WO1999032679A9

WO1999032679A9 - Preparation of a metal matrix composite body by a spontaneous infiltration process

Info

Publication number: WO1999032679A9
Application number: PCT/US1998/026898
Authority: WO
Inventors: Allyn L Mccormick; Michael Aghajanian; John Burke
Original assignee: Lanxide Technology Co Ltd; Allyn L Mccormick; Michael Aghajanian; John Burke
Priority date: 1997-12-19
Filing date: 1998-12-18
Publication date: 1999-10-21
Also published as: WO1999032679A3; WO1999032679A2

Abstract

In the present improved process for producing a metal matrix composite body by spontaneous infiltration, a permeable mass or preform to be infiltrated is exposed to a source of infiltration enhance or infiltration enhance precursor which has been heated to a temperature beyond the expected infiltration temperature. Specifically, the infiltration enhance or its precursor is heated to a temperature at which its vapor pressure is at least 100 millitorr or at least 100 °C greater than the temperature at which subsequent infiltration will occur. This temporary temperature excursion permits the concentration of infiltration enhance or its precursor to more rapidly increase within the permeable mass or preform, thereby more quickly permitting spontaneous infiltration to initiate and proceed.

Description

PREPARATION OF A METAL MATRIX COMPOSITE BODY BY A SPONTANEOUS INFILTRATION PROCESS

TECHNICAL FIELD

The present invention relates to techniques for producing metal matrix composite bodies. In particular, the present invention relates to a technique for more efficiently and reliably producing a metal matrix composite article by a spontaneous infiltration process.

BACKGROUND ART

Composite products comprising a metal matrix and a strengthening or reinforcing phase such as ceramic particulates, whiskers, fibers or the like, show great promise for a variety of applications because they combine some of the stiffness and wear resistance of the reinforcing phase with the ductility and toughness of the metal matrix. Generally, a metal matrix composite will show an improvement in such properties as strength, stiffness, contact wear resistance, and elevated temperature strength retention relative to the matrix metal in monolithic form, but the degree to which any given property may be improved depends largely on the specific constituents, their volume or weight fraction, and how they are processed in forming the composite. In some instances, the composite also may be lighter in weight than the matrix metal per se. Aluminum matrix composites reinforced with ceramics such as silicon carbide in particulate, platelet, or whisker form, for example, are of interest because of their higher stiffness, wear resistance and high temperature strength relative to aluminum.

Although various powder metallurgical techniques have been developed over the years to produce metal matrix composite bodies, the infiltration or casting approaches seem to be preferred by present manufacturers. Particularly preferred is infiltration of a mass of ceramic material by a molten matrix metal which does not require an assist by pressure or vacuum (whether externally applied or internally created). Such infiltration has been termed "spontaneous infiltration."

A preferred spontaneous infiltration system is the aluminum/magnesium/nitrogen system, in which aluminum is the matrix metal, magnesium is the infiltration enhancer precursor and nitrogen is the infiltrating atmosphere. When the magnesium infiltration enhancer precursor is provided to the system solely in the form of an alloying element in the matrix metal, the infiltration process may be slow. Also, if infiltration has to proceed a substantial distance, there is the danger that magnesium may have escaped from the system through volatilization to such a degree that the continued infiltration slows down or even terminates.

One solution to this problem is to provide to the mass of filler material magnesium metal in particulate form. This may be accomplished by, for example, admixing magnesium with the filler material making up the permeable mass or preform. By so doing, it is possible to distribute the infiltration enhancer precursor and infiltration enhancer throughout the permeable mass more quickly. The downside to this approach, however, is that preform processing is restricted somewhat, e.g., no aqueous process or high temperature firing in air. Also, adding the infiltration enhancer precursor to the permeable mass or preform may reduce the available space for filler material.

Although the infiltration enhancer precursor typically is in the liquid state at typical infiltration temperatures, this species also features a significant vapor pressure. This vapor phase is thought to play an important role in seeing to it that infiltration enhancer material ultimately is located on or within the permeable mass. Accordingly, it has been suggested that by increasing the infiltration temperature, the increased vapor pressure of infiltration enhancer precursor material means that the quantity of infiltrating enhancer on or within the permeable mass necessary to cause infiltration of molten matrix metal will be achieved that much faster. At higher infiltration temperatures, however, greater amounts of aluminum nitride are formed within the matrix of the resulting metal matrix composite body. Furthermore, the propensity for matrix metal to "overinfiltrate", leaving an aluminum nitride containing skin on the bulk surfaces of the MMC body, is greater at higher infiltration temperatures. Accordingly, there is a need to provide infiltration enhancer to the permeable mass or preform in an efficient manner, preferably from a vapor phase precursor, without having to increase the infiltration temperature.

DESCRIPTION OF COMMONLY OWNED U.S. PATENTS

Commonly owned U.S. Patent No. 4,828,08 to White et al. teaches a technique for producing an aluminum metal matrix composite body by a spontaneous infiltration process. According to the White et al invention, a permeable mass of ceramic filler material may be infiltrated by a molten aluminum alloy containing at least 1 weight % magnesium in the presence of a gas comprising from about 10 to 100 volume % nitrogen without the requirement for pressure or vacuum whether externally applied or internally created. In one embodiment of the White et al. invention, the formed metal matrix composite body is provided with an aluminum nitride skin or surface. Specifically, if the supply of molten aluminum alloy matrix metal becomes exhausted before complete infiltration of the permeable ceramic filler material, an aluminum nitride layer or zone may form on or along the outer surface of the metal matrix composite. Also, an aluminum nitride skin can be formed at the exterior surface of the permeable mass of ceramic filler material by prolonging the process conditions. In particular, once infiltration of the permeable ceramic material is substantially complete, if the infiltrated ceramic material is further exposed to the nitrogenous atmosphere at substantially the same temperature at which infiltration occurred, the molten aluminum at the exposed surface will nitride. The degree of nitridation can be controlled and may be formed as either a continuous phase or discontinuous phase in the skin layer.

Commonly owned U.S. Patent No. 5,298,339, teaches the production of metal matrix composite bodies using a reservoir feed method. This patent also discloses that admixing at least some of the magnesium in with the mass of filler material might reduce the total amount of magnesium required to achieve infiltration, or might reduce the temperature required for infiltration.

Commonly owned U.S. Patent No. 5,172,747 to Aghajanian et al. teaches a method for producing metal matrix composite bodies by an infiltration process whereby an interface between matrix metal and the permeable mass of filler material or preform to be infiltrated has included in at least a portion thereof an infiltration enhancer and/or an infiltration enhancer precursor. According to Aghajanian et al. the presence of magnesium, for example, on a surface of both the filler material or preform and on a surface of the matrix metal may result in a reduction in the required amount of magnesium to achieve spontaneous infiltration.

DISCLOSURE OF THE INVENTION

The present invention is directed to improvements in fabricating metal matrix composite bodies by an infiltration route. Specifically, the present invention provides a technique for increasing the quantity of infiltration enhancer or infiltration enhancer precursor material supplied to the permeable mass or preform by vapor-phase transport.

