Module 5

• Ceramics


  Came from the Greek word keramos meaning potter's clay and keramikos meaning clay products
• Earliest use of ceramics was in pottery and bricks, dating back to before 4000BC
• Properties of ceramics

  • HIGH temperature strength
  • Hardness at elevated temperatures
  • Elastic modulus
  • Inertness to chemicals
  • Melting point
  • Resistance to wear and corrosion
• Properties of ceramics

  • LOW toughness
  • Density
  • Thermal expansion
  • Thermal and electrical conductivity
• Ceramics are compounds of metallic and nonmetallic elements— large number of possible combinations
• Types of ceramics

  • Traditional Ceramics
  • Industrial Ceramics
• Structure of ceramics

  • Contains various atoms of different sizes, and is among the most complex of all material structures
  • Generally covalent (electron sharing) or ionic (primary bonding between oppositely charged ions) bonding
  • Available in single crystal, or in polycrystalline form, consisting of many grains
  • Grain size has a major influence on the strength and properties of ceramics
  • Finer grain size ► higher strength and toughness
• Clay


  Fine-grained sheetlike structure; when water is added, it attaches itself to the layers (adsorption), making it slippery, soft, and have plastic properties (hydroplasticity) making it formable
• Kaolin


  Consisting of silicate of aluminum with altering weakly bonded layers of silicon and aluminum ions
• Flint


  Rock composed of very fine-grained silica SiO2
• Feldspar


  Group of crystalline minerals consisting of aluminum silicate plus potassium, calcium, or sodium
• Types of ceramics

  • Oxide Ceramics
  • Carbides
  • Nitrides
  • Cermets
  • Silica
  • Silicates
  • Nanophase Ceramics
• Alumina (Aluminum oxide, Al2O3)


  Also known as Corundum or Emery; used either in pure form or as a raw material to be mixed with other oxides; although it exists in nature, it contains unknown amounts of impurities— hence, its behavior is unreliable; manufactured totally synthetically so that their quality can be controlled
• Synthetic Aluminum oxide


  Obtained by the fusion of molten bauxite, iron filings, and coke in electric furnaces; applications include electrical and thermal insulation, cutting tools and abrasives
• Zirconia (Zirconium oxide, ZrO2)


  White in color; good resistance to thermal shock, wear, and corrosion; low thermal conductivity, and low friction coefficient
• Partially Stabilized Zirconia (PSZ)


  Has better reliability in performance than zirconia; typical applications include dies for hot extrusion of metals and zirconia beads for grinding and dispersion media for aerospace coatings
• Tungsten carbide


  Cobalt binder; Toughness increases with cobalt content, whereas hardness, strength, and wear resistance decrease
• Titanium carbide


  Nickel and Molybdenum binders
• Silicon carbide


  Made from silica sand, coke, and small amounts of sodium chloride and sawdust
• Cubic Boron nitride


  Does not exist in nature and was first made synthetically in the 1970's, with techniques similar to those used in making synthetic diamond
• Titanium nitride


  Improves tool life
• Silicon nitride


  Has low thermal expansion, high thermal conductivity; used for high temperature applications
• Sialon


  Derived from silicon, aluminum, oxygen, and nitrogen; has higher strength and thermal-shock resistance than silicon nitride and thus far is used primarily as a cutting tool material
• Cermets


  Also called black ceramics or hot-pressed ceramics; Combinations of ceramic phase bonded with a metallic phase; Combine the high-temperature oxidation resistance of ceramics and the toughness, thermal-shock resistance, and ductility of metals; can be regarded as composite material
• Silica


  Abundant in nature; A polymorphic material— it can have different crystal structures; Most glasses contain more than 50% silica
• Quartz


  Most common form of silica; hard, abrasive hexagonal crystal; used extensively as oscillating crystals of fixed frequency in communications applications, since it exhibits the piezoelectric effect
• Silicates


  Products of the reaction of silica with oxides of aluminum, magnesium, calcium, potassium, sodium, and iron; e.g. clay, asbestos, mica, and silicate glasses
• Nanophase Ceramics


  A recent development which consists of atomic clusters containing a few thousand atoms; exhibits ductility at significantly lower temperatures than conventional ceramics; stronger and easier to fabricate and to machine, with fewer flaws
• Applications of Nanophase Ceramics

  • Automotive industry
  • Jet components
• Mechanical properties of ceramics

  • Strength in tension is approximately one order of magnitude lower than their compressive strength due to its sensitivity to cracks, impurities, and porosity
  • Lack impact toughness and thermal-shock resistance, because of their inherent lack of ductility
  • Exhibit static fatigue, that is the material suddenly fails when subjected to static tensile load over a period of time
• Prestressing


  The shaped ceramic components are subjected to compressive stresses done before subjecting to tensile stresses; Methods include: heat treatment and chemical tempering, laser treatment of surfaces, coating with ceramics having different thermal-expansion coefficients, and surface-finishing operations, such as grinding, in which compressive residual stresses are induced on the surfaces
• Tensile strength of polycrystalline ceramic parts


  Increases with decreasing grain size and porosity, the latter by the expression: UTS=UTSOe-np ; P=volume fraction of pores in the solid;UTSO=tensile strength at zero porosity; n ranges between 4 to 7
• Modulus of elasticity of ceramics

  Approximately related to porosity by the expression:E=EO(1-1.9P+0.9P2); EO =modulus at zero porosity
• Physical properties of ceramics

  • Relatively low specific gravity ranging from about 3 to 5.8 for oxide ceramics, compared to 7.86 for iron
  • Very high melting or decomposition temperatures
  • Lower tendency toward thermal cracking with low thermal expansion and high thermal conductivity
  • Spalling occurs when a piece or a layer from the surface breaks off
  • Porosity influences optical properties and makes the material less transparent and gives it a white appearance
• Applications of ceramics

  • Porcelain
  • Refractories
  • Ceramic Armor
  • Glasses
  • Glass Ceramics
  • Graphite
  • Diamond
• Porcelain


  A white ceramic composed of kaolin, quartz, and feldspar; its largest use is in appliances and sanitary ware
• Refractories


  Used to provide thermal protection of other materials in very high temperatures
• Ceramic Armor


  Ceramic armor systems are used to protect military personnel and equipment; the ceramic material is discontinuous and is sandwiched between a more ductile outer and inner skin; Most of the impact energy is absorbed by the fracturing of the ceramic and any remaining kinetic energy is absorbed by the inner skin, which also serves to contain the fragments of the ceramic and the projectile preventing severe impact with the personnel/equipment being protected; Advantage: low density of the material can lead to weight-efficient armor systems
• Glasses


  Amorphous Ceramics; Defined as an inorganic product of fusion that has cooled to a rigid condition without crystallizing; an amorphous solid with the structure of a liquid; has been supercooled (cooled at a rate too high to allow crystals to form) with no distinct melting or freezing point
• Composition of glasses

  All glasses contain at least 50% silica— known as a glass former; The composition and properties of glasses, except their strength, can be modified greatly by the addition of oxides; oxides are known as intermediates or modifiers