CERAMICS: CHAPTERS 12&13 ISSUES TO ADDRESS... • Definitions and Classification • Structures of ceramic materials: How do they differ from that of metals? • Point defects: How are they different from those in metals? • Impurities: How are they accommodated in the lattice and how do they affect properties? • Mechanical Properties: What special provisions/tests are made for ceramic materials? CERAMICS: DEFINTIONS (1) • The word "ceramics" comes from the Greek word "Keramos" meaning "Pottery," "Potter's Clay," or "a Potter." This Greek word is related to an old Sanskrit root meaning "to burn" but was primarily used to mean "burnt stuff." • Ceramics are defined as products made from inorganic materials having non-metallic properties, usually processed at a high temperature at some time during their manufacture. GLASS-CERAMICS Quartz tubing is fabricated from beach sand Ceramics Crystals: atoms have long range periodic order The lamp applications are shown in the GE product montage High temperature (the torch flame) Low temperature (the ice cube) Highly thermal resistive ceramics Glasses (non-crystalline) atoms have short range order only (amorphous) CERAMICS Common ceramic materials with characteristic resistance to damage at high temperature and corrosive environments A ceramic turbine in the millimeter range for micro-electromechanical systems, termed MEMS A prototype ceramic engine CERAMICS: DEFINTIONS (2) • The technical definition of ceramics involves a much greater variety of products than is normally realized. To most people, the word ceramics means dinnerware, figurines, vases, and other objects of ceramic art. The majority of ceramic products not generally recognized. Examples are bathtubs, washbowls, sinks, electrical insulating devices, water and sewerage pipes, bricks, hollow tile, glazed building tile, floor and wall tile, earthenware, porcelain enamel and glass. • Ceramic products have a number of outstanding properties which determine their usefulness. One of the most unusual of these is their great durability. This durability can be divided into three types: chemical, mechanical and thermal. CERAMICS: PROPERTIES (1) • Chemical Durability - The high chemical durability of the great majority of ceramic products makes them resistant to almost all acids, alkalis, and organic solvents. - Of further importance is the fact that ceramic materials are not affected by oxygen. The materials generally contained in the ceramic products have already combined with all of the oxygen for which they have an affinity, and therefore, are not affected further by the presence of oxygen in their environment. CERAMICS: PROPERTIES (2) • Mechanical Durability The mechanical durability of ceramics is evidenced by their strength and hardness. The compressive strengths of ceramic materials are extremely high, normally 50,000 to 100,000 lbs/sq. in. The hardness makes ceramic materials very resistant to abrasion. It is this property which makes them useful for floors, and for the grinding of metals and other materials. CERAMICS: PROPERTIES (3) • Thermal Durability Most ceramics have the ability to withstand high temperatures. This is why they are useful in the production of all types of heat-containing equipment such as kilns for the ceramic industry, and such products as the inner linings of fireplaces and home heating furnaces. CLASSIFICATION • Technical Ceramics can also be classified into three distinct material categories: Oxides-based: Silicate and non-silicate oxide ceramics (alumina,zirconia, etc) Non-oxides: Carbides, borides, nitrides, silicides Composites: Particulate reinforced, combinations of oxides/nonoxides. OXIDE CERAMICS Properties: - oxidation resistant, - chemically inert, - electrically insulating - generally low thermal conductivity, Notes: - relatively simple manufacturing and low cost for Al2O3 - more complex manufacturing and higher cost for ZrO2 NON-OXIDE CERAMICS Properties: - Low oxidation resistance, - Extreme hardness, - Chemically inert, - High thermal conductivity, - May be electrically conducting, Notes: difficult energy dependent manufacturing and high cost (TiC, ZrN, B4C, BN, Si3N4, SiC etc). CERAMIC-BASED COMPOSITES Properties: - Toughness, - Low and high oxidation resistance (type related), - Variable thermal and electrical conductivity, Notes: complex manufacturing processes; high cost EXAMPLES Some Silicate Ceramics The Body’s Ceramic Hydroxyapatite (HA) Ca10(HPO4)6(OH)2 is the primary mineral content of bone Some Nonsilicate Oxide Ceramics Some Nonoxide Ceramics CERAMIC BONDING • Bonding: - Mostly ionic, some covalent. - % ionic character increases with difference in electronegativity. CaF2 (89%) SiC (12%) Metal Ceramic Nonmetal H Intermediate Li Be He Ne O F Na Mg S Cl Ar K Ca Sc Se Br Kr Rb Sr Te Y Cs Ba I Xe Po At Rn Fr Ra Electropositive elements: Readily give up electrons to become + ions. Electronegative elements: Readily acquire electrons to become - ions. CaF2 MgO NaCl Al2O3 SiO2 Si3N4 ZnS SiC % Ionic Character 89 73 67 63 51 30 18 12 IONIC BONDING & STRUCTURE • Charge Neutrality: - Net charge in the structure should be zero. • General form: CaF2 : AmXp Ca2+ + cation Fanions F- m, p determined by charge neutrality • Stable crystal structures: maximize the # of nearest oppositely charged neighbors, when all anions are in contact with that cation, i.e. special relations between cation (rC) and anion (rA) radius should hold. COORDINATION NUMBER • The coordination number is a number of anions nearest neighbors for a cation. • Coordination number increases with increasing rC/rA ratio AX – type compounds (p=m=1) EXAMPLE: PREDICTION STRUCTURE OF FeO • On the basis of ionic radii, what crystal structure would you predict for FeO? Cation Al3+ Fe 2+ Fe 3+ Ca2+ Anion O2ClF- • Answer: r cation 0 .077 = 0 .550 = r anion 0. 