Specifically, a permeable mass or preform to be infiltrated by a molten matrix metal is exposed to a source of infiltration enhancer or infiltration enhancer precursor which has been heated to a temperature above the infiltration temperature. More specifically, the infiltration enhancer or its precursor is heated to a temperature at least about 100°C greater than the temperature at which subsequent infiltration will occur. In the alternative, the infiltration enhancer or its precursor is heated to a temperature at which its vapor pressure is at least about 100 millitorr. This temperature excursion permits the concentration of infiltration enhancer or its precursor to more rapidly increase within the permeable mass or preform, thereby permitting spontaneous infiltration to initiate and proceed sooner than it might otherwise. Before an appreciable amount, if any, of infiltration has occurred, the temperature is reduced to a temperature better suited for matrix metal infiltration. In a nitrogenous atmosphere, for example, an infiltrating matrix metal tends to produce greater amounts of metal nitride phase as a byproduct, as infiltration temperature is increased.

The infiltration enhancer precursor may be provided from one or more sources. In particular, the infiltration enhancer precursor may be provided within the permeable mass (e.g., magnesium particulate), alloyed within the matrix metal or provided from an external source (e.g., a separate body of magnesium metal).

Thus, the present invention recognizes and appreciates that the spontaneous infiltration process may be improved by providing different temperatures at different critical states of the process. In particular, an infiltration enhancer may be formed in-situ on or within a permeable mass at a high temperature, and that the infiltration of molten matrix metal and the coated mass may occur optimally at a somewhat reduced temperature. In view of this appreciation that the ideal temperatures for forming infiltration enhancer and for infiltrating the molten matrix metal may not coincide, it may be possible to provide the infiltration enhancer not as an in-situ formed material but instead through volatilization and vapor-phase transport of a separately provided source of the infiltration enhancer material.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is a cross-sectional schematic view of a setup used in accordance with Example 1 ; Figure 2 is a cross-sectional schematic view of a setup used to produce the metal matrix composite body of Example 3; and

Figure 3 is a cross-sectional schematic view of a setup used to produce the metal matrix composite brake rotor of Example 4.

DEFINITIONS

"Aluminum", as used herein, means and includes essentially pure metal (e.g., a relatively pure, commercially available unalloyed aluminum) or other grades of metal and metal alloys such as the commercially available metals having impurities and/or alloying constituents such as iron, silicon, copper, magnesium, manganese, chromium, zinc, etc., therein. An aluminum alloy for purposes of this definition is an alloy or intermetallic compound in which aluminum is the major constituent.

"Balance Non-Oxidizing Gas", as used herein, means that any gas present in addition to the primary gas comprising the infiltrating atmosphere, is either an inert gas or a reducing gas which is substantially non-reactive with the matrix metal under the process conditions. Any oxidizing gas which may be present as an impurity in the gas(es) used should be insufficient to oxidize the matrix metal to any substantial extent under the process conditions. "Barrier" or "Barrier Means", as used herein, means any suitable means which interferes, inhibits, prevents or terminates the migration, movement, or the like, of molten matrix metal beyond a surface boundary of a permeable mass of filler material or preform, where such surface boundary is defined by said barrier means. Suitable barrier means may be any such material, compound, element, composition, or the like, which, under the process conditions, maintains some integrity and is not substantially volatile (i.e., the barrier material does not volatilize to such an extent that it is rendered non-functional as a barrier).

Further, suitable "barrier means" includes materials which are substantially non- wettable by the migrating molten matrix metal under the process conditions employed. A barrier of this type appears to exhibit substantially little or no affinity for the molten matrix metal, and movement beyond the defined surface boundary of the mass of filler material or preform is prevented or inhibited by the barrier means. The barrier reduces any final machining or grinding that may be required and defines at least a portion of the surface of the resulting metal matrix composite product. The barrier may in certain cases be permeable or porous, or rendered permeable by, for example, drilling holes or puncturing the barrier, to permit gas to contact the molten matrix metal.

"Carcass" or "Carcass of Matrix Metal", as used herein, refers to any of the original body of matrix metal remaining which has not been consumed during formation of the metal matrix composite body, and typically, if allowed to cool, remains in at least partial contact with the metal matrix composite body which has been formed. It should be understood that the carcass may also include a second or foreign metal therein.

"Filler", as used herein, is intended to include either single constituents or mixtures of constituents which are substantially non-reactive with and/or of limited solubility in the matrix metal and may be single or multi-phase. Fillers may be provided in a wide variety of forms, such as powders, flakes, platelets, microspheres, whiskers, bubbles, etc., and may be either dense or porous. "Filler" may also include ceramic fillers, such as alumina or silicon carbide as fibers, chopped fibers, particulates, whiskers, bubbles, spheres, fiber mats, or the like, and ceramic-coated fillers such as carbon fibers coated with alumina or silicon carbide to protect the carbon from attack, for example, by a molten aluminum parent metal. Fillers may also include metals.

"Infiltrating Atmosphere", as used herein, means that atmosphere which is present which interacts with the matrix metal and/or preform (or filler material) and/or infiltration enhancer precursor and/or infiltration enhancer and permits or enhances spontaneous infiltration of the matrix metal to occur.

"Infiltration Enhancer", as used herein, means a material which promotes or assists in the spontaneous infiltration of a matrix metal into a filler material or preform. An infiltration enhancer may be formed from, for example, a reaction of an infiltration enhancer precursor with an infiltrating atmosphere to form (1) a gaseous species and/or (2) a reaction product of the infiltration enhancer precursor and the infiltrating atmosphere and/or (3) a reaction product of the infiltration enhancer precursor and the filler material or preform. Moreover, the infiltration enhancer may be supplied directly to at least one of the preform, and/or matrix metal, and/or infiltrating atmosphere and function in a substantially similar manner to an infiltration enhancer which has formed as a reaction between an infiltration enhancer precursor and another species. Ultimately, at least during the spontaneous infiltration, the infiltration enhancer should be located in at least a portion of the filler material or preform to achieve spontaneous infiltration.

"Infiltration Enhancer Precursor" or "Precursor to the Infiltration Enhancer", as used herein, means a material which when used in combination with the matrix metal, preform and/or infiltrating atmosphere forms an infiltration enhancer which induces or assists the matrix metal to spontaneously infiltrate the filler material or preform. Without wishing to be bound by any particular theory or explanation, it appears as though it may be necessary for the precursor to the infiltration enhancer to be capable of being positioned, located or transportable to a location which permits the infiltration enhancer precursor to interact with the infiltrating atmosphere and/or the preform or filler material and/or metal. For example, in some matrix metal/infiltration enhancer precursor/infiltrating atmosphere systems, it is desirable for the infiltration enhancer precursor to volatilize at, near, or in some cases, even somewhat above the temperature at which the matrix metal becomes molten. Such volatilization may lead to: (1) a reaction of the infiltration enhancer precursor with the infiltrating atmosphere to form a gaseous species which enhances wetting of the filler material or preform by the matrix metal; and/or (2) a reaction of the infiltration enhancer precursor with the infiltrating atmosphere to form a solid, liquid or gaseous infiltration enhancer in at least a portion of the filler material or preform which enhances wetting; and/or (3) a reaction of the infiltration enhancer precursor within the filler material or preform which forms a solid, liquid or gaseous infiltration enhancer in at least a portion of the filler material or preform which enhances wetting. "Infiltration Pathway", as used herein, means the route that is taken by molten matrix metal in moving from a body or source of said matrix metal through a permeable or porous layer to the object preform to be infiltrated.

