CA1307097C

CA1307097C - Method of making ceramic composites

Info

Publication number: CA1307097C
Application number: CA000547457A
Authority: CA
Inventors: H. Daniel Lesher; Christopher R. Kennedy; Danny R. White; Andrew W. Urquhart
Original assignee: Lanxide Technology Co LP
Current assignee: Lanxide Technology Co LP
Priority date: 1986-09-17
Filing date: 1987-09-15
Publication date: 1992-09-08
Anticipated expiration: 2009-09-08
Also published as: TW199137B; CS8706615A2; IL83859A; HU204239B; IE872485L; PL156558B1; PH25679A; AU602355B2; FI874026A; NZ221755A; MX166445B; PT85735B; AU7860287A; EP0261068B1; YU170787A; DE3784123T2; NO177224B; BR8704768A; DE3784123D1; DK481887A

Abstract

ABSTRACT
The invention relates to a method for producing ceramic composites obtained by oxidation of an aluminum parent metal to form a polycrystalline ceramic material by providing a filler having a coating of a silicon source on at least a portion of said filler different in composition from the primary composition of said filler, said silicon source possessing intrinsic doping properties. A body of molten parent metal, adjacent a mass of the filler material, reacts with an oxidant to form an oxidation reaction product which infiltrates the adjacent mass of filler thereby forming the ceramic composite.

Description

-` 13070~

METHOD OF MAKING CERAMIC COMPOSITES

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 an aluminum parent metal precursor.

DescriDtion 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 1ntimately 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 patents 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 Canadian Patent Application No. 476,692 (EP0 Publication No. 0,155,831, published on September 25, 1985), filed on March 15, 1985, now Canadian Patent No. 1,257,300, which issued on July 11, 1989, in the names of Marc S. Newkirk et al and entitled ~Novel Ceramic Materials and Methods for Making the Same.~ This patent discloses 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 w~th magnesium and silicon ~n air to form alpha-alumina ceram~c structures. This method was improved upon by the application of dopant materials to the surface of the ; precursor metal, as described in commonly owned Canadian Patent Application ~;
; ~

No. 487,146 (EPO Publication No. 0,169,067, published on January 22, 19~86), filed on July 19, 1985, in the names of Marc S. Newkirk et al. and entitled NMethods of Making Self-Supporting Ceramic Materials~.
This oxidation phenomenon was utilized in producing composite ceramic bodies as described in commonly owned Canadian Patent Application No. 500,994 (EP0 Publication No. 0,193,292, published on September 3, 1986), filed on February 3, 1986, now Canadian Patent No. 1,271,783, which issued on July 17, 1990, in the names of Marc S. Newkirk et al and entitled NComposite Ceramic Articles and Methods of Making Same." This patent discloses 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 Canadian Patent Application No. 536,646 (EP0 Publication No. 0,245,192, published on November 11, 1987), filed May 8, 1987, in the names of Marc S. Newkirk et al.
and entitled ~Shaped Ceramic Composites and Methods of Making the Same". In accordance with the method in this Canadian 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 provid~ng the preform with a barrier means, as disclosed in commonly owned Canadian Patent Appllcation No. 536,645 (EP0 Publication No. 0,245,193, published on November 11, 1987), filed May 8, 1987, 1n the names of Marc S. Newkirk et al. and entitled "Method of Making Shaped Ceramic Composites with the Use of a Barrier" This method produces shaped self-supporting ceramic bodies, including shaped ceram~c 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 Canadian Patent Application No. 528,275 (EPO
Publication No. 0,234,704, published on September 2, 1987), filed January 27, 1987, in the names of Marc S. Newkirk et al. and entitled l'Inverse Shape Replication Method of Making Ceramic Composite Articles and Articles Obtained Therebyt', and in commonly owned Canadian Patent Application No. 542,270-1 (EPO
Publication No. 0,259,239, published on March 9, 1988), filed July 16, 1987, in the name of Marc S. Newkirk and entitled "Method of Making Ceramic Composite Articles With Shape Replicated Surfaces and Articles Obtained Thereby".

Summarv 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 aluminum parent metal with an oxidant, including a vapor-phase oxidant, and a filler coated, at least in~tially, 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 partlally 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 intr~nsic 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.

