Askeland D.R., Fulay P.P. Essentials of Materials Science & Engineering

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Hybrid composites are composed of two or more types of ﬁbers. For instance,

Kevlar

ﬁbers may be mixed with carbon ﬁbers to improve the toughness of a sti¤

composite, or Kevlar

may be mixed with glass ﬁbers to improve sti¤ness. Partic-

ularly good tailoring of the composite to meet speciﬁc applications can be achieved by

controlling the amounts and orientations of each ﬁber.

Tough composites can also be produced if careful attention is paid to the choice of

materials and processing techniques. Better fracture toughness in the rather brittle

composites can be obtained by using long ﬁbers, amorphous (such as PEEK and PPS)

rather than crystalline or cross-linked matrices, thermoplastic-elastomer matrices, or

interpenetrating network polymers.

Metal-Matrix Composites These materials, strengthened by metal or ceramic ﬁbers,

provide high-temperature resistance. Aluminum reinforced with borsic ﬁbers has been

used extensively in aerospace applications, including struts for the space shuttle.

Copper-based alloys have been reinforced with SiC ﬁbers, producing high-strength

propellers for ships.

Aluminum is commonly used in metal-matrix composites. Al

ﬁbers reinforce

the pistons for some diesel engines; SiC ﬁbers and whiskers are used in aerospace ap-

plications, including sti¤eners and missile ﬁns; and carbon ﬁbers provide reinforcement

for the aluminum antenna mast of the Hubble telescope. Polymer ﬁbers, because of

their low melting or degradation temperatures, are not normally used in a metallic

matrix. Polymets, however, are produced by hot extruding aluminum powder and high-

melting-temperature liquid-crystalline polymers. A reduction of 1000 to 1 during the

extrusion process elongates the polymer into aligned ﬁlaments and bonds the aluminum

powder particles into a solid matrix.

Specific strength (3 10

cm)

0.5

1.5

2.5

3.5

Figure 17-26

The specific strength versus

temperature for several composites

and metals.

C HA P T E R 1 7 Composites: Teamwork and Synergy in Materials570

Metal-matrix composites may ﬁnd important applications in components for rocket

or aircraft engines. Superalloys reinforced with metal ﬁbers (such as tungsten) or

ceramic ﬁbers (such as SiC or B

N) maintain their strength at higher temperatures,

permitting jet engines to operate more e‰ciently. Similarly, titanium and titanium alu-

minides reinforced with SiC ﬁbers are considered for turbine blades and disks.

A unique application for metal-matrix composites is in the superconducting wire

required for fusion reactors. The intermetallic compound Nb

Sn has good super-

conducting properties but is very brittle. To produce Nb

Sn wire, pure niobium wire is

surrounded by copper as the two metals are formed into a wire composite (Figure 17-27).

The niobium-copper composite wire is then coated with tin. The tin di¤uses through the

copper and reacts with the niobium to produce the intermetallic compound. Niobium-

titanium systems are also used.

Ceramic-Matrix Com posites Composites containing ceramic ﬁbers in a ceramic matrix

are also ﬁnding applications. Two important uses will be discussed to illustrate the

unique properties that can be obtained with these materials.

Carbon-carbon (C—C) composites are used for extraord inary temperature resist-

ance in aerospace applications. Carbon-carbon composites can operate at temperatures

of up to 3000



C and, in fact, are stronger at high temperatures than at low temperatures

(Figure 17-28). Carbon-carbon composites are made by forming a polyacrylonitrile or

carbon ﬁber fabric into a mold, then impregnating the fabric with an organic resin, such

as a phenolic. The part is pyrolyzed to convert the phenolic resin to carbon. The com-

posite, which is still soft and porous, is impregnated and pyrolyzed several more times,

continually increasing the density, strength, and sti¤ness. Finally, the part is coated with

silicon carbide to protect the carbon-carbon composite from oxidation. Strengths

of 2070 MPa and a Young’s modulus (measure of sti¤ness) of 345  10

MPa can be

Figure 17-27 The manufacture of composite superconductor wires: (a) Niobium wire is

surrounded with copper during forming. (b) Tin is plated onto Nb-Cu composite wire. (c) Tin

diffuses to niobium to produce the Nb

Sn-Cu composite.

