BACKGROUND ART The automobile industry has made great strides in recent years to reduce the weight of cars and light trucks. Reduced weight not only is desirable from a fuel efficiency point of view, it permits lighter suspension and brake components and requires less power for the same acceleration. Within the engine itself, a lighter valve train may permit higher revving engines.

In spite of the weight savings efforts, there is still room for considerable improvement. The use of aluminum in automobiles, in particular, is still relatively new, at least as a structural material. The use of aluminum often requires extensive re-designing, as the properties of aluminum are so different from those of steel. Because aluminum is considerably softer and weaker than steel, larger quantities are often required, reducing the amount of the weight savings. Some property differences, however, cannot be made up through increased quantities: (lack of) hardness, for example. Thus, bare aluminum usually cannot be used in applications involving sliding contact against steel because the aluminum will wear excessively rapidly. This fact has necessitated, for example, the use of steel inserts to line the combustion chambers of aluminum engine blocks. Such combustion liners solve the problem of excessive wear of aluminum by the piston rings, but add weight and complexity (and therefore manufacturing cost) and reduce thermal conductivity.

Metal matrix composite (MMC) materials offer the lightweight attributes of aluminum, as well as sufficient wear and abrasion resistance to stand up to the hardened steel of valve train parts and piston rings. Further, the stiffness and thermal expansion coefficient are usually closer to those of steel than to aluminum, which offers a degree of familiarity to designers. Unfortunately, the most popular reinforcement (e. g., filler) materials for metal matrix composites, aluminum oxide and silicon carbide, can render the

composite material excessively abrasive of the steel components with which it comes into contact. Even when the contact is not sliding contact, vibrations ubiquitous to reciprocating engines create the potential for fretting wear of the contacting parts. Moreover, most components require some amount of machining. Although"exotic"forms of machining such as water jet or laser machining are available, the machining infrastructure is still based on"traditional"machining using cutting tools. Further, sometimes only traditional machining can be used to machine certain geometries or provide a particular surface finish.

The problem with many MMCs based upon aluminum oxide or silicon carbide reinforcements is that they tend to be difficult to machine, at least with traditional cutting tools. Specifically, not only can machining stock not be removed quickly, the cutting tools become dull extremely rapidly. Whether the tools can be resharpened or must be disposed of, a cost is imposed in terms of down-time to change or resharpen tools.

U. S. Patent No. 5,511,603 to Brown et al. discloses a machinable metal matrix composite material. The inventors state that small sized particles for the reinforcement phase, no greater than about three microns in diameter and preferably less than one, in conjunction with relatively low particle loading, and a substantially uniform distribution of ceramic particles in a sintered preform are all important for achieving a machinable composite material. Such metal matrix composites suffer, however, from the expense of such ultra-fine powders, the relative difficulty encountered in distributing them uniformly throughout a preform and infiltrating such preforms expeditiously.

DESCRIPTION OF COMMONLY OWNED U. S. PATENTS Commonly owned U. S. Patent No. 4,828,008 to White et al. teaches a technique for producing a 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 percent magnesium in the presence of a gas comprising from about 10 to 100 volume percent 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. 5,040,588 to Newkirk et al. teaches the production of macrocomposite bodies comprising one or more metal matrix composite bodies bonded to one or more second bodies. The second body may comprise ceramic, metal or composite bodies of ceramic and metal. In a preferred embodiment of the invention, a permeable mass or preform is placed in contact with the second body. A molten matrix metal is caused to infiltrate the permeable mass or preform up to the second body, the infiltrated mass or preform thereby becoming a metal matrix composite body.

Upon solidifying the matrix metal, the formed MMC remains bonded to the second body.

Commonly owned U. S. Patent No. 5,020,584 to Aghajanian et al. teaches the addition of matrix metal in powdered form to a permeable mass to filler material or a preform. The presence of powdered matrix metal in the preform or filler material reduces the relative volume fraction of filler material to matrix metal.

DISCLOSURE OF THE INVENTION The present invention addresses and solves the machinability and abrasiveness problems often associated with MMCs, while maintaining the desirable strength and stiffness attributes of composites. Specifically, an aluminum/aluminum nitride (Al/AlN) composite material is formed on or supplied to those surfaces of the MMC body which are to be machined or which will be in contact with unreinforced metals. In a preferred embodiment, the Al/A1N layer can be formed on the desired surfaces during infiltration of a permeable mass of filler material or a preform to produce a metal matrix macrocomposite body. Moreover, the Al/A1N layer can be formed on a surface of a pre-existing MMC body.

