METHODS FOR ALLOYING A METAL-CONTAINING MATERIAL INTO A DENSIFIED CERAMIC OR CERMET BODY AND ALLOYED BODIES PRODUCED THEREBY

Abstract: Place an alloying metal or an inorganic compound containing an alloying metal in contact with at least a portion of the exterior surface of a densified cermet or ceramic body and then heat the cermet body or the ceramic body to a temperature that allows alloying to occur. The temperature for the cermet body is close enough to the melting temperature of the metal portion of the cermet to allow alloying to occur. The temperature for the ceramic body is close enough for the body's glassy-phase component to soften and allow alloying to occur. The resultant alloyed bodies have a composition gradient that extends into the body perpendicular to the exterior surface portion if alloying stops prior to completion.

Description: METHODS FOR ALLOYING A METAL-CONTAINING MATERIAL INTO A DENSIFIED CERAMIC OR CERMET BODY AND ALLOYED BODIES PRODUCED THEREBY This invention relates to processes for alloying metal, or an inorganic compound containing such a metal, into a ceramic or ceramic-metal (cermet) body that has a density greater than or equal to 95% of the body's theoretical density. It is well known that metal alloys often have more desirable properties than pure metals. It would follow that alloyed cermets and ceramics may have more desirable properties than non-alloyed cermets and ceramics. It would therefore be desirable to have methods for making such alloyed cermets and ceramics. It would also be desirable to be able to alter the properties of a ceramic or cermet body by alloying without significantly changing the shape or density of the body. It would further be desirable to be able to make a cermet or ceramic body that has different properties such as improved oxidative resistance or wear resistance at separate locations on the body. One aspect of the present invention is a process for alloying a metal or a metal-containing material into a densified cermet body comprising: (a) placing an alloying metal or an inorganic compound containing an alloying metal in contact with at least a portion of an exterior surface of a cermet body that has a density of at least 95% of the theoretical full density and is formed of a ceramic that has a matrix metal dispersed therein, the matrix metal extending from the exterior surface portion at least partially into the cermet body; and (b) heating the cermet body, while it is in contact with the alloying metal or the inorganic compound containing the alloying metal, to a temperature that is greater than or equal to 50°C below the lesser of the matrix metal's melting point and the temperature at which the matrix metal and the alloying metal form an eutectic, to diffuse the alloying metal into the cermet body and form an alloy between the alloying metal and the matrix metal. A second aspect of the invention includes a method for alloying a metal or a metal-containing material into a densified ceramic body comprising: (a) placing an alloying metal or an inorganic compound containing an alloying metal in contact with at least a portion of an exterior surface of a ceramic body that has a density of at least 95% of the theoretical full density and is formed of a glassy-phase component and a non-glassy-phase component, the glassy-phase component extending from the exterior surface portion at least partially into the ceramic body; and (b) heating the ceramic body to a temperature at least near that at which the glassy-phase component softens while the alloying metal or the inorganic compound containing an alloying metal is in contact with the ceramic body to diffuse the alloying metal into the ceramic body and form an alloy between the alloying metal and the glassy-phase component. Bodies prepared by the methods described immediately above are also disclosed. The first aspect of the invention concerns a method for alloying a metal, or a material containing a metal, into a densified cermet body. The term "metal- -containing material" collectively refers to alloying metals and inorganic compounds containing an alloying metal. The metal-containing material is suitably selected from pure metals, alloys of two or more metals, and inorganic compounds containing at least one metal. A "metal" is an element from the Periodic Table of the Elements that forms positive ions when its compounds are in solution and whose oxides form hydroxides rather than acids with water. Desirable metals include nickel, chromium, copper, vanadium, titanium, silicon, magnesium, hafnium, tungsten, molybdenum, manganese, iron, copper or tantalum. Satisfactory inorganic compounds release or give up metal under conditions of the present invention to form a metal alloy with the matrix metal of the cermet. Inorganic compounds that are stable, in that they do not release metal to form an alloy under conditions of the present invention, are not suitable. Illustrative inorganic compounds include nitrides, oxides, carbides, borides or suicides of the metals. Carbides, nitrides and oxides are preferred inorganic compounds. The metal component