BACKGROUND OF THE INVENTION Fiber reinforced composite materials comprise fibrous or filamentary material embedded in a matrix. The fiber is the load bearing component and the matrix dissipates loads to the fibers. maintains fiber orientation, and protects the fiber from damaging environmental conditions. The externally applied load is transferred to the fibers by the matrix via the fiber-matrix interface. If the interface is weak and can easily be fractured. effective load transfer cannot be achieved. and the mechanical properties of the composite are impaired. Easv propagation of the interfacial crack can lead to easv fiber pull-out, premature failure. and low overall fracture toughness of the composite. On the other hand. a stroma. fracture resistant interface can assure that the composite is able to bear load even when some fibers are broken. The load will be transferred through the intact portions of the interface to the damaged as well as undamaed fibers.

Chemical fiber sizing has been applied to improve the strength of fiber-matrix interface in composites. The size creates a strong chemical bond between the fiber and matrix. However, the high strength often leads to embrittlement of the interface. In such composites, the propagation of the interfacial cracks is still easy, and the overall fracture toughness of the composite is low. The use of fiber sizing to improve the bonding between fiber and matrix is discussed, for example, in U. S. Patent Nos. 4,364,993 and 4,990,549.

Elastomeric coating of fibers has been applied to reduce stress concentrations at the fiber-matrix interface and to improve interfacial fracture toughness. However, such a coating impairs stress transfer between the matrix and fiber. The thick coating layer leads to reduction of fiber volume fraction and an associated reduction of properties of the composite.

Elastomer coated fibers are taught, for example, in U. S. Patent Nos. 3,943,090,4,737,527, and 5,080.968.

Typical reinforcing fibers have a smooth surface. One way to improve interfacial fracture toughness in composites would be to roughen the fiber surface by an appropriate treatment such as mechanical roughening, chemical etching, and so forth. However, such a roughening would introduce defects on the fiber surface thus reducing fiber strength and other mechanical properties. Such fiber surface treatment is taught, for example, in U. S.

Patent No. 4,664,936.

Whiskered fibers are also known in the art for increasing fiber matrix bonding. For example, U. S. Patent No. 5,187,021 teaches growing whiskers on fibers for use in preparing composites. The whiskers are taught to increase the strength of the fiber matrix interface, the whiskers are also taught to maintain fiber separation and provide uniform fiber distribution. However, the size of the whiskers is generally on the order of the fiber diameter and larger, and the disclosed fiber separation causes a reduction in fiber volume fraction and an associated reduction of properties of the composite.

It would, therefore, be desirable to provide improved composites with better debonding resistance and interfacial fracture toughness without impairing the load transfer between the matrix and fiber, without reducing the fiber strength and other properties, and without reducing the fiber volume fraction and associated composite properties.

SUMMARY OF THE INVENTION The present invention is directed to improved fiber-reinforced composite products and methods formanufacturing composite materials. Briefly, in accordance with the present invention, a method and product are disclosed for providing improved composite systems having increased interfacial fracture toughness. The present invention employs small particles to reinforce and/or toughen the interface between the matrix and fibers. The small reinforcing particles used in accordance with the present invention are smaller than the diameter of core reinforcing fibers.

In one aspect, the present invention provides a composite material comprising a matrix material, a fibrous material embedded within the matrix material, and particles located at or near the fiber-matrix interface, wherein the particles are not uniformly distributed throughout the matrix, but rather are concentrated at or near the fiber-matrix interface.

Preferably, the particles are small, i. e., having a size smaller than the diameter of the fibrous material. In some embodiments, the small particles are attached to the fibrous material. In some embodiments, the particles are located at or near the fiber-matrix interface, but are unattached to the fibrous material. In still other embodiments, some particles are attached to the fibrous material and some particles are unattached to the fibrous material.

In another aspect, the present invention provides a fiber for use in a composite material comprising a core fiber and small particles attached to the core fiber, wherein the small particles have a size smaller than the diameter of the fibrous material.

In still another aspect, the present invention provides a method of fabricating a composite material comprising the steps of attaching small particles to a fibrous material, wherein the small particles have a size smaller than the diameter of said fibrous material, and consolidating the fibrous material into a matrix.

In some embodiments, the small particles are distributed uniformly over the entire fiber-matrix interface. In other embodiments, the small particles may be distributed over one or more individual parts of a fiber-matrix interface, such as areas likely to undergo stress in a given application.

