Chapter 10 Chapter 10 Ceramics and Glass Objectives • Upon

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Chapter 10 Chapter 10 Ceramics and Glass Objectives • Upon Powered By Docstoc
					 Chapter 10

Ceramics and Glass
                       Objectives
•   Upon completion of this chapter you should understand:
•   Structure of ceramics and glass.
•   Composition of ceramics.
•   Classification of ceramics.
•   Physical properties of ceramics.
                 Key Concepts
• About 10,000 years ago people began forming and
  heating clay to make containers to store food and
  water.
• Ceramics are used in the space shuttle (fig 10-1, page
  176) to protect the shuttle and its passengers from
  high reentry temperatures.
• Ceramics are hard brittle compounds.
• Ceramics is derived from a Greek word meaning
  burnt earth.
                    Introduction
• Ceramics are composed of the most abundant elements on
  earth: silica, alumina, and magnesia.
• Heat is used to cure the ceramic – this is called firing.
• Ceramics are classified as crystalline or non crystalline.
• Crystalline ceramics form lattice structures of repeating
  patterns (fig 10-2, page 177).
• Non crystalline ceramics have low porosity and their atomic
  structure is not ordered – example is glass.
• Traditional ceramics are made from naturally occurring earth
  elements referred to as clay.
                     Composition
• The principal elements found in these clays are oxygen,
  silicon, and alumina.
• Often quartz and feldspar are added to improve its properties.
• Feldspar is a universal flux that cleans the clay of oxides and
  other impurities.
• Commonly used clays in ceramics include: Kaolin (China clay),
  Ball Clay (used in white ware), Fire Clay (used for lining
  furnaces, flues, and fire brick), Slip Clay (used as glaze on
  ceramic products), and Flint Clay (used for mixing in ceramics).
              Classifying Ceramics
• Ceramics are commonly classified according to use (fig 10-4,
  page 179) as: structural clay, refractory, porcelain enamel,
  white ware, glass, and advanced engineering ceramics.
• 1. Structural Clay: Representing approximately 2% of the total
  ceramic sales, structural clay products include brick, drain tile,
  flue linings, and floor tile.
• Bricks were manufactured and used as early as 3000 BC to
  build temples and fortifications.
• 2. Refractories: Refractory ceramics represent around 7% of
  total ceramic sales. Refractory ceramics are used in crucibles
  used in melting metals, and for brick linings of large steel
  making furnaces or kilns used to fire ceramic whiteware.
                    Classifying Ceramics
•   3. Porcelain Enamel: They are glassy castings applied to kitchen and bathroom
    appliances (fig 10-7, page 181).
•    They represent approximately 9% of annual sales in ceramics.
•   4. Whiteware: They are used to manufacture dinnerware, ceramic bathroom tile,
    fine china, and earthenware (fig 10-8, page 181).
•    They represent approximately 10% of annual sales in ceramics.
•   Ceramic whiteware are formed by slip casting or dry pressing followed by a firing
    process called vitrification.
•   Vitrification used a high temperature to cause partial melting and produces a
    dense, hard structure.
•   Vitrification is followed by glazing. Glazing is applying a glassy coating to seal the
    surface of the whiteware product.
•   The glaze prevents absorption of liquids and improves its appearance.
•   Pigments may be added to the glaze to change the color of the product.
                 Classifying Ceramics
• 5. Glass: The basic ingredient of glass is silica.
• They represent approximately 55% of annual sales in ceramics.
• Glass is used to make containers, light bulbs, TV tubes, lab glassware, plate
  glass for windows, and glass fiber for insulation and reinforced plastic.
• Plate glass represents around 32% of sales.
• 6. Advanced Engineered Ceramics: Growing demand for special
  applications in electronics, transportation, nuclear power, and
  communication.
• Optical fibers, capacitors, electrical porcelain, and electrical insulators are
  some applications (fig 10-10, page 183).
• They represent approximately 17% of annual sales in ceramics.
                               Properties
•   Ceramics are hard, brittle materials that do not readily conduct heat or electricity.
•   1. Mechanical Properties: Important mechanical properties include tensile
    strength, compressive strength, flexing strength, stiffness, fracture toughness, and
    hardness.
•   Tensile and compressive strength: Ceramics, especially concrete have been used
    where resistance to compressive forces are required.
•   Generally the compressive strength of ceramics is many times greater than their
    tensile strength.
•   Because of their brittle nature ceramics do not have plastic deformation.
•   Ceramics are the stiffest of all engineering materials.
