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:
(IMAGE)
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.
