Addition of alkali silicate to grain size-controlled ceramic coatings for dense film

Densification of a ceramic coating based on SiO₂-Al ₂O₃-TiO₂ using wet processing was investigated by controlling microscopic grains of materials used. The grains of the materials used in coating slurry were reduced from their initial size of about 10 μm to less than 5μm in diameter using ball-milling. The ceramic composite films thus prepared were deposited about 100 μm in thickness on a steel plate by slurry coating with a doctor blade, followed by drying and calcination in air. It was found that the coating prepared was stable to heating up to 800°C in air, although the use of overly small grains easily caused cracks on the surface. By optimizing the amount of alkali silicate, however, such cracks could be eliminated from the coating, and the number of through-holes in the coating could be drastically decreased. The ceramic coating obtained by both the control of grain size in powders and the optimal addition of the alkali silicate showed excellent densification, thermal stability and adhesion strength.