Method of making ceramic composites
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of application Ser. No. 070,006, filed July 6, 1987, now allowed in the names of H. Daniel Lesher et al., and entitled &#34;A Method of Making Ceramic Composites&#34;, which was a continuation-in-part of application Ser. No. 908,473, filed Sept. 17, 1986, now abandoned in the names of H. Daniel Lesher et al., and entitled &#34;A Method of Making Ceramic Composites&#34;. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention broadly relates to a method of making ceramic composites. More particularly, this invention relates to a method for producing ceramic composites by infiltrating a filler, which is coated with a silicon source having intrinsic doping properties, with an oxidation reaction product grown from a parent metal (e.g., aluminum) precursor. 
     DESCRIPTION OF THE PRIOR ART 
     In recent years, there has been an increasing interest in the use of ceramics for structural applications historically served by metals. The impetus for this interest has been the superiority of ceramics to metals with respect to certain properties, such as corrosion resistance, hardness, modulus of elasticity, and refractory capabilities. 
     Current efforts at producing higher strength, more reliable, and tougher ceramic articles are largely focused upon (1) the development of improved processing methods for monolithic ceramics and (2) the development of new material compositions, notably ceramic matrix composites. A composite structure is one which comprises a heterogeneous material, body or article made of two or more different materials which are intimately combined in order to attain desired properties of the composite. For example, two different materials may be intimately combined by embedding one in a matrix of the other. A ceramic matrix composite incorporates one or more diverse kinds of filler materials such as particulates, fibers, rods, and the like. 
     Various suitable materials have been employed as a filler in the formation and manufacture of ceramic matrix composites. These fillers have been used in the form of fibers, pellets, particulates, whiskers, etc. These materials include, for example, some of the oxides (single or mixed), nitrides, carbides or borides of aluminum, hafnium, titanium, zirconium, yttrium, and silicon. Certain known materials that have been utilized as a filler, such as silicon carbide and silicon nitride, are not intrinsically stable in a high temperature oxidizing environment (e.g. over 850° C.), but exhibit in such an environment degradation reactions having relatively slow kinetics. 
     There are several known limitations or difficulties in substituting ceramics for metals, such as scaling versatility, capability to produce complex shapes, satisfying the properties required for the end use application, and costs. Several copending patent applications and one issued Patent assigned to the same owner as this application, overcome some of these limitations or difficulties and provide novel methods for reliably producing ceramic materials, including composites. An important method is disclosed generically in commonly owned U.S. Pat. No. 4,713,360, which issued on December 15, 1987 from U.S. application Ser. No. 818,943, filed January 15, 1986, in the same of Marc S. Newkirk et al and entitled &#34;Novel Ceramic Materials and Methods for Making the Same.&#34; These applications disclose the method of producing self-supporting ceramic bodies grown as the oxidation reaction product from a parent metal precursor. Molten metal is reacted with a vapor-phase oxidant to form an oxidation reaction product, and the metal migrates through the oxidation product toward the oxidant thereby continuously developing a ceramic polycrystalline body. The process may be enhanced by the use of an alloyed dopant, such as is used in the case of oxidizing aluminum doped with magnesium and silicon in air to form α-alumina ceramic structures. This method was improved upon by the application of dopant materials to the surface of the precursor metal, as described in commonly owned U.S. patent application Ser. No. 822,999, filed January 27, 1986, in the names of Marc S. Newkirk et al and entitled &#34;Methods of Making Self-Supporting Ceramic Materials.&#34; Application Ser. No. 822,999 was abandoned in favor of a Rule 62 Continuation application Ser. No. 220,935 filed on June 23, 1988, and now allowed in the names of Marc S. Newkirk et al., and entitled &#34;Methods of Making Self-Supporting Ceramic Materials&#34;. 
     This oxidation phenomenon was utilized in producing composite ceramic bodies as described in commonly owned and copending U.S. patent application Ser. No. 819,397, filed Jan. 17, 1986, now allowed in the names of Marc S. Newkirk et al and entitled &#34;Composite Ceramic Articles and Methods of Making Same.&#34; Application Ser. No. 861,025 was abandoned in favor of a Rule 62 Continuation application Ser. No. 338,471, filed on Apr. 14, 1989, in the names of Marc S. Newkirk et al. and entitled &#34;Shaped Ceramic Composites and Methods of Making the Same.&#34; These applications disclose novel methods for producing a self-supporting ceramic composite by growing an oxidation reaction product from a parent metal precursor into a permeable mass of filler, thereby infiltrating the filler with a ceramic matrix. The resulting composite, however, has no defined or predetermined geometry, shape, or configuration. 
