engineering material lec#3

7/30/2019 Engineering Material LEC#3

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CERAMICS

The word Ceramic is derived from a Greek word keramikos which means burnt

earth. In traditional ceramics (untill mid of 21st century), the primary raw material was

clay. The products considered in traditional ceramics are china-ware, porcelain, bricks,

tiles, and, in addition, glasses and high-temperature ceramics. But industrial ceramics(or advanced ceramics ) are non-metallic inorganic materials, including metal oxides,

borides, carbides, and nitrides as well as complex mixture of these materials.

Because most of the ceramics are composed of at least two elements, and often more,

their crystal structures are generally more complex than those for metals.

Ceramics can be crystalline, amorphous or mixture of both. Crystalline ceramics

have a characteristically brittle behaviour.

The atomic bonding in ceramics ranges from purely ionic to totally covalent; many

ceramics exhibit a combination of these two bonding types, the degree of ionic

character being dependent on the electronegativities of the atoms.

For those ceramic materials for which the atomic bonding is predominantly ionic, thecrystal structures may be thought of as being composed of electrically charged ions

instead of atoms.


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GENERAL PROPERTIES OF CERAMICS

High Youngs Modulus and high melting points(Strong bonds (covalent and /or ionic))

Limited electrical and thermal conductivity

Low thermal shock resistance (Coefficients of thermal expansion and thermal

conductivity are low)

Refractory (Stability at high temperature (NO CREEP))

Resistance to oxidation/corrosion (Chemical stability)


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Glass1. Based on SiO2 + additives

Traditional Ceramics1. Porous ceramics (bricks, pottery, china)

2. Compact ceramics (porcelain, earthware)

3. Refractory ceramics (SiC, Al2O3, ZrO2, BeO, MgO).Industrial / Advanced Ceramics1. Magnetic Ceramics

2. Electronics (Piezoelectric, capacitor dielectric, spark plugs and Ferroelectrics)

3. Electro-optics: LiNbO3

4. Abrasive ceramics: nitrides and carbides Si3N4, SiC

5. Superconductive ceramics ( yttrium barium copper oxide ceramic, YBa2Cu3O7)

6. Biomaterials : Hydroxyapatite

7. Automotive ceramics

8. Nuclear Ceramics

9. Tribological ceramics (resistant to wear and friction)

Classification of Ceramics


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Alumina (Al2O3) is used in applications where a material must operate at hightemperatures with high strength. Alumina is also used as a low dielectric constant

substrate for electronic packaging that houses silicon chips. One classic application is

insulators in spark plugs. Some unique applications are being found in dental and

medical use.

Diamond (C) is the hardest naturally occurring material. Industrial diamonds are usedas abrasives for grinding and polishing. It is, of course, also used in jewelry.

Silica (SiO2) is probably the most widely used ceramic material. Silica is an essentialingredient in glasses and many glass-ceramics. Silica-based materials are used in

thermal insulation, refractories, abrasives, as fiber-reinforced composites, and laboratory

glassware. In the form of long continuous fibers, silica is used to make optical fibers for

communications. Powders made using fine particles of silica are used in tires, paints,and many other applications.

Silicon carbide (SiC) provides outstanding oxidation resistance at temperatures evenabove the melting point of steel. SiC often is used as a coating for metals, carbon-

carbon composites, and other ceramics to provide protection at these extreme

temperatures. SiC is also used as an abrasive in grinding wheels and as particulate and

fibrous reinforcement in both metal matrix and ceramic matrix composites. It is also usedto make heating elements for furnaces.


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Silicon nitride (Si3N4) has properties similar to those of SiC, although its oxidation

resistance and high temperature strength are somewhat lower. Both silicon nitride andsilicon carbide are likely candidates for components for automotive and gas turbine

engines, permitting higher operating temperatures and better fuel efficiencies with less

weight than traditional metals and alloys.

Titanium dioxide (TiO2) is used to make electronic ceramics such as BaTiO3. Fineparticles of TiO2 are used to make suntan lotions that provide protection against

ultraviolet rays.Zirconia (ZrO2) is used to make many other ceramics such as zircon. Zirconia is alsoused to make oxygen gas sensors that are used in automotives and to measure

dissolved oxygen in molten steels. Zirconia is used as an additive in many electronic

ceramics as well as a refractory material. The cubic form of zirconia single crystals is

used to make jewelry items.

Boron Nitride (BN) Because of excellent thermal and chemical stability, boron nitrideceramics are traditionally used as parts of high-temperature equipment. Boron nitride

has a great potential in nanotechnology. Nanotubes of BN can be produced that have a

structure similar to that of carbon nanotubes. The carbon nanotubes can be metallic or

semiconducting, whereas a BN nano-tube is an electrical insulator. BN nanotubes are

more thermally and chemically stable than carbon nanotubes which favors them for

some applications.


