The effect of interfacial variables on metal-porcelain bonding.

Abstract

The strength of the metal-procelain bond determines the life of dental restorations, protective coatings and metal-ceramic composites. There is considerable controversy concerning the relative importance of strengthening by oxide films and by surface roughness. The objective of this thesis was to make a quantitative comparison of the effect of variables on shear strength and then to analyze the fracture path in the microstructure. A simple single phase alloy with 85% palladium, 10% copper, and 5% gallium was cast into pins 45 mm long and 3.2 mm in diameter. After various surface treatments, porcelain was fired onto the samples and they were shear tested. To explore the effects of oxides, Al$\sb2$O$\sb3$, CuO, Mn$\sb2$O$\sb3$, and SnO$\sb2$ coatings were applied by rf sputtering. Of these, the Al$\sb2$O$\sb3$ coated specimens gave 45% increase in bond strength over uncoated control samples. A "natural" oxide film developed by oxidation in air gave an improvement of 150%. However, surfaces which were roughened mechanically before porcelainizing showed improvements as high as 486%. To explain these improvements, the interface was examined by SEM and TEM, and the fracture path was investigated. These examinations disclosed that the principal effect of oxidation was the development of oxide cavities in the metal. These cavities were penetrated by the porcelain and provided mechanical interlocking. In no case was any vestige of the sputtered oxide film found at the interface. However oxidation during firing is important because samples porcelainized in a reducing atmosphere exhibited only 10% of the typical strength. Because the shear test galled the structure at the interface, a new test, the interface separation test (IST), was developed to spall off the porcelain layer from the interface for microexamination. These specimens showed clearly that the strength was proportional to the degree of mechanical interlocking. The results indicate that mechanical interlocking, whether produced by oxide cavities or mechanical working, is a major factor in bond strength, and that the role of oxide films is probably just an aid to provide wetting.