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31 Cards in this Set
- Front
- Back
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4 things which a traditional ceramic contains
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Kaolin - draws particles together in inital binding
Feldspar - draws particles together in final binding Silica -filler, good translucency Alumina - filler, strengthens but makes opaque |
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What is the fine powder which a ceramic is supplied as called?
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Frit
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What prevents ceramic sticking to the die?
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Pt foil
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Why is water blotted away from the layers of ceramic before it is fired?
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Reduces shrinkage - better compaction with lots of different particle sizes
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Describe the firing of porcelain. 3 steps.
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1 - Slowly heat to drive off water
2 - heat to 1200 degrees to sinter (kaolin pulls everything together), product at biscuit stage 3 - heat up more so that feldspar flows and draws it all together. shrinkage here |
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What is used to strengthen porcelain? How does this work?
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Alumina is added, if a crack hitsit then it stops dead
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How do sintered alumina cores work?
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0.5mm thick alumina core which is made by painting straight onto die and firing.
the crown is then built as normal on top of this core, and the alumina is infiltrated by the porcelain. Very strong and very high flexural strength, as cracks can't propagate from the inside |
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What 2 things are added to ceramics to make them compatable to be bonded to metal?
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Na2O and K2O (leucite)
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Why is more Na2O and K2O added to ceramics than is needed to make the coefficients of thermal expansion the same?
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If the coefficient of thermal expansion of ceramic is higher than that of the metal then the ceramic is held in compression and any surface defects are closed, as well as bonding them together
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3 ways in which porcelain binds to metal
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Thermal contraction
Mechanical Chemical |
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Explain mechanical bonding of ceramic to metal
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Metal finished by sandblasting so that when the ceramic is cast on a better interlock is given
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Explain chemical bonding of ceramic to metal
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Oxide layer of metal bonds to ceramic
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Explain the differences in the chemical bonding of gold alloys and base metal alloys to ceramics
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Gold alloys do not oxidise so an oxide forming metal such as In, Sn or Fe have to be added
Base metal alloys can oxidise too much, making this bond very weak |
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Difference between gold alloys for PFM and normally?
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Contain no copper, either high gold (85%) or low gold (50%).
Contain high Pd to increase Mpt Contain an oxide forming metal |
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Which type of god alloy best for PFM crown? Why?
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Low gold, less sag on firing
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What is sag?
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When ceramic is fired metal can flow under its own weight when heated, so moves and changes shape
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Bonding characteristics of Pd/Ag alloys?
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Good bonding due to thin oxide layer
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Which alloy used for PFM has weak bonding due to a thick oxide layer?
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Ni/Cr
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Which allow used for PFM has poor castability and high shrinkage?
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Ni/Cr
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Describe captek
bonding? |
A gold/platinum composite shell which is very thin, strong and stiff with excellent aesthetics.
Uses a pt impregnated, then a gold impregnated wax strip. No oxide layer bonding |
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Describe Pt foil method of PFM
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2 thin layers of Pt and then a tin outer foil which forms an oxide layer suitable for bonding
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Difference between cast glass and normal ceramics?
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Cast glass produced using lost wax technique like alloys are
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What's stronger, pressed or cast cast glass?
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Pressed
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Why is it an issue if you need to adjust surface of cast glass?
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Glazed - would need to be reglazed
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Which type of ceramic could be used to produce an inlay?
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CAD/CAM or Cast glass
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What type of investment material would need to be used for a cast glass crown?
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Phosphate
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What is 'ceramming'?
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Heat treatments used to initiate crystal growth in cast glass
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Why is a very thin layer of cement needed for veneers?
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Cement more flexible than veneer and a thick layer would bend cracking the veneer
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3 structures of zircona
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Cubic - High temp
Tetragonal - As cooling Monoclinic - room temp |
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What is added to zircona and what does this do?
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Ytrium oxide
Forces it to take up tetragonal structure at room temp. If tetragonal structure starts to crack then it collapses into monoclinic structure and stops crack. Called transformation toughening. |
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What is zircona sold as?
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A solid block, called LAVA
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