Download - Composite Unit 1

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UNIT-I

Definition of Composite materials – Classification of composites, Need and General characteristics –

advantages and limitations.

MODU! - "

Introd#ction to Composite Materials

A composite material can be defined as a combination of two or more materials that results in bett

properties than those of the individual components used alone. In contrast to metallic alloys, each mater

retains its separate chemical, physical, and mechanical properties. The two constituents are reinforcement an

a matrix. The main advantages of composite materials are their high strength and stiffness, combined with lo

density, when compared with bulk materials, allowing for a weight reduction in the finished part.

Composite materials

The term “composite materials” broadly refers to a material system which is composed of

discrete constituent the reinforcement! distributed in continuous phase the matrix! and which derives i

distinguishing characteristics from the properties of its constituents, from the geometry and architecture of th

constituent and from the properties of the boundaries interface! between different constituent "urappa, #$$%

A composite material consists of two or more chemically distinct macro constituents, separated by a distin

interface. &omposite material has properties or! characteristics that could not be obtained from any on

material. A material is considered composite only if the constituent material or! phases has significant

different properties and the resulting combination give a significant change in the properties.The field of composite materials technology as a whole, which includes metal matrix composite

has been and is growing so rapidly that no single individual can be aware of all its developments, not

mention being able to assess the relative merits of each endeavor and place.

Definition of Composite Materials A composite is a structural material that consists of two or more combined constituents that a

combined at a macroscopic level and are not soluble in each other.

$einforcing phase' fibers, particles, or flakes

Matri% phase' polymers, metals, ceramics

Characteristics of Composite Materials

(igh specific strength and modulus, as well as high fatigue strength and fatigue damage toleran

Anisotropic )esignable or tailorable materials for both microstructure and *roperties *roduction of bo

material and structure or component in a single operation + manufacturing flexible, net+shape, compl


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geometry &orrosion resistance and durable ther uni-ue functional properties + damping, low &T coefficie

of thermal expansion!

T&pes of Composites

*articulate composites

/lake composites

/iber composites

0anocomposites

MODU! – '

Classification of Composite Materials


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MODU! – (

)ol&mer Matri% Materials

*olymers make ideal materials as they can be processed easily, possess lightweight, and desirab

mechanical properties. It follows, therefore, that high temperature resins are extensively used in aeronauticapplications.

Two main kinds of polymers are thermosets and thermoplastics. Thermosets have -ualities such as

well+bonded three+dimensional molecular structure after curing. They decompose instead of melting o

hardening. 1erely changing the basic composition of the resin is enough to alter the conditions suitably f

curing and determine its other characteristics. They can be retained in a partially cured condition too ov

prolonged periods of time, rendering Thermosets very flexible. Thus, they are most suited as matrix bases f

advanced conditions fiber reinforced composites. Thermosets find wide ranging applications in the chopp

fiber composites form particularly when a premixed or moulding compound with fibers of specific -uality an

aspect ratio happens to be starting material as in epoxy, polymer and phenolic polyamide resins.

Thermoplastics have one+ or two+dimensional molecular structure and they tend to at an elevate

temperature and show exaggerated melting point. Another advantage is that the process of softening at


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elevated temperatures can reversed to regain its properties during cooling, facilitating applications conventional compress techni-ues to mould the compounds.

2esins reinforced with thermoplastics now comprised an emerging group of composites. The them

of most experiments in this area to improve the base properties of the resins and extract the greatest function

advantages from them in new avenues, including attempts to replace metals in die+casting processes. crystalline thermoplastics, the reinforcement affects the morpholog& to a considerable extent, prompting t

reinforcement to empower nucleation. 3henever crystalline or amorphous, these resins possess the facility

alter their creep over an extensive range of temperature. 4ut this range includes the point at which the usage o

resins is constrained, and the reinforcement in such systems can increase the failure load as well as cree

resistance. /igure shows kinds of thermoplastics.

A small -uantum of shrinkage and the tendency of the shape to retain its original form are also to be accounte

for. 4ut reinforcements can change this condition too. The advantage of thermoplastics systems ov

thermosets are that there are no chemical reactions involved, which often result in the release of gases or hea

1anufacturing is limited by the time re-uired for heating, shaping and cooling the structures.

Thermoplastics resins are sold as moulding compounds. /iber reinforcement is apt for these resins. "ince th

fibers are randomly dispersed, the reinforcement will be almost isotropic. (owever, when sub5ected

moulding processes, they can be aligned directionally.

