Lecture-Polymeric and Composite materials.ppt

Polymeric and Composite
Materials

POLYMERS AND
COMPOSITE MATERIALS
1. Fundamentals of Polymer Technology
2. Thermoplastic Polymers
3. Thermosetting Polymers
4. Elastomers
5. Composites--Technology and Classification
6. Composite Materials
7. Guide to the Processing of Polymers and Composite
Materials

Polymer
A compound consisting of long-chain molecules, each
molecule made up of repeating units connected
together
 There may be thousands, even millions of units in a
single polymer molecule
 The word polymer is derived from the Greek words
poly, meaning many, and meros (reduced to mer),
meaning part
 Most polymers are based on carbon and are
therefore considered organic chemicals

Types of Polymers
 Polymers can be separated into plastics and rubbers
 As engineering materials, it is appropriate to divide
them into the following three categories:
1. Thermoplastic polymers
2. Thermosetting polymers
3. Elastomers
where (1) and (2) are plastics and (3) are rubbers

Thermoplastic Polymers - Thermoplastics
Solid materials at room temperature but viscous liquids
when heated to temperatures of only a few hundred
degrees
 This characteristic allows them to be easily and
economically shaped into products
 They can be subjected to heating and cooling cycles
repeatedly without significant degradation
 Symbolized by TP

Thermosetting Polymers - Thermosets
 Cannot tolerate repeated heating cycles as
thermoplastics can
 When initially heated, they soften and flow for
molding
 Elevated temperatures also produce a chemical
reaction that hardens the material into an
infusible solid
 If reheated, thermosets degrade and char rather
than soften
 Symbolized by TS

Elastomers (Rubbers)
Polymers that exhibit extreme elastic extensibility when
subjected to relatively low mechanical stress
 Some elastomers can be stretched by a factor of 10
and yet completely recover to their original shape
 Although their properties are quite different from
thermosets, they share a similar molecular structure
that is different from the thermoplastics

Market Shares
 Thermoplastics are commercially the most important
of the three types
 About 70% of the tonnage of all synthetic
polymers produced
 Thermosets and elastomers share the remaining
30%
 On a volumetric basis, the current annual usage of
polymers exceeds that of metals

Examples of Polymers
 Thermoplastics:
 Polyethylene, polyvinylchloride, polypropylene,
polystyrene, and nylon
 Thermosets:
 Phenolics, epoxies, and certain polyesters
 Elastomers:
 Natural rubber (vulcanized)
 Synthetic rubbers, which exceed the tonnage of
natural rubber

Reasons Why Polymers are Important
 Plastics can be molded into intricate part shapes,
usually with no further processing
 Very compatible with net shape processing
 On a volumetric basis, polymers:
 Are cost competitive with metals
 Generally, require less energy to produce than
metals
 Certain plastics are transparent, which makes them
competitive with glass in some applications

General Properties of Polymers
 Low density relative to metals and ceramics
 Good strength-to-weight ratios for certain (but not all)
polymers
 High corrosion resistance
 Low electrical and thermal conductivity

Limitations of Polymers
 Low strength relative to metals and ceramics
 Low modulus of elasticity (stiffness)
 Service temperatures are limited to only a few
hundred degrees
 Viscoelastic properties, which can be a distinct
limitation in load-bearing applications
 Some polymers degrade when subjected to sunlight
and other forms of radiation

Synthesis of Polymers
 Nearly all polymers used in engineering are synthetic
 They are made by chemical processing
 Polymers are synthesized by joining many small
molecules together into very large molecules, called
macromolecules, that possess a chain-like structure
 The small units, called monomers, are generally
simple unsaturated organic molecules such as
ethylene C2H4

Polyethylene
 Synthesis of polyethylene from ethylene monomers:
(1) n ethylene monomers, (2a) polyethylene of chain
length n; (2b) concise notation for depicting polymer
structure of chain length n

Polymerization
 As a chemical process, the synthesis of polymers
can occur by either of two methods:
1. Addition polymerization
2. Step polymerization
 Production of a given polymer is generally
associated with one method or the other

Addition Polymerization
 In this process, the double bonds between carbon
atoms in the ethylene monomers are induced to open
up so they can join with other monomer molecules
 The connections occur on both ends of the
expanding macromolecule, developing long chains of
repeating mers
 It is initiated using a chemical catalyst to open the
carbon double bond in some of the monomers

