Dental Ceramics Flashcards
Is all porcelain ceramic?
yes
Is all ceramic porcelain?
no
What is the key ingredient of decorative ceramics?
KAOLIN
-kaolin is a clay
-hydrated aluminium silicate
- opaque
- opacity is important for the appearance of the final product
How do dental ceramics become translucent?
kaolin is removed and feldspar and silica replace it
What are the components of dental ceramics?
kaolin <5%
silica 12-25%
feldspar 70-80%
metal oxides 1%
glass - up to 15%
make card from this
How are conventional dental ceramics supplied? and how is this component made
as powder
by heating the constituents to a high temperature >1000 degrees celcius
What are the stages of making conventional dental ceramics?
- constituents heated to over 1000 degrees celcius
- cool rapidly (fritting) - in water
- mill the frit to a fine powder
- add binder which is often starch
- the powder is mixed with distilled water and built up into the restoration
How is a crown built up with conventional dental ceramic?
the crown is build up using different procelains for dentine and enamel (these are not tooth coloured)
How is the built up crown turned from a powder to a ceramic?
the crown is heated in a furnace to coalesce the power into ceramic
how much does the dental ceramic material contract during sintering?
about 20%
What does heating the powder and water ceramic mix lead to?
sintering
What is sintering and when does this occur with regards to creating a dental ceramic crown?
sintering is when the ceramic particles begin to fuse into a single mass
during sintering the glass phase softens and will coalesce
there is controlled diffusion over time and a ceramic mass is formed
(sintering occurs just after the glass transition phase)
What are the key properties of conventional dental ceramics? (6)
- aesthetics
- chemical stability
- biocompatibility
- thermal properties
- dimensional stability
- mechanical properties
Why are the aesthetics of dental ceramics so favourable?
- colour stable
- very smooth surface
- retain their surface better than other materials -> less staining long term
What are 5 optical properties which favour the aesthetics of dental ceramics?
- reflectance
- translucency
- opacity
- transparency
- opalescence
Why are dental ceramics seen as chemically stable?
- generally unaffected by the wise pH range found in the mouth
- dont stain
- good biocompatibility - minimal adverse effects of biological tissues
What are the thermal properties of dental ceramics?
- similar to tooth substance
- coefficient of thermal expansion is similar to dentine (low stressed to the restoration in the mouth during use)
- thermal diffusivity is low
protective of remaining tooth
Is a ceramic crown a dimensionally stable material?
yes- once it is fully fired the material is very stable
HOWEVER during fabrication shrinkage is a problem and must be accommodated for by the technician
20% shrinkage is normal
What are the mechanical properties of dental ceramics?
- high compressive strength
- high hardness (abrasion of opposing teeth if not glazed)
- LOW tensile strength
- LOW flexural strength
- LOW fracture toughness
the last 3 can all lead to failure during loading
What can occur during manufacture, finishing or occlussal wear of dental ceramics
surface micro -cracks
Why can slow crack growth occur?
cyclic fatigue under occlusal forces in a wet environment over time
What causes surface fatigue?
time dependant decrease in strength even in the absence of any applied load.
What area should dental conventional feldspathic ceramics only be used in?
they can only be used in low stress areas
only anterior crowns
not in all patients (never if they have a bruxist habit)
what core material was used to reinforce feldspathic core ceramics
alumina core
What was the flex strength of alimuna core ceramics
> 120Mpa
What were alumina core ceramics used as a core material for
porcelain jacket crowns (PJC’s)
What was the purpose of alumina core prior to PJC’s?
to prevent cracks forming and causing the material to fracture
What colour is aluminous porcelain
opaque
Is an alumina core strong enough for posterior use?
no
What is the maximum percentage of alumina in conventional aluminous cores?
50%
what is the next stage after these core types to produce the final crown?
all of these core types are then veneered with conventional feldspathic porcelain to produce the final crown
What were the problems with alumina cored crowns?
lack of flexural strength
good aesthetics but not suitable for anything other than single crowns
more successful anteriorly
What has become the most popular ceramic core material?
zirconia crown - very hard
What is the zirconia that is used in dentistry called?
Yttria-stabilised
What temperature does zirconia power sinter?
1600 degrees celcius
What enabled the use of zirconia as a core material?
CAD-CAM
Can pure zirconia crack ?
yes - it can crack on cooling
How much yttria is present in zirconia? and what happens if you increase the amount of yttria
3-5%
- if you increase yttria the more translucency
- if you inrease yttria this will reduce the physical properties
what crystal structure is normal zirconia at room temperature?
monoclinic crystal
what crystal structure is yttria stabilised zirconia at room temperature?
tetragonal crystal structure
