Engineering Materials-istanbul .technical University
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Dr.C.Ergun Mak214E
MAK214E Summer 2006-2007 Lecture notes 5
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Ceramic Materials
• Ceramics consist of metallic and non metallic elements joint by ionic or covalent bonds Characteristic properties: – High hardness, – High brittleness, – High melting point, – Low thermal and electrical conductivity, – Good chemical and thermal stability, – High compressive strength.
Applications: – Pottery, – Bricks, – Tiles, – Cooking ware, – Soil pipe to glass, – Refractory, – Magnets, electrical devices, – Abrasives or wear resistant materials. 2
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Applications
• Ceramics are used in a wide range of technologies such as: – Refractory, spark plugs, dielectrics capacitors in electronics, sensors/actuators, abrasives, biomaterials, magnets, magnetic recording media, etc. • Example: The space shuttle makes use of ~25,000 reusable, lightweight, highly porous ceramic tiles that protect the aluminum frame from the heat generated during re-entry into the Earth’s atmosphere.
© 2003 Brooks/Cole Publishing / Thomson Learning™
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Types of Ceramics
a) Crystalline ceramics: Most of ceramics except glasses. • Convensitonal ceramics; (clay based) for whiteware, etc. • Advanced ceramics; oxides, nitrides, carbides, etc. b) Ceramic Glasses: Non crystalline glasses - silica base. c) Glass Ceramics: Crystalline glasses – also silica base.
a) Crystalline ceramics: Good High T strength. b) Ceramic Glasses: Good forming characteristics. c) Glass Ceramics: Combining both properties 4
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Glasses
Glass structures may be 1. Ceramic Glasses: Non crystalline has short range ordered (SRO) structure; 2. Glass Ceramics: Crystalline long range order (LRO) crystalline structures.
Ceramic Glasses
Glass Ceramics 6
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Glass transition (fictive) temperature
Glass temperature (fictive or glass transition temperature)- The point where solid glass start to behave as an undercooled liquid.
• • • •
During crystallization, an abrupt change in the density occurs. The change in slope indicates the formation of a glass. Glass does not have melting T (Tm). Glass ceramics (Crystalline materials) show a fixed Tm and not a Tg.
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Ceramic Glasses
√ Non crystalline materials with random network (short range ordered (SRO)) structure below glass transition temperature. √ Best example; window glass. √ Main constituents: fused silica (pure SiO2)
Ceramic Glasses 8
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Modifiers
The modifiers, such as Sodium oxide (Na2O) break up the glassy network and reduce the ability to form a glass. Melting range considerable decreases and thus processing becomes easier. But not good for high Temperature applications. 9
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High Temperature Prop of Glasses
√ No dislocation motion in cermics or glasses; no plastic deformation at room T. √ At high Temperatures: ¾ Grain boundary sliding in crystalline ceramics. ¾ Viscous flow in glass ceramics and ceramic glasses. Creep in crystalline ceramic important in high temperature application. Crystalline ceramics good creep resistance due to high melting T, high activation energy for diffusion. Factors effecting creep resistance in crystalline ceramics: • Smaller grain size: more grain boundary, more creep. • Porosity: decrease in cross section, increase in stress, increase in grain boundary, more creep. • Impurities: leading to glassy phase along grain boundaries, viscous flow -more creep. • Higher Temperature: reducing the strength in grain boundaries, increasing the diffusion and glassy phase formation, more creep. 11
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High Temperature Prop of Glasses Viscous flow in glass ceramics: •At high T, viscous flow. •Depends on shear stress and velocity.
¾ Melting range: Glass in a liquid form, viscosity 50-500 poise. ¾ Working range: Glass can be deformed to desired geometries, viscosity 104-107 poise. ¾ Annealing range: To relieve residual stresses, viscosity 1013 poise. ¾ Strain point, Glass in completely rigid state, viscosity 1015 poise.
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Melting range: Glass in a liquid form, viscosity 50-500 poise. Working range: Glass can be deformed to desired geometries, viscosity 104-107 poise. Annealing range: To relieve residual stresses, viscosity 1013 poise. Strain point, Glass in completely rigid state, viscosity 1015 poise.
¾ ¾
η=
τ dν dz
η = ηo exp
Qn RT
η: τ: dυ/dz: ηwater: ηglycerin:
viscosity Poise (Pa s g/cm.) shear stress, velocity gradient. 0.01poise at 20oC 15 poise
•Qn activation energy for viscosity as T increase η decrease. •The addition of modifiers breaks up the structure and reduce the Qn.
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⋅
σ dε ε= = f( ) η dt
dε/dt ε η
: Creep rate : Applied stress : Viscosity
From this equation, we can say that; • Creep rate increase exponentially with increasing stress, (more creep) • Glass modifiers decrease the creep resistance, (more creep) • Glassy phase decrease the creep resistance due to the grain boundary sliding. (more creep) • Crystalline phase in all glasses increase the creep resistance. (less creep) • In pure silica glass (high silica), maximum creep resistance, (less creep) • Modifying oxides such as MgO, SrO, PbO reduces the creep resistance. (more creep) 14
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Porosity in Ceramics
¾ Ceramics with a small grain size are stronger than those with coarse-grain sizes. Finer grain sizes help reduce stresses that develop at grain boundaries due to anisotropic expansion and contraction. ¾ Apparent porosity - The percentage of a ceramic body that is composed of interconnected porosity (open to surface) ¾ True porosity - The percentage of a ceramic body that is composed of both closed and interconnected porosity. ¾ Bulk density - The mass of a ceramic body per unit volume, including closed and interconnected porosity. 15
Dr.C.Ergun Mak214E Silicon carbide particles are compacted and fired at a high temperature to produce a strong ceramic shape. The specific gravity of SiC is 3.2 g/cm3. The ceramic shape subsequently is weighed when dry (360 g), after soaking in water (385 g), and while suspended in water (224 g). Calculate the apparent porosity, the true porosity, and the fraction of the closed pore volume.
The closed-pore percentage
= the true porosity - the apparent porosity = 30% - 15.5% = 14.5%
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Glass Ceramics
√ Partially crystalline materials with long range ordered (LRO)) structure below glass transition temperature. √ Produces by nucleation and growth. √ Better High T strength. √ Li2O-Al2O3-SiO2 system. √ Applications: cooking wares, utensils, stove and iron parts, etc.
