William A. Brantley Leon W. Laub
The formation of a strong bond between the opaque porcelain layer and the cast alloy is essential for the longevity of the metal-ceramic restoration. Extensive research over the past three decades has provided insight into the important factors for achieving metal-ceramic bonding. Early work18 established the importance of wetting the alloy surface by the porcelain at the firing temperature. Although similar measurements of contact angles have not been reported for current dental porcelains and casting alloys, good wetting is essential to minimize porosity at the metal-ceramic interface. A detailed relationship between the elevated-temperature contact angle and metal-ceramic bonding has not been established, but the research by O'Brien and Ryge18 indicates that perfect wetting (a contact angle of 0°) does not occur.
The model by Borom and Pask" considers idealized continuous lattice structure across the metal-ceramic interface for chemical bonding. This is achieved by incorporating certain oxidizable ele ments that become dissolved in the porcelain into the casting alloy composition. Recent research? has shown that the structure of the oxidized regions for high-palladium alloys is highly complex, and similar results would be anticipated for detailed studies of other types of oxidized casting alloys. The existence of multiple phases in the oxidized region of the alloy indicates that the proposed continuity" of atomic bonds cannot generally be achieved across the metal-ceramic interface, except possibly at sites where the glass matrix of the porcelain is in contact with the solid solution matrix of the alloy.
Manufacturers incorporate in the casting alloy composition small amounts of certain base metals that form oxides2223 and contribute chemical bonding to the metal-ceramic adherence. Studies using the electron microprobe and scanning electron microscope (SEM) have shown that these elements accumulate at the metal-ceramic interface and form an interfacial oxide layer. For noble metal alloys, elements having a major role for porcelain adherence are iron (high-gold alloys), tin and indium (lower gold content, palladium-silver, silver-palladium, and high-palladium alloys), and gallium (high-palladium alloys). For base metal alloys where the principal elements are nickel and cobalt, chromium oxidation provides chemical bonding for porcelain adherence, whereas titanium oxidation fulfills this role for the titanium casting alloys.
Figure 24-16, A, is an SEM photomicrograph of the interface for a high-palladium alloy bonded to dental porcelain. This alloy undergoes complex external and internal oxidation during the porcelain firing cycles. The internal oxide particles in the palladium solid solution grains are too small (less than 1 to 2 Am diameter) for accurate compositional determinations with x-ray energy-dispersive spectro-scopic analysis with the SEM. X-ray diffraction 20 has shown that CuGa2O3, SnO2, and Cu2O are present in the oxidation region when the alloy surface receives standard airborne particle abrasion with 50 um aluminum oxide before oxidation. Figure 24-16, B shows line scans obtained with the SEM for major elements in the metal and ceramic near the interface. Variations in the x-ray counts occurred when the line scan crossed a region of internal oxidation.
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