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Composite Materials Asst. Prof. Dr. Ayşe KALEMTAŞ Composite Materials Asst. Prof. Dr. Ayşe KALEMTAŞ

COMPOSITE

MATERIALS

Asst. Prof. Dr. Ayşe KALEMTAŞ

Office Hours: Tuesday, 16:30-17:30

[email protected], [email protected]

Phone: +90 – 252 211 19 17

Metallurgical and Materials Engineering Department

mailto:[email protected]







ISSUES TO ADDRESS

Ceramic Materials

Ceramic Matrix Composites

Properties


What is "ceramic"?

• from Greek meaning: "burnt earth"

• non-metal, inorganic

• Ceramic materials are inorganic compounds consisting of metallic and nonmetallic elements which are held together with ionic and/or covalent bonds.


Introduction to Ceramic Materials

Ceramics are

inorganic, nonmetallic, solids, crystalline,

amorphous (e.g. glass), hard, brittle, stable

to high temperatures, less dense than

metals, more elastic than metals, and very

high melting.

Ceramics can be covalent network and/or ionic

bonded.



Ductile versus Brittle Fracture

Fracture Behavior: Very Ductile Modulate Ductile Brittle

Ductile Fracture is

Desirable

Ductile warning

before fracture

Brittle no warning

before fracture

% RA or %EL: Large Moderate Small



A comparison of the properties of ceramics, metals, and polymers



Bonding:

Mostly ionic, some covalent.

% ionic character increases with difference electronegativity.

CaF2

SiC



Ceramic Materials

Advanced Ceramics Traditional Ceramics

Advanced ceramics

Made from artificial or chemically modified raw

materials.

Traditional ceramics

Mainly made from natural raw materials such as kaolinite (clay mineral), quartz and

feldspar.



Ceramic Materials

Advanced Ceramics

Structural Ceramics

Bioceramics

Ceramics used in automotive industry

Nuclear ceramics

Wear resistant ceramics (tribological)

Functional Ceramics

Electronic substrate, package ceramics

Capasitor dielectric, piezoelectric ceramics

Magnetic ceramics

Optical ceramics

Conductive ceramics

Traditional Ceramics

Whitewares

Cement

Abrasives

Refractories

Brick and tile

Structural clay products



The technology of ceramics is a rapidly developing applied science in

today’s world. Technological advances result from unexpected material

discoveries. On the other hand, the new technology can drive the

development of new ceramics.

Currently many new classes of materials have been devised to satisfy

various new applications. Advanced ceramics offer numerous

enhancements in performance, durability, reliability, hardness, high

mechanical strength at high temperature, stiffness, low density, optical

conductivity, electrical insulation and conductivity, thermal insulation

and conductivity, radiation resistance, and so on.

Ceramic technologies have been widely used for aircraft and

aerospace applications, wear-resistant parts, bio-ceramics, cutting

tools, advanced optics, superconductivity, nuclear reactors, etc.

M. Rosso, Ceramic and metal matrix composites: Routes and properties, Journal of Materials Processing Technology 175 (2006) 364–375



Ceramics application could be categorised as structural

ceramics, electrical ceramics, ceramic composites, and

ceramic coatings.

These materials are emerging as the leading class of

materials needed to be improved to explore further

potential applications.

An advanced ceramics application tree, which classifies its

current and potential application areas and related

advantageous properties, has been developed.




Advanced

ceramic

application

tree

Limitations due to

- High cost

- Low toughness

- Low reliability M. Rosso, Ceramic and metal matrix composites: Routes and properties, Journal of Materials Processing

Technology 175 (2006) 364–375



OXIDES

The raw materials used for oxide ceramics

are almost entirely produced by chemical

processes to achieve a high chemical purity

and to obtain the most suitable powders for

component fabrication.

NONOXIDES

Most of the important nonoxide ceramics

do not occur naturally and therefore must

be synthesized. The synthesis route is

usually one of the following:

Combine the metal directly with the

nonmetal at high temperatures.

