SURFACE BONDING OF CERAMIC BODIES

Abstract: Ceramic bodies (2,4) are bonded together via a layer of an oxidation reaction product of a molten metal, which metal is present in one or both of the ceramic bodies (2,4) prior to bonding. At least one of the ceramic bodies (2,4) comprises a ceramic product formed by the oxidation reaction of molten parent metal (e.g., alumina from molten aluminum) and grown as molten metal is trans­ ported through, and oxidized on the surface of, its own oxidation product. One or both of the ceramic bodies (2,4) used in the bonding process contains surface-accessible channels of residual metal, i.e., metal channels which have resulted from molten-metal transport during the ceramic growth process. When the suitably assembled cera­ mic bodies (2,4) are heated in an oxidizing atmosphere at a temperature above the melting point of the residual metal, molten metal at the surface of the ceramic body reacts with the atmospheric oxidant so as to bond the facing surfaces together by a layer of the oxidation reaction product of the molten metal formed therebetween.

Description: The present invention relates to a method of bonding the planar or otherwise congruent ceramic faces of abutting bodies such as plates, disks, or the like. In accordance with the present invention, there is provided a method of bonding the congruent surfaces of ceramic bodies of which at least one body is a polycrystalline ceramic material comprising the oxidation reaction product of a parent metal with a vapor-phase oxidant, and having interconnected metallic constituents derived at least in part from the parent metal, and optionally one or more filler materials, as described below in detail. For this ceramic body, the polycrystalline ceramic material is interconnected in three dimensions and the interconnected metal, distributed through at least a portion of the ceramic body, is at least partially open or accessible or rendered accessible, from at least one bonding surface. Said bonding surface of the ceramic body can now be bonded to a congruent surface of an abutting ceramic body. In the method of this invention, the two ceramic bodies to be bonded (e.g., two oxidation reaction products as described above, or one product which is an oxidation reaction product as described above and another ceramic product made by known or conventional techniques other than by the oxidation of a molten parent metal) are assembled so that the surfaces to be bonded substantially abut, although there may be a slight separation as explained below. The assembled ceramic bodies are heated in an oxidizing atmosphere at a temperature above the melting point of the interconnected metal, but below the melting point of the oxidation reaction product, and on reaction, an oxidation reaction product is grown between the abutting surfaces causing them to bond together. Generally, in accordance with the present invention, there is provided a method of bonding ceramic bodies along substantially congruent surfaces thereof, the method comprising the following steps. There is provided a first body of ceramic comprising a ceramic product formed by the oxidation reaction of molten parent metal, e.g., aluminum, and a vapor-phase oxidant, e.g., air, and grown as molten metal is transported through, and oxidized on the surface of, its own oxidation reaction product. This first ceramic body comprises a polycrystalline oxidation reaction product, e.g., alumina, and interconnected residual metal, e.g., aluminum, and optionally may comprise a composite formed by infiltrating a filler with the oxidation reaction product. The first body of ceramic is assembled adjacent to a second body of ceramic in a manner such that a pair of surfaces of the first and second bodies to be bonded together face one another. The assembled ceramic bodies are then heated in the presence of a vapor-phase oxidant at a temperature above the melting point of the residual metal to induce transport of the residual metal toward the bonding surfaces where oxidation reaction product continues to grow as described above, thereby effecting a bond between the first and second bodies. As used in 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 classical sense, that is, in the sense that it consists entirely of non-metallic and inorganic materials, but rather refers to a body which is predominantly ceramic with respect to either composition or dominant properties, although the body contains minor or substantial amounts of one or more metallic constituents and/or porosity (interconnected and isolated) most typically within a range of from about 1-40% by volume, but may be higher. "Oxidation reaction product" generally means one or more metals 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 one or more metals with a gaseous oxidant such as those described herein. "Oxidant" or "vapor-phase oxidant", which latter term identifies the oxidant as containing or comprising a particular gas, means one or more suitable electron acceptors or electron sharers. "Parent metal" is intended to refer to relatively pure metals, commercially available metals with impurities and/or alloying constituents therein, and alloys and intermetallic compounds of