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Questions and Answers
Which of the following is a characteristic of polymers?
Which of the following is a characteristic of polymers?
What type of bonding is most commonly found in ceramics?
What type of bonding is most commonly found in ceramics?
Which of the following best describes the typical electrical properties of ceramics?
Which of the following best describes the typical electrical properties of ceramics?
Which statement is true regarding the mechanical properties of ceramics compared to metals?
Which statement is true regarding the mechanical properties of ceramics compared to metals?
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What does the term 'refractory' mean in the context of ceramics?
What does the term 'refractory' mean in the context of ceramics?
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Which of the following is an example of an engineering ceramic?
Which of the following is an example of an engineering ceramic?
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What factor primarily influences the degree of ionic character in ceramic bonding?
What factor primarily influences the degree of ionic character in ceramic bonding?
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In oxide ceramic structures, how are oxygen anions typically arranged?
In oxide ceramic structures, how are oxygen anions typically arranged?
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Which of the following is a biopolymer?
Which of the following is a biopolymer?
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What is the primary source of most synthetic polymers?
What is the primary source of most synthetic polymers?
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Which of the following polymers is commonly used in clothing?
Which of the following polymers is commonly used in clothing?
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What type of bond is found in saturated hydrocarbon molecules?
What type of bond is found in saturated hydrocarbon molecules?
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Which of the following is an example of a saturated hydrocarbon?
Which of the following is an example of a saturated hydrocarbon?
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What is the repeating unit for the polymer Polyvinyl chloride (PVC)?
What is the repeating unit for the polymer Polyvinyl chloride (PVC)?
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Which of these applications is LEAST likely to use Polypropylene?
Which of these applications is LEAST likely to use Polypropylene?
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What is the key characteristic of an unsaturated hydrocarbon?
What is the key characteristic of an unsaturated hydrocarbon?
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Which factor does NOT influence the formation of stable ionic structures?
Which factor does NOT influence the formation of stable ionic structures?
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In the general formula $A_mX_p$ for ionic compounds, what do 'm' and 'p' represent?
In the general formula $A_mX_p$ for ionic compounds, what do 'm' and 'p' represent?
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What is the coordination number of $Cs^+$ in the Cesium Chloride (CsCl) structure?
What is the coordination number of $Cs^+$ in the Cesium Chloride (CsCl) structure?
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In the fluorite structure, where are the cations typically located?
In the fluorite structure, where are the cations typically located?
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Which of the following is an example of a compound with the antifluorite structure?
Which of the following is an example of a compound with the antifluorite structure?
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What is a key characteristic of the Perovskite crystal structure?
What is a key characteristic of the Perovskite crystal structure?
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What is the relationship between the radius of $Cs^+$ and $Cl^-$ ions in the Cesium Chloride structure?
What is the relationship between the radius of $Cs^+$ and $Cl^-$ ions in the Cesium Chloride structure?
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Why are ceramics generally more brittle than metals?
Why are ceramics generally more brittle than metals?
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What is the primary effect of firing on a ceramic piece?
What is the primary effect of firing on a ceramic piece?
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What is the main process that occurs during sintering?
What is the main process that occurs during sintering?
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What occurs at the contact regions between adjacent particles during sintering?
What occurs at the contact regions between adjacent particles during sintering?
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Which of the following describes the hardening process of cement?
Which of the following describes the hardening process of cement?
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During the making of Portland cement, what process takes place at 1400°C?
During the making of Portland cement, what process takes place at 1400°C?
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Which ceramic material is described as being doped with magnesium oxide and used for electrical applications?
Which ceramic material is described as being doped with magnesium oxide and used for electrical applications?
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What is the primary reason that zirconia (ZrO2) is combined with 9% MgO?
What is the primary reason that zirconia (ZrO2) is combined with 9% MgO?
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How does an engineering ceramic differ from a traditional ceramic in terms of material composition?
