Introduction
Dental ceramics are divided into four groups by chemical composition: feldspar, aluminous, glass-ceramics and zirconium dioxide. Most contain two phases. The glassy phase gives beauty and translucency, and it can be etched. The crystal phase gives strength, because cracks stop when they reach a crystal.
Feldspar porcelain
This contains 75 to 85% feldspar, with small amounts of quartz and kaolin. Modern types hold 20 to 30% leucite crystals. It has the best optical properties, but is brittle, with a fracture toughness of 0.9 MPa·m½.
Aluminous porcelain
- About 50% alumina (1965): twice as strong as feldspar porcelain, but with poorer optics.
- About 85% alumina (1985): flexural strength of 450 MPa and fracture toughness of 3 to 5 MPa·m½, for single-crown cores.
- 99.5% alumina: fully crystalline, with no transparency, so it needs a feldspar veneer.
Glass-ceramics
These are formed as glass, then heat-treated so that part of the structure crystallizes.
- Infiltrated glass-ceramics: about 55% crystals of leucite, alumina and zirconia. Flexural strength is about 200 MPa.
- Lithium disilicate: pressed or milled, then heat-treated until about 70% is needle-like crystals. Flexural strength is 350 to 400 MPa and toughness 3.5 to 4.5 MPa·m½. It is usually used as a single piece, can be etched with hydrofluoric acid, and is bonded with composite cement.