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Materials Science 1 > Ceramics & Glasses > Flashcards

Ceramics & Glasses Flashcards

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1
Q

what are some general applications of ceramics and glasses?

A
  • facing materials (non-load bearing, used for appearance like texture/colour)
  • load bearing products like bricks (low cost, insulating, fire proof)
  • paving units (abrasion resistance)
  • roofing tiles
  • chemically resistant products like sewers and piping and industrial chimneys
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2
Q

what are the 3 main broad classes of ceramics?

A
  • traditional (clay bodies)
  • advanced/technical (using engineered ceramics)
  • glasses (not always ‘ceramic’ but brittle solids)
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3
Q

what are some of the properties of ceramics?

A
  • brittle solids
  • no means for plastic deformation, as dislocations cannot relieve stress
  • can be high strength but dominated by microstructure
  • low toughness and elongation at failure (poor ductility)
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4
Q

why is it hard to plastically deform a ceramic?

A

has directional covalent bonding and so dislocation movement is difficult

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5
Q

what types of thing form ceramics?

A
  • ionicly bonded compounds with a large difference in electronegativity
  • covalently bonded compounds with similar electronegativities (oxides, nitrides etc.)
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6
Q

what structure does glass or glassy materials have?

A

amorphous structure (non-crystalline), the atoms have no periodic packing and is a result of quick cooling

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7
Q

what are the mechanical properties of glasses?

A
  • glasses are brittle
  • low toughness (low resistance to crack propagation)
  • amorphous - limited dislocation movement as there is no periodic structure
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8
Q

what is the modulus of rupture?

A

a way of measuring the tensile strength of a ceramic - it’s like tensile strength but determined by bending (because ceramics cannot be tested in the same way as metals as they will crush and fracture)

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9
Q

what is the chemical symbol for glass? (common window glass)

A

SiO2

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10
Q

what does adding impurities to glass do?

A

adding impurities like sodium atoms reduces the melting point of the glass by disrupting the structure

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11
Q

what is a main difference between a glassy material and a crystalline material?

A

crystalline materals:
- crystallise at melting temp, Tm
- have an abrupt change in spec. volume at Tm
glasses:
- do not crystallise
- spec. volume varies smoothing with increase in T
- glass transition temp, Tg

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12
Q

what are some advantages of glass?

A
  • optical transparency (windows to optical fibre)
  • corrosion reisstance
  • electrical and thermal insulator
  • ease of fabrication (low softening point) - east to mass produce bottles on production line
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13
Q

what acid can dissolve glass?

A

hydrofluoric acid or you can boil sodium hydroxide and water with the glass

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14
Q

what are the three main components for forming glass?

A

silica soda and lime (plus any ionic solids to lower the melting point), heat these raw materials above Tm for a few hours to get it into viscous form

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15
Q

what are ideal conditions when forming glass for best optical transparency?

A
  • homogenous (completely mixed)

- pore free (avoid bubbles and lower viscosity)

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16
Q

what temp is glass heated up to when processing?

A

glass is heated up to 800 degrees C to achieve a viscosity similar to syrup

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17
Q

what are the 3 main glass fabrication techniques?

A
  • pressing
  • blowing
  • drawing
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18
Q

what is the relationship between temp and viscosity of a glass?

A

viscosity decreases with increasing T - becomes more liquidy with increasing temp
(viscous = thick, sticky)

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19
Q

what is T deform?

A

T deform is the temp. at which the glass is soft enough to deform or “work”

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20
Q

what is the relationship between impurities in glass and T deform?

A

impurities lowers T deform

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21
Q

what is pressing in terms of a glass fabrication technique?

A
  • used to make thick walled pieces (plates)

- pressure in a heated mould of a desired shape

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22
Q

what is blowing in terms of a glass fabrication technique?

A
  • used to make automated jars, light bulbs, bottles
  • think blow moulding
  • temporary shape formed by pressing and blow moulded after to conform to mould contours using compressed air
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23
Q

what is drawing in terms of a glass fabrication technique?

A
  • used to make sheets, tubing or rod glass (anything with a constant cross section)
  • fibre forming technique where temp and viscosity are critical
  • glass is extruded through a small die
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24
Q

how do you make float glass?

A

glass floats on top of molten tine which allows for a very high quality finish and flat surface

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25
Q

how do we generally strengthen glass?

A

we generally enhance the strength by inducing compressive surface stresses (tempering)

26
Q

how does tempering work?

A
  • heat above Tg, just below Ts - then cool with jets of air
  • surface cools more rapidly and becomes rigid
  • interior continues to cool and contract, drawing in the rigid surface
  • surface compressive stresses must now be overcome to cause tensile failure
  • suppresses growth of cracks from surface scratches because of the compressive force on the surface of the material (toughens the glass)
27
Q

what is tempering of glass used for?

A
  • large doors
  • windshields
  • eyeglasses
  • safety glasses
28
Q

what are some other ways we strengthen glass?

A
  • chemical strengthening (replace Na+ ions on glass surface with larger K+ ions)
  • reinforcement in steel mesh or plastic laminates
29
Q

what is annealing?

A

annealing removes internal stress caused by uneven cooling

30
Q

what does laminating do to glass?

A

laminating strengthens the glass

31
Q

why do materials weaken slowly over time?