How does an engineering ceramic differ from a traditional ceramic in terms of material composition?
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What happens to a linear polymer when it is stretched and then released?
What happens to a linear polymer when it is stretched and then released?
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What is the primary characteristic of cross-linked polymers regarding their mechanical behavior?
What is the primary characteristic of cross-linked polymers regarding their mechanical behavior?
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Why are polymers rarely 100% crystalline?
Why are polymers rarely 100% crystalline?
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What effect does annealing have on polymer crystallinity?
What effect does annealing have on polymer crystallinity?
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What tends to happen as the crystallinity of a polymer increases?
What tends to happen as the crystallinity of a polymer increases?
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What is a key characteristic of thermoplastic polymers when heated?
What is a key characteristic of thermoplastic polymers when heated?
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Which of the following is a thermosetting plastic?
Which of the following is a thermosetting plastic?
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What is a key difference between vulcanized and unvulcanized elastomers?
What is a key difference between vulcanized and unvulcanized elastomers?
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What characterizes the initial state of chains in elastomers before deformation?
What characterizes the initial state of chains in elastomers before deformation?
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How does decreasing temperature generally affect the tensile properties of thermoplastics?
How does decreasing temperature generally affect the tensile properties of thermoplastics?
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What happens to the ductility of thermoplastics when increasing the strain rate?
What happens to the ductility of thermoplastics when increasing the strain rate?
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In elastomers, what type of failure occurs due to the deformation of cross-linked structures?
In elastomers, what type of failure occurs due to the deformation of cross-linked structures?
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Compared to other polymers, elastomers primarily display which type of stress-strain behavior?
Compared to other polymers, elastomers primarily display which type of stress-strain behavior?
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What is indicated by the term '%EL' with respect to polymer materials?
What is indicated by the term '%EL' with respect to polymer materials?
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Which of the following would likely happen with a decrease in temperature for a semicrystalline polymer like PMMA?
Which of the following would likely happen with a decrease in temperature for a semicrystalline polymer like PMMA?
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In the context of elastomers, what does it mean when the final chains are described as 'straight'?
In the context of elastomers, what does it mean when the final chains are described as 'straight'?
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Study Notes
Engineering Materials
- Materials are classified into metals, polymers/plastics, and ceramics.
- Metals are strong and ductile, have high thermal and electrical conductivity, and are opaque and reflective.
- Polymers/plastics have covalent bonding, are soft and ductile, have low strength and low density, and are thermal and electrical insulators. They are often optically translucent or transparent.
- Ceramics have ionic bonding (refractory), are brittle, glassy, elastic, and are non-conducting insulators. They are compounds of metallic and non-metallic elements (oxides, carbides, nitrides, sulfides).
Ceramics: Internal Characteristics
- Ceramics are inorganic compounds.
- Bonding is ionic or covalent.
- Electrical properties are insulating.
- Ductility is lower than metals.
- Chemical stability is high.
- Traditional ceramics consist of clay, silica, and feldspar.
- Engineering ceramics include compounds like Al2O3, Si3N4, and SiC (oxides, nitrides and carbides).
Ceramic Bonding
- Bonding is mostly ionic, with some covalent character.
- Percentage ionic character increases with the difference in electronegativity values.
- The ionic bond character of some ceramics is large (e.g., CaF2), while others are small (e.g., SiC).
Ceramic Crystal Structures
- Oxide structures typically involve oxygen anions that are much larger than the metal cations.
- Oxygen anions form close-packed FCC lattices, and metal cations occupy the holes within this lattice.
- Packing of ions depends on the relative size of cations and anions, and magnitude of their electrical charge.
Ionic Bonding & Structure
- Stable structures maximize the number of nearest oppositely charged neighbors.
- Charge neutrality ensures that the net charge in the structure is zero.
- General form of an ionic crystal is AmXp.
AX Crystal Structures
- AX-type crystal structures include NaCl, CsCl, and zinc blende.
