Material science

• Ceramic material


  An inorganic compound consisting of a metal (or semi metal) and one or more non metals
• Silica


  • Main ingredient in most glass products
• Alumina


  • Used in applications ranging from abrasives to artificial bones
• Kaolinite


  • The principle ingredient in most clay products
• General properties of ceramics for engineering applications

  • High hardness
  • Good electrical and thermal insulating characteristics
  • Chemical stability
  • High melting temperatures
  • Some are translucent, window glass being the clearest example
  • They are brittle and posses virtually no ductility
• Typical ceramic products


  • Clay construction products, such as bricks, clay pipes and building tiles
  • Refractory ceramics, which are capable of high temperature applications such as furnace walls, crucibles and molds
  • Cement used in concrete, used for construction and roads (concrete is a composite material, but its components are ceramics)
  • Whiteware products including pottery, stoneware, fine china, porcelain, and other table ware based on mixtures of clay and other minerals
  • Glass used in bottles, glasses, lenses, window panes and light bulbs
  • Glass fibres for thermal insulating wool, reinforced plastics (fibreglass) and fibre optics for communication lines
  • Abrasives such as aluminum oxide and silicon carbides
  • Cutting tool materials including tungsten carbide, aluminum oxide and cubic boron nitride
  • Ceramic insulators used in electrical transmission components, spark plugs, microelectronic chip substrates
  • Magnetic ceramics used in computer memories
  • Nuclear fuels based on the uranium oxide
  • Bioceramics which includes materials used in artificial teeth and bones
• Classification of most ceramic materials

  • Glasses
  • Structural clay products
  • Whitewares
  • Refractories
  • Abrasives
  • Cements
  • Advanced ceramics
• Three basic types of ceramic materials

  • Traditional Ceramics; includes silicates used for clay products such as pottery and bricks, common abrasives and bricks
  • New Ceramics; more recently developed ceramics based on non-silicates such as oxides and carbides, and generally possessing mechanical and physical properties that are superior or unique compared to traditional ceramics
  • Glasses: primarily based on silica and distinguished from the other ceramics by their noncrystalline structure. Glass ceramics are glasses that have been transformed into a largely crystalline structure by heat treatment.
• Structure and properties of ceramics

  • Ceramics have covalent and ionic bonding. These bonds are stronger than metallic bonding in metals, which accounts for high hardness, stiffness and low ductility of ceramics
  • High bonding of electrons makes ceramics poor conductors as opposed to free electrons in metals which makes metals good thermal and electrical conductors
  • The strong bonding also provides ceramics with high melting point
  • Some ceramics decompose instead of melting
  • Most ceramics have crystalline structures, which are more complex than those of metals. The structures of ceramics are more complex than those of metals because Ceramic molecules consist of atoms that are significantly different in size, Ion charges are different, Most ceramic materials consist of more than two elements leading to further complexity in the molecular structures
  • Mechanical and physical properties are affected by Grain size (high strength + toughness are in fine grained materials)
  • Some ceramics assume an amorphous structure or glassy phase rather than a crystalline form, e.g. glass
• Mechanical properties of ceramics

  • Brittle
  • Hardness and elastic modulus for many of the new ceramics are better/greater than for metals
  • Stiffness and hardness of traditional ceramics and glasses are significantly less than for new ceramics
  • Theoretically, ceramics should have high strength than metals because of the covalent and ionic bonds (stronger than metallic bonding) But metallic bonding is better in that it allows slip mechanisms by which metals deform plastically. Bonding in ceramics is so rigid that it does not allow slip. Inability of ceramics to slip reduces ability to withstand stress. Yet ceramics contain similar imperfections like metals, i.e. vacancies, interstitialcies, displaced atoms, and microscopic cracks. These internal flaws concentrate stress. The random nature of the imperfections and the influence of the processing variables especially for traditional ceramics make their performance much less predictable.
• Ceramics are substantially stronger in compression than in tension

  • For structural and engineering applications, they are loaded in compression rather than tension or bending.
• Common methods for strengthening ceramics

  1. Making the starting materials more uniform

  2. Decreasing grain size in polycrystalline ceramic products

  3. Minimizing porosity

  4. Introducing compressive surface stresses; for example, through application of glazes with low thermal expansions, so that the body of the product contracts after firing more than the glaze, thus putting the glaze in compression

  5. Using fibre reinforcement

  6. Heat treatments such as quenching alumina from temperatures in the slightly plastic region to strengthen it.
• Physical properties of ceramics

  • Most ceramics are lighter than metals but heavier than polymers
  • Melting temperatures are higher than for metals. Some ceramics decompose instead
  • Electrical and thermal conductivities are lower than metals, but range of values is greater (some ceramics insulators, others conductors)
  • Thermal expansion coefficients less than for metals (more damaging effects of thermal shock and thermal cracking)
• Glasses


  Noncrystalline silicates containing other oxides, notably CaO, Na2O, and K2O which influence the glass properties
• Typical soda–lime glass


  • Approximately 70 wt% SiO2, the balance being mainly Na2O (soda) and CaO (lime)
• Prime assets of glass materials

  • Optical transparency
  • Relative ease of fabrication
• Glass ceramics


  Most inorganic glasses can be made to transform from a non crystalline state to one that is crystalline by the proper high-temperature heat treatment. This process is called devitrification, and the product is a fine-grained polycrystalline material which is often called a glass–ceramic.
• Desirable characteristics of glass–ceramics