of the metal-containing material is preferably different from the matrix metal of the cermet body. The metal-containing material may be either solid or molten when it is initially placed in contact with the cermet body. When the metal-containing material is solid, it may be in any form including powder or foil. When the metal-containing material is in the form of a foil, the foil is desirably less than 0.75 mm thick, more desirably, less than 0.5 mm thick, and, preferably, less than 0.1 mm thick. Examples of cermet/alloying metal combinations suitable for this invention include those provided in Table 1. Table 1 Cermet Possible Alloying Metals B C/A1 Ni, Cr, Cu, V, Ti, Si, Mg, Hf, W SiC/Al Ni, Cr, Cu, Mn, V, Ti, Mg, Si, Hf, W TiB 2 /Al Ni, Cr, Cu, Mn, Ti TiB 2 /Cu Al, Ti, Si, Ta, Pe, Co Si 3 N 4 /Al Cr, Mn, Ti Si 3 _ι/ i Ti, Fe, Co, Mn SiB 6 /Al Ni, Cr, Cu, Mn, Ti, Si The cermet body, prior to contact with the metal-containing material, has a ceramic component and a matrix metal component. The cermet body desirably contains from 15 to 99 weight percent ceramic component and from 85 to 1 weight percent matrix metal component. The cermet body preferably contains from 60 to 80 weight percent ceramic component and from 40 to 20 weight percent matrix metal component. The weight percentages total 100 percent. The ceramic component is desirably boron carbide, silicon carbide, titanium boride, alumina, or silicon nitride. The matrix metal component is desirably aluminum, titanium, copper or nickel. In order for the metal-containing material to diffuse into the cermet body, two conditions must exist. First, the matrix metal must be in physical contact with the metal- -containing material at least at an exterior surface portion of the cermet body. Second, the matrix metal must extend at least partially into the cermet body from that surface portion. The matrix metal may be continuous throughout the bulk of the cermet body with the ceramic being the discontinuous phase. The non-alloyed cermet body may be prepared by any conventional method. For example, the cermet body may be prepared by densifying a mixture of ceramic and matrix metal powders into a desired shape. The densification may be accomplished by conventional processes such as cold pressing or slip casting. If desired, these processes may be followed by sintering to further density the cermet body. Another method of preparing a non-alloyed cermet body includes a first step of forming a porous ceramic body. Conventional procedures, such as slip casting a ceramic dispersion or cold pressing ceramic powder, produce porous bodies with a desired shape. Molten matrix metal is then placed in contact with the porous ceramic body. The molten metal infiltrates into the porous ceramic body by capillary action. Such methods are disclosed in U.S. Patent Nos. 4,702,770, 4,718,941, and 4,834,938. U.S.-A 4,702,770 describes, at column 6, lines 5-25, a four step method for producing low density boron carbide-aluminum composites. The first three steps yield a satisfactory cermet body. In step one, a colloidal consolidation technique is used to form a porous boron carbide compact from a homogeneous dispersion of boron carbide. In step two, the compact in enriched with carbon by heat treating or sintering the boron carbide in the presence of graphite. In step three, aluminum is infiltrated into the enriched compact. U.S.-A 4,718,941 discloses the infiltration of molten reactive metals into chemically pretreated boron carbide, boron or boride starting materials. The chemical pretreatment procedure is described in detail at column 7, line 8 through column 8, line 12. The starting materials are immersed in a chemical substance that reacts with B2O3, BH3O3, or both, for as long as necessary to change the surface chemistry by forming boron-carbon-hydrogen-oxygen complexes that pyrolyze upon heating, either prior to, or during, infiltration. Suitable chemical substances include various alcohols, such as methanol, isopropyl alcohol and denatured ethanol. U.S.-A 4,834,938 outlines a process for making a composite article having an internal surface or cavity. A porous compact is formed about an insert body that has an external surface corresponding to the internal surface of the composite. The article is heated to the wetting temperature of the insert body to melt the insert and cause it to infiltrate into the porous compact. The non-alloyed cermet body required for purposes of the present invention has a density of at least 95% of theoretical density. The density is more preferably at least 99% of theoretical density. Alloying, or the diffusion of an alloying metal into a matrix metal, is desirably initiated by heating the cermet body to a temperature greater than or equal to that at which the matrix metal melts. If an eutectic alloy is formed between the matrix metal and the alloying metal, the eutectic alloy typically has a melting point lower than that of the matrix metal. If an eutectic alloy is formed, the temperature may be decreased to equal or exceed the melting point of the eutectic alloy. If no eutectic alloy is