There are a few options to increase heat resistance in thermoplastics. Addition of fillers raises the he

resistance. 4ut all thermoplastic composites tend loose their strength at elevated temperatures. (owever, the

redeeming -ualities like rigidity, toughness and ability to repudiate creep, place thermoplastics in the importa

composite materials bracket. They are used in automotive control panels, electronic products encasement etc.

0ewer developments augur the broadening of the scope of applications of thermoplastics. (uge sheets

reinforced thermoplastics are now available and they only re-uire sampling and heating to be moulded into th

re-uired shapes. This has facilitated easy fabrication of bulky components, doing away with the mo

cumbersome moulding compounds.


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Thermosets are the most popular of the fiber composite matrices without which, research an

development in structural engineering field could get truncated. Aerospace components, automobile part

defense systems etc., use a great deal of this type of fiber composites. poxy matrix materials are used

printed circuit boards and similar areas. /igure 6.% shows some kinds of thermosets.

)irect condensation polymeri7ation followed by rearrangement reactions to form heterocyclic entiti

is the method generally used to produce thermoset resins. 3ater, a product of the reaction, in both method

hinders production of void+free composites. These voids have a negative effect on properties of the composit

in terms of strength and dielectric properties. *olyesters phenolic and poxies are the two important classes

thermoset resins.

poxy resins are widely used in filament+wound composites and are suitable for moulding prepres

They are reasonably stable to chemical attacks and are excellent adherents having slow shrinkage durin

curing and no emission of volatile gases. These advantages, however, make the use of epoxies rath

expensive. Also, they cannot be expected beyond a temperature of 68$9&. Their use in high technology are

where service temperatures are higher, as a result, is ruled out.

*olyester resins on the other hand are -uite easily accessible, cheap and find use in a wide range

fields. :i-uid polyesters are stored at room temperature for months, sometimes for years and the mere additio

of a catalyst can cure the matrix material within a short time. They are used in automobile and structur

applications.

The cured polyester is usually rigid or flexible as the case may be and transparent. *olyesters withstan

the variations of environment and stable against chemicals. )epending on the formulation of the resin

service re-uirement of application, they can be used up to about ;


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MODU! – *

Metal matri% materials

1etal matrix composites, at present though generating a wide interest in research fraternity, are not

widely in use as their plastic counterparts. (igh strength, fracture toughness and stiffness are offered by met

matrices than those offered by their polymer counterparts. They can withstand elevated temperature

corrosive environment than polymer composites. 1ost metals and alloys could be used as matrices and th

re-uire reinforcement materials which need to be stable over a range of temperature and non+reactive to

(owever the guiding aspect for the choice depends essentially on the matrix material. :ight metals form th

matrix for temperature application and the reinforcements in addition to the aforementioned reasons a

characteri7ed by high moduli.

1ost metals and alloys make good matrices. (owever, practically, the choices for low temperatu

applications are not many. nly light metals are responsive, with their low density proving an advantagTitanium, Aluminium and magnesium are the popular matrix metals currently in vogue, which are particular

useful for aircraft applications. If metallic matrix materials have to offer high strength, they re-uire hig

modulus reinforcements. The strength+to+weight ratios of resulting composites can be higher than most alloys

The melting point, physical and mechanical properties of the composite at various temperatures determine th

service temperature of composites. 1ost metals, ceramics and compounds can be used with matrices of lo

melting point alloys. The choice of reinforcements becomes more stunted with increase in the meltin

temperature of matrix materials.

Ceramic Matri% Materials

Ceramics can be described as solid materials which exhibit very strong ionic bonding in general and

few cases covalent bonding. (igh melting points, good corrosion resistance, stability at elevated temperatur

and high compressive strength, render ceramic+based matrix materials a favourite for applications re-uiring

structural material that doesn=t give way at temperatures above 6


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This carbon+carbon composite is fabricated through compaction of carbon or multip

impregnations of porous frames with li-uid carboniser precursors and subse-uent pyroli7ation. They can al

be manufactured through chemical vapour deposition of pyrolytic carbon. &arbon+carbon composites are n

be applied in elevated temperatures, as many composites have proved to be far superior at these temperature

(owever, their capacity to retain their properties at room temperature as well as at temperature in the range #8$$9& and their dimensional stability make them the oblivious choice in a garnut of applications related

aeronautics, military, industry and space.

