Ceramic-metal composites
This application is a continuation-in-part of PCT patent application Ser. No. PCT/US93/08835, filed Sep. 17, 1993, which is a continuation-in-part of U.S. patent application Ser. No. 07/946,972, filed Sep. 17, 1992, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to ceramic-metal composite materials, or cermets, and methods of producing such composites. More particularly, the invention relates to a method for producing ceramic-metal composites by spontaneously infiltrating a metal into a porous ceramic and the composites produced thereby. 
     2. Description of Related Art 
     During the last few decades, ceramics have been investigated for use in many structural applications, particularly in high temperature environments. However, ceramic materials are not always well suited since they are brittle, have a limited ductility and low values of fracture toughness at low temperatures. In addition, the fracture strength of ceramics is not very reproducible since the average strength usually varies from one lot of parts to the next, which is attributed to the presence of processing flaws which can initiate fractures. A great deal of effort has been expended in an attempt to increase the fracture reliability of ceramic materials and to develop tough and creep-resistant ceramic composites. 
     One possible solution is the fabrication of a ceramic-metal composite, also commonly referred to as a cermet. Traditionally, ceramic metal composites have been produced in one of two ways; (1) by heating mixtures of ceramic and metal materials to obtain a metal matrix having a discrete ceramic phase, or (2) as disclosed in U.S. Pat. No. 2,612,443 by Goetzel at al., issued Sep. 30, 1952, by forming a sintered porous body that can be a metal, metal-carbide or metal-nitride, and infiltrating the porous body with a molten metal by the use of mechanical squeeze-casting or other means of applying pressure to force the molten metal into the voids within the porous body. 
     Other approaches for forming cermets have been used due to a lack of success in obtaining adequate control of cermet composition and form with traditional processes. For example, use of accelerated oxidation reactions and &#34;combustion wave&#34; exothermic reaction processes have been utilized to form cermets. See, for example, U.S. Pat. No. 4,988,645 by Holt et al., issued Jan. 29, 1991. 
     The LANXIDE process, such as that disclosed in U.S. Pat. No. 4,853,352 by Newkirk et al., issued Aug. 1, 1989, relates to a method for forming cermets whereby a molten parent metal is oxidized, usually in the presence of oxidation enhancing dopants, to create a three-dimensional interconnected ceramic-metal composite material which contains between 1% and 40% of the parent metal by volume. This process is limited in that only the parent metal is infiltrated into the oxide reaction product and the process takes extended periods of time, such as 48 hours or more. 
     Infiltration of molten metals into porous ceramic preforms by squeeze casting and by applying pressure to the molten metal is known, for example, see Verma and Dorcic, &#34;Performance Characteristics of Metal-Ceramic Composites Made by the Squeeze Casting Process&#34;, Ceramic Engineering Science Proc., Vol. 9, pp. 579-596 (1988). However, it is difficult to achieve near complete infiltration of the void space within the preforms without use of substantial pressure. In addition, when ceramic preform materials contain a high volume of porosity, the use of pressure in squeeze casting techniques can crumble the ceramic structure. The use of pressure can also preclude the formation of metal-matrix composites having complex shapes. Further, these processes require complex pressure dies and frequently require extensive flash removal, that is, removal of excess metal. 
     Infiltration using vacuum furnaces and using infiltration enhancers are also described in the art. U.S. Pat. No. 3,864,154 by Gazza et al., issued Feb. 4, 1975, discloses a method for the infiltration of aluminum or silicon into a cold-pressed compact of boron-containing ceramics (e.g., aluminum boride or silicon boride) in a vacuum furnace. It is disclosed that the infiltration process takes about 2 hours. 
     U.S. Pat. No. 4,828,008 by White et al. issued on May 9, 1989. White et al. disclose a method for infiltrating aluminum alloys into a permeable mass of loose ceramic powder, such as alumina. A nitrogen gas atmosphere must be used and magnesium must be alloyed into the aluminum metal to achieve spontaneous infiltration. U.S. Pat. No. 5,016,703 by Aghajanian et al. and issued on May 21, 1991, discloses a process for the spontaneous infiltration of aluminum into a ceramic preform that comprises a mass of particles, platelets, whiskers or fibers. An infiltration enhancer, such as magnesium turnings, is placed between the molten metal and the preform to enhance the infiltration. The infiltration time is on the order of about 5 hours. 
     U.S. Pat. No. 5,004,035 by Burke et al. issued Apr. 2, 1991, discloses the use of infiltration enhancers for infiltrating aluminum alloys into alumina or silicon carbide preforms that comprise loose particles of materials such as alumina or silicon carbide. After infiltration, which can take on the order of about 10 hours, the metal composite can be reheated and worked to vary the properties of the composite. 
     U.S. Pat. No. 5,017,533 by Newkirk et al. issued on May 21, 1991. Newkirk et al. disclose a method for producing a self-supporting ceramic body by oxidation of a molten precursor metal with a vapor-phase oxidant to form an oxidation reaction product. A second metal is incorporated into the molten flux during the oxidation reaction. For example, copper can be alloyed into aluminum which is then oxidized to form an alumina oxidation product. However, the oxidation process takes on the order of 48 hours or more. 
