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CHAPTER 11

CHAPTER 11. Ceramics. 11-1. Introduction. Ceramics are inorganic and nonmetallic . Bounded by ionic or covalent bonds. Good electrical and heat insulation property. Brittle, and lesser ductility and toughness than metals. High chemical stability and high melting temperature.

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CHAPTER 11

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  1. CHAPTER 11 Ceramics 11-1

  2. Introduction • Ceramics are inorganic and nonmetallic. • Bounded by ionic or covalent bonds. • Good electrical and heat insulation property. • Brittle, and lesser ductility and toughness than metals. • High chemical stability and high melting temperature. • Traditional Ceramics: Basic components (Clay and Silica). • Engineering Ceramics: Pure compounds (Al2O3, SiC). 11-2

  3. Ionic and Covalent Bonding in Simple Ceramics • Mixture of Ionic and Covalent Types. • Depends on electronegativity difference. Table 10.2 11-3

  4. Simple Ionic Arrangements • Packing of Ions depends upon • Relative size of ions. • Need to balance electron charges. • If the anion does not touch the cation, then the arrangement is unstable. • Radius ratio = rcation/ranion • Critical radius ratio for stability for coordination numbers 8,6 and 3 are >0.732, >0.414 and > 0.155 respectively. Unstable Stable Figure 10.2 11-4

  5. Cesium Chloride Crystal Structure • CsCl is ionically bonded with radius ratio = 0.94 and CN = 8. • Eight chloride ion surround a central cesium cation at the ( ½ , ½ , ½ ) position. • CsBr, TlCl and TlBr have similar structure. Figure 10.5 11-5

  6. Sodium Chloride Crystal Structure • Highly Ionically bonded with Na+ ions occupying interstitial sites between FCC and Cl- ions. • Radius ratio = 0.56, CN = 6. • MgO, CaO, NiO and FeO have similar structures. Figure 10.7 11-6

  7. Interstitial Sites in FCC and HCP Crystal Lattices • Octahedral interstitial sites: Six nearest atoms or ions equidistant from central void. • Tetrahedral Interstitial Sites: Four nearest atoms or ions equidistant from central void. • There are four octahedral sites and eight tetrahedral sites per unit cell of FCC. Figure 10.9 Figure 10.11 After W. D. Kingery, H. K. Bowen, D. R. Uhlmann, “ Introduction to Ceramics,”2nd ed., Wiley, 1976. 11-7

  8. Zinc Blende (ZnS) Crystal Structue • Four zinc and four sulfur atoms. • One type (Zn or S) occupies lattice points and another occupies interstitial sites of FCC unit cell. • S Atoms (0,0,0) ( ½ ,½ ,0) ( ½ , 0, ½ ) (0, ½ , ½ ) Zn Atoms ( ¾ ,¼ ,¼ ) ( ¼ ,¼ ,¾ )( ¼ ,¾,¼ ) ( ¾ ,¾ ,¾ ) • Tetrahedrally covalently bonded • (87% covalent character) with CN = 8. • CdS, InAs, InSb and ZnSe have similar structures. Figure 10.12 After W. D. Kingery, H. K. Bowen, D. R. Uhlmann, “ Introduction to Ceramics,”2nd ed., Wiley, 1976. 11-8

  9. Calcium Fluorite (CaF2) Crystal Structure • Ca2+ ions occupy the FCC lattice sites while the F- ions are located at eight tetrahedral sites. • UO2, BaF2, PbMg2 have similar structures. • Large number of unoccupied octahedral sites in UO2 allow it to be used as nuclear fuel. • Fission products are accommodated in these vacant positions. Figure 10.14 11-9 After W. D. Kingery, H. K. Bowen, D. R. Uhlmann, “ Introduction to Ceramics,”2nd ed., Wiley, 1976.

