1. 2012/9/13 1ISSUES TO ADDRESS... Structure of ceramics materials:- How do the crystal structures of ceramic materials differ fromthose for metals? Point defects:- How do point defects in ceramics differ from those defects found in metals? Impurities:- How are impurities accommodated in the ceramic lattice? Mechanical Properties:- How are the mechanical properties of ceramics measured, and how do they differ from those for metals? 21

2. 2012/9/13Learning ObjectivesAfter careful study of this chapter you should be able to dothe following:1. Sketch/describe unit cells for sodium chloride,cesiumchloride, zinc blende, diamond cubic, fluorite, and perovskitecrystal structures. Do likewise for the atomic structures ofglass.graphite and a silica glass.2. Given the chemical formula for a ceramic compound andthe ionic radii of its component ions, determine the crystalstructure.structure.3. Name and describe eight different ionic point defects thatare found in ceramic compounds.Learning Objectives4. Briefly explain why there is normally significantscatter in the fracture strength for identical specimensof the same ceramic material.5. Compute the flexural strength of ceramic specimensthat have been bent to fracture in three point loading.6. On the basis of slip considerations, explain whycrystalline ceramic materials are normally brittle.2 3. 2012/9/13IntroductionThe term ceramic comes from the the Greek word keramikos, which mean burnt stuff indicating that desirable properties of these materials are normally achieved through a high- temperature heat treatment process called firing. Inorganic and nonmetallic mostly ionic bonding structure metal + nonmetal elementsAtomic Bonding in Ceramics (1/4) Bonding: -- Can be ionic and/or covalent in character. -- % ionic character increases with difference in electronegativity of atoms. Degree of ionic character may be large or small: CaF2: largeSiC: small Adapted from Fig. 2.7, Callister & Rethwisch 8e. (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.)63 4. 2012/9/13Recall :Ionic Bonding (chapter 2) (2/4) Occurs between + and - ions. Requires electron transfer. Large difference in electronegativity required.Ionic bonds are Non Directional Example: NaClNa (metal)Cl (nonmetal) unstableunstable electronNa (cation) +- Cl (anion)stableCoulombic stableAttraction7Recall: Covalent Bonding (chapter 2) (3/4) similar electronegativity share electrons bonds determined by valence s & p orbitalsdominate bonding OverlappingElectron Clouds Example: CH4 shared electrons HC: has 4 valence e-, CH 4 from carbon atomneeds 4 moreH: has 1 valence e-,HCHneeds 1 more shared electronsElectronegativitiesH from hydrogenare comparable.atoms Adapted from Fig. 2.10, Callister & Rethwisch 8e.8 4 5. 2012/9/13Recall : Bonding (chapter 2) (4/4)9Ionic and Covalent Bonding in SimpleCeramicsCeramics are composed at least two elementsMixture of Ionic and Covalent Types.The degree of ionic character is dependent on theelectronegativities of the atoms.105 6. 2012/9/13Ceramic Crystal Structures (1/14) Two characteristics of the component ions in crystalline ceramic materials influences the crystal structure: the magnitude of the electrical charge on each of the component ions, and the relative sizes of the cations () and anions () . Their crystal structures are generally more complex than those of metals. 11Factors that Determine Crystal Structure (2/14)1. Relative sizes of ions Formation of stable structures:--maximize the # of oppositely charged ion neighbors. - - - -- - + ++Adapted from Fig. 12.1, Callister & Rethwisch 8e . - - - -- - unstablestablestable 2. Maintenance ofCharge Neutrality :F- --Net charge in ceramic CaF 2 :Ca 2+ +cation anionsshould be zero. --Reflected in chemical F-formula: A m Xpm, p values to achieve charge neutrality 126 7. 2012/9/13Coordination # and Ionic Radii (3/14)r cation Coordination # increases with ranionTo form a stable structure, how many anions can surround around a cation? r cationCoordZnS r anion #(zinc blende) Adapted from Fig. 12.4, < 0.155 2 linearCallister & Rethwisch 8e. 0.155 - 0.225 3 triangular NaCl(sodium 0.225 - 0.414 4 tetrahedralchloride) Adapted from Fig. 12.2, Callister & Rethwisch 8e. 0.414 - 0.732 6 octahedral CsCl(cesiumchloride) 0.732 - 1.0 8 cubic Adapted from Fig. 12.3, Callister & Rethwisch 8e.The most common coordination number for ceramics are 4, 6, and 8.13Computation of Minimum Cation-Anion RadiusRatio (4/14)Determine minimum rcation/ranion for an octahedral site(C.N. = 6)2ranion + 2rcation = 2a a = 2ranion 2ranion + 2rcation = 2 2ranionranion + rcation = 2ranion rcation = ( 2 1)ranion rcation = 2 1 = 0.414 ranion147 8. 