BRIEF DESCRIPTION OF THE DRAWINGS The detailed description of the invention may be best understood when read in reference to the accompanying drawings wherein: FIG. 1 illustrates the construction of a reinforcing fiber having small particles attached thereto in accordance with the present invention; FIG. 2 illustrates the path of interfacial crack propagation in fiber reinforced composites according to the present invention; FIGS. 3 and 4 illustrate possible damage mechanisms in fiber reinforced composites according to the present invention; and FIG. 5 illustrates the construction of a fractal fiber-matrix interface in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION Here follows a description of various embodiments of the invention. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed.

In FIG. 1, there is shown a fibrous material 10 having small reinforcing particles 12 thereon. The particles create multiple obstacles for the interfacial (debonding) crack propagation and will cause the crack to deflect. This will make the crack path more tortuous and the resulting fracture surfaces rougher. As a result, the energy required for the interfacial crack propagation (interfacial fracture toughness) will be increased. The key in this fiber- matrix reinforcement concept is the small size of particles used for the modification. The reinforcement effect is achieved without reduction of the core fiber strength and other mechanical properties, as the reinforcing particles do not create stress concentrations in the core fiber. This effect is also achieved without substantial reduction of the core fiber volume fraction in the composite and the associated reduction of composite properties. FIG. 2 shows the path 14 of interfacial crack propagation between fiber 10 and matrix material 20 as it is deflected around reinforcing particles 12. In addition to the increase of interfacial fracture toughness, increased friction between the rougher fracture surfaces will cause increased

energy consumption during fiber pull-out, subsequent to debonding, thus further increasing the overall fracture toughness of the composite.

FIGS. 3 and 4 illustrate potential damage mechanisms of composite materials according to the present invention. FIG. 3 illustrates a potential damage mechanism of a composite material employing small particle reinforcement at the fiber matrix interface in accordance with the present invention, wherein multiple discrete microcracks and/or nanocracks 16 develop before main crack propagation, leading to increased energy required for interfacial crack propagation as the fractures develop, and thus, increased toughness of the interface. FIG. 4 illustrates another potential damage mechanism of a composite material employing small particle reinforcement at the fiber matrix interface in accordance with the present invention, wherein breakup of particles in the path of crack propagation increases the energy consumption by the material as it fractures and thus, increases interfacial toughness.

The present invention is not limited to the use of any particular core reinforcing fiber.

As the core fiber 10, there may be used any suitable fiber or filamentary material. Such reinforcement fibers are generally known in the art and include, but are not limited to, alumina, aluminosilicate, aramid (such as Kevlar Twaron (D, or other aramid fibers), black glass ceramic, boron and boron containing fibers (e. g., boron on titania, boron on tungsten, and so forth), boron carbide, boron nitride, carbonaceous fibers, such as carbon or graphite fibers, ceramic fibers, glass fibers (such as A-glass, AR-glass, C-glass, D-glass, E-glass, R- glass, S-glass, S 1-glass, S2-glass, and other suitable types of glass), high melting polyolefins (e. g., Spectra OO fibers), high strength polyethylene, liquid crystalline polymers, metal fibers, metal coated filaments, such as nickel, silver, or copper coated graphite fiber or filament, and the like, nylon, paraphenylene terephthalamide, polyetheretherketone (PEEK), polyetherketone (PEK), polyacrylonitrile, polyamide, polyarylate fibers, polybenzimidazole (PBI), polybenzothiazole (PBT), polybenzoxazole (PBO), polybenzthiazole (PBT), polyester, polyethylene, polyethylene 2, 6 naftalene dicarboxylate (PEN), polyethylene phthalate, polyethylene terephthalate, polyvinyl halides, such as polyvinyl chloride, other specialty polymers, quartz, rayon, silica, silicon carbide, silicon nitride, silicon carbonitride, silicon oxycarbonitride, titania, titanium boride, titanium carbide, zirconia toughened alumina, zirconium oxide, and so forth. Mixtures of any such suitable fibers may also be employed.

The types of fiber most commonly used in advanced composites are carbon/graphite, aramid, and glass fibers.

The core fiber may be discontinuous or continuous fiber, or combinations thereof, and may be present in any of the various conventional forms. Discontinuous fiber includes, for example, milled fibers, chopped fibers, whiskers, acicular particles, etc. Continuous fibers include short discontinuous and long discontinuous fibers. The fiber may be present in various forms, including but not limited to, monofilament fiber, multifilament yam, woven fabric, stitched fabrics, braids, unidirectional tapes and fabrics, non-woven fabric, roving, chopped strand mat, tow, random mat, woven roving mat, and so forth.