"Matrix Metal" or "Matrix Metal Alloy", as used herein, means that metal which is utilized to form a metal matrix composite (e.g., before infiltration) and/or that metal which is intermingled with a filler material to form a metal matrix composite body (e.g., after infiltration). When a specified metal is mentioned as the matrix metal, it should be understood that such matrix metal includes that metal as an essentially pure metal, a commercially available metal having impurities and/or alloying constituents therein, an intermetallic compound or an alloy in which that metal is the major or predominant constituent.

"Matrix Metal/Infiltration Enhancer Precursor/Infiltrating Atmosphere Svstem" or "Spontaneous Svstem", as used herein, refers to that combination of materials which exhibit spontaneous infiltration into a preform or filler material. It should be understood that whenever a "/" appears between an exemplary matrix metal, infiltration enhancer precursor and infiltrating atmosphere that the "/" is used to designate a system or combination of materials which, when combined in a particular manner, exhibits spontaneous infiltration into a preform or filler material.

"Metal Matrix Composite" or "MMC", as used herein, means a material comprising a two- or three-dimensionally interconnected alloy or matrix metal which has embedded a preform or filler material. The matrix metal may include various alloying elements to provide specifically desired mechanical and physical properties in the resulting composite. A Metal "Different" from the Matrix Metal means a metal which does not contain, as a primary constituent, the same metal as the matrix metal (e.g., if the primary constituent of the matrix metal is aluminum, the "different" metal could have a primary constituent of, for example, nickel).

"Object or Objective Preform", as used herein, refers to the preform the infiltration of which by matrix metal yields a metal matrix composite body intended as an article of commerce.

"Preform" or "Permeable Preform", as used herein, means a porous mass of filler or filler material which is manufactured with at least one surface boundary which essentially defines a boundary for infiltrating matrix metal, such mass retaining sufficient shape integrity and green strength to provide dimensional fidelity prior to being infiltrated by the matrix metal. The mass should be sufficiently porous to accommodate spontaneous infiltration of the matrix metal thereinto. A preform typically comprises a bonded array or arrangement of filler, either homogeneous or heterogeneous, and may be comprised of any suitable material (e.g., ceramic and/or metal particulates, powders, fibers, whiskers, etc., and any combination thereof). A preform may exist either singularly or as an assemblage.

"Reservoir", as used herein, means a separate body of matrix metal positioned relative to a mass of filler or a preform so that, when the metal is molten, it may flow to replenish, or in some cases to initially provide and subsequently replenish, that portion, segment or source of matrix metal which is in contact with the filler or preform. "Sacrificial Bed" or "Sacrificial Preform", as used herein, means a permeable mass of filler material which is to be infiltrated by molten matrix metal to form a metal matrix composite body whose role is to assist in the formation of a different metal matrix composite body.

"Sacrificial MMC", as used herein, means the metal matrix composite body produced as a result of the infiltration of a sacrificial bed or preform by a molten matrix metal.

"Spontaneous Infiltration", as used herein, means the infiltration of matrix metal into the permeable mass of filler or preform occurs without requirement for the application of pressure or vacuum (whether externally applied or internally created).

"Stress Control Layer", as used herein, means a sacrificial bed or preform whose function includes reducing the extent of thermal expansion coefficient mismatch between the body of matrix metal and the object preform.

MODES FOR CARRYING OUT THE INVENTION

The present invention relates to forming a metal matrix composite by spontaneously infiltrating a filler material or preform with molten matrix metal. Particularly, an infiltration enhancer and/or an infiltration enhancer precursor and/or an infiltrating atmosphere are in communication with the filler material or preform, at least at some point during the process, which permits molten matrix metal to spontaneously infiltrate the filler material or preform. In the present improved process for producing metal matrix composite bodies by an infiltration process, an improved coating or deposition of infiltration enhancer on and/or within a permeable mass or preform is achieved.

Specifically, a permeable mass or preform to be infiltrated by a molten matrix metal is exposed to a source of infiltration enhancer or infiltration enhancer precursor which has been heated to a temperature higher than that at which spontaneous infiltration is intended to occur. More specifically, the infiltration enhancer or its precursor preferably is heated to a temperature at least about 100°C greater than the temperature at which subsequent infiltration will occur. More generally, the infiltration enhancer or its precursor preferably is heated to a temperature at which its vapor pressure is at least about 100 millitorr. This extended heating permits the concentration of infiltration enhancer or its precursor to more rapidly increase within the permeable mass or preform, thereby more quickly permitting spontaneous infiltration to initiate and proceed. The actual infiltration of molten matrix metal into the permeable mass or preform, however, is not conducted at this enhanced temperature, but instead is carried out a lower temperature which may be more conducive to the infiltration process. Conducting the actual infiltration at the higher temperature, for example, could cause excessive nitridation of the matrix metal. With reference to Figure 1 , a simple lay-up for forming a spontaneously infiltrated metal matrix composite is illustrated. Specifically, a filler or preform 1 1 , which may be of any suitable material, as discussed in detail below, is placed in a non-reactive vessel 21 for housing matrix metal and/or filler material. A matrix metal 13 is placed on or adjacent to the filler or preform 1 1. The lay-up is thereafter placed in a furnace to initiate spontaneous infiltration.

Although the above discussion occurs largely in the context of the aluminum/magnesium/nitrogen spontaneous infiltration system, it should be understood that the concept contained herein should be applicable to other spontaneous infiltration systems. Among those that have been identified to date include the aluminum/strontium/nitrogen system, the aluminum/calcium/nitrogen system and the aluminum/zinc/oxygen system.

