Metal matrix composite
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A metal matrix composite (MMC) is composite material with at least two constituent parts, one being a metal necessarily, 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.
MMCs are made by dispersing a reinforcing material into a metal matrix. The reinforcement surface can be coated to prevent a chemical reaction with the matrix. For example, carbon fibers are commonly used in aluminium matrix to synthesize composites showing low density and high strength. However, carbon reacts with aluminium to generate a brittle and water-soluble compound Al4C3 on the surface of the fibre. To prevent this reaction, the carbon fibres are coated with nickel or titanium boride.
The matrix is the monolithic material 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. In structural applications, the matrix is usually a lighter metal such as aluminum, magnesium, or titanium, and provides a compliant support for the reinforcement. In high-temperature applications, cobalt and cobalt–nickel alloy matrices are common.
The reinforcement material is embedded into a 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 can be isotropic, and can be worked with standard metalworking techniques, such as extrusion, forging, or rolling. In addition, they may be machined using conventional techniques, but commonly would need the use of polycrystaline diamond tooling (PCD).
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. The most common reinforcing materials in this category are alumina and silicon carbide.
Manufacturing and forming methods
MMC manufacturing can be broken into three types—solid, liquid, and vapor.
Solid state methods
- Powder blending and consolidation (powder metallurgy): Powdered metal and discontinuous reinforcement are mixed and then bonded through a process of compaction, degassing, and thermo-mechanical treatment (possibly via hot isostatic pressing (HIP) orextrusion)
- Foil diffusion bonding: Layers of metal foil are sandwiched with long fibers, and then pressed through to form a matrix
Liquid state methods
- Electroplating and electroforming: A solution containing metal ions loaded with reinforcing particles is co-deposited forming a composite material
- Stir casting: Discontinuous reinforcement is stirred into molten metal, which is allowed to solidify
- Pressure infiltration: Molten metal is infiltrated into the reinforcement through use a kind of pressure such as gas pressure
- Squeeze casting: Molten metal is injected into a form with fibers pre-placed 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
Semi-solid state methods
- Semi-solid powder processing: Powder mixture is heated up to semi-solid state and pressure is applied to form the composites.
- Physical vapor deposition: The fiber is passed through a thick cloud of vaporized metal, coating it.