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 with respect to certain properties, such as corrosion resistance, hardness, modulus of elasticity, and refractory capabilities, when compared with metals.
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 structure typically comprises a ceramic matrix which incorporates one or more diverse types of filler materials such as particulates, fibers, rods, and the like.
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 (hereinafter sometimes referred to as Commonly Owned Patent Applications), overcome these limitations or difficulties and provide novel methods for reliably producing ceramic materials, including composites. The method is disclosed generically in Commonly Owned U.S. Pat. No. 4,713,360, issued on Dec. 15, 1987 in the names of Marc S. Newkirk et al and entitled "Novel Ceramic Materials and Methods for Making Same" (a foreign counterpart to this application was published in the EPO on Sept. 25, 1985 as application Ser. No. 0,155,831). This Patent discloses a method of producing self-supporting ceramic bodies grown as the oxidation reaction product of a molten parent precursor metal which is reacted with a vapor-phase oxidant to form an oxidation reaction product. Molten metal migrates through the formed oxidation reaction product to react with the oxidant thereby continuously developing a ceramic polycrystalline body which can, if desired, include an interconnected metallic component. The process may be enhanced by the use of one or more dopants alloyed with the parent metal. For example, in the case of oxidizing aluminum in air, it is desirable to alloy magnesium and silicon with the aluminum to produce .alpha.-alumina ceramic structures. This method was improved upon by the application of dopant materials to the surface of the parent metal, as described in Commonly Owned and Copending U.S. patent application Ser. No. 220,935, filed on Jun. 23, 1988 and allowed on Nov. 17, 1988, which is a Rule 62 Continuation of U.S. Ser. No. 822,999, filed Jan. 27, 1986, which is a continuation-in-part of Ser. No. 776,965, filed Sept. 17, 1985, which is a continuation-in-part of Ser. No. 747,788, filed Jun. 25, 1985, which is a continuation-in-part of Ser. No. 632,636, filed Jul. 20, 1984, all in the names of Marc S. Newkirk et al and entitled "Methods of Making Self-Supporting Ceramic Materials" U.S. application Ser. Nos. 822,999; 776,965; 747,788; and 632,636 have been abandoned in favor of allowed U.S. application Ser. No. 220,935 (a foreign counterpart to U.S. application Ser. No. 747,788, was published in the EPO on Jan. 22, 1986, as application Ser. No. 0,169,067).
This oxidation phenomenon was utilized in producing ceramic composite bodies as described in Commonly Owned and Copending U.S. patent application Ser. No. 819,397, filed Jan. 17, 1986, and allowed on Aug. 1, 1988 which is a continuation-in-part of Ser. No. 697,876, filed Feb. 4, 1985, now abandoned both in the names of Marc S. Newkirk et al and entitled "Composite Ceramic Articles and Methods of Making Same" (a foreign counterpart to this application was published in the EPO on Sept. 3, 1986 as application Ser. No. 0,193,292). These applications disclose novel methods for producing a self-supporting ceramic composite body by growing an oxidation reaction product from a parent metal into a permeable mass of filler, (e.g., a silicon carbide particulate filler or an alumina particulate 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 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. In accordance with the method in this U.S. Patent Application, the developing oxidation reaction product infiltrates a permeable preform of filler material (e.g., a silicon carbide preform material) in a direction towards a defined surface boundary. It was discovered that high fidelity 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, and allowed on Oct. 11, 1988 in the names of Marc S. Newkirk et al. This method produces shaped self-supporting ceramic bodies, including shaped ceramic composites, by growing the oxidation reaction product of a parent metal to a barrier means spaced from the metal for establishing a boundary or surface. Ceramic composites having a cavity with an interior geometry inversely replicating the shape of a positive mold or pattern are disclosed in Commonly Owned and Copending U.S. patent application Ser. No. 823,542, filed Jan. 27, 1986, in the names of Marc S. Newkirk, et al, and in U.S. patent application Ser. No. 896,157, filed Aug. 13, 1986, in the name of Marc S. Newkirk. U.S. application Ser. No. 823,542 was allowed on Sept. 20, 1988 and U.S. application Ser. No. 896,157 was allowed on Dec. 14, 1988.
