Ceramics

CERAMICS

Ceramics are a material often used in construction, made from a mixture of minerals, typically silica sand, with a clay binder and some impurities, and up to 30% water. They are fired at a higher temperature than bricks, so that the silica re- crystallizes to form a glassy material that has greater density, strength, hardness, resistance to chemicals and frost and a greater dimensional stability. During firing, the water is driven off, though this may be reduced from 30% to 2-5% by drying before firing. At this reduced water content products are moulded as powder before being fired at 1,800-2,000 degrees for days, depending on the ceramic and process details. Ceramics may have an as-fired appearance or be glazed. These materials are environmentally stable and will not oxidize further in the atmosphere, Hence, they are economical in terms of maintenance costs. Problems are likely to occur when they are combined with other materials, typically fixings which are highly stressed and subject to corrosion. Unlike metals, ceramics are not capable of ductile behaviour. They fail in a brittle manner, directly after their elastic limit.

TYPES OF CERAMICS

❖ Fire clays and shales: These products include ordinary bricks, clay roof bailoge tiles, flooring tiles, flooring quarries and pavers.

❖ Terracotta: This is literally 'burnt earth'. It is made from yellow to brownish-red clays with a uniformity and fineness between brick and vitrified wall tiles. Terracotta is often used for unglazed chimney pots, air bricks, copings and planters.

❖ Faience: This is a glazed form of terra-cotta or stoneware. The base material may be fired to the 'biscuit' stage before glazing and re-firing, or a 'once-fired' process may be used. The latter improves resistance of the glaze to crazing, but reduces the range of colours available.

❖ Fireclay: This contains a high proportion of clay resistant to high temperatures (kaolin). It is used for chimney flue linings and fire backs. Stoneware: This is similar in composition to fireclay, but is fired at a higher temperature than fireclay and contains a higher proportion of glass. As a result it is harder and less absorbent. Modern manufacturing processes mean that stoneware no longer has to be glazed for use in drainage pipes.

❖ Earthenware: The raw materials are blended and may contain a considerable proportion of limestone. It is a finer product than stoneware and is used as the body for glazed wall tiles and table 'china'. Water absorption may be up to 15%, however, making it less suitable for sanitary ware than vitreous china.

❖ Vitreous china: This has a higher glass content than earthenware, and its water absorption is only about 0.5%, which makes it suitable for sanitary fittings. It is stronger than earthenware.

❖ Porcelain: Porcelain is very similar to vitreous china, but is often made from purer materials under more strictly controlled conditions. It is used for special uses, such as electrical insulators.

❖ New ceramics: These are also called 'technical' or 'engineering' ceramics. Their purity is far higher than traditional ceramics, not using raw clay mined directly from the ground. Powders are formed which are then cast, pressed, extruded or moulded into shape. The powders may be set in organic binders. The combination of pure materials and exacting production techniques ensures the very high strength of these materials.

USES OF CERAMICS

Some of the uses of ceramics are listed below:

❖ Ceramic products are hard, porous, and brittle. As a result, they are used to make pottery, bricks, tiles, cements, and glass.

❖ They are used in automobiles (sparkplugs and ceramic engine parts found in racecars), and phone lines.

❖ They can also be found on space shuttles, appliances (enamel coatings), and airplanes (nose cones).

❖ Ceramics are also used at many places in gas turbine engines.

❖ Bio-ceramics are used as dental implants and synthetic bones.