     A method for producing ceramic composite bodies having a predetermined geometry or shape is disclosed in the commonly owned and copending U.S. patent application Ser. No. 861,025, filed May 8, 1986, in the names of Marc S. Newkirk et al and entitled &#34;Shaped Ceramic Composites and Methods of Making the Same&#34;. In accordance with the method in this U.S. patent application, the developing oxidation reaction product infiltrates a permeable preform in the direction towards a defined surface boundary. It was discovered that high fidelity shape forming is more readily achieved by providing the preform with a barrier means, as disclosed in commonly owned and copending U.S. patent application Ser. No. 861,024, filed May 8, 1986, now allowed in the names of Marc S. Newkirk et al and entitled &#34;Method of Making Shaped Ceramic Composites with the Use of a Barrier&#34;. This method produces shaped self-supporting ceramic bodies, including shaped ceramic composites, by growing the oxidation reaction product of a precursor metal to a barrier means spaced from the metal for establishing a boundary or surface. A method of forming ceramic composites having a cavity with an interior geometry inversely replicating the shape of a positive mold or pattern of the parent metal is disclosed in commonly owned and copending U.S. Pat. No. 4,828,785 which issued on May 9, 1989 in the names of Marc S. Newkirk et al and entitled &#34;Inverse Shape Replication Method of Making Ceramic Articles and Articles Obtained Thereby&#34;, and in commonly owned and copending U.S. patent application Ser. No. 896,157, filed Aug. 13, 1986, now allowed in the names of Marc S. Newkirk et al and entitled &#34;Method of Making Ceramic Composite Articles with Shape Replicated Surfaces and Articles Obtained Thereby&#34;. 
     The entire disclosures of all of the foregoing commonly owned Patent Applications and Patent are expressly incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     The present invention broadly provides a method for producing a ceramic composite comprising a ceramic matrix obtained by the oxidation reaction of molten parent metal, e.g., aluminum, with an oxidant, and a filler coated, at least initially, with a silicon source (defined below) infiltrated by the matrix. The silicon source has a composition different from the primary composition of the filler and the silicon source is at least partially reducible by or dissolved by the molten parent metal under the process conditions. This coating of silicon source, when heated to a suitable temperature preferably, but not necessarily, in an oxygen-containing environment, possesses intrinsic doping properties for enhancing the oxidation reaction, and the essentially unaltered or remaining portion of the filler, serving as filler, is incorporated into the developing matrix, as explained below in greater detail. 
     The self-supporting ceramic composite is produced by initially forming a bed or mass of filler material, part or all of whose constituents are coated with a silicon source. The silicon source is different in composition from the primary composition of the filler. The filler may be overlaid at least partially with a barrier means which is at least partially spaced from the parent metal for establishing a surface or boundary of the ceramic matrix. 
     The filler bearing the silicon source, which may be used in combination with other filler materials, either in the form of a layup, packed bed or preshaped as a preform, is positioned or oriented adjacent the parent metal such that formation of the oxidation reaction product will occur in a direction towards the oxidant and filler, and towards the barrier means, if the same is utilized. The bed of filler material or preform should be sufficiently permeable to permit or accommodate growth of the oxidation reaction product within the bed, and to permit the gaseous oxidant (if a gaseous oxidant is used) to permeate the preform and contact the molten metal. The parent metal is heated to a temperature above its melting point but below the melting point of the oxidation reaction product to form a body of molten metal. At this temperature, or within this temperature range, the molten metal reacts with the oxidant to form the oxidation reaction product. At least a portion of the oxidation reaction product is maintained in contact with and between the molten metal and the oxidant to draw molten metal through oxidation reaction product towards and into contact with the oxidant such that the oxidation reaction product continues to form at the interface between the oxidant and previously formed oxidation reaction product, thus infiltrating the adjacent filler material. The reaction is continued for a time sufficient to infiltrate at least a portion of the filler material with a polycrystalline material consisting essentially of the oxidation reaction product and one or more metallic constituents, such as unoxidized constituents of the parent metal or of the dopant, dispersed or distributed through the polycrystalline material. It should be understood that the polycrystalline matrix material may exhibit voids or porosity in place of the metal phase, but the volume percent of voids will depend largely on such conditions as temperature, time, dopants, and type of parent metal. If a barrier means has been used, the ceramic body will continue to grow to the barrier, provided sufficient parent metal is present. 
     As explained in the aforesaid Commonly Owned Patent Applications and Patent, the use of dopant materials can favorably influence or promote the oxidation reaction process. Silicon is a useful dopant with an aluminum parent metal, especially in combination with other dopants, and can be applied externally of the parent metal, and a useful source for such a dopant is silica. Under the process conditions of this invention, a silicic compound as the silicon source (e.g. silica) is reduced by the molten parent metal to form alumina and silicon. Thus, the silicic compound coating on the filler material is a useful dopant in promoting the development or growth of the oxidation reaction product. For example, silicon carbide will oxidize at or on the surface at elevated temperatures in air to form a silica film, and therefore silicon carbide is a particularly useful filler in that it serves not only as a filler material but also as a dopant source. The silica film is reduced by the molten parent metal to yield a silicon dopant which promotes growth of the polycrystalline matrix through the silicon carbide filler. In addition, when aluminum is used as the parent metal, the silica coating on silicon carbide particles is advantageous in that during the matrix formation reaction it increases the local silicon concentration in the unoxidized parent metal and thereby reduces the tendency for formation of Al 4  C 3  during the matrix growth process. Al 4  C 3  is undesirable because it is unstable in the presence of moisture levels normally present in ambient air, resulting in the evolution of methane and the degradation of the structural properties of the resulting composite. 
