Condensation And Firing In Porcealin

Condensation And Firing In Porcealin

Write a short note on the porcelain metal bonding mechanism.
Or
Write a short note on the bonding of porcelain to metals.
Or
Write a short note on metal-ceramic bond.
Or
Write a short note on porcelain-metal bonding.
Answer:

The primary requirement for the success of a metal-ceramic prosthesis is the development of a durable bond between the porcelain and the alloy.

Porcelain metal binding falls in three groups:

1. Chemical bonding across the porcelain metal interphase.
2. Mechanical interlocking between porcelain and metal.
3. Compressive (Thermal) bonding.

1. Chemical Bonding:
   • Currently regarded as the primary bonding mechanism.
   • The adherent oxide layer is essential for good bonding.
   • In base metal alloys, chromic oxide is responsible for a good bond.
   • Noble metal alloys do not have an oxide layer so they primarily depend on mechanical interlocking for bonding.
   • Here addition of a small amount of tin to noble metal alloys leads to the formation of oxides on their surface. This is done by electrodeposition.
   • In electrodeposition, a layer of pure gold is electrodeposited onto the cast metal surface.
   • This is quickly followed by a quick flashing deposition of tin over the gold. This tin helps in chemical bonding through the formation of tin oxide.
2. Mechanical Bonding:
   • In some systems, mechanical interlocking provides the principal bond.
   • The fused ceramic flows over the metal covering adapts to minute irregularities present on the metal surface and forms micromechanical bonds.
   • Sandblasting is often used to prepare the metal surface.
   • Irregularities on the coping surface can be produced by sandblasting.
   • The presence of the surface roughness on the metal oxide layer gives retention especially if undercuts are present.
   • The ability of the fused porcelain to intimately adapt to the metal surface is called wetting which is important for bonding.
3. Compressive (Thermal) bonding:
   • A critical requirement for the adhesion is thermal expansion compatibility between the ceramic and metal.
   • Ceramo-metallic systems are designed with a very small degree of mismatch in order to leave the porcelain in a state of compression.
     • α Porcelain — 13 to 14 × 10-6/°C
     • α Metal —13.5 to 14.5 × 10-6/°C
   • The difference of 0.5 × 10-6/°C causes the metal to contract slightly more than the porcelain on cooling from firing temperature.
   • This mismatch leaves porcelain in residual compression and makes it less sensitive to apply tensile forces which increases the bond strength.