Metals and Alloys W3 Flashcards
Where are metals used in dentistry?
Metal prostheses: inlay, onlay, partial crowns, full crowns, bridges, endodontic posts
Cast metal substrcutures: metal-resin protheses
What forms are metals generally used in for dentistry
Alloy form - We want strong metals in the oral cavity, need compressive and tensile material and control of the phases formed for corrosion resistance…
therefore we alloy a pure metal = STRONG
What form do metals take in their solid state?
Crystaline - highly ordered 3D arrangement of atoms
What are the different types of Unit cells for crystal structures**
- *FCC** (face centred cubic) - 4 atoms per lattice point (box)
- *BCC** (body centred cubic) - 2 atoms per lattice point (box)
- *HCP** (hexagonal close packed) ex. Ti
What is the difference between single crystal and polycrystalline material - what would be the prefered material in dentistry and why?
Single crystal: repeated arrangement of atoms = Flat regular faced crystalline material
Polycrystalline: Made of crystals of different size, orientation, shape ie. GRAINS
Dentistry = polycrystalline because grain boundaries and point defects stop cracking propagation and increase the strength of the material.
What two things strengthen a material (metal)?*
- Point defects
- Grain boundries
What size grains provide strength*
Smaller grains = High strength material
- Strengthen the material due to localised point defects that impede dislocation propagation.
How do you identify if a metal will be readily oxidised or reduced
The ‘activity series’ which is based on standard reducion potentials (order of what is easily oxidised (more -ve reduction potential) and what isnt as easily oxidised (more +ve reduction potential) ie. measure of ease of oxidation.
Active = easily oxidised, Inactive = oxidation difficult
What does the term ‘active metals’ refer to
Metals that are inclined towards rapid oxidation
Patient has Amalgam fillings. What happens when they bite on a piece of alfoil?
Due to Al (anode) reacting with the filling/Hg (cathode) in the presence of saliva (electrolyte)
= reaction circuit creates current sensed by nerve
= pain/buzzing sensation
What is the definition of corrosion*
Chemical or electrochemical reactions between a metal based material and its environment that produces a deterioration of the material and its properties
- Redox process where electrons are lost (oxidised) by one chemical species and gained by another (reduced)
What is the predominant form of corrosion for the oral cavity and how does this differ from the other type of corrosion
Electrochemical corrosion - requires presence of fluid electrolyte (saliva)
Chemical corrosion- forms through oxidation reactions. Also called dry corrosion as doesn’t require fluid for electrical current.
What happens when ion rusts and what speeds up this process?
- oxidation of Fe to Fe2+
- reduction of O2 to O2-
- oxidation of Fe2+ to Fe3+
Fe acts as both the anode (where pitting occurs) and a catode (elsewear).
Electrolyte solutions such as salt water. These enhance electrical circuit allowing passage of a current.
What are dissimilar metals
Metals at close proximity (in salvia - electrolyte) which have a difference in electrode potential
= voltaic cell established
Explain what happens in the mouth when gold crowns and amalgam fillings are in close proximity*
Amalgam oxidised (corrosion) and gold reduced with saliva acts as electrolyte allowing set up of voltaic cell = creates current
What is stress corrosion cracking and what 3 things are required for this to occur?
corrosion forms pits -> crack initiation -> crack propagation
Requires: susceptible material, corrosive environment and tensile stress- compressive stress is common in mouth.
What are phase equilibrium diagrams**
A map of the phases and phase equilibria of a system against variables such as temp, pressure, composition ect.
Define sublimation
From solid to gas (vapour) - without liquid intermediate
Opposite = deposition (gas to solid)
How do you freeze bone long term so that it is useable?**
Freeze drying is a example of an application of phase diagrams
This is how bone allografts are prepared for storage at room temp.
Sublimation: Ice is removed from frozen bone without melting to liquid. Goes from solid to Gas by lowering temp, then pressure, then rasing temp.
Example of exploiting phase diagrams
Where does equilibria exist on phase diagrams?
AND
What is the Equilibrium Boundary and where is it found?
Equilibria exists on phase boundary lines.
Equilibrium boundary- the conditions where two phases exist in equilibrium/where its changing between 2 phases
Found on sublimation curve, boiling curve and freezing point curve.
Difference between Triple Point and Critical Point
- *Critical Point::** Temperature above which higher pressure (applied pressure) does not liquify
ie. On gas to liquid phase boundry (vapour pressure line), usually pressure applied to gas causes a liquid.
Triple Point: where all three phases are at equilibrium, used for calibrating thermometers
Draw and label a phase diagram for H20. What are the triple point, critical point and sublimate pressure?
Triple point = 0.01 degrees, 0.006 atms
Critical point = 374 degrees, 218 atms
Sublimates at pressures above 0.006 atm
What is the eutectic point?**
Mixture of substances (in fixed proportions) which melt at a temp that is lower than the melting points of each of the seperate components
Ex. lidocaine + prilocaine
What are polymorphs
compounds with different crystalline forms that are in the same state/phase
Describe what is happening on the left and right of this diagram.
Ie. describe the difference in cooling with regard to grain size, corrosion and grain boundries*
Left: Fast cooling- small grains = coring = less corrosion resistant = more grain boundaries
Right: Slow cooling - larger grain = homogeneous = more corrosion resistant = less grain boundaries
Which is the reduction and which is the oxidation in this diagram?
Reduction = O2 + 4H + 4e- → 2H2O (GOLD)
Oxidation = Sn8Hg → Sn2+ (dental filling)