2012/9/13Ex #1: Predicting the Crystal Structure of FeO (5/14) On the basis of ionic radii, what crystal structurewould you predict for FeO? Answer: Cation Ionic radius (nm)rcation 0.077Al 3+ 0.053= ranion 0.140Fe 2+ 0.077= 0.550Fe 3+ 0.069Ca2+0.100 based on this ratio,-- coord # = 6 because Anion0.414 < 0.550 < 0.732O2- 0.140 -- crystal structure is NaClCl -0.181F-0.13315 Data from Table 12.3, Callister & Rethwisch 8e . Ceramic Crystal Structures (6/14)AX-type Crystal structures (often referred to thosewith equal numbers of cations and anions) Rock Salt (NaCl) Cesium Chloride structure (CsCl) Zinc Blende (or sphalerite) structure (ZnS)AmXp Type crystal structures ( m p ) Fluorite structure include CaF2, UO2, PuO2, ThO2AmBnXp-Type crystal structures perovskite crystal168 9. 2012/9/13AX-Type crystal structure (7/14)Example: NaCl (rock salt) structure rNa = 0.102 nm rCl = 0.181 nm rNa/rCl = 0.564 cations (Na+) prefer octahedral sitesRadius ratio = 0.56, CN = 6.17AX-Type crystal structure (8/14) MgO and FeO also have the NaCl structure O2-rO = 0.140 nm Mg2+rMg = 0.072 nm rMg/rO = 0.514 cations prefer octahedral sites Adapted from Fig. 12.2, Callister & Rethwisch 8e.So each Mg2+ (or Fe2+) has 6 neighbor oxygen atoms189 10. 2012/9/13 AX-type Crystal Structures (9/14)Cesium Chloride structure: rCs + 0.170 = = 0.939 r 0.181Cl Since 0.732 < 0.939 < 1.0,cubic sites preferredSo each Cs+ has 8 neighbor Cl- .Adapted from Fig. 12.3, Callister & Rethwisch 8e CsBr, TlCl and TlBr havesimilar structure. 19Zinc Blende (ZnS) Crystal Structure (10/14) Four zinc and four sulfur atoms. One type (Zn or S) occupies lattice points and anotheroccupies interstitial sites of FCC unit cell. S Atoms (0,0,0) ( , ,0) ( , 0, ) (0, , )Zn Atoms ( , , ) ( , , )( ,, ) ( , , ) Tetrahedrally covalently bonded (87% covalent character) withCN =. CdS, InAs, InSb and ZnSe havesimilar structures. 2010 11. 2012/9/13AmXp Type crystal structures ( m p ) (11/14)Ca2+ ions : centers of cubeFluorite structureF- ions : corner of cubeOne unit cell consists of eightcubes Calcium Fluorite (CaF2) Cations in cubic sites UO2, ThO2, ZrO2, CeO2 Antifluorite structure positions of cations andanions reversedAdapted from Fig. 12.5, Callister & Rethwisch 8e.21 AmBnXp Crystal Structures (12/14) Perovskite structure Ex: complex oxide BaTiO3 Ba2+ and O2- ions form FCC unit cell.Ba2+ Ions occupy corners O2- Ions occupy face centers. Ti4+ ions are at octahedralsites.22 11 12. 2012/9/13Ceramic Crystal Structure (13/14)23Summary of some common ceramiccrystal structure (14/14)24 12 13. 2012/9/13Crystal Structures From The Close Packing Of Anions(1/2) Label T: tetrahedral positions : Four atoms surround one type , label T C.N=4 Label O: octahedral positions : Six atoms surround one type label O C.N=6 25Crystal Structures From The Close Packing Of Anions(2/2) Ceramic crystal structures of this type depend on: The stacking of the close-packed anions layers (both FCC and HCP arrangements are possible, which correspond to ABCABCand ABABABsequences, respectively ), and The manner in which the interstitial sites are filled with cation.The manner in which the interstitial sites are filled with cations, e.g. NaCl, CN=6. 2613 14. 2012/9/13Recall: Theoretical Density, (chapter 3 )(1/2)Mass of Atoms in Unit CellDensity = =Total Volume of Unit CellnA =VC NA wheren = number of atoms/unit cellA = atomic weightVC = Volume of unit cell = a3 for cubicNA = Avogadros number = 6.022 x 1023 atoms/mol27 Density Computations for Ceramics (2/2)Number of formula units/unit celln(AC + AA ) =VC N AAvogadros numberVolume of unit cell AC = sum of atomic weights of all cations in formula unit AA = sum of atomic weights of all anions in formula unit28 14 15. 2012/9/13Silicate Ceramics (1/6) Silicates are materials composed primarily of silicon and oxygen, the two most abundant elements in the earths crust. It is more convenient to use various arrangements of an SiO44- tetrahedron. Each atom of silicon is bounded to four oxygen atoms, which are situated at the corners of the tetrahedron; the silicon atom is positioned at the center. Si4+Adapted from Figs.O2- 12.9-10, Callister &Rethwisch 8e29Silica (2/6)Chemically, the most simple silicate material is silicondioxide, or silica (SiO2)There are three primary polymorphic crystalline formsof silica: quartz(), crystobalite (), &tridymite()The strong Si-O bonds lead to a high meltingtemperature (1710C) for this material crystobalite30 15 16. 2012/9/13Silica Glass (3/6) Glass is noncrystalline (amorphous) Basic Unit: Fused silica is SiO2 to which no 4-impurities have been addedSi0 4 tetrahedron Other common glasses containSi 4+impurity ions such as Na+, Ca2+, O2- Al3+, and B3+ The addition of modifiers and intermediates lowers the melting temperature and viscosity Quartz is crystalline of a glass and makes it easier to form at lower SiO2: temperatures.)Na+Si4+ O2- (soda glass) Adapted from Fig. 12.11, Callister & Rethwisch 8e. 31 Silicates (4/6) Bonding of adjacent SiO44- accomplished by the sharingof common corners, edges, or facesAdapted from Fig.12.12, Callister &Rethwisch 8e.Mg2SiO4 Ca2MgSi2O7Presence of cations such as Ca2+, Mg2+, & Al3+ 1. maintain charge neutrality, and 2. ionically bond SiO44- to one another 3216 17. 