     It has been discovered for purposes of this invention that, in conducting the oxidation reaction, preferably in an oxygen-containing environment, the coating of silicon source serves as a dopant material for the oxidation reaction of the parent metal. The remaining portion of the filler beneath the coating, being of a different composition, is substantially unchanged and serves as a filler in the composite. For example, the filler may bear a silicic compound which is reducible by the molten metal, or the filler may be coated with silicon which is dissolvable by the molten metal. It should be understood that substantially all of the silicon source may be utilized as a dopant, or a portion only is utilized as dopant, the remainder being with the filler and becoming embedded by the matrix. Certain fillers, such as silicon carbide, will oxidize to form SiO 2  at the elevated temperatures of the process, and the conditions are controlled to limit the oxidation so as to produce a silica coating reducible by the molten parent metal. When desired, a separate coating material may be applied to the filler which, on heating, produces the silicic compound. The coating of silicon source, e.g. silicic compound, may be formed or produced by first prefiring or heating a suitable filler in an oxygen-containing atmosphere. The prefired filler having thereon the coating is employed subsequently as a filler material. For example, a preform can be fabricated of silicon carbide particulate or alumina particulate coated with a siliceous precursor or compound such as tetraethyl orthosilicate. The preform is then prefired or heated in air to form an oxide skin of silica on the silicon carbide particles or alumina particles of the preform. The preform then can be employed as a ceramic composite raw material possessing an intrinsic source of silicon dopant material. Alternatively, the particulate of silicon carbide or alumina with the siliceous coating can be used in the setup of filler material and parent metal, and the silica film or coating is formed in situ during the oxidation reaction process in the presence of an oxygen-containing gas. The primary composition of the filler particulate (e.g. silicon carbide or alumina particulate) remains intact and serves as the filler material for the composite. 
     The materials of this invention can exhibit substantially uniform properties throughout their cross-section to a thickness heretofore difficult to achieve by conventional processes for producing ceramic structures. The process which yields these materials also obviates the high costs associated with conventional ceramic production methods, including the preparation of fine, high purity, uniform powders and their densification by such methods as sintering, hot pressing, or isostatic pressing. 
     The products of the present invention are adaptable or fabricated for use as articles of commerce which, as used herein, is intended to include, without limitation, industrial, structural and technical ceramic bodies for such applications where electrical, wear, thermal, structural, or other features or properties are important or beneficial; and is not intended to include recycle or waste materials such as might be produced as unwanted by-products in the processing of molten metals. 
     As used in this specification and the appended claims, the terms below are defined as follows: 
     &#34;Ceramic&#34; is not to 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 derived from the parent metal or produced from the oxidant or a dopant, most typically within a range of from about 1-40% by volume, but may include still more metal. 
     &#34;Oxidation reaction product&#34; generally refers to the parent metal in any oxidized state wherein the metal has given up electrons to or shared electrons with another element, compound, or combination thereof. Accordingly, an &#34;oxidation reaction product&#34; under this definition includes the product of reaction of the metal with an oxidant such as those described in this application. 
     &#34;Oxidant&#34; means one or more suitable electron acceptors or electron sharers and may be a solid, a liquid or a gas (vapor) or some combination of these (e.g., a solid and a gas) at the process conditions. 
     &#34;Parent metal&#34;, refers to that metal, e.g., aluminum, which is the precursor for the polycrystalline oxidation reaction product, and includes that metal as a relatively pure metal, a commercially available metal with impurities and/or alloying constituents, or an alloy in which the metal precursor is the major or most significant constituent; and when a specified metal is mentioned as the parent metal, e.g., aluminum, the metal identified should be read with this definition in mind unless indicated otherwise by the context. 
     &#34;Silicon source&#34; refers to elemental silicon or a silicic compound which will provide a dopant material and/or promote wetting of the filler by the molten parent metal under the process conditions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A, 1B, 2A, 2B, 3A and 3B are photographs in plan and side views, respectively, of a composite prepared in accordance with Example 2. In each of these figures a portion of the grown composite has been removed by sectioning for further analysis. 
     FIG. 4 is a photomicrograph at 50X of a composite structure showing coated filler particles embedded by a ceramic matrix prepared in accordance with Example 3. 
     FIGS. 5A-5D are photomicrographs taken at 50X of cross-sections of an oxidation reaction product prepared in accordance with Example 4. 
     FIGS. 6A-6D are photomicrographs taken at 50X of cross-sections of an oxidation reaction product prepared in accordance with Example 4. 
     FIGS. 7A-7D are photomicrographs taken at 50X of cross-sections of an oxidation reaction product prepared in accordance with Example 4. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
     In practicing the process of the present invention, the parent metal, which may be doped with additional dopant material (as explained below in greater detail) and is the precursor to the oxidation reaction product, is formed into an ingot, billet, rod, plate, or the like. A mass or body of filler material comprising a particulate, powders, fibers, whiskers, or other suitable shapes having a coating of a silicon source is oriented relative to the parent metal so that the direction of growth of the oxidation reaction product will be towards and into the filler. The composition of the coating is different from the primary composition of the filler, and if a silicic compound, also is reducible by the molten parent metal, thereby promoting the oxidation reaction by serving as a dopant. It is also believed that the silicon source also serves to enhance wettability of the filler by the parent metal. The bed is permeable to the vapor-phase oxidant (e.g. air) if a vapor-phase oxidant is employed, and to the growth of oxidation reaction product matrix in order to permit development of the oxidation reaction product and infiltration thereby of the filler. As explained in the Commonly Owned Patent Applications and Patent, dopant materials favorably influence the oxidation reaction process of parent metals, and silicon, silicon dioxide and similar silicon-containing compounds are useful dopant sources in systems employing aluminum as the parent metal. In accordance with one preferred embodiment of this invention, a silicic compound, when heated to a suitable temperature in an oxygen-containing atmosphere, forms an oxide coating which serves as a dopant material. The formation of the oxide coating on the filler may be accomplished in a prefiring step or in situ during formation of the ceramic body in the presence of an oxygen-containing gas as oxidant. Unless otherwise stated, the terms &#34;filler&#34; or &#34;filler material&#34; are intended to mean either a mass, bed or a preform comprising the filler material at least partially coated with a silicon source, which may be used in combination with other filler materials bearing no such coating. 
