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(CERAMIC FABRICATION TECHNIQUES) - Coggle Diagram

- - - - 1.)Preparation of raw materials and melting
        
        The principal component in nearly all glasses is silica (SiO2).Primary source is natural quartz in sand
        
        Other components, such as soda ash (source of Na2O),limestone (source of CaO), aluminium oxide (Al2O3)andpotash (source of K2O) are added in proportions to achieve the desired composition.
        
        Recycled glass is usually added to the mixture too- to preserving the environment - to facilitates melting
      - 2.)Shaping /forming,
        
        Continuous processes for making flat glass (sheet and
        plate glass) and tubing (fluorescent lights, laboratory wares)
        
        Drawing
        
        1. Rolling of flat plate :Starting glass from melting furnace is squeezed through opposing rolls whose gap determines sheet thickness, followed by grinding and polishing for parallelism and smoothness. .
        
        2. Float Process. Molten glass flows onto the surface of a molten tin bath, where it spreads evenly across the surface, achieving a uniform thickness and smoothness - no grinding or polishing is needed.
        
        3. Danner process.- Molten glass flows around a rotating hollow mandrel through which air is blown while the glass is drawn.
        
        Fiber-making process to produce fibers (for insulation and
        fiber optics)
        
        Fibrous glass for thermal insulation, acoustical insulation, and air filtration, in which the fibers are in a random, wool-like condition Ø Produced by centrifugal spraying
        
        Centrifugal Spraying Principle: The molten glass flows into a rotating bowl with many small orifices (opening) around its peripheryl (outer edge). – Centrifugal forces causes the glass to flow through the holes to become a fibrous glass.
        
        Long continuous filaments suitable for fiber reinforced plastics, yarns, fabrics, and fiber optics - Produced by drawing
        
        Glass Fiber Drawing Principle: Continuous glass fibers of small diameter (lower limit is about 0.0025 mm) are produced by drawing (pulling) strands of molten glass (by gravity) through small orifices in a heated plate made of a platinum alloy. As it drops, it cools and forms a thread.
        
        Discrete processes for piece ware (i.e. bottles, light bulbs,
        jars)
        
        Pressing
        
        for mass production of flat products such as plates, dishes, bake ware, and TV tube faceplates- glass formed by application of pressure- mold is steel with graphite lining
        
        Pressing of flat glass pieces: (1) glass hob is fed into mold from furnace; (2) pressing into shape by plunger; and (3) plunger is retracted and finished produced is removed (v indicates motion (velocity) and F indicate applied force.
        
        Blowing
        
        For production of wide-mouth containers such as jars
        
        Press-and-blow forming sequence: (1) molten gob is fed into mold cavity; (2) pressing to form a parison; (3) the partially formed parison, held in a neck ring, is transferred to the blow mold, and (4) blown into final shape.
        
        Casting
        
        for relatively massive objects such as large astronomical (telescope) lenses and mirrors that must cool very slowly to avoid cracking. - smaller lenses are usually made by pressing. - Process: If molten glass is sufficiently fluid, it can be poured into a mold. After cooling and solidifying, the piece must be finished by lapping and polishing
      - 3.) Heat treatment
        
        Annealing: Principle: Heating to elevated temperature and holding for a time to eliminate stresses and temperature gradients; then slow cooling to suppress stress formation, followed by more rapid cooling to room temperature • Function: removes internal stress caused by uneven cooling rate between the surface and interior regions. - same function in glassworking as in metalworking • Annealing temperature : 500 °C • Annealing is performed in tunnel-like furnaces, called lehrs, in which the products flow slowly through the hot chamber on conveyors.
        
        Tempered GlassTempered glass is a glass that has been subjected to an additional heat treatment after annealing in order to increase its mechanical strength. • Properties of tempered glass: q More resistant to scratching and breaking due to compressive stresses on its surface. q 4 to 5 times stronger than annealed glass (without tempering). q When fracturing, it breaks into small fragments – often referred to as “safety glass”. :
        