17-6 Fiber-Reinforced Systems and Applications 571

obtained. Carbon-carbon composites have been used as nose cones and leading edges

of high-performance aerospace vehicles such as the space shuttle, and as brake discs

on racing cars and commercial jet aircraft.

Ceramic-ﬁber–ceramic-matrix composites provide improved strength and fracture

toughness compared with conventional ceramics (Table 17-4). Fiber reinforcements

improve the toughness of the ceramic matrix in several ways. First, a crack moving

through the matrix encounters a ﬁber; if the bonding between the matrix and the ﬁber is

poor, the crack is forced to propagate around the ﬁber in order to continue the fracture

process. In addition, poor bonding allows the ﬁber to begin to pull out of the matrix

[Figure 17-29(a)]. Both processes consume energy, thereby increasing fracture tough-

ness. Finally, as a crack in the matrix begins, unbroken ﬁbers may bridge the crack,

providing a compressive stress that helps keep the crack from opening [Figure 17-29(b)].

Figure 17-28

A comparison of the specific

strength of various carbon-carbon

composites with that of other

high-temperature materials

relative to temperature.

TABLE 17-4 9 Effect of SiC-reinforcement fibers on the properties of

selected ceramic materials

Material

Flexural Strength

(MPa)

Fracture Toughness

(MPa

ﬃﬃﬃﬃ

)

550 5.5

/SiC 790 8.8

SiC 500 4.4

SiC/SiC 760 25.3

ZrO

210 5.5

ZrO

/SiC 450 22.2

470 4.4

/SiC 790 56.0

Glass 60 1.1

Glass/SiC 830 18.7

Glass ceramic 210 2.2

Glass ceramic/SiC 830 17.6

C HA P T E R 1 7 Composites: Teamwork and Synergy in Materials572

Figure 17-29 Two failure modes in ceramic-ceramic composites: (a) Extensive pull-out of SiC fibers in a glass matrix provides good composite

toughness (20). (From Metals Handbook, American Society for Metals, Vol. 9, 9th Ed., 1985.) (b) Bridging of some fibers across a crack

enhances the toughness of a ceramic-matrix composite (unknown magnification). (From Journal of Metals, May 1991.)

17-6 Fiber-Reinforced Systems and Applications 573

Unlike polymer and metal matrix composites, poor bonding—rather than good

bonding—is required! Consequently, control of the interface structure is crucial. In a

glass-ceramic (based on Al

 SiO

 Li

O) reinforced with SiC ﬁbers, an interface

layer containing carbon and NbC is produced that makes debonding of the ﬁber from

the matrix easy. If, however, the composite is heated to a high temperature, the inter-

face is oxidized; the oxide occupies a large volume, exerts a clamping force on the ﬁber,

and prevents easy pull-out. Fracture toughness is then decreased. The following exam-

ple illustrates some of the cost and property issues that come up while working with

composites.

EXAMPLE 17-10

Design of a Composite Strut

Design a unidirectional ﬁber-reinforced epoxy-matrix strut having a round

cross-section. The strut is 3 m long and, when a force of 2224 N is applied, it

should stretch no more than 0.25 cm. We want to assure that the stress acting

on the strut is less than the yield strength of the epoxy matrix, 83 MPa. If the

ﬁbers should happen to break, the strut will stretch an extra amount but may

not catastrophically fracture. Epoxy costs about $1.8/kg and has a modulus of

elasticity of 3450 MPa.

SOLUTION

Suppose that the strut were made entirely of epoxy (that is, no ﬁbers):

max

0:25 cm

300

¼ 0:00083 cm=cm

max

¼ Ee ¼ð3450Þð0:00083Þ¼2:9 MPa

strut

2224 N

2:9 MPa

¼ 7:67 cm

or d ¼ 3:12 cm

Since r

epoxy

¼ 1:25 g/cm

Weight

strut

¼ð1:25ÞðpÞð3:12=2Þ

ð300Þ¼2:87 kg

Cost

strut

¼ð2:87 kgÞð$1:8=kgÞ¼$5:17

With no reinforcement, the strut is large and heavy; the materials cost is high

due to the large amount of epoxy needed.