     The filler itself may react, as in the case of silicon carbide, to form a coating of a silicic compound which is reducible by the molten parent metal. Thus, in such a case the filler, per se, has intrinsic doping properties as provided by the coating, and the remaining unchanged or unaltered portion serves as the filler upon infiltration by the oxidation reaction product. Particularly suitable fillers of this type include silicon carbide and silicon nitride. With materials of this type, a silica coating or silicate coating is formed on heating in air or other suitable oxygen-containing gases. Where desired, the filler may have a suitable coating of a silicon source or silicic precursor applied to a filler of an entirely different composition. For example, a particularly useful system of this type is tetraethyl orthosilicate applied to zirconium oxide fibers, which on drying or mild heating to dissociate the material, will form a silica coating. As a further example, ethyl silicate glass may be applied to an alumina particulate, which on heating forms a coating of silica. 
     In accordance with one embodiment of the invention, a suitable filler such as silicon carbide or silicon nitride in particulate form is prefire or heated in an oxygen-containing atmosphere, e.g. air, at a temperature sufficient to form a coating of silica on a substantial portion of the particulate. For example, in using silicon carbide as the filler, prefiring desirably is at a temperature of from about 1000° C. to about 1450° C., and preferably 1200°-1350° C. The time required for producing the oxide coating on such fillers depends on such factors as particle size, shape, porosity and availability of oxidant. For silicon carbide having a mesh size of about 250 to 750, a suitable prefiring time is about 5 to 40 hours. Another advantage of prefiring is to produce a thicker silica coating than could be produced during the time required for matrix formation. 
     In another embodiment, the silicon dioxide film or coating is formed in situ during the oxidation reaction process with an oxygen-containing gas as oxidant. The setup of parent metal and filler, placed in a suitable refractory vessel, is heated to a temperature not only above the melting point of the parent metal, but also high enough to form a sufficient silica coating on the filler. In this embodiment, the vapor-phase oxidant utilized in the matrix-forming oxidation reaction also reacts with the filler to form the silicon dioxide skin. For example, in using silicon carbide filler infiltrated by an alumina matrix as the oxidation reaction product of aluminum parent metal and an oxygen-containing vapor-phase oxidant, preferably air, the oxide coating will form on the silicon carbide particles at a suitable temperature. The setup first is heated to a temperature between about 1000°-1450° C., and more preferably between about 1200°-1350° C. After the silica film or coating has formed on the silicon carbide filler material, this temperature range can be maintained or can be altered for continuing the oxidation reaction process and to develop the ceramic composite structure. 
     In still another embodiment of the invention, the filler material may be coated with silicon such as by chemical vapor deposition. This may be particularly useful in the case of fillers, typically as fibers, particles, or whiskers, which need to be protected against degradation under the proces conditions. For example, boron nitride particles need to be protected from oxidation and reaction with molten aluminum, and the silicon coating affords this protection and yet meets the other criteria of the invention. 
     The filler materials such as silicon carbide and silicon nitride are preferably in particulate form, and may include an admixture of different grit or mesh sizes, preferably about 10-1000 mesh, but finer particulate may be used. However, in the case of silicon nitride, it is desirable to use relatively coarse material to prevent excessive oxidation or reaction to form aluminum nitride and silicon. In this manner, the admixed filler can be tailored to produce a filler material possessing desired end properties, such as permeability, porosity, density, etc. 
     The filler typically is bonded as a bed or preform with any suitable binding material, agent, compound, or the like, which does not interfere with the reactions of this invention, or leave a significant amount of undesirable residual by-products within the ceramic composite product. Suitable binders have been found to include, for example, polyvinyl alcohol, epoxy resins, natural and synthetic latex, and the like, which are well known in the art. The filler, with or without a binding agent, may be formed into any predetermined size and shape by any conventional method, such as slipcasting, injection molding, transfer molding, vacuum forming, etc. 
     It is preferable that the filler material be preshaped, having at least one surface boundary, and should retain sufficient shape integrity and green strength, as well as dimensional fidelity, during processing and formation of the ceramic body. The filler bed or preform, however, should be permeable enough to accommodate the growing polycrystalline matrix material. For example, a silicon carbide or silicon nitride preform useful in this invention has a porosity of between about 5 and about 90% by volume, and, more preferably, between about 25 and about 75% by volume. 
     In conducting the process, the filler, which may be prefired and/or preshaped, is placed adjacent to one or more surfaces, or a portion of a surface, of the parent metal. The filler material preferably is in contact with an areal surface of the parent metal; but where desired, may be partially immersed, but not totally immersed, in the molten metal because complete immersion would cut off or block access of the vapor-phase oxidant into the filler material for proper development of the polycrystalline matrix. Formation of the oxidation reaction product will occur in a direction towards and into the filler material. 
