Process for producing profiled ceramic composite body

The invention relates to a process for the manufacture of shaped self-supporting ceramic composites.

V čsl. put. No. 271,322 discloses a process for producing self-supporting ceramic composites by incorporating the oxidation reaction product of a parent metal and an oxidant into a permeable filler mass. However, the resulting composite has neither a defined nor a predetermined geometry, but a configuration.

U.S. Pat. No. 4,353,352 also discloses a method for producing ceramic composites, wherein a base metal dopant is used in the oxidation of a dopant which promotes the oxidation phenomenon and allows the production of self-supporting ceramic bodies growing as an oxidation reaction product from a metal precursor. This method utilizes impurities that are deposited on the surface of the metal precursor or in the filler bed. '

In recent years, there has been an increasing interest in the use of ceramic materials for construction purposes, where metals have always been used. The reason for this interest is certain better ceramic properties compared to metals, such as corrosion resistance, hardness, modulus of elasticity and heat resistance.

Current attempts to produce stronger, more reliable and toughened ceramic articles are focused mainly on the development of improved working methods for the production of monolithic ceramic parts and on the development of the composition of new materials, in particular ceramic matrix composites. A composite structure is one that comprises a heterogeneous material, body or article made of two or more distinct substances that are intensely associated to achieve the desired properties of the composite. For example, two different materials can be thoroughly combined by placing one in a matrix of the other. Ceramic matrix composite structures comprise a ceramic matrix that encloses one or more different kinds of filler materials such as grains, fibers, sticks and the like.

Traditional methods of producing ceramic articles generally include the following processes:

preparing ceramic material in powder form, crushing or grinding powders into very fine particles, shaping powders into a body of the desired shape with shrinkage addition during subsequent processing, such as uniaxial compression, isostatic pressing, injection, slurry application, slurry casting or any other known technique densifying said body by heating to an elevated temperature such that the individual powder particles are joined together to form a continuous structure, the heating is preferably carried out without pressure, for example by non-pressurized sintering, but in some cases an additional force is needed to form such an external pressure which can be exerted either uniaxially, for example by hot pressing or isostatic, i.e. hot isostatic pressing and finishing, usually diamond grinding. ,

When these traditional methods are used to produce ceramic matrix composites, various difficulties arise, with the most serious problems relating to densification of the preform.

The normal conventional method, i.e., non-pressurized sintering, can be difficult or even impossible in preparing granular composites if their materials are not highly compatible. In addition, conventional sintering is impossible in most cases when fiber composites are processed, even if the materials are compatible with each other, since grain bonding is precluded by fibers that prevent the necessary movement of powder particles to be densified. In some cases, these difficulties can be overcome in part by promoting the densification process by applying external pressure at higher temperatures. However, such processes can cause additional problems, such as breaking or damaging the reinforcing fibers under external forces, the limited possibility of creating complex shapes, in particular by uniaxial hot pressing, and generally high costs resulting from low process productivity and extensive finishing operations.

Further difficulties may arise in mixing powders with wires or fibers and in forming a body where it is important to maintain a uniform distribution of the second phase of the composite within the matrix. In the preparation of a wire-reinforced ceramic composite, both components flow together when mixed, and may result in non-uniformity when forming the body and

EN 276180 8 6 unwanted orientation of the reinforcing wires so that the resulting ceramic structure does not have the desired performance.

A method for producing refractory materials from metal oxides by the oxidative-reduction reaction of a metal with silica is disclosed in U.S. Pat. No. 2,702,750, according to which the silica body is either immersed in a molten bath of metal, for example aluminum, or the metal powder is dispersed inside the silica body and then heated. An inert material such as alumina may be added to the body as desired. The refractory product is formed by oxidizing a metal to its oxide while reducing the silica that liberates silicon. US Pat. file no.

973 977 describes a process for producing a cermet, consisting primarily of a spinel type magnesium aluminate, in which an agglomerate of several oxides is immersed in a bath of molten aluminum. None of these stalemates. however, it does not disclose the directional growth of the oxidation reaction product resulting from the oxidation of a metal precursor with a vapor-phase oxidant, nor the growth and infiltration of such a product into a preformed preform.

The patents cited in the introduction describe new methods which solve certain problems associated with traditional ceramic technology but do not remove the other limitations common in ceramic technology, namely the formation of complex structures of precise or approximately exact shape.

The invention solves this problem and its object is a method for producing a shaped ceramic composite body by oxidizing the parent metal and growing the oxidation reaction product into the filler. SUMMARY OF THE INVENTION The object of the present invention is to provide at least one permeable preform of the filler material which, together with the parent metal body, is in contact with each other in an inert bed in a refractory vessel, the parent metal is heated to a temperature above its melting point but below melting the oxidation reaction product, and maintaining the reaction for a period of time sufficient to infiltrate the ceramic matrix formed by the oxidation reaction product and optionally containing non-oxidized metal constituents of the parent metal, into the entire parison.

According to the invention, the preform can be formed from two or more parts which are superimposed on their respective surfaces. In order to form the hollow ceramic composite body, the preform parts are provided with recesses on their contact surfaces. A barrier is preferably applied to the walls of the preform recess and optionally to the free surfaces of the preform and the parent metal body to prevent further growth of the oxidation reaction product.

