For the past several years, extensive research has been devoted to the development of metal-ceramic composites such as aluminum reinforced with carbon, boron, silicon carbide, silica, or alumina fibers, whiskers, or particles. Metal-ceramic composites with excellent high temperature yield strengths and creep resistance have been fabricated by the dispersion of very fine (less than 0.1 micron) oxide or carbide particles throughout the metal or alloy matrix.
Prior art techniques for the production of metal-ceramic composites may be broadly categorized as powder metallurgical approaches, molten metal techniques and internal oxidation processes. The powder metallurgical type production of such dispersion-strengthened composites would ideally be accomplished by mechanically mixing metal powders of approximately 5 micron diameter or less with the oxide or carbide powder (preferably 0.01 micron to 0.1 micron). High speed blending techniques or conventional procedures such as ball milling may be used to mix the powder. Standard powder metallurgy techniques are then employed to form the final composite. Conventionally, however, the ceramic component is large, i.e. greater than 1 micron, due to a lack of availability, and high cost, of very small particle size materials since their production is energy intensive, time consuming and costly in capital equipment. Furthermore production of very small particles inevitably leads to contamination of the particles with oxides, nitrides and materials from the attritor such as steel. The presence of these contaminants inhibits particle to metal bonding which in turn compromises the mechanical properties of the resultant composite. Further, in many cases where the particulate materials are available in the desired size, they are extremely hazardous due to their pyrophoric nature.
Alternatively, molten metal infiltration of a continuous ceramic skeleton has been used to produce composites. In most cases elaborate particle coating techniques have been developed to protect ceramic particles from molten metal during molten metal infiltration and to improve bonding between the metal and ceramic. Techniques such as this have resulted in the formation of silicon carbide-aluminum composites, frequently referred to as SiC/Al, or SiC aluminum. This approach is only suitable for large particulate ceramics (e.g. greater than 1 micron). The ceramic material, such as silicon carbide, is pressed to form a compact, and liquid metal is forced into the packed bed to fill the interstices. Such a technique is illustrated in U.S. Pat. No. 4,444,603, of Yamatsuta et al, issued Apr. 24, 1984. Because of the necessity for coating techniques and molten metal handling equipment capable of generating extremely high pressures, molten metal infiltration has not been a practical process for making metal-ceramic composites, particularly for making composites of submicron particles where press size and pressures needed would be extremely large.
As mentioned previously, the presence of oxygen in ball-milled powders used in the prior art metallurgy techniques, or in molten metal infiltration, can result in oxide formation at the interface of the ceramic and the metal. The presence of such oxides may inhibit interfacial binding between the ceramic phase and the matrix, thus adversely effecting ductility of the composite. Such weakened interfacial contact may also result in reduced strength, loss of elongation, and facilitated crack propagation.
Internal oxidation of a metal containing a more reactive component has also been used to produce dispersion strengthened metals, such as internally oxidized aluminum in copper. For example, when a copper alloy containing about 3 percent aluminum is placed in an oxidizing atmosphere, oxygen may diffuse through the copper matrix to react with the aluminum, precipitating alumina. This technique, although limited to relatively few systems since the two metals must have a wide difference in chemical reactivity, has offered a feasible method for dispersion hardening. In addition, the highest possible level of dispersoids formed in the resultant dispersion strengthened metal is generally insufficient to impart significant changes in properties such as modulus, hardness and the like.
Because of the above-noted difficulties with conventional processes, the preparation of metal-ceramic composites with submicron ceramic dispersoids for commercial applications has not been economically feasible nor practical.