     The setup, comprising filler and parent metal, is placed in a furnace that is supplied with a suitable vapor-phase oxidant, if a vapor-phase oxidant is to be employed, and the setup is heated to or within a temperature region above the melting point of the parent metal but below the melting point of the oxidation reaction product. The process temperature range for aluminum parent metal using air as the vapor-phase oxidant generally is between about 700°-1450° C., and more preferably between about 800°-1350° C. Within the operable temperature interval or preferred temperature range, a body or pool of molten metal forms, and on contact with the oxidant, the molten metal will react to form a layer of oxidation reaction product. Upon continued exposure to the oxidizing environment, within an appropriate temperature region, the remaining molten metal is progressively drawn into and through the oxidation reaction product in the direction of the oxidant. On contact with the oxidant, the molten metal will react to form additional oxidation reaction product. At least a portion of the oxidation reaction product is maintained in contact with and between the molten parent metal and the oxidant so as to cause continued transport of the molten metal through the formed oxidation reaction product toward the oxidant such that the polycrystalline oxidation reaction product infiltrates at least a portion of the filler material. The coating of silicon source produced on the filler material, as by prefiring in situ, or precoating, enhances the growth of the polycrystalline oxidation reaction product by supplying a continuous source of silicon dopant material throughout the volume of filler. 
     The process is continued until the oxidation reaction product has infiltrated at least a portion of the bed of filler material. If a preform is used, the process is continued until the oxidation reaction product has infiltrated and embedded the constituents of the preform to a defined surface boundary, and desirably not beyond, which would be an &#34;over-growth&#34; of the polycrystalline matrix material. 
     It should be understood that the resulting polycrystalline matrix material may exhibit porosity which may be a partial or nearly complete replacement of the metal constituents, 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 or pore constituents may be at least partially interconnected. The resulting ceramic composite product will possess the dimensions and geometric configuration of the original preform if used, and especially good fidelity is achieved by the use of a barrier means. 
     When a vapor-phase oxidant is used in the oxidation reaction process, such oxidant is normally gaseous or vaporized at the process conditions, which provides an oxidizing atmosphere such as atmospheric air. However, when a prefired or precoated filler is used, the oxidant need not be an oxygen-containing gas. A solid oxidant or an oxidant which is a vapor-phase oxidant or a liquid under the process conditions may be used in the practice of the present invention. Further, other typical vapor-phase oxidants, the use of which may depend on whether the filler has been prefired or precoated, include additionally, for example, nitrogen or a nitrogen-containing gas, and mixtures such as air, H 2  /H 2  O and CO/CO 2 , the latter two (i.e., H 2  /H 2  O and CO/CO 2 ) 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 more 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 the sole oxidizer of the parent metal under the conditions obtained in the oxidizing environment utilized. Air therefore falls within the definition of an &#34;oxygen-containing gas&#34; oxidant but not within the definition of a &#34;nitrogen-containing gas&#34; oxidant. An example of a nitrogen-containing oxidant as used herein and in the claims is &#34;forming gas&#34;, which typically contains about 96 volume percent nitrogen and about 4 volume percent hydrogen. 
     As mentioned earlier, an oxidant which is liquid or solid at the process conditions may be employed alone or in conjunction with a vapor-phase oxidant. Such oxidants may be particularly useful in enhancing oxidation of the parent metal preferentially within the filler material rather than beyond its surfaces. That is, the use of such oxidants may create an environment within the filler material more favorable to oxidation kinetics of the parent metal than the environment outside the filler bed or preform. With respect to the silicon carbide filler material employed as a preform, this enhanced environment is beneficial in promoting matrix development within the preform to the boundary and minimizing overgrowth. 
     When a solid oxidant is employed, it may be dispersed through the entire volume of the filler material, or through only a portion of filler material adjacent the parent metal, such as in particulate form and admixed with the filler material. Any suitable solid oxidant may be employed depending upon its compatibility with the vapor-phase oxidant, if such oxidant is used in conjunction with the solid oxidant. Such solid oxidants may include suitable elements, such as boron, or suitable reducible compounds, such as certain borates, borate glasses, silicates and silicate glasses of lower thermodynamic stability than the oxidation reaction product of the parent metal with the solid oxidant. 
     If a liquid oxidant is employed, the liquid oxidant may be dispersed throughout the entire volume of bed of filler material or a portion of the filler material adjacent to the molten metal, provided such liquid oxidant does not prevent access of the vapor-phase oxidant to the molten parent metal, if such vapor-phase oxidant is employed in conjunction with the liquid oxidant. Reference to a liquid oxidant means one which is 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, 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. 
     If a shaped preform is used, the preform should be sufficiently porous or permeable to allow the vapor-phase oxidant to permeate the preform and contact the molten parental metal, if a vapor-phase oxidant is employed. The preform also should be sufficiently permeable to accommodate growth of the oxidation reaction product within its boundaries without substantially disturbing, upsetting or otherwise altering its configuration or geometry. In the event the preform includes a solid oxidant and/or liquid oxidant, the preform then should be sufficiently porous or permeable to permit and accept growth of the oxidation reaction product originating from the solid and/or liquid oxidant. 