- 1~07097 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 aluminum 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 lay-up, packed bed or preshaped as a preform, is positioned or oriented adjacent to the aluminum 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 reactton product within the bed, and to permit the gaseous oxidant (if a gaseous oxldant ~s 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 th~s temperature range, the molten metal reacts w~th the oxidant to form the ox~dation 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 oxidat10n reaction product towards and into contact with the oxidant such that the oxidation reaction product contlnues to form at the ~nterface between the ox~dant and previously formed oxidat~on reaction product, thus ~nf~ltrating the ad~acent f111er material. The reaction is continued for a time sufficient to infiltrate at least a portion of the filler mater~al w~th a polycrystall~ne material consisting essentially of the oxidat~on reaction product and one or ~307097 more metallic cons~ituents, such as unoxidized constituents of the parent metal or of the dopant, dispersed or distributed through the polycr~stalline 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 patents, 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 sllicon source (e.g. silica) is reduced by the molten aluminum 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 ox1dation reaction product. For example, sillcon carbide will oxidize at or on the surface at elevated temperatures in alr to form a sillca 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 aluminum parent metal to yield a silicon dopant which promotes growth of the polycrystalline matrix through the silicon carbide filler. In additlon, the silica coating on silicon carbide particles is advantageous in that during the matrlx format~on react~on it lncreases the local silicon concentration in the unoxidized aluminum parent metal and thereby reduces the tendency for formatlon of Al4C3 during the matrlx growth process. A14C3 is undeslrable 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 s11icic 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 ut111zed as a dopant, or a portion only is utilized as dopant, the remainder be1ng with the filler and becoming embedded by the matrix. Certain fillers, such as s11icon carbide, will oxid1ze to form S102 at the elevated temperatures of the process, and the cond1tions are controlled to lim1t the ox1dat10n so as to produce a s111ca coat1ng reducible by the molten parent metal. When des1red, a separate coat1ng mater1al may be appl1ed to the ftller wh1ch, on hèat1ng, produces the s111cic compound. The coating of s11icon source, e.g. sil1cic compound, may be formed or produced by first prefir1ng or heat1ng a su1table f111er in an oxygen-conta1n1ng atmosphere. The prefired f111er hav1ng thereon the coating 1s employed subsequently as a filler mater1al. For example, a preform can be fabr1cated of silicon carbide part1culate or alum1na part1culate coated w1th a s111ceous precursor or compound such as tetraethyl orthos11icate. The preform is then pref1red or heated 1n a1r to form an ox1de sk1n of s111ca on the s111con carb1de particles or alum~na particles of the preform. The preform then can be employed as a ceram1c compos1te raw material possess1ng an 1ntrins1c source of silicon , ~''''' , . ': ,' :
. .

.~, , ., . ,.. ~.~

1~07097 dopant material. Alte~rnatively, 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 11mitation, lndustrial, structural and technical ceramic bodies for such applications where electrical, wear, thermal, structural, or other features or properties are lmportant or beneficlal; and is not intended to ~nclude recycle or waste materials such as might be produced as unwanted by-products in the processing of molten metals.
As used ~n this specification and the appended claims, the terms below are defined as follows:
~ Ceramic~ is not to be unduly construed as being limited to a ceramic body in the class~cal sense, that ~s, in the sense that it consists entirely of non-metallic and inorganic mater~als, but, rather refers to a body which is predominantly ceram~c with respect to e~ther compositlon or dom~nant properties, although the body may conta~n minor or substantial amounts of one :,... -. .....

g 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.
~ Oxidation reaction product~ generally means aluminum as 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 ~oxidation reaction product~ under this definition includes the product of reaction of the aluminum metal with an oxidant such as those described in this application.
~ Oxidant~ means one or more suitable electron acceptors or electron sharers and may be a solid, a liauid or a gas (vapor) or some combination of these (e.g., a solid and a gas) at the process conditions.
~ Parent metal~, refers to aluminum, which is the precursor for the polycrystalline oxidation reaction product, and includes relatively pure aluminum, commercially available aluminum with impurities and/or alloying constituents, or an alloy of aluminum in which aluminum as the precursor is the maJor or most significant constituent in forming the oxidation reaction product.
~ Silicon source~ refers to elemental silicon or a s~licic compound which will prov1de a dopant material and/or promote wetting of the filler by the molten parent metal under the process conditions.

~lef DescriDtion of the Drawinas Figure 1, 2, and 3 are photographs in plan and side views, respectively, of a composite prepared in accordance with Example 2. In each of these f~gures a portion of the grown composite has been removed by sectionlng for further analysis.

~30709~

Figure 4 is a photomic~rograph at SOX of a composite structure showing coated filler particles embedded by a ceramic matrix prepared in accordance with Example 3.

