Ceramic Their Properties and Material Behavior Engr 2110 Dr. R. Lindeke

Taxonomy of Ceramics • Properties: • Applications: • Fabrication Glasses Clay products Refractories Abrasives Cements Advanced ceramics -optical - composite reinforce containers/ household -whiteware bricks -bricks for high T (furnaces) -sandpaper cutting polishing -composites structural engine rotors valves bearings -sensors Adapted from Fig. 13.1 and discussion in Section 13.2-6, Callister 7e. • Properties: -- Tm for glass is moderate, but large for other ceramics. -- Small toughness, ductility; large moduli & creep resist. • Applications: -- High T, wear resistant, novel uses from charge neutrality. • Fabrication -- some glasses can be easily formed -- other ceramics can not be formed or cast.

Ceramic Bonding • Bonding: • Large vs small ionic bond character: -- Mostly ionic, some covalent. -- % ionic character increases with difference in electronegativity (remember!?!). • Large vs small ionic bond character: CaF2: large SiC: small Adapted from Fig. 2.7, Callister 7e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.

Ceramic Crystal Structures Oxide structures oxygen anions much larger than metal cations close packed oxygen in a lattice (usually FCC) cations in the holes of the oxygen lattice The same ideas apply to all “ceramics” Principles of Ceramic Architecture: Size relationships Cation to Anion Electrical Neutrality of the overall structure Crystallographic Arrangements Stoichiometry Must Match

Silica Glass A “Dense form” of amorphous silica Charge imbalance corrected with “counter cations” such as Na+ Borosilicate glass is the pyrex glass used in labs better temperature stability & less brittle than sodium glass

GLASSES – transparent and easily shaped Noncrystalline Silicates + oxides (CaO, Na2O, K2O, Al2O3) E.g. Soda lime glass = 70wt% SiO2 + 30% [Na2O (soda) and CaO(lime) Table 13.1

GLASS PROPERTIES • Specific volume (1/r) vs Temperature (T): • Crystalline materials: --crystallize at melting temp, Tm --have abrupt change in spec. vol. at Tm • Glasses: --do not crystallize --spec. vol. varies smoothly with T --Glass transition temp, Tg Adapted from Fig. 13.5, Callister, 6e. • Viscosity: --relates shear stress & velocity gradient: --has units of (Pa-s) 9

GLASS VISCOSITY VS T AND IMPURITIES • Viscosity decreases with T increase • Impurities lower Tdeform from E.B. Shand, Engineering Glass, Modern Materials, Vol. 6, Academic Press, New York, 1968, p. 262. 10

Important Temperatures Melting point = viscosity of 10 Pa.s Working point= viscosity of 1000 Pa.s Softening point= viscosity of 4x107Pa.s Temperature above which glass cannot be handled without altering dimensions) Annealing point= viscosity of 1012 Pa.s. Strain point = viscosity of 3x1013Pa.s Fracture occurs before deformation • Viscosity decreases with T • Impurities lower Tdeform

Silicates Combine SiO44- tetrahedra by having them share corners, edges, or faces Cations such as Ca2+, Mg2+, & Al3+ act to neutralize & provide ionic bonding Mg2SiO4 Ca2MgSi2O7

Layered Silicates Layered silicates (clay silicates) = SiO4 tetrahedra connected together to form 2-D plane (Si2O5)2- So need cations to balance charge =

Layered Silicates Kaolinite clay alternates (Si2O5)2- layer with Al2(OH)42+ layer Adapted from Fig. 12.14, Callister 7e. Note: these sheets loosely bound by van der Waal’s forces

Layered Silicates Can change the counterions Micas: KAl3Si3O10(OH)2 this changes layer spacing the layers also allow absorption of water Micas: KAl3Si3O10(OH)2 Bentonite used to seal wells packaged dry swells 2-3 fold in H2O pump in to seal up well so no polluted ground water seeps in to contaminate the water supply. Used in bonding Foundry Sands and Taconite pellets