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Composites are produced to give a combination of properties that cannot be attained by

a single material but when two or more materials or phases are used together. A

composite material is a microscopic or macroscopic combination of two or more distinct

materials with a recognizable interface between them. A common example of a

composite is concrete. It consists of a binder (cement) and a reinforcement (gravel).

Composite materials may be selected to give unusual combinations of stiffness,

strength, weight, high-temperature performance, corrosion resistance, hardness, or

conductivity. Composites highlight how different materials can work in synergy. Materials

that have specific and unusual properties are needed for a host of high-technologyapplications such as those found in the aerospace, underwater, bioengineering, and

transportation industries.

The individual materials that make up composites are called constituents. Most

composites have two constituent materials: a binder ormatrix, and a reinforcement. The

reinforcement is usually much stronger and stiffer than the matrix, and gives thecomposite its good properties. The matrix holds the reinforcements in an orderly pattern.

Because the reinforcements are usually discontinuous, the matrix also helps to transfer

load among the reinforcements.

In composites , the constituent materials must be chemically dissimilar and separated by

a distinct interface.

COMPOSITE MATERIALS


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Composites can be placed into three categories: particle-reinforced, fiber-reinforced, and

structural composites based on the shapes of the materials. Concrete, a mixture of

cement and gravel, is a particulate composite; fibreglass, containing glass fibres

embedded in a polymer, is a fibre-reinforced composite; and plywood, having alternatinglayers of wood veneer, is a laminar composite.

Classification of Composite Materials


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PARTICLE-REINFORCED COMPOSITES

In particle-reinforced composites, particles of distinct materials are embedded together

to form the composite. The particulates can be large particles (such as gravels, as in

case of concrete) or very small particles (< 0.25 microns). Thus large-particle

composites and dispersion-strengthened composites are the two sub classifications of

particle-reinforced composites.

Large-Particle Composites: In large particle composites, particlematrix interactions isnot treated on the atomic or molecular level. Here the matrix refers to the bonding

medium which is continuous and surrounds the other phase. The degree of

reinforcement or improvement of mechanical behaviour depends on strong bonding at

the matrixparticle interface. A very common large-particle composite is concrete, whichis composed of cement (the matrix) and sand and gravel (the particulates).

Dispersion-Strengthened Composites: For dispersion-strengthened composites,

particles are normally much smaller, with diameters between 0.01 and 0.1 .m (10 and

100 nm). Particlematrix interactions that lead to strengthening occur on the atomic or

molecular level. Dispersion-strengthened composites are examples of nano-compositesthat consists of nano-scale particles distributed in a matrix phase. Many glass ceramics

are nano-scale composites of different ceramic phases. Many plastics can be

considered composites as well.


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Cermets are example of large-particle composites. These are ceramicmetal

composites, containing hard ceramic particles dispersed in a metallic matrix.

The most common cermet is cemented carbide, which is composed of extremely hard

particles of a refractory carbide ceramic such as tungsten carbide (WC) or titaniumcarbide (TiC), embedded in a matrix of a metal such as cobalt or nickel. These

composites are used extensively as cutting tools for hardened steels. The hard carbide

particles provide the cutting surface but, being extremely brittle, are not themselves

capable of withstanding the cutting stresses. Toughness is enhanced by their inclusion in

the ductile metal matrix, which isolates the carbide particles from one another and

prevents particle-to-particle crack propagation.

CERMETS


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FIBER-REINFORCED COMPOSITESTechnologically, the most important composites are those in which the dispersed phase

is in the form of a fiber. Straw has been used to strengthen mud bricks for centuries.

Steel-reinforcing bars are introduced into concrete structures. Glass fibers in a polymer

matrix produce fiber glass for transportation and aerospace applications. Design goals of

fiber-reinforced composites often include high strength and/or stiffness on a weightbasis.

1. With fiber-reinforced composites, an applied load is transmitted to and distributed

among the fibers via the matrix phase, which in most cases is at least moderately

ductile.

2. Significant reinforcement is possible only if the matrixfiber bond is strong.

On the basis of fiber length and orientation, three different types of fiber-reinforcedcomposites are possible:

Continuous and aligned (Figure a)mechanical properties are highly anisotropic. In

the alignment direction, reinforcement and

strength are maximum; perpendicular to the

alignment, they are a minimum.

Discontinuous and aligned (Figure b)

significant strengths and stiffnesses are

possible in the longitudinal direction.

Discontinuous and randomly oriented(Figure c) despite some limitations on

reinforcement efficiency, properties are isotropic


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FIBER-REINFORCED COMPOSITES (contd.)