- Cesium Chloride structure shows Cs+ at the corners of a cubic unit cell and Cl- in the center of the unit cell.
- Fluorite structure (CaF2) has Ca2+ ions in cubic sites and F- ions in tetrahedral sites.
- The antifluorite structure reverses the positions of the cations and anions.
ABX3 Crystal Structures
- Perovskite structure is a complex oxide structure exemplified by BaTiO3.
Mechanical Properties of Ceramics
- Ceramics are more brittle than metals due to the difficulty in slippage along slip planes in ionic solids.
- Moving one anion past another anion requires high energy due to lattice distortion.
- External forces cause repulsion between like charges, leading to crystal cracks.
Taxonomy of Ceramics
- Ceramic materials encompass glasses, clay products, refractories, abrasives, cements, and advanced ceramics.
- Applications include optical components, whiteware, high-temperature components (bricks, furnaces), abrasive materials (sandpaper, polishing), cements, engine parts/components, and sensors..
- The transition temperature for glasses is moderate, while it is high for other ceramics.
Clay Composition
- Clay is a mixture of clay, filler, and fluxing agent.
- Clay (approximately 50%) provides workability.
- Filler (approximately 25%, eg., quartz) improves temperature resistance.
- Fluxing agent (approximately 25%, e.g., feldspar) binds the components together.
- Aluminosilicates (with K+, Na+, Ca+) are its primary constituent.
Kaolinite
- Kaolinite is a common, inexpensive clay mineral.
- Its chemical composition is Al2Si2O5(OH)4.
- It's a layered silicate mineral with a tetrahedral sheet linked to an octahedral sheet of alumina octahedra.
- Water added to clay reduces van der Waals bonding, allowing particles to more easily move past one another.
- Processes like extrusion, and slip casting use water to create various shapes for kaolinite.
Ceramic Fabrication Methods-I
- Methods for creating ceramic objects include glass forming, particulate forming, and cementation.
Pressing and Glass Forming
- Some glass blowing is done manually.
- The processes for producing glass jars, bottles, and light bulbs are automated.
- A raw gob of glass is shaped into a parison using a mold via mechanical pressing.
- The parison is then inserted into a finishing or blow mold to take on the final contours.
Drawing
- Drawing is used for making long glass parts such as sheets, rods, tubes, and fibers.
- The process involves drawing molten glass through small orifices.
Particulate Forming
- Ceramics are formed by compacting powdered or particulate materials and heating them to create cohesion.
- Material preperation involves grinding, mixing, possibly blending wet or dry ingredients including particles and binders or lubricants.
- Forming happens in dry, plastic or liquid mediums.
- Slip casting, compaction, and extrusion are common cold forming processes.
Traditional Ceramic Processing
- Preparing powders via milling and screening to desired size.
- Shaping of wet clay is formed.
- Drying of clay article.
- Firing to bind particles together.
Material Preparation
- Milling and screening create desired particle size.
- Mixing particles with water produces a slip.
- Slip composition usually contains 25-40% water.
- Ball milling and roller milling are two methods of slip preparation.
Shaping of Wet Clay
- Slip casting is a method where ceramic powders in water are poured into a porous plaster mold which draws out excess water.
- Variants include drain casting (producing hollow items) or solid casting (producing solid items).
- Hydroplastic forming involves extruding the slip through a die.
Drying
- Water must be removed from the clay product prior to firing.
- Two stages-1. rapid evaporation from surface then 2. moisture content continues to reduced to a stable condition limiting further shrinkage.
- Shrinkage during drying can lead to warping or cracking.
Firing
- Firing is a heat treatment process performed in a kiln to sinter ceramic materials.
- This leads to densification of the material and reduction of porosity.
- In higher temperature operations, vitrification is associated with changes to liquid glass phases occurring at lower temperatures.
