Metals in Dentistry Flashcards
Different types of materials used in the mouth and the bonding between these materials
-Polymers
Covalent
Long-chains
-Metals
Metallic
-Ceramics
Ionic and covalent
-Semi-conductors
Covalent
-Composites= mixture of 2 or more materials
Resin Based composite
Organic polymerisation and inorganic filler particles
Dental amalgam
Metal alloy
Metallic bonding definition
- Least understood
- Leads to ordered crystalline structures
- Atoms sit in a sea of delocalised electrons
- Outer electronic cloud around and is non-specific to the positive charged atoms
- Accounted for its optical, electronic and thermal behaviour
-Either processed in cast or wrought
Cast v wrought
- Cast iron has been melted, poured into a mould and then allowed to cool
- Wrought iron has been heated and then worked with tools
- Same chemistry but different microstructure
Metals and alloys used in dentistry examples and properties
Requirements of materials
- Very strong
- Good interaction with bone (titanium alloys)
- Ductility
- Easy to shape
Restorations
- Must have a high corrosion resistance (pH range, chemical composition)
- Biologically inert
- Co-Cr
- Amalgam
- Titanium implants
Instruments/Equipment
Explain the lost wax casting technique
-Objective is to take a wax pattern and reproduce it in metal while showing for:
Wax shrinkage
Metal casting shrinkage
Using investment (setting and thermal expansion)
1) Take a wax pattern on a die
2) Place wax pattern on a wax sprue and attach to a sprue base
3) Place an investing layer around the wax and sprue
4) Burnout: heat up the sand and allow wax to melt and fall through to the bottom leaving a hollow structure
5) Fill up with molten metal (casting)
6) Breakout from the investing layer
7) Pickle casting (oxides removed)
8) Remove the sprue and polish
9) Deliver to the patient
How can metal prosthesis be produced
- Employ lost wax technique to cast very complex shapes
- Then bond white and pink porcelains to make it look more natural
- CAD-CAM production of prosthesis
- Subtractive process
- Cutting or grinding from a blank shape
- Losing and wasting a lot of metal though, so not very cost effective
- Emerging CAD-CAM Process
- Imploys additive manufacturing (metal-based 3D printing )
- Deposits materials usually in layers
- Layer sintering of powdered metal/alloy
- Layer by layer process
- Metal crowns or partial denture framework
- Very good in compression but bad in shear
Describe and explain the cooling curve of a pure metal
-Transformation from liquid to solid occurs at a well-defined, discrete temperature
- As you cool it down, it reaches a certain point where you initiate nuclei
- Thermodynamically, you get regions where atoms do not have enough energy to keep vibrating as a liquid so it solidifies on these nuclei
- Nuclei grow to form grains until you get a solid structure
- Crystallisation releases heat so thermal arrest occurs at melting temperature
- Energy released is known as latent heat of fusion
Definition of a phase
A structurally homogenous part of a system with clear physical boundaries
State of matter
Forming a metal
1) as the temperature of the metallic melt decreases, muclei of crystallisation form within the melt
Either by homogenous nucleation (4 atoms lose suficient energy to form a unit cell)
Or
By heterogenous nuclation (metal atoms deposit on impurities on the melt at melting temperature)
2) Crystals continue to deposit on these nuclei of crystallisation to form grains
3) Grains continue to grow until all of the metal has solidified
- During growth, grains will begin to impinge on one another forming grain boundaries (defect in the crystal structure of the metal)
Where does growth of a solid metal begin from and how can these be produced
as the temperature of the metallic melt decreases, muclei of crystallisation form within the melt
Either by homogenous nucleation (4 atoms lose suficient energy to form a unit cell)
Or
By heterogenous nuclation (metal atoms deposit on impurities on the melt at melting temperature)
Homogenous nucleation explanation
- The smallest atomic unit in a crystal structure
- Repeating unit that builds grains in metals/alloys
- Repeated in 3 directions
Crystalline phase definition
- Local atomic arrangement is repeated at regular intervals millions of times in the 3-dimensions of space
- Crystals have a long range order
3D Unit cell and examples
-Consider the simplest cube containing only a portion of the atoms within it
Body centered cubic
Face centered cubic
Hexagonal close packed
Packing factor definition and lattice approximates. Significance to metals
Packing factor: Volume of atoms inside cube/Volume of cube
Simple cubic: 0.52
Body centred cubic 0.68
Hexagonal close packed 0.74
Face centered cubic 0.74
Metals seek the lowest energy state= best packing factor
Significance of etching and grain boundaries
-If you etch a substance with different types of grains, then look under a microscope, the grains will all appear different shades of grey
- If you etch a substance, with the same type of grain, then light will only scatter in places where there is imperfect packing
- This would be the grain boundaries, so the grains will all appear the same colour but with separate grain boundaries
Solidification of metal in a mold. Orientation and variation in size of grains
- Faster cooling leads to finer grain size
- Higher concentration of nuclei of conentration leads to a finer grain size
-Metal solidifies at the mold walls first creating this weird pattern
Metal Alloys definition and classification of alloys
-Blend of one or more metals
-Binary system
For example brass containing zinc and copper
-Ternary system
For example dental gold alloy- gold, copper, silver
Implant alloy- titanium, vanadium and aluminium
- Combinations investigated by study of phase diagrams, physical properties and microstructure
- Study of cooling curves
Cooling curve or pure metals v alloys
- Look at slides
- At melting temperature, there is a discrete temperature at which the pure metal turns from a liquid to a solid
-During the cooling curve an alloy, crystallization begins to occur at T1 and is complete at T2. Crystallisation takes place over a range of temperatures
Between T1 and T2, one metal is still cooling without solidifying while one is solidifying
How do you go from a cooling curve to a phase diagram
Important terms on the diagram
- Make cooling curve for all the different proportions of the alloy and join them up
- Liquidus line is the transition from a pure liquid to a mixture of a solid and liquid
- Solidus line is the line that separates a mixture of solid and liquid from the solid state
Melting temperature of a pure metal v alloy
- Pure metal would be a discrete temperature
- Alloy would be a range of temperatures depending on the composition of that alloy
Definition of phase and
How many phases in:
- Water and Ice mixture
- Oil and Water
- Sand
- A structurally homogenous part of a system with clear physical boundaries
- State of matter
- Water and Ice is 2 phases but each with a distinct arrangement of atoms
- Oil and Water is 2 phases
- Sand is 1 phase because all sand granules are the same
RTQ
For a particular composition of an alloy (50% Metal A 50% B), describe the changes in the microstructure of that metal as you go from high temperatures (above the liquidus) to low temperatures (below the solidus)
- As you begin at high temperatures, the alloy will present as a melt which is fully liquid
- As you reach the liquidus point, you will get nuclei of crystallisation forming
- These nuclei of crystallisation will grow as atoms begin to crystallise between the liquidus and solidus lines
- Below the solidus line, grains will be present, separated by grain boundaries. It will be fully solid