The following is a broader, more generalized discussion of the phenomenon of spontaneous infiltration. In order to effect spontaneous infiltration of the matrix metal into the filler material or preform, an infiltration enhancer should be provided to the spontaneous system. An infiltration enhancer could be formed from an infiltration enhancer precursor which could be provided (1) in the matrix metal; and/or (2) in the filler material or preform; and/or (3) from the infiltrating atmosphere; and/or (4) from an external source into the spontaneous system. Moreover, rather than supplying an infiltration enhancer precursor, an infiltration enhancer may be supplied directly to at least one of the filler material or preform, and/or matrix metal, and/or infiltrating atmosphere. Ultimately, at least during the spontaneous infiltration, the infiltration enhancer should be located in at least a portion of the filler material or preform. Without wishing to be bound by any particular theory or explanation, when an infiltration enhancer precursor is utilized in combination with at least one of the matrix metal, and/or filler material or preform and/or infiltrating atmosphere, the infiltration enhancer precursor may react to form an infiltration enhancer which induces or assists molten matrix metal to spontaneously infiltrate a filler material or preform. Moreover, it appears as though it may be necessary for the precursor to the infiltration enhancer to be capable of being positioned, located or transportable to a location which permits the infiltration enhancer precursor to interact with at least one of the infiltrating atmosphere, and/or the preform or filler material, and/or molten matrix metal. For example, in some matrix metal/infiltration enhancer precursor/infiltrating atmosphere systems, it is desirable for the infiltration enhancer precursor to volatilize at, near, or in some cases, even somewhat above the temperature at which the matrix metal becomes molten. Such volatilization may lead to: (1) a reaction of the infiltration enhancer precursor with the infiltrating atmosphere to form a solid, liquid or gaseous infiltration enhancer which enhances wetting of the filler material or preform by the matrix metal; and (2) a reaction of the infiltration enhancer precursor within the filler material or preform which forms a solid, liquid or gaseous infiltration enhancer in at least a portion of the filler material or preform which enhances wetting.

Thus, for example, if an infiltration enhancer precursor were included or combined with, at least at some point during the process, molten matrix metal, it is possible that the infiltration enhancer precursor could volatilize from the molten matrix metal and react with at least one of the filler material or preform and/or the infiltrating atmosphere. Such reaction could result in the formation of a solid species, if such solid species is stable at the infiltration temperature, said solid species being capable of being deposited on at least a portion of the filler material or preform as, for example, a coating. Moreover, it is conceivable that such solid species could be present as a discernible solid within at least a portion of the preform or filler material. If such a solid species were formed, molten matrix metal may have a tendency to react (e.g., the molten matrix metal may reduce the formed solid species) such that infiltration enhancer precursor may become associated with (e.g., dissolved in or alloyed with) the molten matrix metal. Accordingly, additional infiltration enhancer precursor may then be available to volatilize and react with another species (e.g., the filler material or preform and/or infiltrating atmosphere) and again form a similar solid species. It is conceivable that a continuous process of conversion of infiltration enhancer precursor to infiltration enhancer followed by a reduction reaction of the infiltration enhancer with molten matrix metal to again form additional infiltration enhancer precursor, and so on, could occur, until the result achieved is a spontaneously infiltrated metal matrix composite. In a preferred embodiment of the invention, it is possible that the infiltration enhancer precursor can be at least partially reacted with the infiltrating atmosphere such that the infiltration enhancer can be formed in at least a portion of the filler material or preform prior to or substantially contiguous with contacting the filler material or preform with the matrix metal (e.g., if magnesium were the infiltration enhancer precursor and nitrogen were the infiltrating atmosphere, the infiltration enhancer could be magnesium nitride which would be located in at least a portion of the preform or filler material).

Under the conditions employed in the method of the present invention, in the case of an aluminum/magnesium/nitrogen spontaneous infiltration system, the preform or filler material should be sufficiently permeable to permit the nitrogen-containing gas to penetrate or permeate the filler material or preform at some point during the process and/or contact the molten matrix metal. Moreover, the permeable filler material or preform can accommodate infiltration of the molten matrix metal, thereby causing the nitrogen- permeated preform to be infiltrated spontaneously with molten matrix metal to form a metal matrix composite body and/or cause the nitrogen to react with an infiltration enhancer precursor to form infiltration enhancer in the filler material or preform and thereby result in spontaneous infiltration. The extent of spontaneous infiltration and formation of the metal matrix composite will vary with a given set of process conditions, including magnesium content of the aluminum alloy, magnesium content of the preform or filler material, amount of magnesium nitride in the preform or filler material, the presence of additional alloying elements (e.g., silicon, iron, copper, manganese, chromium, zinc, and the like), average size of the filler material (e.g., particle diameter) comprising the preform or the filler material, surface condition and type of filler material or preform, nitrogen concentration of the infiltrating atmosphere, time permitted for infiltration and temperature at which infiltration occurs. For example, for infiltration of the molten aluminum matrix metal to occur spontaneously, the aluminum can be alloyed with at least about 1 percent by weight, and preferably at least about 3 percent by weight, magnesium (which functions as the infiltration enhancer precursor), based on alloy weight. Auxiliary alloying elements, as discussed above, may also be included in the matrix metal to tailor specific properties thereof. Additionally, the auxiliary alloying elements may affect the minimum amount of magnesium required in the aluminum matrix metal to result in spontaneous infiltration of the filler material or preform. Loss of magnesium from the spontaneous system due to, for example, volatilization should not occur to such an extent that no magnesium was present to form infiltration enhancer. Still further, the presence of magnesium in both of the preform (or filler material) and matrix metal or the preform (or filler material) alone may result in a reduction in required amount of magnesium to achieve spontaneous infiltration. Additionally, such placement of the magnesium may permit lowering of the temperature at which infiltration occurs, and/or may reduce the amount of undesirable magnesium- containing intermetallics.

It has been noted that it is possible to supply to the spontaneous system infiltration enhancer precursor and/or infiltration enhancer on a surface of the alloy and/or on a surface of the preform or filler material and/or within the preform or filler material and/or from an external source prior to infiltrating the matrix metal into the filler material prior to infiltrating the matrix metal into the filler material or preform. For example, in the aluminum/magnesium/nitrogen system, if the magnesium was applied to a surface of the matrix metal it may be preferred that the surface should be the surface which is closest to, or preferably in contact with, the permeable mass of filler material or vice versa. In the embodiment in which an infiltration enhancer such as magnesium nitride is produced in situ by chemically reacting a magnesium containing infiltration enhancer precursor with a nitrogen containing infiltrating atmosphere, it is not necessary to maintain the presence of the nitrogen containing atmosphere throughout the entire infiltration process. Specifically, once sufficient infiltration enhancer has been produced, the infiltration of the molten matrix metal into the permeable mass or preform may be conducted under vacuum or under a different infiltrating atmosphere such as an atmosphere which is chemically inert with respect to at least the infiltration enhancer precursor, for example, argon.

The method of forming a metal matrix composite is applicable to a wide variety of filler materials, and the choice of filler materials will depend on such factors as the matrix alloy, the process conditions, the reactivity of the molten matrix alloy with the filler material, and the properties sought for the final composite product. For example, when aluminum is the matrix metal, suitable filler materials include (a) oxides, e.g., alumina, magnesia, zirconia; (b) carbides, e.g., silicon carbide; (c) borides, e.g., aluminum dodecaboride, titanium diboride, and (d) nitrides, e.g., aluminum nitride, and (e) mixtures thereof. Ceramics which are preferred for use in the present method include alumina and silicon carbide.