Addition Polymerization
 Model of addition (chain) polymerization: (1) initiation,
(2) rapid addition of monomers, and (3) resulting long
chain polymer molecule with n mers at termination of
reaction

Step Polymerization
 In this form of polymerization, two reacting monomers
are brought together to form a new molecule of the
desired compound
 As reaction continues, more reactant molecules
combine with the molecules first synthesized to form
polymers of length n = 2, then length n = 3, and so on
 In addition, polymers of length n1 and n2 also
combine to form molecules of length n = n1 + n2, so
that two types of reactions are proceeding
simultaneously

Step Polymerization
 Model of step polymerization showing the two types
of reactions occurring: (left) n-mer attaching a single
monomer to form a (n+1)-mer; and (right) n1-mer
combining with n2-mer to form a (n1+n2)-mer.

Some Examples
 Polymers produced by addition polymerization:
 Polyethylene, polypropylene, polyvinylchloride,
polyisoprene
 Polymers produced by step polymerization:
 Nylon, polycarbonate, phenol formaldehyde

Degree of Polymerization
 Since molecules in a given batch of polymerized
material vary in length, n for the batch is an average
 The mean value of n is called the degree of
polymerization (DP) for the batch
 DP affects the properties of the polymer
 Higher DP increases mechanical strength but also
increases viscosity in the fluid state, which makes
processing more difficult

Molecular Weight
 The sum of the molecular weights of the monomers
in the molecule
 MW = n times the molecular weight of each
repeating unit
 Since n varies for different molecules in a batch,
the molecular weight must be interpreted as an
average

Typical Values of DP and MW
Polymer
Polyethylene
Polyvinylchloride
Nylon
Polycarbonate
DP(n)
10,000
1,500
120
200
MW
300,000
100,000
15,000
40,000

Polymer Molecular Structures
 Linear structure – chain-like structure
 Characteristic of thermoplastic polymers
 Branched structure – chain-like but with side
branches
 Also found in thermoplastic polymers
 Cross-linked structure
 Loosely cross-linked, characteristic of
elastomers
 Tightly cross-linked, characteristic of thermosets

Polymer Molecular Structures
Linear
Branched
Loosely cross-linked Tightly cross-linked

Effect of Branching on Properties
 Thermoplastic polymers always possess linear or
branched structures or a mixture of the two
 Branches increases entanglement among the
molecules, which makes the polymer
 Stronger in the solid state
 More viscous at a given temperature in the
plastic or liquid state

Effect of Cross-Linking on Properties
 Thermosets possess a high degree of cross-linking;
elastomers possess a low degree of cross-linking
 Thermosets are hard and brittle, while elastomers are
elastic and resilient
 Cross-linking causes the polymer to become
chemically set
 The reaction cannot be reversed
 The polymer structure is permanently changed;
if heated, it degrades or burns rather than melt

Crystallinity in Polymers
 Both amorphous and crystalline structures are
possible, although the tendency to crystallize is much
less than for metals or non-glass ceramics
 Not all polymers can form crystals
 For those that can, the degree of crystallinity (the
proportion of crystallized material in the mass) is
always less than 100%

Crystalline Polymer Structure
 Crystallized regions in a polymer: (a) long molecules
forming crystals randomly mixed in with the
amorphous material; and (b) folded chain lamella, the
typical form of a crystallized region

Crystallinity and Properties
 As crystallinity is increased in a polymer
 Density increases
 Stiffness, strength, and toughness increases
 Heat resistance increases
 If the polymer is transparent in the amorphous
state, it becomes opaque when partially
crystallized

Low Density & High-Density Polyethylene
Polyethylene type Low density High density
Degree of crystallinity 55% 92%
Specific gravity 0.92 0.96
Modulus of elasticity 140 MPa
(20,000 lb/in2)
700 MPa
(100,000 lb/in2)
Melting temperature 115C
(239F)
135C
(275F)

Some Observations About Crystallization
 Linear polymers consist of long molecules with
thousands of repeated mers
 Crystallization involves folding back and forth of the
long chains upon themselves
 The crystallized regions are called crystallites
 Crystallites take the form of lamellae randomly mixed in
with amorphous material
 A crystallized polymer is a two-phase system
 Crystallites interspersed in an amorphous matrix

