Ceramics & Glasses Flashcards
what are some general applications of ceramics and glasses?
- 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
what are the 3 main broad classes of ceramics?
- traditional (clay bodies)
- advanced/technical (using engineered ceramics)
- glasses (not always ‘ceramic’ but brittle solids)
what are some of the properties of ceramics?
- 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)
why is it hard to plastically deform a ceramic?
has directional covalent bonding and so dislocation movement is difficult
what types of thing form ceramics?
- ionicly bonded compounds with a large difference in electronegativity
- covalently bonded compounds with similar electronegativities (oxides, nitrides etc.)
what structure does glass or glassy materials have?
amorphous structure (non-crystalline), the atoms have no periodic packing and is a result of quick cooling
what are the mechanical properties of glasses?
- glasses are brittle
- low toughness (low resistance to crack propagation)
- amorphous - limited dislocation movement as there is no periodic structure
what is the modulus of rupture?
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)
what is the chemical symbol for glass? (common window glass)
SiO2
what does adding impurities to glass do?
adding impurities like sodium atoms reduces the melting point of the glass by disrupting the structure
what is a main difference between a glassy material and a crystalline material?
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
what are some advantages of glass?
- 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
what acid can dissolve glass?
hydrofluoric acid or you can boil sodium hydroxide and water with the glass
what are the three main components for forming glass?
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
what are ideal conditions when forming glass for best optical transparency?
- homogenous (completely mixed)
- pore free (avoid bubbles and lower viscosity)
what temp is glass heated up to when processing?
glass is heated up to 800 degrees C to achieve a viscosity similar to syrup
what are the 3 main glass fabrication techniques?
- pressing
- blowing
- drawing
what is the relationship between temp and viscosity of a glass?
viscosity decreases with increasing T - becomes more liquidy with increasing temp
(viscous = thick, sticky)
what is T deform?
T deform is the temp. at which the glass is soft enough to deform or “work”
what is the relationship between impurities in glass and T deform?
impurities lowers T deform
what is pressing in terms of a glass fabrication technique?
- used to make thick walled pieces (plates)
- pressure in a heated mould of a desired shape
what is blowing in terms of a glass fabrication technique?
- 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
what is drawing in terms of a glass fabrication technique?
- 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
how do you make float glass?
glass floats on top of molten tine which allows for a very high quality finish and flat surface
how do we generally strengthen glass?
we generally enhance the strength by inducing compressive surface stresses (tempering)
how does tempering work?
- 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)
what is tempering of glass used for?
- large doors
- windshields
- eyeglasses
- safety glasses
what are some other ways we strengthen glass?
- chemical strengthening (replace Na+ ions on glass surface with larger K+ ions)
- reinforcement in steel mesh or plastic laminates
what is annealing?
annealing removes internal stress caused by uneven cooling
what does laminating do to glass?
laminating strengthens the glass
why do materials weaken slowly over time?
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
what is the equation for the time dependent strength of glass? MEMORISE
( 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
what are some general properties of ceramics? good in…
- thermal stability
- chemical / environmental resistance
- high compressive strength
- high stiffness
- excellent wear properties
what are some general properties of ceramics? poor in…
- ductility and toughness = very brittle!
- can be difficult to manufacture
- high density
where are ceramics used in terms of civil applications?
- from facades to structural components
can still be damaged by the environment e.g freeze-thaw
what is an example of an igneous rock?
- granite
- similar to SiO2 - Al2O3 alloys
- subjected to enough heat to melt the two components and fuse them into a dense solid
what is an example of a sedimentary rock?
- sandstone
- contains silica and calcium carbonate
- precipitated from solution in ground water rather than melting
- tends to be porous
- weaker than granite
what are traditional ceramics made from? some properties?
- generally formed from local clays
- cheap and abundant
- easily formed and shaped
- may have some additives (grits, sand, etc.)
where are traditional ceramics often used?
- bricks, tiles, sewer pipes
- whitewares: crockery, plumbing fixtures, etc.
what are some common clays? what is the structure of clays?
- aluminosilicates = (Al2O3 and SiO2) and chemically bound water
- layered structure of ‘platelets’
how does the drying and firing process of clay work?
- 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
what is the process of slip casting?
- 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
what is hydroplastic forming?
- 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
what is firing?
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
what is vitrification?
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
what are some general properties of advanced ceramics?
- 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
what are some applications of advanced ceramics and why?
- 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
what are some advanced ceramics processing techniques?
- uniaxial pressing
- isostatic pressing
- sintering
what is uniaxial pressing?
- 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
what is isostatic pressing?
- 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)
what is sintering?
- 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
what is a green body?
unfired ceramic
what are problems with sintering?
- you will always get flaws
- the higher the sintering temp. the longer we sinter, increases porosity which decreases the strength
what is tensile strength of ceramics and glasses determined by?
flaw size - therefore cracks reduce the strength of ceramics (limited by the largest flaw)
what is the equation that relates to fracture toughness of ceramics? MEMORISE
- a measure of resistance to crack propagation
σTS = Kic / (πa)^0.5
where Kic = toughness of material
what does a high fracture toughness mean?
high resistance to the propagation of cracks
when using the fracture toughness equation what does a and 2a relate to?
a = surface flaw/scratch/crack 2a = pore
what are some methods that reduce the distribution of strengths when testing ceramics?
- proof testing: (removing weakest samples)
- introduce defects of a known dimension (e.g by surface grinding)
how can we improve the strength of a ceramic?
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)
what is transformation toughening?
- 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