     U.S. Pat. No. 5,007,475 by Kennedy et al. issued on Apr. 16, 1991. This patent discloses the formation of a metal matrix composite body by the spontaneous infiltration of a molten matrix metal into a three-dimensional interconnected material. The metal is an aluminum alloy and the three-dimensional matrix is preferably alumina. The aluminum alloy is placed on top of the three-dimensional interconnected material and the assembly is placed in a containing vessel, which is then heated to infiltrate the metal into the three-dimensionally interconnected material. The typical infiltration time is on the order of about 7 hours or more. 
     U.S. Pat. No. 4,868,143 by Newkirk et al. and issued on Sep. 19, 1989, discloses a process for making a composite wherein an oxidation reaction product (e.g., alumina) is formed with aluminum parent-metal interconnected therethrough. The composite is then contacted with a second molten metal such as copper or nickel which infiltrates the interconnected parent metal by interdiffusion. The resulting product is a composite having a mixture of two metals interconnected throughout the composite. 
     U.S. Pat. No. 5,267,601 by Dwivedi, issued on Dec. 7, 1993, discloses a process wherein a permeable mass is formed into a preform having a cavity. The preform having the cavity is placed at least partially into a molten matrix metal such that an infiltrating atmosphere can communicate with the cavity in order to obtain spontaneous infiltration of the molten matrix metal. Extended periods of time (e.g., 25 to 100 hours) are used to complete infiltration and the preform is only infiltrated to the level that the preform is immersed in the molten metal. 
     There exists a need for a simple and inexpensive process to form ceramic-metal composites. It would be particularly advantageous if such ceramic-metal composites could be formed using a process that is reasonably fast and produces substantially dense and non-porous composites that include substantially continuous metal and ceramic phases. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to ceramic-metal composite materials and methods for making such materials. 
     According to one aspect of the present invention, a method for making a ceramic-metal composite is provided. The method can include the steps of contacting a sintered ceramic body having at least about 5 volume percent open porosity with molten copper metal and maintaining the ceramic body in contact with the copper metal in the absence of substantial overpressure for an amount of time sufficient to infiltrate at least a portion of the open porosity in the sintered ceramic body with the copper metal. The metal-infiltrated ceramic body is then cooled to solidify the copper metal and form a ceramic metal composite. 
     According to certain embodiments of this aspect of the invention, the sintered ceramic can have from about 10 volume percent to about 90 volume percent open porosity. The sintered ceramic body can be selected from the group of alumina, aluminum titanate, magnesium oxide and nickel oxide. The molten copper metal can include oxygen as an infiltration additive, such as from about 1.5 weight percent to about 10 weight percent oxygen. The molten copper metal can alternatively include titanium as an infiltration enhancer, such as from about 1 weight percent to about 10 weight percent titanium. In another embodiment, the open porosity of the sintered ceramic body can be coated with a material selected from nickel oxide and copper oxide to enhance infiltration. The molten copper metal can be heated to a temperature of at least about 1250° C. and the infiltration can occur in a vacuum atmosphere. The method according to the present invention is a fast technique, and in one embodiment substantially all of the open porosity in the sintered ceramic body is infiltrated with copper metal in less than about 1 hour. 
     According to another aspect of the present invention, a method for making a ceramic-metal composite is provided. The method can include the steps of forming a sintered alumina ceramic body having from about 10 volume percent to about 90 volume percent open porosity, contacting the sintered ceramic body with molten copper metal, maintaining the ceramic body in contact with the metal for an amount of time sufficient to infiltrate substantially all of the open porosity in the sintered ceramic body with copper metal and cooling the ceramic body to solidify the copper metal and form a ceramic-metal composite. 
     According to certain embodiments of this aspect of the invention, the sintered alumina ceramic has an open porosity of from about 15 volume percent to about 80 volume percent. The molten copper metal can include oxygen as an infiltration additive, such as from about 1.5 weight percent to about 10 weight percent oxygen. The molten copper metal can alternatively include titanium as an infiltration additive, such as from about 1 weight percent to about 10 weight percent titanium. The open porosity of the sintered ceramic body can be coated with a material selected from nickel oxide and copper oxide to enhance infiltration. The molten copper metal can have a temperature of at least about 1250° C. The infiltration can occur in a vacuum atmosphere and the sintered ceramic body can be heated to a temperature of at least equal to the temperature of the molten copper metal before contacting with the molten metal. 
     According to another aspect of the present invention, a method for making a ceramic-metal composite including aluminum titanate is provided. A sintered aluminum titanate ceramic body having an open porosity of at least about 5 volume percent is formed and is contacted with molten copper metal. The sintered aluminum titanate is maintained in contact with the copper metal for an amount of time sufficient to infiltrate at least a portion of the open porosity in the sintered aluminum titanate with copper metal. The infiltrated aluminum titanate is then cooled to solidify the copper metal and form a ceramic-metal composite. 
     According to certain embodiments of this aspect of the invention, the copper metal is heated to a temperature of at least about 1250° C. The sintered aluminum titanate can have an open porosity of from about 15 volume percent to about 80 volume percent. The molten copper metal can include from about 1.5 weight percent to about 10 weight percent oxygen as an infiltration additive. The copper metal can alternatively include from about 1 weight percent to about 10 weight percent titanium as an infiltration additive. 