the metals. When a specific metal is mentioned, the metal identified should be read with this definition in mind unless indicated otherwise by the context. In the drawings: FIGURE 1 is a schematic view, partially in cross-section, showing an assembly of a first and second ceramic body and a barrier means, in accordance with one embodiment of the present invention; and Figure 2 is a schematic view, partially in cross-section, showing an assembly of a first and second ceramic body, a reservoir metal body, and a barrier means, in accordance with another embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION A first ceramic body is produced by the method as disclosed in EP-A-155 831. According to the method, a parent metal precursor, e.g., aluminum, is heated in the presence of a vapor-phase oxidant, e.g. air, 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. The molten parent metal is reacted with the vapor-phase oxidant to form an oxidation reaction product, which product is maintained at least partially in contact with, and extends between, the body of molten parent metal and the vapor-phase oxidant. In this temperature range, molten parent metal is transported through the previously formed oxidation reaction product, towards the vapor-phase oxidant. As the molten parent metal contacts the vapor-phase oxidant at the interface between the vapor-phase oxidant and previously formed oxidation reaction product, it is oxidized by the vapor-phase oxidant, and thereby grows or forms a progressively thicker layer or body of oxidation reaction product. The process is continued for a time sufficient to produce a ceramic body having interconnected metallic constituents including nonoxidized parent metal. This metal is at least partially open or accessible, or can be rendered accessible by fracturing, machining, etc. This ceramic body is hereinafter identified as the "first ceramic body". The process may be enhanced by the use of an alloyed dopant, such as in the case of an aluminum parent metal oxidized in air. The method of EP-A-0 155 831 was improved by the use of external dopants applied to the surface of the precursor metal as disclosed in commonly owned EP-A-169 067. Thus, generally, a dopant may be used in conjunction with the parent metal in forming the first ceramic body. Commonly owned EP-A-193 292 discloses a novel method for producing self-supporting ceramic composites by growing an oxidation reaction product from a parent metal into a permeable mass of filler, thereby infiltrating the filler with a ceramic matrix. The first ceramic body thus may comprise a composite formed by infiltrating a filler with the oxidation reaction product. A method for producing ceramic composite bodies having a geometry or shape is disclosed in the commonly owned EP-A-245 192 (not published). In accordance with the method of this invention, the developing oxidation reaction product infiltrates a permeable preform in the direction towards a defined surface boundary. A solid or liquid oxidant may be used in conjunction with the vapor-phase oxidant, and the preform is permeable to the gaseous oxidant and to infiltration by the developing oxidation reaction product product. The resulting ceramic composite has the geometry of the preform. In the present method, a first ceramic body is bonded to another ceramic body, either of like kind or of a different ceramic (hereinafter "second ceramic body") by the development of a bond layer derived from the first ceramic body as a result of the oxidation reaction of molten parent metal contained in the first ceramic body. Two or more ceramic bodies can be so bonded in a single operation provided that, at each pair of facing surfaces, at least one of the surfaces is a surface of the first ceramic body formed by the oxidation of molten parent metal, and grown as molten metal is transported through, and oxidized on the surface of, its own reaction product. The interconnected metal of the first ceramic body is the source of metal required for the formation of the ceramic bonding layer. More particularly, the first ceramic contains surface-accessible residual metal present as a result of molten parent metal transport during the ceramic growth process. In the case of a bond between two ceramic bodies of like kind, in that each ceramic body contains interconnected parent metal as described above, then both ceramic bodies may participate in the growth of the bond layer at their common interface. The ceramic bodies are assembled with each pair of surfaces to be bonded facing one another, either in intimate contact, or at a small standoff or separation. For example, a single pair of first and second ceramic bodies can be arrayed, or a first ceramic body can be assembled between two second ceramic bodies. An array of multiple surfaces, such as plates, can be used, provided that at least every other surface or layer is a body of oxidation reaction product containing interconnected metal. FIGURES 1 and 2 each show typical assemblies used in accordance with the invention in which a first ceramic body 2 (FIGURE 1) or 2′ (FIGURE 2) is positioned with a surface thereof facing a corresponding surface of a second ceramic body 4 (FIGURE 1) or 4′ (FIGURE 2). The