     The present invention provides in one embodiment a composite filler material which, when heated to a suitable temperature in an oxygen-containing atmosphere, provides an intrinsic source of a dopant material; that is, for example, silicon carbide as filler is an intrinsic source of silicon dioxide as a dopant. In certain applications it may be necessary or desirable to employ an additional dopant material to supplement that intrinsically supplied by the silicon source. When one or more dopants are used in addition to the silicon source, they: (1) may be provided as alloying constituents 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 incorporated into part or all of the filler material, or any combination of two or more of techniques (1), (2) and (3) may be employed. For example, a dopant alloyed with the parent metal may be used solely or in combination with a second externally applied dopant in conjunction with the silicic compound coating. In the case of technique (3), where additional dopant or dopants are applied to the filler material, the application may be accomplished in any suitable manner as explained in the Commonly Owned Patent Applications and Patent. The function or functions of a dopant material can depend upon a number of factors other than the dopant material itself. Such factors include, for example, the particular combination of dopants when two or more dopants are used, the use of an externally applied dopant in combination with a dopant alloyed with the parent metal, the concentration of the dopant, the oxidizing environment, and the process conditions. 
     Dopants useful in combination with a silicon source as dopant for the aluminum parent metal, particularly with air as the oxidant, include magnesium and zinc, which may be used in combination with other dopants, as described below. These metals, or a suitable source of the metals, may be alloyed into the aluminum-based parent metal at concentrations for each of between about 0.1-10% by weight based on the total weight of the resulting doped metal. Where desired, silicon metal may be alloyed with the parent metal to supplement the silicon source as coating on a filler. In such examples, a preferred magnesium concentration falls within the range of from about 0.1 to about 3% by weight, for silicon in the range of from about 1 to about 10% by weight, and for zinc when used with magnesium in the range of from about 1 to about 6% by weight. These dopant materials or a suitable source thereof (e.g. MgO and ZnO) may be used externally to the parent metal. Thus an alumina ceramic structure is achievable for the aluminum parent metal using air as the oxidant by using MgO as a dopant in an amount greater than about 0.0008 gram per gram of parent metal to be oxidized and greater than 0.003  gram per square centimeter of parent metal upon which the MgO is applied. 
     Additional examples of dopant materials effective with aluminum parent metals reacting with an oxygen-containing atmosphere include sodium, germanium, tin, lead, lithium, calcium, boron, phophorous, and yttrium which may be used individually or in combination with one or more other dopants depending on the oxidant and process conditions. 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. All of the dopant materials as explained in the Commonly Owned Patent Applications and Patent, in addition to the coating of silicon source, are effective in promoting polycrystalline oxidation reaction product growth for the aluminum-based parent metal systems. 
     The ceramic composite product obtained by the practice of the present invention will usually be a coherent product wherein between about 5% and about 98% by volume of the total volume of the ceramic composite product is comprised of filler embedded by a polycrystalline ceramic matrix. The polycrystalline ceramic matrix is usually comprised of, when air or oxygen is the oxidant and an aluminum parent metal is employed, about 60% to about 99% by volume (of the volume of polycrystalline matrix) of interconnected α-aluminum oxide and about 1% to 40% by weight (same basis) of metallic constituents such as non-oxidized constituents of the parent metal or reduced metal from the dopant or the oxidant. 
     As disclosed in copending U.S. patent application Ser. No. 861,024, filed May 8, 1986, and assigned to the same assignee, a barrier means may be used in conjunction with the filler material to inhibit growth or development of the oxidation reaction product beyond the barrier. 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 a vapor-phase oxidant while being capable of locally inhibiting, poisoning, stopping, interfering with, preventing, or the like, continued growth of oxidation reaction product. Calcium sulfate (Plaster of Paris), calcium silicate, and portland cement and mixtures thereof, which are particularly useful with aluminum as the parent metal and an oxygen-containing gas oxidant, typically are applied as a slurry or paste to the surface of the filler material. These 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 following examples illustrate the practice of certain aspects of the invention. 
     EXAMPLE 1 
     In accordance with the present invention, a cermic structure was fabricated comprising an alumina oxidation reaction product embedding β-SiC whiskers supplied by NIKKEI TECHNO-RESEARCH COMPANY, LTD., initially coated with either a commercial colloidal silica (Ludox HS-30 from Du Pont Company) or a sodium silicate solution (40°-42° Baume) as supplemental sources of silicon. 
     Three preforms, measuring 2 inches in diameter and 3/8 inch thick, were made by mixing three separate batches of β-SiC whiskers with a liquid medium, pouring the resulting slurry into a mold, and then degassing and drying in a vacuum dissicator. The liquid media that were mixed with the β-SiC whiskers included distilled water as a control, colloidal silica, and sodium silicate solution. The preforms were placed on a bed of 90 grit E1 Alundum (from Norton Company) contained in a refractory boat. Aluminum alloy ingots (No. 712.2) with the same diameter as the preforms had one side coated with a thin layer of sand, and the coated side of each ingot was placed in contact with the upper surface of a preform. This setup was placed in a furnace and heated to 900° C. in 5 hours. This temperature was held for 36 hours, and the setup was cooled to ambient temperature in 5 hours. The infiltration of the alumina oxidation reaction product was negligible in the preform containing only the β-SiC whiskers (the control using distilled water). The β-SiC whiskers coated with colloidal silica were infiltrated through the entire thickness of the preform. The infiltration of the β-SiC whiskers with sodium silicate solution occurred to approximately the center of the preform. 
