Chapter 3: Ceramics & Composites Flashcards
what are ceramics?
complex inorganic compounds consisting of metal & non-metal elements joined by ionic or covalent bonds
properties of ceramics
high hardness, m.p
high creep & corrosion resistance
high compressive strength
low thermal conductivity
brittle
low electrical conductivity
low toughness
difference between ionic and covalent bonding
ionic: metal & non-metal
electrons are transferred
covalent: non-metal bonding
electrons are shared
why are ceramics hard and brittle?
hard:
ionic bonds make moving easy on some planes but hard on others,
covalent bonds have large resistance to movements
brittle:
small defects (cracks, voids)
energy to fracture ceramics is thus reduced
how are ceramics processed?
compacting powders/particles into shapes, then heated to high temp to bond particles together
describe the 3 basic steps in processing ceramics
1) powder production: grinding (traditional)
vapour-phase deposition (eng. ceramics)
2) pressing: particles are pressed into a die to form “green” shaped products
3) sintering: rigidity and strength will increases, causes additional shrinkage as pore size is reduced
what is cold isostatic pressing
rubber container,
pressurising medium: water or oil
free of die frictional forces
2 types of ceramics pressing
cold isostatic pressing
hot isostatic pressing
what is hot isostatic pressing
container made of metal/glass
pressuring medium is gas
container deforms plastically to compact powder at high temp.
process of slip casting
1) slip poured into porous mould
2) leave inside for a layer to form
3) excess slip is poured away
4) top is trimmed
what does sintering do?
increases rigidity and strength of ceramics, causes additional shrinkage as pore size between particles decreases
3 stages of sintering
1) neck growth proceeds rapidly
2) densification occurs, structure recrystallise, particles diffuse into each other
3) isolated pores become spheroidal
what ceramics properties are needed for furnace lining?
high thermal stability (temp. does not fluctuate)
high mp, (hot environment)
low thermal conductivity (does not lose heat fast)
ceramics properties needed for cutting tools
high hot hardness (retain hardness even at high temp)
biomedical properties of ceramics
biocompatibility, lower thermal conductivity
applications of ceramics
cutting tools, medical implants, glass, electrical, vases/mugs/plate, building materials
why is ceramics better in compression than tension?
voids and cracks inside are squeezed when compression, flaws will be stable
flaws will expand under tension which may open up
what are composites?
a mixture of 2 or more materials on a macroscale, properties which are superior than those of its individual components
3 categories of composites
particulate, laminate, fibre
factors that control behaviour of composites
- properties of components
- size & shape of components
- volume fraction of components
- strength of bond between components
composite strength formula
σc = σ1V1 + σ2V2
σ - strength
V - volume fraction
volume fraction points
whole value must add to 1 (V1 + V2 + V3 = 1)
to only use 1 fraction e.g. V1, 1 - the others)
example:
V1 + V2 = 1
V2 = 1 - V1 (sub back into eqn)
what is particulate composites
consist of fine, hard particles evenly distributed into softer, tougher matrix to improve, mechanical properties
how are particulate composites stronger and stiffer?
particles are stronger than parent matrix material
restrict movements of dislocation and cracks
how does adding rubber particles toughen particulate composites
rubber particles acts as springs, clamping the crack shut, increasing the load needed to propagate
types of laminate composites
plywood, safety glass, honeycomb
how does laminate composite help to not shatter glass?
when 2 glass plates are stuck together by polymer adhesive, strength will increase on 1 plate and will also prevent the glass from shattering everywhere.
what do fibre composites improve?
strength, stiffness, fatigue properties, strength to weight ratio
how does fibre composites improve properties?
by incorporating strong, stiff fibres into a softer & more ductile matrix
matrix transmits the load to the fibres which fibres carry most of the applied load
functions of matrix in fibre composites
- bind fibres together
- protect fibre surfaces
- separate individual fibres
- prevent brittle cracks from spreading
- medium to transfer and distribute load to fibres
3 types of fibre alignment
1) continuous & aligned
2) discontinuous & aligned
3) discontinuous & randomly orientated fibers
how does fibre alignment affect performance?
if the load direction is 1 direction and known, aligned is better (stronger in same direction (longitudinal than transverse)
why is fibre-reinforced composite stronger in 1 direction?
fibres will take the load when applied along fibre direction (fibre is stronger than matrix)
3 types of fibres used in composites & their usage
1) glass:
used in plastic due to low cost
2) carbon:
aerospace/sports due to high strength-to-weight ratio
3) kevlar:
lightweight, impact resistance
what is the strength of fibre composites determined by?
- strength of fibres
- orientation of fibres wrt applied load
- continuity of fibres
- properties of matrix
- strength and nature of bond between fibre and matrix
factors to consider when choosing fibres
1) matrix should wet fibres to reduce void chances
2) extensive detrimental reactions should not take place
3) difference in thermal expansion coefficient should not be large due to excessive thermal stresses on cooling/heating
types of processing methods for composites
1) hand lay-up
2) spray lay-up
3) filament widing
4) pultrusion
hand lay-up points
most common,
rolling ensures good contact and freedom of porosity
product is cured
spray lay-up points
continuous strands are fed through a chopper, mixed with resin from spray gun
product is cured
filament winding points
1 or more continuous fibres are wrapped around a form to build a solid hollow shape (cylindrical parts)
pultrusion method points
extrude polymer matrix around fibres to form simple shape products with constant cross-section (rods/tubes)
failure modes of fibre composites
fibre fracture
fibre pull-out
what is fibre fracture?
fibre breaks in 1 plane, composite will fail in that plane since matrix is soft and cannot carry load
what is fibre pull-out?
adhesion between fibres and matrix is not strong enough, fibres pulled out of the matrix
why does plywood have an odd number of layers?
so that top & bottom layers of plywood have the same finish (reduced warping)