Reduce the oxide with carbon at high

temperature (carbothermal reduction) and

subsequently react it with the nonmetal.


Ceramic Matrix Composites (CMCs)

• A ceramic primary phase imbedded with a

secondary phase, which usually consists of fibers.

• Attractive properties of ceramics: high stiffness,

hardness, hot hardness, and compressive

strength; and relatively low density.

• Weaknesses of ceramics: low toughness and bulk

tensile strength, susceptibility to thermal cracking .

• CMCs represent an attempt to retain the desirable

properties of ceramics while compensating for their

weaknesses .



• The matrix is relatively hard and brittle

• The reinforcement must have high tensile

strength to arrest crack growth

• The reinforcement must be free to pull out as a

crack extends, so the reinforcement-matrix bond

must be relatively weak

Composite Materials Asst. Prof. Dr. Ayşe KALEMTAŞ


Ceramic matrix composites (CMC) are used in

applications where resistance to high temperature and

corrosive environment is desired. CMCs are strong

and stiff but they lack toughness (ductility).

Matrix materials are usually silicon carbide, silicon

nitride and aluminum oxide, and mullite (compound of

aluminum, silicon and oxygen). They retain their

strength up to 1650C.

Fiber materials used commonly are carbon and

aluminum oxide.

Applications are in jet and automobile engines, deep-

see mining, cutting tools, dies and pressure vessels.



Monolithic ceramics have reasonably

high strength and stiffness but are brittle.

Thus one of the main objectives in

producing ceramic matrix composites is

to increase the toughness.

Naturally it is also hoped, and indeed

often found, that there is a concomitant

in strength and stiffness.

Typical stress–strain curves for

composites with that for a monolithic

ceramic; the area under the stress–strain

curve is the energy of fracture of the

sample and is a measure of the

toughness. It is clear from this figure that

the reinforcement with particulates and

continuous fibres has lead to an increase

in toughness but that the increase is

more significant for the latter.

Schematic force–displacement curves for a

monolithic ceramic and CMCs illustrating the

greater energy of fracture of the CMCs

M. Rosso, Ceramic and metal matrix composites: Routes and properties,

Journal of Materials Processing Technology 175 (2006) 364–375



Both the monolithic and the

particulate-reinforced composite fail in

a catastrophic manner, which contrast

with the failure of the continuous fibre

composite where a substantial load

carrying capacity is maintained after

failure has commenced.

Therefore not only has the continuous

fibre composite a better toughness

but the failure mode is more

desirable.

However, fibres are a more expensive

reinforcement than particles and the

processing is more complex, therefore

the improvement in toughness is

associated with an extra cost burden.

Schematic force–displacement curves for a

monolithic ceramic and CMCs illustrating the

greater energy of fracture of the CMCs



Ceramic matrix composites

Ceramic matrix composite (CMC) development has lagged behind other

composites for two main reasons.

First more of the processing routes for CMCs involve high temperatures

and can only be employed with high temperature reinforcements. It

follows that it was not until fibres and whiskers of high temperature

ceramics, such as silicon carbide, were readily available was there

much interest in CMCs. The high temperature properties of the

reinforcement are also of importance during service. A major attribute of

monolithic ceramics is that they maintain their properties to high

temperatures and this characteristic is only retained in CMCs if the

reinforcements also have good high temperature properties. Hence,

there is only limited interest in toughening ceramics by incorporation of

reinforcements of materials, such as ductile metals, that lose their

strength and stiffness at intermediate temperatures.




The second factor that has hindered the progress of CMCs is also

concerned with the high temperatures usually employed for

production. Differences in coefficients of thermal expansion, ,

between the matrix and the reinforcement lead to thermal stresses

on cooling from the processing temperature.

However, whereas the thermal stresses can generally be relieved in

metal matrix composites by plastic deformation of the matrix, this is

not possible for CMCs and cracking of the matrix can result.

