Dental Ceramics Flashcards
What is Kaolin?
- Decorative ceramics contain Kaolin- It is a clay- Hydrated Aluminium silicate- Opaque- Opacity imp for appearance of final product
Why does Kaolin need to be removed from Dental ceramics?
- Ceramics need to be translucent so Kaolin removed- Feldspar and silica replace it
What are the components of Decorative ceramic?
- Kaolin 50+%- Quartz (silica) 15-25%- Feldspar 15-25%- Metal oxides <1%- Glass 0
What are the components of Dental ceramics?
- Kaolin <5%- Quartz (silica) 12-25%- Feldspar 70-80%- Metal oxides 1%- Glass up to 15%
What are the two types of Feldspar?
- Potash Feldspar (potassium alumina silicate)- Soda Feldspar (sodium alumina silicate)
What does Feldspar do in dental ceramics?
- Acts as a flux- Lowers fusion and softening temp of glass- Has lowest fusing component and flows during firing forming a solid mass around other components
What are dental ceramics considered as ?
- Glasses
What property do metal oxides gives the ceramic?
- Metallic oxide determine colour of ceramic
What colour does Chromium convey to ceramic?
- Green
What colour does Cobalt convey to ceramic?
- Blue
What colour does Copper convey to ceramic?
- Green
What colour does Iron convey to ceramic?
- Brown
What colour does Manganese convey to ceramic?
- Lavender
What colour does Nickel convey to Ceramic?
- Brown
How are conventional dental ceramics used to build up into a restoration?
- Supplied as a powder- Powder made by heating constituents to high temp >1000oC- Cool rapidly (Fritting) in water - This creates cracks and crazing of ceramic mass- Mill the fritt to fine powder- Add binder (usually starch)- Powder is mixed with distilled water and built up into restoration
What do feldspathic ceramics form when heated to 1150oC-1500oC?
- Form leucites around glass phase of ceramic- Gives powder of known physical and thermal properties- No further chemical reaction needed during fabrication of restoration- Powder melts together to form the crown
What is Leucite?
- Potassium aluminium silicate
How are conventional dental ceramics fabricated?
- Ceramic powder mixed with water and applied to die with brush- Crown built up using different porcelains for dentine and enamel- They are not tooth coloured- Crown is heated in furnace to coalesce the powder into ceramic
What is Sintering?
- Heating the crown leads to Sintering- When the ceramic particles begin to fuse into a single mass- Occurs just above glass transition temp
What happens during sintering of dental ceramic?
- The glass softens and will coalesce- Over time controlled diffusion occurs and solid ceramic mass formed- The material contracts by about 20%
What is meant by the term Coalesce?
- Come together to form one mass or whole
What are some important properties required of conventional dental ceramics?
- Aesthetics- Chemical Stability- Biocompatibility- Thermal Properties- Dimensional Stability- Mechanical Properties
Why do ceramics have the best aesthetics properties of any dental restorative material?
- Colour stable- Very smooth surface- Retain their surface better than other materials leads to less staining long term
What are the optical properties of dental ceramics?
- Reflectance- Translucency- Opacity- Transparency- Opalescence
What is the chemical stability of dental ceramics?
- Chemically very stable- Unaffected by wide pH range found in mouth- Do not take up stain from food/drink- Good Biocompatibility so minimal adverse effects on biological tissues
What are the thermal properties of Dental ceramics?
- Sim to tooth substance- Coefficient of thermal expansion sim to dentine- Results in low stresses to restorations in mouth during use- Thermal diffusivity low which protects remaining tooth
What is the dimensional stability of dental ceramics?
- Once fully fired material is very stable- During fabrication shrinkage occurs which is problem and must be accommodated by technician- Shrinkage of 20% during firing normal
What are the mechanical properties of conventional dental ceramics?
- High compressive strength- High hardness (Can lead to abrasion of opposing teeth especially if not glazed)- Tensile strength – very low- Flexural strength – very low- Fracture toughness – very low (All lead to failure during loading)
What is Static fatigue?
- Decrease in strength even in absence of any applied load, it is time dependent