Detailed DescriDtion of the Invention and Preferred Embodiments In practicing the process of the present invention, the aluminum 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 aluminum 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 aluminum parent metal, thereby promoting the oxidation reaction by serving as a dopant. It is also belleved that the silicon source also serves to enhance wettability of the flller by the parent metal. The bed is permeable to the vapor-phase oxidant (e.g. air), and to the growth of oxldation reactlon product matrix in order to permlt development of the oxldation reaction product and infiltration thereby of the filler. As explained in the commonly owned patent applications and patents, 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 wlth one preferred embodlment of this invention, a slliclc compound, when heated to a suitable temperature in an oxygen-containing atmosphere, forms an oxlde coating whlch serves as a dopant material. The formation of the oxlde 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 "filler~ or "filler material" 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 aluminum parent metal. Thus, in such a case the filler, ~er 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 dlfferent composition. For example, a particularly useful system of this type is tetraethyl orthosilicate applied to zirconium oxide fibers, which on drying or mild heating ta dissociate the material, will form a silica coat~ng. 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 prefired 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 aluminum 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 coat1ng has formed on the silicon carbide f~ller material, th~s temperature range can be malntained 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 degradat~on under the process conditions. For example, boron nitrlde particles need to be protected from oxidation and reaction with molten aluminum, and the silicon coating affords this protectlon and yet meets the other criteria of the invention.

13~7097 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 slip casting, ln~ection molding, transfer molding, vacuum forming, etc.
It is preferable that the filler material be preshaped, having at least one surface boundary, and should reta~n 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 aluminum. 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 aluminum as the parent metal, is placed in a furnace that is supplied with a suitable vapor-phase oxidant, 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 700-1450C, 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 reactlon product. Upon continued exposure to the oxidizing environment, w~th~n an appropriate temperature region, the remaining molten metal ~s progressively drawn into and through the oxidation reaction product ln the d~rect~on of the oxidant. On contact with the oxidant, the molten metal w~ll react to form additional oxidation reaction product. At least a port10n 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 react~on product toward the oxidant such that the polycrystall~ne oxidation reaction product ~nfiltrates at least a portion of the filler material. The coating of silicon source produced on the filler mater~al, 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 "over-growth" 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 temperature1 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 constttuents may be at least partially interconnected. The resulting ceramic composite product will possess the dimensions and geometric configuration of the orlginal preform if used, and especially good fidelity is achieved by the use of a barrier means.
The vapor-phase oxidant used in the oxidation reaction process is normally gaseous or is 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.
Typical vapor (gas) oxldants include additionally, for example, nitrogen or a nitrogen-containing gas, and mixtures such as air, H2/H20 and CO/C02, the latter two (i.e., H2/H20 and CO/C02) being useful in reducing the oxygen actlvity of the environment relative to desirable oxldizable constituents of the preform. Oxygen or gas mixtures containing oxygen (including air) are ~''i 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 "oxygen-containing gas~ oxidant but not within the definition of a ~nitrogen-containing gas"
oxidant. An example of a nitrogen-containing oxidant as used herein and in the claims is "forming gas", which typically contains about 96 volume percent nitrogen and about 4 volume percent hydrogen.
An oxidant which is liquid or solid at the process conditions may be employed in conjunction with the vapor-phase oxidant. Such additional ox1dants may be partlcularly useful in enhancing oxidation of the parent metal preferentially within the filler material rather than beyond its surfaces.
That is, the use of such additional 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 sillcon carbide filler material employed as a preform, this enhanced env~ronment is beneficial in promoting matrix development within the preform to the boundary and minimizing overgrowth.
When a solid oxldant is employed additionally to the vapor-phase oxidant, lt may be dispersed through the entire volume of the filler material, or through a portion only of filler material ad~acent the parent metal, such as in particu1ate form and admixed with the flller material. Any suitable solid oxidant may be employed depending upon its compatibility with the vapor-phase . . .
.' . ' ':~ , ' -. : . ., i307097 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.
If a liquid oxidant is employed additionally to the vapor-phase oxidant, 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. 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 parent metal. 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 conf~guration or geometry. In the event the preform includes a solid oxidant and/or liquid oxidant which may accompany the vapor-phase oxidant, the preform then should be sufficiently porous or permeable to permit and accept growth of the oxidation reaction product or~ginating from the solid andlor liquid oxidant.
The present invention provides in one embodiment a composite filler material wh~ch, when heated to a suitable temperature ~n an oxygen-containing atmosphere, provides an intrinsic source of a dopant material; that is, for example, silicon carbide as filler is an intrins~c 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 aluminum 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 (l), (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 s11icic 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 patents. 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 particu1ar 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 oxidiz~ng environment, and the process conditions.
Dopants useful ~n 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 O.l-lOY. by weight based on the total weight of the resulting doped metal.
Where desired, s~licon metal may be alloyed w~th the parent metal to supplement the silicon source as coating on a filler. In such examples, a ,9 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, phosphorus, 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. A11 of the dopant materials as explained in the commonly owned patent applications and patents, 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 invent10n will usually be a coherent product wherein between about 5% and about 98% by volume of the total volume of the ceramic compos~te product is comprised of filler embedded by a polycrystalline ceramic matrix. The polycrystalllne ceramic matrix is usually comprised of, when air or oxygen is the oxidant, about 60Y. to about 99% by volume (of the ~olume of polycrystalline matrix) of interconnected alpha-aluminum oxide and about 1% to 40% by weight (same basis) of metallic constituents such as non-oxidized - ~07097 constituents of the parent metal or reduced metal from the dopant or the oxidant.
As disclosed in copending Canadian Patent Application No. 536,645, filed May 8, 1987, 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 the 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 ind permeability of the barrier means. Still further, the barrier means may lnclude a suitable refractory particulate to reduce any possible shr1nkage 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.