Glass Ceramics 17
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GlassesCeramic: Crystallization of Glasses
(a) Rapid Cooling to avoid the start of crystallization. (b) Slower cooling for crystallization. (c) Higher nucleation rate of precipitates at lower temperatures, (d) Higher growth rate of precipitates at higher temperatures. 18
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Making of Glass Ceramic Parts
(a) Melting for composition adjustment. (b) Forming-plastic deformation into desired geometries (c) Reheating to a temperature for max necleation rate of crystallites. (d) Heating to a temperature for max growth rate of necleated crystallites
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Failure in Ceramics
• Both crystalline and non-crystalline ceramics are very brittle due to the presence of flaws. • Any flaw or crack or imperfection limits tensile strength, causes stress concentration and magnification of applied stress. Facotrs effecting the brittle fracture; 1. Small cracks, 2. Porosity, 3. Foreign inclusions, 4. Glassy phases, 5. Large grain size. 20
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• For very thin cracks: (small r) or long cracks (large a) the ratio of (σ actual / σ) very high. • If (σ actual > σfracture), the crack grows • Eventually causes failure, even tough the actual applied stress is small. If σ is a tensile stress is applied then actual stress at the crack tip
σ actual = 2σ
a r
a Ü ⇒ σmax Ü R Þ ⇒ σmax Ü
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Treatment of Fracture in Ceramics
σ critical
2 Eγ = πa
σ=
K f πa
a : The length of the surface crack (or one half the length of an internal crack) γ : The surface energy (per unit area). σ : Allowable max. stress which does not cause brittle or sudden fracture KIC :Fracture toughness
If
K1 ≤ K1C
Ö
No brittle fracture.
If
K1 ≤ K1C
Ö
Brittle fracture.
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• Malzemenin ani zorlamalara karşı dayanımını ifade eden büyüklük “kırılma tokluğu” dur. • Bu değer K1C ile ifade edilir • K1C azaldıkça malzemenin gevrek kırılma eğilimi artar. • Parçanın tasarımda herhangi bir zorlama altında ani ve gevrek kırılmaması için aşağıdaki şart sağlanmalıdır.
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Toughening Mechanisms in Ceramics
1. Cermets: Brittle ceramic particles in softer and tougher metal matrix: 9 WC in Co matrix for cutting tools: hardness and cutting ability due to WC, softer Co matrix for ductility and energy absorption. 9 TiC in Ni matrix, 9 TiB2 in Co matrix: high T strength and corrosion resistance in rocket engines, 9 UO2 in Al for nuclear fuels. 2. Ceramic matrix composites; reinforced with ceramic fibers or agglomerates: 3. Toughening by zirconia (Addition of PSZ): Compressive effect of phase transformation of tetragonal partially stabilized zirconia toughenes the ceramics. 4. Adding oxide to prevent undesired phase transformations. 5. Microcracks: Blunts the sharp crack tip and prevents or delay crack propagation. 6. Fine grain, high purity, high density. 24
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Transformation Toughening with PSZ
Partially stabilized ZrO2 (PSZ) 1. Has meta-stable tetragonal phase. 2. This meta-stable phase transforms to monoclinic phase under pressure. 3. Since monoclinic phase has bigger unit cell volume, therefore increases the volume during transformation. 4. Since the pressure is very high at the crack tip, tetragonal phase transforms to monoclinic phase 5. New monoclinic phase closes (blunts) the crack and increase the fracture toughness and strength of the ceramic.
K: (8000psi√in)
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Processing of Ceramics / Glasses
a) Crystalline ceramics: Most of ceramics except glasses. • Conventional ceramics; (clay based) for white ware, etc. • Advanced ceramics; b) Glasses: • Ceramic Glasses: Non-crystalline glasses. • Glass Ceramics: Crystalline glasses. As general, processing may be considered in two categories: 1. Particulate processing for Traditional and advanced ceramics: 2. Solidification processing for Glasses 26
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Processing of Crystalline Ceramics Important
Conventional ceramics; (clay based) for white ware, etc. • Raw materials are fine powders. • These powders are shaped or compacted in to the geometries of desired articles • Then, shaped powders are bonded by 9Chemical reaction 9Partial or complete vitrification (melting of glassy phase between grains) 9Sintering. • Typical Applications: Bricks, pipes, cooking wares, other pottery, etc. 27
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(a) (b)
Article Microstructure
• • • •
Steps Preparation of raw materials, Shaping, Drying, and Firing (sintering) 28
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Drying of Conventional Ceramics Important (Clay based) Large dimensional changes can causes residual stresses and therefore cracking during drying
Types of water in the structure • Interparticle water • Pore water.
During drying • Drying of interparticle moisture Ö decrease in volume. High risk for crack or break. • Dimensional change continues until particles touches each other. • Drying of pore moisture ÖNo dimensional changes. Min. risk for cracking. 29
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Processing of Conventional Ceramics Important (Clay based) • Pressing • Isostatic pressing • Extrusion • Jiggering • Slip Casting • Hand modeling (manual method)
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Pressing Important
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Isostatic Pressing
Extrusion Important
Jiggering 32
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Jiggering (Sıvama)
Symmetrical products
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Slip Casting Important
Slip; a suspension of ceramic powders in liquid, Slip is poured into a porous plaster of paris mold so that water from the mix is absorbed into the plaster to form a firm layer of ceramic at the mold surface. • The slip composition is 25% to 40% liquid • Two principal variations: – Drain casting - the mold is inverted to drain excess slip after a semi-solid layer has been formed, thus producing a hollow product – Solid casting - to produce solid products, adequate time is required. 34
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Slip Casting: drain casting
Green ceramic
Next step: Firing (sintering)
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Processing of Advanced Ceramics
Important
Producing a green ceramic (green body) 9 Slip casting 9 Compaction 9 Tape casting 9 Hot isostatic pressing 9 Extrusion 9 Injection
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Tape Casting
Important
Tape casting – Used to make thin sheets of ceramics using a ceramic slurry
•The ceramic slurry used in both tape casting and slip casting •Slurry consists of also binders, plasticizers, etc. •The slurry is cast in to a mold in slip casting •The slurry is coated on a carrier film (platic substrate) in tape casting. 37
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Sintering / Firing
Important
Green ceramic - A ceramic that has been shaped into a desired form but not sintered yet. Sintering (Firing) Heating a ceramic body at a high temperature (0.7-0.9 Tm) to cause a ceramic bond to form with diffusion. Increase in rigity and strength, Volume shrinkage and densification: Due to the reduce in proe size or elimination of pores,
Bonding of ceramic particles 1.Directly bonding between particles by diffusion 2.Liquid phase sintering (ceramic bonds): During firing, clay and other fluxing materials react with coarser particles to produce a glassy bond and reduce porosity. 3.Reaction bonding with a chemical reaction between grains Elimination of pores from the body: With grain boundary diffusion and/or bulk diffusion. 38
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Direct bonding of the particles duing sintering. Crystalline advanced ceramics Liquid phase sintering (ceramic bond): bonding of particles wiht a glassy phase. In clay based ceramics. Not desired in crystalline ceramics
Small movie
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Different Sintering Mechanism
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Important Processing of Ceramic Glasses
At high Temperatures; viscosity of glasses can be controled Processing techniques; a) Sheet and plate manufacturing • Hot rolling • Floating on molten tin bath b) Glass products • Pressing • Press and Blow • Drawing of fibers 41
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Important Sheet or Plat Manufacturing
Hot Rolling
Floating
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Pressing
Press and Blow
Drawing of fibers
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Tempered Glass
Important
• Heating the glass above annealing temperature into the plastic range, • Quenching of surfaces by air jets; the surfaces cool first, contract and harden while interior is still plastic. • As the internal glass cools and contracts Ö compressive stresses on surfaces; toughness extremely increases. • Surface becomes resistant to scratching and breaking. • Products: windows for tall buildings, all-glass doors, safety glasses, and other products requiring toughened glass.