Factors for Crystallization
 Slower cooling promotes crystal formation and
growth
 Mechanical deformation, as in the stretching of a
heated thermoplastic, tends to align the structure and
increase crystallization
 Plasticizers (chemicals added to a polymer to soften
it) reduce crystallinity

Thermal Behavior of Polymers
 Specific volume
(density)-1 as a
function of
temperature

Additives
 Properties of a polymer can often be beneficially
changed by combining it with additives
 Additives either alter the molecular structure or
 Add a second phase, in effect transforming the
polymer into a composite material

Types of Additives by Function
 Fillers – strengthen polymer or reduce cost
 Plasticizers – soften polymer and improve flow
 Colorants – pigments or dyes
 Lubricants – reduce friction and improve flow
 Flame retardents – reduce flammability of polymer
 Cross-linking agents – for thermosets and elastomers
 Ultraviolet light absorbers – reduce degradation from
sunlight
 Antioxidants – reduce oxidation damage

Thermoplastic Polymers (TP)
 Thermoplastic polymers can be heated from solid state
to viscous liquid and then cooled back down to solid
 Heating and cooling can be repeated many times
without degrading the polymer
 Reason: TP polymers consist of linear and/or
branched macromolecules that do not cross-link
upon heating
 Thermosets and elastomers change chemically when
heated, which cross-links their molecules and
permanently sets these polymers

Mechanical Properties of Thermoplastics
 Low modulus of elasticity (stiffness)
 E is much lower than metals and ceramics
 Low tensile strength
 TS is about 10% of metal
 Much lower hardness than metals or ceramics
 Greater ductility on average
 Tremendous range of values, from 1% elongation
for polystyrene to 500% or more for polypropylene

Strength vs. Temperature
 Deformation
resistance
(strength) of
polymers as a
function of
temperature

Physical Properties of Thermoplastics
 Lower densities than metals or ceramics
 Typical specific gravity for polymers are 1.2
(compared to ceramics (~ 2.5) and metals (~ 7)
 Much higher coefficient of thermal expansion
 Roughly five times the value for metals and 10
times the value for ceramics
 Much lower melting temperatures
 Insulating electrical properties

Commercial Thermoplastic
Products and Raw Materials
 Thermoplastic products include
 Molded and extruded items
 Fibers and filaments
 Films and sheets
 Packaging materials
 Paints and varnishes
 Starting plastic materials are normally supplied to the
fabricator in the form of powders or pellets in bags,
drums, or larger loads by truck or rail car

Thermosetting Polymers (TS)
 TS polymers are distinguished by their highly
cross-linked three-dimensional, covalently-bonded
structure
 Chemical reactions associated with cross-linking are
called curing or setting
 In effect, formed part (e.g., pot handle, electrical
switch cover, etc.) becomes a large macromolecule
 Always amorphous and exhibits no glass transition
temperature

General Properties of Thermosets
 Rigid - modulus of elasticity is two to three times
greater than thermoplastics
 Brittle, virtually no ductility
 Less soluble in common solvents than thermoplastics
 Capable of higher service temperatures than
thermoplastics
 Cannot be remelted - instead they degrade or burn

Cross-Linking in TS Polymers
 Three categories:
1. Temperature-activated systems
2. Catalyst-activated systems
3. Mixing-activated systems
 Curing is accomplished at the fabrication plants that
make the parts rather than the chemical plants that
supply the starting materials to the fabricator

Temperature-Activated Systems
Curing caused by heat supplied during part shaping
operation (e.g., molding)
 Starting material is a linear polymer in granular form
supplied by the chemical plant
 As heat is added, material softens for molding,
but continued heating causes cross-linking
 Most common TS systems
 The term “thermoset" applies best to these
polymers

Catalyst-Activated Systems
Cross-linking occurs when small amounts of a catalyst
are added to the polymer, which is in liquid form
 Without the catalyst, the polymer remains stable and
liquid
 Once combined with the catalyst it cures and
changes into solid form

Mixing-Activated Systems
Mixing of two chemicals results in a reaction that forms
a cross-linked solid polymer
 Elevated temperatures are sometimes used to
accelerate the reactions
 Most epoxies are examples of these systems

