COMPOSITE MATERIALS: Classification of Composite Materials, Advantages and Disadvantages

COMPOSITE MATERIALS

Due to recent advances in technology, a demand has been felt for materials with extraordinary combination of properties. A material should have the strength of steel but lightness of magnesium. It should have hardness of tungsten carbide but should have toughness of steel. Obviously such combination of properties cannot be satisfied by ordinary materials available. This is specially true for materials needed for aerospace, marine and transportation industries.

Material scientists and engineers have found a way to solve this problem by devising composite materials. Consider Portland cement concrete with which we have already dealt with. It can be considered as aggregate composite whereas reinforced concrete can be regarded as a prototype of a composite material.

A composite material is often made up of two phases. One is called the matrix phase and the other is the dispersed phase. In reinforced cement concrete, cement concrete is the matrix and the steel rods which are used for reinforcement form the dispersed phase.

The matrix phase or the constituent is commonly a polymer material, whereas the reinforcing agents are carbon fibre, glass fibre or ceramics depending upon the properties which are required in the finished composite material.

Classification of Composite Materials

A simple method to classify composite material is shown in Fig.

Classification of Composite Materials
Classification of Composite Materials

An example of particle reinforced composite is CERMET. One of the well known cermet contains extremely hard particles of tungsten carbide and titanium carbide embedded in a matrix of cobalt. The name cermet comes from a combination of words ceramic (WC) and metal (cobalt). This cermet is used as a cutting tool material.

Fibre reinforced composites are well known e.g. fibre glass. Fibre glass is a composite consisting of glass fibres laid in a matrix of a plastic material (resin). Glass is easily drawn into high strength fibres when in molten state. These glass fibres when used as reinforcement add strength to this composite.

Sometimes in addition to glass fibres, carbon fibres are used, which are even stronger but stiff.

Fibre glass reinforced plastics are used in making small boats, car-bodies, acid containers/tanks, and specially papers. Carbon fibre composites are used to make tennis and badminton racquets, other sports equipment and light weight orthopaedic components.

An example of structural composites is sunmica or formica sheets which we use in house hold furniture and cabinets. Structural composite is made by using two dimensional sheets which are cemented together. While stacking the sheets together, it is ensured that the orientation of high strength direction (such as in aligned fibre-reinforced plastics) varies with each successive layer.

Very often, a protective coating which is hard and inert is given on the top surface of a structural composite so as to increase it service-life.

Advantages and Disadvantages

Composites provide exceptional strength to weight and stiffness to weight ratios. They can provide great heat resistance, improved fatigue resistance, improved toughness and reduced notch sensitivity composites are generally non-corrosive.

The main disadvantage is the high cost of composites.

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