Download - introduction to ceramics
Name: Fayza Shemsu
Jimma Institute of Technology
Department of Materials Science and
Engineering
TITLE: CERAMICS
CERAMICS
Thermal properties of ceramics Mechanical properties of ceramics Electrical properties of ceramics
Outline
Introduction Atomic bonding in ceramics Ceramics crystal structure Defects in ceramics General properties of ceramics
Classification of ceramics Electronic ceramics Processing of ceramics
The word ‘ceramic’ is originated from Greek word
“keromikos”, which means ‘burnt stuff’. Ceramics are compounds of metallic and non-metallic
elements.
Introduction
Are wide-ranging group of materials whose
ingredients are clays, sand and feldspar.
Are Inorganic non-metallic materials obtained by
the action of heat and subsequent cooling.
Always composed of more than one element (e.g., Al2O3,
NaCl, SiC, SiO2)
Bonds are partially or totally ionic, and can have
combination of ionic and covalent bonding
Generally hard and brittle
Generally electrical and thermal insulators
Can be optically opaque, semi-transparent, or Transparent
• Periodic table with ceramics compounds indicated by a combination of one or more metallic elements (in light color) with one or more nonmetallic elements (in dark color).
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Atomic Bonding in Ceramics Bonding:
Degree of ionic character may be large or small:
SiC: small
CaF2: large
Can be ionic and/or covalent in character. % ionic character increases with difference
in electronegativity of atoms.
Ceramic Crystal Structures
Crystal structure is defined by -Magnitude of
the electrical charge on each ion.
Oxide structures
– oxygen anions larger than metal cations
– close packed oxygen in a lattice (usually FCC)
– cations fit into interstitial sites among oxygen ions
Stable ceramic crystal structures: anions surrounding a
cation are all in contact with that cation.
For a specific coordination number there is a critical or
minimum cation anion radius ratio rC/rA for which this
contact can be maintained.
Two interpenetrating FCC lattices
NaCl, MgO, LiF, FeO have this crystal structure
Rock Salt Structure Cesium Chloride Structure
Examples of crystal structures in ceramics
Zinc Blende Structure: typical for compounds where covalent bonding dominates. C.N. = 4
ZnS, ZnTe, SiC have this crystal structure
Fluorite (CaF2):
FCC structure with 3 atoms per lattice point
Silicate Ceramics• Most common elements on earth are Si & O
• SiO2 (silica) polymorphic forms are quartz, crystobalite, &
tridymite
• The strong Si-O bonds lead to a high melting temperature
(17100C) for this material 12
Si4+
O2-
Adapted from Figs. 12.9-10, Callister & Rethwisch 8e crystobalite
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Defects in Ceramics
• Vacancies -- vacancies exist in ceramics for both cations and anions
• Interstitials
Adapted from Fig. 12.20, Callister & Rethwisch 8e. (Fig. 12.20 is from W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. 1, Structure, John Wiley and Sons, Inc., p. 78.)
Cation Interstitial
Cation Vacancy
Anion Vacancy
interstitials exist for cations interstitials are not normally observed for anions
because anions are large relative to the interstitial sites
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Point Defects in Ceramics Point defects in ionic crystals are charged. The Coulombic forces are very large and any charge
imbalance has a strong tendency to balance itself. To maintain charge neutrality several point defects can be created:
Shottky Defect a paired set of cation and anion vacancies.
Shottky Defect:
Frenkel Defect
Frenkel Defect a cation vacancy-cation interstitial pair.
Low ductility– Very brittle– High elastic modulus
Low toughness– Low fracture toughness– Indicates the ability of a crack or flaw to produce a catastrophic failure
Low density– Porosity affects properties
High strength at elevated temperatures
General Properties of ceramics
Thermal properties
1)Thermal expansion The coefficients of thermal
expansion depend on the bond
strength between the atoms that
make up the materials.
Strong bonding (diamond,
silicon carbide, silicon nitrite) →
low thermal expansion
coefficient
Weak bonding ( stainless steel)
→ higher thermal expansion
coefficient in comparison with
fine ceramics
Comparison of thermal expansion coefficient between metals and fine ceramics
generally less than that of metals such as steel or copper
ceramic materials, in contrast, are used for thermal insulation due to their low thermal conductivity (except silicon carbide, aluminium nitride)
2)Thermal conductivity
A large number of ceramic materials are sensitive to thermal shock
Some ceramic materials → very high resistance to thermal shock is despite of low ductility (e.g. fused silica, Aluminium titanate )
The thermal stresses responsible for the response to temperature stress depend on:-geometrical boundary conditions-thermal boundary conditions-physical parameters (modulus of elasticity, strength…)
3)Thermal shock resistance
STRESS-STRAIN BEHAVIOR of selected materials
Al2O3
thermoplastic
http://www.keramvaerband.de/brevier_engl/5/5_2.htm
Mechanical Properties of Ceramics
Elastic modulus
The elastic modulus E [GPa] of almost all oxide and non-oxide ceramics is consistently higher than that of steel.