TS vs. TP Polymers
 TS plastics are not as widely used as the TP
 One reason is the added processing costs and
complications involved in curing
 Largest market share of TS = phenolic resins with 
6% of the total plastics market
 Compare polyethylene with  35% market share
 TS Products: countertops, plywood adhesives,
paints, molded parts, printed circuit boards and other
fiber reinforced plastics

Elastomers
Polymers capable of large elastic deformation when
subjected to relatively low stresses
 Some can be extended 500% or more and still
return to their original shape
 Two categories:
1. Natural rubber - derived from biological plants
2. Synthetic polymers - produced by
polymerization processes like those used for
thermoplastic and thermosetting polymers

Characteristics of Elastomers
 Elastomers consist of long-chain molecules that are
cross-linked (like thermosetting polymers)
 They owe their impressive elastic properties to two
features:
1. Molecules are tightly kinked when unstretched
2. Degree of cross-linking is substantially less
than thermosets

Elastomer Molecules
 Model of long elastomer molecules, with low degree
of cross-linking: (left) unstretched, and (right) under
tensile stress

Elastic Behavior of Elastomer Molecule
 When stretched, the molecules are forced to uncoil
and straighten
 Natural resistance to uncoiling provides the initial
elastic modulus of the aggregate material
 Under further strain, the covalent bonds of the
cross-linked molecules begin to play an increasing
role in the modulus, and stiffness increases
 With greater cross-linking, the elastomer becomes
stiffer, and its modulus of elasticity is more linear

Stiffness of Rubber
 Increase in stiffness as a function of strain for three
grades of rubber: natural rubber, vulcanized rubber,
and hard rubber

Vulcanization
Curing to cross-link most elastomers
 Vulcanization = the term for curing in the context of
natural rubber (and certain synthetic rubbers)
 Typical cross-linking in rubber is one to ten links per
hundred carbon atoms in the linear polymer chain,
depending on degree of stiffness desired
 Considerably less than cross-linking in
thermosets

Natural Rubber (NR)
 NR = polyisoprene, a high molecular-weight polymer
of isoprene (C5H8)
 It is derived from latex, a milky substance produced
by various plants, most important of which is the
rubber tree that grows in tropical climates
 Latex is a water emulsion of polyisoprene (about 1/3
by weight), plus various other ingredients
 Rubber is extracted from latex by various methods
that remove the water

Vulcanized Natural Rubber
 Properties: High tensile strength, tear strength,
resilience (capacity to recover shape), and resistance
to wear and fatigue
 Weaknesses: degrades when subjected to heat,
sunlight, oxygen, ozone, and oil
 Some of these limitations can be reduced by
additives
 Market share of NR  22% of total rubber volume
(natural plus synthetic)

Natural Rubber Products
 Largest single market for NR is automotive tires
 Other products: shoe soles, bushings, seals, and
shock absorbing components
 In tires, carbon black is an important additive
 It reinforces the rubber, serving to increase tensile
strength and resistance to tear and abrasion
 Other additives: clay, kaolin, silica, talc, and calcium
carbonate, as well as chemicals that accelerate and
promote vulcanization

Synthetic Rubbers
 Development of synthetic rubbers was motivated
largely by world wars when NR was difficult to obtain
 Tonnage of synthetic rubbers is now more than three
times that of NR
 The most important synthetic rubber is
styrene-butadiene rubber (SBR), a copolymer of
butadiene (C4H6) and styrene (C8H8)
 As with most other polymers, the main raw material
for synthetic rubbers is petroleum

Thermoplastic Elastomers (TPE)
A thermoplastic that behaves like an elastomer
 Elastomeric properties not from chemical cross-links,
but from physical connections between soft and hard
phases in the material
 Cannot match conventional elastomers in elevated
temperature, strength and creep resistance
 Products: footwear; rubber bands; extruded tubing,
wire coating; molded automotive parts, but no tires

COMPOSITE MATERIALS
1. Technology and Classification of Composite
Materials
2. Metal Matrix Composites
3. Ceramic Matrix Composites
4. Polymer Matrix Composites
5. Guide to Processing Composite Materials

Composite Material Defined
A materials system composed of two or more distinct
phases whose combination produces aggregate
properties different from those of its constituents
 Examples:
 Cemented carbides
 Plastic molding compounds with fillers
 Rubber mixed with carbon black
 Wood (a natural composite as distinguished from
a synthesized composite)

