Base Metal Casting Alloys

• Price of gold has increased by x100 since the early 70s

  • Platinum, palladium and silver also increased

  Alternative alloys required

  • Alloys based on Co, Cr and Ni - most successful
  • All these metals corrode readily - termed base metals
  • Unlike gold and other noble metals that had been used previously

  Dentists want alloys to match gold alloy types

  • Properties must at least match the equivalent gold alloy
  • Some alloys now perform better than gold alloys
• Categorisation of base metal alloys - standards cover 2 applications:

  • Removable appliances
  • Two ISO standards (BE EN ISO 6871) - one for Co/Cr and one for Ni/Cr
  • Fixed appliances
  • Only one ISO standard (BS EN ISO 16744) - covers all alloys from Co/Cr to Ni/Cr
• Categorisation of base metal alloys - alloys' metal concentrations not as fixed as for gold alloys:

  • Cost not as much of an issue now; doesn't matter quantities, whereas if precious metals are involved, you want to ensure that the value of the product (how much precious metal is in there) is worth it
  • Manufacturers produce a wide array of alloys to perform the same function
  • Alloy properties very dependent on minor element concentrations
• Co/Cr alloys: removeable appliances

  • Main constituent is cobalt (Co) - at least 50% has to be cobalt
  • Chromium (Cr) added for corrosion resistance
  • Chromium oxide forms very quickly and bonds strongly to Cr
  • Passivation - v strongly bonded oxide layer on top surface of Cr
  • Gives good properties
  • Forms a solid solution with chromium
  • Forms large grains even when quenched
  • Means thin sections (eg clasps) may have few grains
  • Leads to low mechanical properties

  
• Co/Cr alloys: Removable appliances

  • Molybdenum (Mo) added to make smaller grains
  • Improves yield strength; more grain boundaries
  • Carbon added in small quantities
  • Forms hard carbides at grain boundaries - improves strength
  • Creates more obstacles for dislocations
  • Too much carbide leads to brittle alloys
  • Leads to in-service failures
  • Can contain nickel - total compensation according to ISO standard Co + Cr + Ni > 85%

  
• Ni/Cr alloys: removable appliances

  • Main constituent is nickel (Ni)
  • Chromium (Cr) added for corrosion resistance - similar action to the Co/Cr alloys
  • Molybdenum and Carbon added for same reason as Co/Cr alloys
  • Grains can be still large so beryllium added
  • Can contain cobalt - total composition according to ISO standard Ni + Cr + Co > 85%

  
• Safety considerations:

  • Nickel is commonly a cause of allergic dermatitis
  • All alloys may contain nickel
  • Beware, even Co/Cr alloys may contain nickel!
  • A full medical history must be taken before prescribing a Ni containing device
  • Check the manufacturer's information for Ni concentration
  • Ni/Cr alloys contain beryllium (Be)
  • Added to refine grain structure
  • Highly toxic element
  • Long term exposure to Be dust can lead to chronic beryllium disease
  • More relevant to technicians - dust mostly found during device production
• Base metal casting alloys for fixed prosthesis:

  • Only one ISO standard (BS EN ISO 16744)
  • Recognises that a wide range of alloys are possible
  • Easier for manufacturers, technicians and dentists to keep up with new legislation
  • Can be close to either Co/Cr or Ni/Cr
• Base metal casting alloys for fixed prosthesis:

  • Composition is not limited other than
  • Beryllium, cadmium (added to get smaller grains but carcinogenic, so not ideal) and nickel are indicated to be hazardous
  • Beryllium and cadmium <0.04% max
  • For nickel >0.1% a warning is required
  • All of the above are due to the device being "fixed" and so a permanent device
  • So, there is a greater risk of exposure to the patient
• Alloys for fixed restorations:

  • Type 1 - for low stress applications such as inlays
  • Type 2 - for moderate stress applications such as larger inlays, onlays and full crowns
  • Type 3 - for high stress applications including bridge pontics and implant superstructures
  • Type 4 - for very high stress applications including long span bridges and implant superstructures

  Types indicated are equivalent to gold casting alloys
• Mechanical properties:

  • Increases in proportional limit mean the device can withstand greater stress without deformation

  BUT

  • Decrease in % elongation mean that the device is much harder to adjust without it breaking
  • Therefore casting would need to be really accurate with these materials - but difficult because the bigger the structure, the more likely a casting error will be made worse

  
• Comparison of Co/Cr alloys with gold alloys = harder to get accurate casting with basement alloys than with gold alloys

  
• Comparison of Co/Cr alloys with gold alloys = harder to get accurate casting with basement alloys than with gold alloys

  
• Comparison of Co/Cr alloys with gold alloys = gold alloys softer, so easier to polish, but also easier to scratch

  
• Crown and bridge alloys:

  • Accuracy and detail
  • Casting - gold alloys: HIGHER density, lower shrinkage - therefore give more accurate castings
  • Adjustments
  • Burnishing inlay margins - gold alloys: lower stiffness, lower yield strength
  • Function
  • Stiffness and resistance to deformation - base metal alloys: higher stiffness
  • Safety
  • Biocompatibility - gold alloys: lower incidence of problems
• Partial denture alloys:

  • Connectors
  • Must be rigid
  • Must resist permanent deformation in mouth
  • Base metal alloys - higher stiffness
  • Clasps
  • Must be flexible enough to engage undercuts in teeth
  • Gold alloys - lower stiffness
  • Must resist permanent deformation in mouth
  • Base metal alloys - higher yield strength
  • Should allow adjustments
  • Gold alloys - lower yield strength (heat treatments)