     EXAMPLE 2 
     In accordance with the present invention, a ceramic composite structure was fabricated comprising an alumina oxidation reaction product embedding particles of silicon carbide filler material (39 Crystolon, 500 grit from Norton Co.) initially coated with colloidal silica (Ludox HS-30, from Du Pont Company, 30% solution) as a source of silicon. 
     The colloidal silica coating on the silicon carbide particles was done by preparing two preforms measuring 2×2×1/2 inches by sediment casting into a rubber mold a mixture of silicon carbide particles (500 grit) and colloidal silica at a two-to-one powder to liquid ratio. Upon setting up and drying, one of the preforms was crushed and passed 100% through 100 mesh. This crushed colloidal silica coated silicon carbide was then sediment cast again utilizing a 2% acrylic latex binder (Elmer&#39;s Wood Glue, Borden Co.). A preform identical to those above was prepared with silicon carbide not coated with colloidal silica utilizing only the latex binder. 
     Three bars of aluminum alloy 712 (having a nominal composition by weight of 0.15% Si, 0.6% Mg, 6% Zn) were placed into a refractory bed of Wollastonite fibers (NYAD FP from Petty-Rowley Chemical Co.) which was contained in a refractory vessel, such that one 2×2 face of each bar was exposed to the atmosphere and substantially flush with the bed. The three above-described preforms were placed one each on top of the alloy bars such that one 2×2 face of each respective preform and alloy were substantially aligned. A layer of Wollastonite fibers was dispersed over the top of the preforms to mitigate overgrowth of the ceramic matrix beyond the preform boundaries. This setup was placed into a furnace and heated up over 10 hours to 1000° C. The furnace was held at 1000° C. for 80 hours, and cooled down to ambient over 10 hours. The setup was removed from the furnace, and the resulting ceramic composite structures were recovered. The resulting composites were sandblasted lightly to remove unembedded preform materials. FIGS. 1(a), 1(b), 2(a), and 2(b) are photographs of the resulting composite materials utilizing preforms with the colloidal silica coating (FIGS. 2(a) and 2(b) involving the recast preform), which illustrate good growth; and FIGS. 3(a) and 3(b) show the resulting composite utilizing no silica coating. As apparent from the figures, the preforms employing the silica coated particles were embedded substantially to their dimensional boundaries, while the non-silica containing preform showed substantially less infiltration by the ceramic matrix. 
     EXAMPLE 3 
     In accordance with the present invention, a ceramic composite structure was fabricated comprising an alumina oxidation reaction product embedding particles of boron nitride which were coated with silicon. 
     A bar of aluminum alloy 380.1 (from Belmont Metals, having a nominally identified composition by weight of 8-8.5% Si, 2-3% Zn, and 0.1% Mg as active dopants, and 3.5% Cu as well as Fe, Mn, and Ni, but the Mg content was sometimes higher as in the range of 0.17-0.18%) was submerged into a bed of boron nitride particles (approximately 50 mesh size). The boron nitride particles were coated with silicon (accomplished by chemical vapor deposition) to protect the boron nitride from degradation and to serve as a source of silicon dopant, which supplemented the silicon source in the alloy. This bed was contained in a refractory vessel. This setup was placed into a furnace which had an opening to facilitate the passage of air, and heated up over 5 hours to 1100° C. The furnace was held at 1100° C. for 48 hours, and cooled down to ambient. The resulting ceramic composite was recovered. FIG. 4 is a photomicrograph at 50X of the composite which shows the alumina matrix 2 embedding the particles of boron nitride 4 still bearing some of the silicon coating 6. 
     The above examples demonstrate the utility of a filler material with a silicon source having intrinsic doping properties for enhancing composite formation. Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that the present invention embraces many variations other than those exemplified. 
     EXAMPLE 4 
     This example illustrates the use of a prefired preform in the technique of the instant invention. 
     A preform was prepared by first mixing 1000 grit green SiC particles (supplied by Exolon-ESK Company of Tonawanda, N.Y. under the tradename &#34;Carbolon&#34; F-1000), Polyvinyl Alcohol (supplied by DuPont of Wilmington, De under the tradename Elvanol 75-15), distilled H 2  O and (optionally) small amounts of kerosene. The Polyvinyl Alcohol and distilled H 2  O were mixed together and then added to the SiC particles which were contained within a beaker. The mixture, consisting of the Polyvinyl Alcohol, distilled H 2  O and SiC particles were then stirred for approximately 5 minutes before the kerosene was added. The final mixture was stirred for an additional 10 minutes and was composed by weight of approximately 90.5% SiC particles, 1.2% polyvinyl alcohol, 8.3% distilled H 2  O and optionally small amounts of kerosene (e.g., less than 1 weight percent). Subsequent to the stirring step, the mixture was sieved through a 20 mesh screen and the resultant powder was uniaxially dry pressed in a die with a pressing load of 19000 kbs force (thus resulting in a pressure of about 15,000 psi). The die had a 32 mm inner diameter. The pressure was maintained for approximately 10 seconds resulting in a preform being 7 mm thick and having a  32 mm diameter. The preform was then removed from the die. Twenty-one preforms were made from the above-described mixture and process. These preforms were placed on top of &#34;Fiberfrax&#34; strips supplied by Carborundum Company, Niagara Falls, N.Y. which were situated on top of a refractory tray. These strips prevented contact between the preforms and the refractory tray. The assembly, consisting of the Fiberfrax strips and preforms, was placed within a resistance heated furnace and heated in air from ambient temperature to 1100° C. over 10 hours. The preforms were held at 1100° C. in air for 5 hours and then ramped to ambient over 8 hours. The preforms were weighed before and after this prefiring step to determine the amount of silica that had developed as a coating on the outside of the SiC particles. The average preform, after prefiring, increased 7.9% by weight, volatiles excluded. This increase was due to the formation of silica which developed during the prefiring step, and which amounted to 22% of the sample weight after prefiring. 
