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
