Materials Engineering (Week 4) Flashcards
What are Ceramics?
Non-metallic and inorganic solids
Ceramics examples (6)
– Rocks & minerals
– Clays (vitreous ceramics)
– Cements & concrete
– Glasses & Glass-ceramics
– Engineering Ceramics
* Oxides, Nitrides, Carbides, Superconductors….
– Bioceramics
* Oxides, Calcium-Phosphates, Bioglasses
Rocks & Minerals
Oldest of construction materials
– Sedimentary
* Silica bonded by Silica or Calcium
Carbonate
– Igneous
* Natural Silica-alumina (SiO2
-Al2O3) glass
ceramics
Vitreous Ceramics
Crystalline silicate phases bonded
together during firing by a glassy phase
* Contain natural organic binders in their
wet state which give them the plasticity to
be easily moulded and formed
* Often relatively porous – require glazing to
make water tight
Cements & Concrete
Cement
– Eg: Lime (CaO), Silica & Alumina – sets
when mixed with water
Concrete
– Sand and aggregate (shingle) in a
cement matrix
Glasses
Non-crystalline
– Disordered structure
* Most common are silicate based
– Soda-lime glass (70SiO2
, 10CaO,
15Na2O)
– Borosilicate glass (80SiO2
, 15B2O3
,
5Na2O)
Glass Ceramics
Ceramics formed by controlled
crystallization (through heat treatment or
heterogeneous nucleation) of a ceramic
phase from a quenched glassy matrix
* Can be very dense
and tough
Why Engineer Ceramics
High performance
– Tightly controlled phase composition
and pore size distribution
* Alumina Al2O3
, Zirconia ZrO2
,
Silicon carbide SiC, Diamond C,
Silicon nitride Si3N4
Traditional Applications of Ceramics (6)
Whitewares
Glass
Concrete and Cement
Structural Clay products
Refactories in Industrial processing
Abrasives
Advance Ceramics Application (11)
Electronics
Electrochemical
Medical and Bioengineering
Optical
Cements and sealing
Composites
Structural Ceramics (bearings and gears)
Environmental and Chemical
Coatings
Nuclear
Thermal Management
Application abrasives
The abrasive industry consists of four major segments:
Bonded Abrasives
Loose Abrasives
Super Abrasives
Coated Abrasives
Bonded Abrasives
(Including grinding and polishing wheels), are made
up of grains of aluminium oxide, zirconium oxide or silicon carbide.
Super Abrasives
Involve high quality grains such as diamond or cubic
boron nitride (CBN), which are bonded with ceramics, metals or resins
and applied to a metallic, ceramic or other core.
Loose Abrasives
are used for applications such as sandblasting
(roughen or clean (a surface) with a jet of sand driven by compressed air or steam).
Coated Abrasives
consist of grains of aluminium oxide, zirconium
oxide or silicon carbide laminated to paper, fabric (e.g. cotton,
polyester) or tape backing and come in belts, discs, rolls and sheets
Ceramics Application
Bioceramics
High purity, non-toxic
* Near-Inert
– Alumina & Zirconia
– Selected Glass ceramics
– Porcelains
Biocompatible/bioactive
– Selected Calcium-Phosphates,
– Bioglasses,
– Apatite wollastonite Glass Ceramics
– Various cements (Ca-PO4
, glass ionomer, Ca-SO4)
Ceramics
Application: Refractories
High purity, non-toxic
* Inert in severe environments
– Thermal insulation
– Corrosive or hot fluids
– Rapid temperature changes
– Furnaces, cement kilns, power
generators
* Composition
– Range of components
* Alumina,
* Silica,
* Periclase (magnesia)
– Marketed as
* powders or plastic masses
* Cast, sprayed, poured
Ceramics
Application: Die Blanks
Die blanks:
– Need wear resistant properties!
Die surface:
– 4 mm polycrystalline diamond
particles that are sintered onto a
cemented tungsten carbide
substrate.
