SAS 20 - DENTAL PORCELAIN

Dental Porcelain

Low-melting glass with embedded leucite crystals

leucite crystals

crystal structure of porcelain

1. Crystalline Materials
2. Fluxes

Composition of Dental Porcelain:

1. Silica
2. Feldspar
3. Alumina

Crystalline materials:

1. Sodium Carbonate
2. Lithium Carbonate

Fluxes:

Sodium Carbonate ( Na2Co3)

Helps reduce the fusion temperature of ceremic, facilitating easier processing

Lithium carbonate (Li2Co3)

Effective in improving thermal experation properties of ceremic thus reducing the risk of cracking during fining and cooling.

Crystalline materials and flux

Melted together ro form Dental Procelain

Leucite

A dispersed crystalline material

Matrix

Non-crystalline low-melting glass

RIG
1. Refiring the dental porcelain
2. Incorporating metal oxide pigments
3. Grounding the porcelain to powder

Dental laboratory porcelain powder is made by:

1. High Fusing 1288-1371°C
2. Medium Fusing 1093-1260°C
3. Low Fusing 871-1066°C

Classification of Porcelain according to fusing temperature:

1288-1371°C

Temperature of High fusing porcelain

1093-1260°C

Temperature of Medium fusing porcelain

871-1066°C

Temperature of Low fusing porcelain

1. Transverse Strength
2. Coefficient of Thermal Expansion
3. Color

Properties of Porcelain:

Transverse strength

The ability of a porcelain bar to resist fracture when loaded from above

Better resistance to fracture

What does high transverse strength mean?

56-446 MPa

Transverse strength of dental porcelains

12x10^-6/°C

LCTE of most common porcelain

0.5x10^-6/°C

When porcelain will be bonded to metal or other porcelain, the LCTE of the substructure should be properly matched with porcelain, which is:

Shade guide

Useful for selection of porcelain color

Mid-day or morning

Best time to do a shade guide

natural light

Requirement for shade guide

Oxide layer

Porcelain-Metal bonding is achieved through what?

ITFI
1. Inadequate oxide layer
2. Too thick oxide layer that is brittle
3. Failure within ceramic layer near metal-ceramic interface, through microscopic cracks
4. Incompatible LCTE of metal and porcelain

Causes of Failures of Porcelain-Metal Bond:

BETH
1. Biocompatible as it is chemically inert
2. Excellent esthetic
3. Thermal properties similar to enamel and dentin
4. High hardness and compressive strength

Advantages of Dental Porcelain:

BLLM
1. Brittleness
2. Low fracture toughnes
3. Low tensile strength
4. Make abrasion to antagonist natural dentitions and difficult to adjust and polish

Disadvantages of Dental Porcelain:

Feldspar

serves as a binding flux for quarts and clay