A metal matrix composite (MMC) is a type of composite material with at least two constituent parts, one being a metal. The other material may be a different metal or another material, such as a ceramic or organic compound. When at least three materials are present, it is called a hybrid composite. An MMC is complementary to a cermet.

Composition

Each part of the material is either the matrix or a reinforcement. The reinforcement surface can be coated to prevent a chemical reaction with the matrix.

Matrix

The matrix is the "frame" into which the reinforcement is embedded, and is completely continuous. This means that there is a path through the matrix to any point in the material, unlike two materials sandwiched together. The matrix is usually a lighter metal such as aluminum, magnesium, or titanium, and provides a compliant support for the reinforcement.

Reinforcement

The reinforcement material is embedded into the matrix. The reinforcement does not always serve a purely structural task (reinforcing the compound), but is also used to change physical properties such as wear resistance, friction coefficient, or thermal conductivity. The reinforcement can be either continuous, or discontinuous. Discontinuous MMCs are isotropic, and can be worked with standard metalworking techniques.

Continuous reinforcement uses monofilament wires or fibers such as carbon fiber or silicon carbide. Because the fibers are embedded into the matrix in a certain direction, the result is an anisotropic structure in which the alignment of the material affects its strength. One of the first MMCs used boron filament as reinforcement. Discontinuous reinforcement uses "whiskers", short fibers, or particles.

Manufacturing and forming methods

MMC manufacturing can be broken into three types: solid, liquid, and vapor.

Solid

• Powder blending and consolidation: Powdered metal and dicontinuous reinforcement are mixed and then bonded through a process of compaction, air extraction, and heating (possibly via hot isostatic pressing (HIP) or extruding).
• Foil diffusion bonding: Layers of metal foil are sandwiched with long fibers, and then melted through to form a matrix.

Liquid

• Stir casting: Discontinuous reinforcement is stirred into molten metal, which is allowed to solidify.
• Squeeze casting: Molten metal is injected into a form with fibers preplaced inside it.
• Spray deposition: Molten metal is sprayed onto a continuous fiber substrate.
• Reactive processing: A chemical reaction occurs, with one of the reactants forming the matrix and the other the reinforcement.

Vapor

• Physical Vapor Deposition: The fiber is passed through a thick cloud of vaporized metal, coating it. [1]

Applications

• Carbide drills are often made from a soft cobalt matrix with tough tungsten carbide particles inside.
• Some tank armors may be made from metal matrix composites, probably steel reinforced with boron nitride. Boron nitride is a good reinforcement for steel because it is very stiff and it does not dissolve in molten steel.
• Some automotive disc brakes use MMCs. Early Lotus Elise models used Aluminum MMC rotors, but they have less than optimal heat properties and Lotus has since switched back to cast-iron, Modern high-performance sport cars, such as those built by Porsche, use rotors made of carbon fiber within a silicon carbide matrix because of its high specific heat and thermal conductivity. 3M sells a preformed aluminum matrix insert for strengthening cast aluminum disc brake calipers [2], allowing them to weigh as much as 50% less while increasing stiffness.
• Lotus has since used metal matrix composites in the 2ZZ-GE engine block, and the Porsche Boxter and 911 use MMCs to reinforce the engine's cylinder sleeves.
• The F-16 Fighting Falcon uses monofilament silicon carbide fibres in a titanium matrix for a structural component of the jet's landing gear.

MMCs are nearly always more expensive than the more conventional materials they are replacing. As a result, they are found where improved properties and performance can justify the added cost. Today these applications are found most often in aircraft components, space systems and high-end or "boutique" sports equipment. The scope of applications will certainly increase as manufacturing costs are reduced.

See also

• organic matrix composite
• cermet

External links

• MMC overview
• Closeup photos of MMCs
• Assessment of Metal Matrix Composites for Innovations
• Space application of MMCs