Systems
High Performance
Composites
Ray Loszewski
03/09/05
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Purpose of Presentation
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Overview of boron, carbon, and silicon carbide
fibers, prepregs and composite fabrication
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Differences in fiber structures, how made and used
Performance characteristics; strengths/limitations
Tailored coatings, surface treatments, and sizing
Prepregs, preforms, and composite fabrication
Hybrids; design and synergistic combinations
Aging characteristics and composite repair
Specialized applications; friction, re-entry, and etc.
Important to understand the micromechanics
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Disclaimer/Information Sources
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Requirement to show/discuss only information
or hardware that is in the public domain
All photos/illustrations are from Internet sources
or current owners (Textron originally), e.g.
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Nat'l Academies Press, High Performance Synthetic
Fibers for Composites (1992)
Some information is taken directly from
websites and/or edited to fit slide format, e.g.
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http://www.nap.edu/execsumm/0309043379.html
http://www.specmaterials.com/
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Methods of Reinforcing
Plastics, Metals, and Ceramics
Particulates
Short or long
fibers, flakes, fillers
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Continuous fibers
or monofilaments
Source of sketches: http://www.nd.edu/~manufact/pdfs/Ch09.pdf
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Fiber Types Covered Herein
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Boron (B) and silicon carbide (SiC) fibers are relatively
large diameter (typically 2 – 8 mils) monofilaments
produced by chemical vapor deposition onto a core
material, usually a 0.5 mil tungsten-filament or a 1.3 mil
CMF (carbon monofilament).
Carbon fibers are produced by the pyrolysis of an
organic precursor fiber, such as PAN (polyacrylonitrile),
rayon or pitch, in an inert atmosphere at temperatures
above 982°C/1800°F, typically 1315°C/2400°F, and
contain 93-95% carbon. Carbonized fibers can be
converted to graphite fibers by graphitization at 1900°C
to 2480°C (3450°F to 4500°F) to yield >99% carbon.
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Definitions adapted from: www.compositesworld.com
High-Performance Composites Sourcebook 2004 Glossary
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Fiber Size Comparison Chart
.5 Dia 1.0 Dia
1.3 mil
.28 mil
(7µ)
Carbon Fibers
CVD
Fibers
5.6 mil
4 mil
.47 mil
( 12 µ )
Kevlar Fibers or
Tungsten Filaments
1.3 mil
( 33 µ )
Carbon Monofilaments (CMF)
(Scale 1000/1)
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Fiber Spinning Process Steps
Melt or
Solution
V1>V0
V1
Spinneret
Heat or
Chemical
Treatment
Stretch
(Orient)
and
Solidify
V0
Take-up
or Idler
1st Step
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V2
V2≈V1
Packaging
2nd Step
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Orientation During Spinning
(e.g. Nylon)
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(e.g. Kevlar)
(e.g. Vectran)
(Source: Dupont Kevlar® and Celanese Vectran ® Brochures)
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PAN Based Carbon Fiber Process
Polymerization
Spinning
Precursor
Stabilization
Carbonization
1000-3000°C
Graphitization
Surface Treat
Sizing
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Carbon Fiber
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PAN/Pitch Process Comparison
Polyacrylonitrile
(PAN)
Pitch
Carbon/Graphite
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(Source: A. R. Bunsell, Fibre Reinforcements for Composite Materials,
Amsterdam, The Netherlands: Elsevier Science Publishers B.V., 1988, p. 90.)
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Complete PAN Based Process
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(Source: http://www.harperintl.com/carbon2.htm)
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Carbon Fiber Properties
Treatment Carbon Nitrogen Hydrogen Oxygen
Step (PAN) (wt%)
(wt%)
(wt%)
(wt%)
Untreated
68
26
6
Thermoset
65
22
5
8
Carbonized
>92
<7
<0.3
<1
Graphitized 100
-
Fiber Grade
Modulus
6
GPa
10 psi
Strength
GPa
ksi
High
228-283 33-41 3.45-4.83 500-700
Strength /
Intermediate
Modulus
High
379
55
2.41
350
Modulus
Very High
517
75
2.07
300
Modulus
Ultrahigh
690-827 100-120 2.24-2.41 325-350
Modulus
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(Photo Source: A. R. Bunsell, Fibre Reinforcements for Composite Materials,
Amsterdam, The Netherlands: Elsevier Science Publishers B.V., 1988, p. 203.)
