Ceramics Flashcards
General Properties of Ceramics
high compressive strength, relatively low tensile strength, brittle, low toughness, high hardness, good thermal + electrical insulator,
resistant to environmental conditions such as oxidation, relatively low cost.
Give examples of ceramic use (general uses)
clay products, whitewares (decorative), refractories, glass and abrasives (tough materials used for cutting grinding etc)
advantages of brick over stone
lower application costs and relatively low skill required
What ceramic products are made in a similar way to 2000 years ago
Pottery, porcelain, tiles, bricks and refractory bricks
what are the six categories of clay used in industry
brick, bentonite, common, fire, Fuller’s Earth, Kaolin
how does water make clay slippery
The polar water molecules come between layers of clay
What is sintering
the process of forming a solid mass of material through heat and pressure without melting to the point of liquefaction. Used to fuse the separate ceramic powder particles together.
What are typical sintering temperatures
850 for tiles to 1650 for engineering ceramics such as silicon nitride, silicon carbide, zirconium oxide
What form of sintering forms good high temperatures properties and conversely bad high temperature properties
liquid phase may cool to glass which has bad properties
liquid phase may crystallise which has good properties.
What phases do most ceramics consist of
glassy phases, crystalline phases and lots of holes
What does etching a ceramic mean
Polish the surface then proceed to heat in a furnace at a high temperature
What drives sintering
surface area. By heating up the particles, the surface area is reduced
what is liquid phase sintering
A technique of sintering that utilises a liquid to accelerate the process since transport through liquid is faster than through solid and capillary forces of liquids can rapidly rearrange particles and draw them together.
What can be a weakness in the microstructure of ceramic
Grain boundaries (the boundary or gap between two crystal grains)
What is a dislocation and what is special about them in ceramics
An extra plane of atoms which is an irregularity in the structure. They can generally move through the structure in metals (by breaking and reforming bonds) , allowing them to be malleable. This is only possible due to the nature of metallic bonding. Ceramics’ dislocations cannot move so there is no way to relieve stress. This means cracks will continue to propagate.
Why are ceramics said to have low toughness
Any applied stress in the flaws of the structure are amplified, which means any cracks will propagate
What is Griffith’s equation
An equation that relates the tensile fracture stress to the toughness and the size of the largest flaw
How can one increase the density of ceramics when sintering
Increase the temperature of the sintering process to a certain maximum (too much will reduce density) and reducing the size of the particles being sintered
what are the three main ways of bringing the unsintered particles as close as possible before sintering
powder pressing (spray dried powders are pressed) , extrusion (squeezing the particles through a small aperture), slip casting (slurry of particles are cast into a mould that absorbs the water)
What is adobe
A ceramic made from mud or clay that does not use sintering. They are dried in the sun
What is one advantage of brick over concrete
It can last even longer than concrete due to being sintered and not chemically bound
what is efflorescence of bricks
salts dissolved in the brick when it is wet deposit on its surface while drying as white discolouration.
What is iron staining in bricks
The iron inside the bricks leaches out then oxidises into iron oxide
What is lime staining of bricks
The cement in the mortar binding the bricks contains CaO that leaches out due to rain and becomes CaCO3
