Ceramics and glasses

Question 1

What are 6 classification of ceramics?

Answer 1

Glasses
Clay products
refractories
abrasives
cements
advanced ceramics

Question 2

What are glasses? (3)

Answer 2

• optical
• composite reinforce
• containers/household

Question 3

What are clay products? (2)

Answer 3

• whiteware

- structure

Question 4

What are refractories? (1)

Answer 4

• brick for high T (furnaces)

Question 5

What are abrasives? (3)

Answer 5

• sandpaper
• cutting
• polishing

Question 6

What are cements? (2)

Answer 6

• composites

- structures

Question 7

What are advanced ceramics? (2)

Answer 7

• engine rotors valves bearings

- sensors

Question 8

What are ceramics?

Answer 8

A compound of metallic and non‐metallic elements, in which the interatomic bonding is ionic (predominant) or covalent. The atomic structure is ordered or crystalline

Question 9

What are glasses?

Answer 9

A combination of metallic and non‐metallic elements, in which the interatomic bonding is ionic or covalent. The atomic structure is random or amorphous (usually silicate based).

Question 10

What materials do ceramics encompass?

Answer 10

Ceramics encompass materials with highest hardness and melting point in nature – diamond.

Question 11

What are ionic ceramics?

Answer 11

Typically compounds of metal with non‐ metal eg MgO, Al2O3, ZrO2.

Question 12

What are the two types of ceramics?

Answer 12

ionic ceramics

covalent ceramics

Question 13

What are covalent ceramics?

Answer 13

Typically compounds of metalloid or non‐metals eg SiO2, SiC

Question 14

What are the mechanical properties of ceramics?

Answer 14

High values of Young’s Modulus:
-Diamond approximately 3 × alumina & Alumina approximately 2 × steels

Low ductility, low or no tendency for plastic deformation due to the nature of the atomic bonding.

Brittle nature, related to the presence of flaws limits “engineering” strength.

Question 15

What is the Weibull Modulus?

Answer 15

M indicates how rapidly strength falls (confidence) approaching σ0.

Question 16

What does a Low M indicate? (2)

Answer 16

Low M
– greater variability
– low design strength.

Question 17

What does a High M indicate?

Answer 17

High M

– more stability, more confidence.

Question 18

What do stress/strain plot for ceramics at room temp indicate?

Answer 18

no plasticity

Question 19

Why do ionic and covalent produce much more resistance to dislocations than metallic bonds? (3)

Answer 19

– Covalent bonds would require breaking of interatomic bonds and reforming
–Ionic bonds: Would in most cases, require intimate contact of like charged ions – thus limited slip systems.
– Easier to propagate a crack than to move a dislocation.

Question 20

What kind of deformation can ceramics exhibit at an elevated temperature? (3)

Answer 20

● Dislocation climb
● Grain boundary sliding
● Herring-Nabarro creep – “diffusion of vacancies”

Question 21

What is the toughness like in ceramics? (3)

Answer 21

● Low – relates to brittle nature.
● Fracture toughness values – KIC ≈ 0.2 of metals.
● Nature of manufacture – powder consolidation – leads to flaws.

Question 22

How can you improve the toughness of ceramics? (2)

Answer 22

• Alloying with other ceramics.

- Produce composite structures.

Question 23

Describe the physical property of density in ceramics? (2)

Answer 23

• Lower than most metals ( ≈ aluminium)

- Porosity can mislead.

Question 24

Describe the physical property of electrical in ceramics? (3)

Answer 24

• Conductivity is variable(semiconductors are ceramics)
• Type specific – insulating at room temperature, conducting at elevated temperatures.
• Some superconductive at low (cryogenic) temperatures
  (Specifically formulated).

Question 25

Describe the physical property of thermal properties in ceramics? (1)

Answer 25

- CoE (Coefficient of Thermal Expansion) is lower than metals and plastics, tends to zero.

Question 26

Describe the physical property of thermal conductivity in ceramics?(2)

Answer 26

- Low, ceramics act as insulators. | - Refractory properties due to high melting points.

