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Youngstown State University2
Fundamentals of Material Properties
- Part 3-Non-Metallic Materials for Manufacturing
Darrell Wallace
Youngstown State UniversityDepartment of Mechanical and Industrial
Engineering
January 14, 2006
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Non-Metals in Manufacturing Long History
Organics Wooden Tools Textiles Rope
Ceramics Pottery
Very Different Properties from Metals Some Overlap of Processes Key to many “cutting edge” manufacturing
processes
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Ceramics
What is a ceramic? Narrow Definition:
A compound composed of both metallic and non-metallic components
Broader Definition: Everything that is not a metal or organic and that
is subjected to very high temperature during manufacture or use.
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Where do we find Ceramics?
Naturally Occuring:
Silica SiO2
Silicates SiO4
Oxides
Man-Made Carbides
Nitrides
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Bonding and Structure
Ceramic materials are predominantly bound by covalent and ionic bonds
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Covalent Bonds in Ceramics Covalent Bonds - Electrons are shared by adjacent
atoms Very Strong Has associated directionality Significant factor in atomic
spacing and crystalline structure
Associated Characteristics High melting point, strength, brittleness and hardness Low thermal expansion, thermal and electrical conductivity
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Ionic Bonds in Ceramics Ionic Bonds: Electron transfer leads to ionization of
atoms. Attraction based on opposing electrical charges.
•Creates a smaller (denser) molecule than covalent bonding
•Brittle and nonconductive at lower temperatures, but exhibits some movement of dislocations and charge carriers at elevated temperatures.
•Deformation is particularly possible under elevated temperature and hydrostatic pressure
•Example: Na+Cl-
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Crystalline Structure Most ceramics exhibit a crystalline structure in
their solid state Some ceramics exhibit different crystalline
structures (polymorphs) under different pressure or temperature conditions. Changes in crystalline structure lead to changes in
properties, especially density Volumetric changes tend to be more pronounced in
ceramics than in allotropic metals Ceramics that don’t have a crystalline structure
(amorphous) are called “glasses”
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Glasses Glasses are formed when a ceramic is heated
above its melting point and cooled at a rate faster than the crystallization can occur.
Ceramic glasses can be held at elevated temperature for extended periods to allow stable crystalline structures to form. This is called “devitrification”
Amorphous glasses tend to be isotropic whereas crystalline ceramics can be very anisotropic.
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Mechanical Properties of Ceramics Ceramics are VERY sensitive to stress risers
(notch sensitivity) Material tests must take great care not to damage the
surface Cracks are naturally occurring, so tests must be statistical
in nature.
Ceramics are less sensitive to crack formation in compression than in tension (including bending)
Excellent hot-hardness and dimensional stability
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Improving Mechanical Properties of Ceramics
Reduce Particle Size Retard the Propagation of Large Cracks
Incorporate particles that suffer phase transformation
Introduce microfractures Guide the crack propagation with fibers
Induce Compressive Residual Stresses Reduce Creep (improve hot hardness)
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Polymers and Plastics
From the Greek: Polymer:
Poly = many Meros = parts
Plastic: Plastikos = able to be molded or formed
Most polymers are based on Carbon chains and are, therefore, organic compounds.
