5. Engineering ceramics and glasses Flashcards
5 typical properties of ceramics
Strong (in compression) Brittle Hard Insulator (electricity and heat) Inert (high chemical stability)
2 Factors that dictate the structure of ceramics
Balanced charges, structure is electrically neutral
Relative size of anions and cations
What determines the co-ordination number on a ceramic, and how is this calculated?
Ionic radius ratio = radius of cation/radius of anion
Describe briefly the structure of NaCl
two interlocking FCC structures
What is the co-ordination number for each ion in NaCl?
6
Why are metals much tougher than ceramics? 2 reasons
metals have plastic deformation, ceramics do not
high chance of porosity in ceramics,
(even a small pore will have huge effects, not so much in ceramics)
Two reasons why porosity weakens ceramic strength
reduces (effective) cross sectional area
concentrates stress around the pores
Two toughening mechanisms in ceramics
Removing porosity/Densifying (in processing)
Reinforcing (adding another material to make a COMPOSITE) to achieve a balance of properties
Hydroplastic forming is what?
Adding a little water to clay (which is made of sheets of silicates) allows the water to ‘float’ these sheets past each other making the whole thing very plastic
What is slip or slurry?
Suspension of clay particles in water (when you add more water than in hydroplastic forming)
Why does unidirectional powder pressing have a relatively low pressure limit?
powder is bad at transmitting pressure (powder at the bottom will experience very little pressure)
Best method of powder pressing?
Isostatic pressing (pressuring the powder from all directions)
Describe the process of sintering
Heat treatment (nothing melts) to reduce porosity, by diffusion of particles to fill up the pores
What is vitrification?
In firing, when some of the components melt and flow around the ceramic particles, solidifying as glass (makes very strong ceramic)
What is the driving force of sintering?
Reducing surface area
Glass is not a crystal because
it has no LONG RANGE ORDER (which is what all crystals have)
What are glass forming oxides?
oxides that can form networks with no long range order
Intermediate oxides
can’t form networks on their own but can join into existing networks
Network modifiers
disrupt glass networks to give the glass particular properties (Na2O, CaO used for soda lime silica, most common type of glass)
Cooling molten silica slowly results in what kind of structure?
A crystal structure as you give the molecules time to line up
Cooling molten silica fast results in what kind of structure?
A glass (disordered structure)
What is the effect of the cooling rate on the final volume?
faster cooling rate = larger volume (less time for molecules to pack)
How to improve fracture toughness in glass?
Tempering the glass, causing the surface region to be in compression (thermal or chemical techniques), this technique is known as residual compressive stress.
Describe how thermal tempering works
Glass heated to transition temperature, then cold water is sprayed onto surface. Inside layer cools slower, so shrinks more and pulls surfaces into compression.
