Chapter 13 (Applications and Processing of Ceramics) Flashcards
what are die blanks made of
polycrystalline diamond particles
why do is polycrystalline diamond used?
helps control fracture and gives a uniform hardness in all directions
list the advantages of using ceramics for automobile engines
high operating temperatures
low frictional losses
operate without a cooling system
lower weights than current engines
list the disadvantages
ceramics are brittle
difficult to remove internal voids
ceramic parts are difficult to form and machine
what are bottles windows and lenses all made from?
silica based glass
what are the main properties of glass
optical transparency easy manufacturing and shaping of objects hard vaccum tight chemically resistant
what is particular about glasses in comparison to crystalline materials
- they don’t solidify at particular temperature
- they increase in viscocity as temperature drops
- they don’t show sudden change in volume upon solidifying.
what can be said about the change in volume for glasses as temperature decreases
they gradullly shrink
what is the basic structure of glass
(SiO4)4- tetrahedron
what do impurities do for glass
they interfere with the formation of crystalline structure, which then causes non-crystlline strucutre (amorphous)
describe the behaviour of crystalline materials as they are cooled
they crystallize at melting temperature an have and abrupt change in specific volume at melting temp.
describe the behaviour of glasses as they are cooled
they don’t crystallize. And the specific volume varies smoothly with temperature
what is the glass viscosity
the temp at which glass is fluid enough to be considered a liquid
what is the working point
the temp at which the glass is easily deformed
what is the softening point
the maximum temp at which a glass piece may be handled without causing significant dimensional alterations
what is the annealing point
temp at which atomic diffusion is sufficiently rapid that any residual stresses may be removed within 15 mins
what are the units of viscosity?
Pa-s
what is the relationship between viscosity and temperature
as temp increases, viscosity decreases
which is the most common type of glass
soda-lime
what is sheet forming
continuous casting: sheets are formed by floating the molten glass on a pool of molten tin
What can happen during annealing
internal stresses build up during the cooling phase, can cause spontaneous fracture
how can you avoid spontaneous fracture
slow cooling
what is glass tempering
deliberate introduction of residual stresses by rapid surface cooling of hot glass
what are the benefits of glass tempering
it suppresses the growth of cracks from surface scratches
describe glass tempering
- surface contracts and hardens
- inside stays warm and soft
- inside cools and shrinks to try and pull surface (which is already rigid) inwards
- this puts the surface into compression and the interior into tension
what is chemical toughening
- replacing surface Na+ ions in soda-lime glass with larger K+ ions by soaking in hot salt bath
- when the glass is cooled, the larger k+ ions induce compressive stress at surface, and tensile stress at the centre
how can we “devitrify” glass, and what material does this yield
by heat treatment to produce a crystalline material called glass ceramic
what is hydroplastic forming
the process of milling and screening the constituents to obtain a desired particle size
why is is impractical to melt and cast ceramics, and what is used to get around this?
because they have very high melting points, instead, we form compacts from powders
briefly explain the drying and firing process?
drying: as water is removed the inter particle spacing decrease
firing: a heat treatment, this causes vitrification: liquid glass forms from clay and flux.
what would drying the cermaic body too fast cause?
it would cause the sample to warp or crack due to non-uniform shrinkage.
what is sintering useful for?
both clay and non-clay compositions
describe the sintering process?
- grind to produce ceramic or glass particles
- inject into the mold
- press at elevated temp to reduce pore size
list the types of powder pressing and describe them
- uniaxial compression: compacted in a single directed direction
- isostatic compression- pressure applied by fluid-powder is in a rubber envelope
- hot pressing: pressure+heat
what is tape casting, and name 2 of its applications
thin sheets of green ceramic are cast as flexible tape. used for ICs and capacitors
describe the cementation process
- involves the hardening of a paste. the paste is formed by mixing cement material with water
- Hardening process: hydration
- production of cement powder
- addition of water sand and stones-» cementation
what are refractories?
the materials used for producing/refining metals and glasses and other high temperature applications
what properties do refractories need to have? (3)
- high melting temps (withstand melting/deforming)
- must be inert
- thermal insulators
what are the 4 types of refractories?
- silica/acidic
- basic
- neutral
- special
how would one make diffusion of ions fast
add Ca impurity
If we add impurities to glass, what will happen to the deformation Temp
it will lower
if we want to remove a surface crack or scratch, what process can we use
glass tempering
what is the typical composition of porcelain
50% clay
25% filler
25% fluxing agent
which process forms a neck
sintering process
what are three factors what would affect the rate of DRYING?
- Humidity
- Temperature
- Rate of Air flow
What are 3 factors that would affect the rate of SHRINKAGE?
- thickness of the body
- clay particle size
- water content
