Pierson Hugh O. Handbook of carbon, graphite, diamond and fullerene. New Jersey / carbon

216 Carbon,

Graphite,

Diamond,

and Fullerenes

A CO2laser-roller

densification

process is used to fabricate a unidirec-

tional carbon-fiber-aluminum-matrix

composite,[41) Although good densi-

fication

is obtained, the

mechanical

properties of the composites are low,

probably

due to fiber

degradation,

Aluminum

and copper are the most common matrix materials inves-

tigated so far ,[9)Other

metals include nickel, niobium, and beryllium,[40)[42)

Figure

9.6.

Cross-section

ofgraphite-fjber/coppercomposites

with

NbC interface

after

thermal

exposure.

(Photograph

courtesy

of Rocket

dyne,

Canoga

Park,

CA.)[39]

important variables such as fiber-matrix ratio and fiber orientation

are not

mentioned. As a rule, the mechanical properties of present composites

are still

far short of the potential predicted by the rule-of-mixtures.

The thermal

conductivity

can be increased by the proper utilization of

high-conductivity

fibers such

as Pitch-130 or CVD fibers (see Ch. 8, Secs.

3.0 and 5.0) .The

thermal conductivity

of these fibers is considerably

higher

than that of copper and other

metals

as shown in Fig. 9.7.[431

The

major

drawback

of ceramics

is their

intrinsic

brittleness.

For

example,

most

metals

have

a fracture

toughness

forty

times

greater

than

conventional

ceramics

and glasses.

This brittleness is related on the atomic

level to the strong hybrid-ionic-covalent

bonds of ceramics.

These strong

bonds

prevent

deformation

such

as

occurs

in

ductile

metals.

Applied

stresses tend to concentrate

at the sites of flaws, at voids

and chemical

impurities,

and

at grain

interfaces.

The

result

is catastrophic

brittle

failure.

Ceramics,

reinforced

with carbon fibers or whiskers,

are less brittle and

have

increased

fracture

toughness

and improved thermal-shock

resis-

tance.

These

composites

have excellent

potential

but, as with metal-matrix

composites,

many problems

must

be solved

before they

become

reliable

engineering

materials.

Applications of Carbon Fibers

219

Oxides

such

as

alumina

(AI,O,)

are

generally

not

suitable matrix

materials

in

carbon-fiber

composites

as the

carbon

reduces

the

oxide

to

form

metal carbide

and

CO

during

the

fabrication

process.

The

oxidation

of the

carbon

fiber

may

be sufficient

to

generate

a high

partial

pressure

of

CO which

results

in the formation of gas bubbles

and cracks

in the oxide.t45)

As opposed

tooxides,

carbidessuch

assilicon

carbide

(Sic) and boron

carbide

(B4C) are compatible

with

carbon fibers,

and

satisfactory

compos-

ites are

produced

with

these

matrices

and

PAN-based

yarn

by chemical

vapor

infiltration

(CVi).t461 A boron-carbon

intermediate

coating

provides

optimum

strength

and

toughness

as it prevents

fiber

degradation.

The

tensile

strength data are

shown in Table

9.9.

The

carbon-fiber

composites

show

substantially

higher strength

than

the silicon

carbide

materials

but degrade

more rapidly

in air at 1100°C.

The

fracture

modes

of

a

carbon

fiber/silicon-carbide

matrix

composite

are

shown

in Fig. 9.9.

Table 9.9.

Tensile

Strength of

Carbon Fiber, SIC-

and

B4C-

Matrix

Composites

Tensile

Standard

Strength

Deviation

Composite

MPa

(ksi)

MPa

(ksi)

Carbon

fiber,

Sic

matrix

515

(74.7)

58

(8.4)

Carbon

fiber,

B,C

matrix

380

(55)

26

(3.8)

Sic

fiber,SiC

matrix

310

(45)

62

(9)

SIC

fiber,

B,C

matrix

314

(45.6)

68

(9.8)

Notes:

1.

Carbon

fibers areT-

(Amoco Performance

Products).

2.

Silicon

carbide

fibers are Nicalon (Nippon

Carbon Co.).

3.

Both fibers

are

coated

with B-C intermediate.

4.

Fibers

are

unidirectional

and composite

was

tested in

fiber direction.

Figure

9.9. Fracture of a carbon

fiber/silicon-carbide

matrix composite.

(Photo-

graph

courtesy Rockwelllnternational,

Canoga Park,

CA.)

6.2

Applications

Applications

of carbon-fiber,

ceramic-matrix

composites

are

still

es-

sentially in the development

stage and many fabrication

problems

must be

solved

before the

full

potential of these

materials

is realized.

Among

the

more

successful applications

to date are the following.

Carbon-Fiber,

Cement

Composites.

Work carried out in Japan

has

shown

the potential

of carbon

fibers

as

an effective

reinforcement

for

Applications

of Carbon

Fibers

221

cement.t47] In one composite application,

thefiberswere

pitch-based,

either

chopped,

continuous,

or in a mat form,

and comprised

2 - 4% by volume

of

the

composite.

As shown

in Fig. 9.10,

the fibers

impart

a marked

increase

in tensile

strength

and

pronounced

change

in fracture

mode.

No deteriora-

tion

of the fiber

occurs

due to the high resistance

of the

carbon

fiber

to alkali

solutions.

When

cured

in an

autoclave

(as

opposed

to air

cure),

tensile

strength

more

than

triples to 6.8

MPa

with

4% carbon.

These

composites

are

now

produced

commercially

on a modest

scale in Japan

with applica-

tions in walland

floor-panels,

foot bridges,

and

other

architectural

compo-

nents.t4sl

Niobium

Nitride

(NbN)

Superconductor:

Thin

films

of

NbN

are

applied on carbon

fibers

by vapor

deposition

followed

by thermal-sprayed

oxygen-free high-conductivity

copper

(OFHC).

A transition

temperature

of

the

composite

of

16

K

and

a critical

current

of

1O7 A/cm2 have been

observed.

Applications

are in high-energy

lasers,

particle-beam

weapons,

and

electromagnetic

guns.t4g]

0

4

8

12

Strain,

x 10m3

Figure 9.10. Tensile stress-strain

curves of carbon-reinforced

cement with various

volume-fractions

of chopped carbon fibers.[47]

Carbon fibers

in felted

or woven

form are the preferred materials

for

high-temperature

insulationforfurnaces

operating

above 1500°C invacuum

or inert

atmosphere.

However, these

materials

have a tendency

to sag and

compact

with

time

which

eventually

results

in

non-uniform

insulation

characteristics.

This

problem

is partially solved

by the

use of rigid

insulation

which

consists

of

a felt impregnated

with

a polymer,

subsequently

pyro-

lyzed,

and fired

and

outgassed

at 2000°C

under vacuum.

The

material

is

available

in the

form

of cylinders,

boards,

disks

and special

shapes

to suit

furnace

demands.fWt

Carbon

fibers

are

being considered for electromagnetic

interference

(EMI) shields.

Thefibersaresometimes

coated with nickel by electroplating

to increase

the electrical

conductivity,

and molded in the form

of sheets with

a polymer

such as

polycarbonate.f4*)

Many potential

applications

of

carbon fibers

in batteries have

been

reported

but their inherent

high cost

compared

to

conventional

electrode

materials such as graphite

powder,

porous carbon, and molded carbon have

precluded

their use

in commercial

applications

so far (see Ch.

10,

Sec.

3.0) .[*I

Applications

of Carbon

Fibers

223

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