Ceramics or Porcelains are chemically intimate mixtures of metallic and non metallic elements, that allow ionic (K2O) and/or covalent bonding (SiO2) to occur.
The most common ceramics used in dentistry are alloys of 3 main metallic oxides (SiO2, Al2O3, K2O).
Composition of Dental Ceramics and Dental Porcelain:
1) Feldspar: 60 to 80%. this is a mixture of Sodium Aluminium Silicate. Na2O.Al2O3.6SiO2 (Albite) and potassium aluminum Silicate K2O.Al2O3.6SiO2.
Use: Reduces Firing (fusing) temperatures and increases the viscosity of the molten glass
These when melted (at 1250 – 1500 C) combines to form a glass with a free crystalline silica phase. Fusion temperature is lower if Na2O/K2O ratio is less.
2) Quartz (or) Silica: 15%to 25%. Used in Porcelain as a strengther. It remains unchanged during firing.Forms of Silica are Quartz, Cristobalite and tridymite. It is a a very hard, infusable and stable compound.
3) Kaolin (Al2O3.2SiO2.2H2O): 3 to 45%. This is a variety of Clayacts as flux and a binder, gives opacity to the mass. Increases the mouldability of the unfired porcelain.
Use: Acts as a Flux
4) Alumina: 8 to 20%. Only in Aluminous Porcelain. Due to Brittleness it can easily be powdered to very fine particles.
5) Opacifiers: Zirconium, tin or titanium oxides – small quantity
6) Metallic pigments: Less than 1%
7) Zinc Oxide(ZnO), Silicon Carbide(SiC): Used in Dental cements, and as Abrasive and Polishing agent.
Classification of Ceramics:
1) According to their Application:
- Artificial Teeth
- Jacket Crown and inlayss
- Enamel Veneer over cast metal crown
- Abrasive and Polishing agents
2) According to their maturing or Fusion Temperature:
- Low Fusing 871 to 1066 C
- Medium Fusing 1093 to 1260 C
- High Fusing 1288 to 1371 C
3) According to their types:
- Felspathic or Conventional porcelain
- Leucite – reinforced porcelain
- Aluminious Porcelain
- Glass – infiltrated alumina
- Glass Ceramic
4) According to their Substructure Method:
- Cast metal
- Swaged metal
- Glass Ceramic
- CAD-CAM porcelain
- Sintered Ceramic core
5) According to their Method of Firing:
- Air fired at atmospheric pressure
- Vacuum fired at reduced pressure