Still further, the layer can be formed on unreinforced metal bodies. It has been discovered that the Al/AlN surface layer is sufficiently soft as to be machinable without costly diamond tipped or diamond coated cutting tools, sufficiently abrasion resistant to hardened steel (such as engine valve train components), yet not so abrasive as to unduly accelerate the wear of these contacting steel components. Significantly, the Al/A1N layer is highly engineerable, which provides for extensive tailorability of properties. Further, the Al/AlN layer is compatible both chemically and physically (e. g., thermal expansion coefficient) with the MMC or unreinforced metal substrate to which it is bonded.

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.

"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. 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. When the matrix metal comprises aluminum, a suitable barrier comprises carbon. Particularly preferred is graphite, especially in paper form or as a slurry comprising colloidal-size particles.

"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.

"Fugitive Material", as used herein, means a material which permits and results in a reduced volumetric loading of ceramic filler material in the MMC surface layer by displacing such filler material in the permeable surface coating.

"Fugitive Metal", as used herein, means a fugitive material which possesses at least one property characteristic of metals and which is capable of reaction with or substantial dissolution of or into the matrix metal. Fugitive metals include for example, the semimetals silicon, germanium, boron, and arsenic but exclude carbon.

"Infiltration", as used herein, means the bulk transport of matrix metal into a permeable mass or permeable surface layer, with or without pressure or vacuum assist.

"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 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.

"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 System"or "Spontaneous System", 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.

"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. 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.

"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).

"Substrate"or"MMC Substrate", as used herein, means the body to which the surface layer is applied, and which defines the basic size and shape of the desired article.

"Surface Layer"or"MMC Surface Layer", as used herein, means the MMC material deposited or formed on at least a portion of at least one surface of the substrate and having reduced abrasiveness and/or enhanced machinability with respect to an MMC substrate.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a side view of a connecting rod for an internal combustion engine; Figure 2 is a side view of a bearing cap or bearing cap stiffener for an internal combustion engine; Figure 3 is a valve lifter or tappet for the valve train portion of an internal combustion engine; Figure 4 is a cross-sectional schematic view of a brake caliper piston; Figure 5 is a cross-sectional schematic view of a hydraulic clutch master cylinder; Figure 6 is a cross-sectional schematic view of a cylinder liner for the combustion chamber of an internal combustion engine; and Figure 7 is a cross-sectional schematic view of a brake disc or brake rotor.

Figure 8 is a cross-sectional schematic view of a setup used to produce an Al/AlN MMC surfaced brake caliper piston in accordance with Example 1; and Figure 9 is a cross-sectional schematic view of a setup used to form an MMC brake rotor featuring an Al/A1N surface in accordance with Example 2.

MODES FOR CARRYING OUT THE INVENTION The present invention addresses and solves many of the above-identified difficulties with making and using MMC materials, particularly when such materials are fabricated into specific end-use components. Machinability of MMC materials is of considerable

importance, particularly for components requiring machining of large surfaces, such as brake rotors and clutch plates.

Specifically, it has been discovered that a MMC material whose reinforcement phase consists predominantly or essentially of aluminum nitride possesses both improved machinability and reduced abrasiveness in comparison to MMCs featuring aluminum oxide or silicon carbide as the reinforcement phase.

The machinability of a material is dependent upon many variables. In general however, one can say that the machinability of MMC materials decreases as the amount and hardness of the reinforcement phase of the MMC increases. It has been discovered that a MMC material whose ceramic reinforcement phase consists essentially of aluminum nitride is relatively machinable. By"relatively machinable"what is meant is that such material may be machined using non-diamond or non-diamond-coated cutting tools and the cutting tool can still possess a reasonable lifetime before it must be resharpened or discarded.

