Dental Ceramics
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What is porcelain a type of?
ceramic
What is removed from dental ceramics and what replaces it?
Dental Ceramics need to be translucent so Kaolin (opaque) is removed and feldspar and silica replace it
What. material are dental ceramics?
glasses
What components is feldpathic porcelain made of?
• feldspar • borax • silica • metallic oxides
What does feldspar do?
Acts as a flux Lowers the fusion and softening temperature of the glass It is the lowest fusing component and flows during firing forming a solid mass around the other components
What form are conventional dental ceramics? what do they require?
powder mixed with distilled water and built up into the restoration
How is the powder made?
The powder is made by heating the constituents to a high temperature >1000oC Cool rapidly (Fritting) In water creating cracks and crazing of the ceramic mass Mill the Frit to a fine powder Add binder (often starch)
What happens to feldspathic ceramics when heated to 1150-1500 degrees?
form leucite (potassium aluminium silicate)
What phase is leucite formed and what does this determine?
This forms around the glass phase of the ceramic. Gives a powder of known physical and thermal properties. No further chemical reaction is required during fabrication of the restoration The powder melts together to form the crown
How is the crown fabricated?
Ceramic powder is mixed with water and applied to the die with a brush The crown is built up using different porcelains for dentine and enamel These are not tooth coloured The crown is heated in a furnace to coalesce the powder into ceramic
What is sintering?
Heating leads to SINTERING This occurs just above the glass transition temperature It is when the ceramic particles begin to fuse into a single mass. During sintering the glass phase softens and will coalesce Over time there is controlled diffusion and a solid ceramic mass is formed
What happens to the material during sintering?
contrcts by 20%
What are the favourable properties of conventional dental ceramics?
• smooth, translucent aesthetics • less susceptible to staining • chemically stable - unaffected by ph • good biocompatibility • similar thermal properties to tooth • dimensional stability (except during fabrication • high compressive strength • high hardness
What are the negative mechanical properties of conventional feldspathic ceramics?
• Tensile strength – very low • Flexural strength – very low • Fracture toughness – very low • Static fatigue • Surface micro-cracks • Slow crack growth • Brittle
Where should conventional feldspathic ceramics only be used?
only anterior crowns low stress areas
How does alumina core work? why can it only be used as a core
Alumina particles act as crack stoppers preventing cracks propagating through the material and causing fracture Aluminous porcelain is opaque and can only be used as a core material
How are conventional ceramics made stronger?
metal coping using alumina core zirconia core
How much alumina is in a conventional aluminous core?
max 50% alumina
What covers the core?
All of these core types are then veneered with conventional feldspathic porcelain to produce the final crown
What is the problem of alumina cores?
lack of flexural stregth cannot be used for posterior crowns or bridgework
When does zirconia powder sinter?
heated to over 1600
Why is zirconia stabilised with yttria?
pure zirconia can crack on cooling stablises tetragonal phase
What does more yttria mean?
as yttria increases, zirconia becomes more translucent (good for aesthetics) but less tough and strong (less suitable for high-stress areas). change from tetragonal > cubic
What type of crystal is normal zirconia at room temperature?
monoclinic crystal