Question 27

Describe the physical property of light transmission in ceramics?(2)

Answer 27

Some ceramics developed as transparent/opaque.

Question 28

What is thermal shock? (2)

Answer 28

● When material passes through temperature range, fracture can occur. ● Due to stresses resulting from different shrinkage or expansion of surface layer and inner part on cooling or heating (poor conductivity).

Question 29

What are the chemical properties of ceramics? (2)

Answer 29

● Inert to most environments – already in low energy state (oxides, nitrides, etc). ● Severe chemical activity – hot acids can cause corrosion – leaching of one or more component.

Question 30

What are glasses?

Answer 30

Glass is a (inorganic) product of fusion which has been cooled to a rigid condition without crystallising

Question 31

What does Tg mean?

Answer 31

● Tg – Glass Transition Temperature (when viscosity is so high, the glass can be considered solid)

Question 32

What does Tg' mean?

Answer 32

● Tg’ – Lower GTT – can be achieved by slower cooling rate | – stabilised glass.

Question 33

What are the two steps of heat treating glass?

Answer 33

• Annealing: -- removes internal stresses caused by uneven cooling. • Tempering: -- puts surface of glass part into compression -- suppresses growth of cracks from surface scratches. -- sequence:

Question 34

What is the annealing step of heat treating glass?

Answer 34

• Annealing: | -- removes internal stresses caused by uneven cooling

Question 35

What is the tempering step of heat treating glass?

Answer 35

• Tempering: - - puts surface of glass part into compression - - suppresses growth of cracks from surface scratches. -- sequence:

Question 36

Why do you compress the glass?

Answer 36

surface crack growth is suppressed, strengthening.

Question 37

What is thermal toughening? (3)

Answer 37

Surfaces of glass are in compression but subsurface in tension. If compressive stress in surface penetrated glass fails catastrophically. Compression is produced to finished size/shape prior to process.

Question 38

What is chemical toughening? (2)

Answer 38

● By infusion of non‐network ions (larger) (Na+,K+) in surface ● Thickness of compressive layer smaller – easier penetration, but reduced tensile stress – no catastrophic failure.

Question 39

What are 4 physical and chemical properties of glass?

Answer 39

• Transparent to visible spectrum of light (or translucent) • Insulating ‐ Electrically (ρ > 1020μΩm), compare to metals (100× lower than steel) ‐ Thermally (radiation) * Low CoE – 3‐6×10‐6m/m°C * Chemically inert – resist acids, alkalis in general. HF can leach out network modifiers.

Question 40

What are die blanks surface? (2)

Answer 40

-- 4 m polycrystalline diamond particles that are sintered onto a cemented tungsten carbide substrate. Adapted from Fig. 11.8(d), -- polycrystalline diamond gives uniform hardness in all directions to reduce wear.

Question 41

What are some advantages for using ceramics for materials for automobile engines?

Answer 41

– Operate at high T_high efficiencies – Low frictional losses – Operate without a cooling system – Lower weights than current engines

Question 42

What are some disadvantages for using ceramics for materials for automobile engines?

Answer 42

- Ceramic parts are difficult to form and machine - Difficult to remove internal voids (that weaken structures) - Ceramic materials are brittle

Question 43

What is the principle of using ceramics for sensors?

Answer 43

Increase diffusion rate of oxygen | to produce rapid response of sensor signal to change in oxygen concentration

Question 44

What is the approach for using ceramics for sensors? (2)

Answer 44

Add Ca impurity to ZrO2: - - increases O2- vacancies - - increases O2- diffusion rate

Question 45

What is the operation for using ceramics for sensors? (2)

Answer 45

-- voltage difference produced when O2- ions diffuse from the external surface through the sensor to the reference gas surface. gas with an unknown, higher oxygen content reference O2- gas at fixed -- magnitude of voltage difference  partial pressure of oxygen at the external surface

Question 46

How are ceramics used as materials for ceramic armour? (2)

Answer 46

-- Facing plates -- hard and brittle — fracture high-velocity projectile — Al2O3, B4C, SiC, TiB2 -- Backing sheets -- soft and ductile — deform and absorb remaining energy — aluminum, synthetic fiber laminates