The invention allows the formation of composite moldings having a relatively complex geometry or arrangement, for example, rounded mold surfaces with holes or holes. In addition, the invention allows the production of ceramic composites with a predetermined shape by means of an oxidation phenomenon, which eliminates the difficulties and limitations associated with known processes. With the method of the invention, ceramic bodies with high strength and toughness can be produced by a mechanism that is more straightforward, adaptable and cheaper than conventional methods. The invention also makes it possible to reliably manufacture ceramic articles as integral bodies of a predetermined shape and bodies with a dimension and thickness which has hitherto been impossible to produce by prior art.

In the process according to the invention, the ceramic matrix is ​​formed by the oxidation reaction of a base metal precursor and a vapor-phase oxidant to a polycrystalline material which permeates the preform and consists predominantly of the resulting oxidation reaction product and optionally one or more metal components. The vapor phase oxidant may be used in conjunction with a solid or liquid oxidant, in which case the polycrystalline matrix may comprise the reaction product of the metal precursor with the additional oxidant and the oxidized or reduced components of such oxidants.

When the parent metal is heated, the operating range or preferred temperatures need not extend over the entire temperature range indicated. The molten metal is contacted with a permeable preform, for example by placing it next to the preform, whereby the formation or growth of the oxidation reaction product takes place in the preform and towards the boundary surface. The molten metal reacts with the oxidant to form a layer of oxidation reaction product and gradually penetrates the oxidation reaction product towards the oxidant. As a result, the oxidation reaction product continually forms and forms at the interface between the oxidant and the previously formed oxidation product layer.

CS 276100 D 6 the reaction of another product that grows through the preform. The reaction is maintained until the polycrystalline oxidation reaction product grows through the preform to the boundary surface such that the resulting matrix encloses the preform and forms a ceramic composite having the shape or geometry of the preform.

Materials with uniform properties throughout the cross-section may grow by the process according to the invention to a thickness which has hitherto been virtually impossible to achieve by conventional methods of producing dense ceramic structures. The process avoids the disadvantages of the prior art, which include high costs, in particular for the preparation of fine, very pure powders, the formation of the green body, the binder firing and the sintering, hot pressing and / or isostatic hot pressing.

The products are usable or directly manufactured as commercial articles which include, without limitation, industrial, structural and technical ceramic articles for purposes where certain properties, such as electrical, abrasion, thermal or structural properties, are important or advantageous. However, the invention does not encompass recycled or waste materials, such as those which may arise as undesirable by-products in the processing of molten metals.

Within the scope of the invention, the following terms may be specified as follows:

Ceramic does not only mean a ceramic body in the classical sense, that is to say a body consisting exclusively of non-metallic and inorganic materials, but includes a body which is predominantly ceramic in either composition or principal properties, although the body may contain less or more metal , derived from a parent metal or formed from an oxidant or dopant, typically in the range of from 1 to 40 ⁵volume, but may also contain a greater proportion of metal.

The oxidation reaction product generally means one or more metals in an oxidized state, where the metal has given up electrons or shares electrons with another element, compound or combination thereof. Thus, the oxidation reaction product means the product of the reaction of one or more metals with an oxidizing agent.

Oxidant means one or more electron acceptors or electron-sharing substances and can denote an element, combination of elements, compounds or combination of compounds including reducible compounds, the oxidant being in the gas, solid or liquid phase conditions.

Base metal means that metal, for example, aluminum, which is a precursor of the oxidation reaction product and includes such a metal as a relatively pure metal, as a commercial metal with impurities and / or clinker components of the nobo alloy in which the metal precursor forms a lilac component. Thus, wherever a metal, for example aluminum, is referred to as a parent metal, the meanings given are intended to be included in this term unless otherwise indicated.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained with reference to the accompanying drawings, in which: FIG. 1 is a cross-sectional view showing a base metal ingot on which a system of two preforms having a rectangular cavity is disposed; 2a is a plan view of a preform shaped as a toothed wheel for producing a composite of the invention; FIG. Fig. 2b is a cross-sectional view taken along line 2b-2b of Fig. 2a; Fig. 3 is a cross-sectional view illustrating the preform assembly of Fig. 2a above the parent metal; Fig. 4 is an assembly of Fig. 3 embedded in an inert bed inside the crucible; Fig. 5 is a photograph of the composite produced; Figs. 6a, 6b are cross-sectional photographs of the composite produced by the method of Example 2; Figs. 7a, 7b are side and plan views of the preform used in the composite of Example 3; 7c is a photograph of a composite made according to Example 3.

According to the invention, the parent metal, which may be doped with the dopant (s) and is a precursor of the oxidation reaction product, is formed in the form of an ingot, edge, rod, plate and the like and inserted into the inert bed of filler, crucible or other refractory vessel. At the same time

CS 276180 B 6 produces a permeable, preformed preform so that it has at least one defined boundary area and permits a vapor-phase oxidant and an infiltrating oxidation reaction product. The preform is positioned adjacent to the parent metal and preferably in contact with one or more surfaces and portions of the parent metal surface such that at least a portion of the defined boundary surface of the preform lies outside or outside or at some distance from the parent metal surface. The preform preferably contacts the surface of the parent metal. If desired, the preform may be partially but not completely immersed in the molten metal, since complete immersion would prevent the vapor phase oxidant from entering the preform and hence the formation of a polycrystalline matrix. The oxidation reaction product forms in the direction of the defined boundary area of ​​the preform. The parent metal and permeable preform assembly, placed in a suitable container, is placed in the furnace and the gaseous oxidant is introduced into the furnace while heating to working temperature.

