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)