The size, shape, chemistry and volume percent of the filler material (or preform) can be any that may be required to achieve the properties desired in the composite. Thus, the filler material may be in the form of particles, whiskers, platelets or fibers since infiltration is not restricted by the shape of the filler material. The fibers can be discontinuous (in chopped form) or in the form of continuous filament, such as multifilament tows. Other shapes such as spheres, tubules, pellets, refractory fiber cloth, and the like may be employed. Further, the filler material or preform may be homogeneous or heterogeneous. Average particle diameters as small as a micron or less to about 1 100 microns or more can be successfully utilized in the present invention, with a range of about 2 microns through about 1000 microns being preferred for a vast majority of commercial applications.

Further, the constituency of the matrix metal within the metal matrix composite may be modified by controlling the cooling rate of the metal matrix composite. For example, the metal matrix composite may be directionally solidified by any number of techniques including: placing the container holding the metal matrix composite upon a chill plate; and/or selectively placing insulating materials about the container. Such directional solidification can also reduce the size and extent of defects (e.g., porosity). Further, the constituency of the metal matrix may be modified after formation of the metal matrix composite as, for example, by a heat treatment.

Moreover, it is possible to use a reservoir of matrix metal to assure complete infiltration of the filler material and/or to supply a second metal which has a different composition from the first source of matrix metal. It has been observed that for aluminum infiltration and matrix formation around a ceramic filler, wetting of the ceramic filler by the aluminum matrix metal may be an important part of the infiltration mechanism. Further, the wetting of the filler by molten matrix metal may permit a uniform dispersion of the filler throughout the formed metal matrix composite body and improve the bonding of the filler to the matrix metal.

Moreover, at low processing temperatures, a negligible or minimal amount of metal nitriding occurs resulting in a minimal discontinuous phase of aluminum nitride dispersed in the matrix metal. However, as the upper end of the temperature range is approached, nitridation of the metal is more likely to occur. Thus, the amount of the nitride phase in the metal matrix can be controlled by varying the processing temperature at which infiltration occurs.

The specific process temperature at which nitride formation becomes more pronounced also varies with such factors as the matrix aluminum alloy used and its quantity relative to the volume of filler or preform, the filler material to be infiltrated, and the nitrogen concentration of the infiltrating atmosphere. For example, the extent of aluminum nitride formation at a given process temperature is believed to increase as the ability of the matrix metal to wet the filler decreases and as the nitrogen concentration of the atmosphere increases.

By the same token, when infiltration enhancer is formed in situ in the permeable mass of filler material prior to infiltration, the amount of aluminum nitride subsequently formed may depend on the amount of magnesium nitride infiltration enhancer formed. As discussed above, the concentration of infiltration enhancer formed may depend on the amount of magnesium infiltration enhancer precursor which can be reacted with nitrogen infiltrating atmosphere. This quantity, in turn, will depend upon the magnesium vapor pressure (temperature dependent) and the amount of time that magnesium is permitted to react with nitrogen.

Thus, the present invention recognizes and appreciates that the temperature or temperature range which is ideal for volatilization or formation of infiltration enhancer may not overlap the ideal temperature range for infiltration of molten matrix metal into the permeable mass. Specifically, optimal formation of infiltration enhancer, more specifically in-situ formed infiltration enhancer, may take place at a temperature which is higher than optimal for infiltration. Thus, in accordance with the present invention, the infiltration enhancer is formed at its optimal temperature or within its optimal range, then the temperature is adjusted to be within the optimal range for matrix metal infiltration. In the aluminum/magnesium/nitrogen system, the optimal temperature range for infiltration enhancer formation generally is above the optional temperature range for infiltration of matrix metal. For example if infiltration optimally occurs at about 800°C, infiltration enhancer formation may be improved at a temperature somewhat higher than 800°C, for example, at about 900°C.

Although the present invention focuses largely on the volatilization of the infiltration enhancer precursor, it should not be construed as necessarily being limited to such materials. Specifically, it may be possible to employ the "pre-infiltration temperature excursion" of the present invention to an external source of the infiltration enhancer to yield a sufficient vapor pressure of the infiltration enhancer (e.g., at least about 100 millitorr) to effectively vapor coat a permeable mass of filler material or a body of matrix metal within a commercially reasonable period of time. For example, it may be possible to coat a permeable mass or preform with magnesium nitride infiltration enhancer which is deposited from the vapor phase. In the aluminum/magnesium/nitrogen system, however, it would seem that an external source of magnesium nitride would have to be heated to a temperature substantially above any reasonable infiltration temperature due to the expected low vapor pressure of magnesium nitride. Thus, while technologically possible, economic considerations may argue for forming the infiltration enhancer in situ in the permeable mass.

The following Examples further illustrate the present invention. EXAMPLE 1

This Example demonstrates the present improved method for producing a metal matrix composite body. In particular, this Example demonstrates that infiltration of a permeable mass or preform by a molten matrix metal is accomplished more quickly when the processing involves a relatively brief temperature excursion somewhat above the subsequent temperature employed for infiltration. Figure 1 shows a side view in schematic form of the setup employed in producing the metal matrix composite body of the present Example.

A hollow golf club head preform 11 was fabricated by an injection molding operation using conventional techniques known to those skilled in the art. The preform composition comprised by weight about 2% clay, 1% fumed silica and the balance 38 Alundum® aluminum oxide particulate (Norton- St. Gobain Co., Worcester, MA) having an average particle size of about 25 microns. The hollow preform was supplied in two pieces, among other reasons, to permit a body of matrix metal to be positioned within the hollow interior of the club head. Upon demolding the golf club head preform or portion thereof from the injection molding die, the preform pieces were bisque fired in air to a maximum temperature of about 1200°C. A body of matrix metal 13 having a mass of about 250 grams and comprising by weight about 10.5% magnesium, balance aluminum, was placed into the hollow interior of the golf club head preform whose pieces had a combined weight of about 31 1 grams. The two pieces of the golf club head preform were then bonded together with a grout mixture.

A cement of the grout was prepared as follows: First, a dry grout mixture was prepared by adding to a 20-25 gram quantity of 38 Alundum® aluminum oxide particulate (Norton-St. Gobain, 25 microns) about 2 percent by weight of a first liquid comprising by weight about 2 percent Lupersol 231 peroxide (Aldrich Chemical, Milwaukee, WI) and the balance CERASET™ SN inorganic polymer (Lanxide Performance Material, Inc., Newark, DE). To this dry grout mixture was added a second liquid in an amount sufficient to form a wet grout mixture of a suitable consistency for cementing the preform pieces together. Typically the wet grout mixture is about 67-83 percent solids by weight. The composition of the second liquid was about 30 percent by weight of Q-Pac 40 (poly) propylene carbonate (PAC Polymers, Inc., Greenville, DE) and the balance acetone.

