Editorial Feature

Composite Metal Matrix Materials

Metal matrix composites (MMCs) are composite materials containing at least two constituent parts – a metal part and a material or a different metal part. The metal matrix is reinforced with the other material to improve strength and wear. Most metals and alloys make good matrices.

In structural applications, the matrix is usually composed of a lighter metal such as magnesium, titanium, or aluminum. In high-temperature applications, cobalt and cobalt-nickel alloy matrices are common. Continuous carbon, silicon carbide, or ceramic fibers are some of the materials that can be embedded in a metallic matrix material.

For example, MMCs are increasingly found in the automotive industry. These materials use a metal such as aluminum as the matrix, and reinforce it with fibers such as silicon carbide. The space era has increased the demand for MMCs that possess high specific stiffness and near-zero coefficient of thermal expansion (CTE).

Material Properties and Characteristics

Metal matrices posses the advantage of being suitable for use in applications requiring a long-term resistance to severe environments over polymeric matrices. It is a fact that the yield strength and modulus of most metals are higher than those for polymers. Another advantage of using metals is that they can be plastically deformed and strengthened by many thermal and mechanical treatments.

The characteristics of MMCs can be designed into the material based on specific applications.

The following are some key material properties benefits of MMCs:

  • Fire resistant
  • Operate in wider range of temperatures
  • Do not absorb moisture
  • Better electrical and thermal conductivity
  • Resistant to radiation damage
  • Do not display outgassing
  • Low CTE and light weight
  • Good damping and high compression strength
  • High specific stiffness and strength.

Manufacturing Process

The following are the key composite metal matrix material manufacturing processes:

  • High-pressure diffusion bonding process - is used for producing continuous- fiber reinforced MMCs. Here, layers of metal foil are sandwiched with long fibers, and then pressed through to form a matrix.
  • Casting process - Used for producing continuous - fiber reinforced MMCs. In this process, molten metal is injected into a form with fibers pre-placed inside it.
  • Powder-metallurgy process – Used for producing discontinuously reinforced MMCs. Powdered metal and discontinuous reinforcement are mixed together and bonded via a process of compaction, degassing, and thermo-mechanical treatment.
  • Physical vapor deposition: In this process, the fiber is passed through a thick cloud of vaporized metal, thereby coating it.

The process for producing discontinuous, particle reinforced or continuous fiber reinforced part begins with a preform that is shaped as the final part. The preform is infiltrated with a molten metal, e.g. aluminum, to produce a metal matrix composite component.

Care should be taken while manufacturing MMCs to maintain the reinforcing fibers strength, to ensure a strong bond of fibers with matrices and between the matrix layers, to provide the correct fiber length, which is greater than the critical length, to ensure uniform distribution of fibers in the matrix, to ensure orientation of fibers in the direction of the applied load, and to achieve the required shape and dimensions of the MMC.

Applications

The following are some of the most common application areas of composite metal matrix materials:

  • Pushrods for racing engines
  • Carbide drills
  • Tank armors
  • Automotive industry - disc brakes, driveshaft, engines
  • Aircraft components - structural component of the jet's landing gear
  • Bicycle frames
  • Space systems
  • High density multi-chip modules in electronics
  • Parts in particle accelerators
  • Attach plates for high-speed robots

Sources and Further Reading

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