1. CERAMIC MATERIALS
Dr. N. Ramesh Babu
Assistant Professor
Dept. of Metallurgical and Materials Engineering
National Institute of Technology
1
2. Ceramics derived from the original Greek word keramos,
meaning 'burned stuff or 'kiln-fired material', has long
been directly appropriate.
keramos, meaning "pottery", which in turn is derived
from an older Sanskrit root, meaning "to burn".
Modern ceramics, however, are often made by
processes that do not involve a kiln-firing step (e.g. hot-
pressing, reaction sintering,
glass-devitrification, etc.).
Introduction
3. GLASSES
CERAMIC
MATERIALS
CLAY REFRACTORIES ABRASIVES CEMENTSGLASSES
ADVANCED
CERAMICS
GLASS
CERAMICS
STRUCTURAL
CLAY
PRODUCTS
WHITEWARES
FIRECLAY
SILICA
BASIC
SPECIAL
Ceramics may be generally classified, according to type or function,
in various ways. In industrial terms, they may be listed as
7. The modulus of elasticity of ceramics can be exceptionally high
(Table 10.1). This modulus expresses stiffness, or the amount
of stress necessary to produce unit elastic strain, and, like
strength, is a primary design consideration.
However, it is the combination of low density with this stiffness
that makes ceramics particularly attractive for structures in
which weight reduction is a prime consideration.
8. General properties of ceramics
The constituent atoms in a ceramic are held together by
very strong bonding forces which may be ionic, covalent or
a mixture of the two.
As a direct consequence, their melting points are often
very high, making them eminently suited for use in energy-
intensive systems such as industrial furnaces and gas
turbines.
Alumina primarily owes its importance as a furnace
refractory material to its melting point of 2050°
C.
9. The type of inter-atomic bonding is responsible for the
relatively low electrical conductivity of ceramics. For
general applications they are usually regarded as excellent
electrical insulators, having no free electrons.
However, ion mobility becomes significant at temperatures
above 500-600°C and they then become progressively more
conductive.
The strength of ceramics under compressive stressing is
excellent; In contrast, the the tensile strength of ceramics
is not exceptional, sometimes poor, largely because of the
weakening effect of surface flaws.
10. Ceramics consist largely of elements of low atomic mass,
hence their bulk density is usually low, typically about 2000-
4000 kg m-3
. Ceramics such as dense alumina
accordingly tend to become pre-eminent in listings of specific
moduli.
The strong interatomic bonding means that ceramics are hard
as well as strong. That is, they resist penetration by
scratching or indentation and are potentially suited for use as
wear-resistant bearings and as abrasive particles for metal
removal.
Grinding of ceramics is possible, albeit costly.
11. During the consolidation and densification of a 'green' powder
compact in a typical firing operation, sintering of the particles
gradually reduces the amount of pore space between
contiguous grains.
The final porosity, by volume, of the fired material ranges from
30% to nearly zero. Pore spaces, particularly if interconnected,
also lower the resistance of a ceramic structure to penetration
by a pervasive fluid such as molten slag.
On the other hand, deliberate encouragement of porosity, say
25-30% by volume, is used to lower the thermal conductivity of
insulating refractories.
12. Ceramics are often already in their highest state of
oxidation. Not surprisingly, they often exhibit low
chemical reactivity when exposed to hot oxidizing
environments.
Their refractoriness, or resistance to degradation
and collapse during service at high temperatures,
stems from the strong inter-atomic bonding.
However, operational temperatures are subject to
sudden excursions and the resulting steep
gradients of temperature within the ceramic body
can give rise to stress imbalances.
13. As the ceramic is essentially non-ductile, stresses are not
relieved by plastic deformation and cracking may occur
in planes roughly perpendicular to the temperature
gradient, with portions of ceramic becoming detached
from the hottest face.
The severity of this disintegration, known as spalling,
depends mainly upon thermal expansivity (α) and
conductivity (k).
Silica has a poor resistance to spalling whereas silicon
nitride can withstand being heated to a temperature of
10000
C and then quenched in cold water.