A

from the moment of manufacture, slow crack growth will occur by:

  • internal stresses
  • chemical interaction (e.g water vapour Si-O bonds broken by H2O)
  • mechanical interaction (e.g erosion by abrasive particles in the atmosphere)
  • tensile strength decreases over time
32
Q

what is the equation for the time dependent strength of glass? MEMORISE

A 
( σ / σ TS)^n = t test / t
where
σ TS = initial strength measured at time t test
σ = strength determined at time t
n = time exponent (material property)
t test = time when tested (inital t0)
t = new time
33
Q

what are some general properties of ceramics? good in…

A
  • thermal stability
  • chemical / environmental resistance
  • high compressive strength
  • high stiffness
  • excellent wear properties
34
Q

what are some general properties of ceramics? poor in…

A
  • ductility and toughness = very brittle!
  • can be difficult to manufacture
  • high density
35
Q

where are ceramics used in terms of civil applications?

A
  • from facades to structural components

can still be damaged by the environment e.g freeze-thaw

36
Q

what is an example of an igneous rock?

A
  • granite
  • similar to SiO2 - Al2O3 alloys
  • subjected to enough heat to melt the two components and fuse them into a dense solid
37
Q

what is an example of a sedimentary rock?

A
  • sandstone
  • contains silica and calcium carbonate
  • precipitated from solution in ground water rather than melting
  • tends to be porous
  • weaker than granite
38
Q

what are traditional ceramics made from? some properties?

A
  • generally formed from local clays
  • cheap and abundant
  • easily formed and shaped
  • may have some additives (grits, sand, etc.)
39
Q

where are traditional ceramics often used?

A
  • bricks, tiles, sewer pipes

- whitewares: crockery, plumbing fixtures, etc.

40
Q

what are some common clays? what is the structure of clays?

A
  • aluminosilicates = (Al2O3 and SiO2) and chemically bound water
  • layered structure of ‘platelets’
41
Q

how does the drying and firing process of clay work?

A
  • clay contains significant liquid (low strength)
  • if you dry clay you remove water, avoiding shrinkage cracking and allows handling
  • layers size and spacing decreases for the clay platelets as they dry
  • it is called green ceramic when it has been fully dried
42
Q

what is the process of slip casting?

A
  • slip is formed with high water content (liquid clay-water)
  • poured into porous mould (plaster of Paris)
  • mould is rotated so the slip coats the entire mould
  • water is absorbed away into the mould leaving a solid layer
  • remove mould to reveal ceramic material]
  • take the green body and fire it
43
Q

what is hydroplastic forming?

A
  • mix slip with water t make a plastic and pliable solid
  • then can mould and extrude the slip (e.g into a pipe) using a ram
  • dry and fire the coponent
  • process used to make bricks
44
Q

what is firing?

A

the process of heating the ceramic to 900-1400 degrees C, density and mechanical properties is increased, as green body is transformed into a fired solid

45
Q

what is vitrification?

A

formation of liquid glass (normally from SiO2 particles), which flows into pores and produces dense ceramic - kind of acts like a glue and keeps everything together

46
Q

what are some general properties of advanced ceramics?

A
  • stiffness normally exceeds metals (ionic bonding)
  • light elements (C, O, Al) are not close packed and so have generally low density
  • hard and wear resistant - difficult to plastically deform
  • still brittle (low toughness and ductility(
  • poor strength in tension
47
Q

what are some applications of advanced ceramics and why?

A
  • high Tm so used in high temp engines (increased fuel efficiency in engines)
  • good wear resistance so useful in bearing surfaces or abrasives
  • used in spark plugs because they are electrical and thermal insulators
  • used in gold clubs
48
Q

what are some advanced ceramics processing techniques?

A
  • uniaxial pressing
  • isostatic pressing
  • sintering
49
Q

what is uniaxial pressing?

A
  • uniaxial pressing is pressure in one axis
  • uniaxial pressing is when you put a ceramic powder with binder placed into a ‘die ‘and pressure is applied via a ram
  • simple / rapid / cheap
  • automated
  • limited to simple shapes
50
Q

what is isostatic pressing?

A
  • hydrostatic pressure (same magnitude in all directions hence isostatic)
  • powder with binder placed in a rubber preform
  • pressure applied by a fluid (hydraulic oil)
  • time consuming and expensive
  • labour intensive
  • can make complex shapes (e.g spark plugs)
51
Q

what is sintering?

A
  • heating up the powder to 0.6Tm
  • diffusion occurs such that the particles sinter and form necks which grow, reducing surface area and energy and forming dense solids
52
Q

what is a green body?

A

unfired ceramic

53
Q

what are problems with sintering?

A
  • you will always get flaws

- the higher the sintering temp. the longer we sinter, increases porosity which decreases the strength

54
Q

what is tensile strength of ceramics and glasses determined by?

A

flaw size - therefore cracks reduce the strength of ceramics (limited by the largest flaw)

55
Q

what is the equation that relates to fracture toughness of ceramics? MEMORISE

A
  • a measure of resistance to crack propagation
    σTS = Kic / (πa)^0.5
    where Kic = toughness of material
56
Q

what does a high fracture toughness mean?

A

high resistance to the propagation of cracks

57
Q

when using the fracture toughness equation what does a and 2a relate to?

A 
a = surface flaw/scratch/crack
2a = pore
58
Q

what are some methods that reduce the distribution of strengths when testing ceramics?

A
  • proof testing: (removing weakest samples)

- introduce defects of a known dimension (e.g by surface grinding)

59
Q

how can we improve the strength of a ceramic?

A 
by reducing ceramic flaw size by...
- using fine powders
- quality control / clean room processing
- uniform/controlled grain size
- limit porosity (high density)
or by improving the toughness...
- addition of fibre reinforcement (but expensive)
- transformation toughening 
- tempering (for glasses)
60
Q

what is transformation toughening?

A
  • using zirconia (ZrO2)
  • change from tetragonal particles turning into monoclinic
  • associated with a change in volume
  • crack tip is put into compression
  • toughens the material

Materials Science 1 (10 decks)

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