Thick sol—gel coatings based on the B 2O 3SiO 2 system

Abstract

Sol—gel coatings composed of B 2O 3SiO 2 (10, 20 and 30 mol% B 2O 3) were prepared using boron trimethoxide (B[OCH 3] 3), silicon tetraethoxide (TEOS) and methyltriethoxysilane (SiCH 3[OCH 2CH 3] 3) as precursors. Soda-lime glass slides were used as substrates. Coatings were obtained at room temperature and atmospheric pressure by dip-coating into an automatically controlled glove box with withdrawal rates ranging from 8 to 33 cm min −1. Densification was carried out at 500°C during 1 h in air. The thickness of the obtained coatings as a function of withdrawal rate was measure with a profilometer and the presence of cracks was monitored by optical microscopy. The effect of relative humidity on the thickness and transparency of the coatings was measured. Critical thickness varied from 1 μm to 0.6 μm depending on the B 2O 3 content. On the other hand, multilayer coatings were prepared to obtain crack-free and transparent samples with thickness greater than 1 μm. As an interesting application, oxidation resistance of borosilicate coatings on SiC/C composite substrates was measured by a stepwise test between 400 and 1500°C.