Without wishing to be bound by any particular theory or explanation, it may be that the "relative softness"of aluminum nitride enhances machinability. Although still quite hard compared to most metals, aluminum nitride is not as hard as many ceramic materials, particularly those ceramic materials which are popular as reinforcements for MMCs, such as silicon carbide and aluminum oxide. In addition to friction components, another broad potential application area for such MMCs bearing an aluminum nitride rich surface layer is in the field of contact wear. In the absence of such a layer, it has been observed that an MMC component featuring, for example, a silicon carbide reinforcement tends to cause accelerated wear to an adjoining component. Such wear can be pronounced when two components are sliding past one another in the absence of a lubricant such as oil to provide an oil film (e. g., a hydrodynamic layer) which would otherwise keep the components physically separated from one another. Thus, an MMC body featuring the present aluminum nitride containing surface layer may find additional applications in machines which call for unlubricated sliding contact of their various components.

Like the MMC substrate to which it is bonded, the MMC surface layer is highly engineerable. First, the composition of the MMC surface layer may be varied widely. The MMC surface layer typically comprises aluminum nitride embedded in a matrix metal. The aluminum nitride phase may be continuous or discontinuous. The matrix metal embedding the aluminum nitride may have substantially the same chemical composition as the matrix metal in the underlying MMC substrate, or it may have a substantially different chemical composition. Further, the MMC surface layer may feature a reinforcement phase in addition to the aluminum nitride. This optional reinforcement phase may be the same material as the reinforcement phase making up the underlying MMC substrate, or it may be an entirely different material, or it may be the same material chemically but feature a

morphology different from that of the reinforcement phase of the substrate MMC. For example, if the reinforcement phase in the substrate comprised alumina particulates, in the MMC surface layer it might comprise platelets. In a preferred embodiment, the optional reinforcement phase in the surface layer may be present in concentrations (loadings) up to about 70 volume percent.

It may be important to engineer the composition of the MMC surface layer such that its coefficient of thermal expansion is similar to or equal to that of the underlying MMC substrate. It is a basic tenet of material science that two articles joined to one another having different thermal expansion coefficients create stresses on each other as the temperature to which they are exposed changes. These stresses can become sufficient to cause physical deformation of the articles or even cracking of one or both articles. In the present as-infiltrated MMC surface coatings, the thermal expansion coefficient can be engineered in a number of ways--by tailoring the amount of aluminum nitride present, by tailoring the amount and kind of any filler material which might be added, and by tailoring the composition of the matrix metal, for example.

An MMC surface layer featuring aluminum nitride dispersed in a metallic matrix also features relatively high thermal conductivity, a property which can be used to advantage in friction components. High thermal conductivity is frequently desirable, especially for friction components. The heat generated by friction is thereby capable of being rapidly dissipated away from its point of origin. Although the matrix metal in the surface layer typically already possesses high thermal conductivity, the high thermal conductivity of aluminum nitride may become important when the matrix of the surface layer comprises a not-so-thermally conductive metal such as iron.

A number of techniques may be employed to produce an MMC surface layer containing aluminum nitride. In perhaps the simplest embodiment aluminum nitride particulate is placed or coated onto a permeable mass (or preform) or onto a formed MMC body and subsequently infiltrated with a molten matrix metal. In the case of coating aluminum nitride onto a permeable mass or preform, it is possible to produce both the MMC substrate and MMC coating from a single infiltration of molten matrix metal through the permeable mass and aluminum nitride coating. In the case of coating aluminum nitride particulate onto an already formed MMC body, the matrix metal for the coating layer may be supplied from the source of matrix metal within the formed MMC body, or it may be supplied from an external source.

In a preferred embodiment, the aluminum nitride phase within the surface layer is formed in situ during a spontaneous infiltration process. Commonly Owned U. S. Patent No. 5,249,621 provides a thorough description of spontaneous infiltration, and is incorporated herein by reference. For purposes of the present disclosure, a body of molten

matrix metal may be caused to infiltrate a permeable mass of one or more filler materials (e. g., ceramic particulates) without the requirement for the application of pressure or vacuum when the body of molten matrix metal contacts the permeable mass, for example, in the presence of an infiltration enhancer, and/or an infiltration enhancer precursor and/or an infiltrating atmosphere combination. 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.

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 was the infiltration enhancer precursor and nitrogen was 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).

A particularly preferred embodiment of spontaneous infiltration features a matrix metal comprising aluminum, an infiltration enhancer precursor comprising magnesium, and an infiltrating atmosphere comprising nitrogen. In this particularly preferred embodiment aluminum nitride is formed in situ in the developing metal matrix composite body as a by- product of the process. This in-situ formed aluminum nitride manifests itself in at least two forms: first, as discrete discontinuous bodies contacted substantially only by matrix metal; and second, as a layer covering at least a portion of the filler material bodies making up the

permeable mass. Generally speaking the higher the processing temperature, the greater the amount of in situ formed aluminum nitride produced.