formed, the temperature is preferably maintained at or above the melting temperature of the matrix metal until alloying reaches a desired level. Temperature plays an important role in determining the rate at which alloys between the matrix metal and the alloying metal form. Diffusion occurs so slowly at room temperature that it is not readily detected over a period of several days. As the temperature approaches the melting temperature of the matrix metal, diffusion becomes more discernible. At the melting temperature, diffusion is rapid enough to yield a readily detectable amount of alloy. At temperatures in excess of the melting temperature, diffusion occurs very rapidly. The temperature must not, however, be so high that the ceramic portion of the cermet begins to decompose or dissociate. The term "at least near" means greater than or equal to a temperature that is 50°C below the melting temperature of the matrix metal. The temperature preferably equals or exceeds the melting temperature of the matrix metal, but is less than the dissociation temperature of the ceramic portion of the cermet. During the alloying process, the metal- -containing material may or may not become molten, depending on its melting point. A satisfactory degree of alloying is attainable if only the matrix metal becomes liquefied. When foils are used and the metal- -containing material is to become molten during the alloying process, thin foils are preferred as they melt faster and result in more uniform diffusion across the contacted surface of the cermet body. The matrix metal preferably has a melting point that is less than or equal to the melting point of the metal-containing material. The second aspect of the invention is a method for alloying a metal-containing material into a densified ceramic body. The ceramic body contains a glassy—phase component and a non-glassy-phase component. The ceramic body desirably contains from 70 to 99 weight percent of non-glassy phase and from 30 to 1 weight percent glassy phase. The ceramic body preferably contains from 85 to 97 weight percent non-glassy phase and from 15 to 3 weight percent glassy phase. The non-glassy-phase component of the ceramic body is desirably silicon nitride. The glassy-phase component of the ceramic body is desirably formed of a metallic oxide such as yttria, magnesia, alumina, silica, zirconia, tantalum oxide, calcium oxide or a mixture of two or more of such oxides. In order for the metal-containing material to diffuse into the ceramic body, two conditions must be met. First, the glassy-phase component must be in physical contact with the metal-containing material at least at an exterior surface portion of the ceramic body. Second, the glassy-phase component must extend at least partially into the ceramic body from that surface portion. The glassy-phase component may be continuous throughout the bulk of the ceramic body while the non- -glassy phase component may be discontinuous throughout the remainder of the bulk of the ceramic body. The ceramic body may be prepared by densif ing a mixture of powders that will constitute the glassy- phase and non-glassy-phase components. For instance, silicon nitride, yttria, magnesia, and zirconia powders may be densified into a desired shape by cold pressing, or slip casting, the powders into a body and then sintering the body. The ceramic bodies desirably have a density of at least 95% of theoretical density. The density is preferably at least 98% of theoretical density. Metal-containing materials suitable for purposes of the second aspect of the present invention, like those of the first aspect, are metals, inorganic compounds containing a metal or having a metallic constituent, a metal alloy, or a metal oxide. Metal oxides are commonly available metal-containing materials. The metal or metallic constituent is suitably yttrium, magnesium, chromium, zirconium, molybdenum, vanadium, titanium or a rare earth metal (elements 57-71 of the Periodic Table). The metal or metallic constituent must differ from that contained in the glassy-phase component if alloying is to occur. The metal-containing material may be solid or liquid when it is initially placed in contact with the ceramic body. As in the first aspect, it may be in the form of a powder or a foil when it is a solid. As the ceramic body is heated to soften the glassy-phase component, the metal-containing material may retain its initial form or be converted to a liquid. If desired, two or more different metal- -containing materials may be placed in contact with exterior surface portions of either the cermet of the first aspect of the invention or the ceramic of the second aspect. If, for example, two different materials are placed in contact with exterior surface portions of a cermet or ceramic body, they are preferably not applied to the same surface portion. During the heating step of the invention, the metal-containing material gradually diffuses into the cermet or ceramic body from the exterior surface portion in contact with said material. Before alloying is complete, alloying occurs in a gradient manner. The term "gradient manner" means that alloying occurs to a greater extent at or near the exterior surface portion and to a progressively lesser extent as distance from that portion increases as one travels in a direction perpendicular to the surface portion into the ceramic or cermet body. Alloying may change only the chemistry of the matrix metal or glassy-phase component. It may also change the chemistry of the ceramic or non-glassy phase component thereby forming new phases. The alloying process may be continued until the body is substantially free of chemical composition gradients. More typically, alloying is terminated while gradients are still present. Termination occurs by conventional means, such as cooling the cermet body or the ceramic body to a temperature well below the respective matrix metal liquidus temperature or the glassy-phase component softening temperature. After termination of alloying, any excess metal-containing material on the surface of the cermet or the ceramic body may be removed by conventional processes. These processes include peeling or grinding. The alloyed bodies of this invention are especially useful for making abrasion-resistant parts, such as cutting tools and drilling tools, and wear parts, such as automobile brakes. The following examples are illustrative only and should not be construed as limiting the invention which is properly delineated in the appended claims. Example 1 A block-shaped porous body of boron carbide, measuring about 1 cm thick, was prepared by slip casting a dispersion of boron carbide powder. The slip-cast body had a density of about 64% of the theoretical density of boron carbide. The porous body was impregnated with aluminum by contacting the boron carbide body with aluminum and heated to 1180°C to form a cermet body. At that temperature, sintering was effected and the cermet body comprised about 64 weight % boron carbide, about 25 weight % aluminum and about 11 weight % reaction products containing aluminum. The body had a density of greater than 99% of the theoretical density. Table 2 shows typical reaction products. Copper foil measuring about 0.75 mm thick was placed on one side of the cermet body, while a foil of nickel measuring about 0.75 mm thick was placed on the opposite side. The respective melting points of the matrix metal (aluminum), the copper, and the nickel are 660°C, 1085°C, and 1445°C. The cermet body and the associated copper and nickel foils were placed in a graphite crucible. The crucible was placed in an oven with a flowing argon atmosphere. The temperature of the oven was increased from room temperature to 1100° in about two hours and held at 1100°C for one hour. The cermet body was then removed from the furnace and allowed to cool naturally to ambient temperature. Cooling occurred over a period of about 1 1/2 hours. Chemical analysis of the alloyed cermet body was completed using an MBX-CAMECA microprobe, available from Cameca Co., France. The results are provided in Table 2 which shows the volume percent amounts of the various phases listed on the side of the table at three locations in the alloyed cermet body. The first location was in copper side, the second location was in the bulk of the boron carbide cermet body, and the third location was in the nickel side. The results summarized in Table 2 demonstrate that diffusion of the metal-containing materials occurred. The nickel component ranged from a high of 20 volume percent at the "nickel end" through an intermediate value of 12 volume percent in the middle of the cermet body to a low of not being detectable in the "copper end". The copper component (CUAI 2 ) showed a similar gradient. Furthermore, the boron carbide volume percent remained fairly consistent throughout the cermet body. Table 2 A boron carbide-aluminum cermet body was prepared by the method described in Example 1. A copper foil, measuring about 1 ram thick, was placed on the cermet body. The cermet body was then heated to 1100°C, and the temperature was maintained for 1 hour. After cooling the cermet body, the copper foil was removed. It was found that the copper content of the foil was reduced by 3.5 wt%. Example 3 This example illustrates the use of a powder form of a metal-containing material. A densified ceramic body having a composition of 86.3 wt% Si3N , 8.5 wt% Y2O3, 4.7 wt% MgO and 0.5 wt.% CaO was prepared by hot-pressing a mixture of the Si3 4 and oxide powders at 1825°C for 1 hour. The ceramic body had a density of about 99.5% of the maximum theoretically possible density. The ceramic body was buried in a Hfθ2 powder bed in a quartz container. The ceramic body and Hfθ 2 bed were heated to 1400°C and held at this temperature for 1 hour while in a flowing nitrogen atmosphere. The ceramic body and the Hfθ2 bed were then allowed to cool. After cooling, the ceramic body was separated from the Hfθ2 powder, sectioned, and analyzed. On the surface of the ceramic body, an alloyed layer rich in Hfθ2 measuring approximately 50 micrometers deep was observed. The diffusion of Hfθ2 altered the ceramic body's composition, but did not alter the chemistry or shape