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Temper Cam
• Camlaşma sıcaklığının üzerindeki cam malzemenin yüzeyine soğuk hava üflenerek yüzeyi ani soğutulur. • İç bölge, halen sıcakken, vizkoz akış ile şekil değiştirebilir. Daha sonra soğuyan iç bölgeler büzülür ve kendini çeker. • Bu şekilde camın yüzeyinde basma gerilmeleri oluşturulur. • Böylece, çekme dayanımı ve kırılmalara karşı direnci arttırılması ile daha dayanıklı camlar elde edilir.
Ani soğumuş yüzey
Yavaş soğumuş merkez
Basma Çekme
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Defination of some concepts
9 Green ceramic - A ceramic that has been shaped into a desired form but has not yet been sintered. 9 Parison - A crude glassy shape that serves as an intermediate step in the production of glassware. The parison is later formed into a finished product. 9 Refractories: A group of ceramic materials capable of withstanding high temperatures for prolonged periods of time. 9 Slip casting - Forming a hollow ceramic part by introducing a pourable slurry into a mold. 9 Tempered glass - A high-strength glass that has a surface layer where the stress is compressive, induced thermally during cooling or by the chemical diffusion of ions. 9 Vitrification - Melting, or formation, of a glass. 9 Devitrification - The crystallization of glass. 46
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Polymers
Commercial Polymers: • Plastics; Thermosets, Thermoplasts • Elastomers; Rubbers •Light weight, • Adhesives •Corrosion resistance, •Chemical resistance, Engineering polymers: • Improved strength and performance •Recyclable, • Better high T properties (350oC), •Electrical insulation, • Kevlar (aramid), very high strength. • Plexiglas: A transparent, a substitute •Low strength, for glasses. •Low stiffness, • Teflon: high frictions coef.: non•Not good for high T, sticking cookware.
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Structure of the Polymers
• Polymers are materials with Giant chain-like organic molecules. Structure of PE: • Backbone of C atoms • Two H are bonded to one C atom. • All bonds are covalent. • Van der Waals bonds between chains The lines between the atoms show the covalent bonds. (a) Solid three-dimensional model, (b) Three-dimensional “space” model (c) Simple two-dimensional model.
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Classification of Polymers ¾ Thermoplastics (Amorphous, crystalline) ¾ Thermosetting polymers ¾ Elastomers In this lecture: ¾ Molecular structure, ¾ Mechanical and thermal behavior.***** ¾ Production method.
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Classification of Polymers Important
9 Thermoplastics – Flexible Linear (w/o branched) chains. Softening and viscous flow is possible with heating; recyclable. Behaves in ductile and plastic manner 9 Linear polymer - giant molecules with a spaghetti-like form. 9 Thermosetting polymers –heavily cross-linked chains forming 3D network. Stronger but brittle than thermoplasts. Can not be processed after cross linking. No melting or softening upon heating after cross linking. But degradation (burning), not recyclable. 9 Elastomers – Linear cross linked chains (intermediate structure; thermoplastics or lightly cross-linked Thermosets). Very high elastic deformation (> 200%) without plastic deformation. Can not be processed after cross-linking.
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Classification of Polymers Important
Thermoplasts: flexible linear chains
Thermosets: Rigid 3D network
Elastomer: Linear cross linked chains.
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Branches
Cross links (a) (b) (c) (d)
Linear unbranched polymer Linear branched polymer Thermosets without branching Thermosets with branches
Copolymer: linear addition of two or more different molecules. Such as; ABS: acrylonitrile+butadiene+styrene. Excellent combination of strenght, rigidity and 52 toughness.
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How to make long chains
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Polymerization
Important
Polymerization: Production of long chains in which atoms have covalent bonds Small molecules (monomers) > giant molecules (polymers). 1. By the Addition Mechanism (chain growth) a. Initiation of addition Polymerization b. Growth of the addition chain c. Termination of addition polymerization a. (a) Combination b. (b) Disproportiation 2. By the Condensation Mechanism (step growth)
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Additional PolymerizationImportant Ö
Monomer
Mer
Polimerizasyon
Polietilen 55
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THERMOPLASTİCS
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Thermoplastics
Important
Two types: • Disordered chain arrangements: Amorphous • Ordered chain arrangements: Crystalline Parameters: •Cooling rate. •Waiting at Tm.
The effect of temperature on the modulus of elasticity for an amorphous thermoplastic. Note that Tg and Tm are not fixed.
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Amorphous Thermoplastics
9 Strong covalent bonds within the chains 9 Weak secondary bonds (Van der Waals-hydrogen bonds) between the chains 9 Application of stress can overcome weak secondary bonds 9 During deformation; rotation and slide between chains relative to one another depending on temperature and structure of polymer.
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Degradation and MeltingImportant
9 Melting temperature (Tm) –Liquid form- The bonding between the chains becomes weak, resulting flow of polymer without elastic deformation. The strength and elasticity modulus become near zero. The polymer becomes suitable for casting or many other forming processes. 9 Degradation temperature( Td) – a complete destruction of the covalent bonds in the backbone of the polymer at above certain temperature –Td- resulting that a polymer burns, chars, or decomposes.
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Glass Transition Temperature. Important
Glass temperature - The temperature range below which the amorphous polymer behaves like a rigid glassy structure. Tg= (0.5 -0.75) Tm.
• T < Tg ; Glass like behavior; –Hard, brittle, higher strength and elasticity. Still amorphous. • T > Tg ; Chains can twist • Near Tm > Rubbery behavior • Near Tg > Leathery behavior.
Tg and Tm are not fixed.
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Effect of Temperature onImportant Thermoplastics
9 Rubbery: (twisting and inter-twining in chains in amorphous structure) Just below Tm, behaves in a Rubbery manner; elastic and plastic deformation under stress. When stress is removed, elastic strains recover but permanent deformation remains: Large permanent deformation permitting shaping with molding and extrusion. 9 Leathery polymers: At lower T, bonding between chains is strong, polymer becomes stiffer and stronger. In this condition, polymer has a usable strength 9 Glassy polymers: Below Tg, hard, brittle and glass-like, and still amorphous. Density and elasticity modulus change at different rates with T. 61
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Crystalline Polymers
Important
• Some thermoplasts partially crystallize. • Chains closely aligned in an order • Higher density • Improved chemical resistance, mechanical properties. • This ordered structure is explained with Folded chain model Factors influencing the crystal formation: Important 9Complexity: negative effect 9Cooling rate: if increase, negative effect 9Annealing: encourages the crystallization 9Degree of Polymerization: (long chains) negative effect. 9Deformation: If slow (between Tm – Tg), positive effect. 62
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Folded Model
Important
Amorphous region
Spherulite crystals in an amorphous matrix of nylon
The folded chain model, (a)Two dimensions (b)Three dimensions.
