The idea
A metal matrix composite, often shortened to MMC, is a metal with a second material mixed in. The second material is usually a hard ceramic in the form of particles, short fibers or continuous fibers. The metal supplies toughness, ductility and the ability to conduct heat and electricity. The ceramic supplies stiffness, hardness, wear resistance or a lower coefficient of thermal expansion.
The result is a material that can differ noticeably from the base metal in ways that matter for particular parts. It is a member of the wider family of composite materials, but it behaves differently from the polymer composites most people picture, because the matrix is metallic.
Why use a metal rather than a polymer
Polymer matrices are light and easy to process, but they limit temperature, conduct heat poorly and can be affected by moisture and ultraviolet light. A metal matrix offers other benefits:
- Higher operating temperatures than most polymers.
- Good thermal and electrical conductivity.
- Better resistance to fire and to many fluids.
- Higher strength in directions other than the fiber direction, especially with particle reinforcement.
- Little or no moisture absorption.
The price is usually higher density than a polymer composite, more difficult processing and higher cost.
Common matrices
- Aluminum. The most widespread matrix. It is light and relatively inexpensive, and it is easy to cast. Many applications target stiffness or wear resistance with weight savings.
- Magnesium. Even lighter, used in some specialized applications, though it is more reactive and more difficult to protect from corrosion.
- Titanium. Used with continuous ceramic fibers in aerospace research and select applications, where temperature capability and strength-to-weight matter.
- Copper and silver. Chosen when thermal or electrical conductivity is the priority, such as heat spreaders. Heat spreader materials based on diamond in copper or aluminum are one example of this approach.
- Nickel and iron-based alloys. Considered for higher temperature uses, sometimes with ceramic particles for wear.
Common reinforcements
- Silicon carbide. The most common particle reinforcement in aluminum. It increases stiffness and wear resistance. Silicon carbide is better known as an abrasive and a structural ceramic in its own right.
- Alumina. Used as particles or short fibers. Alumina ceramics are well established in industry, and in an MMC the same compound serves as a hard filler.
- Boron carbide. A hard, light ceramic, sometimes used where neutron absorption or hardness is wanted.
- Carbon and graphite. Fibers or particles can lower weight and thermal expansion and raise conductivity, though chemical reactions with the metal can be a concern. Aluminum can react with carbon at high temperature to form brittle carbides.
- Diamond. Used for exceptional thermal conductivity, as in diamond-filled copper, although cost and bonding challenges apply.
What the reinforcement does
Inside the metal, hard particles interfere with the movement of the defects that allow metals to deform. This tends to raise strength and stiffness. Particles also improve resistance to abrasive wear because the hard phase stands proud as the softer metal wears away. Another effect is on thermal expansion: ceramics generally expand less than metals, so adding them pulls the composite’s expansion down. This helps where a part must stay dimensionally stable or must match an electronic device.
Continuous fibers do something different. Aligned along the main load direction, they carry much of the load, giving high strength and stiffness in that direction in the way described in the pillar article on fibers, matrices and interfaces.
The trade-off is toughness. Compared with the base metal, particle-reinforced metals tend to have lower ductility and lower fracture toughness, and they can be sensitive to particle clusters and cracks around particles.
The interface again
As in every composite, the boundary between reinforcement and matrix controls much of the outcome. A good bond transfers load and passes heat across; a poor one acts as a weak point. In metals there are added complications. The molten metal may not wet the ceramic, so it will not flow into the gaps, and chemical reactions can produce brittle compounds at the boundary. Producers use coatings on the reinforcement, alloying additions or controlled processing temperatures to manage this.
How they are made
- Stir casting. Particles are stirred into molten metal and the mixture is cast. It is relatively economical, though the particles must be kept evenly distributed and wetted.
- Pressure or gas-pressure infiltration. Molten metal is forced into a porous preform of ceramic, giving high ceramic fractions.
- Powder metallurgy. Metal and ceramic powders are blended, pressed and sintered. This route allows good control of composition.
- Spray deposition and other methods. Molten metal droplets are sprayed together with particles to build up material.
- Diffusion bonding of fiber layers. Used for continuous fiber MMCs, where foils and fibers are consolidated under heat and pressure.
After forming, MMCs are sometimes extruded, rolled or forged to improve particle distribution and reduce porosity.
Where they are used
Typical applications include brake rotors and other automotive parts, engine components, electronic packaging and heat sinks, aerospace structures, and sporting goods. The automotive and aerospace sectors are interested largely for the combination of low weight and wear resistance, while electronics uses them for matching expansion and conducting heat. The details in each case depend on the specific alloy and reinforcement, and many of these uses are specialized rather than widespread.
Machining and joining
Hard ceramic particles wear cutting tools quickly. Conventional tool materials can wear out rapidly on silicon carbide-reinforced aluminum, so diamond-based tooling is often preferred. This ties into the practical points about aluminum machining and other non-ferrous metals, with the added abrasiveness of the reinforcement. Welding is harder than for plain alloys, because the ceramic can react with molten metal or segregate, and joining often relies on mechanical fasteners, brazing or adhesives.
Weighing the options
MMCs fit when a specific combination of properties is needed that neither plain metals nor polymer composites provide. They compete against other solutions such as ceramic matrix composites for the highest temperatures and polymer composites for the lowest weight. Because of the cost and complexity, they tend to be selected for particular jobs rather than as general-purpose replacements.