     According to another aspect of the present invention, a ceramic-metal composite is provided. The composite includes a substantially continuous phase of sintered alumina ceramic and a substantially continuous metallic phase of copper metal. 
     According to certain embodiments of this aspect of the invention, the ceramic-metal composite includes from about 10 volume percent to about 90 volume percent alumina. The copper metal can include from about 1.5 weight percent to about 10 weight percent oxygen or from about 1 weight percent to about 10 weight percent titanium. The composite can have a compressive strength of greater than about 110 ksi. 
     According to another aspect of the present invention, a ceramic-metal composite is provided that includes a substantially continuous phase of sintered aluminum titanate ceramic and a substantially continuous metallic phase of copper metal. 
     According to certain embodiments of this aspect of the invention, the ceramic-metal composite includes from about 10 volume percent to about 90 volume percent aluminum titanate. The sintered aluminum titanate can also include silica and iron oxide as sintering aids or stabilizers. The metallic copper phase can also include from about 1.5 weight percent to about 10 weight percent oxygen. 
     In yet another aspect of the present invention, a method for making a ceramic-metal composite is provided. The method can include the steps of placing a metal in a refractory vessel, heating the metal to a temperature in excess of the melting temperature of the metal such that the metal is in the form of a molten pool having a top surface, contacting a sintered ceramic body having at least about 5 volume percent open porosity with the top surface of the molten pool for a time sufficient to infiltrate at least a portion of the ceramic body, removing the ceramic body from contact with the molten pool and cooling the metal-infiltrated ceramic to form a ceramic-metal composite having substantially continuous metal and ceramic phases. 
     According to certain embodiments of this aspect of the invention, the contacting step can include contacting the sintered ceramic body with the molten pool for a period of time sufficient to infiltrate substantially all of the open porosity in the sintered ceramic body. In one embodiment, the period of time is less than about 1 hour and is preferably less than about 15 minutes. In another embodiment, the sintered ceramic body is heated to at least the temperature of the molten metal. In another embodiment, the sintered ceramic body consists essentially of alumina and the metal consists essentially of copper metal. In yet another embodiment, the molten metal infiltrates the sintered ceramic body to a point above the portion of the ceramic body that is contacted with the molten metal. In another embodiment, the sintered ceramic body has from about 10 volume percent to about 90 volume percent open porosity. In yet another embodiment, the sintered ceramic is silicon nitride and the metal is a nickel-titanium alloy. In another embodiment, the sintered ceramic is alumina and the metal is a silver-copper alloy. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a compression stress-strain diagram comparing a composite produced according to the present invention with the component materials of the composite. 
     FIG. 2 illustrates an apparatus useful for carrying out a method according to one embodiment of the present invention. 
     FIG. 3 illustrates an apparatus useful for carrying out a method according to one embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention is generally directed to a method for making a ceramic-metal composite material with interconnecting and substantially continuous ceramic and metal phases. The composite is formed by infiltrating molten metal into a porous ceramic body having a substantially interconnected and continuous pore structure, referred to as open porosity. The infiltration is preferably accomplished in the absence of significant overpressures and the ceramic is infiltrated by capillary action. Substantially all of the open porosity in the ceramic can be infiltrated, if desired. 
     According to the present invention, the ceramic matrix material is selected to be compatible with the infiltrant metal. That is, it is preferred that the ceramic matrix is capable of being wetted and infiltrated by the molten metal without the use of any substantial overpressure. It is more preferred that, at the temperature of infiltration, the ceramic matrix is wet by the molten metal. Complete wetting of the ceramic matrix by the molten metal will enhance the spontaneous infiltration of the metal into the ceramic matrix without the use of substantial overpressure to force the metal into the ceramic matrix. 
     The ceramic matrix is a sintered matrix that comprises a substantially continuous ceramic phase. Among the preferred ceramic matrix materials are the sintered ceramic oxides, carbides and nitrides. In a preferred embodiment, a sintered oxide ceramic is selected from the group of alumina (Al 2  O 3 ), aluminum titanate (Al 2  TiO 5 ), nickel oxide (NiO), or magnesium oxide (MgO) ceramic. In addition to the primary ceramic oxide, the sintered ceramic can include other materials, such as sintering aids (e.g., SiO 2 ). Sintered aluminum titanate, for example, can also include additives, such as SiO 2  or Fe 2  O 3 , which can be added to aluminum titanate as stabilizers. In addition to the above-described oxides, non-oxide ceramics such as silicon carbide (SIC) or silicon nitride (Si 3  N 4 ) can be used in some applications. 
     The ceramic matrix is a sintered matrix that has a substantially continuous ceramic phase. The sintered ceramic has an identifiable shape and a free-standing form in the absence of constraining supports. This is to be distinguished from a vessel filled with loose or lightly packed particles or fibers of material. One of the advantages of using a cohesive sintered ceramic matrix is that the cohesive sintered ceramic matrix can be prefabricated into the shape of a selected article of manufacture (a preform). Further, the cohesive sintered ceramic matrix can be gripped by one portion (e.g., a top portion) while another portion of the matrix (e.g., a bottom portion) is contacted with the molten metal. The molten metal will then infiltrate by capillary action into substantially all of the open porosity of the ceramic, including those portions not immersed in the molten metal. 