Many factors must be considered when designing a fiber-reinforced composite, including

properties of the fibers; the properties of the matrix; and the bonding between the fibers

and the matrix.

Characteristics of Fiber Phase: While designing a fiber-reinforced composite, variouscharacteristics of fiber are considered; including the length, diameter, orientation,

amount, and properties of the fiber.

Fiber Length and Diameter: Fibers can be short, long, or even continuous. Theirdimensions are often characterized by the aspect ratio l/d, where l is the fiber length and

d is the diameter. The strength of a composite improves when the aspect ratio is large.

On the basis of diameter and character, fibers are grouped into three differentclassifications: whiskers, fibers, and wires. Whiskers are very thin single crystals that

have extremely large aspect ratios. As a consequence of their small size, they have a

high degree of crystalline perfection and are virtually flaw-free, which accounts for their

exceptionally high strengths. They are extremely expensive, thus are not used

extensively as a reinforcement medium. Materials that are classified as fibersare either

polycrystalline or amorphous and have small diameters. Fine wires have relatively largediameters; typical materials include steel, molybdenum, and tungsten. Wires are used as

a radial steel reinforcement in automobile tires, in filament-wound rocket casings, and in

wire-wound high-pressure hoses.


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FIBER-REINFORCED COMPOSITES (contd.)

Amount of Fiber: A greater volume fraction of fibers increases the strength andstiffness of the composite, as we would expect from the rule of mixtures. The maximum

volume fraction is about 80%, beyond which fibers can no longer be completely

surrounded by the matrix.

Orientation Of Fibers: The reinforcing fibers may be introduced into the matrix in anumber of orientations. Short, randomly oriented fibers having a small aspect ratio-

typical of fiberglass-give relatively isotropic behaviour in the composite. Long,

continuous fibers can be introduced in several directions within the matrix.

Unidirectional arrangements of fibers produce anisotropic properties, with Long, or even

continuous, particularly good strength and stiffness parallel to the fibers. Unidirectionalorientations provide poor properties if the load is perpendicular to the fibers.


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STRUCTURAL COMPOSITES

A structural composite is normally composed of both homogeneous and composite

materials, the properties of which depend not only on the properties of the constituent

materials but also on the geometrical design of the various structural elements. Laminar

composites and sandwich panels are two of the most common structural composites.

Laminar composite is composed of two-dimensional sheets or panels that have apreferred high-strength direction, such as is found in wood and continuous and aligned

fiberreinforced plastics. The layers are stacked and subsequently cemented together

such that the orientation of the high-strength direction varies with each successive layer.

For example, adjacent wood sheets in plywood are aligned with the grain direction at

right angles to each other.


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Sandwich panels, considered to be a class of structural composites, are designed to belightweight beams or panels having relatively high stiffnesses and strengths. A sandwich

panel consists of two outer sheets, or faces, that are separated by and adhesively

bonded to a thicker core. The outer sheets are made of a relatively stiff and strong

material, typically aluminum alloys, fiber-reinforced plastics, titanium, steel, or plywood;

they impart high stiffness and strength to the structure and must be thick enough to

withstand tensile and compressive stresses that result from loading. The core material is

lightweight and normally has a low modulus of elasticity.

Gl Fib R i f d P l (GFRP) C i


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Glass FiberReinforced Polymer (GFRP) Composites

Fiber glass is simply a composite consisting of glass fibers, either continuous or

discontinuous, contained within a polymer matrix. Glass fiber composites are the most

widely used and the least expensive of all fibers. GFRP composite may contain between

30 and 60% glass fibers by volume. Fibers are made by drawing molten glass through

small openings in a platinum die. The molten glass is then elongated mechanically,cooled, and wound on a roll. A protective coating or sizing may be applied to facilitate

their passage through the machinery.

The following are the principal types of glass fibers:

E-type: a calcium aluminoborosilicate glass, the type most commonly used.

S-type: a magnesia aluminosilicate glass, offering higher strength and stiffness, butat a higher cost. E-CR-type: a high-performance glass fiber, offering higher resistance to elevated

temperatures and acid corrosion than does the E glass.

Surface flaws are easily introduced on glass fibers by rubbing the surface with another

hard material or surface layer is weakened if glass fibers are exposed to the normal

atmosphere for even short time periods. Thus Newly drawn fibers are normally coatedduring drawing.

REFRACTORY MATERIALS


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REFRACTORY MATERIALS

Refractory materials are important components of the equipment used in the production,

refining, and handling of metals and glasses, for constructing heat-treating furnaces, and

for other high-temperature processing equipment. The refractories must survive at high

temperatures without being corroded or weakened by the surrounding environment.

Refractory materials are marketed in a variety of forms, but bricks are the most common.Typical applications include furnace linings for metal refining, glass manufacturing,

metallurgical heat treatment, and power generation.