of the S13N4 grains. Analysis indicated that the alloyed layer contained Si3N grains in two glassy phases: a phase relatively high in Mg, relatively low in Y, and having no Hf; and a phase having a relatively intermediate levels of Mg and Y and a relatively high level of Hf. Example 4 A densified ceramic body was prepared as in Example 3 and placed in a bed of Cr2U3 powder. After heating the ceramic body and Cr 2 U3 powder to 1400°C, maintaining this temperature for 1 hour, then cooling, three alloyed layers near the surface of the ceramic body were observed. The first, or outside layer, consisted of Cr3Si, Cr 2 N, a relatively high level of yttria silicate crystals, and a mixture of two glassy phases. The glassy phases were: a phase having Y and Mg in a weight ratio of about 1:1; and a phase containing a relatively low level of Y, and relatively high levels of Mg and Ca. The second, or middle layer, consisted of Si3N , Cr3Si and a relatively low level of CaO glass. The third, or innermost, layer consisted of Si3N4 and glass containing a relatively intermediate amount of Y, a relatively intermediate amount of Mg, and no Ca. The weight ratio of Y to Mg in this innermost layer was about 0.6:1. The total thickness of all three layers was about 200 micrometers. This example illustrates that even though diffusion occurred through the glassy phase, the majority of the Cr formed new crystalline phases and the glass chemistry remained unchanged. Therefore, the properties of the Si3 4 body were changed without changing the chemistry of the glassy phase. The examples show that the processes of the present invention produce alloyed cermet or ceramic bodies. Some of the resultant bodies optionally have chemical composition gradients. The main advantage is that the processes can alter the chemistry of a cermet or ceramic body without significantly altering the density or shape of the original body. While our invention has been described in terms of a few specific embodiments, it will be appreciated that other embodiments could readily be adapted by one skilled in the art. Accordingly, the scope of our invention is to be considered limited only by the following claims.

Claims: CLAIMS: 1. A process for alloying a metal-containing material into a densified cermet body comprising: (a) placing an alloying metal or an inorganic c compound containing an alloying metal in contact with at least a portion of an exterior surface of a cermet body that has a density of at least 95% of the theoretical full density and is formed of a ceramic that has a matrix metal dispersed therein, the matrix metal 0 extending from the exterior surface portion at least partially into the cermet body; and (b) heating the cermet body, while it is in contact with the alloying metal or the inorganic 5 compound containing the alloying metal, to a temperature that is greater than or equal to 50°C below the lesser of the matrix metal's melting point and the temperature at which the matrix metal and the alloying metal form an eutectic, to diffuse the alloying metal into the cermet 0 body and form an alloy between the alloying metal and the matrix metal. 2.A process as claimed in claim 1, wherein the ceramic is boron carbide, alumina, silicon carbide, 5 titanium boride, or silicon nitride; the matrix metal is aluminum, copper, titanium, or nickel; the alloying metal or the inorganic compound containing an alloying metal has a metal component that is nickel, chromium, copper, vanadium, titanium, silicon, magnesium, hafnium, tungsten, molybdenum, manganese, iron, copper, or tantalum; and the matrix metal and the alloying metal are different metals. 3. A process as claimed in claim 1 or claim 2 10 wherein the density of the cermet body is at least about 99% of the theoretical full density. 4.A process as claimed in any of the preceding claims wherein the alloying metal or the inorganic ,_. compound is in the form of a foil having a thickness less than 0.75 mm during step (a). 5. A process as claimed in any of the preceding claims wherein the matrix metal has a lower melting 0 point than that of the alloying metal or the inorganic compound. 6.A process as claimed in any of the preceding claims wherein a first alloying metal or inorganic 5 compound containing an alloying metal is in contact with a first exterior surface portion of the cermet body and at least one additional alloying metal or inorganic compound containing an alloying metal is in contact with at least one additional exterior surface portion of the 0 cermet body, the first alloying metal or inorganic compound differing from each additional alloying metal or inorganic compound, and the exterior surface portions are distinct from each other. 7.A process as claimed in any of the preceding claims wherein the heated cermet body is subsequently cooled to stop diffusion and provide a chemical composition gradient that penetrates into the cermet body perpendicular to the exterior surface portion in contact with the alloying metal. 8.An alloyed cermet body formed by the process of any of claims 1-7. 