Unit-V: Ceramics, Plastics, Composites & Performance of Materials in Service
Unit-V: Ceramics, Plastics, Composites & Performance of Materials in Service
Complete Revision Notes
1. Ceramics
Structure of Ceramics
- Mostly ionic + covalent bonding
- Crystal structures: Rock salt (NaCl), CsCl, Zinc blende (ZnS), Fluorite (CaF₂), Perovskite (BaTiO₃), Spinel (MgAl₂O₄)
- Silicate ceramics → SiO₄⁴⁻ tetrahedra linked (chains, sheets, 3D network)
Properties of Ceramics
| Property | Value/Behavior | Reason |
|---|---|---|
| Hardness | Very high | Strong ionic/covalent bonds |
| Brittleness | High | No slip systems, flaws |
| Melting point | Very high | Strong bonding |
| Thermal conductivity | Low (traditional), high in AlN, SiC | Phonon scattering |
| Electrical | Mostly insulators | Large band gap |
| Chemical resistance | Excellent | Inert bonds |
Types & Applications
| Type | Examples | Applications |
|---|---|---|
| Traditional | Clay, bricks, pottery | Construction, tiles |
| Whiteware | Porcelain, china | Tableware, insulators |
| Refractories | Al₂O₃, MgO, SiC | Furnace linings |
| Advanced Ceramics | Al₂O₃, ZrO₂, Si₃N₄, Sialon | Cutting tools, engine parts |
| Electro-ceramics | BaTiO₃, PZT, ZnO | Capacitors, piezoelectrics, varistors |
| Bioceramics | Al₂O₃, Hydroxyapatite | Hip implants, dental |
Processing of Ceramics
- Powder preparation (ball milling)
- Shaping: Dry pressing, Slip casting, Tape casting, Injection molding
- Drying & Binder burnout
- Sintering (1200–1800°C) → densification
2. Plastics & Polymers
Classification of Polymers
| Basis | Type-1 | Type-2 |
|---|---|---|
| Origin | Natural (rubber, cellulose) | Synthetic (PE, PVC) |
| Structure | Linear → ductile | Branched/Cross-linked → rigid |
| Thermal behavior | Thermoplastics (soften on heating) PE, PP, PVC, PS, Nylon | Thermosets (do not soften) Phenolic, Epoxy, Bakelite |
| Tacticity | Isotactic, Syndiotactic, Atactic | — |
Mechanical Behavior
- Viscoelastic → Show both viscous & elastic behavior
- Creep under constant load
- Stress relaxation under constant strain
- Glass transition temperature (Tg) → Amorphous polymers become rubbery above Tg
Processing of Plastics
- Injection molding (thermoplastics)
- Compression & Transfer molding (thermosets)
- Extrusion (pipes, sheets)
- Blow molding (bottles)
- Thermoforming, Rotational molding
Future of Plastics
- Biodegradable polymers (PLA, PHA)
- Recyclable & bio-based plastics
- High-performance polymers (PEEK, Kevlar)
- Smart polymers (shape memory, self-healing)
3. Other Important Materials
Composite Materials
- Combination of two or more materials → better properties
- Matrix + Reinforcement
| Type | Matrix | Reinforcement | Examples & Uses |
|---|---|---|---|
| PMCs | Polymer | Glass, Carbon, Aramid | FRP, Aircraft, Sports goods |
| MMCs | Metal | SiC, Al₂O₃ particles/fibers | Engine pistons, brake rotors |
| CMCs | Ceramic | SiC, C fibers | Gas turbines, heat shields |
Concrete
- Composite: Cement + Sand + Aggregate + Water
- Reinforced with steel (RCC), Prestressed concrete
- High compressive strength, low tensile → needs reinforcement
Optical & Thermal Materials
- Optical: Glass, Quartz, Optical fibers, Sapphire, Lasers (Nd:YAG)
- Thermal: Insulators (aerogel, glass wool), Conductors (Cu, Al), Refractories
4. Performance of Materials in Service
Fracture
- Brittle fracture: Little plastic deformation (glass, ceramics)
- Ductile fracture: Cup-and-cone, necking
- Griffith theory: Fracture stress ∝ 1/√c (c = crack length)
- Fracture toughness (KIc) → resistance to crack propagation
Fatigue
- Failure under cyclic loading at stress < yield strength
- Stages: Crack initiation → Propagation → Final fracture
- S-N curve (stress vs cycles)
- Endurance limit (ferrous), Fatigue strength (non-ferrous)
- Fatigue strength improved by: shot peening, surface hardening, avoiding stress raisers
Corrosion & Its Control
- Electrochemical degradation of metals
- Types: Uniform, Galvanic, Pitting, Crevice, Stress corrosion cracking (SCC)
- Prevention:
- Material selection (SS, Al alloys)
- Cathodic protection (sacrificial anode or impressed current)
- Coatings (paint, galvanizing, anodizing)
- Inhibitors
Unit-V Quick Revision Summary
| Material | Key Feature | Typical Use |
|---|---|---|
| Ceramics | Hard, brittle, heat resistant | Cutting tools, insulators |
| Thermoplastics | Re-meltable | Bottles, pipes |
| Thermosets | Heat resistant, rigid | Electrical parts |
| Composites | High strength/weight | Aircraft, sports |
| Concrete | High compression | Buildings, dams |
| Fracture | Griffith → crack size | Brittle materials |
| Fatigue | S-N curve | Shafts, aircraft wings |
| Corrosion | Electrochemical | Control by coating/cathodic protection |
Unit-V Complete! You now have notes for the entire Materials Science syllabus.
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