Carbon Forms Carbon black – amorphous – surface area ca. 1000 m2/g Diamond tetrahedral carbon hard – no good slip planes brittle – can cleave (cut) it large diamonds – jewelry small diamonds often man made - used for cutting tools and polishing diamond films hard surface coat – cutting tools, medical devices, etc. (C likes to have covalent bonds & tetravalent)

Carbon Forms - Graphite layer structure – aromatic layers weak van der Waal’s forces between layers planes slide easily, good lubricant Like connected benzene rings

Carbon Forms – Fullerenes and Nanotubes Fullerenes or carbon nanotubes wrap the graphite sheet by curving into ball or tube Buckminister fullerenes Like a soccer ball C60 - also C70 + others Adapted from Figs. 12.18 & 12.19, Callister 7e.

Defects in Ceramic Structures • Frenkel Defect --a cation is out of place. • Shottky Defect --a paired set of cation and anion vacancies. Shottky Defect: Frenkel Defect Adapted from Fig. 12.21, Callister 7e. (Fig. 12.21 is from W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. 1, Structure, John Wiley and Sons, Inc., p. 78.) • Equilibrium concentration of defects

Mechanical Properties We know that ceramics are more brittle than metals. Why? Consider method of deformation slippage along slip planes in ionic solids this slippage is very difficult too much energy needed to move one anion past another anion (like charges repel)

Measuring Elastic Modulus • Room T behavior is usually elastic, with brittle failure. • 3-Point Bend Testing often used. --tensile tests are difficult for brittle materials! F L/2 d = midpoint deflection cross section R b d rect. circ. Adapted from Fig. 12.32, Callister 7e. • Determine elastic modulus according to: F x linear-elastic behavior d slope = E = L 3 4 bd 12 p R rect. cross section circ.

Measuring Strength s = 1.5Ff L bd 2 Ff L pR3 x F F • 3-point bend test to measure room T strength. F L/2 d = midpoint deflection cross section R b d rect. circ. location of max tension Adapted from Fig. 12.32, Callister 7e. • Flexural strength: • Typ. values: Data from Table 12.5, Callister 7e. rect. s fs = 1.5Ff L bd 2 Ff L pR3 Si nitride Si carbide Al oxide glass (soda) 250-1000 100-820 275-700 69 304 345 393 Material (MPa) E(GPa) x F Ff d

Mechanical Issues: Properties are significantly dependent on processing – and as it relates to the level of Porosity: E = E0(1-1.9P+0.9P2) – P is fraction porosity fs = 0e-nP -- 0 & n are empirical values Because the very unpredictable nature of ceramic defects, we do not simply add a factor of safety for tensile loading We may add compressive surface loads We often choose to avoid tensile loading at all – most ceramic loading of any significance is compressive (consider buildings, dams, brigdes and roads!)

Application: Refractories • Need a material to use in high temperature furnaces. • Consider the Silica (SiO2) - Alumina (Al2O3) system. • Phase diagram shows: mullite, alumina, and crystobalite as candidate refractories. Composition (wt% alumina) T(°C) 1400 1600 1800 2000 2200 20 40 60 80 100 alumina + mullite + L Liquid (L) crystobalite alumina + L 3Al2O3-2SiO2 Adapted from Fig. 12.27, Callister 7e. (Fig. 12.27 is adapted from F.J. Klug and R.H. Doremus, "Alumina Silica Phase Diagram in the Mullite Region", J. American Ceramic Society 70(10), p. 758, 1987.)

Application: Die Blanks -- Need wear resistant properties! tensile force A o d die Adapted from Fig. 11.8 (d), Callister 7e. Courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission. • Die surface: -- 4 mm polycrystalline diamond particles that are sintered onto a cemented tungsten carbide substrate. -- polycrystalline diamond helps control fracture and gives uniform hardness in all directions. Courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission.

