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. Traditional Ceramics: Basic components (Clay and Silica). Engineering Ceramics: Pure compounds (Al2O3, SiC). 11-2

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

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 Figure 10.2 Stable 11-4

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

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

Interstitial Sites in FCC and HCP Crystal Lattices Interstitial sites are empty spaces or voids among the atoms or ions that are packed into a crystal structure lattice. In these voids or interstitial sites, atoms or ions other than those of parent lattice can be fitted in. In the FCC and HCP crystal structures, there are two types of interstitial sites: octahedral and tetrahedral

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

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

Mass of unit cell = Number of atoms in a unit cell x the mass of each atom

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. http://www.morgantechnicalceramics.com/downloads/animations/material-preperation Forming: Formed in dry, plastic or liquid conditions. Cold forming process is predominant. Pressing, slipcasting and extrusion are the common forming processes. 11-15

Pressing Wide variety of shapes can be formed rapidly and accurately. Dry Pressing: Simultaneous uniaxial compaction and shaping of powder along with binder. Wide variety of shapes can be formed rapidly and accurately. http://www.morgantechnicalceramics.com/downloads/animations/die-pressing Isostatic 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. http://www.morgantechnicalceramics.com/downloads/animations/isostatic-pressing Figure 10.25 Figure 10.24 11-16 After J. S. Reed and R. B Runk, “ Ceramic Fabrication Process,” vol 9: “1976, p.74.

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.

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. http://www.morgantechnicalceramics.com/downloads/animations/extrusion https://www.youtube.com/watch?v=ENyNINLFFB4 Figure 10.28 11-18 After W. D. Kingery, “ Introduction to Ceramics,” Wiley, 1960.

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. http://www.morgantechnicalceramics.com/downloads/animations/sintering 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

Traditional Ceramics Clay: Provide workability and hardness. 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

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

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

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

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

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

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

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. For example, aluminosilicate glasses can withstand higher temperature than common glasses 11-42

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