CHAPTER 12 & 13: CERAMICS Ceramic materials are inorganic nonmetallic materials which consist of metallic and nonmetallic elements bonded primarily by ionic and/or covalent bonds. Traditional ceramics - clay is the primary raw material Engineering ceramics - typically consist of pure or nearly pure compounds such as aluminum oxide, silicon carbide and silicon nitride.

Typical Properties: 1. good thermal and electrical insulators -- due to absence of “free” electrons 2. many ceramics are transparent -- due to absence of “free” electrons that absorb photons 3. high hardness and brittleness; low ductility and toughness 4. weak in tension and strong in compression -- due to brittleness 5. can undergo loss of strength over time -- static fatigue 6. can crack/fracture due to sudden changes in temperature (thermal shock)

Applications: 1. bricks, tiles 2. porcelain used in electronics 3. glasses and lenses 4. refractories -- high temperature resistant materials 5. Silicon carbide (SiC) used in high temperature gas turbine engines protective coating – enamels Main Categories: A. Crystalline ceramics -- with crystalline atomic structure B. Glasses -- noncrystalline C. Glass-Ceramics -- initially formed as glasses and then recrystallized

Crystalline Ceramics: 1) Silicates -- based on SiO2 - Ex. cement, pottery, clay, porcelain, bricks - Si and O are very plentiful 2) Nonsilicate oxide ceramics - Ex. alumina (Al2O3) - used for electronic; magnesia (MgO) - used as refractory; uranium dioxide (UO2) - nuclear fuel; zirconia dioxide (ZrO2) - metal substitute 3) Nonoxides - Ex. silicon carbide, SiC; silicon nitride, Si3N4

Crystal Structure • Charge Neutrality: • Stable structures: --Net charge in the structure should be zero. --General form: • Stable structures: --when anions surrounding a cation are all in contact with the cation .

Coordination Number Coordination number -- the number of equidistant neighbors to an atom or ion in a unit cell crystal structure Radius ratio -- ratio of the radius of the central cation to that of the surrounding anions; determines coordination number and coordination geometry (Table 12.1). Critical or minimum radius ratio -- when all surrounding anions just touch each other and the central cation; determined by pure geometric considerations

Ex. Find the minimum cation-anion radius ratio for a coordination number of 3

Common Structures: a) AX-type rock salt structure CN = 6 Like FCC for Na and Cl Superimposed FCC’s

Common Structures: AX-Type ii) cesium chloride CsCl: CN = 8 Like BCC

Common Structures: AX-Type iii. zinc blende ZnS: CN = 4 tetragonal Like diamond

Crystal Structure: AmXp-Type Fluorite (CaF2) CN = 8 Like CsCl but with only half of center atoms

Perovskite: Barium Titanate AmBnXp - Type Perovskite: Barium Titanate BaTiO3

Ex: Predicting the structure of FeO • On the basis of ionic radii, what crystal structure would you predict for FeO? • Answer: based on this ratio, --coord # = 6 --structure = NaCl

Density Computation Density = mass/volume Where n’ is the number of formula units in a unit cell. Ex. Calculate the theoretical density of NiO, knowing that it has a rock salt crystal structure. Since the crystal structure is rock salt, n' = 4 = 6.79 g/cm3

Defects in Ceramic Crystals • Frenkel Defect --a cation is out of place. • Shottky Defect --a paired set of cation and anion vacancies. • Equilibrium concentration of defects

IMPURITIES • Impurities must also satisfy charge balance • Ex: NaCl • Substitutional cation impurity • Substitutional anion impurity

Glasses Noncrystalline silicates (SiO2 is the main component) containing other oxides, notably CaO, Na2O, K2O, and Al2O3. Constituents are heated to an elevated temperature above which melting occurs, and then cooled to the rigid condition without crystallization. Glass Transition Temperature -- the center of the temperature range in which a non-crystalline solid changes from being glass-brittle to being viscous.

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)

GLASS STRUCTURE • Basic Unit: • Glass is amorphous • Amorphous structure occurs by adding impurities (Na+,Mg2+,Ca2+, Al3+) • Impurities: interfere with formation of crystalline structure. • Quartz is crystalline SiO2: (soda glass)

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: --Result: surface crack growth is suppressed. 11

Mechanical Properties of Ceramics: Brittleness Brittleness of ceramics is due to their ionic and covalent chemical bonds. Covalent Bonds are very strong directional bonds. Due to the directional nature of its bonds, the material can only change shape by breaking bonds. This leads to brittle fracture due to the separation of electron-pair bonds without their subsequent reformation. Ionic Bonds result in limited number of slip planes. Family of planes that result in ions of the same charge being in contact tend to separate and hence do not allow slip.

Mechanical Properties: Strength Weak in tension and relatively strong in compression! Mechanical failure occurs mainly from structural defects (surface cracks, voids, inclusions, and large grains) produced during processing. Ductile materials have the ability to deform plastically in the vicinity of a crack tip to redistribute high stress distributions. For brittle materials, once cracks starts to propagate, unstable growth happens rapidly. ---> Tensile strength of ceramic material is LOW. Compressive stresses tend to close (not open) the cracks and consequently does not diminish the inherent strength of the material.

3. Static Fatigue -- Loss off strength over time at room temperature without cyclic loading ; due to chemical degradation -- Water attacks SiO2 at the surface, creating cracks. 4. Creep - deformation with time Sliding of adjacent grains along the grain boundary. Ex. Old windows - glass flows, the thickness of glass is greater at the bottom of the pane than at the top.

Elastic Modulus • 3-Point Bend Testing often used. • Room T behavior is usually elastic, with brittle failure. • 3-Point Bend Testing often used. --tensile tests are difficult for brittle materials. Adapted from Fig. 12.29, Callister 6e. • Determine elastic modulus according to:

Flexural Strength • 3-point bend test to measure room T strength. Adapted from Fig. 12.29, Callister 6e. • Flexural strength: • Typ. values: Si nitride Si carbide Al oxide glass (soda) 700-1000 550-860 275-550 69 300 430 390 69 Data from Table 12.5, Callister 6e.

Thermal Properties of Ceramics: 1. Low thermal conductivities and are good thermal insulators - due to the strong ionic-covalent bonding; High heat resistance and high melting points: used as refractories - materials that resist the action of hot environments. 2. Thermal Shock Resistance Heating or cooling results in an internal temperature distribution. Thermal stresses may be established as a result of thermal gradients. For ductile materials, thermally induced stresses may be relieved by plastic deformation. For brittle materials, rapid cooling may cause thermal shock since the induced stresses are tensile. Thermal Shock Resistance parameter TSR: TSR = (f k)/(E) where f = fracture stress k = thermal conductivity E = Young’s modulus of elasticity  = coefficient of thermal expansion