     The ceramic matrix material is a sintered, coherent body that has an open and substantially continuous pore structure to facilitate the infiltration of molten metal into the matrix without the use of any substantial overpressure to force the metal into the matrix. As used herein, the term &#34;substantial overpressure&#34; refers to pressure in excess of about 3 atmospheres (about 45 psi) applied during the infiltration process, either mechanically or through the use of pressurized gas. 
     The total open porosity in the ceramic matrix must be at least about 5 volume percent, and is preferably from about 10 volume percent to about 90 volume percent. More preferably, the open porosity is from about 15 volume percent to about 80 volume percent of the sintered ceramic matrix. The average pore size of the open porosity is preferably from about 1 micrometer to about 10 micrometers, however, it is not believed that the average pore size is critical to the practice of the present invention. Porous ceramics having open and continuous porosity as described above can be formed in a number of ways. 
     For example, a green body comprising a ceramic powder can be formed and sintered in an enhanced vapor phase sintering process to form a porous body having a substantially continuous and interconnected pore structure. The total porosity and average pore size of the sintered ceramic matrix can be controlled by controlling the porosity of the green body and the sintering conditions. Vapor phase sintering is a convenient process to produce porous ceramics having controlled porosities and pore sizes. As such, the porous ceramics form suitable matrices for infiltration with molten metals to produce ceramic-metal composites having interpenetrating three-dimensional structures. 
     In the vapor phase sintering process, a green body is formed from ceramic powder. The starting particle size is not critical to the practice of the present invention, however, a smaller average particle size can be used to produce a sintered body having a lower average pore size. 
     It may be desirable to form agglomerates of the powder as a means of controlling the porosity of the green body formed from the powder. For example, aluminum hydroxide (Al(OH) 3 ) or aluminum sulfate (Al 2  (SO 4 ) 3 ) particles having a diameter of about 50 micrometers can be calcined to form alumina agglomerates that have a porosity of about 50 volume percent. If these agglomerates are then formed into a body with about 50 volume percent packing, the net result will be about 75 volume percent porosity. The agglomerates can also be formed with additional powder to produce green bodies with a wider range of porosity than can be obtained with milled powder alone. 
     After ceramic powder having a desired particle size range has been obtained, the powder can be formed into a green body. As used herein, the term green body refers to an unsintered body of ceramic powder. For example, the powder can be uniaxially pressed into a green body at a pressure of from about 48 MPa to about 69 MPa (7 ksi to 10 ksi) or isostatically pressed at similar pressures. In addition, forming additives can be used to improve the mechanical strength of the green body formed by pressing the ceramic powder. Additives can include binders such as polyvinyl alcohol, plasticizers such as polyethylene glycol, and lubricants such as aluminum stearate. In addition, other forming methods such as injection molding, extrusion, slip casting and similar processes can be used to form green bodies according to the present invention. 
     Further, some green bodies with high levels of forming additives may have sufficient strength to enable the green body to be machined. Thus, intricate parts may advantageously be formed by machining processes while the piece is in the soft green state. 
     One method for controlling the total porosity of the sintered ceramic matrix formed by vapor phase sintering is to control the total porosity of the green body. This can be done, for example, by varying the pressing pressure. Typically, green bodies formed by uniaxially pressing finely-divided ceramic powder have porosities ranging from about 50 volume percent to about 65 volume percent. The total porosity can be increased to from about 70 volume percent to about 80 volume percent by using agglomerated powder, as is discussed hereinabove. 
     After forming, the green body can be sintered to obtain a sintered ceramic body. If organic binders or other organic materials are used in the green body forming process, these additives can advantageously be removed prior to fully sintering the ceramic powder. This is commonly referred to as &#34;binder burnout.&#34; For example, the green body can be placed in a furnace and slowly heated to a temperature of, for example, about 600° C. to volatilize organic additives. Since these organic additives comprise a large amount of carbon, it is usually preferable to volatilize these materials under a flowing gas such as oxygen. 
     The green body can be presintered prior to vapor phase sintering. Presintering is a convenient and economical method of controlling the total porosity of the final sintered body. Presintering conveniently lowers the porosity of the green body to a range that is desirable for the sintered body, since the vapor phase sintering technique does not substantially affect the total porosity of the sintered body. 
     Preferably, the presintering step is done at a temperature that is slightly below the normal solid-state sintering temperature of the ceramic material. For example, alumina can be presintered at a temperature of from about 1300° C. to about 1600° C., more preferably from about 1450° C. to about 1550° C. The optimum temperature and sintering time will depend on the particle size of the starting powder or agglomerates. The sintering atmosphere is not critical and, therefore, air is preferred. However, certain atmospheres may be undesirable due to reactions between the atmosphere and the ceramic material at the presintering temperature. The presintering step preferably produces a presintered body having a total porosity of from about 10 percent to about 70 percent. The total porosity can be controlled by varying the time at the presintering temperature, such as from about 1 minute to about 300 minutes. The presintering step can determine the total porosity of the final sintered body, however, presintering may not be necessary if the green body has the desired total porosity for the final sintered product. The presintered body may advantageously be machined to its appropriate final shape and dimensions. 