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Structure and properties of Thermoplasts: ¾ Branching: Attachment of another chain to the backbone. Less destity, ability to crystallize, strength, and stiffness. ¾ Copolymer: linear addition of two or more different molecules. ABS: acrylonitrile+butadiene+styrene. Excellent combination of strenght, rigidity and toughness. ¾ Blending and alloying: Improved mechanical properties by mixing immiscible plastics producing two or multiple phase structure. Such as ABS, polycarbonate+elastomer alloys
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MECHANICAL PROPERTIES OF THERMOPLASTICS
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Mechanical Properties: Amorphous Thermoplastics
Important
a) Tensile properties 1.Elastic behavior: 2.Plastic (permanent) deformation b) Creep c) Stress relaxation d) Impact
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1. Elastic behavior
Important
Elastic behavior: Below yield point. Two types; ¾ Instant recovery: Stretching and distortion of the covalent bonds within the chains resulting elastic elongation in the chains. Instant recovery upon removing of the load. Just like the other materials such as metals and ceramics. ¾ Viscous recovery: Relative movement of chains with respect to one another. Distortion of entire segments. Elastic recovery may take time (several hours even days) upon removing the load. (Viscoelasticity -A time dependent behavior)
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2. Plastic behavior
Important
2. Plastic behavior • Plastic (permanent) deformation above the yield strength. • Not due to the dislocation movements 1. First stretching, rotation, sliding and disentanglements. 2. Then Necking: local neck forms and chains start to align. 3. Chains get close together. Increase in van der Waals forces between the chains. 4. Thus, strength increases until fracture • Effect of Time and strain rate • Effects the sliding of chains relative to one another • Higher rates, higher resistance- higher strength • Therefore testing speed is important 68
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Tensile Test
Important
All chains allign.
6,6-nylon, a typical thermoplastic polymer.
Necks are not stable in amorphous polymers, because local alignment strengthens the necked region and reduces its rate of deformation. 69
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Viscoelasticity
Viscoelasticity - The deformation of a material by elastic deformation and viscous flow of the material when stress is applied. ¾ At low T and High rates of loading: Fully elastic behavior like other solids such as metals: stress and strain are linear. ¾ At high T or low rates of loading: viscoelastic behavior.
η=
τ dν
⎛ Qn ⎞ ⎟ ⎝ RT ⎠
η = ηo ⋅ exp⎜ dz
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(b) Creep (Viscoplasticity)Important
¾Due to low Tm, low viscosity and activation energy for viscosity ¾Creep even at room T: Increase in strain with time due to the slide between chains. ¾For an applied stress and operating T, determine the service life. ¾Measure the strain as a function of time and applied stress and find the max. allowable strain during the service. Constant load: Creep
ε (t ) = at n a and n : constants for a given stress and T Constant strain: Stress relaxation
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c) Stress relaxation
Important
9 Stress relaxation: For a fixed strain, decrease in the stress with time in a prestressed material. Elastic deformation changes to plastic deformation. Time and temperature is important. 9 Relaxation time (λ)– The rate at wiht stress relaxation occurs.
( )
σ = σ o ⋅ exp − t λ
λ = λo ⋅ exp⎛⎜ Qn RT ⎞⎟ ⎝
⎠
σo= original stress λo=constant
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d) Impact
¾ ¾ ¾ ¾
Important
At very high rates of strain, No time for sliding or relative movement in chains No time for plastic deformation Therefore brittle fracture occurs.
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Deformation in Crystalline Polymers Important
¾No complete crystalline structure: Amorphous transition region between in crystalline lamellae regions. ¾ Under load, crystalline lamellae breaks into smaller units and slide and begin to separate. ¾ The folds in the lamellae become aligned. ¾ The crystallites become elongated ¾ The chains between crystallites (in the amorphous region) disentangle and break causing the polymer to fail.
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o o
Crystalline ThermoplastsImportant
Crazing: Stretching the molecules in glassy region and micro void formation. After crazing; coalescence of micro voids resulting fibril deformation and finally macro crack and failure.
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THERMOSETS
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Thermosets
Important
9 Highly cross linked polymer chains with 3-D network structure 9 The slip and/or rotation in molecules is not possible 9 Good strength, stiffness and hardness 9 High Tg, but brittle 9 Starting polymers may be solid or liquid or two or more components 9 Heating, mixing and/or pressure triggers condensation and formation of cross linking 9 Cross linking not reversible so not recyclable The amount and strength of cross-links determines the properties of polymers including mechanical properties.
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ELASTOMERS
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Elastomers (Rubbers)
Important
¾ Large amount of elastic deformation under stress. ¾ Highly coiled (chain molecule) structure: Behaves viscoelastic manner. ¾ Applied stress – First uncoils the chains then stretches the bonds: elastic deformation, ¾ Increased stress cause slipping between chains: plastic deformation. ¾ To prevent viscous plastic deformation: Cross-linking Attaching chains of polymers together with sulfur. ¾ Cross linking process: Vulcanization - by introducing sulfur 0.5-5%. Higher S increase the hardness. 82
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The σ-ε curve for an elastomer. Virtually all elastic but “E” varies with ε
(a) If no cross-linked, permanently deformed. (b) If cross-linked occurs, returning to its original shape. 83
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The effect of vucanisation on 1/T-E curve.
The σ-ε curve for a polymer. 84
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Important
Additives for Plastics ¾ ¾ ¾ ¾ ¾ ¾ ¾
Fillers: increase toughness, reduce price. Pigments: for coloring. Stabilizers: Prevent degradation from environment (UV or sunlight) Antistatic Agents: to reduce the static electricity and the risk of sparking or discharge. Flame Retardants: to delay the flaming Plasticizers: to reduce Tg, internal lubrication, Reinforcements: to modify the mechanical properties. 85
(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
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Figure 6.9 Tensile stress-strain curves for different materials. Note that these are qualitative
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Processing of Polymeric Materials
Forming Processes for Thermoplastics: ¾Extrusion (Continues section (hollow or solid) tubes, pipes, etc. ¾Extrusion and blowing: Plastic bags, etc. ¾Blow Molding: bottles, etc. ¾Injection Molding; different single parts. ¾Thermoforming; Sheet forming to single products. ¾Calendaring (Rolling); Sheet or flat products, printing on to sheets, etc. Foiles, etc. ¾Spinning; Fiber production
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Forming Processes for Thermosets: ¾ Compression Molding (from semi solids to solid products) ¾ Transfer Molding (from liquids to solid products)
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Thermoplast polymers
Extrusion 89
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(a)
Thermoplast polymers (b)
(c)
(a)Extrusion and blowing (b)Extrusion (c)Blow molding 90
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Thermoplast polymers
(a)
(b)
(c)
(d)
Typical forming processes for thermoplastic: (a) injection molding, (b) thermoforming, (c) calendaring, (d) spinning. 91
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Thermoset polymers
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
1. Compression molding, 2. Transfer molding.
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Quiz
• Why is ceramic materials are very brittle? • Why is bending strength higher than tensile strength in crystalline ceramics?
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Questions
• Which properties are required for the materials used in manufacturing turbine blades for jet engines? • Describe a method for the production of a cheap scissor made of cast iron. • Design a high-performance connecting rod for the engine of a racing automobile.
