Yttrium Silicate Coatings for Oxidation Protection of Carbon–Silicon Carbide Composites

Yttrium silicate (Y₂SiO₅) coatings complement SiC coatings for protecting ceramic multilayer composite materials based on carbon-fiber-reinforced SiC composites (C-SiC). Thick (100 μm), dense Y₂SiO₅ coatings were prepared by dip coating, using concentrated aqueous slips. The resulting phases were studied by taking into account the simultaneous presence of oxide and non-oxide materials, which affected the chemical stability of the coatings. Thick, mechanically stable coatings were obtained by sintering in carbon crucibles and a SiC bed in an argon-flow furnace. Pure Y₂SiO₅ coatings completely separated from the SiC substrates. A high percentage of Y₂Si₂O₇ was necessary to fit the thermal expansion coefficients and ensure the stability of the coatings. Oxidation resistance of the coated substrates was investigated by isothermal and stepwise oxidation tests.