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Engineering materials 06

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Usman Shah

This document discusses different types of ceramic materials and their applications. It describes glasses, glass-ceramics, clay products, refractories, and abrasives. Glasses are used for containers and lenses. Glass-ceramics have high strength and temperature capabilities and are used for ovenware and electronics. Clay products include bricks, tiles, and pottery. Refractories withstand high temperatures and include fireclay, silica, and basic bricks. Abrasives like silicon carbide are used for grinding and require hardness.

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Engineering Materials Instructors: Zafar Shakoor
Ceramics Introduction • The preceding discussions of the properties of materials have demonstrated that there is a significant disparity between the physical characteristics of metals and ceramics. • Consequently, these materials are utilized in totally different kinds of applications and, in this regard, tend to complement each other and also the polymers. • Most ceramic materials fall into an application- classification scheme that includes the following groups: glasses, structural clay products, whitewares, refractories, abrasives, cements, and the newly developed advanced ceramics.
Classification of ceramic materials on the basis of application.
Types and Applications of Ceramics GLASSES • The glasses are a familiar group of ceramics; containers, lenses, and fiberglass represent typical applications. • As already mentioned, they are noncrystalline silicates containing other oxides, notably CaO, Na2O, K2O, and Al2O3, which influence the glass properties. • A typical soda–lime glass consists of approximately 70 wt% SiO2, the balance being mainly Na2O (soda) and CaO (lime). • Possibly the two prime assets of these materials are their optical transparency and the relative ease with which they may be fabricated.
GLASS–CERAMICS • Most inorganic glasses can be made to transform from a noncrystalline state to one that is crystalline by the proper high- temperature heat treatment. • This process is called crystallization, and the product is a fine-grained polycrystalline material which is often called a glass–ceramic.
Properties and Applications of Glass– Ceramics • Glass-ceramic materials have been designed to have the following characteristics: • Relatively high mechanical strengths; low coefficients of thermal expansion (to avoid thermal shock); relatively high temperature capabilities; good dielectric properties (for electronic packaging applications); and good biological compatibility. • Some glass–ceramics may be made optically transparent; others are opaque. • Possibly the most attractive attribute of this class of materials is the ease with which they may be fabricated; conventional glass-forming techniques may be used conveniently in the mass production of nearly pore-free ware.
Properties and Applications of Glass– Ceramics • Glass–ceramics are manufactured commercially under the trade names of Pyroceram™, Corningware™, Cercor™, and Vision™. • The most common uses for these materials are as ovenware, tableware, oven windows, and rangetops—primarily because of their strength and excellent resistance to thermal shock. • They also serve as electrical insulators and as substrates for printed circuit boards, and are used for architectural cladding, and for heat exchangers and regenerators.
CLAY PRODUCTS • One of the most widely used ceramic raw materials is clay. • This inexpensive ingredient, found naturally in great abundance, often is used as mined without any upgrading of quality. • Another reason for its popularity lies in the ease with which clay products may be formed; 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.
CLAY PRODUCTS • Most of the clay-based products fall within two broad classifications: the structural clay products and the whitewares. • Structural clay products include building bricks, tiles, and sewer pipes—applications in which structural integrity is important. • The whiteware ceramics become white after the high-temperature firing. • Included in this group are porcelain, pottery, tableware, china, and plumbing fixtures (sanitary ware). • In addition to clay, many of these products also contain nonplastic ingredients, which influence the changes that take place during the drying and firing processes, and the characteristics of the finished piece.
REFRACTORIES • Another important class of ceramics that are utilized in large tonnages is the refractory ceramics. • The salient properties of these materials include the capacity to withstand high temperatures without melting or decomposing, and the capacity to remain unreactive and inert when exposed to severe environments. • In addition, the ability to provide thermal insulation is often an important consideration. • Refractory materials are marketed in a variety of forms, but bricks are the most common. • Typical applications include furnace linings for metal refining, glass manufacturing, metallurgical heat treatment, and power generation.