Why Composites are Important
 Composites can be very strong and stiff, yet very light in
weight
 Strength-to-weight and stiffness-to-weight ratios
are several times greater than steel or aluminum
 Fatigue properties are generally better than for common
engineering metals
 Toughness is often greater
 Possible to achieve combinations of properties not
attainable with metals, ceramics, or polymers alone

Disadvantages and Limitations
 Properties of many important composites are
anisotropic
 May be an advantage or a disadvantage
 Many polymer-based composites are subject to attack
by chemicals or solvents
 Just as the polymers themselves are susceptible
 Composite materials are generally expensive
 Manufacturing methods for shaping composite materials
are often slow and costly

Possible Classification of Composites
1. Traditional composites – composite materials that
occur in nature or have been produced by
civilizations for many years
 Examples: wood, concrete, asphalt
2. Synthetic composites - modern material systems
normally associated with the manufacturing
industries
 Components are first produced separately and
then combined to achieve the desired
structure, properties, and part geometry

Components in a Composite Material
Most composite materials consist of two phases:
1. Primary phase - forms the matrix within which the
secondary phase is imbedded
2. Secondary phase - imbedded phase sometimes
referred to as a reinforcing agent, because it usually
strengthens the composite material
 The reinforcing phase may be in the form of
fibers, particles, or various other geometries

Classification of Composite Materials
1. Metal Matrix Composites (MMCs) - mixtures of
ceramics and metals, such as cemented carbides
2. Ceramic Matrix Composites (CMCs) - Al2O3 and SiC
imbedded with fibers to improve properties
3. Polymer Matrix Composites (PMCs) - polymer resins
imbedded with filler or reinforcing agent
 Examples: epoxy and polyester with fiber
reinforcement, and phenolic with powders

Functions of the Matrix Material
 Primary phase provides the bulk form of the part or
product made of the composite material
 Holds the imbedded phase in place, usually
enclosing and often concealing it
 When a load is applied, the matrix shares the load
with the secondary phase, in some cases deforming
so that the stress is essentially born by the
reinforcing agent

Reinforcing Phase
 Function is to reinforce the primary phase
 Reinforcing phase (imbedded in the matrix) is most
commonly one of the following shapes: fibers,
particles, or flakes

Physical Shapes of Imbedded Phase
Possible physical shapes of imbedded phases in
composite materials: (a) fiber, (b) particle, and (c)
flake

Fibers
Filaments of reinforcing material, usually circular in
cross section
 Diameters from ~ 0.0025 mm to about 0.13 mm
 Filaments provide greatest opportunity for strength
enhancement of composites
 Filament form of most materials is significantly
stronger than the bulk form
 As diameter is reduced, the material becomes
oriented in the fiber axis direction and probability
of defects in the structure decreases significantly

Continuous Fibers vs.
Discontinuous Fibers
 Continuous fibers - very long; in theory, they offer a
continuous path by which a load can be carried by
the composite part
 Discontinuous fibers (chopped sections of continuous
fibers) - short lengths (L/D = roughly 100)
 Whiskers = discontinuous fibers of hair-like
single crystals with diameters down to about
0.001 mm (0.00004 in) and very high strength

Fiber Orientation – Three Cases
 One-dimensional reinforcement, in which maximum
strength and stiffness are obtained in the direction of
the fiber
 Planar reinforcement, in some cases in the form of a
two-dimensional woven fabric
 Random or three-dimensional in which the composite
material tends to possess isotropic properties

Fiber Orientation
Fiber orientation in composite materials: (a)
one-dimensional, continuous fibers; (b) planar,
continuous fibers in the form of a woven fabric; and (c)
random, discontinuous fibers

Materials for Fibers
 Fiber materials in fiber-reinforced composites
 Glass – most widely used filament
 Carbon – high elastic modulus
 Boron – very high elastic modulus
 Polymers - Kevlar
 Ceramics – SiC and Al2O3
 Metals - steel
 Most important commercial use of fibers is in polymer
composites

Particles and Flakes
 A second common shape of imbedded phase is
particulate, ranging in size from microscopic to
macroscopic
 Flakes are basically two-dimensional
particles - small flat platelets
 Distribution of particles in the matrix is random
 Strength and other properties of the composite
material are usually isotropic