- Addition of fluxing agents like feldspar can cause the creation of a liquid glass phase that flows and fills voids in the ceramic article.
New Ceramic Processing
- The processing sequence involves material preparation, powder pressing, drying, and firing.
Material Preparation (cont')
- Milling and screening are used to achieve the desired particle size.
- Uniaxial pressing compacts the powder in a metal die in a single direction.
- Isostatic pressing uses a fluid to apply pressure isotropically on the powder.
- Hot pressing combines pressure and heat for simultaneous application.
Drying (cont')
- Drying of clay articles is done at temperatures less than the melting point.
- Warping and cracking are possible if the temperature is too high.
Firing (cont')
- Sintering involves bonding of particles through solid-state diffusion processes occuring at temperatures below the material's melting point.
- Sintering time and particle size factors impact outcome in the final product.
Sintering
- Powder particles touch each other.
- Necks develop at contact regions between adjacent particles.
- A grain boundary forms within each neck.
- Interstices between particles become pores.
- Pores become progressively smaller and more spherical in shape during sintering
Cementation
- Paste hardening occurs after cement mixing with water.
- Rigid structures form with varied shapes.
- During hardening, hydration is the complex chemical process between water and cement particles.
Engineering Ceramics
- Alumina (Al2O3) is often doped with magnesium oxide.
- Silicon Nitride (Si3N4) is created by compacting silicon powder then nitriding in nitrogen gas.
- Silicon Carbide (SiC) is a very hard refractory carbide sintered at high temperatures.
- Zirconia (ZrO2) can be combined with a material (e.g MgO) to increase toughness and resist cracking.
Ceramic Comparison
- Traditional ceramics typically contain clay, filler, and a fluxing agent.
- Engineering ceramics often comprise a compound oxide, nitride, or carbide.
- Traditional ceramics uses forming techniques such as slip casting, while engineering ceramics use different fabrication methods.
Polymer Composition
- Most polymers are organic.
- Formed from hydrocarbon molecules.
- C atoms participate in four bonds, H participates in one.
- Examples of hydrocarbon molecules: methane, ethane, and propane (saturated).
- Unsaturated hydrocarbon molecules have double or triple bonds between C atoms.
Chemistry of Polymers
- Polymer molecules are large macromolecules.
- Most polymers are chains of C atoms (backbone).
- Side bonding involving carbon-hydrogen atoms also occurs.
- A repeat unit in a polymer chain is a "mer."
- A single mer is called a monomer.
Molecular Shape
- The angle between singly bonded carbon atoms is approximately 109 degrees.
- Polymer form a zigzag pattern.
- Chain rotations occur around C-C bonds, except for double and triple bonds which are rigid.
- Random kinks and coils lead to entanglements in polymer structures.
- Mechanical and thermal characteristics depend on the ability of chain segments to rotate and shift in their position.
Molecular Structures
- Covalent chain configurations and strength differ based on whether chains are linear, branched, cross-linked, or network.
- Van der Waals bonding between chains is reduced in branched or cross-link polymers.
Physical Properties of Polymers
- Linear polymers can be stretched to deform chains that flow past one another, but they do not retain the original shape when stress is released.
- Cross-linked polymers maintain their shape even after deformation, because these chains are held together by cross-links.
Polymer Crystallinity
- Polymer molecules are often partially crystalline (semicrystalline) with interspersed amorphous regions
- The percentage of crystallinity in a polymer material is the percentage of crystalline material.
- Annealing causes crystalline regions to grow, increasing crystallinity.
- Increase in crystallinity leads to higher density, stiffness, strength, and toughness.
- Amorphous state is usually transparent.
- Increase in crystallinity tends to obscure transparency.
Taxonomy of Polymers
- Polymer materials are classified as Thermoplastics, Thermosets, and Elastomers.
- Specific types are listed under each category
Thermoplastics
- Thermoplastic polymers become soft and deformable upon heating.