The preform suitable for carrying out the process of the invention must be sufficiently porous or permeable to allow the vapor phase of the oxidant and its contact with the molten metal. At the same time, the preform must be sufficiently permeable to permit the formation or ingrowth of the oxidation reaction product as a matrix of the preform without causing distortion or change in the shape of the preform.

The vapor-phase oxidant is normally gaseous or vaporizes under operating conditions to produce an oxidizing atmosphere. Air is a suitable oxidant. Typical types of vapor-phase oxidants include, for example, elements or compounds, combinations of elements or compounds of the following, including volatile or vaporizable elements of compounds or components: oxygen, nitrogen, halogen, sulfur, phosphorus, arsenic, carbon, boron, selenium, tellurium, methane, ethane, propane, acetylene, ethylene, propylene (hydrocarbons as carbon sources) and mixtures such as air, Τ ^ / Τ ^ Ο and CO / CO 2, the latter two compositions being useful in reducing oxygen activity to the oxidizable components of the preform. Suitable vapor-phase oxidants are oxygen and oxygen-containing mixtures, including air, with air usually being preferred for obvious economic reasons. When it is stated herein that the gas phase oxidant comprises or comprises a gas or vapors, it is an oxidant wherein said gas or vapor is the only major or at least substantial oxidizing agent for the parent metal under the process conditions of the invention. For example, although the main air component is nitrogen, the oxygen contained in air is normally the only parent metal oxidant under the process conditions of the invention. Thus, air falls within the definition of an oxygen-containing gas, but is not a nitrogen-containing oxidant. An example of a nitrogen-containing gaseous oxidant is a forming gas, which typically contains about 96% nitrogen by volume and 4% hydrogen by volume.

In conjunction with a vapor-phase oxidant, oxidants which are liquid or solid under the operating conditions of the invention may be used. Such additional oxidants are particularly useful in promoting oxidation of the parent metal within the preform and not beyond its boundary surfaces. That is, the use of such additional oxidants can create conditions in the preform that are more favorable to the parent metal oxidation kinetics than conditions outside the preform. Such a support action is advantageous in that it promotes the formation of the matrix within the preform towards the boundary surface and minimizes its overgrowth across the boundary surface. When a solid phase oxidant is used, it may be dispersed throughout the preform or in a portion of the preform adjacent to the parent metal, for example, in the form of grains mixed with the preform, or may form a coating on the preform particles. Any suitable solid oxidant may be used depending on its compatibility with the vapor-phase oxidant. Solid oxidants may include suitable elements such as boron or carbon, or suitable reducible compounds such as silica, forming an oxygen source, or some borides with lower thermodynamic stability than the boride forming product of the parent metal oxidation reaction.

CS 276180 8 6

When used in the liquid phase, the oxidant may be dispersed throughout the preform or in the portion of the preform adjacent to the molten metal, provided that it does not prevent the oxidant gas from accessing the molten parent metal. The term liquid oxidant refers to a substance which is liquid under oxidation reaction conditions such that the liquid oxidant may have a solid precursor, for example, a salt that melts or is liquid under oxidation reaction conditions. Alternatively, the liquid oxidant may have a liquid precursor, for example, a solution that serves to coat all or part of the preform and melt or decompose under operating conditions to form an oxidizing residue. Examples of such liquid oxidants are, for example, low melting glasses.

The assembly consisting of the parent metal and the permeable preform is placed in a furnace to which the oxidant in the gas phase is fed and heated to a temperature or temperature range above the melting point of the parent metal but below the melting point of the oxidation reaction product. In the case of aluminum as the parent metal and air as the oxidant gas, the temperature is generally in the range of 850 to 1450 ° C and preferably between about 900 and 1300 ° C.

Within this operating temperature range, a molten metal bath is formed which, upon contact with the oxidant (s), reacts to the oxidation reaction product. Under continuous action of the oxidizing medium and within a suitable temperature range, the remaining molten metal is gradually drawn into the resulting oxidation reaction product in the direction of the oxidant. In contact with the oxidant, the molten metal forms an additional oxidation reaction product. At least a portion of the oxidation reaction product is maintained in contact with the molten metal and the gaseous oxidant therebetween so that the oxidation reaction product continuously grows and grows through the preform. The oxidation reaction product is constantly growing and formed within the preform and closes its components when it is constantly in contact with the oxidant gas and in the presence of another parent metal. When the gaseous oxidant is air, the oxidant supply is most preferably realized by providing air supply means within the furnace.