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MAK214E Summer 2006-2007 Lecture notes 5
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Ceramic Materials
• Ceramics consist of metallic and non metallic elements joint by ionic or covalent bonds Characteristic properties: – High hardness, – High brittleness, – High melting point, – Low thermal and electrical conductivity, – Good chemical and thermal stability, – High compressive strength.
Applications: – Pottery, – Bricks, – Tiles, – Cooking ware, – Soil pipe to glass, – Refractory, – Magnets, electrical devices, – Abrasives or wear resistant materials. 2
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Applications
• Ceramics are used in a wide range of technologies such as: – Refractory, spark plugs, dielectrics capacitors in electronics, sensors/actuators, abrasives, biomaterials, magnets, magnetic recording media, etc. • Example: The space shuttle makes use of ~25,000 reusable, lightweight, highly porous ceramic tiles that protect the aluminum frame from the heat generated during re-entry into the Earth’s atmosphere.
© 2003 Brooks/Cole Publishing / Thomson Learning™
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Types of Ceramics
a) Crystalline ceramics: Most of ceramics except glasses. • Convensitonal ceramics; (clay based) for whiteware, etc. • Advanced ceramics; oxides, nitrides, carbides, etc. b) Ceramic Glasses: Non crystalline glasses - silica base. c) Glass Ceramics: Crystalline glasses – also silica base.
a) Crystalline ceramics: Good High T strength. b) Ceramic Glasses: Good forming characteristics. c) Glass Ceramics: Combining both properties 4
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Glasses
Glass structures may be 1. Ceramic Glasses: Non crystalline has short range ordered (SRO) structure; 2. Glass Ceramics: Crystalline long range order (LRO) crystalline structures.
Ceramic Glasses
Glass Ceramics 6
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Glass transition (fictive) temperature
Glass temperature (fictive or glass transition temperature)- The point where solid glass start to behave as an undercooled liquid.
• • • •
During crystallization, an abrupt change in the density occurs. The change in slope indicates the formation of a glass. Glass does not have melting T (Tm). Glass ceramics (Crystalline materials) show a fixed Tm and not a Tg.
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Ceramic Glasses
√ Non crystalline materials with random network (short range ordered (SRO)) structure below glass transition temperature. √ Best example; window glass. √ Main constituents: fused silica (pure SiO2)
Ceramic Glasses 8
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Modifiers
The modifiers, such as Sodium oxide (Na2O) break up the glassy network and reduce the ability to form a glass. Melting range considerable decreases and thus processing becomes easier. But not good for high Temperature applications. 9
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High Temperature Prop of Glasses
√ No dislocation motion in cermics or glasses; no plastic deformation at room T. √ At high Temperatures: ¾ Grain boundary sliding in crystalline ceramics. ¾ Viscous flow in glass ceramics and ceramic glasses. Creep in crystalline ceramic important in high temperature application. Crystalline ceramics good creep resistance due to high melting T, high activation energy for diffusion. Factors effecting creep resistance in crystalline ceramics: • Smaller grain size: more grain boundary, more creep. • Porosity: decrease in cross section, increase in stress, increase in grain boundary, more creep. • Impurities: leading to glassy phase along grain boundaries, viscous flow -more creep. • Higher Temperature: reducing the strength in grain boundaries, increasing the diffusion and glassy phase formation, more creep. 11
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High Temperature Prop of Glasses Viscous flow in glass ceramics: •At high T, viscous flow. •Depends on shear stress and velocity.
¾ Melting range: Glass in a liquid form, viscosity 50-500 poise. ¾ Working range: Glass can be deformed to desired geometries, viscosity 104-107 poise. ¾ Annealing range: To relieve residual stresses, viscosity 1013 poise. ¾ Strain point, Glass in completely rigid state, viscosity 1015 poise.
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Melting range: Glass in a liquid form, viscosity 50-500 poise. Working range: Glass can be deformed to desired geometries, viscosity 104-107 poise. Annealing range: To relieve residual stresses, viscosity 1013 poise. Strain point, Glass in completely rigid state, viscosity 1015 poise.
¾ ¾
η=
τ dν dz
η = ηo exp
Qn RT
η: τ: dυ/dz: ηwater: ηglycerin:
viscosity Poise (Pa s g/cm.) shear stress, velocity gradient. 0.01poise at 20oC 15 poise
•Qn activation energy for viscosity as T increase η decrease. •The addition of modifiers breaks up the structure and reduce the Qn.
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⋅
σ dε ε= = f( ) η dt
dε/dt ε η
: Creep rate : Applied stress : Viscosity
From this equation, we can say that; • Creep rate increase exponentially with increasing stress, (more creep) • Glass modifiers decrease the creep resistance, (more creep) • Glassy phase decrease the creep resistance due to the grain boundary sliding. (more creep) • Crystalline phase in all glasses increase the creep resistance. (less creep) • In pure silica glass (high silica), maximum creep resistance, (less creep) • Modifying oxides such as MgO, SrO, PbO reduces the creep resistance. (more creep) 14
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Porosity in Ceramics
¾ Ceramics with a small grain size are stronger than those with coarse-grain sizes. Finer grain sizes help reduce stresses that develop at grain boundaries due to anisotropic expansion and contraction. ¾ Apparent porosity - The percentage of a ceramic body that is composed of interconnected porosity (open to surface) ¾ True porosity - The percentage of a ceramic body that is composed of both closed and interconnected porosity. ¾ Bulk density - The mass of a ceramic body per unit volume, including closed and interconnected porosity. 15
Dr.C.Ergun Mak214E Silicon carbide particles are compacted and fired at a high temperature to produce a strong ceramic shape. The specific gravity of SiC is 3.2 g/cm3. The ceramic shape subsequently is weighed when dry (360 g), after soaking in water (385 g), and while suspended in water (224 g). Calculate the apparent porosity, the true porosity, and the fraction of the closed pore volume.
The closed-pore percentage
= the true porosity - the apparent porosity = 30% - 15.5% = 14.5%
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Glass Ceramics
√ Partially crystalline materials with long range ordered (LRO)) structure below glass transition temperature. √ Produces by nucleation and growth. √ Better High T strength. √ Li2O-Al2O3-SiO2 system. √ Applications: cooking wares, utensils, stove and iron parts, etc.
Glass Ceramics 17
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GlassesCeramic: Crystallization of Glasses
(a) Rapid Cooling to avoid the start of crystallization. (b) Slower cooling for crystallization. (c) Higher nucleation rate of precipitates at lower temperatures, (d) Higher growth rate of precipitates at higher temperatures. 18
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Making of Glass Ceramic Parts
(a) Melting for composition adjustment. (b) Forming-plastic deformation into desired geometries (c) Reheating to a temperature for max necleation rate of crystallites. (d) Heating to a temperature for max growth rate of necleated crystallites
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Failure in Ceramics
• Both crystalline and non-crystalline ceramics are very brittle due to the presence of flaws. • Any flaw or crack or imperfection limits tensile strength, causes stress concentration and magnification of applied stress. Facotrs effecting the brittle fracture; 1. Small cracks, 2. Porosity, 3. Foreign inclusions, 4. Glassy phases, 5. Large grain size. 20
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Dr.C.Ergun Mak214E a : Crack of length. r : The radius of the crack.