Application: Cutting Tools -- for grinding glass, tungsten, carbide, ceramics -- for cutting Si wafers -- for oil drilling • Solutions: oil drill bits blades -- manufactured single crystal or polycrystalline diamonds in a metal or resin matrix. coated single crystal diamonds -- optional coatings (e.g., Ti to help diamonds bond to a Co matrix via alloying) polycrystalline diamonds in a resin matrix. -- polycrystalline diamonds resharpen by microfracturing along crystalline planes. Photos courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission.

Application: Sensors • Example: Oxygen sensor ZrO2 2+ impurity removes a Zr 4+ and a O 2 - ion. • Approach: Add Ca impurity to ZrO2: -- increases O2- vacancies -- increases O2- diffusion rate • Example: Oxygen sensor ZrO2 • Principle: Make diffusion of ions fast for rapid response. reference gas at fixed oxygen content O 2- diffusion gas with an unknown, higher - + voltage difference produced! sensor • Operation: -- voltage difference produced when O2- ions diffuse from the external surface of the sensor to the reference gas.

Alternative Energy – Titania Nano-Tubes "This is an amazing material architecture for water photolysis," says Craig Grimes, professor of electrical engineering and materials science and engineering. Referring to some recent finds of his research group (G. K. Mor, K. Shankar, M. Paulose, O. K. Varghese, C. A. Grimes, Enhanced Photocleavage of Water Using Titania Nanotube-Arrays, Nano Letters, vol. 5, pp. 191-195.2005 ), "Basically we are talking about taking sunlight and putting water on top of this material, and the sunlight turns the water into hydrogen and oxygen. With the highly-ordered titanium nanotube arrays, under UV illumination you have a photoconversion efficiency of 13.1%. Which means, in a nutshell, you get a lot of hydrogen out of the system per photon you put in. If we could successfully shift its bandgap into the visible spectrum we would have a commercially practical means of generating hydrogen by solar energy.

Ceramic Fabrication Methods-I PARTICULATE FORMING CEMENTATION GLASS FORMING • Pressing: plates, dishes, cheap glasses --mold is steel with graphite lining Gob Parison mold Pressing operation • Fiber drawing: wind up • Blowing: suspended Parison Finishing mold Compressed air Adapted from Fig. 13.8, Callister, 7e. (Fig. 13.8 is adapted from C.J. Phillips, Glass: The Miracle Maker, Pittman Publishing Ltd., London.)

Sheet Glass Forming Sheet forming – continuous draw originally sheet glass was made by “floating” glass on a pool of mercury – or tin Adapted from Fig. 13.9, Callister 7e.

Modern Plate/Sheet Glass making: Image from Prof. JS Colton, Ga. Institute of Technology

Heat Treating Glass • Annealing: • Tempering: --removes internal stress caused by uneven cooling. • Tempering: --puts surface of glass part into compression --suppresses growth of cracks from surface scratches. --sequence: before cooling hot surface cooling hot cooler further cooled tension compression --Result: surface crack growth is suppressed.

Ceramic Fabrication Methods-IIA GLASS FORMING PARTICULATE FORMING CEMENTATION • Milling and screening: desired particle size • Mixing particles & water: produces a "slip" ram billet container force die holder die A o d extrusion --Hydroplastic forming: extrude the slip (e.g., into a pipe) Adapted from Fig. 11.8 (c), Callister 7e. • Form a "green" component solid component --Slip casting: Adapted from Fig. 13.12, Callister 7e. (Fig. 13.12 is from W.D. Kingery, Introduction to Ceramics, John Wiley and Sons, Inc., 1960.) hollow component pour slip into mold drain mold “green ceramic” absorb water • Dry and fire the component

Clay Composition A mixture of components used (50%) 1. Clay (25%) 2. Filler – e.g. quartz (finely ground) (25%) 3. Fluxing agent (Feldspar) binds it together aluminosilicates + K+, Na+, Ca+

Features of a Slip • Clay is inexpensive • Adding water to clay charge weak van der Waals bonding charge neutral Si 4+ Al 3 + - OH O 2- Shear • Clay is inexpensive • Adding water to clay -- allows material to shear easily along weak van der Waals bonds -- enables extrusion -- enables slip casting • Structure of Kaolinite Clay: Adapted from Fig. 12.14, Callister 7e. (Fig. 12.14 is adapted from W.E. Hauth, "Crystal Chemistry of Ceramics", American Ceramic Society Bulletin, Vol. 30 (4), 1951, p. 140.)