The process is continued until the oxidation reaction product encloses the preform components up to, but not preferably beyond, the defined boundary surface, which would mean an overgrowth of the polycrystalline matrix material. Thus, the resulting ceramic composite product comprises a preform infiltrated to the boundary surfaces with a ceramic matrix of polycrystalline material consisting essentially of the oxidation reaction product of the parent metal and the oxidant gas and optionally one or more metal components, for example unoxidized parent metal components or reducible metal components oxidizing agents. Generally, the boundary surfaces of the preform and the polycrystalline matrix coincide, but the individual components on the surface of the preform may be exposed or protrude from the matrix such that the preform is not completely surrounded or closed by the matrix. It should further be noted that the resulting polycrystalline matrix may comprise pores which may partially or almost completely replace the metal phase, but the void volume depends mainly on conditions such as temperature, time, type of parent metal and the concentration of the dopant. Typical of polycrystalline ceramic structures is that the oxidation reaction product crystals are interconnected in more than one dimension, preferably in three dimensions, and the metal phase or pores may be at least partially interconnected. The ceramic composite product of the invention has generally well defined boundaries and has the dimensions and geometric shape of the original preform.

Although the invention has been described with particular reference to aluminum as the parent metal, it is only an example, and it is to be understood that other metals such as silicon, titanium, tin, zirconium and the like that meet or meet the criteria of the invention subsidized to suit them. Materials suitable for making a preform for forming a self-supporting polycrystalline body according to the invention include, depending on the parent metal and oxidation system selected, one or more of alumina, silicon carbide, alumino-silicon oxynitride, zirconia, zirconium boride, titanium nitride, bariumtitanate, boron nitride, silicon nitride, various iron alloys, for example iron-chromium-aluminum alloy, carbon, aluminum, various clays and mixtures thereof. However, any suitable material may be used to form the preform. For example, when the parent metal is aluminum and the desired oxidation reaction product is aluminum nitride,

CS 276180 Β 6 is a suitable material for the production of grain preforms of aluminum nitride and / or alumina. When the zirconium parent metal and the desired oxidation reaction product are zirconium nitride, they form a suitable mixture; for the production of zirconium diboride grain preform. When the parent metal is titanium and the desired oxidation reaction product is titanium nitride, a suitable preform is a body comprising alumina and / or titanium diboride particles. When the parent metal is tin and the desired oxidation reaction product is tin oxide, a suitable preform is an alumina grain body. If the silicon nitride parent metal and the desired oxidation reaction product are silicon nitride, a titanium nitride grain preform is suitable.

The permeable preform of the invention may be formed or manufactured to any predetermined or desired size and shape in any manner, for example, by slurry casting, injection molding, extrusion, vacuum forming or otherwise, i.e. by processing any suitable materials which will be described in further detail. Thus, preforms within the meaning of the description are intended to include all re-molded articles made by any of these methods, not just by molding. The permeable preform may, as already mentioned, comprise a solid oxidant and / or a liquid oxidant which can be used in conjunction with the vapor phase oxidant. The preform should be made with at least one boundary surface and should retain sufficient shape cohesiveness and wet strength in order to form a body with sufficient dimensional fidelity after infiltration with a ceramic matrix. However, the permeable preform must be sufficiently permeable to allow polycrystalline matrices to penetrate it. The preforms of the invention preferably have a porosity of between 5 and 90 4 volumes and preferably between 25 and 50 4 volumes. The porous preform should conveniently be wettable by the molten metal under operating conditions, thereby promoting the formation of a polycrystalline matrix within the preform and the formation of a solid ceramic composite with well defined edges and sides.

The preform of any shape and size has at least one boundary surface that substantially forms the boundary for the ingrown polycrystalline matrix. For example, the preform may have a hemispherical shape and its flat side may contact the surface of the parent metal, while the convex surface forms the boundary surface to which the polycrystalline matrix is ​​to penetrate. Another example is a cuboid preform whose one square wall contacts the surface of the parent metal, while the other five sides form boundary surfaces to which the polycrystalline matrix grows. The matrix of the polycrystalline material formed by the oxidation reaction grows into the permeable preform, infiltrates and encloses its constituents and reaches its boundary surface without substantially breaking or relocating the surface. Thus, in the method of the invention, external forces that could damage or deform the preform are not applied, there is no shrinkage that could cause cracks in the preform and cause loss of fidelity over the original shape, and there is no need for high temperatures, high pressure and special equipment to produce a composite ceramic product. In addition, the invention bypasses otherwise necessary requirements for chemical and physical compatibility of materials, which is a prerequisite for non-pressurized sintering of granular materials.

The permeable preform of the invention may be comprised of any material such as ceramic and / or metal particles, powders, fibers, wires, wires, grains, hollow bodies or spheres, wire or refractory fabrics, solid spheres, and combinations thereof. The preform materials typically form a bonded assembly having gaps, apertures, intermediate spaces, and the like to pass the oxidant and allow infiltration of the growing oxidation reaction product without altering its shape. The preform may comprise a lattice of reinforcing bars, rods, tubes, tubes, plates, wires, spheres or other materials, wire fabrics, ceramic refractory fabrics or the like, or a combination of any of these, arranged in a desired shape. The preform materials may be homogeneous or heterogeneous. The preform materials, for example ceramic powders or grains, may be bonded together by any suitable binder, for example polyvinyl alcohol and the like, which neither obstruct the reaction of the invention nor leave undesirable residual by-products within the ceramic composite. Suitable grains, for example silicon carbide or alumina, have a grain size or mesh size of 10 to 1000 or finer, and mixtures of these grain sizes and particle types can also be used. The grains may be brought to a suitable shape such as 7

CS 276100 Β 6 by conventional techniques, for example by forming a slurry of grains in an organic binder, pouring the slurry into a mold and solidifying it, for example by drying at an elevated temperature.

