Metal Ceramic - Dental Ceramic-Metal
Posted by John Doe at Dental Assistant on January 1, 1970.
Categories: Dental Materials
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All-ceramic anterior restorations can appear very natural. Unfortunately, the ceramics used in these restorations are brittle and subject to fracture from high tensile stresses. Conversely, all-metal restorations are strong and tough but, from an esthetic viewpoint, acceptable only for posterior restorations. Fortunately the esthetic qualities of ceramic materials can be combined with the strength and toughness of metals to produce restorations that have both a natural toothlike appearance and very good mechanical properties. As a result they are more successful as posterior restorations than all-ceramic crowns. A cross section of a ceramic-metal anterior crown is shown in Fig. 19-1. The cast metal coping provides a substrate on which a ceramic coating is fused. The ceramics used for these restorations are porcelains, hence the common name, porcelain-fused-to-metal restorations. These ceramic-metal restorations are highly popular and are used for most of the crown and bridge restorations made today.
REQUIREMENTS FOR A CERAMIC-METAL SYSTEM
Fig. 19-1 Cross section of a ceramic-metal crown showing a sold alloy or base metal copins, the opaque body (dentin), and enamel ceramic layers.
- High fusing temperature of the alloy. The fusing temperature must be substantially higher (>100° C) than the firing temperature of the ceramic and solders used to join segments of a bridge.
- Low fusing temperature of the ceramic. The fusing temperature must be lower than ceramic used for all-ceramic restorations so no distortion of the coping takes place during fabrication.
- The ceramic must wet the alloy readily when applied as a slurry in order to prevent voids forming at the interface. In general, the contact angle should be 60 degrees or less.
- A good bond between the ceramic and metal is essential and is achieved by the interactions of the ceramic with metal oxides on the surface of metal (Fig. 19-2) and by the roughness of the metal coping.
- Compatible coefficients of thermal expansion of the ceramic and metal so the ceramic does not crack during fabrication. The system is designed so the value for the metal is slightly higher than for the ceramic, thus putting the ceramic in compression (where it is stronger) during cooling (Fig. 19-3).
- Adequate stiffness and strength of the alloy core. This requirement is especially important for fixed bridges and posterior crowns. High stiffness in the alloy reduces stresses in the ceramic by reducing deflection and strain. High strength is essential in the interproximal regions in fixed bridges.
- High sag resistance is essential. The alloy copings are relatively thin; no distortion should occur during firing of the ceramic or the fit of the restoration will be compromised.
- An accurate casting of the metal coping is required even with the higher fusing temperature of the alloy.
- Adequate design of the restoration is critical. The preparation should provide for adequate thickness of alloy (see 5 on p. 576) and also provide enough space for an adequate thickness of ceramic to yield an esthetic restoration. In some instances, a ceramic-metal restoration has an advantage over an all-ceramic restoration because less tooth structure needs to be removed to provide adequate bulk for the all-ceramic restoration. However, in cases of small, lower, anterior teeth, an all-ceramic restoration has an advantage with respect to esthetics, because with a ceramic-metal restoration it is difficult to remove enough tooth structure to provide space for the coping and the esthetic ceramic layer. The geometry of the shoulder should be flat with a rounded angle or a chamfer to allow enough bulk of ceramic and avoid fracture in this area. If full ceramic coverage is not used (e.g., a metal occlusal) the position of the ceramic-metal joint should be located as far as possible from areas of contact with opposing teeth.
Fig. 19-2 Electron micrograph of replicated oxidized surface of a Au-Pt-Pd alloy (x8000).
Fig. 19-3 Diagram of the ceramic-metal bond at the firing temperature and at room temperature when the thermal coefficient of expansion of the metal is 0.5 x 10-6/° C greater than the ceramic, thus placing the ceramic in compression at room temperature.
CERAMIC-METAL BONDING
The bond strength between the ceramic and metal is perhaps the most important requirement and thus will be given special attention. In general, the bond is a result of chemisorption by diffusion between the surface oxides on the alloy and in the ceramic. These oxides are formed during wetting of the alloy by the ceramic and firing of the ceramic. The most common mechanical failure of these restorations is ceramic de-bonding from the metal. Many factors control metal-ceramic adhesion: the formation of strong chemical bonding, mechanical interlocking between the two materials, and residual stresses. In addition, as noted earlier the ceramic must wet and fuse to the surface to form a uniform interface with no voids. These factors are also important for ceramic coatings on metallic implants.
An interface between a metal and a ceramic with many strong chemical bonds between them, with the bonds acting as tags that hold the two materials together, would obviously lead to strong bonding. However, methods producing a ceramic-metal interface with strong chemical bonding have not been developed. But the formation of oxides on the surface of the metal have been proven to contribute to the formation of strong bonding. Noble metals, which are resistant to oxidizing, must have other, more easily oxidized elements added, such as indium and tin, to form surface oxides. When these more easily oxidized elements are added, bonding is improved. The common practice of "degassing" or preoxidizing the metal coping before ceramic application creates surface oxides that improve bonding.