2012/9/13Layered Silicates- (Si2O5)2- (5/6) Layered silicates (e.g., clays, mica, talc)SiO4 tetrahedra connectedtogether to form 2-D plane A net negative charge is associated with each (Si2O5)2- unit Negative charge balanced by adjacent plane rich in positively charged cations Adapted from Fig. 12.13, Callister & Rethwisch 8e.33 Layered Silicates (6/6) Kaolinite clay () alternates (Si2O5)2- layer with Al2(OH)42+ layer Adapted from Fig. 12.14, Callister & Rethwisch 8e. Note: Adjacent sheets of this type are loosely bound to one another by van der Waals forces.34 17 18. 2012/9/13Polymorphic Forms of Carbon (1/9)Carbon is an element that exists in various polymorphicforms, as well as in the amorphous state. Diamondtetrahedral bonding of carbonhardest material knownvery high thermal conductivityVery low electrical conductivityOptically transparent in the visibleand infrared regions. Adapted from Fig. 12.15,large single crystals gemCallister & Rethwisch 8e.stonessmall crystals used togrind/cut other materialsdiamond thin filmshard surface coatings used forcutting tools, medical devices, etc.35Polymorphic Forms of Carbon (2/9) Graphite ( layered structure parallel hexagonal arrays of carbon atoms Other desirable properties of graphite include the following:high strength good chemical stability at elevated temperaturesAdapted from Fig. and in nonoxidizing atmospheres 12.17, Callister & Rethwisch 8e. high thermal conductivity higher electrical conductivity low coefficient of thermal expansion high resistance to thermal shock high adsorption of gases good machinability weak van der Waals forces between layers planes slide easily over one another -- good lubricant36 18 19. 2012/9/13Polymorphic Forms of Carbon -Fullerenes andNanotubes (3/9)Fullerenes discovered in 1985. spherical cluster of 60carbon atoms, C60 Like a soccer ballCarbon nanotubes sheet of graphite rolled into a tube Ends capped with fullerene hemispheresAdapted from Figs. 12.18 & 12.19, Callister& Rethwisch 8e. 37Nano-wire (4/9)Scanning electron microscope picture of an array of ultravioletnanowire nanolasers grown on a sapphire substrate. The wires areabout one thousandth the diameter of a human hair, and their length, about 10 microns, is one tenth the diameter of a hair.Photo: Peidong Yang/UC Berkeley, courtesy of Science) 19 20. 2012/9/13(5/9)(6/9)20 21. 2012/9/13 (7/9)Graphenesp2[1][1]20042010[2] (8/9)4221 22. 2012/9/13 (9/9)Point Defects in Ceramics (1/4) Vacancies -- vacancies exist in ceramics for both cations and anions Interstitials -- interstitials exist for cations-- interstitials are not normally observed for anions because anions are large relative to the interstitial sitesCationInterstitial Cation VacancyAdapted from Fig. 12.20, Callister& Rethwisch 8e. (Fig. 12.20 isfrom W.G. Moffatt, G.W. Pearsall,and J. Wulff, The Structure andProperties of Materials, Vol. 1,Structure, John Wiley and Sons,Inc., p. 78.) Anion Vacancy 4422 23. 2012/9/13Point Defects in Ceramics (2/4) Frenkel Defect-- a cation vacancy-cation interstitial pair. Shottky Defect-- a paired set of cation and anion vacancies. Shottky Defect: Adapted from Fig.12.21, Callister & Rethwisch 8e. (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.) Frenkel Defect Equilibrium concentration of defects e QD /kT45Point Defects in Ceramics (3/4) Nonstoichiometric: if the compound has any deviation from the exact ratio. Schematic representation for cation and anion substitutional as well as interstitial impurities 23 24. 2012/9/13Imperfections in Ceramics (4/4) Electroneutrality (charge balance) must be maintainedwhen impurities are present Ex: NaCl Na + Cl -cation Substitutional cation impurityvacancy Ca 2+ Na +Na +Ca 2+without impurity Ca 2+ impurity with impurity Substitutional anion impurity anion vacancy O2- Cl - Cl -without impurityO2- impuritywith impurity 47Ceramic phase diagrams (1/3)The Al2O3-Cr2O3 SystemThe MgO-Al2O3 systemSpinel: MgAl2 O424 25. 2012/9/13 Spinel- Spinel-structure) () (2/3) MgAl2O4Ceramic phase diagrams (3/3) The ZrO2-CaO systemThe SiO2-Al2O3 system25 26. 2012/9/13Mechanical Properties (1/13)Ceramic materials are more brittle than metals.Why is this so?Consider mechanism of deformation In crystalline, by dislocation motion In highly ionic solids, dislocation motion is difficultfew slip systemsresistance to motion of ions of like charge (e.g., anions) past oneanotherStrength of ceramics vary greatly but they aregenerally brittle.Tensile strength is lower than compressive strength. 51Ideal vs Real Materials (2/13) Stress-strain behavior (Room T): perfect matl-no flaws E/10 TSengineering Y a28 29. 