        Tempering: Principle: (Re)heating to a temperature somewhat above annealing temperature (typically above 600-650 °C) into the plastic range, followed by quenching of surfaces, usually by air jets, to create a desired stress profile within the material. • When the surfaces cool, they contract and harden while interior is still plastic. As the internal glass cools, it contracts, putting the hard surfaces in compression .
  - - - The raw ceramic material usually occurs in nature as rocky lumps, and reduction (from rocky lumps) to powder (particle size is reduced) is the purpose of this step. (i) raw materials go through milling or grinding operation using crushers such as roll crusher. (ii) the product of crusher is screening to make powder product having desired shape of particle size. Shaping processes for traditional ceramics require the starting material to be a plastic paste (ceramic powder + water). (iii) the fine powdered product must be thoroughly mixed with water to make plastic paste.
    - - Ingredients:. 1. Clay (hydrous aluminum silicates) - plastic material Ø usually the main ingredient because of ideal forming characteristics when mixed with water 2. Water Ø creates clay-water mixture with suitable plasticity for shaping 3. Non-plastic raw materials, such as alumina and silica Ø reduce shrinkage in drying and firing but also reduce plasticity of the mixture during forming 4. Other ingredients/agents, such as fluxes Ø fluxes melt(vitrify) during firing and promote sintering, and wetting agents to improve mixing of clay.
      - Dry pressing and (ii)Semi-dry pressing. Process sequence: Uses high pressure to overcome the clay’s low plasticity and force it into a die cavity.
      - Plastic forming
        
        The starting mixture must have a plastic consistency, with 15% to 25%water. Variety of manual and mechanized methods Manual methods use clay with more water because it is more easily formed, i.e. hand modeling  More water means greater shrinkage in drying Ø
        
        Mechanized methods (work with machinery) generally use a mixture with less water so starting clay is stiffer. - Jiggering - Plastic pressing - Extrusion
      - Hand modelling:Hand throwing on a potter's wheel is another refinement of handcraft methods. Potter's wheel = a round table that rotates on a vertical spindle, powered either by motor or foot operated treadle- Products of circular cross-section can be formedby throwing and shaping the clay, sometimes using a mold to provide the internal shape .
      - Jiggering or jolleying: This is extension of the potter's wheel methods, but hand
        throwing is replaced by mechanized techniques.
      - Slip casting
        
        Drain casting – (1) the slip is poured into the mold; (2) the mold is then inverted to drain excess slip after a semi-solid layer has been formed, thus producing a hollow product in the mold; (3) the mold is opened and the part is removed.
        
        Solid casting - (1) the slip is poured into the mold, to produce solid products, adequate time is allowed for entire body to become firm; (2) the mold is periodically resupplied with additional slip to account for shrinkage because of absorbed water; (3) the part is removed by breaking the mold through the neck.
    - - Water plays an important for better binding and shaping purpose. (water contributes volume to the piece) Therefore, during drying, the volume is reduced as the interparticle spacings decrease when water is removed - cause shrinkage.
    - - Firing is the heat treatment process that sinters the ceramic material. It is performed in a furnace called a kiln. During sintering, bonds are developed between the ceramic grains, and this is accompanied by densification and reduction of porosity. Therefore, additional shrinkage occurs in the polycrystalline material in addition to that which has already occurred in drying. In the firing of traditional ceramics, a glassy phase forms among the crystals which acts as a binder
    - - Application of a ceramic surface coating to make the piece more impervious to water and enhance its appearance (graze can serve to color, decorate or waterproof of a ceramic product).
      - The usual processing sequence with glazed ware is: 1. Fire the piece once before glazing to harden the body of the piece 2. Apply the glaze 3. Fire the piece a second time to harden the glaze.
  - - - Preparation of the starting powders include crushing and grinding (mechanical method) to the desired size range and chemically treating (chemical method) to separate phases and compounds and to achieve specified purity
    - - Powder Injection Molding (PIM):Ceramic particles are mixed with a thermoplastic polymer, then heated and injected into a moldcavity. The polymer acts as a carrier and provides flow characteristics for molding. Upon cooling which hardens the polymer, the mold is opened and the part is removed. The plastic binder is removed and the remaining ceramic part is sintered.
      - Powder pressing
        
        (i) Uniaxial compression:The die cavity is filled with powder. The powder is compacted by means of pressure applied to the top die. The compacted piece is ejected by rising action of the bottom punch.
        
        (ii) Isostatic compression:Pressure applied by fluid - powder in rubber envelope Uses hydrostatic pressure to compact the ceramic powders from all directions. Avoids the problem of nonuniform density in the final product that is often observed in conventional uniaxial pressing.
        
