Metal Ceramic - Dental Ceramic-Metal
Posted by John Doe at Dental Assistant on February 2, 2012.
Categories: Dental Materials
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CERAMICS FOR CERAMIC-METAL RESTORATIONS
The ceramics used for porcelain-fused-to-metal restorations must fulfill five requirements: (1) they must simulate the appearance of natural teeth, (2) they must fuse at relatively low temperatures, (3) they must have thermal expansion coefficients compatible with the metals used for ceramic-metal bonding, (4) they must withstand the oral environment, and (5) they must not unduly abrade opposing teeth. The ceramic is carefully formulated to achieve these requirements. These ceramics are composed of crystalline phases in an amorphous and glassy (vitreous) matrix. They comprise primarily Si02, Al203, Na20, and K20 (Table 19-1). Opacifiers (Ti02, Zr02, Sn02) and various heat-stable pigments are also added to the ceramic. Because of their composition, they can be considered a type of glass. To match the appearance of tooth structures, small amounts of fluorescing pigments such as rare earth oxides (Ce02) are added. The nature of ceramics, with their glassy matrix and crystalline phases, produces a translucency much like that of teeth, whereas pigments and opacifiers control the color and translucency of the restoration. The ceramic is supplied as a fine powder.
| Component | Opaque Powder (o/o) | Dentin (Body) Powder (%) |
|---|---|---|
| Si02 | 50-59 | 57-62 |
| A1203 | 9-15 | 11-16 |
| Na20 | 5-7 | 4-9 |
| K20 | 9-11 | 10-14 |
| Ti02 | 0-3 | 0-0.6 |
| Zn02 | 0-5 | 0.1-1.5 |
| Sn02 | 5-15 | 0-0.5 |
| Rb20 | 0-0.1 | 0-0.1 |
| Ce02 | - | 0-3 |
| Pigments | - | Trace |
In developing ceramics for ceramic-metal bonding, a major breakthrough was formulating products that had sufficiently high thermal expansion coefficients to match those of dental alloys. This higher expansion was made possible by the addition of potassium oxide and the formation of a high-expansion phase called leucite (KAlSijOg). This phase increased the thermal expansion of the porcelain so it could match that of dental alloys.
These materials have other qualities that make them well suited for ceramic-metal restorations. They fuse at lower temperatures than do many other ceramic materials, lessening the potential of distorting the metal coping. Sodium and potassium oxides in the glassy matrix are responsible for lowering the fusing temperatures to the range 930° to 980° C; low-fusing ceramics have hydroxyl groups and more Na20 to lower fusing temperatures to as low as 660°C. These ceramics do not corrode, and are also resistant to the fluids present in the oral environment. They can, however, be abrasive to opposing teeth because of their hardness; this becomes a significant problem if the porcelain surface is rough due to improper processing or becomes rough in the oral environment. Newer products have been shown to be less abrasive to natural teeth. These new ceramics are also strong in compression, which permits their use on the occlusal surfaces of the restorations. The ceramics used to bond to metals have tensile strengths of 35 MPa, compressive strengths of 860 MPa, shear strengths of 120 MPa, and flexural strengths of 60 MPa.
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