The fibers may be sized or unsized. When the fibers are sized, the sizing may be any conventional sizing agent and may be applied according to any conventional sizing process.

The sizing may be applied to the fibers before or after the particles are attached to the fiber.

In one embodiment, the sizing is applied to improve the adhesion of the of the particles to the fiber, and in such case, is preferably applied before the particles are attached. In one embodiment, the particles are coated/sized prior to application to the fiber, with the fiber itself being either sized or unsized. In still other embodiments, the particles are uncoated prior to application to the fiber, with the fiber itself being either sized or unsized.

The present invention is not limited to any particular matrix material. Any suitable matrix material may be employed. In one embodiment, the matrix material is a polymer, such as thermosets and thermoplastics. Polymer matrix composite manufacturing processes generally involve the combining of a resin, a curing agent, and some type of reinforcing fiber. Typically, heat and pressure are used to shape and cure the mixture into a finished part. Thermosets require a curing step to produce a cured or finished part. Once cured, the part cannot be changed or reformed. Thermoplastics currently represent a relatively small part of the PMC industry. They are typically supplied as nonreactive solids (no chemical reaction occurs during processing) and require only heat and pressure to form the finished part. Unlike the thermosets, the thermoplastics can usually be reheated and reformed into another shape, if desired.

The matrix material may be, for example, thermosetting materials based on epoxy resins, biscitraconicimide (B CI), bismaleimides (BMI), bismaleimide/triazine/epoxy resins,

cyanate esters, cyanate resins, furanic resins, phenolic resins, urea-formaldehyde resins, melamine-formaldehyde resins, phthalocyanine resins, polyacrylates, polybenzoxazole resins, polybutylene, polyester resins, polyimides, including high temperature polyimides such as PMR, PMR-15, and DMBZ polyimides), acetylene terminated polyimide resins, polvurethanes, silicones, tetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymers, triazines, alkyds, unsaturated polyester (UP) resins, vinyl ester resins, vinyl esters, xylene resins, specialty polymers, and so forth.

The matrix material may also be thermoplastic materials based on acrylonitrile butadiene styrene (ABS) copolymers, aromatic polycarbonates, aromatic polyesters, carboxymethylcellulose, ethyl cellulose, ethylene vinyl acetate copolymers, polyacetals, polyacetates, polyacrylonitrile and other nitrile resins, polyacrylonitrile-vinyl chloride copolymer, polyamides, aromatic polyamides (aramids), polyamide-imide, polyarylates, polyarylene oxides, polyarylene sulfides, polyarylsulfones, polybenzimidazole, polybutylene terephthalate, polycarbonates, polyester imides, polyether sulfones, polyetherimides, polyetherketones, polyetheretherketones, polyethylene terephthalate, polyimides, polymethacrylate, polyolefins (polyethylene, polypropylene), polyallomers, polyoxadiazole, polyparaxylene, polyphenylene oxides (PPO), modified PPOs, polystyrene, polysulfone, polytetrafluoroethylene, polyvinyl acetate, polyvinyl alcohol, polyvinyl halides such as polyvinyl chloride, polyvinyl chloride-vinyl acetate copolymer, polyvinyl pyrrolidone, polyvinylidene chloride, specialty polymers, and so forth.

Also, other elastomeric or rubber materials may be employed as the matrix material, including, but not limited to, butadiene-acrylonitrile copolymers, ethylene butadiene block copolymer, ethylene-propylene base copolymer, natural rubber, polychloroprene rubber, polvisoprene-isobutylene coploymers, silicone rubber, styrene-acrylonitrile copolymers, styrene-butadiene copolymers, styrene-maleic anhydride copolymers, and so forth.

In one embodiment, the matrix material may be a toughened resin material. Methods of improving the toughness of resins, such as the thermoset resins listed above, wherein elastomeric or thermoplastic particles are incorporated into the resin matrix system, are well known in the art and are attractive for use with the small particle fiber-matrix reinforcement according to the present invention, such where it is desired to produce a composite material

with increased toughness, such as toughness against impact induced damage. In one embodiment, the matrix material may be a thermosetting resin toughened with elastomeric or thermoplastic particles. In another embodiment, the matrix material may be an epoxy, cyanate, or bismaleimide resin toughened with elastomeric or thermoplastic particles. In yet another embodiment, the matrix material may be an epoxy resin toughened with elastomeric or thermoplastic particles.

