DENSE OUTER LAYERS FORMED BY PLASMA TREATMENTS OF SILICA COATINGS PRODUCED BY THE SOL-GEL METHOD

Silica gel coatings prepared by the sol-gel method were subjected to l ow-temperature plasma treatments in air and argon. This was found to g ive rise to the formation of a dense outer layer, whose thickness incr eased with the duration of the treatment and decreased when the pressu re in the plasma chamber was reduced. The formation of a dense layer i n the coatings was confirmed from measurements of the overall thicknes s and the refractive index by ellipsometry, and also from TEM examinat ion and light transmission experiments. The coatings were found to con tain an uppermost layer of alumina, which was formed through secondary sputtering of aluminium from the electrodes in the plasma chamber. Th e concentration of alumina in the mixture with silica decreased throug h the thickness of the coating and became zero at a distance slightly smaller than the overall thickness of the dense layer. The thickness o f the dense layer was found to be slightly higher with argon than with air plasma treatments. Whereas the aluminium concentration through th e thickness of the coating was the same for the two types of treatment , a carbon residue was found only in the case of argon plasma treatmen ts. The com position of the underlying silica layers was found to corr espond to SiO1.6.