The performance of a refractory ceramic depends mainly on its composition. On this

basis, there are several classifications: fireclay, silica, basic, and special refractories.

Fireclay RefractoriesThe primary ingredients for the fireclay refractories are high-purity fireclays, alumina andsilica mixtures usually containing between 25 and 45 wt% alumina. Fireclay bricks are

used principally in furnace construction, to confine hot atmospheres, and to thermally

insulate structural members from excessive temperatures. For fireclay brick, strength is

not ordinarily an important consideration, because support of structural loads is usually

not required.Silica RefractoriesThe prime ingredient for silica refractories, sometimes termed acid refractories, is silica.

These materials, well known for their high-temperature load-bearing capacity, are

commonly used in the arched roofs of steel- and glass-making furnaces; for these

applications, temperatures as high as 1650.C (3000.F) may be realized.

Basic Refractories


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Basic RefractoriesThe refractories that are rich in magnesia (MgO), are termed basic refractory. They may

also contain calcium, chromium, and iron compounds. The presence of silica is

deleterious to their high-temperature performance. Basic refractories are especially

resistant to attack by slags containing high concentrations of MgO and CaO and find

extensive use in some steel-making open hearth furnaces.Special RefractoriesThese ceramic materials are used for specialized refractory applications. Some of these

are relatively high-purity oxide materials, many of which may be produced with very little

porosity. Included in this group are alumina, silica, magnesia, beryllia (BeO), zirconia

(ZrO2), and mullite (3Al2O32SiO2). Others include carbide compounds, in addition to

carbon and graphite. Silicon carbide (SiC) has been used for electrical resistance

heating elements, as a crucible material, and in internal furnace components. Carbon

and graphite are very refractory, but find limited application because they are susceptible

to oxidation at temperatures in excess of about 800.C (1470.F).


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Refractory metals are those having a melting temperature above 1925C. Examples

include niobium (Nb), molybdenum (Mo), tungsten (W), and tantalum (Ta). Meltingtemperatures range between 2468.C (4474.F) for niobium and 3410.C (6170.F) for

tungsten, the highest melting temperature of any metal. Interatomic bonding in these

metals is extremely strong, which accounts for the melting temperatures, and, in

addition, large elastic moduli and high strengths and hardnesses, at ambient as well as

elevated temperatures. The applications of these metals are varied. For example,

tungsten alloys are used for incandescent light filaments, x-ray tubes, and weldingelectrodes. Molybdenum alloys are utilized for extrusion dies and structural parts in

space vehicles. Tantalum and molybdenum are alloyed with stainless steel to improve its

corrosion resistance.

REFRACTORY METALS

GLASS


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GLASS

Glass is an amorphous solid with the structure of a liquid obtained by supercooling.Technically, glass is defined as an inorganic product of fusion that has cooled to a rigidcondition without crystallizing. The glasses are a familiar group of ceramics;containers, lenses, and fiber glass represent typical applications.

GLASS-CERAMICS

Glass-ceramics are crystalline materials that are derived from amorphous glasses.Usually, glass-ceramics have a substantial level of crystallinity (70-99%). Most inorganic

glasses can be made to transform from a noncrystalline state to one that is crystallineby the proper high-temperature heat treatment. This process is called crystallization, and

the product is a fine-grained polycrystalline material that is often called a glass-ceramic.The first step in producing a glass-ceramic is to ensure that crystallization does not

occur during cooling from the forming temperature. A continuous and isothermal cooling

is required. If glass cools too slowly, nucleation and growth of the crystals begin, but in

an uncontrolled manner. Nucleation of the crystalline phase is controlled in two ways.

First, the nucleating agents such as TiO2

are added to promote crystallization. TiO2react

with other oxides and form phases that provide the nucleation sites. Second, a heattreatment is designed to provide the appropriate number of nuclei; the temperature

should be relatively low in order to maximize the rate of nucleation. The formation of

these small glass-ceramic grains is, in a sense, a phase transformation, which involves

nucleation and growth stages.

With glass ceramics e can take ad antage of the formabilit and densit of glass


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With glass-ceramics, we can take advantage of the formability and density of glass.

Also, a product that contains very low porosity can be obtained by producing a shape

with conventional glass-forming techniques, such as pressing or blowing.

In comparison with glasses, glass-ceramic materials have been designed to have the

following characteristics: relatively high mechanical strength and toughness; low

coefficients of thermal expansion; high-temperature corrosion resistance; good dielectricproperties; and good biological compatibility. Some glass-ceramics may be made

optically transparent; others are opaque. Possibly the most attractive attribute of this

class of materials is the ease with which they may be fabricated; conventional glass-

forming techniques may be used in the mass production of nearly pore-free ware.

engineering material lec#3

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