Other suitable matrix materials operable to embody the present invention include any metal containing material, including metals, metal alloys, intermetallic compounds, with exemplary metals including aluminum and titanium. The matrix material may also be a ceramic, such as oxides, borides, carbides, nitrides. Likewise, glass matrixes, carbon matrixes, and the like, may also be employed.

Any other conventional additives may optionally be present in the matrix system which include, but are not limited to, flame retardants, catalysts, promoters, dopants, or hardeners, such as curing or cross-linking catalysts or catalysts to promote the growth of conductive materials, fillers such as quartz powder to reduce thermal expansion or other relatively inert materials which may be added to reduce cost, or other fillers or extenders used to modify mechanical properties, serve as a base for color effects, or to improve surface texture, extenders, or to dilute or extend high cost resins without significant lessening of properties, inhibitors, thixotropic agents, adhesion promoters, any other additive capable of exerting a positive effect on the substrate and during processing such as finishing agents to improve matrix to fiber coupling or a noble metal or noble metal compound to make the material catalytic for electroless deposition of copper, and so forth. Such additives and their use are generally known to those skilled in the art.

The fiber-matrix reinforcement particles 12 may be of any shape or geometrical configration, including, but not limited to, round, angular, fibrous, flaky, etc., or any combinations thereof. The small particles may be of any material, hard or soft, including, but not limited to, ceramic, carbon (including graphite and diamond), metal (including metals, metal alloys, and intermetallic compounds), mineral, glass, polymer (including thermoplastic and thermoset polymer materials), rubber, etc., and any combinations thereof.

Small particles 12 to be used at the fiber-matrix interface can be produced by any suitable method, including but not limited to: (1) Breaking-down methods of size reduction and comminution, e. g., blending, crushing, grinding, homogenizing, milling, disintegration under the action of radiation, thermal decomposition, combustion, disintegration due to electrical charging, physico-chemical and chemical reactions, etc.; and (2) Building-up methods of particle growth, e. g., any physical or chemical method of particle growth from gaseous, liquid, or solid phase or plasma, chemical vapor deposition, chemical spray deposition, photo-assisted chemical vapor deposition, plasma-assisted chemical vapor deposition, laser chemical vapor deposition, physical vapor deposition, sputtering, ion planting, physical and chemical condensation from vapor, particle growth by assisted crystallization, nucleation solidification, pyrolysis, precipitation, hydrolysis, laser synthesis, polymerization, etc. Examples of building-up reaction techniques which have produced fine particles include: (1) gas phase techniques, e. g., thermal decomposition, evaporation- condensation, gas evaporation, hydrogen reduction, nitrogen and hybrid plasma, electric arc, thermal hydrolysis, flame reaction methods; (2) solution techniques, e. g., precipitation, hydrolysis, solvent evaporation; and (3) solid-state reactions, e. g., thermal decomposition.

Alternatively, small particles of various shapes can be prepared by methods based on the coalescence of even smaller particles, including: coagulation in aerosols, e. g., Brownian, laminar, turbulent, acoustic, electrostatic, coagulation due to velocity difference under gravity or centrifugal force, etc.; coagulation in solutions, e. g., coagulation with electrolytes, coagulation with polymers and surfactants, etc.; agglomeration; aggregation; flocculation; sintering ; fusion; solid and liquid bridging ; interlocking ; and so forth.

Small particles can also be prepared from droplets obtained by any method of atomization of a liquid (mechanical, electric, magnetic, ultrasonic, etc) with the consequent conversion of droplets into particles by any suitable method, e. g., solvent evaporation, solidification by cooling, crystallization, hydrolysis, etc. Examples of droplet-based techniques which have produced fine particles include: atomization into flame, spray drying, condensation with subsequent solidification, atomized hydrolysis. etc.

Small particles can be separated and/or collected by any suitable method, e. g., sieving, filtering, sedimentation, electrostatic, magnetic, magnetohydrostatic, or dielectric separation, electrophoresis, wet scrubbing, segregation, stratification, etc.

