5. Engineering ceramics and glasses

Question 1

5 typical properties of ceramics

Answer 1

Strong (in compression)
Brittle
Hard
Insulator (electricity and heat)
Inert (high chemical stability)

Question 2

2 Factors that dictate the structure of ceramics

Answer 2

Balanced charges, structure is electrically neutral

Relative size of anions and cations

Question 3

What determines the co-ordination number on a ceramic, and how is this calculated?

Answer 3

Ionic radius ratio = radius of cation/radius of anion

Question 4

Describe briefly the structure of NaCl

Answer 4

two interlocking FCC structures

Question 5

What is the co-ordination number for each ion in NaCl?

Answer 5

6

Question 6

Why are metals much tougher than ceramics? 2 reasons

Answer 6

metals have plastic deformation, ceramics do not
high chance of porosity in ceramics,
(even a small pore will have huge effects, not so much in ceramics)

Question 7

Two reasons why porosity weakens ceramic strength

Answer 7

reduces (effective) cross sectional area

concentrates stress around the pores

Question 8

Two toughening mechanisms in ceramics

Answer 8

Removing porosity/Densifying (in processing)

Reinforcing (adding another material to make a COMPOSITE) to achieve a balance of properties

Question 9

Hydroplastic forming is what?

Answer 9

Adding a little water to clay (which is made of sheets of silicates) allows the water to ‘float’ these sheets past each other making the whole thing very plastic

Question 10

What is slip or slurry?

Answer 10

Suspension of clay particles in water (when you add more water than in hydroplastic forming)

Question 11

Why does unidirectional powder pressing have a relatively low pressure limit?

Answer 11

powder is bad at transmitting pressure (powder at the bottom will experience very little pressure)

Question 12

Best method of powder pressing?

Answer 12

Isostatic pressing (pressuring the powder from all directions)

Question 13

Describe the process of sintering

Answer 13

Heat treatment (nothing melts) to reduce porosity, by diffusion of particles to fill up the pores

Question 14

What is vitrification?

Answer 14

In firing, when some of the components melt and flow around the ceramic particles, solidifying as glass (makes very strong ceramic)

Question 15

What is the driving force of sintering?

Answer 15

Reducing surface area

Question 16

Glass is not a crystal because

Answer 16

it has no LONG RANGE ORDER (which is what all crystals have)

Question 17

What are glass forming oxides?

Answer 17

oxides that can form networks with no long range order

Question 18

Intermediate oxides

Answer 18

can’t form networks on their own but can join into existing networks

Question 19

Network modifiers

Answer 19

disrupt glass networks to give the glass particular properties (Na2O, CaO used for soda lime silica, most common type of glass)

Question 20

Cooling molten silica slowly results in what kind of structure?

Answer 20

A crystal structure as you give the molecules time to line up

Question 21

Cooling molten silica fast results in what kind of structure?

Answer 21

A glass (disordered structure)

Question 22

What is the effect of the cooling rate on the final volume?

Answer 22

faster cooling rate = larger volume (less time for molecules to pack)

Question 23

How to improve fracture toughness in glass?

Answer 23

Tempering the glass, causing the surface region to be in compression (thermal or chemical techniques), this technique is known as residual compressive stress.

Question 24

Describe how thermal tempering works

Answer 24

Glass heated to transition temperature, then cold water is sprayed onto surface. Inside layer cools slower, so shrinks more and pulls surfaces into compression.