2012/9/13Brittle Fracture of Ceramics (7/13)There is usuallyconsiderable variation andscatter in the fracturestrength for manyspecimens of a specificbrittle ceramic materialThe fracture strength of a brittle ceramic many be enhanceddramatically by imposing residual compressive stress at itssurface. One way this may be accomplished is by thermaltempering. Ceramics display much higher strengths in compression than in tension (on the order of a factor 10).Stress-Strain behavior (8/13)The stress at fracture using Flexural Strengththis flexure test, is know asthe flexural strength,modulus of rupture, fracturestrength, or the bendstrength, an importantmechanical parameter forbrittle ceramics. For a rectangular cross section, the flexural strength fs is equal3toL = sFfs 2bd 2 When the cross section is circular fs = Fs LR329 30. 2012/9/13Stress-Strain behavior (9/13) Room T behavior is usually elastic, with brittle failure. 3-Point Bend Testing often used. -- tensile tests are difficult for brittle materials. cross sectionFL/2L/2d R b = midpointrect. circ.deflectionlocation of max tensionThe stress at fracture using this flexure test: flexural strength,modulus of rupture, fracture strength, or the bend strength. For a rectangular cross section, the flexural 3Fs L fs =strength fs is equal to2bd 2 Fs L When the cross section is circular fs =R 3 59Measurement of Elastic Modulus (10/13) cross sectionFL/2 L/2Adapted from Fig. 12.32, Callister & Rethwisch 8e .d R b = midpointrect. circ. deflection Determine elastic modulus according to:FF L3xE= (rect. cross section)F 4bd 3slope =F L3 E= (circ. cross section) 12R 4linear-elastic behavior 6030 31. 2012/9/13Stress-Strain behavior (11/13) Characteristic flexural strength values for several ceramic materialsMechanism of plastic deformation(12/13) For crystalline ceramics, plastic deformation occurs by motion of dislocation the bonding is predominantly ionic, electrical repulsion High hardness & brittleness: difficult of slipFor ionic bonding: bring like ions togetherFor covalent bonding: strong bond, limited slip systems &complex dislocation structure62 31 32. 2012/9/13Mechanism of plastic deformation (13/13) For non-crystalline ceramics, Plastic deformation dose not occur by dislocation motion for noncystalline ceramics because there is no regular atomic structure. Rather, these materials deform by viscous flow The rate of deformation is proportional to the applied stress Atoms or ions slide past one another by the breaking &reforming the interatomic bonds F/A==dv / dy dv / dyThe units for viscosity are poises (P) As the temperature is raised, the magnitude of the bonding is diminished, the sliding motion or flow of the atoms or ions is facilitatedMiscellaneous Mechanical-considerations influence of porosity (1/6) The influence of volume fraction porosity on the modulus of elasticity for aluminum oxide is shown: (E = E o 1 1.9 P + 0.9 P 2) E0 is the modulus of elasticity of the nonporous material 32 33. 2012/9/13Miscellaneous Mechanical-considerations influence of porosity (2/6) The degree of the influence of pore volume on flexural strength is demonstrated in Fig 13.32, for aluminum oxide fs = o exp(np) o and n are experimental constants. (1) Pores reduce the cross-sectional area across which the load is applied, (2) they also act as stress concentratorsMiscellaneous Mechanical Hardness (3/6) The hardest known materials are ceramics Ceramics are hard and can be used as abrasives.Examples:- Al2O3, SiC. By combining ceramics, improved abrasives can be developed.Example:- 25% ZrO2 + 75% Al2O333 34. 2012/9/13Miscellaneous Mechanical -Creep in Ceramics(4/6)Ceramic materials experience creep deformation as aresults of exposure to stressed (usually compressive atT> 0.4 Tm) Time-deformation creep behavior of ceramic is similar to that of metal Creep occurs at higher temperature in ceramics67Miscellaneous Mechanical -Creep in Ceramics (5/6) Primary Creep: slope (creep rate) decreases with time. Secondary Creep: steady- state i.e., constant slope (/t). Tertiary Creep: slope (creep rate) increases with time, i.e. acceleration of rate.68 34 35. 2012/9/13Miscellaneous MechanicalFatigue Failure in Ceramics ()- (6/6) Fatigue = failure under applied cyclic stress.A form of failure that occurs in structures subjected to dynamic and fluctuatingstresses. (at a relatively low stress than YS) specimen compression on top maxSbearing bearing motor counter m flex coupling min timetension on bottomFatigue fracture in ceramics is rare due to absence of plasticdeformation.Straight fatigue crack has been reported inalumina after 79,000compression cycles. 69SUMMARYCeramics are inorganic and nonmetallic.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).70 35 36. 