Additionally, magnesium particulate 17 (Hart Metals, Tamaqua, PA) having substantially all particles between about 75 microns and 150 microns in size was stucco coated to the inside of the hosel or "stem" portion of the golf club head preform, as illustrated in Figure 1. A paste of the magnesium particulate suitable for stucco coating was fabricated by admixing with the magnesium particulate sufficient amounts of the above- mentioned Q-Pac 40 (poly) propylene carbonate/acetone solution. The paste was applied with a long, thin wooden dowel rod. The cemented and stucco coated golf club head preform was then subjected to bisque firing in air. Specifically, the preform was placed onto a refractory setter tray which in turn was placed into the heating chamber of an air atmosphere furnace. The heating chamber and its contents were then heated from substantially ambient (e.g. about 20°C) temperature to a temperature of about 300°C at a rate of about 100°C per hour. After maintaining a temperature of about 300°C for about 4 hours, the temperature was then increased to a temperature of about 450°C at a rate of about 100°C per hour. After maintaining a temperature of about 450°C for about 5 hours, the temperature was then decreased to a temperature of 80°C at a rate of about 200°C per hour and maintained at this approximate 80°C temperature. Upon bisque firing, the preform (less the matrix metal inside) had a mass of about 312 grams.

The exterior surfaces of the golf club head preform were then coated with barrier coatings 19. In particular, the outside surface was first spray coated with Krylon™ acrylic (Borden Co., Columbus, OH). Next a suspension comprising equal weight fractions of ethanol and DAG 154 colloidal graphite (Acheson Colloids Co., Port Huron, MI) was applied to the outside surface using an air brush. Finally, a thin layer of Dylon CW colloidal graphite (Dylon Industries, Cleveland, OH) was applied over the previous colloidal graphite layer using a foam brush. With further reference to Figure 1 , a setup for matrix metal infiltration was prepared as follows. A shallow graphite boat 21 was lined on its floor surface with a single sheet of GRAFOIL™ graphite sheet material 23 (Union Carbide Co., Danbury, CT). A graphite setter ring 25 (previously painted with Dylon CW colloidal graphite and then dried) was centered in the graphite boat on the graphite sheet material, and the coated golf club head preform containing the body of matrix metal was placed onto this setter ring to complete the setup.

The graphite boat and its contents were then placed into the heating chamber of a controlled atmosphere furnace at about ambient temperature. Upon closing and sealing the door, the heating chamber was isolated from the ambient atmosphere. A nitrogen gas flow rate of about 20 standard liters per minute (slpm) was then established straightaway, that is without first evacuating and backfilling the furnace. The temperature of the heating chamber was then increased to a temperature of about 900°C. After maintaining a temperature of about 900°C for about 4 hours, the temperature was then decreased to a temperature of about 750°C. After maintaining a temperature of about 750°C for about 10 hours, the temperature was further decreased to a temperature of about 650°C which temperature was then maintained until an operator could attend to the setup (about 1 to 16 hours at 650°C. All heating and cooling was done at a rate of about 200°C per hour except for the final cooling step to ambient temperature which was effected by opening the about 650°C furnace and physically removing the graphite boat and its contents. Inspection of the setup revealed that the matrix metal within the golf club head preform had infiltrated the latter to produce a metal matrix composite golf club head.

Thus, the present Example demonstrates the use of a brief temperature excursion above the about 750°C infiltration temperature for producing a metal matrix composite body.

EXAMPLE 2

The techniques of Example 1 were substantially repeated but with the following notable exceptions. The injection molded golf club head preform had a mass of about 203 grams. The body of matrix metal had a mass of about 162 grams. Further, the infiltration schedule was such that the setup was in the furnace for a total of only about 12 hours.

Specifically, the furnace chamber and its contents were heated from about ambient temperature (e.g., about 20°C) to temperature of about 250°C. After maintaining a temperature of about 250°C for about 5 hours, the temperature was further increased to a temperature of about 700°C. After maintaining a temperature of about 700°C for about 2 hours, the temperature was further increased to a temperature of about 900°C. After maintaining a temperature of about 900°C for about 1 hour, the temperature was decreased to a temperature of about 800°C. After maintaining a temperature of about 800°C for about 4 hours, the temperature was further decreased to a temperature of about 675°C. Again, as in the previous Example, all temperature increases and decreases within the furnace were conducted at a heating or cooling rate of about 200°C per hour. At a temperature of about 675°C, the furnace was opened to the ambient atmosphere and the graphite tray and its contents were removed and permitted to cool to ambient temperature. Complete infiltration of the golf club head preform by the matrix metal to produce a metal matrix composite golf club head was accomplished.

By way of comparison, when the heating schedule does not include the step of heating to 900°C prior to infiltration, about 15 to 20 hours at the infiltration temperature of about 800°C is required to achieve complete infiltration of a similar preform.

Thus, the present Example illustrates the processing improvements made possible by the present invention ~ specifically, that a brief excursion above the infiltration temperature permits subsequent infiltration to be completed faster.

EXAMPLE 3

This Example demonstrates, among other things, a thermal processing schedule which features a temperature excursion prior to infiltration and above the temperature used for infiltration. The temperature excursion assists in the infiltration of a permeable mass or preform to form a metal matrix composite article and the facilitated recovery thereof. This Example furthermore demonstrates the application of infiltration enhancer precursor to the preform to be infiltrated as a stucco layer to enhance the effective contact area between the preform and the molten matrix metal which is to infiltrate the preform.

A preform 85 having a cross section substantially as shown in Figure 2 and measuring roughly 6 inches (152 mm) by 6 inches (9152 mm) by 4 inches (102 mm) was fabricated by a sedimentation casting process. In particular, a slurry suitable for sedimentation casting was prepared by thoroughly blending about 3 parts by weight solids to 1 part liquid. The liquid component comprised about 5% by weight of Nyacol 2040 colloidal silica (Nyacol Products, Inc., an affiliate of PQ Corporation, Ashland, MA), balance de-ionized water. The solid component comprised by weight about 6% Grade KS44 graphite particulate (Lonza Inc., Fair lawn, NJ) having substantially all particles smaller than about 44 microns in size, and the balance milled, 38 Alundum® fused alumina particulate (Norton-St. Gobain, Worcester, MA) having an average particle size of about 25 microns. The slurry was cast into a rubber mold supported by an aluminum plate placed on a vibration table. As sedimentation proceeded the excess liquid at the top of the casting was first decanted, then daubed with a sponge. After sedimentation was substantially complete, vibration was ceased. The rubber mold and its contents were then frozen to form an inorganic binder network. After freezing and removal from the rubber mold, the preform was then buried in 90 grit 38 Alundum® alumina (216 microns) contained within a refractory boat. The refractory boat and its contents were then dried and fired in an air atmosphere furnace according to the following heating schedule. First the furnace and its contents was heated from substantially ambient temperature (e.g., about 20°C) to a temperature of about 35°C at a rate of about 100°C per hour. After maintaining a temperature of about 35°C for about 5 hours, the temperature was then further increased to a temperature of about 85°C at a rate of about 50°C per hour. After maintaining a temperature of about 85°C for about 5 hours, the temperature was then further increased to about 1000°C at a rate of about 100°C per hour. After maintaining a temperature of about 1000°C for about 15 hours, the temperature was then decreased to a temperature of about 50°C at a rate of about 200°C per hour. The preform was maintained at a temperature of about 50°C pending further processing.

