|
Materials: Ceramics
Introduction
to Ceramics
Ceramics are inorganic non-metallic
materials. Metal oxides (Al2O3, FeO, etc.)
are common examples of ceramics, but other
compounds such as carbides and nitrides
are also included. Porcelain, glass, bricks
and refractory materials are some examples
of traditional ceramics. In the last 30
years, advances in material science have
transformed formerly brittle ceramics into
materials tough enough to withstand engine
environments. Ceramics are used in a variety
of applications including window glass,
implantable teeth, brick, ceramic bones,
nuclear fuel, tennis racquets, solid-state
electronic devices, engine components, cutting
tools, valves, bearings, and chemical-processing
equipment.
The properties for which
ceramics are most often selected include:
- High-temperature resistance
(High melting temperatures.)
- High electrical resistivity
(Although some ceramics are superconductors.)
- Broad range of thermal
conductivity (Some ceramics are excellent
insulators)
- High hardness (Many ceramics
are brittle.)
- Good chemical and corrosion
resistance.
- Low cost of raw materials
and fabrication for some ceramics.
- Good appearance control
through surface treatments, colorization,
etc.
Ceramics are generally more
brittle than metals and can have similar stiffness
(modulus of elasticity) and similar strength,
particularly in compression. But in a tensile
test they are likely to fail at a much lower
applied stress. This is because the surfaces
of ceramics nearly always contain minute cracks
("Griffith cracks"), which magnify the applied
stress.
Since ceramics often have
very high wear-resistance and hardness,
most ceramic parts are formed as near net
shape as possible. Ceramics are most often
produced by compacting powders into a body
which is then sintered at high temperatures.
During sintering the body shrinks, the grains
bond together and a solid material is produced.
Other ceramic forming processes include:
Dry Pressing, Isostatic Pressing, Roll Compaction,
Continuous Tape Casting, Slip Casting, Extrusion,
Injection Molding, Pre-Sinter Machining,
Hot-Pressing, Hot Isostatic Pressing, Grinding,
Lapping and Polishing.
Ceramics
are generally separated into the following
categories.
- Metallic
Oxides
- Glass
Ceramics
- Nitrides
and Carbides
- Glass
- Carbon
and Graphite
- Porcelain
- Ceramic
Fibers
|
Ceramic
Type
|
Characteristics
|
| Metallic
Oxides |
| Alumina |
- Abundant and easily fabricated.
- Good strength and hardness.
- Wear and Temperature Resistant.
- Good electrical insulators.
- Low dielectric loss.
|
| Beryllium Oxides |
- Exceptionally high thermal conductivities
(for ceramics) at low to moderate
temperatures.
|
| Zirconia |
- Extreme inertness to most metals.
- Good toughness and strength.
|
| Glass
Ceramics
|
| Glass-Ceramics |
- Low, medium or high thermal expansion
depending on composition type.
- Good electrical insulators.
- Transparent
- One can be machined with steel
tools.
|
| Nitrides
and Carbides
|
| Silicon Nitrides |
- Resistant to high temperatures,
to thermal stress and shock.
- High strength and oxidation resistant.
- Good electrical insulators.
|
| Boron Carbide |
- High hardness and low density.
- Best abrasion resistance of any
ceramic.
- Low strength at high temperatures.
|
| Silicon Carbides |
- Low electrical resistivity.
- High strength and resistance to
chemical attack, high temperature
and thermal stress.
|
| Tungsten Carbides |
- Used for tool tips.
- Excellent hardness and mechanical
strength.
- Good thermal conductivity.
- Good wear and abrasion resistance.
|
| Glass
|
| Glasses
- Oxide (silica)
- Silicates
- Phosphates
- Borosilicates
|
- Good resistance to thermal shock.
- Large range of special optical
characteristics.
- Transperent.
- Low thermal expansion and high
dielectric strength.
- Good chemical resistance.
|
| Carbon
and Graphite
|
| Carbons and Graphites |
- Poor strength except when produced
as fibre.
- Good electrical and thermal conductivity
- Creep resistant at high temperatures
in non-oxidizing conditions.
- Self-lubricating.
- Good refractoriness and thermal
shock resistance.
- Low density and chemically inert.
|
| Carbon/Carbon Composites |
- High strength and low coefficient
of thermal expansion at temperatures
above 2000C.
- Excellent thermal shock resistance.
- Superior toughness, excellent
thermal and electrical conductivity
- Resistance to corrosion and abrasion.
- High cost.
|
| Porcelain
|
| Porcelain |
- Good chemical and thermal resistance.
- High density, strength, resistivity
and dielectric strength
- Good thermal shock, wear and hot
strength.
- Chemically inert.
|
| Ceramic
Fibers
|
| Ceramic Fibers |
- Oxides spun to fiber and bulked
to felt.
- Used for high temperature insulation
including former applications of
asbestos.
|
|
Ceramic
Links
|
|
Resource
Sites
|
| Introduction
to Ceramics
A good article on
the background of the ceramics industry
provided by The American Ceramics
Society.
Advanced
Ceramic Technology Center
The Advanced Ceramic
Technology Center has the resources
to help U.S. industry overcome the
high cost of machining and certifying
structural ceramics, especially for
stringent applications such as heat
engines.
Ceramics
Industry Magazine - Reference Tables
This site has some
great references including: Material
Properties, Drying/Firing Tables,
Forming/Finishing Info., Refractories
and Testing Methods.
Dynamic-Ceramic
- Design Guides
A series of guides
are intended to act as an aid to engineers
and designers wishing to produce cost
effective designs for advanced ceramic
manufacture. The design tips also
refer primarily to components which
have to be finish ground to tight
tolerances.
Dynamic-Ceramic
- Material Properties
This site was designed
to aid in the selection of advanced
ceramics with a selection of pages,
which provide both comparative data
on the available materials in addition
to a more in-depth analysis of the
individual materials and their mechanical
and physical properties.
Dynamic-Ceramic
- Manufacturing Processes
Ceramic forming processes
may be classified as traditional -
die pressing, cold isostatic pressing,
slip casting and extrusion - or as
new and emerging, such as injection
moulding and tape casting.
|
|
Magazines,
Periodicals, Newsletters
|
|
Ceramic
Industry Magazine
Ceramic Industry
magazine is the exclusive voice of
ceramic manufacturing, serving the
advanced ceramics, glass, whitewares,
structural clay, refractories and
decorating technology industries.
Ceramic
Bulletin
Ceramic Bulletin
provides authoritative coverage of
established and emerging processing
technology that will impact ceramic
manufacturing around the world.
|
Societies
and Organizations
|
|
The
American Ceramic Society
ACerS, an international
association that provides the latest
technical, scientific and educational
information to its Members and others
in the ceramics and related materials
field, structures its services, staff
and capabilities to meet the needs
of the ceramics community, related
fields, and the general public.
Swedish
Ceramic Institute
Informative site
with a 2-minute introduction to ceramics,
material data and other useful information.
|
|
Meta
Sites
|
|
Metallurgical
and Materials Engineering Global Links
Ceramics
and Industrial Minerals
Metallurgical
and Materials Engineering Global Links
The
WWW Virtual Library: Technical Ceramics
| |
|
