Lecture 3 - Ceramics Flashcards
(35 cards)
What type of bonding is predominantly found in ceramics?
Ionic bonding + Covalent
Advanced ceramics like:
SiC (silicon carbide)
Si₃N₄ (silicon nitride)Ionic bonding involves the transfer of electrons from metals to non-metals, creating cations and anions.
What is the coordination number in ceramics?
The number of anion nearest neighbors for a cation (and vice versa)
This indicates how many oppositely charged ions surround a given ion in a lattice.
Define polymorphism
Materials that have more than one crystal structure
What is allotropy?
Polymorphism that occurs in elemental solids
Example: Carbon exists as both graphite and diamond.
What characterizes ionic bonding?
The transfer of electrons between atoms, leading to the formation of oppositely charged ions (cations and anions) that are strongly attracted to each otherElectrons are transferred, forming cations and anions that create a tight electrostatic lattice
This results in strong, non-directional bonds with high melting points but brittleness.
Explain covalent bonding in ceramics.
Atoms share valence electrons in a directional manner, forming networks
Covalent bonds are very strong, as seen in diamond or SiC.
What is the purpose of the transverse bending test?
To test brittle materials by bending them until they fracture
Unlike tensile tests, bending tests are used because ceramics are too brittle.
Describe brittleness in ceramics.
Ceramics tend to fracture under tensile load without plastic deformation at room temperature
This is a key characteristic of both crystalline and amorphous ceramics.
What is the definition of flexural strength?
The maximum stress a material can withstand at the moment of fracture during a bending (flexural) test.
It is measured using a transverse bending test.
What is viscous flow in amorphous ceramics?
Sliding atoms and ions (by breaking and re-forming of interatomic bonds) due to applied shear stress
What is the equation for flexural strength?
Flexural strength = 3FL/2bd
Variables: F = load at fracture, L = distance between support points, b = width, d = height.
How does porosity affect ceramics?
Porosity reduces flexural strength by acting as stress concentrators.
This makes it easier for cracks to initiate and propagate. Porosity reduces modulus of elasticity and acts as stress concentrator
What are typical crack configurations in ceramics?
Impact or point loading, bending, torsion, internal pressure
These configurations indicate how ceramics may fail under different stresses.
What is heat capacity (C)?
Amount of heat needed to produce a unit rise in temperature
(for 1 mole of a substance)
It indicates the material’s ability to absorb heat.
What does the linear coefficient of thermal expansion (αl) indicate?
The extent to which a material expands upon heating
It is a material property relevant to thermal behavior.
What is electrical conductivity in ceramics?
Often low due to the unavailability of free electrons for conduction
This is a typical characteristic of traditional ceramics.
What characterizes corrosion in ceramics?
A process involving oxidation and reduction reactions in an aqueous environment
Ceramics can be resistant to corrosion but some can dissolve in water.
List the properties of ceramics.
- High melting temperatures
- Brittle
- Chemically resistant
- High thermal shock resistance
- Highly chemically inert
These properties make ceramics suitable for various applications.
What are glasses in ceramics?
Non-crystalline, often silicate-based materials containing other oxides
Soda-lime glass is a common example, consisting of SiO2 and other oxides.
What are the properties of glass-ceramics?
Fine-grained, polycrystalline materials made by crystallization of glass through a high-temperature heat treatment.
- Relatively easy fabrication process
* Relatively high mechanical strengths
* resistant to thermal shock
* Good high-temperature capabilities
* Good dielectric properties
* Good biological compatibility
* Sometimes optically transparen
They undergo high-temperature heat treatment to form crystals.
What are clays?
Naturally found, often silicate-based layered materials
Clays are inexpensive and easy to fabricate.
What are refractories?
Substances resistant to heat, remaining unreactive in severe environments
They provide thermal insulation and are classified based on composition.
What defines abrasives?
Hard, wear-resistant materials used for grinding
Examples include diamonds and aluminum oxide.
What is Portland cement?
A hydraulic cement formed by grinding and mixing raw materials, then heating
It sets and hardens through cementation bonds at room temperature.