Interface between Constituent Phases in
Composite Material
 For the composite to function, the phases must bond
where they join at the interface
 Direct bonding between primary and secondary phases

Interphase
 In some cases, a third ingredient must be added to
bond primary and secondary phases
 Called an interphase, it is like an adhesive

Alternative Interphase Form
Formation of an interphase consisting of a solution of
primary and secondary phases at their boundary

Properties of
Composite Materials
 In selecting a composite material, an optimum
combination of properties is often sought, rather than
one particular property
 Example: fuselage and wings of an aircraft must
be lightweight, strong, stiff, and tough
 Several fiber-reinforced polymers possess
these properties
 Example: natural rubber alone is relatively weak
 Adding carbon black increases its strength

Three Factors that Determine Properties
1. Materials used as component phases in the
composite
2. Geometric shapes of the constituents and resulting
structure of the composite system
3. How the phases interact with one another

Example: Fiber Reinforced Polymer
 Model of fiber-reinforced
composite material
showing direction in
which elastic modulus is
being estimated by the
rule of mixtures

Example: Fiber Reinforced Polymer
(continued)
 Stress-strain relationships
for the composite material
and its constituents
 The fiber is stiff but brittle,
while the matrix
(commonly a polymer) is
soft but ductile

Variations in Strength and Stiffness
Variation in elastic modulus and tensile strength as
function of direction relative to longitudinal axis of
carbon fiber-reinforced epoxy composite

Importance of Geometric Shape: Fibers
 Most materials have tensile strengths several times
greater as fibers than as bulk materials
 By imbedding the fibers in a polymer matrix, a
composite material is obtained that avoids the
problems of fibers but utilizes their strengths
 Matrix provides the bulk shape to protect the fiber
surfaces and resist buckling
 When a load is applied, the low-strength matrix
deforms and distributes the stress to the
high-strength fibers

Other Composite Structures
 Laminar composite structure – conventional
 Sandwich structure
 Honeycomb sandwich structure

Laminar Composite Structure
 Conventional laminar
structure - two or more
layers bonded together
in an integral piece
 Example: plywood, in
which layers are the
same wood, but grains
oriented differently to
increase overall strength

Sandwich Structure: Foam Core
 Relatively thick core of
low-density foam
bonded on both faces to
thin sheets of a different
material

Sandwich Structure:
Honeycomb Core
 Alternative to foam
core
 Foam or
honeycomb achieve
high ratios of
strength-to-weight
and
stiffness-to-weight

Other Laminar Composite Structures
 FRPs - multi-layered, fiber-reinforced plastic panels for
aircraft, boat hulls, other products
 Printed circuit boards - layers of reinforced copper and
plastic for electrical conductivity and insulation,
respectively
 Snow skis - layers of metals, particle board, and
phenolic plastic
 Windshield glass - two layers of glass on either side of
a sheet of tough plastic

Metal Matrix Composites (MMCs)
Metal matrix reinforced by a second phase
 Reinforcing phases:
1. Particles of ceramic
 These MMCs are commonly called cermets
2. Fibers of various materials
 Other metals, ceramics, carbon, and boron

Cermets
MMC with ceramic contained in a metallic matrix
 The ceramic often dominates the mixture, sometimes
up to 96% by volume
 Bonding can be enhanced by slight solubility between
phases at elevated temperatures used in processing
 Cermets can be subdivided into
1. Cemented carbides – most common
2. Oxide-based cermets – less common

Cemented Carbides
One or more carbide compounds bonded in a metallic
matrix
 Common cemented carbides are based on tungsten
carbide (WC), titanium carbide (TiC), and chromium
carbide (Cr3C2)
 Tantalum carbide (TaC) and others are less
common
 Metallic binders: usually cobalt (Co) or nickel (Ni)

 Photomicrograph (about 1500X) of cemented carbide
with 85% WC and 15% Co (photo courtesty of
Kennametal Inc.)
Cemented Carbide

 Typical plot of
hardness and
transverse
rupture strength
as a function of
cobalt content
Cemented Carbide Properties

