In materials science, a metal matrix composite (MMC) is a composite material with fibers or particles dispersed in a metallic matrix, such as copper, aluminum, or steel. The secondary phase is typically a ceramic (such as alumina or silicon carbide) or another metal (such as steel[1]). They are typically classified according to the type of reinforcement: short discontinuous fibers (whiskers), continuous fibers, or particulates. There is some overlap between MMCs and cermets, with the latter typically consisting of less than 20% metal by volume. When at least three materials are present, it is called a hybrid composite. MMCs can have much higher strength-to-weight ratios,[2] stiffness, and ductility than traditional materials, so they are often used in demanding applications. MMCs typically have lower thermal and electrical conductivity and poor resistance to radiation[citation needed], limiting their use in the very harshest environments.
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 fiber. To prevent this reaction, the carbon fibers 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 complete 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 polycrystalline 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.[3]
MMC manufacturing can be broken into three types—solid, liquid, and vapor.
MMCs are fabricated at elevated temperatures, which is an essential condition for diffusion bonding of the fiber/matrix interface. Later on, when they are cooled down to the ambient temperature, residual stresses (RS) are generated in the composite due to the mismatch between the coefficients of the metal matrix and fiber. The manufacturing RS significantly influence the mechanical behavior of the MMCs in all loading conditions. In some cases, thermal RS are high enough to initiate plastic deformation within the matrix during the manufacturing process.[8]
The addition of ceramic particles in general increases the strength of the material while having a tradeoff on material ductility. For example, a Al-Al2O3 composite can increase the yield strength of cast Al 6061 alloys from 105 to 120 MPa and increase the young’s modulus from 70 to 95 GPa.[9] However, the composite had negative effects on the ductility decreasing it from 10% to 2%. Ultimately, the increase in elastic modulus is significant because the metals get the benefit of the higher specific stiffness of ceramics while retaining some ductility.[10][11] Metal-matrix composites can also significantly increase the wear resistance and hardness of aluminum alloys. Al2O3 particles were found to significantly increase the wear resistance of an Al-Si alloy, and SiO2 particles increased the hardness of a Al-Mg alloy significantly.[12][13] The application of this is in light, wear-resistant alloys for wear components such as piston liners in automobile engines. Current aluminum alloys are soft and often require hard, heavy cast iron liners which reduces the benefits of the lightweight aluminum engines.
Fracture toughness of the composites is typically dominated by the metal phases; however, it can also be dominated by the ceramic phase or delamination depending on the material system.[14] For example the Cu/Al2O3 system has a high thermal expansion mismatch causing localized stresses encouraging crack propagation in the form of delamination. This significantly inhibits its fracture toughness compared to other compositions. In an Al/Al2O3 co-continuous system the crack propagated through the ceramic phase and was deflected upon reaching interfaces with the metallic phases. As a result, more energy was needed to deflect the crack around the phases and the composite was significantly toughened. Overall, fracture toughness is largely dependent on MMC composition due to thermal mismatch and crack modes but can toughen composites with low thermal mismatch.
MMCs strengthen materials against plasticity for a variety of reasons. The first is direct load transfer to the stronger ceramic particles.[15] The second is due to the difference in plastic deformation of the two components. This causes a dislocation to become pinned on the stronger particles and bow around them to continue moving. Dislocations typically drive plastic deformation due to the lower energy to move them rather than moving an entire plane of atoms. Therefore, pinning them causes a large increase in the energy and stress required for plastic deformation (see Precipitation hardening). The final mechanism is caused by the stress from thermal and coherency mismatch.[16] This creates a stress field which traps dislocations creating a pileup further inhibiting plastic deformation.
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.
In comparison with conventional polymer matrix composites, MMCs are resistant to fire, can operate in wider range of temperatures, do not absorb moisture, have better electrical and thermal conductivity, are resistant to radiation damage, and do not display outgassing. On the other hand, MMCs tend to be more expensive, the fiber-reinforced materials may be difficult to fabricate, and the available experience in use is limited.