Ceramics Flashcards
What are 2 types of sintered cermaicc
Alumina
-Leucite reinforced
What happens in sintering
Pressed powdered form which is moulded and fired
What is sintered ceramic combined with in final restoration
glassy translucent porcelain
How good is aluminas bond to the glass
- Bonds well
- Coefficent of thermal expansion is similar between glass and alumina
What is the benefit of large alumina crystals being present?
They act as a crack deflecting mechanism
Disadvangtages of alumina (sintered)
shrinkage of 10-20%
-Opacity of alumina
Why is heat pressing better than sintering?
- Improves crystal dispersion
- Higher crystallinity
- Decreased crystal size
How is heat pressed and sintered restorations similar
-They both have a veneering glass porcelain layer
2 Types of Heat pressed ceramics
- Lithium disilicate
- Leucite reinforced
What is the thermal coefficient of leucite reinforced compared to glass? (sintered)
3x higher (mismatch)
What happens to the restoration when it is cooled with the mismatch
leucite cools quicker than the glass and this puts residual stress on the glass, but this acts as a CRACK DEFLECTING mechanism
Benefit of Leuite reinforced sintered
-Aesthetic
What is the thermal coefficient of Lithium disilcate compared to glass (Heat pressed)
- There is a mismatch between the 2.
lithium cools quicker and this puts residual stress on the glass which acts as a crack deflecting mechanism
Why is the crack delfection in lithum disilicate better than lecuite (heat pressed)
-As the crystals are elongated and interlocked in lithum disilicate
Why is heat pressed leucite reinforced better than sintered?
- Less operator variability
- Stronger as improved crystallinity and better dispersion of crystal
- Higher flexural strength
What materials can we soft machine?
Alumina
Zirconia
Spinel
Why do we design the alumina oversized in soft machining process?
To accommodate for shrinkage
What materials can we hard machine
- Felspar porcelain
- Lithium Disilcate
- Leucite reinforced
What makes a metal ceramic restoration
-Metal framework with 2+ layers of ceramic
What is the first layer of the MCC
=-Ceramic which contains opacifying oxide. This helps bond to metal (ceramic oxide +metal oxide)
-Also aesethically hides the metal
What are the additional layers on the MCC
-Translucent porcelains for aesthetics and colour match
Why is the restoration vibrated after the porcelain has been applied
to condense the material which reduces shrinkage and increases packing factor
Why is a vacuum of 0.01 mPa needed?
To decrease porosity
Characteristics of what makes the metal ceramic restoration
- Metal must have higher melting temp than the ceramic (so when ceramic is applied metal doesnt melt)
- Metal should not lose its shape when the ceramic is sintered or it can reduce fit
- metal have higher thermal coeffiecnet of expansion than ceramic
- Metal should have chemical bond via oxides and mechanical bond via bur roughening of surface
- Metal to have high elastic modulus to mitigate the stress the ceramic experiences
- Veneering porcelain to have low fusion temperature so that it doesn’t distort in sintering
5 Key properties of ceramics
- High Strength but fracture toughness is lower than metals
- Optically favourable
- Stable over long period
- Doesnt react with acids,alkalis liquids and gases
- High harness
Advantages and disadvantages of feldspar
A- Translucent, matches shade/lustre and addition of crystal phases improves mechanical properties,
D- low strength and prone to mechanical failure
What is different about theoretic strength of ceramics and IRL
Theoretical strength = High as it can handle tensile loads
IRL= It is never defect free so the tensile strenth can vary.
What limits strength in ceramics
- Defect size
- Distribution of defect
Define fracture toughness
Measures the resistance to a crack propagating through a material under tensile stress
Why are glassy ceramics brittle
Due to the microstructure, they experience little plastic deformation before failing
Why are crystals added to glassy ceramics
-To toughen the glassy matrix, for a crack to progagate it will have to take a path around the glass particles as it doesnt propgate through the crystals.
Whats the problem with adding crystals to the matrix
- Shrinkage during sintering, so dimensional accuracy goes down
- decreased transparency