With respect to suitable materials for making the permeable preform of the invention, there are essentially three suitable types.

The first type of preform materials includes those chemical individuals that do not evaporate at the temperatures and oxidation conditions of the invention, are thermodynamically stable, and do not react with or excessively dissolve in the molten metal. A variety of materials are known to those skilled in the art to meet these conditions when the parent metal is aluminum and the oxidant is air or oxygen. These materials include single metal oxides such as aluminum oxide Al2 O3, sulfur dioxide Cr0₂, Hfdium oxide HfD₂, lanthanum oxide la₂0j, neodymium oxide NdjOj, various praseodymium oxides, Samarium oxide SnijO ^, scandium oxide Sc₂0₃, ThO₂, uranium dioxide U0₂, Yttrium oxide Y₂0₃^{and ox}zirconium ZrO₂. In addition, a large number of binary, ternary and higher metal compounds, for example magnesium aluminate MgO.Al2 O3, belong to this class.

A second type of suitable materials are those which are not themselves stable in an oxidizing hot environment, but which, due to the relatively low kinetics of the decomposition reactions, may function as a preform phase when they infiltrate the polycrystalline ceramic matrix. silicon. This substance would completely oxidize under conditions necessary for oxidation of aluminum by oxygen or air, in the absence of a protective layer of silica that forms and covers the silicon carbide particles and thus limits their further oxidation.

A third class of materials suitable for producing a preform are those materials which, for thermodynamic or kinetic reasons, are not expected to survive the action of oxidizing media and molten metal. Such a preform can be made compatible with the process of the invention, either by reducing the activity of the environment, e.g.₂/ H₂O or C0 / C0₂ as an oxidizing gas, or by applying a coating thereto, for example alumina, which deactivates the material under operating conditions. An example of such a material is the carbon fibers used in aluminum as the parent metal. When aluminum is to be oxidized by air or oxygen at a temperature of, for example, 1250 ° C on a matrix containing a carbon fiber preform, carbon tends to react with both aluminum to aluminum carbide and the oxidizing environment to carbon monoxide or carbon dioxide. These undesirable reactions can be avoided by coating carbon fibers, such as alumina, to prevent the reaction of carbon with both the parent metal and the oxidant, and optionally using a mixture of carbon dioxide and carbon monoxide as the oxidant that oxidizes aluminum but not aluminum fibers.

The poodle of the invention can be used in the form of a single body or as an assembly of several bodies to form more complex shapes. It has been found that the polycrystalline matrix can grow through adjacent, interlocking walls of such a preform assembly and thus join these bodies at their interface to form a single, integral ceramic composite.

The array of bodies forming the preforms is positioned so that the growth of the oxidation reaction product is directed to the interior of the assembly such that the reaction product infiltrates and encloses the entire assembly within the boundary surfaces forming the walls of the individual preforms. In this way, complex ceramic composite shapes can be produced as a unitary body, which is impossible in conventional ceramic manufacturing technology. It is noted that wherever the term "preform" is used in the text, it means a single preform or set of preforms, unless otherwise indicated.

As an example of such a preform assembly, FIG. 1 shows a vertical cross-section of a crucible 10 comprising an inert base metal bearing 12 on which a system consisting of a recess 16 with a recess 11 and a preform 20 having an upper boundary surface 21 and The preform 20 is laid on the preform 16 lacquer that the edges of the two winches III, 22

CS 276180 Β 6 are superimposed so that the two recesses 18, 22 complement each other and form a rectangular cavity 24. The surfaces of the two recesses 18, 22 may be provided with a barrier, for example, plaster, as described in U.S. Pat. No. 4,923,832. Such a barrier prevents the oxidation reaction product from overgrowing the surfaces of the cavity 24 into its interior. The polycrystalline matrix grows through both preforms 16, 20 up to the upper boundary surface 21 of the upper preform 20, so that it joins the two preforms 16, 20 into a single body to form a ceramic composite having a rectangular cavity 24.

In order for the ceramic composite body to have an exact leg of at least an almost exact shape that retains the original shape and dimensions of the preform, the growth of the ceramic matrix should extend to at least one defined boundary surface of the preform. Limiting surface growth can be prevented, inhibited or controlled by a combination of the following measures:

- creating such conditions within the preform, for example by introducing a solid oxidant, to preferentially occur internal growth of the matrix in addition to the growth of the preform, the liquid being bounded by a substantially accurate, predetermined amount of parent metal so as to be fully consumed or converted a polycrystalline structure so that the oxidation reaction product lies at the perimeter of the permeable preform,

- by regulating or limiting the amount of oxidant initially available,

forming a barrier on the area or areas of the preform, and

- stopping the process at a suitable time, for example by draining or removing the oxidizing atmosphere or by changing the temperature reaction from the operating temperature range, for example by lowering it below the melting point of the parent metal. As a rule, the temperature is lowered by lowering the temperature in the furnace, and then the material is removed from the furnace. This last measure requires a lot of attention so that the matrix does not overlap any boundary area.