•   The greatest weakness of traditional ceramics is their brittle nature.
•   Ceramics are the hardest of all industrial materials. They make some of the best
    abrasive and cutting material. Tungsten Carbide is one of the most popular
    materials for blades and cutting tools.
                    Properties
• 2. Physical Properties: Physical properties are
  defined as characteristics of materials resulting from
  interaction with various forms of energy.
• Ceramics are a poor conductor of heat and
  electricity.
• Ceramics have the lowest thermal expansion of all
  materials (fig 10-16, page 189).
• Ceramics are excellent insulators (fig 10-17, page
  190).
                      Forming Ceramics
•   In traditional ceramics, silica, clay, fluxes, and refractory material in powder forms
    are carefully blended.
•   Most ceramic products are formed by casting.
•   Slip casting is used to form hollow shapes such as toilets and sinks. Slip is a low
    viscosity ceramic slurry. The slip is poured into a Plaster of Paris mould. When the
    desired thickness is achieved the remaining slip is poured from the mold leaving
    the desired product (fig 10-19, page 191).
•   Extrusion: During extrusion, the uncured ceramic body is forced through a die. The
    extruded ceramic is fed onto a conveyor belt where it is cut to length (fig 10-20,
    page 193). Bricks, pipes, ceramic tubing, and electrical ceramic components are
    formed by extrusion.
•   Jiggering: The uncured ceramic body is placed between matching halves of a mold.
    The clay and one half of the mold rotates as the other half is pressed against them.
    Dinner plates are made using this process (fig 10-21, page 193).
                   Forming Ceramics
• Die Pressing: ceramic floor tiles, ceramic wall tile, and many electronic
  components are formed by die pressing (fig 10-22, page 193). The uncured
  clay is placed in steel or carbide dies and hydraulic force is applied to
  compact the clay.
• Powder pressing uses fine ceramic powders forced under load and
  heated. This causes the powder to stick together.
• Tape Casting: A powdered slurry is formed on a thin layer of moving
  plastic belt (fig 10-24, page 194). This is the process used to form ceramic
  substrates for integrated electronic circuits.
• Injection Molding: Similar to injection molding of plastics (fig 10-25, page
  195). The mold has a cavity the shape of the desired product. Injection
  molding is used to form complex solid shapes.
            Post Forming Processes
• The purpose of drying is to remove moisture from the green ceramic body.
• Densification is achieved by sintering, hot pressing, or hot isostatic
  pressing.
• Sintering involves applying a high temperature that reduces porosity and
  causes densification. Sintering is usually called firing or vitrification.
• Hot pressing is used to densify ceramics that cannot be sintered. Hot
  pressing involves the use of a graphite die, pressure, and heat to reduce
  porosity, thus increasing density.
• Hot isostatic pressing (HIP) involves pressing done at high temperatures
  using gas as the pressurizing fluid instead of liquid.
                                    Summary
•   Ceramics are composed of the most abundant elements on earth: silica, alumina, and
    magnesia.
•   Traditional ceramics are made from naturally occurring earth elements referred to as clay.
•   Ceramics are commonly classified according to use as: structural clay, refractory, porcelain
    enamel, white ware, glass, and advanced engineering ceramics.
•   Glass: The basic ingredient of glass is silica. They represent approximately 55% of annual
    sales in ceramics.
•   Mechanical Properties: Important mechanical properties include tensile strength,
    compressive strength, flexing strength, stiffness, fracture toughness, and hardness.
•   Ceramics are the hardest of all industrial materials. They make some of the best abrasive and
    cutting material. Tungsten Carbide is one of the most popular materials for blades and
    cutting tools.
•   Slip casting is used to form hollow shapes such as toilets and sinks. Slip is a low viscosity
    ceramic slurry. The slip is poured into a Plaster of Paris mould. When the desired thickness is
    achieved the remaining slip is poured from the mold leaving the desired product.
•   Sintering involves applying a high temperature that reduces porosity and causes
    densification. Sintering is usually called firing or vitrification.
                  Home Work
• 1. How are ceramics used in the space shuttle?
• 2. Where is structural clay used?
• 3. What is glazing, and what is its advantage?
• 4. What are some applications of advanced
  (engineered) ceramics?
• 5. Give an example of a popular ceramic cutting
  material.
• 6. How are most ceramics formed?

				
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