Thus, when AIN is formed in-situ, for example, in the luminum/magnesium/nitrogen spontaneous infiltration system, the need for providing one or more ceramic filler materials in the permeable coating layer is greatly reduced, possibly even eliminated. But to reduce or eliminate the filler material in the coating layer, a substitute material termed a"fugitive material"should be used to displace the filler material. The fugitive material is one which does not remain in the form in which it was applied during processing conditions. Common examples of fugitive materials include materials which decompose on heating such as Styrofoam, materials which may combust on heating such as sawdust, and materials which may volatilize on heating such as polymers. The definition also includes metals which may dissolve or be dissolved by the matrix metal, or may react with the matrix metal. When the fugitive material comprises a fugitive metal, the filler material in the coating layer can be eliminated completely, and the resulting MMC layer after spontaneous infiltration will comprise matrix metal and an in-situ formed reinforcement such as aluminum nitride.

To elaborate on this concept in more detail, a fugitive material is applied to the permeable mass (or preform) to a desired thickness. In a preferred embodiment the fugitive material comprises a metal in particulate form. In a particularly preferred embodiment the fugitive metal comprises a metal having a melting point higher than the melting point of the infiltrating matrix metal. For example, if the matrix metal comprises aluminum, the fugitive metal could comprise particulate copper. Under spontaneous infiltration conditions (e. g., in the presence of an infiltration enhancer or infiltration atmosphere/infiltration enhancer precursor combination) the molten matrix metal comprising aluminum infiltrates through the particulate copper layer to form a metal matrix composite surface layer comprising aluminum, copper and aluminum nitride. Using such particulate metals as fugitive materials, MMC surface layers up to about 3 millimeters have been produced.

Further, when the fugitive metal comprises a metal having a chemical composition which is different from that of the matrix metal, the possibility of"metal phase tailoring"exists.

Such metal phase tailoring affords the opportunity of further modifying or engineering the properties of at least the surface layer through control of the extent of alloying or diffusion of the fugitive metal into the matrix metal, or of the extent of reaction of the fugitive metal with the matrix metal (e. g., to form intermetallics). Commonly owned U. S. Patents 5,518,061 and 5,287,911 provide a detailed discussion of metal phase tailoring, particularly as applied to metal matrix composites. The entire disclosures of these two commonly owned U. S. patents are incorporated herein by reference.

In yet another embodiment for forming the present MMC surface layers, the fugitive material need not be applied to a permeable mass or preform. Instead, the fugitive material

may be applied to a surface of a MMC which has been formed already by spontaneous infiltration. Once spontaneous infiltration conditions have been re-established, matrix metal (e. g., from within the MMC) may begin to infiltrate the fugitive material layer. It may be desirable to provide a reservoir of additional matrix metal in contact with the MMC to replenish that from the original MMC lost to infiltration of the fugitive material.

Alternatively, a separate body of matrix metal may be provided in direct contact with the fugitive material layer.

Extending this embodiment further, a fugitive material layer may be applied to a surface of a MMC body not produced by spontaneous infiltration. When spontaneous infiltration conditions are present or created, matrix metal, whether from the original MMC body or from some other source, can infiltrate the fugitive material layer to produce the MMC surface layer. In this embodiment, it may be necessary to supply the source of infiltration enhancer precursor such as magnesium, because such a substance may not have been present to produce the original MMC body. A particularly preferred technique for providing such magnesium is to mix it in particulate form with particulates of the fugitive metal. When there are no ceramic materials present in the fugitive metal layer, the magnesium may not be necessary to achieve infiltration of one molten metal into particulates of another; however, little aluminum nitride can be formed in-situ in the surface layer without an infiltration enhancer or infiltration enhancer precursor.

In still yet another embodiment, the MMC surface layer can be applied to an unreinforced metal body. The matrix metal for the MMC surface layer may be supplied by the unreinforced metal body, or it may be supplied from an additional source contacted to the fugitive material layer. An infiltration enhancer or infiltration enhancer precursor such as magnesium should be present in the system at least at some point to create the spontaneous infiltration conditions necessary to form in-situ aluminum nitride in the MMC surface layer. Spontaneous infiltration conditions are also desirable to achieve infiltration when ceramic materials are present in the fugitive material layer. Absent such conditions, most ceramic materials are difficult to infiltrate without having to resort to pressure or vacuum infiltration assists.

