CSAR 14.1 + 14.2 - Dental ceramics for indirect restorations Flashcards
Define ceramic material.
Non-metallic inorganic material
Formed by ionic bond
Crystalline
Define glass material.
No regular structure - amorphous
Metamorphous material
What are the mechanical properties of glasses and ceramics?
Hard
Strong in compression, weak in tension
Notch sensitive
Brittle
What are the 2 main ingredients of dental porcelain?
Where are they found and what are they like in temperatures?
Feldspar - naturally forming glass - flows at elevated temps
Silica - naturally forming glass - stable at high temps - good translucency, like enamel
What material isn’t used in dental porcelain anymore?
Kaolin - lacks translucency and adds orange colour
How are the dental porcelain materials made into frit powder for the lab?
Feldspar and silica heated to a high temperature to created a molten mass
Mass quenched in water, causing it to shatter
Then ground into fine powder to create frit powder
How is the frit powder of dental porcelain made into a crown?
Frit forms paste with water which can be moulded well
Can be died with gypsum materials
Sublayer - dentine - more opaque and yellow
Layer on top - enamel
Translucent incisal edge
Paste compacted/blotted to remove water to reduce shrinkage up to 30% during firing
Put in oven to cause sintering of the material, allowing glass particles to flow and join, which is cohesive and stronger
Needs to be slow cooled as ceramics are insulators, so stress concentrations can form causing fractures
If there are exterior cracks on porcelain crown, how can they be filled?
Glazed and fired at high temperature
What is the most common failure of anterior porcelain crowns?
Fitting surface cracks
What are advantages of the PFM layers, 1. porcelain exterior layer and 2. alloy sub-layer?
- Aesthetics, chemical resistance
2. High stiffness, strength and reduced tensile stresses forming in ceramic
How is the metallic sub-layer hidden in PFM crowns?
Opaque porcelain layers
What are the 3 ways of bonding the porcelain and metal alloy layers of PFM crown?
Thermal contraction - matching thermal coefficients
Chemical bonding - ceramic reacts with the oxide layer on alloy - good interdigitation
Mechanical interlocking - roughen alloy and alloy ceramic to flow in undercuts during firing
What are 3 alloys that are commonly used for PFM crowns?
Gold
Pd/Ag (60:30)
Ni/Cr (80:15)
Why are gold alloys not favourable for PFMC?
If not copper free -> stain ceramic blue/green
Stiffness problem –> therefore use low gold alloys for greater stiffness
Can deform during firing causing loss of fit
What are the + and - of Pd/Ag alloys for PFMC?
\+ stiffer than gold alloys \+ good bonding, stable thin oxide layer \+ high melting point, therefore do not deform \+ cheaper than gold - susceptible to casting faults
What are the + and - of Ni/Cr alloys for PFMC?
+ very high melting point, therefore do not deform
+ very high stiffness
+ can be used for long spans in the mouth
- bonding is weak, oxide layer is too thick
- casting difficulties, shrinkage difficult to compensate
What materials are used as ceramic cores? where and why?
Alumina (Al2O3) - only for anterior crowns for palatal portion, much stronger than porcelain
Ceramic core - strong, tough and aesthetics
Why is alumina added to porcelain crowns?
Improves flexural strength and better at stopping cracks than silica
What is the downfall of using alumina?
It is opaque, therefore poor aesthetics
Fine for core, however need to make a veneer
What is the difference in amount of alumina added to porcelain in 1. aluminous porcelain and 2. sintered alumina?
- 40-50%
2. 85%
What is the technique of slip casting?
Building up layers on a refractory die
How is sintered alumina crown placed on to a tooth?
Needs rough fitting surface
Dual cure resin for satisfactory polymerisation - limited light transmission through ceramic
Name 3 sintered materials and their strengths?
Sintered alumina - 400-500MPa Sintered spinel (spinel is complicated Mg salt) - 350MPa Sintered zirconia - 700MPa
Which has better aesthetics sintered spinel or sintered zirconia?
Sintered spinel
What is the disadvantage when a crown material is sintered?
Inaccurate fit
Once a block is milled in crown production, it is sintered, how much shrinkage needs to be compensated and by what process can this be done?
20-25% and added on by CADCAM
What are the properties of zirconia?
White ceramic
Polycrystalline - when cooled
Strong
High flexural strength
What are the different crystal structures of zirconia?
Monoclinic zirconia
Stabilised tetragonal zirconia - Yttrium oxide trapped some as tetragonal (which takes up more volume)
What are the disadvantages of zirconia?
Needs veneer layer and delamination is a problem
Really hard to remove once placed in the mouth