~' ~ 1 3 0 7 0 9 7 ExamDle 1 In accordance with the present invention, a ceramic structure was fabricated comprising an alumina oxidation reaction product embedding beta-SiC
whiskers supplied by NIKKEI TECHN0-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 beta-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 beta-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 beta-SiC whiskers (the control using distilled water). The beta-SiC whiskers coated with colloidal silica were infiltrated through the entire thickness of the preform. The infiltration of the beta-SiC
whiskers with sodium silicate solution occurred to approximately the center of the preform.

~t ExamDle 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~O solution) as a source of silicon.
The colloidal silica coating on the silicon carbide particles was done by preparing two preforms measuring 2 x 2 x 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'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 .15% S1, .6% Mg, 6% Zn) were placed into a refractory bed of Wollastonite fibers (NYAD FP from Peltz-Rowley Chemical Co.) which was contained in a refractory vessel, such that one 2 x 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 x 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 sandblas~ed lightly to remove unembedded preform materials.
Figures l(a), 1(b)7 2(a), and 2(b) are photographs of the resulting composite materials utilizing preforms with the colloidal silica coating (Figures 2(a) and 2(b) involving the recast preform), which illustrate good growth; and Figures 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 part1cles (approximately 50 mesh size). The boron nitride particles were coated with silicon (accomplished by chemical vapor deposition) to protect the boron r.itride from degradation and to serve as a source of sil1con 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 llOO-C. The furnace was held at llOO~C for 48 hours, and cooled down to ambient. The resulting ceramic composite was recovered. Figure 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.

Claims

1. A method for producing a self-supporting ceramic composite, comprising (1) a ceramic matrix obtained by oxidation of an aluminum parent metal to form a polycrystalline material comprising oxidation reaction product of the parent metal with at least one oxidant including a vapor-phase and a filler infiltrated by said matrix, said method comprising: (A) orienting said aluminum parent metal and a filler material relative to each other so that formation of the oxidation reaction product will occur in a direction towards and into said filler, said filler material bearing a coating of a silicon source on at least a portion of said filler different in composition from a primary composition of said filler, said silicon source possessing intrinsic doping properties; (B) heating said aluminum parent metal to a temperature above its melting point but below the melting point of the oxidation reaction product to form a body of molten aluminum parent metal; (C) reacting the molten aluminum parent metal with said oxidant at said temperature to form the oxidation reaction product:
(D) maintaining at said temperature at least a portion of said oxidation reaction product in contact with and between said body of molten metal and said oxidant, to progressively draw molten metal through the oxidation reaction product towards the oxidant and into the filler material so that fresh oxidation reaction product continues to form within said filler at an interface between the oxidant and previously formed oxidation reaction product; and (E) continuing said reaction for a time sufficient to infiltrate at least a portion of said filler with said polycrystalline material.

2. The method as claimed in claim 1, wherein said silicon source comprises a silicic compound reducible by said molten parent metal under the process conditions.

3. The method as claimed in claim 1, wherein said coating of said silicon source is produced by oxidation or dissociation of a silicic precursor.

