METAL MATRIX COMPOSITES

Classification of Metal Matrix Composites

Classification of Composite Materials by Primary Phase (Matrix Material)

1. Metal Matrix Composites (MMCs) - mixtures of ceramics and metals, such as cemented carbides and other cermets

2. Ceramic Matrix Composites (CMCs) - Al2O3 and SiC imbedded with fibers to improve properties, especially in high temperature applications - The least common composite matrix

3. Polymer Matrix Composites (PMCs) - thermosetting resins are widely used in PMCs

Examples: epoxy and polyester with fiber reinforcement, and phenolic with powders

Functions of the Matrix Material (Primary Phase)

Ø  Provides the bulk form of the part or product made of the composite material

Ø  Holds the imbedded phase in place, usually enclosing and often concealing it

Ø  When a load is applied, the matrix shares the load with the secondary phase, in some cases deforming so that the stress is essentially born by the reinforcing agent

Functions of the Reinforcing Phase (Secondary Phase)

1. Function is to reinforce the primary phase

2. Imbedded phase is most commonly one of the following shapes:

Ø  Fibers

Ø  Particles

Ø  Flakes

3. In addition, the secondary phase can take the form of an infiltrated phase in a skeletal or porous matrix

Ø  Example: a powder metallurgy part infiltrated with polymer

Metal Matrix Composites (MMCs)

A metal matrix reinforced by a second phase

Ø  Reinforcing phases:

1.     Particles of ceramic (these MMCs are commonly called cermets)

2.     Fibers of various materials: other metals, ceramics, carbon, and boron

Ø  Materials consisting of metallic matrices, reinforced with ceramic particles or fibers, are known as metal matrix composites or MMCs

Ø  The volume fraction of the reinforcement is typically in the range 10-70%.

Ø  MMCs can offer a range of property enhancement over monolithic alloys.

Ø  Metal matrix composite materials have found application in many areas of daily life for quite some time.

Ø  Materials like cast iron with graphite or steel with a high carbide content, as well as tungsten carbides, consisting of carbides and metallic binders, also belong to this group of composite materials.

Ø  Metal matrix composites become interesting for use as constructional and functional materials, if the property profile of conventional materials either does not reach the increased standards of specific demands, or is the solution of the problem.

The reinforcement of metals can have many different objectives;

Ø  Increase in yield strength and tensile strength at room temperature and above while maintaining the minimum ductility or rather toughness,

Ø  Increase in creep resistance at higher temperatures compared to that of conventional alloys,

Ø  Increase in fatigue strength, especially at higher temperatures,

Ø  Improvement of thermal shock resistance,

Ø  Improvement of corrosion resistance,

Ø  Increase in Young’s modulus,

Ø  Reduction of thermal elongation.

Advantages

Ø  Light Weight

Ø  Performance at higher temperatures

Ø  High Strength

Ø  Low Density, Better wear resistance

Classification of Metal Matrix Composites

Reinforcements for Metal Matrix Composites

Reinforcements for metal matrix composites have a manifold demand profile, which is determined by production and processing and by the matrix system of the composite material. The following demands are generally applicable

Ø  low density

Ø  mechanical compatibility (a thermal expansion coefficient which is low but adapted to the matrix)

Ø  chemical compatibility

Ø  thermal stability

Ø  high Young’s modulus

Ø  high compression and tensile strength

Ø  good processability

Ø  economic efficiency

 

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