Dual-layer coatings for polyimide composite thermo-oxidative stability

Abstract

Dual-layer coatings consisting of layers of both aluminum oxide and silicon dioxide have been deposited by plasma-enhanced chemical vapor deposition onto polymide-based composites for thermo-oxidative protection. The silica was deposited from tetraethoxysilane, and the alumina was deposited from aluminum triisopropoxide. Deposition temperatures ranged between 573 and 598 K, and film thicknesses ranged from 100 to 200 nm. In all cases, alumina was deposited first, followed by the silica. Results indicate that such coatings improve the stability of these composites at elevated temperatures (to 673 K) for up to 300 h exposure to ambient air at 101.3 kPa (1 atm). Film systems with an alumina layer of intermediate thickness (150 nm) gave the greatest degree of resistance to thermo-oxidative mass loss (about half the mass loss rate of an uncoated control sample). Comparisons with earlier work with silica-only coatings showed the silica coating to be the best of all coatings studied, but there is considerable variation in the batch-to-batch performance of uncoated composite.