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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

Particulates

Short or long fibers, flakes, fillers

Continuous fibers or monofilaments

Source of sketches: http://www.nd.edu/~manufact/pdfs/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 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.

Definitions adapted from: www.compositesworld.com High-Performance Composites Sourcebook 2004 Glossary

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Fiber Size Comparison Chart

1.3 mil( 33 µ )

.5 Dia

.47 mil( 12 µ )

.28 mil( 7 µ )

1.3 mil1.0 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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Fiber Spinning Process Steps

Melt or Solution

Spinneret

Stretch(Orient)

and Solidify

Take-upor Idler

V0

V1

V1>V0

V2

V2≈V1 Packaging

Heat or Chemical Treatment

1st Step 2nd Step

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(e.g. Nylon) (e.g. Kevlar) (e.g. Vectran)

(Source: Dupont Kevlar® and Celanese Vectran ® Brochures)

Orientation During Spinning

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PAN Based Carbon Fiber Process 

Polymerization

Spinning

Precursor

Stabilization

GraphitizationCarbonization

Surface Treat

Sizing

Carbon Fiber

1000-3000°C

 

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PAN/Pitch Process Comparison

(Source: A. R. Bunsell, Fibre Reinforcements for Composite Materials, Amsterdam, The Netherlands: Elsevier Science Publishers B.V., 1988, p. 90.)

Polyacrylonitrile (PAN)

Pitch

Carbon/Graphite

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Complete PAN Based Process

(Source: http://www.harperintl.com/carbon2.htm)

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Carbon Fiber PropertiesTreatment Step (PAN)

Carbon (wt%)

Nitrogen (wt%)

Hydrogen (wt%)

Oxygen (wt%)

Untreated 68 26 6 -Thermoset 65 22 5 8Carbonized >92 <7 <0.3 <1Graphitized 100 - - -

GPa 106 psi GPa ksi

High Strength / Intermediate Modulus

228-283 33-41 3.45-4.83 500-700

High Modulus

379 55 2.41 350

Very High Modulus

517 75 2.07 300

Ultrahigh Modulus

690-827 100-120 2.24-2.41 325-350

Modulus StrengthFiber Grade

(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 Density

(GPa) (ksi) (GPa) 106 psi (g/ccm) (GPa) 106 psiStandard Grade Carbon Fiber

3.5 500 230 33 1.75 2 290

High Tensile Steel

1.3 190 210 30 7.87 0.17 25

Tensile Strength Tensile Modulus Specific Strength Material

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*

http://www.advancedcomposites.com/technology.htm#properties

*Note: composite materials at 60% fiber volume with epoxy

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(Source: Dupont Kevlar® Brochure 12/92)

Kevlar® Fiber Structure

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(Internet Source – Lost Reference)

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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(Internet Source – Lost Reference)

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 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 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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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)

0

10

20

30

40

50

300 450 600 750

Strengths (ksi)

Tensile Histogram

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Fibers/Monofilaments/Hybrids

4 mil dia (100μ)0.5 mil dia (12μ)MatrixBoronTungsten

MatrixKevlar Fibers0.5 mil dia (12 μ)

Carbon Fibers0.3 mil dia (7 μ)

Conventional Boron/Graphite (Carbon) Hybrid

HyBor®

Versus

Void

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

Ti-6Al-4V SCS-6™Composite @ 35 v/o Ultra SCS

Composite* @ 29 v/o

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

Source: Vassel A., Pautonnier F., “Mechanical Behavior of SiC Monofilaments in Orthorhombic Titanium Aluminide Composites”, ICCM, Pékin (Chine), 25-29 June 2001

SCS-6/Ti-22Al-27Nb Composite. Ultra SCS Metal Matrix Composite

Source: Textron Specialty Materials

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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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Carbon/Carbon Process Flow

High char yield polymer

or pitch

Carbon fiber

Preform fabrication

First Carbonization

(~1000°C)

Impregnation (CVD or RD)

Intermediate Graphitization 2500-3000°C

Carbonization 1000°C

Curing of polymer or Carbonization of pitch

under pressure

Impregnation with liquid

polymer or pitch

Final graphitization 2500-3000°C

C/C composite 1000°C

C/C composite

2500-3000°C

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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