Preparation and characterization of ceramic sol–gel composite coatings - densification temperature optimisation -

Thick (~ 25 µm) ceramic coatings on porous ceramic substrates were elaborated by means of dispersing alumina and rutile powders in a silica sol-gel solution. Resulting coatings, which present a composite structure consisting in Al2O3 and TiO2 grains embedded in an amorphous SiO2 matrix, demonstrate a good adhesion to the substrate and a real improvement of its surface by closing the porosity and also reducing the roughness. Mechanical characterization by micro-indentation showed an increase of the coating hardness when the thermal treatment temperature increases. This hardness increase is attributed to the densification of the coating that proceeds by grain rearrangement thanks to the sol-gel derived silica phase viscosity decrease during the thermal treatment. In order to decrease the coating densification temperature, the silica intergranular phase was modified by incorporating Na+ ions in its structure. This was done by adding NaCl salt in the sol-gel solution and leads to the decrease of the densification temperature