Surface finishing of enamels by sol-gel coating

Sol-gel coatings composed of SiO2, SiO2-TiO2, SiO2-ZrO2 and SiO2-TiO2-ZrO2 were prepared on different technical enamels as substrates. Thin (from 80–300 nm) and thick (up to 2 μm) films were deposited by the dip coating method. The chemical durability of the coated and uncoated enamels was tested against acidic and basic attack in accordance with DIN ISO standards. The experimental results show that the thin SiO2 coatings increase the chemical resistance of the coated enamels against oxalic acid attack by a factor of 4 to 22 compared to the uncoated ones. Especially efficient were SiO2 films of 1 μm thickness, based on MTEOS-TEOS solutions. SiO2 films do not protect the enamels against NaOH attack; however, an SiO2-ZrO2 coating on a non-resistant low-network former enamel significantly increases its stability against this basic attack. SNMS in-depth profiles show that in this case Na, Ca and Ba diffuse from the enamel substrate into the sol-gel layer, apparently stabilizing it.