Transcript Slide 1

Fundamentals of
Material Properties
- Part 3Non-Metallic Materials for Manufacturing
Darrell Wallace
Youngstown State University
Department of Mechanical and Industrial
Engineering
January 14, 2006
Youngstown State University
2
Non-Metals in Manufacturing

Long History

Organics




Ceramics




Wooden Tools
Textiles
Rope
Pottery
Very Different Properties from Metals
Some Overlap of Processes
Key to many “cutting edge” manufacturing
processes
Youngstown State University
3
Ceramics

What is a ceramic?

Narrow Definition:


A compound composed of both metallic and nonmetallic components
Broader Definition:

Everything that is not a metal or organic and that
is subjected to very high temperature during
manufacture or use.
Youngstown State University
4
Where do we find Ceramics?


Naturally Occuring:

Silica SiO2

Silicates SiO4

Oxides
Man-Made

Carbides

Nitrides
Youngstown State University
5
Bonding and Structure

Ceramic materials are predominantly
bound by covalent and ionic bonds
Youngstown State University
6
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
Youngstown State University
7
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+ClYoungstown State University
8
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”
Youngstown State University
9
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.
Youngstown State University
10
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
Youngstown State University
11
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)
Youngstown State University
12
Polymers and Plastics
From the Greek:
 Polymer:



Plastic:


Poly = many
Meros = parts
Plastikos = able to be molded or formed
Most polymers are based on Carbon chains and
are, therefore, organic compounds.
Youngstown State University
13
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
Youngstown State University
14
Chain Polymerization - Polyethylene
Polyethylene
Monomer
Youngstown State University
15
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)
Youngstown State University
16
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)
Youngstown State University
17
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
Youngstown State University
18
Straight Chain Polymers
Youngstown State University
19
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.
Youngstown State University
20
Pendant-Forming Polymers
Youngstown State University
21
Naming Conventions for PendantForming Polymers

Isotactic – all pendants form on one side of the
molecule


Can develop highly ordered, compact, crystalline structure
Wide use in engineering applications
Youngstown State University
22
Naming Conventions for PendantForming Polymers

Syndiotactic – pendants alternate sides in a
pattern
Youngstown State University
23
Naming Conventions for PendantForming Polymers

Atactic – pendants alternate sides randomly



Tight packing is not achievable
Amorphous
Generally poor properties
Youngstown State University
24
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
Youngstown State University
25
Crosslinked Polymers (Thermosets)


Occurs when bonds between molecules are
covalent
Polymer becomes “cured” and process cannot be
reversed
Youngstown State University
26
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
Youngstown State University
27
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
Youngstown State University
28
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
Youngstown State University
29
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
Youngstown State University
30
Shear Stress, t
Polymer Rheology
.
Shear Strain rate, g
Youngstown State University
31
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.
Youngstown State University
32
Matrix Composites

Matrix Material




Polymer
Metal
Ceramic
Embedded Material


Particulate Composites
Fiber Reinforcement
Youngstown State University
33
Composites that Utilize Deliberate
Orientation

Unidirectional
composites
Youngstown State University
34
Composites that Utilize Deliberate
Orientation

Biaxial
Composite


Designed to resist
stresses In two
axes
Not designed to
be strong in the
third direction
Youngstown State University
35
Composites that Utilize Deliberate
Orientation

Laminate
Composites


Stacks of planar
material
Planar
subcomponents are
usually varied in
orientation to
compensate for
directionality.
Youngstown State University
36
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
Youngstown State University
37