3 properties of carbon fibre - smithersrapra.com · based on carbon fibre properties...

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3 PropertiesofCarbonFibre

Carbon fibre(s) (CF) possesses a unique combination of properties (Table 3.1).

Table 3.1 Properties of CF

High strength-to-weight ratio

Good rigidity

Resistant to corrosion

Conducts electricity

Resistant to fatigue

Good tensile strength but brittle

Fire resistance/not flammable

High thermal conductivity

Low coefficient of thermal expansion and low abrasion

Non-poisonous

Biologically inert and permeable to X-rays

Self-lubricating

Excellent shielding against electromagnetic interference

Relatively expensive

Requires specialised experience and equipment for use

High damping

Electromagnetic properties

Adapted from [4, 5]

CF are the stiffest and strongest reinforcing fibres for polymer composites, the most used after glass fibres [1-7]. They have low density and a negative coefficient of longitudinal thermal expansion.

42

Update on Carbon Fibre

CF has a high modulus of elasticity that results from the fact that the carbon layers tend to be parallel to the fibre axis [2, 4, 5]. Fibre ‘texture’ is a term applied to this preferred orientation the crystal structure. The modulus of elasticity of CF is higher parallel to the fibre axis than perpendicular to the axis. The stronger the ‘fibre texture’, the greater the degree of alignment of the carbon layer parallel to the fibre axis. CF with high fibre texture has high strength and high tensile energy absorption [2, 4]. The tensile energy absorption refers to energy stored in the fibre when the fibre is under tension with force and undergoes all extension or changes in length

CF are very expensive and can give galvanic corrosion in contact with metals [2, 4, 7]. They are generally used together with epoxy, where high strength and stiffness are required, i.e., race cars, automotive and space applications, sport equipment. Depending on the orientation of the fibre, the CF composite can be stronger in a certain direction or equally strong in all directions. A small piece can withstand an impact of many tons and still deform minimally. The properties of a CF part are close to that of steel and the weight is close to that of plastic. Thus the strength to weight ratio and stiffness to weight ratio of a CF part is much higher than either steel or plastic. The specific details depend on the matter of construction of the part and the application. For instance, a foam-core sandwich has extremely high strength to weight ratio in bending, but not necessarily in compression or crush. In addition, the loading and boundary conditions for any components are unique to the structure within which they reside. The modulus of CF is typically 20 msi and its ultimate tensile strength is typically 500 ksi. High stiffness and strength CF materials are also available through specialised heat treatment processes with much higher values. In comparison with 2024-T3 Aluminum, which has a modulus of only 10 msi and ultimate tensile strength of 65 ksi, and 4130 Steel, which has a modulus of 30 msi and ultimate tensile strength of 125 ksi.

CF can be classified based on CF properties; precursor fibre materials and final heat treatment temperature (Tables 3.2 and 3.3).

43

Properties of Carbon Fibre

Tab

le 3

.2 C

lass

ifica

tion

of

CF

Cla

ssifi

cati

onT

ype

Bas

ed o

n ca

rbon

fibr

e pr

oper

ties

Ult

ra-h

igh-

mod

ulus

(U

HM

)

Hig

h-m

odul

us (

HM

)

Inte

rmed

iate

-mod

ulus

(IM

)

Low

mod

ulus

and

hig

hly

tens

ile

Supe

r hi

gh-t

ensi

le

Bas

ed o

n pr

ecur

sor

fibre

m

ater

ials

Poly

acry

loni

trile

(PA

N)-

base

d C

F

Pitc

h-ba

sed

CF

Mes

opha

se p

itch

-bas

ed C

F

Isot

ropi

c pi

tch-

base

d C

F

Ray

on-b

ased

CF

Gas

-pha

se-g

row

n C

F

Bas

ed o

n fin

al h

eat

trea

tmen

t te

mpe

ratu

reH

igh-

heat

-tre

atm

ent

CF,

whe

re fi

nal h

eat

trea

tmen

t te

mpe

ratu

re s

houl

d be

>2,

000

°C

and

can

be a

ssoc

iate

d w

ith

high

-mod

ulus

-typ

e fib

re.

Inte

rmed

iate

-hea

t-tr

eatm

ent

CF,

whe

re t

he fi

nal h

eat

trea

tmen

t te

mpe

ratu

re s

houl

d be

ar

ound

or

>1,5

00 °

C a

nd c

an b

e as

soci

ated

wit

h hi

gh-s

tren

gth-

type

fibr

e.

