Atomic layer deposition onto fabrics of carbon and silicon carbide fibers: Preparation of multilayers comprising alumina, titania-furfuryl alcohol hybrid, and titanium phosphate

High temperature-resistant fabrics can be used as a reinforcement structure in ceramic matrix composites. They often need a coating for oxidation protection and mechanical decoupling from the matrix. Atomic layer deposition (ALD) provides very thin conformal coatings even deep down into complex or porous structures and thus might be a suitable technique for this purpose. Carbon fiber fabrics (size 300 mm × 80 mm) and SiC fiber fabrics (size 400 mm × 80 mm) were coated using ALD with a multilayer system: a first layer made of 320 cycles of alumina (Al₂O₃) deposition, a second layer made of 142 cycles of titania-furfuryl alcohol hybrid (TiO₂-FFA), and a third layer made of 360 cycles of titanium phosphate (Ti_xPO_y). Scanning electron microscopy reveals that the coatings are uniform and that the thickness of each layer is almost independent of the place in the reactor while coating. Appearance and thickness do not show any dependence on the type of fiber used as a substrate. Energy dispersive x-ray spectroscopy confirmed the expected elemental composition of each layer. Thermogravimetric analysis under oxidizing environment revealed that the first layer increases the onset temperature of fiber oxidation significantly, while the following two layers improve the oxidative protection only to a much smaller degree. Varying the geometry and size of the sample holder and especially the stacking of several fabric specimens on top of each other allowed increasing the total area of coated fabric up to 560 cm² per batch. It was demonstrated that four-layered fiber coatings could be obtained with high uniformity even on these much more complicated geometries.