thesis
Glass-ceramic coatings for metals
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Abstract
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