&omponents, that are exposed to higher temperature and on which the demands for high standa

performance are many, are most likely to have carbon+carbon composites used in them.

Glass Matrices

In comparison to ceramics and even considered on their own merit, glass matrices are found to be mo

reinforcement+friendly. The various manufacturing methods of polymers can be used for glass matrices.

>lasses are meant to improve upon performance of several applications. >lass matrix composite wi

high strength and modulus can be obtained and they can be maintained upto temperature of the order of ?


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MODU! –+

$einforcements

2einforcements for the composites can be fibers, fabrics particles or whiskers. /ibers are essential

characteri7ed by one very long axis with other two axes either often circular or near circular. *articles have n

preferred orientation and so does their shape. 3hiskers have a preferred shape but are small both in diamet

and length as compared to fibers. /igure 6.8 shows types of reinforcements in composites.


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2einforcing constituents in composites, as the word indicates, provide the strength that

makes the composite what it is. 4ut they also serve certain additional purposes of heat resistance or

conduction, resistance to corrosion and provide rigidity. 2einforcement can be made to perform all

or one of these functions as per the re-uirements.

A reinforcement that embellishes the matrix strength must be stronger and stiffer than the

matrix and capable of changing failure mechanism to the advantage of the composite. This means

that the ductility should be minimum or even nil the composite must behave as brittle as possible.

ier $einforcement

/ibers are the important class of reinforcements, as they satisfy the desired conditions and

transfer strength to the matrix constituent influencing and enhancing their properties as desired.

>lass fibers are the earliest known fibers used to reinforce materials. &eramic and metal fibers weresubse-uently found out and put to extensive use, to render composites stiffer more resistant to heat.

/ibers fall short of ideal performance due to several factors. The performance of a fiber composite is

5udged by its length, shape, orientation, composition of the fibers and the mechanical properties of

the matrix.

The orientation of the fiber in the matrix is an indication of the strength of the composite and

the strength is greatest along the longitudinal directional of fiber. This doesn=t mean the longitudinal

fibers can take the same -uantum of load irrespective of the direction in which it is applied.

ptimum performance from longitudinal fibers can be obtained if the load is applied along its

direction. The slightest shift in the angle of loading may drastically reduce the strength of the

composite.

@nidirectional loading is found in few structures and hence it is prudent to give a mix of

orientations for fibers in composites particularly where the load is expected to be the heaviest.

1onolayer tapes consisting of continuous or discontinuous fibers can be oriented unidirectional

stacked into plies containing layers of filaments also oriented in the same direction. 1ore

complicated orientations are possible too and nowadays, computers are used to make pro5ections of

such variations to suit specific needs. In short, in planar composites, strength can be changed from

unidirectional fiber oriented composites that result in composites with nearly isotropic properties.


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There are several methods of random fiber orientations, which in a two+dimensional one, yield

composites with one+third the strength of an unidirectional fiber+stressed composite, in the direction

of fibers. In a %+dimension, it would result in a composite with a comparable ratio, about less than

one+fifth.

In very strong matrices, moduli and strengths have not been observed. Application of the

strength of the composites with such matrices and several orientations is also possible. The

longitudinal strength can be calculated on the basis of the assumption that fibers have been reduced

to their effective strength on approximation value in composites with strong matrices and non+

longitudinally orientated fibers.

It goes without saying that fiber composites may be constructed with either continuous or

short fibers. xperience has shown that continuous fibers or filaments! exhibit better orientation,

although it does not reflect in their performance. /ibers have a high aspect ratio, i.e., their lengths

being several times greater than their effective diameters. This is the reason why filaments are

manufactured using continuous process. This finished filaments.

1ass production of filaments is well known and they match with several matrices in different

ways like winding, twisting, weaving and knitting, which exhibit the characteristics of a fabric.

"ince they have low densities and high strengths, the fiber lengths in filaments or other fibers

yield considerable influence on the mechanical properties as well as the response of composites to

processing and procedures. "horter fibers with proper orientation composites that use glass, ceramic

or multi+purpose fibers can be endowed with considerably higher strength than those that use

continuous fibers. "hort fibers are also known to their theoretical strength. The continuous fiber

constituent of a composite is often 5oined by the filament winding process in which the matrix

impregnated fiber wrapped around a mandrel shaped like the part over which the composite is to be

placed, and e-uitable load distribution and favorable orientation of the fiber is possible in the

finished product. (owever, winding is mostly confined to fabrication of bodies of revolution and the

occasional irregular, flat surface.

