CERAMICS Part 1: Structure and Properties. MSE 206-Materials Characterization I Lecture-7

Transcription

1 CERAMICS Part 1: Structure and Properties MSE 206-Materials Characterization I Lecture-7

2 Classification of Materials

3 Ceramics Ceramics comes from Greek word keramikos, means burnt stuff Compounds between metallic and nonmetallic elements; i.e CaF 2, MgO, NaCl, Al 2 O 3, SiO 2, Si 3 N 4, ZnS, SiC Bonding: -- Can be ionic and/or covalent in character. -- % ionic character increases with difference in electronegativity of atoms.

4 Ceramics Degree of ionic character may be large or small: CaF 2 : large SiC: small

5 Ceramics PROPERTIES: Hard and brittle with low toughness and ductility Good electrical and thermal insulator because of absence of conduction electrons High melting point and high chemical stability We know that ceramics are more brittle than metals. Why? Consider method of deformation slippage along slip planes in ionic solids (ceramics) this slippage is very difficult too much energy needed to move one anion past another anion

6 Ceramics TYPES OF CERAMICS: 1-) Traditional ceramic materials (glasses, refractories, cement and concrete, bricks and tiles) 2-) Advanced ceramic materials (consist of pure or nearly pure compounds such as Al 2 O 3, SiC, Si 3 N 4.

7 Ceramics 1-TRADITIONAL CERAMICS Whiteware 2-ADVANCED CERAMICS Structural: bioceramics, cutting tools, engine components, armour. Electrical: Capacitors, insulators, magnets and superconductors Gas turbine rotor, Si 3 N 4 Brake disc SiC engine components SiC body armour Cutting tools

8 Ceramics

9 Ceramics: Crystal Structures The ionic ceramics crystal structure can be thought of as being composed of electrically charged ions: - The metallic positively charged ions are called cations. (give valence electrons) - The non-metallic ions are negatively charged and called anions (accept electrons) i.e. CaF 2 : Ca 2+ ion: has two positive charges (cation) F - ion: has one negative charge (anion) Ionic radii of the cations and anions: - Metallic elements give up electrons - Non metallic elements accept electrons Cations are smaller than anions

10 Ceramics: Crystal Structures Fluorite structure AX Type Crystal Structures include NaCl, CsCl, and zinc blende AX 2 Crystal Structures UO 2, ThO 2, ZrO 2, CeO 2 Perovskite structure Ex: complex oxide BaTiO 3 ABX 3 Crystal Structures

11 Silicate Ceramics: Silica (SiO 2 ) Most common elements on earth are Si & O Depending on the arrangement of SiO 4 4- different silicate structures arise. SiO 4 4- Si 4+ O 2- crystobalite SiO 2 (silica) polymorphic forms are quartz, crystobalite, & tridymite The strong Si-O bonds lead to a high melting temperature (1710ºC) for this material

12 Silica Glasses Basic Unit: 4- Si0 4 tetrahedron Si 4+ O 2-1) CRYSTALLINE 2) AMORPHOUS (NON-CRYSTALLINE) No impurities are added Network modifiers are added, i.e. CaO, Na 2 O Na + Si 4+ O 2 - Fused silica is SiO 2 Borosilicate glass is the pyrex glass used in labs better temperature stability & less brittle than sodium glass Sodium silicate glass (soda glass) Used for containers, windows

13 Silicates: Simple Silicates Complex structures are formed by sharing of oxygen atoms of SiO 4 4- on the corners, edges, or faces Addition of Mg Ca, Mg Adapted from Fig , Callister & Rethwisch 8e. Mg 2 SiO 4 Ca 2 MgSi 2 O 7 Presence of cations such as Ca 2+, Mg 2+, & Al maintain charge neutrality, and 2. ionically bond SiO 4 4- to one another

14 Silicates: Layered Silicates Layered silicates (e.g., clays, mica, talc) SiO 4 tetrahedra connected together to form 2-D plane A net negative charge is associated with each (Si 2 O 5 ) 2- unit Negative charge balanced by adjacent plane rich in positively charged cations Adapted from Fig , Callister & Rethwisch 8e. 14

15 Silicates: Layered Silicates Kaolinite (Al 2 (Si 2 O 5 )(OH) 4 ) clay alternates (Si 2 O 5 ) 2- layer with Al 2 (OH) 4 2+ layer Adapted from Fig , Callister & Rethwisch 8e. Note: Adjacent sheets of this type are loosely bound to one another by van der Waal s forces.

16 CARBON: Diamond Diamond - Hardest material known - Covalently bonded tetrahedral carbon hard no good slip planes brittle can cut it Diamond cubic structure - High thermal conductivity and optically transperant - large diamonds jewelry - small diamonds-often man made - used for cutting tools and polishing Adapted from Fig , Callister 7e. - diamond films- hard surface coat tools, medical devices, etc.

