Types of Composite Materials
These have superior mechanical properties and yet are lightweight. The reinforcing fibers are usually glass, graphite, boron, etc. Epoxies and polyester commonly serve as a matrix material. Reinforced plastics are being developed rapidly. New developments concern metal-matrix and ceramic-matrix composites and honeycomb structure (Honeycomb structure consists of a core of honeycomb or other corrugated shapes bonded to two thin outer skins. Ceramic-matrix cutting tools are being developed, made of silicon carbide reinforced alumina, with greatly improved tool life. A composite material, as stated above, contains more than one component. The compound materials are incorporated into the composite to take advantage of their attributes, thus obtaining improved material. They become cohesive structures made by physically combining two or more compatible materials. Fiber-reinforced composites are heterogeneous materials prepared by associating and bonding in a single structure of materials possessing different properties. Due to complementary nature, the composite material possesses additional and superior properties. These thus become ideal materials for structural applications requiring high strength-to-weight and stiffness-to-weight ratios. Fiber-reinforced materials exhibit anisotropic properties. Glass fibers are strong but if notched they fracture readily. By encapsulating them in a polyester resin matrix, they can be from damage. Fibers of graphite and boron are also used in composites. Commonly used fibers for composite materials are-glass, silica, and boron for amorphous structure, ceramic and metallic for single crystals as well as polycrystals, carbon, and boron (amorphous) materials for multiphase structure, and organic material for macromolecular structure. For two-dimensional structural applications such as in plates, walls, shells, cylinders, pipes, etc. a planar reinforcement is much more advantageous as compared to the linear reinforcement.
Duplex Composite Components:
Components subjected to severe wear and high contact stresses can be made of duplex composite, the composite layer is located on the outer or inner. Surface depending on the requirement. Aluminum composite alloys reinforced by ceramic have been developed and these have a relatively high strength to weight ratio, high modulus of elasticity, and good wear characteristics.
Silicon carbide particles are incorporated into the surface of aluminum alloy heated to its mushy state and pressure is applied to get a good wetting between the aluminum alloy and the silicon carbide particles. Experiments can be carried out to determine the semi-solid forming conditions. Specimens surrounded by SiC particles are heated up to this temperature for about 45 minutes in order to homogenize the temperature through the specimen. A hydraulic press is used to apply the necessary low pressure for the semi-solid forming process. There is an optimum combination of temperature and pressure values to obtain optimum mechanical properties. In this way, a composite layer of about 2.5 mm width can be formed with uniformly distributed particles having a good bond with the aluminum matrix, with no separation or porosity at the composite layer/matrix interface.
The surface composite layer has the hardness and wears resistance about 1.75 and 10 times those of as-received aluminum matrix alloy.
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