Ceramics

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Ceramics

  1. 1. CERAMICSDefinition: A compound of metallic and non-metallic elements prepared by the action of heat and subsequent cooling. There are two general categories of ceramic; Traditional ceramics – tiles, brick, sewer pipe, pottery Industrial ceramics (engineering, high-tech, or fine ceramics) – turbine, semiconductors, cutting tools The structure of ceramics is maybe crystalline or partly crystalline structure, or may be amorphous. Generally atoms in ceramics are covalent or ionic bonded and the much stronger is metallic bonds. The hardness and thermal and electrical resistance in ceramics are better than in metals. The grain size influences the structure of ceramics (finer grain size has give higher strength and toughness). The oldest materials to make ceramics is clay (fine-grained sheet like structure) i.e. kaolinite (a white clay of silicate of aluminum with alternating weakly bonded layers of silicon and aluminum ions). The other common materials are flint (a rock composed of very fine grained SiO₂) and feldspar (a group of crystalline minerals of aluminum silicate and potassium, calcium or sodium). Porcelain is a white ceramic made of kaolin, quartz, and feldspar used mostly in kitchen appliance and bath ware.
  2. 2. Mechanical properties
  3. 3. Physical properties Most ceramics have low specific gravity. They also have very high melting or decomposition temperatures. The thermal conductivity of ceramics decrease with increasing temperature and porosity because air is a poor thermal conductor. k = kₒ(1 – P) kₒ = thermal conductivity at zero porosity P = the porosity as a fraction of the total volume Thermal shock or thermal fatigue may be caused by internal stresses formed during thermal expansion and thermal conductivity. Thermal cracking or spalling (a small piece or layer from the surface break off) will not occur when combine with lower thermal expansion and high thermal conductivity. Anisotropy of thermal expansion • that varies with different direction which lead to cracking.
  4. 4. Alumina Also called corundum or emery Most widely used Used in pure form or as raw material High hardness and moderate strength Alumina + other oxides are used as refractory materials for high-temp applications Suitable as electrical and thermal insulation, cutting tools/abrasives, etc.
  5. 5. Zirconia Good toughness, good resistance to thermal shock, wear and corrosion Have low friction coefficient Used in hot extrusion die, grinding beads/dispersion media for aerospace coatings, etc. Have thermal stability and low thermal conductivity
  6. 6. Carbides Made of tungsten and titanium,silicon Examples : Tungsten carbide (WC), titanum carbide (TiC), silicon carbide (SiC)
  7. 7. Nitrides Cubic boron nitride (CBN) Titanum nitride (TiN) Silicon nitride (Si3N4)
  8. 8. Glass Amorphous solid Super-cooled liquid (cooled at a rate too high for crystal formation) Content •more than 50% silica (glass former) Types of commercial glasses •¨ sodalime glass (most common), lead alkali glass, borosilicate glass, aluminosilicate glass, 96% silica glass, fused silica glass Thermal classification - hard (greater heat, e.g., borosilicate) or soft glass (e.g., soda lime glass •¨ lampworking)
  9. 9. Mechanical properties Physical properties Perfectly elastic and  Low coefficient of brittle thermal expansion Bulk form has strength  High electrical resistivity +/- 140MPa  Dielectric strength Strength measurement  CTE lower than metals → bending and plastic, may Static fatigue (same approach zero with ceramics)
  10. 10. Glass ceramics High crystalline microstructure Stronger than glass Shaped and then heat treated Treatment •process known as devitrification(recrystallization of glass) Near •zero coefficient of thermal expansion, high thermal shock resistance
  11. 11. Graphite Crystalline form of carbon •layered structure Basal planes or sheets of close packed C atoms Weak when sheared along the layers Also known as lampblack •(pigment High electrical and thermal conductivity Good resistance to thermal shock and high temperature
  12. 12. Types of graphite Fibers •- important use in reinforced plastics and composite materials Foams - high service temperature, chemical inertness, low coefficient of thermal expansion and electrical properties Carbon foams - graphitic or non-graphitic structures Buckyballs - carbon molecules in the shape of soccer balls. Also called fullerents, chemicallyinert, and act like solid lubricant particles
  13. 13. Diamond Diamond-Like Carbon (DLC) •developed as diamond film coating Can be coated with Ni, Cu, or Ti for improved performance Cutting tools materials (single or polycrystalline) Abrasive in grinding Dressing of grinding wheels (abrasive sharpening) Dies for wire drawing Cutting tools and dies coating

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