2. Classes of Materials Flashcards
(25 cards)
How can we classify materials
Material classes -> similar types of bonds -> similar properties
Bonding: what type of bonding do metals have
Metallic bonding
- allows atoms to rearrange
- electrons move freely
- easily deformed from this
- bonding is strong therefore metals are
Bonding: What type of bonding do ceramics have
Ionic or covalent bonds
- storngly bound together
- electrons localised
- so ceramics strong
- cannot be deformed
- wont conduct well (but can find a few that do conduct)
- high melting points
Bonding: What type of bonding do polymers have
chains
- weak forces
- low strength
- low melting temp
- poor conductors (but can find exceptions)
What is steel usually made up of
iron and carbon
can add other things
what can you add to steel to mkae it stainless
Chromium
- less likely to oxidise
- 10% to make it stainless
what internal structure do metals have
crystal
- don’t act like crystal
- atoms shuffle when hit to rearrange themselves
- when hit, they become stronger as blockages are formed so less likely for atoms to shuffle
why do we need alloys
can create stonger metals by combining properties of other metals
- sometimes adding to soft metals together e.g. in bronze, even if both tin and copper are soft, can create a strong metal as tin atoms replace some copper atoms so harder to shuffle
What are “Blockages”
shuffling atoms collide, cause dislocation, harder for other atoms to move around
What is an alloy
A mixture of two or more elements inw hich the main component is a metal
Why is concrete brittle
down to it’s inner strucutre
- isn’t entirely solid as has small air gaps
- when compressed its strong
- when its stretched, holes get bigger as stress builds on the edges of the holes, causing it to fracture
- reinforced to stop cracking but cannot control the concrete
why should the coefficients of thermal expansion (CTE) of steel and concrete be similar
if steel is used to reinofrce concrete, when cooling and heating, the steel and concrete should expand and contract at the same rate so no futher pressure builds
What is a superconductor and why aren’t they used more
- at extreme low temperatures, there is no electrical resistance
- low temperatures are near absolute zero so not useful in every day
What are examples of engineering ceramics
- Aluminium oxide (hard, birttle, poor ocnductivity, used in theral insulation, doesn’t oxidise)
- silicon nitride (high thermal conductivity, tough)
- carbon fibre (high strength to weight ratio, low coefficient of thermal expansion)
What does amorphous mean (in terms of glass)
without regular crystal structure,
no alignment
weak bonds between molecules
What are examples of glasses
-soda-lime glasses (sodium oxide + calcium oxide + silica, low melt point)
- borosilicate glasses (heat resisitant, low expansion)
- E-glass (aluminium oxide, calcium oxide, magnesium oxide, fibre forming)
what is vulcanisation
curing rubbers
what does adding sulfur and white lead do to rubber
transform the rubber
creates links between molecules
What is a polymer
A chain of thousands of monomers that are linked together by the covalently-bonded carbon atoms
- can be visualised as tangled chains to form low density structures
- attractive forces between polymer chains
What are thermoplastics
Polymers with weak forces within the chains, that are melted (have low melting temperatures), cooled slowly (to form ordered strucutres)
What is thermosetting
polymers with strong chemical cross linking so higher tensile stregnths, can’t form ordered strucutres so burn instead of soften with heating
What is a partially crystalline strucutre
aligned regions
stronger bonds between molecules
what is a drawn structure
highly aligned molecule
weak bonds between molecules (transverse to alignement)
strong in direction of alignement
what are examples of polymers
nylon (abrasion resistant, self lubricating, resistant to most chemicals)
Kevlar (high tensile, verys tiff)
Perspex (moderate strength, optical properties, resistant to chemicals)
Polyethylene (moderate strength, easily formed, low density)
Epoxies (resins, good emchanical and adhesive properties, resistant to heat)
Phenolics (brittle, high resistance to heat, electricity and chemicals)