bonding layer will be grown between the facing surfaces to bond ceramic bodies 2 and 4 of FIGURE 1; and 2′ and 4′ of FIGURE 2. The facing surfaces can be essentially in contact with one another, provided that the vapor-phase oxidant required for the molten metal oxidation can contact the surface of the first ceramic body. However, inasmuch as the oxidation reaction product of the bonding layer is able to grow by transport of molten metal therethrough and oxidation of molten metal adjacent thereto (as has occurred in the formation of the first ceramic body itself), an initial separation between facing surfaces can be used, provided that sufficient molten metal is available, and the process conditions maintained for a sufficient time, that the growth process can continue to the degree required for the pre-separated surfaces to be joined. When such a standoff is used, it may be beneficial to provide a small angle, e.g., of about 5-10°, between surfaces to minimize the possibility of the formation of voids in the resulting bonding layer due to growth irregularities, which could make access of oxidant difficult as the growing ceramic bond layer comes in contact with the adjacent ceramic. For bonding, the assembled ceramic bodies as illustrated in FIGURES 1 and 2 are heated in an oxidizing atmosphere at a temperature above the melting point of the residual metal in the first ceramic body but below the melting point of the oxidation reaction product. Molten metal accessible from the bonding surface (the surface of ceramic body 2 or 2′ which faces the corresponding surface of ceramic body 4 or 4′) is oxidized on contact with the oxidant, and then growth of the oxidation reaction product is induced as described above so as to form a bonding layer of sufficient thickness. A strong bond can be achieved even with relatively thin bonding layers, and thus it may be unnecessary, and in some cases undesirable, to permit extensive growth of the bonding layer. Any one of various parent metals, e.g., aluminum, titanium, tin, zirconium, hafnium, or silicon, can be used in the practice of the invention, although the invention is described herein with particular reference to aluminum, especially oxidized in air, as to preferred embodiment. Also, the oxidation reaction product may be an oxide, nitride, or carbide, depending on the choice of oxidant. When a first ceramic body is to be bonded to another first ceramic body, the two bodies may be of the same or different composition, and if the metals in both ceramic bodies are derived from the same parent metal, the interconnected metal still may differ with respect to the purity, grade or alloy composition. Ceramic products of other types useful as a second ceramic body which can be bonded to a first ceramic body include densified ceramic powders, e.g. a metal oxide, boride, carbide, or nitride which have been pressed and sintered or otherwise processed by conventional methods. The assembled ceramic bodies to be bonded are heated above the melting point of the residual metal (but below the melting point of the oxidation reaction product to be formed), and an appropriate temperature within this range is maintained for a period sufficient for a bonding layer of the required thickness to grow. The operable and preferred temperature ranges vary depending on the metal, dopant(s) used, time, and oxidant. In the case of a molten aluminum parent metal and air as the oxidant, the reaction temperature may be about from 850°C to 1450°C, or preferably from about 900°C to about 1350°C. In this system, and particularly in the case in which magnesium and one or more of the Group 4B elements, silicon, germanium, tin, and lead are alloyed with the aluminum to act as dopants, a heating time at the selected temperature of only a few hours, e.g., about five hours at about 1100°C, usually is sufficient to produce a strong bond about 0.02 mm or more thick between two ceramic bodies. The oxidizing atmosphere in which the assembly is heated is provided by a vapor-phase oxidant, i.e., a vaporized or normally gaseous material. For example, oxygen or gas mixtures containing oxygen (including air) are desirable vapor-phase oxidants, as in the case where a molten aluminum parent metal is to be oxidized to form an alumina reaction product, with air usually being preferred for obvious reasons of economy. The flow rate of the vapor-phase oxidant should be sufficient to assure good metal oxidant contact between the assembled ceramic bodies. The molten metal consumed in the formation of the bonding layer is carried within channels of at least the first ceramic body, and the metal channels have open access to the surface of the ceramic. In producing the first ceramic body, interconnected metal will remain in the structure if the growth process is stopped prior to or just at the depletion of the pool of molten metal that provides the parent metal for the reaction. If the growth process is continued beyond this point, interconnected metal within the ceramic body is drawn to the surface to form additional polycrystalline growth at the interface with the oxidant, thereby resulting in interconnected porosity in the vacated metal channels. Thus, the first ceramic body used