4. Ceramics And Glasses

Question 1

What are ceramic materials?

Answer 1

• Ceramics are inorganic and non- metallic materials. Most ceramics are compounds with metallic and non metallic elements for which the interatomic binds are either totally ionic or predominantly ionic but with some covalent character
• desired properties in ceramics are usually achieved through high temp heat treatment process called firing
• ceramics are crystalline or semi crystalline.
• glasses are amorphous- random, not repeated stack of atoms

Question 2

Ceramics and glasses are considered together due to having similar properties. What are these?

Answer 2

• strong
• stiff
• hard
• brittle
• low conductivity
• excellent temp resistance

Question 3

Ceramics and glasses have strong ionic and covalent bonds. What properties does this give them?

Answer 3

• poor electron mobility, cannot carry a charge, hence good electrical and thermal insulation
• no plastic deformation- ionic and covalent bonds are not interchangeable, no dislocation movements.
• strong bonds mean High MP, therefore excellent retention of properties at high temp

Question 4

Applications of ceramics and glasses?

Answer 4

Glasses

• windows
• reinforcing composites
• kitchen ware

Structural

• bricks
• pipes
• tableware
• floor/roof tiles

Refractories

• furnace lining
• metal and glass making crucibles

Advanced

• ballistics/body armour
• bearings
• brakes
• jet engine parts

Question 5

Manufacturing?

Answer 5

• brittle so not easily machined or cut
• changes in volume or poor thermal conductivity mean castings often crack
• hence powder processing is most common
• a powder liquid slurry is shaped, dried and sintered under heat and pressure- diffusion

Question 6

Defects in ceramics?

Answer 6

Glass fibres are brittle, hard and suffer from defects

• strength is dependent on volume
• big volumes can have big defects. The larger the volume the more likely is that a large defect exists and vice versa. See diagram

Question 7

the effects of surface defects can be reduced through a range of techniques. What are they?

Answer 7

• flame polishing
• acid polishing
• compressive stresses from coating and glazes

However we cannot remove the internal defects and we must account for them in design

Question 8

Weibul statistics?

Answer 8

We use weibul distribution to estimate likelihood of failure. See diagram

• the width and shape of the weibul distribution are controlled by the weibul modulus- indication of quality
• large modulus: narrow distribution, small spread ->reliable material m~10-20
• small modulus: wide distribution, large spread -> unreliable material. M~5-10

Question 9

Mechanical testing of ceramics?

Answer 9

See diagram

-3 or 4 bending tests required due to lack if formability

• ceramics are weak in tension
• much stronger in compression
• the larger the volume the higher the likelihood of failure is
• bending gives lower likelihood of failure than pure tension- only half volume is in tension