Material Science: Ceramics

Definitions:

Ceramics are known for their high temperature melting points, high mechanical strengths, electrical, magnetic, optical and thermal properties.

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The word ceramic is derived from the Greek word κεραμικός (keramikos). The term covers inorganic non-metallic materials which are formed by the action of heat.

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The percentage ionic character: of a bond between elements A and B (A being the most electronegative) may be approximated by the expression

% ionic character = {1 – exp[-(0.25)(XA – XB)2]} x 100

where XA and XB are the electronegativities for the respective elements.

Two characteristics of the component ions in crystalline ceramic materials influence the crystal structure:

– the magnitude of the electrical charge on each of the component ions, and,

– the relative sizes of the cations and anions.

Question and Answers:

1. Why are ceramics brittle and most metals not?

In ceramic materials, the atoms are not free to move under stress as they are in metals.

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Ceramics have very high yield strengths, and thus very little plastic deformation takes place at crack tips in these materials. The local stress ahead of the crack tip reaches excess of the ideal strength and is thus large enough to break apart the interatomic bonds there. The crack then spreads between a pair of atomic planes giving rise to an atomically flat surface by cleavage. The energy required simply to break the interatomic bonds is much less than that absorbed by ductile tearing in a tough material, and this is why materials like ceramics and glasses are so brittle. The crack mechanism in Ceramics is called Cleavege.

– Engineering Materials 1

The atoms in ceramic materials are held together by a chemical bond which will be discussed a bit later. Briefly though, the two most common

chemical bonds for ceramic materials are covalent and ionic. Covalent and ionic bonds are much stronger than in metallic bonds and, generally speaking, this is why ceramics are brittle and metals are ductile.

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Brittle Fracture of Ceramics

Stress–Strain Behavior

At room temperature, virtually all ceramics are brittle. Microcracks, the presence

of which is very difficult to control, result in amplification of applied tensile stresses

and account for relatively low fracture strengths (flexural strengths). This amplification

does not occur with compressive loads, and, consequently, ceramics are

stronger in compression. Fractographic analysis of the fracture surface of a ceramic

material may reveal the location and source of the crack-producing flaw, as well as

the magnitude of the fracture stress. Representative strengths of ceramic materials

are determined by performing transverse bending tests to fracture.

Mechanisms of Plastic Deformation

Any plastic deformation of crystalline ceramics is a result of dislocation motion; the

brittleness of these materials is explained, in part, by the limited number of operable

slip systems. The mode of plastic deformation for noncrystalline materials is by

viscous flow; a material’s resistance to deformation is expressed as viscosity.At room

temperature, the viscosities of many noncrystalline ceramics are extremely high.

Miscellaneous Mechanical Considerations

Many ceramic bodies contain residual porosity, which is deleterious to both their moduli

of elasticity and fracture strengths. In addition to their inherent brittleness, ceramic

materials are distinctively hard. Also, since these materials are frequently utilized at

elevated temperatures and under applied loads, creep characteristics are important.

For crystalline ceramics, plastic deformation occurs, as with metals, by the motion

of dislocations (Chapter 7). One reason for the hardness and brittleness of these

materials is the difficulty of slip (or dislocation motion). For crystalline ceramic materials

for which the bonding is predominantly ionic, there are very few slip systems

(crystallographic planes and directions within those planes) along which dislocations

may move.This is a consequence of the electrically charged nature of the ions.

For slip in some directions, ions of like charge are brought into close proximity to

one another; because of electrostatic repulsion, this mode of slip is very restricted,

to the extent that plastic deformation in ceramics is rarely measurable at room temperature.

By way of contrast, in metals, since all atoms are electrically neutral, considerably

more slip systems are operable and, consequently, dislocation motion is

much more facile.

On the other hand, for ceramics in which the bonding is highly covalent, slip is

also difficult and they are brittle for the following reasons: (1) the covalent bonds

are relatively strong, (2) there are also limited numbers of slip systems, and (3) dislocation

structures are complex.

This causes detriment to fracture strength thus stress redistribution do not occur to any appreciable extent around flaws and discontinuities.

The brittle fracture process consists of the formation and propagation of cracks

through the cross section of material in a direction perpendicular to the applied

load. Crack growth in crystalline ceramics may be either transgranular (i.e., through

the grains) or intergranular (i.e., along grain boundaries); for transgranular fracture,

cracks propagate along specific crystallographic (or cleavage) planes, planes of high

atomic density.

Ceramics have very high yield strengths, and thus very little plastic deformation takes place at crack tips in these materials. The local stress ahead of the crack tip reaches excess of the ideal strength and is thus large enough to break apart the interatomic bonds there. The crack then spreads between a pair of atomic planes giving rise to an atomically flat surface by cleavage. The energy required simply to break the interatomic bonds is much less than that absorbed by ductile tearing in a tough material, and this is why materials like ceramics and glasses are so brittle. The crack mechanism in Ceramics is called Cleavege.

very high yield strengths, and thus very little plastic deformation takes place at crack tips in these materials.

Stress redistribution do not occur to any appreciable extent around flaws and discontinuities in brittle materials

The effect of a stress raiser is more significant in brittle than in ductile materials. For a ductile material, plastic deformation ensues when the maximum stress exceeds the yield strength. This leads to a more uniform distribution of stress in the vicinity of the stress raiser and to the development of a maximum stress concentration factor less than the theoretical value. Such yielding and stress redistribution do not occur to any appreciable extent around flaws and discontinuities in brittle materials; therefore, essentially the theoretical stress concentration will result.

