Ceramics Flashcards
What is the definition of a ceramic?
Inorganic, non-metallic materials
They are typically crystalline, with some glassy phases
What compounds are ceramics made from? Give some examples
Formed from compounds of metallic and non-metallic elements e.g. aluminium and oxygen (alumina) (Al2O3) Calcium and oxygen (CaO) Silicon and Nitrogen
Give some examples of ceramic products
Clay products, whitewares, refractories, glass, abrasives
Where are ceramics used in civil engineering?
Bricks and glass
What are the general properties of ceramics?
- High compressive strength, relatively low tensile strength
- Low toughness/ brittle
- Relatively high hardness
- Good thermal and electrical insulators
- very good durability under environmental conditions
- Relatively low cost, despite thermal processing
What are the advantages of brick over stone?
Processing advantages out weigh higher energy costs
Lower application costs
Rerlatively low skill to install
How are traditional ceramics manufactured?
This applies to pottery, porcelain, tiles…
- Moulded in plastic state
- then fired (sintered or ‘burnt’)
What are the phases of tratditional ceramics (clays)?
Glassy phate (melts and ''glues'' poly christalline multiphase body) This is done through multiphase sintering
Clays are complex hydrated alumino silicates, give some examples
Kaolinite
Montmorrilonite
With silicates feldspar and quartz sand/ flint
What are the six categories of clays used in industry?
Brick clay bentonite Common clay Fire clay Fuller's earth kaolin
What is the size range of ceramic powders?
0.5 - 5 um
What makes up a clay?
Ceramic powder with water
What makes up most traditional ceramic materials?
A mixture of clays
What are the three phases of sintering?
- initial green body
- intermediate
- final sintered body
What happens in liquid phase sintering?
- Glassy phhase may melt and wet higher melting point consituents
- Liquid draws the solid together by viscous flow and capillary pressure
- Liquid phase may cool to a glass or chrystallise
If the liquid phase in ceramics cools to a glass what does it mean for the high temperature properties?
Poor, if it chrystallises, the thermal properties are improved
Give a range of temperatures for ceramics
850 C (tiles) to >1650 C for engineering ceramics
Give the names of 3 engineering ceramics
Silicon nitride
Silicon carbide
Zirconium oxides
Describe the microstructure of alumina
Large chrystalline particles with glassy phase in between, There are also microscopic holes
Describe typical ceramic microstructure
Chrystalline phases
Amorphous (glassy) phase
porosity
Why are dislocations in ceramics so dangerous?
Dislocations in ceramic and glassy materials are not mobile
What do pores and surface pores act as in ceramics?
Stress concentrators (Griffith flaws)
What are the properties of ceramics?
- High hardness
- Low toughness (brittle)
- Low tensile strength
- High compressive strength
What is Griffith’s equation?
sigma_f = K_Ic/(alpha*sqrt(pi*a)) K_Ic = Fracture toughness alpha = geometrical factor (~1) a = size of the biggest crack under stress
What does Griffith’s equation say about ceramics?
The strength is much less than expected as they have many flaws - this makes ceramics brittle
What is the equation for the stress intensity factor?
K1 = alpha*sigma*(sqrt(pi*a)) sigma = nominal stress a = crack depth alpha = the constant dependent on the crack and test piece geometry
What does the stress intenisty factor K_1c describe?
The intesnity of the elastic crack tip stress field in a thick
What is the effect of oversintering on the microstructure?
It opens up the pores - reducing the strength
What is the general structure of ceramic processing technologies?
- Particle forming processes
- Powder pressing/ extrusion/ slip casting
- drying
- firing (sintering)
- ceramic product
Give some examples of particle forming processes
Slurry formation
Moisture optimisation
spray drying
Give some examples of powder pressing
Hot pressing
Uniaxial pressing
Isostatic pressing
How does spray drying work?
Slurry is sprayed and dried to form agglomerated particles that flow
How does slip-casting ceramics work?
- Slip is poured into mould
- Water absorbed
- Finished piece
How does thin walled slip casting work?
- slip is poured into mould
- mould is drained
- top trimmed
- finished piece
How does extrusion of ceramic bodies work?
A ram goes into a dummy block that pushes the wet ceramic in a container through a die, forming the final shape
What ceramic products are made by extrusion?
Bricks and pipes
What are Adobe bricks?
Unfired clay bricks
Where are adobe blocks used?
Latin America, Africa, The Indian subcontinent
When do Adobe bricks date back to?
8000 BC
How many bricks are manufactured in the UK per annum?
2.7 billion
How many bricks were made at peak production in the Uk and when?
10-12 billion (1930s)
How are bricks manufactured?
pressed at a high temperature Clay helps act as a fuel during firing Using coarse granular material from quarry Extruded at very high rates Then wire cut
What are the benefits of masonry?
Durability
What are the main destructive agents for bricks?
Water frost (Repeated freeze/ thaw cycles do most damage) Temperature change
What signs of wear do bricks exhibit?
Flaking, spalling and mortar joint crumbling
What makes up the white efflorescence on bricks, and when does it occur?
Sodium, potassium and magnesium sulphates
Occurs in the early life of a bulding, after wet conditions
The deposit of salt solution on the surface of the water
What causes Iron staining of bricks?
Occurs in wire bricks and those with high iron content
Iron salts migrate to the surface
It is cause by the saturation of immature bricks
Disappears with natural weathering
What causes lime staining on bricks?
Free lime in mortar leaches
Occurs when work is left without being covered
Triggered by water - e.g. rain