As explained in the above-described Commonly Owned Patent Applications, 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 onto the parent metal for example, as elemental silicon or as silica. As discussed in Commonly Owned patent application Ser. No. 070,006, filed Jul. 6, 1987, and now abandoned which is a continuation-in-part of Ser. No. 908,473, filed Sept. 16, 1987, both in the names of H. Daniel Lesher et al, and entitled "A Method of Making Ceramic Composites", a silicon carbide particulate can be a filler material because of the favorable properties of the resultant composite and because the silicon carbide filler is particularly compatible with the matrix growth process. This compatibility arises from the formation, at elevated temperatures, of a silica film on the external surfaces of the silicon carbide particles. Thus, a silicon carbide preform can be particularly useful because it serves not only as a filler material, but also provides a source of dopant material because of its intrinsic doping properties. Stated more particularly, a silica film formed on a silicon carbide material can be reduced by molten aluminum parent metal to yield a silicon dopant which promotes growth of the polycrystalline matrix through the silicon carbide filler. In addition, the silica coating on silicon carbide particles is advantageous in that there is a reduction of the tendency for the formation of Al.sub.4 C.sub.3 during growth of the oxidation reaction product of the parent metal. Such Al.sub.4 C.sub.3 is undesirable because it is an unstable product in the presence of moisture levels normally present in ambient air, resulting in the evolution of methane and the degradation of mechanical properties of the resulting composite body.
Moreover, as described in Commonly Owned and Copending U.S. patent application Ser. No. 908,117, filed on Sept. 16, 1986, and allowed on Nov. 14, 1988 in the names of Stanley J. Luszcz, et al, and entitled "Dense Skin Ceramic Structure and Method of Making Same", a terminal region can be formed integral with a first region. Stated more particularly, a first region is formed by reacting a molten parent metal with an oxidant to form an oxidation reaction product which, optionally, can embed a filler material, thereby forming a ceramic composite body. Transport of molten parent metal used for formation of the first region is thereafter attenuated or discontinued and the first region is then induced to subsequently react with an oxidant to form a terminal region of oxidation reaction product on at least a portion of a surface of the first region. The formed terminal region may have a finer microstructure and/or a different chemical composition from the first region from which the terminal region was formed.
The above-discussed Commonly Owned Patent Applications, and Patent, disclose methods for producing ceramic and/or ceramic composite articles which overcome some of the traditional limitations or difficulties in producing ceramic articles as substitutes for metals in end-use applications.
Common to each of these Commonly Owned Patent Applications, and Patent, is the disclosure of embodiments of a ceramic body comprising an oxidation reaction product interconnected in one or more dimensions (usually in three dimensions) and, if desired, one or more metallic constituents or components. The volume of metal, which typically includes non-oxidized constituents of the parent metal and/or metal reduced from an oxidant or a reactive filler, depends on such factors as the temperature at which the oxidation reaction product is formed, the length of time during which the oxidation reaction is allowed to proceed, the composition of the parent metal, the presence of dopant materials, the presence of reduced constituents of any oxidant or reactive filler materials, etc. Some of the metallic components can be isolated or enclosed, but also a substantial volume percent of metal can be interconnected and accessible, or rendered accessible, from an external surface of the ceramic body. It has been observed for these ceramic bodies that this metal-containing component or constituent (both isolated and interconnected) can range from about 1 to about 40 percent by volume, and sometimes higher. The metallic component can impart certain favorable properties to, or improve the performance of, the ceramic articles in many product applications. For example, the presence of metal in the ceramic structure may have a substantial benefit with respect to imparting fracture toughness, thermal conductivity, or electrical conductivity to the ceramic body.
The entire disclosures of all of the foregoing Commonly Owned Patent Applications and U.S. Patent are expressly incorporated herein by reference.
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