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Carbon Fiber Vs High Tensile Steel
Material
Tensile Strength Tensile Modulus Density Specific Strength
(GPa)
(ksi)
(GPa) 106 psi (g/ccm) (GPa) 106 psi
Standard Grade
3.5
500
230
33
1.75
2
290
Carbon Fiber
High Tensile
1.3
190
210
30
7.87
0.17
25
Steel
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Carbon fibers per se are not very useful
A matrix is needed to transfer load from fiber to fiber
and to hold everything together to form a composite
An oxidative surface treatment is often needed to
provide functionality or attachment points for bonding
A coating or “sizing” protects fiber and facilitates wetting
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Specific Property Comparison*
*Note: composite materials at 60% fiber volume with epoxy
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http://www.advancedcomposites.com/technology.htm#properties
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Kevlar® Fiber Structure
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(Source: Dupont Kevlar® Brochure 12/92)
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Kink Bands and Fibrillation
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Microfibril is the fundamental
building block in highly
oriented, high modulus fibers.
These fibers typically have ten
times weaker compressive
strength than tensile strength.
Local high angle bending or
folding causes compressive
strain and results in local,
microfibrillar misorientation or
kink bands.
Once enough microfibrils are
broken within the kink band,
the entire fiber will fail.
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(Internet Source – Lost Reference)
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Photomicrograph of Kink Band
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(Internet Source – Lost Reference)
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Why Boron or Boron Hybrids?
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Typically, graphite or microfibrillar unidirectional
lamina are compression strength limited
High tensile strength is unavailable when cyclic
loads and stresses limit the strength to the
compression strength allowable
Graphite fiber + Boron fiber are often matched
to yield improved balance between tension and
compression strength and modulus
Increased strength efficiency translates to
weight and cost savings
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Boron Fiber Structure
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The fiber surface is nodular, with nodules oriented
axially along the length. Fiber crystal structure is fine
and complex with crystallite size on the order of 2
nanometers (amorphous).
Large diameter and lack of well-defined crystalline
structure leads to high compression properties.
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Boron Reactor Schematic
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Boron fiber is produced via
CVD using the hydrogen
reduction of boron
trichloride on a tungsten
filament in a glass tube
reactor. The basic
reaction, carried out at
1350°C, is as follows:
2BCl3(g) + 3H2 (g) = 2 B (s)
+ 6HCl
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Boron Filament Production
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CVD Fiber Structural Limitation
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CVD fibers are actually micro-composites
Fiber structure depends on deposition parameters
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Theoretically, mechanical properties are limited by the
strength of the atomic bonds that are involved
Practically, strengths are limited by residual stresses
and structural defects that are built in during CVD
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temperatures, gas composition, flow dynamics, etc.
Residual stresses caused by volume differences in chemical
reaction products, CTE mismatches during cool-down, etc.
Structural defects caused by temperature gradients, power
fluctuations, impurities/inclusions, gas flow instabilities, etc.
Must maintain compressive stresses on fiber surface
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Boron Fiber Properties
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Tensile Strength
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Tensile Modulus
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~1000 ksi (6900 MPa)
Coefficient of Thermal
Expansion
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58 msi (400 GPa)
Compression Strength
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520 ksi (3600 MPa)
2.5 PPM/°F (4.5 PPM/°C)
Density
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0.093 lb/in³ (2.57 g/cm³)
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Tensile Histogram
50
40
30
20
10
0
300
450
600
750
Strengths (ksi)
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Fibers/Monofilaments/Hybrids
Matrix
Kevlar Fibers
0.5 mil dia (12 μ)
4 mil dia (100μ)
0.5 mil dia (12μ)
Matrix
Boron
Tungsten
Carbon Fibers
0.3 mil dia (7 μ)
Void
Conventional
Boron/Graphite
(Carbon) Hybrid
Versus
HyBor®
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Source of Top Photos: http://www.nd.edu/~manufact/pdfs/Ch09.pdf
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Understanding Hy-Bor®
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Hy-Bor® is a mixture of Boron and Graphite
fibers commingled as a single ply
High compression properties of Boron fiber
improve Graphite fiber micro buckling stability
Individually, each material is strain limited by
the fiber properties
Commingled, each fiber contributes and shares
load according to principles of micromechanics
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Hy-Bor® Prepregging Process
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Hy-Bor® Compression Strength
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Compression
Strength of Hy-Bor®
directly relates to
Shear Modulus*
Increasing Boron
fiber count increases
compression strength
towards theoretical
600 ksi limit
* “The Influence of Local Failure Modes on the Compressive Strength of Boron/Epoxy
Composites”, ASTM STP 497, J.A. Suarez, J.B. Whiteside & R.N. Hadcock, 1972
“Influence of Boron Fiber Count on Compressive and Shear Properties of HyBor”, Alliant
Techsystems, J.W. Gillespie,1986
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Benefits of Hy-Bor®
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Provides the Maximum Compression Strength
of any continuous filament-based composite
material
Tailored to meet specific materials properties
and design objectives (Graphite fiber type and
Boron fiber ratio)
Prepregged to customer resin preferences
