Particulate composite materials

Particulate composite materials consist of particles of one or more materials suspended in a matrix of another material. The particles can be either metallic or nonmetallic as can the matrix. The four possible combinations of these constituents are described in the following paragraphs. 

Nonmetallic Particles in Nonmetallic Matrix Composite Materials 

The most common example of a nonmetallic particle system in a nonmetallic matrix, indeed the most common composite material, is concrete. Concrete is particles of sand and gravel (rock particles) that are bonded together with a mixture of cement and water that has chemically reacted and hardened. The strength of the concrete is 

Metal flakes in a suspension are common. For example, aluminum paint is actually aluminum flakes suspended in paint. Upon application, the flakes orient themselves parallel to the surface and give very good coverage. Similarly, silver flakes can be applied to give good electrical conductivity. 

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Particulate composite materials that are composed of particles in a matrix... 

Numerous multiphase composite materials exhibit more than one characteristic of the various classes, fibrous, laminated, or particulate composite materials, just discussed. For example, reinforced concrete is both particulate (because the concrete is composed of gravel in a cement-paste binder) and fibrous (because of the steel reinforcement). 

The mechanics of materials approach to the micromechanics of material stiffnesses is discussed in Section 3.2. There, simple approximations to the engineering constants E., E2, arid orthotropic material are introduced. In Section 3.3, the elasticity approach to the micromechanics of material stiffnesses is addressed. Bounding techniques, exact solutions, the concept of contiguity, and the Halpin-Tsai approximate equations are all examined. Next, the various approaches to prediction of stiffness are compared in Section 3.4 with experimental data for both particulate composite materials and fiber-reinforced composite materials. Parallel to the study of the micromechanics of material stiffnesses is the micromechanics of material strengths which is introduced in Section 3.5. There, mechanics of materials predictions of tensile and compressive strengths are described. 

COMPARISON OF APPROACHES TO STIFFNESS 3.4.1 Particulate Composite Materials... 

Common traditional fillers may require a new generation of fillers. Engineered fillers with tunable properties may be one way of improving the performance of particulate composite materials. In the future, the performance of particulate composites may be strongly dependent on the advanced filler system, which is far superior to the conventional fiUer system. 

Particulate Composites. These composites encompass a wide range of materials. As the word particulate suggests, the reinforcing phase is often spherical or at least has dimensions of similar order ia all directions. Examples are concrete, filled polymers (18), soHd rocket propellants, and metal and ceramic particles ia metal matrices (1). 

Sohd rocket propellants represent a very special case of a particulate composite ia which inorganic propellant particles, about 75% by volume, are bound ia an organic matrix such as polyurethane. An essential requirement is that the composite be uniform to promote a steady burning reaction (1). Further examples of particulate composites are those with metal matrices and iaclude cermets, which consist of ceramic particles ia a metal matrix, and dispersion hardened alloys, ia which the particles may be metal oxides or intermetallic compounds with smaller diameters and lower volume fractions than those ia cermets (1). The general nature of particulate reinforcement is such that the resulting composite material is macroscopicaHy isotropic. 

We now examine the properties of fibrous and particulate composites and foams in a little more detail. With these materials, more than any other, properties can be designed-in the characteristics of the material itself can be engineered. 

For particulate-reinforced composite materials, Paul derived upper and lower bounds on the composite modulus [3-4]. His approximate mechanics of materials solution agrees fairly well with experimental data for tungsten carbide particles in cobalt. 

Particulate composites consist of particles dispersed in a matrix. These particles are divided into two classes, skeletal and flakes. The first one consists of continuous skeletal structures filled with one or more additional materials. Flakes consist generally of flat flakes oriented parallel to each other. These particles may have any... 

Particulate composites consist of the reinforcing materials being dispersed throughout the resin. Unlike fibrous composites, the reinforcing material is more bulky and not fibrous in nature. 

At times the difference between fillers and particulate composites is small. For particulate composites, the emphasis is on the added strength due to the presence of particulates within the resin. Particle board is often used as an example of particulate composites where the particulates are pieces of wood, woodlike, or derived material. Unlike plywood, particle board generally uses flakes or chips of wood rather than layers. 

Highly porous membranes with an inter-connected pore structure were produced using this solvent-casting and particulate-leaching technique (Fig. 4a, b). The porosity of porous PLLA membranes could be controlled by varying the amount of salt used to construct the composite material (Fig. 5a). 

Particulate Composites. Particulate composites encompass a wide range of materials, from cement reinforced with rock aggregates (concrete) to mixtures of ceramic particles in metals, called cermets. In all cases, however, the particulate composite consists of a reinforcement that has similar dimensions in all directions (roughly spherical), and all phases in the composite bear a proportion of an applied load. The percentage of particulates in this class of composites range from a few percent to 70%. 

One important class of particulate composites is dispersion-hardened alloys. These composites consist of a hard particle constituent in a softer metal matrix. The particle constituent seldom exceeds 3% by volume, and the particles are very small, below micrometer sizes. The characteristics of the particles largely control the property of the alloy, and a spacing of 0.2-0.3 tim between particles usually helps optimize properties. As particle size increases, less material is required to achieve the desired interparticle spacing. Refractory oxide particles are often used, although intermetallics such as AlFes also find use. Dispersion-hardened composites are formed in several ways, including surface oxidation of ultrafine metal powders, resulting in trapped metal oxide particles within the metal matrix. Metals of commercial interest for dispersion-hardened alloys include aluminum, nickel, and tungsten. 

Finally, metal- and resin-bonded composites are also classified as particulate composites. Metal-bonded composites included structural parts, electrical contact materials, metal-cutting tools, and magnet materials and are formed by incorporating metallic or ceramic particulates such as WC, TiC, W, or Mo in metal matrixes through traditional powder metallurgical or casting techniques. Resin-bonded composites are composed of particulate fillers such as silica flour, wood flour, mica, or glass spheres in phenol-formaldehyde (Bakelite), epoxy, polyester, or thermoplastic matrixes. 

Over the last decade advances have occurred very rapidly in the area identified as composite materials. In general, a composite material is the combination of any two or more materials, one of which has superior mechanical properties but is in a difficult to use form (e.g. fiber, powder, etc.). The superior component is usually the reinforcement while the other component serves as the matrix in which the reinforcement is dispersed. The resultant composite is a material whose properties are near those of the reinforcement element but in a form which can be easily handled and can easily function as a structural element. Included in this definition are all of the reinforced materials including particulate, fiber, flake and sheet reinforcements. Adhesive joints for, example, would be a planar or two dimensional composite 1). 

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