In producing MMC bodies to a desired size and shape, a barrier material typically is applied to the outer surfaces of a preform or a permeable mass to be infiltrated for the purpose of terminating the infiltration process at these outer surfaces. In the absence of such a barrier material layer, it has been observed that the spontaneous infiltration process can extend beyond the original outer surfaces of a permeable mass or preform. Such a phenomenon has been termed"over-infiltration"and was first disclosed in U. S. Patent 4,828,008 to White et al. as an aluminum nitride skin. It is not completely clear how this over-infiltration process occurs. The over-infiltration manifests itself as a surface layer on

the underlying or substrate MMC body. This surface layer consists of aluminum nitride embedded by interconnected matrix metal. The aluminum nitride may range from substantially discontinuous bodies to a substantially interconnected co-matrix component.

Generally speaking, the amount and continuity or connectivity of the aluminum nitride phase increases as the processing temperature increases.

This over-infiltration zone typically is less than about 1 millimeter thick. For many applications a thicker layer would be desirable. Specifically, where the surface layer is to be machined, a thicker layer would provide a greater amount of machining stock.

The"fugitive metal"approach is particularly preferred for producing the present surface engineered metal matrix macrocomposites. Specifically, a permeable layer comprising equal weight fractions of copper and magnesium particulates, generally ranging in size from about 75 microns to about 150 microns for the magnesium component, and less than about 45 microns for the copper, optionally containing a binder material, may be applied to one or more surfaces of a preform, an existing MMC or an unreinforced metal by painting, spray coating, stuccoing, or most any other preforming technique. Upon contact with a molten matrix metal comprising aluminum in a nitrogen atmosphere at a temperature of about 800°C, for example, the molten matrix metal will spontaneously infiltrate the permeable layer and any preform to which the layer may be applied to produce a MMC surface layer. The MMC surface layer compositionally comprises matrix metal and in-situ formed aluminum nitride. The copper and magnesium particulates may dissolve into or react with the matrix metal, e. g., to form intermetallic compounds.

Figures 1-7 present an illustrative but nonexhaustive listing of components for various vehicles. Figures 1-3 and 6 illustrate components which might be found in a typical internal combustion engine. Figures 4,5, and 7 illustrate components which may be present in a hydraulic braking system. Specifically, Figure 1 is a side view of a connecting rod whose function is to act in concert with the crankshaft to convert the reciprocating action of the piston in an internal combustion engine to rotational movement. If the connecting rod were fabricated from, for example, a reinforced aluminum MMC material, surface 13 which contacts the crankshaft and surface 15 which contacts a piston pin which is connected to a piston, might comprise an MMC material comprising aluminum nitride and aluminum.

Figure 2 is a side view of a bearing cap or bearing stiffener whose function it is to support the crankshaft of an internal combustion engine. Surface 23 may be fabricated from MMC material featuring aluminum nitride dispersed in a matrix metal comprising aluminum. Surface 23 is in contact with the bearing portion of the crankshaft which typically comprises a hardened steel. Substrate material 21 may comprise reinforced or unreinforced aluminum. A bore 25 is provided so that a fastener may attach the bearing cap to the engine block.

Figure 3 is a cross-sectional schematic view of a valve lifter. The valve lifter is part of the valve train whose function it is to convert the rotating motion of a camshaft into a reciprocating motion which opens and closes the valves for the fuel mixture and the exhaust gases in an internal combustion engine. Surfaces 33,35, and 37 contact the camshaft, the pushrod and the valve guide, respectively. Each surface may be fabricated from the aluminum nitride reinforced aluminum MMC of the present invention.

Figure 4 is a cross-sectional schematic view of a brake caliper piston. The brake caliper piston fits inside a barrel or chamber within the brake caliper and pushes the brake pad against a brake rotor or brake drum in response to the hydraulic pressure of brake fluid.

Here, the aluminum nitride reinforced aluminum MMC material is shown as a layer 43 adhered to substrate material 41. O-ring 45 provides a positive seal between the piston and the barrel through which the piston travels. Depressions 47 engage with corresponding pins on a brake pad (not shown).