- These processes are reversible and repeatable (recyclable).
- The high temperature plasticity is due to the ability of polymer chains to slide past one another.
- Thermoplastics are usually fabricated by simultaneous application of heat and pressure.
- On a molecular level, at higher temperatures, secondary bonding forces diminish and chains slide more readily under applied stress.
- Injection molding, blow molding, thermoforming, and vacuum molding are common processing methods.
Recycling Symbols
- Recycling symbols for various polymers are provided.
Commodity (Amorphous) Thermoplastics
- Polyethylene (PE), Polypropylene (PP), Polyvinylchloride (PVC), and Polystyrene (PS) are four common amorphous thermoplastics.
- They are used in diverse applications thanks to their good dimensional stability, low cost, temperature resistance and strength.
- They typically, but not always, are transparent.
Thermoplastic Processing
- Injection molding and blown film molding are two types of thermoplastic processing method.
- Extrusion involves pushing molten thermoplastic through a die, creating shapes based on the die design.
Thermosetting Plastics
- These polymers become hard and rigid upon heating, and this state is irreversible (non-recyclable).
- Network molecular structures result from step growth mechanisms.
- Chemical reaction enhancements occur at high temperatures.
- Covalent cross-links form between adjacent molecular chains during processing.
- This process creates more rigid structures by anchoring the chains together to resist vibration and rotational chain motion at higher temperatures.
- Excessive heating can break crosslinks which changes the polymer properties.
- Compression molding and transfer molding processing are used in thermosets.
Thermosetting Examples & Applications
- Some examples of thermosets include epoxies, phenolics, and polyesters.
- Epoxies are often applied in adhesive formulations.
Thermosetting Processing (cont')
- Compression molding involves placing the raw thermosetting material into a die that is heated.
- Pressure is applied on the material to set the shape of the object.
- Transfer molding involves using a plunger to transfer the resin from a separate container to the heated mold.
- The plunger forces the resin through the cavity of the mold which forms the object.
Elastomers
- Elastomers are capable of large deformations under stress, and return to their original shape when the stress is released.
- They must meet several criteria, including resistance to crystallization (they are typically amorphous), and relatively free chain rotations.
- Crosslinking achieved through vulcanization increases resistance to plastic deformation.
- The temperatures must be above the glass transition temperature (Tg).
Key Figures in Polymer History
- Charles Goodyear, inventor of vulcanization of rubber in the 1830s.
- Vulcanization involves introducing sulfur compounds into the structure during the heating process.
- This makes the material less likely to soften at higher ambient temperatures, but also harder to soften at lower ambient temperatures.
Vulcanization (I)
- Crosslinking is essential for elastomeric behavior.
- It involves an irreversible chemical reaction, usually at high temperatures.
- Sulfur atoms bond to double-bonded carbons which restricts molecular movement hence forms crosslinks.
- Unvulcanized rubber softens at high ambient temperatures and hardens at lower temperatures
Vulcanization (I) (cont')
- Elastomeric materials, being cross-linked, are thermosetting polymers. Their elastic modulus, tensile strength and oxidation resistance increases with vulcanization.
- Too much crosslinking reduces extensibility.
Mechanical Properties
- Stress-strain behavior for polymers varies: ductile polymers have a gradual elastic deformation-to-plastic deformation region and brittle polymers fracture suddenly in a limited elastic region.
Polymers: Deformation
- Bond breaking requires high energy.
- Chain sliding requires low energy and occurs easily at elevated temperatures.
- Side group ordering requires high energy at higher temperatures.
Tensile Response: Brittle & Plastic
- Brittle polymers exhibit sudden fracture after a limited elastic region.
- Plastic polymers deform elastically and then plastically before failure.
- Elastomers exhibit an entirely elastic response.
- The different behaviors of the structures are related to their chain configurations.
Tensile Response: Elastomer Case
- Deformation in response to load is usually considered reversible.