– polycrystalline diamond helps control
fracture and gives uniform hardness
in all directions.
Ceramics
Application: Cutting Tools
Tools:
– for grinding glass, tungsten,
carbide, ceramics
– for cutting Si wafers
– for oil drilling
Solutions:
- manufactured single crystal
or polycrystalline diamonds
in a metal or resin matrix.
– optional coatings (e.g., Ti to help
diamonds bond to a Co matrix
via alloying)
– polycrystalline diamonds
resharpen by microfracturing
along crystalline planes.
Ceramics
Applications: Advanced Ceramics
Heat engines, advantages and disadvantages
Advantages:
– Run at higher
temperature
– Excellent wear &
corrosion resistance
– Low frictional losses
– Ability to operate without
a cooling system
– Low density
Disadvantages:
– Brittle
– Too easy to have voidsweaken the engine
– Difficult to machine
Ceramics
Applications: Advanced Ceramics
Electronic Packaging
- Chosen to securely hold microelectronics & provide
heat transfer - Must match the thermal expansion coefficient of the
microelectronic chip & the electronic packaging
material. Additional requirements include:
– good heat transfer coefficient
– poor electrical conductivity - Materials currently used include:
- Boron nitride (BN)
- Silicon Carbide (SiC)
- Aluminum nitride (AlN)
– thermal conductivity 10x that for Alumina
– good expansion match with Si
Ceramic Bonding
– Mostly ionic, some covalent.
– % ionic character increases with difference in
electronegativity
Ceramic (ionic) bonding examples (3)
– Alumina Al2O3
– Zirconia ZrO2
– Hydroxyapatite,
Ceramic (covalent) bonding examples (3)
- Silicon carbide SiC
– Silicon nitride Si3N4
– Glasses (silicate or phosphate based)
Ceramic Crystal Structures
Oxide structures (3)
– oxygen anions much larger than metal cations
– close packed oxygen in a lattice (usually FCC)
– cations in the holes of the oxygen lattice
* The same ideas apply to all “ceramics”
Principles of Ceramic Architecture: (4)
– Size relationships Cation to Anion, Stable structures:
maximize the # of nearest oppositely charged neighbors.
Size - ratio of anion to cation size
– Electrical Neutrality of the overall structure (Net charge in the structure should be zero).
– Crystallographic Arrangements
– Stoichiometry (the determination of the proportions in which elements or compounds react with one another) Must Match
Defects in Ceramic Structures
Frenkel Defect
–a cation is out of place
Defects in Ceramic Structures
Schottky Defect
–a paired set of cation and anion vacancies
Impurities must also satisfy …
Impurities must also satisfy charge balance = Electroneutrality
(if you add an impurity such as a new anion e.g oxygen with -2e charge then you will have two Cl- atoms vacant as Cl has charge of -1e). The no. of cations remains unchanged.
We know that ceramics are more brittle than
metals. Why?
The bonding of atoms together is much stronger in covalent and ionic bonding than in metallic. That is why, generally speaking, metals are ductile and ceramics are brittle.
Consider method of deformation
– slippage along slip planes
* in ionic solids this slippage is very difficult
* too much energy needed to move one anion
past another anion (like charges repel)
Covalent Bonding in Ceramics
(no. of neighbouring atoms)
The number of electrons available for
sharing per atom fix the number of
neighbours
– E.g. SiO2
(Si4+ + 2.O2- )
* Si4+ has 4 neighbours
* O2- has 2 neighbours
Ceramic Fabrication Methods
Fabrication Techniques: (3)
– glass forming (impurities affect forming temp).
– particulate forming (needed if ductility is limited)
– cementation (large volume, room T process)
Cooling Ceramics
Defects can be generated during cooling:
– As the result of differential thermal
expansion in glassy and crystalline
phases
– Due to differences in thermal
expansions with crystallographic axis
* Residual stresses at grain boundries
* Microstructural defects
* Thermal shock
