Chapter 3: Ceramics & Composites

Question 1

what are ceramics?

Answer 1

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

Question 2

properties of ceramics

Answer 2

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

brittle
low electrical conductivity
low toughness

Question 3

difference between ionic and covalent bonding

Answer 3

ionic: metal & non-metal
electrons are transferred

covalent: non-metal bonding
electrons are shared

Question 4

why are ceramics hard and brittle?

Answer 4

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

Question 5

how are ceramics processed?

Answer 5

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

Question 6

describe the 3 basic steps in processing ceramics

Answer 6

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

Question 7

what is cold isostatic pressing

Answer 7

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

Question 8

2 types of ceramics pressing

Answer 8

cold isostatic pressing
hot isostatic pressing

Question 9

what is hot isostatic pressing

Answer 9

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

Question 10

process of slip casting

Answer 10

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

Question 11

what does sintering do?

Answer 11

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

Question 12

3 stages of sintering

Answer 12

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

Question 13

what ceramics properties are needed for furnace lining?

Answer 13

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

Question 14

ceramics properties needed for cutting tools

Answer 14

high hot hardness (retain hardness even at high temp)

Question 15

biomedical properties of ceramics

Answer 15

biocompatibility, lower thermal conductivity

Question 16

applications of ceramics

Answer 16

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

Question 17

why is ceramics better in compression than tension?

Answer 17

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

flaws will expand under tension which may open up

Question 18

what are composites?

Answer 18

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

Question 19

3 categories of composites

Answer 19

particulate, laminate, fibre

Question 20

factors that control behaviour of composites

Answer 20

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

Question 21

composite strength formula

Answer 21

σ_c = σ1V1 + σ2V2

σ - strength
V - volume fraction

Question 22

volume fraction points

Answer 22

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)

Question 23

what is particulate composites

Answer 23

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

Question 24

how are particulate composites stronger and stiffer?

Answer 24

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

Question 25

how does adding rubber particles toughen particulate composites

Answer 25

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

Question 26

types of laminate composites

Answer 26

plywood, safety glass, honeycomb

Question 27

how does laminate composite help to not shatter glass?

Answer 27

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.

Question 28

what do fibre composites improve?

Answer 28

strength, stiffness, fatigue properties, strength to weight ratio

Question 29

how does fibre composites improve properties?

Answer 29

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

Question 30

functions of matrix in fibre composites

Answer 30

- bind fibres together - protect fibre surfaces - separate individual fibres - prevent brittle cracks from spreading - medium to transfer and distribute load to fibres

Question 31

3 types of fibre alignment

Answer 31

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

Question 32

how does fibre alignment affect performance?

Answer 32

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

Question 33

why is fibre-reinforced composite stronger in 1 direction?

Answer 33

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

Question 34

3 types of fibres used in composites & their usage

Answer 34

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

Question 35

what is the strength of fibre composites determined by?

Answer 35

- strength of fibres - orientation of fibres wrt applied load - continuity of fibres - properties of matrix - strength and nature of bond between fibre and matrix

Question 36

factors to consider when choosing fibres

Answer 36

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

Question 37

types of processing methods for composites

Answer 37

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

Question 38

hand lay-up points

Answer 38

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

Question 39

spray lay-up points

Answer 39

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

Question 40

filament winding points

Answer 40

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

Question 41

pultrusion method points

Answer 41

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

Question 42

failure modes of fibre composites

Answer 42

fibre fracture fibre pull-out

Question 43

what is fibre fracture?

Answer 43

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

Question 44

what is fibre pull-out?

Answer 44

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

Question 45

why does plywood have an odd number of layers?

Answer 45

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