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CERAMICS

CERAMICS.

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CERAMICS

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  1. CERAMICS

  2. Ceramics are compounds between metallic and nonmetallic elements which can be oxides, nitrides, and carbides. The wide range of materials that falls within this classification includes ceramics that are composed of clay minerals, cement and glass. Up until past 40 or so years, the most important materials in this class were termed the “traditional ceramics”, those for which the primary raw material is clay. Products that are considered to be traditional ceramics are china, porcelain, bricks, tiles and in addition, glasses and high-temperature ceramics.

  3. Over the years, significant progress has been made in understanding the fundamental character of ceramics and of the phenomena that occur in them that are responsible for their unique properties. Consequently, a new generation of these materials has evolved, and the term “ceramic” has taken on much broader meaning. These new materials have a rather dramatic effect on our lives; electronics, computer, communication, aerospace and a host of other industries rely on their use.

  4. Since the atomic bonding in ceramic materials is either partially or totally ionic, most ceramic crystal structures may be thought of as being composed of electrically charged ions instead of atoms. The metallic ions are positively charged cations and the nonmetallic ions are negatively charged anions. Ceramics are composed of at least two elements and their crystal structures are far more complex than those of metals

  5. Na+ Cl - Unit Cell for the rock salt (NaCl) structure

  6. Examples for Ceramic Materials • Alumina • Boron Carbide • Chromium Carbide • Graphite • Magnesia • Silicon Carbide • Tungsten Carbide • Zirconia • Clay • Brick • Limestone • Granite

  7. Properties of Ceramic Materials • Crystalline and noncrystalline states • High melting temperatures (varying from 3500 to 7000 o F) • All ceramics are brittle at room temperatures • Very low resistance to tensile loads. Very low fracture strengths. Microcracks are formed very easily under tensile stresses. • Stronger under compressive loads and microcracks are not formed as easily as in tension.

  8. Properties of Ceramic Materials (Cont’d) • High hardness nd good wear resistance. • High toughness • Low thermal and electrical conductivity. • High creep resistance at elevated temperatures • Capacity to remain unreactive and inert when exposed to severe environments • Can be magnetized and demagnetized, some can be permanently magnetized

  9. At room temperature ceramics almost always fracture before any plastic deformation can occur in response to an applied tensile load. The measure of ceramic material’s ability to resist fracture when a crack is present is specified in terms of fracture toughness. Table 1 shows a comparison between the toughness of ceramics and other materials.

  10. Material Table 1

  11. Stress-Strain Behavior of Ceramics Instead of standard tensile test which is applied to metals, a transverse bending test (three-or four-point loading) is employed. In this test a rod specimen having either a circular or a rectangular cross section is bent until fracture. LOAD (F) SUPPORT L/2 L/2

  12. Stress-Strain Behavior of Ceramics The maximum stress, or stress at fracture is known as the Modulus of Rupture (mr), which is an important mechanical parameter for ceramics. Modulus of rupture is given by the following equations: CIRCULAR RECTANGULAR d 2R b

  13. Table 2. Characteristic modulus of rupture and elastic modulus values for various ceramic materials.

  14. 40,000 30,000 20,000 10,000 ALUMINUM OXIDE STRESS (Psi) GLASS 0.0002 0.0004 0.0006 0.0008 STRAIN

  15. (Classification based on application)

  16. Glasses • Containers • Windows • Lenses • Fiberglass • Most commercial glasses are combination of silica + soda + limestone

  17. Clay Products: • Structural products (Bricks, tiles, sewer pipes) • Whitewares (Porcelain, pottery, tableware, china, plumbing fixtures) These products are composed of: Alumina (Al2O3) + Silica (SiO2)

  18. Refractories: • Furnace linings • Heat treatment equipment • Power generation equipment Types of refractories: • Fireclay • Silica • Basic (Open hearth furnaces) • Special refractories (Zirconia, magnesia, alumina) Used as electrical resistance elements, cruicible materials, internal furnace components.

  19. Abresives: • Grinding wheels • Polishing wheels • Lapping wheels Types of abrasives: • Diamond • Silicon carbide • Aluminum carbide • Aluminum oxide • Tungsten carbide

  20. Other Applications: • Soft magnets • Hard magnets • Electrical insulators • Semiconductor • Internal combustion engine blocks • Valves • Rotors • Electronic packaging (Boron nitride, aluminum nitride, silicon carbide) • Composite materials (matrix or fiber)

  21. Back to Table of Contents

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