Ceramics and glasses Flashcards
What are 6 classification of ceramics?
Glasses Clay products refractories abrasives cements advanced ceramics
What are glasses? (3)
- optical
- composite reinforce
- containers/household
What are clay products? (2)
- whiteware
- structure
What are refractories? (1)
- brick for high T (furnaces)
What are abrasives? (3)
- sandpaper
- cutting
- polishing
What are cements? (2)
- composites
- structures
What are advanced ceramics? (2)
- engine rotors valves bearings
- sensors
What are ceramics?
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
What are glasses?
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).
What materials do ceramics encompass?
Ceramics encompass materials with highest hardness and melting point in nature – diamond.
What are ionic ceramics?
Typically compounds of metal with non‐ metal eg MgO, Al2O3, ZrO2.
What are the two types of ceramics?
ionic ceramics
covalent ceramics
What are covalent ceramics?
Typically compounds of metalloid or non‐metals eg SiO2, SiC
What are the mechanical properties of ceramics?
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.
What is the Weibull Modulus?
M indicates how rapidly strength falls (confidence) approaching σ0.
What does a Low M indicate? (2)
Low M
– greater variability
– low design strength.
What does a High M indicate?
High M
– more stability, more confidence.
What do stress/strain plot for ceramics at room temp indicate?
no plasticity
Why do ionic and covalent produce much more resistance to dislocations than metallic bonds? (3)
– 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.
What kind of deformation can ceramics exhibit at an elevated temperature? (3)
● Dislocation climb
● Grain boundary sliding
● Herring-Nabarro creep – “diffusion of vacancies”
What is the toughness like in ceramics? (3)
● Low – relates to brittle nature.
● Fracture toughness values – KIC ≈ 0.2 of metals.
● Nature of manufacture – powder consolidation – leads to flaws.
How can you improve the toughness of ceramics? (2)
- Alloying with other ceramics.
- Produce composite structures.
Describe the physical property of density in ceramics? (2)
- Lower than most metals ( ≈ aluminium)
- Porosity can mislead.
Describe the physical property of electrical in ceramics? (3)
- Conductivity is variable(semiconductors are ceramics)
- Type specific – insulating at room temperature, conducting at elevated temperatures.
- Some superconductive at low (cryogenic) temperatures
(Specifically formulated).
Describe the physical property of thermal properties in ceramics? (1)
- CoE (Coefficient of Thermal Expansion) is lower than metals and plastics, tends to zero.
Describe the physical property of thermal conductivity in ceramics?(2)
- Low, ceramics act as insulators.
- Refractory properties due to high melting points.
Describe the physical property of light transmission in ceramics?(2)
Some ceramics developed as transparent/opaque.
What is thermal shock? (2)
● 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).
What are the chemical properties of ceramics? (2)
● 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.
What are glasses?
Glass is a (inorganic) product of fusion which has been cooled to a rigid condition without crystallising
What does Tg mean?
● Tg – Glass Transition Temperature (when viscosity is so high, the glass can be considered solid)
What does Tg’ mean?
● Tg’ – Lower GTT – can be achieved by slower cooling rate
– stabilised glass.
What are the two steps of heat treating glass?
• Annealing:
– removes internal stresses caused by uneven cooling.
• Tempering:
– puts surface of glass part into compression
– suppresses growth of cracks from surface scratches. – sequence:
What is the annealing step of heat treating glass?
• Annealing:
– removes internal stresses caused by uneven cooling
What is the tempering step of heat treating glass?
• Tempering:
- puts surface of glass part into compression
- suppresses growth of cracks from surface scratches. – sequence:
Why do you compress the glass?
surface crack growth is suppressed, strengthening.
What is thermal toughening? (3)
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.
What is chemical toughening? (2)
● 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.
What are 4 physical and chemical properties of glass?
• 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.
What are die blanks surface? (2)
– 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.
What are some advantages for using ceramics for materials for automobile engines?
– Operate at high T_high efficiencies
– Low frictional losses
– Operate without a cooling system
– Lower weights than current engines
What are some disadvantages for using ceramics for materials for automobile engines?
- Ceramic parts are difficult to form and machine
- Difficult to remove internal voids (that weaken structures)
- Ceramic materials are brittle
What is the principle of using ceramics for sensors?
Increase diffusion rate of oxygen
to produce rapid response of sensor signal to change in oxygen concentration
What is the approach for using ceramics for sensors? (2)
Add Ca impurity to ZrO2:
- increases O2- vacancies
- increases O2- diffusion rate
What is the operation for using ceramics for sensors? (2)
– 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
How are ceramics used as materials for ceramic armour? (2)
– 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