Next, a series of barrier and separation facilitator coatings comprising colloidal graphite were applied to the preform. Specifically, all surfaces of the object preform were coated with DAG 154 colloidal graphite (Acheson Colloids Co., Port Huron, MI) using a foam brush. After the coating had air dried, those surfaces to be subsequently stucco coated were further brush coated with a slurry comprising equal weight fractions of DAG 154 colloidal graphite and magnesium particulate (Hart Metals, Tamaqua, PA) having substantially all particles between about 150 and 300 microns in size. Those surfaces not being stucco coated were brush coated with Dylon CW colloidal graphite (Dylon Industries,

Inc., Cleveland, OH).

Next, a stucco layer 86 comprising an infiltration enhancer precursor material was applied to several outside surfaces of the preform as shown in Figure 2. Specifically, the stucco material comprised by weight about 1 percent of a binder, about 3.5 percent of magnesium particulate (Hart Metals) having substantially all particles between about 150 and 300 microns in size and the balance 90 grit (216 microns) 38 Alundum® alumina particulate (Norton-St. Gobain, Worcester, MA). The binder material comprised by weight about 2 percent Lupersol 231 peroxide (Aldrich Chemical Co., Milwaukee, WI) and the balance CERASET SN inorganic polymer (Lanxide Corp., Newark, DE). This stucco mixture was applied to the preform surfaces using a tongue depressor to an approximate thickness of about 0.5 inch (13 mm). The stucco coated preform was then placed into an air atmosphere furnace already heated to and maintained at a temperature of about 170°C. After soaking at this 170°C temperature for about 20 minutes, the temperature was increased to a temperature of about 300°C at a rate of about 100°C per hour. After maintaining a temperature of about 300°C for about 4 hours, the temperature was then further increased to a temperature of about 425°C at a rate of about 100°C per hour. After maintaining a temperature of about 425°C for about 5 hours, the temperature of the furnace and its contents was decreased to substantially ambient temperature at a rate of about 200°C per hour. The coated preform was then removed from the furnace and placed into a graphite boat to prepare a setup for matrix metal infiltration.

A setup for conducting the infiltration of molten matrix metal into the stucco coated preform was prepared as follows. First, a particulate bedding material 63 comprising by weight about 5 percent Grade F69 glass frit (Fusion Ceramics, Inc., CarroUton, OH) and the balance 90 grit (216 microns ave. particle size) 38 Alundum® alumina particulate (Norton- St. Gobain, Worcester, MA) was placed onto the bottom of a graphite boat 81. A graphite foil box 62 was then placed on top of the particulate bedding material. Next, a sacrificial preform 82 having a mass of about 500 grams was placed onto the floor of the graphite foil box close to one wall of the box. The sacrificial preform comprised by weight about 3 percent magnesium particulate (Hart Metals, Tamaqua, PA) having substantially all particles between about 150 microns and 300 microns in diameter and the balance 90 grit (216) 38 Alundum® alumina particulate (Norton-St. Gobain, Worcester, MA). A graphite foil "window" 83 was then placed on top of the sacrificial preform. The stucco coated preform 85 was then centered over the opening 87 of the graphite foil window 83 substantially as shown in the figure. An ingot of matrix metal 88 comprising by weight about 7 percent magnesium and the balance aluminum was placed on the floor of the graphite foil box adjacent to the wall opposite that of the preform. The ingot of matrix metal weighed about 1 130 grams and the stucco coated preform weighed about 467 grams.

The completed setup was then placed into a controlled atmosphere furnace at substantially ambient temperature. After evacuating the furnace chamber with a mechanical roughing pump and backfilling with commercially pure nitrogen gas, a nitrogen gas flow rate of about 15 standard liters per minute (slpm) was established and maintained. The setup was then thermally processed substantially according to the furnace schedule used in Example 2 with the exception that the infiltration temperature was about 790°C (as opposed to 800°C in Example 2), and this temperature was maintained for about 21 hours (as opposed to 4 hours in Example 2). Once the furnace and its contents had stabilized at a temperature of about 700°C following infiltration, the setup was removed from the furnace and air was directed at the stucco coated preform using Model 606 "cold guns" (Vortec

Corp., Cincinnati, OH) to bring about directional solidification. Once the setup had cooled substantially to ambient temperature, the setup was disassembled to reveal that matrix metal had infiltrated the preform substantially completely to produce a metal matrix composite article. Thus, the present Example demonstrates the infiltration of a preform with a molten matrix metal in a relatively short period of time to form a metal matrix composite article. EXAMPLE 4

This Example demonstrates among other things the fabrication of a metal matrix composite brake rotor utilizing the techniques of the present invention. Figure 3 is a cross- sectional schematic view of a setup used to infiltrate a preform with a molten matrix metal to produce the metal matrix composite body.

First, a preform blank of the brake rotor was fabricated by a compression molding process. The press mixture comprised by weight about 1.5% of an inorganic binder material, the balance consisting of aluminum oxide particulate (Norton-St. Gobain, Worcester, MA) having an average particle size of about 25 microns. The binder material consisted by weight of about 1% Lupersol 231 peroxide (Aldrich Chemical Co.,

Milwaukee, WI ) and the balance CERASET™ SN inorganic polymer (Lanxide Corporation, Newark, DE). The mixing process was performed in the mixing chamber of a Model RV02 Eirich high intensity mixer (Eirich Machines, Maple, Ontario, Canada).

Next, about 6000 grams of the admixture was placed into the compression molding die which measured about 8.5 inches (216 mm) in diameter by about 6 inches (152 mm) in height. The admixture was pressed uniaxially under an applied pressure of about 100 psi (690 kPa). After maintaining this applied pressure for about 1 minute, the die and its contents were heated to a temperature of about 170°C. This cure temperature of about 170°C was maintained for about 2 hours. The die and its contents were then transferred to an oven maintained at a temperature of about 80°C for about 16 hours.

After cooling the die and its contents substantially to ambient temperature, the pressed preform was removed from the die, and green machined to the desired dimensions. The machined preform was then placed onto a cordierite plate containing regularly spaced holes. The supporting plate and the preform were then placed into an air atmosphere furnace for bisque firing. The preform was heated to a temperature of about 300°C. After maintaining a temperature of about 300°C for about 4 hours, the temperature was further heated to a temperature of about 1000°C. After maintaining a temperature of about 1000°C for about 5 hours, the preform was cooled to a temperature of about 50°C. At a temperature of about 50°C the preform was removed from the furnace and transferred to the 80°C holding oven pending further processing. The heating was conducted at rates of about

100°C per hour, and the cooling occurred at a rate of about 200°C per hour.

Next a series of coatings were applied to the preform. First, a mixture comprising 90 parts by weight of dibasic ester and 10 parts of QPAC® 40 poly (polypropylene carbonate) (PAC Polymers, Inc., Greenville, DE) was brush painted over all surfaces of the preform and permitted to dry in air. Then Dylon CW colloidal graphite (Dylon Industries,

Inc.) was brush coated on those interior surfaces 37 comprising the "hub region" of the preform (see Figure 3). Finally, DAG 154 colloidal graphite (Acheson Colloids Co.) was coated over all surfaces of the preform left uncoated. After air drying the coated preform 38 had a weight of about 844 grams.