2012/9/13SUMMARY Interatomic bonding in ceramics is ionic and/or covalent. Ceramic crystal structures are based on: -- maintaining charge neutrality -- cation-anion radii ratios. Imperfections -- Atomic point: vacancy, interstitial (cation), Frenkel, Schottky -- Impurities: substitutional, interstitial -- Maintenance of charge neutrality Room-temperature mechanical behavior is elastic, with brittlefracture and negligible ductility.Elevated T creep properties are generally superior to those ofmetals (and polymers)71 72 36 37. 2012/9/13ISSUES TO ADDRESS... ISSUES TO ADDRESS... How do we classify ceramics? What are some applications of ceramics? How is processing of ceramics different than for metals?73Learning Objectives Describe the process that is used to produce glass-ceramics.Name the two types of clay products, and then give two examples of each. Cite three important requirements that normally must be met by refractory ceramic and abrasive ceramics Describe the mechanism by which cement hardens when water is added.Name and briefly describe four forming methods that are used to fabricate glass pieces. Briefly describe and explain the procedure by which glass pieces are thermally tempered. Briefly describe processes that occur during the drying and firing of clay- based ceramic ware. Briefly describe/diagram the sintering process of powder particle aggregates. 37 38. 2012/9/13Introduction One chief concern in the application of ceramic material isthe method of fabrication. Ceramic material materials have relatively high meltingtemperatures , casting them is normally impractical.75Ceramics Application: Die Blanks () Die blanks: die Ad -- Need wear resistant properties! Aotensileforce die Die surface:Adapted from Fig. 11.8(d), Callister & Rethwisch 8e. -- 4 m polycrystalline diamondparticles that are sintered onto acemented tungsten carbidesubstrate. -- polycrystalline diamond gives uniformhardness in all directions to reducewear. Courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission.7638 39. 2012/9/13Ceramics Application: Cutting Tools () Tools:-- for grinding glass, tungsten, carbide, ceramics-- for cutting Si wafers-- for oil drilling Materials: oil drill bits blades -- manufactured single crystalor polycrystalline diamondsSingle crystal diamondsin a metal or resin matrix. -- polycrystalline diamonds polycrystallineresharpen by microfracturing diamonds in a resinalong cleavage planes. matrix. Photos courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission.77 Ceramics Application: Sensors () Example: ZrO2 as an oxygen sensor Ca2+ Principle: Increase diffusion rate of oxygen to produce rapid response of sensor signal to change in oxygen concentration Approach:A substituting Ca2+ ionremoves a Zr 4+ ion and Add Ca impurity to ZrO2: an O2- ion.-- increases O2- vacancies-- increases O2- diffusion rate Operation: sensor-- voltage difference produced when gas with anO2- ions diffuse from the external unknown, higher reference gas at fixedsurface through the sensor to theoxygen contentO2- oxygen content diffusionreference gas surface. -- magnitude of voltage difference partial pressure of oxygen at the + -external surface voltage difference produced!7839 40. 2012/9/13 Properties Structure Fabrication and process 7913.2 &13.3 Glasses & Glass ceramics The glasses are noncrystalline silicates containing other oxides, notablyCaO, Na2O and Al2O3, which influence the glass properties.Soda-lime glass: 70wt% SiO2, Balance: Na2O (soda), CaO (lime)Two prime assets of these materials are their optical transparency and the relative ease with which they may be fabricated.8040 41. 2012/9/13 13.2 &13.3 Glasses & Glass ceramics Crystallization: Most inorganic glasses can be made totransform from a noncrystalline state to one that is crystallineby the high-temperature heat treatment. The product is a fine-grained polycrystalline material (glass-ceramics). A nucleating agent (TiO2) must be added to induce thecrystallization process. Desirable properties :low thermal expansion coefficient (avoid thermal shock)high mechanical strengththermal conductivities (for ovenware)Good dielectric properties (for electronic packaging applications) 8113.9 Glass Structure Basic Unit: Glass is noncrystalline (amorphous)4- Fused silica is SiO2 to which no Si0 4 tetrahedronimpurities have been added Si 4+ Other common glasses contain O2-impurity ions such as Na+, Ca2+,Al3+, and B3+ Quartz is crystallineNa +SiO2:Si 4+O2- (soda glass) Adapted from Fig. 12.11, Callister & Rethwisch 8e. 8241 42. 