In a composite, the maximum strain is still 0.00083 cm/cm If we make the

strut as small as possible—that is, it operates at 83 MPa—then the minimum

modulus of elasticity E

of the composite is:

max

0:00083

¼ 100  10

MPa

Let’s look at sever al possible composite systems. The modulus of glass ﬁbers is

less than 100  10

MPa; therefore, glass reinforcement is not a possible choice.

For high modulus carbon ﬁbers, E ¼ 531  10

MPa; the density is 1.9 g/cm

and the cost is about $66/kg. The minimum volume fraction of carbon ﬁbers

needed to give a composite modulus of 100  10

MPa is:

¼ f

ð531  10

Þþð1  f

Þð3:45  10

Þ > 100  10

¼ 0:183

C HA P T E R 1 7 Composites: Teamwork and Synergy in Materials574

The volume fraction of epoxy remaining is 0.817. An area of 0.817 times the

total cross-sectional area of the strut must support a 2224 N load with no more

than 83 MPa if all of the ﬁbers should fail:

epoxy

¼ 0:817A

total

2224 N

83 MPa

¼ 0:268 cm

total

0:268

0:817

¼ 0:328 cm

or d ¼ 0:646 cm

Volume

strut

¼ A

total

¼ð0:328 cm

Þð300 cmÞ¼98:4cm

Weight

strut

¼ rV ¼½ð1:9Þð0:183Þþð1:25Þð0:817Þð98:4Þ

¼ 0:135 kg

Weight fraction carbon ¼

ð0:183Þð1:9g=cm

ð0:183Þð1:9Þþð0:817Þð1:25Þ

¼ 0:254

Weight carbon ¼ð0:254Þð0:135 kgÞ¼0:034 kg

Weight epoxy ¼ð0:746Þð0:135 kgÞ¼0:101 kg

Cost of each strut ¼ð0:034Þð$66Þþð0:101Þð$1:80Þ¼$2:42

The carbon-ﬁber reinforced strut is less than one-quarter the diameter of an all-

epoxy structure, with only 5% of the weight and half of the cost.

We might also repeat these calculations using Kevlar

ﬁbers, with a mod-

ulus of 124  10

MPa, a density of 1.44 g/cm

, and a cost of about $44/kg. By

doing so, we would ﬁnd that a volume fraction of 0.8 ﬁbers is required. Note that

0.8 volume fraction is at the maximum of ﬁber volume that can be incorporated

into a matrix. We would also ﬁnd that the required diameter of the strut is

1.288 cm and that the strut weighs 0.573 kg and costs $20.94. The modulus of

the Kevlar

is not high enough to o¤set its high cost.

Although the carbon ﬁbers are the most expensive, they permit the lightest

weight and the lowest material cost strut. (This calculation does not, how-

ever, take into consideration the costs of manufacturing the strut.) Our design,

therefore, is to use a 0.646-cm-diameter strut containing 0.183 volume fraction

high-modulus carbon ﬁber.

17-7 Laminar Composite Materials

Laminar composites include very thin coatings, thicker protective surfaces, claddings,

bimetallics, laminates, and a host of other applications. In addition, the ﬁber-reinforced

composites produced from tapes or fabrics can be considered partly laminar. Many

laminar composites are designed to improve corrosion resistance while retaining low

cost, high strength, or light weight. Other important characteristics include superior

wear or abrasion resistance, improved appearance, and unusual thermal expansion

characteristics.

