Example 20. Each slurry was diluted with sufficient water to produce a solids content of about 9 percent by weight. In Examples 22 and 23, the silica fibers were first coated with colloidal alumina (e.g.. boehmite) according to the technique described in the aforementioned U.S. Patent No. 5, 165.996. Each formulation successfully oxidized in-situ to produce a thin, rigid, self-supporting ceramic body. The silica fiber component of Example 21 had reacted substantially with the aluminum parent metal during thermal processing; the coated silica fibers of Examples 22 and 23 reacted to a much lesser degree. TABLE 4 Ingredient Example 21 Example 22 Example 23 pulp 1 12.3 kg 22.8 kg 10.5 kg silica fiber 2 4.56 kg 3 8.21 kg 4 21.7 kg 4 alumina paniculate 5 68.4 kg 171 kg 171 kg aluminum particulate^ 68.4 kg 171 kg 171 kg VICTAWET® 12 7 200 ml 750 ml 1000 ml Kymene® 557H 8 600 g 1500 g 1500 g Reten® 235 9 30 1 91 kg 91 kg 1 softwood pulp (James River Coφ. , Richmond, VA) ^ Silfa (Ametek, Haveg Division, Wilmington, DE) J uncoated 4 coated in accordance with U.S. Patent No. 5,165,996 5 Grade A-10, 2-3 microns ave. particle size (Alcoa Industrial Chemicals Div. , Bauxite AR) 6 Grade 101 L (Alcoa Specialty Metals, Rockdale, TX) -7 Dentsply International, Inc. , Ransom and Randolph, Maunee, OH o ° Hercules, Inc. , Wilmington, DE y Hercules, Inc. , Wilmington, DE Example 24 This Example demonstrates another technique for producing a complex, arch- shaped ceramic body. The techniques described in Example 20 were substantially repeated. Instead of dabbing the spacing material in slurry form between the pairs of thin curved sheets to be bonded, spacers were cut from the same roll of ceramic paper used to produce the arch-shaped sections. The arch-shaped sections and the spacer strips were assembled in the mold and thermally processed substantially as described in Example 20. The complex body produced featured an aligned pair of ceramic plates, each measuring about 65 cm long by about 10 cm wide wherein the width dimension is curved to a radius of about 50 cm. About four strips of spacer material, each measuring about 65 cm long by about 3 mm wide, bonded the plates to each other and provided an approximate 1 mm space between the two. Example 25 This example demonstrates another embodiment for producing an aligned pair of ceramic plates having a cavity therein. Aligned ceramic plates were produced substantially in accordance with Example 24, except that the spacer material, prior to placement on the arch-shaped sections in the mold, was soaked in a slurry having a composition of equal weight fractions of the Example 4 parent metal and the Example 4 filler material and sufficient CERASET™ SN inorganic polymer to give the slurry a viscosity approximately that of house paint. Further, the arch-shaped sections had a width of about 20 cm, and about six of the thin strips of spacer material having varying lengths ranging from about 40 to about 65 cm were employed to support the interior regions of the pair of arch-shaped rectangular forms. Example 26 This example demonstrates another embodiment of the present invention. A roll of ceramic paper was produced substantially in accordance with Example 20 and soaked with the CERASET™ SN inorganic polymer/ 1 % dicumyl peroxide solution. After squeezing out excess liquid, the ceramic paper was shaped by hand by making strategically placed folds. The hand-shaped ceramic paper was then thermally processed in accordance with the Example 20 procedure. A photograph of the formed body is shown in Figure 1. The above examples are intended solely to illustrate the invention and are not intended to limit the spirit and scope of the invention as understood by an artisan of ordinary skill and as defined by the claims appended hereto.

