High Performance Composites

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High Performance Composites

• Ray Loszewski

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Purpose of Presentation

• Overview of boron, carbon, and silicon carbide
  fibers, prepregs and composite fabrication
• 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

• 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.
• 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.
• http//www.nap.edu/execsumm/0309043379.html
• http//www.specmaterials.com/

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Methods of Reinforcing Plastics, Metals, and
Ceramics
Source of sketches http//www.nd.edu/manufact/pd
fs/Ch09.pdf
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Fiber Types Covered Herein

• 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 982C/1800F,
  typically 1315C/2400F, and contain 93-95
  carbon. Carbonized fibers can be converted to
  graphite fibers by graphitization at 1900C to
  2480C (3450F to 4500F) to yield gt99 carbon.

Definitions adapted from www.compositesworld.com
High-Performance Composites Sourcebook 2004
Glossary
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Fiber Size Comparison Chart
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Fiber Spinning Process Steps
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Orientation During Spinning
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PAN Based Carbon Fiber Process
 
 
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PAN/Pitch Process Comparison
Carbon/Graphite
(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
(Source http//www.harperintl.com/carbon2.htm)
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Carbon Fiber Properties
(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

• 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
http//www.advancedcomposites.com/technology.htmp
roperties
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Kevlar Fiber Structure
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Kink Bands and Fibrillation

• 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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Photomicrograph of Kink Band
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Why Boron or Boron Hybrids?

• 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

• 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

• 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 1350C, 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

• CVD fibers are actually micro-composites
• Fiber structure depends on deposition parameters
• temperatures, gas composition, flow dynamics,
  etc.
• 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
• 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/inclusio
  ns, gas flow instabilities, etc.
• Must maintain compressive stresses on fiber
  surface

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Boron Fiber Properties

• Tensile Strength
• 520 ksi (3600 MPa)
• Tensile Modulus
• 58 msi (400 GPa)
• Compression Strength
• 1000 ksi (6900 MPa)
• Coefficient of Thermal Expansion
• 2.5 PPM/F (4.5 PPM/C)
• Density
• 0.093 lb/in³ (2.57 g/cm³)

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Fibers/Monofilaments/Hybrids
Source of Top Photos http//www.nd.edu/manufact/
pdfs/Ch09.pdf
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Understanding Hy-Bor

• 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

• 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

• 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

• 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

• Boron was ineffective in metal matrices
• CVD SiC made by similar process using less costly
  gases
• SCS offers
• Improved strength at higher temperatures
• Optimized surface for handling and bonding

• SCS-6 (5.6 mil)
• Developed for titanium and ceramics
• SCS-9A (3.1 mil)
• Developed for thin-gauge face sheets for NASP
• SCS-ULTRA (5.6 mil)
• Developed to achieve highest strength

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SCS SiC Fiber Process

• 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

• 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
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Transverse Optical Micrographs
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Carbon/Carbon Composites

• Unimpressive properties at ambient but offers
  combination of high-temperature resistance to
  2760C (5000F), light weight, and stiffness
• Expensive due to difficult processing, pore
  closure
• Rapid Densification (RD)
• Applications
• 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
Carbonization 1000C
Impregnation with liquid polymer or pitch
High char yield polymer or pitch
First Carbonization (1000C)
Final graphitization 2500-3000C
C/C composite 2500-3000C
Preform fabrication
Intermediate Graphitization 2500-3000C
C/C composite 1000C
Impregnation (CVD or RD)
Carbon fiber
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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

• Aerospace/Defense applications emphasize enabling
  technologies and performance
• 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
• Commercial applications emphasize availability
  and cost, i.e value for the dollar
• Competitive edge and marketability are important
• e.g. Sports equipment, fuel cells, solar, and
  etc.

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Closing Comments

• Composite design starts with the reinforcement
• 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.
• Key to solving most problems is knowledge of
• 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