Introduction to Ceramics Flashcards
Give 7 dental uses of ceramic.
- Inlays
- Onlays
- Veneers
- Crowns
- Implant components
- Fixed partial prostheses
- Denture Teeth
Define ceramics.
Inorganic, non-metallic materials which are compounds formed between metallic elements and non-metallic elements.
What bonds exist in ceramics?
Ionic and covalent bonds
What are the 4 types of ceramics used in dentistry?
- Silicate glasses
Entirely non crystalline - used as glazes.
- Porcelains
Predominantly non-crystalline- used as a veneering material.
- Glass ceramics
Mixture of crystalline and noncrystalline phases - proportions vary - used for inlays only as crowns and anterior bridges.
- Highly crystalline materials
Polycrystalline - used for crowns bridges and implant components
- Do not readily react with most liquids, gases, alkalis and weak acids.
- Remain stable over long periods of time.
- Can exhibit high strength but their fracture toughness is much lower than metals.
- Have high hardness.
- Are optically favourable for cosmetic applications.
Why are ceramics termed as brittle and what does this result in?
- They have a low fracture toughness which makes it brittle.
- They have very little plastic deformation at failure and failure occurs rapidly.
What is fracture toughness a measure of?
The resistance to crack growth under a state of tensile strength.
Why is it that ceramics have poor tensile strength, but very high theoretical tensile strength?
- Ceramics are never defect free.
- And these defects (and their statistical distribution) determine the actual strength of the material.
What is another name for porcelain ceramics?
Feldspathic ceramics.
Why are porcelain ceramics good?
Can match the:
- Shade
- Translucency
- Luster
Of the natural dentition.
What are the disadvantages of porcelain ceramics?
- Low strength
- Prone to mechanical failures
How can glassy ceramics be toughened and how does this work?
Crystalline reinforcement:
- Higher strength crystals distribruted throughout the glassy matrix.
- This results in the crack having to go around the crystals in order to propagate.
What is the one condition that must be present for crystalline
- The crystal phase must have compatibility with the glass to be effective.
- If it is not compatible, it will serve as a defect itself.
Give 2 examples where crystalline reinforcement has been used.
- Leucite reinforced glasses.
- Lithium disilicates.
- What are the positive and negatives of crystal phases in ceramic?
+ves: Reinforces the ceramic.
-ves: Increasing the crystalline content has a draw back in terms of dimensional accuracy during fabrication (shrinkage during sintering) and also in decreasing transparency.
What are the 4 different manufacturing methods for all-ceramic restorations?
- Sintering
- Heat-pressing
- Slip-casting
- Computer Aided Drafting - Computer Aided Manufacturing (CAD-CAM)
What is the process for sintering of dental ceramics?
The final object is formed by:
- First pressing the constituents in powdered form under pressure into a mold.
- Then firing the pressed object at high temperatures.
What are the 2 main types of crystalline reinforced sintered dental ceramics?
- Alumina-based
- Leucite-reinforced
What is alumina composed of?
Aluminium oxide : Al2O3
What are 2 positives of alumina-based materials and What are 2 negatives of alumina-based materials?
- Forms excellent bond with glass phase.
- Flexural strength of 600MPa.
however
1. During sinistering procedure shrinkage between 10-20% can occur. This leads to dimensional inaccuracies.
2. The opacity of the core necessitates veneering with glassy material to achieve esthetics
What are leucite-reinforced materials composed of?
Compound of potassium, aluminium, silicone and oxygen, K[AlSi2O6]
What are 4 positives of leucite-reinforced materials?
- 100MPa flexural strength
- Leucite crystals have up to a 3-fold larger thermal coefficient of expansion compared to the glassy phase.
- During cooling the leucite contracts more than the glass resulting in residual stresses. These residual stresses act as a crack deflecting mechanism and increase strength.
- Highly aesthetic - have been extensively used for anterior crowns and inlays.
What is the process for heat-pressing of dental ceramics?
The simultaneous application of both heat and pressure to form and sinter the ceramic:
- Employs a lost wax process
- Contrast with sintering, where the constituent powders are first pressed then fired
- Pressures generally in range of 0.3-0.4MPa for 10-20 minutes.
- Temperature will depend on the ceramic used.
Why is heat pressing preferred over classical sintering?
- Improves crystal dispersion.
- Can obtain higher crystallinity.
- Decreases crystal size.
After heat pressing what is done to the ceramic?
It is veneered with translucent porcelain.
- Give 2 ceramic materials used in heat pressing.
- Leucite-reinforced ceramics.
- Leucite-reinforced ceramics.
- Lithium disilicate-based ceramics.
What is the composition of a heat pressed leucite-reinforced ceramic?
35-55% K[AlSi2O6]
What are 5 positives of heat pressed leucite-reinforced ceramics?
- Increase over the strength of sintered leucite-reinforced ceramics (100MPa)
- 9% porosity
- Reduced operator induced variability compared with sintered leucite ceramics
- Improved strength a result of increased crystallinity and improved dispersion.
- Crack-deflecting mechanism due to the coefficient of thermal expansion mismatch between crystal and glass phases.
What is the composition of a heat pressed lithium disilicate ceramic?
Major crystal phase 65% by volume of Li2Si2O5.
Give 3 positives of heat pressed lithium disilicate ceramics.
- 1% porosity
- 300-400MPa flexural strength.
- Crack deflection with coefficient of thermal expansion mismatch with the crystal phase and interlocking of elongated crystals.
Used in Guy’s - IPS e.max
What is the process for slip-casting of dental ceramics?
An interpenetrating phase composite:
- A ceramic frit layer is first condensed on a refractory die or a layer is machined from a preformed block.
- A glass phase is then drawn into the porous ceramic at high temperatures after the crystal phase has been sintered.
Give 3 types of materials used in slip-casting.
- Alumina-based
- Spinel-based
- Zirconia-toughened alumina
What is the process for slip-CAD-CAM of dental ceramics?
The desired restoration is scanned to obtain a digital reconstruction (CAD) and then imported to a milling device for machining of a bulk ceramic material (CAM)
What are the 2 pathways with CAD-CAM to do with sintering?
- Hard Machining
When the ceramic is fully sintered before machining.
- Soft Machining
Machining is performed on a partially sintered ceramic. Full sintering occurs after machining.
What 3 types of materials are used in the full sintered state in CAD-CAM restorations and why are they used in this state?
- Feldspar ([Na, K]AlSi3O8)
30% concentration by volume.
Flexural strength of 120MPa - Leucite-reinforced
35-55% by volume
120MPa - Lithium disilicate
Can be machined fully and partially crystallised (contains lithium metasilicate and disilicate).
Crystallisation procedure post machining will depend on concentration of lithium disilcates.
360MPa
THEY ALL HAVE LOW FRACTURE TOUGHNESS
Why are CAD-CAM restoration productions used?
Rapid production in clinic.
What 3 types of materials are used in the partially sintered state in CAD-CAM restorations and why are they used in this state?
- Alumina
99% Aluminium Oxide
Designed in oversized form to accomodate shrinkage during sinistering.
500-650MPa
Opaque to needs a veneering layer.
- Spinel
- Zirconia
Tetragonal zirconia are partially stabilised with yttrium (Y-TZP)
900-1500MPa HIGHEST FLEXURAL STRENGTH OF ALL DENTAL CERAMICS.
