Dental Porcelain: Composition, Properties, and Bonding

Porcelain: Aesthetic Refractory Ceramic Material

Porcelain is an aesthetic refractory ceramic material constituted by a combination of a crystalline mineral phase and a glass phase.

Chemical Composition

It features a cored structure comprising a crystalline mineral phase and a glass (vitreous) phase.

Crystalline Mineral Phase Components:

• Feldspar (80%): Provides the glassy phase, crystalline phase matrix, and contributes to a low fusion temperature.
• Quartz Silica (15%): Enhances resistance and refractory properties.
• Alumina (1%): Further contributes to resistance and refractory characteristics.
• Kaolin Clay (3%): Adds strength, color, and acts as a linking agent.
• Metallic Pigment (1%): Used for color.

Glassy Phase Components:

• Silica (65%): Provides strength and translucency.
• Alumina (15%): Lowers the melting point.
• Borosilicate Glass (15-20%): Acts as a flux.

Metallic Pigments

Various metallic pigments are used to achieve specific colors:

• Titanium: Yellow-brown
• Uranium: Orange-yellow
• Iron: Brown
• Copper: Green
• Magnesium: Lavender
• Cobalt: Blue
• Tin: Increases opacity

Opacifiers

Opacifiers are used to reduce translucency:

• Tin oxide
• Zirconium oxide
• Titanium oxide
• Other white oxides

Manufacturing Process

1. Pyrochemical Reaction: Crystalline minerals, carbonate, and borax are subjected to a high-temperature pyrochemical reaction.
2. Glassy Phase Formation: This reaction forms a glassy phase.
3. Quenching: The glassy phase is rapidly cooled.
4. Grinding: The quenched material is ground into a fine powder.

Classification

Porcelain can be classified in two main ways:

By Use:

• Artificial porcelain teeth
• Jacket crowns and inlays
• Porcelain-to-metal restorations (on a metal structure)

By Fusion Temperature:

• High Fusion: 1288 to 1371 ºC
• Medium Fusion: 1090 to 1260 ºC
• Low Fusion: 870 to 1066 ºC

Commercial Forms

Porcelain is typically supplied in:

• Powder form
• Liquid form

Processing

1. Paste Preparation: Powder is mixed with water to form a paste.
2. Condensing: The paste is dried through vibration and the addition of more powder.
3. Firing: The material undergoes several stages:
   • Sintering
   • Bisque firing
   • Glazing
   • Controlled contractions
   • Pigmentation

Properties

• Fusion: Glass transition temperature typically around 600 ºC.
• Resistance:
  • Low tensile strength (prone to cracks; Griffith's theory applies, with elongation typically >0.1%).
  • Good compressive strength.
• Interfacial: Characterized by surface tension, wetting, and contact angle.
• Bonding to Metals: Essential for many dental applications.
• Optical Properties: Aesthetic and translucent qualities.
• Biological Effects: Inert to both soft and hard tissues, making it biocompatible.

Aluminous Porcelain

A specialized type of porcelain:

• Core: Contains 50% or more recrystallized alumina.
• Matrix: Has a similar composition to the core.

Uses

Porcelain is widely used in dentistry for:

• Artificial teeth
• Crowns (both full and metal-based)
• Simple inlays
• Veneers
• Fixed prostheses (especially porcelain-to-metal)

Porcelain-Metal Bonding Requirements

For successful bonding, specific conditions must be met:

• The thermal expansion coefficient of the porcelain must be similar to that of the alloy.
• The fusion point of the porcelain must be compatible with that of the alloy.
• The alloy must contain metals capable of producing stable metal oxides.

Mechanism of Metal-Porcelain Bonding

The bonding process involves several key steps:

1. Diffusion of metal elements from the alloy.
2. Oxidation of these metal elements on the alloy surface.
3. Dissolution of metal oxides into the porcelain.
4. Further diffusion of metal elements within the porcelain.
5. Interfacial reactions between the metal oxides and the porcelain matrix.