Dental Ceramics Flashcards
What is the difference between decorative and dental ceramics ?
Decorative ceramics contain kaolin - give opacity.
Dental ceramics have to be translucent so kaolin volume reduced.
What is added to dental ceramics in place of kaolin ?
Feldspar and silica.
What are the 5 components of dental ceramics ?
70-80% feldspar glass.
12-25% silica (quartz).
15% glass.
<5% kaolin.
1% metal oxides.
What is the function of silica (quartz) ?
Translucency.
What is the function of metal oxides ?
Colouration.
What is the function of feldspar ?
Acts as flux - allows for fusion of glass into solid mass on firing by reducing fusion and softening temperatures of glass.
How is conventional ceramic made ?
- Powder created by heating all constituents >1000 degrees.
- Powder cooled rapidly - called fritting.
- Frit milled into fine powder.
- Binder added - starch.
- Mixed with distilled water.
- Restoration made.
- Fired in furness - 1150-1500 degrees.
What does feldspathic ceramic form when fired to 1150-1500 degrees ?
Leucite - potassium alumina silicate.
Glass phase of ceramic.
What is sintering ?
Heating of potassium alumina silicate ceramic particles to melt and fuse to form one mass i.e. glass phase softens and coalesces.
What is the percentage of contraction on sintering ?
20%.
What is the benefit of sintering ?
Improves thermal and physical properties.
What are the two ways in which mechanical faults of dental ceramic can be overcome ?
Metal coping i.e.
Alumina core - usually 50%.
Zirconia core.
What is the benefit of an alumina core crown ?
Alumina acts as crack stoppers - increasing fracture toughness.
>120mPA flexural strength.
What is the problems with a zirconia core crown ?
CAD-CAM and milling out of monolithic block required as >1600 degrees required for material to sinter - too expensive.
What is added to zirconia to prevent cracking on cooling and improve transparency ?
Yttria 3-5%.