  10. Other Crystal Structures • Antifluorite: Anions occupy lattice points and cations occupy eight tetrahedral sites of FCC. Examples: Li2O, Na2O • Corundum: Oxygen ions in lattice points of HCP unit cell. • Two Al3+ ions in octahedral sites for every three O- ions distortion of structure. • Spinel (MgAl2O4): Oxygen ions form FCC lattice and Mg and Ml ions occupy interstitial sites . • These are nonmetallic magnetic materials. Figure 10.15 11-10

  11. Other Crystal Structures • Perovskite (CaTiO3) : Ca2+ and O2- ions form FCC unit cell. • Ca2+ Ions occupy corners • O2- Ions occupy face centers. • Ti4+ ions are at octahedral sites. • Graphite : Polymorphic form of compound. • Layered structure with carbon atoms in hexagonal arrays. • Good lubricating properties. Figure 10.16 Figure 10.17 11-11 After W. D. Kingery, H. K. Bowen, D. R. Uhlmann, “ Introduction to Ceramics,”2nd ed., Wiley, 1976.

  12. Silicate Structures • Silicate (SiO44-) is building block of silicates. • 50% Ionic and 50% covalent. • Many different silicate structures can be produced. • Island structure: Positive ions bond with the oxygen of SiO44- tetrahedron. • Chain/ring structure: Two corners of each SiO44- tetrahedron bonds with corners of other tetrahedron. Figure 10.18 Figure 10.19a 11-12 After M. Eisenstadt, “Mechanical properties of Materials,” Macmillan, 1971, p.82.

  13. Sheet Structures of Silicates • Sheet structure: Three corners of same planes of silicate tetrahedron bonded to the corners of three other silicate tetrahedra. • Each tetrahedron has one unbounded oxygen and hence chains can bond with other type of sheets. • If the bondings are weak, sheets slide over each other easily. Figure 10.19b Figure 10.20 11-13 After M. Eisenstadt, “Mechanical properties of Materials,” Macmillan, 1971, p.83.

  14. Silicate Networks • Silica: All four corners of the SiO44- tetrahedra share oxygen atoms. • Basic structures: Quartz, tridynute and cristobarlite. • Important compound of many ceramic and glasses. • Feldspars: Infinite 3D networks. • Some Al3+ Ions replace Si4+ Ions Net negative charge. • Alkaline and alkaline fit into interstitial sites. Figure 10.22 After W. D. Kingery, H. K. Bowen, D. R. Uhlmann, “ Introduction to Ceramics,”2nd ed., Wiley, 1976. 11-14

  15. Processing of Ceramics • Produced by compacting powder or particles into shapes and heated to bond particles together. • Material preparation: Particles and binders and lubricants are (sometimes ground) and blend wet or dry. • Forming: Formed in dry, plastic or liquid conditions. • Cold forming process is predominant. • Pressing, slipcasting and extrusion are the common forming processes. 11-15

  16. Pressing • Dry Pressing: Simultaneous uniaxial compaction and shaping of power along with binder. • Wide variety of shapes can be formed rapidly and accurately. • Isolatic pressing: Ceramic powder is loaded into a flexible chamber and pressure is applied outside the chamber with hydraulic fluid. • Examples: Spark plug insulators, carbide tools. Figure 10.25 Figure 10.24 11-16 After J. S. Reed and R. B Runk, “ Ceramic Fabrication Process,” vol 9: “1976, p.74.

  17. Slip Casting • Powdered ceramic material and a liquid mixed to prepare a stable suspension (slip). • Slip is poured into porous mold and liquid portion is partially absorbed by mold. • Layer of semi-hard material is formed against mold surface. • Excess slip is poured out of cavity or cast as solid. • The material in mold is allowed to dry and then fired. Figure 10.27 11-17 After W. D. Kingery, “ Introduction to Ceramics,” Wiley, 1960, p.52.

  18. Extrusion • Single cross sections and hollow shapes of ceramics can be produced by extrusion. • Plastic ceramic material is forced through a hard steel or alloy die by a motor driven augur. • Examples: Refractory brick, sewer pipe, hollow tubes. Figure 10.28 11-18 After W. D. Kingery, “ Introduction to Ceramics,” Wiley, 1960.