17 CARBON FORMS: Graphite In layers, between carbon atoms the bond type is covalent; Between layers weak van der Waal s forces planes slide easily, good lubricant Electrical conductivity is high in crystallograhic directions parallel to the hexagonal sheets High thermal conductivity Good machinability Adapted from Fig , Callister 7e. Applications: heating elements for furnaces, casting molds, rocket nozzles, electrical contacts

18 CARBON FORMS: Nanotubes and Fullerenes wrap the graphite sheet by curving into ball or tube Buckminister fullerenes Like a soccer ball C 60 - also C 70 + others Carbon nanotube - Extremely strong and stiff (tensile strength : GPa, elastic modulus : 1 TPa) - Very low density - May behave as conductor or semiconductor C 60

19 Mechanical Properties of Ceramics The main drawback is catastrophic fracture in a brittle manner with very low energy σ ceramics X X metals ε

20 Mechanical Properties of Ceramics Metals Crystalline Metallic bond (non-directional) Dislocations move under relatively low stresses Ceramics Crystalline (covalently or ionically bonded) or Non-crystalline 1-In covalently bonded ceramic: - Bond is directional, - limited slip system, - Brittle fracture due to seperation of electron-pair bonds 2- In Ionically bonded ceramics: - Limited slip systems - Cracking occurs at the grain boundaries 3-In Non-crystalline ceramics: -Deform by viscous flow (rate of deformation is proportional to stress) -Atoms or ions slide past one another by breaking and reforming of interatomic bonds.

21 Mechanical Properties of Ceramics Elastic modulus and Flexural Strength Stress-strain behavior is not determined by a tensile test (difficult to shape and grip the ceramic) Transverse bending test, in which the ceramic material is bent until fracture - Rod specimen having either a circular or rectengular cross-section - Three or four point loading technique cross section d R b rect. circ. F L/2 L/2 d = midpoint deflection

22 Mechanical Properties of Ceramics Elastic modulus cross section b rect. d R circ. F L/2 L/2 δ = midpoint deflection Determine elastic modulus according to: F x slope = F d linear-elastic behavior d E = F d L 3 4 bd 3 rect. cross section = F d L 3 12 p R 4 circ. cross section 22

23 Mechanical Properties of Ceramics Flexural Strength cross section b rect. d R circ. F L/2 L/2 location of max tension δ= midpoint deflection The stress at fracture using this flexure test is konown as the flexural strength, modulus of rupture, or bend strength s fs = 1.5F f L = F f L F f F x bd 2 rect. pr 3 d fs d

24 Mechanical Properties of Ceramics

25 Mechanical Properties of Ceramics Factors Effecting the Strength Surface cracks Voids (porosity) Inclusions Large grains EFFECT OF POROSITY: Pores act as stress risers. When pores reach a critical value, a crack forms and propogates.

26 Mechanical Properties of Ceramics Brittle Fracture of ceramics The measured fracture strengths of ceramics are smaller than predicted Flaws in ceramics act as stress riser SURFACE CRACK INTERNAL CRACK Maximum stress at the crack tip; s m = 2s o (a/ t ) 1/2 σ m σ 0 ρ t a : maximum stress at the crack tip : magnitude of the nominal applied tensile stress : radius of curvature of the crack tip : length of a surface crack or half of the length of an internal crack

27 Mechanical Properties of Ceramics Fracture toughness Ability of material to resist fracture when a crack is present Ceramics have relatively low fracture toughness K Ic = Ys pa σ : applied stress a : crack length Y : dimensionless parameter and its value depends on both crack and specimen sizes and geometries, and load application. Y=1 for aplate of infinite width having a through-thickness crack Y= 1.1 for aplate of semi-infinite width having an edge crack lentgh of a.

28 Mechanical Properties of Ceramics Hardness Hardness is important when the material is going to be used in abrasive or grinding action

29 Mechanical Properties of Ceramics Abrasive ceramics that are used to grind or cut away other materials: - High hardness or wear resistance is needed - High degree of tougness is also needed to ensure that the abrasive particles don t easily fracture The abresive wear resistance (AWR) of a cutting tool AWR α K IC 3/4 H 1/2 K IC : Fracture toughness H : hardness number Al 2 O 3 -SiC composites are used primarily for cutting tool applications

30 Mechanical Properties of Ceramics Material Density, ρ (g/cm 3 ) Hardness vickers (GPa) Elastic Modulus (GPa) Fracture toughness (MPa(m) -1/2 ) Sintered Alumina Al 2 O 3-30%TiC (hp) Al 2 O 3-30%TiC (lps) HP-Si 3 N Al 2 O 3 -SiC (lps) Sintered SiALON

31 Mechanical Properties of Ceramics Elevated Temperature Tensile Test (T > 0.4 Tm). creep test s s e x. slope = e ss = steady-state creep rate time 31