Metal Ceramic - Dental Ceramic-Metal
Posted by John Doe at Dental Assistant on February 4, 2012.
Categories: Dental Materials
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ALLOYS FOR CERAMIC-METAL RESTORATIONS
Chronologically the alloys developed for ceramic-metal restorations were Au-Pt-Pd, Ni-Ci, Co-Cr, Au-Pd-Ag, Pd-Ag, Au-Pd, Pd-Cu, and Ti.
COMPOSITION AND PROPERTIES OF NOBLE METAL ALLOYS
The composition ranges and color of five types of noble alloys for ceramic-metal restorations are listed in Table 19-2. The properties of these alloys are given in Table 19-3.
Au-Pt-Pd Types These alloys contain a very high noble metal content, mainly gold with platinum and palladium to increase the melting range. The high-noble content provides good corrosion resistance. Indium, tin, and iron are present and form oxides to produce a ceramic-metal bond. Rhenium is added as a grain refiner. Hardening of Au-Pt-Pd type alloys results from solid solution hardening and the formation of an FePtj precipitate. Optimum heat treatment for hardening is 30 minutes at 550° C, but practically the hardening occurs during firing of the ceramic. During the casting of these alloys some of these base elements are lost; it is therefore recommended that 50% new alloy be used with a sprue button if it is used to make a second casting. The new alloy will provide enough of the base elements so adequate oxides and hardening result.
From Table 19-3 it is seen that these alloys have high stiffness (elastic modulus), strength, and hardness and reasonable elongation; however, they have somewhat low sag resistance. The alloys are very costly because of their high noble-metal content and high density; they are sold on a weight basis but used on a volume basis. The casting temperature is reasonably high, and although reasonably easy to solder, care must be taken because the soldering temperature is only about 50 0 C below the melting temperature of the alloys. Finally, although considerable Pt and Pd are present, these alloys are still yellow, which makes producing pleasing esthetics with the ceramic easier than with white alloys.
| Type | Au (%) | Pt (%) | Pd (%) | Ag (%) | Cu (%) | Other (%) | Total Noble Metal Content (%) | Color |
|---|---|---|---|---|---|---|---|---|
| Au-Pt-Pd | 84-S6 | 4-10 | 5-7 | 0-2 | - | Fe, In, Re, Sn 2-5 | 96-98 | Yellow |
| Au-Pd | 45-52 | - | 38-45 | 0 | - | Ru, Re, In 8.5, Ga 1.5 | 89-90 | White |
| Au-Pd-Ag | 51-52 | - | 26-31 | 14-16 | - | Ru, Re, In 1.5, Sn 3-7 | 78-83 | White |
| Pd-Ag | - | - | 53-88 | 30-37 | - | Ru, In 1-5, Sn 4-8 | 49-62 | White |
| Pd-Cu | 0-2 | - | 74-79 | 10-15 | In, Ga 9 | 76-81 | White |
| Type | Ultimate Tensile Strength (MPa) | 0.2% Yield Strength (MPa) | Elastic Modulus (GPa) |
|---|---|---|---|
| Au-Pt-Pd | 480-500 | 400-420 | 81-96 |
| Au-Pd | 700-730 | 550-575 | 100-117 |
| Au-Pd-Ag | 650-680 | 475-525 | 100-113 |
| Pd-Ag | 550-730 | 400-525 | 95-117 |
| Pd-Cu | 690-1300 | 550-1100 | 94-97 |
| Type | Elongation (%) | Hardness (DPH, kg/mm2) | Density (g/sm3) | Casting Temperature (°C) |
|---|---|---|---|---|
| Au-Pt-Pd | 3-10 | 175-180 | 17.4-18.6 | 1150 |
| Au-Pd | 8-16 | 210-230 | 13.5-13.7 | 1320-1330 |
| Au-Pd-Ag | 8-18 | 210-230 | 13.6-13.8 | 1320-1350 |
| Pd-Ag | 10-14 | 185-235 | 10.7-11.1 | 1310-1350 |
| Pd-Cu | 8-15 | 350400 | 10.6-10.7 | 1170-1190 |
Au-Pd Types This high-noble type with good corrosion resistance has decreased gold but increased palladium content. These alloys contain no platinum or iron and thus are solution- rather than precipitation-hardened. They contain In for bonding, gallium to decrease the fusion temperature, rhenium for grain refining, and ruthenium for castability. Because of their high palladium content, the alloys are white (some call it gray) rather than yellow, even though they contain about 50% gold. This color causes increased difficulty in producing esthetic restorations. These alloys are stronger, stiffer, and harder than the Au-Pt-Pd type and have higher elongation (more ductile) and casting temperatures (easier to solder). They have lower densities, and when they were introduced were less costly than the Au-Pt-Pd type, because Pd was cheaper than Au. With Pd being more costly than Au, there is no longer a cost advantage. The decrease in density indicates more care should be taken during casting because of the decrease in the force with which the alloy enters the casting ring. However, these alloys are easy to cast, and soldering is easy because of the higher casting temperature.