Drying and Firing • Drying: layer size and spacing decrease. • Firing: Adapted from Fig. 13.13, Callister 7e. (Fig. 13.13 is from W.D. Kingery, Introduction to Ceramics, John Wiley and Sons, Inc., 1960.) wet slip partially dry “green” ceramic Drying too fast causes sample to warp or crack due to non-uniform shrinkage • Firing: --T raised to (900-1400°C) --vitrification: liquid glass forms from clay and flows between SiO2 particles. Flux melts at lower T. Adapted from Fig. 13.14, Callister 7e. (Fig. 13.14 is courtesy H.G. Brinkies, Swinburne University of Technology, Hawthorn Campus, Hawthorn, Victoria, Australia.) Si02 particle (quartz) glass formed around the particle micrograph of porcelain 70 mm

Ceramic Fabrication Methods-IIB GLASS FORMING PARTICULATE FORMING CEMENTATION Sintering: useful for both clay and non-clay compositions. • Procedure: -- produce ceramic and/or glass particles by grinding -- place particles in mold -- press at elevated T to reduce pore size. • Aluminum oxide powder: -- sintered at 1700°C for 6 minutes. 15 m Adapted from Fig. 13.17, Callister 7e. (Fig. 13.17 is from W.D. Kingery, H.K. Bowen, and D.R. Uhlmann, Introduction to Ceramics, 2nd ed., John Wiley and Sons, Inc., 1976, p. 483.)

Powder Pressing Sintering - powder touches - forms neck & gradually neck thickens add processing aids to help form neck little or no plastic deformation Uniaxial compression - compacted in single direction Isostatic (hydrostatic) compression - pressure applied by fluid - powder in rubber envelope Hot pressing - pressure + heat Adapted from Fig. 13.16, Callister 7e.

Tape Casting thin sheets of green ceramic cast as flexible tape used for integrated circuits and capacitors cast from liquid slip (ceramic + organic solvent) Adapted from Fig. 13.18, Callister 7e.

Ceramic Fabrication Methods-III GLASS FORMING PARTICULATE FORMING CEMENTATION • Produced in extremely large quantities. • Portland cement: -- mix clay and lime bearing materials -- calcinate (heat to 1400°C) -- primary constituents: tri-calcium silicate di-calcium silicate • Adding water -- produces a paste which hardens -- hardening occurs due to hydration (chemical reactions with the water). • Forming: done usually minutes after hydration begins.

Applications: Advanced Ceramics Disadvantages: Brittle Too easy to have voids- weaken the engine Difficult to machine Heat Engines Advantages: Run at higher temperature Excellent wear & corrosion resistance Low frictional losses Ability to operate without a cooling system Low density Possible parts – engine block, piston coatings, jet engines Ex: Si3N4, SiC, & ZrO2

Applications: Advanced Ceramics Ceramic Armor Al2O3, B4C, SiC & TiB2 Extremely hard materials shatter the incoming projectile energy absorbent material underneath

Applications: Advanced Ceramics Electronic Packaging Chosen to securely hold microelectronics & provide heat transfer Must match the thermal expansion coefficient of the microelectronic chip & the electronic packaging material. Additional requirements include: good heat transfer coefficient poor electrical conductivity Materials currently used include: Boron nitride (BN) Silicon Carbide (SiC) Aluminum nitride (AlN) thermal conductivity 10x that for Alumina good expansion match with Si