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. So da-lime glass slides were used as substrates. Coatings were obtained a t room temperature and atmospheric pressure by dip-coating into an aut omatically controlled glove box with withdrawal rates ranging from 8 t o 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 w ithdrawal rate was measured with a profilometer and the presence of cr acks was monitored by optical microscopy. The effect of relative humid ity on the thickness and transparency of the coatings was measured. Cr itical 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 obtai n 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 tes t between 400 and 1500 degrees C. (C) 1997 Elsevier Science B.V.