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xxxEmats 2 > Chapter 3: Ceramics & Composites > Flashcards

Chapter 3: Ceramics & Composites Flashcards

1
Q

what are ceramics?

A

complex inorganic compounds consisting of metal & non-metal elements joined by ionic or covalent bonds

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2
Q

properties of ceramics

A

high hardness, m.p
high creep & corrosion resistance
high compressive strength
low thermal conductivity

brittle
low electrical conductivity
low toughness

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3
Q

difference between ionic and covalent bonding

A

ionic: metal & non-metal
electrons are transferred

covalent: non-metal bonding
electrons are shared

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4
Q

why are ceramics hard and brittle?

A

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

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5
Q

how are ceramics processed?

A

compacting powders/particles into shapes, then heated to high temp to bond particles together

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6
Q

describe the 3 basic steps in processing ceramics

A

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

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7
Q

what is cold isostatic pressing

A

rubber container,
pressurising medium: water or oil
free of die frictional forces

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8
Q

2 types of ceramics pressing

A

cold isostatic pressing
hot isostatic pressing

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9
Q

what is hot isostatic pressing

A

container made of metal/glass
pressuring medium is gas
container deforms plastically to compact powder at high temp.

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10
Q

process of slip casting

A

1) slip poured into porous mould
2) leave inside for a layer to form
3) excess slip is poured away
4) top is trimmed

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11
Q

what does sintering do?

A

increases rigidity and strength of ceramics, causes additional shrinkage as pore size between particles decreases

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12
Q

3 stages of sintering

A

1) neck growth proceeds rapidly
2) densification occurs, structure recrystallise, particles diffuse into each other
3) isolated pores become spheroidal

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13
Q

what ceramics properties are needed for furnace lining?

A

high thermal stability (temp. does not fluctuate)
high mp, (hot environment)
low thermal conductivity (does not lose heat fast)

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14
Q

ceramics properties needed for cutting tools

A

high hot hardness (retain hardness even at high temp)

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15
Q

biomedical properties of ceramics

A

biocompatibility, lower thermal conductivity

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16
Q

applications of ceramics

A

cutting tools, medical implants, glass, electrical, vases/mugs/plate, building materials

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17
Q

why is ceramics better in compression than tension?

A

voids and cracks inside are squeezed when compression, flaws will be stable

flaws will expand under tension which may open up

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18
Q

what are composites?

A

a mixture of 2 or more materials on a macroscale, properties which are superior than those of its individual components

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19
Q

3 categories of composites

A

particulate, laminate, fibre

20
Q

factors that control behaviour of composites

A
  • properties of components
  • size & shape of components
  • volume fraction of components
  • strength of bond between components
21
Q

composite strength formula

A

σc = σ1V1 + σ2V2

σ - strength
V - volume fraction

22
Q

volume fraction points

A

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)

23
Q

what is particulate composites

A

consist of fine, hard particles evenly distributed into softer, tougher matrix to improve, mechanical properties

24
Q

how are particulate composites stronger and stiffer?

A

particles are stronger than parent matrix material
restrict movements of dislocation and cracks

25
Q

how does adding rubber particles toughen particulate composites

A

rubber particles acts as springs, clamping the crack shut, increasing the load needed to propagate

26
Q

types of laminate composites

A

plywood, safety glass, honeycomb

27
Q

how does laminate composite help to not shatter glass?

A

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.

28
Q

what do fibre composites improve?

A

strength, stiffness, fatigue properties, strength to weight ratio

29
Q

how does fibre composites improve properties?

A

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

30
Q

functions of matrix in fibre composites

A
  • bind fibres together
  • protect fibre surfaces
  • separate individual fibres
  • prevent brittle cracks from spreading
  • medium to transfer and distribute load to fibres
31
Q

3 types of fibre alignment

A

1) continuous & aligned
2) discontinuous & aligned
3) discontinuous & randomly orientated fibers

32
Q

how does fibre alignment affect performance?

A

if the load direction is 1 direction and known, aligned is better (stronger in same direction (longitudinal than transverse)

33
Q

why is fibre-reinforced composite stronger in 1 direction?

A

fibres will take the load when applied along fibre direction (fibre is stronger than matrix)

34
Q

3 types of fibres used in composites & their usage

A

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

35
Q

what is the strength of fibre composites determined by?

A
  • strength of fibres
  • orientation of fibres wrt applied load
  • continuity of fibres
  • properties of matrix
  • strength and nature of bond between fibre and matrix
36
Q

factors to consider when choosing fibres

A

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

37
Q

types of processing methods for composites

A

1) hand lay-up
2) spray lay-up
3) filament widing
4) pultrusion

38
Q

hand lay-up points

A

most common,
rolling ensures good contact and freedom of porosity
product is cured

39
Q

spray lay-up points

A

continuous strands are fed through a chopper, mixed with resin from spray gun
product is cured

40
Q

filament winding points

A

1 or more continuous fibres are wrapped around a form to build a solid hollow shape (cylindrical parts)

41
Q

pultrusion method points

A

extrude polymer matrix around fibres to form simple shape products with constant cross-section (rods/tubes)

42
Q

failure modes of fibre composites

A

fibre fracture
fibre pull-out

43
Q

what is fibre fracture?

A

fibre breaks in 1 plane, composite will fail in that plane since matrix is soft and cannot carry load

44
Q

what is fibre pull-out?

A

adhesion between fibres and matrix is not strong enough, fibres pulled out of the matrix

45
Q

why does plywood have an odd number of layers?

A

so that top & bottom layers of plywood have the same finish (reduced warping)

xxxEmats 2 (5 decks)

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