Engineering Materials: Polymers and Ceramics

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.