Particles can be placed on fiber surface prior to embedding the fiber into the matrix material. Previously prepared particles can be placed on fiber surface by any suitable method, e. g., adsorption from a suspension, deposition from gas or liquid fluidized bed or slurry, electrodeposition from suspension (particles can be electrified by contact electrification, electrification by impaction, electrification through breakage), etc. Particles can also be grown directly on the fiber surface by any method of particle growth on a substrate (see the methods of particle preparation by growth described above), e. g., chemical or physical vapor deposition, surface polymerization, surface precipitation from solutions, etc. Particles can also be formed from droplets deposited directly on the fiber surface by any method of liquid atomization with subsequent solidification (see the methods of particle preparation from droplets described above), e. g., spray congealing, vacuum evaporation, electrostatic engulfing, spray drying, mechanochemical deposition (high-speed-impact deposition), etc.

Alternatively, small particles can be placed at the fiber-matrix interface from a suspension in matrix or grown from a solution in matrix by methods of interfacial precipitation, interfacial deposition, interfacial polymerization, etc. These methods are based on the fact that in many suspensions and solutions, the concentration of a component is higher in a thin interfacial layer near the surface.

Small particles can be attached to the fiber surface by any suitable force or combination of forces, including but not limited to forces of : (1) Physical adsorption: Van der Waals, electrostatic, magnetic, liquid bridge, capillary forces, forces of dispersion or London attraction, polarizability attraction, dipole-dipole interactions, magnetic dipole interactions, hydrogen bonding (sometimes called chemisorption), etc. The physical adsorption can be assisted by mechanochemical activation of particles during size reduction, charge pick-up during high-temperature growth in flame or plasma or during electrostatic precipitation, etc. The forces of physical attraction

dramatically increase with the decrease of particle size [Handbook of Powder Science and Technology, M. E. Fayed, L. Otten, Eds., Van Nostrand, 1984, p. 232].

(2) Chemical adsorption: forces of ionic bonding, metallic bonding, covalent bonding, interfacial polymerization, etc.

(3) Cohesive and adhesive forces in solid bridges. The solid bridges between the fiber and particles can form during any process of particle growth on fiber surface, e. g., solidification and crystallization from melts, crystallization of dissolved substances, precipitation from solutions, chemical and physical vapor deposition, etc. The bridges can also form as a result of various treatments of the fiber with small particles, e. g., resolidification or recrystallization as a result of thermal treatment, sintering, brazing, welding (cold, ultrasonic, explosion, diffusion welding and bonding), fusion, etc. The third phase bridges can be formed by hardening binders and particle and/or fiber coatings, soldering, etc. The bridges can also form as a result of surface diffusion, volume diffusion, etc.

(4) Mechanical interlocking: small fibers, platelets, or bulky particles can interlock or fold around the fiber and/or each other.

(5) Alternatively, small particles may produce positive effect on interfacial fracture toughness without being intimately attached to the fiber surface. This will happen, for instance, when interfacial cracking takes place not at the interface (for example, due to a very strong adhesion between the fiber and matrix), but in a thin layer of matrix near the interface (this type of fracture will still be classified as an interfacial fracture). Small particles dispersed in this thin layer will deflect the crack and produce benefits similar to the benefits described above for the interfacial crack.

The mechanisms and strength of attachment of small particles to the fiber can be controlled by particle and/or fiber surface treatment, coating, microencapsulation, any various surface modifications.

The fiber surface can be modified by chemical treatment, mechanical treatment, thermal treatment, thermochemical treatment, coating, etc. before or after small particle modification. For example, chemical sizing to improve fiber-matrix adhesion can be performed on a virgin fiber or after small particle were deposited/grown on fiber surface.

The small particles at the interface in accordance with the present invention can be oriented or disoriented, arranged randomly or in patterns, chains, etc. The small particles can be coated, sized, or otherwise treated both separately, before the deposition, or on the fiber.

Because the small particle reinforcement according to this teaching does not substantially decrease the volume fraction of the core fiber, the small particles may advantageously be placed at the entire fiber-matrix interface. However, it is not necessary that all of the fiber be coated with the small particle reinforcement of the present invention.

For example, in some embodiments, only a portion of the fibers in the composite are coated with the small particles according to the present invention. Likewise, in some embodiments, the small particles may be distributed over one or more individual parts of a fiber-matrix interface, such as areas likely to be stress-prone in given application.

The reinforcement fiber 10 content by volume in the matrix will usually be from about 5 percent to about 90 percent, typically from about 40 percent to about 70 percent, and may be varied appropriately depending on the particular purpose for which the composite is to be used. It will be recognized that given the small particle size of the reinforcing particles, no substantial decrease in fiber volume fraction will result due to the presence of the small reinforcing particles in accordance with this teaching.