Low

-hea

t-tr

eatm

ent

carb

on fi

bres

, whe

re t

he fi

nal h

eat

trea

tmen

t te

mpe

ratu

res

is

≤1,

000

°C. T

hese

are

low

-mod

ulus

and

low

-str

engt

h m

ater

ials

.

Ada

pted

fro

m [

4, 5

]

44


Table 3.3 Mechanical properties of different types of CF

Type of fibres Tensile strength (GPa) Young modulus (GPa)

High strength 3.3–6.9 200–250

IM 4.0–5.8 280–300

HM 3.8–4.5 350–600

UHM 2.4–3.8 600–960

Adapted from [4, 5, 7]

Most common uses for CF are in applications where high strength to weight and high stiffness to weight are desirable [2, 4, 5]. These include aerospace, military structures, robotics, wind turbines, manufacturing fixtures, sports equipment, and many others. High toughness can be accomplished when combined with other materials. Certain applications also exploit CF electrical conductivity, as well as high thermal conductivity in the case of specialised CF. Finally, in addition to the basic mechanical properties, CF creates a unique and beautiful surface finish. Although CF has many important benefits over other materials, there are also tradeoffs one must weigh against. First, solid CF will not yield. Under load CF bends but will not remain permanently deformed. Instead, once the ultimate strength of the material is exceeded, CF will fail suddenly and catastrophically. In the design process it is critical that the engineer understand and account for this behavior, particularly in terms of design safety factors. CF composites are also significantly more expensive than traditional materials. Working with CF requires a high skill level and many intricate processes to produce high quality building materials (for example, solid carbon sheets, sandwich laminates, tubes, and so on). Very high skill level and specialised tooling and machinery are required to create custom-fabricated, highly optimised parts and assemblies.

45


When designing composite parts, one cannot simply compare properties of CF versus steel, aluminum, or plastic, since these materials are in general homogeneous (properties are the same at all points in the part), and have isotropic properties throughout (properties are the same along all axes). By comparison, in a CF part the strength resides along the axis of the fibres, and thus fibre properties and orientation greatly impact mechanical properties [8-16]. CF parts are in general neither homogeneous nor isotropic [2, 4].

CF typically exhibit a skin-core texture that has been confirmed using optical microscopy [2, 5, 17, 18]. The skin can result from higher preferred orientation and a higher density of material at the fibre surface. The formation of the skin is also associated with the coagulation conditions during PAN precursor fibre spinning. The fine structure of CF consists of basic structural units of turbostratic carbon planes. The distance between turbostratic planes and perfect graphite planes is generally >0.34 nm, 0.3345 nm respectively [19, 20]. Typical structural parameters for the selected pitch and PAN based CF are given in Table 3.4. The crystallite size in the high-modulus pitch-based fibres is as high as 25 nm along the c-axis direction and is 64 nm along the a-axis parallel to the fibre axis and 88 nm along the a-axis perpendicular to the fibre axis. Crystallite dimensions in fibres such as K-1100 are expected to be even larger.

46


Tab

le 3

.4 T

ypic

al s

truc

tura

l par

amet

ers

for

sele

cted

pit

ch-

and

PAN

-bas

ed C

F

Srtu

ctur

al p

aram

eter

p-25

P-55

P-10

0P-

120

T-3

00IM

-8

Cry

stal

siz

e pa

ralle

l to

c-ax

is2.

6 12

.4

22.7

25

.1

1.5

8

Cry

stal

siz

e pa

ralle

l to

a-ax

is a

nd p

erpe

ndic

ular

to

the

fibr

e ax

is d

irec

tion

411

4964

2.2

1.9

Cry

stal

siz

e pa

ralle

l to

a-ax

is a

nd p

aral

lel t

o th

e fib

re a

xis

dire

ctio

n6

3080

884.

13.

1

Ori

enta

tion

par

amet

er, f

ull-

wid

th-a

t-ha

lf-

max

imum

of

the

(002

) az

imut

hal s

can

in d

egre

es31

.914

.15.

65.

635

.15.

1

d(00

2)0.

344

0.34

20.

3382

0.33

760.

342

0.34

3

Scan

ning

ele

ctro

n m

icro

scop

y (S

EM

) m

orph

olog

ySh

eet-

like

Shee

t-lik

eSh

eet-

like

Shee

t-lik

eN

oN

o

Ada

pted

fro

m [

5, 6

]

47


The crystallite size in the PAN-based CF (T-300 and IM-8) is in the 1.5-5 nm range. HM pitch-based CF exhibit high orientation (Z ¼ 5.6), whereas the orientation of the pitch-based CF is relatively low (Z ¼ 35.1). HM pitch-based CF (P-100 and P-120) also exhibit graphitic sheet-like morphology from SEM and, as well as clear evidence of the three-dimensional order from X-ray diffraction [21]. Due to the formation of microdomains, which can bend and twist, CF contain defects, vacancies, dislocations, grain boundaries, and impurities [17]. Low interlayer spacing, large crystallite size, high degree of orientation parallel to the fibre axis, low density of defects, and high degree of crystallinity are characteristics of the high tensile modulus and high thermal and high electrical conductivity fibres. Porosity in CF is measured using SAXS [22], and this data can be used to estimate the size, shape, and orientation of the pores. Pore size, pore size distribution, and pore orientation change as the fibre undergoes increasing heat-treatment and tension.

