Thick sol-gel coatings based on the B2O3-SiO2 system

Sol-gel coatings composed of B2O3-SiO2 (10, 20 and 30 mol% B2O3) were prepared using boron trimethoxide (B[OCH3](3)), silicon tetraethoxide (TEOS) and methyltriethoxysilane (SiCH3[OCH2CH3](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 degrees C during 1 h in air. The thickness of the obtained coatings as a function of withdrawal rate was measured 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 mu m to 0.6 mu m depending on the B2O3 content. On the other hand, multilayer coatings were prepared to obtain crack-free and transparent samples with thickness greater than 1 mu m. As an interesting application, oxidation resistance of borosilicate coatings on SiC/C composite substrates was measured by a stepwise test between 400 and 1500 degrees C. (C) 1997 Elsevier Science B.V.