140 based on this ratio: -coord # = 6 -structure = NaCl-type CERAMIC DENSITY COMPUTATION IMPURITIES IN CERAMICS • Impurities must also satisfy charge balance • Ex: NaCl • Substitutional cation impurity cation vacancy Ca2+ Na+ Na+ initial geometry Ca2+ impurity • Substitutional anion impurity O2- initial geometry ClClO2- impurity Ca2+ resulting geometry anion vacancy resulting geometry GLASS STRUCTURE • Basic Unit: 4Si04 tetrahedron Si4+ O2- • 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) 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 • Viscosity: --relates shear stress & velocity gradient: --has units of (Pa-s) dv = dy GLASS VISCOSITY VS TEMPERATURE Important temperatures in glasses are defined in terms of viscosity • Melting point: viscosity <102 P-s, above this temperature glass is liquid • Working point: viscosity ~ 103 P-s, glass is easily deformed • Softening point: viscosity = 6x106 P-s, maximum T at which a glass piece maintains shape for a long time • Annealing point: viscosity = 1012 P-s, relax internal stresses (diffusion) • Viscosity decreases with T • Impurities lower Tdeform • Strain point: viscosity = 5x1013 P-s, above this viscosity, fracture occurs before plastic deformation Glass forming operations - between softening and working points VISCOSITY-TEMPERATURE CHARACTERISTICS Important temperatures in glasses are defined in terms of viscosity • Melting point: viscosity = 100 P, above this temperature glass is liquid • Working point: viscosity = 104 P, glass is easily deformed • Softening point: viscosity = 4x107 P, maximum T at which a glass piece maintains shape for a long time • Annealing point: viscosity = 1013 P, relax internal stresses (diffusion) • Strain point: viscosity = 3x1014 P, above this viscosity, fracture occurs before plastic deformation Glass forming operations - between softening and working points and working points TO REMEMBER The glass transition temperature is, for a noncrystalline ceramic, that temperature at which there is a change of slope for the specific volume versus temperature curve . The melting temperature is, for a crystalline material, that temperature at which there is a sudden and discontinuous decrease in the specific volume versus temperature curve. MECHANICAL PROPERTIRES MECHANICAL PROPERTIRES: BRITTLE FRACTURE • In solids with ionic-type bonds, slip (dislocation motion) is difficult because ions of like charge must be brought into close proximity which forms a large barrier for dislocation motion. • Similarly, in ceramics with covalent bonding, slip is not easy (covalent bonds are strong). • Thus at room temperature ceramics fracture before any plastic deformation occurs – brittle fracture • The mechanism of brittle structure involves the formation and propagation of cracks • The measure of a ceramic’s ability to resist fracture when a crack is present is the fracture toughness. • For example a plane strain fracture toughness equals: • Kic=Ys(pa)0.5 Y -dimensionless parameter, which depends on sample geometry; a -crack’s half length. •Non-crystalline ceramics: there is no regular crystalline structure, thus no dislocations. Materials deform by viscous flow, i.e. by breaking and reforming atomic bonds, allowing ions/atoms to slide past each other (like in a liquid). •Viscosity is a measure of glassy material’s resistance to deformation. WEIBULL MODULUS • • It appears that for brittle materials (e.g. ceramics) the maximum stress that they can withstand, varies unpredictably from specimen to specimen even under identical testing conditions Thus the strength of brittle material is not a well define value and has to be described with respect to fracture statistics •A Weibull distribution of strength with a flexible twoparameter analytic formula has been found to describe a brittle body fracture. The probability (P) of failure for a brittle material is given by: P(s) = 1-exp(-[s/so]m) where s-a failure strength, so - a scaling constant and m is a the Weibull modulus that is a measure of a degree of strength dispersion MEASURING STRENGTH • A three-point bend test to measure the flexural strength, sfs Typical values for different ceramics sfs(MPa) Material Si nitride 700-1000 Si carbide 550-860 Al oxide 275-550 glass (soda) 69 E(GPa) 300 430 390 69 SUMMARY • Ceramic materials have mostly covalent & some ionic bonding. • Structures are based on: • • • • --charge neutrality --maximizing # of nearest oppositely charged neighbors. Structures may be predicted based on: --ratio of the cation and anion radii. Defects --must preserve charge neutrality --have a concentration that varies exponentially w/T. Room T mechanical response is elastic, but fracture brittle, with negligible ductility. Elevated T creep properties are generally superior to those of metals (and polymers). TAXONOMY OF CERAMICS • Properties: --Tmelt 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. APPLICATION: REFRACTORIES • Need a material to use in high temperature furnaces. • Consider Silica (SiO2) - Alumina (Al2O3) system. • Phase diagram shows: mullite, alumina, and crystobalite (made up of SiO2) tetrahedra as candidate refractories. 