     The presintered or green ceramic body is then vapor phase sintered to maintain control over the total porosity and average pore size of the sintered body and form a porous sintered ceramic. 
     Enhanced vapor phase sintering has been studied for some ceramic materials. According to this process, volatile transport gases at a high partial pressure are produced during sintering. Preferably, the partial pressure of the transport gas is at least about 10 -4  atm at the sintering temperature and more preferably at least about 10 -3  atm. It has been found that for some ceramic materials, the vapor phase sintering process may be enhanced by the presence of a reaction gas or a material that produces such a gas, particularly a gas comprising a halide, in the sintering atmosphere. For example, vapor phase sintering of magnesia can be enhanced by the addition of hydrogen chloride (HCl) gas: 
     
         MgO.sub.(s) +2HCl.sub.(g) ⃡MgCl.sub.2(g) +H.sub.2 O.sub.(g)(1) 
    
     In one embodiment of the present invention, an alumina-containing body is sintered in the presence of hydrogen chloride gas (HCl), thereby promoting the reaction: 
     
         Al.sub.2 O.sub.3(s) +6HCl.sub.(g) ⃡2AlCl.sub.3(g) +3H.sub.2 O.sub.(g)                                                 (2) 
    
     Alternatively, alumina may be sintered in the presence of hydrogen fluoride gas (HF) in which case the vapor phase transport occurs primarily via the process: 
     
         Al.sub.2 O.sub.3(s) +6HF.sub.(g) ⃡2AlF.sub.3(g) +3H.sub.2 O.sub.(g)                                                 (3) 
    
     The reaction gas (e.g., HCl or HF) can be added to the sintering furnace directly in the form of commercially available bottled gas. In this embodiment, the gas should be dry and contain minimal residual moisture. Residual water (H 2  O) can drive the reverse reaction and inhibit formation of the vapor transport species. Preferably, the partial pressure of the reaction gas is at least about 0.25 atm and is more preferably from about 0.4 atm to about 1 atm. 
     Alternatively, the gas may be formed in-situ within the sintering furnace. For example, aluminum fluoride (AlF 3 ) powder can be placed in a closed furnace. As the furnace is heated, hydrogen gas is added to the furnace to promote an insitu reaction to form hydrogen fluoride gas over the alumina. This procedure is particularly advantageous when environmentally difficult gases such as hydrogen fluoride are used. 
     Sintering temperatures can vary depending on the ceramic material being sintered. For example, alumina powder is preferably sintered at a temperature from about 1400° C. to about 1600° C. to form a sintered ceramic body. The pore size and pore size distribution can be controlled by adjusting the amount of time that the body is sintered at the sintering temperatures. Table 1 lists the mean pore diameter for alumina compacts sintered at 1600° C. for varying amounts of time under 1 atm HCl. For each sample, the starting particle size was 1 micrometer and the porosity of the sample remained at about 50 volume percent regardless of the sintering time. 
     
                       TABLE 1                                                     
______________________________________                                    
TIME         MEAN PORE SIZE                                               
______________________________________                                    
10 min.      2.1 microns                                                  
80 min.        3 microns                                                  
250 min.       4 microns                                                  
1080 min.    7.5 microns                                                  
______________________________________                                    
 
    
     As Table 1 illustrates, as the sintering time increases, the average pore diameter also increases. However, the total porosity remains substantially constant. 
     The ceramic body may be sintered in any system in which the partial pressure of the reaction gas, and hence the transporting gas can be controlled. For example, a simple tube furnace having a sealed end with an inlet for the reaction gas may be used. 
     As an alternative to vapor phase sintering, it is known to those skilled in the art that the open porosity of a ceramic can be controlled by controlling variables such as the starting particle size of the ceramic powder, the composition of the ceramic powder and/or the sintering temperature. Specifically, ceramic powder having a larger average particle size, the absence of sintering aids in the composition, and lower sintering temperatures can all produce a sintered ceramic body having open porosity. 
     For example, porous sintered alumina can be formed from alumina particles having an average particle size in the range of from about 0.5 micrometers to about 70 micrometers by forming the particles into a green body and sintering. The green body can be sintered at a temperature of from about 900° C. to about 1800° C. to form a porous sintered ceramic if the sintering temperature is somewhat below the normal sintering temperature for obtaining dense alumina. The absence of sintering aids, such as silica, will also increase the porosity of the ceramic. 
     Other methods for forming porous ceramics include the use of pore formers. Pore formers are materials which have a volatilization temperature that is lower than the sintering temperature of the ceramic. Thus, materials such as walnut shells or latex spheres can be added to the ceramic mix. Upon sintering, the material volatilizes, leaving behind open pore space. Aluminum titanate porous bodies can be produced by using pore formers, particularly walnut shells. 
