Glass-ceramic coatings for metals
An investigation was conducted into the coating of metal substrates with a glass-ceramic enamel. Two metal types were coated, a 17% chrome-iron and a low carbon mild steel. The glass-ceramic was based on a complex lithium aluminosilicate glass. The enamel was applied using a vitreous enamelling coating technique, followed by conversion to a glass-ceramic. The coating process strongly influenced the microstructural form developed. For both metal substrate types it was possible to produce coatings which wet well and exhibit good adhesion. Crystalline substrate oxide is observed at the interfaces of these coatings. Abrasion prior to preoxidation is an essential requirement for good coating adhesion on a chrome-iron substrate. The interface region for a coating on chrome-iron exhibits little interaction or microstructural change. -However the coating on mild steel exhibits extensive interaction, with a reaction zone extending into the coating. The marked difference between the two coated substrate types can be explained by the different substrate oxide formed, solubility of the substrate oxide in the coating and nucleating ability of the substrate oxide surface. Addition of adherence oxides (NiO, CoO) to the coating on mild steel was examined. The adherence oxides participate in complex reactions which result in the formation of metallic alloys adjacent the interface. Both adherence oxides promote wetting under conditions where wetting is not possible if they are absent. Nickel oxide is detremental to coating adhesion. This may be related to its ability to cause a rapid dissolution of iron oxide present at the interface.