14-06-2012, 02:45 PM
COMPOSITE MATERIALS
Composite:
A composite is a solid material that results when two or more materials are combined (physically, not chemically ) with strong bonding between them to create a new material whose properties are superior in a specific application those of the original materials. There are two different phases in composites. They are
1. Matrix phase
2. Reinforcing phase
Matrix phase:
The primary phase, having a continuous character, is called matrix. Matrix is usually more ductile and less hard phase. It holds the dispersed phase and shares a load with it.
Properties of a Matrix matrial:
The needs or desired properties of the matrix which are important for a composite
structure are as follows
Reduced moisture absorption.
Low shrinkage.
Low coefficient of thermal expansion.
Good flow characteristics so that it penetrates the fibre bundles completely and
eliminates voids during the compacting/curing process.
Reasonable strength, modulus and elongation (elongation should be greater than fibre).
Must be elastic to transfer load to fibres.
Strength at elevated temperature (depending on application).
Low temperature capability (depending on application).
Excellent chemical resistance (depending on application).
Should be easily processable into the final composite shape.
Dimensional stability (maintains its shape).
Functions of a matrix material:
In a composite material, the matrix material serves the following functions
Holds the fibres together.
Protects the fibres from environment.
Distributes the loads evenly between fibres so that all fibres are subjected to the same
amount of strain.
Enhances transverse properties of a laminate.
Improves impact and fracture resistance of a component.
Helps to avoid propagation of crack growth through the fibres by providing
alternate failure path along the interface between the fibres and the matrix.
Carry interlaminar shear.
General types of Matrix Materials:
In general following types of matrix materials are available
Thermosetting material
Thermoplastic material
Carbon
Metals
Ceramics
Glass Matrix.
Reinforcing phase:
The second phase (or phases) is embedded in the matrix in a discontinuous form. This secondary phase is called dispersed phase. Dispersed phase is usually stronger than the matrix, therefore it is also called as reinforcing phase.
Examples of composite systems include concrete rein-forced with steel and epoxy reinforced with graphite fibers, etc.
Examples of naturally occurring composites include wood, where the lignin matrix is reinforced with cellulose fibers and bones in which the bone-salt plates made of calcium and phosphate ions reinforce soft collagen.
Classification systems of composite materials:
There are two classification systems of composite materials. They are as follows
1. Classification of composites I
2. Classification of composite materials II
Classification of composites I:
This classification system is done based on the matrix material. There are three types of composite materials they are
1. Metal Matrix Composites (MMC)
2. Ceramic Matrix Composites (CMC)
3. Polymer Matrix Composites (PMC)
Metal Matrix Composites (MMC):
Metal Matrix composites are composed of a metallic matrix(aluminum, magnesium, iron, cobalt, copper) and a dispersed ceramic (oxides, carbides) or metallic (lead, tungsten, molybdenum) phase. 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 aluminum matrix to synthesize composites showing low density and high strength.
The following properties are typical for Aluminum Matrix Composites:
High strength even at elevated temperatures;
High stiffness (modulus of elasticity);
Low density;
High thermal conductivity;
Excellent abrasion resistance.
Ceramic Matrix Composites (CMC) :
Ceramic matrix composites (CMCs) are a group of materials within composite materials or within technical ceramics. They consist of ceramic fibers embedded in a ceramic matrix, thus forming a ceramic fiber reinforced ceramic (CFRC) material. The matrix and fibers can consist of any ceramic material.