• For very thin cracks: (small r) or long cracks (large a) the ratio of (σ actual / σ) very high. • If (σ actual > σfracture), the crack grows • Eventually causes failure, even tough the actual applied stress is small. If σ is a tensile stress is applied then actual stress at the crack tip
σ actual = 2σ
a r
a Ü ⇒ σmax Ü R Þ ⇒ σmax Ü
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Treatment of Fracture in Ceramics
σ critical
2 Eγ = πa
σ=
K f πa
a : The length of the surface crack (or one half the length of an internal crack) γ : The surface energy (per unit area). σ : Allowable max. stress which does not cause brittle or sudden fracture KIC :Fracture toughness
If
K1 ≤ K1C
Ö
No brittle fracture.
If
K1 ≤ K1C
Ö
Brittle fracture.
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• Malzemenin ani zorlamalara karşı dayanımını ifade eden büyüklük “kırılma tokluğu” dur. • Bu değer K1C ile ifade edilir • K1C azaldıkça malzemenin gevrek kırılma eğilimi artar. • Parçanın tasarımda herhangi bir zorlama altında ani ve gevrek kırılmaması için aşağıdaki şart sağlanmalıdır.
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Toughening Mechanisms in Ceramics
1. Cermets: Brittle ceramic particles in softer and tougher metal matrix: 9 WC in Co matrix for cutting tools: hardness and cutting ability due to WC, softer Co matrix for ductility and energy absorption. 9 TiC in Ni matrix, 9 TiB2 in Co matrix: high T strength and corrosion resistance in rocket engines, 9 UO2 in Al for nuclear fuels. 2. Ceramic matrix composites; reinforced with ceramic fibers or agglomerates: 3. Toughening by zirconia (Addition of PSZ): Compressive effect of phase transformation of tetragonal partially stabilized zirconia toughenes the ceramics. 4. Adding oxide to prevent undesired phase transformations. 5. Microcracks: Blunts the sharp crack tip and prevents or delay crack propagation. 6. Fine grain, high purity, high density. 24
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Transformation Toughening with PSZ
Partially stabilized ZrO2 (PSZ) 1. Has meta-stable tetragonal phase. 2. This meta-stable phase transforms to monoclinic phase under pressure. 3. Since monoclinic phase has bigger unit cell volume, therefore increases the volume during transformation. 4. Since the pressure is very high at the crack tip, tetragonal phase transforms to monoclinic phase 5. New monoclinic phase closes (blunts) the crack and increase the fracture toughness and strength of the ceramic.
K: (8000psi√in)
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Processing of Ceramics / Glasses
a) Crystalline ceramics: Most of ceramics except glasses. • Conventional ceramics; (clay based) for white ware, etc. • Advanced ceramics; b) Glasses: • Ceramic Glasses: Non-crystalline glasses. • Glass Ceramics: Crystalline glasses. As general, processing may be considered in two categories: 1. Particulate processing for Traditional and advanced ceramics: 2. Solidification processing for Glasses 26
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Processing of Crystalline Ceramics Important
Conventional ceramics; (clay based) for white ware, etc. • Raw materials are fine powders. • These powders are shaped or compacted in to the geometries of desired articles • Then, shaped powders are bonded by 9Chemical reaction 9Partial or complete vitrification (melting of glassy phase between grains) 9Sintering. • Typical Applications: Bricks, pipes, cooking wares, other pottery, etc. 27
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(a) (b)
Article Microstructure
• • • •
Steps Preparation of raw materials, Shaping, Drying, and Firing (sintering) 28
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Drying of Conventional Ceramics Important (Clay based) Large dimensional changes can causes residual stresses and therefore cracking during drying
Types of water in the structure • Interparticle water • Pore water.
During drying • Drying of interparticle moisture Ö decrease in volume. High risk for crack or break. • Dimensional change continues until particles touches each other. • Drying of pore moisture ÖNo dimensional changes. Min. risk for cracking. 29
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Processing of Conventional Ceramics Important (Clay based) • Pressing • Isostatic pressing • Extrusion • Jiggering • Slip Casting • Hand modeling (manual method)
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Pressing Important
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Isostatic Pressing
Extrusion Important
Jiggering 32
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Jiggering (Sıvama)
Symmetrical products
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Slip Casting Important
Slip; a suspension of ceramic powders in liquid, Slip is poured into a porous plaster of paris mold so that water from the mix is absorbed into the plaster to form a firm layer of ceramic at the mold surface. • The slip composition is 25% to 40% liquid • Two principal variations: – Drain casting - the mold is inverted to drain excess slip after a semi-solid layer has been formed, thus producing a hollow product – Solid casting - to produce solid products, adequate time is required. 34
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Slip Casting: drain casting
Green ceramic
Next step: Firing (sintering)
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Processing of Advanced Ceramics
Important
Producing a green ceramic (green body) 9 Slip casting 9 Compaction 9 Tape casting 9 Hot isostatic pressing 9 Extrusion 9 Injection
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Tape Casting
Important
Tape casting – Used to make thin sheets of ceramics using a ceramic slurry
•The ceramic slurry used in both tape casting and slip casting •Slurry consists of also binders, plasticizers, etc. •The slurry is cast in to a mold in slip casting •The slurry is coated on a carrier film (platic substrate) in tape casting. 37
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Sintering / Firing
Important
Green ceramic - A ceramic that has been shaped into a desired form but not sintered yet. Sintering (Firing) Heating a ceramic body at a high temperature (0.7-0.9 Tm) to cause a ceramic bond to form with diffusion. Increase in rigity and strength, Volume shrinkage and densification: Due to the reduce in proe size or elimination of pores,
Bonding of ceramic particles 1.Directly bonding between particles by diffusion 2.Liquid phase sintering (ceramic bonds): During firing, clay and other fluxing materials react with coarser particles to produce a glassy bond and reduce porosity. 3.Reaction bonding with a chemical reaction between grains Elimination of pores from the body: With grain boundary diffusion and/or bulk diffusion. 38
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Direct bonding of the particles duing sintering. Crystalline advanced ceramics Liquid phase sintering (ceramic bond): bonding of particles wiht a glassy phase. In clay based ceramics. Not desired in crystalline ceramics
Small movie
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Different Sintering Mechanism
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Important Processing of Ceramic Glasses
At high Temperatures; viscosity of glasses can be controled Processing techniques; a) Sheet and plate manufacturing • Hot rolling • Floating on molten tin bath b) Glass products • Pressing • Press and Blow • Drawing of fibers 41
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Important Sheet or Plat Manufacturing
Hot Rolling
Floating
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Pressing
Press and Blow
Drawing of fibers
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Tempered Glass
Important
• Heating the glass above annealing temperature into the plastic range, • Quenching of surfaces by air jets; the surfaces cool first, contract and harden while interior is still plastic. • As the internal glass cools and contracts Ö compressive stresses on surfaces; toughness extremely increases. • Surface becomes resistant to scratching and breaking. • Products: windows for tall buildings, all-glass doors, safety glasses, and other products requiring toughened glass.
