Ceramics Flashcards
What is an Ashby diagram?
Is a scatter plot used to showcase properties of different materials.
What is embodied energy in materials?
The total energy consumed during all processes of the material’s life cycle.
- Used in mining, extraction, transportation, processing and manufacturing
What are two sources of raw material?
- Mining and quarrying
- Agriculture
What is concretes embodied energy like compared to other materials?
Concrete is a low-energy material relative to other materials like low-carbon steel and silicon.
How cna ceramics be defined?
Ceramics can be defined as inorganic, non-metallic materials.
What are some properties of ceramics?
Typically crystalline, with some glassy phase
What elements are ceramics formed from?
- Silicon and nitrogen
- Calcium and oxygen
What is included in the ceramic industry?
Clay products → roofing tiles
Whitewares → Dinnerware
Refractories → Brick and monolithic products
Glass → Flat glass
Abrasives → Natural and synthetic, diamond
What are the general properties of ceramics?
- High compressive strength
- Tensile strength is relatively low
- Low toughness/brittle materials
– no significant mechanisms to stop cracks from propagating - Relatively high hardness (some are very hard)
- Relatively low cost, despite the need for thermal processing
What are the key points to do with using stone in construction?
- Stone of various types: lowest energy and carbon dioxide emissions
- Needs a combination of cheap and skilled labor
- High energy input during construction
- Property limitations on design and use
What are the advantages of brick over stone?
- Lower application costs
- Relatively low skill
What is involved in ceramic manufacturing?
- Sintering
- Clays moulded on a plastic state and fired
- Glassy phases which melts and ‘glues’ together a complex polycrystalline multiphase body
What are the six categories of clay materials used in industry?
- Brick clay
- Bentonite
- Common clay
- Fire clay
- Fuller’s earth
- Kaolin
What is the structure of clay?
- Silica in the middle and four oxygens bonds
What is waters role in the clay platelet structure?
Polar water molecules acts as a lubricant between layers and make clay slippery
What are usually the saize of ceramic powders?
Ceramic powders are typically in the size range of 0.5 - 5.0 µm
What is clay?
Clays are weathered minerals of sizes 0.5 - 5.0 µm mixed with water
What are traditional ceramic materials made from?
mixtures of clays
What optimises sintering?
Engineering ceramic powders are synthesized
What is a green body?
- Used for raw pre mature ceramics
- An object whose main constituent is weakly bound clay material
What temperatures are used for sintering?
Sintering temperatures vary from 850°C for tiles to >1650°C for engineering ceramics
What are the key features of the ceramic microstructure alumina?
- Crystals of aluminium oxide
- 96% alumina with MgO, CaO and SiO_2
- Thermally etched surface
What does the ceramic microstructure usually consist of?
- Crystalline phases
- Amorphous (glassy) phase - helps in the sintering process
- Porosity - cannot get out of material
What are dislocations?
- Dislocations contribute to plastic deformation under high temperatures or stress
- Dislocations in crystalline and glassy materials are not mobile → brittle materials cannot move to overcome applied stresses
What is the roles of pores and surface flaws?
- act as stress concentrators
What is a disadvantages of ceramics?
No stress relieving mechanism to inhibit crack propagation
Where are the flaws in brittle materials?
Flaw in the middle, stress at defect is higher
What does amplification depend on?
Amplification depends on orientation, geometry, and dimensions.
What is tensile frcture stress controlled by?
Controlled by defects present either from fabrication or surface damage
What is griffiths equation?
tensile fracture stress = fracture toughness/geometrical factorsqrt(pia)
What does griffiths calculation show?
Explains why a brittle material strength is much lower than expected
What does the stress intensity factor describe?
Describes the intensity of the elastic crack tip stress field in a thick, deeply cracked specimen loaded perpendicular to the crack plane.
What is the stress intensity factor equation?
K_1 = α σ (πa)1/2
- a is the crack depth
- α is a constant
- σ is the nominal stress
What happens to the properties as the microstructure changes?
Changes
What does sintering lead to?
Causes shrinkage to occur
What happens if sintering temps increase?
- Density increase and so does shrinkage
What are the steps in producing ceramics?
- Particle forming processes
- Slurry formation, moisture optimisation, spray drying
- Powder pressing/ extrusion/ slip casting
- Drying
- Firing(sintering)
- Ceramic product
What are the 3 types of pressing?
- Hot pressing
- Uniaxial pressing
- Isostatic pressing
What is involved in spray drying of ceramic powders?
- Slurry is sprayed and dried to form agglomerated particles that flow
- Packs much better
What moulds are used for slip casting?
Porous gypsum moulds
How does slip-casting work?
- Mix particles in slurry form → mill in a solution
- Pour slip into the cavity
- Pour the water out of the slurry and leave a residue of powder inside the mould
- Sinter
What are the stages of cermaic products in terms of its structure?
- Granular powder
- Compacted green body
- Ceramic component
What are adobe mud blocks?
- Adobe mud blocks are one of the oldest and most widely used building materials
- Use of these sun-dried blocks dates back to 8000 B.C
- Very common in Latin America, Africa and India
How many bricks are manufactured in the UK currently?
2.7 billion bricks currently manufactured in the UK per annum
Whats involved in the manufacture of bricks?
- Extraction of clay
- Varies in color
- Extruded bricks and wire cut
- Fired for about 7 days at max temp 1100˚C
What are detsructive agents affecting masonry?
- Water
- Frost
- Temperature change
What happens when brick work absorbs water?
Interact with mortar
What can bricks surfaces suffer from?
Brick surfaces may suffer from flaking or spalling and mortar joints will crumble when frost failure occurs
What does most damage to masonry?
- Repeated freeze/thaw cycles do most damage.
What is effloresence on bricks?
- Occurs during early life of a building, after wet conditions
- Usually composed of sodium, potassium, and magnesium sulfates
- White powdery substance
How does efflorensence occur?
- Salts in brickwork are dissolved by water
- As the wall dries the salt solution becomes more concentrated
- They deposit on the surface as white discoloration
What is iron staining of bricks?
Clays have high iron content and under certain conditions can migrate out and can oxidise to form brown stain
How does iron staining occur?
Iron staining is caused by the saturation of immature bricks
How can you get rid of iron staining?
It disappears with natural weathering, but this can take many years
What is lime staining?
Free lime present in mortar leaches out and leaves lime staining
How does lime staining occur?
- Occurs when work left without being covered
- Exposure to rain triggers the process
- White deposit forms where water removes lime from cement
Sintered ceramics are typically characterised by…
The presence of crystalline phases
What are some properties of the microstructure of ceramics?
- high hardness, low toughness
- low tensile strength, high compressive strength