Referring again to Figure 3, a setup was prepared. First, a bedding material 32 comprising by weight about 13% grade F-69 glass frit (Fusion Ceramics, Inc.) and the balance 38 ALUNDUM® aluminum oxide particulate (Norton- St. Gobain, 216 microns average particle size) was distributed evenly over the floor of a graphite boat to a depth of about 1 inch (25 mm). The graphite boat measured about 10 inches (250 mm) square by about 8 inches (203 mm) in height. Then a GRAFOIL® graphite foil box 23 (Union Carbide Co., Cleveland, OH) having substantially the same dimensions as the interior of the boat was positioned on the bedding material. Next, a ring or tube 34 was fabricated from

GRAFOIL® graphite foil and centered in the boat. The graphite foil tube 34 had dimensions of about 4.75 inches (121 mm) in diameter by about 2.5 inches (64 mm) in height. Additional bedding material 32 having the above-described composition was poured into the annular space between the graphite foil box 23 and the graphite foil tube 34 almost to the top of the tube. Next a "sacrificial" permeable material was sprinkled on top of the bedding material 32. The purpose of this applied layer is to enhance or assist infiltration as well as to assist with disassembly of the setup, specifically to assist with separation of the infiltrated preform from any attached carcass of matrix metal or bedding material. The sacrificial layer actually consisted of two adjacent layers. The first layer 33 in contact with the preform consisted of about 40 grams of 38 ALUNDUM® aluminum oxide particulate (Norton-St. Gobain, 90 grit) having an average particle size of about 216 microns. The second sacrificial layer 35 adjacent to and in contact with the first sacrificial layer consisted of about 80 grams of an admixture comprising by weight about 3% magnesium particulate (Hart Metals, Tamaqua, PA) having substantially all particles between about 150 and 300 microns in diameter, and the balance 38 ALUNDUM® aluminum oxide particulate having an average particle size of about 216 microns (90 grit). Several pieces of matrix metal 36 were then placed into the center of the graphite foil tube 34. The composition of the matrix metal was about 9.5% by weight magnesium and the balance aluminum. Collectively the pieces of matrix metal had a mass of about 1527 grams. The coated preform 38 was then placed over the graphite foil tube 34 and the particulate bedding material as shown in the figure. Finally, a sheet of graphite foil was placed over the preform and glued to the edges of the graphite foil box with RIGIDLOCK® colloidal graphite (Polycarbon Corp., Valencia, CA) to complete the setup.

A spontaneous infiltration process was then undertaken by placing the setup comprising the graphite boat and its contents into the heating chamber of a controlled atmosphere furnace preheated to a temperature of about 150°C. After isolating the furnace from the ambient atmosphere, the heating chamber was evacuated and backfilled with commercially pure nitrogen gas. A gas flow rate of about 15 slpm of nitrogen was thereafter established and maintained. Then the furnace and its contents were heated to a temperature of about 250°C. After maintaining a temperature of about 250°C for about 10 hours, the furnace was heated to a temperature of about 480°C. After maintaining a temperature of about 480°C for about 5 hours, the temperature was further increased to a temperature to about 580°C. After maintaining a temperature of about 580°C for about 5 hours, the temperature was further increased to a temperature of about 900°C. After maintaining a temperature of about 900°C for about 6 hours, the temperature was reduced to a temperature of about 800°C. After maintaining a temperature of about 800°C for about 30 hours, the temperature was further reduced to a temperature of about 700°C. At a temperature of about 700°C the furnace was opened and the setup was removed and directionally solidified using Model 606 "cold guns" (Vortec Corp., Cincinnati, OH). During thermal processing the heating and cooling was conducted at a rate of about 200°C per hour, except for the temperature increases between 480°C and 900°C, which were conducted at a rate of about 100°C per hour. After the setup and its contents had cooled to substantially ambient temperature, the setup was disassembled to reveal a completely infiltrated preform, thereby yielding a metal matrix composite brake rotor.

Thus, the present Example features the production of an MMC article in which the thermal processing initially features enhanced heating for a period of time to generate magnesium vapor. The geometry of the setup is such that the magnesium vapor is well confined, thereby permitting magnesium nitride to form on the walls defining the confinement zone. Molten matrix metal is able to infiltrate through this magnesium nitride layer into the preform against the force of gravity.

The preceding Examples are by no means exhaustive; instead they are illustrative of the present invention. An artisan of ordinary skill will readily appreciate that numerous modifications of the above-identified Examples can be made without departing from the scope and spirit of the present invention, as set forth in the following claims.

Claims

WHAT IS CLAIMED IS:

1. A method for making a metal matrix composite body, comprising: providing a body of matrix metal, a source of infiltration enhancer precursor and a permeable mass or preform; heating said source of infiltration enhancer precursor to a temperature at least 100┬░C higher than said temperature at which subsequent infiltration is to be carried out; exposing said permeable mass or preform to a vapor of said infiltration enhancer precursor; communicating an infiltrating atmosphere to said permeable mass or preform; reacting said infiltration enhancer precursor with said infiltrating atmosphere to form an infiltration enhancer within said permeable mass or preform; contacting said permeable mass or preform to said body of matrix metal in molten form; and spontaneously infiltrating molten matrix metal into said permeable mass or preform.

2. A method for making a metal matrix composite body, comprising: providing a body of matrix metal, a source of infiltration enhancer, and a permeable mass or preform comprising at least one filler material; heating said source of infiltration enhancer to a temperature sufficient to yield a vapor pressure of said infiltration enhancer of at least 100 millitorr; causing said infiltration enhancer to coat at least a portion of said filler material; heating at least said body of matrix metal to an infiltration temperature, thereby rendering said body molten; and spontaneously infiltrating molten matrix metal into said permeable mass or preform at said infiltration temperature.

3. A method for making a metal matrix composite body, comprising: providing a body of matrix metal comprising aluminum and magnesium, a permeable mass or preform comprising at least one filler material, and an infiltrating atmosphere comprising nitrogen; heating said body of matrix metal to a temperature of at least about 900┬░C, thereby establishing a vapor comprising magnesium; communicating said vapor comprising magnesium to said permeable mass or preform; communicating said infiltrating atmosphere to said permeable mass or preform; heating said body of matrix metal to a temperature sufficient to render said body molten but not substantially greater than about 800┬░C; contacting said body of matrix metal to said permeable mass or preform; and spontaneously infiltrating molten matrix metal into said permeable mass or preform.

4. A method for making a metal matrix composite body, comprising: providing a permeable mass comprising at least one filler material, an infiltration enhance precursor and an infiltrating atmosphere; heating at least said infiltration enhance precursor to a first temperature sufficient to cause formation of an infiltration enhance, and deposition of a quantity of said infiltration enhance reasonable for a spontaneous infiltration process on at least a portion of said at least one filler material within a time reasonable for conducting said infiltration process, and spontaneously infiltrating said molten matrix metal into said permeable mass at a second temperature lower than the first temperature.