16. Pauling’s Rules
Pauling’s first rule states that coordination polyhedra are formed.
coordination polyhedra are three-dimensional geometric constructions such as
tetrahedra and octahedra. Which polyhedron will form is related to the radii
of the anions and cations in the compound
Pauling’s second rule on the packing of ions states that local electrical
neutrality is maintained
Pauling’s third rule tells us how to link these polyhedra together.
Pauling’s fourth rule is similar to the third, stating that polyhedra formed
about cations of low coordination number and high charge tend to be linked
by corners.
The fifth and final rule states that the number of different constituents in a
structure tends to be small; that is, it is difficult to efficiently pack different-
sized polyhedra into a single structure.
1616
38. The combination of strength and a low coefficient of thermal expansion
(approximately 3.2 x 10-6
°C-1
over the range 25-1000°C) in hot-pressed silicon
nitride confer excellent resistance to thermal shock.
Small samples of HPSN are capable of surviving 100 thermal cycles in which
immersion in molten steel (1600°C) alternates with quenching into water.
39. Scientific basis of SAILONS
Although silicon nitride possesses extremely useful properties, its
engineering exploitation has been hampered by the difficulty of
producing it in a fully dense form to precise dimensional tolerances.
Hot-pressing offers one way to surmount the problem but it is a costly
process and necessarily limited to simple shapes.
The development of SAILONS provided an attractive and feasible
solution to these problems. The material is based on upon the Si-Al-
O-N system.
On the basis of structural analyses of silicon nitrides, it was predicted
that substituting oxygen (O2-) in nitrogen (N3-) was a promising
possibility if silicon (Si 4+) in the tetrahedral network could be
replaced by aluminium (Al3+), or by some other substituent of
valency lower than silicon.
40. SiAlONs properties and applications:
· low density,
· high strength
· superior thermal shock resistance,
· moderate wear resistance
· fracture toughness,
· mechanical fatigue and creep resistance,
· oxidation resistance.
Shot Blast Nozzles
Milling Media
Thermocouple Protection Sheaths
Weld Location Pins
Extrusion & Drawing Dies
Cutting Tips
Chemical & Process Industry Applications
46. The term CSZ refers to material with a fully-
stabilized cubic (not tetragonal) crystal structure
which cannot take advantage of the toughening
transformation. It is used for furnace refractory's
and crucibles.
FSZ ceramics have a high coefficient of thermal
expansion, higher than that of pure zirconia. This,
together with low thermal conductivity, leads to
poor resistance against thermal shock. The
mechanical strength and fracture toughness are
also lower than those of PSZ and TZP ceramics.
47. The version known as tetragonal zirconia polycrystal (TZP) contains the least
amount of oxide additive (e.g. 2-4 mol% Y2O3) and is produced in a fine-grained
form by sintering and densifying ultra-fine powder in the temperature range 1350-
1500°C; such temperatures are well within the phase field for the tetragonal solid
After cooling to room temperature, the structure is essentially single-phase,
consisting of very fine grains (0.2-1 micron) of t-ZrO2 which make this material
several times stronger than other types of zirconia-toughened ceramics.
48. Metastable tetragonal zirconia particles can
also be used in other matrices such as
Al2O3, SiC, Si3N4, TiB2, mullite, etc. in which
case the materials are referred to as DZC
ceramics (Dispersed Zirconia Ceramics).
The most well known example of this group
is zirconia toughened alumina (ZTA).
An intergranular distribution of the
metastable phase results when
conventional processing methods are
used but it has also been found
possible to produce an intragranular
distribution. As with PSZ materials, the
size of metastable particles and matrix
grains must be carefully controlled and
balanced.
49. In addition to these established applications, it has been
found practicable to harness the structural transitions of
zirconia, thereby reducing notch-sensitivity and raising
fracture toughness values into the 15-20 MN m-3/2
band, thus
providing a new class of toughened ceramics.
The other approaches to increasing the toughness of a
ceramic by either
(1) adding filaments or
(2) introducing micro-cracks that will blunt the tip of a
propagating crack.
51. Silicon CarbideSilicon Carbide
Body armour and otherBody armour and other
components chosen for theircomponents chosen for their
ballistic propertiesballistic properties..