2012/9/1313.9 Glass PropertiesOne of the distinctions between crystalline and noncrystalline materials lies in the dependence of specific volume Specific volume (1/) vs Temperature (T):Specific volume Crystalline materials: -- crystallize at melting temp, Tm Supercooled Liquid-- have abrupt change in spec.Liquid (disordered) vol. at Tm Glasses: Glass(amorphous solid) -- do not crystallize-- change in slope in spec. vol. curve atCrystalline glass transition temperature, Tg(i.e., ordered)solid-- transparent - no grain boundaries to TgTm T scatter light8313.9 Glass Properties-Viscosity () Viscosity, :-- relates shear stress () and velocity gradient (dv/dy):dy dv glass dvdy =dv / dy velocity gradient has units of (Pa-s)8442 43. 2012/9/13 13.9 Glass PropertiesLog Glass Viscosity vs. Temperature soda-lime glass: 70% SiO2balance Na2O (soda) & CaO (lime) Viscosity decreases with T borosilicate (Pyrex):13% B2O3, 3.5% Na2O, 2.5% Al2O3 Vycor: 96% SiO2, 4% B2O3 fused silica: > 99.5 wt% SiO2Viscosity [Pa-s] 10 14strain pointannealing point 10 10 10 6 Working range:glass-forming carried out10 2Tmelt1 200 600 1000 1400 1800 T(C) 85 13.9 Glass Properties On the viscosity scale several specific points that are important in the fabrication and processing of glasses are labeled:1.Melting point : the glass is fluid enough to be considered a liquid , v= 100 P2.Working point : the glass is easily deformed at this viscosity , v = 104P3.Softening point : the maximum temperature at which a glass piecemay be handled without causing significant dimensionalalterations, v = 4x107 P4.Annealing point : at this temperature, atomic diffusion issufficiently rapid that any residual stresses may be removed withinabout 15 min, v = 1013 P5.Strain point :for temperatures below the strain point ,fracture willoccur before the onset of plastic deformation, v = 3x1014 P86 43 44. 2012/9/13 13.9 Fabrication and processing of glasses & glass-ceramics: Glass FormingGLASSPARTICULATECEMENTATION FORMINGFORMING Blowing of Glass Bottles: Pressing: plates, cheap glasses Pressing -- glass formed by application ofGob operation pressureParison-- mold is steel with graphitemold lining Fiber drawing: Compressed airSuspendedparison Finishing moldwind up Adapted from Fig. 13.8, Callister & Rethwisch 8e. (Fig. 13.8 is adapted from C.J. Phillips, Glass: The Miracle Maker, Pittman Publishing Ltd., London.)8713.9 Fabrication and processing of glasses &glass-ceramics: Glass Forming Sheet Glass Forminghttp://www.youtube.com/watch?v=tDyeiePort0 Sheet forming continuous casting sheets are formed by floating the molten glass on a pool of molten tin Adapted from Fig. 13.9, Callister & Rethwisch 8e.8844 45. 2012/9/1313.9 Fabrication and processing of glasses &glass-ceramics: Heat treating Glass Annealing:-- removes internal stresses caused by uneven cooling. Tempering:-- puts surface of glass part into compression-- suppresses growth of cracks from surface scratches.-- sequence: before cooling initial coolingat room temp.cooler compression hothottensioncooler compression-- Result: surface crack growth is suppressed. 89 Properties Structure Fabrication and process 9045 46. 2012/9/13 13.4 Clay products One of the most widely used ceramic raw material is clay. Clay is inexpensive. When mixed in the proper proportion, clay and water form a plastic mass that is very amenable to shaping. The formed piece is dried to remove some of the moisture, after which it is fired at an elevated temperature to improve its mechanical strength. Applications: structural clay product: include building bricks, tiles, and sewer pipes- applications in which structure integrity is important. White ware 9113.10 characteristics of Clay -Hydroplasticity Clay are aluminosilicates, beingShear composed of alumina (Al2O3)& silica(SiO2),that contain chemically bound water.charge When water is added to clayneutral -- water molecules fit in between layered sheets -- Allow material to shear easily along weakvan der Waals bondingweak van -- when external forces applied clayder Waals particles free to move past one bonding another becomes hydroplastic 4+ charge Si 3+ neutralAl Structure of Kaolinite OH- Clay: [Al2(Si2O5)(OH)4 2-O Shear 92 46 47. 2012/9/1313.10 characteristics of Clay Product Many of these products (ex.whitewares) also contain some nonplastic ingredients; quartz is used primarily as a filler material, being inexpensive ,relatively hard, and chemically unreactive.When mixed with clay , a flux forms a glass that has a relatively low melting point . The feldspars are some of the more common fluxing agents; group of aluminosilicate material that contain k+, Na+ and Ca2+ ions. The characteristics of the finished piece, are influenced by the proportions of these three constituents: clay, quartz, and flux. A typical porcelain might contain approximately 50% clay, 25%quartz, and 25% feldspar. 9313.10 Fabrication and processing of ClayproductsThe as-mined raw materials usually have to go through amilling or grinding operation in which particle size is reduced; This is followed by screening or sizing to yield a powderedproduct having a desired range of particle size .Two common shaping techniques are utilized for forming clay-based compositions : hydroplastic forming and slip casting .47 48. 