The ceramic composite produced by the process of the invention is a dense cohesive product wherein between 5 and 90% by volume of the total volume of the composite is formed by one or more preform materials enclosed in a polycrystalline ceramic matrix. When the parent metal is aluminum and the oxidant is air or oxygen, the polycrystalline matrix generally consists of about 60% to 99% by weight, based on the weight of the polycrystalline matrix, of the interconnected pC-alumina and about 1 to 40% by weight. non-oxidized metal components.

The dopants in conjunction with the parent metal favorably influence the oxidation reaction. The function of the impurities also depends on a number of factors other than the impurity material itself. These factors include, for example, the parent metal selected, the desired end product, a particular combination of impurities, if more than one is used, the use of an external impurity in combination with the alloying impurity, the concentration of impurities, the oxidizing environment and working conditions.

The additive (s) in association with the parent metal may be alloying constituents of the parent metal, may be applied to at least a portion of the parent metal surface, or may be applied to at least a portion of the parent metal surface, or may be deposited or introduced into the preform or portion of the preform, or a combination of these measures. For example, alloying dopants may be used in combination with an externally applied dopant. When applied to or introduced into the preform, it can be applied in any conventional manner, for example by dispersing the admixtures in all or part of the preform, as a coating or in the form of grains, preferably in the nobo preform on the preform. with base metal. Application of the admixture to the preform or preform can also be accomplished by applying a layer of one or more admixtures to the preform or preform, including its internal openings, gaps, channels, intermediate spaces, and the like, to ensure its permeability. A suitable method of applying the dopant is to soak the entire bed of filler in the dopant solution or precursor thereof. The dopant source may also be placed in the form of a solid body in contact with at least a portion of the parent metal and the preform. For example, a thin sheet of silicon glass may be placed on the base metal surface, which is useful as an additive for oxidizing the aluminum base metal. When aluminum as the parent metal, possibly alloyed internally with magnesium and covered with a silicon-containing material, it melts

In an oxidizing environment, for example in air at a temperature of about 850 ° C to 1450 ° C, preferably between 900 ° C and 1350 ° C, the polycrystalline ceramic matrix grows into a permeable article. When the additive is applied externally to at least a portion of the surface of the parent metal, the oxide structure generally grows considerably beyond the additive layer, i.e. over the depth of the additive layer, in the preform. In all cases, one or more impurities may be applied externally to the surface of the parent metal and / or permeable preform. In addition, the dopants constituting the alloying constituents of the parent metal and / or deposited externally on the parent metal may be supplemented and thus increased by the amount of dopants or impurities deposited on the preform. Thus, a too low concentration of dopants alloyed with the parent metal and / or deposited on the parent metal from outside can be increased by the appropriate concentration of dopants introduced into or applied to the preform and vice versa.

Suitable dopants for the aluminum parent metal, especially in conjunction with air as the oxidant, include magnesium metal and zinc metal, in combination with each other or in combination with other dopants. These metals or their suitable sources can be alloyed with an aluminum-based parent metal in a concentration for each of them in the range of 0.1 to 10% by weight, based on the total weight of the doped metal. Concentrations within this range appear to initiate the growth of the ceramic, promote metal movement and favorably influence the growth morphology of the oxidation reaction product formed.

Other dopants that effectively promote the growth of the oxidation reaction product in an aluminum-based parent metal system include, for example, silicon, germanium, tin and lead, particularly in conjunction with magnesium or zinc. One or more of these admixtures or a source thereof is alloying the aluminum parent metal at a concentration for each of them in the range of about 0.5 to 15% by weight. total alloys. Better growth kinetics and improved morphology of the resulting product can be achieved at admixture concentrations in the range of about 1 to 10% by weight based on the weight of the total doped base alloy. Lead as an admixture is alloyed with an aluminum-based parent metal at a temperature of at least 1000 ° C because lead is poorly soluble in aluminum. However, the addition of other alloying agents, such as tin, improves the solubility of the lead and thus allows it to be added at lower temperatures.

Depending on the circumstances, one or more impurities may be used, as mentioned above. For example, in the case of aluminum as the parent metal and air as the oxidizing agent, combinations of magnesium and silicon and zinc and silicon are particularly suitable admixture combinations. In these cases, the preferred magnesium concentration is in the range of about 0.1 to 3 wt%, the zinc concentration in the range of about 1 to 6 wt%. and a silicon concentration in the range of about 1 to 10 wt.

Other examples of dopants useful in treating aluminum parent metal with an air oxidant include sodium, lithium, calcium, boron, phosphorus, and yttrium, which may be used singly or in combination with one or more dopants, depending on the oxidant and operating conditions. Sodium and lithium may be used in very small amounts of the order of ppm, typically about 100 to 200 ppm, each of which may be used singly or in combination with each other or in combination with other additives. Rare earth elements such as cerium, lanthanum, praseodymium, neodymium, and samarium are also suitable admixtures, especially in combination with other admixtures.

