Last modified: November 25, 2012
Composites are materials that are created by design and built at a macroscopic level by combining two or more component materials having different physical and chemical properties which are not lost or altered in the final product.
The component materials are generally polymers, metal or ceramics used as a base or a matrix to hold the reinforcement materials that could be fibres, particles, flakes or fillers. The matrix performs the function of a mould of the desired shape while the reinforcement materials provide the strength and increased mechanical properties to the final composite.
The matrix should be malleable and pliable to keep the fibres in their place and to avoid compression, flexing or shearing of the material by transferring the stresses through uniform distribution of the forces acting on it. The matrix also functions as a protective cover from adverse and damaging influences of chemical substances, humidity, erosion, or loads.
Depending on their matrix composition, composites materials can be classified into three main categories: composites with a metallic matrix (MMC), those with a ceramic matrix (CMC) and those with a polymer matrix (PMC)
In the Metallic Matix Composities, metals such as lead, tungsten or molybdenum could serve as the matrix for ceramics like oxides or carbides. Light metals like aluminium, titanium and magnesium also used as the matrix with fibres of carbon, silicon carbide, aluminium oxide, boron or tungsten embedded in them. Comparative mechanical properties in strength, hardiness and rigidity and higher operating temperatures are the advantages of MMCs but they also need higher temperatures for processing. Mechanical and thermal management techniques are used in shaping and strengthening of MMCs.
Ceramic Matrix Composites consist of ceramic oxides or carbides in a dispersed stage within a ceramic matrix. CMC have properties such as low density, stability at high temperatures, high modulus, solidity, resistance to thermal shock and corrosion. But they are brittle and crack easily.
Polymer Matrix Composites have fibrous reinforcing materials in a polymer resin matrix. Their positive properties include stability in room temperature low-cost and easy fabrication processes. But they have poor mechanical properties like tensile strength and impact strength. Thermoplastic resins like polypropylene, polyethylene, polyphenylene sulfone, polyamide and thermoset resins like polyesters, phenolics, polyurethanes and epoxies are examples of PMCs.
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