Study of the Bond between Gold Alloys and Porcelain

¹ J. F. Jelenko Co., New York, New York

Certain difficulties encountered in brittle-material bond testing have been surmounted for dental porcelain-to-dental gold. Cylindrical shear tests can be made with no porcelain or metal cohesive fracture, but rather with the separation occurring precisely at the interface. The shear strengths thus obtained appear to be the highest reported in the literature on ceramic-to-metal bonding.

The data indicate that noble metals bonded to porcelain have relatively poor bond strength. When certain trace metals are added to the noble metals, the bond strength may increase by a factor of 3. In our experiments the maximum strengths were obtained when the porcelain was also chemically modified with oxides of the same trace metals added to the gold alloy. The shear strength of the bond appeared to be somewhat independent of the normal stress component at the interface. Roughness, whether longitudinal or transverse to the shear direction, appeared to have no influence on the shear strength measurements.

From these results it was concluded that the dental porcelain-to-dental gold bond, if properly made, was primarily one of interatomic bonding. The total bond had a shear resistance of about 10,000-13,000 psi. About one-third of this resistance appeared to be due to the van der Waals forces of wetting, and two-thirds appeared to be due to chemical bonding, perhaps a mixture of ionic, covalent, and metallic bonding. A mechanical bond did not play an important role in the bond strength proper. Roughening per se did not add to the shear resistance at the bond.

One of the important factors in a good bond is the freedom of residual shear stresses at the bond. This is produced by close matching of the porcelain and the metal expansion coefficients. A study of the residual stresses and their control for porcelain-to-metal bonds in dental restorations in general appears warranted. The precise function of the metal roughening prior to porcelain baking also needs close study. In general, the porcelain-to-metal bond strengths are of high order. Only good wetting prior to baking of a suitably modified gold alloy with a dimensionally matched porcelain is necessary to achieve the rather high bond strengths that appear to be the natural adhesion tendencies of ceramics for metals.