A survey of Engineering Materials August 27, 2003

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A survey of Engineering MaterialsAugust 27, 2003

• Dr. Richard Chung
• Department of Chemical and Materials Engineering
• San Jose State University

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Types of Engineering Materials

• Metals (alloying elements, processing techniques,
  refractory metals, superelastic /superplastic
  metals, metallic glass, shape memory alloys,
  etc.)
• Ceramics (glass, metal oxides, metal carbides,
  metal nitrides, etc.)
• Polymers (thermoplastics, thermosets, elastomers,
  copolymers, plastics alloy, polyblend)
• Composites (laminated, braided, pultruded,
  nanocompsoites)

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Definitions

• Plastics alloy refers to two or more different
  polymers that are physically mixed during a
  melting process. This should not be considered as
  copolymer.
• Polyblend refers to a polymer that has been
  modified by adding an elastomer to it.
• No primary bonds are developed between two
  dissimilar polymer chains.

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Factors Affecting Material Performance

• Structure-property-processing relationships
• Hot work, cold work,
• Solid solution strengthening
• Precipitation hardening
• Inclusions
• Imperfections (e.g. number of dislocations)
• Crystal structures Crystalline versus amorphous
• Toughening
• Heat treatment (such as annealing, normalizing,
  and quenching)
• Residual stresses

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Things Related to Mechanical Behavior

• Material nature
• Ductile versus brittle fracture
• Static versus cyclic loading
• Deformation mechanisms
• Creep deformation (stress and temperature
  effects)
• Fatigue fracture (high cycle vs. low cycle)
• Notch sensitivity and loading directions

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Material Configurations (Composites)

• Ductile - Brittle
• Ductile - Ductile
• Brittle - Brittle
• Brittle - Ductile
• Fiber orientation (reinforcing directions)
• Continuous versus short fiber reinforcement
• Random, intercalation, exfoliation

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Superplasticity

• Superplasticity is a phenomenon in which some
  materials will demonstrate remarkably high strain
  to failure at a strain rate roughly around
  0.001/s. This phenomenon generally happens at
  high temperatures gt 0.5 Tm.
• Superplastic deformation can be achieved at high
  temperatures to form complex shapes which are
  impossible to obtain using traditional processes
  such as rolling, forging, extrusion, or drawing
  processes.

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History of Nanocomposites

• 1993 a versatile synthesis technique developed
• direct mixing of modified clay with melted
  polymer
• by workers at Cornell University
• stimulated vigorous research (Manius et al.,
  2001)
• 1999 market analysis performed by Principal
  Partners
• global market was 2 million pounds (Rossi, 2000)
• predicted to reach 1.2 billion pounds in 2009

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Structure - Clays

• Clays are a class of minerals consisting of
  mainly silica (siO2) and alumina (Al2O3).
• Silicate sheets are stacked in sequence using van
  der Waal forces or hydrogen bonds.
• Typical clays used are montmorillonites,
  smectites, and kaolinites.
• Montmorillonite structures have an alumina
  octahedral sheet sandwiched between two silicate
  tetrahedral sheets (21 ratio)
• Natural Smectites are missing trivalent aluminum
  atoms. Instead, they are replaced by divalent
  magnesium, lithium and silicon atoms resulting in
  an overall negative charge in structure.
• Kaolinites consist of many unit layers sandwiched
  together the unit layers are held together by
  hydrogen bonds.

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Introduction to Clays

• Clays are minerals
• Consist of silica (SiO2) and alumina (Al2O3)
• Basic structural component is silicate sheets
  with silicon-oxygen 6-membered rings

Figure 1 Silicate Sheet (Si2O5)2- (Callister,
2003)
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Introduction to Clays

• Smectites are a group of clay minerals with
  useful properties for making nanocomposites

Figure 2 Smectite Structure (Worral, 1986)
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Intercalation and Exfoliation

• Van der Waals gap between Smectite unit layers
• contains H2O and cations such as Na
• gap is naturally hydrophilic
• can be made organophilic by exchanging cations
  with organic cation salts
• Nanoclays can be purchased from Nanocor and
  Southern Clay Products (Cloisite)
• Intercalation diffusion of polymer chains into
  gaps, resulting in finite expansion (limited
  dispersion). Multilayer structure is retained.
• Exfoliation extensive polymer penetration,
  resulting in delamination and dispersal.

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Tetrahedral

• Figure 3. Theoretical formula and structure for
  montmorillonites
• (G-105 Nanocor, technical note)

Octahedral
Tetrahedral
Charged inorganic cation layer (Al3,Ca2, Mg2,
Na, Li, etc.)
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• Figure 4. Schematic representation of polymer
  -layered silicate composite structures

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Structure of Nanoparticles

• Silica filler or reinforcement
• The chemical name is Polyhedral Oligometric
  Silsesquioxanes (POSS)
• It will improve Tg, HDT, Creep, compression,
  hardness, fire resistance, permeability

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Types of Polymer Matrixes

• Thermosets polyurethane, phenolic, epoxy,
  polyester, vinyl ester
• Thermoplastics Nylon, PET, ABS, PI, PP, PC, PEEK
• Elastomers silicone, EPDM

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Properties

• Mechanical properties tensile, toughness,
  hardness, impact, fatigue and creep properties.
• Physical properties glass transition temperature
  (Tg), permeability, dielectric properties,
  optical, thermal and electrical properties.
• Chemical properties reactivity, corrosion
  resistance, polymerization, biochemical reactions

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Material Selection

• Process
• Analysis of material application problem
• Translation of the material application
  requirements to material property values
• Selection of candidate materials
• Evaluation of candidate materials
• Decision making
• Properties
• Mechanical, thermal, chemical, electrical
• Constraints
• Existing facilities
• Compatibility
• Marketability
• Availability
• Disposability and recyclability

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Selection Procedure for a Cantilever Beam

• Requirements L (length), P (load), X (safety
  factor)
• Geometry variable r (radius)
• Material properties ? (density), ?o (max.
  stress)
• Quantity to minimize m
• Q f1 (requirements) f2 (materials)
• ? m f1 (L, P, X) f2 (?, ?o )

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Summary

• Pure metals in bulk form demonstrate low yield
  stresses. Their applications in industry can be
  enhanced by introducing various
  parameters/obstacles to them such as
  dislocations, work hardening, strengthening
  effects, heat treatment, etc.
• Various materials depending on their geometry and
  structural configurations will perform
  differently.
• Advanced engineering applications such as
  superplasticity and nanocompsoites are briefly
  discussed to delineate the relationships between
  material construction and material performance.