in the process of this invention is one which has been made without substantial depletion of its metal content, by suitable control of process time and temperature. Inasmuch as the first ceramic body contains channels of interconnected metal, oxidation of the molten metal and growth of the oxidation product can be expected to occur not only at the bonding surface, but at all free (exposed) surfaces of the body, as well as on exposed surfaces of any additional parent metal being used (as described below in connection with FIGURE 2) to augment the interconnected metal of the first ceramic body. Growth of oxidation reaction product can be confined to the surface(s) to be bonded by applying a barrier means to the other surfaces. As described in EP-A-245 193 (not prepublished), a suitable barrier means inhibits growth or development of the oxidation reaction product within defined boundaries or zones. Suitable barrier means may be a material compound, element, composition, or the like, which, under the process conditions of this invention, maintains some integrity, is not volatile, and may be permeable or impermeable to the vapor-phase oxidant while being capable of locally inhibiting, poisoning, stopping, interfering with, preventing, or the like, growth of the oxidation reaction product. Suitable barriers for use with aluminum parent metal using air as an oxidant include calcium sulfate (plaster of Paris), calcium silicate, Portland cement, tricalcium phosphate, and mixtures thereof, which typically are applied as a slurry or paste to the surface of the ceramic body and parent metal as shown in the drawings. These barriers means are well suited for confining or preventing growth of alumina oxidation reaction product from molten aluminum in air, and thereby growth toward the bonding zone. FIGURE 1 shows a barrier means 6 (partially broken away for clarity of illustration) which is applied to all free or exposed surfaces of first ceramic body 2, so that oxidation of residual metal and growth of oxidation reaction product from first ceramic body 2 is confined to the bonding surface of ceramic body 2, i.e., the surface thereof facing or abutting a corresponding surface of second ceramic body 4. In the present method, wherein the molten metal required to produce the bonding layer is supplied by the first ceramic body, this body may have been formed originally under such process conditions that it is depleted of interconnected metal, and consequently is porous or at least partially porous. The first ceramic body can be augmented with parent metal by contacting an exposed surface of the ceramic with an additional body of parent metal, which may be the same or different from that used in producing the original first ceramic body. This technique is illustrated in FIGURE 2 in which a parent metal body 8 is positioned adjacent to a free surface of the first body of ceramic 2′, i.e., adjacent to a surface thereof other than a bonding surface which faces or abuts a surface of second ceramic body 4′. All the surfaces of first ceramic body 2′ except its bonding surface and the portion of its surface in contact with parent metal body 8 are covered by a barrier means 6′, which is also applied to all the exposed surfaces of parent metal body 8. The bonding process is carried out as described above, and molten parent metal, as it reacts to form oxidation reaction product, is transported therethrough and to the bonding surface where oxidation reaction product then forms as the bonding layer. Even when the first ceramic body contains interconnected metal, additional parent metal may be supplied to prevent the generation of porosity in the body as metal is drawn to the surface to form the bond layer. Example In order to show the utility of this invention, two 4.8 mm thick ceramic plates were bonded end-to-end at surfaces measuring 4.8 mm x 7.9 mm. The plates both originated from a single piece of alumina ceramic which had been formed by the oxidation reaction of molten aluminum parent metal (aluminum alloy 5052 containing nominally 2.4% magnesium), externally doped with a thin layer of SiO₂ and exposed for 120 hours at 1175°C to form the alumina ceramic. These ceramic bodies contained interconnected aluminum in dispersed channels which extended to the surfaces. The ceramic plates were positioned end-to-end and laid on edge in a high purity alumina boat where they were heated at 1175°C for five hours in flowing air. When cooled, the total weight of the assembly was found to have increased by 2.4%. The plates were firmly bonded together, end-to-end, by a 0.018-mm-thick layer of newly grown alumina ceramic. In addition, a 0.05-mm-thick layer of alumina ceramic had also grown on the other exposed surfaces including the surfaces in contact with the boat which was also strongly bonded to the ceramic plates. The new growth had a finer microstructure than that of the original ceramic plates, with finely dispersed aluminum in evidence therein. An attempt was made to recover the bonded plates by hammer blows to the boat to break it away. All of the bond zones remained intact, indicating a high degree of bonding both between the ceramic plates and from the plates to the high purity alumina boat.