This results in an elastic deformation of only about 50 to 70 % of what is found in steel components.
http://www.keramverband.de/brevier_engl/5/3/4/5_3_4.htm
Material Class Vickers Hardness (HV) GPa
Glasses 5 – 10
Zirconias, Aluminium Nitrides 10 - 14
Aluminas, Silicon Nitrides 15 - 20
Silicon Carbides, Boron Carbides
20 - 30
Cubic Boron Nitride CBN 40 - 50
Diamond 60 – 70 >
Some typical hardness values for ceramic materials are provided below:
The high hardness of technical ceramics results in favourable wear resistance.
Ceramics are thus good for tribological applications. http://www.dynacer.com/hardness.htm
Hardness
http://www.subtech.com/dokuwiki/doku.php?id=fracture_toughness22
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Toughness
Material KIc (MPa-m1 / 2)
Metals
Aluminum alloy (7075) 24
Steel alloy (4340) 50Titanium alloy 44-66Aluminum 14-28CeramicsAluminum oxide 3-5Silicon carbide 3-5Soda-lime-glass 0.7-0.8Concrete 0.2-1.4PolymersPolystyrene 0.7-1.1Composites
Mullite fiber reinforced-mullite composite 1.8-3.3
Porosity can be generated through the appropriate selection of raw materials, the manufacturing process, and in some cases through the use of additives.
This allows closed and open pores to be created with sizes from a few nm up to a few µm.
http://www.ucl.ac.uk/cmr/webpages/spotlight/articles/colombo.htm
Change in elastic modulus with the amount of porosity in SiOC ceramic foams obtained from a preceramic polymer
http://www.keramverband.de/brevier_engl/5/3/5_3_2.htm23
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Porosity
Electrical properties of ceramic
• Most of ceramic materials are dielectric. (materials, having very low electric conductivity, but supporting electrostatic field).
• Dielectric ceramics are used for manufacturing capacitors, insulators and resistors.
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Superconducting properties
• Despite of very low electrical conductivity of most of the
ceramic materials, there are ceramics, possessing
superconductivity properties (near-to-zero electric resistivity).
• Lanthanum (yttrium)-barium-copper oxide ceramic may be
superconducting at temperature as high as 138 K.
• This critical temperature is much higher, than
superconductivity critical temperature of other
superconductors (up to 30 K).
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Ceramics are classified in many ways. It is due to
divergence in composition, properties and applications.
Based on their composition, ceramics are:
1.Classification –Ceramics
Carbides
Nitrides
Sulfides
Fluorides
etc.
CERAMICS
Oxides
Nonoxides
Composite
Oxide Ceramics:
Oxidation resistant
chemically inert
electrically insulating
generally low thermal conductivity
slightly complex manufacturing
low cost for alumina
more complex manufacturing
higher cost for zirconia.
zirconia
• Non-Oxide Ceramics:
Low oxidation resistance
extreme hardness
chemically inert
high thermal conductivity
electrically conducting
difficult energy dependent
manufacturing and high cost.
Silicon carbide cermic foam filter (CFS)
• Ceramic-Based Composites:
Tough
low and high oxidation
resistance (type related)
variable thermal and electrical
conductivity
complex manufacturing processes
high cost.
Ceramic Matrix Composite (CMC) rotor
Based on their engineering applications ,ceramics
are classified in to two groups as: traditional and
advanced ceramics.
Traditional ceramics–most made up of clay,
silica and feldspar
Advanced ceramics–these consist of highly
purified aluminum oxide(Al2O3), silicon carbide
(SiC) and silicon nitiride (Si3N4)
2.Classification –Ceramics
Classification of ceramics
The older and more generally known types
(porcelain, brick, earthenware, etc.)
Based primarily on natural raw materials of
clay and silicates
Applications; building materials (brick, clay pipe, glass)
household goods (pottery, cooking ware)
manufacturing ( abbrasives, electrical
devices, fibers)
Traditional Ceramics
Traditional Ceramics
1) Clay Ceramics
Made from natural clays and mixtures of clays and added crystalline ceramics.
These include:
Whitewares Crockery Floor and wall tiles Sanitary-ware Electrical porcelain Decorative ceramics
Whitewares Structural Clay Products
Whiteware: Bathrooms
2)Refractories Firebricks for furnaces and ovens.
Have high Silicon or Aluminium oxide content.
3)Amorphous Ceramics (Glasses)
Main ingredient is Silica (SiO2) If cooled very slowly will form crystalline structure. If cooled more quickly will form amorphous structure
consisting of disordered and linked chains of Silicon and Oxygen atoms.
This accounts for its transparency as it is the crystal boundaries that scatter the light, causing reflection.
Glass can be tempered to increase its toughness and resistance to cracking.
Three common types of glass: Soda-lime glass - 95% of all glass, windows
containers etc. Lead glass - contains lead oxide to improve
refractive index Borosilicate - contains Boron oxide, known as
Pyrex.
Glass ContainersLeaded Glass
4)Abrasives Natural (garnet, diamond, etc.) Synthetic abrasives (silicon carbide, diamond, fused
alumina, etc.) are used for grinding for cutting Si waferspolishing for oil drilling
lapping, or pressure blasting of materials.