As mentioned, it is not necessary to alloy the parent metal with dopants. Thus, for example, selective application of one or more thin film admixtures to all or part of the parent metal surface allows or improves local growth of the ceramic from the parent metal surface or portions thereof, making it suitable for local growth of the polycrystalline matrix into a permeable preform. Thus, the growth of the polycrystalline ceramic matrix can be favorably influenced by the localized deposition of the dopant material on the parent metal surface. The deposited coating or dopant layer is thin compared to the thickness of the parent metal body and the growth or formation of the oxidation reaction product into the permeable preform extends far beyond the dopant layer, i.e. the depth of the dopant layer applied. The admixture layer can be applied by painting, dipping, screen printing, steaming, or otherwise applying the admixture in the form of a liquid

Or by pasting or simply depositing a layer of solid grain of the dopant or a thin sheet or film of dopant on the surface of the parent metal. The dopant material may, but need not, include an organic or inorganic binder, carrier, solvent and / or thickener. Most preferably, the dopant product materials are applied in the form of powders to the surface of the parent metal or dispersed in at least a portion of the filler. One particularly advantageous method is to spray the admixture of the admixture in a mixture of water and an organic binder onto the surface of the parent metal, thereby forming an adhesive coating which facilitates the application of the admixture. handling of the doped base metal before processing.

The externally applied dopants are usually deposited on a portion of the parent metal surface as a uniform coating. The amount of dopant is effective over a wide range with respect to the amount of parent metal to which the dopant is applied, and in the case of aluminum parent metal, experiments have shown neither upper nor lower limits. For example, when an aluminum-based base metal silicon dioxide is used as an external dopant of silicon and air or oxygen as the oxidant, as little as 0.00003 g of silicon and 1 g of parent metal is sufficient to induce the growth of the polycrystalline ceramic matrix. 0.0001 g of silicon per cm of parent metal surface, together with a second dopant which forms a source of magnesium and / or zinc. It has also been found that a ceramic structure can be induced by using magnesium oxide as an additive for an aluminum-based parent metal with air or oxygen as the oxidant in an amount of greater than 0.0008 g of admixture per g of parent metal and greater than 0.003 g of dopant per cm of parent metal to which magnesium oxide is applied. To some extent, increasing the amount of material appears to reduce the reaction time required to form the ceramic composite, but this also depends on the type of dopant, parent metal, and reaction conditions.

When aluminum as the parent metal is internally doped with magnesium and the oxidizing medium is air or oxygen, it has been observed that magnesium is at least partially oxidized from alloys at temperatures from 820 to 950 ° C. In such cases of magnesium-doped systems, magnesium forms magnesium oxide and / or magnesium aluminate on the surface of the molten base alloy and, during the growth of the ceramic matrix, retain the magnesium compounds at the initiating, i.e., original, oxide surface of the parent metal alloy in the growing ceramic structure. Thus, in such a magnesium-doped system, an alumina-based structure and a relatively thin layer of magnesium aluminate are formed on the initiation surface. Where desired, this initiation surface can be removed, for example by abrasion, machining, polishing, or grain blasting.

The invention will be explained by means of several examples, which are intended to illustrate and not limit it.

Example 1

Referring to Figures 2 to 5, where like reference numerals designate like components, a ceramic sprocket 38 has been made from a preform 30, the shape of which is shown in Figures 2a, 2b. The preform 30 had an outside diameter of 76.2 mm and a thickness of 4.76 mm and had a central opening 31. The preform 30 was manufactured in conventional manner from silicon carbide grains. A well mixed mixture containing 80 wt. % of silicon carbide grains, which was a mixture consisting of 70 wt. 500 and 30 wt. 220, and 20 wt. The organic binder solution, which consisted of a 4: 1 solution of glue in water, was poured into a silicone rubber mold and allowed to solidify. The solid preform 30 was then removed from the mold.

A cylindrical plate 32 having a diameter of 76.2 mm of aluminum alloy 380.1 having a nominal weight composition of 8 to 8.5% Si, 2 to 3% Zn and 0.1% Mg as active ingredients was placed on the surface 33 of the preform 30. and 3.5% Cu, and further contained Fe, Mn and Ni, the magnesium content being slightly higher, about 0.17 to 0.18%. This alloy was alloyed with lead in an amount of 6% by weight. The same alloy ingot 34 was laid on the plate surface 28

The system consisting of the cylindrical plate 32 and the ingot 34 had a weight of 100 g. This system, consisting of the preform 30, the plate 32 and the ingot 34, assembled according to FIG. coated on

All the exposed areas of the calcined gypsum slurry containing about 35 wt. calcium carbonate. This coating served to prevent the ceramic matrix from penetrating through the boundaries of the preform. The gypsum coating 35 was allowed to solidify, and the entire coated assembly was then completely embedded in the interior of a bed of 90 alumina grains. This bed was in a refractory crucible 37 as shown in FIG. 4.

The assembly shown in Figure 4 was heated in air from an initial temperature of 200 ° C at a rate of 250 ° C / h to a final temperature of 1000 ° C for which it was maintained for 66 h in air.