2200 T(°C) 2000 3Al2O3-2SiO2 Liquid (L) 1800 1400 0 alumina + L mullite +L crystobalite +L 1600 mullite mullite + crystobalite 20 alumina + mullite 40 60 80 100 Composition (wt% alumina) APPLICATION: DIE BLANKS • Die blanks: --Need wear resistant properties! die Ao die Ad tensile force Courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission. • Die surface: --4 mm polycrystalline diamond particles that are sintered on to 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 • Tools: --for grinding glass, tungsten, carbide, ceramics --for cutting Si wafers --for oil drilling • Solutions: oil drill bits --manufactured single crystal or polycrystalline diamonds in a metal or resin matrix. --optional coatings (e.g., Ti to help diamonds bond to a Co matrix via alloying) --polycrystalline diamonds resharpen by microfracturing along crystalline planes. blades coated single crystal diamonds polycrystalline diamonds in a resin matrix. Photos courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission. APPLICATION: SENSORS • Ex: Oxygen sensor: ZrO2 • Principle: Make diffusion of ions fast for rapid response. • Approach: Add Ca impurity to: --increase O2- vacancies --increase O2- diffusion • Operation: --voltage difference produced when O2- ions diffuse between external and references gases. sensor gas with an reference unknown, higher gas at fixed O2oxygen content oxygen content diffus ion + - voltage difference produced! SUMMARY • Basic categories of ceramics: --glasses --clay products --refractories --cements --advanced ceramics • Fabrication Techniques: --glass forming (impurities affect forming temp). --particulate forming (needed if ductility is limited) --cementation (large volume, room T process) • Heat treating: Used to --alleviate residual stress from cooling, --produce fracture resistant components by putting surface into compression. CERAMIC FABRICATION METHODS-I GLASS FORMING • Pressing: Gob • Fiber drawing: Pressing operation Parison mold • Blowing: wind up GLASS FORMING Continuous drawing of sheet glass Hot-rolling! The press – and –blow technique for glass bottle production THERMAL STRESSES • Residual thermal stresses are introduced into a glass piece when it is cooled because surface and interior regions cool at different rates, and, therefore, contract different amounts; since the material will experience very little, if any deformation, stresses are established. • The thinner the thickness of a glass ware the smaller the thermal stresses that are introduced when it is either heated or cooled. The reason for this is that the difference in temperature across the cross-section of the ware, and, therefore, the difference in the degree of expansion or contraction will decrease with a decrease in thickness. HEAT TREATING GLASS • Annealing: --removes internal stress caused by uneven cooling. • Tempering: --puts surface of glass part into compression --suppresses growth of cracks from surface scratches. Tempered glass --sequence: before cooling hot surface cooling cooler hot cooler further cooled compression tension compression --Result: surface crack growth is suppressed - The strength is enhanced !! CERAMIC FABRICATION METHODS-IIB PARTICULATE FORMING • Sintering: useful for both clay and non-clay compositions. • Procedure: --grind to produce ceramic and/or glass particles --inject into mold --press at elevated T to reduce pore size. • Aluminum oxide powder: --sintered at 1700C for 6 minutes. CERAMIC FABRICATION METHODS-IIA PARTICULATE FORMING • Milling and screening: desired particle size • Mixing particles & water: produces a "slip“ – highly plastic media • Form a "green" component --Hydroplastic forming: extrude the slip (e.g., into a pipe) --Slip casting: solid component • Dry and Fire the component pacificator hollow component Clay Products CERAMIC FABRICATION METHODS-III CEMENTATION • Produced in extremely large quantities. • Portland cement: --mix clay and lime bearing materials --calcinate (heat to 1400C) Example of hydration reaction: --primary constituents: tri-calcium silicate 2CaO-SiO2+xH2O=2CaO-SiO2-xH2O 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. SUMMARY • Basic categories of ceramics: --glasses --clay products --refractories --cements --advanced ceramics • Fabrication Techniques: --glass forming (impurities affect forming temp). --particulate forming (needed if ductility is limited) --cementation (large volume, room T process) • Heat treating: Used to --alleviate residual stress from cooling, --produce fracture resistant components by putting surface into compression.
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