1. High Performance Composites
Ray Loszewski
Frequently, presenters must deliver material of a technical nature to an audience unfamiliar with the topic or vocabulary. The material may be complex or heavy with detail. To present technical material effectively, use the following guidelines from Dale Carnegie Training®.
Consider the amount of time available and prepare to organize your material. Narrow your topic. Divide your presentation into clear segments. Follow a logical progression. Maintain your focus throughout. Close the presentation with a summary, repetition of the key steps, or a logical conclusion.
Keep your audience in mind at all times. For example, be sure data is clear and information is relevant. Keep the level of detail and vocabulary appropriate for the audience. Use visuals to support key points or steps. Keep alert to the needs of your listeners, and you will have a more receptive audience.
03/09/05

2. 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
In your opening, establish the relevancy of the topic to the audience. Give a brief preview of the presentation and establish value for the listeners. Take into account your audience’s interest and expertise in the topic when choosing your vocabulary, examples, and illustrations. Focus on the importance of the topic to your audience, and you will have more attentive listeners.
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3. 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.
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4. Methods of Reinforcing Plastics, Metals, and Ceramics
Particulates
Short or long fibers, flakes, fillers
Continuous fibers or monofilaments
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Source of sketches:

5. 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 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:
High-Performance Composites Sourcebook 2004 Glossary

6. Fiber Size Comparison Chart
1.3 mil
( 33 µ )
.5 Dia
.47 mil
( 12 µ )
.28 mil
( 7 µ )
1.0 Dia
4 mil
5.6 mil
CVD Fibers
Carbon Fibers
Kevlar Fibers or Tungsten Filaments
Carbon Monofilaments (CMF)
(Scale 1000/1)
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7. Fiber Spinning Process Steps
Melt or Solution
Spinneret
Stretch
(Orient)
and Solidify
Take-up
or Idler V0 V1
V1>V0 V2
V2≈V1
Packaging
Heat or Chemical Treatment
1st Step
2nd Step
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8. Orientation During Spinning
(e.g. Nylon) (e.g. Kevlar) (e.g. Vectran)
(Source: Dupont Kevlar® and Celanese Vectran ® Brochures)
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9. PAN Based Carbon Fiber Process
PAN Based Carbon Fiber Process
Polymerization
Spinning
Precursor
Stabilization
Graphitization
Carbonization
Surface Treat
Sizing
Carbon Fiber °C
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10. 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.)

11. Complete PAN Based Process
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(Source:

12. Carbon Fiber Properties
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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.)

13. 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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14. Specific Property Comparison*
*Note: composite materials at 60% fiber volume with epoxy
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15. Kevlar® Fiber Structure
(Source: Dupont Kevlar® Brochure 12/92)
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16. 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.
(Internet Source – Lost Reference)
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17. Photomicrograph of Kink Band
(Internet Source – Lost Reference)
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18. 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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19. 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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20. 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 1350°C, is as follows:
2BCl3(g) + 3H2 (g) = 2 B (s) HCl
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21. Boron Filament Production
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22. 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/inclusions, gas flow instabilities, etc.
Must maintain compressive stresses on fiber surface
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23. 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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24. Fibers/Monofilaments/Hybrids
4 mil dia (100μ)
0.5 mil dia (12μ)
Matrix
Boron
Tungsten
Kevlar Fibers
0.5 mil dia (12 μ)
Carbon Fibers
0.3 mil dia (7 μ)
Conventional Boron/Graphite (Carbon) Hybrid
HyBor®
Versus
Void
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Source of Top Photos:

25. 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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26. Hy-Bor® Prepregging Process
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27. 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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28. 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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29. 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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30. Boron Doubler Reinforcement
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31. Boron Doubler Installation
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32. 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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33. 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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34. Construction of SCS Fiber for Strength and Matrix Compatibility
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35. Schematic of SCS-6 CVD SiC
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36. 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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37. Comparison of SCS SiC Fibers
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38. Comparison of SCS SiC Fibers
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39. SCS-6 Strength Vs. Temperature
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40. Comparison of Strength Vs. Temperature for SiC Fibers
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41. Properties of Ti-6-4 Composites
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42. Transverse Optical Micrographs
Source: Vassel A., Pautonnier F., “Mechanical Behavior of SiC Monofilaments in Orthorhombic Titanium Aluminide Composites”, ICCM, Pékin (Chine), June 2001
SCS-6/Ti-22Al-27Nb Composite.
Ultra SCS Metal Matrix Composite
Source: Textron Specialty Materials
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43. Carbon/Carbon Composites
Unimpressive properties at ambient but offers combination of high-temperature resistance to 2760°C (5000°F), 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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44. Carbon/Carbon Process Flow
Curing of polymer or Carbonization of pitch under pressure
High char yield polymer or pitch
Impregnation with liquid polymer or pitch
Carbonization 1000°C
Preform fabrication
First Carbonization (~1000°C)
Intermediate Graphitization °C
C/C composite 1000°C
Final graphitization °C
C/C composite °C
Carbon fiber
Impregnation (CVD or RD)
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45. 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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46. 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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47. 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
Determine the best close for your audience and your presentation. Close with a summary; offer options; recommend a strategy; suggest a plan; set a goal. Keep your focus throughout your presentation, and you will more likely achieve your purpose.
03/09/05