Figure 5 is a cross-sectional schematic view of a clutch master cylinder. The clutch master cylinder, often in cooperation with a clutch slave cylinder, converts the movement of the clutch pedal in the passenger compartment of the vehicle into a hydraulic pressure which operates the clutch mechanism. Here, a pushrod 51 attached to the clutch pedal mechanism (not shown) pushes on piston 53 which moves to the left down bore 55. The bore 55 is defined by an interior wall of master cylinder 57 which may comprise an aluminum nitride reinforced aluminum MMC material as a surface layer 59. Pressurized hydraulic fluid 52 passes through orifice 54 in fitting 56 and exits through tubing 58.

Figure 6 is a cross-sectional schematic view of a liner for the combustion chamber of an internal combustion engine. Substrate material 91 may comprise unreinforced or reinforced metal (e. g., a metal matrix composite material). Surface layer 93 may comprise aluminum nitride reinforced aluminum MMC material. Surface layer 93 defines the cylinder 95 through which the piston (not shown) moves back and forth during the combustion cycle of the engine.

Figure 7 is a cross-sectional schematic view of a brake disc or brake rotor. Braking surfaces 71 may feature an aluminum nitride reinforced aluminum MMC material 73 as a friction surface which contacts the brake pads (not shown). Friction surface 73 is bonded to substrate material 75 which itself may comprise a MMC material. The brake rotor illustrated in this figure is of the vented variety, with the vent being illustrated as 77.

The following Examples further illustrate the present invention.

EXAMPLE 1 This Example demonstrates, among other things, the infiltration of a cup-shaped preform to produce a metal matrix composite brake caliper piston. In particular, the present Example features an overinfiltration zone whereby a metal matrix composite surface layer is formed on the exterior of the caliper piston. The reinforcement phase for this MMC surface layer consists essentially of in-situ formed aluminum nitride. Figure 8 is a cross- sectional schematic view of the setup used to accomplish the infiltration.

A cup shaped preform was fabricated by a wax injection molding process as practiced by a commercial vendor (Certech, Inc., Woodridge, NJ). The composition utilized for injection molding comprised by weight about 1% fumed silica and the balance grade T64 aluminum oxide particulate (-100 mesh, Alcoa Industrial Chemicals Division, Bauxite, Arkansas) having substantially all particles smaller than about 150 microns in size.

The cup shaped preform was then bisque fired in an air atmosphere furnace at a maximum temperature of about 1200°C for about 2 hours. This bisque fired preform retained a mass of about 128 grams.

Next, a sacrificial preform was prepared. Specifically, a ring having approximately the same diameter and height as the cup shaped preform was wax injection molded (Certech, Inc.). The filler material component of the injection molding composition consisted essentially of grade T64 tabular alumina particulate (-100 mesh, Alcoa).

Both the objective and the sacrificial preform were heated in an air atmosphere furnace to a temperature of about 825°C. After maintaining this temperature of about 825°C for about 2 hours, the preforms were cooled to a temperature of about 150°C at which temperature they were held pending further processing.

Following the air bake, a series of barrier coatings were applied to the sacrificial preform 84. Specifically, the exterior circumferential surface of the ring shaped preform was brush coated with two layers of undiluted Dylon CW colloidal graphite (Dylon Industries, Cleveland, OH). The surface to be adjacent to the cup shaped preform 86 was brush coated with one layer of undiluted Dag 154 colloidal graphite (Acheson Colloids Company, Port Huron, MI).

Next, a setup for matrix metal infiltration was assembled. With reference to Figure 8, the floor or base of a shallow steel boat 81 was covered with several sheets of GRAFOILO graphite sheet material 83 (Union Carbide Company, Carbon Products Division, Cleveland, Ohio). A cylindrical body of matrix metal 85a having a mass of about 150 grams was centered on the top graphite foil sheet. A second cylindrical body of matrix metal 85b having a mass of about 90 grams was then placed atop the first body. Both cylindrical ingots of matrix metal had a composition comprising by weight about 10.5% magnesium, and the balance aluminum. The sacrificial ring preform 87 was then placed

