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
What are drug polymorphs*
Solid-state drugs in different crystalline conformations, due to intermolecular forces = different physical properties (solubility, bioavailability)
What is a clinical example of phase diagrams, specifically exploiting the eutectic point**
- *Lidocaine and prilocaine**
- normally solid at room temp, need liquid to apply
- once the eutectic point is found the emulsion (at that proportion) will be liquid at room temp
Define a metal*
A class of solid substances characterised by high electrical and thermal conductivity as well as by malleability, ductility, and high reflectivity of light.
Define an alloy*
A material with metallic properties resulting from the combination of > 1 element (2 or more)
Homogenous solutions (solution alloys) include, substitutional and interstitial alloys, compare*
Substitutional: crystal structure of solvent retained, solute atoms enter the lattice and substitute for some solvent atoms
- atoms are similar size and have similar chemical bonding
Example: Form binary metal alloys ex.
Cu+Zn (brass)
Interstitial: One atom type is smaller and fits into the interstices of the structure. E.g. Fe+C (stainless steel - small carbon atoms fit into interstices of the iron matrix.
What properties can alloying change*
Physical properties: Lustre, malleability, ductility, hardness, melting point, corrosion
What is a solid solution?*
Both metals are completely soluble in each other in all combining proportions
What are noble metals and why are they used in dental alloys*
Like noble gases
- relatively chemically inert, resistant to oxidation
Therefore good for dental alloys!
ex. gold, platinum
What are base metals
- *Non-noble, more easily oxidised**
ex. Fe, Ti, Co
Compare and give an example of homogeneous and heterogeneous alloys*
- *Heterogeneous**: components not dispersed uniformly ie. 2 distinct phases/microstructures/products
ex. in pearlite steel - ferrite (phase 1) cementite (phase 2) - *Homogenous**: components dispersed uniformly but randomly at ALL proportions. Same crystal type (BCC ect.)
ex. stainless steel (interstitial)
What are intermetallics and why are they special*
Components have discrete compositions ie. repeating unit cell
Gamma phase in dental amalgam
ex. Ag3Sn - y (gamma) phase dental amalgam
What is amalgam
Substance formed by mixing Hg with another metal = alloy
What elements are in dental amalgam
Ag, Sn, Cu, Zn, Hg
What is the process of amalgamation (generally)
Mixing liquid mercury with the powedered intermetallic solid alloy (silver tin) to form amalgam
What is the setting reaction for a low Cu alloy (Amalgamation)**
Ag3Sn + Hg —> Ag3Sn + Ag2Hg3 + Sn8Hg
Copper increases strength and reduces corrosion
All products in gamma phase (y, y1 and y2 respectively) - These are all solid phases but are structurally different to each other, hence the use of the word microstructure.
Why do we add copper in amalgamation*
Copper added to the alloy > 5% removes the Sn8Hg (y2) phase and you get a secondary reaction (another reaction) forming Copper-tin.
Ie. favourable as y2 phase was the weakest and least corrosion resistant
= increases strength, reduces corrosion
What is the Pd-Ag (paladium silver alloy) used for?
Homogeneous alloy at all proportions
Used for cast metal prosthesis
Noble metal alloy & similar size atoms = solution alloy
What is the best cooling rate to achieve homogeneous alloys
Slow cooling = grains become homogeneous
What happens when we cool things to quickly
heterogenous composition = layered grains of differing compositon ie. coring = more susceptible to corrosion
Whats special about titanium (Ti)*
- corrosion resistant due to surface of TiO2 film!
- offers strength/weight advantages
- physical properties easily modified by alloying
- is used for biocompatible implant materials (osseointegration of live bone and implant)
What is osseointegration, give an example of a metal used**
A direct structural and functional interface between the live bone and Ti impact surface (without any intervening fibrous C.T)
What phases can Ti exist in*
- *a (alpha) phase** - commercially ‘pure’ form - HCP structural form
- *B (beta) phase** - BCC structural form
Adding oxygen to Ti makes a significant difference to the strength*
What happens when you alloy Ti and Aluminium and Vanadium*
- *Aluminium** promotes the alpha phase (decreases density, increases strength)
- *Vanadium** promotes the beta phase
Together/both phases exist at the same time = substitutional alloy
= Ti 6Al 4V
What is the benefit of alloying Ti with Aluminium and Vanadium (substitutional alloy)*
Combination of alpha and beta phase enable heat treatment and precipitate hardening which strengthens the structure…
= Heat resistant, low density, strong, resistant to corrosion and fatigue, good elasticity and biocompatible
Binary metal alloys can be defined as?
What does their formation depend on?
Substitutional solution alloys- Homogenous and random
- proportion of elements
- rates of cooling from liquid state
What is an example of an inter metallic compound and what does it mean?
components have discrete concentrations- every unit cell is the same
Ag3Sn gamma phase dental amalgam
What is coring and when does it occur?
Layered grains of differing compositions. Occurs in fast cooling, results in higher grain boundaries, high corrosion chance.
What is osseointegration and what metal does it relate to?
Titanium fuses directly to bone:
Direct structural and functional interface between live bone and Ti implants surface
Doesn’t intervene with connective tissue.
What is the allotropic phase change Ti undergoes and at what temp does this occur?
alpha HCP → Beta BCC
For pure titanium above 882 degrees undergoes this phase change ie, when you heat
What does formation of a homogenous solution alloy depend on and how might coring be avoided?
- The formation of a homogenous binary metal solution alloy depends on the proportions of the combining metal elements and rates of cooling from liquid state.
- For slow cooling, diffusion of atoms in solid grains at temperature allows grains to become homogenous.
- If fast cooling, we can end up with layered grains of differing composition (coring)
What 4 conditions in the oral cavity contribute to corrosion?
Conditions: moisture, electrolytes, temp variance and pH changes.
What is an electrode potential?
Why are effective electrode potentials altered?
measurement of potential for equlibrium- differences between electrode and electrolyte
Differences in materials, deformations in homegeneties across structures. Electrolyte concentration variances.
What is the negative effect of cold working metal alloys in relation to stress?*
Causes stress corrosion - oxide layer is removed = deformed (stressed) regions
= anodic regions where oxidation can occur. Less stressed regions- cathode.
What enables corrosion of grain boundaries?
heat treatment
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)
What is metallic bonding and what are the forces involved?*
Metallic bonding: Metal + Metal
Electron sea model: delocalised ‘sea’ of mobile valence electrons around metal ions
