Gold Alloys and Semi-Precious Alloys

• Noble metals are metals that do not oxidise easily. Examples include: gold, platinum and palladium.
• The name 'precious' metals is not related to a chemical property, it's related to how valuable the metal is. Examples include: gold, platinum and silver. Silver is the only one that is precious but not noble; it corrodes very easily.
• Pure gold:

  • Requires pure gold - i.e. 24 carat gold
  • Gold = very ductile - it's easily shaped so can fill the cavity
  • Relatively soft
  • Easily distorted - useful for fitting to cavity
  • Can distort in service - needs support so can only be used for small cavities; needs lots of tooth to support it, otherwise will deform
  • Stable/unreactive - doesn't corrode/tarnish - good aesthetics
  • Coefficient of thermal expansion similar to tooth
  • Therefore expansion & contraction v similar to the surrounding tooth when hot/cold drinks consumed - therefore no/few gaps created at margins due to temp changes
• Gold foil for cohesive gold restorations:

  • Require very thin sheets (25 μm thick) of pure gold (foils)
  • Can be joined together under pressure - termed cold welding
  • Gold foil needs to be very clean for cold welding to work - flame in Bunsen flame prior to use
  • Need to do this because otherwise impurities could get on the foil and stop the welding process occurring
  • Can be carried out by hand - not possible for everyone due to high pressures required
  • Some instruments available - but patients find them uncomfortable
• Gold foil for cohesive gold restorations:

  • No cement required to retain material in the cavity
  • Foil is v ductile so can conform to cavity easily
  • Potential advantage; majority of crown failures are cement layer failures
  • However, there are limitations
  • Gold is expensive, pure gold is really expensive!
  • Even with good technique, restoration is never fully dense
  • Typically only 75% - 82% fully cold-welded together - potential weakness
  • Possible marginal staining if you don't get full condensation of materials around margins
  • Poor mechanical properties so applications limited to v small cavities
• Gold alloys:

  • To extend the range of treatment options gold alloys developed
  • Cheaper than pure gold - gold replaced by e.g. Ag, Cu
  • More commonly used than pure gold
  • Harder than pure gold so able to use in more than small cavities
  • Solution hardening due to being alloys
  • Different atoms, therefore harder for dislocations to flow, so the yield strength improves and we get solution hardening
  • Heat treatments can improve mechanical properties even more
  • Because know how to control properties by cooling them down during casting
• Gold alloys:

  • Alloys are less ductile than pure gold
  • Shaping to the cavity not possible
  • Shaping must now be done by casting
  • Need to heat materials up to melt them, pour them into a mould, then cool down in the desired shape
  • Less dense than pure gold
• Traditional gold alloys:

  • Range of different alloys possible
  • Properties dependent on concentration of metals
  • Potentially confusing for dentists so standardisation needed
  • ISO standard covers composition ranges
  • Divided into 4 groups related to mechanical properties
  • Termed: soft, medium, hard and extra-hard
  • Each group has specific indications for use

  
• Metallurgy of traditional dental gold alloys:

  • Gold forms solid solutions with other metals
  • Improved mechanical strength & hardness compared to gold; solution hardening
  • Devices (crowns, inlays, onlays, etc.) produced by casting
  • Equiaxed grain structure formed
  • Quenching (cooled down quickly) produces small grains - improve yield strength so it can survive in pt's mouth
  • Quenching can lead to coring - makes it v susceptible to corrosion
  • Problem in Type III and & IV alloys
  • Due to Pt/Pd content widening the solid + liquid phase region
  • Homogenisation required to prevent coring

  
• Hardening of traditional dental gold alloys:

  • Properties of gold alloys can be improved
  • Solution hardening: controlled by composition and atomic sizes

  
• Hardening of traditional dental gold alloys:

  • Properties of gold alloys can be improved
  • Precipitation hardening:
  • Controlled by Cu + Ag
  • Because silver & copper don't form complete solid solutions - they're partially soluble within each other - therefore potential for precipitation hardening to occur, where if after cooling the casting down, it's heated to around 450-500 °C to get some silver and copper to precipitate out of the grains to form regions of pure silver and pure copper - they provide more barriers to dislocations - therefore improvement in mechanical properties
  • Heat treatment

  
• Hardening of traditional dental gold alloys:

  • Properties of gold alloys can be improved
  • Order hardening - most important method
  • Controlled by Cu and Au content (>11% Cu) - relies on having at least 11% copper in the alloy
  • Particularly useful for partial denture frameworks, where when we cool things down we get ordered intermetallics forming (Cu₃Au or CuAu) - and because we have an ordered structure, dislocations find it harder to flow, therefore gives it better mechanical properties
  • Heat treatment

  
• Hardening of traditional dental gold alloys:

  • Properties of gold alloys can be improved
  • Work hardening
  • Requires a mechanical work
  • Not a heat treatment

  
• Properties of traditional dental gold alloys:

  • Melting point increases as we go from high amounts of gold to low amounts
  • Recrystallisation temperature increases as we add other atoms - the more other metals we add, the less likely we are to have recrystallisation occurring
  • Casting ability - the more gold we have, the higher the density - tends to make casting easier
  • Corrosion resistance v good when just got mostly gold with a few other elements added, but copper & silver corrode readily, so as we increase the amount of them we have a higher chance of corrosion - affects biocompatibility

  
• Properties of traditional dental gold alloys:

  • Hardness = resistance to polishing & scratching - harder to polish a type IV alloy than type I - but also, easier to scratch type I alloy than type IV, so type IV alloy appearance = more consistent w/ time
  • Proportional limit = related to yield strength & how easy it can deform - can be increased w/ solution hardening & heat treatment
  • Modulus & strength relate to how easily it can resist deformation/breaking - improves as gold content decreases
  • Ductility is related to how easily adjustments can be made - less gold, harder to adjust; less ductile

  
• Applications of traditional dental gold alloys:

  • Low stress bearing applications
  • Small occlusal restorations
  • Pure gold
  • Type I alloys (inlays) - easier to cast and fit to desired anatomy
  • Medium stress bearing applications
  • Larger inlays
  • Type II alloys - mechanical properties a bit better, so better for larger inlays
• Applications of traditional dental gold alloys:

  • High stress bearing applications
  • Full crowns, denture components
  • Type III and type IV alloys
  • Type III alloys can be heat treated
  • Type IV alloys almost always heat treated
  • Hardening can increase yield strength up to two-fold - significant advantage 
  • Hardening can decrease ductility up to ten-fold - so if any changes need to be made after hardening then the product will just break
  • Therefore need to be careful when using these - try-in stage v important for this reason
• Semi-precious alloys:

  • Gold is very expensive and is increasing in cost
  • Alternative alloys designed
  • Medium/low gold content
  • Silver/palladium
• Medium/low gold content alloys:

  • Normally <50% gold - significant cost saving
  • High in palladium - white-ish appearance
  • Similar properties to type III or IV gold alloys - strong, rigid, limited ductility
  • Lower density than high gold content alloys - less accurate casting - may be difficult to get device to fit properly
  • Used extensively for PFM (platinum-fused-to-metal) bonded restorations - because very good properties and behave well
• Silver/palladium alloys:

  • Lower density than gold alloys - less accurate casting
  • Oxygen dissolves in molten alloy
  • Because higher casting temperatures needed due to silver and palladium having high melting points
  • Porous castings result because oxygen can't escape as we cool the alloy down - if we get these then it'll be a source of weakness
  • Therefore care needed in casting technique
  • Adequate corrosion resistance
  • Lower than gold alloys
  • Because lower precious metal content
  • Properties similar to type III gold alloys