        (iii) Hot pressing Similar to dry pressing except it is carried out at elevated temperatures so sintering of the product is accomplished simultaneously with pressing. Hot pressing is mainly for densification of ceramics. This eliminates the need for a separate firing step. Higher densities and finer grain size are obtained, but die life is reduced by the hot abrasive particles against the die surfaces.
      - Slip Casting- 1. Preparation of a powdered ceramic material and a liquid (usually clay and water) into a stable suspension called a slip. 2. Pour the slip into a porous mold made of plaster of and allowing the liquid portion of the slip to be partially absorbed by the mold. As the liquid is removed from the slip, a solid layer is formed against the mold surface. 3. When the sufficient wall thickness has been formed, the excess slip is poured out of the cavity (Drain casting). 4. The material in the mold is allowed to dry to provide adequate strength for handling and the subsequent removal of the part from the mold.
      - Tape Casting: Thin sheets of green ceramic are casted as a flexible tape. • These sheets are casted from liquid slip (ceramic + organic solvent containing binders and plasticizers to enhance strength and flexibility to cast) • Used for integrated circuits and capacitors.
    - - During firing the formed piece shrinks, and experiences a reduction of porosity and an improvement in mechanical integrity. o These changes occur by the coalescence of the particle into a more dense mass. o The 3 stages in the sintering process - Powder touches to form neck - The neck gradually thickens - The grain boundary develop.
    - - Parts made of new ceramics sometimes require finishing, which has one or more of the following purposes: 1. Increase dimensional accuracy 2. Improve surface finish 3. Make minor changes in part geometry -Finishing usually involve abrasive processes - Diamond abrasives must be used to cut the hardened ceramic materials.
  - - - Glass is an inorganic solid material that is made from natural and abundant raw materials (sand, soda ash and limestone). 2. Main component of glass is silica.
      - Properties of glass are: a) Hard but brittle b) Chemically inert c) Not permeable to gas and liquid (fluid) d) Does not conduct electricity or heat e) Transparent
      - Type of glass
        
        a) Fused glass
        
        Fused quartz glass is the simplest glass. 2. Main component of glass is silica. 3. Properties of fused glass: a) High purity and optical transparency b) High softening point c) Low coefficient of thermal expansion d) Chemical durability e) Difficult and expensive to produce.
        
        b) Soda lime glass
        
        Soda-lime glass is the most common glass. 2. Produced by heating silica with sodium oxide and calcium oxide. 3. Properties of soda-lime glass: a) Low softening point- easy to make into different shapes b) High thermal coefficient of expansion c) Low resistance to chemical attacks
        
        c) Borosilicate glass
        
        Borosilicate glass is formed when boron oxide is added to soda-lime glass. 2. Was first developed by German glassmaker Otto Schott in the late 19th century. 3. Properties of borosilicate glass: a) Very low thermal expansion coefficient b) High softening point c) Resistant to thermal shock d) Chemically resistant
        
        d) Lead crystal glass
        
        Commonly known as lead crystal. 2. Made by using lead oxide and potassium oxide. 3. Lead glass has a relatively soft surface that is easy to grind, cut and engrave. 4. Properties of lead glass are: a) High refractive index b) High density c) Attractive glittery/shiny appearance
    - - Ceramics are inorganic non-metallic solid compounds which containing of metallic and non metallic elements, such as clay. 2. Ceramic is made from clay. 3. Main component of clay is aluminosilicate.
      - Properties a) Very hard and strong but brittle. b) Chemically inert. c) Has very high melting point. d) Heat and electrical insulators. e) Opaque and porous
      - Ceramics are used in many areas such as: a) Construction materials, e.g. bricks, roof tiles, sinks and toilet bowls. b) Decorative items, e.g pottery, plates, cups and vases and wall tiles c) Electrical insulators, e.g. insulators for toasters, refrigerators and electric plugs. d) Medical field, e.g. dentures and prosthetic limbs. 6. Ceramics can also be improved to new composite materials such as superconducters
    - - Glass-ceramics are mostly produced in two steps: (i) A glass is formed by a glass-manufacturing process (heating and forming operations used in glassworking create product shape). (ii) The glass is cooled down and then reheated to cause a dense network of crystal nuclei. - during this heat treatment, the glass partly crystallizes - mostly, nucleation agents such as TiO2, P2O5, and ZrO2 are added to control the crystallization process - once nucleation is started, heat treatment is continued at a higher temperature to cause growth of crystalline phases - high density of nucleation site inhibits grain growth and leads to fine grain size
      - Properties of glass-ceramics are: a) hermetic sealing properties of glass b) special properties of ceramics, e.g. high mechanical strength and toughness (stronger than glass) c) low coefficients of thermal expansion (due to absence of porosity) d) high resistance to thermal shock e) good dielectric properties (electrical insulators) f) special optical properties – can be made optically transparent g) good biological properties h) low porosity