- The material can handle high amounts of strain without breakage.
T and Strain Rate: Thermoplastics
- Temperature effects on elastic modulus, tensile strength (% elongation) as strain rate alters properties.
Summary
- General polymer drawbacks are small Ε , σy, and Kc; deformation dependent on temperature, and time.
- Thermoplastics are easier to process and recycle, with small Ε, σy and large Kc.
- Elastomers have large reversible strains at elevated temperatures with larger Ε, σy and smaller Kc.
- Thermosets are hard, stronger, and more brittle; however, processes are more complex.
Introduction to Composites
- Composites are materials consisting of two or more different materials.
- They exhibit better properties (e.g., strength-to-weight, fatigue resistance, toughness) than the constituent materials individually.
- Typical applications include aerospace, automotive, and construction.
Composites
- Composites consist of two or more different materials (matrix and reinforcement), in which the resultant properties of the material are superior to the constituent materials involved.
- The components interact to produce materials with more desirable characteristics than the constituent materials have on their own.
Uses of Composites
- Composites have numerous applications, including composite bicycles, graphite snowboards, and laminated fiberglass bows.
Composites: Getting the Best of All Worlds
- Comparing properties of composites with metals, ceramics, and polymers
- Composites combine the benefits of all three material types in one material, but the exact composite will require careful selection and design.
Why Composites are Important
- Composites are strong and stiff and very light.
- Fatigue resistance and toughness are generally better for composites than other common engineering materials.
- Composites can be designed to not corrode (e.g., metals).
- Composites can be designed to have unique combinations of properties unavailable in other material types.
Composites in Industry
- Aerospace, automotive, pressure vessels, and any applications requiring high-performance materials.
Nature of Composites
- Advantages: High strength-to-weight ratio, high creep resistance, high elevated temperature strength, high toughness, better fatigue properties, higher wear resistance, and lower dimensional changes with temperature.
- Disadvantages: High material cost, fabrication difficulties, repair issues, high temperature requirements (for some types).
Classification based on Reinforcement
- Particle-reinforced composites (e.g., spheroidite in steel), Dispersion-strengthened composites, Fiber-reinforced composites (continuous and discontinuous), Structural composites (laminates, sandwich panels) are examples of various composite types.
Polymer Matrix Composites
- Polymer matrix composites (PMCs) use polymers as their matrix.
- Different types of fibers are reinforced, providing varying properties depending on the type of fiber and matrix material.
- Examples of fibers include glass, carbon, and aramid (Kevlar).
Metal Matrix Composites
- Metal matrix composites (MMCs) are typically used in aerospace, automotive, and sports equipment due to high modulus of elasticity, ductility, and resistance to elevated temperatures.
- Fibers like graphite, boron, alumina, silicon carbide are used as reinforcement phases.
- Examples include aluminum-alloy-boron fiber composite, used in driveshafts.
Ceramic Matrix Composites
- Ceramic matrix composites (CMCs) are chosen when resistance to elevated temperatures and harsh environments is critical.
- Common matrix materials involve silicon carbide, aluminum oxide, and mullite; fibers frequently use carbon and aluminum oxides.
- These are strong and stiff, but usually lack toughness.
- Fabrication occurs via hot isostatic pressing and chemical vapor deposition.
Composite Benefits
- Increased toughness, strength, high elastic modulus, creep resistance, and low density in various types of composites
- Comparisons of composite properties with other material counterparts are shown.
Structural Composites
- Consist of homogeneous and composite material; properties depend on the materials involved and geometric design
- Laminar Composites: Stacked sheets/panels in preferred directions to achieve high strength.
- Sandwich Panels: Employ a core with a less dense material between strong face sheets providing high stiffness and strength.
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Test your knowledge on the characteristics and properties of polymers and ceramics. This quiz includes questions on bonding, electrical properties, mechanical properties, and applications of these materials. Suitable for students studying engineering materials.