Au-Pd-Ag Types These alloys contain less palladium than the Au-Pd type; the decrease is made up by adding silver. However, they still have good corrosion resistance. Again, In and Sn are added for bonding with the ceramics, ruthenium (Ru) for castability, and rhenium (Re) for grain refining. Hardening results from solution hardening. As seen in Table 19-3, the properties of the Au-Pd-Ag type are similar to those of the Au-Pd type.
Pd-Ag Types These alloys, which contain no gold and have a moderately high silver content, have the lowest noble-metal content of the five noble alloys. They contain In and Sn for bonding and Ru for castability. Their properties are similar to the Au-Pd-Ag type, except they are less dense (-11 g/cm3 vs. 14 g/cm3). They were developed at a time when the cost of Au was about $800/oz and the cost of Pd was low; those conditions no longer exist. Some ceramics used with these high-Ag alloys resulted in what was called "greening," really a color shift toward yellow. Contamination and technique was blamed to some extent for this problem.
Pd-Cu Types These alloys contain very high Pd content with 10% to 15% Cu. They contain In for bonding and Ga for controlling casting temperature. These alloys have high strength and hardness, moderate stiffness and elongation, and low density. However, they have low sag resistance and form dark oxides. They are white alloys, like all the other types except the yellow Au-Pt-Pd type.
COMPOSITION AND PROPERTIES OF BASE-METAL ALLOYS
The range of compositions of base metal alloys for ceramic-metal restorations are given for Ni-Cr, Co-Cr, and Ti type in Table 19-4, and typical properties of these alloys are listed in Table 19-5. Considerable variation in composition and properties are shown in these tables.
Ni-Cr Types Chromium provides tarnish and corrosion resistance, whereas alloys containing Al and Ti are strengthened by the formation of coherent precipitates of Ni3Al or Ti3Al. Mo is added to decrease the thermal coefficient of expansion, and Be to improve castability (by reducing the melting point) and hardening. Note because of the wide differences in atomic weight of Be, Ni, and Cr, 2 wt% is roughly equal to 6 at%. The use of Be may cause some toxicity problems and surface oxidation at high temperatures. These alloys are harder than noble alloys but usually have lower yield strengths. They also have higher elastic moduli, and it was hoped thinner copings and frameworks could result. They have much lower densities (7 to 8 g/cm3) and generally higher casting temperatures. Adequate casting compensation is at times a problem, as is the fit of the coping.
Co-Cr Types Again, Cr provides tarnish and corrosion resistance. Unlike Co-Cr partial-denture alloys, the alloys for ceramic-metal restorations are strengthened by solution hardening rather than carbide formation. Mo helps lower the coefficient of expansion and Ru improves castability. They are stronger and harder than noble and Ni-Cr alloys and have roughly the same densities and casting temperature as Ni-Cr alloys. Casting and soldering of these alloys is more difficult than noble alloys, as is obtaining a high degree of accuracy in the castings.
Ti Types Pure Ti and Ti-6Al-4V may become important for ceramic-metal restorations, but they present processing difficulties, as indicated by casting temperatures of 1760° to 1860° C and their ease of oxidation. However, newer techniques, such as machine duplication and spark erosion to fabricate copings, may increase the use of these metals. In summary, the noble alloys have good corrosion resistance, but only the Au-Pt-Pd alloys have a desirable yellow color. The other types are white (gray), which is more difficult to mask with the ceramic. The Au-Pd, Au-Pd-Ag, and Pd-Ag types have excellent mechanical properties coupled with high fusion temperatures and ease of casting and soldering. However, the Pd-Ag type has caused some problems with discoloration of the ceramic. The Pd-Cu types are characterized by the formation of dark oxides, which may cause problems with masking by the ceramic. The Ni-Cr and Co-Cr types are noted for high hardness and elastic modulus, although some Ni-Cr alloys have lower yield strengths. They are also noted for high casting temperatures. In general the Ti types have lower mechanical properties than the other base metal alloys but notably lower density and higher casting temperatures. Good bonding of ceramic and alloy can be achieved with all alloys, but bonding with some base metal alloys is more technique sensitive.
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