13,14- Ceramics 47-92 Flashcards
All-ceramic restorations contain what percentage of the crystalline phase?
99% crystalline phase.
What are the 4 processing techniques used to fabricate all-ceramic restorations?
- Sintering
- Heat Pressing
- Slip casting
- CAD/CAM
What are the two main types of all-ceramic materials available for the SINTERING technique?
- Alumina-based ceramic
- Leucite-reinforced ceramic
Compared to Feldspathic Porcelain, Alumina ceramic has a _______ modulus of elasticity and __________ fracture toughness.
Compared to Feldspathic Porcelain, Alumina ceramic has a HIGH modulus of elasticity and HIGH fracture toughness.
the increased strength in Alumina Ceramic over Feldspathic is due to the:
Excellent bond between the alumina and the glass phase.
True or False Aluminous core porcelains have flexural strengths twice that of Feldspathic procelains?
TRUE
In order to compensate for sintering shrinkage what can be done with Aluminous Ceramics?
in order to compensate for sintering shrinkage, ENLARGED DIES are generated by computer aided design.
High purity amunia-based ceramics are often fabricated by:
Dry pressing and sintering at a high temperature.
After sintering to produce a highly crystalline ceramic core with great strength what else must be done before the restoration is completed?
After sintering the restoration must be veneered with a translucent porcelain, stained and then glazed.
The performance of these crowns 15 years after being placed is EXCELLENT
What benefits does reinforcing ceramic restorations with leucite add?
Leucite gives:
- higher flexural strength
- increased compressive strength
- increased resistance to crack propagation
Heat-processed restorations are starting to replace sintered restoration. What is the difference?
Heat pressed restorations rely on application of eternal pressure at high temperatures to sinter and shape the ceramic.
This process can be used to produce all-ceramic crowns, inlays, onlays, veneers and fixed partial prosthesis.
How does Heat pressing work?
During heat pressing ceramic ingots are heated to high temperatures in investment mold produced by lost-wax technique.
how long is high temperature held for in heat processed ceramics?
10-20 minutes in special pressing furnace
In lithium disilicate based ceramics, what is the main crystalline phase? what tempearture range are they heat pressed at and what happens to the restorations after heat pressing?
The main crystalline phase is lithium disilicate…duh
they are heat pressed at 890-920 degrees C
After heat pressing they are veneered with ceramics of MATCHING THERMAL EXPANSION
What is the main advantage of lithium disilicate based ceramics?
Enhanced flexural strength and fracture toughness.
Why do lithium disilicates demonstrate greater fracture toughness?
- thermal expansion mismatch between the crystals and glass matrix is responsible for crack deflection
- microstructure of interlocking crystals leads to multiple crack deflections and increases resistance to crack propagation.
- heat pressing promotes crystal alignment along direction of pressing and so they have high resistance to cracks perpendicular to crystal alignment.
What is slip casting?
slip casting is the process of forming ceramic shapes by putting a slurry of ceramic into a porous die material and then removing all the water, which densifies the ceramic into a “green body” which is then sintered to increase density and strength.
how is water removed from the slip in slip casting?
water is removed by capillary action via the porosities in the slip cast.
After being sintered the porous core of slip cast ceramics are glass infiltrated. WHY?
Glass infiltration happens through capillary action at high temperatures and causes the glass to be drawn into the tiny pores of the core. This makes the two networks interpenetrate one another.
What are the 3 types of ceramics that can be used for slip-casting?
- Alumina based
- Spinel based
- Zirconia toughened Alumina
Slips for alumina based ceramics contain around what percentage of alumina?
90%
what kind of glass are alumina based ceramics infiltrated with during slip casting?
lanthanum
alumina-based slip cast restorations have very high flexural strengths and high core strength.
HOWEVER, these alumina crystals have a high refractory index which results in _________
OPACITY. The alumina crystals have a high refractive index and 5% porosity, this causes some opacity.
What is one advantage and one disadvantage of SPINEL slip cast ceramics?
Advantage: more translucent
Disadvantage: lower felxural strength than alumina based
What are the two types of strengthening mechanisms seen from the combination of alumina and zirconia?
- Stressed induced transformation of ceria and associated increase in volume produces stresses within zirconia grains
- large alumina grains promote crack deflection
this produces very high flexural strength (630mPa)
What is the main advantage of slip-cast ceramics and what is their main disadvantage?
main advantage: HIGH STRENGTH
main disadvantage: HIGH OPACITY AND PROCESSING TIMES
Machining of all-ceramic materials occurs in the _________state
machining of all-ceramic materials occurs in the fully-sintered state directly to the final size.
What is soft machining?
soft machining is where a ceramic is machined in a partially sintered state.
THIS REQUIRES MILLING OF AN ENLARGED RESTORATION TO COMPENSATE FOR SINTERING SHRINKAGE.
what is Hard Machining good for?
Hard machining is often carried out by CAD/CAM milling machine in the office after digital scanning to produce inlays, onlays, veneers and crowns in one office visit.
If you wanted a good hard milled ceramic would you want it done in your office with your brand new CAD/CAM or at the lab with their fancy shmancy CAD/CAM milling machine?
At the lab, they spend TONS of money on their machines and they produce WAY higher quality in only a few minutes (it would still be back the same day)
After hard milling how are ceramics placed?
with a composite cement
What is “Copy milling”?
Copy milling is where a hard resin pattern is made on a traditional stone die, the handmade pattern is then copied and machined from ceramic using a pantographic device which works similar to the machine used to copy house keys.
what 3 ceramics are commonly used in copy milling?
- feldspar
- leucite
- lithium based.
Because of their toughness lithium disilicate ceramic blocks are often machine in which state?
they are machined in their partially crystallized state because it is more easily machined.
how can translucency of ceramic be adjusted?
translucency can be adjusted by varying the crystallization heat treatment.
CAD/CAM machines can also be used for presintered alumina, spinel or zirconia-toughened alumina to fabricate copings for crowns and fixed partials.
Copings are then glass infiltrated.
no answer. just the info on the question card.
What is DCM and when is it used?
DCM stands for DIRECT CERAMIC MACHINING, and it is used with presintered blocks to produce single or muilti unit restorations.
What are some advantages of DCM?
in DCM
-Blocks are easy to mill
-produces subsantial savings in time and tool wear
What is one way DCM and CAD/CAM are different?
In DCM- process involves fabrication of a full contour wax-up that is then laser scanned, restoration is oversized to compensate for sintering shrinkage.
In CAD/CAM- wax-up isn’t needed because the preparation is made and the restoration is designed by computer. then its sintered.
