Ceramics and Glasses Flashcards
Lecture 13
What are the typical compounds?
Involving metal and non-metal. In construction, materials usually (metal) oxides due to metal bonding to oxygen.
What are typical bond types?
Covalent and Ionic
What is the mechanical behaviour of ceramics and glass?
Brittle materials
-> little or no plasticity before sudden, catastrophic failure
-> Fracture occurs BEFORE yield
-> strength =/= yield strength
low toughness
-> flaw sensitive
-> not well-defined failure strength
-> Okay in compression
-> low fraction toughness -> brittle
What are the applications of ceramics in structures?
bricks
roof tiles
cerement and concrete
NB: need to be loaded in compression
What are the other applications of ceramics in buildings?
floor and wall tiles
sanitary ware
What are the applications of glass in structures?
-> windows
-> fibre insulation
-> Reinforcements in GFRP
-> communications
What happens to ceramics in structures?
Generally oxides, typically several crystal phases will be present (glassy grain boundary phases, complex microstructures - details depend on firing temperature) outer surface may be glazed.
What is “bricks”?
Clay-based products. Typical compositions:
What is engineering ceramics?
Higher grade materials
-> significantly higher cost-high value-added products
-> unlikely to be used in construction
Typically one major phase
-> e.g. alumina, zirconia, silicon carbide, silicon nitride
What is ceramics processing?
formed into a near net shape “green” body
-> Wet processing or powder pressing
Fired to form final project
-> sintering and densification
- shrinking occurs
- these processes may be accompanied by chemical
reactions
What is a glass?
Non-crystalline solid that retains the atomic structure of the liquid
-> changes continuously from a solid to a liquid (and vice versa)
What is the glasses transition region?
Typically characterised by a single temperature Tq
T «_space;Tq - Elastic SOLID
T ~ Tq - Viscoeastic solid/liquid
T > Tq - Liquid
Why does glass usually break?
Glass typically breaks from defects at the surface or an edge
What are some major methods of strengthening glass?
Major methods of strengthening involve including compressive residual stresses on the glass surface.
-> thermal tempering
-> chemical tempering
Laminating is also used - 2 or more layers separated by polymeric interlayers.
-> polyvinyl brutyral (PVB)
-> ethylene vinyl acetate (EVA)
-> lonoplast - copolymer of ethylene & metha-acrylic acid
Often in conjunction with thermal tempered glass
What is thermal tempering?
Heat glass above Tq. Rapidly cool;
-> using jets of cold air
-> surface rapidly becomes rigid
-> exterior in compression ~ 100MPa compression
-> interior in tension
Hence counteracts surface flaws -> making the glass more difficult to break.
If the glass breaks tensile stresses cause multiple fragmentation ~ safety glass, but nickel sulphide (NiS) can be an issue.
What is chemically tempered glass?
Surface compressive stresses. Produced using a diffusional ion exchange process:
-> Very high surface compressive stresses can be achieved
~ several 100MPa to ~ 1GPa
~ Enables the production of fragile strong glass but slow
and expansive
-> major application is in mobile devices screens
What is glass fibres?
Reinforcing elements in polymer matrix composites ~ E glass
Other than glass fibres, what else can be used for thermal insulation?
Glass wool
Stonewool ~ greater fire resistance
What are reinforcing fibres?
Structural fibres e.g. E-glass
-> Down drawn from a melt-through bushing
Fibres coating with a size immediately after drawing
What is glass wool?
It may be produced from recycled SLS glass
-> With added B2O2
-> Conventional glass melting furnace
Or from minerals
-> e.g. Basalt fibres (rock wool/stone wool)
-> requires higher production temperatures
~ Cupola furnace
* essentially a blast furnace system
* improve iron is a by-product
How is glass wool produced?
2 possibilities:
- melt poured onto the centrifugal cascade
- melt can be passed through a spinning fibres disk containing ~ 50 x 10^3 holes
Produces chopped fibres of varying length
Spraying with organic binders
Cured
Product density ~ 10 - 30 kgm^-3
-> compressed for transport
What composition is bulk glass?
What composition is glass fibre reinforcements?
What raw materials include glass compositions?
