Post 346 – by Gautam Shah
Composite materials have at least two materials, with distinct roles. One acts as MATRIX and the other functions as a FILLER. The structure and properties of the filler-matrix interface play a major role in the mechanical properties of composite materials. The stresses acting on the matrix are transmitted to the fillers across the interface. A series of strips held together offers strength equal to one strip but when clamped together all layers take the load.
A metal matrix composite (MMC) is composite material with at least two constituents, one is a Metal whereas the other may be a different metal or another material, such as a ceramic or organic compound. Matrix, metals like Al, Be, Mg, Ti, Fe, Ni, Co, and Ag. By far the largest usage is in aluminium matrix composites. Filler or reinforcements are largely provided by ceramics for their favourable combination of stiffness, strength, and low density. These reinforcement materials include SiC, Al2O3, B4C, TiC, TiB2, graphite, number of advanced ceramics and also metallic materials as reinforcements such as steel fibres.
Some engineering parts have to operate at temperatures high enough to melt or degrade a polymer, so a polymer matrix is not useful. In such a situation Metal matrices offer high-temperature resistance, and strength and ductility, or bend-ability. The main problem with metal-matrix composites (MMC) is that even the lightest metals are heavier than polymers, and they are very complex to process.
MMC can be used in such areas as the skin of a hyper-sonic aircraft, and Space shuttle, commercial airliners, electronic substrates, bicycles, automobiles, golf clubs, and a variety of other applications. Metal matrix composites have good thermal conductivity, high shear strength, very high abrasion resistance, high-temperature bearing capacity, non-flameability, not affected by solvents. These composites can be machined or processed further through conventional engineering tools and equipments.
Majority of commercially used metal matrix composites have Aluminum as the matrix, but in speciality metal sections, large number of applications employ matrix properties of super-alloys, titanium, copper, magnesium, or iron. Aluminium-matrix composites are not a single material, but a group of materials whose stiffness, strength, density, and thermal and electrical properties can be defined. The matrix alloy, the reinforcement material, the volume and shape of the reinforcement, the location of the reinforcement, and the fabrication method can all be varied to achieve required properties. Regardless of the variations, however, aluminium composites offer the advantage of low cost over most other MMCs.
Aluminum MMCs are produced through several routes such as casting, powder metallurgy, in situ development of reinforcements, and foil-and-fiber pressing techniques. For Metal matrix composites fillers as reinforcements are like, continuous fibres, discontinuous fibres, whiskers, particulates, and wires. With the exception of wires, which are metals, reinforcements generally are ceramics. Metal wires include tungsten, beryllium, titanium, and molybdenum. Metal matrix composites are typically made by infiltrating liquid metal into a fabric or a prearranged fibrous configuration called a preform.
MMCs are susceptible to corrosion of dispersed reinforcing material into the metal matrix. Corrosion occurs in the presence of air and chloride ions. The effect of corrosion is governed by the geometry and volume percent of reinforcements. Reinforcement above about 30% of content are rarely used as high hardness and low ductility makes it difficult to process, form or machine the items.