around the cylindrical bodies of matrix metal and adhered to the top graphite foil sheet using RigidLock colloidal graphite cement (Polycarbon Corp., Valencia, California). To prevent the formation of a hermetic seal during processing, the sacrificial graphite ring preform featured a vent hole 89 positioned above the highest expected level of molten matrix metal. The cup shaped preform 82 was then placed on top of the sacrificial ring preform 87 and contacting the Dag 154 colloidal graphite coated surface 86. The placement of the cup shaped preform on top of the sacrificial ring preform substantially enclosed the bodies of matrix metal. Finally, a bedding material 88 comprising by weight about 10% grade F69 glass frit (Fusion Ceramics, Carrollton, Ohio) and the balance 90 grit (216 microns) 38 Alundum alumina particulate (Norton-St. Gobain, Worcester, MA) was piled up around the base of the sacrificial ring preform 87 to a maximum height of about 0.5 inch (13 mm) to complete the setup.

The setup comprising the steel boat and its contents were then placed into a controlled atmosphere furnace at substantially ambient temperature. After isolating the heating chamber from the ambient atmosphere, the heating chamber was evacuated and backfilled with commercially pure nitrogen gas. A gas flow rate of about 20 standard liters per minutes (slpm) was then established and maintained throughout the subsequent thermal processing. The furnace chamber and its contents were then heated to a temperature of about 250°C. After maintaining a temperature of about 250°C for about 19 hours, the furnace chamber was then heated to a temperature of about 450°C. After maintaining a temperature of about 450°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 then 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 about 800°C. After maintaining a temperature of about 800°C per hour, the temperature was further reduced to about 600°C. Up to this point, the temperature increases and decreases were carried out at a rate of about 200°C per hour. At a temperature of about 600°C, the furnace chamber was opened and the setup was removed and placed onto a Fiberfrax insulating ceramic sheet (Carborundum Co., Niagara Falls, NY) and allowed to cool in air at its natural cooling rate. Upon cooling to substantially ambient temperature, the setup was disassembled. Specifically, the cup shaped preform was separated from the rest of the setup using low force impacts. This preform had been completely infiltrated by matrix metal thereby producing a cup shaped MMC body. The sacrificial ring preform had also been completely infiltrated by molten matrix metal during processing to produce a sacrificial MMC body. Because the caliper piston preform had not been coated with a barrier material, some matrix metal infiltrated slightly beyond the preform boundaries to produce a metal matrix composite surface layer several mils (several dozen microns) thick.

This MMC surface layer contained about 15 percent by volume of aluminum nitride as the reinforcement phase, the aluminum nitride being formed in-situ as a byproduct of the spontaneous infiltration process.

EXAMPLE 2 This Example demonstrates, among other things, an improved technique for infiltrating a preform with a molten matrix metal to form a metal matrix composite (MMC) brake rotor for a bicycle. Specifically, the present Example demonstrates the formation of a surface layer of desired thickness on the metal matrix composite article having a reduced hardness with respect to the underlying MMC material.

A disc-shaped brake rotor preform 67 having a cross-section substantially as shown in Figure 9 was produced by compression molding. The particulate admixture for forming the preform consisted of 38 Alundum alumina particulate (Norton-St. Gobain, Worcester, MA) having an average particle size of about 25 microns, to which had been added about 2 weight percent of magnesium particulate having substantially all particles smaller than about 45 microns in diameter, plus about 1.5 weight percent of a binder based on polyureasilazane. The binder consisted of CERASETTM SN polyureasilazane inorganic polymer (Lanxide Corporation, Newark, Delaware) to which had been added about 1 % by weight of Lupersol 231 peroxide (Aldrich Chemical Co., Milwaukee, WI).

The mixing was accomplished as follows: The alumina and magnesium particulates were hand mixed in a metal can, then transferred to the bowl or mixing chamber of a Model RV02 Eirich high intensity mixer (Eirich Machines, Inc., Uniontown, PA). The binder solution components were stirred together, then about half of the solution was added to the mixing chamber bowl. After mixing on the fast speed setting for a few minutes, the rest of the binder solution was added. After additional mixing, the mixture was screened through a 25 mesh screen and stored in a sealed container.

A brake rotor preform was formed by compression molding the particulate admixture at a temperature of about 170°C, applying a pressure of about 420 psi (2900 kPa) and maintaining this temperature and pressure for about 20 minutes.