  19. Thermal Treatments • Drying: Parts are dried before firing to remove water from ceramic body. • Usually carried out at or below 1000C. • Sintering: Small particles are bonded together by solid state diffusion producing dense coherent product. • Carried out at higher temperature but below MP. • Longer the sintering time, larger the particles are. • Vetrification: During firing, glass phase liquefies and fills the pores. • Upon cooling liquid phase of glass solidifies and a glass matrix that bonds the particles is formed. 11-19

  20. Traditional Ceramics • Made up of clay, silica and fledspar. • Clay: Provide workability and hardness. • Silica: Provide better temperature resistance and MP. • Potash Fledspar: Makes glass when ceramic is fired. Table 10.6 SEM of Porcelain Quartz grain High-silica glass Figure 10.33 Source: F. Norton, Elements of Ceramics, 2nd ed., Addision-Wesley,1974, p.140. 11-20

  21. Engineering Ceramics • Alumina (Al2O3): Aluminum oxide is doped with magnesium oxide, cold pressed and sintered. • Uniform structure. Used for electric applications. • Silicon Nitride (Si3N4): Compact of silicon powder is nitrided in a flow of nitrogen gas. • Moderate strength and used for parts of advanced engines. • Silicon Carbide (SiC):Very hard refractory carbide, sintered at 21000C. • Used as reinforcement in composite materials. • Zirconia (ZrO2): Polymorphic and is subject to cracking. • Combined with 9% MgO to produce ceramic with high fracture toughness. 11-21

  22. Electrical Properties • Basic properties of dielectric: • Dielectric constant:- Q = CV Q = Charge V = Voltage C = Capacitance C = ε0A/d ε0 = permeability of free space = 8.854 x 10-12 F/m • When the medium is not free space C = Kε0A/d Where K is dielectric constant of the material between the plates Figure 10.35 11-22

  23. Dielectric Strength and Loss Factor • Dielectric strength is measure of ability of material to hold energy at high voltage. • Defined as voltage gradient at which failure occurs. • Measured in volts/mil. • Dielectric loss factor: Current leads voltage by 90 degrees when a loss free dielectric is between plates of capacitor. • When real dielectric is used, current leads voltage by 900 – δ where δ is dielectric loss angle. • Dielectric loss factor = K tan δ measure of electric energy lost. 11-23

  24. Ceramic Insulator Materials • Ionic and covalent bonding restricts the mobility of ions and electrons and hence ceramics are good insulators. • Electrical porcelain: 50% Clay + 25 % Fledspar. • Good plasticity, wider firing temperature range, cheap. • High power loss factor. • Steatite: 90% talc + 10 % clay • Good insulator, low power loss factor, impact strength • Fosterite: Mg2SiO4 no alkali ions • Higher resistivity, low electrical loss • Alumina: Al2O3 Crystalline phase bounded to glassy matrix. • High dielectric strength, low dielectric loss 11-24

  25. Ceramic Materials for Capacitors • Ceramics are used as dielectric materials for capacitors. • Example: Disk ceramic capacitors. • BaTiO3 + additive • Very high dielectric constant • Used in ceramic based thick film hybrid electronic circuit • Higher capacitance per unit area Figure 10.38a Courtesy of Sprague Products Co 11-25

  26. Ceramic Semiconductors • Ceramics can be used as semiconducting materials. • Thermistor: Thermally sensitive resistor. • NTC thermistor: Conductivity raises with temperature. • Solid solution oxides of Mn, Ni, Fe, Co and Cu are used to obtain necessary property ranges. • By combining low conducting metal oxide with low conducting oxides intermediate properties are obtained. • Example: Conductivity of Fe3O4 is reduced gradually by adding increasing amounts in solid solution of MgCr2O4 11-26

  27. Ferroelectric Ceramics • Ferroelectric Domains: If unit cells do not have center of symmetry, dipole moments arise. • Dipole moment of unit volume = sum of all dipole moments of cell. • Example: BaTiO3 unit cell is symmetric above 1200C but below 1200C (Cutie temperature) dipole moment is created due to shifting of Ti4+ and O2- ions. • If cooling takes place in electric field, dipoles align in the direction of the field. Figure 10.40 11-27 After K. M. Ralls, T. H. Courtney, and J. Wulff, “An Introduction to Material Science and Engineering,” Wiley, 1976, p.610.