REFRACTORIES • Of course, the performance of a refractory ceramic, to a large degree, depends on its composition. • On this basis, there are several classifications—namely, fireclay, silica, basic, and special refractories. • For many commercial materials, the raw ingredients consist 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.
REFRACTORIES • 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. • Of course, the optimum porosity depends on the conditions of service.
Fireclay Refractories • The primary ingredients for the fireclay refractories are high-purity fireclays, alumina and silica mixtures usually containing between 25 and 45 wt% alumina. • Fireclay bricks are used principally in furnace construction, to confine hot atmospheres, and to thermally insulate structural members from excessive temperatures. • For fireclay brick, strength is not ordinarily an important consideration, because support of structural loads is usually not required. • Some control is normally maintained over the dimensional accuracy and stability of the finished product.
Silica Refractories • The prime ingredient for silica refractories, sometimes termed acid refractories, is silica. • These materials, well known for their high-temperature load- bearing capacity, are commonly used in the arched roofs of steel- and glass-making furnaces; for these applications, temperatures as high as 1650 C may be realized. • Under these conditions some small portion of the brick will actually exist as a liquid. The presence of even small concentrations of alumina has an adverse influence on the performance of these refractories. • Substantial amounts of liquid may be present at temperatures in excess of 1600 C. • Thus, the alumina content should be held to a minimum, normally to between 0.2 and 1.0 wt%
Silica Refractories • These refractory materials are also resistant to slags that are rich in silica (called acid slags) and are often used as containment vessels for them. • On the other hand, they are readily attacked by slags composed of a high proportion of CaO and/or MgO (basic slags), and contact with these oxide materials should be avoided.
Basic Refractories • The refractories that are rich in periclase, or magnesia (MgO), are termed basic; they may also contain calcium, chromium, and iron compounds. • The presence of silica is deleterious to their high- temperature performance. • Basic refractories are especially resistant to attack by slags containing high concentrations of MgO and CaO, and find extensive use in some steel- making open hearth furnaces.
Special Refractories • There are yet other ceramic materials that are used for rather specialized refractory applications. • Some of these are relatively high-purity oxide materials, many of which may be produced with very little porosity. • Included in this group are alumina, silica, magnesia, beryllia (BeO), zirconia (ZrO2), and mullite (3Al2O3–2SiO2). • Others include carbide compounds, in addition to carbon and graphite. • Silicon carbide (SiC) has been used for electrical resistance heating elements, as a crucible material, and in internal furnace components. • Carbon and graphite are very refractory, but find limited application because they are susceptible to oxidation at temperatures in excess of about 88 C. • As would be expected, these specialized refractories are relatively expensive.
ABRASIVES • Abrasive ceramics are used to wear, grind, or cut away other material, which necessarily is softer. • Therefore, the prime requisite for this group of materials is hardness or wear resistance; in addition, a high degree of toughness is essential to ensure that the abrasive particles do not easily fracture. • Furthermore, high temperatures may be produced from abrasive frictional forces, so some refractoriness is also desirable.
ABRASIVES • Diamonds, both natural and synthetic, are utilized as abrasives; however, they are relatively expensive. • The more common ceramic abrasives include silicon carbide, tungsten carbide, aluminum oxide (or corundum), and silica sand.
ABRASIVES • Abrasives are used in several forms—bonded to grinding wheels, as coated abrasives, and as loose grains. • In the first case, 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 prevents excessive heating. • Coated abrasives are those in which an abrasive powder is coated on some type of paper or cloth material; sandpaper is probably the most familiar example.
ABRASIVES • Wood, metals, ceramics, and plastics are all frequently polished using this form of abrasive. • 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.

