Sol–Gel Coating of Thin Display Glasses - Problems and Remedy

Evaporation cooling is an inevitable but often neglected concomitant effect of sol–gel dip coating and related techniques, which is responsible for occasional coating problems. The phenomenon, however, emerges mostly for thin substrates of less than 1 mm thickness as the heat capacity of the substrate - the main source of heat for the evaporation - is reduced. The cooling of the substrate results in a reduced evaporation rate and an increased humidity in the surroundings of the film, leading to demixing of liquid components and an uncontrolled hydrolysis. The resulting coatings exhibit a hazy and cloudy appearance with crater-like structures in the micron range and consequently, functional properties such as the electrical conductivity of ATO coatings (antimony-doped tin oxide, SnO₂:Sb) are unsatisfactory. Among a variety of measures to overcome this cooling problem, a moderate heating of the coating liquid to only 25_°C was found to be the method of choice to obtain high quality sol–gel thin films on substrates with a thickness down to 0.4 mm as required for display applications. This slight modification of the dip coating processing helps to reduce the susceptibility of the wet film to humidity and to improve the coating quality and the reproducibility in general - both on thin and thick substrates.