MSEN flash topics

chat 12-17 describe or cite or explain

Describe the differences between silica (SiO₂) crystalline materials and silica glass.

Silica materials:

• polymorphic forms.

• high melting temperature because of strong Si-O bonds.

Silica glass:

• noncrystalline (amorphous).

why there is normally a significant scatter in the fracture strength for  identical specimens of the same ceramic material. 

scatters happen because fails occur at flaws and flaws are never identical in each sample.

explain why crystalline ceramic materials are  normally brittle. ( slip) (3)

Crystalline ceramics are normally brittle because dislocation motion (slip) is very difficult in their crystal structures.

Ceramics are brittle

• strong bonding,

• charge constraints,

• limited slip systems prevent dislocation motion,

they fracture before they can plastically deform.

Describe the process that is used to produce glass-ceramics

1. forming a glass

2. heat-treating it to induce controlled crystallization (nucleation and growth).

This creates a fine-grained crystalline structure within the glass. 

Name the two types of clay products, and give two examples of each.

coarse :

• bricks

• tiles

whitewares:

• porcelain

• china

Coarse = construction

whitewares = household or decorative items

Cite three important requirements that normally must be met by refractory ceramics and  abrasive ceramics.

Refractory ceramics:

• high temperature resistance

• chemical stability

• strength at elevated temperatures

Abrasive ceramics:

• high hardness

• wear resistance

• sufficient toughness. 

Describe the mechanism by which cement hardens when water is added

Cement hardens through hydration reactions between water and cement compounds, forming products like C-S-H gel. These products grow and interlock, binding the material into a solid structure. .

Name three forms of carbon discussed in this chapter and note at least two distinctive  characteristics for each.

Diamond:

• extremely hard

• electrical insulator.

Graphite:

• soft with a layered structure

• conducts electricity.

Fullerene:

• spherical or tubular molecules

• unique electrical/mechanical properties.

four forming methods that are used to fabricate glass pieces.

1. pressing (molding)

2. blowing (air expansion)

3. drawing (pulling into sheets/fibers)

4. casting (pouring into molds)

procedure by which glass pieces are thermally tempered

heated, then rapidly cooled (so the surface goes into compression while the interior is in tension.)

*improves strength and resistance to fracture.  

Briefly describe methods to form a ceramic shape before thermal processing.

During drying : water is removed (shrinkage occurs) which may cause cracking.

During sintering : particles bond by diffusion, reducing porosity and increasing density.

explain the sintering process of powder particle aggregates

Sintering:

1. neck formation between particles

2. growth of these necks and pore shrinkage

3. pores close and the material densifies ( from surface energy reduction)

Describe a typical polymer molecule in terms of its chain structure and, in addition, how  the molecule may be generated from repeat units.

a long chain molecule made of repeating structural units (monomers) covalently bonded together.

These repeat units link end-to-end to form backbone structures that may be linear or branched.

the four general types of polymer molecular structures

• Linear: Long straight chains; can pack closely → higher density and strength.

• Branched: Side-chain branches reduce packing efficiency → lower density.

• Crosslinked: Chains connected by covalent bonds → more rigid structure.

• Network: Highly crosslinked 3D structure → very strong and rigid (e.g., thermosets).

differences in behavior and molecular structure for thermoplastic and  thermosetting polymers.

Thermoplastics: Linear/branched chains, soften when heated and can be reshaped (recyclable).

Thermosets: Heavily crosslinked networks, do not melt upon reheating and are permanently rigid.

describe the crystalline state in polymeric materials

polymer chains align in an ordered, repeating arrangement.

Briefly describe/diagram the spherulitic structure for a semicrystalline polymer.

Spherulites are spherical aggregates of crystalline lamellae radiating outward from a nucleation point. Amorphous material exists between lamellae, giving a mixed structure of ordered and disordered regions.

(they contain both crystalline and amorphous regions)

Make schematic plots of the three characteristic stress-strain behaviors observed for  polymeric materials.

Brittle: Fractures at low strain with little plastic deformation.

Plastic (ductile): Yields and undergoes significant plastic deformation.

Elastomeric: Large elastic deformation with recovery after load removal

Describe/sketch the various stages in the elastic and plastic deformations of a  semicrystalline (spherulitic) polymer.

Elastic deformation begins with chain stretching, followed by yielding where lamellae start to slip.

Plastic deformation, chains align and disentangle, leading to strain hardening as alignment increases. 

Discuss the influence of the following factors on polymer tensile modulus and/or  strength:  

a. molecular weight, 

b. degree of crystallinity, 

c. predeformation, and 

d. heat treating of undeformed materials.

(a) Molecular weight: Higher weight → more entanglements → higher strength.

(b) Crystallinity: Higher crystallinity → increased stiffness and strength.

(c) Predeformation: Aligns chains → increases strength and modulus.

(d) Heat treating: Can increase crystallinity → improves strength and stiffness.

 Describe the molecular mechanism by which elastomeric polymers deform elastically

Elastomers stretch by uncoiling and aligning polymer chains; when stress is removed, entropy drives chains back to their original coiled state, restoring shape. 

List four characteristics or structural components of a polymer that affect both its melting  and glass transition temperatures.