Claims: CLAIMS 1. A method for making a shaped thin form, comprising: a) preparing at least one thin form comprising at least one parent metal and at least one fiber; b) contacting the at least one thin form with at least one liquid or fusible composition; c) shaping the at least one thin form; and d) rigidizing the at least one shaped thin form to render said shaped thin form self-supporting. 2. The method of claim 1 , further comprising heating the at least one shaped thin form to a temperature above the melting point of the at least one parent metal but below the decomposition temperature of its oxidation reaction product, and reacting the molten parent metal with at least one oxidant to convert at least a portion of the molten parent metal to an oxidation reaction product, thereby forming a thin ceramic body. 3. The method of claim 1 , wherein said liquid or fusible composition comprises at least one silicon-containing composition. 4. The method of claim 1 , wherein said liquid or fusible composition comprises at least one preceramic polymer composition. 5. The method of claim 4, wherein said preceramic polymer composition comprises at least one composition selected from the group consisting of polysiloxane, polysilazane, polycarbosilazane, and polyureasilazane. 6. The method of claim 1 , wherein said rigidizing comprises at least one of drying and heating. 7. The method of claim 1 , wherein said liquid or fusible composition comprises at least one sol. 8. A thin form, comprising: (a) at least one composition comprising at least one parent metal and at least one fiber; and (b) at least one silicon-containing composition contacting at least a portion of said at least one composition. 9. The thin form of claim 8, wherein said at least one composition further comprises at least one filler material . 10. The thin form of claim 8, wherein said at least one fiber comprises at least one organic fiber. 11. The thin form of claim 8, wherein said at least one fiber comprises at least one ceramic fiber. 12. The thin form of claim 8, wherein said at least one fiber comprises at least one debond coating . 13. The thin form of claim 8, wherein said at least one parent metal comprises aluminum. 14. The thin form of claim 8, wherein said at least one silicon-containing composition comprises at least one preceramic polymer. 15. The thin form of claim 14, wherein said at least one preceramic polymer comprises at least one composition selected from the group consisting of polycarbosilane, polysilazane and polyureasilazane. 16. The thin form of claim 10, wherein said at least one organic-fiber comprises wood pulp. 17. A method for making a thin ceramic body, comprising: (a) dispersing in water a plurality of bodies of at least one parent metal, at least one inorganic filler material and at least one fiber; (b) flocculating the dispersion; (c) de- watering the flocculated dispersion to form a thin form; (d) drying the thin form; (e) contacting the dried thin form with a liquid or fusible composition; and (f) heating said thin form to a temperature sufficient and for a time sufficient to render said thin form self-supporting. 18. The method of claim 17, wherein said thin form is heated to a temperamre above the melting point of the parent metal but below the decomposition temperature of its oxidation reaction product, and reacting the molten parent metal with an oxidant to form an oxidation reaction product. 19. The method of claim 17, wherein said thin form is heated to a temperamre above the melting point of said parent metal to render said parent metal molten; reacting the molten parent metal with an oxidant at said temperamre to form said oxidation reaction product, and at said temperature maintaining at least a portion of said oxidation reaction product in contact with and extending between said body of molten metal and said oxidant, to draw molten metal through the oxidation reaction product towards the oxidant so that fresh oxidation reaction product continues to form within the permeable mass at an interface between the oxidant and previously formed oxidation reaction product, thereby forming a void at each site at which a parent metal body has oxidized; and continuing said reacting for a time sufficient to enable the developing polycrystalline material to form an at least partially interconnected strucmre which embeds at least a portion of said at least inorganic one filler material within said polycrystalline material. 20. The method of claim 17, wherein said inorganic filler material comprises at least one ceramic material. 21. The method of claim 20, wherein said ceramic material comprises at least one material selected from the group consisting of silicon carbide and aluminum oxide. 22. The method of claim 18, wherein said oxidant comprises at least one vapor- phase oxidant and wherein said thin form is permeable to said vapor-phase oxidant. 