Non-metallic Materials Flashcards
Why are ceramics typically stiff?
- high Young’s modulus
- high E strain
- elastic deformation
- bonds stretch not break
Why is theoretical strength in ceramics never reached?
Imperfections
- porosity, holey, cracks
What ways does porosity affect strength (2 points)
- reduces cross sectional area, less material
- pores act as stress concentrators
Why is the toughness of ceramics less than that of metals?
- No plastic deformation
- highly likely to contain pores
- even small pores can have a big effect
What are toughening mechanisms for ceramics
- reduce porosity as much as possible
- reinforce with something that has high tensile strength (composite)
What are the ways of processing ceramics
- hydroplastic forming
- slip casting
What is hydroplastic forming
There are clay particles and water
Water gets between the sheets
Clay particles slide past each other
With little water clay becomes very plastic
What is slip casting?
Suspension of clay particles that can be poured into a mould
The plaster mould is slightly porous so water leaves through it
Requires drying and firing
What is vitrification
One component melts while the rest stays solid, liquid can flow into the gaps between the solid particles and fill the pores, liquid then solidifies as glass
Occurs during the firing process of slip casting
What are the processing routes for advanced ceramics (3)
- Powder pressing ( limited to to the density)
- uniaxial pressing ( powder at top gets compressed more)
- isostatic pressing ( pressing from all sides)
What is sintering?
Powders are not easily compacted which results in porosity
During sintering solid state diffusion allows for the atoms in a material to move and reduce porosity
What is glass?
A type of ceramic with no long range order (random triangles)
How to form crystalline or glassy
- depends a lot on cooling rate
- when molten SiO2 is cooled slowly it forms a crystalline solid
- cooled rapidly it forms an amorphous or glass
Draw graph of volume V temp of glassy solid and crystalline solid
Draw bitch
What is viscosity
- As temp increases viscosity decreases
What are the properties of glass? (5)
- excellent optical properties
- high chemical stability
- electrical insulator
- thermal properties (expansion) can be altered by varying composition
- Extremely brittle (low fracture toughness)
What is toughened glass (also called tempered glass)
- the surface is made so it is harder for cracks to grow by thermal or chemical techniques
What is the process of thermal tempering
- heat glass above glass transition temp
- spray with water
- outer layer cools very fast resulting in low shrinkage
- the inside cools more slowly resulting in more shrinkage
- pulls outside into compression
What is annealing?
Heating to get rid of residual stress
Examples of engineering applications (review on notes
- Fused silica
- Soda-lime-silica
- Borosilicate
What are polymers?
Materials with structures that are made up of many repeating molecular units, often based on carbon
What are long chains of polymers
- Chains become all tangled
- weak secondary bongs between tangled chains
- strong bonds holding chains together
- longer chains are stronger
What does the mechanical strength of polymer structures depend on? Draw graph + labels
- depends on chain length
- look at notes
Name and draw polymer chain structure? (3)
- linear (HDPE) secondary bonds between chains are more alignment
- branched (cannot pack together closely due to branches) (LDPE)
- cross-linked (strong covalent bonds)
What are thermoplastics
- weak secondary bonds
- c-c bond vibrates with temp thus can untangle and slide past each other, when load is applied
- plastic deformation
- cross-linked polymer
- linear or branched
Looks like a river kinda
What are thermosetting plastics (thermosets)
- strong cross-links
- do not soften with temp (melt) so no plastic deformation
- get rubbery or char
- Epoxy resin
- brick wall
Elastomers
- rubber
- part way between a thermoset and a thermoplastic
- long chain molecules all tangled up together
- only a few cross-links
- causes enormous elastic deformation
Polymers amorphous vs crystalline
- Can be completely amorphous (no long range order)
- Can be partially crystalline (regions of long range order)
Draw the influence of temp on thermoplastics
Draw ya bitch
What do the mechanical properties of polymers depend on (2 Lines)
Depends on structure, including degree of crystallinity
Also temp and rate of loading
What are the 2 forms of polyethylene?
High density (HDPE) has high strength (chains line up, lots of secondary bonding
Low density (LDPE) has lower strength (fewer secondary bonds)
Why is PMMA completely amorphous?
Side groups stop chains from lining up
Mechanical response of elastomers
Elastomeric polymers (rubber) can have very high elastic strain
What happens when load is applied to elastomers?
Cross-links stretch and don’t break
Heat is required
Mechanical response of thermoplastics
Necking occurs
Orientation strengthening
Look at graph
What is orientation strengthening?
Long tangled molecules are pulled into the neck
Chains become more aligned
More crystalline
More secondary bonding in the neck
Rate of loading of polymers
Fast loading - chains cannot untangle so elastic behaviour
Slow loading - chains can untangle and slide past each other so viscous behaviour
What are the viscoelastic models used to describe polymer behaviour
Elastic behaviour = spring
Viscos behaviour = dash pot
What is stress relaxation in polymers
If a polymer is held at high stress it will gradually reduce due to chains sliding
What are the 2 phases composites consist of
- discrete
- continuous (matrix)
What is anisotropy
Different properties in different directions
- box with different layers
What are fibre reinforced composites
Strong, high modulus fibres embedded in a more ductile lower strength matrix
To obtain optimum stiffness, strength and toughness
What is wood comprised of?
Cellulose (semi-crystalline polymer of glucose)
Lignin (amorphous, highly branched and cross-linked polymer)
Hemicellulose (amorphous, slightly branched polymer)
What are the typical properties of ceramics
- strong in compression ( stepping on a cup)
- brittle ( no ductility- no plastic deformation)
- hard
- insulators
- high chemical stability
- high melting points
What is the structure and bonding of ceramics?
- ionic and/or covalent
- strongest of all atomic bonds
- also directional
What are the 2 factors controlling ceramics
- charges must be balanced
- relative size of the cations and anions ( anions tend to be bigger, anions accept electrons)
- cations are surrounded by anions
Why are ceramics typically brittle?
Plastic deformation (slip) is very limited (cannot happen)
What are silicate structures? (Review notes)
- structures are based on joining up SiO4 - tetrahedral
How does adding sodium to glass reduce the viscosity?
When Na+ is added Si4- is removed, resulting in non-bridging oxygens, so less rigid structure
What is isostres
Load this way, one component with stretch more than the other
Box thing
What is isostrain
Load this way, the deformation will be limited by the stiffer components
