cermamics strctr and properties Flashcards
genral fcts about Ceramics?
Ceramics:
* Inorganic, non-metallic materials. Most are compounds of metallic and
non-metallic elements.
* BONDING is Ionic or Covalent or a mixture of both.
* hard, brittle (low toughness and ductility)
* Good electrical insulators and good thermal insulators because no conduction electrons.
* High melting points and good chemical stability
name some ceramic materials
glasses
clay products
cements
abrasives
carbons
advnced
crmcs
glasses type
glass ]
glass crmc
carbons
- Polymorphic: Diamond, Graphite
- Nano carbons: fullerenes, carbon nanotubes, graphene
Crystal Structure of Ionic Ceramics
- As usually have two types of atomic species (e.g. Na and Cl) so crystal
structure is made up of both ions.
size/coordination number requirements. - Cations (+ve) usually smaller than anions (-ve). Coordination number reflects size ratio.
- “Standard” type of crystal structures (AX)
- Also more complex structures (AmXp)
- Yet more complex: (AmBn Xp)
Diamond
Diamond
* Polymorphic:
* (ZnS) type structure.
* Strong covalent bonds
* Hardest known material
* very low electrical conductivity
* very high thermal conductivity
* optically transparent
* Used as:
* gemstones
* industrial cutting/grinding
* diamond thin films (surface coatings for wear, etc)
Graphite
Graphite
- Polymorphic:
- Layered crystal structure
- in-plane covalent bonding, (strong)
- between sheets - v.d.Waals (weak)
- gives easy interplanar cleavage - lubrication
- high electrical conductivity in plane.
- Relatively good strength, chemical stability
- low thermal expansion, good thermal shock
- easily machined
- Used for:
- heating elements, electrodes, crucibles, insulators
Point Defects in Ceramics
vacancies
interstitials
Frenkel Defect
Shottky Defect
talk about
Vacancies
* vacancies exist in ceramics for both cations and anions
* Interstitials
* interstitials exist for cations
* interstitials are not normally observed for anions because anions are
large relative to the interstitial sites
talk about
Frenkel Defect
Shottky Defect
- Frenkel Defect
- a cation vacancy-cation interstitial pair.
- Shottky Defect
- a paired set of cation and anion vacancies.
Impurities in Ceramics
Impurities in Ceramics
* Impurity atoms can form solid solutions - both substitutional and
interstitial. Substitution would be for the atom that forms the electrically
similar ion (cation or anion). Interstitial atom must be small.
Mechanical Properties of Ceramics
Mechanical Properties of Ceramics
* CERAMICS ARE BRITTLE!
* Tensile strengths usually very low
* Formation and propagation of cracks
* Compressive strengths usually very high
* High HARDNESS
* Reduce number of pores and pore size and will increase strength and
stiffness.
to improve strength of ceramics
- So to improve strength of ceramics increase K1c or decrease crack size
“ɑ
Stress-Strain Behavior
Tensile samples very expensive and difficult to make and test.
Use FLEXURE (bending) tests instead to measure Flexural Strength ( Modulus
of Rupture).
Values higher than for tensile test.
- Elastic behaviour is usually linear (like metals)
Plastic deformation
Dislocations can be present in ceramics BUT:
* in ionic crystals very few slip systems and charge neutrality problems
* in covalent crystals; strong bonding, limited slip systems, complex
dislocations.
* Thus dislocations cannot move easily at room Temp.
* NO PLASTIC DEFORMATION (Some ceramics can deform at high
temperatures).