     As discussed above, it is preferable to obtain a ceramic matrix having an open porosity that is at least about 5 volume percent, preferably from about 10 volume percent to about 90 volume percent, and more preferably from about 15 volume percent to about 80 volume percent open porosity. The open porosity preferably has an average pore size of at least about 1 micrometer and more preferably from about 1 micrometer to about 10 micrometers. 
     When a porous ceramic matrix having the desired total porosity and pore size is obtained, molten metal can be infiltrated into the open porosity of the ceramic matrix. In a preferred embodiment of the present invention, the ceramic matrix is brought into contact with the molten metal and the metal infiltrates the ceramic matrix by capillary action without the assistance of substantial overpressure. Thus, when contacted with the sintered ceramic, the molten metal enters the pore structure of the ceramic and fills substantially all of the open porosity or a selected portion of the open porosity. 
     According to a preferred embodiment of the present invention, a portion of the sintered ceramic body is contacted with the top surface of a molten pool of infiltrant metal for a time sufficient to infiltrate at least a portion of the sintered ceramic body. According to this embodiment, the infiltrant metal is placed in a refractory vessel and is heated to a temperature in excess of the melting temperature of the metal such that the metal is in the form of a molten pool having a top surface. Preferably, the metal is heated to a temperature that is sufficiently high to melt all constituents of the alloy and is high enough to yield a liquid metal with a low enough viscosity to permit infiltration in the absence of substantial pressure. For example, if oxygen is used as an infiltration additive, the temperature of the molten metal should be above the melting point of copper oxide, e.g., at least about 1230° C. The sintered ceramic body is brought into contact with the molten metal and the molten metal infiltrates the open porosity of the ceramic and substantially fills the open porosity. The metal-infiltrated ceramic is then cooled to form the ceramic-metal composite having substantially continuous metal and ceramic phases. It is also possible to heat the composite in a reducing atmosphere to reduce any excess copper oxide to copper metal. 
     The pool of molten metal infiltrates the ceramic by capillary action which overcomes the forces of gravity and pulls the molten metal into the ceramic matrix. The advantage of using such a process is that the molten metal infiltrates the ceramic structure and no excess metal will flow out of the ceramic structure. Further, since capillary action is the driving mechanism for infiltrating the metal into the ceramic structure, the metal will infiltrate substantially all portions of the ceramic structure, regardless of the shape of the structure, without having to completely immerse the matrix into the molten metal. Further, the infiltrating metal can force out gas as the metal infiltrates so that little or no gas is trapped in the composite. 
     An embodiment of this method is illustrated in FIGS. 2 and 3. Referring to FIG. 2, a ceramic tube 205 is provided having a lower end 210 that is closed and an upper end 215 that is open. The open upper end 215 is sealed, such as by using a teflon seal 220. A gas inlet/outlet 225 is provided such that gases can be extracted from or purged into the ceramic tube 205. 
     A ceramic control rod 230 extends through the seal 220 and is attached to a sintered ceramic body 235 having open and continuous porosity. The control rod 230 can be raised or lowered to control the vertical position of the sintered ceramic body 235 within the ceramic tube 205. 
     The lower end 210 of the ceramic tube 205 contains a refractory vessel 240. The lower end 210 is placed in a &#34;hot zone&#34; wherein heating means (not illustrated) can heat the lower end 210 to elevated temperatures. The refractory vessel 240 contains a quantity of molten metal 245 having an upper surface 250. Thus, the sintered ceramic body 235 can be lowered by the control rod 230 such that at least a portion of the sintered ceramic body 235 contacts the upper surface 250 of the molten metal 245. 
     Referring to FIG. 3, the hot zone is heated to a temperature of at least the melting temperature of the molten metal 245. Preferably, the sintered ceramic body 235 is in the hot zone for a sufficient amount of time that the sintered ceramic body 235 is heated to at least the melting temperature of the molten metal 245. A portion of the sintered ceramic body 235 is contacted with the top surface 250 of the molten metal 245. The molten metal 245 then infiltrates the entire sintered ceramic body 235 to form a ceramic-metal composite 255. Preferably, the amount of molten metal 245 available in the refractory vessel 240 to infiltrate the sintered ceramic body 235 is sufficient in quantity such that the sintered ceramic body 235 completely infiltrates without the top surface 250 lowering to a point below the portion of the sintered ceramic body 235 that is in contact with the molten metal 245. 
     The method according to this embodiment of the present invention also permits the composite to be formed in a very quick and efficient manner. For example, the infiltration time typically takes less than about 1 hour, preferably less than about 15 minutes and more preferably less than about 1 minute. For example, a 0.75 inch diameter cylinder of porous alumina can be infiltrated to a depth of 3 inches by copper metal in less than about 1 minute. 
     It is also preferable to heat the ceramic to at least about the temperature of the molten metal. This is desirable since, if the ceramic is substantially cooler, the metal may cool and solidify upon infiltrating and contacting the ceramic matrix and thereby prevent further infiltration of the ceramic matrix by the metal. 
     The amount of infiltration into the ceramic matrix can be controlled by controlling the amount of time that the matrix is in contact with the molten pool. The desired amount of time will depend upon the ceramic matrix and metal being infiltrated, and the specific amount of time to infiltrate to a specific degree can be determined by routine experimentation. 