2012/9/1313.10 Fabrication and processing of Clayproducts Ceramic Fabrication MethodsGLASSPARTICULATECEMENTATION FORMING FORMINGHydroplastic forming: Mill (grind) and screen constituents: desired particle size The most common hydroplastic forming technique is extrusion,(e.g., into a brick)Aocontainerdie holder forceAdapted fromram billet extrusionAdFig. 12.8(c),Callister &containerdieRethwisch 8e. Dry and fire the formed piece9513.10 Fabrication and processing of Clayproducts http://www.youtube.com/watch?v=gQN9TJiuAn0Slip casting: (http://www.youtube.com/watch?v=8u_SABU_8d0) Mill (grind) and screen constituents: desired particle size Mix with water and other constituents to form slip Slip casting operation pour slip absorb water pour slipdrain green into mold into moldinto moldmoldceramic Adapted from Fig. green 13.12, Callister & ceramic Rethwisch 8e. (Fig.13.12 is from W.D.Kingery, Introductionto Ceramics, JohnWiley and Sons,Inc., 1960.) solid component hollow component Dry and fire the cast piece96 48 49. 2012/9/1313.10 Fabrication and processing of Clayproducts Drying: as water is removed - interparticle spacings decrease shrinkage .Adapted from Fig. 13.13, Callister & Rethwisch 8e.(Fig. 13.13 is from W.D. Kingery, Introduction toCeramics, John Wiley and Sons, Inc., 1960.) wet bodypartially dry completely dryDrying too fast causes sample to warp or crack due to non-uniform shrinkage micrograph of porcelainSi02 particle Firing:(quartz) -- heat treatment betweenglass formed900-1400Caroundthe particle -- vitrification: liquid glass formsfrom clay and flux flowsbetween SiO2 particles. (Flux70mAdapted from Fig. 13.14, Callister & Rethwisch 8e. (Fig. 13.14 is courtesy H.G. Brinkies, Swinburnelowers melting temperature).University of Technology, Hawthorn Campus, Hawthorn, Victoria, Australia.)97 Fireclay refractories Silica refractories Basic refractories Special refractories98 49 50. 2012/9/1313.5 RefractoriesAnother important class of ceramics that are utilized inLarge tonnages is the refractory ceramics.The salient properties: 1. The capacity to withstand high temperatures withoutmelting or decomposing. 2. The capacity to remain unreactive and inert whenexposed to severe environments. 3. The ability to provide thermal insulation.Typical applications:1. Furnace linings for metal refining.2. Glass manufacturing.3. Metallurgical heat treatment.4. Power generation.13.5 Refractories The performance of a refractory ceramic, depends on itscomposition. For many commercial materials, the rawingredients consist of both large(or grog) particles and fineparticles, which may have different compositions.10050 51. 2012/9/1313.5 Refractories Porosity: is one microstructural variable that mustbe controlled to produce a suitablerefractory brick. Strength, load-bearing capacity, and resistance to attack by corrosive materials are all increased with porosity reduction. But thermal insulation characteristics and resistance to thermal shock are diminished. The optimum porosity depends on the conditions of service13.5 Fireclay refractoriesThe primary ingredients for the fireclay refractoriesare high-purity fireclays, alumina and silica mixtures,usually containing between 25 and 45 wt% alumina.Fireclay bricks are used principally in furnaceconstruction, to confine hot atmospheres, and tothermally insulate structural members from excessivetemperatures.For fireclay brick, strength is not ordinarily animportant consideration, because support ofstructural loads is usually not required.10251 52. 2012/9/13 13.5 Silica refractoriesThe prime ingredient for silica refractories,sometimes termed acid refractories, is silica. Mainly for their high-temperature load-bearingcapacity.The alumina content should be held to a minimum,normally to between 0.2 and 1.0 wt%These refractory materials are also resistant to slagsthat are rich in silica (called acid slags) and are oftenused as containment vessels for them.103)13.5 Silica refractories ( Use at high temperatures (e.g., in high temperature furnaces). Consider the Silica (SiO2) - Alumina (Al2O3) system. Silica refractories - silica rich - small additions of alumina depress melting temperature (phase diagram): 2200 3Al2O3-2SiO2T(C)mullite2000 Liquid (L)alumina + L1800 crystobalite mullitealuminaFig. 12.27, Callister & Rethwisch 8e. (Fig.12.27 adapted from F.J. Klug and R.H.+L +L +Doremus, J. Am. Cer. Soc. 70(10), p. 758,1987.)1600 mullitemullite+ crystobalite14000 20406080100 Composition (wt% alumina)10452 53. 2012/9/1313.5 Basic refractories Basic refractories: that are rich in periclase, or magnesia(MgO).They may also contain calcium, chromium, and iron compounds. The presence of silica is deleterious to their high-temperature performance. Basic refractories are especially resistant to attack by slags containing high concentrations of MgO and CaO, and find extensive use in some steel-making open hearth furnaces. 