  28. Piezoelectric Effect • If compressive force is applied to piezoelectric ceramic, it changes dimension and results in net dipole moment. • Change in dipole moment changes the charge density at the ends and changes voltage difference between the ends. • If electric field is applied to the sample, charge density changes resulting in change of dimension of the sample. • Peizolectric effect Electric response Mechanical force 11-28

  29. Piezoelectric Materials • PZT ceramics: Solid solutions of lead zirconate (PbZrO3) and lead titanate (PbTiO3) • Have broader range of piezoelectric properties. • High curie temperature. • Barium Titanate: ( BaTiO3 ) Commonly used. • Low curie temperature. • Applications: Piezoelectric compression accelerometer, ultrasonic cleaning transducer and underwater sound transmitter. 11-29

  30. Mechanical Properties of Ceramics • Strength of ceramics vary greatly but they are generally brittle. • Tensile strength is lower than compressive strength. Table 10.9 11-30

  31. Mechanism of deformation • Covalently bonded ceramics: Exhibit brittle fracture due to separation of electron-pair bonds without their subsequent reformation. • Ionically bonded ceramics: Single crystal show considerable plastic deformation. Polycrystalline ceramics are brittle. • Example: NaCl crystal • Slip in {100} family of planes is rarely observed as same charges come into contact. • Cracking occurs at grain boundaries. Figure 10.44 11-31

  32. Factors Affecting Strength • Failure occurs mainly from surface defects. • Pores gives rise to stress concentration and cracks. • Pores reduce effective cross-sectional area. • Flaw size is related to grain size. • Finer size ceramics have smaller flaws and hence are stronger. • Composition, microstructure, surface condition, temperature and environment also determine strength. 11-32

  33. Toughness of ceramic Materials • Ceramics have low strength. • Research has been conducted to improve toughness. • Hot pressing with additives and reaction bonding improve toughness. • KIC values obtained by four point bend test. = fracture stress (MPa) a = half size of target internal flaw Y = dimensionless constant Figure 10.46 11-33

  34. Transformation Toughening of Partially Stabilized ZrO2 • Transformation of Zirconia combined with some other refractory oxides (MgO) can produce very high toughness ceramics. • ZrO2 exists in 3 structures. • Monoclinic Up to 11700C • Tetragonal 1170 – 23700C • Cubic above 23700C • Adding 10% mol ofMgO stabilizes cubic form so that it can exist in metastable state in room condition. 11-34

  35. Toughness of Zirconia (Cont..) • If a mixture of ZrO2 – 9 mol% MgO is sintered at about 18000C and rapidly cooled, it will be in metastable state. • If reheated to 14000C and held for sufficient time tetragonal structure precipitates. • Under action of stress, this tetragonal structure transforms to monoclinic increasing volume and hence retarding crack growth. Figure 10.47a 11-35 After A. H. Heuer, “ Advances in Ceramics,” vol. 3, “Science and Technology of Zirconia,” American Ceramic Society, 1981.

  36. Fatigue Failure • Fatigue fracture in ceramics is rare due to absence of plastic deformation. • Straight fatigue crack in has been reported in alumina after 79,000 compression cycles. • Ceramics are hard and can be used as abrasives. • Examples:- Al2O3, SiC. • By combining ceramics, improved abrasives can be developed. • Example:- 25% ZrO2 + 75% Al2O3 Figure 10.48 11-36 After Suresh and J. R. Brockenbrough, Acta Metall. 36:1455, 1988.

  37. Thermal Properties of Ceramics • Low thermal conductivity and high heat resistance. • Many compounds are used as industrial refractories. • For insulating refractories, porosity is desirable. • Dense refractories have low porosity and high resistance to corrosion and errosion. • Aluminum oxide and MgO are expensive and difficult to form and hence not used as refractories. 11-37

  38. Acidic and Basic Refractories • Acidic refractories: • Silica refractories have high mechanical strength and rigidity. • Fireclays: Mixture of plastic fireclay, flint clay and grog. Particles vary from coarse to very fine. • High aluminum refractories: Contain 50-90% alumina and have higher fusion temperature. • Basic refractories: consists mainly of MgO and CaO. • Have high bulk densities, melting temperature and resistance to chemical attack. • used for lining in basic-oxygen steelmaking process. 11-38