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Temper Cam
• Camlaşma sıcaklığının üzerindeki cam malzemenin yüzeyine soğuk hava üflenerek yüzeyi ani soğutulur. • İç bölge, halen sıcakken, vizkoz akış ile şekil değiştirebilir. Daha sonra soğuyan iç bölgeler büzülür ve kendini çeker. • Bu şekilde camın yüzeyinde basma gerilmeleri oluşturulur. • Böylece, çekme dayanımı ve kırılmalara karşı direnci arttırılması ile daha dayanıklı camlar elde edilir.
Ani soğumuş yüzey
Yavaş soğumuş merkez
Basma Çekme
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Defination of some concepts
9 Green ceramic - A ceramic that has been shaped into a desired form but has not yet been sintered. 9 Parison - A crude glassy shape that serves as an intermediate step in the production of glassware. The parison is later formed into a finished product. 9 Refractories: A group of ceramic materials capable of withstanding high temperatures for prolonged periods of time. 9 Slip casting - Forming a hollow ceramic part by introducing a pourable slurry into a mold. 9 Tempered glass - A high-strength glass that has a surface layer where the stress is compressive, induced thermally during cooling or by the chemical diffusion of ions. 9 Vitrification - Melting, or formation, of a glass. 9 Devitrification - The crystallization of glass. 46
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Polymers
Commercial Polymers: • Plastics; Thermosets, Thermoplasts • Elastomers; Rubbers •Light weight, • Adhesives •Corrosion resistance, •Chemical resistance, Engineering polymers: • Improved strength and performance •Recyclable, • Better high T properties (350oC), •Electrical insulation, • Kevlar (aramid), very high strength. • Plexiglas: A transparent, a substitute •Low strength, for glasses. •Low stiffness, • Teflon: high frictions coef.: non•Not good for high T, sticking cookware.
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Structure of the Polymers
• Polymers are materials with Giant chain-like organic molecules. Structure of PE: • Backbone of C atoms • Two H are bonded to one C atom. • All bonds are covalent. • Van der Waals bonds between chains The lines between the atoms show the covalent bonds. (a) Solid three-dimensional model, (b) Three-dimensional “space” model (c) Simple two-dimensional model.
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Classification of Polymers ¾ Thermoplastics (Amorphous, crystalline) ¾ Thermosetting polymers ¾ Elastomers In this lecture: ¾ Molecular structure, ¾ Mechanical and thermal behavior.***** ¾ Production method.
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Classification of Polymers Important
9 Thermoplastics – Flexible Linear (w/o branched) chains. Softening and viscous flow is possible with heating; recyclable. Behaves in ductile and plastic manner 9 Linear polymer - giant molecules with a spaghetti-like form. 9 Thermosetting polymers –heavily cross-linked chains forming 3D network. Stronger but brittle than thermoplasts. Can not be processed after cross linking. No melting or softening upon heating after cross linking. But degradation (burning), not recyclable. 9 Elastomers – Linear cross linked chains (intermediate structure; thermoplastics or lightly cross-linked Thermosets). Very high elastic deformation (> 200%) without plastic deformation. Can not be processed after cross-linking.
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Classification of Polymers Important
Thermoplasts: flexible linear chains
Thermosets: Rigid 3D network
Elastomer: Linear cross linked chains.
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Branches
Cross links (a) (b) (c) (d)
Linear unbranched polymer Linear branched polymer Thermosets without branching Thermosets with branches
Copolymer: linear addition of two or more different molecules. Such as; ABS: acrylonitrile+butadiene+styrene. Excellent combination of strenght, rigidity and 52 toughness.
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How to make long chains
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Polymerization
Important
Polymerization: Production of long chains in which atoms have covalent bonds Small molecules (monomers) > giant molecules (polymers). 1. By the Addition Mechanism (chain growth) a. Initiation of addition Polymerization b. Growth of the addition chain c. Termination of addition polymerization a. (a) Combination b. (b) Disproportiation 2. By the Condensation Mechanism (step growth)
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Additional PolymerizationImportant Ö
Monomer
Mer
Polimerizasyon
Polietilen 55
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THERMOPLASTİCS
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Thermoplastics
Important
Two types: • Disordered chain arrangements: Amorphous • Ordered chain arrangements: Crystalline Parameters: •Cooling rate. •Waiting at Tm.
The effect of temperature on the modulus of elasticity for an amorphous thermoplastic. Note that Tg and Tm are not fixed.
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Amorphous Thermoplastics
9 Strong covalent bonds within the chains 9 Weak secondary bonds (Van der Waals-hydrogen bonds) between the chains 9 Application of stress can overcome weak secondary bonds 9 During deformation; rotation and slide between chains relative to one another depending on temperature and structure of polymer.
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Degradation and MeltingImportant
9 Melting temperature (Tm) –Liquid form- The bonding between the chains becomes weak, resulting flow of polymer without elastic deformation. The strength and elasticity modulus become near zero. The polymer becomes suitable for casting or many other forming processes. 9 Degradation temperature( Td) – a complete destruction of the covalent bonds in the backbone of the polymer at above certain temperature –Td- resulting that a polymer burns, chars, or decomposes.
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Glass Transition Temperature. Important
Glass temperature - The temperature range below which the amorphous polymer behaves like a rigid glassy structure. Tg= (0.5 -0.75) Tm.
• T < Tg ; Glass like behavior; –Hard, brittle, higher strength and elasticity. Still amorphous. • T > Tg ; Chains can twist • Near Tm > Rubbery behavior • Near Tg > Leathery behavior.
Tg and Tm are not fixed.
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Effect of Temperature onImportant Thermoplastics
9 Rubbery: (twisting and inter-twining in chains in amorphous structure) Just below Tm, behaves in a Rubbery manner; elastic and plastic deformation under stress. When stress is removed, elastic strains recover but permanent deformation remains: Large permanent deformation permitting shaping with molding and extrusion. 9 Leathery polymers: At lower T, bonding between chains is strong, polymer becomes stiffer and stronger. In this condition, polymer has a usable strength 9 Glassy polymers: Below Tg, hard, brittle and glass-like, and still amorphous. Density and elasticity modulus change at different rates with T. 61
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Crystalline Polymers
Important
• Some thermoplasts partially crystallize. • Chains closely aligned in an order • Higher density • Improved chemical resistance, mechanical properties. • This ordered structure is explained with Folded chain model Factors influencing the crystal formation: Important 9Complexity: negative effect 9Cooling rate: if increase, negative effect 9Annealing: encourages the crystallization 9Degree of Polymerization: (long chains) negative effect. 9Deformation: If slow (between Tm – Tg), positive effect. 62
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Folded Model
Important
Amorphous region
Spherulite crystals in an amorphous matrix of nylon
The folded chain model, (a)Two dimensions (b)Three dimensions.