CF properties are related to the fibre microstructure and morphology. Properties of some commercial CF are listed in Table 3.5.

Table 3.5 Properties of commercial CF

Tensile strength (GPa)

Tensile modulus

(GPa)

Elongation to break

(%)

Density, r (g/cm3)

Thermal conductivity

(W/mK)

Electrical conductivity

(S/m)

Toray Torayca1 PAN-base

T300 3.53 230 1.51 1.76

T700SC 4.90 230 2.1 1.80

M35JB 4.70 343 1.4 1.75

M50JB 4.12 475 0.9 1.88

M55J 4.02 540 0.8 1.91

M30SC 5.49 294 1.9 1.73

Cytec Thomel1 PAN-based

T300 3.75 231 1.4 1.76 8 5.56Eþ04

T650/35 4.28 255 1.7 1.77 14 6.67Eþ04

T300C 3.75 231 1.4 1.76 8 5.56Eþ04

Adapted from [4, 5]

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The axial compressive strength of PAN-based CF is higher than those of the pitch-based fibres and it decreases with increasing modulus in both cases. It is understood that higher orientation, higher graphitic order, and larger crystal size all contribute negatively to the compressive strength. PAN-based CF typically fail in the buckling mode, whereas pitch based fibres fail by shearing mechanisms [23]. This suggests that the compressive strength of intermediate modulus PAN-based CF may be higher than what is being realised in the composites. Changes in the fibre geometry, effective fibre aspect ratio, fibre/matrix interfacial strength, as well as matrix stiffness can result in fibre compressive strength increase, until the failure mode changes from buckling to shear. High compressive strength fibres also exhibit high shear modulus [21]. Compressive strength dependence of pitch- and PAN-based CF on various structural parameters has been studied [21] and the compressive strength of high-performance fibres as well as compression test methods have been reviewed [24]. The electrical and thermal conductivities increase with increasing fibre modulus and carbonisation temperature [25, 26]. The electrical conductivity of PAN-based CF is in the range of 104-105 S/m, whereas that of the pitch-based CF is in the range of 105-106 S/m. The electrical conductivity increases with temperature because as the temperature is raised, the density and carrier (electrons and holes) mobility increases. Defects are known to cause carrier scattering. An increase in modulus is due to increased orientation of the carbon planes; this decreases the concentration of defects and subsequently decreases carrier scattering. The thermal conductivity of pitch-based CF is in the range of 20–1000 W/mK. CF resistance to oxidation increases with the degree of graphitisation. For CF, thermal gravimetric analysis in air shows the initial weight loss above 400 oC, sharp weight loss in the 500–600 oC range, and total weight loss by 850 oC. Axial coefficient of thermal expansion of the 200-300 GPa modulus CF is in the range of 0.4 to 0.8 × 106/C. For the HM (700-900 GPa) CF, it is about 1.6 × 106/C [5, 6].

CF reinforced composites can be used in the design of advanced materials and systems. The properties of the fibre-reinforced plastic articles are governed mainly by the properties of the fibre,

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in particular the CF, and the form of textile into which the fibre is processed. Pre-impregnated materials (prepregs) offer a precise and economical way of combining reinforcements with a resin matrix. Prepregs consist of high-quality textile fabrics impregnated with curable resins. The fibre type is the main factor governing the strength, Young’s modulus and other important properties of fibre composite products. High strength, rigidity and pronounced anisotropy are achieved by a unidirectional arrangement of the fibres or the prepregs themselves (Table 3.6). As the fibres are arranged in dense bundles, the unidirectional prepregs contain at least 60% fibres by volume. In principle, prepregs made from woven fabrics are employed for components that have to be isotropic in one plane (orthotropic). This can be achieved with plain-weave fabrics, in which warp and weft are arranged at angles of +45°/-45°and 0°/90° to the main axis of the laminate. In general, the fibre content of such elements will be about 50% by volume. Not only does the resin influence the essential properties of the resulting products, but it also determines their processibility, manufacturing time.