  39. Insulation for Space Shuttle Orbital • About 70% of external surface is protected from heat by 24000 ceramic tiles. • Material: Silica fiber compound. • Density is 4kg/ft3 and withstands temperature up to 12600C. Figure 10.51 Courtesy of NASA 11-39

  40. Glasses • Combination of transparency, strength, hardness and corrosion resistance. • Glass is an inorganic product of fusion that has cooled to a rigid condition without crystallization. • Glass does not crystallize up on cooling. • Up on cooling, it transforms from rubbery material to rigid glass. Figure 10.52 11-40

  41. Structure of Glasses • Fundamental subunit of glass is SiO44- tetrahedron. • Si 4+ ion is covalently ionically bonded to four oxygen atoms. • In cristobalite, Si-O tetrahedron are joined corner to corner to form long range order. • In simple silica glass, tetrahedra are joined corner to corner to form loose network. Figure 10.53 Cristobailite Simple silica glass 11-41 Courtesy of Corning Glass Works

  42. Glass Modifying Oxides and Intermediate Oxides • Network modifiers: Oxides that breakup the glass network. • Added to glass to increase workability. • Examples:- Na2O, K2O, CaO, MgO. • Oxygen atom enters network and other ion stay in interstices. • Intermediate oxides: Cannot form glass network by themselves but can join into an existing network. • Added to obtain special properties. • Examples: Al2O3, Lead oxide. 11-42

  43. Composition of Glasses • Soda lime glass: Very common glass (90%). • 71-73% SiO2, 12-14% Na2O, 10-12% CaO. • Easier to form and used in flat glass and containers. • Borosilicate glass: Alkali oxides are replaced by boric oxide in silica glass network. • Known as Pyrex glass and is used for lab equipments and piping. • Lead glass: Lead oxide acts as network modifier and network former. • Low melting point – used for solder sealing. • Used in radiation shields, optical glass and TV bulbs. 11-43

  44. Viscous Deformation of glasses. • Viscous above Tg and viscosity decreases with increase in temperature. η* = η0e+Q/RT η* η0 Q = Activation energy = Viscocity of glass (PaS) = preexponential constant (PaS) • Working point: 103 PaS – glass • fabrication can be carried out • Softening point: 107 PaS – glass • flows under its own weight. • Annealing point: 1012 PaS – Internal • stresses can be relieved.. • Strain point: 10 13.5 PaS – glass is • rigid below this point. Figure 10.55 11-44 After O. H. Wyatt and D. Dew-Hughes, “Metals Ceramics and Polymers,” Cambridge, 1974, p.259.

  45. Forming Methods • Forming sheet and plate glass: Ribbon of glass moves out of furnace and floats on a bath of molten tin. • Glass is cooled by molten tin. • After it is hard, it is removed and passed through a long annealing furnace. Figure 10.56 11-45 After D. C. Boyd and D. A. Thompson, Kirk-Othmer Encyclopedia of Chemical Technology,” 3rd ed., Wiley, 1980, p.862.

  46. Blowing, Pressing and Casting • Blowing: Air blown to force molten glass into molds. • Pressing: Optical and sealed beam lenses are pressed by a plunger into a mold containing molten glass. • Casting: Molten glass is cast in open mold. • Centrifugal casting: Glass globs are dropped into spinning mold. • Glass first flows outward towards wall of mold and then upward against the mold wall. Figure 10.57 After W. Giegerich and W. Trier, “ Glass Machines Construction …..,” Spring-Verlag, 1969. 11-46

  47. Tempered Glass • Glass is heated into near softening point and rapidly cooled. • Surface cools first and contracts. • Interior cools next and contracts causing tensile stresses in the interior and compressive stress on the surface. • Tempering strengthens the glass. • Examples: Auto side windows and safety glasses. Figure 10.58 11-47

  48. Chemically Strengthened Glass • Special treatment increases chemical resistance of glasses. • Example:- Sodium aluminosilicate glasses are immersed in a bath of potassium nitrate at 500C for 6 to 10 hours • Large potassium ions are induced into surface causing compressive stress. • Compressive layer is much thinner than that in thermal tempering. • Used for supersonic aircraft glazing and ophthalmic lenses. 11-48

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