LESSON 10: CERAMICS

• Ceramic - An inorganic compound consisting of a metal (or semi-metal) and one or more non-metals.
• Silica, the main ingredient in most glass products
• Alumina, used in various applications from abrasives to artificial bones
• hydrous aluminum silicate, the main ingredient in most clay products
• Ceramic Materials have high hardness, electrical and thermal insulating, chemical stability, and high melting temperatures
• Brittle, virtually no ductility - can cause problems in both processing and performance of ceramic products
• Some ceramics are translucent, window glass (based on silica) being the clearest example.
• Traditional Ceramics: clay products such as pottery and bricks, common abrasives, and cement
• New Ceramics: more recently developed ceramics based on oxides, carbides, etc., and generally possessing mechanical or physical properties superior or unique compared to traditional ceramics
• Glasses: based primarily on silica and distinguished by their non-crystalline structure and transformed into a largely crystalline structure by heat treatment.
• the strength of ceramics should be higher than metals because their covalent and ionic bonding types are stronger than metallic bonding.
• metallic bonding allows for slip, the basic mechanism by which metals deform plastically when subjected to high stresses
• Bonding in ceramics is more rigid and does not permit slip under stress.
• The inability to slip makes it much more difficult for ceramics to absorb stresses.
• Ceramics contain the same imperfections in their crystal structure as metals - vacancies, displaced atoms, interstitialcies, and microscopic cracks.
• Internal flaws tend to concentrate stresses, especially tensile, bending, or impact.
• ceramics fail by brittle fracture much more readily than metals.
• The frailties that limit the tensile strength of ceramic materials are not nearly so operative when compressive stresses are applied.
• Ceramics are substantially stronger in compression than in tension.
• DENSITY: in general, ceramics are lighter than metals and heavier than polymers
• MELTING TEMPERATURES: higher than for most metals. Some ceramics decompose rather than melt
• ELECTRICAL AND THERMAL CONDUCTIVITIES: lower than for metals; but the range of values is greater, so some ceramics are insulators while others are conductors
• THERMAL EXPANSION: somewhat less than for metals, but effects are more damaging because of brittleness
• Traditional Ceramics, Primary products are fired clay (pottery, tableware, brick, and tile), cement, and natural abrasives such as alumina
• Glass is also a silicate ceramic material and is sometimes included among traditional ceramics
• Mineral silicates, such as clays of various compositions, and silica, such as quartz, are among the most abundant substances in nature and constitute the principal raw materials for traditional ceramics.
• Another important raw material for traditional ceramics is alumina
• Clays consist of fine particles of hydrous aluminum silicate
• When mixed with water, clay becomes a plastic substance that is formable and moldable
• When heated to a sufficiently elevated temperature (firing), clay fuses into a dense, strong material
• clay can be shaped while wet and soft, and then fired to obtain the final hard product
• The main source of quartz is sandstone
• Silica, Low in cost; also hard and chemically stable
• Silica, principal component in glass, and an important ingredient in other ceramic products including whiteware, refractories, and abrasives
• Bauxite - most alumina is processed from this mineral, which is an impure mixture of hydrous aluminum oxide and aluminum hydroxide plus similar compounds of iron or manganese
• Bauxite is also the principal source of metallic aluminum
• Corundum - a more pure but less common form of Al2O3, which contains alumina in massive amounts
• Alumina Ceramic is used as an abrasive in grinding wheels and as a refractory brick in furnaces.
• New Ceramics also refers to improvements in processing techniques that provide greater control over structures and properties of ceramic materials
• new ceramics are based on compounds other than variations of aluminum silicate, which form most of the traditional ceramic materials.