Table 3.6 Advantages of CF-reinforced carbon composites

Resistance to high temperatures and weathering, low flammability, low smoke density, low toxicity of decomposition products. Temperature resistance depends on choice of resin.

High chemical stability.

Large variety of possible component shapes and sizes.

High durability due to long storage life of prepreg.

Prepregs comprise a range of reinforcements and resin matrix combinations. They are manufactured at a state-of-the-art fusible resin plant. Fusible resins have fewer volatile constituents and increase the composite mechanical strength of the material. The prepreg manufacturing plant is accredited to DIN AND ISO 9001 quality assurance standards.

Adapted from [4, 6]

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Carbon nanotubes (CNT) have extraordinary mechanical, electrical and thermal properties [27-32] Single wall nanotubes can be thought of as the ultimate CF due to their perfect graphitic structure, low density, and alignment with respect to each layer which gives them exceptional engineering properties and light weight. The elastic modulus parallel to the nanotubes axis is estimated to be ~640 GPa and the tensile strength to be ~37 GPa [33, 34]. Single wall nanotube electrical and thermal conductivity at 300 K are 106 S/m [35] and ~3000 W/mK [36)], respectively. The combination of density, mechanical, thermal, and electrical properties of single wall nanotubes is unmatched, as there are no other materials with this combination of properties. The translation of these properties into macroscopic structures is the subject of current challenge for the material scientists and engineers. The intrinsic mechanical and transport properties of CNT make them the ultimate CF [37]. Tables 3.7 and Table 3.8 compare these properties to other engineering materials. Overall, CNT show a unique combination of stiffness, strength, and tenacity compared to other fibre materials which usually lack one or more of these properties. Thermal and electrical conductivity are also very high, and comparable to other conductive materials. The properties of nanotubes have caused researchers and companies to consider using them in several fields. For example, because CNT have the highest strength to weight ratio of any known material, researchers at NASA are combining CNT with other materials into composites that can be used to build lightweight spacecraft.

Table 3.7 Properties of various engineering fibres

Fibre material Specific density

E (TPa) Strength (GPa)

Strain at break (%)

High strength steel 7.8 0.2 4.1 <10

CF - PAN 1.7–2 0.2–0.6 1.7–5 0.3–2.4

CF - pitch 2-2.2 0.4-0.96 2.2–3.3 0.27–0.6

E/S - glass 2.5 0.07/0.08 2.4/4.5 4.8

Kevlar 49 1.4 0.13 3.6–4.1 2.8

CNT 1.3–2 1 10–60 10

Adapted from [37]

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Table 3.8 Thermal conductivity and electrical conductivity of CF and CNT

Fibre material Thermal conductivity (W/m.k) Electrical conductivity

CF - pitch 1000 2–8.5 × 106

CF - PAN 8–105 6.5–14 × 106

CNT >3000 106–107

Adapted from [37]

Another property of nanotubes is that they can easily penetrate membranes such as cell walls. In fact, nanotubes long, narrow shape make them look like miniature needles, so it makes sense that they can function like a needle at the cellular level. Medical researchers are using this property by attaching molecules that are attracted to cancer cells to nanotubes to deliver drugs directly to diseased cells.

Another interesting property of CNT is that their electrical resistance changes significantly when other molecules attach themselves to the carbon atoms. Companies are using this property to develop sensors that can detect chemical vapours such as carbon monoxide or biological molecules. Researchers and companies are working to use CNT in various fields.

References

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2. D.L. Chung in Carbon Fibre Composites, Butterworth-Heinemann, Boston, MA, USA, 1994.

3. W. Watt in Handbook of Composites–Volume I, Eds., A. Kelly and Yu.N. Rabotnov, Elsevier Science, Holland, 1985, p.327.

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4. J.B. Donnet and R.C. Bansal in Carbon Fibres, 2nd Edition, Marcel Dekker, New York, NY, USA, 1990, p.1.

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19. W.P. Hoffman, W.C. Hurley and P.M. Liu, Journal of Material Research, 199, 16, 1685.

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25. J.P. Issi and B. Nysten in Carbon Fibres, Eds., J.B. Donnet, T.K. Wang, S. Rebouillat and J.C.M. Peng, Marcel Dekker, New York, NY, USA, 1998, p.371.

26. Cytec Industries/Toray Global. http://www.cytec.com/http://www.toray.com

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28. R.E. Smalley, Science, 1996, 273, 5274, 483.

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33. G. Gao, Nanotechnology, 1998, 9, 3, 184.

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37. Carbon Nanotubes. http://www.nanocyl.com/en/CNT-Expertise-Centre/Carbon-Nanotubes

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