"hort+length fibers incorporated by the open+ or close+mould process are found to be less

efficient, although the input costs are considerably lower than filament winding.


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1ost fibers in use currently are solids which are easy to produce and handle, having a circular

cross+section, although a few non+conventional shaped and hollow fibers show signs of capabilities

that can improve the mechanical -ualities of the composites.

>iven the fact that the vast difference in length and effective diameter of the fiber are assets to a

fiber composite, it follows that greater strength in the fiber can be achieved by smaller diameters due

to minimi7ation or total elimination of surface of surface defects.

After flat+thin filaments came into vogue, fibers rectangular cross sections have provided new

options for applications in high strength structures. wing to their shapes, these fibers provide

perfect packing, while hollow fibers show better structural efficiency in composites that are desired

for their stiffness and compressive strengths. In hollow fibers, the transverse compressive strength is

lower than that of a solid fiber composite whenever the hollow portion is more than half the total

fiber diameter. (owever, they are not easy to handle and fabricate.

M#ltiphase iers

"poilable filaments made by chemical vapour deposition processes are usually the multiphase

variety and they usually comprise materials like boron, silicon and their carbides formed on surface

of a very fine filament substrate like carbon or tungsten. They are usually good for high temperature

applications, due to their reduced reaction with higher melting temperature of metals than graphite

and other metallic fibers. 4oron filaments are sought after for structural and intermediate+

temperature composites.

A poly+phase fiber is a core+sheath fiber consisting of a poly+crystalline core.

his/ers

"ingle crystals grown with nearly 7ero defects are termed whiskers. They are usually

discontinuous and short fibers of different cross sections made from several materials like graphite,

silicon carbide, copper, iron etc. Typical lengths are in % to


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affect the strength of materials, they came to be incorporated in composites using several methods,

including power metallurgy and slip+casting techni-ues.

1etal+whisker combination, strengthening the system at high temperatures, has been

demonstrated at the laboratory level. 4ut whiskers are fine, small si7ed materials not easy to handle

and this comes in the way of incorporating them into engineering materials to come out with a

superior -uality composite system.

arly research has shown that whisker strength varies inversely with effective diameter. 3hen

whiskers were embedded in matrices, whiskers of diameter upto # to 6$m yielded fairly good

composites.

aminar Composites

:aminar composites are found in as many combinations as the number of materials. They can

be described as materials comprising of layers of materials bonded together. These may be of several

layers of two or more metal materials occurring alternately or in a determined order more than once,

and in as many numbers as re-uired for a specific purpose.

&lad and sandwich laminates have many areas as it ought to be, although they are known to

follow the rule of mixtures from the modulus and strength point of view. ther intrinsic values

pertaining to metal+matrix, metal+reinforced composites are also fairly well known.

*owder metallurgical processes like roll bonding, hot pressing, diffusion bonding, bra7ing and

so on can be employed for the fabrication of different alloys of sheet, foil, powder or sprayed

materials. It is not possible to achieve high strength materials unlike the fiber version. 4ut sheets and

foils can be made isotropic in two dimensions more easily than fibers. /oils and sheets are also made

to exhibit high percentages of which they are put. /or instance, a strong sheet may use over B#C in

laminar structure, while it is difficult to make fibers of such compositions. /iber laminates cannot

over ;


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There are many combinations of sheet and foil which function as adhesives at low

temperatures. "uch materials, plastics or metals, may be clubbed together with a third constituent.

*re+painted or pre+finished metal whose primary advantage is elimination of final finishing by the

user is the best known metal+organic laminate. "everal combinations of metal+plastic, vinyl+metal

laminates, organic films and metals, account for upto Blass flakes tend to have notches or cracks around the edges, which weaken the final

product. They are also resistant to be lined up parallel to each other in a matrix, causing uneven

strength. They

illed Composites

illed composites result from addition of filer materials to plastic matrices to replace a portion

of the matrix, enhance or change the properties of the composites. The fillers also enhance strength

and reduce weight.

Another type of filled composite is the product of structure infiltrated with a second+phase filler

material. The skeleton could be a group of cells, honeycomb structures, like a network of open pores.

The infiltrant could also be independent of the matrix and yet bind the components like powders or

fibers, or they could 5ust be used to fill voids. /illers produced from powders are also considered as

particulate composite.