23. The method of claim 18, wherein said dispersion further comprises at least one oxidant which is solid or liquid at said reaction temperamre. 24. The method of claim 23, wherein said solid oxidant comprises at least one material selected from the group consisting of silicon dioxide and silicon nitride. 25. The method of claim 18, wherein said parent metal comprises aluminum, said oxidant comprises oxygen and said oxidation reaction product comprises aluminum oxide . 26. A green body, comprising: an admixture comprising a plurality of discrete bodies of at least one ceramic filler material, at least one fiber, at least one parent metal composition, and at least one dopant material for said parent metal; and dispersed through at least a portion of said admixture at least one substance which renders said admixture shape-formable. 27. The green body of claim 26, wherein said parent metal comprises aluminum and said dopant material comprises at least one of magnesium, silicon, tin, lead and zinc. 28. The green body of claim 26, wherein said substance comprises a silicon- containing material. 29. The green body of claim 26, wherein said substance comprises tetra ethyl ortho silicate. 30. The green body of claim 26, wherein said substance and said dopant are identical. 31. The green body of claim 28, wherein said silicon-containing material comprises a preceramic polymer. 32. The green body of claim 26, wherein said ceramic filler material and said fiber are identical. 33. The green body of claim 26, wherein said fiber comprises an organic material. 34. The green body of claim 33, wherein said organic material comprises a material selected from the group consisting of wood pulp, polymeric pulp and cotton. 35. The green body of claim 33, wherein said organic material comprises carbon. 36. The green body of claim 26, wherein said fiber comprises an inorganic material. 37. The green body of claim 36, wherein said inorganic material comprises at least one material selected from the group consisting of ceramic, metal and glass. 38. The green body of claim 26, wherein said admixture further comprises at least one fugitive material. 39. The method of claim 1 , wherein said parent metal comprises at least one metal selected from the group consisting of aluminum, silicon, titanium, tin, molybdenum, zinc and zirconium. 40. The method of claim 19, further comprising contacting at least one barrier material to at least a portion of a surface of said thin form, whereby said reacting locally terminates when said developing oxidation reaction product contacts said barrier material . 41. The thin form of claim 8, wherein said at least one fiber comprises at least one protective coating. 42. The thin form of claim 8, wherein said at least one fiber comprises a ceramic substrate, at least one protective coating disposed over at least a portion of said ceramic substrate, and at least one debond coating located between said ceramic substrate and said at least one protective coating. 43. The thin form of claim 12, wherein said debond coating comprises a material selected from the group consisting of carbon and boron nitride. 44. The thin form of claim 41 , wherein said protective coating comprises a material selected from the group consisting of aluminum oxide, aluminum nitride, silicon nitride, silicon carbide and calcium aluminate. 45. A method for making a thin ceramic body, comprising: (a) forming a green body by at least one process selected from the group consisting of slip casting, tape casting, blow molding, slush molding and paper-making, said green body comprising an admixture comprising a plurality of discrete bodies of at least one parent metal, at last one fiber and at least one ceramic material; (b) introducing at least one substance into at least a portion of said green body, thereby rendering said green body moldable; (c) introducing at least one dopant material into said green body; (d) molding said green body to a desired shape; (e) rigidizing said green body at said desired shape; (f) introducing at least one oxidant to said green body; (g) heating said green body to a temperamre at least above the melting point of said parent metal, and at said temperamre reacting said parent metal with said oxidant to form an oxidation reaction product; and (h) continuing said heating at least until said oxidation reaction product becomes at least partially interconnected.

Inventors: GARNIER JOHN EDWARD, MATSUMOTO ROGER LEE KEN, GARNIER, JOHN, EDWARD, MATSUMOTO, ROGER, LEE, KEN
Applicants: GARNIER JOHN EDWARD, LANXIDE TECHNOLOGY CO LTD, MATSUMOTO ROGER LEE KEN, GARNIER, JOHN, EDWARD, LANXIDE TECHNOLOGY COMPANY, L.P., MATSUMOTO, ROGER, LEE, KEN
Application year: 1996
Main classification: C04B35
IPC codes: C04B35