Applications of
Cemented Carbides
 Tungsten carbide cermets (Co binder)
 Cutting tools, wire drawing dies, rock drilling bits,
powder metal dies, indenters for hardness testers
 Titanium carbide cermets (Ni binder)
 Cutting tools; high temperature applications such as
gas-turbine nozzle vanes
 Chromium carbide cermets (Ni binder)
 Gage blocks, valve liners, spray nozzles

Ceramic Matrix Composites (CMCs)
Ceramic primary phase imbedded with a secondary
phase, usually consisting of fibers
 Attractive properties of ceramics: high stiffness,
hardness, hot hardness, and compressive strength;
and relatively low density
 Weaknesses of ceramics: low toughness and bulk
tensile strength, susceptibility to thermal cracking
 CMCs represent an attempt to retain the desirable
properties of ceramics while compensating for their
weaknesses

Ceramic Matrix Composite
 Photomicrograph (about 3000X) of fracture surface of
SiC whisker reinforced Al2O3 (photo courtesy of
Greenleaf Corp.)

Polymer Matrix Composites (PMCs)
Polymer primary phase in which a secondary phase is
imbedded as fibers, particles, or flakes
 Commercially, PMCs are more important than MMCs
or CMCs
 Examples: most plastic molding compounds,
rubber reinforced with carbon black, and
fiber-reinforced polymers (FRPs)

Fiber-Reinforced Polymers (FRPs)
PMC consisting of a polymer matrix imbedded with
high-strength fibers
 Polymer matrix materials:
 Usually, a thermosetting plastic such as
unsaturated polyester or epoxy
 Can also be thermoplastic, such as nylons
(polyamides), polycarbonate, polystyrene, and
polyvinylchloride
 Fiber reinforcement is widely used in rubber
products such as tires and conveyor belts

Fibers in PMCs
 Various forms: discontinuous (chopped), continuous,
or woven as a fabric
 Principal fiber materials in FRPs are glass, carbon,
and Kevlar 49
 Less common fibers include boron, SiC, and
Al2O3, and steel
 Glass (in particular E-glass) is the most common fiber
material in today's FRPs
 Its use to reinforce plastics dates from around
1920

Common FRP Structures
 Most widely used form of FRP is a laminar structure
 Made by stacking and bonding thin layers of fiber
and polymer until desired thickness is obtained
 By varying fiber orientation among layers, a
specified level of anisotropy in properties can be
achieved in the laminate
 Applications: boat hulls, aircraft wing and fuselage
sections, automobile and truck body panels

FRP Properties
 High strength-to-weight and modulus-to-weight ratios
 A typical FRP weighs only about 1/5 as much as
steel
 Yet strength and modulus are comparable in fiber
direction
 Good fatigue strength
 Good corrosion resistance, although polymers are
soluble in various chemicals
 Low thermal expansion for many FRPs

FRP Applications
 Aerospace – much of the structural weight of today’s
airplanes and helicopters consist of advanced FRPs
 Example: Boeing 787
 Automotive – some body panels for cars and truck cabs
 Low-carbon sheet steel still widely used due to its
low cost and ease of processing
 Sports and recreation
 FRPs used for boat hulls since 1940s
 Fishing rods, tennis rackets, golf club shafts,
helmets, skis, bows and arrows

Other Polymer Matrix Composites
 Other PMCs contain particles, flakes, and short fibers
 Called fillers when used in molding compounds
 Two categories:
1. Reinforcing fillers – used to strengthen or
otherwise improve mechanical properties
2. Extenders – used to increase bulk strength and
reduce cost per unit weight, with little or no effect
on mechanical properties

Guide to Processing Composite Materials
 The two phases are typically produced separately
before being combined into the composite part
 Processing techniques to fabricate MMC and
CMC components are similar to those used for
powdered metals and ceramics
 Molding processes are commonly used for
PMCs with particles and chopped fibers
 Specialized processes have been developed for
FRPs

Guide to the
Processing of Polymers
 Polymers are nearly always shaped in a heated,
highly plastic state
 Common operations are extrusion and molding
 Molding of thermosets is more complicated because
of cross-linking
 Thermoplastics are easier to mold, and a greater
variety of molding operations are available
 Rubber processing has a longer history than plastics,
and rubber industries are traditionally separated from
plastics industry, even though processing is similar

Lecture-Polymeric and Composite materials.ppt

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Lecture-Polymeric and Composite materials.ppt