The nature of the cracking depends on the whether the

reinforcement contracts more or less than the matrix on cooling as

their determines the character (tensile or compressive) of the local

thermal stresses.




If R for a particulate reinforcement is great than that for the

matrix M then the circumferential cracks may be produced in the

matrix, and for R < M radial cracks may be found.

With a fibre reinforcement, when R > M the axial tensile

stresses induced in the fibres produce an overall net residual

compressive stresses in the matrix and, as the fibres contract,

there is a tendency for them to pull away from the matrix.

The stress situation is reversed when R < M and cracking of

the matrix due to the axial tensile stresses may occur. Clearly

there has to be some matching of the coefficients of thermal

expansion in order to limit these problems.




A variety of ceramic particulate, whiskers high-strength single crystals with

length/diameter ratios of 10 or more), and fibers may be added to the host

matrix material to generate a composite with improved fracture toughness.

The presence of these reinforcements appears to frustrate the propagation

of cracks by at least three mechanisms.

First, when the crack tip encounters a particle or fiber that it cannot easily

break or get around, it is deflected off in another direction. Thus, the crack

is prevented from propagating cleanly through the structure.

Second, if the bond between the reinforcement and the matrix is not too

strong, crack propagation energy can be absorbed by pullout of the fiber

from its original location.

Third, fibers can bridge a crack, holding the two faces together, and thus

prevent further propagation.



CMCs

Ceramic Matrix Composite (CMC) is a material consisting of a ceramic matrix combined with a

ceramic (oxides, carbides) dispersed phase.

Ceramic Matrix Composites are designed to improve toughness of conventional ceramics, the

main disadvantage of which is brittleness.

Ceramic Matrix Composites are reinforced by either continuous (long) fibers or discontinuous

(short) fibers.

Short-fiber (discontinuous) composites are produced by conventional ceramic processes from

an oxide (alumina) or non-oxide (silicon carbide) ceramic matrix reinforced by whiskers of

silicon carbide (SiC), titanium boride (TiB2), aluminum nitride (AlN), zirconium oxide (ZrO2) and

other ceramic fibers. Most of CMC are reinforced by silicon carbide fibers due to their

high strength and stiffness (modulus of elasticity).

Whiskers incorporated in a short-fiber Ceramic Matrix Composite improve its toughness

resisting to cracks propagation. However a character of failure of short-fiber reinforced

materials is catastrophic.

Long-fiber (continuous) composites are reinforced either by long monofilament of long

multifilament fibers.


CMCs

The best strengthening effect is provided by dispersed

phase in form of continuous monofilament fibers, which are

fabricated by chemical vapor deposition (CVD) of silicon

carbide on a substrate made of tungsten (W) or carbon

(C) fibers.

Monofilament fibers produce stronger interfacial

bonding with the matrix material improving its toughness.

Failure of long-fiber Ceramic Matrix Composites is not

catastrophic.


CMCs

Typical properties of long-fiber Ceramic Matrix

Composites:

• High mechanical strength even at high temperatures;

• High thermal shock resistance;

• High stiffness;

• High toughness;

• High thermal stability;

• Low density;

• High corrosion resistance even at high temperatures.


CMCs

Ceramic composites may be produced by traditional ceramic

fabrication methods including mixing the powdered matrix material with

the reinforcing phase followed by processing at elevated

temperature: hot pressing, sintering.

Such fabrication routs are successfully employed for preparing

composites reinforced with a discontinuous phase (particulate or short

fibers).

However the composites reinforced with continuous or long fibers are

rarely fabricated by conventional sintering methods due to mechanical

damage of the fibers and their degradation caused by chemical

reactions between the fiber and matrix materials at high sintering

temperature. Additionally sintering techniques result in high porosity of

the fiber reinforced composites.


CMCs

Ceramic matrix composites reinforced with long fibers are

commonly fabricated by infiltration methods.

In this group of fabrication techniques the ceramic matrix is

formed from a fluid (gaseous or liquid) infiltrated into the

fiber structure (either woven or non-woven).

