Ceramics and Glasses
Chapter 14
History
Ceramics were some of the earliest of
mankind’s structural materials
Pots
 Bricks

Low Tech
High Tech
Composition
Two or more elements
At least one metal and one non-metal
 Ionic and Covalent Bonding

Metal oxides
Al2O3
 FeO

Silicon Oxides

Basis of glass
Properties
High melting point
Electrical Insulators
Thermal Insulators
Hard (though many are brittle)
Resistance to Chemical Attack
Many are low cost (bricks)
Wide range of appearance
Unit Cells
FCC
Common
SCC
HCP
Must have a “basis” greater than one
There is more than one atom associated with
each lattice point
 Low packing factors

Why are ceramics brittle?
It is hard for dislocations to move
Low density planes and directions
 Ions

Clay
Natural source of ceramic material
Silica Sheets

Hydrated alumina

Alumina is Al2O3
Bonding within the sheets is covalent
Bonding between the sheets is Van der Wall
Water disrupts the Van der Waal forces, allowing
the sheets to slip past each other
That’s why wet clay is slippery, and can be made
into pottery
Silicates
Silicon and Oxygen are the two most
common elements on the earth’s surface
Bonding is mostly covalent
Bond angles are fixed
 Produce a tetrahedral structure

Orthosilicate
4-
O
Si O
O
Geometry
Tetrahedron
All silicates are based on the tetrahedron
A single tetrahedron (SiO4-4) requires cations to
balance the charge
Two tetrahedrons can share a single vertex

Si2O7 –6
Or an edge

Si2O6 –4
Or a face

Si2O5-2
Chains
The tetrahedra can form chains or rings or
sheets
Very complex structures can result

See the animation on the CD
Pure SiO2 results from a network structure,
where the tetrahedra share faces
There are several allotropes of SiO2
Allotropes
Cristobalite is the simplest
Quartz is the room temperature allotrope
Alpha
 Beta

Check out the models
Phase Diagrams
We have already used some ceramic phase
diagrams
MgO-FeO is an example of solid solubility
See the CD for more complicated phase
diagrams
Defects
In metals, defects such as vacancies and
substitutional atoms increase the strength
Why?

Because they inhibit slip
But in ceramics slip is already difficult, so
defects have little effect on strength
Sintering
Many ceramic parts are made by sintering
Its what happens when you fire clay in a kiln
The clay consists on many fine ceramic particles
held together by van der Waal forces
When it is heated, the grains that touch each other
grow together
See the animation
Glass
Usually a term applied to ceramics
Metals can be glasses too
Any material that has solidified and become
rigid without forming a regular crystal
structure
There is no long range order, although the
silicate tetrahedra are still linked together
Glass Transition Temperature
If you melt a crystal the melting point is
usually well defined
It is easy though to cool a liquid past the
“melting” point – to undercool it
If you cool it far enough, it eventually
becomes rigid. That point is the glass
transition temperature
Specific Volume
Melting Point
Glass Transition
Temperature
Temperature
Modifiers
Silicate glass – pure SiO2
Melts at a very high temperature
 Very Brittle
 High Viscosity

Hard to Fabricate
Modifiers are added to open up the net work
and improve the properties
Mechanical Properties
Weibull modulus

Be sure to watch the video!!
Tempered Glass
Glass is brittle
Sensitive to surface cracks
If the surface is in compression, the cracks
close up
Manufacture the glass with residual
compressive stress at the surface
Balanced by a residual tensile stress
someplace else