AutomotiveAutomotive
Components ofComponents of
Silicon CarbideSilicon Carbide
Chosen for its heatChosen for its heat
and wear resistanceand wear resistance
52.
53. NitridesNitrides
The important nitride ceramics are silicon nitrideThe important nitride ceramics are silicon nitride
(Si3N4), boron nitride (BN), and titanium nitride(Si3N4), boron nitride (BN), and titanium nitride
(TiN)(TiN)
Properties: hard, brittle, high meltingProperties: hard, brittle, high melting
temperatures, usually electrically insulating, TiNtemperatures, usually electrically insulating, TiN
being an exceptionbeing an exception
Applications:Applications:
– Silicon nitride: components for gas turbines,Silicon nitride: components for gas turbines,
rocket engines, and melting cruciblesrocket engines, and melting crucibles
– Boron nitride and titanium nitride: cutting toolBoron nitride and titanium nitride: cutting tool
material and coatingsmaterial and coatings
57. The Structure of Glasses
A 3-co-ordinated
crystalline network is
shown at (a).
But the bonding
requirements are still
satisfied if a random (or
glassy) network forms,
as shown at (b).
5757
59. Radial distribution function [RDF; the quantity is ρ(r)].
ρ(r) = atom density in a spherical shell of radius r from the center of any
selected atom.
5959
70. The four-point loading method is often preferred because it subjects a greater volume
and area of the beam to stress and is therefore more searching. MoR values from
four-point tests are often substantially lower than those from three-point tests on the
same material. Similarly, strength values tend to decrease as the specimen size is
increased. To provide worthwhile data for quality control and design activities, close
attention must be paid to strain rate and environment, and to the size, edge finish and
surface texture of the specimen.
71.
72. Geometry of a compact tensile specimen
used in fracture toughness tests
Fracture Toughness Test
The picture is of a Single-Edged, Notched Beam configuration set up for three-point
loading. Various-sized beams can be used on the same fixture. Also note that the
lower pins are free to roll as the beam increases in length as it bends. Two little
springs hold the roller pins in place until the sample is loaded.
78. Rochelle salt (NaKC4H4O6 · 4H2O), potassium dihydrogen phosphate
(KH2PO4 ), potassium niobate (KNbO3 ), and lead zirconate–titanate
(Pb[ZrO3 ,TiO3 ]).
79. Piezoelectric materials are utilized in transducers, devices that convert
electrical energy into mechanical strains, or vice versa.
Familiar applications that employ piezoelectrics include phonograph pickups,
microphones, ultrasonic generators, strain gages, and sonar detectors.
80. Soft Magnetic Materials - Ferromagnetic materials are often used
to enhance the magnetic flux density (B) produced when an
electric current is passed through the material.
Applications include cores for electromagnets, electric motors,
transformers, generators, and other electrical equipment.
Data Storage Materials - Magnetic materials are used for data
storage.
Permanent Magnets - Magnetic materials are used to make
strong permanent magnets
Power - The strength of a permanent magnet as expressed by
the maximum product of the inductance and magnetic field.
Magnetic materials
81. Ferromagnetism - Alignment of the magnetic moments of atoms
in the same direction so that a net magnetization remains after
the magnetic field is removed.
Ferrimagnetism - Magnetic behavior obtained when ions in a
material have their magnetic moments aligned in an antiparallel
arrangement such that the moments do not completely cancel
out and a net magnetization remains.
Diamagnetism - The effect caused by the magnetic moment due
to the orbiting electrons, which produces a slight opposition to
the imposed magnetic field.
Antiferromagnetism - Arrangement of magnetic moments such
that the magnetic moments of atoms or ions cancel out causing
zero net magnetization.
Hard magnet - Ferromagnetic or ferrimagnetic material that has
a coercivity > 104
A . m-1
.
82.
83. Calculate the total magnetic moment per cubic centimeter in magnetite.
Calculate the value of the saturation flux density (Bsat) for this material.
Magnetization in Magnetite (Fe3O4)
Figure 19.15 (b) The
subcell of magnetite.