This causes detriment to fracture strength and very high yield strengths, and thus very little plastic deformation takes place at crack tips in these materials. For a

The local stress ahead of the crack tip reaches excess of the ideal strength and is thus large enough to break apart the interatomic bonds there.

The crack then spreads between a pair of atomic planes giving rise to an atomically flat surface by cleavage. The energy required simply to break the interatomic bonds is much less than that absorbed by ductile tearing in a tough material, and this is why materials like ceramics and glasses are so brittle. The crack mechanism in Ceramics is called Cleavege.

These stress-raisers account for relatively low fracture strengths (flexural strengths).

Due to presence of stress raisers, Ceramics have significantly lower fracture strength.

When the magnitude of a tensile stress at the tip of one of these flaws exceeds the value of this critical stress, a crack forms and then propagates, which results in fracture.

The atoms in ceramic materials are most commonly held together by the two covalent and ionic primary bonds (or) mix of them. Covalent and ionic bonds are much stronger than metallic bonds. Both these bond types have a very few/limited slip systems (crystallographic planes and directions within those planes) as a consequence dislocations are limited and results in a negligible or no plastic deformation at room temperature. Ceramics, have very small and omnipresent flaws in the material that serve as stress raisers-points at which the magnitude of an applied tensile stress is amplified. These stress raisers may be minute surface or interior cracks (microcracks), internal pores, and grain corners, which are

virtually impossible to eliminate or control.

The measured fracture strengths for most brittle materials are significantly lower

than those predicted by theoretical calculations based on atomic bonding energies.

This discrepancy is explained by the presence of very small, microscopic flaws

or cracks that always exist under normal conditions at the surface and within the

interior of a body of material.These flaws are a detriment to the fracture strength

because an applied stress may be amplified or concentrated at the tip, the magnitude

of this amplification depending on crack orientation and geometry. This

phenomenon is demonstrated in Figure 8.8, a stress profile across a cross section

containing an internal crack. As indicated by this profile, the magnitude of this

localized stress diminishes with distance away from the crack tip. At positions far

Furthermore, the effect of a stress raiser is more significant in brittle than in

ductile materials. For a ductile material, plastic deformation ensues when the maximum

stress exceeds the yield strength. This leads to a more uniform distribution

of stress in the vicinity of the stress raiser and to the development of a maximum

stress concentration factor less than the theoretical value. Such yielding and stress

redistribution do not occur to any appreciable extent around flaws and discontinuities

in brittle materials; therefore, essentially the theoretical stress concentration

will result.

The brittle fracture process consists of the formation and propagation of cracks

through the cross section of material in a direction perpendicular to the applied

load. Crack growth in crystalline ceramics may be either transgranular (i.e., through

the grains) or intergranular (i.e., along grain boundaries); for transgranular fracture,

cracks propagate along specific crystallographic (or cleavage) planes, planes of high

atomic density.

Brittle materials, for which appreciable plastic deformation is not possible in

front of an advancing crack, have low values and are vulnerable to catastrophic

failure. On the other hand, values are relatively large for ductile materials. Fracture

mechanics is especially useful in predicting catastrophic failure in materials

having intermediate ductilities

Using principles of fracture mechanics, it is possible to show that the critical

stress required for crack propagation in a brittle material is described by the

expression

(8.3)

where

All brittle materials contain a population of small cracks and flaws that have a

variety of sizes, geometries, and orientations.When the magnitude of a tensile stress

at the tip of one of these flaws exceeds the value of this critical stress, a crack forms

and then propagates, which results in fracture. Very small and virtually defect-free

metallic and ceramic whiskers have been grown with fracture strengths that approach

their theoretical values.

Thus Ceramics are brittle. Whereas,

bonding is predominantly ionic, there are very few slip systems

(crystallographic planes and directions within those planes) along which dislocations

may move.This is a consequence of the electrically charged nature of the ions.

For slip in some directions, ions of like charge are brought into close proximity to

one another; because of electrostatic repulsion, this mode of slip is very restricted,

to the extent that plastic deformation in ceramics is rarely measurable at room temperature.

By way of contrast, in metals, since all atoms are electrically neutral, considerably

more slip systems are operable and, consequently, dislocation motion is

much more facile.

On the other hand, for ceramics in which the bonding is highly covalent, slip is

also difficult and they are brittle for the following reasons: (1) the covalent bonds

are relatively strong, (2) there are also limited numbers of slip systems, and (3) dislocation

structures are complex.

2. What are the two general classes of ceramics and how are they different?

+ Ceramics

+ Based on crystal structure

– 1. Crystalline – regular structure

– 2. Noncrystalline (amorphous)-irregular structure

+ Based on material used

– 1. Traditional – clay, cement & glass

– 2. Advanced – newer, high strength, high temperature materials

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3. List the parts of your body that are ceramic materials. How do you know that they are?

Bones and teeth – hard, brittle and temperature resistant.

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General properties Of Ceramics

1. High melting points

2. Tend to be brittle

3. Have both ionic and covalent bonds

4.

Advantages Of Ceramics

1. Hard

2. Temperature resistance

3. Corrosion resistance

4. Inexpensive

Dis-advantages Of Ceramics

1. Brittle

2. Hard to machine (mold/shape)

3.

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