1. Chain stiffness (rigid chains → higher temperatures)

2. Molecular weight

3. Degree of crystallinity

4. Intermolecular forces (stronger bonding → higher temperatures)

Briefly describe addition and condensation polymerization mechanisms.

Addition: Monomers add together without byproducts (e.g., double bonds open).

Condensation: Monomers react and release small molecules (e.g., water) as byproducts.

Name the five types of polymer additives and, for each, indicate how it modifies the  properties.

1. Fillers: Improve strength, reduce cost

2. Plasticizers: Increase flexibility and ductility

3. Stabilizers: Protect against UV/thermal degradation

4. Colorants: Add color

5. Flame retardants: Reduce flammability

Name and briefly describe five fabrication techniques used for plastic polymers.

1. Injection molding: Molten polymer injected into mold

2. Extrusion: Polymer forced through die to create continuous shapes

3. Blow molding: Used to form hollow objects (e.g., bottles)

4. Compression molding: Polymer pressed into shape under heat

5. Rotational molding: Powder rotated in mold to coat interior evenly

Name the four main divisions of composite materials and cite the distinguishing feature  of each.

Particle-reinforced: Particles improve strength/stiffness by load sharing.

Fiber-reinforced: Fibers provide high strength and stiffness along their length.

Structural composites: Combine materials/macroscopic geometry (e.g., laminates, sandwich panels).

Nanocomposites: Reinforcement at nanoscale → improved mechanical and functional properties.

Cite the difference in strengthening mechanism for large-particle and dispersion strengthened particle-reinforced composites.

Large-particle: Strengthening by load transfer; particles impede deformation but don’t block dislocations strongly.

Dispersion-strengthened: Very fine particles hinder dislocation motion → significantly increase strength.

Distinguish the three different types of fiber-reinforced composites on the basis of fiber  length and orientation; comment on the distinctive mechanical characteristics for each  type.

1. Continuous aligned fibers: Highest strength/stiffness in fiber direction; anisotropic.

2. Discontinuous (short) aligned fibers: Moderate strength; less efficient than continuous.

3. Randomly oriented fibers: Nearly isotropic properties but lower overall strength.

the three common fiber reinforcements used in polymer-matrix composites and, for  each, cite both desirable characteristics and limitations.

1. Glass fibers: Cheap, corrosion-resistant; lower stiffness.

2. Carbon fibers: High strength/stiffness, lightweight; expensive, brittle.

3. Aramid fibers (e.g., Kevlar): Tough, impact-resistant; poor compression strength.

Cite the desirable features of metal-matrix composites.

MMCs have high strength, good high-temperature performance, improved wear resistance, and better thermal/electrical conductivity than polymers.

Note the primary reason for the creation of ceramic-matrix composites

CMCs are designed to improve the low fracture toughness of ceramics, making them less brittle and more damage-tolerant.

Name and briefly describe the two subclassifications of structural composites.

Laminates: Layers bonded together with varying orientations for improved strength.

Sandwich panels: Strong outer faces with lightweight core → high stiffness-to-weight ratio.

Distinguish between oxidation and reduction electrochemical reactions.

Oxidation: Loss of electrons.

Reduction: Gain of electrons.

 Describe the following: galvanic couple, standard half-cell, and standard hydrogen  electrode.

Galvanic couple: Two different metals electrically connected in an electrolyte → one corrodes preferentially.

Standard half-cell: One side of an electrochemical cell used to measure potential.

Standard hydrogen electrode (SHE): Reference electrode with zero potential

 For each of the eight forms of corrosion and hydrogen embrittlement, describe the nature  of the deteriorative process and then note the proposed mechanism.

Uniform corrosion: Even material loss; electrochemical reaction over entire surface.

Galvanic corrosion: Dissimilar metals → anodic metal corrodes.

Crevice corrosion: Localized in stagnant regions due to oxygen concentration differences.

Pitting corrosion: Small pits form from breakdown of protective film.

Intergranular corrosion: Along grain boundaries due to compositional differences.

Selective leaching: One element removed (e.g., dezincification).

Erosion-corrosion: Fluid motion accelerates material removal.

Stress corrosion cracking (SCC): Tensile stress + corrosive environment → cracks.

Hydrogen embrittlement: Hydrogen diffuses in, reduces ductility → brittle fracture.

List five measures that are commonly used to prevent corrosion. 

1. Coatings (paint, plating)

2. Cathodic protection

3. Material selection (corrosion-resistant alloys)

4. Environmental control (reduce moisture, oxygen)

5. Inhibitors

Explain why ceramic materials are, in general, very resistant to corrosion.

Ceramics are chemically stable and already in an oxidized state, so they do not readily undergo further electrochemical reactions.

For polymeric materials, discuss 

(a) two degradation processes that occur when they are  exposed to liquid solvents and 

(b) the causes and consequences of molecular chain bond  rupture.

(a) Solvent effects:

• Swelling: Solvent penetrates polymer → expands structure.

• Dissolution: Chains separate and dissolve in solvent.

(b) Chain bond rupture:

• Caused by heat, UV radiation, or oxidation → reduces molecular weight, leading to brittleness and loss of strength.