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Engineering materials 06

  • 2. Ceramics Introduction • The preceding discussions of the properties of materials have demonstrated that there is a significant disparity between the physical characteristics of metals and ceramics. • Consequently, these materials are utilized in totally different kinds of applications and, in this regard, tend to complement each other and also the polymers. • Most ceramic materials fall into an application- classification scheme that includes the following groups: glasses, structural clay products, whitewares, refractories, abrasives, cements, and the newly developed advanced ceramics.
  • 3. Classification of ceramic materials on the basis of application.
  • 4. Types and Applications of Ceramics GLASSES • The glasses are a familiar group of ceramics; containers, lenses, and fiberglass represent typical applications. • As already mentioned, they are noncrystalline silicates containing other oxides, notably CaO, Na2O, K2O, and Al2O3, which influence the glass properties. • A typical soda–lime glass consists of approximately 70 wt% SiO2, the balance being mainly Na2O (soda) and CaO (lime). • Possibly the two prime assets of these materials are their optical transparency and the relative ease with which they may be fabricated.
  • 5. GLASS–CERAMICS • Most inorganic glasses can be made to transform from a noncrystalline state to one that is crystalline by the proper high- temperature heat treatment. • This process is called crystallization, and the product is a fine-grained polycrystalline material which is often called a glass–ceramic.
  • 6. Properties and Applications of Glass– Ceramics • Glass-ceramic materials have been designed to have the following characteristics: • Relatively high mechanical strengths; low coefficients of thermal expansion (to avoid thermal shock); relatively high temperature capabilities; good dielectric properties (for electronic packaging applications); and good biological compatibility. • Some glass–ceramics may be made optically transparent; others are opaque. • Possibly the most attractive attribute of this class of materials is the ease with which they may be fabricated; conventional glass-forming techniques may be used conveniently in the mass production of nearly pore-free ware.
  • 7. Properties and Applications of Glass– Ceramics • Glass–ceramics are manufactured commercially under the trade names of Pyroceram™, Corningware™, Cercor™, and Vision™. • The most common uses for these materials are as ovenware, tableware, oven windows, and rangetops—primarily because of their strength and excellent resistance to thermal shock. • They also serve as electrical insulators and as substrates for printed circuit boards, and are used for architectural cladding, and for heat exchangers and regenerators.
  • 8. CLAY PRODUCTS • One of the most widely used ceramic raw materials is clay. • This inexpensive ingredient, found naturally in great abundance, often is used as mined without any upgrading of quality. • Another reason for its popularity lies in the ease with which clay products may be formed; 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.
  • 9. CLAY PRODUCTS • Most of the clay-based products fall within two broad classifications: the structural clay products and the whitewares. • Structural clay products include building bricks, tiles, and sewer pipes—applications in which structural integrity is important. • The whiteware ceramics become white after the high-temperature firing. • Included in this group are porcelain, pottery, tableware, china, and plumbing fixtures (sanitary ware). • In addition to clay, many of these products also contain nonplastic ingredients, which influence the changes that take place during the drying and firing processes, and the characteristics of the finished piece.
  • 10. REFRACTORIES • Another important class of ceramics that are utilized in large tonnages is the refractory ceramics. • The salient properties of these materials include the capacity to withstand high temperatures without melting or decomposing, and the capacity to remain unreactive and inert when exposed to severe environments. • In addition, the ability to provide thermal insulation is often an important consideration. • Refractory materials are marketed in a variety of forms, but bricks are the most common. • Typical applications include furnace linings for metal refining, glass manufacturing, metallurgical heat treatment, and power generation.
  • 11. REFRACTORIES • Of course, the performance of a refractory ceramic, to a large degree, depends on its composition. • On this basis, there are several classifications—namely, fireclay, silica, basic, and special refractories. • For many commercial materials, the raw ingredients consist 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.
  • 12. REFRACTORIES • 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. • Of course, the optimum porosity depends on the conditions of service.
  • 13. Fireclay Refractories • The primary ingredients for the fireclay refractories are high-purity fireclays, alumina and silica mixtures usually containing between 25 and 45 wt% alumina. • Fireclay bricks are used principally in furnace construction, to confine hot atmospheres, and to thermally insulate structural members from excessive temperatures. • For fireclay brick, strength is not ordinarily an important consideration, because support of structural loads is usually not required. • Some control is normally maintained over the dimensional accuracy and stability of the finished product.
  • 14. Silica Refractories • The prime ingredient for silica refractories, sometimes termed acid refractories, is silica. • These materials, well known for their high-temperature load- bearing capacity, are commonly used in the arched roofs of steel- and glass-making furnaces; for these applications, temperatures as high as 1650 C may be realized. • Under these conditions some small portion of the brick will actually exist as a liquid. The presence of even small concentrations of alumina has an adverse influence on the performance of these refractories. • Substantial amounts of liquid may be present at temperatures in excess of 1600 C. • Thus, the alumina content should be held to a minimum, normally to between 0.2 and 1.0 wt%
  • 15. Silica Refractories • These refractory materials are also resistant to slags that are rich in silica (called acid slags) and are often used as containment vessels for them. • On the other hand, they are readily attacked by slags composed of a high proportion of CaO and/or MgO (basic slags), and contact with these oxide materials should be avoided.
  • 16. Basic Refractories • The refractories that are rich in periclase, or magnesia (MgO), are termed basic; they may also contain calcium, chromium, and iron compounds. • The presence of silica is deleterious to their high- temperature performance. • Basic refractories are especially resistant to attack by slags containing high concentrations of MgO and CaO, and find extensive use in some steel- making open hearth furnaces.
  • 17. Special Refractories • There are yet other ceramic materials that are used for rather specialized refractory applications. • Some of these are relatively high-purity oxide materials, many of which may be produced with very little porosity. • Included in this group are alumina, silica, magnesia, beryllia (BeO), zirconia (ZrO2), and mullite (3Al2O3–2SiO2). • Others include carbide compounds, in addition to carbon and graphite. • Silicon carbide (SiC) has been used for electrical resistance heating elements, as a crucible material, and in internal furnace components. • Carbon and graphite are very refractory, but find limited application because they are susceptible to oxidation at temperatures in excess of about 88 C. • As would be expected, these specialized refractories are relatively expensive.
  • 18. ABRASIVES • Abrasive ceramics are used to wear, grind, or cut away other material, which necessarily is softer. • Therefore, the prime requisite for this group of materials is hardness or wear resistance; in addition, a high degree of toughness is essential to ensure that the abrasive particles do not easily fracture. • Furthermore, high temperatures may be produced from abrasive frictional forces, so some refractoriness is also desirable.
  • 19. ABRASIVES • Diamonds, both natural and synthetic, are utilized as abrasives; however, they are relatively expensive. • The more common ceramic abrasives include silicon carbide, tungsten carbide, aluminum oxide (or corundum), and silica sand.
  • 20. ABRASIVES • Abrasives are used in several forms—bonded to grinding wheels, as coated abrasives, and as loose grains. • In the first case, 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 prevents excessive heating. • Coated abrasives are those in which an abrasive powder is coated on some type of paper or cloth material; sandpaper is probably the most familiar example.
  • 21. ABRASIVES • Wood, metals, ceramics, and plastics are all frequently polished using this form of abrasive. • 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.