Metal matrix composite

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.

Composition

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 fiber 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.

Matrix

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.

Reinforcement

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.^[1]

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) or extrusion)
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
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.^[2]^[3]

Vapor deposition

Physical vapor deposition: The fiber is passed through a thick cloud of vaporized metal, coating it.

In-situ fabrication technique

Controlled unidirectional solidification of a eutectic alloy can result in a two-phase microstructure with one of the phases, present in lamellar or fiber form, distributed in the matrix.^[4]

Applications

Carbide drills are often made from a tough cobalt matrix with hard tungsten carbide particles inside.
Some tank armors may be made from metal matrix composites, probably steel reinforced with boron nitride, which 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, allowing them to weigh as much as 50% less while increasing stiffness. 3M has also used alumina preforms for AMC pushrods.
Ford offers a Metal Matrix Composite (MMC) driveshaft upgrade. The MMC driveshaft is made of an aluminum matrix reinforced with boron carbide, allowing the critical speed of the driveshaft to be raised by reducing inertia. The MMC driveshaft has become a common modification for racers, allowing the top speed to be increased far beyond the safe operating speeds of a standard aluminum driveshaft.
Honda has used aluminum metal matrix composite cylinder liners in some of their engines, including the B21A1, H22A and H23A, F20C and F22C, and the C32B used in the NSX.
Toyota has since used metal matrix composites in the Yamaha-designed 2ZZ-GE engine which is used in the later Lotus Lotus Elise S2 versions as well as Toyota car models, including the eponymous Toyota Matrix. Porsche also uses MMCs to reinforce the engine's cylinder sleeves in the Boxster and 911.
The F-16 Fighting Falcon uses monofilament silicon carbide fibers in a titanium matrix for a structural component of the jet's landing gear.
Specialized Bicycles has used aluminum MMC compounds for its top of the range bicycle frames for several years. Griffen Bicycles also made boron carbide-aluminum MMC bike frames, and Univega briefly did so as well.
Some equipment in particle accelerators such as Radio Frequency Quadrupoles (RFQs) or electron targets use copper MMC compounds such as Glidcop to retain the material properties of copper at high temperatures and radiation levels.^[5]^[6]
Copper-silver alloy matrix containing 55% by volume diamond particles, known as Dymalloy, is used as a substrate for high-power, high-density multi-chip modules in electronics for its very high thermal conductivity.
Aluminium-Graphite composites are used in power electronic modules because of their high thermal conductivity, the adjustable coefficient of thermal expansion and the low density.

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.

References

↑ Materials science and Engineering, an introduction. William D. Callister Jr, 7th Ed, Wiley and sons publishing
↑ Yufeng Wu, Gap Yong Kim, Carbon nanotube reinforced aluminum composite fabricated by semi-solid powder processing ,Journal of Materials Processing Technology, Volume 211, Issue 8, August 2011, Pages 1341–1347, http://www.sciencedirect.com/science/article/pii/S0924013611000707
↑ Yufeng Wu, Gap Yong Kim, et alFabrication of Al6061 composite with high SiC particle loading by semi-solid powder processing, Acta Materialia, Volume 58, Issue 13, August 2010, Pages 4398–4405, http://www.sciencedirect.com/science/article/pii/S0924013611000707
↑ University of Virginia's Directed Vapor Deposition (DVD) technology
↑ Ratti, A.; R. Gough; M. Hoff; R. Keller; K. Kennedy; R MacGill; J. Staples (1999). "The SNS RFQ Prototype Module" (PDF). Particle Accelerator Conference, 1999. 2 (1): 884–886. doi:10.1109/PAC.1999.795388. ISBN 0-7803-5573-3.
↑ Mochizuki, T.; Y. Sakurai; D. Shu; T. M. Kuzay; H. Kitamura (1998). "Design of Compact Absorbers for High-Heat-Load X-ray Undulator Beamlines at SPring-8" (PDF). Journal of Synchrotron Radiation 5 (4): 1199–1201. doi:10.1107/S0909049598000387. PMID 16687820.