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Structure and properties of Thermoplasts: ¾ Branching: Attachment of another chain to the backbone. Less destity, ability to crystallize, strength, and stiffness. ¾ Copolymer: linear addition of two or more different molecules. ABS: acrylonitrile+butadiene+styrene. Excellent combination of strenght, rigidity and toughness. ¾ Blending and alloying: Improved mechanical properties by mixing immiscible plastics producing two or multiple phase structure. Such as ABS, polycarbonate+elastomer alloys
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MECHANICAL PROPERTIES OF THERMOPLASTICS
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Mechanical Properties: Amorphous Thermoplastics
Important
a) Tensile properties 1.Elastic behavior: 2.Plastic (permanent) deformation b) Creep c) Stress relaxation d) Impact
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1. Elastic behavior
Important
Elastic behavior: Below yield point. Two types; ¾ Instant recovery: Stretching and distortion of the covalent bonds within the chains resulting elastic elongation in the chains. Instant recovery upon removing of the load. Just like the other materials such as metals and ceramics. ¾ Viscous recovery: Relative movement of chains with respect to one another. Distortion of entire segments. Elastic recovery may take time (several hours even days) upon removing the load. (Viscoelasticity -A time dependent behavior)
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2. Plastic behavior
Important
2. Plastic behavior • Plastic (permanent) deformation above the yield strength. • Not due to the dislocation movements 1. First stretching, rotation, sliding and disentanglements. 2. Then Necking: local neck forms and chains start to align. 3. Chains get close together. Increase in van der Waals forces between the chains. 4. Thus, strength increases until fracture • Effect of Time and strain rate • Effects the sliding of chains relative to one another • Higher rates, higher resistance- higher strength • Therefore testing speed is important 68
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Tensile Test
Important
All chains allign.
6,6-nylon, a typical thermoplastic polymer.
Necks are not stable in amorphous polymers, because local alignment strengthens the necked region and reduces its rate of deformation. 69
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70
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Viscoelasticity
Viscoelasticity - The deformation of a material by elastic deformation and viscous flow of the material when stress is applied. ¾ At low T and High rates of loading: Fully elastic behavior like other solids such as metals: stress and strain are linear. ¾ At high T or low rates of loading: viscoelastic behavior.
η=
τ dν
⎛ Qn ⎞ ⎟ ⎝ RT ⎠
η = ηo ⋅ exp⎜ dz
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(b) Creep (Viscoplasticity)Important
¾Due to low Tm, low viscosity and activation energy for viscosity ¾Creep even at room T: Increase in strain with time due to the slide between chains. ¾For an applied stress and operating T, determine the service life. ¾Measure the strain as a function of time and applied stress and find the max. allowable strain during the service. Constant load: Creep
ε (t ) = at n a and n : constants for a given stress and T Constant strain: Stress relaxation
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c) Stress relaxation
Important
9 Stress relaxation: For a fixed strain, decrease in the stress with time in a prestressed material. Elastic deformation changes to plastic deformation. Time and temperature is important. 9 Relaxation time (λ)– The rate at wiht stress relaxation occurs.
( )
σ = σ o ⋅ exp − t λ
λ = λo ⋅ exp⎛⎜ Qn RT ⎞⎟ ⎝
⎠
σo= original stress λo=constant
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d) Impact
¾ ¾ ¾ ¾
Important
At very high rates of strain, No time for sliding or relative movement in chains No time for plastic deformation Therefore brittle fracture occurs.
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Deformation in Crystalline Polymers Important
¾No complete crystalline structure: Amorphous transition region between in crystalline lamellae regions. ¾ Under load, crystalline lamellae breaks into smaller units and slide and begin to separate. ¾ The folds in the lamellae become aligned. ¾ The crystallites become elongated ¾ The chains between crystallites (in the amorphous region) disentangle and break causing the polymer to fail.
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o o
Crystalline ThermoplastsImportant
Crazing: Stretching the molecules in glassy region and micro void formation. After crazing; coalescence of micro voids resulting fibril deformation and finally macro crack and failure.
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THERMOSETS
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Thermosets
Important
9 Highly cross linked polymer chains with 3-D network structure 9 The slip and/or rotation in molecules is not possible 9 Good strength, stiffness and hardness 9 High Tg, but brittle 9 Starting polymers may be solid or liquid or two or more components 9 Heating, mixing and/or pressure triggers condensation and formation of cross linking 9 Cross linking not reversible so not recyclable The amount and strength of cross-links determines the properties of polymers including mechanical properties.
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ELASTOMERS
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Elastomers (Rubbers)
Important
¾ Large amount of elastic deformation under stress. ¾ Highly coiled (chain molecule) structure: Behaves viscoelastic manner. ¾ Applied stress – First uncoils the chains then stretches the bonds: elastic deformation, ¾ Increased stress cause slipping between chains: plastic deformation. ¾ To prevent viscous plastic deformation: Cross-linking Attaching chains of polymers together with sulfur. ¾ Cross linking process: Vulcanization - by introducing sulfur 0.5-5%. Higher S increase the hardness. 82
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The σ-ε curve for an elastomer. Virtually all elastic but “E” varies with ε
(a) If no cross-linked, permanently deformed. (b) If cross-linked occurs, returning to its original shape. 83
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The effect of vucanisation on 1/T-E curve.
The σ-ε curve for a polymer. 84
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Important
Additives for Plastics ¾ ¾ ¾ ¾ ¾ ¾ ¾
Fillers: increase toughness, reduce price. Pigments: for coloring. Stabilizers: Prevent degradation from environment (UV or sunlight) Antistatic Agents: to reduce the static electricity and the risk of sparking or discharge. Flame Retardants: to delay the flaming Plasticizers: to reduce Tg, internal lubrication, Reinforcements: to modify the mechanical properties. 85
(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
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Figure 6.9 Tensile stress-strain curves for different materials. Note that these are qualitative
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Processing of Polymeric Materials
Forming Processes for Thermoplastics: ¾Extrusion (Continues section (hollow or solid) tubes, pipes, etc. ¾Extrusion and blowing: Plastic bags, etc. ¾Blow Molding: bottles, etc. ¾Injection Molding; different single parts. ¾Thermoforming; Sheet forming to single products. ¾Calendaring (Rolling); Sheet or flat products, printing on to sheets, etc. Foiles, etc. ¾Spinning; Fiber production
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Forming Processes for Thermosets: ¾ Compression Molding (from semi solids to solid products) ¾ Transfer Molding (from liquids to solid products)
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Thermoplast polymers
Extrusion 89
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(a)
Thermoplast polymers (b)
(c)
(a)Extrusion and blowing (b)Extrusion (c)Blow molding 90
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Thermoplast polymers
(a)
(b)
(c)
(d)
Typical forming processes for thermoplastic: (a) injection molding, (b) thermoforming, (c) calendaring, (d) spinning. 91
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Thermoset polymers
©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.
1. Compression molding, 2. Transfer molding.
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Quiz
• Why is ceramic materials are very brittle? • Why is bending strength higher than tensile strength in crystalline ceramics?
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Questions
• Which properties are required for the materials used in manufacturing turbine blades for jet engines? • Describe a method for the production of a cheap scissor made of cast iron. • Design a high-performance connecting rod for the engine of a racing automobile.
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