Two Kyocera ceramic knives (Y:ZrO2)
oil drill bits
5) Cements Used to produce concrete roads, bridges, buildings,
dams.
have been developed over the past half century.
Include artificial raw materials, exhibit specialized
properties, require more sophisticated processing
Advanced ceramics are also referred to as “special,”
“technical,” or “engineering” ceramics.
They exhibit superior mechanical properties,
corrosion/oxidation resistance, or electrical, optical, and/or
magnetic properties.
Advanced Ceramics
laser host materials
piezoelectric ceramics
ceramics for dynamic random access
memories (DRAMs), often produced in small
quantities with higher prices.
as thermal barrier coatings to protect metal
structures, wearing surfaces
Engine applications :Si3N4, SiC, Zirconia
(ZrO2), Alumina (Al2O3))
Advanced ceramics include newer materials such as
Engine Components
Rotor (Alumina)
Gears (Alumina)
Turbocharger
Ceramic Rotor
Ceramic Si3N4 bearing parts
Radial rotor made from Si3N4 for a gas turbine engine
The Porsche Car silicon carbide disk brake
Structural ceramics
Silicon Carbide
Automotive Components in Silicon Carbide
Chosen for its heat and wear resistance
Body armour and other components chosen for their ballistic properties.
Ceramics in the field Biomaterials
Metallic framework
Angry gums
Ceramic framework
Why ceramics ?
Dental implant
The first use of ceramics in the electrical industry
took advantage of their stability when exposed to
extremes of weather and to their high electrical
resistivity, a feature of many siliceous materials.
Ceramics with higher resistivities also had high
negative temperature coe cients of resistivity, fficontrasting with the very much lower and positive
temperature coe cients characteristic of metals.ffi
Electronic Ceramics
Electronic Ceramics
Ferroelectric
Pyroelectric
Piezoelectric
Dielectric Dielectric Property
Piezoelectricity
Pyroelectricity
Ferroelectricity
Piezoelectricity Mechanical and electrical energy conversion phenomena,
discovered by France Scientist Pierre and Jacques Curie
brother in 1880.
They showed that crystals of tourmaline, quartz, topaz, cane
sugar, and Rochelle salt generate electrical polarization from
mechanical stress.
Piezoelectric Material will generate electric potential when
subjected to some kind of mechanical stress.
Crystal Structure of piezoelectric ceramics
A traditional piezoelectric ceramic is a mass of perovskite crystals.
Pervoskite structure,
Each crystal consists of a small tetravalent metal ion, usually titanium or zirconium, in a lattice of larger divalent metal ions, usually lead or barium, and O2- ions
with the chemical formula as ABO3
e.g. : BaTiO3, , CaTiO 3
Above the Curie point each perovskite crystal in the fired ceramic element exhibits a simple cubic symmetry
At temperatures below the Curie point, however, each crystal has tetragonal or rhombohedral symmetry and a dipole moment.
Applications of piezoelectric ceramics
Piezoelectric ceramics used as the resonator and filter in
communication system with frequency lower than 100MHz。The ceramic filter and resonator are made of high stability
piezoelectric ceramics that functions as a mechanical resonator.
The frequency is primary adjusted by the size and thickness of the
ceramic element.
Typical application includes telephones, remote controls and radios.
Processing of ceramics
powder compact or“green”
ceramic
Forming
Sintering ordensification or
firing
T 2Tm/3
Ceramic powder processing route: synthesis of
powder , followed by fabrication of green product
which is then consolidated to obtain the final
product.
Synthesis of powder involves
1)Ceramic powder processing
crushing,
grinding
Separating impurities
blending different powders.
Grinding refers to the reduction of small pieces after crushing to fine powder
Accomplished by abrasion, impact, and/or compaction by hard media such as balls or rolls
Examples of grinding include: Ball mill Roller mill Impact grinding
Ball mill Roller mill
Grinding
Green component can be manufactured in
different ways:
Green component is then fired/sintered to get
final product.
tape casting slip casting extrusion injection molding and cold-/hot-compaction.
Shaping Processes
Slip casting
• A suspension of ceramic powders in water , slip, is poured into a porous plaster mold .
• Water from the mix is absorbed into the plaster to form a firm layer of clay at the mold surface
http://global.kyocera.com/fcworld/first/process06.html
Raw materials are mixed with resin to provide the necessary fluidity degree.
Then injected into the molding die The mold is then cooled to harden the binder and produce a "green"
compact part (also known as an unsintered powder compact).
Drying process• Water must be removed from clay piece before
firing• Shrinkage is a problem during drying. Because
water contributes volume to the piece, and the volume is reduced when it is removed.
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• Sintering step is still very much required• Driving force for sintering–reduction in total surface area
and thus energy. • Functions of sintering are the same as before:
1. Bond individual grains into a solid mass2. Increase density3. Reduce or eliminate porosity
Sintering of Ceramics
Finishing Operations for Ceramics
• Parts made of ceramics sometimes require finishing, with one or more of the following purposes: 1. Increase dimensional accuracy 2. Improve surface finish3. Make minor changes in part geometry
• Finishing usually involves abrasive processes – Diamond abrasives must be used to cut the
hardened ceramic materials
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