9.A process for alloying a metal-containing 10 material into a densified ceramic body, comprising: (a) placing an alloying metal or an inorganic compound containing an alloying metal in contact with at least a portion of an exterior surface of a ceramic body 15 that has a density of at least 95% of the theoretical full density and is formed of a glassy-phase component and a non-glassy-phase component, the glassy-phase component extending from the exterior surface portion at 20 least partially into the ceramic body; and (b) heating the ceramic body to a temperature at least near that at which the glassy-phase component its softens while the alloying metal or the inorganic «e compound containing an alloying metal is in contact with the ceramic body to diffuse the alloying metal into the ceramic body and form an alloy between the alloying metal and the glassy-phase component. 0 10.A process as claimed in claim 9 wherein the ceramic body is made of silicon nitride and the alloying metal or the inorganic compound has a metal component that is selected from metals, metal alloys, and metal oxides. 11. A process as claimed in claim 9 or claim 10 wherein the glassy-phase component has a softening point that is lower than the melting point of the alloying metal or the inorganic compound containing the alloying metal. 12. A process as claimed in any one of claims 9- 11 wherein a first alloying metal or inorganic compound containing an alloying metal is in contact with a first exterior surface portion of the ceramic body and at least one additional alloying metal or inorganic compound containing an alloying metal is in contact with at least one additional exterior surface portion of the ceramic body, the first alloying metal or inorganic compound differing from each additional alloying metal or inorganic compound, and the exterior surface portions are distinct from each other. 13.A process as claimed in any one of claims 9- 12 wherein the heated ceramic body is subsequently cooled to stop infiltration and provide a chemical composition gradient that penetrates into the ceramic body perpendicular to the exterior surface portion in contact with the alloying metal. 14. An alloyed ceramic body formed by the process of any of claims 9-13. 15. A process as claimed in claim 1 wherein the density of the cermet body is at least about 99% of the theoretical full density. 16.A process as claimed in claim 1 wherein the alloying metal or the inorganic compound is in the form of a foil having a thickness less than about 0.75 mm during step (a). 17.A process as claimed in claim 1 wherein the matrix metal has a lower melting point than that of the alloying metal or the inorganic compound. 18.A process as claimed in claim 1 wherein a first alloying metal or inorganic compound containing an alloying metal is in contact with a first exterior surface portion of the cermet body and at least one 10 additional alloying metal or inorganic compound containing an alloying metal is in contact with at least one additional exterior surface portion of the cermet body, the first alloying metal or inorganic compound ,5 differing from each additional alloying metal or inorganic compound, and the exterior surface portions are distinct from each other. 19.A process as claimed in claim 1 wherein the 0 heated cermet body is subsequently cooled to stop diffusion and provide a chemical composition gradient that penetrates into the cermet body perpendicular to the exterior surface portion in contact with the alloying metal. 5 20.An alloyed cermet body formed by the process of claim 1. 21.A process as claimed in claim 9 wherein the 0 ceramic body is made of silicon nitride and the alloying metal or the inorganic compound has a metal component that is selected from the group consisting of metals, metal alloys, and metal oxides. 22.A process as claimed in claim 9 wherein the glassy-phase component has a softening point that is lower than the melting point of the alloying metal or the inorganic compound containing the alloying metal. 23. A process as claimed in claim 9 wherein a first alloying metal or inorganic compound containing an alloying metal is in contact with a first exterior surface portion of the ceramic body and at least one additional alloying metal or inorganic compound containing an alloying metal is in contact with at least one additional exterior surface portion of the ceramic body, the first alloying metal or inorganic compound differing from each additional alloying metal or inorganic compound, and the exterior surface portions are distinct from each other. 24.A process as claimed in claim 9 wherein the heated ceramic body is subsequently cooled to stop infiltration and provide a chemical composition gradient that penetrates into the ceramic body perpendicular to the exterior surface portion in contact with the alloying metal. 25. An alloyed ceramic body formed by the process of claim 9.

Inventors: BEAMAN DONALD R, OTT JACK J, PYZIK ALEKSANDER J, BEAMAN, DONALD, R., OTT, JACK, J., PYZIK, ALEKSANDER, J.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1992
Main classification: C23C10
IPC codes: C22C29, C22C32, C23C10, C23C26, F16D69