METAL-MATRIX COMPOSITES

METAL-MATRIX COMPOSITES

Post 346 ⇒   by Gautam Shah 

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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.

Metal matrix composite components

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.

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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.

Air craft shell -metal matrix composites

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Majority of commercially used metal matrix composites have Aluminum as the matrix, but in specialty metal sections, large number of applications employ matrix properties of super-alloys, titanium, copper, magnesium, or iron. Aluminum-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, aluminum composites offer the advantage of low cost over most other MMCs.

Tungsten_carbide

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 fibers, discontinuous fibers, 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.

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BURNISHING

BURNISHING

Post 288 ⇒   by Gautam Shah  →

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Burnishing is a material surface finishing process. It polishes and hardens a surface, so that the endowed finish lasts longer. Surfaces that depend on the smoothness for reduced friction, and take lots of wear, need to be burnished. A burnishing rubs the rough surface texture and makes it shinier, but it is not intended as a polishing process. Polishing removes all excess (protruding) materials to level out the surface, whereas Burnishing removes minimum surface material and hardens the surface. It is true that a well polished or a smooth surface takes better burnishing.

Burnishing

Burnishing is mainly a Metal surface alteration process. It is used in various versions for Ceramics, Wood, Leather Paper Cement, Textiles and Artwork. Burnishing occurs on a surface, where another surface sliding on it creates a contact stress which locally exceeds the yield strength of the material. It induces plastic deformation of the surface component, hardens the surface by generating compressive stresses.

Bearing Surface -burnished metal face

Burnishing is not always desirable process for all metal items. It affects the behaviour quality of the surface significantly and often unpredictably. A burnished face, visually seems smoother but with repeated sliding marks grooves over the surface in the sliding direction. Heavy burnishing forces separation of top layer causing peeling of it. Burnishing generates heat which is greater than rubbing or polishing. This excess heat deforms thin body parts. A part deformed due to heavy burnishing, takes greater friction, creating a ‘runaway’ situation where the part fails.

Ceramics burnishing are a treatment in which the green mass of the pot (before drying for baking) is polished and compressed. Compressing the mass and allows excess water to come to the surface, increases the density of the mass and provides a glossy surface. Burnishing is also done after coating the raw item with the slip. As part of surface compaction, sometimes patterns are embossed on the surface. Hard smooth surfaces like wood, bone, glass, metal, or ceramics are rubbed on the surface.

Tripod vessel with lid, Maya culture, Mexico or Guatemala, 4th-5th C, hand-built ceramic with incised decoration and burnished slip, Honolulu Museum of Art

Wood Burnishing is done by rubbing hard grained wood piece along the surface of the wood. Burnishing generates heat, to dry out the surface, melt and fuse the resinous substances or additive substances such as oils, wax etc. Burnished surfaces retain the natural feel (grain and colour) of the wood, and is more natural looking then any coating treatment. Lacquer coated woods are burnished with wool fabrics to gain a natural sheen. Sometimes rubbing compounds that have very fine abrasive grains, wax, oils, lubricants like silicone oil and colouring dyes are used.

Leather Burnishing is used for top and under surfaces as well as edges of leather products. Hard wood pieces are rubbed over leather with or without rubbing materials like oil or wax to achieve a compressed mass and glossy face. The heat also facilitates penetration of rubbing material. The process is also conducted at leather sheet and product formation level. At a leather sheet level-heated roller with pattern compresses the leather. A process, reverse of burnishing is done to produce suede surfaces.

Leather edge finishing

Paper Burnishing is a post paper forming process. It is done to compact the grain-mass and provide sheen, by heavy calendering. Calendering is accompanied by bodying with starch, minerals or resins. It is done to emboss textures or patterns. Photographic mount-boards have such ingrained textures.

Plaster Trowelling -burnishing

Cement Burnishing is done to plasters and cast concrete surfaces. Cement plasters are re-trowelled after the initial setting of the cement. Trowels of wood or metal sheets are rubbed to compress the mass, bring the excess water to the surface and polish it. In case of Tri-mix concrete floors, post setting vibration compacts the surface bringing out the excess water, which is than suctioned out.

Tri-mix concrete -burnishing process

Textile Burnishing is a fabric finishing and texturizing process. Fabrics are hot pressed and passed through rollers. Sized and chemical treated fabrics get a sheen and smoother surface. Fabrics are singed during the process to burn standing or loose fibres and to compact the mass. Shrinking also reduces the mass.

Art-Work Burnishing involves applying colours and than rubbing them to level the surface. The technique was used for Encaustic or wax colour painting. Wax colours were rubbed and polished to achieve a saturated effect. Tempera paintings were also treated or touched with same techniques. Modern day application uses wax crayons or pencils to fill in colours, which are then rubbed with smooth glass or stone. The surface gets warm to melt and fuse the colours.

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