     The prefired preforms were then entirely coated with a barrier material except for one face. This barrier material comprised by weight 53% fine grained Wollastonite (10 micron mean diameter) 1% Polyvinyl Alcohol Elvanol 75-15, and 46% distilled H 2  O. The face which was not coated with a barrier material was coated with an initiator layer comprising a slurry of 29% by weight 500 grit silicon metal, obtained from Atlantic Equipment Engineers, Bergenfield, N.J., and 71% distilled water. The coated preforms were then dried in an oven for 2 hours at 150° F. Four 1/2&#34; by 2&#34; by 9&#34; ingots of a parent metal aluminum alloy having the approximate composition by weight 7.5-9.5% Si, 3.0-4.0% Cu, &lt;2.9% Zn, &lt;1.5% Fe, &lt;0.5% Mn, &lt;0.5% Ni, &lt;0.35% Sn and &lt;0.18% Mg were placed on top of a bed of coarse grain wollastonite particulate &#34;Nyad SP&#34; , NYCO, Inc., contained within a refractory boat comprised of 3000 Castable obtained from New Castle/Corundite Refractories, New Castle, D.E. The ingots were disposed within the wollastonite so that only the uppermost surfaces thereof were exposed to the atmosphere. 
     The assembly, consisting of the refractory boat and its contents, was placed within a resistance heated furnace at ambient temperature. Fourteen of the preforms were placed within the furnace but separate from the assembly. The furnace temperature, was ramped from ambient to 900° C. over 5 hrs. At this temperature the parent metal ingots formed a molten pool of parent metal. Seven of the 14 preforms that were in the furnace were immediately placed on the surface of the molten parent metal (after the surface had been scraped of any air grown oxidation reaction product) upon the furnace reaching 900° C. After 5 hours at 900° C., the furnace temperature was reduced to 875° C. and the remaining 7 preforms were placed on the surface of the molten parent metal. At this time, the last 7 preforms, which had been held at ambient room temperature, were also placed on the molten parent metal surface. The furnace was held at 875° C. for 100 hours and then ramped to 700°  C. over 2 hours. The preforms, which had now been infiltrated by the polycrystalline oxidation reaction product of the molten parent metal and the air atmosphere, were removed from the molten parent metal surface and placed on a rack inside the same furnace. The furnace, containing the infiltrated preforms, was permited to cool for about 5 hours to a temperature of approximately 300° C. before the infiltrated preforms were removed. The infiltrated preforms were then sandblasted to remove any remaining barrier material and residual parent metal. What remained after the sandblasting step were self-supporting ceramic composite bodies which substantially duplicated the shape of the original preforms. 
     FIGS. 5A through 5D are photomicrographs taken at 50X of a cross-section in the oxidation reaction product growth direction (7 mm cross-section) of a sample selected from the group of preforms that were placed on the molten parent metal surface at 900° C. after being preheated within the furnace. 
     FIGS. 6A through 6D are photomicrographs taken at 50X of a cross-section in the oxidation reaction product growth direction (7 mm cross-section) of a sample selected from the group of preforms that were placed on the molten parent metal surface at 875° C. after being preheated within the furnace. 
     FIGS. 7A through 7D are photomicrographs taken at 50X of a cross-section in the oxidation reaction product growth direction (7 mm cross-section) of a sample selected from the group of preforms that were held at ambient temperature before being placed on the molten parent metal surface at 875° C. 
     Each set of the above-described photomicrographs, when connected A-B-C-D, form an approximately continuous photomicrograph at 50X of the 7 mm cross-section of the preform described. The &#34;A&#34; end is the face which was in contact with the parent metal alloy and the D end is the face which was in contact with the air oxidant and covered by barrier material. 
     The black areas represent porosity, the dark grey areas Al 2  O 3 , the light grey areas SiC and the white areas Al alloy. 
     As illustrated in Example 4, and described elsewhere in the specification, it is possible to prefire a preform comprising a silicon compound or silicic precursor and thereby create a silicon source coating on at least a portion of the particles comprising the preform. 
     It is believed that a silicon source concentration gradient may result within the prefired preform due to varying the length of time that the preform is prefired or due to varying the temperature at which the preform is prefired, or both. Such concentration gradients could occur because the prefiring atmosphere may not have sufficient time to penetrate to the center regions of the preform thus resulting in a greater conversion near the surface relative to the center regions of the preform; or a temperature gradient may develop within the preform which also may lead to concentration gradients therein. It is expected that this silicon source concentration gradient would be most prevalent in larger (especially thicker) preforms. Such a silicon source concentration gradient could result in a final product having graded properties. 
     The above examples demonstrate the utility of a filler material with a silicon source having intrinsic doping properties for enhancing composite formation. Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that the present invention embraces many variations other than those exemplified.