  • A low coefficient of thermal expansion, such that the glass–ceramic ware will not experience thermal shock
  • Relatively high mechanical strengths
  • Relatively high thermal conductivities
  • Some may be made optically transparent; others are opaque
• Fabrication of glass-ceramics


  Conventional glass-forming techniques may be used conveniently in the mass production of nearly pore-free ware.
• Commercial trade names of glass-ceramics

  • Pyroceram, Corningware, Cercor, and Vision
• Common uses of glass-ceramics

  • Ovenware and tableware, primarily because of their strength, excellent resistance to thermal shock, and their high thermal conductivity
  • Electrical insulators and as substrates for printed circuit boards
  • Architectural cladding
  • Heat exchangers and regenerators
• Clay


  One of the most widely used ceramic raw materials, found naturally in great abundance, often is used as mined without any upgrading of quality
• Forming of clay products

  When mixed in the proper proportions, clay and water form a plastic mass that is very amenable to shaping. The formed piece is dried to remove some of the moisture, after which it is fired at an elevated temperature to improve its mechanical strength.
• Two broad classifications of clay-based products

  • Structural clay products
  • Whitewares
• Structural clay products


  Include building bricks, tiles, and sewer pipes—applications in which structural integrity is important
• Whitewares


  Become white after the high-temperature firing. Included in this group are porcelain, pottery, tableware, and plumbing fixtures (sanitary ware). In addition to clay, many of these products also contain other ingredients, each of which has some role to play in the processing and characteristics of the finished piece.
• Important properties of refractories

  • The capacity to withstand high temperatures without melting or decomposing, and the capacity to remain unreactive and inert when exposed to severe environments
  • The ability to provide thermal insulation is often an important consideration
• Forms of refractory materials


  • Bricks
  • Other forms
• Typical applications of refractories

  Furnace linings for metal refining, glass manufacturing, metallurgical heat treatment, and power generation
• Factors affecting performance of refractories

  • Composition
  • Raw ingredients consisting of both large (or grog) particles and fine particles, which may have different compositions
  • Upon firing, the fine particles normally are involved in the formation of a bonding phase, which is responsible for the increased strength of the brick; this phase may be predominantly either glassy or crystalline
  • The service temperature is normally below that at which the refractory piece was fired
  • Porosity is one microstructural variable that must be controlled to produce a suitable refractory brick. Strength, load-bearing capacity, and resistance to attack by corrosive materials all increase with porosity reduction. At the same time, thermal insulation characteristics and resistance to thermal shock are diminished. The choice of the optimum porosity depends on the conditions of service.
• Prime requisites for abrasive ceramics


  • Hardness or wear resistance
  • A high degree of toughness to ensure that the abrasive particles do not easily fracture
  • Refractoriness to withstand high temperatures produced from abrasive frictional forces
• Common ceramic abrasives


  • Silicon carbide
  • Tungsten carbide (WC)
  • Aluminum oxide (or corundum)
  • Silica sand
• Forms of abrasives


  • Bonded to grinding wheels
  • Coated abrasives
  • Loose grains
• Bonded abrasives


  The abrasive particles are bonded to a wheel by means of a glassy ceramic or an organic resin. The surface structure should contain some porosity; a continual flow of air currents or liquid coolants within the pores that surround the refractory grains will prevent excessive heating.
• Coated abrasives

  An abrasive powder is coated on some type of paper or cloth material e.g. sandpaper. Wood, metals, ceramics, and plastics are all frequently ground and polished using this form of abrasive.
• Loose abrasives


  Grinding, lapping, and polishing wheels often employ loose abrasive grains that are delivered in some type of oil- or water-based vehicle. Diamonds, corundum, silicon carbide, and rouge (an iron oxide) are used in loose form over a variety of grain size ranges.
• Cements


  Several familiar ceramic materials are classified as inorganic cements e.g cement, plaster of paris, and lime. The characteristic feature of these materials is that when mixed with water, they form a paste that subsequently sets and hardens.
• Role of cements


  Solid and rigid structures having just about any shape may be expeditiously formed. Also, some of these materials act as a bonding phase that chemically binds particulate aggregates into a single cohesive structure. Under these circumstances, the role of the cement is similar to that of the glassy bonding phase that forms when clay products and some refractory bricks are fired. One important difference, however, is that the cementitious bond develops at room temperature.
• Portland cement


  Produced by grinding and intimately mixing clay and lime-bearing minerals in the proper proportions, and then heating the mixture to about 1400°C in a rotary kiln; this process, called calcination, produces physical and chemical changes in the raw materials. The resulting ''clinker'' product is then ground into a very fine powder to which is added a small amount of gypsum (CaSO4·2H2O) to retard the setting process.
• Properties of portland cement

  • Setting time and final strength depend on its composition
  • The setting and hardening of this material result from hydration reactions that occur between the various cement constituents and the water that is added. The hydrated products are in the form of complex gels or crystalline substances that form the cementitious bond.
  • Hydration reactions begin just as soon as water is added to the cement. These are first manifested as setting (i.e., the stiffening of the once-plastic paste), which takes place soon after mixing, usually within several hours. Hardening of the mass follows as a result of further hydration, a relatively slow process that may continue for as long as several years.
  • The process by which cement hardens is not one of drying, but rather, of hydration in which water actually participates in a chemical bonding reaction.