The furnace was then cooled at the same rate and the sample was removed therefrom at a temperature of about 600 ° C. The described process resulted in a ceramic composite consisting of a matrix of alumina as shown by X-ray analysis which completely enclosed the silicon carbide grains of the preform 30 to the gypsum-covered boundary surfaces. The excess aluminum that adhered to the surface 33 of the toothed disk and the dehydrated gypsum layer were mechanically removed from the composite. The resulting ceramic sprocket 38 accurately reproduced the preform 30, as shown in FIG. 5, and had an average Rockwell hardness of 79.8.

Example 2

Two preforms having a size of 14.5 cm and a thickness of 6.35 to 9.52 mm and containing 95% by weight were prepared. % alumina particles of 90 mesh and 5 wt. silica. Both preforms were formed by first mixing the alumina with an organic binder, then compressing the mixture to a pressure of 55.3 MPa, and finally pre-firing at 13 to 75 ° C for 24 h. Each of the preforms was placed on the surface of a 24 mesh alumina bed placed in a refractory vessel. Two aluminum blocks of different composition were used as the base metal, each block having a size of 12.9 cm and a thickness of 12.7 mm. Each of the two blocks was placed on one preform. The first block was of pure aluminum 99% and the second one was of alloy 380.1 having the composition of Example 1 but without an additional 6% lead.

The two systems were heated to 900 ° C in air for 36 hours, the time required to completely infiltrate the preform with a cO-alumina ceramic matrix up to the opposite boundary surface. The formation of the? 6-alumina ceramic matrix was confirmed by X-ray. Giant. 6a, 6b show sections of the resulting ceramic products. Examination of a body 45 made of 99% pure aluminum and a body 47 made of alloy 380.1 revealed that the ceramic matrix of aluminum (III) oxide had completely penetrated into the interior of the preform. The overgrowth of the ceramic matrix over the boundaries of the preform was limited to that. the area of ​​the preform facing the alumina bed was different in both systems. The sample, which was made of pure aluminum with a purity of 99%, showed only a slight overgrowth of the boundary surface with a ceramic matrix, the overgrown material being easily removed by light turning or abrading. Giant. 6a shows a very thin overgrowth layer of material 46 on this body 5, since the ceramic matrix formed by oxidation of alloy 380.1 apparently required less time to penetrate the preform, and the reaction time for both samples lasted equally long, the ceramic body 47 had substantially more overgrown material 46 Thus, fidelity can be nitralized by controlling and monitoring the reaction, in order to prevent the ceramic matrix from overgrowing beyond the boundary surface of the preform.

Example 3

Referring to Figs. 7a, 7b, 7c, a preform 60 having a trapezoidal side view, a thickness of 44.45 mm, a smaller rectangular area of ​​214.31 x 63.5 mm, and a larger rectangular area 52 of 219.07 x was manufactured. 69.85 mm. The preform 50 was made from a blend containing 32 wt. 1000 mesh, 35 wt. 500 mesh silica, 0.5 wt. % of silicon, 0.5% by weight of sodium silicate contained in the water used to form a paste from the mixture, and 32% by weight of silicon; Binders with a grain size of 100 mesh and less. The mixture was stirred in water containing the abovementioned amount of dissolved sodium silicate and poured into the molds of the dimensions described. The mixture was allowed to air dry and was removed from the mold as a solid trapezoid body in the mold

The word Lanxide was engraved on the rectangular surface 52 of the preform 50, as shown in Figure 7b, and the preform was fired in an air atmosphere at 1000 ° C for 1 hour.

Both commercial alloy bars 50, 52; having a nominal weight composition of 2.5% mg and approximately 1% of other elements such as Fe, Cr, Si, Cu, and one 99% pure aluminum bar were stacked so that the pure aluminum bar lay between the bars All rods were 215.9 mm long, 63.5 mm wide and 12.7 mm thick. The assembly was then laid on the surface of a 24 mesh silicon carbide grain layer placed in a refractory vessel. The trapezoidal preform was placed on top of the aluminum rods such that the smaller rectangular surface 51 completely touched the rectangular area of ​​215.9 x 63.5 mm aluminum rods so that the entire weight of the preform rested on the metal bars. The refractory vessel was then filled with 14 mesh silicon carbide grains that completely covered the aluminum rods, but left five preform surfaces that did not touch the metal aluminum free and thus exposed to the atmosphere.

The assembly was placed in an oven having airflow openings and heated to a reaction temperature of 1000 ° C for 5 h. The furnace temperature was maintained at this temperature for 144 h, then the furnace was cooled to ambient temperature and reheated to 1000 ° C for an additional 6 h to completely infiltrate the ceramic matrix by the preform.

The molten aluminum has reacted with oxidizing agents, i.e., gaseous oxidant and gaseous oxidants such as silica, to form a ceramic matrix of alumina, which has infiltrated the preform and thus enclosed the preform mixture particles. The formation of the ceramic matrix continued to the boundary surfaces of the preform and the matrix was substantially contained within these boundary surfaces. The composite product 53 was very true to the shape of the preform, as shown by the clear imprint of FIG. 7c, and the ceramic matrix grew only slightly.

This example shows an embodiment of the invention wherein the preform resistor composition is oxidation of the molten parent metal preferably within the boundary surfaces of the preform. Such preferred oxidation contributes to controlling the overgrowth of the boundaries of the preform by the ceramic matrix. Thus, the composite product 53 is a shaped ceramic article that retains the shape of a trapezoidal preform 50.