In recent years, numerous ceramics have been formed using a process referred to as self-propagating high-temperature synthesis (SHS), which involves an exothermic, self-sustaining reaction which propagates through a mixture of compressed powders. Generally the SHS process is ignited by electrical impulse, laser, thermite, spark, etc. The SHS process involves mixing and compacting powders of the constituent elements, and igniting the green compact with a suitable heat source. On ignition, sufficient heat is released to support a self-sustaining reaction, which permits the use of sudden, low power initiation of high temperatures, rather than bulk heating over long times at lower temperatures. Exemplary of these techniques are the patents of Merzhanov et al. In U.S. Pat. No. 3,726,643, there is taught a method for producing high-melting refractory inorganic compounds by mixing at least one metal selected from groups IV, V, and VI of the Periodic System with a non-metal such as carbon, boron, silicon, sulfur, or liquid nitrogen, and heating the surface of the mixture to produce a local temperature adequate to initiate a combustion process. In U.S. Pat. No. 4,161,512, a process is taught for preparing titanium carbide by ignition of a mixture consisting of 80-88 percent titanium and 20-12 percent carbon, resulting in an exothermic reaction of the mixture under conditions of layer-by-layer combustion. These references deal with the preparation of ceramic materials, in the absence of a second non-reactive metallic phase.
Similarly, U.S. Pat. No. 4,431,448 teaches preparation of a hard alloy by intermixing powders of titanium, boron, carbon, and a Group I-B binder metal, such as copper or silver, compression of the mixture, local ignition thereof to initiate the exothermic reaction of titanium with boron and carbon, and propagation of the reaction, resulting in an alloy comprising titanium diboride, titanium carbide, and the binder metal. This reference, however, is limited to the use of Group I-B metals such as copper and silver, as binders, and requires local ignition. As is set forth in the patent, products made by this method have low density, requiring subsequent compression and compaction.
It has been recognized for a considerable period of time that certain types of exothermic reactions can be used in forming welds. For example, U.S. Pat. No. 1,872,254 to deGolyer discloses that refractory metals and alloys or metals and alloys having high thermal conductivity can be welded by means of utilizing the exothermic values of the reaction of certain intermediate compounds containing boron, such as borides or chemical compounds of boron with metal, with metallic oxides in conjunction with heat supplied by an electric arc, oxyacetylene flame or mechanical means. The heat supplied to the weld area aids in melting the metal being welded and and metal being added, and also raises the temperature of the metal oxide and boride to a point at which the boride will act to reduce the oxide.
This patent further discloses that when, it is desirable to use a boride which is substantially insoluble in the metal welded, the weld metal, or both, intermetallic compounds of boron with barium, calcium, lithium, silicon and magnesium are particularly valuable. When a boride is employed which is not soluble in either the welded metal or the weld metal, no residual metallic impurities can result from the use of the intermetallic boron compound, and consequently the composition and character of the metals entering into the weld will not be altered.
U.S. Pat. No. 3,415,697 to Bredzs et al discloses the fluxless brazing of aluminum and aluminum alloys by employing an unreacted mixture of a particulate aluminum alloy of a first element and a particulate component containing a second element. The first and second elements are capable of reacting exothermically to form a high-melting intermetallic compound when the mixture is heated to the molten state. In brazing operations, wherein the surfaces to be joined are not subjected to melting, the mixture is placed in contact with the aluminum surfaces to be joined and is heated to melt the mixture and produce an exothermic reaction. The filler metal thus formed is permitted to solidify to form a brazed joint. This reference is limited to brazing, as opposed to welding, however, and does not teach the formation of ceramic materials.
In U.S. Pat. No. 3,890,168 to Shumway, there is disclosed an apparatus, process and composition for producing a weldable joint between ferrous parts. In the process, the opposing wall surfaces of the parts to be joined are supported so as to define a gap therebetween and a solid weld element is nested in the gap. The weld element includes a suitable ignitable or pyro composition adapted to substantially instantaneously ignite to produce molten ferrous weld material and to heat opposing wall surfaces so as to receive and fuse with the molten weld material. The weld element, in one embodiment, was in the form of a rod having a core of flux material, the rod being composed of suitable exothermic reactants, slag control materials, ferrous weld material sources and alloying agents.