Post 181 – by Gautam Shah
A ceramic may be defined as a product manufactured by the heat treatment to a material or mix of materials. Ceramics are generally inorganic and non metallic solids in nature, however, small but appreciably affective metal compounds may be present in them. Ceramics are mainly crystalline, but could also be partly crystalline structured or like a glass -amorphous (non-crystalline) in nature.
Ceramics are produced for many different uses. Ceramic products such as pottery, bricks etc. are formed or shaped before firing, while other ceramic products such as cement and glass are formed after firing. Some ceramics are used in their manufactured form, while others are crushed or powdered before use. Pozzolana is a natural ceramic powder, produced by the lava activity. Cements are ceramic products, where initially a ceramic called clinker is produced and then powdered. Surkhi is a burnt clay product, used to replace cement in low strength structures. Ceramics like bricks, roofing tiles, flooring tiles, cooking pots etc. can be produced cheaply at all geographic locations, from all sorts of clays. Earthenware ceramics made with very plastic or wet material are very porous. Bricks, water pots, planters etc. are conceived for this quality.
Ceramic materials are used in toilet products such as the water closets, bath tubs, wash basins, soil pipes, etc. These items have vitreous and a non-porous surface with good gloss and excellent abrasion resistance. Similarly household tableware and oven-ware in addition have good thermal shock resistance and low thermal expansion.
Most ceramics are dielectric and except for ferrite, generally lack magnetic properties. Ceramics products’ properties like better thermal insulation, fire resistance, electric insulation and very high density make them the ideal material in electric, electronics and super conductivity field.
Ceramics unlike metals do not oxidize and are fairly stable in extreme environments, so are used for storage of alkaline and acidic substances in industries. Ceramics withstand extremely high operating temperatures, with high radiation resistance, which make them suitable for engine and turbine parts. High abrasion resistance and the capacity to withstand high temperature are two prime qualities, why ceramics are used for saturated steam and super heated gas nozzles and burner ends.
Refractory products have superior resistance to high temperatures and thermal shock, and are used in applications such as furnace linings. Ceramics’ creep resistance or ability to withstand high stress at elevated temperature is better then any material yet produced and its applications include turbine wheels, jet engines, dies etc.
Speciality ceramics are much harder than metals, and can easily cut steel and glass. Speciality ceramics because of their extremely fine-gained poly crystalline micro structures are almost free of residual pores and defects. Such materials are extremely hard and cannot easily be machined or cut, except by a laser. Toughened zirconium ceramics are used in the home as non magnetic scissors and kitchen knives, cutting tool bits, cutter edges, etc. Silicon Nitride ceramic’s engine parts can run more efficiently than do parts made of nickel based super alloys. Engine parts including combustion chamber walls, cylinder liners and heads, piston crowns and intake / exhaust parts can be made of ceramic. Ceramic bearings can operate at high speed without lubricants.
Body implants of ceramics in orthopaedic surgery have superior wear and erosion resistant characteristics compared with other materials. Ceramic implants for human body show not only high strength to weight ratio, but where required, these ceramics can be deliberately made porous, enabling regenerating bone to grow into and bond with the implant.
Ceramic air tiles consisting of an open cellular micro structure of extremely fine coated silica fibres, so loosely packed, that the tiles consist of 95% air, as lightweight as cotton wool, are used in rocketry. The ceramic tiles not only withstand temperature of 1500 C (above melting point of steel) but insulate the space module within. Micro-porous ceramics’ candles are used for filtration.
Ceramic products show various levels of glassification and depending on the raw materials used and the process of crystallization. Higher glassification levels impart better density, low porosity and in most cases better homogeneity.
Earthenware are soft porous ceramic products fired at low temperatures (950 -1050 °C) Stoneware fired at higher temperatures (1100 -1300 °C ), show fusing of the clay body, and are fairly dense and non-porous. Porcelains are almost glass like bodies, fairly homogeneous, dense and non-porous (fired at 1200 -1,400 °C). Glass (1400 -1650 °C ) is a formation of non crystalline solid, referred to as super-cooled liquid.