3. Ceramic Materials
• The term ceramics has its origin in the Greek
word 'keramos', meaning burnt matter. Probably
associated with 'Cerami', an ancient district in
Athens.
• The term ceramics covers a wide variety of
inorganic materials, which are generally non-
metallic and frequently processed at high
temperatures.
4. • Ceramic structures have survived longer than
any other works.
• The great pyramids of Giza is solid ceramic
(nearly 1,000,000 tonnes of it) and pottery from
5000 BC survives to the present day.
• Ceramics may not be as tough as metals, but for
resistance to corrosion, wear, decay, they are
incomparable.
5. • In view of the advances made in
the last thirty years, it is
convenient to categorize the
ceramic materials into two classes:
• a- Traditional ceramics
• b- Engineering ceramics
6. Traditional Ceramics
Traditional or conventional
ceramics:
These are generally in
monolithic (uniform) form.
These include bricks, pottery,
tiles and variety of objects.
8. Engineering Ceramics
Advanced, engineering or high
performance ceramics:
these represent a new and improved
class of ceramic materials where some
sophisticated chemical processing route
is used to obtain them. Generally their
characteristics are a sensitive function
of the high quality and purity of the
raw materials used.
12. Importance in some applications
• Cutting tools made of ‘sialons’ or of
dense ‘alumina, can cut faster and last
longer than the best metal tools.
Engineering ceramics are highly wear-
resistant: they are used to clad the
leading edges of agricultural machinery
like harrows, increasing the life by 10
times.
13. • They are inert and
biocompatible, so they are good
for making artificial joints
(where wear is a big problem)
and other implants.
• A major attraction of ceramics is
its relatively high mechanical
strength at high temperatures.
14. Structure of ceramics
•A ceramic, like a metal, has
structure at the atomic scale:
crystal structure (crystalline),
or its amorphous structure
(glassy).
15. Types of Ceramic Structures
Ceramics is classified into two
main structure patterns:
• 1- Ionic Ceramics
• 2- Covalent Ceramics
16. Ionic Ceramics
Ionic ceramics are, typically,
compounds of a metal with a
non-metal;
Examples of ionic ceramics:
sodium chloride, NaCl;
magnesium oxide, MgO;
alumina Al2O3;
zirconia ZrO2.
17. Bonding in Ionic ceramics
The metal and nonmetal
have unlike electric charges.
For example in sodium chloride,
the sodium atoms have one
positive charge and the chlorine
atoms have one negative charge
each.
18. The electrostatic attraction between the
unlike charges gives most of the bonding.
So the ions pack densely (to get as many plus
and minus charges close to each other as
possible), but with the constraint that ions of
the same type (and so with the same
charge) must not touch.
19. This leads to certain basic ceramic
structures, typified by:
Rock salt, NaCl, or by alumina
Al2O3
22. Covalent Ceramics
• Covalent ceramics are different.
• They are compounds of two non-
metals (like silica SiO2),
• or, some times, are just pure
elements (like diamond, C, or
silicon, Si).
23. • An atom in this class of ceramic
(Covalent Ceramics) bonds by
sharing electrons with its
neighbours to give a fixed
number of directional bonds.
27. Properties of Ceramic Materials
Crystalline and non-crystalline states
High melting temperatures (varying from 3500
to 7000 o F)
All ceramics are brittle at room temperatures
Very low resistance to tensile loads.
Very low fracture strengths. Microcracks are
formed very easily under tensile stresses.
Stronger under compressive loads and
microcracks are not formed as easily as in tension.
28. Properties of Ceramic Materials (Cont’d)
High hardness and good wear resistance.
High toughness
Low thermal and electrical conductivity.
High creep resistance at elevated temperatures
Un-reactive and inert when exposed to severe
environments (chemically stable)
Can be magnetized and demagnetized, some can
be permanently magnetized
29. Fracture Properties
At room temperature, ceramics almost
fracture before plastic deformation occur
in tensile loading. The measure of
ceramic material’s ability to resist
fracture when a crack is present is
specified in terms of fracture toughness.
30. Material Material Fracture
Toughness
(psiin x103
)
Metals
Alloy steel (4340 tempered) 46
Titanium alloy (Ti-6Al-4V) 40-60
Ceramics
Aluminum Oxide 2 -5
Soda-lime glass 0.7
Polymers
Polymethylmethacrylate (PMMA) 0.9
Polystyrene (PS) 0.7 -1.0
comparison between the toughness of ceramics and other materials.
31. Stress-Strain Behavior of Ceramics
Instead of standard tensile test which is
applied to metals, a transverse bending test
(three-or four-point loading) is employed. In
this test a rod specimen having either a
circular or a rectangular cross section is bent
until fracture.
LOAD (F)
L/2 L/2
SUPPORT
32. Stress-Strain Behavior of Ceramics
The maximum stress, or stress at fracture is
known as the Modulus of Rupture (mr),
which is an important mechanical parameter
for ceramics. Modulus of rupture is given by
the following equations:
2
2
3
bd
FL
mr
RECTANGULAR
d
b
CIRCULAR
3
R
FL
mr
2R
33. MATERIAL Modulus of
Rupture
(Ksi)
Modulus of
Elasticity
(Ksi)
Aluminum Oxide 30-50 53
Silicon Carbide 25 68
Titanium Carbide 160 45
Glass 10 10
Table 2. Characteristic modulus of rupture
and elastic modulus values for various
ceramic materials.
42. Ceramic Materials Drawback
• Ceramics have high strength but
low fracture toughness.
• The low fracture toughness has
its origin in the extreme
sensitivity of ceramics to the
presence of flaws in them.
43. Flaw types in ceramics
• Various flaw types can occur in ceramics. They
could be categorized into three broad types
• a- Processing induced flaws such as inclusions,
pores, isolated large grains, laminations induced
during pressing, machining induced necks and
thermal stresses.
• b- Design induced flaws like sharp corners,
burrs, etc.
c- Service induced flaws such as environmental
degradation, thermal stresses, impact and wear.
