Dental Casting Alloys Question And Answers

Dental Casting Alloys

Question 1. Enumerate base metals used in dentistry. Describe the composition, advantages, and disadvantages of chromium-cobalt alloy as a denture base material.
Answer:

Base metals:

These are non-noble metals. They are important components of dental casting alloys because of their influence on physical properties, control of amount, type of oxidation, and their strengthening effects.

• Such metals are reactive with their environment and are referred to as “Base metals”.
• Some of the base metals can be used to protect an alloy from corrosion by a property known as “Passivation”.
• They are also called, “Non-precious metals”.
• Examples: Chromium, cobalt, nickel, iron, copper, manganese, etc.

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 An alloy of Base metal is:

• Nickel: Chrome alloys.
• Cobalt-chromium alloys.
• Pure titanium and its alloys.
• Aluminum bronze alloys.

Cobalt-Chromium Alloy:

• They possess high strength; their excellent corrosion resistance especially at high temperatures makes it useful for a number of applications.
• These alloys are also known as “satellites” because of their shiny, star-like appearance.
• They have bright lustrous, hard, strong, and non-tarnishing qualities.

Uses Of Cobalt-Chromium Alloy:

• Denture base
• Cast removable partial denture framework
• Crown and bridges
• Bar connectors.

Composition Of Cobalt-Chromium Alloy: 

Functions of Alloying Elements:

• Cobalt:
  • Increase the hardness, strength, and rigidity of the alloy.
  • It has a high melting point.
• Chromium:
  • Its passivating effect ensures corrosion resistance.
  • It reduces the melting point.
  • It also acts in solid solution hardening with other elements.
• Nickel:
  • It decreases strength, hardness, modulus of elasticity, and fusion temperature.
  • It increases ductility.
• Molybdenum, Tungsten, Iron, Copper:
  • They are effective hardeners.
• Beryllium:
  • Hardener and reduce fusion temperature and refine grain structure.
• Manganese and Silicon:
  • Primarily oxide “scavengers” to prevent oxidation of other elements during melting.
  • They also act as hardeners.
• Boron:
  • Deoxidize and hardliner, but reduce ductility.
• Carbon:
  
  • Carbon content is most critical in small amounts.
  • It increases strength, hardness, and ductility.
  • However, excess carbon increases brittleness.

Advantages Of Cobalt-Chromium Alloy:

• Lighter in weight.
• Better mechanical properties.
• Corrosion resistance due to the passivating effect of chromium.
• Less expensive than gold alloys.

Disadvantages Of Cobalt-Chromium Alloy: 

• More techniques is sensitive.
• Complexity in the production of dental appliances.
• High fusion temperature.
• Greater casting shrinkage.
• Extremely hard thus cutting, grinding, and finishing is difficult so require special equipment for finishing.
• The high hardness can cause excessive wear of restorations and natural teeth contacting the restorations.