Rule of Mixtures Some prope rties of the laminar composite materials parallel to the

lamellae are estimated from the rule of mixtures. The density, electrical and thermal

17-7 Laminar Composite Materials 575

conductivity, and modulus of elasticity parallel to the lamellae can be calculated with

little error using following formulas:

Density ¼ r

c;k

ðf

Þð17-13Þ

Electrical conductivity ¼ s

c;k

ðf

Thermal conductivity ¼ K

c;k

ðf

;

ð17-14Þ

Modulus of elasticity ¼ E

c;k

ðf

The laminar composites are very anisotropic. The properties perpendicular to the

lamellae are:

Electrical conductivity ¼

c;L



Thermal conductivity ¼

c;L



;

ð17-15Þ

Modulus of elasticity ¼

c;L



However, many of the really important properties, such as corrosion and wear resist-

ance, depend primarily on only one of the components of the composite, so the rule of

mixtures is not applicable.

Producing Laminar Composites Several methods are used to produce laminar com-

posites, including a variety of deformation and joining techniques used primarily for

metals. (Figure 17-30).

Individual plies are often joined by adhesive bonding, as is the case in producing

plywood. Polymer-matrix composites built up from several layers of fabric or tape pre-

pregs are also joined by adhesive bonding; a ﬁlm of unpolymerized material is placed

between each layer of prepreg. When the layers are pressed at an elevated temperature,

polymerization is completed and the prepregged ﬁbers are joined t o produce composites

that may be dozens of layers thick.

Most of the metallic laminar composites, such as claddings and bimetallics, are

produced by deformation bonding, such as hot- or cold-roll bonding. The pressure

exerted by the rolls breaks up the oxide ﬁlm at the surface, brings the surfaces into

0.0075 cm

Figure 17-30 Techniques for producing laminar composites: (a) roll bonding, (b) explosive

bonding, (c) coextrusion, and (d) brazing.

C HA P T E R 1 7 Composites: Teamwork and Synergy in Materials576

atom-to-atom contact, and permits the two surfaces to be joined. Explosive bonding

can also be used. An explosive charge provides the pressure required to join metals.

This process is particularly well suited for joining very large plates that will not ﬁt into

a rolling mill. Very simple lami nar composites, such as coaxial cable, are produced by

coextruding two materials through a die in such a way that the soft material surrounds

the harder material. Metal conductor wire can be coated with an insulating thermo-

plastic polymer in this manner.

Brazing can join composite plates (Chapter 9). The metallic sheets are separated by

a very small clearance—preferably, about 0.0075 cm—and heated above the melting

temperature of the brazing alloy. The molten brazing alloy is drawn into the thin joint

by capillary action.

17-8 Examples and Applications of Laminar Composites

The number of laminar composites is so varied and their applications and intentions are

so numerous that we cannot make generalizations concerning their behavior. Inste ad

we will examine the characteristics of a few commonly used examples.

Laminates Laminates are layers of materials joined by an organic adhesive. In lami-

nated safety glass, a plastic adhesive, such as polyvinyl butyral (PVB), joins two pieces

of glass; the adhesive prevents fragments of glass from ﬂying about when the glass is

broken (Chapter 15). Laminates are used for insulation in motors, for gears, for printed

circuit boards, and for decorative items such as Formica1 countertops and furniture.

Microlaminates include composites composed of alternating layers of aluminum

sheet and ﬁber-reinforced polymer. Arall (aramid-alumin um laminate) and Glare (glass-

aluminum laminate) have been developed as possible skin materials for aircraft. In

Arall, an aramid ﬁber such as Kevlar

is prepared as a fabric or unidirectional tape,

impregnated with an adhesive, and laminated between layers of aluminum alloy (Figure

17-31). The composite laminate has an unusual combination of strength, sti¤ness, cor-

rosion resistance, and light weight. Fatigue resistance is improved, since the interface

between the layers may block cracks. Compared with polymer-matrix composites, the

microlaminates have good resistance to lightning-strike damage (which is important in

aerospace applications), are formable and machinable, and are easily repaired.

Figure 17-31 Schematic diagram of an aramid-aluminum laminate, Arall, which has potential

for aerospace applications.

17-8 Examples and Applications of Laminar Composites 577

Clad Metals Clad materials are metal-metal composites. A common example of clad-

ding is United States silv er coinage. A Cu-80% Ni alloy is bonded to both sides of a

Cu-20% Ni alloy. The ratio of thicknesses is about 1/6:2/3: 1/6. The high-nickel alloy is

a silver color, while the predominantly copper core provides low cost.

