Definitions:
Ceramics are known for their high temperature melting points, high mechanical strengths, electrical, magnetic, optical and thermal properties.
– http://www.mse.uiuc.edu/home/introduction.html
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.
– http://en.wikipedia.org/wiki/Ceramic
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.
– http://matse1.mse.uiuc.edu/ceramics/quiz.html
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.
http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Introduction/ceramics.htm
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
– http://matse1.mse.uiuc.edu/ceramics/quiz.html
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.
– http://matse1.mse.uiuc.edu/ceramics/quiz.html
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.
