Study of Crystallization of Glass Coatings on Metal by Electron Microscopy

Controlled growth of crystals and immiscible phases in glass coatings on steel increased their resistance to mechanical and thermal shock. The electron microscope was used to follow crystal and immiscible phase development during heat treatment and to study the effect of crystals on fracture propagation in several complex glass compositions applied to metals by standard enameling techniques. Initial firing temperature, heating rates, heat treatment, and other enamel processes were found to affect the nature of the phases developed in the coatings. Crystals and immiscible phases caused fracture paths to be disrupted, resulting in nonconchoidal types of failure. A correlation is made between resistance to thermal shock and fracture properties.