Prior to the infiltration with a ceramic derived fluid the

reinforcing fibers surface is coated with a debonding

interphase providing weak bonding at the interface

between the fiber and matrix materials. Weak bonding

allows the fiber to slide in the matrix and prevents brittle

fracture.


CMCs

Matrix material for long-fiber (continuous fiber) composite may be

silicon carbide ceramic, alumina (alumina-silica) ceramic or carbon.

Silicon carbide matrix composites are fabricated by chemical vapor

infiltration or liquid phase Infiltration methods of a matrix material into

a preform prepared from silicon carbide fibers.

Alumina and alumina-silica (mullite) matrix composites are produced

by sol-gel method, direct metal oxidation or chemical bonding.

Carbon-Carbon composites are fabricated by chemical vapor

infiltration or Liquid phase infiltration methods of a matrix material

into a preform prepared from carbon fibers.



Following table presents the fracture toughness and critical flaw sizes (assuming a

typical stress of 700 MPa, or about 100,000 psi of a variety of ceramics and

compares them with some common metals.

Toughness of monolithic ceramics generally falls in the range of 3 to 6 MPa.m1/2,

corresponding to a critical flaw size of 18 to 74 µm. With transformation toughening

or whisker dispersion, the toughness can be increased to 8 to 12 MPa.m1/2 (the

critical flaw size is 131 to 294 µm); the toughest ceramic matrix composites are

continuous fiber-reinforced glasses, at 15 to 25 MPa.m1/2. In these glasses, strength

appears to be independent of preexisting flaw size and is thus an intrinsic material

property. By comparison, metal alloys such as steel have toughnesses of more than

40 MPa.m1/2, more than 10 times the values of monolithic ceramics; the toughness of

some alloys may be much higher.

The critical flaw size gives an indication of the minimum flaw size that must be

reliably detected any nondestructive evaluation (NDE) to ensure reliability of the

component. Most NDE techniques cannot reliably detect flaws smaller than about

100 µm (corresponding to a toughness of about 7 MPa.m1/2). Toughnesses of at least

10 to 12 MPa.m1/2 would be desirable for most components.



Fracture Toughness and

Critical Flaw Size of

Monolithic and Composite

Ceramic Materials Compared

With Metals.

LAS: lithium aluminosilicate;

CVD: chemical vapor deposition

a: Assumes a stress of 700 MPa (-100,000 psi).

b: The strength of these composites is

independent of preexisting flaw size.

c: The toughness of some alloys can be much

higher

SOURCES: David W. Richerson, “Design, Processing

Development, and Manufacturing Requirements of

Ceramics and Ceramic Matrix Composites,” contractor

report for OTA December 1985; and Elaine P. Rothman,

“Ultimate Properties of Ceramics and Ceramic Matrix

Composites,” contractor report for OTA, December 1985



Ceramic fibres such as SiC and Si3N4 use polysilane as the base material. CMCs,

in which ceramic or glass matrices are reinforced with continuous fibres, chopped

fibres, whiskers, platelets or particulates, are emerging as a class of advanced

engineering structural materials. They currently have limited high-temperature

applications but a large potential for much wider use in military, aerospace and

commercial applications such as energy-efficient systems and transportation.

There are also other specialty CMCs such as nanocomposites (made from

reactive powders) and electroceramics. CMCs are unique in that they combine

low density with high modulus, strength and toughness (contrasted with

monolithic ceramics) and strength retention at high temperatures. Many have

good corrosion and erosion characteristics for high temperature applications.




CMC Development

The 1970’s-1990’s

The Honeymoon

composite synthesis and characterisation via ceramics

technology

toughness and strength via fibers and interfaces

The Realization

“realistic” environments and realistic tests

interfaces and interphases control performance

the fibers manufacturers are key

CMC’s are not materials, they are structures

The 30 Year “Long Haul”

continuous improvement

ever changing industrial business base


Thanks for your kind

attention

THE END


Any

Questions

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