Claims: A method of bonding ceramic bodies along substantially congruent surfaces, comprising: (a) providing a first body of ceramic, said first body comprising a ceramic product formed by the oxidation reaction of molten parent metal and a vapor-phase oxidant and grown as molten metal is transported through, and oxidized on, the surface of its own previously formed oxidation reaction product, said ceramic body comprising a polycrystalline oxidation reaction product and interconnected residual metal; (b) assembling said first body of ceramic adjacent a second body of ceramic in a manner such that a pair of surfaces of said first and second bodies to be bonded together face one another at bonding surfaces of said first and second bodies of ceramic; and (c) heating the assembled ceramic bodies in the presence of a vapor-phase oxidant to a temperature above the melting point of said residual metal but below the melting point of the oxidation reaction product to be formed and below the melting point of the first and second body of ceramic, to induce transport of said residual metal toward said bonding surfaces where oxidation reaction product continues to grow as in step (a) thereby effecting a bond between said ceramic bodies. The method of claim 1 wherein the parent metal comprises aluminum parent metal, the oxidant comprises air, the oxidation reaction product comprises alumina, and the residual metal comprises aluminum. The method of claim 1 or claim 2 wherein said first body of ceramic, said second body of ceramic, or both, comprises a composite formed by infiltrating a filler with said oxidation reaction product. The method of claim 1 or claim 2 wherein a dopant is used in conjunction with said parent metal. The method of claim 1 wherein said first and second bodies are assembled in intimate contact. The method of claim 1 wherein said first and second bodies are assembled with their bonding surfaces at a small angle to one another. The method of claim 1 wherein a barrier means is applied to all exposed surfaces of said first body of ceramic except a surface thereof to be bonded to a surface of said second body of ceramic. The method of claim 7 wherein said barrier comprises a material selected from the group consisting of calcium sulfate, calcium silicate, portland cement, tricalcium phosphate, and mixtures thereof. The method of claim 1 wherein a body of parent metal is positioned adjacent a free surface of said first body of ceramic and a barrier means is applied to part or all of the remaining exposed surfaces of said body of parent metal and said first body of ceramic, whereby said body of parent metal melts during the heating step and molten metal is transported into said first body of ceramic toward said bonding surfaces. The method of claim 1 wherein said second body of ceramic has substantially the same composition as said first body of ceramic. The method of claim 1 wherein said second body of ceramic is made by a method other than the oxidation of a molten parent metal. The method of claim 1 wherein said second body of ceramic is made by the oxidation of a molten parent metal but is of a different composition than said first body of ceramic. The method of claim 1 wherein said oxidation reaction product comprises a material selected from the group consisting of an oxide, a nitride, or a carbide. The method of claim 1 wherein said parent metal comprises a material selected from the group consisting of titanium, silicon, zirconium, hafnium, or tin. A ceramic article obtained by bonding ceramic bodies in accordance with a method of any of the foregoing claims 1 to 14.

Inventors: LUSZCZ, STANLEY J., NEWKIRK, MARC S., URQUHART, ANDREW W.
Applicants: LANXIDE TECHNOLOGY COMPANY, LP.
Application year: 1987
Main classification: C04B37
IPC codes: C04B35, C04B37