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Continuous Multifilament

Whisker/Short Fiber
Continuous Monofilament

Fig. 9.1. Metal Matrix Composite Reinforcements1

fibers which are usually random but can contain some degree of alignment; or long aligned multifilament or monofilament fibers. Particulate reinforced composites (Fig. 9.2), primarily silicon carbide (SiC) or alumina (Al2O3) ceramic particles in an aluminum matrix, are known as discontinuously reinforced aluminum (DRA). They exhibit high stiffness, low density, high hardness, adequate toughness at volume percentages less than 25%, and relatively low cost. Normal volume percentages are 15-25% with SiCp particle diameters of 3-30^m. DRA is usually manufactured by melt incorporation during casting, or by powder blending and consolidation. MMCs can also be reinforced with single crystal SiCw whiskers (Fig. 9.3), made by either vapor deposition processes or from rice hulls. They have better mechanical properties than particulate reinforced MMCs, but whiskers are more expensive than particles; it is more difficult to obtain a uniform dispersion in the matrix; and there are health hazard concerns for whiskers. Short fibers are also used to reinforce MMCs; for example, Saf-fil short alumina fibers are used in aluminum matrices. Short fiber reinforced MMCs can also be produced by melt infiltration, squeeze casting, or by powder blending/consolidation. Typical fiber diameters are a few micrometers to several hundred micrometers in length but are broken-up during processing so that their aspect ratios range from 3 to about 100.

Large stiff boron and silicon carbide monofilament fibers (Fig. 9.4) have been evaluated for high value aerospace applications. Early work was conducted in the 1970s with boron, or Borsic (a silicon carbide-coated boron fiber), reinforced aluminum for aircraft/spacecraft applications. Later work in the 1990s concentrated on SiC monofilaments in titanium (Fig. 9.5) for the National

Fig. 9.3. Silicon Carbide Whiskers
Fig. 9.4. Silicon Carbide Monofilament

Aerospace Plane. Potential applications are extremely high temperature airframes and engine components. Multifilament fibers, such as carbon and the ceramic fibers Nextel (alumina based) and Nicalon (silicon carbide), have also been used in aluminum and magnesium matrices; however, the smaller and more numerous multifilament tows are difficult to impregnate using solid state processing techniques, such as diffusion bonding, because of their small size and tightness of their tow construction. In addition, carbon fiber readily reacts

Fig. 9.5. Silicon Carbide Monofilament/Titanium Matrix Composite

with aluminum and magnesium during processing and can cause these matrices to galvanically corrode during service.

Most of the commercial work on MMCs has focused on aluminum as the matrix metal. Aluminum's combination of lightweight, environmental resistance, and useful mechanical properties are attractive. The melting point is high enough to meet many applications, yet low enough to allow reasonable processing temperatures. Also, aluminum can accommodate a variety of reinforcing agents. Although much of the early work on aluminum MMCs concentrated on continuous fibers, most of the present work is focused on discontinuously reinforced (particulate) aluminum MMCs, because of their greater ease of manufacture, lower production costs, and relatively isotropic properties. The most common reinforcement materials in discontinuously reinforced aluminum composites are SiC and Al2O3, although silicon nitride (Si3N4), TiB2, graphite, and others have also been used in some specialized applications. For example, graphite/aluminum MMCs have been developed for tribological applications due to their excellent anti-friction properties, wear resistance, and anti-seizure characteristics. Typical fiber volumes of discontinuous DRAs are usually limited to 15-25%, because higher volumes result in low ductility and fracture toughness.

The primary MMC fabrication processes are often classified as either liquid phase or solid state processes. Liquid phase processing is generally considerably less expensive than solid state processing. Characteristics of liquid phase processed discontinuous MMCs include low cost reinforcements, such as silicon carbide particles, low temperature melting matrices such as aluminum and magnesium, and near net shaped parts. Liquid phase processing results in intimate interfacial contact and strong reinforcement-to-matrix bonds, but also can result in the formation of brittle interfacial layers as a result of interactions with the high temperature liquid matrix. Liquid phase processes include various casting processes, liquid metal infiltration, and spray deposition. However, since continuous aligned fiber reinforcement is normally not used in liquid state processes, the strengths and stiffness are lower.

Solid state processes, in which no liquid phase is present, are usually associated with some type of diffusion bonding to produce final consolidation, whether the matrix is in a thin sheet or powder form. Although the processing temperatures are lower for solid state diffusion bonding, they are often still high enough to cause significant reinforcement degradation. In addition, the pressures are almost always higher for the solid state processes. The choice of a fabrication process for any MMC is dictated by many factors, the most important being: preservation of reinforcement strength; preservation of reinforcement spacing and orientation; promotion of wetting and bonding between the matrix and reinforcement; and minimization of reinforcement damage, primarily due to chemical reactions between the matrix and reinforcement.

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