Base-metal alloys contain elements, such as nickel, chromium, and beryllium, which form oxides easily during degassing, and care must be taken to avoid too thick an oxide layer. Manufacturers' specify conditions to form the optimal oxide and often indicate the color of the oxide. Oxides rich in NiO tend to be dark gray, whereas those rich in Cr203 are greenish. These oxides dissolve in the ceramic during fixing and may discolor it or be visible through thin layers of ceramic near the gingival edge of the restoration.
The oxides are not completely dissolved during the fusion of the ceramic and thus the oxide-alloy interface can be the site of mechanical failure. This situation is especially true with some alloys that form layers rich in Cr203, which does not adhere well to the alloy. These alloys typically require the application of a bonding agent to the alloy surface to modify the type of oxide formed.
Alloys containing Be normally form well-adhering oxides. BeO is a slow-growing oxide that does not delaminate from the surface of the alloy. Rare earth elements such as yttrium can be added to the alloy to improve adherence by forming oxides, tying the alloy to the oxide layer.
From both theoretical and practical standpoints, the roughness, or more generally the topography, of a ceramic-metal interface plays a large part in adhesion. The ceramic penetrating into a rough metal surface can mechanically interlock with the metal, like Velcro, improving adhesion. The increased area associated with a rougher interface also provides more room for chemical bonds to form. However, rough surfaces can reduce adhesion if the ceramic does not penetrate into the surface and voids are present at the interface; this may happen with improperly fired porcelain or metals that are poorly wetted by the porcelain. Sandblasting is often used to remove excess oxide and to roughen the surface of the metal coping to improve the bonding of the ceramic.
High residual stresses between the metal and ceramic can lead to failure. If the metal and ceramic have different thermal expansion coefficients, the two materials will contract at different rates during cooling and strong residual stresses will form across the interface. If these stresses are strong enough the ceramic on the restoration will crack or separate from the metal. Even if the stresses are less strong and do not cause immediate failure, they can still weaken the bond. To avoid these problems the ceramics and alloys are formulated to have closely matched thermal expansion coefficients. Most porcelains have coefficients of thermal expansion between 13.0 and 14.0 x 10-6/° C, and metals between 13.5 and 14.5 x 10-6/° C. The difference of 0.5 x 10-6/° C in thermal expansion between the metal and ceramic causes the metal to contract slightly more than does the ceramic during cooling after firing. This condition puts the ceramic under slight residual compression, which makes it less sensitive to applied tensile forces.
Wetting is important to the formation of good ceramic-metal bonding. During firing, the ceramic must wet and flow over the metal surface. The contact angle between the ceramic and metal is a measure of the wetting and, to some extent, the quality of the bond that forms. The wetting of the alloy surface by the fused ceramic indicates an interaction between surface atoms in the metal with the ceramic. Low contact angles indicate good wetting. The contact angle of ceramic on a gold type of ceramic alloy is about 60 degrees. The surface of the noble alloys containing tin and indium after heating have these oxides present, and they diffuse into and interact with the ceramic, forming an adhesive bond. The oxide surface of an Au-Pt-Pd 98% noble alloy is shown at high magnification in Fig. 19-2.
EVALUATION OF CERAMIC-METAL BONDING
Many tests have been used to determine the bond strength between ceramics and metals; however, the ideal test currently does not exist. In addition, data obtained from different tests are often not comparable. One of the established bond-strength tests is the planar shear test. Other commonly used tests are the flexural tests. The flexural tests require layers of ceramic to be bonded to a strip or plate of metal. The coated metal plate is flexed in a controlled manner until the ceramic fractures off. In the 3-point flexure bend test, ceramic is fired to one side of a rectangular strip of metal. The metal-ceramic strip is supported by two knife edges, and the specimen is loaded in the center with the ceramic surface down until failure of the ceramic occurs.
An adequate bond occurs when the fracture stress is > 25 MPa; however, with many metal-ceramic systems values of 40 to 60 MPa are common. In a variant of this test, opaque and body ceramics are applied and fired to a thickness of approximately 1 mm on a 20-mm x 5-mm x 0.5-mm alloy sheets. The specimen is then bent over a 1-cm-diameter rod (with the ceramic on the outside) and then straightened. The surface is viewed under low magnification and the percent of the surface with retained ceramic is reported. Tests based on tensile and torsional loading schemes have also been used.
Fig. 19-4 Diagram showing three observed types of bond failure in ceramic-metal systems: A, metal-metal oxide, B, metal oxide-metal oxide; and C, ceramic-ceramic. Note. The dimensions of the layers are not to scale.
A ceramic-metal bond may fail in any of three possible locations (Fig. 19-4). Knowing the location of the fracture provides considerable information. The highest strength metal-ceramic specimens will fracture in the ceramic when tested (see Fig. 19-4, O; this is observed with some alloys that were properly prepared and had ceramic applied and fused. Testing these high-strength specimens using the push-through shear test shows the maximum strengths are approximately the same as the shear strength of the ceramic. Fractures through the oxide (see Fig. 19-4, B) and metal-metal oxide fracture (see Fig. 19-4, A) are commonly observed with poor bonding. Base-metal alloys commonly fracture through the oxide (Fig. 19-5) if an excessively thick oxide layer is present. Interfacial fracture is observed with metals that are resistant to forming surface oxides, such as pure gold or platinum.
Fig. 19-5 Bond failure through the metal oxide layer of a nickel-based ceramic-metal crown.
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