     Although the present invention is applicable to many combinations of matrix and infiltrant metals, it has been found particularly applicable to certain ceramic-metal composites. For example, in one preferred embodiment, copper is the infiltrant metal. As used herein, the term &#34;copper metal&#34; refers to any metal alloy that is predominantly copper, i.e., at least about 50 percent copper. Copper can be alloyed with a number of other elements, and in certain preferred embodiments discussed in detail below, copper can be alloyed with oxygen or titanium. 
     According to one preferred embodiment, the copper is infiltrated into a ceramic body that comprises a sintered ceramic oxide selected from the group of alumina, aluminum titanate, magnesium oxide and nickel oxide. In a more preferred embodiment, the ceramic matrix is alumina. Copper infiltrated alumina can advantageously provide high conductivity along with good hardness and strength provided by the ceramic matrix. Such combinations are useful for producing articles such as welding electrodes and electrical contacts. 
     To improve the capillary action between the ceramic and the molten metal, it may be desirable to modify the wetting or spreading characteristics of the ceramic and metal. One way to do this is to coat the open porosity of the ceramic with a coating that is more easily wet by the molten metal. For instance, the interior surfaces of a sintered alumina ceramic can be modified by vapor phase coating, such as by chemical vapor deposition (CVD), the ceramic with copper oxide or nickel oxide to enhance the infiltration of copper metal. 
     Another way of enhancing the wetting characteristics is to modify the chemical composition of the molten metal with an infiltration additive. For instance, the wetting of molten copper on alumina can be enhanced by the addition of oxygen to the copper or with the addition of titanium (Ti). Such additions enhance the wetting and permit infiltration to occur rapidly and easily. When oxygen is added, it is preferred to add the oxygen to the copper metal in an amount of from about 1.5 weight percent to about 10 weight percent, more preferably from about 2 weight percent to about 5 weight percent. When titanium is used as an infiltration additive, it is preferably added in an amount of from about 1 weight percent to about 10 weight percent. 
     After one or more of the surface modifications and chemical alterations noted above, if necessary, the molten metal will wet the ceramic and infiltrate substantially all of the void space of the ceramic through capillary action. 
     In a preferred embodiment, the metal infiltration step is performed in a vacuum atmosphere. As used herein, vacuum atmosphere refers to an atmospheric pressure of about 10 millitorr or less. The evacuation of air from the ceramic void space reduces the likelihood that air pockets will be entrapped in the composite. 
     The ceramic-metal composites produced by the present invention have relatively high strengths and toughness. In one embodiment, a composite comprising about 65 percent alumina and about 35 percent copper/copper oxide has a compressive strength of at least about 110 ksi, as illustrated in FIG. 1. 
     According to one embodiment of the present invention, the sintered ceramic matrix material has a porosity gradient. That is, the porous ceramic matrix has regions of different porosity. For example, one portion of the ceramic matrix can be substantially 100 percent dense while another portion can have a high porosity, for example about 60 volume percent or greater. When the porous end is contacted with molten metal, the metal will infiltrate throughout the ceramic porosity, resulting in an article having a dense ceramic portion and a composite portion. The porosity gradient may be gradual through the material or it may include one or more abrupt changes in porosity, such as a ceramic matrix formed by layers of material having different porosity characteristics. The advantages of a gradient composite material can include the alleviation of the effects of an abrupt thermal expansion gradient, the ability to attach the composite to a variety of materials and the ability to have an article with a dense ceramic surface intimately attached to a composite surface. This embodiment of the invention is described in more detail in U.S. Pat. No. 5,525,374, issued Jun. 11, 1996. 
     One particular application where the process of the present invention has found particular utility is in the field of filters. This embodiment is disclosed in more detail in commonly assigned U.S. patent application Ser. No. 08/220,558, filed Mar. 31, 1994, pending. Ceramic filters comprise long cylindrical bodies of porous ceramic, such as alumina. The cylinders have a plurality of channels parallel to the cylindrical axis for receiving the material (i.e., a liquid) to be filtered. Since each end of the cylinder is typically sealed to prevent leakage, the liquid is forced through the porous ceramic and is thereby filtered. An example of such a filter is illustrated in U.S. Pat. No. 4,069,157 by Hoover et al., which is incorporated herein by reference in its entirety. 
     One of the problems associated with manufacturing these filters is sealing the ends of the filter. Typically, the end of the filter must form a seal with a metal component, such as a stainless steel ring. According to one embodiment of the present invention, the end of the filter is infiltrated with metal to assist in sealing the filter. For example, the metal can be selected from copper or a reactive braze such as a copper/silver/titanium alloy. When copper is used to infiltrate into an alumina filter, the resulting composite can be nickel coated to promote adhesion to a stainless steel ring using, for example, a Cu/Ag braze. When using a reactive braze, no brazing material is necessary and the stainless steel ring can be adhered to the filter by heating the components while in contact. 
     Another application of the present invention is in the area of engine component liners, such as exhaust port liners for diesel engines. See, for example, the port liners described in U.S. Pat. No. 5,066,626 by Fukao et al., which is incorporated herein by reference in its entirety. Aluminum titanate is a preferred ceramic material due to its resistance to corrosion and its low thermal expansion characteristics. One of the problems associated with these port liners is that a compliant layer between the metal (e.g., aluminum or cast iron) and the aluminum titanate is necessary to absorb stresses resulting from the contracting metal. 