The preform was then bisque fired as follows: The piece was placed flush on a setter tray made from refractory material. The setter tray and its contents were then placed into an air atmosphere furnace at about 20°C. The furnace temperature was then raised at a rate of about 100°C per hour to a temperature of about 300°C. After maintaining a temperature of about 300°C for about 2 hours, the furnace temperature was increased to a temperature of about 425°C at about 100°C per hour. After maintaining a temperature of about 425°C for about 4 hours, the furnace temperature was decreased to about 20°C at a rate of about 200°C per hour. The preform was then removed to a dry box until further

processing. A particulate admixture 60 comprising equal weight fractions of copper particulate (-325 mesh) eand magnesium particulate (-100+200 mesh) were contacted to one of the surfaces of the preform and held in place with a graphite foil ring 101, as shown in the Figure.

With further reference to Figure 9, a graphite boat 61 measuring about 13 inches (330 mm) long by about 9 inches (229 mm) wide by about 3.5 inches (89 mm) in height was lined on its interior surfaces with a single sheet of GRAFOIL graphite foil material 62 (Union Carbide Co., Danbury, CT). A particulate admixture 63 comprising by weight about 5 percent Grade F69 glass frit (Fusion Ceramics, Inc., Carrollton, OH) and the balance 90 grit (216 microns ave. particle size) 38 Alundum alumina particulate (Norton- St. Gobain, Worcester, MA) was poured onto the floor of the graphite foil lined boat 61 to a uniform depth of about 0.5 inch (13 mm). A GRAFOIL graphite foil ramp 64 and platform 65 were fabricated by cutting and folding another single sheet of graphite foil and positioned near one of the interior walls of the graphite foil lined boat. The graphite foil platform measured about 5 inches (127 mm) square by about 0.75 inch (19 mm) in height.

A second particulate admixture 66 comprising by weight about 3 percent magnesium particulate (Hart Metals, Tamaqua, PA) having substantially all particles between about 150 microns and about 300 microns in size and the balance 90 grit (216 microns) 38 Alundum alumina was uniformly distributed over the first particulate admixture 63 and over the graphite foil platform 65 and ramp 64 to a thickness of about 0.25 inch (6 mm). The preform to be infiltrated was then placed on top of this second particulate admixture above the graphite foil platform 65 and an ingot of matrix metal 68 was placed on the second particulate admixture 66 near the wall of the graphite boat opposite that adjacent to the preform. The ingot of matrix metal comprised by weight about 5 percent magnesium, balance aluminum and had a mass of about 460 grams. The opening of the graphite foil lined boat was loosely covered with another sheet of graphite foil 69 to complete the setup.

The setup was then placed into a controlled atmosphere furnace at substantially ambient (e. g., about 20°C) temperature. The furnace atmosphere was first evacuated, then backfilled with commercially pure nitrogen. A continuous flow of nitrogen was established. The furnace and its contents were then heated to a temperature of about 250°C at a rate of about 200°C per hour. After maintaining a temperature of about 250°C for up to about 30 hours, the temperature was increased to about 480°C, again, at a rate of about 200°C per hour. After maintaining a temperature of about 480°C for about 5 hours, the temperature was increased to about 580°C. After maintaining a temperature of about 580°C for about 5 hours, the temperature was further increased to about 800°C at a rate of about 100°C per hour. After maintaining a temperature of about 800°C for about 4 hours, the temperature was decreased to about 700°C at a rate of about 200°C per hour. The setup was

removed from the furnace at a temperature of about 700°C and placed on a graphite slab to continue cooling in air to ambient temperature at the setup's natural cooling rate.

Disassembly of the graphite boat and its contents revealed that the brake disc preform had been fully infiltrated (including the applied coating) to produce a metal matrix composite body.

The infiltrated surface layer was characterized by image analysis and Knoop microhardness. Analysis of 96 frames at a magnification of about 200X yielded the following results--ceramic : 34 volume percent; metal: 60 percent; other: 6 percent. Five Knoop hardness indentations on the MMC surface layer produced the following hardness numbers: maximum 347; minimum 252; average 288. By comparison the average Knoop hardness of the MMC substrate is in the vicinity of 1000.

Thus, the present Example demonstrates that a metal matrix composite body can be formed having an engineered surface. Specifically, the present Example demonstrates that a MMC surface layer can be formed on a MMC substrate body by infiltrating a fugitive metal layer under spontaneous infiltration conditions. Further, an aluminum nitride reinforcement of the MMC surface layer is formed in-situ.

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 minor 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.