In the open matrices of a porous or spongy composite, the formation is the natural result of

processing and such matrices can be strengthened with different materials. 1etal impregnates are

used to improve strength or tolerance of the matrix. 1etal casting, graphite, powder metallurgy parts

and ceramics belong to this class of filled composites.

In the honeycomb structure, the matrix is not naturally formed, but specifically designed to a

predetermined shape. "heet materials in the hexagonal shapes are impregnated with resin or foam

and are used as a core material in sandwich composites.


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/illers may be the main ingredient or an additional one in a composite. The filler particles may be

irregular structures, or have precise geometrical shapes like polyhedrons, short fibers or spheres.

3hile their purpose is far from adding visual embellishment to the composites, they occasionally

impart colour or opacity to the composite which they fill.

As inert additives, fillers can change almost any basic resin characteristic in all directions

re-uired, to tide over the many limitations of basic resins as far as composites are concerned. The

final composite properties can be affected by the shape, surface treatment, blend of particle types,

si7e of the particle in the filler material and the si7e distribution.

/illed plastics tend to behave like two different constituents. They do not alloy and accept the

bonding. They are meant to develop mutuallyD they desist from interacting chemically with each

other. It is vital that the constituents remain in co+ordination and do not destroy each others desired

properties.

1atrix in a few filled composites provides the main framework while the filler furnishes almost

all desired properties. Although the matrix forms the bulk of the composite, the filler material is used

in such great -uantities relatively that it becomes the rudimentary constituent.


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The benefits offered by fillers include increase stiffness, thermal resistance, stability, strength

and abrasion resistance, porosity and a favorable coefficient of thermal expansion.

(owever, the methods of fabrication are very limited and the curing of some resins is greatly

inhibited. They also shorten the life span of some resins and are known to weaken a few composites.

)artic#late $einforced Composites

1icrostructures of metal and ceramics composites, which show particles of one phase strewn in

the other, are known as particle reinforced composites. "-uare, triangular and round shapes of

reinforcement are known, but the dimensions of all their sides are observed to be more or less e-ual.

The si7e and volume concentration of the dispersoid distinguishes it from dispersion hardened

materials.

The dispersed si7e in particulate composites is of the order of a few microns and volume

concentration is greater than #EC. The difference between particulate composite and dispersion

strengthened ones is, thus, oblivious. The mechanism used to strengthen each of them is also

different. The dispersed in the dispersion+strengthen materials reinforces the matrix alloy by

arresting motion of dislocations and needs large forces to fracture the restriction created by

dispersion.

In particulate composites, the particles strengthen the system by the hydrostatic coercion of

fillers in matrices and by their hardness relative to the matrix.

Three+dimensional reinforcement in composites offers isotropic properties, because of the three

systematical orthogonal planes. "ince it is not homogeneous, the material properties ac-uire

sensitivity to the constituent properties, as well as the interfacial properties and geometric shapes of

the array. The composite=s strength usually depends on the diameter of the particles, the inter+particle

spacing, and the volume fraction of the reinforcement. The matrix properties influence the behaviour

of particulate composite too.

Cermets

&ermets are one of the premier groups of particle strengthened composites and usually comprises

ceramic grains of borides, carbides or oxides. The grains are dispersed in a refractory ductile metal

matrix, which accounts for #$ to E


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varied depending on the relative volumes and compositions and of the metal and ceramic

constituents. Impregnation of a porous ceramic structure with a metallic matrix binder is another

method used to produce cermets. &ermets may be employed as coating in a power form. The power

is sprayed through a gas flame and fused to a base material. A wide variety of cermets have been

produced on a small scale, but only a few have appreciable value commercially.

Microspheres

1icrospheres are considered to be some of the most useful fillers. Their specific gravity, stable

particle si7e, strength and controlled density to modify products without compromising on

profitability or physical properties are its their most+sought after assets.

"olid 1icrospheres have relatively low density, and therefore, influence the commercial value

and weight of the finished product. "tudies have indicated that their inherent strength is carried over

to the finished molded part of which they form a constituent.

(ollow microspheres are essentially silicate based, made at controlled specific gravity. They are

larger than solid glass spheres used in polymers and commercially supplied in a wider range of

particle si7es. &ommercially, silicate+based hollow microspheres with different compositions using

organic compounds are also available. )ue to the modification, the microspheres are rendered less

sensitive to moisture, thus reducing attraction between particles. This is very vital in highly filled

li-uid polymer composites where viscosit& enhancement constraints the -uantum of filler loading.