4. The method as claimed in claim 3, wherein said oxidation or dissociation to produce said coating of said silicon source is conducted prior to said orienting step in (A).

5. The method as claimed in claim 3 or 4, wherein said oxidation to form said coating is conducted by heating said silicon source in the presence of an oxygen-containing atmosphere to form a coating of silica.

6. The method as claimed in claim 1, 2 , or 3, wherein said filler comprises a material selected from the group consisting of silicon carbide, silicon nitride, alumina, zirconia, and boron nitride.

7. The method as claimed in claim 4, wherein said filler comprises a material selected from the group consisting of silicon carbide, silicon nitride, alumina, zirconia, and boron nitride.

8. The method as claimed in claim 1, 2 or 3, wherein said heating step in (B) is at a temperature of from about 700°C
to about 1450°C.

9. The method as claimed in claim 4, wherein said heating step in (B) is at a temperature of from about 700 C to about 1450°C.

10. The method at claimed in claim 1, 2 or 3, additionally comprising using at least one dopant material, in addition to said silicon source, in conjunction with said parent metal.

11. The method as claimed in claim 4, additionally comprising using at least one dopant material, in addition to said silicon source, in conjunction with said parent metal.

12. The method as claimed in claim 1, 2 or 3, additionally comprising shaping said filler material into at least one permeable, self-supporting preform.

13. The method as claimed in claim 4, additionally comprising shaping said filler material into at least one permeable, self-supporting preform.

14. The method as claimed in claim 1, 2 or 3, additionally comprising shaping said filler material into at least one permeable, self-supporting preform, additionally comprising overlaying at least a portion of said preform with a barrier means for inhibiting the formation of said oxidation reaction product therebeyond.

15. The method as claimed in claim 4, additionally comprising shaping said filler material into at least one permeable, self-supporting preform, additionally comprising overlaying at least a portion of said preform with a barrier means for inhibiting the formation of said oxidation reaction product therebeyond.

16. The method as claimed in claim 1, 2 or 3 further comprising incorporating at least one additional oxidant selected from the group consisting of a solid oxidant and liquid oxidant into at least a portion of said filler material, and reacting said molten metal with said at least one additional oxidant, so that said polycrystalline material further comprises the oxidation reaction product of said parent metal with said at least one additional oxidant.

17. The method as claimed in claim 4 further comprising incorporating at least one additional oxidant selected from the group consisting of a solid oxidant and liquid oxidant into at least a portion of said filler material, and reacting said molten metal with said at least one additional oxidant, so that said polycrystalline material further comprises the oxidation reaction product of said parent metal with said at least one additional oxidant.

18. The method as claimed in claim 1 or 2, wherein said silicon source comprises at least one material selected from the group consisting of silica, a silicate, and silicon.

19. The method as claimed in claim 4, wherein said oxidation to produce said coating utilizes an oxidant comprising an oxidant selected from the group consisting of a nitrogen-containing gas, an oxygen-containing gas and mixtures thereof.

20. A method for producing a self-supporting ceramic composite comprising a ceramic matrix obtained by oxidation of an aluminum parent metal to form a polycrystalline material comprising oxidation reaction product of the parent metal with at least one oxidant including a vapor-phase oxidant and a filler infiltrated by said matrix, said method comprising:
(a) orienting said aluminum parent metal and a filler material relative to each other so that formation of the oxidation reaction product will occur in a direction towards and into said filler;
(b) heating said aluminum parent metal and said filler to a temperature above the melting point of the parent metal but below the melting point of the oxidation reaction product to form a body of molten aluminum parent metal, said filler oxidizing or dissociating at said temperature to produce a coating of a silicon source on at least a portion of said filler different in composition from a primary composition of said filler, said silicon source possessing intrinsic doping properties;
(c) reacting the molten aluminum parent metal with said oxidant at said temperature to form the oxidation reaction product;
(d) maintaining at said temperature at least a portion of said oxidation reaction product in contact with and between said body of molten metal and said oxidant, to progressively draw molten metal through the oxidation reaction product towards the oxidant and into the filler material so that fresh oxidation reaction product continues to form within said filler at an interface between the oxidant and previously formed oxidation reaction product; and (e) continuing said reaction for a time sufficient to infiltrate at least a portion of said filler with said polycrystalline material.

21. The method as claimed in claim 20, wherein said oxidation to form said coating is conducted by heating said silicon source in the presence of an oxygen-containing atmosphere to form a coating of silica.