10513.5 Special refractoriesSome of these ceramic materials are relatively high-purity oxide materials, many of which may beproduced with very little porosity, and are relativelyexpensive. Included: alumina, silica, magnesia, beryllia (BeO),zirconia (ZrO2), and mullite (3Al2O3-2SiO2); othersinclude carbide compounds, in addition to carbon andgraphite. Carbon and graphite are very refractory, butfind limited application because they are susceptibleto oxidation at temperatures in excess of about 800oC(1470oF). 106 53 54. 2012/9/13 Applications: Abrasives () Applications: Cements () Processing methods: Powder Pressing () Processing methods: Tape casting ()10713.6 AbrasivesAbrasive ceramics are used to wear, grind, or cutaway other material, which necessarily is softer.Therefore, the prime requisite for this group ofmaterials is hardness or wear resistance.The more common ceramic abrasive includesilicon carbide (SiC), tungsten carbide (WC),aluminum oxide (or corundum), and silica sand.10854 55. 2012/9/1313.6 Abrasives Abrasives are used in several forms bounded to grinding wheels, as coated abrasives. The surface structure should contain some porosity. Microstructures of two bonded abrasives, revealing abrasive grains, the bounding phase, and pores. 10913.6 CementsSeveral familiar ceramic materials are classified asinorganic cements: cement, plaster of paris, and limeHardening of a paste paste formed by mixing cement material with waterFormation of rigid structures having varied and complexshapesThe role of the cement is similar to that of the glassy bondingphase that forms when clay products and some refractorybrick are fired.The setting and hardening of this material results fromrelatively complicated hydration reactions that occur betweenthe various cement constituents and the water that is added. 110 55 56. 2012/9/1313.6 Cements- Cementation For example, one hydration reaction involving dicalcium silicate is as fellows: 2CaO SiO2 + xH2O = 2CaO SiO2 xH2O Hardening process hydration (complex chemicalreactions involving water and cement particles) (notdrying) Portland cement hydraulic cement -- hardness develops by chemical reaction with water Other cement materials nonhydraulic cement -- example: Lime -- compounds other than water (e.g., CO2) are involved inthe hardening reaction 13.11 Powder PressingPowder Pressing: used for both clay and non-clay compositions. A powdered mass, usually containing a small amount ofwater or other binder, is compacted into the desired shape. Binder is to lubricate the powder particles as they move pastone another in the compaction process Powder (plus binder) compacted by pressure in a mold-- Uniaxial compression - compacted in single direction-- Isostatic (hydrostatic) compression - pressure applied by fluid - powder in rubber envelope-- Hot pressing - pressure + heat 112 56 57. 2012/9/13 13.11 Powder Pressing Uniaxial compression11313.11 Powder Pressing -sinteringhttp://www.youtube.com/watch?v=48Is5ENhkGE&NR=1http://www.youtube.com/watch?v=4ijIUdQe3M4Sintering: for both uniaxial and isostatic procedures, afiring operation is required after the powder pressingoperation. -- powder particles coalesce and reduction of pore sizeAdapted from Fig. 13.16,Callister & Rethwisch 8e.11457 58. 2012/9/1313.11 Powder Pressing -sinteringThe driving force for sintering is the reduction intotal particle surface area; surface energies are largerin magnitude then grain boundary energies. Aluminum oxide powder: -- sintered at 1700Cfor 6 minutes. Adapted from Fig. 13.17, Callister & Rethwisch 8e. (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.)15 m (b) (a) 11513.12 Tape Casting Thin sheets of green ceramic cast as flexible tape Used for integrated circuits and capacitors Slip = suspended ceramic particles + organic liquid (contains binders, plasticizers) 116 58 59. 2012/9/13Summary Ceramic MaterialsGlassesClay Refractories AbrasivesCementsAdvanced productsceramics-optical-whiteware -bricks for-sandpaper-composites-engine-composite-structural high T- cutting - structural rotors reinforce(furnaces)- polishingvalves-containers/ Adapted from Fig. 13.1 and discussion in Section 13.2-8, Callister & Rethwisch 8e. bearings household -sensors117Summary Categories of ceramics:-- glasses -- clay products-- refractories-- cements-- advanced ceramics Ceramic Fabrication techniques: -- glass forming (pressing, blowing, fiber drawing). -- particulate forming (hydroplastic forming, slip casting, powder pressing, tape casting) -- cementation Heat treating procedures -- glassesannealing, tempering -- particulate formed piecesdrying, firing (sintering)11859