/ormerly, hollow spheres were mostly used for thermosetting resin systems. 0ow, several new

strong spheres are available and they are at least five times stronger than hollow microspheres in

static crush strength and four times long lasting in shear.

2ecently, ceramic alumino silicate microspheres have been introduced in thermoplastic systems.

>reater strength and higher density of this system in relation to siliceous microspheres and their

resistance to abrasions and considerable strength make then suitable for application in high pressure

conditions.

(ollow microspheres have a lower specific gravity than the pure resin. This makes it possible

to use them for lightning resin dominant compounds. They find wide applications in aerospace and

automotive industries where weight reduction for energy conservation is one of the main

considerations.

4ut their use in systems re-uiring high shear mixing or high+pressure moulding is restricted as

their crush resistance is in no way comparable to that of solid spheres. /ortunately, 5udicious


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applications of hollow spheres eliminate cra7ing at the bends in the poly+vinyl chloride plastisol

applications, where the end component is sub5ected to bending stresses.

1icrospheres, whether solid or hollow, show properties that are directly related to their

spherical shape let them behave like minute ball bearing, and hence, they give better flow properties.

They also distribute stress uniformly throughout resin matrices.

In spherical particles, the ratio of surface area to volume is minimum. In resin+rich surfaces of

reinforced systems, the microspheres which are free of orientation and sharp edges are capable of

producing smooth surfaces.

4ased on the of type of the matrix used, composite can be classified as

". )ol&mer Matri% Composites 0)MC1

'. Metal Matri% Composites 0MMC1

(. Ceramic Matri% Composite 0CMC1

)ol&mer Matri% Composites 0)MC1

*olymer matrix composites have the advantage of easy fabrication, low cost, low

density and chemical inertnessD however, most advanced resin cannot be used above %$$o& for

prolonged period.

Metal Matri% Composites 0MMC1

1etal matrix composite can be used at higher temperature ranges. The maximum

temperature that can withstand by such material is about

6$$$o& as in the case of fiber reinforced super alloy.

Ceramic Matri% Composite 0CMC1

&eramic composites have the advantage over metal matrix composites at even higher

temperature because of their lower chemical reactivity and greater oxidation reaction resistance

however, their main disadvantages are'

6. :ow strain to failure, which limit the stress in the fiber at low levels.

#. 2elatively high modulus and lack of ductility, which prevents accommodation of thermal

stresses from, any mismatch in thermal expansion.


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Metal matri% composites 0MMC1

1etal matrix composites in general consist of two or more chemically distinct

macroconsituents, separated by distinct interface. ne obviously the primary phase is the metal

matrix and the second component is the reinforcement. In the production of the composites, the

matrix and the reinforcement are mixed together. This is to distinguish a composite from a two phase

alloy where the second phase forms as a particulate, eutectic or eutectoid reaction, etc., in other

words a composite initially begins as separate components, i.e., the metal matrix and the

reinforcement.

MODU! – 234

Classification of the Metal Matri% Composites

The metal matrix composites may be classified in many ways, depending on the

concept of interest. ne useful classification system is based on micro structural aspects,

differentiating between composite materials according to the morphology of the constituent phase.

@sing this system composite materials fall into following categories'

6. )ispersion strengthened composites.

#. *article strengthened composites.

%. /iber reinforced composites.

Dispersion 5trengthened Composites

This composite is characteri7ed by a microstructure consisting of an element matrix

within which fine particles are uniformly dispersed. The particle diameter ranges from $.$6 to $.6

Fm and the volume fraction of particle ranges from 6 to 6


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as hardness, wear resistance and compressive strength. /ibers are expected to effect better

improvements in rigidity and tensile strength /ranck A. >irot et al, 6BE;!. "ilicon &arbide has

advantages over other reinforcement such as thermal conductivity, densitity, relative cost, corrosion

resistance, machinability and workability of the aluminum alloy+ silicon carbide composites

"rivatsan et al, 6BB

as the reinforcement in aluminium alloy are graphite, "i #, 1g, "i% 08, Ti&, 40, 1ica, Jr#, 48&,

Ti#, Al#% and >lass.

