L04 Types Of Dental Materials: Crystal Defects, Ceramics Flashcards
What are ceramics?
Compounds of metallic and non-metallic elements, most frequently oxides, nitrides and carbides
A crystalline or amorphous material (glass) held together by ionic and covalent bonds
What are the properties of ceramics?
Hard, high melting point, brittle, chemically inert, wear resistant
What’s bad about ceramics being brittle?
If critical load is applied, they will snap, fracture or chip
Why is it good for a material to be chemically inert?
Improves biocompatibility with the body
How does the bonding in ceramics relate to the properties?
Covalent bond is directional
Ionic bond is strong
Thus ceramics tend to be hard and have high melting points, but NOT ALL CERAMICS MELT. So can be difficult to form them into the shape we want.
What materials can crystal defects occur in?
All crystalline materials. Metallic bonds, ionic and covalent bonds within crystalline or metal structures.
What can crystal defects do?
Manipulate properties
What is a point defect?
Missing atoms (vacancies)
Substitutional atoms (eg Zn in Cu)
Interstitial atoms (eg C in Fe)
What are linear defects?
Slightly larger, more 2D. Running in lines through a lattice. Dislocations are introduced doing solidification or work-hardening.
What must occur for plastic deformation?
For plastic deformation, slip of atomic planes must occur by dislocation motion.
Bonds are incrementally broken and reformed. If dislocations do not move, plastic deformation cannot happen.
What is a slip plane?
The atomic plane along which the dislocation line traverses
How do you strengthen a crystal?
Make it harder for dislocations to move
Give an example of linear defects?
Bending a paper clip will snap it as we are adding dislocations.
What are planar defects?
More 3 dimensional. Stacking faults. Grain boundaries are defects.
Why are ceramics brittle?
Because you can introduce more and more deformations which prevent slip deformation, so shatter without bending = catastrophic failure.
Why can slip deformation not occur in ceramics?
As the top layer moves, like charged ions get closer, causing mutual repulsion. So no ability for slip deformation (as there would be for metallic structure)
What is the basis for traditional ceramics/feldspathic glass?
Silica SiO2
What is polymorphism?
When a compound has many different forms
Uses of silica (ceramic raw materials)
With Al2O3 - aluminosilicate glasses
Feldspathic porcelains
Fillers in resin-based composites
What is glass?
A supercooled amorphous liquid
How does glass transition?
From a crystalline solid, discrete solid to liquid transition with a sudden change in volume
Amorphous glass has a gradual transition. Change in rate of volume at Tg
Does silica form glass or crystalline structures?
Silica can form crystalline or glass form
What is devitrification?
Can reverse and create a crystal from a glass. (Graph pg15)
What is dental porcelain based on?
Silicate glass - SiO4 tetrahedra
1 silicone atom to 4 oxygen atoms
What are the requirements of dental porcelain?
Good aesthetics
Appropriate thermal expansion coefficient
Low fusion temperature
High viscosity
Why does dental porcelain need an appropriate thermal expansion coefficient?
If used for veneers on top of a metallic base it has to be fired on
Why does dental porcelain need to have high viscosity (resistance to pyroplastic flow)
So the material doesn’t slump at high temperatures
How does the addition of metal oxides change the properties of porcelain?
The flux. They will affect/get in the way of bridges between the silicon and oxygen atoms. Act as network modifiers.
What is flux?
The action of flowing. Treat with a flux to promote melting.
What is essential to the formation of glass (or crystalline solids)?
Cations (eg Si2+, Al3+) are capable of forming tetrahedra or polyhedra microstructures and thus glass networks.
Why can’t we process crystalline silica?
We can’t process crystalline silica due to time, temperature and expense. Pure silica has a high melting temperature.
How can we control the properties of silica?
Use of a flux - helps control the properties.
It interrupts the bridge between the silica atoms and the oxygen atoms. Changes the ratio of silica to oxygen.
What elements can form mixed oxide glasses (flux)
Na, Ca, K, Zn, Pb
Give some examples of how mixed oxide glasses change the properties of silica
NaO and CaO give optical translucency
PbO changes viscosity and density of glass, changing refractive index
What do positive ions do to silica?
Positive ions disrupt the oxygen tetrahedra. Formation of non-bridging oxygen.
What does the Si:O ratio influence?
Thermal expansion
Softening temperature
Viscosity of the glass
Devitrification
What types of ceramics are used in dentistry?
Feldspathic glasses (and alumina reinforced feldspathic glasses)
All ceramic crowns (eg alumina and zirconia)
Glass ceramics (eg lithium disilicate)
Investments for casting (metal alloys)
Implants and bone substitutes
Fillers in composite (eg silica, alumina, zirconia)
Glass ionomer cements
Broad classifications of dental ceramics
Predominantly glass
Particle filled
Polycrystalline
Describe predominantly glass dental ceramics
Amorphous glass network
Traditional feldspars
Use of network modifiers
Describe particle filled dental ceramics
Composites
Glass with crystalline fillers
Describe polycrystalline dental ceramics
No glsss component
Complex processing
CAD/CAM
How has processing of dental ceramics developed?
Traditional processing - layer by layer build up types of porcelain
‘Net-shape’ engineering
Hot-pressing
Slip-casting
CAD-CAM
