Overview of the Precipitates in a Ground-Coat Vitreous Enamel Using Analytical TEM

Abstract

Precipitates in the ground-coat vitreous enamel, which form when the dried coating is fired and cooled on sheet steel, are essential for the function of the enamelling. They lead to an interlocked metal-glass interface and are triggered by redox-driven processes in a glass layer saturated with iron. Using analytical transmission electron microscopy, it is shown that alloys of the Fe-Co-Ni-Cu system not only precipitate near the interface, where they contribute to adhesion through an interlocking interface, but also form nanocrystallites that are finely distributed in the enamelled glass to a depth determined by the diffusivity of Fe2+ (30 μm after four minutes of firing) and probably contribute to the toughening of this glass layer. The saturation with iron also prevents the segregation of the glass flux and opacifier calcium fluoride, which only occurs far from the interface in the chemically unmodified base glass. At the metal-glass interface, however, phosphorus traces diffuse from the steel and precipitates in the form of calcium phosphate needles that grow into the enamelled glass.