Deposition of meso-porous γ-alumina coatings on ceramic honeycombs by sol-gel methods

The deposition of meso-porous γ-alumina coatings on multi-channeled cordierite honeycombs via sol–gel methods was investigated with the aim to correlate the deposition characteristics such as loading percentage, thickness and integrity of the coating to the support pore structure properties. Even though the mean pore size of the honeycomb supports was much higher than the size of the deposited particles, proper adjustment of sol viscosity prevented penetration of the sol into the support and led to the formation of a smooth coating of uniform, adjustable thickness. Sol viscosity was adjusted with the addition of poly-vinyl-alcohol (PVA) and with sol concentration by controlled evaporation, and fine-tuned in order to control loading percentage from 2 to 8 wt.% per impregnation, corresponding to coating thickness from 2 to approximately 10 μm respectively. The mean pore diameter of the support was found to affect the loading percentage. However, scanning electron microscopy observations have revealed that a very high loading percentage almost inevitably induces cracks on the coatings surface. The combination of sols and slurries of powders as coating media seems to be the optimum technical solution that can provide for satisfactory loading percentage per impregnation together with structural integrity of the coating.