SiC fiber reinforced SiC matrix (SiC/SiC) composites are one of the most promising
materials for high temperature structural applications such as power generation and propulsion
systems. SiC/SiC composites are, however, susceptible to accelerated attacks in water vapor
environments through oxidation and volatilization reaction. For protection from such attacks,
Environmental Barrier Coatings (EBCs) are indispensable.
We have investigated some oxides and rare-earth silicates as topcoat candidate materials for
EBCs. Topcoat materials must be stable in the high-water-vapor pressurized environments at high
temperatures. Also, it is important that the thermal expansion coefficient of topcoat materials is
similar to that of the SiC/SiC composites.
In this study, first, zirconium oxides, lutetium silicates and yttrium silicates were selected as
topcoat candidate materials. They were exposed in a water-containing atmosphere at a temperature of
1673 K for 100 h under a total pressure 0.96 MPa. Mass changes, structure of crystals and
microstructures were investigated after the exposure experiments in order to evaluate the thermal
stability of these materials. After their estimation, lutetium silicates were considered to be promising
for topcoat materials.
Then, lutetium silicates were coated as the topcoat of an EBC system on SiC/SiC composites, and
their fracture toughness and microstructures were investigated after exposure to an oxidizing
atmosphere. The evaluation results of the topcoat materials are reported in this paper.
Rear Earth Oxide Multilayer Deposited by Plasma Spray-Physical Vapor Deposition for Envisaged Application as Thermal/Environmental Barrier Coating