DENTAL CERAMICS Flashcards
What do decorative ceramics contain?
Kaolin
What is Kaolin?
An opaque clay composed of hydrated aluminium silicate
What are the constituents of dental ceramic?
- Kaolin <5%
- Quartz (silica) 12-25%
- Feldspar 70-80%
- Metal oxides 1%
- Glass (up to) 15%
What is Feldspar?
Potash feldspar = potassium alumina silicate
Soda feldspar = sodium alumina silicate
what is the function of Feldspar?
- Acts as a flux
- Lowers the fusion and softening temperature of the glass
How does feldspar act during firing of ceramics?
flows during firing forming a solid mass around the other components
What is the function of metallic oxides in ceramics?
They convey colour to the ceramic
How are conventional dental ceramics supplied?
Supplied as a powder
How is conventional dental ceramic powder made?
- heating the constituents to a high temperature (>1000 degrees)
- cool rapidly
- mill the frit to a fine power
- add binder
- power is mixed with distilled water & built up into the restoration
What is the rapid cooling of conventional dental ceramic power known as?
Fritting
- creates cracks and crazing of the ceramic mass
What is the binder commonly added to conventional dental ceramics?
often starch
What do Feldspathic ceramics form when heated to 1150-1500 degrees celsius?
Leucite (potassium aluminium silicate)
- forms around the glass phase of the ceramic
- gives a powder
- powder melts together to form crown
What are the properties of conventional dental ceramics?
- brilliant aesthetics
- chemically stable
- thermally similar to tooth substance
- dimensionally stable
- good mechanical properties
Why are dental ceramics very aesthetic?
- colour stable
- very smooth surface
- less staining long term
- good optical properties
The coefficient of thermal expansion of dental ceramics is similar to dentine, why is this good?
results in low stresses to the restoration in the mouth during use
The thermal diffusivity of dental ceramics is low, why is this good?
protective of the remaining tooth
What occurs during firing of a conventional feldspathic ceramic crown?
shrinkage of about 20%
What are the mechanical properties of dental ceramics?
- high compressive strength
- high hardness
- very low tensile strength
- very low flexural strength
- very low fracture toughness
The mechanical properties of dental ceramics mean that conventional feldspathic ceramics can only be used in low stress areas, why?
- Stress fatigue
- Surface micro-cracks
- Slow crack growth
What can feldspathic ceramic crowns be used for?
Anterior crowns
How are dental ceramics made stronger?
By metal coping with
- alumina
- zirconia
What is the flex strength of alumina core ceramic crowns?
> 120Mpa
What is the function of the Alumina in dental ceramic crowns?
Alumina particles act as crack stoppers preventing cracks propagating through the material and causing fracture
What are the downsides of alumina core dental ceramic crowns?
- lack of flexural strength
- probably not suitable for anything other than single crowns
- cannot be used posteriorly
What is the most popular ceramic core material?
Zirconia core
At what temperature does Zirconia powder sinter?
over 1600 degrees celsius
What zirconia is used in dentistry? Why?
Yttria-stabilised zirconia
- pure zirconia can crack on cooling
How much Yttria is present in yttria stabilised zirconia?
3-5%
Higher levels of Yttria in zirconia crowns lead to what?
More translucency & reduced physical properties
What is the flexural strength of Yttria Stabilised Zirconia?
1000MPA
What are the problems associated with zirconia cored crowns?
- expensive equipment required
- potential for veneering porcelain to deboned from core
- inert fitting surface (cannot etch or bond)
How are Cast & Pressed Ceramics made?
- restoration is waxed up
- invested
- cast from a heated ingot of ceramic
- once divested and cleaned the restoration is heated to improve its crystal structure
- this is called CERAMING
What ceramic is used in cast and pressed ceramics?
Glass ceramics
- lithium disilicate glass
- leucite reinforced glass
What is CERAMING?
Stage one = crystal formation maximum number of crystal nuclei are formed
Stage two = crystal growth to maximise the physical properties
What ceramic crowns should be used on posterior teeth?
Monolithic Zirconia
(can be used for single crowns and shorter span bridges)
What ceramic crowns should be used on anterior teeth where aesthetics are the most important factor?
LiDiSi
Explain what translucency means with regard to optic properties of materials:
Translucency refers to the ability of a material to allow light to pass through but be scattered at one of the surfaces or internally leading to a blurring of the transmitted light
Explain what opalescence means with regard to optic properties of materials:
The ability of a material to appear
- blue in reflected light
- orange/yellow in transmitted light
Chemically how does a silane coupling agent work when bonding composite to ceramic?
- Creates a strong bond between oxide groups on the ceramic surface and the silane
- Other end of the silane molecule has a C=C bond which reacts with the composite resin luting agent