     It is believed that the problem can be reduced by infiltrating an aluminum titanate material having a gradient porosity. The infiltrated metal gradient can assist in alleviating the stresses caused by the contraction of the metal. For example, copper can be infiltrated into the aluminum titanate porous body by the process described above. 
     EXAMPLE 1 
     An alumina powder (CERALOX HPA, Ceralox Corp., Tucson, Ariz.) is obtained having an average particle size of about 1.0 micrometers. The alumina powder is formed into a green body by uniaxially pressing the powder at a pressure of about 50 MPa to obtain a green body having a porosity of about 55 volume percent. 
     The green body is then presintered in air at a temperature of about 1500° C. for about 3 minutes. The presintered body has a porosity of about 35 volume percent. 
     The presintered body is then placed in a sintering furnace that comprises an alumina tube. The temperature of the furnace is raised as the furnace is evacuated. Before reaching about 800° C., the furnace is purged with argon gas to remove impurities in the furnace atmosphere. At about 800° C., the furnace is filled with HCl gas having a pressure of about 1 atm. The alumina body is then sintered under HCl gas at a temperature of about 1600° C. for about 80 minutes. 
     The sintered alumina ceramic has a total porosity of about 35 volume percent and has an average pore size of about 3 micrometers. The alumina forms a continuous three-dimensional structure and there is substantially no closed porosity. 
     The sintered alumina ceramic is placed in a furnace and heated to about 1275° C. and is contacted with a molten copper bath containing about 3 weight percent oxygen at a temperature of about 1275° C. The bottom surface of the alumina ceramic is contacted with the molten metal and the molten metal infiltrates through the entire alumina matrix by capillary action. The composite is then cooled. The composite comprises about 65 volume percent of a substantially continuous alumina phase and about 35 volume percent copper/copper oxide as a substantially continuous phase. 
     The stress strain diagram for the sample is illustrated in FIG. 1. The composite has a compressive strength of about 112 ksi. 
     EXAMPLE 2 
     To demonstrate the feasibility of infiltrating copper into aluminum titanate, two disks of a porous aluminum titanate were obtained. The aluminum titanate disks had a porosity of about 20 volume percent and about 70 volume percent. Copper metal was contacted with the aluminum titanate disks and the assemblies were heated to about 1300° C. in air and substantially complete infiltration occurred in about 20 minutes. Copper metal infiltrated the aluminum titanate and contained about 4.5 weight percent oxygen. The composite can be heated in a reducing atmosphere to reduce residual copper oxide to copper. 
     EXAMPLE 3 
     Reaction bonded silicon nitride was infiltrated with 99.5 percent pure aluminum. Aluminum was contacted with a reaction bonded silicon nitride body and the assembly was heated in a vacuum furnace at 1100° C. for 60 minutes. Upon cooling, a portion of the silicon nitride was infiltrated with aluminum. There was some fracture of the composite portion from the silicon nitride body due to the thermal expansion mismatch. 
     EXAMPLE 4 
     This example illustrates the infiltration of silicon nitride with a nickel-titanium alloy. A nickel-titanium alloy containing 10 weight percent titanium was placed in contact with a porous silicon nitride body and was heated in a vacuum furnace to a temperature of 1400° C. with a partial pressure of oxygen of about 10 -8  atmospheres. The nickel-titanium alloy infiltrated the silicon nitride. A similar result was obtained by substituting chromium for titanium. 
     EXAMPLE 5 
     Alumina was infiltrated with a silver-copper alloy composition including about 95 percent silver and about 5 percent copper/copper oxide. The advantage of this composite is that the silver maintains a high ductility in the composite. 
     EXAMPLE 6 
     Aluminum having a purity of 99.5 percent was infiltrated into reaction bonded silicon carbide having silicon metal as a second phase. The system was maintained at 1100° C. for 60 minutes to insure complete mixing in the liquid state and the aluminum infiltrated the silicon carbide. 
     EXAMPLE 7 
     The following example illustrates the application of the present invention to the infiltration of porous ceramic matrices, such as filter elements. 
     Alumina powder having an average particle size of about 10 micrometers is mixed with organic additives and extruded into a cylindrical chamber having a plurality of channels therethrough. The cylindrical chamber was sintered at a temperature of about 1640° C. for about 2 hours to obtain a sintered ceramic matrix having a total open porosity of about 40 volume percent. The average pore size was from about 6 micrometers to about 10 micrometers. The ceramic cylider had a length of about 39 inches and a diameter of about 1 inch. 
     Copper metal including about 3 weight percent oxygen was heated to a temperature of about 1250° C. to form a molten pool of metal. The filter element was also heated to about 1250° C. and was then lowered such that the very bottom portion of the filter element contacts the molten pool of copper. The copper infiltrated and wicked up the ceramic matrix. After less than about one minute, the ceramic matrix is removed from the copper metal bath and the height of infiltration is about 1 inch. 
     While various embodiments of the present invention have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present invention.