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Ceramics Flashcards

1
Q

What is the definition of a ceramic?

A

Inorganic, non-metallic materials

They are typically crystalline, with some glassy phases

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2
Q

What compounds are ceramics made from? Give some examples

A 
Formed from compounds of metallic and non-metallic elements
e.g. 
aluminium and oxygen (alumina) (Al2O3)
Calcium and oxygen (CaO)
Silicon and Nitrogen
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3
Q

Give some examples of ceramic products

A

Clay products, whitewares, refractories, glass, abrasives

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4
Q

Where are ceramics used in civil engineering?

A

Bricks and glass

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5
Q

What are the general properties of ceramics?

A
  • 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
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6
Q

What are the advantages of brick over stone?

A

Processing advantages out weigh higher energy costs
Lower application costs
Rerlatively low skill to install

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7
Q

How are traditional ceramics manufactured?

A

This applies to pottery, porcelain, tiles…

  • Moulded in plastic state
  • then fired (sintered or ‘burnt’)
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8
Q

What are the phases of tratditional ceramics (clays)?

A 
Glassy phate (melts and ''glues'' poly christalline multiphase body)
This is done through multiphase sintering
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9
Q

Clays are complex hydrated alumino silicates, give some examples

A

Kaolinite
Montmorrilonite

With silicates feldspar and quartz sand/ flint

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10
Q

What are the six categories of clays used in industry?

A 
Brick clay
bentonite
Common clay
Fire clay
Fuller's earth
kaolin
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11
Q

What is the size range of ceramic powders?

A

0.5 - 5 um

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12
Q

What makes up a clay?

A

Ceramic powder with water

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13
Q

What makes up most traditional ceramic materials?

A

A mixture of clays

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14
Q

What are the three phases of sintering?

A
  • initial green body
  • intermediate
  • final sintered body
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15
Q

What happens in liquid phase sintering?

A
  • 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
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16
Q

If the liquid phase in ceramics cools to a glass what does it mean for the high temperature properties?

A

Poor, if it chrystallises, the thermal properties are improved

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17
Q

Give a range of temperatures for ceramics

A

850 C (tiles) to >1650 C for engineering ceramics

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18
Q

Give the names of 3 engineering ceramics

A

Silicon nitride
Silicon carbide
Zirconium oxides

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19
Q

Describe the microstructure of alumina

A

Large chrystalline particles with glassy phase in between, There are also microscopic holes

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20
Q

Describe typical ceramic microstructure

A

Chrystalline phases
Amorphous (glassy) phase
porosity

21
Q

Why are dislocations in ceramics so dangerous?

A

Dislocations in ceramic and glassy materials are not mobile

22
Q

What do pores and surface pores act as in ceramics?

A

Stress concentrators (Griffith flaws)

23
Q

What are the properties of ceramics?

A
  • High hardness
  • Low toughness (brittle)
  • Low tensile strength
  • High compressive strength
24
Q

What is Griffith’s equation?

A 
sigma_f = K_Ic/(alpha*sqrt(pi*a))
K_Ic = Fracture toughness
alpha = geometrical factor (~1)
a = size of the biggest crack under stress
25
What does Griffith's equation say about ceramics?
The strength is much less than expected as they have many flaws - this makes ceramics brittle
26
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 ```
27
What does the stress intenisty factor K_1c describe?
The intesnity of the elastic crack tip stress field in a thick
28
What is the effect of oversintering on the microstructure?
It opens up the pores - reducing the strength
29
What is the general structure of ceramic processing technologies?
1. Particle forming processes 2. Powder pressing/ extrusion/ slip casting 3. drying 4. firing (sintering) 5. ceramic product
30
Give some examples of particle forming processes
Slurry formation Moisture optimisation spray drying
31
Give some examples of powder pressing
Hot pressing Uniaxial pressing Isostatic pressing
32
How does spray drying work?
Slurry is sprayed and dried to form agglomerated particles that flow
33
How does slip-casting ceramics work?
1. Slip is poured into mould 2. Water absorbed 3. Finished piece
34
How does thin walled slip casting work?
1. slip is poured into mould 2. mould is drained 3. top trimmed 4. finished piece
35
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
36
What ceramic products are made by extrusion?
Bricks and pipes
37
What are Adobe bricks?
Unfired clay bricks
38
Where are adobe blocks used?
Latin America, Africa, The Indian subcontinent
39
When do Adobe bricks date back to?
8000 BC
40
How many bricks are manufactured in the UK per annum?
2.7 billion
41
How many bricks were made at peak production in the Uk and when?
10-12 billion (1930s)
42
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 ```
43
What are the benefits of masonry?
Durability
44
What are the main destructive agents for bricks?
``` Water frost (Repeated freeze/ thaw cycles do most damage) Temperature change ```
45
What signs of wear do bricks exhibit?
Flaking, spalling and mortar joint crumbling
46
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
47
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
48
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

Materials (13 decks)

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