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Chain Polymerization Monomer (“one part”) Initiator is used to open up double bonds and allow it to
bond to adjacent atoms Polymerization occurs in the entire batch almost
simultaneously Most commonly forms hydrocarbon chains (aliphatic
hydrocarbons) or benzene rings (aromatic hydrocarbons) Additional elements may bond covalently
in place of a carbon atom (N, O, S, P, Si) In place of a hydrogen atom (Cl, F, Br)
Some of these polymers can be recycled through a process called high-temperature cracking
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Chain Polymerization - Polyethylene
Polyethylene Monomer
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Step-Reaction Polymerization Joining of two dissimilar monomers into short groups Pattern increases, usually releasing a low molecular
weight byproduct (for example, water in the case of nylon-6,6)
Such polymers can sometimes be recycled by depolymerization (unless cross-linked)
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Degree of Polymerization The polymers form lengthy chains. The length of these chains has a significant
influence on mechanical properties. Measures of this characteristic include:
Molecular weight – average weight in grams of 1 mole (6.02x1023 molecules)
Degree of Polymerization – average number of mers in a molecule
Typical degrees of polymerization range from about 700 (LDPE) to 170,000 (UHMWPE)
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Linear Polymers (Thermoplastics) “Straight” chains
Not truly straight, since bond angle of C-C bonds is 109.5˚
Chains twist and tangle together like sticky spaghetti
Shorter chains will not develop sufficient order to create crystalline patterns, thus amorphous (simple PE has lengths of only about 18nm)
Long straight chains (HDPE) may allow for more entanglement
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Straight Chain Polymers
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Linear Polymers (Thermoplastics)
Some polymers form pendant groups Polypropylene (PP), for example These pendant groups grow off of the sides of
the backbone of the polymer and increase “tangling”
Such polymers are characterized by the pattern of these pendant groups.
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Pendant-Forming Polymers
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Naming Conventions for Pendant-Forming Polymers
Isotactic – all pendants form on one side of the molecule Can develop highly ordered, compact, crystalline structure Wide use in engineering applications
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Naming Conventions for Pendant-Forming Polymers
Syndiotactic – pendants alternate sides in a pattern
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Naming Conventions for Pendant-Forming Polymers
Atactic – pendants alternate sides randomly Tight packing is not achievable Amorphous Generally poor properties
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Bonding Between Polymer Molecules
Entanglement (mechanical bonding) Adds limited strength
Secondary Bonds Van der Waals (weak) Dipole bonds (polar molecules) Hydrogen bonds (strong)
H with O, N, or F
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Crosslinked Polymers (Thermosets)
Occurs when bonds between molecules are covalent
Polymer becomes “cured” and process cannot be reversed
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Characteristics of Thermosets
Strong High elastic modulus High temperature resistance Relatively brittle Bonds can only be broken by overheating,
and result is burning with carbon residue Scrap cannot be recycled except as filler
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Elastomers
Capable of elastic deformation of 200% or more Thermoset Elastomers – crosslinked amorphous
linear polymers (e.g. natural rubber crosslinked with sulfer – ‘vulcanized’)
Thermoplastic Elastomers – semi-crystalline with glassy regions
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Fillers and Additives
Polymer properties are often enhanced by the addition of other compounds Additives: agents designed to change properties
UV stabilization, flame retardant, plasticizers, dyes, lubricants
Fillers: reinforcing agents Add structural stability in a two-phase structure Effectively a composite material
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Typical Mechanical Characteristics of Polymers Strength
Stress-strain characteristics are widely varied and typically are very sensitive to temperature
Range from pure elastic to nearly perfect-plastic Creep
Polymers are generally susceptible to creep, especially at elevated temperatures
Deflection temperature Residual Stresses
Anisotropy, particularly related to thermal expansion, often leads to residual stress considerations in polymer processing
Rheology Polymers can exhibit a wide range of viscosity behaviors
depending on formulation and applied process
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Polymer RheologyS
hea
r S
tres
s,
.Shear Strain rate,
Newtonian
Bingham
Dilata
nt
Pseudoplastic
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Composites Two or more distinct materials combined
such that the identities and properties of the constituent materials are retained.
Composites are usually “engineered” materials
Utilize materials with materials with complementary properties to compensate for weaknesses individually.
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Matrix Composites Matrix Material
Polymer Metal Ceramic
Embedded Material Particulate Composites Fiber Reinforcement
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Composites that Utilize Deliberate Orientation
Unidirectional composites
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Composites that Utilize Deliberate Orientation
Biaxial Composite Designed to resist
stresses In two axes
Not designed to be strong in the third direction
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Composites that Utilize Deliberate Orientation
Laminate Composites Stacks of planar
material Planar
subcomponents are usually varied in orientation to compensate for directionality.
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Familiar Composites
Fiberboard, OSB, and Plywood Fiberglass Concrete / Steel-reinforced concrete Steel-belted radial tires Carbon-fiber
Bike frames, fishing poles, skis
Rice Krispy Treats