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The bimaterial strip, particularly in the form of a split ring, has been used since 1930 (Schurecht and Pole) to determine the thermal expansion compatibility between porcelain and metal substrates. An analysis by Timoshenko (1925) relates the expansion difference to the radius of curvature and stress in the composite. Tuccillo and Nielsen (1972) used that theory to predict the stress from the curvature in such strips. The change in the gap in the split ring is a sensitive measure of the change in curvature, but there are end-effects (Tesk et al., 1981) that introduce a small error when curvature is calculated from gap width. DeHoff and Anusavice (1985) suggest using a half-ring to avoid those effects, and provide an analytical solution for the stresses that agrees well with a finite element analysis. All of this work has been based on the assumption that the materials in the composite are elastic. In many cases, the authors were well aware of the fact that dental porcelains are viscoe-lastic, but used the elastic analyses to obtain results in terms of readily measured properties, and to establish useful "rules of thumb". The purpose of this note is to emphasize that the viscoelastic character of the porcelain or glass layer invalidates the elastic analysis, and can lead to erroneous predictions of stress. In particular, it is not valid to use the elastic equations to predict the stress from the curvature of the composite. The stress (o'j) from thermal expansion mismatch in an elastic body has the form X=-KAEf (1) where K is a constant involving the elastic properties of the materials and a geometric factor, and-Efn = Ef2-Ef 1 (2) where Efn is the free strain in material n. The free strain is usually approximated by fTr Efn-fT on dT (3) where oXn is the linear thermal expansion coefficient of material n, Tr is room temperature, and Ts is called the setting temperature. It is assumed that the glass is fluid above Ts, so that it sustains no stress, and is elastic below TS. This approximation replaces the gradual increase of viscosity in the glass transition region with an instantaneous freezing process. It is assumed that the setting temperature is a property of the glass, so that the same value of Ts can be used in any composite of the same materials. However, it has been shown (Rekhson and Mazurin, 1977) that Ts depends on the geometry of the composite. Consider a thin layer of glass on a thick metal substrate. The more massive metal will force the glass to flow even when its viscosity is high, so the setting temperature will be low. On the other hand, if the glass is much thicker than the metal, it will stop flowing at a relatively high temperature, because the metal can exert little force on it, and the setting temperature will be high. If Ts is measured on a composite with a thick glass member, and the corresponding value of ACf is used to predict the stress in a seal with a thin glass member, the stress will be overestimated. In fact, even the sign of the stress may be incorrectly predicted (Scherer and Rekh...

A method has been reported for evaluating adherence of an oxide to its substrate metal to a maximum value of about 40 MPa. Oxidized alloy plates were cemented between two aluminum cylinders with a high-strength cyanoacrylate cement and loaded in tension until failure occurred either at the oxide/metal interface, within the oxide layer, or in the cement itself. Significant differences were found among the oxide adherence values obtained from different PFM alloys. The oxides formed on five of the alloys exhibited adherence strengths in excess of the published value for cohesive strength of dental opaque porcelain, indicating that they possess sufficient adherence to act as the transition zone between the porcelain and the alloy. In addition, a correspondence was found between the quality of porcelain bond for a given alloy and its oxide adherence strength. These results remove the principal objection to the oxide-layer theory of porcelain bonding in dental alloy systems and emphasize the importance of oxide adherence in the establishment of a bond. It is therefore suggested that future work devoted to porcelain-metal bonding should seek to elucidate the mechanism of oxide adherence to PFM alloys and explore the development of new alloys which form adherent oxides.

The creep rates of six alloys for porcelain-fused-to-metal (PFM) restorations were determined as a function of flexural stress and temperature. Although two Pd-Cu alloys demonstrated excellent resistance to creep at low-stress, high-temperature conditions, they were especially susceptible to deformation at high stresses and temperatures near the glass transition temperature of dental porcelains. In comparison, a Ni-Cr alloy and a Pd-Co alloy demonstrated superior creep resistance at high-stress, low-temperature conditions. This indicates a relatively low potential for deformation due to stresses which may result from a thermal contraction differential between these two alloys and incompatible dental porcelains.

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