ier $einforced Composites

The reinforcing phase in fiber composite materials' a! spans the entire si7e range, from

a fraction of a Fm to several mm in diameterD b! spans the length from mm to continuous fibersD

and c! spans an entire range of volume concentration, from a few percent to greater than E$C.

ver the years, a larger number of fibers have been investigated as possible

reinforcements for metals. The most important during the mid of 6BB$=s are alumina, boron, carbon

and silicon carbide. ther fibers used at various times include boron carbide, silica, 7irconia,

magnesia, alumina+boria+silica, silicon nitride, mullite, boron nitride and titanium diboride. In

addition, new fibers are continually being developed. ne example is “tryanno fibers” a silicon

carbide based material, which is composed of titanium, silicon, carbon and oxygen.

&oatings are often applied to fibers for a variety of reasons' to promote wetting by

matrix, to reduce handling damage, to bridge or fill in surface defects to improve strength, to

optimi7e fiber+matrix bonding characteristics and to prevent deleterious fiber+matrix interaction at

high temperatures.

The reinforcing phase in fiber reinforced composites span the entire si7e range, from

$.6 to #


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applications emerge with material re-uirements that can not be met by monolithic metals and other

composites.

There are very few commercial applications of 11&s at the present time. Industry

observers believe that the level of development effort in the @nited "tates is not large enough to lead

to significant near+term commercial use of 11&s. (owever, the Toyota truck diesel engine piston

has spurred the ma5or @" automakers to undertake preliminary activity in 11&s. Although

considerable interest in these materials is being generated in the @nited "tates, it will be decades

before they are likely to have an appreciable impact on production levels of competing materials.

The space shuttle orbiter was the first aerospace system to use 11&s in a production

application. The fuselage and nose landing gear use a total of #8# unidirectional boron fiber

reinforced aluminium circular tubes, providing a weight saving of about 8


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MODU! – 6

Tale "." 7pplications of MMC

5l.

No

7rea of

application

5ome of the components 7dvantages

6 Aerospace

Industry

"pace structures, Antennas, Aircraft

primary and "econdary structures,

ngine static and rotating

components, transmission static and

rotating components, satellite K

helicopter structures, compressor

blades.

(igh thrust to weight ratio for

engines, (igh stiffness, low

density, controlled thermal

expansion, (igh wear resistance

etc.

# Automotive

Industry

*iston K &ylinder blocks for I.&

engines, drive shaft, connecting rod,

wheels &hassis, 4rake rotors.

(igh wear resistance, :ower cost,

:ower density, levated

temperature strength, fatigue

resistance, &ontrolled thermal

expansion

% lectronic

packaging

"ubstrate and housing for electronic

microcircuits.

(igh stiffness, (igh heat

dissipation capacity, controlled

thermal expansion, low density.

8 "ports Industry 4icycle frames, Tennis rackets (igh stiffness, :ow density, (igh

fatigue resistance


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MODU! – 8

7dvantages and imitations of Composite Materials

The ma5or advantages of A1& compared to unreinforced materials are as follows

6. >reater strength

#. Improved stiffness

%. 2educed density weight!

8. Improved high temperature properties

*a to #8$ >*a by reinforcing with ?$+volC continuous

aluminium fiber. n the other hand incorporation of ?$+volC alumina fibers in pre aluminium leads

to decrease in coefficient of thermal expansion ?$C approximately. "imilarly it is possible to process

Al+BC"i+#$volC "i& p composites having wear resistance e-uivalent or better than of gray cast iron.

All these examples illustrates that it is possible to alter several technological properties of

aluminiumLaluminium alloy by more than two three orders of magnitude by incorporating

appropriate reinforcement in suitable volume fraction "urappa, #$$#!. The primary disadvantages of

all aluminium 11& is, they suffer from low ductility, inade-uate fracture toughness and inferior fatigue crack growth and fracture resistance compared to that of the constituent matrix material.

Disadvantages and imitations of Composite Materials

6. *roperties of many important composites are anisotropic + the properties differ depending

on the direction in which they are measured M this may be an advantage or a disadvantage

#. 1any of the polymer+based composites are sub5ect to attack by chemicals or solvents, 5ust

as the polymers themselves are susceptible to attack

%. &omposite materials are generally expensive

8. 1anufacturing methods for shaping composite materials are often slow and costly

0ot often environmentally friendly.


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;. &an be damaged.

E. Anisotropic properties.

B. 1atrix degrades.

6$. :ow reusability.

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