Metals Flashcards
Definition of precious and noble alloys
-Precious= related to how valuable the metal is, its not a chemical property. Likely to be expensive and used for jewellery
-semi-precious= less expensive
-Noble alloys=do not corrode easily
Is silver and gold noble
-silver is not noble because it is likely to corrode with oxygen
-gold is noble as does not corrode/tarnish
Properties of pure gold
-very ductile=easily shaped so can fill cavity
-relatively soft=easily distorts, so only used for small cavities
-stable= does not corrode or tarnish (noble)
-coefficent of thermal expansion is like enamel and dentine, so expand/ contract together when temp changes
-expensive
-good corrosion resistance
Gold foil used to be used. what are its Properies. why not used anymore
-long lasting, conforms to cavity well, no luting required
-very thin sheets that need to be cleaned using Bunsen burner before hand for cold welding to work
-cold welding uses high pressure to press sheets together so they permanently stick. This is technique sensitive and to comfortable for patient
-expensive
-possible margin staining
-only for small cavities as poor mechanical properties
-ductile
Why gold alloys more commonly used than pure gold. The disadvantage
-harder
-less ductile
-cheaper, more accessible
-solution hardening, heat treatments
-but less dense, so less accurate
Properties of gold alloys are dependent on metal concentrations. Adding what metals increases hardness. What is added to reduce other metals oxidising
hardness increases if:
-gold decreases (content of palladium and platinum increased to compensate this, but more of this causes poor corrosion)
-copper increases (but poorer aesthetics)
-silver increases
adding zinc means the other metals don’t oxidise (as it sacrificially oxidises)
What content of noble metals must be in dental alloys
overall content must be at least 75% noble metals (don’t corrode)
Why gold alloys are quenched
*Quenching (cooling quickly) causes finer grains – which increases yield stress which is important as gold alloys on their own are not strong
*Dislocations get trapped at boundaries – increases yield strength so more difficult to deform
How dislocations affect properties
- Dislocations means atoms are harder to move when stress applied so strengthens the material and makes it hard to deform. causing plastic deformation.= less ductile. Increases hardness and yield strength
What is yield stress. What happens when force is above yield stress
- the stress required to cause dislocations to move, to cause plastic deformation
-smaller grain size means more dislocations, requires more energy to move, increasing yield stress. Harder to deform
When force is above yield stress (during work hardening):
-Dislocations can move and more dislocations form and start to become trapped at grain boundaries
-Stress required to move dislocations increases – more barriers
Collected dislocations start to form pores
Necking occurs and then failure
What is hot work and cold work hardening
-Hot: Increasing temperature above recrystallisation temp. (hot work) decreases yield strength. Fully ductile.
-Cold working: below RcT. Grains become fibrous, more dislocation, yield strength & hardness increases, but ductility reduces so can lead to failure.
What is solution hardening, precipitation hardening, order hardening and work hardening
-solution: all metals in alloy have similar crystal structure forming a solid solution but differing size of atoms obstruct the slip planes.
-precipitation: controlled by Cu and Ag. Heat treatment to 400 degrees
-order: forms superlattice, harder for dislocations to flow so yield strength increases. Controlled by Cu and gold. Heat treatment
-work: mechanical work to deform things. Not a heat treatment. Dentist bends alloy chairside.
Difference in hardness and strength for type I to IV gold alloys and their uses
-type I = more gold (85%). soft, less strong, very ductile. For small occlusal restorations.
-type II = large inlays
-type III =crowns
-type IV= less gold (65%). harder, stronger, less ductile. For full crowns, partial dentures
Explain 1) medium/low gold content, and 2) silver/paladium semiprecious alloys [decreasing gold content to decrease cost]
- Medium/low gold content= normally <50% gold, high in palladium, increases strength with limited ductility, lower density than high gold content so less accurate. Used extensively for PFMC. Similar properties to type III or IV gold alloys
- Silver/paladium =low density so less accurate, porosity more likely in casting, low corrosion resistance. Similar properties to type III gold alloys
The roles of Co, Cr, Mo and C in Cobalt chromium metal, for removal appliances
-Co (main constituent) doesn’t react quickly=preventative effect
-Co forms solid solutions
-Chromium oxide added for corrosion resistance – stable oxide layer forms very quickly and bonds strongly to Cr underneath = passivation
-Mo -Makes smaller grains to improve yield strength
-Carbon (trace) -forms carbide at grain boundaries which hardens it and makes it difficult to deform, but too much makes it brittle
Roles of Ni, Cr, Mo, C and Beryllium in nickel chromium alloys for removal appliances
-Ni (>50) = forms solid solutions (but common for allergic dermatitis so ensure pt does not have allergy)
-Cr = corrosion resistance
-Mo =smaller grain size to improve yield strength
-Be= reduces grains (but long exposure can be toxic however)
-C= hardens, but makes it brittle
Why beryllium, cadmium and nickel are indicated to be hazardous
-Beryllium and cadmium <0.04% max (as toxic & will be luted onto tooth)
-Cadmium is being phased out
-For nickel >0.1 % a warning is required. Allergies
Explain the differences and uses of type 1-4 for alloy fixed restorations
-Type 1 - for low stress applications (inlays)
-Type 2 - for moderate stress applications (larger inlays, onlays and full crowns)
-Type 3 - for high stress applications (bridge pontics and implant superstructures)
-Type 4 - for very high stress applications (long span bridges and implant superstructures)
Do you want proportional limit and ductility to be high for Cobalt chromium
-proportional limit=how easily it is to deform permanently. Want it small enough so you can make small adjustments but large enough that it will resist forces in the mouth and when taking it out
-ductility= how much strain before lateral breaks. Want this to be large
How elongation affects ability to adjust Co/Cr removal appliances. What elongation % is for type 1-4 and for Co/Cr
-decreases in elongation means it is harder to adjust without it breaking
-it is 3%, compared to: 18% for type 1
-10% for type 2
-6% for type 3
-3% for type 4
it is difficult to adjust T4 alloys
How properties of gold alloys and base metal alloys (Co/Cr and Ni/Cr) differs [melting points, density, hardness, strength, ductility]
Base alloys:
-melt at higher temperatures
-less dense, so more shrinkage and less accurate
-much harder
-allergies more common
-low material cost, but may have higher manufacturing cost due to equipment and need to heat more
-higher strength
-higher stiffness
-lower ductility
Reasons for using gold or base alloys for crowns and bridges
-Gold= higher density so more accurate and less shrinkage. Lower yield strength and stiffness so adjustments can be made. More biocompatible
-Base metal alloys= higher stiffness and resistance to deformation
so gold alloys usually better
Are gold alloys or base metal alloys better for partial denture connectors
-base metal alloys (co/cr) due to higher stiffness which is important as it must be rigid so patient can chew without it deforming