Clad materials provide a combination of good corrosion resistance with high

strength. Alclad is a clad composite in which commercially pure aluminum is bonded to

higher-strength aluminum alloys. The pure aluminum protects the higher-strength alloy

from corrosion. The thickness of the pure aluminum layer is about 1% to 15% of the

total thickness. Alclad is used in aircraft construction, heat exchangers, building con-

struction, and storage tanks, where combin ations of corrosion resistance, strength, and

light weight are desired.

Bimetallics Temperature indicators and controllers take advantage of the di¤erent

coe‰cients of thermal expansion of the two metals in laminar composite. If two pieces

of metal are heated, the metal with the higher coe‰cient of thermal expansion becomes

longer. If the two pieces of metal are rigidly bonded together, the di¤erence in their

coe‰cients causes the strip to bend and produce a curved surface. The amount of

movement depends on the temperature. By measuring the curvature or deﬂection of the

strip, we can determine the temperature. Likewise, if the free end of the strip activates

a relay, the strip can turn on or o¤ a furnace or air conditioner to regulate temper-

ature. Metals selected for bimetallics must have (a) very di¤erent coe‰cients of thermal

expansion, (b) expansion characteristics that are reversible and repeatable, and (c) a

high modulus of elasticity, so that the bimetallic device can do work. Often the low-

expansion strip is made from Invar, an iron-nickel alloy, whereas the high-expansion

strip may be brass, Monel, or pure nickel.

Bimetallics can act as circuit breakers as well as therm ostats; if a current passing

through the strip becomes too high, heating cau ses the bimetallic to deﬂect and break

the circuit.

Multilayer capacitors Similar geometry is used to make billions of multi-layer capaci-

tors. Their structure is comprisesd of thin sheets of BaTiO

-based ceramics separated by

Ag/Pd or Ni electrodes.

17-9 Sandwich Structures

Sandwich materials have thin layers of a facing material joined to a lightweight ﬁller

material, such as a polymer foam. Neither the ﬁller nor the facing material is strong or

rigid, but the composite possesses both properties. A familiar example is corrugated

cardboard. A corrugated core of paper is bonded on either side to ﬂat, thick paper.

Neither the corrugated core nor the facing paper is rigid, but the combination is.

Another important example is the honeycomb structure used in aircraft applica-

tions. A honeycomb is produced by gluing thin aluminum strips at selected locations.

The honeycomb material is then expanded to produce a very low-density cellular panel

that, by itself, is unstable (Figure 17-32). When an aluminum facing sheet is adhesively

bonded to either side of the honeycomb, however, a very sti¤, rigid, strong, and excep-

tionally lightweight sandwich with a density as low as 0.04 g/cm

is obtained.

The honeycomb cells can have a variety of shapes, including hexagonal, square,

rectangular, and sinusoidal, and they can be made from aluminum, ﬁberglass, paper,

C HA P T E R 1 7 Composites: Teamwork and Synergy in Materials578

aramid polymers, and other materials. The honeycomb cells can be ﬁlled with foam or

ﬁberglass to provide excellent sound and vibration absorption. Figure 17-33 describes

one method by which the honeycomb can be fabricated.

SUMMARY

V Composites are composed of two or more materials or phases joined or connected

in such a way so as to give a combination of properties that cannot be attained

otherwise. The volume fraction and the connectivity of the phases or materials in a

composite and the nature of the interface between the dispersed phase and matrix

are very important in determining the properties.

Figure 17-32 (a) A hexagonal cell honeycomb core, (b) can be joined to two face sheets by

means of adhesive sheets, (c) producing an exceptionally lightweight yet stiff, strong

honeycomb sandwich structure.

Figure 17-33 In the corrugation method for producing a honeycomb core, the material (such

as aluminum) is corrugated between two rolls. The corrugated sheets are joined together with

adhesive and then cut to the desired thickness.

Summary 579

Askeland D.R., Fulay P.P. Essentials of Materials Science &amp; Engineering

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Askeland D.R., Fulay P.P. Essentials of Materials Science & Engineering