Analytically treated as another lamina within a
laminate stack per Classical Lamination Theory
Can be mixed with carbon plies or it can be the
total laminate (maximum fiber volume)
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Aging and Composite Repair
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Properties may deteriorate over time by
exposure to high temperatures, moisture, UV
radiation, or other hostile environments
Degradation may be reversible or permanent;
chemical (oxidation) or mechanical (fatigue)
Cracks may be patched using “doublers” or
adhesively bonded reinforced epoxies
Aluminum structures cannot be repaired using
graphite/epoxy due to galvanic corrosion issues
Boron/epoxy doublers gaining acceptance
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Boron Doubler Reinforcement
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Boron Doubler Installation
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SCS Family of SiC Fibers
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Boron was ineffective
in metal matrices
CVD SiC made by
similar process using
less costly gases
SCS offers
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Improved strength at
higher temperatures
Optimized surface for
handling and bonding
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SCS-6 (5.6 mil)
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SCS-9A (3.1 mil)
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Developed for titanium
and ceramics
Developed for thingauge face sheets for
NASP
SCS-ULTRA (5.6 mil)
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Developed to achieve
highest strength
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SCS SiC Fiber Process
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CMF vs. tungsten
Pyrolytic graphite
Complex chemistry
and glassware
High maintenance
Multistage reactor
Integral surface
coating region
Each run optimized
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Construction of SCS Fiber for
Strength and Matrix Compatibility
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Schematic of SCS-6 CVD SiC
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Brittle Fracture Characteristics
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Distribution of strengths
rather than single value
Imperfections lead to
stress concentrations
Fracture initiates
because material cannot
deform plastically
Cracks typically originate
at defects on the core, at
interfaces or the surface
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Comparison of SCS SiC Fibers
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Comparison of SCS SiC Fibers
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SCS-6 Strength Vs. Temperature
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Comparison of Strength Vs.
Temperature for SiC Fibers
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Properties of Ti-6-4 Composites
Composite
Composite*
@ 35 v/o Ultra SCS @ 29 v/o
Ti-6Al-4V
SCS-6™
Strength
120 Ksi
550 Ksi
225-250 Ksi
940 Ksi
318-324 Ksi
Modulus
16 Msi
56 Msi
28-30 Msi
60 Msi
28-29 Msi
Density .16 lb./in.³ .12 lb./in.³ .14 lb./in.³ .12 lb./in.³ .14 lb./in.³
* Similar properties were obtained for Ultra SCS/Ti-22Al-23Nb
for improved oxidation and creep resistance
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Transverse Optical Micrographs
SCS-6/Ti-22Al-27Nb Composite.
Source: Vassel A., Pautonnier F., “Mechanical
Behavior of SiC Monofilaments in
Orthorhombic Titanium Aluminide Composites”,
ICCM, Pékin (Chine), 25-29 June 2001
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Ultra SCS Metal Matrix Composite
Source: Textron Specialty Materials
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Carbon/Carbon Composites
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Unimpressive properties
at ambient but offers
combination of hightemperature resistance
to 2760°C (5000°F), light
weight, and stiffness
Expensive due to difficult
processing, pore closure
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Rapid Densification (RD™)
Applications
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Rocket nozzles, Re-entry
Brake linings, discs, torque
converters (wet friction)
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Carbon/Carbon Process Flow
Curing of polymer or
Carbonization of pitch
under pressure
High char
yield polymer
or pitch
Preform
fabrication
Carbon fiber
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Impregnation
with liquid
polymer or pitch
First
Carbonization
(~1000°C)
Carbonization
1000°C
Intermediate
Graphitization
2500-3000°C
C/C
composite
1000°C
Final
graphitization
2500-3000°C
C/C
composite
2500-3000°C
Impregnation
(CVD or RD)
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Ceramic-Matrix Composites
Major hurdle is to overcome brittleness
 Traditional reinforcements are not very
effective because cracks still propagate
 Conversely, SCS-6 fibers impart strength
and toughness to ceramics because their
carbonaceous surface coating layer
arrests and/or deflects the energy, which
allows for bridging of any cracks
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Applications Drive Technology
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Aerospace/Defense applications emphasize
enabling technologies and performance
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Competition is more effective than consortia
Many promising technologies languish due to
funding cuts or satisfaction with status quo
• e.g. NASP and Superconducting Supercollider
• “chicken/egg” cost dilemma and public apathy
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Commercial applications emphasize availability
and cost, i.e value for the dollar
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Competitive edge and marketability are important
• e.g. Sports equipment, fuel cells, solar, and etc.
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Closing Comments
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Composite design starts with the reinforcement
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Key to solving most problems is knowledge of:
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Fiber choice depends upon the application; must
weigh advantages/disadvantages, cost, etc.
Matrix selection (polymeric, metal, carbon, ceramic)
often dictates fiber type and material form, i.e.
whether to use tow, fabric, tape, and etc.
How fibers are made; why they behave as they do
Role of coatings, surface treatments, and sizing
Reactions at the fiber surface during processing
Focus on the micromechanics at interfaces
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