22. The method as claimed in claim 20, wherein said heating step in (b) is at a temperature of from about 700 C to about 1450°C.

23. The method as claimed in claim 20, wherein said heating step in (b) is conducted in the presence of an oxygen-containing atmosphere at a temperature of from about 1000°C
to about 1450°C to produce said coating of said silicon source, an after said coating has formed in situ on a substantial portion of said filler, altering the temperature to continue formation of said oxidation reaction product.

24. The method as claimed in claim 20, further comprising using at least one dopant material, in addition to said silicon source, in conjunction with said parent metal.

25. The method as claimed in claim 20, further comprising shaping said filler material into at least one permeable, self-supporting preform.

26. The method as claimed in claim 20, further comprising overlaying at least a portion of said preform with a barrier means for inhibiting the formation of said oxidation reaction product therebeyond.

27. The method as claimed in claim 20, further comprising incorporating at least one additional oxidant selected from the group consisting of a solid oxidant and a liquid oxidant into at least a portion of said filler material, and reacting said molten metal with said at least one additional oxidant, so that said polycrystalline material further comprises the oxidation reaction product of said parent metal with said at least one additional oxidant.

28. The method as claimed in claim 1 or 20, wherein said vapor-phase oxidant used to form said oxidation reaction product comprises an oxidant selected from the group consisting of a nitrogen-containing gas, an oxygen-containing gas and mixtures thereof.

29. The method as claimed in claim 1 or 20, wherein said vapor-phase oxidant used to form said oxidation reaction product comprises an oxidant selected from the group consisting of a nitrogen-containing gas, an oxygen-containing gas and mixtures thereof, wherein said vapor-phase oxidant comprises an oxidant selected from the group consisting of air, a H2/H2O mixture, a CO/CO, mixture, and mixtures thereof.

30, A self-supporting ceramic composite body comprising a ceramic matrix obtained by oxidation of a parent metal to form a polycrystalline material comprising an oxidation reaction product of the parent metal with at least one oxidant including a vapor-phase oxidant and a filler infiltrated by said matrix, formed by a method comprising:
(A) orienting said parent metal and a filler material relative to each other so that formation of the oxidation reaction product will occur in a direction towards and into said filler, said filler material bearing a coating of a

31 silicon source on at least a portion of said filler different in composition from a primary composition of said filler, said silicon source possessing intrinsic doping properties;
(B) heating said parent metal to a temperature above its melting point but below the melting point of the oxidation reaction product to form a body of molten parent metal; (C) reacting the molten parent metal with said oxidant at said temperature to form the oxidation reaction product: (D) maintaining at said temperature at least a portion of said oxidation reaction product in contact with and between said body of molten metal and said oxidant, to progressively draw molten metal through the oxidation reaction product towards the oxidant and into the filler material so that fresh oxidation reaction product continues to form within said filler at an interface between the oxidant and previously formed oxidation reaction product; and (E) continuing said reaction for a time sufficient to infiltrate at least a portion of said filler with said polycrystalline material, thereby forming said self-supporting ceramic composite body.

31. A self-supporting ceramic composite body comprising a ceramic matrix obtained by oxidation of an aluminum parent metal to form a polycrystalline material comprising an oxidation reaction product of the parent metal with at least one oxidant including a vapor-phase oxidant and a filler infiltrated by said matrix, formed by a method comprising:
(a) orienting said aluminum parent metal and a filler material relative to each other so that formation of the oxidation reaction product will occur in a direction towards and into said filler;
(b) heating said aluminum parent metal and said filler to a temperature above the melting point of the parent metal but below the melting point of the oxidation reaction product to form a body of molten aluminum parent metal, said filler at least partially oxidizing or dissociating at said temperature to produce a coating of a silicon source possessing intrinsic doping properties on at least a portion of said filler, said coating having a different composition

32 from a primary composition of said filler;
(c) reacting the molten aluminum parent metal with said oxidant at said temperature to form the oxidation reaction product;
(d) maintaining at said temperature at least a portion of said oxidation reaction product in contact with and between said body of molten metal and said oxidant, to progressively draw molten metal through the oxidation reaction product towards the oxidant and into the filler material so that fresh oxidation reaction product continues to form within said filler at an interface between the oxidant and previously formed oxidation reaction product; and (e) continuing said reaction for a time sufficient to infiltrate at least a portion of said filler with said polycrystalline material, thereby forming said self-supporting ceramic composite body.