As stated above, small size reinforcement particles are needed to implement the present invention. As used herein, the term"small"refers to the size of the particle relative to the transverse or diameter dimension of the reinforcement fibers. Thus, the size of the reinforcing particles depends on the diameter of the reinforcement fibers. As used herein, the term"size"in reference to the reinforcing particles, unless it is otherwise clear from the context, generally refers to a diameter or traverse dimension of the particle, or, where the particle has a geometrical configuration wherein a longitudinal dimension thereof exceeds a transverse or diameter dimension thereof, to the longitudinal dimension thereof. The small particles should be sufficiently small so as to provide fiber-matrix interface reinforcement without substantial reduction of core fiber volume fraction and associated reduction in properties. In an embodiment, the small particles according to the present invention generally have a size of up to about one-half of the diameter or transverse dimension of the core fiber. In an embodiment, the small particles according to the present invention

generally have a size of up to about one-third, or less, of the diameter or transverse dimension of the core fiber. In an embodiment, the small particles according to the present invention generally have a size of up to approximately one-fourth, or less, of the diameter or transverse dimension of the core fiber. In an embodiment, the small particles according to the present invention generally have a size of up to approximately one-fifth, or less, of the diameter or transverse dimension of the core fiber. In an embodiment, the small particles according to the present invention generally have a size of up to approximately one-sixth, or less, of the diameter or transverse dimension of the core fiber. In an embodiment, there is a difference of about one order of magnitude or greater between the size of the reinforcing particle and the core fiber diameter or transverse dimension. As an example. typical reinforcing fibers in composites are about 5-10 micrometers in diameter. In such cases, submicron (nanoscopic) particles may be especially effective. It will be recognized that the small particles need not be uniform in size and small particles having different sizes or a range of sized may be used.

Referring now to FIG. 5, there is shown an embodiment wherein reinforcement particles of at least two different sizes are employed to provide a fractal fiber-matrix interface. In the illustration of FIG. 5, there appears core fiber 10 treated with small particles 22,24, and 26, of decreasing size. Although three different size particles are shown in FIG.

5 for illustrative purposes, the number of different sizes of particles may be 2 or more. In an alternative embodiment, not shown, it is not necessary to use generally discretely sized particles, and particles having a generally continuous size distribution range may be used as well. The differently sized particles 22,24, and 26 may be contacted with the core fiber sequentially in the order of decreasing particle size, beginning with the largest particle size, or. alternatively, may be applied simultaneously. The plurality of particle sizes creates a fractal fiber-matrix interface. The propagation of an interfacial crack will be deflected along a path 14 that is likewise fractal in nature and thus more tortuous. Also, after debonding, the resulting fracture surfaces will be rougher. As a result, the energy required for the interfacial crack propagation is increased, as is energy consumption during fiber pull-out subsequent to debonding. In an embodiment, all or the differently sized particles are smaller than the diameter of the core fiber. In another embodiment, some of the reinforcing particles may be

larger than the diameter of the core fiber, so long as at least one of the differently sized particles is smaller than the diameter of the reinforcing fiber.

The fiber modified by small particles in accordance with the present invention may be employed to produce composite materials at the fiber-matrix interface by any conventional composite fabrication process wherein a fibrous material is embedded or consolidated in a matrix material. Such process include, but are not limited to, composite fabrication using prepregs, filament winding processes, including wet filament winding processes, processes involving the use of preforms, resin transfer molding processes, including reaction resin transfer molding processes, or other techniques as may be employed in preparing polymer matrix. metal matrix, ceramic matrix, glass matrix, and carbon matrix composites.

Having described the invention of fiber-matrix reinforcement using small particles by way of reference to particular materials and techniques, many additional variations and modifications will now become apparent to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims. For example, it will be recognized that the specific matrix materials, core fiber materials, and small particle materials disclosed herein are provided for exemplary, illustrative, and explanatory purposes only. Any such listings of specific materials are not intended to be exhaustive and are not in any way limiting of the present invention as defined in the appended claims. Also, it will be noted that specific methods and techniques of forming composite materials and parts have been discussed as being readily adaptable to employ the novel concept of small particle reinforcement of the fiber-matrix interface. It will be recognized that any specific fabrication methods and techniques mentioned are exemplary only provided to illustrate and explain the principles of the invention. Such methods and techniques specifically mentioned are not intended in any way to provide a comprehensive or exhaustive listing and are not in any way limiting of the invention as claimed. It is the intention of the appended claims to encompass and include such changes. Accordingly, the scope of the invention should be determined solely bv the appended claims and their legal equivalents.