L14 Dental ceramics Flashcards
what are ceramics ?
nonmetallic , inorganic structures primarily containing compounds of oxygen with one of more metallic or semi metallic elements
what are porcelains ?
ceramics produced with a significant amount of kaolinite
advantages of ceramics :
-excellent appearance
-transparent
-insoluble in saliva but absorbs water
-biocompatible
-high compression strength and hardness
-insulating nature (low coefficient of thermal expansion )
-brittle
-low tensile and shear strength
disadvantages of ceramics :
-wear of opposing natural teeth
-heavy bite
- less shear and impact strength
-high shrinkage on firing
-high melting point
metal thickness of metal ceramic :
0.3-0.5mm
porcalin thickness of metal ceramic :
1.3-1.5mm
how is the metal bonded to outer-ceramic porcelain ?
metal oxide layer formation, chemical reaction between oxide layer and ceramic occurs . -bonding is dependent on the composition and the thickness of the oxide layer
conventional dental porcelain is based on
silica
what is the primary requirement for success of a metal ceramic ?
durable bond between metal and porcelain
fabrication of metal ceramic involves a 2 step process :
casting of metal and firing of veneering ceramic
compaction :
firing :
glazing :
compaction : porcelain powder is mixed with water and made into paste to form slurry
firing: porcelain is placed in furnace which causes it to shrink and dehydrates it –> makes porcelain set
glazing : final step to produce a smooth , shiny layer to prevent plaque accumulation and fuses to superficial ceramic layer
when is cooling done ?
after firing
what are the three mechanisms responsible for bonding porcelain to metal?
mechanical retention –by air abrasive or grinding
compression fit —vacuum compressing
chemical bonding – oxide forming elements are added to alloy
